Wood Composites. http://dx.doi.org/10.1016/B978-1-78242-454-3.00003-2Copyright © 2015 Elsevier Ltd. All rights reserved.

3Adhesives for wood compositesG.A. Ormondroyd

3.1 Introduction – A brief history of wood composites resins

The development of resins and adhesives for the wood composite industry was ini-tially driven by the need to produce composites economically and quickly and to con-form to the use standards as laid down by various standards bodies. Early composites such as early plywood (Muller, 1992) were produced by hand and therefore the speed of the setting of the resin was not paramount so casein resins could be used in their manufacture, whereas in today’s factories for the manufacture of medium density fi-breboard, the curing time and therefore the speed at which the panels can be made is the critical point for the industry. The selection of the resins will also depend on the end use of panel products. Urea formaldehyde, developed in the 1930s, is a good cheap commodity resin and was suited for the development of particleboard and MDF. However, as the performance criteria for the panels was increased and a requirement for panels that are suitable for moist conditions introduced, melamine was added to the resins and a more moisture resistant board was produced.

Phenol formaldehyde (PF) resin was originally developed in the late nineteenth century; however, it was not until the 1930s it was commercially developed and then used in the wood panel industry. PF resin is used in the manufacture of plywood and engineered wood products. The use of PF resins is due to its high resistance to moisture and comparative cheapness. As with UF resin, the formulations have been developed to speed up production and to keep costs down; however, the basics of the resins have remained the same.

In recent years, the drivers for resin development have changed. In the 1980s, some European countries began to regulate against the emission of formaldehyde in panel products. In 1985 E1 (<0.1 ppm formaldehyde in boards) became obligatory across Austria, Denmark, Germany and Sweden and in 2002 this was adopted across Europe within the standard EN 13986. However, the standard did give two classifications, E1 and E2. The standard does not apply to boards with no added formaldehyde and therefore, in theory, boards with no added formaldehyde can be classed as E1 with-out any testing. In 2006, E1 was deemed the limit for panels produced by European Panel Federation members and an additional environmental label ‘Blue Angel’ was developed for boards with less than 0.05 ppm formaldehyde. Other testing methods and limits have influenced the resins used in panel products, including the Japanese F limits and the legislation of the state of California in the USA.

The increased legislation on formaldehyde has led to some companies investi-gating a move away from formaldehyde-based resins. This has included the use of other synthetic resins, such as MDI, or the investigation and investment into bio-based

48 Wood Composites

alternatives. However, the investment in new resins and their use in panel product manufacture are not insignificant, and only when a niche premium product can be made and marketed is the move away from commodity resins possible.

3.2 Common resins for current composite technologies

3.2.1 Urea formaldehyde

Urea formaldehyde resin was developed in the 1930s (Dinwoodie, 1979) and is widely used in the composites industry. Ninety percent of the world’s particleboard is pro-duced using UF resin (Dinwoodie, 1979). The advantages of UF resins were listed by Pizzi (1994a,b) as follows:

1. The hardness of the resin.2. The low flammability of the resin.3. The good thermal properties of the resin.4. The absence of colour in the cured polymer.5. The adaptability of the resin to a variety of curing conditions.

The initial water solubility renders UF resins suitable for bulk and inexpensive pro-duction. However, UF resin has disadvantages, the major problem being that UF resin is subject to hydrolytic degradation when in the presence of moisture and/or acids. This deg-radation is mainly due to the hydrolysis of the amino plastic and the methylene bridges.

3.2.1.1 The manufacture of urea formaldehyde resin

The manufacture of UF resin is complex. Urea is manufactured from carbon dioxide and ammonia at a temperature of 135–200 °C and at a pressure of 70–230 atmospheres. Formaldehyde is manufactured by the oxidation of methanol which can be produced from the reaction of carbon dioxide with hydrogen or can be derived from petroleum.

The combination of the urea and the formaldehyde gives both branched and linear polymers as well as the three-dimensional matrix that can be found in the cured resin. These different structures are due to the functionality of the urea and the formalde-hyde. Urea has a functionality of four (due to the presence of four replaceable hydro-gen atoms) and formaldehyde has a functionality of two (Figure 3.1).

The most important factors affecting the properties of the reaction products are:

● The relative molarities or the reactants.● The reaction temperature.● The pH at which the condensation reaction takes place.

FormaldehydeNH2

NH2

O

O

HUrea

H CC

Figure 3.1 Urea and formaldehyde.

Adhesives for wood composites 49

These factors influence the rate of increase of the molecular weight of the resin (Pizzi and Mittel, 1994), therefore the reaction products vary widely with the changes in reaction criteria. Solubility, viscosity, water retention and final rate of cure all vary with molecular weight.

The reaction of urea and formaldehyde is divided into two stages. The first stage is alkaline condensation to form mono-, di- and trimethylolureas (Figure 3.2).

