Лако-красочные материалы — производство

Understanding the causes and mechanisms of fracture in adhesive-bonded joints and mate­rials is important to improving their performance, developing products based on new com­binations of materials and adhesives, predicting the performance of new materials, and developing design methods for structural joints. In this chapter I have briefly discussed some aspects of wood and adhesive fracture, the influence of wood and adhesive properties upon joint fracture, the effects of environment and joint geometry on fracture, and the attempts to develop design methods for bonded joints and materials based on fracture mechanics.

Microstructure, in particular the discontinuities in the walls of thick-walled cells, is a controlling factor in the fracture of well-made joints bonded with rigid, thermosetting adhesives. The properties of the adhesive, however, play a major role in ameliorating the weaknesses of thin-walled cells and the discontinuities in thick-walled cells. Good wetting and chemical adhesion are important to bond performance, but they are not in themselves sufficient for maximum fracture toughness of bonded wood joints and materials. Hard, brittle adhesives, especially those that do not effectively penetrate the wood cell cavities and the cell wall, promote transwall cracking of thin-walled cells and microcracking and intra­wall fracture of thick-walled cells. Less rigid adhesives that penetrate the cell lumens and cell wall distribute stress and inhibit microcracking in the wood.

The best adhesive for improved fracture toughness (1) does not develop shrinkage stresses during cure, (2) has a modulus close to that of wood perpendicular to the grain, (3) has a modulus that changes in parallel with the wood modulus as moisture content changes, (4) penetrates small-lumen, thick-walled cells but does not overpenetrate large — lumen thin-walled cells, and (5) can infiltrate the cell wall to reinforce the weak interphase between cell-wall layers.

The behavior of bonded joints and materials can be predicted successfully on the basis of material properties through applying the principles of fracture mechanics. However, much research is still required to achieve a method that is generally applicable to all adhesives, species, and joint geometries or material constructions. One field of particular importance and complexity revolves around the important effects of time, moisture, and temperature, and their interactions. At present, without extensive and long-term testing, there is no way to predict or evaluate the trade-offs between high short-term fracture toughness in joints or materials bonded with semirigid adhesives and reduced stress-rupture resistance of these adhesives under conditions of elevated moisture, temperature, or pro­longed loading. An understanding of these relationships and the development of a model to predict the effects of trade-offs could lead to a new generation of wood-based materials and efficient adhesive-bonded wood structures.