Fluoropolymers with shell-modified particles, and processes for their preparation

Aqueous polymer dispersion containing colloidal particles of a partially modified tetrafluoroethylene polymer with a multi-shell particle structure of shells of different composition, said particles consisting essentially of

• (a) a particle core comprising a polymer of 0.05 to 6% by weight of units of at least one modifying fluoro-olefinic comonomer of the formulae ##STR7## wherein R.sub.1 is H, Br or Cl;
• R.sub.2 is F or R.sub.f.sup.1, R.sub.f.sup.1 denoting a perfluoroalkyl radical with 1 to 4 C atoms;
• and R.sub.3 is R.sub.1 or R.sub.2 ;
• ##STR8## wherein R.sub.f.sup.1 is a perfluoroalkyl radical with 1 to 4 C atoms and R.sub.f.sup.2 is F or a perfluoromethyl radical;
• R.sub.f.sup.3 --O--CF.dbd.CF.sub.2 (a.sup.3)
• wherein R.sub.f.sup.3 is a perfluoroalkyl radical with 1 to 5 C atoms;
• or
• R.sub.4 --O--CF.dbd.CF.sub.2 (a.sup.4)
• wherein R.sub.4 is a radical XCF.sub.2 --(CF.sub.2).sub.m -- in which X is H or Cl and m denotes an integer from 0 to 4, and units of tetrafluoroethylene, the proportion of the particle core being 5 to 20% by weight of the total particle,
• (b) an inner particle shell, immediately adjacent to the particle core, of a polymer consisting of tetrafluoroethylene units, and
• (c) an outer particle shell, immediately adjacent to the inner particle shell, of a polymer comprising 0.1 to 15% by weight of units of at least one modified, fluoro-olefinic comonomer of the formulae (a.sup.1), (a.sup.2), (a.sup.3), and (a.sup.4), these formulae being as defined under (a), and units of tetrafluoroethylene, the weight ratio of the inner particle shell to the outer particle shell being 75:25 to 99.5:0.5.