The reaction also produces cyclic derivatives such as uron, monomethyloluron and dimethyloluron.

The second stage is an acid condensation of the methylolureas to form firstly solu-ble and then insoluble cross-linked resins.

When acid condensation takes place, the products that precipitate from an aqueous solution of urea and formaldehyde, or from methylolureas, are low molecular weight methyleneureas (Figure 3.3).

NH

CO

NH CH2 OH

NH

N

CO

CH2 OH

CH2 OH

CH2 OH

CH2 OH

CH2 OH

NH2

CCCO O

H

H

+ O

NH

NH2

NH

CO

NH2

NH2

OH−

CH2 OH

C O

H

HCO

NH

NH

CH2 OH

CH2 O CH2 NH

H2N

C O

CH2

CO

NH

NH

CH2 O

CH2

CH2 CH2OH HO

OH HN

NH

C OCO

NH

N

CH2 O CH2 NH

HN

H2N

C OCO

NH2

NH

CH2 O

H2C

CH2 NH

HO

NH

NH

CH2 OH

CO C O

CH2 OH

CH2 OH

CO

NH

N

OH+−

OH−

C O

H

H

Figure 3.2 Alkaline condensation of urea and formaldehyde.

50 Wood Composites

These contain methylol end groups in some cases, through which it is possi-ble to continue the hardening process. The monomethylolureas copolymerise by acid catalysis and produce polymers and then highly branched and cured networks (Figure 3.4).

The kinetics of the formation of mono and dimethylolureas and of the simple con-densation products have been studied extensively. The formation of the monometh-ylolurea molecules in a weak acid or alkaline solutions is characterised by an initial fast phase followed by a slow bimolecular reaction. The rate of reaction varies with the pH of the system. A minimum rate of reaction is achieved with a pH of 5–8 for a urea/formaldehyde ratio of 1:1 and a pH value of ±6.5 for a 1:2 molar ratio (Figure 3.5).

The rate of formation of the methylenebisurea molecules by the condensation of urea with monomethyleneurea is also pH dependent. The rate of reaction decreases exponentially from a pH of 2–3 to a neutral pH. The reaction does not take place in alkaline conditions.

The initial addition of formaldehyde to urea is reversible. The rates of introduction of the one, two and three methylol groups have been estimated to be 9:3:1, respec-tively. The formation of N,N′-dimethylolurea to monomethylolurea is three times that of monomethylolurea to urea.

The methylenebisurea and higher oligomers undergo further condensation with formaldehyde and monomethylurea, which behaves like urea (Pizzi and Mittel, 1994). The capacity of methylenebisurea to hydrolyse to urea and methylolurea in weak acid solutions (pH 3–5) indicates the reversibility of the aminomethylene link and its proneness to chemical change in weak acid moisture.

H2NCONH(CH2NHCONH)nH

Figure 3.3 Low molecular weight methyeneurea.

HN C

ONH2H2C

+

NH2

n

HN C

ONH2CHO

+

H

H+

H2HN C

ONH2CHO

H2

HN

HNC

OCH3CH2 H

NHNC

OCH2 H

N CO

OC N

H +CH2H2N

Figure 3.4 Copolymerisation of monomethylolureas.

Adhesives for wood composites 51

3.2.1.2 Commercial production of urea formaldehyde resins

In the commercial production of UF resin the most important property that has to be controlled is the size of the molecules. As the size of the molecules increases, the properties of the resin change, the most perceptible being the increase in viscosity (Pizzi and Mittel, 1994). The increase in molecular weight is due to water molecules splitting off the resin molecules at random thus presenting reactive groups for further condensation. However, the condensation reaction is not favoured in aqueous condi-tions. Once the viscosity has been established and the pH, concentration and solubility have been determined the resin can be used.

The most common method of preparation for commercial UF resin is the addition of a second amount of urea during the reaction. The ratio of urea to formaldehyde is between 1:2 and 1:2.2 and therefore methylolation can take place at in a short amount of time at temperatures between 90 and 95 °C, with a mixture being maintained under reflux. The formation of the resin is completed after the exotherm has subsided. Acid is then added to decrease the pH to allow the polymer building stage to begin (usually with a pH of 5.0–5.3). As soon as the correct viscosity, is reached the pH is increased to stop the polymers increasing in size. The second urea is added to mop up any free formaldehyde until a ratio of 1:1.1 to 1:1.7 has been established. The resin is then left to react for another 24 h at a temperature of 25–30 °C after which the resin solids con-tent is adjusted appropriately and the pH is altered to give maximum shelf life.

