PHENOL FORMALDEHYDE RESIN CURING AND BONDING IN STEAM-INJECTION PRESSING .1. RESIN SYNTHESIS, CHARACTERIZATION, AND CURE BEHAVIOR

Two different phenol-formaldehyde (PF) resole resins are serving as models in a study aimed at establishing the effects of moisture, temperature, pressure, and time on resin cure and bonding during the pressing of wood flakeboard. This phase of the program had two goals: first, to characterize the two resins in terms of their structure and chemistry during synthesis, aging, and cure-using viscosity measurement, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA); second, to make a preliminary evaluation of the utility of DSC, FTIR, and DMA for measuring the degree of resin cure. The two resins differed significantly in relative amounts of hydroxymethyl groups and methylene linkages (NMR), in molecular weight and its distribution (GPC), and in reaction rate (as measured by viscosity, DSC, FTIR, or DMA). The degree of cure developed during constant heating rate DSC scans was calculated for a series of maximum DSC temperatures from both the loss in hydroxymethyl groups (FTIR) and the decrease in available exothermic heat (DSC). Agreement between the two methods was quite good, considering the inherent difficulties in quantifying infrared data. For comparison, the degree of cure developed during constant heating rate DMA scans was calculated for a series of maximum DMA temperatures from both the increase in storage modulus (DMA) and the decrease in exothermic heat (DSC after rewetting). Samples that apparently achieved complete cure in the DMA still exhibited significant residual cure potential in the DSC. We attribute the lower apparent cure in the DMA to loss of moisture from samples during the DMA scan, with consequent loss in plasticization and molecular mobility.