SYNOPSISWe are interested in using dynamic mechanical analysis (DMA) to measure the degree of cure achieved by aqueous phenol-formaldehyde (PF) resole resins as a result of previous exposures to various temperature-humidity-time regimes. We developed procedures that permit the obtaining of data that properly characterize and quantify the DMA behavior of these aqueous systems. Particularly important factors are substrate selection, sample clamping geometry, and sample humidification prior to analysis. In this report, we illustrate the problems encountered and the steps that we adopted to resolve them. We also present preliminary DMA data that illustrate some approaches for measuring the degree of cure achieved during previous resin exposures. One especially promising approach employs the area under the tan delta curve during isothermal scanning as an inverse measure of precure.

SYNOPSISTwo different phenol-formaldehyde ( P F ) resole resins are serving as models in a study aimed a t 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. INTRODUCTIONSteam-injection pressing is a recent development for manufacturing reconstituted wood panel products. Compared with conventional panel pressing, steam-injection pressing permits more rapid cure of thicker panels and yields more uniform density cross sections.' However, the very different time-temper-* To whom correspondence should be addressed. ature-moisture regime during steam injection may cause profound differences in the chemistry and physics of resin cure and resin-wood bonding. Unfortunately, the present understanding of the chemistry and physics of resin cure and bonding is limited, even for conventional pressing of reconstituted wood products. As a result, the industry optimizes adhesives and the bonding process primarily on an empirical basis.An investigation is underway at the Forest Products Laboratory in cooperation with the University of Wisconsin-Madison to establish how the cure and wood bonding of different phenol-formaldehyde Table I ) for our initial investigations, primarily on the basis of the significant differences between their compositions and molecular weight distributions. Formula 2 is a conventional alkaline resole PF flakeboard adhesive, whereas...

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onSummary The effects of temperature and relative humidity on phenol-formaldehyde resin bonding were evaluated. Two flakes in a lap-shear configuration were bonded under an environment of controlled temperature (110 ~ 120 ~ 130 ~ 140 ~ and relative humidity (41%, 75%, 90%) for a series of time periods (0.25 to 16 min). The lap-shear specimens were then shear-tested on a mechanical testing machine and the results were used to establish a family of bond strength development curves at each temperature and level of relative humidity. At 1 I0 ~ the higher relative humidity appeared to retard resin bonding. The effects of relative humidity diminished as temperature increased to 140 ~ Bond strength development was chemical ratecontrolled. The rate of bond strength development at each relative humidity follows a first order reaction mechanism. The activation energy of resin-wood bonding, determined by bonding kinetics, was higher than that of resin alone, determined by differential scanning calorimetry. This comparison indicates that to form a strong resin-wood bond, a higher energy level might be required. 253

Dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) were applied to samples of phenol-formaldehyde resole resins that had been previously exposed to dry precure conditions. The area under tan delta curves during DMA isothermal scans of such samples decreased as the degree of precure increased; this area was used as an empirical measure of the rate of mechanical cure for phenolic resins. Similarly, the DSC exotherm area decreased with greater sample precure; this area was used to measure the rate of chemical cure. Rates of mechanical and chemical cure differed for different resins and with temperature. Under dry precure conditions at 115° and 140°C, a high degree of mechanical cure was achieved at a low degree of chemical cure.