An investigation on the postneck drawing of high modulus polypropylene filaments

The effects of using a variable drawing rate in the postneck drawing (superdrawing) of polypropylene filaments on the microstructure, thermal stability and tensile modulus have been investigated. Experimental studies on the effect of gauge length and testing rate on the measurement of Young's modulus of highly anisotropic filaments were conducted. The microstructure of the drawn filaments was characterized using scanning electron microscopy as well as wide and small angle x-ray diffraction. Differential scanning calorimetry and thermal shrinkage tests were carried out to study the melting characteristics, crystallinity and thermal stability of the drawn samples.

A two stage drawing process in the solid state is used to superdraw the filaments. The first stage of the drawing process (natural drawing) consisted of rapidly drawing (at a constant rate) the filament as prepared from melt extrusion to the completion of the necking process at elevated temperatures. The subsequent superdrawing process involved the postneck drawing of the natural drawn filament at constant load and at high temperatures. The natural drawing process converts the initial unoriented spherulitic structure to a fibrillar structure while the subsequent superdrawing process transforms the latter into a matrix of microfibrils consisting of segregated amorphous regions and a great number of intramolecular and intermolecular taut tie molecules (or stress bearing elements).

Superdrawn filaments produced at the optimum conditions (130c to 140°c, 45 MPa) have an extrapolated modulus of 24 GPa at 32X and exhibited no discrete meridional scattering in small angle x-ray scattering patterns. The crystallinity and peak melting point of superdrawn filaments are found to increase with draw ratio and to a lesser extent with the superdrawing temperature. The highest peak melting temperature and crystallinity obtained in this study is 168°c and 84% respectively. Annealed superdrawn samples at high draw ratios (greater than 20x) exhibited minimal shrinkage (less than 2%) and minimal decrease in modulus (less than 13%) as well as no discrete meridional

the natural drawn samples which showed a shrinkage of 20% and a drastic decrease in modulus of about 40%. It is postulated that the taut tie molecules produced in the superdrawing process are crystalline in nature. SEM micrographs of superdrawn filaments revealed the formation of longitudinal voids (0.2 to l um in width) at high draw ratios and thus explains the opacity of the superdrawn samples. It was observed that voids are more prone to form at a lower temperature of draw and thus limited the drawability of the filaments.

The modulus of superdrawn filaments was found to be insensitive to the drawing temperatures between 105°c and 140°c. However, a higher draw ratio and hence modulus was attained by drawing at a higher temperature in that temperature range. Superdrawing at a temperature above 145°c would not produce filaments of equivalent modulus as obtained from superdrawing at a lower drawing temperature and at the same draw rate.

The effect of gauge length (end effects) on the measurement of modulus is substantial when compared with the effect of testing rate. An analysis shows that the true modulus of highly anisotropic filaments

can be determined from the extrapolated data of the plot of reciprocal modulus versus reciprocal gauge length of the superdrawn filaments.