3.2.1.3 The curing mechanism of urea formaldehyde

Although the curing of urea formaldehyde can take place at room temperature using the addition of an acid catalyst (such as citric or formic acid) to drive the reaction, the manufacture of panel products is generally driven by speed or production and therefore the reactions take place in the presence of heat. During the hot curing of UF resin two

4

3

4

log

k5

6

6 8

U + F

T = 35 °C

U + UF

UF

UMU

pH10

Figure 3.5 Influence of pH on the addition and condensation reactions of urea and formaldehyde (Pizzi and Mittel, 1994).

52 Wood Composites

condensation reactions take place and a ridged three-dimensional structure is created. The first condensation reaction occurs between adjacent polymers with the adjacent nitrogen within the amide group (originating on the formaldehyde molecule) forming methylene bridges. The second condensation reaction is between the methylol groups, and these form an ether bridge. As the UF resin cures, it first increases in viscosity and gels until finally complete cross linking has taken place.

3.2.2 Melamine urea formaldehyde

Melamine formaldehyde resins are widely used in applications in which the product may come in to contact with water, such as exterior grade panel products and kitchen furnishings. This is due to its high resistance to water attack which distinguishes it from UF resins.

However, melamine formaldehyde is expensive (approximately 2.5 times the price of urea formaldehyde) and therefore a varying amount of urea is added to the resin so that a compromise between cost and performance is met.

3.2.2.1 The manufacture of melamine formaldehyde resin

The initial reaction in the formation of MF resin is the condensation of melamine with formaldehyde. The formaldehyde first attacks the amino groups of the melamine, forming methylol compounds. This reaction is similar to the initial reaction of formal-dehyde with urea; however, the reaction between formaldehyde and melamine occurs more freely and completely than the reaction with urea. It has been noted by Pizzi (1983) and Pizzi and Mittel (1994) that complete methylolation of melamine is possi-ble which is not the case with urea. The condensation will lead to a series of methylol compounds with between two and six methylol groups attached (Figure 3.6).

Due to reduced solubility in water of melamine, when compared to urea, the hydro-philic stage of the reaction proceeds more rapidly in the formation of MF than in the formation of UF, therefore hydrophobic intermediaries appear early in the reaction. An important difference between the condensation of MF resin (and also the curing) and the condensation of UF resin is that the resin condenses not only in acid conditions but also in neutral and alkaline conditions (Pizzi and Mittel, 1994).

The reaction mechanism continues as with urea formaldehyde, methylene and ether bridges form and the molecular weight of the resin increases rapidly. The in-termediates that are formed at this stage of the reaction make up the bulk of com-mercially available resins. The final curing process transforms the intermediates to

NC C

N

CH2OH

CH2OH CH2OH

HOH2C

HOH2C CH2OH

N

N N

N

C

C CC

O

H

H6+

N

NH2

NH2H2N

N

N

C

Figure 3.6 Melamine resin manufacture.

Adhesives for wood composites 53

the desired insoluble, infusible resins through the reaction of amino and methylol groups which are still available for reaction. Koehler (1941) and Frey (1935) noted that ether bridges formed next to un-reacted methylol groups and methylene bridges. This is because when MF resin is cured at temperatures of up to 100 °C no substantial amounts of formaldehyde are liberated whereas urea formaldehyde liberates signif-icant amounts.

3.2.2.2 Commercial production of melamine formaldehyde resin

Generally the commercial production of MF resin is in fact similar to the production of UF resin. The specifics of the production of the MF resin system depend on the application for which the resin is intended.

Resins that are intended for the impregnation of paper or fibres have to be modi-fied with other compounds such as acetoguonamine and E-caprolactame (Pizzi and Mittel, 1994). These modifying compounds are usually added at around 3–5% (w/w) and decrease the cross linking in the cured resin, thus making the resin less brittle. In the manufacture of wood panel products, the additives are not usually needed. Sugars have been used as modifiers in the wood panel adhesive industry but these are added to lessen the cost of the resin. However, the addition of sugars means that with age the resin will yellow and crack and has a detrimental effect on long-term resin properties.

Resins intended for use on the wood panels industry are generally designed with a higher viscosity than those intended for the infusion of paper, in order to prevent over penetration into the wood substrate.

Resins with good penetration can be created in several ways; a resin with a low level of condensation and high methylol group content will create a low viscosity resin with fast curing rate. A resin with a low level of condensation and a melamine to formaldehyde ratio of 1:1.8 to 2.0 will give the desired result. A second approach to creating the resin is to form a resin with a higher degree of condensation and a lower methylol group content, and add a second batch of melamine to the mix (usually giving a total melamine content of 3–5%) towards the end of the reaction. Typical total melamine formaldehyde ratios are in the region of 1:1.5 to 1:1.7 for this system.

3.2.2.3 The curing of melamine formaldehyde-based resins

The curing of MF-derived resins, as with its manufacture, is similar to that of UF resins. The curing of MF resins can only occur under hot press conditions and cannot be driven by the addition of an acid catalyst (Marra, 1992). The curing completes the cross linking by the formation of the methylene bridges to form the solid resin. Koehler (1941) and Frey (1935) also observed that ether bridges were formed next to the methylene bridges and the unreacted methylol groups.

