TRANSIENT RHEOLOGICAL BEHAVIOR OF A LONG DISCONTINUOUS FIBER-MELT SYSTEM

The transient elongational behavior of a highly-aligned 60% fiber volu me fraction long discontinuous-fiber filled poly-ether-ketone-ketone ( PEKK) melt system was investigated with a computer controlled extensio nal rheometer at 370 degrees C. Long discontinuous graphite fibers, 7 mu m in diameter, and PEKK thermoplastic constituted the melt system. Experiments conducted under conditions to approximate controlled strai n rate and controlled stress produced tau(E)(+) (t, epsilon) and epsil on(-) (t, tau(E)) similar to a shear dominated flow of a non-linear vi scoelastic fluid. Increasing strain rate resulted in an increasing pea k stress and in an increasing ration between peak stress and plateau s tress. Increasing controlled stress level resulted in decreasing elong ational viscosity and increasing post-elastic-step strain rate. The ra tio of apparent eta(E)/eta(PEKK) was 3.67 x 10(8) to 3.94 x 10(10) for average strain rates from 6.81 x 10(-7) to 1.49 x 10(-3) s(-1). A var iable taper shear cell analysis provided the range of expected eta(E)/ eta, which agreed well at high strain rates with the experimental data . (C) 1996 Society of Rheology.