Лако-красочные материалы — производство

A precondition for the gluing of two wood surfaces is the wetting of these surfaces by the liquid or liquified adhesive. Wetting here includes the value for the contact angle, the spreading of the liquid on the surface, and the partial penetration of the liquid into the porous adherend. Good wetting enables the creation of high adhesion forces between the wood surface and the adhesive. However, direct correlations between the contact angle and the bonding strength achieved are rather rare [401] or seem not to be universal [402]. Low contact angles (© < 45°) indicate good wetting behavior. Contact angles greater than 90° lead to incomplete wetting, which might cause low bond strengths.

The main parameters that influence the surface tension of the adhesive, when on the substrate, and therefore the possible bond strengths are:

wood species [24-26,403] roughness of the surface [404-406] cutting direction (radial/tangential) [24-26] earlywood, latewood [24-26,407,408]

direction of the spreading of the droplet during measurement of the contact angle (along or lateral to the direction of the fibers) [409] wood moisture content [410-412] fiber angle [413]

age of the wood surface [414,415] pH of the wood surface [416-418] type and amount of wood extractives [401,419,420]

pretreatment of the surface, e. g., by extraction with various solvents [421] type of adhesive: UF resins [24-26]; PF resins [406-408]

During the production of wood-based panels a certain portion of the adhesive penetrates into the wood surface. An overpenetration causes starved glue lines, whereas too low a penetration limits the contact surface between the wood and the adhesive; low penetration often is the consequence of a poor wetting behavior. 2 or to yield a higher resistance against physical, chemical, and biological degradation. To render the wood substrate hydrophobic, e. g., by acetylation, decreases the number of hydrophilic sites [422]. The OH groups of the cellulose react with acetic anhydride forming an ester. The hygroscopicity of the wood substrate decreases, and hence swelling and shrinking of the panel can be lowered [423]. Use of acetylated fibers for the production of MDF boards showed marked reduction in their thickness swelling [424,425]. It has also been reported that wood acetylation can yield reactions of the anhydride with the aromatic ring of the lignin, although the exact reaction paths are not known [426]. This chemical attack at the aromatic rings can yield some crosslinking of the constituents of the wood substrate and can, therefore, contribute to the improved wood dimensional stability.