Viscoelastic properties of the phenolic resin–polyvinyl butyral system

Abstract The viscoelastic properties of the phenolic resin–polyvinyl butyral system were studied over a wide range of temperature by means of the vibrating reed method, at about 50 cycles/sec., and dielectrically, at 60 cycles/sec. Two mechanical loss peaks due to polyvinyl butyral in the mixed system, occurring at about 100 and 160°C., were readily observed. These peaks are influenced by the curing temperatures, being readily detected in specimens cured in the range 180–200°C. but not in those cured below 170°C. The effect of cure temperature on the appearance of a dielectric loss peak at about 100°C. for the system agrees in trend with the effect of cure temperature on the mechanical loss peak in the same temperature range. This result may lend some support to the hypothesis that the polar functional groups of the system play some important role in the appearance of a loss peak. The loss peak at about 100°C. coincides approximately with the maximum loss peak for polyvinyl butyral in the mixed system, and hence this peak may be due to the second‐order transition of the polyvinyl butyral which seems to be in a heterogeneous state in the specimen. Another peak, found at about 160°C., may be attributed to grafting or crosslinking between the phenolic resin and the polyvinyl butyral, resulting from chemical reaction between the two materials during the curing process, as was shown by results for the mixtures of the phenolic resin with PVB and with two PVB derivatives. A model of the state of dispersion of the two constituents in the system is proposed on the basis of these results. Finally, the loss peak occurring at about −42°C. for the system is clearly attributable to the phenolic resin alone. The molecular mechanism for this dispersion, on the basis of results of viscoelastic and dielectric measurements, may be the local movement of side chains, especially unreacted methylol and amino groups on the phenolic resin molecules, although there is no additional clear support for this conclusion.