Weak boundary layers on wood surfaces

Adoption of the weak boundary layer (WBL) concept in wood adhesion science and a general extension of the concept are suggested. A distinction is made between a chemical weak boundary layer (CWBL) and a mechanical weak boundary layer (MWBL). Definitions of both CWBL and MWBL are proposed. On wood, a CWBL is caused mainly by extractives migrating to the wood surface after machining, whereas an MWBL is caused mainly by the machining operation itself or by degradation of the fibres at the surface, in practice mainly by natural (UV) light irradiation. Sawn endgrain and planed side-grain surfaces of softwoods have been studied by scanning electron microscopy [SEM and environmental SEM (ESEM)]. Both sawn and planed wood surfaces are deformed to a depth of 0.1-0.2 mm to such an extent that one cannot expect the same strength at the surface as in the intact wood material. The crushed and damaged surface can be expected to contain failure initiation sites when the glueline, especially in an end-grain joint, or a coating is exposed to stresses. Furthermore, this crushed and damaged surface can be expected to be a barrier for adhesives and coatings to penetrate into the material and anchor to intact wood material. A comparison between microtomed and sawn end-grain surfaces has shown that the damaged fibres seem, to some extent, to improve the glue joint performance, using a conventional glue. Reasons for this are discussed.