than 1 800 psi and with hc 141b such solutions of polyethylene can be formed in the halocarbon liquid only at pressures greater than the autogeneous pressure of the solvent such solutions of polypropylene can be formed in the halocarbon spin liquids of this invention at pressures greater than the autogeneous pressure of the solvent satisfactory solutions of polyethylene and halocarbon can be formed at pressures as low as 1 000 psi when co solvents of high solvent power are present in the halocarbon spin liquid the combination of halocarbon characteristics have been discovered to be met substantially by only the ten halocarbons listed above to be an equivalent of any of the halocarbons of the invention a newly developed or discovered halocarbon would also have to meet substantially all of these characteristics in order to be suitable for flash spinning high quality plexifilamentary film fibril strands of fiber forming polyolefin even among the halocarbons suitable for use in the process of the invention care must be taken with these halocarbons to avoid certain disadvantageous characteristics which may be present for example excessive heating times are avoided with hc 123a hc 132a hc 132b and hc 141b to minimize decomposition that can arise from dehydrohalogenation or hydrolysis of the halocarbon care must also be taken with hc 132b because there have been some indications that this chemical may be a male animal reproductive toxin because of its relative freedom from all of these stability and toxicity problems hc 123 is the preferred halocarbon for use in the process of the invention in forming a solution of fiber forming polyolefin in the halocarbon liquids of the invention a mixture of the fiber forming polyolefin and halocarbon is raised.