30. Furan-Based Adhesives

30 

Furan-Based Adhesives 

Mohamed Naceur Belgacem and Alessandro Gandini 

Ecole Française de Papeterie et des Industries Graphiques (INPG), 

St. Martin d’Hères, France 

I. INTRODUCTION 

The dwindling availability of fossil reserves constitutes a driving force towards finding 

alternative resources which can substitute them, totally or partially, in order to prepare 

chemicals and materials that are normally produced from petroleum and coal. In this 

context, vegetable biomass represents a very promising source since it offers a large variety 

of potential monomers, oligomers, and polymers, some of which can be extracted and used 

as such (namely, products such as terpenes, tannins, rosins, lignins, and cellulose) and 

others which can be suitably transformed to give monomers, solvents, surfactants, and a 

variety of polymeric materials (e.g., modified sugars, saponified oils, furfural and its 

derivatives, and cellulose acetates). We tried to show [1] that, besides its extensive use 

as a source of fibers for papermaking and textiles, vegetable biomass can also lead to 

interesting chemicals and materials. In a recent review [2], we focused on the use of furanic 

monomers for the preparation of polymeric materials and showed that different petroleum-based 

monomers (especially aromatic derivatives) could be replaced by their furanic 

counterparts. Thus, a variety of totally furanic, aromatic–furanic, and aliphatic–furanic 

polymers display properties similar to (and sometimes better than) those of currently used 

polymers derived from petroleum, proving that a whole area of biomass-based materials 

can be developed from two first-generation compounds which are readily available from a 

wide spectrum of renewable resources. 

Furanic monomers can be obtained from hemicelluloses which are among the main 

constituents of vegetal biomass and are abundant in trees and agricultural residues of 

annual plants, such as sugarcane bagasse, oat hulls, corn husks, rice, and wheat straw. 

The precursors of most industrial furan derivatives are obtained directly from hemicelluloses 

through the acid-catalyzed hydrolysis of pentosans (e.g., xylans) followed by 

dehydration and cyclization of the ensuing pentoses leading to the formation of furfural 

(1), which is today the most important first-generation furan derivative, produced 

industrially at a rate of ca. 200,000 tonnes per year. This output is spread widely 

among numerous countries, including both industrialized and developing economies, 

because the process is particularly simple and the raw materials are available and plentiful 

virtually everywhere and are renewable often on short cycles. An additional advantage 

of this approach is that it calls upon a rational exploitation of agricultural wastes. Furfural 

Copyright © 2003 by Taylor & Francis Group, LLC

can be used as such, but is mostly (more than 80%) converted into furfuryl alcohol 

(2) using either liquid-phase or vapor-phase hydrogenation in the presence of copper 

catalysts which were found to be very selective in avoiding the hydrogenation of 

the heterocycle ring [3]. 

Furfuryl alcohol finds numerous applications as monomer (see below) and has, 

therefore, been for decades the most important second-generation furan derivative. 

It is also used to prepare 2,5-bis(hydroxymethyl furan) (3) through its reaction with 

formaldehyde [3], namely: 

Compound 3 can also be prepared by the hydrogenation of 5-hydroxymethyl 

furfural (4) which, in turn, is obtained from hexoses following the same acid-catalyzed 

process described above for furfural [2]. 

Compounds 1, 2, and 3 are among the most relevant monomers or co-monomers for 

furan-based adhesives, but so also are furfurylidene acetone (5) and its bis-adduct 6. The 

synthesis of 5 involves the base-catalyzed reaction between 1 and acetone [2] and, in the 

same context, the use of an excess of 1 leads to the formation of 6: 

This chapter is devoted to adhesives and resins prepared from totally furanic 

monomers or formulations in which furanic compounds are added. In this realm, only 


a few furanic monomers and resins are involved, namely: 1, 2, 3, 5, and 6, as well as liquid 

oligomers of 2 (poly2) and3 (poly3). The properties of these monomers together with 

the mechanisms of their resinification and the composition of poly2 and poly3 will 

be briefly dealt with before discussing their use in the manufacture of resins for binders 

and adhesives. 

II. PROPERTIES OF FURANIC MONOMERS 

The relevant properties of furanic compounds covered in this review are summarized in 

Table 1. 

The compositions of poly2 and poly3 were studied by several groups [2,3] and shown 

to have mainly the following structures: 

Their relative abundance depends, of course, on the conditions used for their 

syntheses. A typical composition [3] is given in Table 2. 

Table 1 Properties of Furanic Compounds Used in Adhesives 

Compound type 1 2 3 5 6 

Molecular weight 96.09 98.10 128.10 136.15 214.22 

Boiling point ( C) 161 170 — 116 a 

— 

Melting point ( C) 39.7 

Density at 20 C (kg/dm 3 ) 1.16 1.13 — 1.06 b 

— 

Refractive index at 20 C 1.53 1.49 — — 

Viscosity at 25 C (mPa s) 1.48 4.62 — — — 

Surface tension (mN/m) 40 c 

38 d 

— — — 

a At 10 mm Hg. 

b at 45 C. 

c at 30 C. 

d at 25 C. 

