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 penetration and (b) cell wall penetration. Gross penetration is the flow of liquid adhesive into the porous structure of wood in order to fill the lumens. For cell wall penetration, the adhesive enters the woody cell wall. Adhesive gross penetration and cell wall penetration are both critical to the performance and durability of adhesive joints [32–35].
1.5.1.1 Penetration in Different Size Scales
Laborie proposed a categorization of adhesive penetration in wood in terms of different size scales of adhesive penetration [32]. Accordingly, in the following classification, four scales of penetration are envisioned:
1 Macroscopic penetration (millimeters)
2 Microscopic penetration (microns to tens of microns)
3 Nanoscale penetration (nanometers to tens of nanometers)
4 Angstrom scale penetration (up to tens of angstroms)
Wood as a porous, cellular material displays roughness on the micron scale but can also exhibit roughness on the millimeter scale, depending on how a particular wood element to be bonded is produced. For example, production of rotary-peeled veneer can produce roughness on a millimeter scale due to the creation of lathe checks. Pores or free volume also occur within the amorphous regions of the cell wall material on the molecular level. The length scales over which the wood–adhesive interactions occur are given in Table 1.2 [36].
Table 1.2 Comparison of wood–adhesive interactions relative to length scale.
| Component | μm | nm |
| Adhesive force | 0.0002–0.0003 | 0.2–0.3 |
| Cell wall pore diameter | 0.0017–0.002 | 1.7–2.0 |
| PF resin molecular length | 0.0015–0.005 | 1.5–5.0 |
| Diameter of particles that can pass through a pit | 0.2 | 200 |
| Tracheid lumen diameter | 4–25 | |
| Glue line thickness | 50–250 |
In order to understand the adhesion phenomenon operating at different size scales in wood, the values of size scales shown in Table 1.2 should be viewed in conjunction with the length scale (as shown in Table 1.3) [37] over which different wood–adhesive interactions take place.
Table 1.3 Comparison of adhesion interactions relative to length scale [36].
| Category of adhesion mechanism | Type of interaction | Length scale |
| Mechanical | Interlocking or entanglement | 0.01–1000 μm |
| Diffusion | Interlocking or entanglement | 10 nm-2 nm |
| Electrostatic | Charge | 0.1–1.0 μm |
| Covalent bonding | Charge | 0.1–0.2 nm |
| Acid-base interaction | Charge |