SynopsisDifferential scanning calorimetry was used on a range of synthesized phenol-formaldehyde (PF) resols to discover relationships between formulation parameters or physical properties of resols, and their thermal behavior during cure. The thermograms showed either one or two exothermic reactions. The lower exothermic peak temperature varied between 98 and 129°C with changes in the free formaldehyde content. This exotherm is caused by the addition of free formaldehyde to phenolic rings. The upper exothermic peak temperature varied from 139 to 151"C, with the higher temperatures occurring when the formaldehyde-to-phenol molar ratio was low or the total amount of sodium hydroxide relative to phenol was high. These two factors led to resins which contain a somewhat higher level of unreacted ortho or para aromatic ring positions and no free formaldehyde. Consequently, condensation is probably not solely by the faster self-condensation through hydroxymethyl groups, but also includes the slower condensation of hydroxymethyl groups with unreacted ring positions. Gel times show trends with changes of formulation parameters somewhat similar to trends of the upper exothermic peak temperatures.

The structure of polypropylene crystallized at pressures up to 5000 atm. has been studied. Upon slow cooling from the melt at 320 atm., the γ modification, previously found only in low molecular weight and stereoblock fractions, begins to appear in small amounts in addition to the normal α monoclinic form. As the pressure is increased further, a larger proportion of the sample crystallizes in the γ form until, at 5000 atm., only the γ modification is present. X‐ray and DTA studies show that the γ form of polypropylene transforms to the normal α modification at a temperature only slightly below the γ melting point. Evidence is presented which favors the occurrence of a solid‐state transition as a model of transformation to the α form. Results from isothermal crystallizations at low supercoolings and annealing experiments under high pressure show that the melting point of the γ modification of polypropylene is very sensitive to crystallite perfection.

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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...