Table 2 Typical Composition (w/w %) of poly2 and poly3 

7 8 9 10 

poly2 25 12 35 28 

poly3 — — 5 95 


III. HISTORY, ADVANTAGES, AND LIMITATIONS ASSOCIATED WITH 

THE USE OF FURAN-BASED ADHESIVES 

The first synthetic thermosets used as adhesives were phenol–formaldehyde resins 

produced at the end of the nineteenth century, historically linked to Baekeland’s process 

which attained industrial status at the beginning of the twentieth century [4]. Furanic 

condensates appeared much later as a result of the marketing of 2. They were first used 

as foundry binders by Quaker Oats in 1960. The use of furanic resins in the aerospace 

industry began ten years later. Although furanic resins represent a mere 1% of the total 

thermoset production, the high added-value of these materials amply justifies their use. 

In fact, furan-based adhesives and binders are fire-, solvent-, and acid- or alkali-resistant. 

They are known, however, to display two main drawbacks related to their sensitivity to 

shrinkage and oxidation. 

IV. RESINIFICATION MECHANISMS 

The acid- or heat-initiated cross-linking mechanisms of 1 were extensively studied for 

decades, but because of the complexity of the reactions involved and the effect of 

atmospheric conditions (e.g., light, oxygen, and water vapor) intermediate products 

were not identified until 1975. In that study, 1 was polymerized at 100–250 C in the 

absence of air and the following intermediates were isolated [5,6]: 

And for the final product, the following structure was proposed [5,6]: 

The polycondensation of 2 in acidic media has also been studied for a long time, 

but only recently was a clear-cut reaction mechanism established from a study in our 


laboratory [2,7,8]. The success of this investigation stemmed from the fact that a large 

number of model compounds were synthesized which helped to establish the mechanisms 

of both cross-linking and color formation in this process. The use of mild catalysts 

confirmed that the first steps of the polymerization reactions occurred as follows: 

This initial mechanism does not explain, however, these anomalies since both 

macromolecular structures should give rise to colorless and thermoplastic materials. 

It was then shown that only several units actually condensed following this mechanism, 

since the average degree of polymerization (DP) never exceeded about 5, and crosslinking 

and color formation rapidly took place thereafter. In the mechanism of color 

formation, sketched in Scheme 1, we postulated that the formation of highly conjugated 

sequences resulted from successive hydride-ion/proton abstraction cycles [7]. This 

mechanism was confirmed by using different model compounds which were treated with 

an excess of hydride-ion (H ) abstractors (such as dioxolenium or triphenylmethyl 

cations) and the ensuing reactions followed by both ultraviolet (UV)–visible and 

1 H nuclear magnetic resonance (NMR) spectroscopies. This mechanism also explained 

the presence of methyl groups already observed by several authors [9–11]. The reaction 

of poly2 (obtained at early stages of the polycondensation) with hydride-ion abstractors 

was again followed by UV–visible spectroscopy and the results confirmed the proposed 

mechanism. Thus, the presence of conjugated sequences of different lengths was 

established, since the corresponding carbenium ions absorbed at different wavelengths, 

namely around 420, 450, 540, 600, and 800 nm. 

Having solved the long-standing puzzle related to color formation, we switched to the 

problem of the occurrence of branching and/or cross-linking reactions [2,7,8]. It was 

argued that these events could start either from the ‘‘irregular’’ units formed by 

the mechanism shown in Scheme 1, as illustrated in Scheme 2, and/or by Diels–Alder 

reactions between two chains, as proposed in Scheme 3. In fact, since the participation 

of furanic hydrogen atoms at C3 and C4 and those of methylene bridges had been clearly 

excluded on the basis of model reactions, it seemed reasonable to attribute the branching 

and cross-linking reactions to these two mechanisms. The second alternative, involving 

the cross-linking through Diels–Alder reactions, was recently confirmed by using 

2,5-dimethyl furan as a solvent for the acid-catalyzed polycondensation of 2. In this 

experiment, the large excess of dimethyl furan played the role of predominant diene 

trap for the exo-dihydrofuran dienophiles and thus prevented their coupling with 

the regular units of poly2 (Scheme 3). The fact that in these conditions the polymers 

remained soluble up to long reaction times and high yields was taken as clear evidence 

of the validity of Scheme 3. 


Scheme 1 

Scheme 2 


V. FURAN RESINS AS FOUNDRY BINDERS 

Furan resins have been extensively used as foundry binders in combination with 

formaldehyde, urea, phenol, and casein, for decades [12,13]. The main two monomers 

used in this field are 1 and 2. Table 3 summarizes their proportions in different commercial 

phenolic resins [12]. 

The main advantages of furan resins are due to their excellent thermal stability, and 

remarkable resistance to acidic conditions, as well as to fire and corrosion. These resins 

Scheme 3 

Table 3 Proportions of 1 and 2 in Commercial Phenolic Resins 

Supplier 

Amount 

added 

(% w/w) 

1 2 

Amount 

retained after 

curing (% of the 

amount added) 

Amount 

added 

(% w/w) 

Amount 

retained after 

curing (% of the 

amount added) 

Bakelite 0215 Quaker Oats Co. 10 90 10 94 



Durez 7031 OxyChem 20 88 20 85 

Durez 8045 OxyChem — — 20 77 

Durez 14000 OxyChem — — 20 87 

Durite 278 Contenti Inc. 10 96 10 91 





Monsanto 795 Monsanto 20 88 — — 

Varcum 1364 OxyChem 20 87 20 84 

Varcum 1192 OxyChem 20 69 20 80 


have found widespread industrial applications as witnessed by the large number of 

both patents covering their uses and scientific publications dealing with their chemistry, 

structures, and properties [3,6,12,14–16]. 

