11 General effect of conditions on adhesive penetration. The temperature makes the adhesive more fluid until too much causes polymerization. At low wood moisture the water is drawn from the adhesive, while at high wood moisture the water retards the penetration. As the water content of the adhesive increases, the viscosity of the adhesive is lower and penetration increases. Both an increase in bond pressure and a longer time promote adhesive penetration. 

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... be proposed as to how these adhesive components may influence bond strength. The simplest is that the oligomers and monomers are simply soluble in the cell walls, but do not react, being too diluted by the cell wall components. In this case, they would maintain the cell walls in the expanded state due to steric constraint (bulking effect); thus, the process would reduce the stresses due to less dimensional change. A second model is that the adhesives react with cell wall components and possibly crosslink some of the components, thereby increasing the strength properties of the surface wood cells, as shown in Figure 9.10. A third model is that the adhesives polymerize to form molecular scale fingers of the adhesive in the wall, providing a nanoscale mechanical interlock. The fourth is that they form an interpenetrating polymer network within the wood, providing improved strength (Frazier 2002). All of these models have the adhesive reducing the dimensional changes of the surface cells, and therefore reducing the stress gradient between the adhesive and the wood, thereby improving the bond strength. Knowing that adhesive components do migrate into the cell wall, the next questions is: Are they associated with any specific cell layer or the middle lamella, and are they more in the cellulose, hemicellulose or lignin domains? One study indicates that the isocyanates seem to be more concentrated in the lignin domains (Marcinko et al. 2001). Peeling experiments have shown that an epoxy adhesive gave failure in the S 3 layer while a phenol-formaldehyde adhesive resulted in failure deeper in the S layer (Saiki 1984). Once an adhesive is applied to wood, the adhesive needs to set to form a product with strength. Set is “to convert an adhesive into a fixed or hardened state by chemical or physical action, such as condensation, polymerization, oxidation, vulcanization, gelation, hydration, or evaporation of volatile solvent.” Although the ASTM terminology uses solvent to refer to organic solvents, this chapter uses it in the more general sense of both water and organics because wood adhesives are usually water-borne. Water-borne adhesives often contain some organic solvent to help in the wetting of wood surfaces. For some of the polymeric adhesives, including polyvinyl acetate, casein, blood glue, etc., the loss of solvent sets the adhesive. For many others, including the formaldehyde-cured adhesives, the set involves both the loss of water and polymerization to form the bond. For polymeric diphenylmethane diisocyanate, the set is by polymerization. For hot melt adhesives, cooling to solidify the polymer is sufficient. In wood bonding, all of these mechanisms are applicable, dependent upon the adhesive system that is being used. The original wood adhesives were either hot-melt or water-borne natural polymers (Keimel 2003). These had several limitations in relation to speed of set, formation of a strong interphase region, and environmental resistance. All of the biomass-based adhesives had poor exterior resistance. The use of composites and laminated wood products has greatly expanded with the development of synthetic adhesives with good moisture resistance. Instead of being mainly polymers with limited and reversible crosslinks, these adhesives have strong covalent crosslinks to provide environmental resistance. In addition, these synthetic adhesives generally cure by both polymerization and solvent loss, leading to a faster setting process. Having multiple modes of set allows both the use of lower viscosity polymers for good wetting and polymers with a higher molecular weight for a faster cure. This combination gives a fast set rate that allows for higher production speeds. With many adhesive uses, solvents are a problem because of the non-porous nature of the substrate preventing removal of the solvent by migration into and through the substrate. However, wood is quite effective in allowing solvent to migrate away from the bondline, thus allowing adhesives to set. Of course, this property is very dependent upon the wood species and the moisture level of the wood (Tarkow 1979). It is not surprising that wet wood will less rapidly absorb moisture, thus making it harder for water-borne adhesives to move into the wood. The dynamics of water movement have a large effect on the bonding process. The factors involve penetration of the adhesive into the wood, rate of adhesive cure, flow of heat through composites, and premature drying of the adhesive. Most bonding processes require the wood to be within a set range of moisture content to get an acceptable set rate. The desire is to have the bonded product be near the normal in-use moisture condition to reduce internal