Dissertations in Forestry and Natural Sciences

PUBLICATIONS OF 

THE UNIVERSITY OF EASTERN FINLAND 

Dissertations in Forestry and 

Natural Sciences 

TANELI VÄISÄNEN 

EFFECTS OF THERMALLY EXTRACTED WOOD DISTILLATES 

ON THE CHARACTERISTICS OF WOOD-PLASTIC COMPOSITES


Effects of Thermally 

Extracted Wood Distillates 

on the Characteristics of 

Wood-Plastic Composites 

Publications of the University of Eastern Finland 

Dissertations in Forestry and Natural Sciences 

Number 222 

Academic Dissertation 

To be presented by permission of the Faculty of Science and Forestry for public 

examination in the Auditorium SN201 in Snellmania Building at the University of 

Eastern Finland, Kuopio, on June, 10, 2016, at 12 o’clock noon. 

Department of Applied Physics

Grano Oy 

Jyväskylä, 2016 

Editors: Prof. Pertti Pasanen, 

Prof. Jukka Tuomela, Prof. Pekka Toivanen, Prof. Matti Vornanen 

Distribution: 

Eastern Finland University Library / Sales of publications 

P.O. Box 107, FI-80101 Joensuu, Finland 

tel. +358-50-3058396 

http://www.uef.fi/kirjasto 

ISBN: 978-952-61-2123-9 (printed) 

ISBN: 978-952-61-2124-6 (PDF) 

ISSNL: 1798-5668 

ISSN: 1798-5668 

ISSN: 1798-5676 (PDF)

Author’s address: 

University of Eastern Finland 

Department of Applied Physics 

P.O. Box 1627 

70211 KUOPIO 

FINLAND 

email: taneli.vaisanen@uef.fi 

Supervisors: 

Professor Reijo Lappalainen, Ph.D. 



P.O. Box 1627 

70211 KUOPIO 

FINLAND 

email: reijo.lappalainen@uef.fi 

Laura Tomppo, Ph.D. 



P.O. Box 1627 

70211 KUOPIO 

FINLAND 

email: laura.tomppo@uef.fi 

Reviewers: 

Professor Raimo Alén, Dr.Tech. 

University of Jyväskylä 

Department of Chemistry 

P.O. Box 35 

40014 JYVÄSKYLÄ 

FINLAND 

email: raimo.j.alen@jyu.fi 

Professor Rupert Wimmer, Ph.D. 

University of Natural Resources and Life Sciences, Vienna 

Sustainable Biomaterials Group Institute for Natural Materials Technology 

Konrad Lorenz Strasse 20 

3430 TULLN 

AUSTRIA 

email: rupert.wimmer@boku.ac.at 

Opponent: 

Professor Jyrki Vuorinen, Dr.Tech. 

Tampere University of Technology 

Department of Materials Science 

P.O. Box 589 

33101 TAMPERE 

FINLAND 

email: jyrki.vuorinen@tut.fi

ABSTRACT 

The use of raw materials derived from renewable sources is increasing 

due to the finiteness of crude oil reserves. In wood-plastic composites 

(WPCs), the plastic in a material is partially replaced by wood, which 

is an abundantly available and inexpensive raw material. WPCs are 

materials that encompass a wide range of performance levels such 

that they have diverse applications, e.g., in fencing and decking as 

well as in the manufacture of automobiles. The use of WPCs in indoor 

applications is also becoming increasingly popular. Despite the 

increasing popularity of WPCs, certain inherent limitations mean that 

these materials are unsuitable for some applications. Examples of the 

limitations associated with WPCs are their insufficient mechanical 

strength and their susceptibility to excess water absorption. 

Furthermore, the VOC (volatile organic compound) characteristics of 

WPCs have not been widely studied and therefore, a better 

understanding of these properties of WPCs would be of great 

importance. The properties of WPCs and their constituents can be 

altered by incorporating additives. However, some additives are 

rather expensive and their incorporation into WPCs is not 

straightforward. There is a clear need for novel, affordable and 

effective filler materials, especially those that would minimize the use 

of expensive constituents. 

Wood distillates are products originating from thermal processes 

where the components of wood are partly or completely decomposed 

into charcoal, condensable vapors, and non-condensable gases. 

Although the liquid components of wood have many potential 

applications, large volumes of liquids are still being discarded and not 

exploited in industrial applications. Thus, the incorporation of more 

of wood distillates into WPCs would enhance the use of raw materials 

and secondary products from the wood-processing industries. This 

would be both economically valuable and environmentally friendly 

since it would represent sustainable development by making 

commercial use of a potentially hazardous waste product. 

The main aim of this thesis was to investigate whether wood 

distillates could be used as WPC components. Another aim was to 

assess the possibility to improve the mechanical properties and water

esistance of the WPCs with wood distillates. Furthermore, the 

applicability of proton-transfer-reaction time-of-flight massspectrometry 

(PTR-TOF-MS) in determining the VOC emission 

characteristics of WPCs was studied. The effects of incorporating 

hardwood and softwood distillates into WPCs were examined by 

characterizing the mechanical properties, water resistance and VOC 

emissions of these WPCs modified with the distillates. The distillate 

content varied from 1 wt% to 20 wt%. The suitability of PTR-TOF-MS 

for analyzing VOC emissions from WPCs was assessed by measuring 

VOC emissions from a WPC deck during a 41-day trial and comparing 

VOC emission rates between seven different WPC decks. 

Both hardwood and softwood distillates exerted positive effects on 

the water resistance of the WPC; the addition of hardwood distillate 

decreased the water absorption of the WPC by over 25% whereas at 

least a 16% decrease was observed for the WPC with the softwood 

distillate. Moreover, a 1 wt% addition of hardwood distillate into the 

WPC led to a highly significant increase (11.5%, p < 0.01) in the tensile 

modulus as well as achieving minor enhancements in some other 

mechanical properties. Similarly, when 2 wt% of softwood was added 

to the WPC, a highly significant increase in the tensile strength (5.0%, 

p < 0.01) was observed. Even though the addition of the distillates 

increased the total release of VOCs, the emission rates of harmful 

compounds, such as benzene, remained low. Nonetheless, the results 

from the VOC analyses indicated that some of the compounds 

investigated in this thesis may be smelled from the WPCs because 

their odor thresholds were exceeded. 

Wood distillates displayed good potential as natural additives in 

WPCs as they improved the mechanical properties and water 

resistance. The results of this thesis provide a basis for the further 

development of wood distillates as bio-based additives in WPCs.

Universal Decimal Classification: 66.092, 662.712, 674.048, 674.816 

Library of Congress Subject Headings: Composite materials; Wood distillation; 

Pyrolysis; Hardwoods; Softwood; Volatile organic compounds; Wood—Chemistry; 

Wood—Mechanical properties 

Yleinen suomalainen asiasanasto: komposiitit; puu; muovi; kuivatislaus; fysikaaliset 

ominaisuudet; vetolujuus; kosteudenkestävyys; haihtuvat orgaaniset yhdisteet; 

puukemia; puuteknologia

Acknowledgements 

This thesis summarizes the studies performed in the Department of 

Applied Physics at the University of Eastern Finland during the years 

2014 and 2015. The studies were financially supported by European 

Regional Development Fund (ERDF, granted by the Finnish Funding 

Agency for Technology and Innovation, project 70049/2011), Centre 

for Economic Development, Transport and the Environment (North 

Savo, project S12261), the Academy of Finland (decision no. 252908) 

and Teollisuusneuvos Heikki Väänänen’s Fund. 

First, it is my pleasure to thank my supervisors for their support 

and guidance during the thesis project. I express my thanks to my first 

supervisor Prof. Reijo Lappalainen, Ph.D., for his trust, advice and 

encouragement during this process. I owe my deepest gratitude to 

Laura Tomppo, Ph.D., for her friendship, patience and prompt 

assistance whenever it was needed. I also want to thank all my coauthors 

for their contributions, especially Jorma Heikkinen for his 

expertise in the preparation of the composite granules, and Pasi Yli- 

Pirilä, M.Sc., for the advice in PTR-TOF-MS analyses. 

I am very grateful to the pre-examiners of this thesis, Prof. Raimo 

Alén, Dr.Tech., and Prof. Rupert Wimmer, Ph.D., for their comments 

and constructive feedback. Furthermore, I am grateful to Prof. Jyrki 

Vuorinen for accepting the role of my opponent at the defense of this 

thesis and thus being part in one of the most important events of my 

academic career. I also want to thank Ewen MacDonald, Pharm.D., for 

his linguistic advice. 

I express my special thanks to the co-workers in Panos building 

and in the new premises. The good humor, inspiration, and support 

really helped me to do my best and gave me extra motivation. The 

support from my friends is also highly acknowledged. 

I am thankful to my family, Raija, Matti, Jouni, Arja, Emma, Lauri, 

and Juuso, for the support throughout my life. Thank you for 

believing in me and for encouraging me to always follow my heart.

Finally, I am grateful to my wife Anne. Your support, love, humor, 

and care always make my day and give me strength to carry on. 

Kuopio, June 2016 

Taneli Väisänen

LIST OF ABBREVIATIONS 

ASTM 

CIS 

DMTA 

EBS 

EDS 

FTIR 

GC/MS 

HDPE 

HWD 

ISO 

L/D 

LDPE 

LG 

MAPE 

MAPP 

MFA 

MOE 

PAH 

PE 

PEEK 

PLA 

PP 

ppb 

ppmv 

PS 

PTFE 

PTR-MS 

PTR-TOF-MS 

PVC 

SEM 

SWD 

RH 

RMT 

American Society for Testing and Materials 

Charpy’s impact strength 

Dynamic mechanical thermal analysis 

Ethylene-bis-stearamide 

Energy-dispersive x-ray spectroscopy 

Fourier transform infrared spectroscopy 

Gas chromatography-mass spectrometry 

High-density polyethylene 

Hardwood distillate 

International Organization for Standardization 

Barrel-length-to-diameter 

Low-density polyethylene 

LunaGrain 

Maleated polyethylene 

Maleated polypropylene 

Microfibril angle 

Modulus of elasticity 

Polycyclic aromatic hydrocarbon 

Polyethylene 

Polyether ether ketone 

Polylactide 

Polypropylene 

Parts per billion 

Parts per million by volume 

Polystyrene 

Polytetrafluoro-ethylene 

Proton-transfer-reaction mass-spectrometry 

Proton-transfer-reaction time-of-flight massspectrometry 

Polyvinyl chloride 

Scanning electron microscopy 

Softwood distillate 

Relative humidity 

Reinforced matrix theory

TD-GC-FID/MS Thermal desorption/gas chromatography with 

flame ionization detector and mass spectrometry 

TOF 

Time-of-flight 

UF 

UPM ForMi 

VOC 

Volatile organic compound 

WPC 

Wood-plastic composite 

XPS 

X-ray photoelectron spectroscopy

LIST OF SYMBOLS 

A 

Cross-sectional area 

Asample 

Area of a sample 

b 

Width 

B 

Bending 

c 

Water absorption 

Creal room Real room air concentration of a VOC 

Cvoc 

Concentration of a VOC 

ds 

Thickness 

d 

Deflection 

E 

Modulus of elasticity 

Evoc 

Emission rate of a VOC 

 

Strain 

Fvoc 

Flow rate 

F 

Load/force 

FS 

Flexural strength 

h 

Height 

I, I 

Cellulose polymorphs 

k[r] 

Rate coefficient 

L0 

Initial gauge length 

L 

Final length of the gauge 

Ls 

Length of span 

Lp 

Product loading factor 

Mvoc 

Molar mass of a VOC 

m1 

Mass of a dried specimen 

m2 

Mass of a specimen after water immersion 

n 

Air exchange rate 

S1, S2, S3 Layers of secondary wall 

S 

Maximum surface stress 

 

Stress 

T 

Temperature 

TM 

Tensile modulus 

TS 

Tensile strength 

t 

Reaction time

LIST OF ORIGINAL PUBLICATIONS 

This thesis is based on data presented in the following articles, 

referred to by the Roman numerals I–IV. 

I 

Väisänen T, Tomppo L, Selenius M and Lappalainen R. Effects of 

slow pyrolysis derived birch distillate on the properties of woodplastic 

composites. Eur J Wood Prod. 74 (1), pp. 131-133, 2016. 

II Väisänen T, Laitinen K, Yli-Pirilä P, Tomppo L, Joutsensaari J, 

Raatikainen O and Lappalainen R. Rapid technique for 

monitoring VOC emissions from wood-plastic composites. 

Submitted for publication. 

III Väisänen T, Heikkinen J, Tomppo L and Lappalainen R. 

Improving the properties of wood-plastic composite through 

addition of hardwood pyrolysis liquid. J Thermoplast Compos 

Mater. DOI: 10.1177/0892705716632862. 2016. In press. 

IV Väisänen T, Heikkinen J, Tomppo L and Lappalainen R. 

Softwood distillate as a bio-based additive in wood-plastic 

composites. J Wood Chem Tech. 36 (4), pp. 278-287, 2016. 

The original articles have been reproduced with permission of the 

copyright holders.

AUTHOR’S CONTRIBUTION 

This dissertation is based on four publications that examined the 

treatment and testing of WPCs modified with two types of distillates, 

and the characterization of VOCs from various types of WPCs. Papers 

I, III, and IV were concerned with the characterization of WPCs 

treated with hardwood and softwood distillates. Paper II focused on 

the evaluation of the applicability of PTR-TOF-MS for monitoring 

VOC emissions from WPCs. 

The original idea for the utilization of wood distillates in WPCs 

was presented by Prof. Reijo Lappalainen, who also treated the 

granules with the hardwood distillate in paper I. The author was 

mainly responsible for the sample preparation. Moreover, the sample 

characterization and result analyses were conducted by the author. 

The author was also the main writer for the paper, with contributions 

from other authors. 

In paper II, the comparative measurements of seven different WPC 

decks were conducted by the author with the kind help from Pasi Yli- 

Pirilä, M.Sc. Kimmo Laitinen, M.Sc., conducted the measurements for 

the 41-day trial and wrote a part of the materials and methods -section 

for the paper. The samples for the study were acquired by Dr. Laura 

Tomppo and Prof. Reijo Lappalainen. The author analyzed the results 

and wrote the majority of the research paper. Pasi Yli-Pirilä, M.Sc., Dr. 

Laura Tomppo, Doc. Jorma Joutsensaari, Doc. Olavi Raatikainen, and 

Prof. Reijo Lappalainen gave constructive comments and suggestions 

for the paper. 

The ideas for papers III and IV were devised by the author and Dr. 

Laura Tomppo. Jorma Heikkinen processed the distillates and 

impregnated the granules. The author was mainly responsible for the 

preparation of the samples. In addition, the sample characterization 

and sample analyses were undertaken by the author. The papers were 

written by the author with contributions from Dr. Laura Tomppo and 

Prof. Reijo Lappalainen.

Contents 

1 Introduction .................................................................................. 19 

2 Wood-plastic composites ........................................................... 23 

2.1 Raw materials ....................................................................... 24 

2.1.1 Wood ............................................................................ 25 

2.1.2 Polymers ....................................................................... 28 

2.1.3 Additives ...................................................................... 31 

2.2 Properties .............................................................................. 33 

2.2.1 Mechanical properties .................................................. 34 

2.2.2 Water absorption .......................................................... 38 

2.2.3 VOC emissions ............................................................. 40 

2.3 Manufacturing technologies ............................................... 42 

2.3.1 Extrusion ...................................................................... 42 

2.3.2 Injection molding ......................................................... 44 

2.3.3 Compression molding ................................................... 45 

2.3.4 Choosing appropriate manufacturing method ............. 46 

3 Characterization of wood-plastic composites ........................ 49 

3.1 Mechanical properties ......................................................... 49 

3.1.1 Tensile strength ............................................................ 50 

3.1.2 Flexural strength and modulus .................................... 52 

3.1.3 Impact strength ............................................................ 53 

3.2 Water absorption .................................................................. 55 

3.3 VOC emissions ..................................................................... 56 

3.3.1 TD-GC-FID/MS .......................................................... 56 

3.3.2 PTR-MS ....................................................................... 57 

4 Thermal processing of wood ..................................................... 61 

4.1 ThermoWood ® process ........................................................ 63 

4.2 Slow pyrolysis of wood ....................................................... 65 

4.3 Products obtained from the processes .............................. 67 

4.3.1 Charcoal........................................................................ 67 

Dissertations in Forestry and Natural Sciences No 222 17

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on 

the Characteristics of Wood-Plastic Composites 

4.3.2 Condensable vapors ...................................................... 68 

4.3.3 Non-condensable gases ................................................. 70 

5 Aims and significance ................................................................ 71 

6 Materials and methods ............................................................... 73 

6.1 Sample preparation .............................................................. 73 

6.1.1 Distillates ..................................................................... 74 

6.1.2 Impregnation of WPC granules ................................... 76 

6.1.3 Injection molding ......................................................... 77 


6.2.1 Tensile and flexural properties ..................................... 79 

6.2.2 Charpy’s impact strength ............................................. 80 


6.4 VOC emissions ..................................................................... 81 

6.5 Statistical analyses ................................................................ 85 

7 Results ........................................................................................... 87 



7.3 VOC emissions ..................................................................... 91 

8 Discussion .................................................................................... 97 

8.1 Impregnation of WPC granules with wood distillates ... 97 


8.3 Water absorption ................................................................ 101 

8.4 VOC emissions ................................................................... 103 

8.5 Limitations and future prospects ..................................... 109 

9 Summary and conclusions ...................................................... 113 

Bibliography ................................................................................. 115 

18 Dissertations in Forestry and Natural Sciences No 222

1 Introduction 

The finiteness of crude oil reserves is globally recognized, and 

therefore, new raw material alternatives are being sought from 

renewable sources (Najafi et al. 2010). Wood is an inexpensive 

and abundantly available material that possesses suitable 

characteristics for multiple applications, such as in the 

construction industry (Clemons 2008). On the other hand, the 

combination of wood with the commodity plastics, adhesives, 

and other substances provide unique properties that cannot be 

achieved with either wood or plastic products on their own. 

Thus, wood-plastic composites (WPCs) are ecological, durable, 

and recyclable materials (Kim and Pal 2010). In WPCs, the wood 

fibers are surrounded by a continuous polymer matrix, and the 

compatibility between these two constituents is typically 

enhanced by incorporating coupling agents and other additives 

into the composite. WPCs can be created with a wide range of 

performance levels, and therefore, they have many applications 

not only in decking, and fencing, but also in more sophisticated 

manufacturing, e.g., in the car-making industry (Klyosov 2007, 

Faruk et al. 2012). 

Even though the use of WPCs is becoming more and more 

common, at present, these materials cannot be used in 

applications where high mechanical strength is required. This is 

mainly due to the weak bonding between the hydrophilic wood 

fibers and the hydrophobic polymer matrix (Gao et al. 2008, 

Yuan et al. 2008, Butylina et al. 2011). Moreover, wood fibers 

contain a large amount of hydroxyl groups that can form 

hydrogen bonds with water molecules. Hence, WPCs are 

susceptible to water absorption that induces thickness swelling 

and the creation of microcracks in the material (Li et al. 2014), 

which increases the risk of mold growth. 

Many different approaches have been examined to eliminate, 

or at least reduce, these limitations in the present generation of 




WPCs. There are several ways to modify wood fibers, e.g., heat 

treatment (Ayrilmis et al. 2011), the extraction of hemicelluloses 

(Hosseinaei et al. 2012) and the treatment with coupling agents 

(Müller et al. 2012); these modifications can considerably 

increase the water resistance of the WPCs. On the other hand, in 

some instances, the mechanical properties of WPCs can be 

enhanced by using recycled polymers instead of virgin material 

(Adhikary et al. 2008a). Moreover, the mechanical durability of 

WPCs can be improved by incorporating additives, such as 

maleated polypropylene or polyethylene (MAPP or MAPE) 

(Pérez et al. 2012, Ndiaye et al. 2013), waste charcoal (Li et al. 

2014, Das et al. 2015a), nanoclay (Abdolvahaba et al. 2014), or 

inorganic fillers (Gwon et al. 2012), into the composite. 

WPCs are also increasingly used in indoor applications, such 

as window frames and furniture. However, the impact of WPCs 

on the quality of the indoor air has not been studied widely. 

