Homogeneous, high modulus ultrahigh molecular weight polyethylene composites and processes for the preparation thereof

A process for preparing a homogeneous composite comprising (a) about 0.5 to 90% by weight of an ultrahigh molecular weight linear polyethylene having a molecular weight of at least about 400,000, a crystalline melting point of greater than 143.degree. C., the reduction of said melting point upon remelting being at least 3.degree. C. and a crystal morphology comprising a bimodal distribution of molecular chain fold spacings reflecting significant populations of both folder and very highly extended molecular chains, and (b) about 0.5 to 90% by weight of at least one filler compound having a neutral-to-acidic surface, said polyethylene being polymerized onto the surface of said filler, and said composite being substantially free of polymer-free filler and of filler-free polymer, comprising:

• (a) forming an article of a solid homogeneous particulate composite comprising (i) about 0.5 to 90% by weight of ultrahigh molecular weight polyethylene having a molecular weight of at least about 400,000, and (ii) about 0.5 to 90% by weight of at least one finely-divided filler compound having a neutral-to-acidic surface and having interacted at the surface of said filler a catalytically active amount of transition metal compound, said polyethylene being polymerized onto the surface of said filler, said composite being substantially free of polymer-free filler and of filler-free polymer;
• (b) heating said article in an inert atmosphere to a temperature of at least 180.degree. C. sufficient to melt said polyethylene and, at this temperature, compressing said article at a pressure of at least 280 MPa for at least 2 minutes;
• (c) reducing the temperature to 175.degree. C. or lower while maintaining a pressure of at least 280 MPa, the rate of reduction of temperature being such that temperature gradients in the shaped article are substantially avoided;
• and,
• (d) rapidly cooling to a temperature below 100.degree. C. while maintaining a pressure of at least 280 MPa, then reducing the pressure in such a manner that remelting of the polyethylene constituent of the article is avoided.