Fine powders of modified PTFE having a “core-shell” structure; the fine powder being formed by: a “core” of tetrafluoroethylene (TFE) modified by one or more monomers selected from: a) dioxoles; b) one or more perfluorinated monomers which cyclize during the polymerization; a “shell” of TFE modified by perfluoropropene (PFP); the amount, referred to the total core+shell particle, of comonomers 1) in the “core” being in the range 0.01-0.06% by weight; the amount, referred to the total core+shell particle, of perfluoropropene 2) in the “shell” being in the range 0.002-0.06% by weight; the thermal instability index (TII) of the fine powder being equal to or lower than 5, determined according to ASTM D4895.