Volatile organic compounds (VOCs) are chemicals that have a 

high vapor pressure at room temperature, allowing a great 

number of molecules to evaporate from the material and enter 

the surrounding air. VOCs include chemical compounds that 

occur in nature, and compounds that originate from human 

activity. Some VOCs exert harmful effects on human health and 

the environment, and therefore, their release and maximum 

concentrations in indoor air are regulated. 

Several studies have examined the effects of organic wastes 

and residues on WPCs (Ashori and Nourbakhsh 2010, Hamzeh 

et al. 2011, Madhoushi et al. 2014, Das et al. 2015b). When 

organic waste is added to WPCs, typically there is an 

emphasized need for coupling agents to improve the bonding 

between the fibers and the polymer matrix. The choice, 

acquisition and application of the right coupling agent for 

WPCs is neither straightforward nor inexpensive, and the 

whole process requires time. Therefore, there is a desire to 

minimize the use of coupling agents and other additives with 

similar challenges, especially if they can be substituted with 

more affordable bio-based filler materials that can confer similar 

benefits. 


Introduction 

A new and environmentally friendly approach to improve 

the properties of WPCs is to add the thermal degradation 

products of wood into the composites (Das et al. 2015b). Wood 

can be converted into charcoal, liquids, and non-condensable 

gases in pyrolytic processes (Klass 1998). The yields of these 

products vary depending on the process type (Nachenius et al. 

2013). Since it is the primary product which is sought, the 

secondary products of the processes are commonly considered 

simply as waste. The potential of biochar as an additive in WPCs 

has been investigated previously, and the positive effects of 

incorporation of biochar into WPCs were evident (Li et al. 2014, 

Das et al. 2015a). Even though the liquid components of wood 

have multiple applications, e.g., as biocides, pesticides, material 

coating, and medicine (Bridgwater 1996, Fagernäs et al. 2012a), 

their effects on the characteristics of WPCs have not been 

studied earlier. Nonetheless, since wood distillates have a rather 

versatile nature, one could speculate that they could be utilized 

in WPCs as ecological additives, coupling agents, lubricants, 

biocides or stabilizers after proper processing. 

The present thesis project investigated the effects of liquid 

components of wood on the properties of WPCs. It was 

hypothesized that the utilization of liquid components of wood 

in WPCs could provide many advantages. First, the content of 

rather expensive and petroleum-derived polymers in WPCs 

could be reduced. Second, the raw materials would be exploited 

more effectively as these liquids would otherwise be treated as 

waste and therefore, not be further utilized. The focus of this 

thesis was also on the determination of the VOC characteristics 

of WPCs. Proton-transfer-reaction time-of-flight massspectrometry 

(PTR-TOF-MS) was used to analyze the levels of 

VOCs emerging from WPCs. The effects of hardwood and 

softwood distillates were evaluated in mechanical tests, water 

absorption studies, and VOC analyses. The working hypothesis 

was that WPC granules could be effectively impregnated with 

hardwood or softwood distillates to increase the water 

resistance of WPCs and to strengthen the materials. The VOC 




emission rates were expected to increase due to incorporation 

of these types of distillates. 


2 Wood-plastic composites 

By definition, composite materials are formed by combining 

two or more constituent materials that have substantially 

different chemical or physical properties (Callister 2005). As a 

result, the individual components remain distinct within the 

finished material, and thus composites can possess properties 

that cannot be achieved with the individual constituent 

materials. Composites can be classified into particle-reinforced, 

fiber-reinforced, and structural composites. There are many 

well known composite materials, e.g., metal and ceramic 

composites, cements, concrete, and reinforced plastics. 

In materials science, a fiber is commonly defined as a 

substance that has been drawn into a long and thin filament, i.e., 

the aspect ratio, which is defined as the ratio of fiber length to 

diameter, is at least 100 (Callister 2005). However, the term fiber 

may also refer to the spindle-shaped cells within the wood 

material (Clemons 2008), and in the case of natural fiberpolymer 

composites, fiber can be defined as an object with an 

aspect ratio greater than one (e.g. Stokke et al. 2014). Thus, from 

the viewpoint of materials science, some WPCs can be classified 

as particle-reinforced composites although they are commonly 

referred to as fiber-reinforced composites. In this thesis, the 

definition of a fiber is adopted from the terminology used for 

natural fiber-polymer composites. 

WPCs are fiber-reinforced composites produced by mixing 

wood components and molten thermoplastics. In a WPC, a 

polymer forms a continuous matrix that surrounds the 

reinforcing wood components. The low price and high stiffness 

of wood makes it an attractive reinforcement for the commodity 

plastics. Since the processability of WPCs is similar to the 

plastic, there are several appropriate manufacturing 

technologies available for WPCs. Although the majority of WPC 

products are extruded, injection and compression molding are 




other major technologies used in WPC production. (Godavarti 

2005) 

2.1 RAW MATERIALS 

The properties of WPCs are mainly determined by the 

characteristics of their two main constituents. Even though both 

are polymer-based materials, wood and plastic exhibit 

distinctive properties and have different origins (Clemons 2008). 

Matrix polymers are high-molecular-mass materials created by 

the polymerization of small repeating subunits, monomers. 

Polymers can be either natural or synthetic and furthermore, 

virgin material or recycled based on their origin (Adhikary et al. 

2008a, Adhikary et al. 2008b). Several polymers are used as the 

matrix material in WPCs, e.g., polyethene (PE), polypropene 

(PP), polyvinyl chloride (PVC), polystyrene (PS), and 

polylactide (PLA). Due to the high molecular mass relative to 

the small molecule compounds, polymers possess unique 

physical properties, such as viscoelasticity and toughness. 

Wood is a natural composite consisting primarily of three 

polymeric components: cellulose, hemicelluloses and lignin 

(Pettersen 1984). Cellulose constitutes 40–45%, hemicelluloses 

25–35%, and lignin makes up much of the remaining 20–30% of 

wood. Wood is an attractive material to be incorporated in 

polymer composites not only because it is abundant but also 

due to its light weight in relation to its mechanical properties. 

In WPCs, the wood components are surrounded by the 

continuous polymer matrix. In general, the development of high 

quality WPCs is limited by two physical factors (Godavarti 

2005): the difference between the surface energy of the polymer 

matrix and wood components, and the upper temperature at 

which wood can be processed. There are several ways to offset 

or minimize these limitations and to improve the general 

performance of the WPC. The most common approach involves 

the incorporation of different types of additives. Examples of 


Wood-plastic composites 

additives used in WPCs are coupling agents, lubricants, 

stabilizers, inorganic fillers, biocides, and flame retardants. 

2.1.1 Wood 

Wood has unique and useful properties – it is a recyclable, 

biodegradable, renewable, bendable, and relatively stable 

material. In addition, wood has an important role in carbon 

sequestration; growing trees take up and store considerable 

amounts of atmospheric carbon dioxide (CO2) (Hill 2006a). 

The reinforced matrix theory (RMT) is a concept which can 

help to understand the cell wall structure of wood fibers, and 

ultimately the properties of wood. In short, the RMT describes 

the cell wall structure as follows: the cell wall of a plant consists 

of the thermoplastic matrix (lignin) reinforced by the high 

tensile strength fibers (cellulose) and the hygroscopic material 

(hemicellulose). (Stokke et al. 2014) 

Wood can be anatomically divided into two classes 

(Wiedenhoeft 2010, Wiedenhoeft 2012): softwoods 

(gymnosperms) and hardwoods (angiosperms). When 

examined in the microscope, wood can be observed to be a 

composite of many cell types. It is a complex biological structure 

whose parts act together to fulfill the needs of a living plant: to 

conduct water from the roots to the leaves, to provide 

mechanical support for the plant’s body, and to store and 

synthesize essential biochemicals. Both softwoods and 

hardwoods consist mainly of tracheids – these are elongated 

and hollow cells arranged in parallel to each other along the 

trunk of the tree. In general, softwoods have a simpler structure 

than hardwoods because softwoods have only two cell types 

and less variation in the structure within the cell types 

(Pettersen 1984, Godavarti 2005, Wiedenhoeft 2012). The most 

distinctive difference in the structure between hardwood and 

softwood is the presence of vessel elements in hardwoods; these 

elements are absent in softwoods. Generally, softwoods have 

longer (3–8 mm) wood fibers than hardwoods (0.2–1.2 mm), but 

the length of wood fibers varies between wood species 

(Wiedenhoeft 2010, Clemons et al. 2013). 




The layered structure of wood fibers also explains the unique 

properties of wood. As shown in Figure 1, the cell wall of a 

wood fiber consists of two main parts: the primary and 

secondary wall. The secondary wall consists of three separate 

layers designated as S1, S2, and S3. 

Figure 1. The layered structure of the cell wall of wood. The lines in the primary and 

secondary cell wall layers describe the orientation of microfibrils. 

The middle lamella is a lignin-rich region that binds the fibers 

together. The primary cell wall is made up of a loose and thin 

(0.1 µm) network of randomly oriented cellulose microfibrils. It 

also consists of hemicelluloses, proteins, and pectin. The first 

layer of the secondary cell wall, S1, is approximately 0.2 µm 

thick with a relatively high microfibril angle (MFA). S2 is the 

thickest layer of the cell wall (up to 20 µm thick), and it 

primarily defines the mechanical properties of the fiber. S2 

consists mainly of cellulose and hemicelluloses. S3 is a thin layer 

(0.1 µm) of cellulose microfibrils. (Pettersen 1984, Stokke et al. 

2014) 

The chemical composition of wood also varies from species 

to species. In general, dry wood has an elemental composition 

of approximately 50% carbon, 6% hydrogen, and 44% oxygen. 

In addition, wood contains trace amounts of other elements 

such as calcium, potassium, sodium, magnesium, iron, 

manganese, sulfur, and phosphorous. (Rowell et al. 2013) 



Cellulose is a linear and highly crystalline polymer of D- 

glucopyranose units linked together by -(14)-glucosidic 

bonds (Pettersen 1984, Li 2011). The repeating unit in cellulose 

is a two-sugar unit, cellobiose. When randomly oriented 

cellulose molecules form intra- and intermolecular hydrogen 

bonds, the packing density of cellulose increases, leading to the 

formation of crystalline regions. For example, wood-derived 

cellulose may contain as much as 65% of crystalline regions that 

confer the strength and structural stability to the wood (Rowell 

et al. 2013, Stokke et al. 2014). There are several different 

crystalline structures of cellulose. Cellulose I is the form of 

cellulose found in nature (Thomas et al. 2011). It has structures 

I and I, of which I is enriched in the cellulose produced by 

algae and bacteria, and I in higher plants (Stokke et al. 2014). 

Hemicelluloses are heteropolymers that include 

arabinoxylans, glucomannans, xyloglucans, glucuronoxylans 

and xylans (Rowell et al. 2013). In addition to glucose, 

hemicelluloses can be made of other sugar monomers, such as 

xylose, mannose, and galactose. They are present in plant cell 

walls along with cellulose and lignin. In contrast to the linear 

and crystalline structure of cellulose, hemicelluloses are 

branched and amorphous polymers with little strength. 

Whereas cellulose consists of approximately 10 000 glucose 

molecules per polymer, hemicelluloses have shorter chains of 

about 2 000 sugar units (Pettersen 1984, Clemons 2008). In the 

cell walls of plants, hemicelluloses form a network of crosslinked 

fibers, thus endowing flexibility to the plant. 

Lignin is a complex, amorphous and cross-linked polymer, 

consisting of aromatic alcohols known as monolignols 

(Pettersen 1984, Li 2011, Stokke et al. 2014). There are three 

monolignol monomers incorporated into lignin during its 

biosynthesis in the form of phenylpropanoids: p-coumaryl 

alcohol, coniferyl alcohol, and sinapyl alcohol. The chemical 

composition of lignin varies in the different wood species. For 

example, lignin in the softwoods consists almost entirely of 

guaiacyl moieties. In cell walls, lignin can be considered as a 

chemical adhesive that fills the gaps between hemicelluloses 




and cellulose. Lignin is covalently linked to hemicellulose 

molecules, increasing the mechanical strength of the cell wall. 

Lignin is a non-polar hydrophobic polymer whereas cellulose 

and hemicelluloses are hydrophilic (Thomas et al. 2011). In the 

pulp industry, lignin is normally removed from the pulp 

(chemical pulp) when manufacturing bleached writing paper, 

because lignin is responsible for the yellowing of paper with age 

(Ek et al. 2009). Since lignin yields a considerable amount of 

energy when burned, it is considered as a potential alternative 

to fuels derived from non-renewable sources. In addition, the 

pyrolysis of lignin yields chemical compounds that are thought 

to be potentially useful in many fields of applications (Lora and 

Glasser 2002). For instance, guaiacol, which is a thermal 

degradation product of lignin, has smoky sensory notes and it 

can be used as a flavorant (Goldstein 2002, Dorfner et al. 2003). 

In addition to lignocellulose, wood contains small amounts 

(3–10%) of other organic components (Pettersen 1984, Rowell et 

al. 2013, Stokke et al. 2014). Wood extractives include simple 

sugars, fats, waxes, resins, proteins, terpenes, and gums. The 

extractive compounds are crucial components of the defense 

system of the tree, but they also act as energy reserves and 

support tree metabolism (Clemons 2008). There are also trace 

amounts (about 1%) of inorganic ash in wood (Rowell et al. 

2013). 

2.1.2 Polymers 

A variety of thermoplastic or thermosetting polymers can be 

used as the matrix material in WPCs (Clemons 2008). However, 

the low thermal stability of wood limits the polymers to those 

that have adequately low processing temperatures. The thermal 

degradation of wood components begins at approximately 

120 °C, and major changes take place at over 200 °C. Thus, 

polymers which have a processing temperature lower than 

200 °C need to be used in WPCs (Godavarti 2005). The most 

common polymers include PE, PP, and PVC that are 

thermoplastics. 



The properties of a polymer are primarily defined by its 

molecular structure. Homopolymers contain only one type of 

monomer whereas copolymers or terpolymers consist of several 

kinds of monomers. The branching of polymer chains has 

multiple effects on the polymer. For example, the highly 

branched low-density polyethylene (LDPE) is softer and has a 

lower density and poorer tensile strength than the more linear 

high-density polyethylene (HDPE). (Clemons et al. 2013) 

The properties of polymers also depend on its tacticity – the 

arrangement of monomers along the polymer backbone. 

Polymer tacticity can be divided into three classes: an isotactic 

polymer has all of its substituents on the same side of the 

backbone, and polymers with alternating placements of 

substituents along the backbone are called syndiotactic. Atactic 

polymers lack any consistent arrangement in their substituents. 

(Clemons et al. 2013) 

The monomers of a copolymer can also be organized in a 

variety of ways (Clemons 2008). An alternating copolymer 

consists of two different monomers arranged in an alternating 

sequence within the chain of the molecule (ABABAB…). The 

organization of monomers in random copolymers is not defined 

(ABAABBBA…). Statistical copolymers have monomers 

arranged according to a known statistical rule. Block 

copolymers are made up of polymerized monomer blocks. If a 

copolymer contains side chains that have a different 

composition compared with the main chain, the polymer is 

termed as a graft copolymer. 

The crystallinity of the polymer affects its thermal and 

physical properties because the crystalline regions inside the 

polymer structure increase the interactions between the 

polymer chains. When the structure of the polymer is highly 

ordered, there are fewer possibilities for the polymer chains to 

move relative to one another. Thus, more energy is required to 

transform the polymer into an unordered fluid state, meaning 

that polymers with high crystallinity have higher melting points 

in comparison with their more amorphous counterparts. (Beyler 

and Hirschler 2001) High crystallinity also means that the 




polymer will be strong but brittle, which accounts for the high 

modulus and low impact resistance (Galeski 2003). 

Semicrystalline polymers have both crystalline and amorphous 

regions, i.e., these polymers combine the high strength of 

crystalline polymers with the flexibility of the amorphous types 

(Callister 2005). In composite materials, semicrystalline 

polymers are typically more efficiently reinforced by fibers than 

amorphous ones because the fibers act as nucleation sites for the 

crystallization process with the fiber becoming surrounded by 

a finely divided microcrystalline structure, which improves the 

modulus, especially the flexural modulus (Quan et al. 2005). 

At the moment, PEs are the most commonly used plastics 

because they are easy to produce and modify. PE (Figure 2) is a 

semicrystalline polymer. The polymer chains in PE can branch 

in a different manner, resulting in polymers with different 

properties. HDPE has a variety of applications because of its 

excellent barrier properties and resistance to different solvents. 

LDPE is commonly used in containers, bottles, films, and plastic 

bags because it is a flexible and tough polymer with a good 

resistance to chemicals. In addition, LDPE has good electrical 

properties. (Klyosov 2007, Kim and Pal 2010) 

Figure 2. The chemical structure of PE. 

The properties of PP resemble those of PE. PP is a 

semicrystalline polymer with a methyl group (CH3) attached to 

the polymer backbone (Figure 3), meaning that PP can be either 

isotactic, atactic or syndiotactic. However, over 90% of 

produced PP is isotactic. Like PE, PP finds its applications in 

packaging, containers and films. Furthermore, PP is used in the 

automotive industry and can be found in laboratory equipment 

and textiles. (Kim and Pal 2010) 



Figure 3. The chemical structure of isotactic PP. 

PVC (Figure 4) is commonly used in construction, packaging, 

and insulation because it has an excellent weather resistance 

and electrical properties. Additionally, PVC has good 

mechanical properties. However, the processing of PVC is 

problematic because it releases a toxic compound, hydrochloric 

acid (HCl), when burned or melted. Furthermore, the thermal 

stability of PVC is very poor but can be improved by adding 

heat stabilizers during processing. The presence of a chlorine 

group in PVC means that the polymer may have different 

tacticity. Unlike PP, PVC has mainly an atactic stereochemistry. 

(Klyosov 2007, Kim and Pal 2010) 

Figure 4. The chemical structure of atactic PVC. 

2.1.3 Additives 

Additives are introduced into a polymer to alter its 

processability or performance (Clemons 2008). When a 

polymeric material contains additives, it is usually referred to 

as a “plastic”. Examples of additives are plasticizers, pigments, 

biocides, UV stabilizers, and antioxidants. The additive content 

is usually low because these materials are often rather expensive. 

Moreover, an excessive amount of the additive may deteriorate 

the properties of the material (Clemons et al. 2013). In WPCs, 

additives are used to improve the processability of the 

composites and especially to enhance the coupling between 

chemically different wood fibers and plastics. Furthermore, 




additives provide WPCs with a better surface appearance and 

long-term durability (Sherman 2004). 

Coupling agents are additives that improve the adhesion 

between wood and plastics, and their content in WPCs is 

typically less than 5%. Coupling agents can be classified into the 

surface-active agents and functional modifiers. Surface-active 

agents do not form covalent bonds with either the polymer 

matrix or the wood fiber; instead, they increase the interfacial 

adhesion of these constituents by acting as a solid 

surfactant. MAPP is one of the most commonly 

used coupling agents; its anhydride part forms ester bonds with 

wood’s hydroxyl groups and the long hydrophobic polymer 

incorporates into the polymer network. MAPP is therefore a 

functional modifier. Consequently, the wood fibers and the 

polymer matrix become bonded together, resulting in enhanced 

mechanical properties and reduced moisture absorption. 

Organosilanes, acrylic-modified polytetrafluoro-ethylene 

(PTFE), epoxides, isocyanates, organic acids, inorganic agents, 

and titanates are some other examples of the coupling agents 

used in WPCs (Lu et al. 2000, Godavarti 2005). 

Mineral additives are another major group of additives used 

in WPCs. They include talc (Mg3Si4O10(OH)2), calcium carbonate 

(CaCO3), kaolin clay (Al2Si2O5(OH)4), and silica sand (SiO2). In 

particular, talc and calcium carbonate are commonly utilized in 

WPCs because they are abundantly available, inexpensive, and 

they clearly enhance mechanical properties of the composite. In 

addition, talc has a natural affinity to oil, making it a good filler 

and lubricant for the mineral oil derived plastics (Klyosov 2007). 

Due to its hydrophobicity and ability to close the pathways for 

water in the composite, the addition of talc into WPCs results in 

reduced moisture absorption and less swelling liability. 