There are three techniques associated with their production, mostly covered by 

patent literature, namely: (i) no-bake, (ii) hot-box, and (iii) cold-box processes. The nobake 

technique is simple and relatively cheap. It consists in mixing the resin (based on 2) 

with the sand in the presence of an acidic catalyst. The reaction starts at room temperature 

and the curing is accelerated by the heat generated during the polycondensation reaction. 

The molds thus obtained are withdrawn after 10–30 min and left undisturbed for 3–6 h in 

order to accomplish a total curing. The hot-box technique is used in light (e.g., aluminum) 

and heavy (e.g., copper, bronze) metal casting [4,17]. The resins used for light metals are 

urea-modified furan resins, whereas those used for heavy metals contain only furan components. 

The hot-box process is well suited for mass production and it consists in mixing 

the moist sand with a liquid resin and a curing agent. The ensuing mixture is then cured at 

180–260 C in heated core boxes. The main limitation of this process is its extremely long 

bench life. The cold-box (or SO2–furan) process is based on curing the reactive resin at 

room temperature in a closed-air system with SO2. This gas is converted in situ into a 

mixture of sulfurous and sulfuric acids which catalyze the curing. 

VI. FURAN RESINS AS WOOD ADHESIVES 

Regardless of the fact that numerous investigations exist about the possibility of incorporating 

the furan heterocycle into wood adhesive formulations, their industrial exploitation 

is still modest. The first suggestion concerning the use of 1 in partial substitution of 

formaldehyde in phenolic resins was put forward in 1958 by Baxter and Redfern [18] 

who proposed that the furfural units were incorporated into the polymer skeleton following 

condensation reactions such as: 

The intermediate oligomers such as 13 were then subjected to methylolation with formaldehyde 

to form phenolic–furanic–formaldehyde resins, according to: 


The interest in this type of process was, of course, the decrease of formaldehyde 

content and, therefore, its lower release during the life cycle of the resin. 

This approach was then extended by Pizzi’s group to other phenolic type adhesives 

such as phenol–resorcinol–formaldehyde networks [19]. In this work, it was shown that 

the addition of 1 gave cold setting resins with performances and costs comparable to those 

made using formaldehyde alone. Thus, the phenol–resorcinol–furfural–formaldehyde cold 

sets obtained appeared to have a lower bulk shrinkage compared to those prepared without 

1. Moreover, it was established that the presence of furfural did not slow down the 

curing rate of the resins. 

Stamm [20] studied the dimensional stabilization of different woods with 2. Thus, 

Douglas fir, Engelman spruce, loblolly pine, and yellow poplar woods were treated with 2 

in the presence of zinc chloride, citric acid, or formic acid in order to induce their acidcatalyzed 

polymerization. It was established that the maximum antishrinking efficiency 

(around 72%) could be reached with a resin level of a minimum of 40% with respect to 

oven dried (OD) wood. The optimal amount of each acidic catalyst was also determined. 

The curing time was studied for each system and it was shown that the use of 1% zinc 

chloride and 6 h of curing time at 120 C gave very satisfactory fracture moduli, toughness, 

abrasion resistance, and antishrinking behavior. The only limitation associated with the 

possible uses of these systems is the dark color of the final materials. 

Dhamaney [21] showed that the addition of furfural into cashew nut shell liquid 

adhesives based on phenol–formaldehyde resins, using CuCl2 or CaCO3 as a ‘‘hardener,’’ 

gave good adhesive bonding for ordinary plywood. Johns et al. [22] prepared white fir 

flakeboards using an aqueous solution containing a mixture of ammonium lignosulfonate, 

2, and maleic acid as a binder. Before bonding, the wood surface was activated by a nitric 

acid treatment. It was shown that the panels thus obtained possessed a higher elasticity 

modulus and lower thickness swell and water absorption compared with those prepared 

using classical phenol–formaldehyde binders. Nevertheless, the internal bonding and the 

rupture modulus were higher for panels obtained using conventional resins. It was also 

established that best surface activation was achieved using a 1.5% aqueous solution of 

nitric acid (25–40%) with respect to OD wood, since it gave the optimal mechanical 

properties for both high and low density panels. 

Gupta et al. [23] prepared plywoods from Cedrus deodora and phenol–formaldehyde 

resins. They showed that the addition of 5% of 1 to this adhesive did not result in any 

appreciable improvement, but the concomitant addition of 10% of coconut shell powder 

gave very high failing loads and very low glue failures. Subsequently, in another context, 

Pizzi et al. [24] tested different aliphatic aldehydes and 1, in tannin-based adhesives, and 

showed that furfural could replace formaldehyde in the manufacture of adhesive resins for 

beam lamination. Roczniak [25] studied the thermal properties of phenol–formaldehyde–1 

resins, as catalyzed by dichlorohydrin of glycerol, boric acid, hexamethylenetetramine 

(HMTA), or p-toluene sulfonic acid. Two main conclusions were drawn from this work: 

(i) p-toluene sulfonic acid gave a faster resinification rate and (ii) HMTA led to the highest 

thermal resistant resins. Krach and Gos [26] investigated the gluing of large dimension 

sawn wood structures using urea–melamine–furfural as a binder. They stated that the 

initial wood moisture (8 to 12%) and the time of adhesive spreading (10 to 90 min) did 

not influence significantly the strength properties of the glued junction. 

