Structure and properties of high-speed melt-spun filaments of poly(butylene terephthalate)

Abstract

Filaments of poly(butylene terephthalate) were prepared by melt spinning with take-up velocities in the range 1000–5600 m/min. Two polymers with different molecular weights were used (intrinsic viscosities of 0.75 and 1.0 dL/g). The filaments were characterized using measurements of density, birefringence, shrinkage, thermal properties (differential scanning calorimetry), crystal size, crystalline orientation and phases present (wide angle X-ray diffraction), and tensile mechanical properties. Filaments spun from the 0.75 IV polymer with a mass throughput of 6 g/min at 1000 m/min have essentially amorphous structures, while higher take-up velocities result in α-form crystals or, at the highest take-up velocity, a mixture of α-form and β-form crystals. Only α-form crystals were detected in the higher IV polymer. Crystal size varied with crystallographic direction but generally increased as take-up velocity increased. At the lowest take-up velocities the filaments increased in length during thermal shrinkage measurements. With increasing take-up velocity the shrinkage became positive and continued to increase until reaching a maximum in the range of the highest sprinning speeds. This behavior correlates with the variation of the orientation factors of the amorphous phase. A plateau was observed in stress versus strain curves corresponding to strain-induced transformation from α-form to β-form crystals. The length of this plateau increased with increase of take-up velocity and the α-form crystal content in the sample. Both morphology and physical properties varied with polymer molecular weight and melt spinning conditions.