Isotactic polypropylene (PP) was reinforced with Vectra B950 (VB), a thermotropic liquid crystalline polymer (TLCP), in two novel fiber spinning processes which both employ two extruders to allow separate thermal histories to be applied to PP and VB prior to their combination. In one process, a single VB melt stream was surrounded by a sheath of PP, in a manner comparable to a wire coating process, just prior to melt spinning to generate fibers possessing a sheath-core structure. The other process involved the distribution of continuous VB melt streams within a separately plasticated PP melt to form a composite melt which was drawn to create matrix-fibril fibers consisting of a PP matrix reinforced with VB fibrils. Both processes allow polymers with non-overlapping processing temperatures to be combined and melt spun to form composite fibers. The effect of fiber draw ratio on the tensile mechanical properties was assessed for composite fibers reinforced with 50 wt.-% (39 vol.-%) VB. Numerical simulations of the non-isothermal fiber drawing processes and an investigation of fiber morphology were undertaken in order to explain the mechanical properties. The composite fibers were compression molded at a temperature 80°C below the melting temperature of VB to form unidirectionally reinforced plaques. For both fiber types, the plaques exhibited excellent retention of fiber properties, illustrating the potential for using the fibers to form reinforced composite parts.
© 1997, Carl Hanser Verlag, Munich