Philippou et al. [27] studied the bonding of wood by graft polymerization. They 

produced white fir, Douglas fir, and bishop pine particleboards using 2 as well as mixtures 

of ammonium lignosulfonate with 2 or with formaldehyde as cross-linking agents. Before 

bonding, the wood surface was activated with different amounts of hydrogen peroxide 


(from 0.5 to 4% with respect to OD wood). The amount of the binder was kept constant in 

all experiments (7% with respect to OD wood). The internal bond strength of the materials 

obtained was found to increase with increasing amounts of hydrogen peroxide, whereas 

the thickness swelling followed the inverse trend. The use of both 2 alone and its mixture 

with ammonium lignosulfonate showed very good bonding capability. Bishop pine gave 

the highest internal bonding and white fir yielded the lowest thickness swelling and water 

absorption when the mixture of ammonium lignosulfonate with 2 was used as a binder. 

The least efficient adhesive was found to be the formaldehyde–lignosulfonate system. The 

differences between wood species were attributed to their different contents of extractives. 

In another study, Philippou et al. [28] studied the effect of the composition of the bonding 

materials on the properties of Douglas fir particleboards. Thus, the proportion between 2 

and ammonium lignosulfonate was varied as follows: 10/0, 9/1, 8/2, 7/3, 6/4, 5/5, 2.5/7.5, 

and 0/10. In this work, the wood was also activated by hydrogen peroxide (2% w/w with 

respect to OD wood) and the catalysts used were ferric chloride and maleic acid. 

Ammonium lignosulfonate without 2 failed to develop resistance to boiling water whereas 

2 without ammonium lignosulfonate gave good mechanical and water resistance properties. 

However, the use of a mixture containing six parts of lignosulfonate and four parts of 

2 yielded boards with the highest internal bond strength and water resistance values. 

Increasing the amount of resin with respect to wood was found to produce an increase 

in the elasticity and rupture moduli and a decrease in water absorption and thickness 

swelling. The boards prepared exhibited strength and resistance to cold and boiling water 

comparable to those made using classical phenol–formaldehyde resins. In a third investigation, 

Philippou et al. [29] studied the effect of the processing parameters on the mechanical 

properties of particleboards made from Douglas fir wood treated with ammonium 

lignosulfonate and 2 as a binder in the presence of maleic acid as a catalyst. They showed 

that increasing the pressing temperature from 121 to 177 C or the pressing time from 4 to 

8 min, progressively enhanced the internal bond strength and the water resistance of the 

treated boards. The water resistance was found to be further improved by the addition of a 

small amount of wax (0.5% w/w with respect to OD wood) in the binder mixture. 

Leitheiser et al. [30] prepared water dilutable furan resins as binders for particleboard 

and showed that the resulting composites could be used for exterior applications. 

These resins were readily water dilutable and had low viscosities, which made their application 

with conventional equipment an easy process. Kelley et al. [31] prepared wood 

panels from Acer saccharum var. Marsh. with various binders. They first activated the 

surface of the wood by nitric acid and bonded the particles with tannin, 2, and a mixture of 

the two, with and without maleic acid. In all cases, the particleboards obtained exhibited 

shear strengths as high as that obtained from a control system made with a conventional 

phenol–formaldehyde binder. However, the acidic treatment of wood appeared to have 

only a slight effect on the mechanical properties of the panels bonded with the tannin–2– 

maleic acid system. Subramanian et al. [32] subjected Douglas fir wood flakes to a nitric 

acid treatment followed by a grafting reaction with 2(1-aziridinyl)ethyl methacrylate and 

2. They showed that the amount of carboxylic acid groups at the wood surface had 

increased substantially, thus enhancing its reactivity towards both reagents. 

Philippou and Zavarin [33] studied the interactions between lignocellulosic materials, 

2, and maleic acid in the presence or absence of hydrogen peroxide. They used white fir 

wood flour, microcrystalline cellulose, milled-wood lignin, and ammonium lignosulfonate 

and followed their interactions with the binder by differential scanning calorimetry (DSC) 

and concluded that a graft copolymerization between hydrogen peroxide activated wood, 

2, and ammonium lignosulfonate had occurred. Balaba and Subramanian [34] studied the 


polymerization of 2 catalyzed by the surface acidity resulting from treating wood with 

nitric acid. They followed the polymerization by intrinsic viscosity measurements and 

showed that there were two reaction regimes. The first was found to obey zero order 

kinetics, with an activation energy of 53.4 kJ/mol, whereas the second could not be 

exploited because of polymer precipitation following the formation of network structures. 

In 1985, experiments on an industrial scale were carried out jointly at Quaker Oats 

Chemicals and Collins Pine Company particleboard plants [35]. In these trials 1 was 

used as an extender in a polymeric methylene diphenyl isocyanate (MDI) binder 

(1:MDI ¼ 1:3 w/w). The main conclusions which could be reached from these trials 

were that savings in binder levels, pressing time, and temperature and drying requirements 

could be obtained compared with the corresponding performances of standard phenol– 

formaldehyde and urea–formaldehyde systems. 

Nguyen and Zavarin [36] studied graft polymerization of 2 on cellulosic materials. 

