Theoretical and experimental study of fibre attrition during extrusion of glass-fibre-reinforced polypropylene

Abstract On the basis of the experimental observations described in ref. 1, a model has been proposed to analyse the phenomenon of fibre breakage taking place when granules of short-fibre-reinforced thermoplastics are processed in an extruder. This model essentially determines the bending moment experienced by a single fibre, anchored at one end, due to drag forces produced by the flow of molten polymer past it. This model clearly brings out the effects of molten film thickness, screw speed, viscosity of the polymer melt and the fibre orientation relative to the flow direction. It is postulated that the model provides a mechanism for the breakage of fibres exposed by melting of polymer at the solid bed-molten polymer interface. For fibres which are free to move in the molten polymer, buckling introduced by the shearing motion of the molten polymer, as predicted by the Forgacs and Mason model 2 is responsible for their attrition. Using the extrusion data of ref. 1, the results of the combined use of the two models show good agreement with the experimental observations, in particular the fibre length distribution of various stages of extrusion. This agreement is observed in spite of several assumptions inherent in the model. Finally, the effect of fibre agglomeration on the attrition phenomenon is also studied.