(Huuhilo et al. 2010) 

The processability and surface appearance of WPCs can be 

improved using lubricants, such as zinc stearate, paraffin waxes, 

oxidized PE, and ethylene-bis-stearamide (EBS). However, the 

use of metal stearates together with maleated coupling agents 

can nullify the effects of both additives. Typically, the amount 



of lubricants in WPCs is less than 5%, but it is also dependent 

on the type of the polymer matrix. For example, the lubricant 

content level for a HDPE-wood composite (wood content 

50–60%) is usually 4–5% whereas a similar composite composed 

of PP as a polymer matrix instead of HDPE is typically 

manufactured with 1–2% of lubricant. (Sherman 2004) 

Light stabilizers and colorants (pigments) are also added to 

WPCs to improve the resistance against color fade and UV 

degradation, and to provide the desired appearance (Sherman 

2004, Clemons et al. 2013). The amount of pigments in WPCs 

must be 1–3% or even higher to avoid color staining from the 

wood. Biocides, such as zinc borate, protect composites against 

fungal and microbial attacks and maintain their surface 

appearance. They also reduce the moisture absorption. Flame 

retardants suppress the production of flames and therefore, 

prevent the spread of fire. 

2.2 PROPERTIES 

There are two common reasons to add wood to polymers: 1) to 

lower the price of the final product and 2) to reduce the 

dependency on mineral oil based products (Kim and Pal 2010). 

This, however, means a compromise as the properties of wood 

and plastics are altered, i.e., WPCs possess rather different 

characteristics. For example, WPCs absorb less water than wood 

but have higher tensile strength than plastics. WPCs have 

therefore found use in multiple applications. The low density 

and good processability of WPCs are favored in automobile 

industry, for instance. On the other hand, WPCs are widely 

used in building products, such as siding and decking because 

of their low water absorption and good creep performance. 

However, the properties of WPCs are highly dependent on the 

product formulation, manufacturing, and the quality of the raw 

materials. 




2.2.1 Mechanical properties 

Wood fibers are added to polymers to increase their stiffness 

and strength (Wolcott and Englund 1999). The presence of wood 

fibers in the polymer matrix typically increases the strength and 

modulus of the composite (Bhaskar et al. 2012, Li et al. 2014). 

However, both the polymer matrix and the fiber reinforcement 

are responsible for the mechanical performance of the 

composite. Tensile strength is more sensitive to the properties 

of the polymer matrix whereas the modulus of elasticity of the 

composite is primarily dependent on the properties of the fiber. 

In order to increase tensile strength, a strong fiber-matrix 

interface, oriented fibers, and low stress concentration are 

required whereas the maximization of the tensile modulus 

requires fiber wetting in the matrix phase, a high fiber 

concentration and fibers with a high aspect ratio. (Saheb and Jog 

1999) 

The fiber must have a certain minimum length, i.e., the 

critical fiber length, in order to achieve the fully stressed 

properties to the fiber in the polymer matrix (Stark and 

Rowlands 2003, Sain and Pervaiz 2008). The critical length 

depends on the fiber characteristics and shear strength of the 

fiber-matrix bond. The fiber-matrix interface is likely to fail due 

to the debonding at lower stresses if the length of the fiber is less 

than its critical strength (Stark and Rowlands 2003, Bourmaud 

and Baley 2007). By contrast, exceeding the critical fiber length 

may reduce the strength of the composite because the effective 

stress transfer may be impaired due to fiber curling and fiber 

bending (Sreekumar et al. 2007). 

Interphase and interface are two important concepts in fiberreinforced 

polymer composites. The interface is a twodimensional 

surface between the fiber and the matrix whereas 

the interphase is the three-dimensional intermediate between 

the matrix phase and the fiber phase (Pilato and Michno 1994, 

Oksman Niska and Sanadi 2008, Jesson and Watts 2012). The 

interface in any fiber-polymer composite system is responsible 

for transmitting stresses from the matrix to the fibers, and the 

contribution of surfaces to stress transfer depends on both the 



roughness and the surface chemistry of the constituents. The 

stress in WPCs is transferred not only by shear along the length 

of the fiber, but also by tension at the fiber-matrix interface. The 

stress transfer is limited by the fiber strength, the shear yield 

strength, and the tensile yield strength of the plastic matrix 

polymer. (Sretenovic et al. 2006, Sain and Pervaiz 2008) A 

composite failure can occur through several scenarios, and the 

uneven nature of the surfaces makes the process even more 

complex. However, in the simplest case, an adhesive failure can 

occur in the fiber-interphase interface or in the interphasematrix 

interface. A cohesive failure of the interphase is also 

possible. The typical techniques to evaluate interfacial 

interactions and adhesion between the main constituents 

include surface analysis methods, such as X-ray photoelectron 

spectroscopy (XPS) and Fourier transform infrared 

spectroscopy (FTIR), microscopy, single fiber-pullout and 

microbond tests, and dynamic mechanical thermal analysis 

(DMTA). (Sretenovic et al. 2006, Oksman Niska and Sanadi 2008) 

The mechanical properties of WPCs have been extensively 

investigated. Changes in the amount of the wood component 

exert multiple effects on the characteristics of WPCs. When the 

wood fiber content is increased, the tensile and flexural moduli 

tend to increase because wood, especially cellulose, is a highly 

crystalline material compared to PE, PP, and PVC (Bhaskar et al. 

2012). However, the moduli of WPCs are highly dependent on 

the fiber type and source (Bouafif et al. 2009, Butylina et al. 2011, 

Ashori et al. 2011, Adhikari et al. 2012, Migneault et al. 2015). 

Although an increase in the wood fiber content may also lead to 

the higher hardness, it tends to reduce impact and tensile 

strength (Bledzki et al. 2002, La Mantia et al. 2005, Ndiaye et al. 

2013). In addition, the tensile strain at break decreases 

considerably. Huang and Zhang (2009) and Ashori et al. (2011) 

concluded that higher loadings of wood flour in WPCs induced 

the agglomeration of wood particles, which may impair the 

mechanical durability of WPCs. 

Interestingly, Bledzki et al. (2002) showed that WPCs 

consisting of hardwood fibers had a higher elongation at break 




and better impact strengths compared with WPCs containing 

softwood fibers. Their findings can be explained by the 

compositional differences between hardwoods and softwoods; 

hardwoods contain more cellulose and hemicelluloses than 

softwoods. On the other hand, the higher lignin content in 

softwoods could explain the better stiffness of those WPCs 

consisting of softwood fibers. (Sain and Pervaiz 2008, Lai 2012) 

The durability of WPCs can be considerably modified by 

altering the characteristics of the wood fiber surface, which 

changes the compatibility between wood fibers and coupling 

agents. For example, if WPCs are manufactured with wood 

fibers from bark, then the esterification reactions between 

reinforcing fibers and the coupling agent are inadequate and 

these WPCs are mechanically weaker. Conversely, the 

manufacture of WPCs with pure cellulose fibers leads to 

stronger WPCs because cellulose fibers and polymer matrix can 

be more extensively coupled through coupling agents. The 

underlying reason for this phenomenon is the difference 

between the surfaces of fibers; the surface of pure cellulose fiber 

is more polar than the surface of bark because cellulose contains 

more polar hydroxyl groups. In contrast, bark consists mainly 

of lignin and extractives that are chemically non-polar. 

Furthermore, the coupling between wood fibers and polymers 

can be altered by treating wood fibers with coupling agents, 

acids or alkalis. (Balasuriya et al. 2002, Bouafif et al. 2009, Farsi 

2010, Müller et al. 2012, Zhang et al. 2013, Migneault et al. 2015) 

Thermal treatment of wood fibers is another way to modify 

the properties of WPCs (Ayrilmis et al. 2011). In general, WPCs 

reinforced with thermally treated wood fibers are mechanically 

weaker than those reinforced with non-treated fibers. However, 

the thermal treatment of the wood fibers significantly increases 

the dimensional stability and water resistance of WPCs. The 

thermal degradation of hemicelluloses begins already at 120 °C. 

As mentioned in section 2.1.1, hemicelluloses act as the 

connective bridges between cellulose fibers and lignin, leading 

to the stiffer wood material. The degradation of hemicelluloses, 

therefore, results in weakened mechanical properties. 



Hosseinaei et al. (2012) showed that the extraction of 

hemicelluloses from wood fibers significantly improved the 

tensile properties of WPCs because the resulting wood fibers 

were more hydrophobic and less polar, enhancing the 

compatibility between wood fibers and thermoplastics. 

Bouafif et al. (2009) demonstrated that wood fiber size also 

affected the mechanical properties of WPCs; increasing the fiber 

size improves the modulus of elasticity (MOE) and maximum 

strength in both flexural and tensile tests; the results have been 

confirmed by Kociszewski et al. (2012). Migneault et al. (2008) 

demonstrated that increasing fiber length and maintaining 

constant fiber diameter exerted beneficial effects on the tensile 

and flexural moduli and toughness of WPC. 

The possibility of modifying the polymer matrix also results 

in the preparation of WPCs with distinctive characteristics. For 

example, using recycled polymers instead of virgin materials 

may improve mechanical properties of WPCs (Adhikary et al. 

2008a). However, the use of recycled polymers in WPCs can be 

challenging since the post-consumer plastics waste may contain 

several grades, colors, and contaminants, leading to varying 

outcomes when the plastics are combined with wood fibers 

(Najafi 2013). Sobczak et al. (2013) showed that the flexural and 

impact strength of WPCs increase along with the mass-average 

molecular mass of the polymer. The polymer matrix of WPCs 

does not necessarily consist of one type of polymer; Gao et al. 

(2008) used a PE/PP-blend as a polymer matrix. Clemons (2010) 

investigated WPCs with varying HDPE:PP ratios and observed 

that if the ratio was changed from 75:25 to 25:75, the tensile yield 

stress of the WPC increased considerably whereas the opposite 

effect was observed for impact energies and yield strain. 

There are other ways to optimize further the mechanical 

properties of WPCs, e.g., the incorporation of additives can help 

to overcome an incompatibility between the wood and the 

polymers. The use of MAPP or MAPE is a well-established 

approach to improve the durability of WPCs. Several studies 

have confirmed the effectiveness of MAPP and MAPE 

(Nourbakhsh and Ashori 2009, Pérez et al. 2012, Bhaskar et al. 




2012, Ndiaye et al. 2013). In addition, new types of additives 

have been recently introduced. For example, Li et al. (2014) and 

Das et al. (2015a) used waste charcoal as an additive in WPCs 

and noted improvements in the tensile and flexural properties. 

Abdolvahaba et al. (2014) improved the durability of WPCs by 

using nanoclay as a filler. Gwon et al. (2012) added three 

different types of inorganic fillers (kaolin, talc, and zinc-borate) 

to WPCs and observed that those WPCs containing kaolin or 

talc had higher mechanical strengths than WPCs with zincborate. 

The mechanical performance of WPCs with kaolin filler 

was the highest because kaolin has a staked plate shape, small 

particle size and a surface with highly hydrophilic 

characteristics. 

To summarize, the mechanical properties of WPCs are highly 

dependent on the product formulation. The incorporation of 

additives, such as coupling agents, is usually required to 

produce WPCs with adequate mechanical properties. Thus, 

new potential additives providing higher mechanical strength 

are constantly being discovered and developed. 

2.2.2 Water absorption 

A well-known disadvantage resulting from the addition of 

wood fibers in plastics is the consequent susceptibility to water 

absorption (Adhikary et al. 2008b). Moisture penetrates into the 

composite materials by three different mechanisms. The first 

and the most common process is the diffusion of water 

molecules inside the microgaps between the polymer chains. 

The second mechanism is capillary transport into the gaps and 

flaws at the interfaces between the fibers and polymers. 

Moisture transport by microcracks formed during the 

processing is another mechanism. In general, water absorption 

on natural fiber reinforced composites follows the kinetics of a 

Fickian diffusion process. (Espert et al. 2004) 

Wang et al. (2006) studied moisture absorption in natural 

fiber-plastic composites. They proposed that moisture 

absorption occurred via two mechanisms depending on the 

fiber content of the composite. At higher fiber loadings, when 



the accessible fiber ratio was high and the material more 

homogeneous, the diffusion process was the dominant 

mechanism. At low fiber loading, percolation is the dominant 

mechanism; the fiber loading threshold for percolation is about 

50 wt%. Percolation was applicable for nonhomogeneous 

materials and it takes into account the randomness of the 

composite structure. 

As WPCs absorb water, not only do they become more 

vulnerable to the dimensional changes and microbial attack, but 

they also become mechanically weaker (Espert et al. 2004, 

Sombatsompop and Chaochanchaikul 2004, Tamrakar and 

Lopez-Anido 2011). Several efforts have been made to improve 

the water resistance and dimensional stability of WPCs. Some 

manufacturers have attempted to reduce water absorption of 

WPCs by the addition of zinc borate, which also improves the 

fungal resistance. On the other hand, along with the 

improvements in mechanical properties, the addition of MAPP 

or MAPE also reduces moisture absorption (Adhikary et al. 

2008a, Najafi et al. 2010). Li et al. (2014) improved the water 

resistance of WPCs by incorporating biochar into the composite, 

but other additives have also been proven to decrease water 

absorption of WPCs (Lee and Kim 2009, Huuhilo et al. 2010, 

Turku et al. 2014). 

An increase in wood fiber content or fiber size leads to a 

higher water absorption but like the mechanical durability, this 

property is also highly dependent on the fiber type and source 

(Yang et al. 2006, Migneault et al. 2008, Bouafif et al. 2009, 

Migneault et al. 2009, Ayrilmis et al. 2011, Butylina et al. 2011). 

In addition, the characteristics of the polymer matrix exert a 

considerable impact on water absorption (Adhikary et al. 2008a, 

Najafi et al. 2010, Sobczak et al. 2013); in general, water 

absorption of WPCs with PP as the polymer matrix is higher 

than of those with PE (Najafi et al. 2007). However, the water 

absorption of WPCs is also dependent on the temperature of the 

water, i.e., by increasing the temperature, then one also 

increases the amount of water absorbed (Najafi et al. 2007). 




Water absorption of WPCs can be reduced by modifying the 

wood fibers. For instance, Dányádi et al. (2010) showed that the 

benzylation of wood fibers resulted in decreased water 

absorption. The wood modifications conducted by Müller et al. 

(2012) had similar effects. Hosseinaei et al. (2012) extracted 

hemicelluloses from wood fibers, which also resulted in lower 

water absorption of their WPCs. Wei et al. (2013) modified 

poplar wood fibers chemically by esterification and noted that 

the esterified fibers were more hydrophobic than the 

unmodified fibers. Consequently, the compatibility between 

wood fibers and the plastic matrix increased, leading to lower 

water absorption. Thermal modification of wood also results in 

a considerably lower water absorption of WPCs (Ayrilmis et al. 

2011, Butylina et al. 2011). 

2.2.3 VOC emissions 

VOCs have a low boiling point and therefore, a high vapor 

pressure at room temperature, leading to the evaporation of a 

large number of molecules into the surrounding air. VOCs are 

abundantly present in nature, for example, they play an 

important role in the communication between plants (Ueda et 

al. 2012). However, some VOCs exert adverse effects on human 

health and may cause harm to the environment. Therefore, 

legislative efforts have been made to diminish the release of 

harmful VOCs from commercial products. The regulation of 

indoor VOC emissions aims to limit VOC emissions from 

commercial products into indoor air where concentrations are 

the highest. The major difficulty in the research of VOCs and 

their effects is that their concentrations are usually low and the 

symptoms and illnesses they evoke tend to develop very slowly. 

The VOC emission characteristics of WPCs have not been 

extensively studied because these materials are generally used 

outdoors. Nevertheless, WPCs are increasingly being used 

indoors (Kim and Pal 2010). Therefore, their effects on indoor 

air quality are becoming more relevant. As WPCs consist mainly 

of wood and plastics, it is often presumed that their VOC 

emissions are dominated by these two major constituents. 



However, WPCs have usually been processed at high 

temperatures, which may change the VOC emission 

characteristics of the material. Furthermore, the incorporation 

of additives exert effects on the VOCs. 

Schwarzinger et al. (2008) conducted an elemental analysis 

of different WPCs by two-stage pyrolysis-GC/MS (gas 

chromatography-mass spectrometry). In addition to the 

identification of marker compounds for different wood types, 

they identified pyrolysis products from polymers (PE, PP, and 

PVC), which is important with respect to VOCs. Furthermore, 

their analysis of WPCs with various lignocelluloses provided 

further insights into the fundamental differences between 

WPCs with different reinforcements. 

Félix et al. (2013) examined the release of VOCs from WPCs 

made from landfill-derived plastic and sawdust. Their findings 

were in accordance with those of Schwarzinger et al. (2008); the 

key markers for WPCs were phenols and aldehydes. In general, 

the profile of VOCs displayed alkanes, alkenes, phenols, 

aldehydes, aromatic hydrocarbons, terpenes, carboxylic acids, 

esters, nitrogen compounds, ketones, and alcohols. The most 

abundant VOCs in WPCs were furfural, -pinene, 2-ethyl-1- 

hexanol, 2-methoxyphenol, N-methylphthalimide, butylated 

hydroxytoluene, 2,4-di-tert-butylphenol, and diethylphthalate. 

In addition, Félix et al. (2013) demonstrated that the 

incorporation of additives increased the release of certain VOCs. 

Another important finding was that WPCs have the potential to 

emit off-odor compounds that cannot be completely masked by 

odorizing agents. The identified off-odor compounds in WPCs 

included acetylfuran, hexanal, 4-vinylguaiacol, acetic acid, and 

2-methoxyphenol. 

One way to control VOC emissions from WPCs is to 

incorporate odorants into the composites to mask at least part 

of the off-odor compounds (Félix et al. 2013). Moreover, the 

addition of other types of additives capable of affecting the odor 

characteristics along with the other properties of WPCs could 

provide a simple solution. Another approach, proposed by Yeh 

et al. (2009), is to cover the composite with a thin layer of virgin 




polymer in a co-extrusion process. This layer should be able to 

delay the release of VOCs and therefore, alter the odor profiles 

of WPCs. 

2.3 MANUFACTURING TECHNOLOGIES 

There are several alternative methods for manufacturing WPCs. 

Compounding is a process in which filler and additives are 

added to the molten polymer. The compounded material can be 

formed into pellets or granules prior to future processing, or 

they can be immediately shaped into the final product (in-line 

processing) (Clemons et al. 2013). The processability of WPCs is 

similar to plastics, which is an advantage since WPCs are can 

typically be processed with the same machinery. Extruders are 

the most commonly used systems for WPC compounding. Hotcold 

mixers are also used but mainly for the processing of PVCbased 

WPCs (Schwendemann 2008). 

The product manufacturing technologies for WPCs include 

sheet or profile extrusion, injection molding, and compression 

molding (Stokke et al. 2014). Profile extrusion is the most 

commonly used manufacturing method for a WPC, and it is 

used to produce composites with a continuous profile of the 

desired shape (Gonçalves et al. 2014). WPC panels can be 

produced by sheet extrusion. Injection and compression 

molding produce non-continuous pieces with a more 

complicated shape. 

2.3.1 Extrusion 

Extrusion produces continuous linear profiles by forcing a 

melted WPC through a die. Different types of extruders and 

processing strategies have been used to produce WPCs. For 

example, some processors manufacture WPCs in one step, using 

twin-screw extruders whereas some prefer to adopt several 

extruders in tandem to compound and finally form the desired 

profile of the WPC. (Clemons et al. 2013) 



A typical screw extruder consists of feeders, modular barrels, 

screws, a gearbox, a heating, and cooling unit (Figure 5), and a 

centralized control unit to adjust the extrusion speed, feeding 

rate, temperature, and other process parameters 

(Schwendemann 2008). The extruding screw system, consisting 

of screws and barrels, mixes, devolatilizes, and performs the 

reactions for multiple applications. 

Figure 5. The basic structure of an extruder. 

