Читать книгу Adhesives for Wood and Lignocellulosic Materials - R. N. Kumar - Страница 59

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