Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar

outlined above are not mutually exclusive since one or more of the above mechanisms can occur simultaneously depending on the specific conditions prevailing during bonding. The hierarchical cellular characteristics of wood offer such varied conditions.

      The mechanical interlocking theory has long been used to explain wood bonding [6].

      The electronic or electrostatic theory has been applied to wood in finishing and coating operations, although this adhesion bonding mechanism needs more fundamental research [21]. The adsorption or wetting theory has been exhaustively studied on wood over the past 40 years [7, 8].

      The diffusion theory is appropriate in wood bonding during the production of compressed fibrous materials such as hardboard. The thermoplastic matrix, namely, lignin, can soften beyond its glass-transition temperature during the thermal conditions employed during hot pressing. Under these conditions, lignin can diffuse throughout the fibrous mat and react with the furfural liberated from hemicelluloses (pentosans) and solidify due to chemical reaction and hence function as an adhesive.

      Besides the diffusion and molecular interpenetration of lignin occurring during wet process in the hardboard production as mentioned above, there is also the phenomenon of diffusion of monomers/oligomers of synthetic resin adhesives such as PF or UF into the wood cells followed by subsequent polymerization. This is an important concept that speaks of monomers that penetrate at a molecular level for thermosetting adhesives [9].

      While discussing on the theories of adhesion in wood, one should keep in mind the opposite process (debonding). Weak boundary layers have been identified as the cause for the premature failure of the adhesive bond. In the case of wood bonding, the theory of weak boundary layers has also been proposed and studied. The weak boundary layers can be caused as a result of the mechanical damages occurring during the machining of wood surfaces. Further, the impact of surface aging the consequent inactivating of the wood surfaces [10–12] can also be responsible for the weak boundary layer.

      2.4.1 Mechanical Theory

      McBain and Hopkins [13] first proposed the concept of “mechanical adhesion” in their classical paper “On adhesives and adhesive action”.

      According to McBain and Hopkins, there are two kinds of adhesion: mechanical and specific adhesion. Specific adhesion involved interaction between the adherend surface and the adhesive. This interaction might be chemical interaction or adsorption.

Mechanical adhesion occurs “whenever a liquid adhesive penetrates into the porous adherend and solidifies in situ in the pores”. Examples are adhesion to wood, unglazed porcelain, pumice, and charcoal.

      The surface of a substrate is never truly smooth but consists of a maze of peaks and valleys. This type of topography allows adhesive to penetrate and fill these valleys, displace the entrapped air, and secure mechanically in position inside the substrate similar to the operation of the Velcro.

      Porosity and roughness of the substrate increase the total area of contact between the adhesive and the adherend. Hence, roughening the adherend surface enhances the mechanical interlocking since total effective area over which the forces of adhesion can develop increases. The mechanical adhesion theory does not take into account the intrinsic incompatibility between the adhesive and the substrate.

       2.4.1.1 lllustration of Mechanical Adhesion for Wood

In wood adherends, there is a vast array of void spaces as shown in Figure 2.2. Spontaneous surface wetting and capillary effects allow the flow of the adhesive resin into the cell lumen, vessels, or other interstices followed by subsequent hardening of the resin and resulting in mechanical interlocking. The resin acts to reinforce the surface/interface layers of wood cells. An adhesive penetration of approximately 6–10 cell diameters (fewer than 100 μm, maximum) is regarded as necessary for optimal adhesive bonding. Filling the cell lumen with adhesive provides much larger mechanical interlocks than are available with surface roughness for other substrates. Absorption into the cell wall can provide micromechanical interlocks and interpenetrating networks [1].

Figure 2.2 Various wood elements.

2.5 Electronic Theory

      This theory was mainly promoted by Deryaguin [14–16]. If the adhesive and the substrate have different electronic band structures, there is likely to be electron transfer on contact between the two surfaces. This results in the promotion of a double layer of electrical charges at the adhesive substrate interface. Electrostatic forces are formed at the adhesive–adherent interface. This accounts for the resistance to separation. This theory gathers support from the fact that electrical discharges have been noticed when an adhesive is peeled from a substrate. Electrostatic adhesion is regarded as a dominant factor in biological cell adhesion and particle adhesion. No application of this theory to wood appears possible.

      The electronic theory

       Depends on material properties that allow electron transfer across the interface

       Requires intimate contact/smooth surfaces

       Interactions are very weak and rather insignificant

       Mechanism is not important for wood substrates

2.6 Diffusion Theory

      The diffusion theory was proposed in the early 1960s by Voyutskii [17–19]. It states that the intrinsic adhesion of a resin to a polymeric substrate is due to mutual diffusion of polymer molecules across their interface.

      As a result of this interdiffusion of molecules of the adhesive and adherend, their interface disappears. Hence, the diffusion theory is applicable only when both adhesive and adherend are compatible polymers that possess sufficient mobility and mutual solubility. Solvents or heat welding of thermoplastic substances is caused by diffusion.

      The prerequisite of the diffusion theory is that the polymers of the adhesive and of the substrate should possess similar values of solubility parameters.

Several problems are encountered when an attempt is made to apply the diffusion theory to wood. Basically, wood is not homogeneous in composition. It is a cellular composite of three polymers, namely, cellulose, hemicelluloses, and lignin. Furthermore, cellulose consists of both crystalline and amorphous regions. It is clear then from solubility parameter concepts that some polymers, the amorphous ones such as hemicelluloses and lignin and the amorphous portion of cellulose, could, under some conditions, undergo mutual diffusion with the polymer chains of the synthetic adhesives. The crystalline portion of cellulose is not likely to be involved.

      There is one specific instance in the case of wood adhesion [17] in which interdiffusion appears to exist and is likely to play a significant role in wood bonding. This is the production of fiberboard by the wet process in which no adhesive is added. At high moisture content, high temperature and pressure and long pressing times, the glass transition temperatures of lignin are exceeded. Thus, the lignin in the fibers is mobilized and the interdiffusion between lignin polymers on different fibers contributes to the bonding of the fibers together.

2.7 Adsorption/Covalent Bond Theory

      The adsorption theory of adhesion, and the most widely accepted one, in wood science, which is sometimes also called “specific adhesion” [20], states