The screws mix the components in order to produce a 

homogeneous blending fluid in the barrel. The screws are 

usually made up of three zones: the feeding, melting, and melt 

pumping zone. In the feeding zone, the raw materials for WPC 

are usually solid, but when they move to the melting zone, most 

polymers have melted while fillers and additives remain in a 

solid state. The melt pumping zone forms a continuous fiberpolymer 

blend, which is finally pumped to the pelletizer after 

cooling or extruded through the die. (Stokke et al. 2014) 

A typical barrel-length-to-diameter (L/D) ratio of a singlescrew 

extruder varies from 20 to 30. The screw builds up high 

pressure in the composite melt so that it can be extruded 

through the die. Even though the single-screw extruders are less 

expensive than those with twin-screw systems, they suffer from 

a limited mixing and self-cleaning ability as well as from the 

selective material intake. The twin-screw extruders, whether corotating 

or counter-rotating according to the screw rotation 




directions, are used for compounding, mixing or reactive 

polymer materials. L/D ratios for the twin-screw extruders vary 

from 39 to 48. The advantages of the twin-screw extruders 

include the self-cleaning and high mixing ability. However, 

unlike the single-screw extruders, these systems cannot develop 

a up high pressure in the melt pumping zone. In addition, the 

twin-screw extruders are more expensive. (Schwendemann 

2008, Stokke et al. 2014) 

The barrels are divided into the sections heated with the 

individual control units (Stokke et al. 2014). The temperature of 

the barrel gradually increases from the rear to the front which 

allows the material to melt gradually and to prevent thermal 

degradation or overheating. Sometimes the friction and high 

pressure in the barrel provide the required heat for the system, 

and the heaters can be turned off. 

Multi-layered WPC structures are produced by coextrusion. 

This process utilizes multiple extruders (single or twin-screw) 

to melt and deliver different types of materials to a single 

extrusion die that will extrude the materials in the desired form 

(Stokke et al. 2014). In addition to the reduced material and 

production costs, coextrusion makes the properties of final 

products highly controllable, which is a significant advantage 

over other production technologies. 

2.3.2 Injection molding 

Injection molding is used for producing large quantities of WPC 

pieces with complex geometries (Migneault et al. 2009). The 

molding of WPCs begins by inserting the pelletized raw 

material into the hopper, which feeds the material into the 

heated barrel with a reciprocating screw (Figure 6). The majority 

of the injection molding machines are equipped with single 

screws. The increased thermal energy reduces the viscosity of 

the material, allowing the screw to push the material forward. 

The simultaneous mixing and homogenizing increase the 

friction and heat within the barrel. The material is collected at 

the front of the screw and then injected at high pressure and 

velocity into the mold. The volume of the material that is used 



to fill the mold is known as a shot (Stokke et al. 2014). The high 

packing pressure completes the mold filling and compensates 

for thermal shrinkage. Once the cavity entrance solidifies, no 

more material can enter the cavity. Consequently, the screw 

reciprocates and receives the new material for the next cycle. 

Meanwhile, the material inside the mold is cooled to the preset 

temperature and ejected from the mold. After the prepared 

piece is demolded by an array of pins, the mold closes and the 

process is repeated. 

Figure 6. The structure of an injection molding apparatus. 

Most injection-molded WPCs are produced from pelletized raw 

materials. However, in-line compounding is also possible. It is 

a combination of a two-stage injection unit with a co-rotating 

twin screw. Once the raw materials are fed into the co-rotating 

twin screw, they are compounded and transferred to a shooting 

pot. The shooting pot pushes the material via the machine 

nozzle and hot runner into the mold. (Schwendemann 2008) 

2.3.3 Compression molding 

Compression molding of WPCs is utilized especially in the 

automotive industry due to its capability to produce large and 

complex parts. Moreover, it wastes relatively little raw material, 

and therefore, it is one of the least expensive molding methods. 

However, the product quality is not always consistent and it can 




be problematic to control for leakage between the two surfaces 

of the mold (flashing). (Clemons et al. 2013, Stokke et al. 2014) 

The compression molding process (Figure 7) starts by 

placing the WPC pellets or granules into the preheated mold 

(Stokke et al. 2014). To shorten the molding cycle time, the 

charge is also usually preheated. The material is softened by the 

heat and as the upper half of the mold moves downward, the 

charge is forced to conform to the shape of the mold. After the 

mold is opened, the part is removed by the ejector pin. 

Figure 7. A schematic drawing of compression molding process. 

2.3.4 Choosing appropriate manufacturing method 

Processing methods have significant effects on the properties of 

WPCs. For instance, Migneault et al. (2009) observed that 

injection-molded WPCs have better physical and mechanical 

properties and reduced water absorption than extruded WPCs. 

However, the extruded WPCs had higher densities. These 

investigators also discovered that wood fibers in injectionmolded 

WPCs were aligned in the main flow direction whereas 

the fibers in extruded samples were more randomly oriented. 

This resulted in a better stress transfer between wood fibers and 

the polymer matrix in the injection-molded samples. 

Bledzki et al. (2005a) compared three different compounding 

processes (two-roll mill, high-speed mixer, and twin-screw 

extruder) and claimed that the composites compounded by 

extrusion had the best mechanical strength and lowest water 



absorption. Yeh and Gupta (2008) observed that a change in the 

extrusion parameters did not exert any significant effect on the 

mechanical properties of WPCs, but it did influence the water 

absorption behavior. Namely, the longer residence time and 

higher screw speed resulted in a lower rate of water absorption 

as well as lower density. They speculated that the lower density 

and therefore, the lower rate of water absorption was caused by 

the loss of hydrophilic compounds in WPCs. Goncalves et al. 

(2014) revealed that the design of the extrusion die had a 

considerable effect on the properties of a WPC deck. They used 

a computer system to design a die and then simulated 

experimental conditions, and the comparison between 

numerical and experimental results achieved good qualitative 

agreement. 

Even though there may be considerable differences in the 

properties of WPCs produced with different manufacturing 

methods, it is also important to assess the strengths and 

limitations of the manufacturing processes in a production 

point of view. Table 1 presents a simple comparison between 

extrusion, injection molding and compression molding. 

Table 1. A comparison between extrusion, injection molding, and compression 

molding. 

Injection Compression 

Parameter 

Extrusion 

molding molding 

Setup costs Moderate High Low 

Production costs Low Low Moderate 

Production speed Moderate High Low 

Product consistency High High Low 

Product geometry 

Limited to parts of a 

fixed cross section 

Complex 

Limited to flat or 

curved parts 


3 Characterization of woodplastic 

composites 

Material testing is usually the last step in the manufacturing 

process, and its purpose is to ensure that the material meets the 

requirements of its applications. WPCs are commonly used in 

applications that require adequate mechanical strength and 

resistance to water absorption. The physical and mechanical 

properties of WPC products are typically evaluated with 

standard laboratory tests. In general, WPCs and plastics are 

tested with similar procedures because they are typically 

manufactured with the same technologies. 

The British standard BS EN 15534-1 was validated as a 

standard in the EU in 2014. This standard specifies the 

procedures for the determination of physical and mechanical 

properties of WPCs. In addition, the test methods for durability, 

such as weathering and natural ageing, and thermal properties 

are defined. 

The characterization of WPCs can also be carried out 

according to ISO (International Organization for 

Standardization) and ASTM (America Society for Testing and 

Materials) standards. ASTM standards are widely applied in the 

US whereas ISO standards are typically used in Europe. 

3.1 MECHANICAL PROPERTIES 

The mechanical properties of WPCs are characterized according 

to the standards originally developed for plastics since in most 

cases, these standards are suitable for WPCs. However, it is 

possible that the testing of WPCs according to these standards 

does not provide valid results. For example, the effect of the size 

of wood fibers may be overemphasized at the sample 




dimensions specified in the ISO and ASTM standards. In other 

words, it is possible that the stress required to fracture the WPC 

sample is relatively lower at smaller sample dimensions than it 

would be at the dimensions of the final applications because 

wood fibers are larger in relation to the cross-sectional area of 

the sample and thus undergo fractures more easily. 

According to BS EN 15534-1, WPC samples should be 

conditioned in a standard atmosphere at 23 ± 2 °C and relative 

humidity (RH) of 50 ± 10% before mechanical testing. An 

atmosphere of 20 °C and 65% RH may also be used but those 

conditions should be declared. 

When determining flexural, tensile, and impact strength of 

WPCs, the preferred test specimen according to ISO standards 

should be 80 ± 2 mm in length, 10.0 ± 0.2 mm in width, and 

4.0 ± 0.2 mm in thickness. The corresponding specimen 

dimensions for WPCs in ASTM standards are 125 mm × 12.7 mm 

× 3.2 mm. In total, a set of 10 specimens should be tested unless 

the coefficient of variation has a value less than 5%. In that case, 

a minimum number of five specimens may be sufficient. 

3.1.1 Tensile strength 

Tensile strength is the measure of the maximum amount of 

tensile stress that a material can withstand while being stretched 

before breaking. It is defined as a stress and expressed as force 

per unit area. The most important parameters for tensile testing 

include testing speed, force capacity, precision, and accuracy. 

The testing speed is expressed as mm/min, and according to ISO 

527-1, it can vary between 0.125–500 mm/min depending on the 

sample type. 

At the start of the measurement, the machine slowly extends 

the sample until it breaks. The elongation of the sample is 

measured against the applied force. With the measured 

elongation, it is possible to calculate the strain, , using the 

following equation: 

= L-L 0 

L 0 

, (3.1) 


Characterization of wood-plastic composites 

where L is the final length of the gauge and L0 is the initial gauge 

length. The applied force is used to calculate the stress, , using 

the following equation: 

= F A , (3.2) 

where F is the tensile force (N) and A is the cross-sectional area 

of the sample. The relationship between the stress and strain of 

a material can be displayed on a stress-strain curve (Figure 8). 

The curve also provides the fracture strength of a material, 

which is the final recorded point. 

Figure 8. A typical stress-strain curve for WPCs. 

Tensile properties of WPCs can be determined according to ISO 

527-1 and ASTM D638-14 standards. Although the testing 

procedures presented in these standards are rather similar, 

there are some differences that can significantly influence the 

results obtained. In ISO 527-1, it is stated that the specimens 

must not be pre-stressed considerably prior to testing. Pre- 




stresses can be induced when the specimen is centered in the 

grips or when the clamping pressure is applied. The maximum 

allowable pre-stress must be less than 1% of the measured stress 

results. The induced strain value must be less than 0.05%, 

accordingly. ASTM D638-14 does not contain these 

specifications, and therefore, it lacks the defined status of the 

stress after placing the specimen in the grips. Thus, the 

corrected strain zero point is defined as the point where the 

linear slope of the stress-strain curve crosses the strain axis. This 

correction can exert a significant effect on the measured tensile 

modulus. In ISO 527-1, the point where the tensile modulus is 

measured is precisely defined whereas the definition of tensile 

modulus in ASTM D638-14 is based on the corrected strain zero 

point. If the material lacks the linear region in the stress-strain 

curve, the modulus is determined from a secant modulus that is 

determined between the corrected strain zero point and a freely 

selected point on the curve. Consequently, this may result in 

significant variations between the results carried out according 

to ISO or ASTM standard. 

Another difference between these standards is related to the 

test speed used in determining tensile modulus. In ISO 527-1, 

the tensile modulus is measured with the lower test speeds than 

tensile strength, but the same test speeds are allowed to be used 

throughout the test in ASTM D638-14. There are also differences 

in the requirements for extensometers; ISO 527-1 allows the 

lower measurement uncertainty than ASTM D638-14. 

3.1.2 Flexural strength and modulus 

The material’s ability to resist deformation under load is defined 

as the flexural strength, which is typically measured using a 

three- or four-point flexural test technique. During the test, the 

sample experiences many kinds of stresses throughout its depth. 

At the outside of the bend, the stress is tensile in its nature 

whereas at the load-bearing side, the sample experiences 

compressive stress (Sain and Pervaiz 2008). Usually most of the 

materials fail under tensile stress rather than compressive stress, 

meaning that the maximum tensile stress value that the material 



can withstand before breaking is its flexural strength. 

Mathematically, the formula to calculate the maximum surface 

stress, S, for a rectangular sample in the three-point bending test 

is expressed as: 

S = 3FL s 

2bd s 

2 , (3.3) 

where F is the bending load (N) at the given point, Ls is the 

length of span (mm), b is the width of the sample (mm), and ds 

is the thickness of the sample (mm). In ISO 178, Ls is 

recommended to be 64 mm. 

Flexural modulus, E, is the ratio of stress to strain within the 

elastic region. It is computed from the slope of a stress-strain 

curve (Figure 8) obtained from the flexural strength test. The 

flexural modulus for the three-point test of a rectangular sample 

can be expressed as: 

E = 

L s 3 F 

4bh 3 d , (3.4) 

where h is the height of the sample (mm) and d is the deflection 

(mm). 

The test parameters for flexural testing are defined 

differently in ASTM D790-15 and ISO 178 since the dimensions 

of the specimen are also different. In addition, the point where 

the test is stopped is not the same. In ASTM D790-15, the test is 

stopped when a 5% deflection is reached or if the specimen 

breaks this value. In ISO 178, the test continues until the 

specimen breaks. If the specimen does not break, the stress at 

3.5% strain is reported. Consequently, these standards provide 

different results if the specimen strain is higher than 3.5%. 

3.1.3 Impact strength 

Impact strength can be determined using either the Charpy or 

Izod impact test. Although the principle of these tests is similar, 

there are some differences in the testing procedures. In Charpy 

impact test, a specimen with dimensions of 4.1 mm × 10.1 mm × 

55 mm is positioned horizontally in the middle of two supports. 




A pendulum strikes the middle of the specimen that may be 

unnotched or have a U-notch or V-notch. In the Izod impact test, 

the dimensions of the specimen are 4.1 mm × 10.1 mm × 75 mm. 

The specimen is placed vertically on the support, and it can be 

unnotched or have only a V-notch. In the Izod test, the notch is 

facing the pendulum whereas in the Charpy test the notch side 

of the specimen faces away from the pendulum. 

The Charpy impact test determines the amount of energy 

absorbed by a material during the fracture. The measurement 

apparatus consists of a pendulum of a known mass and length. 

The pendulum is dropped from a known height so that it strikes 

the specimen. The amount of energy absorbed by the material 

at the impact can be determined by comparing the heights of the 

hammer before and after the fracture. The energy absorbed in 

the breaking is expressed as impact energy (J/m). It is calculated 

by dividing the energy by the thickness of the sample. In ISO 

179-1, the Charpy’s impact strength is reported in kJ/m 2 , which 

is derived by dividing the impact energy by the area under the 

notch. In ASTM D6110-10, the results are reported as J/m. 

Notching has a considerable effect on the results of the 

impact test. Thus, the exact geometries and dimensions of 

notches have been determined in ISO 179-1. In addition, the size 

of the sample can affect the results. 

The differences between ISO and ASTM impact tests are 

related to the type of pendulum hammer used in the tests. In 

ISO 179-1, the pendulum hammer may be used in the range 

from 10 to 80% of its nominal potential energy whereas the 

maximum value in ASTM D6110-10 is 85%. In addition, 

according to ISO 179-1, the largest possible hammer must be 

used because the speed loss at the impact must be kept as low 

as possible. ASTM D6110-10 defines that the standard 

pendulum hammer has a rated initial potential energy of 2.7 J, 

and the hammer size is increased by doubling its dimensions. 

However, unlike ISO 179-1, the smallest hammer in the range 

has to be used in the tests. 



3.2 WATER ABSORPTION 

Water absorption of WPCs is typically evaluated using the 

standard methods developed for plastics, wood, and WPCs. 

However, the time to reach the moisture equilibrium is longer 

for WPCs than for plastics or wood (Defoirdt et al. 2010); wood 

reaches an equilibrium in hours or weeks, plastics in weeks and 

WPCs in months. Therefore, the testing methods used for WPCs 

may not allow the material to reach the moisture equilibrium 

and this may affect the results obtained. This, however, does not 

mean that the test methods described in these standards could 

not be used to investigate the differences between various WPC 

types; considerable differences in the water resistance can be 

observed even with only two hours of water immersion. 

Guidelines for the determination of water absorption of 

WPCs are given in BS EN 15534-1. ISO 62 can also be used 

because it applies to the reinforced plastics, to which WPCs 

belong. Similarly, ASTM D570-98(2010)E1 can also be used to 

determine the moisture absorption of WPCs. According to the 

standards, the water absorption of the material can be 

determined by completely immersing the samples in water at 

23 °C or in boiling water. Before the immersions, the samples 

must be dried in an oven at 50 ± 2 °C for at least 24 h and then 

cooled to room temperature in a desiccator before weighing. 

After the immersion, water absorption, c (%), of the materials 

can be determined by using the following formula: 

c = m 2-m 1 

m 1 

100% , (3.5) 

where m1 is the mass of the test specimen after initial drying and 

before the immersion, and m2 is the mass of the test specimen 

after the immersion. 

There are some differences in these standards. In BS EN 

15534-1, the immersion period in the water bath (23 °C) is 28 ± 1 

days whereas the immersion period in ISO 62 should be at least 

24 hours. In addition, the immersion time in boiling test is 

5 h ± 10 min in BS EN 15534-1, but in ISO 62 the immersion 




should last for at least 30 ± 2 min. ASTM D1037-12 specifies two 

methods to determine water absorption of WPCs. In method A, 

the WPCs are first immersed for two hours in fresh water 

(20 ± 1 °C) and then weighed. After weighing, the sample is 

submerged in water for an additional 22 hours and then 

weighed again. Method B is similar to the 24-hour immersion 

procedure described in ISO 62. 

3.3 VOC EMISSIONS 

There are several methods available for the determination of 

VOC emissions from solid products. If the product is primarily 

intended for indoors use, the emissions are commonly 

determined according to ISO 16000-6. In this standard, the 

emissions are analyzed by thermal desorption/gas 

chromatography with flame ionization detector and mass 

spectrometry (TD-GC-FID/MS) using a Tenax TA ® absorbent 

tube as a collector for VOCs. 

Proton-transfer-reaction mass-spectroscopy (PTR-MS) is 

another way to determine and compare VOC emissions 

between different samples. This is an online monitoring 

technique using gas phase hydronium ions as the ion source 

reagents. This technique is used in food science, medicine, and 

biological and environmental research. (Schripp et al. 2014) 

3.3.1 TD-GC-FID/MS 

The guidelines for determination of VOC emissions from 

building products using emissions test chamber system are 

given in ISO 16000-9. In this procedure, the air flow transfers the 

emitted compounds from the chamber to a Tenax TA ® absorbent 

tube. After reaching the tube, the compounds of interest are 

adsorbed onto the surface of the material. When the compounds 

are being analyzed, the tube is heated to a temperature over 

250–300 °C and the adsorbed compounds are released into the 

flow of carrier gas that transfers the compounds to the GC-MS. 



The measurements are conducted under controlled 

conditions as defined in the standard. This states that the 

products should be tested at a temperature of 23 ± 2 °C and RH 

of 50 ± 5%, with an air velocity in the range 0.1–0.3 m/s. In 

addition, the temperature should not vary by more than ± 1.0 °C 

during the measurements, and RH and air flow rate can 

fluctuate by only ± 3%. Before the measurements, the test 

chamber must be cleaned with alkaline detergents and then 

rinsed twice with distilled water. In addition, cleaning by 

thermal desorption is also allowed. To eliminate the possible 

effects of background emissions, an air sample of the empty 

emission chamber is taken before the actual measurements. 

When the sample is placed in the chamber, it should be 

positioned in the center of the chamber to ensure that the air 

flow is evenly distributed over the emitting surface. The 

measurements should be carried out at predefined sampling 

times that depend on the objective of the test. However, 

duplicate air samples should be taken at least at 72 ± 2 h and 

28 ± 2 days after the start of the test. 

For TD-GC-FID/MS, the analysis of VOCs is optimal for the 

range of VOCs eluting between and including n-hexane and 

n-hexadecane (Woolfenden 2009). However, when the tube is 

heated, the adsorbed compounds are released slowly from the 

tube. This may lead to low sensitivity and wide 

chromatographic peaks. In addition, this system is not capable 

of measuring the emissions of certain VOCs, such methane and 

formaldehyde. Modern TD-GC-MS systems avoid wide peaks 

by using cold traps to focus the samples before they reach the 

column, but the properties of column also affect the wideness of 

the peaks. Overall, the sensitivity of TD-GC-MS is highly 

dependent on the absorbent material, system parameters and 

the amount of the sample. 

3.3.2 PTR-MS 

PTR-MS is an easy-to-use online VOC monitoring system with 

high sensitivity and rapid time response. In PTR-MS, the VOC 

trace gases in the sampled air are ionized in proton-transfer 




reactions using hydronium (H3O + ) as the primary reagent ion 

(Lindinger et al. 1998, Lindinger et al. 2001). The concentrations 

of the product ion and the reagent are then measured in a mass 

spectrometer. The system consists of an ion source (hollow 

cathode), a drift tube reactor (reaction chamber) and a mass 

spectrometer. In the ion source, a hollow cathode discharge in 

water vapor produces H3O + ions (de Gouw and Warneke 2007). 

