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Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on the Characteristics of Wood-Plastic Composites 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 38 Dissertations in Forestry and Natural Sciences No 222
Wood-plastic composites 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). Dissertations in Forestry and Natural Sciences No 222 39
Page 1: PUBLICATIONS OF THE UNIVERSITY OF E
Page 4 and 5: Grano Oy Jyväskylä, 2016 Editors:
Page 7 and 8: ABSTRACT The use of raw materials d
Page 9: Universal Decimal Classification: 6
Page 12 and 13: Finally, I am grateful to my wife A
Page 14 and 15: TD-GC-FID/MS Thermal desorption/gas
Page 17 and 18: LIST OF ORIGINAL PUBLICATIONS This
Page 19 and 20: Contents 1 Introduction ...........
Page 21 and 22: 1 Introduction The finiteness of cr
Page 23 and 24: Introduction A new and environmenta
Page 25 and 26: 2 Wood-plastic composites By defini
Page 27 and 28: Wood-plastic composites additives u
Page 29 and 30: Wood-plastic composites Cellulose i
Page 31 and 32: Wood-plastic composites The propert
Page 33 and 34: Wood-plastic composites Figure 3. T
Page 35 and 36: Wood-plastic composites of lubrican
Page 37 and 38: Wood-plastic composites roughness a
Page 39: Wood-plastic composites Hosseinaei
Page 43 and 44: Wood-plastic composites However, WP
Page 45 and 46: Wood-plastic composites A typical s
Page 47 and 48: Wood-plastic composites to fill the
Page 49: Wood-plastic composites absorption.
Page 52 and 53: Taneli Väisänen: Effects of Therm
Page 63 and 64: 4 Thermal processing of wood Therma
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Page 73 and 74: 5 Aims and significance Despite the
Page 75 and 76: 6 Materials and methods Two types o
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Page 81 and 82: Materials and methods 6.2.1 Tensile
Page 83 and 84: Materials and methods 6.3 WATER ABS
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Taneli Väisänen: Effects of Therm
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9 Summary and conclusions In the pr
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Bibliography Abdolvahaba E, Lashgar
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Bibliography Ayrilmis N, Jarusombut
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Bibliography Cernansky R. State-of-
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Bibliography polypropylene composit
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Bibliography with odor assessments
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Bibliography La Mantia FP, Morreale
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Bibliography Mohan D, Pittman CU, a
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Bibliography Rinne J, Taipale R, Ma
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Bibliography Stark NM and Rowlands
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Bibliography Yeh S and Gupta RK. Im
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