They showed that 2 in an aqueous medium at pH 2.0 and 90 C did not copolymerize with 

the cellulose surface in the presence of H2O2/Fe 2þ . However, under the same conditions, 

poly2 was efficiently grafted onto cellulosic fibers and the amount of homopolymer of 2 

was negligible. In these conditions, the amount of grafted poly2 reached 68% w/w with 

respect to OD fibers. They also showed that working at higher temperature and with more 

concentrated media yielded higher grafting efficiency. Sellers [37] prepared plywoods from 

southern pine (major structural species) and yellow poplar (most representative decorative 

species) using polymeric methyl diphenyl diisocyanate adhesive in the presence of 1 as a 

reactive diluent in order to reduce the adhesive costs. These formaldehyde-free plywood 

composites did not suffer delamination after accelerated-aging tests and, although the 

interfacial failure did not satisfy the requirements for structural plywood, they approached 

or exceeded requirements for decorative applications. Schultz [38] prepared an exterior 

plywood resin based on 2 and paraformaldehyde. Three-ply assemblies from yellow pine 

were bonded at different processing conditions and showed that the curing time necessary 

for these systems was longer than that which was generally required for conventional 

gluing systems. The use of veneers with a high moisture content (9.5 instead of 5.1%) 

had very negative effects on the strength properties of the plywood prepared. Pizzi [39] also 

prepared particleboard urea–furfural–formaldehyde binders. He concluded that a partial 

substitution of formaldehyde with 1 led to an enhanced CH2O emission and explained this 

unexpected feature in terms of two competitive reactions. In fact, he showed that in the 

resins which contained both formaldehyde and 1, the higher stability to hydrolysis of the 

1–urea bonds induced the release of formaldehyde from the final product. 

New adhesives from furfural-based diamines and diisocyanates were prepared by 

Holfinger and coworkers [40,41]. They produced flakeboards alternatively bonded with 

phenol–formaldehyde, MDI, and 5,5 0 -ethylidene difurfuryl diisocyanate (14) adhesives 

and showed that the strength properties of flakeboards prepared with 14 were slightly 

lower than those based on MDI and higher than those prepared with phenol–formaldehyde 

resins. Thus, the internal bond strength values of flakeboards bonded with MDI and 

14 at 3% resin content, were 1.33 and 0.97 MPa, respectively [41], which are much higher 

than the value required by American standard ANSI/A208.1 (0.41 MPa). 


Joshi and Singh [42] showed that about 30% of formaldehyde could be replaced by 1 

(obtained from wheat straw) in the formulation of phenol–formaldehyde adhesives. They 

used these phenol–1–formaldehyde resins in the preparation of plywoods from Vateria 

indica and Toona ciliata and obtained materials with good resistance to boiling water. 

These authors mentioned, however, that 1 slowed down the curing rate of the resin and 

recommended longer condensation times compared with conventional phenol–formaldehyde 

thermosets. Motawie et al. [43] prepared 1 by hydrolysis of Egyptian cotton straw and 

prepared different resins by the copolymerization of the in situ formed furfural with phenol, 

epichloridrin–phenol, or a bisphenol A-based epoxy prepolymer. The curing of these resins 

was investigated using phthalic or maleic anhydride at 170–185 C or using diamines at 

room temperature, both in the presence or absence of kaolin as an inorganic filler. Their 

properties appeared to be comparable to those of commercially available wood adhesives. 

Ellis and Paszner [44] investigated the self-bonding of various lignocellulosic materials 

possessing high hemicellulose content through the in situ generation of furanic 

derivatives by acid-catalyzed thermal conversion of some saccharidic units. They used 

seven different raw materials with increasing pentosan content, i.e., elm, aspen, oak, 

and birch woods as well as bagasse, sweetcorn cob, and feed corn cob, with pentosan 

contents of 18.8, 19.4, 20.2, 25.5, 27.2, 39.7, and 42.3%, respectively. The pressing temperatures, 

pressures, and times tested were in the ranges of 160 to 220 C, 14–20 kg/cm 2 and 

2–10 min, respectively. Ammonium sulfate and ammonium chloride were used as catalysts 

and their amounts were varied from 0 to 6% w/w with respect to the vegetable material. 

The bending strength of the materials obtained was directly proportional to the xylan 

content of the initial lignocellulosic source. The optimal amount of catalyst was found to 

be around 1.5% w/w based on the natural raw material and the optimal pressing time was 

established to be around 6 min. Increasing the wood particle size induced a drastic 

decrease in the bending load, whereas an increase in press plate temperature led to a 

substantial increase in the mechanical properties of these self-bonding composites. 

Gos et al. [45] glued spruce wood (Picea excelsa L.) using three different adhesives, 

namely: (i) a phenol–resorcinol binder, (ii) carbamide–melamine–1 resins, and (iii) a 

poly(vinyl acetate) glue. They tested the bending elasticity of these glued woods in the 

temperature range of 20 to 150 C and a minimum loss of bending strength, when the 

temperature increased from 20 to 150 C, was observed when phenol–resorcinol or carbamide–melamine–1 

resins were used. Kim et al. [46] synthesized 1-modified phenol–formaldehyde 

resol resins by partial substitution of formaldehyde by 1. They tested the 

performance of these resins using them as adhesives for oriented strandboards. They 

used 13 C-NMR to establish the reaction mechanism between 1 and the other resin 

components and isolated and identified convincingly structures 15, 16, and 17. The use 

of 1 with 0.25 mole per mole of phenol in phenol–formaldehyde resol resins gave boards 

with properties very similar to those obtained by conventional gluing. 