These ions are then injected into the drift tube where VOCs are 

ionized with H3O + ions according to the following reaction: 

H3O + + R RH + + H2O (3.6) 

Next, a homogeneous electric field in the drift tube transports 

the reagent and the product ions into the second intermediate 

chamber. Most of the air is pumped away and a small fraction 

of ions is extracted for analysis in the mass spectrometer. The 

concentration of trace gas [RH + ] is computed from the following 

equation: 

[RH + ] = [H3O + ]0 (1 – e -k[r]t ) [H3O + ]k[R]t (3.7) 

The approximation of the equation is made assuming that [R] is 

small and, therefore, [H3O + ] is equal to [H3O + ]0. The rate 

coefficient k for the proton-transfer-reaction and reaction time t 

are predefined parameters, and the fraction of [RH + ] and [H3O + ] 

is obtained from the mass spectrometry. 

The sensitivity of PTR-MS can be further improved by 

combining the PTR ion source with a time-of-flight mass 

spectrometer (TOF-MS). In this arrangement, the system can 

separate most atmospherically relevant protonated isobaric 

VOCs and identify their corresponding empirical formulas 

(Müller et al. 2010, Schripp et al. 2014). However, the maximum 

measurable concentration of PTR-MS is limited to 

approximately 10 ppmv (parts per million by volume). If the 

total concentration of VOCs is too high, the concentration 

calculation will be incorrect. In addition, the identification of 

compounds is based on their mass, which is not always unique. 



As PTR-MS is based on the proton-transfer-reaction with H3O + 

as the primary reagent, the system detects only molecules that 

have a higher proton affinity than water (Schripp et al. 2010). 

Moreover, the fragmentation of ions can influence the results 

obtained (Aprea et al. 2007). 


4 Thermal processing of 

wood 

Thermal modification of wood has been conducted on an 

industrial scale since the beginning of the 20 th century to 

improve the dimensional stability and decay resistance of wood. 

Nowadays, there are several commercial thermal modification 

processes, such as ThermoWood ® (Finland), Plato ® (Holland), 

Perdure process, and Retification ® (France). In general, the 

temperatures of the thermal modification processes vary 

between 180 °C and 260 °C. (Hill 2006b, Esteves and Pereira 2008, 

Navi and Sandberg 2011) 

Thermal modification exerts multiple effects on the physical 

and biological properties of wood (Hill 2006b); these are mainly 

induced by the chemical changes in the macromolecular 

constituents. In addition to the improved dimensional stability 

and decay resistance, thermally modified wood absorbs less 

water but has a tendency to form cracks and splits and has 

reduced impact toughness, modulus of rupture, and work to 

fracture. The process variables, such as time and temperature of 

the treatment, wood species, sample dimensions, and the use of 

catalysis, have considerable effects on the resulting changes. 

When the temperature increases to over 300 °C, the 

degradation behavior of wood changes, and it becomes severely 

degraded (Hill 2006b). In slow pyrolysis, wood is slowly heated 

in the absence of oxygen up to a final temperature of 400–500 °C. 

The heating rate of the process is typically in the range 

5–10 °C/min (Klass 1998, Mohan et al. 2006, Dahmen et al. 2010, 

Li and Suzuki 2010). The primary product of this process is 

charcoal, but it also produces condensable vapors and noncondensable 

gases. Charcoal can be utilized as a smokeless solid 

fuel or as pure carbon in chemistry. On the other hand, the 

charcoal-rich biochar could be exploited to boost agricultural 




yields and control pollution (Cernansky 2015). Condensable 

vapors can be condensed into separate fractions according to the 

processing temperature, and the heat from the non-condensable 

gases can be reused in the process to diminish the need for 

external energy (Nachenius et al. 2013). 

The degradation of wood constituents begins at 130–200 °C 

when hemicelluloses start to decompose (Figure 9). However, 

most of the hemicellulose decomposition occurs above 180 °C, 

resulting in the formation of gases and relatively small amounts 

of liquids and charcoal (Finnish Thermowood Association 2003, 

Bhaskar et al. 2011). The decomposition of cellulose occurs in 

multiple phases. First, very short-lived active cellulose is 

formed. It is then dehydrated, decarboxylated, and carbonized 

at a temperature under 300 °C to produce charcoal and noncondensable 

gases. At over 300 °C, cellulose becomes 

depolymerized and cleaved into condensable gases and vapors, 

including tar. In the final stage, cellulose undergoes secondary 

reactions, cracking the vapors into secondary charcoal, tar and 

gases. Lignin decomposition produces primarily charcoal (40%), 

but liquid components (35%), and gases (10%) are also formed. 

The decomposition of lignin occurs over a wide temperature 

range; the process can begin at 200 °C, continuing to 450–500 °C. 

As wood contains many kinds of extractives, the decomposition 

temperatures of these compounds vary extensively; the 

decomposition begins at 150 °C and continues to approximately 

400 °C. Extractives usually evaporate or are cracked into 

secondary products. (Mohan et al. 2006, Basu 2013, Nachenius 

et al. 2013) 


Thermal processing of wood 

Figure 9. Thermal decomposition temperatures of wood constituents. The line at the 

bottom indicates the typical temperature ranges for the processes. 

4.1 THERMOWOOD ® PROCESS 

ThermoWood ® is an industrial scale heat treatment process for 

wood (Finnish Thermowood Association 2003). During the 

process, wood is heated to at least 180 °C while it is protected 

with steam. In addition, the presence of steam affects the 

compositional changes taking place in the wood. 

The process consists of three phases. In the first phase, the 

temperature of the kiln is rapidly raised to approximately 100 °C. 

Then the wood is dried until it has a moisture content of nearly 

0% by elevating the temperature slowly up to 130 °C. In the 

second phase, the temperature of the kiln is increased to 

185–215 °C. Depending on the final application, the temperature 

can remain unchanged for 2–3 hours. In the third and the final 

phase, the temperature is lowered back to below 100 °C using 

water spray systems. At 80–90 °C, the wood moisture content 

increases to 4–7% as re-moisturizing takes place. The total 

duration of the process is typically about 36 hours, depending 

on the raw material and desired outcome. (Navi and Sandberg 

2011) 

ThermoWood ® has two treatment classes: Thermo-S 

(stability) and Thermo-D (durability). In Thermo-S, the 




maximum treatment temperature varies between 185–190 °C 

depending on the wood type. The maximum treatment 

temperature of Thermo-D ranges between 200–215 °C. 

The ThermoWood ® process has multiple effects on wood. As 

shown in Figure 9, extractives are the first components to 

thermally degrade or evaporate from wood. The extractives 

include terpenes, waxes and phenols that are easily evaporated 

during the treatment. In addition, changes occur in 

carbohydrates (cellulose and hemicelluloses). However, the 

majority of the changes take place in the hemicelluloses. When 

wood is heated to the treatment temperatures, acetic acid is 

formed from acetylated hemicelluloses by hydrolysis. The 

formed acetic acid formed serves as a catalyst such that 

hemicelluloses are hydrolyzed to soluble sugars, and it also 

depolymerizes amorphous cellulose microfibrils into shorter 

chains. (Finnish Thermowood Association 2003) 

Consequently, the hemicellulose content in wood is reduced 

and the degree of crystallinity in cellulose increases. These 

changes result in the improved resistance to fungal decay, better 

dimensional stability, and decreased water absorption. Thus, 

heat-treated wood is very suitable for outdoor applications. 

Heat treatment also results in minor changes in lignin, even 

though it is thermally the most stable component of wood. 

During heating, the bonds between phenylpropane units are 

partly broken. When the temperature exceeds 200 °C, -aryl 

ether bonds start to break. At high temperatures, the methoxy 

content in lignin decreases and some non-condensed units are 

transformed into diphenylmethane-type units. These reactions 

exert considerable effects on the properties of lignin. Since 

hardwoods contain more hemicelluloses than softwoods, 

hardwoods are more susceptible to thermal degradation than 

softwoods. (Finnish Thermowood Association 2003) 

In addition to the chemical changes, the ThermoWood ® 

process results in multiple physical changes in wood. Loss of 

mass and the formation of microcracks during the treatment 

decrease the density of treated wood. Consequently, heat 

treated wood has generally a lower strength than its untreated 



counterpart. However, the thermal conductivity of wood after 

heat treatment is reduced by over 20%. Visually, heat treated 

wood has a darker color than untreated wood (Navi and 

Sandberg 2011). 

In total, heat-treated wood has lower VOC emissions. 

However, the release of acetic acid, furfural, and hexanal 

increases significantly after heat treatment. Acetic acid is a 

harmful VOC that causes irritation of the respiratory system. 

Furfural has a smoky odor and it is thought to be partly 

responsible for the characteristic odor of heat treated wood. It is 

therefore uncertain whether heat-treated wood has a better 

VOC profile than untreated wood (Manninen et al. 2002, 

Hyttinen et al. 2010). Despite the increased release of some 

harmful VOCs, heat treated wood can be regarded a safe 

construction material for indoor air quality (Manninen et al. 

2002). 

4.2 SLOW PYROLYSIS OF WOOD 

Slow pyrolysis refers to the thermal decomposition of wood or 

other types of biomass in the absence of oxygen. In this process, 

wood is slowly heated (5–10 °C/min) to 400–500 °C so that the 

heat energy breaks down the long chains of carbon, hydrogen, 

and oxygen into smaller compounds. The products of this 

process include charcoal (35–40%), tar and liquids (30–45%), 

and gases (25–35%). The yields of the products are dependent 

on the process variables, such as the heating rate, final 

temperature and pressure, and on the type and size of the wood 

pieces. When the final temperature is low (~400 °C) and the 

heating rate is slow (< 5 °C/min), pyrolysis will yield mainly 

charcoal. With a rapid or moderate heating rate (> 5 °C/min) and 

a high final temperature (~500 °C), mainly non-condensable 

gases are formed. If one wishes to optimize the yield of bio-oil, 

then an intermediate temperature and relatively high heating 

rate must be used. Torrefaction is a mild or incomplete form of 

slow pyrolysis, and thus, only a partial thermochemical 




conversion and devolatilization take place; the maximum 

temperature of the process typically ranges between 200 °C and 

300 °C. (Williams and Besler 1996, Klass 1998, Nachenius et al. 

2013) 

Pyrolysis and charcoal production have a long history. 

Traditionally, charcoal has been produced in kilns, but modern 

charcoal production utilizes large retorts with capacities of 

100 m 3 or even more. In addition, these systems are typically 

combined with refining facilities to capture the volatile products. 

The most common types of processes used in the industry are 

the Reichert retort process, the SIFIC, and the Lambiotte process. 

A Reichert facility typically consists of multiple retorts (up to 

six), and therefore, has a high rate of production. For example, 

the Reichert production facility operated by proFagus GmbH in 

Bodenfelde, Germany, has been designed to produce 

30 000 tons of charcoal, 5 200 tons of acetic acid, 1 800 tons of 

pyroligneous spirit, and 12 000 tons of bio-oil per year. The 

charcoal production capacity in a traditional Lambiotte retort is 

12 000 tons per year. (Dahmen et al. 2010) 

The decomposition of cellulose and hemicelluloses produces 

primarily condensable vapors and non-condensable gases. 

Lignin decomposes into liquids, gases, and charcoal. The 

decomposition or volatilization of extractives produces liquid 

or gas products. Minerals and ash remain solid in charcoal, and 

they exert a catalytic effect on the pyrolysis reactions; these 

compounds increase the yield of charcoal. (Williams and Besler 

1996, Nachenius et al. 2013) 

The pyrolytic production processes require only small 

amounts of external energy. When the temperature is below 

200 °C, the process is endothermic and it produces primarily 

water, formic acid, and acetic acid. At 200–270 °C, the process 

becomes partly exothermic. At this stage, the main products are 

carbon dioxide, formic acid, and acetic acid. When the 

temperature reaches 350–400 °C, the process reactions are 

highly exothermic, producing a great number of products, such 

as methanol, formaldehyde, and tar compounds. When 400 °C 

is exceeded, the reaction heat becomes weakly endothermic, 



and the process produces non-condensable gases. The energy 

efficiency of pyrolysis can be further optimized by recycling the 

heat energy present in the non-condensable gases. The moisture 

content of the raw material is an important factor affecting the 

energy efficiency of the process; if the moisture content exceeds 

30%, the process will be endothermic. (Dahmen et al. 2010, 

Nachenius et al. 2013) 

4.3 PRODUCTS OBTAINED FROM THE PROCESSES 

Even though the most important and commercially relevant 

product in the ThermoWood ® process is the thermally modified 

wood itself, other products are also formed during the process. 

The maximum temperature used in the ThermoWood ® process 

is 215 °C, and the reactions are carried out in the presence of 

steam. Hemicelluloses and extractives are decomposed at 

temperatures below 215 °C (Figure 9), and some minor changes 

occur in lignin. Therefore, the products formed in this process 

are primarily liquids and gases formed from these constituents. 

The formation of solid products, such as charcoal, is minor. 

The maximum temperature in slow pyrolysis, in turn, 

exceeds the decomposition temperatures of all wood 

constituents (Figure 9), and the products of slow pyrolysis can 

be found in the gaseous, liquid and solid phases. Gaseous 

products include hydrogen, methane, carbon monoxide, and 

carbon dioxide. The liquids include tars and oils that remain in 

the liquid form at room temperature. Solid products are mainly 

composed of charcoal, but other inert materials are also present. 

(Dahmen et al. 2010, Bhaskar et al. 2011, Basu 2013) 

4.3.1 Charcoal 

Charcoal is the main product from slow pyrolysis, consisting 

mainly of carbon (60–90%), hydrogen, and oxygen. Charcoal 

production requires slow heating rate for a long duration but at 

a relatively low temperature (400 °C). Charcoal is used as an 

energy source especially in developing countries, but it has 




recently attracted substantial interest because of its high 

potential in other applications. For example, charcoal can be 

used as biochar in agriculture to improve the water holding 

capacity of soil and to retain nutrients more efficiently, and 

therefore, improve yields. On the other hand, biochar is also 

able to combat pollution by binding the heavy metals in soils 

and liquids, and by reducing the nitrous oxide emissions 

because of its bulk surface area, pore size distribution, particle 

size distribution, density, and packing. In this context, charcoal 

is referred to as activated carbon. However, the properties of 

biochars vary according to the processing method and raw 

materials used in their manufacture. The pyrolysis temperature 

is the most important parameter affecting the properties of 

biochar. (Downie and Van Zwieten 2012, Nachenius et al. 2013, 

Cernansky 2015) 

Charcoal is also used as an ore reductant in the metallurgical 

industry because of its low mercury and sulfur content. Its high 

carbon content makes it a desirable material in chemistry. Other 

applications of charcoal include water and air (and gas) 

purification. The addition of charcoal has also beneficial effects 

on the properties of WPCs. (Li et al. 2014, Das et al. 2015a) 

4.3.2 Condensable vapors 

The decomposition of hemicelluloses, cellulose, and lignin 

produces vapors that can be condensed into several fractions 

(Fagernäs et al. 2012a, Fagernäs et al. 2015). Tars originate 

primarily from lignin and they contain carbohydrates, phenols, 

and aldehydes. Hemicelluloses and cellulose decompose into 

several liquid fractions that consist of acetic acid, formic acid, 

acetone, phenol, and water. The formation of liquid distillates 

begins at approximately 150 °C when the bound water in wood 

starts to evaporate. Methanol, acetic acid, and furfural become 

evaporated at 180–210 °C, and tars are formed at approximately 

330 °C. At 400 °C, the majority of the compounds have been 

distilled. 

The detailed chemical composition of liquid products from 

slow pyrolysis has been determined by Fagernäs et al. (2012a) 



and Miettinen et al. (2015). Fagernäs et al. (2012a) analyzed slow 

pyrolysis products from birch and found that the aqueous 

phases were composed mainly of acetic acid (60% of the 

compounds), methanol (9%), hydroxypropanol (5–6%), furfural 

(3–4%), and acetone (2–5%). The compositions of tars were 

similar to the aqueous phases, but they consisted also of lignin 

monomers phenols, guaiacol, and syringols. The amount of 

polycyclic aromatic hydrocarbons (PAHs) in tars ranged from 

0.1 wt% to 0.4 wt%. PAHs have harmful effects on human health 

and the environment, and therefore, Fagernäs et al. (2012b) 

conducted a further study of the PAHs present in pyrolysis 

products. PAHs are mostly concentrated in the heavy tars which 

collect at to the bottom of the retort, but low PAH contents are 

also found in the tar-free aqueous phases as well as in the noncondensable 

gases. Miettinen et al. (2015) showed that the slow 

pyrolysis oil from unbarked pine was composed mainly of 

various wood extractives and lignin degradation products 

whereas the aqueous phase contained saturated fatty acids, 

degradation products of lignin, anhydrosugars, and other 

oxygen-rich compounds. 

Tars and liquids derived from the thermal processing of 

wood have been stated to possess a huge potential in a vast 

number of applications (Fagernäs et al. 2015). So far, hundreds 

of chemicals have been identified from tars and liquids, and 

new ways to separate valuable chemicals and chemical families 

from these fractions are being developed (Brown and Brown 

2014). Their potential applications include their use as biocides, 

repellents, pesticides, material coating and medicines. However, 

the presence of PAHs may limit the widespread applications of 

tars (Fagernäs et al. 2012a). 

Examples of chemicals with a high commercial potential are 

levoglucosan, furfural, glycolaldehydes, and phenolic 

compounds, such as guaiacol and catechol (Abou-Zaid and 

Scott 2012). Furfural, glycolaldehydes, guaiacol, and catechol 

can be utilized in resin production whereas levoglucosan can be 

used as a pesticide and in the production of antibiotics and 

polymers. Other rather valuable compounds identified in wood 




distillates are vanillin, propionic acid, and syringol, all of which 

can be used in the food processing industry. However, many of 

the compounds listed above are present in rather low 

concentrations and their fractionation and subsequent 

individual identification and separation would be commercially 

impractical. Therefore, the synthesis of chemical compounds 

from fossil resources is currently favored (Brown and Brown 

2014). Nevertheless, there are still many unidentified 

compounds in a wide variety of wood distillates, some of which 

may prove to have very high values and even be easy to 

separate. 

4.3.3 Non-condensable gases 

By definition, non-condensable gases are the gases that remain 

once they have passed the condensation stage in the pyrolysis 

process (Nachenius et al. 2013). Cellulose decomposition 

produces a great amount of gases. In some applications, the heat 

energy from non-condensable gases is utilized to drive the 

pyrolysis process or to dry the biomass feed. It is also possible 

to release these gases to the atmosphere or to burn them and use 

their energy for other purposes. 

The composition of non-condensable gases is largely 

determined by the pyrolysis temperature and the temperature 

at which the condensable vapors become condensed 

(Nachenius et al. 2013). At lower temperatures, the noncondensable 

gases consist primarily of carbon monoxide and 

carbon dioxide. High reaction temperatures result in increased 

hydrogen and methane contents. As mentioned in section 4.2, 

the maximum yield of non-condensable gases can be achieved 

at high reaction temperatures. Gasification is an example of a 

process that primarily aims to convert biomass into noncondensable 

gases (Decker et al. 2007). 


5 Aims and significance 

Despite the fact that WPCs are recognized as potential 

substitutes for the mineral oil based plastics and conventional 

building products, they still lack certain desirable properties. 

For example, one of the major limiting factors of the 

applicability of WPCs is their excess water absorption, and this 

can cause their swelling and increase their susceptibility to 

microbial attack. In addition, as WPCs are increasingly being 

used indoors, their impact on indoor air quality has not been 

assessed. To overcome the limitations associated with WPCs, a 

wide variety of additives has been developed (Sherman 2004). 

Even though the additives, such as coupling agents, usually 

provide at least a partial solution to the problems, additives are 

usually relatively expensive and developed via synthetic routes. 

Thus, the WPC industry needs novel, bio-based and 

inexpensive solutions to replace these materials. 

On the other hand, the efficient utilization of raw materials, 

including industrial by-products, is becoming more relevant. 

For example, considerable amounts of liquid waste are 

generated in charcoal and ThermoWood ® production. The use 

of these liquids in WPCs could provide benefits for both 

industries, as material previously considered as waste would 

become a valuable additive in WPCs. In order to explore this 

possibility, the following aims were set for this thesis: 

1. To explore if WPC granules could be effectively impregnated with 

different types of wood distillates. The impregnation of WPC 

granules with the distillates was tested in studies I, III and 

IV using two types of commercial WPC granules and two 

types of wood distillates. 

