Critical variables in the rapid cure and bonding of phenolic resins

Research conducted over the past 8 years on the curing and bonding characteristics of phenol-formaldehyde (PF) resin is summarized and interpreted. PF resins exposed to controlled temperature and relative humidity (RH) conditions exhibit faster cure rates with increases in RH. Chemical cure (exothermic activity measured by a differential scanning calorimeter), however, develops at a slower rate than mechanical cure (derived from stiffness measurements of a dynamic mechanical analyzer). Bond strength development (shown by lap shear tests of flake pairs) also lags behind mechanical cure but shows a peak rate at some optimum RH. Investigations have shown that boards can have maximum bending and internal bond strengths even though cure levels and flake pair bond strengths are relatively low. Conversely, boards with only marginal properties can result even at high levels of resin cure and flake bond strength. We believe the cured resin is substantially weakened by moisture, thus lowering its ability to counteract the internal forces. Extended press times are required to eliminate excess moisture and produce boards with adequate strength. Recommendations for future research are given.

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