Recently, Schneider et al. [47] fabricated particleboards using poly2–urea–formaldehyde 

adhesives (P2-U-F). They observed that the curing time needed for P2-U-F was 

double that necessary for classical urea–formaldehyde resins. They also established that 

P2-U-F produced boards with lower strength properties, but with higher water resistance, 

if classical processing conditions were used. However, at higher resin contents, P2-U-F 

gave boards with better mechanical properties. The following optimal conditions were 

derived to produce particleboards: a blending time of 10 min, a press platen temperature 

of 150 C, 15% of P2-U-F resin with respect to OD softwood, 1.4 min of pressing time per 

millimeter thickness, and a board density of 0.67 kg/dm 3 . 

Dao and Zavarin [48,49] prepared boards using wood powder and 2 or poly2 as 

binders. The wood species was white fir (Abies concolor) which was used as powder 

screened to 80 mesh. Compound 2, poly2, and wood were subjected to chemical activation 

with hydrogen peroxide/ferrous ions or nitric acid. It was established that an increase in 

the degree of polymerization of poly2 yielded boards with increased strength properties 

and that poly2 gave materials with higher strength and water resistance properties than 

those obtained using 2. They also showed that the addition of the activator to poly2, rather 

than to wood, was more efficient. Finally, they also isolated the acetone-soluble fraction of 

poly2 (about 73%) and used it as a binder for the same wood samples. They found that the 

tensile properties of the corresponding boards exceeded, by over 50%, those of composites 

prepared with conventional phenol–resorcinol–formaldehyde resins. 

Abd El Mohsen et al. [50] modified classical urea–formaldehyde resins by adding 

different amounts of 2 and used them as binders for beech-based plywoods. These modified 

resins gave materials with higher shear strength properties (100% increase) in 

comparison to unmodified adhesives. They also established the following optimal 

formulations: addition of 30, 45, and 60% of 2 to classical urea–formaldehyde resins 

and 3, 4.5, and 6% of p-toluene sulfonic acid as a hardener, respectively. Coppock [51] 

prepared durable wood adhesives from furfural-based diols, diamines, and diisocyanates. 

She then made plywoods or particleboards using modified urea–formaldehyde resins, with 

3 and 4 as binders and found that the materials thus obtained showed acceptable mechanical 

properties. These properties were not improved by the addition of further modifiers, 

such as 5,5 0 -ethylidene furfuryl amine (18). Measurements using DSC showed that 3 did 

not react under alkaline conditions, but readily resinified at pH values below 3.0. These 

materials were found to have lower formaldehyde emission compared with those made 

with unmodified resins. The mechanical performances of flakeboards made with 14 

exceeded the industrial standard requirements and were equivalent to those prepared 

using MDI. Finally, materials based on 14 in the presence of 3 or 18 as modifiers were 

obtained and found to have better performances in comparison to those prepared without 

these additives. 

Suzuki et al. [52] prepared wood-meal/plastic composites with an average thickness of 

4 mm using urea–2 and phenol–1 resins as binders. The molar ratio between urea and 2 was 

varied from 9:1 to 1:9. The amount of formaldehyde emission decreased with increasing 


quantities of added 2 and the optimal ratios were found to lie between 2:1 and 1:2. 

Hexamethylenetetramine was added to phenol–formaldehyde resins which were formulated 

with a molar ratio of 1:3. The bending strengths of composites prepared using urea–2 

adhesives were substantially higher than those made using phenol–formaldehyde binder. 

More recently, Raknes [53] studied the natural aging of 14 different commercial adhesives 

used in plywood manufacturing. He glued spruce (Picea abies) pieces and subjected them to 

30 years of natural aging ! He concluded that the shear strength and the water resistance of 

samples bonded with ‘‘furfurylated’’ urea–formaldehyde resins (Cascorit 1250 and Dynorit 

L166, manufactured by Casco Wood Adhesives, Sweden) were still satisfactory. 

Kim et al. [54] explored the possibility of using 2 as a cobinder in conventional urea– 

formaldehyde adhesives. They successfully prepared water-insoluble poly2 as oil-in-water 

emulsions and added them to urea–formaldehyde in different proportions. The ensuing 

mixtures were used to produce particleboards from a mixture of southern pine and hardwoods 

(75/25). The resin content of these panels was 8% w/w based on OD wood particles 

and the catalyst used was ammonium sulfate at a level of 0.3% w/w with respect to the dry 

resins. The optimal quantity of added 2 was found to be in the range of 20–30% with 

respect to conventional urea–formaldehyde resins. These formulations gave panels with 

increased strength and low formaldehyde emission. Russian investigators [55–58] used 5 as 

a binder for fir (Abies) plywoods and showed that the properties of these materials met the 

Russian standard requirements if pressing time of about 10 min, pressing temperature of 

160 C, and a platen pressure of 1.8 MPa were used. Thus, the shear strength of the 

plywoods reached almost 1.5 MPa, and their water uptake did not exceed 39%. The 

use of clay as a filler (up to 40% w/w with respect to the binder) decreased substantially 

the final properties of the materials [57]. Mezhov et al. [59] also studied the furfural 

emission from plywoods prepared using 5 as a binder (produced in situ by reaction of 1 

with acetone) and showed that it was much lower than that allowed, i.e., 3–5 mg/100 g of 

plywood instead of 10 mg/100 g. 