2. To study whether PTR-TOF-MS is an applicable technique for 

determining VOC emissions from WPCs. The suitability of 

PTR-TOF-MS to determine VOC emissions from WPCs was 




assessed in study II where the emissions of seven different 

commercial WPC decks were determined and compared. In 

general, VOCs are related to the safety of the materials and 

they define the odor profile of the material. PTR-MS has 

been previously used to determine material emission 

signatures from nine common building materials (Han et al. 

2010), but there are no studies where PTR-TOF-MS has been 

used to monitor VOC emissions from WPCs over a more 

prolonged period of time. 

3. To determine the effects of hardwood distillate on the properties of 

WPC. The impact of hardwood distillate on the 

characteristics of WPC was determined in studies I and III. 

In study I, 4.2–4.8 wt% of hardwood distillate was added to 

the WPCs whereas 1–8 wt% of a similar distillate was 

added to the WPC in study III. The effects of the distillate 

addition were evaluated in mechanical tests and water 

immersion assays. 

4. To determine the effects of softwood distillate on the properties of 

a WPC. The impact of softwood distillate on the 

characteristics of a WPC was determined in study IV where 

1–20 wt% of softwood distillate was added to the WPC. The 

effects of the distillate addition were evaluated by 

conducting mechanical tests and water immersion assays. 

As far as the author is aware, this thesis is a pioneering work 

examining the incorporation of wood distillates into WPCs. This 

study can serve as a basis for further studies with similar 

objectives and it is anticipated that it will represent a milestone 

in reaching the ultimate goal of replacing synthetic additives 

with inexpensive, bio-based alternatives. 


6 Materials and methods 

Two types of commercial WPC granules were used in this thesis; 

UPM ForMi (UPM Biocomposites, Lahti, Finland), hereafter 

abbreviated as UF, was composed of cellulose fibers and 

additives in a PP matrix whereas LunaGrain (LunaComp Ltd, 

Iisalmi, Finland), hereafter abbreviated as LG, consisted of 

thermally modified sawdust (Scots pine, 50 wt%), PP and 

additives. UF was available with four different cellulose fiber 

contents: UF20 contains 20 wt%, UF30 30 wt%, UF40 40 wt%, 

and UF50 50 wt% cellulose fibers. After the granules were 

treated with the distillates and dried, the samples for 

mechanical testing and water absorption tests (studies I, III, and 

IV) were prepared by injection molding. 

Seven different commercial WPC decks were used in study 

II to evaluate their VOC characteristics: two decks were 

produced by UPM (UPM ProFi), three by LunaComp, and two 

by unknown Chinese manufacturers. The decks were either 

provided by the manufacturers or obtained from a local retailer. 

Table 2 presents a summary of the materials and methods used 

in this thesis. 

6.1 SAMPLE PREPARATION 

The WPC granules were prepared for distillate impregnations 

and injection molding by drying them in a force-convection 

oven. The temperature in the oven was set to 105 ± 2 °C and the 

granules were dried in the oven for at least four hours. At the 

beginning of the trial, the granules were weighed before and 

after the drying for reference. Once dried, the granules were 

tightly packed in plastic bags to avoid humidity. The packed 

granules were stored under laboratory conditions (T = 22 ± 2 °C 

and RH = 50 ± 10%) before further processing. 




Table 2. An overview of the materials and methods used in the thesis. 

Study Materials * Distillate type and 

content 

I 

Injection-molded 

specimens from LG and 

UF20, UF30, UF40, and 

UF50 


4.1–4.8 w% 

Study methods 

Mechanical properties 


II 

Seven WPC decks from 

four manufacturers 

- VOC emissions 

III 

Injection-molded 

specimens from LG 


1.0–8.0 w% 



VOC emissions 

Injection-molded Softwood distillate 

IV 

specimens from LG 1.0–20.0 w% 

* LG refers to LunaGrain and UF to UPM ForMi 



VOC emissions 

The WPC decks used in study II were sawn into smaller pieces 

to fit into the glass vessels used in the VOC emission 

measurements. The cut surfaces were covered with Kapton ® 

tape immediately after sawing to avoid any VOC emissions 

from the newly-cut surfaces. The samples were then stored 

under laboratory conditions (T = 22 ± 2 °C and RH = 50 ± 10%) 

and characterized shortly after sawing. 

6.1.1 Distillates 

Two types of distillates were used in this thesis. In studies I and 

III, the WPC granules were treated with hardwood distillate 

whereas softwood distillate was used in study IV. 

The conversion of birch into charcoal, liquids, and noncondensable 

gases was conducted using a two-part slow 

pyrolysis retort with an approximate capacity of 10 m 3 . First, 

5–15 cm thick blocks of birch were put inside the inner part of 

the retort. The temperature of the retort was slowly (1 °C/min) 

elevated to about 350 °C in the absence of oxygen. The process 

was endothermic until the temperature reached 270 °C. 

Subsequently, the process became exothermic and the 

formation of non-condensable gases started to increase. The 


Materials and methods 

heat energy from these gases was recycled to maintain the 

process conditions. 

The vapors were condensed into collectable liquids by 

transporting them through cooled pipes. The evaporation of 

moisture from the raw material began below 100 °C. As 

expected, the liquids formed from these vapors consisted 

primarily of water. At 100–300 °C, the formation of condensable 

vapors continued, but the composition of liquids changed; as 

the temperature increased, the share of water decreased and 

that of acetic acid and other organic acids increased. When the 

temperature exceeded 300 °C, the formation of tars began and 

continued until the final temperature (350 °C) was reached. This 

fraction was used for the treatments. 

The softwood distillate originated from industrial 

ThermoWood ® process conducted by LunaWood Ltd (Iisalmi, 

Finland). In the process, primarily Scots pine planks were 

thermally treated as described in section 4.1. During the process, 

the evaporation of condensable compounds resulted in the 

formation of liquids, a part of which was collected into a liquid 

container (V = 1 m 3 ) and further processed. Due to the 

differences in the compositions and densities, there were two 

distinguishable phases in the container; the lighter phase 

consisted primarily of water and water-soluble organic 

compounds, such as acetic acid, and the heavier phase, which 

physically resembled tar, was a mixture of acetic acid, methanol, 

phenols, long chain fatty acids and their esters, squalene, cyclic 

hydrocarbons and PAHs. This heavier phase was separated, 

processed and used for the treatments. 

Both distillates, hardwood and softwood, were processed 

similarly before they were used for the treatments. First, the 

distillates were rinsed three times with water (T 40 °C) to 

extract water-soluble compounds, such as simple carbohydrates, 

from the distillate. In general, simple carbohydrates have low 

melting points (< 200 °C) and therefore, they could start to burn 

during injection molding. When the distillates were rinsed, 

water was added to the distillates in a ratio of 1:1, and the 

resulting blend was carefully mixed for 15 minutes. After the 




mixture, the blend was stabilized and the lighter phase 

containing water and water-soluble compounds was removed. 

Next, the distillates were heated to approximately 105 °C to 

evaporate those compounds with relatively low boiling points 

because the evaporation of these compounds during injection 

molding could affect the product quality. Finally, the distillates 

were filtered to remove the solid particles. 

6.1.2 Impregnation of WPC granules 

The impregnation of the WPC granules with hardwood or 

softwood distillate was carried out in a similar manner. 

However, the softwood distillate was more viscous than the 

hardwood distillate, and therefore, the softwood distillate had 

to be heated to approximately 80–90 °C in order to achieve an 

efficient impregnation. It was also found that the UF granules 

were not suitable for this type of impregnation because they 

were too dense to be thoroughly treated with the distillate. 

Especially the granules with relatively low cellulose fiber 

content (UF20 and UF30) were problematic because they 

resembled plastic granules with a low porosity. Therefore, only 

the LG granules were treated with the distillate and they were 

then mixed with the UF granules. 

The impregnation began by mixing the WPC granules and 

the distillate in a dish until the granules were thoroughly 

covered with the distillate. The excess distillate was then 

removed from the blend by pouring the mixture onto a steel 

sieve, allowing the extra distillate to drip. Once the dripping of 

the distillate ended, the granules were placed into a forceconvection 

oven in aluminum trays at 120–130 °C. The 

temperature in the oven was then raised to 170 °C for 30 minutes 

to polymerize the distillate. The mixture was then cooled to 

room temperature and tightly packed in plastic bags. The final 

distillate content of the treated granules was determined by 

weighing the dried granules before and after the impregnation 

with the distillate. 

The distillate content in the WPC materials used for injection 

molding was controlled by mixing the impregnated granules 



with the untreated granules and changing the mixing ratio 

according to the desired distillate content. Consequently, the 

distillate content varied between 1–8 wt% for the WPCs 

containing hardwood distillate, and 1–20 wt% for the WPCs 

containing softwood distillate. 

6.1.3 Injection molding 

Specimens with dimensions of 4.1 mm × 10.1 mm × 170 mm 

were prepared by injection molding using a Haitian Mars MA 

1600/600 apparatus (Ningbo Haitian Huayuan Machinery Co., 

Ltd, Ningbo, China) as presented in Figure 10. The screw 

diameter was 50 mm with an L/D ratio of 18. The most 

important parameters used in the injection molding are listed in 

Table 3. 

Figure 10. Haitian Mars MA 1600/600. 




Table 3. Some of the parameters used in injection molding. 

Parameter 

Value 

Heater temperature (°C) 

Mold temperature (°C) 

Heater 1 Heater 2 Heater 3 Heater 4 Nozzle 

100–150 140–165 165–175 180 185–195 

Fixed 

Movable 

50–75 50–75 

Cooling time (s) 20–35 

Holding pressure (MPa) 350–800 

Holding time (s) 15.0 

Injection pressure (MPa) 80–100 

In comparison with the specimens prepared from LG, the UF 

specimens were injection molded at higher heater and mold 

temperatures but at lower holding pressures. In general, the 

WPCs containing distillates required lower holding and 

injection pressures for injection molding. However, the addition 

of distillates resulted in smoother surfaces of the specimens, and 

thus, the specimens started to stick to the mold. Therefore, 

longer cooling times were used for the WPCs treated with 

distillates. At least 50 pairs of specimens were prepared from 

each material type, and the specimens were stored in plastic 

bags under laboratory conditions. 


The mechanical properties of the WPCs were determined using 

the injection-molded specimens. The measurements were 

carried out within two weeks after the sample preparation, and 

the samples were conditioned and stored in the laboratory 

between the measurements. 

In studies I, III and IV, the tensile and flexural properties of 

the WPCs were characterized. Moreover, Charpy’s impact 

strengths were determined in studies III and IV. A total of ten 

specimens of each material type were examined in each test. 



6.2.1 Tensile and flexural properties 

Tensile strength, modulus, and strain of the WPCs were 

determined as specified in ISO 527-1. The measurements were 

conducted using an Instron 8874 dynamic mechanical tester 

(Instron Industrial Products, Grove City, Pennsylvania, United 

States) under laboratory conditions (Figure 11). The test speed 

was set to 5.0 mm/min. The system recorded strain, tensile 

modulus, and the force needed to break the sample. Thickness 

and width for each specimen were measured, and the crosssectional 

area was used to calculate the tensile strength as 

described in section 3.1.1. 

The flexural properties of the WPC samples were determined 

according to ISO 178. The measurements were carried out using 

an Instron 8874 dynamic mechanical tester (Figure 11). Ls was 

64 mm and the speed of the loading edge was set to 2.0 mm/min. 

The system was tested with reference samples to verify the 

correct calibration prior the actual measurements. 

The system was set to stop the loading and the measurement 

when the specimen broke. During the measurements, the 

system recorded the force and the corresponding deflection of 

the specimen and provided a complete stress-strain curve. 

Figure 11. Instron 8874 dynamic mechanical tester used in studies I, III, and IV. 




6.2.2 Charpy’s impact strength 

In studies III and IV, Charpy’s impact strengths (unnotched) 

were determined according to ISO 179-1. The measurements 

were carried out using a Ray-ran advanced universal pendulum 

system (JD Instruments Inc., Houston, Texas, United States) as 

presented in Figure 12. The injection-molded samples were cut 

into pieces with dimensions of 4.1 mm × 10.1 mm × 80 mm. 

Before the measurements, the test machine was calibrated to 

determine the frictional losses and to correct for any absorbed 

energy. In addition, the dimensions of one specimen from each 

material type were measured to ensure that the dimensions 

corresponded to the guidelines given in ISO 179-1. The samples 

were then placed edgewise between two supports (Ls = 62 mm). 

The pendulum (m = 0.952 kg) was lifted up its prescribed height 

to achieve a pendulum velocity of 2.9 m/s. The pendulum was 

released and the result was recorded. For all material types, the 

impact resulted in a complete fracture of the specimen. 

Figure 12. Ray-ran advanced universal pendulum system used in studies III and 

IV. 




In studies I, III, and IV, the water absorption of the WPCs was 

determined from the injection-molded samples as outlined in 

ISO 62. Sprues and runners were removed from the specimens 

to ensure that the specimens were similar in shape. A total of 

three samples of each material type was tested. 

The specimens were first dried in a force-convection oven at 

50 ± 2 °C for 24 hours. The specimens were then allowed to cool 

to room temperature in a desiccator before they were weighed 

using a Mettler Toledo AX205 –scale (Mettler-Toledo, LCC, 

Columbus (Ohio), United States). Immediately thereafter, the 

specimens were immersed in distilled water (T = 21.0 °C) for 24 

and 48 hours. After the immersion, any excess water was 

removed from the surfaces of the specimens and they were 

reweighed within 1 minute of their removal from water. The 

specimens were weighed at least three times at each stage, with 

the results being reported as the means of the separate 

weighings. 


The VOC emissions from the WPC samples were measured 

using a high-resolution PTR-TOF-MS (PTR-TOF 8000, Ionicon 

Analytik, Innsbruck, Austria) as presented in Figure 13. The 

PTR-TOF-MS was operated under controlled conditions: 

2.3 mbar drift tube pressure, 600 V drift tube voltage and 60 °C 

temperature. 




Figure 13. PTR-TOF 8000 used in studies II, III, and IV. 

In study II, the VOC emissions from seven different WPC deck 

boards were determined. One of the decks (LunaComp) was 

obtained from the manufacturer immediately after its 

production to investigate the changes in the VOC emissions 

during the first 41 days. Starting from the first day after the 

manufacture, the VOC emissions for this sample were 

characterized 11 times over a 41-day period to monitor the 

changes in the VOC emission rates. The sample was stored 

under laboratory conditions between the measurements. The 

ages of the six remaining decks were unknown, but they 

represented products that a consumer would use. As the WPC 

deck samples were prepared from different products, their 

shapes were irregular and dissimilar. Therefore, the VOC 

emission rates of these samples were determined with respect 

to their masses. 

In studies III and IV, the VOC emission measurements were 

carried out using the injection-molded specimens as samples. 

Five specimens of each material type were cut so that they had 

dimensions of 4.1 mm × 10.1 mm × 80 mm. The areas of the 

samples were determined so that it was possible to convert the 

obtained emissions into area-specific emissions rates. 



The emissions were determined using 1.5 L glass vessels as 

the chambers. The glass vessels were prepared for the 

measurements by cleaning the inner surfaces with detergent 

and then rinsing them with distilled water. Furthermore, to 

ensure the purity of the chambers, the vessels were thermally 

cleaned by heating them in a force-convection oven at 120 °C for 

at least one hour. The tubes that introduce the air to the chamber 

and transport it to the PTR-TOF-MS system were installed on 

the metal lid that covered the glass vessels. The air (RH < 5%) 

that was introduced into the chamber with a flow rate of 0.3 

L/min had been filtered with an active carbon/Purafil ® /HEPA 

filter. The air containing VOCs was then transported into the 

PTR drift tube via a polyether ether ketone (PEEK) tube at a total 

flow rate of 0.1–0.3 L/min. 

Before the samples were analyzed, the emissions from empty 

chambers were measured and this background was subtracted 

from the data during the analysis. Once the samples were 

placed in the chamber, the system immediately started to collect 

the data. As the VOC emissions could be observed online, each 

measurement was continued until the emissions of the 

compounds of interest stabilized. This took approximately 

15 minutes for each material. After each measurement, the 

chamber was carefully flushed with purified air. 

The obtained data were analyzed using PTR-MS Viewer 

3.1.0.27 software (Ionicon Analytik, Innsbruck, Austria). The 

concentrations were calculated by the program using a standard 

reaction rate constant of 2 × 10 -9 cm 3 s -1 molecule -1 . The trace 

gases were detected from the spectral peaks assuming that the 

molecules consisted only of hydrogen, carbon, and oxygen. The 

detection of gases was further supported by the literature data 

on the VOC emissions of wood materials and the matching of 

the isotope patterns, especially in cases where the detection was 

ambiguous. Table 4 lists the VOCs studied in each study. 




Table 4. The VOCs studied in studies II, III and IV. 

VOCs 

Studies 

Formaldehyde (m/z 31.018 CH3O + ) 

Methanol (m/z 33.034 CH5O + ) 

Acetaldehyde (m/z 45.034 C2H5O + ) 

Acetic acid (m/z 61.029 C2H5O2 + ) 

Cyclohexene (m/z 83.0706 C6H11 + ) 

Furan (m/z 69.034 C4H5O + ) 

Benzene (m/z 79.0548 C6H7 + ) 

Furfural (m/z 97.1000 C5H5O2 + ) 

Guaiacol (m/z 125.1233 C7H9O2 + ) 

Monoterpenes (m/z 137.1330 C10H17 + and 81.0704 C6H9 + ) 

II 

III 

II and IV 

II 

II, III, and IV 

II 

III and IV 




The VOC emission values calculated by the software were 

expressed as parts per billion (ppb). In studies III and IV, these 

emission values were converted into emission rates (µg/m 2 h) 

according to the following equation: 

E voc = 0.0409 C vocF voc M voc 

A sample 

, (6.1) 

where Cvoc is the concentration of the VOC (ppb), Fvoc is the flow 

rate (m 3 /h), and Mvoc is the molar mass of the individual VOC 

molecule (g/mol). The unitless constant 0.0409 was obtained 

from the conversion of units using the ideal gas law at 1 atm 

pressure and 25 °C, and Asample is the area of the WPC sample. In 

study II, the emission rates were calculated with respect to the 

mass of the samples; the emission rates were expressed as 

µg/kgh, therefore. 

The emission rates were further converted into real room air 

concentrations (µg/m 3 ) using product loading factor (Lp), which 

is equal to the sample surface area divided by the chamber 

volume. By applying this conversion, it was possible to compare 

the VOC emissions of the material with the odor thresholds of 

VOCs, and thus one could estimate whether a certain VOC 

could be smelled. The real room air concentration is expressed 

as follows: 

C real room = E vocL p 

n 

, (6.2) 



where Evoc is the emission rate of VOC (µg/m 2 h), Lp is the loading 

factor of the sample (m 2 /m 3 ) and n is the air exchange rate (1/h) 

in the chamber. In studies III and IV, Lp was calculated with 

respect to the area of the samples, but in study II, sample mass 

was used to determine Lp. In study II, it was also assumed that 

the samples had similar compositions and shapes. 

6.5 STATISTICAL ANALYSES 

In studies I, III, and IV, the statistical analyses were performed 

using Matlab R2013b (Mathworks, Natick, MA, US) and IBM 

SPSS Statistics 21 software (IBM Corp., Armonk, NY, US). Both 

parametric and non-parametric statistical tests were considered. 

Mann-Whitney U test was found to be suitable for evaluating 

the statistical significance of the differences observed between 

the material types when the number of specimens was more 

than five. The limit for statistical significance was set at p 0.05, 

and p < 0.01 was designated as high statistical significance. 


7 Results 

The most important findings from studies I–IV are summarized 

in Table 5. The results will be presented in more detail in the 

following chapters. 

Table 5. A summary of the main findings in this thesis. 

Study subject Studies The main findings 

Impregnation of WPC 

granules with the 

distillates 

I, III, and IV 

The LG granules were successfully 

impregnated with the distillates. 

Hardwood and softwood distillates 

enhanced the processability of the WPC 

granules. 

