Producing modified high performance polyolefin fiber

A process of forming a high tenacity, high modulus polyethylene or polypropylene fiber, in which (a) a dilute solution of ultrahigh molecular weight polyethylene or polypropylene in a first solvent is spun and cooled to form a first gel fiber, (b) the gel fiber is extracted (if said first solvent is non-volatile) with a volatile solvent to form a second gel fiber, (c) the first gel fiber (if said first solvent is volatile) or the second gel fiber is dried to form a xerogel fiber and (d) at least one of the first gel fiber, the second gel fiber and the xerogel fiber is stretched;

• characterized by the dilute solution comprising the ultrahigh molecular weight polyethylene or polypropylene, the solvent and a polymeric additive selected from the group consisting of copolymers containing monoolefins as primary monomers, polyolefins of molecular weight not greater than 250,000, oxidized polyolefins, graft polyolefin copolymers, and polyoxymethylenes, with the weight of polymeric additive being at least 2% of the weight of ultrahigh molecular weight polyethylene or polypropylene, but not being sufficiently high to lower the main melting temperature of the product fiber (by DSC in argon at 10.degree. C./min) to below 140.degree. C. in the case of polyethylene or to below 162.degree. C. in the case of polypropylene.