The idea of fiber impregnation by powdered resin is well-known. Over the years, the industry has used different techniques for producing yarns or rovings (prepregs) impregnated with various thermoplastic or thermoset polymer powders. One known process passes bundles of filaments (glass, carbon or even Aramid.RTM. fibers) through a fluidized bed of powdered thermoplastic resin. The filaments are kept separate by different means so that the resin can be picked-up by the filaments.
Another process achieves adherence of the polymer powder to the yarn or roving by creating an electrostatic charge on either the yarn or the powder. Another process to adhere the powder to the yarn is by mechanical means. Short-toothed combs, stationary or rotating rods or bars and the like separate the yarns so that the powders may penetrate the filaments. Still other processes sheath the rovings with a flexible polymer sleeve after the powder has penetrated the fibers in an attempt to keep the powder on the fibers before the fibers are used in a molding process. Another alternative process fuses the powder impregnated strands into a rigid ribbon in order to keep the powder on the strands.
Still another process produces fiber reinforced thermoplastic material directly from the bushing by extruding filaments into a bed of dry thermoplastic powder which has a lower melting point than the filaments. The powder sticks to the filaments so that impregnation takes place when the filaments are still separate. These filaments must be cut into granules or fed directly to a heated die.
All of these processes are based on the same principle of dry impregnation of a strand followed by some type of post treatment. Each process has its problems. For example, plain impregnated strands with no sizing or further treatment usually lose much of the impregnated powder before molding, while the flexible sleeve typically creates resin rich, glass poor areas in the molded part and the rigid ribbons are difficult to process.
Further, problems occur when the thermoplastic or thermoset resin powder is put between the filament bundles after the filaments have previously been gathered or glued together into a bundle. The filaments in the bundles must be separated or "unglued" and often good "wet out" or dispersion of the resin between the filaments is not achieved. Rather, the resin forms a sheath or coating around the bundle and in the final injection molded products, these glass bundles remain intact and good glass dispersion is not achieved.
Considerable time and expense would be saved if a method for impregnating strands with a thermoplastic or thermoset resin could be employed which would eliminate non-aqueous solvents, fluidized beds, sheaths, and time-consuming extrusion techniques or other, dangerous methods for applying thermoplastic or thermoset resins to fibers. This invention is directed to that objective.