An advantage of pure MF resins over UF (and MUF) resins is that if it can be cured below 100 °C such that no formaldehyde is released due to the curing process. If the panel is cured between 100 and 150 °C then low amounts of formaldehyde are given off when compared with UF resin cured in the same conditions.

54 Wood Composites

3.2.3 Phenolic resins

PF was discovered as an adhesive at the end of the nineteenth century and was the first true synthetic polymer to be developed. However, it was not until the 1930s that it was produced commercially. Presently the resin is generally used in the plywood and the waferboard (e.g. OSB) industries.

3.2.3.1 The manufacture of phenolic resins

Phenolic resins can be obtained through two manufacturing routes, the Novolac process and the Resol process. Throughout the panel industry, it is the Resol type PF resins that are predominantly used and therefore it is this process that it is focused on here.

Resol resins are manufactured by the reaction of phenol in the presence of an acid catalyst and an excess of formaldehyde to form a quinine methide molecule (Figure 3.7). The reaction is affected by the molar ratio of formaldehyde to phenol with a typical range of ratios being 1.6:1 to 2.5:1. At the lower end of the range, linear structures will be produced whereas if a higher ration of formaldehyde to phenol is used greater cross linking will take place.

Quinone methide is then condensed to form Resol. The reaction is performed un-der alkaline conditions, usually via the addition of caustic soda (NaOH). The alkaline conditions catalyse the formation of the methylol bridges. The methylol bridges are the only strength giving components of the phenolic resin, due to the C–C bonds. Methylol bridges are considered to be the strongest and most durable bonds that can be produced between two organic molecules (Marra, 1992). The condensation can occur in two ways although reaction 2 is favoured (Figure 3.8).

The Resol molecule contains reaction methylol groups, and on heating, these mol-ecules combine to form larger molecules, and eventually gel and form the solid-state resin. A high alkalinity is maintained throughout the manufacture and storage of the resin to aid in stability and shelf life.

OH2

−O O

H O O

H

H

−

−

−

−

−

HO

O

H

H2C O

O O

H

CH2OH

CH2

CH2

O

Quinone methide

OH

+

+−

Figure 3.7 The formation of the quinone methide molecule.

Adhesives for wood composites 55

3.2.3.2 The curing of PF resins

PF resins go through three distinct phase as they cure. In the initial phase (the A phase) the resin has a low molecular weight (<200) (Sellers, 1985). The resin is, in essence, a mixture of the two monomers. The monomer is attracted to the wood cell wall and a high amount of penetration occurs into the cell wall. It has been observed that if strips of wood are immersed in PF resin, after a time the resin becomes less concentrated and this indicated that the wood preferentially takes up the resin mono-mers (Marra, 1992).

Once the resin is heated the monomers begin to polymerise, more Methylol bridges will be created and a B stage resin, known as Resitol, will be formed. The Resitol resin contains molecules of different molecular weight as polymerisation has taken place but is not complete. The Resitol is insoluble in most solvents; however, it can still be in a swollen state and becomes rubbery and soft on heating. This is due to the cross linking that has already taken place.

On heating the Resitol, resin becomes soft and begins to polymerise once again and the third and final C stage is produced. The C stage of the curing is where total polymerisation occurs. In this form, the resin is known as a Resite. Once the resin has reached this stage it can neither be re-melted nor dissolved by solvents.

3.3 Other currently used resins

3.3.1 Poly vinyl acetate

Poly vinyl acetate (PVA) is a thermoforming resin that is usually water carried and generally used for the wood working and DIY industries.

The first record of the manufacture of PVA was a patent registered at the German State patent office in June 1912 by Dr F. Klatte of the Grisheim Elektron chemical works (Pizzi, 1983). Between the years of 1915 and 1930 research into the free radical initiation polymerisation of various vinyl acetates was at its height and by 1930 PVA was being commercially produced.

OH

OH

HO

CH2

OH OH

21

CH2

CH2

O

CH2OH

OH

CH2OH

Figure 3.8 The formation of phenol formaldehyde resin.

56 Wood Composites

Since the Second World War, the production and use of PVA has rapidly expanded because of its substitution for hot pot animal-based glues in the carpentry industry and it has been widely accepted as the DIY woodwork glue.

Once the PVA resin cures, it is a very different polymer to the formaldehyde resins; the polymer is linear with an aliphatic backbone, and is thus very flexible. The resin forms hydrogen bonds through the acetate groups and therefore has good adhesion with the hydroxyl groups in the wood cell walls. The adhesive has the ability to main-tain their bond strength as the wood expands and contracts due to the dissipation of the energy through the flexing of the polymer backbone (Rowell, 2012). However, the resins do not show resistance to moisture and do not resist creep to any extent. The resistance to moisture and loading can be improved by the addition of a cross linker (essentially turning the thermoforming resin into a thermosetting system); however, this has to be added just prior to the application of the resin.

