An Impregnation Model for the Preparation of Thermoplastic Prepregs

The penetration of the polymeric melt into the fine interstices (typically of the order of a few microns or less) between the fibers of a thermoplastic prepreg depends upon the shape, volume fraction and geometry of packing of the fibers, the rheology of the polymeric melt and the interfacial surface tension. A model for preimpregnation would help determine optimal processing variables. In this paper the methods currently used to prepare thermoplastic prepregs are briefly reviewed and a preimpregnation model (which is generic to most processes involving melt impregnation) for describing the flow of resin into fiber-tows is developed. Such flows are usually modelled using Darcy's Law, but the approach of this paper represents an improvement in that empirical parameters (like permeability) are not required and detailed flow profiles can also be calculated. The effect of different geometries of packing on processing variables (fiber spacing, impregnation time, pressure drop) and flow characteristics (velocity, planar elongation and shear rates) is calculated. The fiber spacing (which depends upon the fiber volume fraction and geom etry of packing) has been found to be a critical parameter. The degree of shear-thinning of the polymeric melt on the pressure drop during flow through reinforcement is found to be very significant. Numerical values for the various flow and processing variables are calculated based upon experimental data reported in the patent literature. The value of the model is that it lets one predict the processing parameters required to completely impreg nate the tow and whether effective impregnation is feasible for a given polymer- reinforcement system.