A small (1 wt%) addition of hardwood 

distillate significantly increased the 

tensile modulus of the WPC. A higher 

distillate content (2–8 wt%) reduced 

the mechanical properties of the WPC. 





A minor (2 wt%) addition of softwood 

distillate significantly increased the 

tensile strength of the WPC. Strain and 

bending increased significantly with a 

high distillate content (over 4 wt%) 

whereas the strength of the WPC 

declined. 

The WPCs containing wood distillates 

absorbed less water than those without 

distillates. 

VOCs 


PTR-TOF-MS is an applicable method for 

determining VOCs from WPCs. 

The VOC emissions from the WPCs 

change considerably as a function of 

time. 





The effects of wood distillates on the mechanical properties of 

the WPCs were determined in studies I, III, and IV. In study I, 

the addition of hardwood distillate did not improve the 

mechanical properties of the WPCs studied. UF40 had the 

highest flexural and tensile strength whereas the highest 

flexural modulus was detected for UF50 and the highest tensile 

modulus for UF50 + LG50. 

In studies III and IV, a minor addition (1–2 wt%) of wood 

distillates significantly improved the mechanical properties of 

the WPC studied. In study III, the LG granules were 

impregnated with a similar hardwood distillate as used in study 

I. The distillate content ranged from 1 to 8 wt%. In study IV, 

1–20 wt% of softwood distillate was added to the WPC. The 

results from studies III and IV are summarized in Table 6. 

Tensile modulus increased highly significantly with 1 wt% of 

hardwood distillate. Similar trends, although not statistically 

significant, were observed for tensile and flexural strength and 

modulus of elasticity. 

The addition of softwood distillate had advantageous effects 

on the mechanical properties of the WPC in study IV. With 

2 wt% of softwood distillate, a highly significant increase was 

observed in the tensile strength. Another finding emerging from 

study IV was that when the softwood distillate content 

exceeded 4 wt%, statistically significant or highly significant 

increases were observed in strain and bending. 


Results 

Table 6. The mechanical properties of the WPCs in studies III and IV (mean ± 

standard deviation). The underlined values indicate at least significant (p 0.05) 

difference in comparison with the other WPCs in the same study. 

Property LG LG + HWD1 LG + HWD2 LG + HWD4 LG + HWD8 

TS (MPa) 22.41 ± 0.82 22.85 ± 0.31 22.26 ± 0.30 22.05 ± 0.44 19.67 ± 0.52 ** 

TM (GPa) 2.09 ± 0.20 2.33 ± 0.09 ** 2.24 ± 0.06 2.16 ± 0.06 1.86 ± 0.11 ** 

(mm) 1.86 ± 0.26 1.81 ± 0.12 1.83 ± 0.15 1.92 ± 0.17 1.85 ± 0.13 

FS (MPa) 44.09 ± 2.00 45.27 ± 1.01 43.09 ± 0.83 42.86 ± 1.17 39.36 ± 0.63 ** 

MOE (GPa) 3.05 ± 0.16 3.21 ± 0.08 3.11 ± 0.12 2.89 ± 0.09 * 2.73 ± 0.09 ** 

B (mm) 4.54 ± 0.28 4.42 ± 0.28 4.37 ± 0.30 4.72 ± 0.29 4.67 ± 0.27 

CIS (kJ/m 2 ) 11.57 ± 2.02 10.61 ± 1.27 11.17 ± 0.81 10.89 ± 1.13 9.42 ± 0.69 * 

Property LG + SWD1 LG + SWD2 LG + SWD4 LG + SWD8 LG + SWD20 

TS (MPa) 21.13 ± 1.03 * 23.54 ± 0.66 ** 22.45 ± 0.81 19.51 ± 0.79 ** 15.46 ± 1.56 ** 

TM (GPa) 1.98 ± 0.18 2.16 ± 0.06 1.96 ± 0.10 1.60 ± 0.10 ** 1.07 ± 0.23 ** 

(mm) 1.94 ± 0.12 2.07 ± 0.15 2.13 ± 0.12 * 2.18 ± 0.14 ** 2.60 ± 0.39 ** 

FS (MPa) 43.04 ± 2.48 45.47 ± 1.40 40.81 ± 4.08 39.39 ± 1.41 ** 32.80 ± 2.38 ** 

MOE (GPa) 2.91 ± 0.28 3.10 ± 0.07 2.58 ± 0.33 ** 2.34 ± 0.15 ** 1.70 ± 0.29 ** 

B (mm) 4.71 ± 0.21 4.73 ± 0.27 5.21 ± 0.35 ** 5.59 ± 0.30 ** 6.65 ± 0.71 ** 

CIS (kJ/m 2 ) 10.40 ± 1.52 12.08 ± 2.12 12.22 ± 1.65 11.26 ± 1.41 10.63 ± 1.14 

* p 0.05 (a significant difference compared with the unmodified WPC). 

** p < 0.01 (a highly significant difference compared with the unmodified WPC). 

LG = LunaGrain, HWD = Hardwood distillate, SWD = Softwood distillate, 

TS = tensile strength, TM = tensile modulus, = strain, FS = flexural strength, 

MOE = modulus of elasticity, B = bending, CIS = Charpy’s impact strength. 


Water absorption of WPCs was determined in studies I, III, and 

IV. In study I, the addition of hardwood distillate did not have 

any consistent effects on the amount of water absorbed by the 

UFs; for example, when distillate-treated LG was added to UF20, 

a minor increase was observed in water absorption, but the 

opposite effect was detected for UF30 and UF50. However, 

when 4.2 wt% of distillate was added to LG, water absorption 

reduced by over 30%. 

In study III, the addition of hardwood distillate considerably 

decreased water absorption of the WPCs (Figure 14). The major 

differences occurred during the first 24 hours of immersion as 

the differences between the material types remained more or 




less the same after 48 hours even though there was an increase 

in the values for all the materials. After 48 hours of immersion, 

LG containing 8 wt% of hardwood distillate had absorbed 

approximately 20% less water than unmodified LG. 

Accordingly, the difference between the unmodified LG and LG 

with 1 wt% of hardwood distillate was about 10%. 

The addition of softwood distillate did not decrease water 

absorption of the WPCs to the same extent as hardwood 

distillate even though water absorption decreased as a function 

of the distillate content (Figure 14). The difference between 

water absorption of LG and LG + SWD20 was approximately 

16% whereas no difference was observed between LG and LG 

with 1 wt% of distillate. 

Study IV also shows that the difference in the water 

absorption values between LG and distillate-treated LGs 

decreased after 48 hours of immersion. The WPCs containing 

softwood distillate absorbed more water between 24 and 48 

hours of immersion than those containing hardwood distillate. 

0.55 

0.5 

HWD, 24h 

SWD, 24h 

HWD, 48h 

SWD, 48h 

Moisture content (wt%) 

0.45 

0.4 

0.35 

0.3 

0.25 

0 5 10 15 20 

Distillate content (wt%) 

Figure 14. Moisture content of the WPCs after 24 and 48 hours of water 

immersion in studies III and IV. HWD = hardwood distillate, and SWD = softwood 

distillate. 


Results 


The VOC emission rates of a WPC deck determined in study II 

are presented in Figures 15 and 16. The compounds of interest 

were formaldehyde, acetaldehyde, acetic acid, cyclohexene, 

furan, furfural, guaiacol, and monoterpenes. 

Decreasing trends of the emission rates were observed for 

acetaldehyde, furfural, and monoterpenes. Additionally, a 

minor decline was observed in the guaiacol emission rates. 

Formaldehyde and furan emission rates remained relatively 

stable during the experiment, but acetic acid emission rates 

fluctuated, especially at the beginning of the trial. During the 

first 13 days of the experiment, cyclohexene emission rates 

remained relatively stable. However, starting from the day 16, 

the emission rates of cyclohexene nearly tripled compared with 

the values observed during the first 13 days. 

The VOC emission rates of the seven different WPC decks 

determined in study II are presented in Figures 17 and 18. One 

of the decks (LunaComp 3) was also used in the 41-day trial. 

1.8 

1.6 

1.4 

Emission rate (g/kgh) 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

Formaldehyde Acetaldehyde Acetic acid Cyclohexene 

Days: 1 3 6 10 13 16 21 24 28 34 41 

Figure 15. The emission rates of formaldehyde, acetaldehyde, acetic acid, and 

cyclohexene from a WPC deck during a 41-day period. 




2 

1.8 

1.6 


1.4 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

Furan Furfural Guaiacol Monoterpenes 

Days: 1 3 6 10 13 16 21 24 28 34 41 

Figure 16. The emission rates of furan, furfural, guaiacol, and monoterpenes 

from a WPC deck during a 41-day period. 

4 

3.5 


3 

2.5 

2 

1.5 

1 

0.5 

0 

Formaldehyde Acetaldehyde Acetic acid Cyclohexene 

Manufacturer 1 Manufacturer 2 UPM ProFi 1 UPM ProFi 2 

LunaComp 1 LunaComp 2 LunaComp 3 

Figure 17. The emission rates of formaldehyde, acetaldehyde, acetic acid and 

cyclohexene from seven different WPC decks. 


Results 

0.7 

0.6 


0.5 

0.4 

0.3 

0.2 

0.1 

0 

Furan Furfural Guaiacol Monoterpenes 

Manufacturer 1 Manufacturer 2 UPM ProFi 1 UPM ProFi 2 

LunaComp 1 LunaComp 2 LunaComp 3 

Figure 18. The emission rates of furan, furfural, guaiacol, and monoterpenes 

from seven different WPC decks. 

An overview of the results reveals that the deck from 

Manufacturer 2 had the highest emission rates for cyclohexene, 

furan, furfural, and guaiacol. The highest formaldehyde 

emission rates were observed for the deck from Manufacturer 1. 

UPM ProFi had the highest acetic acid and acetaldehyde 

emission rates. Monoterpenes were most abundantly released 

from UPM ProFi 2. In general, LunaComp decks had the lowest 

VOC emission rates when compared with the other 

manufacturers. 

The emission rates obtained from the comparative study 

were further converted into real room concentrations. This 

conversion revealed that acetaldehyde and guaiacol exceeded 

their odor thresholds and therefore, it was likely that they could 

be smelled from the decks. The values for other VOCs were low 

with respect to their odor thresholds. 

In study III, the effects of hardwood distillate addition on the 

VOC characteristics of the WPCs were assessed by determining 

the emission rates of cyclohexene, furfural, guaiacol, 

monoterpenes, methanol, and benzene (Figures 19 and 20). 




10 

9 

8 

Emission rate (g/m 2 h) 

7 

6 

5 

4 

3 

2 

1 

0 

Cyclohexene Furfural Guaiacol 

LG LG + HWD1 LG + HWD2 LG + HWD4 LG + HWD8 

Figure 19. The emission rates of cyclohexene, furfural, and guaiacol from the 

WPCs treated with hardwood distillate. 

35 

30 


25 

20 

15 

10 

5 

0 

Monoterpenes Methanol Benzene 

LG LG + HWD1 LG + HWD2 LG + HWD4 LG + HWD8 

Figure 20. The emission rates of monoterpenes, methanol, and benzene from the 

WPCs treated with hardwood distillate. 


Results 

The addition of hardwood distillate increased the emission rates 

of the studied compounds, especially for cyclohexene, furfural, 

monoterpenes, and methanol. Monoterpenes were most 

abundantly emitted whereas only trace amounts of guaiacol 

were detected. The emission rates of benzene were also low. The 

conversion of the emission rates into real room concentrations 

indicated that the odor threshold of guaiacol would be exceeded 

for all of these materials. The odor threshold for monoterpenes 

was also exceeded when 8 wt% of hardwood distillate was 

added to the WPC. 

Similar evaluations were done in study IV where WPCs were 

modified with softwood distillate. The emission rates of 

cyclohexene, furfural, guaiacol, monoterpenes, acetaldehyde, 

and benzene were determined (Figures 21 and 22). 

The addition of softwood distillate clearly increased the 

emission rates of cyclohexene, furfural, and monoterpenes. The 

emission rates of benzene and guaiacol, in turn, remained rather 

low although the odor threshold of guaiacol was exceeded in all 

of the materials. Acetaldehyde emission rates decreased below 

the odor threshold when softwood distillate content was 

increased from 1 wt% to 8 wt%. An increase in the distillate 

content from 8 wt% to 20 wt% resulted in a considerably higher 

emission rate of acetaldehyde, and as a consequence, the odor 

threshold of acetaldehyde was exceeded. 




25 


20 

15 

10 

5 

0 

Cyclohexene Furfural Guaiacol 

LG LG + SWD1 LG + SWD2 

LG + SWD4 LG + SWD8 LG + SWD20 

Figure 21. The emission rates of cyclohexene, furfural, and guaiacol from the 

WPCs treated with softwood distillate. 

20 

18 

16 


14 

12 

10 

8 

6 

4 

2 

0 

Monoterpenes Acetaldehyde Benzene 

LG LG + SWD1 LG + SWD2 

LG + SWD4 LG + SWD8 LG + SWD20 

Figure 22. The emission rates of monoterpenes, acetaldehyde, and benzene from 

the WPCs treated with softwood distillate. 


8 Discussion 

In the present thesis, the effects of hardwood and softwood 

distillates on the characteristics of commercial WPCs were 

studied. The first objective of this thesis was to assess the 

suitability of the impregnation method used to incorporate the 

distillates into the WPC granules. The suitability of wood 

distillates as additives in WPCs was determined by conducting 

mechanical tests, water absorption studies, and via a 

characterization of VOC emission rates. In addition, the 

applicability of PTR-TOF-MS for determining the VOC 

emissions from WPCs was assessed. 

8.1 IMPREGNATION OF WPC GRANULES WITH WOOD 

DISTILLATES 

The impregnation of the WPC granules with the distillates was 

successfully performed for the LG granules but not for the UFs. 

Commercially available granules were used in this thesis 

because the impregnation of the WPC granules with the 

distillates was both straightforward and quick and there were 

no suitable facilities with which to produce WPC granules from 

raw materials. Commercialized WPC granules contain multiple 

additives, and theoretically, these could have interfered with 

the modifications investigated here. Nonetheless, the present 

results clearly demonstrated that wood distillates can be added 

to certain types of WPC granules and this can improve their 

processability during injection molding. 

In future studies, the incorporation of distillates into WPCs 

could be carried out at an earlier stage of the WPC production. 

The impregnation of the agglomerate or wood particles with the 

distillates could eliminate at least some of the limitations 

associated with the impregnation of WPC granules. For 




example, the distillates could be directly incorporated onto the 

surfaces of the WPC constituents. Moreover, this would permit 

a more precise control of the amounts of other additives used in 

the WPCs. It would also demonstrate whether wood distillates 

could replace some of the additives currently used in WPCs. 


The results from study I indicated that an increase in cellulose 

fiber content in UFs enhanced the modulus of elasticity and 

tensile modulus, a property attributable to the higher stiffness 

and crystallinity of cellulose fibers compared to those of PP. A 

similar phenomenon, in accordance with the rule of mixtures, 

has also been observed in other studies (Balasuriya et al. 2002, 

Bouafif et al. 2009, Ashori et al. 2011). However, the maximum 

values of flexural and tensile strength for UFs were achieved 

with cellulose fiber content of 40 wt%. A similar finding was 

also reported by Ndiaye et al. (2013), Bhaskar et al. (2012) and 

Yuan et al. (2008). The decrease in flexural and tensile strength 

was probably due to the limited bonding between the cellulose 

fibers and the PP matrix. On the other hand, Huang and Zhang 

(2009) also postulated that when the fiber content exceeded 40 

wt%, WPCs became more susceptible to the formation of fiber 

agglomerates that decrease the mechanical strength of the 

composites. 

When the distillate-treated LGs were added to UF20, UF30, 

and UF40, the resulting composites were mechanically weaker 

than the untreated composites. However, the addition of LG 

and distillate-treated LG in UF50 exerted different effects; for 

example, UF 50 containing LG or distillate-modified LG 

possessed significantly higher flexural and tensile strengths 

than the unmodified material. These findings suggested that the 

distillate improves the mechanical durability of WPCs only 

when there is a higher fiber content (over 40%). On the other 

hand, these results could also be attributed to the chemical 

nature of the wood fibers in LG; as LG has thermally modified 


Discussion 

sawdust as its fiber material, i.e., the fibers are more 

hydrophobic, resulting in stronger interactions between the 

fillers and the polymer matrix – especially if there is a high fiber 

content. 

The LG composites had lower flexural and tensile strength 

than the UFs. In addition, only UF20 had a lower modulus of 

elasticity and tensile modulus than LG. As mentioned in section 

4.1, the degradation of wood components during the thermal 

treatment process results in the formation of organic acids, such 

as acetic acid, and these can catalyze the degradation of wood 

fibers. Therefore, composites having thermally modified wood 

dust as their filler have lower mechanical strengths than 

composites with unmodified wood fibers. Wood fiber size can 

also have a significant effect on the strength of WPCs as 

discussed in chapter 3.1 and demonstrated by Bouafif et al. 

(2009) and Kociszewski et al. (2012). Optical microscopic 

analyses (unpublished) for the samples in study I revealed that 

LG had considerably larger fibers than UFs, which can also 

partly explain the differences in the results. Moreover, no 

distillate agglomeration was observed. The energy required to 

break a specimen is known to be lower for those materials 

containing large fibers or fiber agglomerates because the cracks 

tend to travel through the large particles (Griffith 1921, Weibull 

1951, Bouafif et al. 2009). 

Incorporation of 4.2 wt% of hardwood distillate did not 

improve the mechanical properties for LG. Since the distillate 

acts like a coupling agent or any other additive that enhances 

the bonding between wood fibers and polymer matrix, it is 

possible that only a small amount of distillate would be 

required to enhance the properties of WPCs. The optimum 

amount of coupling agents in WPCs has been reported to be 

approximately 2 wt% (Balasuriya et al. 2002), and as mentioned 

in section 2.1.3, the typical amount of additives in WPCs is less 

than 5%. This theory was tested in study III, and the results 

revealed that the mechanically strongest composite was 

achieved when the hardwood distillate content was 1 wt%. An 

increase in tensile modulus is evidence that the composite had 




become stiffer after the distillate addition. Mathematically, the 

tensile modulus is the slope of the stress-strain curve. An 

increase in the slope means that more force is required to strain 

the material. The higher stiffness, which was also indicated by 

the decreased strain and bending with the higher distillate 

content, could be attributed to the capability of hardwood 

distillate to fill the gaps and pores in WPCs. This leads to 

restricted movement of the polymer chains within the material. 

The restricted movement of the polymer chains and the reduced 

ductility also explains the reduced Charpy’s impact strength. 

On the other hand, as the distillate fills the pores in WPC, this 

may increase the interfacial bonding between the polymer and 

wood fibers. Other studies have also revealed a correlation 

between material porosity and elastic modulus; as the material 

becomes less porous, elastic modulus increases (Patterson 2001, 

Bledzki et al. 2005b). 

The possible inclusion of distillates into the polymer-fiber 

interphase could also explain the improvements to the 

mechanical properties of the WPCs. The distillates are 

chemically hydrophobic but they may be able to bind to 

hydrophilic wood fibers through mechanical interlocking. On 

the other hand, the chemical similarity between the polymer 

matrix and the distillates could create a rather strong polymerdistillate 

interface. The molecular entanglement between the 

polymerized molecules in the distillates and the polymer chains 

may also account for the improved mechanical durability of the 

WPCs. Thus, the compositional differences between hardwood 

and softwood distillate may affect the compatibility 

characteristics and the interactions in the fiber-polymer 

interphase. When the distillate content was higher than 4 wt%, 

the excess distillate may have agglomerated or otherwise 

disrupted the interactions between the additives and other 

WPC constituents. Thus, there was a reduction of the strength 

of the WPCs. For more homogeneous distribution, the distillates 

may have to be added to the WPCs at the earlier stage of the 

production, which could also enhance the positive effect on the 

mechanical properties of the material. 


Discussion 

Softwood distillate had distinct effects in comparison with 

hardwood distillate on the WPCs. The tensile strength of the 

WPC containing 2 wt% of softwood distillate was enhanced by 

over 5% compared to the unmodified WPC, and improvements 

in some other properties were also observed. One of the most 

clear distinctions between the WPCs with hardwood and 

softwood distillates was that the addition of hardwood distillate 

stiffened the composite whereas opposite effect was observed 

for the WPCs supplemented with softwood distillate. The 

improvement in mechanical strength at 2 wt% distillate content 

and a major increase in bending and strain at higher distillate 

contents (over 4 wt%) indicated that softwood distillate may 

improve the interfacial properties of the WPC constituents. 