PVA can be transformed into poly vinyl alcohol via hydrolysis (Jaffe and Rosenblum, 1990). Again due to the water solubility the resin generally needs to be cross linked and this is via reaction with UF or MF or forming ionic bonds with metal salts (Rowell, 2012).

The use of PVA for the production of composites has been very limited and is pri-marily used for academic studies.

Ozaki et al. (2005) manufactured a Sugi (Criptomeria japonica) flour/(PVA/Pathalic anhydride) composite with a ratio of 1:1. The researchers noted that the mod-ulus of elasticity and the modulus of rupture for the optimised modified PVA were similar to some engineered plastics; however, the composites were degraded in soil bed tests to similar weight losses to those of Eucalyptus grandis whilst retaining me-chanical properties higher than those of solid Sugis.

Much of the recent work has been based around the manufacture of PVA films reinforced with timber derived flour and more recently nano- and micro-fibrils. For example, Chakraborty et al. (2005) reinforced PVA with 5% (w/w) bleached soft-wood kraft pulp fibres to give films with a 2.5-fold increase in tensile strength and a theoretical stiffness of 69 GPa. Zimmermann et al. (2004) combined wood derived cellulose fibrils into PVA at ratios up to 10%. The research used both chemically refined and mechanically refined fibrils; however, only the mechanically refined fibrils, at a high loading, gave a statistically significant increase in the MOE of the composite.

It appears from the literature that the future of wood-based PVA composites will be based around PVA reinforced with wood fibrils and wood derived cellulose, rather than wood-based composites.

3.3.2 Isocyanate resins

Isocyanates were discovered as early as 1884 (Pizzi, 1983) but it was not until the Second World War that isocyanates were developed as adhesives. Initially, the isocy-anates were developed as adhesives for the manufacture of tyres and it was not until 1951 and the work of Depp and Ernest (Ball and Redman, 1978) that isocyanates were used as a binder for wood and wood composites.

Adhesives for wood composites 57

Isocyanates are available in many forms; however, the form that is generally used in the wood panel industry is MDI (4,4-diphenylmethane diisocyanate), this is because it has certain advantages over other isocyanates:

1. It has a low vapour pressure at ambient temperatures.2. It has a lower viscosity than other isocyanates.3. Its cost is low when compared with other isocyanates.4. It is moisture tolerant therefore the fibre can have a higher moisture content when resonated

meaning that less energy needs to be used to dry the fibre.

MDI has a large advantage over other resins used in the composites industry as it bonds with the hydroxyl groups within the wood. Therefore, when MDI is used as a wood adhesive it is not only polyurea that acts as a standard adhesive (i.e. in a lock and key mechanism) but the MDI also forms chemical bonds with the wood and thus increases the bond strength (Pizzi, 1994a,b).

However MDIs have their disadvantages. The panels when manufactured adhere to the caul plates and therefore a release agent has to be used, adding to the expense of the panel. MDI when cured is not toxic and does not emit VOCs or formaldehyde; however, when the resin is uncured it can be very toxic, especially if in a gaseous or droplet form, the gas can enter the lungs of unprotected operators and bond with the reactive sites in the lungs, workers within factories using MDI have to have good personal protective equipment and regular health checks. It has been noted that when composites are being manufactured using MDI the pre-pressed material may exhibit low tack and therefore the composites are very delicate when handled.

Currently the use of MDI in the wood composite industry is limited, especially in Europe; however, its use is gaining popularity for certain specialist and premium applications. These applications include ‘no added formaldehyde’ composites and composites for outdoor (wet) applications. MDI resins are mainly used in products such as OSB and strand board; how-ever, one company have successfully manufactured MDF using MDI resins.

3.3.2.1 The manufacture of MDI resin

MDI is produced through a reaction of 1° amines (produced from the condensation of aniline and formaldehyde in varying rations and in varying conditions) and phosgene (Figure 3.9). This reaction was first performed in 1884 by Hentschel (Encyclopedia of Chemical Technology, 1990) and although some refinement of the process has hap-pened over the years the base reaction is essentially the same.

A solution of the 1° amine in an aromatic solvent (i.e. xylene) is mixed with a solution of phosgene in the same aromatic solvent at a temperature under 60 °C. This is digested in stages at a temperature increasing to approximately 20 °C while additional phosgene is added. The solution is then distilled to recover excess phosgene and any impurities.

3.3.2.2 The curing of MDI

The curing mechanism for MDI resin has two parts. Firstly, there is the reaction of the MDI with hydroxyl containing compounds, and the second reaction is the direct reaction of the MDI with water.

