A MODEL LONG-DISCONTINUOUS-FIBER FILLED THERMOPLASTIC MELT IN EXTENSIONAL FLOW

The shear cell model works for dilute fiber filled systems in extensio nal flow. This research investigates the suitability of the idea for h ighly aligned fibers in a concentrated suspension. A model fiber-fille d polymer system made from nylon fibers in low-density polyethylene pr ovided a means of controlling the material parameters. Two systems, wi th fiber aspect ratios of 20 and 100, containing 50% 0.5 mm fibers by volume are investigated. The thickness of the polymer layer, i.e. with fibers this size, allows bulk viscosity data to be compared with the data from the filled fluid. A weaving process created the discontinuou s fiber/polyethylene preforms with high alignment of the fibers and wi th control of the fiber to fiber overlap. Testing the polyethylene in simple shear and extending the nylon/polyethylene provided the data ne eded to check the micro mechanics. A cone and plate rheometer and a ca pillary instrument produced the viscosity/strain rate data that charac terized the specific polyethylene used in the composite. ir furnace in set placed in an Instron hydraulic test machine allowed extension of t he filled system at strain rates from 0.002 to 0.4 s(-1). The shear ex periments show that the low-density polyethylene is a simple shear-thi nning melt that provides a good model fluid. The extension of the fill ed systems I;hows an increase of the apparent extensional viscosity fr om that of neat polyethylene. Apparent viscosity rises two to three or ders of magnitude for the systems investigated. The micromechanics all owed the conversion of the extensional data from the two filled system s to the shear viscosity of the polymer surrounding the fibers. The ca lculated polyethylene viscosity compares well with the data from the s tandard rheometers. The shear cell approach may be applied to highly a ligned, high fiber-volume-fraction suspensions when the viscosity of t he polymer is known at the scale of the Sim surrounding each fiber. (C ) 1997 Elsevier Science B.V.