Читать книгу Adhesives for Wood and Lignocellulosic Materials - R. N. Kumar - Страница 61

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.