58 Wood Composites

The reaction between MDI and water forms a polyurethane that then sets similar to a UF resin. There are several reactions in which the polyurethanes are produced. The first involves the formation of an unstable carbamic acid, which then decomposes to form an amine and carbon dioxide (Figure 3.10).

The amine then reacts with more isocyanate molecules to form a substituted urea compound (Figure 3.11).

Therefore the overall condensation reaction can be seen as shown in Figure 3.12.As well as the formation of the substituted urea in the reaction described above other

reactions take place, including the formation of a carbamic acid aldehyde followed by the decomposition to a substituted urea molecule and carbon dioxide (Figure 3.13).

In addition to the reaction between MDI and water the other significant reaction that takes place is the reaction between the MDI and hydroxyl containing compounds, such as wood. Figure 3.14 shows the reaction of MDI and wood.

The reaction shown in Figure 3.14 takes place whilst the MDI resin is curing and leads to the covalent bonds between MDI and wood, as shown in Figure 3.15.

RNH2

RNHCOCl

RNHCONHR

100–200 °CRNCO 2HCl+

RNH2

HCl+RNH2

RNH2HCl

COCl2

COCl2+Solvent

−20 to 60 °C

Figure 3.9 Reaction of 1° amines and phosgene to make MDI.

R NH R NH2 CO2+R N C O H2O+ OH

O

C

Figure 3.10 Formation of the amine and carbon dioxide.

R+ N C OFast

R NH NHC R

O

R NH2

Figure 3.11 The formation of the substituted urea compound.

H2OR2 N C O + R NH C CO2+

O

NH R

Figure 3.12 The overall reaction of isocyanate with the hydroxyl group.

Adhesives for wood composites 59

3.4 The future of resins for composites applications

3.4.1 Bio-resins – A history and state of the art

Wood adhesives and resins from renewable resources have been a topic of research for a number of years, the interest increased dramatically with the world’s first oil crisis in the 1970s. However, it was not until the beginning of the twenty-first century that the public increased its awareness of the environment and its protection. But it is not the public perception that is driving the research into bio-based resins, rather the increase in both national and international regulations against the traditional compos-ite resins and their constituent components (Pizzi, 2006).

Bio-based adhesives are adhesives that are derived from a natural, non-mineral and organic sources. These adhesives should have the performance of synthetic resins with only a minimal modification. There are a number of bio-based materials that can be

R NH NH R + CO2CH2

R NH C

O

O

C

O

R HN

R NHH2OR N C O + C OH

O

Figure 3.13 The formation of a substituted urea molecule from a carbamic acid anhydride.

R�R N C O + OH NH C

O

ROR

Figure 3.14 The reaction between MDI and the hydroxyl groups in wood.

Wood

Wood Wood

OH OCN R+ NCO Wood OHCONH

OHCONH NHCOOR

R NCO2

Figure 3.15 The overall chemical reaction between MDI and wood.

60 Wood Composites

used to produce resin products, these include tannins, carbohydrates, oils and liquefied cellulosic materials. This list can be supplemented with protein-based resins; however, for the purpose of this book the focus will remain on plant-based products.

3.4.1.1 Tannin-based resins

Tannins can be used as a source of phenols for the production of PF-based resins.Tannin-hexamethylenetetramine has been of interest as a resin for some time, al-

though there was a concern that its condensation yielded formaldehyde. However, this has been proven not to be the case (Pizzi, 1994a,b; Pichelin et al., 1999; Kamoun and Pizzi, 2000; Kamoun et al., 2003). TH resins have shown to give good results with dry particleboard IB strengths of 0.92 and 0.58 MPa when the resin has been used in the laboratory and on an industrial scale, respectively (Pizzi, 1978). It has also been noted by researchers that emissions from panels manufactured with TH resins have been comparable to those of heated virgin wood (Pizzi, 2006). The resin has been shown to have the ability to be used to manufacture boards for both indoor and outdoor applica-tions (Pizzi et al., 1995a,b, 1996, 1998).

Tannin can be used as a pure resin, and the auto-condensation of the polyflavonoids in acid or base conditions will lead to a tannin chain formed through the free C6 and C8 sites on the molecule (Meikleham et al., 1994; Pizzi and Stephanou, 1993). When pure tannins are auto-condensed the reaction leads to a rapid increase of viscosity but no gelling occurs. However, it has been found that either a small amount of silica (Pizzi et al., 1995a,b) or the presence of lignocellulosic materials initiates gelling. Panels manufactured with auto-condensed resins have been found to have internal bond strengths of around 0.65 MPa when the panels have been manufactured using pine particles.

3.4.1.2 Lignin-based resins

Roughly 45 million tonnes of lignin are produced a year by the pulp and paper indus-try (Hu, 2002) and this is primarily used as a low grade fuel.

