Effects of molecular weight on melt spinning and mechanical properties of high-performance poly(ethylene terephthalate) fibers

Effects of the molecular weight (intrinsic viscosity) of poly(ethylene terephthalate) on the formation of fibers and their properties are analyzed. Correlations between molecular weight and various viscosity characteristics are compared. Because of high melt viscosity, effective melt spinning is limited to polymers with intrinsic viscosities [eta] not higher than 0.8-0.9 dl/g. High molecular weight polymers ([eta] = 2.0-4.9 dl/g) have been used for solution (gel) spinning. Molecular weight is not a primary characteristic responsible for mechanical properties, but it may affect tenacity and modulus indirectly. The tenacity of industrial PET fibers manufactured by a variety of processes exhibits positive correlation with intrinsic viscosity; the correlation of the modulus is less evident. Melt spun and drawn fibers exhibit tenacities of up to 12 g/den and moduli of up to 220 g/den, while fibers solution spun from high molecular weight PET ([eta] = 2.6 dl/g) subjected to deep drawing (total draw ratio = 16.4) reach a tenacity of 18.5 g/den and a modulus of 313.2 g/den. A new zone drawing and annealing process, apparently insensitive to molecular weight, yields PET fibers with a tenacity of 14 g/den and a modulus of 246 g/den. The role of molecular weight in obtaining high mechanical performance is discussed.