However, it was possible that unlike the hardwood distillate, 

softwood distillate did not restrict the movement of polymer 

chains within the composite. 

The differences between the distillates can be attributed to 

their compositional differences; the softwood distillate was 

formed at lower temperatures (maximum temperature 215 °C) 

in ThermoWood ® process whereas hardwood distillate was 

obtained from a slow pyrolysis process where the maximum 

temperature was approximately 350 °C. The softwood distillate 

primarily consisted of hemicellulose degradation products 

whereas the hardwood distillate was a mixture of cellulose, 

hemicellulose, extractive and lignin degradation products. 

Moreover, the composition of the distillates was affected by the 

feedstock: softwood distillate was obtained primarily from pine 

whereas hardwood distillate was acquired from the slow 

pyrolysis of birch. In general, hardwoods contain more cellulose 

than softwoods, and the lignin content in softwoods is higher 

than in hardwoods. 


The positive effects of hardwood and softwood distillates on 

water absorption of the WPCs were evident in studies III and IV. 




However, no conclusions can be drawn on the effects of 

hardwood distillate on water absorption of UFs because the 

granules could not be successfully impregnated with the 

distillate. As expected, and as shown previously by Ndiaye et 

al. (2013), water absorption of the composites increased as a 

function of fiber content. However, water absorption did not 

consistently decrease after the addition of distillate-treated LGs. 

An incompatibility of the raw materials may explain the 

inconsistent results as the wood-based fillers used in the 

composites are chemically different. Moreover, the distillate 

may be more effective with WPCs that contain thermally 

modified wood fibers as the reinforcing material. 

The results from study I indicated that the extent of water 

absorption of LG was considerably lower than that of UF50 even 

though they possessed the same fiber content. This finding is in 

accordance with other studies, and it can be attributed to the fact 

that thermally modified wood fibers are less hydrophilic than 

pure cellulose fibers because of the degradation of 

hemicelluloses and other changes occurring in the structure of 

cellulose during the thermal modification process (Balasuriya et 

al. 2002, Ayrilmis et al. 2011). Water absorption of UF50 was 

approximately 0.63%, which was in agreement with the values 

found in the literature; typical water absorption (24 h) for WPCs 

with a wood fiber content of 50–65 wt% has been reported to be 

in the range 0.7–2.0% (Klyosov 2007). According to the 

manufacturer (UPM), water absorption (ISO 62, 24 h) of UF40 is 

0.66% whereas water absorption of UF50 is 0.90%. The 

considerably lower water absorption of the UFs used in this 

thesis may be due to the differences in the manufacture of the 

sample. Moreover, it was unclear whether UPM used similar 

samples in their tests. 

Water absorption of LG can be further decreased by adding 

hardwood or softwood distillates; this was examined in studies 

I, III, and IV. As discussed in the previous section, the distillates 

may fill the pores and voids in the WPC, and affect the fiberpolymer 

interface or interphase. Therefore, the penetration of 

water molecules into the hydrophilic wood fibers may be 


Discussion 

hindered (Oksman Niska and Sanadi 2008). This theory is also 

in accordance with the percolation theory presented by Wang et 

al. (2006). On the other hand, the hydrophobic nature of the 

distillates could also explain the findings because both 

distillates were obtained from the water-insoluble fractions. 

Thus, the addition of distillates resulted in more hydrophobic 

WPCs that absorbed less water. In order to minimize water 

absorption of WPCs, high amounts of wood distillates should 

be used, but this would result in poorer mechanical properties. 

Hardwood distillate (study III) decreased the water 

absorption of LGs more efficiently than softwood distillate 

(study IV). This might be due to compositional differences of the 

distillates but the different densities of the composites might 

also affect the water absorption. Namely, the composites treated 

with hardwood distillate had higher densities than the ones 

treated with softwood distillate. A higher density of the WPC 

indicates lower porosity (Klyosov 2007). Thus, WPCs with a 

high density are less prone to the water absorption than the lowdensity 

WPCs. In addition, the water absorption as well as the 

physical and mechanical properties of WPCs are also 

prominently affected by the manufacturing technology and the 

process parameters (Yeh and Gupta 2008, Migneault et al. 2009). 


The emission rates of a WPC deck in the 41-day trial showed 

that acetaldehyde, furfural, and monoterpenes were the 

compounds most abundantly emitted. However, a trend 

towards decreasing emissions could be identified for these 

compounds. A similar decrease, but not to the same extent, was 

also observed for guaiacol. Furan and formaldehyde were 

emitted at a rather stable rate throughout the trial, but the 

emission rates of acetic acid and cyclohexene changed 

inconsistently. The results, therefore, indicate that the emissions 

of WPCs fluctuate extensively after their manufacture and 




furthermore they can change considerably during the storage 

depending on the conditions. 

The emissions of acetic acid and aldehydes from the WPC 

were not surprising as these compounds are formed during the 

thermal modification and extrusion processes (Peters et al. 2008) 

and they originate primarily from the degradation of 

hemicelluloses. However, the emissions of these compounds 

need to be carefully monitored because they have harmful 

effects on indoor air quality and they can damage human health. 

Acetic acid is an irritant compound with a rather low odor 

threshold (Akakabe et al. 2006). Formaldehyde and 

acetaldehyde, in turn, are carcinogenic compounds with 

mutagenic and irritant properties (Silla et al. 2001). The emission 

rates of formaldehyde remained low during the trial; those of 

acetaldehyde decreased substantially after 41 days, but no signs 

of any trend-like behavior could be identified for acetic acid. 

Low formaldehyde emission levels have also been reported for 

wood (Roffael 2006). The proton affinity of formaldehyde is 

only slightly greater than that of water, which may lead to 

reverse proton transfer reactions between the protonated 

formaldehyde and water molecules (Hansel et al. 1995, Schripp 

et al. 2010). For this reason, it is possible that not all of the 

formaldehyde was detected by PTR-TOF-MS. On the other 

hand, the TD-GC-FID/MS-system has similar limitations as 

described in section 3.3.1. 

Hyttinen et al. (2010) obtained similar results when they 

evaluated the emission rates of acetic acid in their comparison 

of VOC emissions between air-dried and heat-treated wood 

species. They postulated that the fluctuations in acetic acid 

emissions could be caused by the diffusion of the compound 

from the inside of the material to the surface if it was not evenly 

distributed within the sample. A similar explanation could be 

also the case for WPCs. Yrieix et al. (2010) analyzed VOCs from 

wood-based panels and observed that acetaldehyde emissions 

decreased by over 62% during their 25 day sampling period. In 

addition, they did not detect any major changes in 

formaldehyde emissions during the trial. Even though their 


Discussion 

findings mirror the results obtained in study II, the material 

they used is not fully comparable to WPCs. Particleboards and 

wood-based panels sometimes contain formaldehyde-based 

resins that can dramatically alter the emission characteristics. 

This could also be observed in their results; the emissions of 

formaldehyde were over ten times higher than those of 

acetaldehyde after 28 days. Contrasting results were obtained in 

study II: the emissions of acetaldehyde were approximately 3 

times higher than those of formaldehyde after 41 days. 

Cyclohexene is an irritant to humans and it is considered to 

be at least partly responsible for the unpleasant odor of WPCs. 

Cyclohexene and its derivatives are present in the essential oils 

of multiple wood species (Amoore and Hautala 1983, 

Chowdhury et al. 2008, Peters et al. 2008). The emissions of 

cyclohexene remained stable during the first 13 days of the trial. 

However, the detected emissions nearly tripled on day 16 and 

remained at that level for the rest of the trial. The emissions of 

cyclohexene from WPCs have not been studied previously, so 

there is no supporting information for this finding. There are 

several reasons to account for the unexpected change on day 16. 

For example, the unique release kinetics of cyclohexene can 

explain the result. In addition, due to the extremely high 

sensitivity of PTR-TOF-MS, one cannot completely rule out the 

possibility that some minor loosening or movements of the 

Kapton ® tape may have been responsible for the change in the 

emissions. However, this is unlikely because one would have 

expected to detect similar changes in the emission rates of other 

VOCs. 

Furan and guaiacol have similar odor characteristics and 

they are formed in similar processes. They have odors 

reminiscent of burning wood, and both are formed during the 

thermal treatment. (Goldstein 2002, Greenberg et al. 2006, 

Aigner et al. 2009) Furan has been classified as a possible 

carcinogen with a high odor threshold (Bakhiya and Appel 2010) 

whereas guaiacol has a very low odor threshold with no direct 

health effects. The detected emission rates for furan were low 

throughout the trial, and there were no major changes observed 




in the emission rates. Therefore, the results suggested that furan 

does not contribute any major effects to the VOC characteristics 

of WPCs. Guaiacol was also emitted to a low extent, but it is 

possible that it can be smelled from WPCs due to its low odor 

threshold. 

Furfural was emitted most abundantly in the beginning of 

the trial. However, as the trial continued, the emission rates of 

furfural decreased continuously. Previously, the emissions of 

furfural from heat-treated wood species have been investigated 

by Hyttinen et al. (2010). They demonstrated that heat treatment 

caused the formation and higher emissions of furfural. In 

contrast to the present findings, the emission rates of furfural 

did not decline during the test period in their study – in contrast, 

the emission behavior of furfural resembled that of acetic acid. 

Furfural is formed from the degradation of hemicelluloses and 

cellulose, and therefore, it is possible that the degradation 

process had continued in the wood samples during the 

measurements. The inconsistent emissions rates may be caused 

by the uneven diffusion of furfural. The plastic matrix in WPCs 

may decelerate the diffusion of furfural from the inside of the 

material to the surface, and therefore, the detected emission 

rates constantly declined. 

Monoterpenes, such as - and -pinene, are abundant in 

wood (Roffael et al. 2015). They have a pleasant resinous aroma 

with pine-like odors, are highly repellant to insects and possess 

antioxidative properties (Goldstein 2002, Nerio et al. 2010, Silva 

et al. 2012). Monoterpenes do not cause major changes in the 

respiratory tract or lung function, but they have evoked chronic 

reactions in the airways (Eriksson and Levin 1990, Falk et al. 

1990, Eriksson et al. 1997). The presence of monoterpenes is 

therefore permissible only to a certain extent. Monoterpene 

emission rates decreased during the trial. Yrieix et al. (2010) also 

observed a decrease in the monoterpene emissions from their 

wood-based panels. Similarly, Roffael (2006) reported 

decreasing emission rates for monoterpenes from wood. 

LunaComp decks, that have thermally modified sawdust as 

a reinforcement material, had the lowest VOC emission rates in 


Discussion 

the comparative study of the seven different WPC decks. In 

general, thermally modified wood has lower VOC emissions 

than unmodified wood, although there is greater release of 

some irritating compounds, such as acetic acid (Manninen et al. 

2002). This could not be observed in the present experiments, 

but it was probably due to the different ages of the decks. 

Moreover, possible variations in surface quality of the samples, 

the sample contamination and changes in the production can 

explain the differences. This comparison, however, provided 

some interesting aspects. When the emission rates were 

converted into real room concentrations, the results suggested 

that acetaldehyde and guaiacol, which have low odor 

thresholds, could be smelled from the decks. 

In study III, methanol was monitored because it is a toxic 

hydrocarbon that is abundantly emitted from various plant 

species (Rinne et al. 2007). It is found especially in wood 

pyrolysis liquids (Fagernäs et al. 2012a). As expected, the 

addition of hardwood distillate lead to the increased overall 

VOC emissions. The composition of distillate could explain the 

findings; most of the compounds monitored in study III are 

formed through the thermal degradation of the wood 

components. The slow pyrolysis oil from hardwood has been 

extensively analyzed by Fagernäs et al. (2012a), and their 

findings support the present results. The emission rates of 

benzene and guaiacol remained low, even when there was a 

high (8 wt%) distillate content, indicating that these compounds 

were not abundantly present in the hardwood distillate 

presumably due to relatively low process temperature. It is also 

possible that the plastic matrix in WPCs reduces the diffusion of 

these compounds within the material. It is desirable that there 

are low emission rates of these compounds as benzene is a 

carcinogenic compound and guaiacol has an unpleasant odor. 

The odor of monoterpenes (-pinene) was detectable with 

8 wt% of distillate. 

The effects of softwood distillate on the VOC characteristics 

of the WPC were different when compared with the hardwood 

distillate, which could be explained by the compositional 




differences between the raw materials (Schwarzinger et al. 2008). 

A comparison of the VOCs studied showed that monoterpenes 

were the compounds being emitted most from the hardwood 

distillate modified WPCs whereas cyclohexene was emitted 

most abundantly from the softwood distillate modified WPCs. 

Surprisingly, monoterpene emissions were higher from the 

hardwood distillate modified WPCs than from the WPCs 

containing the softwood distillate. In general, hardwoods 

mainly contain sterols, triterpenoids, and higher terpenes 

whereas softwood terpenes possess mono-, sesqui-, and 

diterpenes together with sterols (Björklund Jansson and 

Nilvebrant 2009). It was, therefore, assumed that the WPCs 

treated with softwood distillate would have higher 

monoterpene emissions than those modified with hardwood 

distillate. As terpenes are constructed from units of isoprene, it 

is possible that the higher terpenes in hardwood had thermally 

decomposed during the slow pyrolysis process, which resulted 

in the formation of lower terpenes, such as monoterpenes. 

Yadav et al. (2014) demonstrated that isoprene is one of the 

products of the thermal decomposition of terpenes. 

Both hardwood and softwood distillates contained only 

small amounts of guaiacol and benzene even though guaiacol 

could be smelled from the distillate modified WPCs. 

Interestingly, when the softwood distillate content was 

increased from 1 wt% to 8 wt%, the emissions of acetaldehyde 

declined. This also affected the odor characteristics of the WPCs 

as acetaldehyde could not be smelled from the WPCs with 

2–8 wt% of distillate. This finding suggested that the distillate 

contains chemicals that react with acetaldehyde to form other 

compounds. 

There are no statutory limits for the material VOC emissions 

determined with PTR-MS. Thus, no conclusions could be made 

about whether the emission levels of a certain VOC exceed its 

safety limits. Due to the extremely high sensitivity of PTR-TOF- 

MS, the emission rates presented in this thesis may be 

considerably higher than the levels that would be estimated 

with the standardized chamber test method. However, the 


Discussion 

comparison of VOC emissions between different material types 

using PTR-TOF-MS is feasible and sometimes more practical 

than the traditional chamber test method. 

8.5 LIMITATIONS AND FUTURE PROSPECTS 

In this thesis, the effects of hardwood (birch) and softwood 

(primarily pine) distillates on the properties of WPCs were 

determined. Similar studies have not been previously 

conducted, even though the potential of other types of organic 

waste and residues as additives for WPCs has been extensively 

assessed (Hamzeh et al. 2011, Li et al. 2014, Madhoushi et al. 

2014, Das et al. 2015a). This thesis focused on the determination 

of the effects of different wood distillates on the mechanical 

properties, water resistance and VOC emissions of WPCs. 

The positive effects of both distillates on the mechanical 

properties of the WPCs were evident, and the underlying reason 

for the enhancement was anticipated to be the improvement in 

the interfacial bonding between the polymer matrix and wood 

fibers. Further investigations of WPCs with scanning electron 

microscopy (SEM) and spectroscopic techniques would provide 

more detailed information on the interactions occurring 

between WPC constituents before and after the additions of the 

distillates. 

The considerably lower water absorption values for the 

WPCs treated with distillates were attributed to the 

composition of the distillates and to the ability of the distillates 

to fill the gaps within the WPCs. It was assumed that the 

distillates were hydrophobic as they were obtained from the tar 

phases and all the water-soluble compounds had been extracted 

from the distillates. However, the exact compositions of the 

distillates were not characterized. The chemical composition of 

birch distillates has been determined by Fagernäs et al. (2012a), 

and it was presumed, based on our other studies, that the 

distillate used in this thesis would be composed of similar 

chemical compounds. The composition of softwood distillates 




was known only roughly and the analyses conducted in other 

publications were also used as supporting material (Vitasari et 

al. 2011, Miettinen et al. 2015, Özbay et al. 2015). Nevertheless, 

more information on the exact chemical composition of the 

distillates could provide further insights into the chemical 

interactions between distillates and WPC constituents and 

explain the phenomena observed in this thesis more rigorously. 

In this thesis, water absorption of the WPCs was studied for 

24 and 48 hours. Considerable differences between different 

material types were observed even at these time intervals, but it 

would be interesting to examine the differences after multiple 

weeks or months. It would also be interesting to determine the 

impact of cyclic weather conditions or microbial exposure on 

the properties of WPCs treated with wood distillates. 

Nonetheless, the results of this thesis suggest that the distillates 

had improved the water resistance of WPCs, especially at high 

distillate contents. The decreased water absorption of WPCs 

also suggests that the addition of wood distillates could 

improve the fungal resistance of WPCs, and this property 

should be evaluated in future studies. 

PTR-TOF-MS was tested to assess its suitability for 

determining the VOC emissions from WPCs. The applicability 

of PTR-TOF-MS for monitoring the concentrations of VOCs 

emitting from WPCs was confirmed but further analyses would 

be appropriate. In this thesis, the number of monitored VOCs 

was limited to ten compounds. It is apparent that further 

analyses of VOCs are needed. For example, the monitoring of 

hazardous VOCs, such as styrene, toluene, and naphthalene, 

could provide valuable information on the impact of WPCs on 

indoor air quality. 

In study I, two kinds of WPCs were used whereas only one 

kind of WPC was evaluated in studies III and IV. In the future, 

examinations of the effects of wood distillates on the 

characteristics of other types of WPCs are appropriate. Wood 

distillates may act differently if the polymer matrix or wood 

fiber type is changed. Furthermore, changes in the wood fiber 


Discussion 

content may lead to considerable changes in the properties of 

WPCs modified with wood distillates. 


9 Summary and 

conclusions 

In the present thesis, the effects of thermally extracted wood 

distillates on the properties of WPCs were successfully 

determined. The effects of the distillates were investigated 

through the mechanical tests, water absorption studies, and 

VOC emissions analyses. The main conclusions with respect to 

the aims are: 

1. The impregnation of the WPC granules with the wood 

distillate, originated from either hardwood or softwood, is 

possible with the presented method. However, other 

methods, such as impregnation of the agglomerate before 

the granulation process, could well be more suitable. The 

addition of distillates improved the processability of the 

WPC granules. 

2. PTR-TOF-MS can be applied to determine and compare the 

release of VOCs from WPCs. The advantages of PTR-TOF- 

MS include its rapid time response, high sensitivity, and 

ease of use. In addition, there is no need for special sample 

preparation. However, there are no regulatory limits for 

VOC emissions measured with PTR-MS. 

3. A small (1 wt%) addition of hardwood distillate 

significantly increased the tensile modulus of the WPC. 

Higher distillate contents (2–8 wt%) reduced the 

mechanical properties of the WPC. In addition, the ability 

of the material to absorb water was considerably decreased 

in those containing the hardwood distillate. The VOC 

emissions increased for the WPCs containing hardwood 

distillate. 

4. A minor (2 wt%) addition of softwood distillate 

significantly increased the tensile strength of the WPC. 




Strain and bending increased significantly when there was 

a high distillate content (over 4 wt%) whereas the strength 

of the WPC declined. The water absorption of the WPC can 

be reduced with distillate content higher than 2 wt%. The 

addition of softwood distillate increased the release of 

VOCs studied. 

To conclude, the studies presented in this thesis emphasize that 

the inclusion of wood distillates originating from industrial 

wood processes into WPCs can enhance the properties of these 

materials. In addition, it was shown that PTR-TOF-MS is a 

feasible technique for analyzing VOCs being emitted from 

WPCs. The further developments in wood distillates and the 

discovery of new ways to incorporate the distillates into WPCs 

could provide novel and ecological materials based more on the 

renewable resources. 


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Wood-plastic composites (WPCs) represent 

an ecological alternative to conventional 

petroleum-derived materials. The wood 

distillates studied in this thesis displayed good 

potential as bio-based additives for WPCs 

as they improved the water resistance and 

mechanical properties. It was also shown 

that proton-transfer-reaction time-of-flight 

mass-spectrometry (PTR-TOF-MS) can be 

applied to study the release of volatile organic 

compounds (VOCs) from WPCs. 

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