Process for preparing polytetrafluoroethylene fine powder

A process for preparing polytetrafluoroethylene fine powder by polymerization of tetrafluoroethylene in an aqueous medium comprising a water-soluble polymerization initiator and a non-telogenic surfactant capable of keeping colloidal polytetrafluoroethylene particles at a sufficiently stable state, the process comprising effecting the polymerization at a temperature of 55.degree. to 120.degree. C., and incorporating into the aqueous medium a polymerization retarder selected from the group consisting of aromatic compounds having at least one substituent group selected from the group consisting of hydroxy and amino, and quinone compounds, said retarder having a water-solubility of not less than 2.5.times.10.sup.-6 mol/liter at 25.degree. C., a boiling point of from 94.degree. to 241.degree. C. and not having any reinitiation ability after its addition or chain transfer to a free radical in the polymerization system, said retarder being incorporated into the aqueous medium in an amount of 0.7 to 20 ppm at the stage after the initiation of the polymerization and after the consumption of at least 10% by weight of tetrafluoroethylene to be polymerized but before the consumption of 85% by weight of tetrafluoroethylene to be polymerized so that the polymerization time is extended at least 130% in comparison with that of the case wherein the polymerization is effected under the same conditions as above except not using the polymerization retarder, whereby colloidal polytetrafluoroethylene particles, of which the core portions occupying 50% by weight of the total weight of the finally produced particles have a specific gravity of not more than 2.210, of which the specific gravity of the finally produced particles is not more than that of the core portions and of which the average particle size is 0.1 to 0.5 .mu.m, are obtained.