Thermal stress arising in the polypropylene filament preventing its thermal-shrinkage was measured to study the micro-fine structural character of filament and the stabilizability of the dimension of fiber by thermo-setting.
Four kinds of polypropylene filament ranging in molecular weight from 5.81×104 to 2.51×105 were prepared under the same spinning and stretching conditions except the spinning temperature.
By analyzing thermal stress, fs, defined as force per unit cross-section of sample at room temperature, the following experimental results were obtained;
(1) when the rate of changing temperature is less than 1°C/min., fs -temperature relations is not affected by the rate of changing temperature.
(2) Thermal stress is observed only for stretched filaments not subjected to heat-treatment.
(3) Generally, thermal stress increases with temperature above 65_??_72°C, independently of molecular weight and of the stretching conditions.
(4) (fs)max, maximum value of fs, increases with stretching ratio, (fs)max of filament, prepared under the same stretching conditions, was found to be proportional to molecular weight.
(5) fs on filament which was thermostatted at the fixed lower temperature than that at (fs)max slighthy increases with time, on the rther hand, fs above that the temperature was constant within the limits of experimental precision.
(6) Thermal stress was completely changed reversibly below the temperature, to which filament is preliminary heated.
It is evident from the above results that the internal strain, given in the stretching process, must be responsible to thermal stress. The degree of release of the internal strain may depend on the nature of intermolecular interaction, in other words, on the nature of the states of aggregation, depending on temperature. It should be remarked that thermal stress can not be interpreted immediately with the orientation of polymeric chains. Futhermore, the qualitative concept of thermal stress is discussed in some details on the basis of the molecular theory.
