Rigid poly(vinyl chloride) gelation in a brabender measuring mixer. III. Transformation in the torque maximum

The processing and structural effects occurring in a poly(vinyl chloride) (PVC) compound processed in a plastograph, at the time at which the maximum value of the torque (point X on the torque curve) was reached, were studied. The unplasticized PVC compound was processed at various temperatures (150-200 degrees C) with a kneader operating at rotors speeds of 5-40 min(-1). The changes in the tem perature of the compound due to kneading and its influence on the progress of PVC gelation were analyzed. The gelation progress was studied with differential scanning calorimetry, rheological measurements, and scanning electron microscopy. The maximum value of the torque occurred within the temperature range (170-180 degrees C) of the PVC compound, and it was independent of the adjusted temperature of the mixing, chamber. The processing of the PVC compound in the kneader, with high mechanical charges and a comparatively low adjusted temperature (150-170 degrees C), spontaneously generated an effect of self-heating, resulting in an increasing temperature of the compound. Despite the surprisingly high degree of gelation (80-98%) of the PVC compound processed under these conditions (as determined by differential scanning calorimetry), the scanning electron microscopy observations indicated significant nonhomogeneity of the fracture surface. Consequently, the kneaded PVC compound at point X of the torque curve could be treated as a two-phase system composed of a liquid, amorphous phase and elements of a grain structure. The transformation of both phases (particularly the quantitative and qualitative changes), which was significantly dependent on the PVC temperature and the shear rate applied by the rotors, determined the state of the melted compound at this point of the torque curve. (c) 2007 Wiley Periodicals, Inc.