A process for the preparation of plexifilamentary film-fibril strands of polymer. The process includes the steps of generating a spin fluid containing (a) 5 to 30 wt. % containing one or more polymer types, (b) a primary spin agent selected from the group consisting of dichloromethane, cis-1,2-dichloroethylene and trans-1,2-dichloroethylene, and (c) a co-spin agent comprising 1H,6H-perfluorohexane or 1H-perfluorohexane or 1H-perfluoroheptane. The spin fluid is flash-spun at a pressure that is greater than the autogenous pressure of the spin fluid into a region of lower pressure to form plexifilamentary film-fibril strands of the polymer. The co-spin agent is present in the spin fluid in an amount sufficient to form an azeotrope-like composition with the primary spin agent in the presence of the one or more polymer types. The polymer may be selected from the group consisting of high density polyethylene, polypropylene, polybutene-1, polymethylpentene, polyvinylidene fluoride, poly (ethylene tetrafluoroethylene), and blends of the foregoing.