will adhere to a substrate because of intermolecular and interatomic forces between the atoms and molecules of the two materials. The interatomic and intermolecular forces referred to can be any type of either primary or secondary valency forces. van der Waals forces, hydrogen bond, and electrostatic forces are as much applicable as the primary valence forces such as ionic, covalent metallic coordination bonds. In the case of wood adhesion, however, there is an age-old mistaken notion that covalent linkages must be present to ensure good joint strength. In fact, covalent bonding theory was invoked to explain the durable wood bonding with thermosetting adhesives. But as mentioned by Gardner [21], it is very likely that covalent bonds between the wood and adhesive are not necessary for durable wood adhesive bonds.
Calculations carried out by a number of authors based on the secondary forces involved indicate that the wood bond strength in tension should be over 100 MPa. This is considerably higher than the experimental values obtained in the case of several wood adhesives. This discrepancy could be due to the presence of voids, defects, and the geometrical features of the test specimen. Pizzi concludes that these studies indicate that the secondary valency forces themselves are adequate to explain the practical results and it is not necessary to invoke the involvement of covalent bonds [20]. An elaborate discussion on the relative importance of the primary and secondary valence forces has been furnished by Pizzi based on the adhesive strengths obtained from wood joints and the common wood adhesives such as phenolics, amino resins, and isocyanates [20].
2.8 Adhesion Interactions as a Function of Length Scale
It is useful to know the scale of lengths over which the adhesion interactions as described above do occur (Table 2.2) [21]. It is apparent from Table 2.2 that the adhesive interactions relying on interlocking or entanglement can occur over larger lengths than the adhesion involving charge interactions. Most of the charge interactions occur at the molecular level or the nano-length scale. Electrostatic interactions are the exception to this generalization. For the purpose of adhesive interactions, they are considered to operate from a nano- to a micron-length scale.
Table 2.2 Comparison of adhesion interactions relative to length scale.
| Category of adhesion | ||
| Mechanism | Type of interaction | Length scale |
| Mechanical | Interlocking or entanglement | 0.01–1000 μm |
| Diffusion | Interlocking or entanglement | 10 nm-2 mm |
| Electrostatic | Charge | 0.1–1.0 μm |
| Covalent bonding | Charge | 0.1–0.2 nm |
| Acid-base interaction | Charge | 0.1–0.4 nm |
| Lifshitz van der Waals | Charge | 0.5–1.0 nm |
2.9 Wetting of the Substrate by the Adhesive
It is evident from the previous section that various interactions operate effectively only when the molecules of the adhesives come as close as possible to those of the substrate in order that such a proximity will lead to maximum mutual interaction. Such closeness is possible only when the adhesives wet the substrate.
Wetting is the ability of liquids to form interfaces with solid surfaces. To determine the degree of wetting, the contact angle (θ) that is formed between the liquid and the solid surface is measured. The smaller the contact angle and the smaller the surface tension, the greater the degree of wetting (Figure 2.3).
Figure 2.3 Wetting phenomenon.
For maximum adhesion, the adhesive must completely cover the substrate, i.e., spreading is necessary. The contact angle is a good indicator of adhesive behavior. This is illustrated in Figure 2.4.
Figure 2.4 Wetting, spreading, and dewetting for different contact angles.
2.10 Equilibrium Contact Angle
In 1805, Thomas Young provided the first good approach for describing wettability, spreading, and their relationship to the contact angle.
A drop of adhesive on a surface will come to equilibrium under the action of three forces as shown in Figure 2.5.
Figure 2.5 Equilibrium contact angle based on balance of forces.
Considering the component of γLV along the X-axis, we can write the following force balance:
Thus, when θ = 0, the liquid spreads spontaneously on the substrate; in other words, when cos θ is high (i.e., as it approaches 1), there is spontaneous spreading.
From Equation 2.1, it is clear that wetting will be favored when the surface tension of the liquid is low.
Since the tendency of the liquid to wet and spread spontaneously increases as the contact angle decreases, the contact angle is a useful inverse measure of wetting or the cosine of the contact angle is a direct measure of wetting.
2.11 Thermodynamic Work of Adhesion
Perhaps