peroxidase by hydrogen peroxide.
The first step of the biochemical pathway for building up lignin macromolecules is the enzymatic dehydrogenation of p-hydroxycinnamyalcohols, yielding mesomeric ring systems with a loosened proton. Figure 1.7 shows the formation of phenoxy radicals from coniferyl alcohol by a one-electron transfer:
Figure 1.7 Enzymatic dehydrogenation of coniferyl alcohol yielding phenoxy radicals.
The origin of the hydrogen peroxide was cleared up by discovering cell-wall-bound enzyme systems able to deliver H2O2 [22, 23].
Only 4-phenoxyradical I to IV are actually involved in lignin biosynthesis. Structure V is sterically hindered or thermodynamically not favored [24].
The polymerization of monomeric precursors by random coupling reactions cannot be studied in vivo, but it is known from numerous in vitro experiments to run without enzymatic control as a spontaneous process. The first step in polymerization is the formation of dimeric structures. Some prominent lignol dimers called dilignols are shown in Figure 1.8.
Figure 1.8 Typical dilignol structures [25].
Further polymerization is called end-wise polymerization involving coupling of monolignols with the phenolic end groups of di- or oligolignols or a coupling of two end group free radicals, yielding a branched polymer via tri-, tetra-, penta-, and oligolignols [11].
Summarizing the formation of lignin, as mentioned by Fengel and Wegener [11], it is evident that these macromolecules are not formed by a genetically prescribed regular mechanism, but by a random coupling of lignols to form a nonlinear polymer. The final constitution of lignin is therefore determined mostly by reactivity and the frequency of the building units involved in its polymerization.
Proportions of different types of linkages connecting the phenylpropane units in lignin are given in Table 1.1.
Table 1.1 Proportions of different types of linkages connecting the phenylpropane units in lignin.
| Percent of the total linkages | |||
| Linkage typeb | Dimer structure | Softwooda | Hardwooda |
| β-O-4 | Arylglycerol-β-aryl ether | 50 | 60d |
| α-O-4 | Noncyclic benzyl aryl ether | 2–8c | 7 |
| β-5 | Phenylcoumaran | 9–12 | 6 |
| 5–5 | Biphenyl | 10–11 | 5 |
| 4–0–5 | Diaryl ether | 4 | 7 |
| β-1 | 1,2-Diaryl propane | 7 | 7 |
| β-β | Linked through side chains | 2 | 3 |
a Approximate values based on data of Adler [26] obtained for MWL from spruce (Picea abies) and birch (Betula verrucosa).
b For corresponding structures.
c Values have been reported [25].
d Of these structures, about 40% are of guaiacyl type and 60% are of syringyl type.
1.5 Influence of Hierarchical Structure of Wood on Wood–Adhesive Interaction
Marra [27] describes the process of adhesive bond formation in a wood substrate by five steps: flow, transfer, penetration, wetting, and solidification. The flow involves the spreading of the liquid on the wood surface. This is followed by transfer of adhesive to the adjacent wood surface. Capillary forces within the cell lumens promote penetration, and bulk flow occurs due to applied pressure. Wetting of the wood surface by the adhesive occurs to an optimum extent, which promotes the molecular contact between the adhesive and wood surface. Finally, solidification occurs as a result of the formation of three-dimensional cross-linked structure when the glue line is exposed to high temperature.
Penetration of adhesive into the porous network of wood cells is believed to have a strong influence on bond strength [28–30].
Damaged wood cells may be reinforced by the adhesive, and stresses may be more effectively distributed within a larger interphase region. The optimum depth of penetration is required to ensure mechanical adhesion [31], but excessive penetration causes insufficient adhesive remaining at the interface [27], leading to a starved bondline. This constitutes a weak boundary layer and a weak spot in the chain (see Figure 2.7) of Marra’s chain-link analogy [27] (see Section 2.15). This analogy emphasizes the fact that the overall strength of an adhesive bond in a composite is determined by the weakest portion of the chain. In other words, an adhesive bond is just as good as the weakest link in the chain. In this respect, adhesive penetration plays a vital role in this analogy.
1.5.1 Penetration
The hierarchical structure of wood profoundly influences the adhesion phenomenon over a wide range of “length scales”. The adhesive phenomenon occurs first by transport phenomenon (bulk flow, penetration, and diffusion) followed by a number of possible processes ranging from mechanical adhesion to the formation of chemical bonds as shown in Table 1.3. Adhesive penetration in wood is commonly categorized into (a) gross