Lignins have been studied extensively for the manufacture of bio-based res-ins, although it has been found that to manufacture panels successfully, the lignin resin has to be mixed with a small amount of synthetic, formaldehyde-based resins (Shimatanti et al., 1994; Gardner and Sellers, 1986; Newman and Glasser, 1985). However, these resins have been found not to be commercially viable, this is gener-ally due to the time the board is required to be pressed. The only lignin resin-based composite to be produced commercially is plywood in North America. The resin used was a pre-reacted lignin/formaldehyde resin added at a 30% (w/w) ratio to PF resins.

Some work has been undertaken to minimise the formaldehyde within the resin for-mula, and to this end, work has been undertaken blending pre-methylated lignin with polymeric 4′4′-methane diisocyanate (PMDI) (Newman and Glasser, 1985; Pizzi and Stephanou, 1993). A resin blend of 22/26/52 (PMPI/PF/lignin) led to the manufacture of particleboards with a dry IB strength of 0.85 MPa and a IB strength after a 2 h water soak of 0.53 MPa.

Adhesives for wood composites 61

The resin cures very quickly and therefore is of interest to the industry; however, it will still require the specialist handling required for MDI resins.

3.4.1.3 Starch-based resins

Starch has been modified to produce resinous materials and these are often used in the paper, paperboard and textile industries. The most readily used starch-based adhe-sive is starch-borax adhesives (McPherson and Schmidt, 2000; Snyder, 1990; Bozich, 1990). However, due to the non-water-resistant nature of these resins, they have not been employed in the composite industry. Starch-derived dextrins have also been used in a similar manner; however, they suffer from the same lack of moisture resistance as native starch adhesives.

Starches have been modified in an attempt to improve their moisture resistance and bond strength. For example, starch was cross linked with poly(vinyl alcohol) and hexamethoxymethylmelamine for the use of a bonding agent for engineered wood products (Imam et al., 1999). The resin compared with commercial urea formalde-hyde and PF adhesives in respect to cure time and cure temperature. The Automated Bonding Evaluation System (ABES) system showed that the bond strength was supe-rior to those of urea formaldehyde and PF (Imam et al., 2001).

3.4.1.4 Soy-based resins

Soy-based resins have been used as experimental adhesives for the panels industry. The soy protein is partially hydrolised and panel properties have been reported with in line with panel standards (Zhong et al., 2003; Liu and Li, 2002; Sun and Bian, 1999; Hettiarachchy et al., 1995).

Soy protein is available in three forms: firstly, a crude extract that is not effective as a resin; secondly, a protein rich concentrate; and thirdly, as protein isolates, which are effective in the manufacture of resins. The performance of soy-based resins is dependent on several factors, including particle size, viscosity, morphology and the chemistry of the bonded substrate. Although soy-based resins tend not to be water resistant and therefore have limited use in the manufacture of composites, the proteins can be modified to improve water resistance and the bond strength. Modification of soy protein with alkali, urea and guanidine hydrochloride have led to resins with high water resistance and improved bonding strength when compared with non-modified soy proteins (Graham and Krinski, 1983).

3.4.1.5 Carbohydrate adhesives

Carbohydrates have been used as modifiers for formaldehyde resins for many years; however, it is the formation of degradation compounds and the use of these com-pounds to manufacture resins which is of interest here. The reduction of carbohydrates in an acid environment will lead to the formation of fufuraldehyde and furfuryl alco-hol. These components can then be used to manufacture a resin, which, whilst very dark has been successfully used (Brown, 1952) as a panel adhesive.

The direct liquefaction of carbohydrate materials has been studied extensively. A large variety of lignocellulosic biomass such as wood (Alma et al., 2003), wheat straw

62 Wood Composites

(Chen and Lu, 2009), corn bran and cornstalks have been studied for the production of biopolyols; generally, the produced biopolyols exhibit promising properties for the production of polyurethane (PU) foams, and these foams show comparable properties with their petroleum analogs (Alma et al., 2003; Chen and Lu, 2009). However, all these liquefaction processes require high volumes of petroleum-derived solvents to be used as liquefaction agents, i.e., approximately 100–125 g solvent is required for 20 g of lignocellulosic biomass to obtain high-quality biopolyols (Chen and Lu, 2009; Lee et al., 2000). This high volume use of petroleum-based solvents considerably increases the production cost of biopolyols and consequently hinders future commercialisation efforts. Some work has been undertaken to assess the use of liquefied biomass as a suitable resin for the manufacture of panel products. Medved et al. (2011) showed that with the addition of liquefied wood to an MUF resin gave improved properties up to a 30% substitution rate, after which the properties began to fall.

