Rheokinetical behavior of melamine-formaldehyde resins

The effects of cure temperature and amount of catalyst on the rheokinetical behavior of a melamine-formaldehyde (MF) thermosetting system is investiga ted using a dynamic mechanical technique similar in nature to Torsion Impre gnated Cloth Analysis (TICA) and Torsional Braid Analysis (TBA). The propos ed name of the used technique is Torsional Substrate Analysis (TSA). Isothe rmal cures of the resin are carried out from 115 degrees C to 160 degrees C for varying amounts of catalyst. Each TSA measurement exposes several tran sitions. First, a glass-to-liquid transition during the heatup procedure is seen, indicated by sharp peaks of the loss shear modulus, G ", and loss ta ngent, tan delta. Later, vitrification is seen, indicated by a second G " m aximum. Finally, a completion of shift to a diffusion controlled cure react ion occurs, shown as a storage shear modulus, G', plateau. The rheokinetica l data is used to construct Time-Temperature-Transformation (TIT) cure diag rams, for each level of catalyst. High pressure differential scanning calor imetry (HPDSC) measurements are carried out in order to estimate the fracti onal conversion of samples that have been cured isothermally for times corr esponding to a second tan delta maximum, the second G " maximum, and the G' plateau. The fractional conversion is determined by the residual entalphy technique. The HPDSC measurements do not give a clear answer whether the se cond tans maximum corresponds to gelation or not. It is therefore likely th at TSA, like similar techniques, is not capable to detect gelation. A glass transition temperature of 130 degrees C and 150 degrees C is found to corr espond to a fractional conversion of 0.65 and similar to 0.80, respectively . Preliminary measurements suggest that the maximum glass transition temper ature, T-g infinity, of the investigated MF resin is at least 180 degrees C .