High-temperature aging of Halar film. II. Analysis of melting behavior

Melting behavior of an experimental Halar film, a predominantly alternating 1:1 copolymer of ethylene (E) and chlorotrifluoroethylene (CTFE), has been studied. Differential scanning calorimetry (DSC) reveals single or double melting peaks, depending upon the thermal history. The lower-temperature melting peak T_m1 is produced only by the thermal treatment and shows a strong dependence on annealing time and temperature. On the basis of the DSC and x-ray data it can be suggested that T_m1 represents the melting of relatively small crystallites formed upon annealing. The higher-temperature melting peak T_m2 is always shown at 238°C. (Note: the specification for commercial Halar product is 240°C. The slightly lower melting temperature reported in this study is probably due to the fact that we are dealing with an experimental melt-processed material.) On the basis of the heating rate study we propose that Halar crystallizes with stable crystals (T_m2 = 238°C) regardless of the crystallization conditions, i.e., quenching, slow cooling, or even annealing. Crystals of Halar have a heat of fusion of approximately 35 cal/g or 146 kJ/kg. Detailed analysis of the melting behavior of Halar is presented.