3.4.1.6 Oil-based adhesives

Resins derived from unsaturated oils have been the subject of research as the oils can be purchased for very acceptable commodity prices. Linseed oil, for example, can undergo an epoxidation reaction and then be cross linked with a cyclic polycarboxilic acid anhydride to give a resin of the appropriate molecular weight. Although oil-based binders can be tuned to the requirements of the industry, their utilisation in the wood-based panel industry has yet to be established.

Recently there has been interest in the use of cashew nut liquor, a bi- product of the cashew nut, as a precursor for the manufacture of cardanol-based resins (Tomkinson, 2002; Wool et al., 1998). The process for the manufacture of the resin is based on the ozonolysis of the cardanol to generate an alkenyl chain on the car-danol. These groups can then self-condense to from a hardened network. The CNSL resin has been used to produce good lab results for dry and wet IB strengths, namely 1.05 and 0.58 MPa, respectively. Work is currently being undertaken on this resin system to optimise the manufacture of the resins and the integration into industrial scale panel manufacture.

3.4.2 Advances in synthetic resins

Whilst the development of bio-based resins has been the priority for the academic community, spinouts and for specialist applications, the advances in synthetic res-ins have largely been driven by legislation and markedly by the legislation around formaldehyde emissions. With the formaldehyde-based resins the composite man-ufacturers have a resin that is easy to use, they understand the chemistry and the kinetics of the cure of the resin, it does its job and, importantly, it is cheap to produce. However, government legislation is reducing the permissible formaldehyde releases from buildings (for example, the Clean Air Act, CARB, California, USA) The reduc-tion in formaldehyde emissions can be achieved in several ways, namely, changes in the resin formulations, additions to the furnish and post-manufacture treatment of the composites.

Adhesives for wood composites 63

3.4.2.1 Resin formulation changes

The reduction in the formaldehyde to urea ratios has a drastic effect on the amount of free formaldehyde and therefore the emission of formaldehyde from the panels. Over time the F/U ration has dropped from 1.6 to 1.12, and this has led to a 4-fold reduction in formaldehyde emissions (Mayer, 1985). Although the reduction in the F/U ratios can lead to composites that achieve the standard specifications, it also leads to a less forgiving resin in the factory (i.e. too quick curing and unreliable when there is a variation in atmospheric temperature). The addition of formaldehyde scavengers to the board has also been of interest to researchers and these have taken many forms, from additional urea to nut shells (Mayer, 1984).

3.4.2.2 Addition to the furnish

The addition of pre-treated furnish has been shown to reduce the emissions from panels, whilst mitigating the effects of the scavenging chemicals on the cure of the resin. This methodology has been tried by researchers and been shown to be successful; however, the uptake into industry has been very limited due to the cost implications. Several methods have been prevalent in the literature and are briefly described below:

1. The addition of scavenger impregnated furnish. This method has been proven within the laboratory to reduce emissions significantly (50–75%). However, it has not been adopted by industry because of the cost.

2. Spray in the furnish scavengers. This has been adopted with some composites, mainly plywood where the amount of scavenger can be closely monitored (Minemura et al., 1976).

3. Using a core layer that contains a formaldehyde scavenger and a non-formaldehyde-based resin. This method utilises an isocyanate-based resin for the core layer and relies on the free formaldehyde permeating through the panel to the scavenger material in the core (Roffeal et al., 1980).

3.4.2.3 Post-manufacture composite treatment

Five chemical and physical principles have been reported for the post-manufacture treatment of panels (Mayer, 1986):

1. Formaldehyde reaction with ammonia to form a stable hexamethylene tetramine molecule.

2. The reaction of formaldehyde with an oxygenated sulphur compound to form methylolsul-fonic acid.

3. Formaldehyde reaction with an –NH– group to form stable group.4. Adjustment of the pH of the final board to prevent the hydrolysis of the resin.5. Application of physical barriers such as coatings or paints to prevent the emission of

formaldehyde.

All of the treatments have been adopted in one form or another; however, the cost of implementation still prevents the mass implementation.

64 Wood Composites

3.5 Conclusions

This chapter has covered a range of both commonly used and novel, bio-based resins, other resins are used in more specific applications – for example, polyurethane resin is used in the manufacture of cross-laminated timber and epoxies used in the bonding in of connections. As mentioned, these resins are used for specific tasks, and, whilst worthy of a mention, have not been discussed in detail.

There are two overriding factors in the use of resin in wood composite manufac-ture: firstly legislation, which is becoming tighter; and secondly, emission rates (not just formaldehyde but other VOCs) becoming lower, but primarily cost. As mentioned earlier, the ‘traditional’ resins work and can be manufactured cheaply. For other resins (whether bio-based or otherwise) to enter the market successfully the resin would have to compete on performance, availability and price, whilst adding the ‘green creden-tials’ to the composite. It will only be through legislation and a large rise in oil prices that bio-based resins will gain a foothold within the market.

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