Crosslinked and entanglement networks in thermomechanical analysis of polycondensation resins

Abstract

Thermomechanical analysis (TMA) tests on joints bonded with synthetic phenol–formaldehyde (PF) resins have shown that, frequently, the joint increase in modulus does not proceed in a single step but in 2 steps, yielding an increase of the modulus first-derivate curve presenting 2 peaks rather than a single peak. This behavior has been found to be due to the initial growth of the polycondensation polymer, leading first to linear polymers of critical length for the formation of entanglement networks. Two modulus steps and 2 first-derivate peaks then occur, with the first due to the formation of linear polymers entanglement networks, and the second due to covalent crosslinked networks. The faster the reaction of phenolic monomers with formaldehyde, or the higher the reactivity of a PF resin, the earlier and at lower temperature the entanglement network occurs; more important is its modulus value in relation to the final, crosslinked resin modulus. The accepted methods of calculating the gel point and gel temperature of a polycondensation resin or from the single peak of the first derivate of the modulus increase curve or from the start of the uprise of the modulus increase curve is still acceptable in resins in which the entanglement effect is small or it is not present. In resin systems in which the entanglement effect, instead, is of importance, the question of what is the gel point in such systems had to be addressed, and gel temperature and gel point must be obtained from the modulus and its first-derivate curve in a different manner, which is presented. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1111–1119, 1998