VII. FURAN RESINS AS CEMENT ADHESIVES 

Furan resins have also been extensively used in formulating mortars, grouts, and ‘‘setting 

beds’’ for brick lining destined to be exposed to highly corrosive environments, such as 

concentrated acids or highly alkaline cleaning solutions [3,16,60–62]. Two techniques are 

used in order to realize assemblies, namely tilesetter’s and bricklayer’s methods. The first 

method is based on the use of quarry tiles or pavers with smooth surfaces. The second 

method consists in using acid-resistant brick linings. Depending on the end use, three types 

of bricks are used for the installation of this type of assembly, namely: 

(i) Red shale bricks which have the highest resistance to chemical attack. They are 

relatively fragile towards thermal and mechanical shocks. Typically, standard 

brick dimensions are 20.3 cm by 9.5 cm. 

(ii) Fire clay bricks which are less resistant to chemical attack, but much more 

stable against thermal and physical shocks. Their standard dimensions are 

22.8 cm by 11.4 cm. 

(iii) Carbon bricks which are used to withstand hydrofluoric acid, fluoride salts, 

and hot, strong alkaline media. They are also very resistant to thermal shocks. 

In 1990, 2 was also used in order to prepare low temperature ( 10 C) hardening 

epoxy resin mortar adhesive [63]. For this, 2 was added as a reactive diluent to classical 


epoxy resin based on bisphenol A and the adhesive thus obtained was found to have 

good mechanical properties. These compositions are presently being produced by the 

Chinese Yanan Chemical plant. More recently, 2 was used in polymer compositions 

in building and structural repairs and showed properties similar to those obtained with 

epoxy resins [64]. 

VIII. FURAN RESINS/GLASS FIBER COMPOSITES 

Corrosion-resistant glass fiber reinforced composites were also produced on the basis 

of furfuryl alcohol thermosetting resins [3,16,60]. Thus, many furan-based glass fiber 

reinforced materials have been available for many years, particularly for the storage 

of chlorinated aromatic and aliphatic hydrocarbon solvents. Amongst the commercial 

units available one finds: (i) very large scrubbing towers packed with Raschig rings. 

These containers are resistant to hot (up to about 120 C) HCl and organic chlorides; 

(ii) large brink mist eliminators typically working close to 85 C; (iii) acid wash surge 

tanks used to store waste liquids with a pH of about 2 at temperatures of 55–60 C; 

and (iv) dryer exhaust water driven coolers for incoming hot (230 C) acidic HCl and 

aromatic vapors. These few examples do not cover all the equipment constructed on the 

basis of furan resin reinforced by fiberglass but they show clearly the usefulness of these 

materials in different industrial areas. Other applications include the use of 2 as a matrix 

for fiberglass in the production of wrappings of pipes carrying corrosive liquids and vapors 

[12]. Thus, steel pipes previously coated with bitumen or coal tar pitch can be wrapped 

with a bonded glass fiber mat based on this type of resin. In this context, 2 is mixed 

with water in the presence of an emulsifying agent and an acid catalyst and the ensuing 

emulsion impregnates the mat. Then, the resulting composite is heated in order to 

remove the water and induce the acid-catalyzed polycondensation of the matrix. 

Amongst the composites used one can cite furfuryl alcohol resins reinforced 

with carbon filled woven glass fiber (commercialized under the name of Permanite, 

manufactured by the IKO Group, Canada). The main mechanical properties of such 

composites are: tensile, shear, flexural, and compressive strengths of 15, 20, 39, and 

41 MPa, respectively. Their average density, thermal conductivity, and coefficient of thermal 

expansion are 1.57 kg/dm 3 , 3.44 W/(m 2 K) and 1.8 10 5 / C. Permanite-based pipes 

are hard, tough, and rigid with exceptional resistance to thermal shocks. They can 

be used up to 140 C and should be protected against high tensional, torsional, and 

shear loads. 

The combination of furanic derivatives with formaldehyde is also used in order to 

produce pipes. Haveg 61, manufactured by High Performance Alloys, Inc., Tipton, IN, 

can be cited as an example of these resins which are usually filled with acid-digested 

asbestos [16]. These composites are resistant to thermal shocks and have been used 

continuously at high temperatures (150 C). They have very low electrical and thermal 

conductivities. The main mechanical properties of these composites are: ultimate tensile, 

shear, and compressive strengths, at 26 C, of 28, 109, and 72 MPa, respectively. 

Their coefficient of thermal expansion is 3.2 10 5 / C. The working and hardening 

times of these resinous cements depend strongly on the working temperature [12]. Thus, 

for example, for carbon filled 2-based resin cement, the following critical values are found: 

at 16, 21, and 27 C the working and hardening times are 90, 60, and 30 min and 48, 20, 

and 12 h, respectively [12]. 


