INVESTIGATION ON THE MELTING AND STRUCTUR AL BEHAVIOR OF POLY(ETHYLENE-TEREPHTHALATE) FIBERS IN AIR AT 1 BAR AND IN SUPERCRITICAL CO2 UP TO280 BAR

A comparative study was carried out as to the influence of CO2 on chan ges of the fibre structure of thermofixed and unfixed PETP multifilame nt and monofilament fibres in relation to pressure and temperature. To this aim measurements were carried out on the glass transition-, the pre-melting-and the melting temperature with a dynamic heat flow diffe rence calorimeter (DDC) in air at 1 bar, as well as in CO2 at pressure s up to 280 bar. In the thermograms taken under high pressure the melt ing point was clearly visible, in contrast to the glass transition-and pre-melting temperatures. Owing to its hydrophobic properties, the CO 2 is capable of diffusing into the fibre where it can act as a virtual contamination to the effect that the melting point at 280 bar is lowe red by 13-14 degrees C for all PETP fibres. Measurements of the pre-me lting temperature, stress-strain behaviour and shrinkage after treatme nt of the PETP fibres at temperatures between 80 and 200 degrees C in air and CO2 show that particularly in the case of non-thermofixed PETP yams at 280 bar structural changes are brought about from temperature s as low as 80 degrees C upwards which are attributable to partial cry stallite growth in the imperfect areas of the fibre polymers. In CO2 a t 280 bar, this results in higher pre-melting temperatures, increased shrinkage and higher elasticity of the fibres in contrast to air at 1 bar at comparable treatment temperatures. In the case of thermofixed f ibres these effects are, as a rule, considerably less marked.