| Abstract: | —The processes operative within wood-based composites during hot pressing are briefly reviewed, and the role of adhesion in the system is considered. A new method is described for investigating how temperature and moisture content together affect the kinetics of thermosetting adhesive bond strength development. For this purpose, powdered phenol formaldehyde to wood bonds were formed under highly controlled, nearly steady-state conditions of temperature (between 90 and 120°C) and moisture content (between 4 and 16%); after a range of curing times, the bonds were immediately destructively tested in tension. Plotting accumulated strength versus forming time enables construction of strength-development curves, which reveal an initial delay before the onset of strength development, a period of nearly constant strength development, a small discontinuity in which the bonding rate is reduced, and, finally, a period of well-defined bonding at an ever decreasing rate. All the stages of these curves were found to be affected interactively by local temperature and moisture content. The term 'reactivity index' was coined to describe the dependence of strength-development rates on temperature for a specified moisture content. The derived relationships may be used in simulation models to estimate the development of bonding within composites as hot pressing proceeds. The technique may also be used more generally to aid in the development of adhesive systems with bonding characteristics tailored for specific curing conditions. |