stress and dimensional changes (Marra 1992). Penetration of the adhesive into the wood is an important part of the bonding process. Green wood is difficult to bond with most adhesives because there is little volume into which the adhesive can penetrate. (See Figure 9.11 for the generalized effect of bonding parameters on penetration.) At the other extreme, overly dry wood can also be difficult for the adhesive to penetrate because the wood surface is more hydrophobic and harder to wet (Christiansen 1994). Thus, wood with a 4% to10% moisture range is typically good for optimum penetration and set rates. While green wood hinders the adhesive flow into lumens for forming mechanical and chemical bonds, wood can also be too dry so that there is poor absorption of the water and adhesive. For the adhesive to set, the solvent needs to flow away from the adhesive into the adjoining and further removed cell walls. The sorption of the water into the nearby cell walls allows the formation of the solid, cured adhesive. Although most of the studies on uptake of small molecules into wood have naturally concentrated on water, other solvents are also readily absorbed/adsorbed by wood. For many of adhesives, cure rate is dependent upon the moisture content. Many setting reactions involve condensations that give off water; higher moisture levels can retard the reactions as expected by normal chemical equilibrium theory and from limited collisions due to dilution. The amount of water present also alters the mobility of polymer chains during the curing process, which can change the product distribution for the adhesive polymers. On the other hand, many isocyanates depend on a small amount of water to start the curing process. A very important issue in the rate of setting is the heat flow through composites or laminates to the bond surface, especially since wood is a good insulator. In composites, water in the wood near the surface or added steam helps transfer heat to the core of the composite. Use of core resins that cure at lower temperatures than face resins is important for fast production cycles. Controlling heat transfer and moisture levels is important for fast, reproducible composite production. In laminates, the use of water vapor for heat transfer is not available, thus leading to longer heating cycles. The ability of resorcinol-formaldehyde and phenol-resorcinol-formaldehyde to cure rapidly at room temperature favors them over phenol-formaldehyde resins despite their higher cost. Another way to accelerate cure is to use radiation methods, such as radio frequency curing. With some adhesives, premature drying can be a problem if the open time is too long. This involves too much loss of solvent so that the adhesive does not flow to wet the other surface. Proper control of moisture level and penetration determines the length of open- and closed-assembly times. For a strong bond, higher molecular weight and more crosslinked polymers are generally better. In most cases, adhesives consist of monomers and/or oligomers, which are a small number of monomers linked together. Because adhesives need to have stability prior to application, there needs to be some method for activation of polymerization. This activation can include heat, change in pH, catalyst, addition of a second component, or radiation. Sometimes a combination of methods is used for faster acceleration. The acceleration method is closely tied to the process for making the wood product. Heat is a very common way to accelerate polymerization reactions. Most chemical processes are controlled by the transition state activation energy, using the standard Arrhenius equation. One typical factor is that rates of reaction double for every 10 ̊C increase in temperature, but this does not always apply. This means that if a fast reaction is desired and the normal reaction temperature is not extraordinarily high, there will be appreciable reaction at room temperature limiting storage life of the adhesive. Since wood is a good insulator, uniform heating of the adhesive continues to be a problem for many composites and laminates. Incomplete heating gives poor bond strength as a result of incomplete formation of the polymer. To overcome this problem adhesive producers try to have the adhesive formulation to as advanced a degree of polymerization as is possible while still having good flow and penetration into the wood. Having a more advanced resin means that fewer reactions need to take place to obtain the strength properties needed from the adhesive. This balance between the advancement of the resin for fast curing while still having good bonding properties has been optimized by intense study of reaction mechanisms over the years to allow higher production rates. On the other hand, the understanding of heat and moisture levels within the composites is still being studied to allow further improvement in production rates. Many of the adhesive polymerization rates are sensitive to pH. This is especially true of the formaldehyde polymers, but the effect varies with the individual type of co-reactant and the different steps in the reaction. For urea-formaldehyde (UF) resins, the initial step of addition of formaldehyde to urea is base ...