IX. MISCELLANEOUS APPLICATIONS OF FURAN RESINS 

Azimov et al. [65] compared the performances of furan resins with those of conventional 

phenol–formaldehyde adhesives. They used these binders to assemble aluminumto-aluminum 

and glass-to-glass structures and showed that furanic resins gave 

much higher rupture moduli in both systems studied. Rassokha and Avramenko [66] 

studied similar systems, but gave more information about the furan resin used. They 

used 5- and6-based adhesives both in the presence and absence of zeolite-based fillers, 

and assembled aluminum-to-aluminum and glass-to-glass structures. They showed that 

the use of polyethylene-co-vinylacetate as a filler dispersant gave well dispersed suspensions 

and consequently the best mechanical properties of the assembly. Nikolaev 

et al. [67] studied the thermal stability of furan resins produced by the reaction 

between a furfuryl ether of glycerol and 2,4-toluene diisocyanate. They showed clearly 

that the incorporation of this resin into conventional adhesives improved their thermal 

stability. The mechanical properties of steel-to-steel assemblies based on these compositions 

were found to follow the same tendency as that observed for the thermal 

properties. 

Poly(hydroxymethyl furfurylidene-acetone) adhesive resins were synthesized and 

characterized [68–70] through the 5-formaldehyde adduct (19) and its acid-catalyzed polymerization. 

The catalysts used were sulfuric, phosphoric, or p-toluenesulfonic acid. The 

authors postulated that the first condensation products resulted from the condensation of 

two methylol groups of two 19 molecules (adduct 20). They also proposed a hypothetical 

structure of the network formed after curing (21). It seems, however, difficult to envisage 

the acid-catalyzed resinification of 19 without the participation of hydrogen atoms at the 

C5 position of the furanic ring [2]. 


Macro-diisocyanates based on the reaction of an excess of 2,4-toluene diisocyanate 

with different poly(dimethylsiloxane)diols of different lengths have been prepared 

by Nikolaev et al. [71]. These macro-diisocyanates were reacted with 2 in stoichiometric 

proportions and the resulting adduct (22) was cured with a commercial epoxy 

resin in the presence of what was termed ‘‘poly(ethylene)-poly(amine)’’ at room tempcerature, 

80, and 100 C. The mechanical and thermal properties of steel-to-steel 

assemblies joined by these adhesives were better than those obtained using more 

common binders. 

Bowles et al. [72] studied the copolymerization of different methacrylates with NCOethyl 

methacrylate to obtain dental adhesives. Furfuryl methacrylate (23) was among the 

monomers tested. The main objective of this investigation was to establish a correlation 

between the solubility parameter of the copolymers and their shear strength. It was 

moreover shown that the setting time of the furan-based copolymer was very short compared 

to that of aliphatic homologues, but its shear strength was relatively low. 

Dopico et al. [73] prepared 5- and6-based furan resins which, after acid-catalyzed 

polymerization, were subjected to epoxidation with thiokol in different proportions. 

In a second series of experiments, 6–7% of 2 was added to the epoxidized resins. They 

showed that all these resins presented a lower flexure resistance compared to unmodified 

totally furanic binder. Moreover, the addition of 2 was found to induce negative effects on 

the mechanical properties of metal-to-metal assemblies. 

Furan resins have also been used as binders in grinding wheels [4]. In this field, 

5–20% of phenolic resin in combination with 1 as a special wetting agent is added to 

the abrasive grains and the resulting wheels thereafter coated onto the surfaces of different 

substrates. Paper, cloth as well as composites based on glass fiber reinforced films, have 

been used as grinding wheel supports. Acid-catalyzed poly2 has also been used in the 

aircraft industry as a low-temperature setting adhesive to bond wood and plastic parts. 

This adhesive was found to be suitable for assemblies subjected to warping and other 

deformations at high temperatures [12]. 

X. CONCLUSIONS AND SUMMARY 

From the above survey, it appears that the industrial use of furanic monomers such as 

furfuryl alcohol and furfural, i.e., chemicals based on renewable resources, as binders 

in foundry molds is highly successful. Similar furan-based resins can also be used as 

efficient adhesives in wood–particle composites and thus are interesting alternatives to 

petroleum-based counterparts. The fact that the substitution of formaldehyde by furfural 

has not yet met with a reasonable industrial success probably stems from the 

higher cost of the furan aldehyde. The increasing pressure on the reduction of formaldehyde 

emission and the renewable character of furfural should play in its favor in 

the near future. 

This chapter has dealt with the use of furanic derivatives as adhesives and binders. It 

has been shown that the main industrial applications concern the use of furfural and 


furfuryl alcohol as raw materials for binders for coating different surfaces, namely: 

(i) The storage and the transport of hot and highly corrosive fluids such as 

chlorinated solvents, acids, and bases. For this purpose, the vessels (e.g., 

tanks, pipes, or towers) are coated with a composite material based on filled 

and/or glass fiber reinforced furanic matrix. 

(ii) The molding of liquid metals. In this context, foundry molds are produced 

from furan derivatives, or in combination with phenolic resins, and are utilized 

because of their excellent fire resistance and thermal stability. 

(iii) The preparation of highly resistant cements and concretes which are employed 

when the object is used as a container for chemicals and/or exposed to corrosive 

cleaning agents. 

(iv) The preparation of grinding wheels in which furan resins are used to bond 

abrasive grains. 

In addition to these well known applications, different studies dealing with the use of 

furan derivatives as wood adhesives and other miscellaneous applications are presented 

and discussed. 

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30. Furan-Based Adhesives

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