ABSTRACT
Phenol-resorcinol-formaldehyde (PRF) cold-setting adhesives have always been used in the manufacture of structural glulam, finger joints, and other exterior timber structures They produce bonds not only of high strength but also of outstanding water and weather resistance when exposed to many climatic conditions [1,2]. PRF resins are mainly prepared by grafting resorcinol onto the active methylol groups of the low condensation resols obtained by the reaction of phenol with formaldehyde. Resorcinol is the chemical species that gives to PRF adhesives their characteristic cold-setting behavior. It gives accelerated and improved crosslinking, at ambient temperature and on addition of a hardener, to the phenol-formaldehyde resins onto which it has being grafted by chemical reaction during resin manufacture. Resorcinol is an expensive chemical, produced in only a very few locations around the world (to date only three commercial plants are known to be operative: in the United States, West Germany, and Japan), and as such it is the determining factor in the cost of PRF adhesives. The resorcinol manufacturing process is a difficult one, and the use of certain solvents during production and purification of this chemical enhances the risks of dramatic plant breakdowns. This, from time to time, may endanger world supply, with unwanted critical consequences to the bigger users of resorcinol, namely the adhesives and timber industries depending on it. Disruptions in the world supply of resorcinol have occurred several times in the past, and although the last major crisis was nearly 14 years ago, there is never any guarantee that another crisis in the supply of this chemical is not just around the corner. Thermosetting phenol-formaldehyde resins can be used in place of PRF adhesives during such a crisis, but the cost of equipping a glulam or finger-jointing plant with adequate machinery, such as high temperature, moisture-controlled ovens or high capacity radiofrequency presses, is economically unwarranted. The performance of the PF resins is also not as good as that of PRF resins. Since resorcinol is such an expensive, strategic chemical, it is also the determining factor in the cost of PRF adhesives. Significant reductions in the cost of such adhesives must, by necessity, be coupled with a decrease in their resorcinol content.
