Nucleating agents for thermoplastic resins

In an improved process for producing a foam of a thermoplastic resin composition, the improvement characterized in that the process is carried out with an effective amount of the nucleating agent of the formula

• [Z(CF.sub.2).sub.x (CF.sub.2 CFX).sub.p (R').sub.y (CH.sub.2).sub.z RO.sub.3 ].sub.n M
• wherein:
• Z is CCl.sub.3, CCl.sub.2 H, H, F, Cl or Br;
• each X, independently, is selected from H, F or Cl;
• R is sulfur or phosphorus;
• M is H or a metallic, ammonium, substituted ammonium or quaternary ammonium cation;
• x is an integer and is 0 to 10;
• p is an integer and is 0 to 6;
• y is 0 or 1;
• z is an integer and is 0 to 10;
• x+y+z+p is a positive integer or, if x+y+z+p is 0, Z is CCl.sub.3 or CCl.sub.2 H;
• n is the valence of M;
• and
• R' is a C.sub.1-16 straight or branched chain perfluoroalkylene diradical containing a perfluorinated alicyclic ring;
• a C.sub.1-16 perfluorinated aliphatic polyether diradical with repeat units selected from [CF.sub.2 O], [CF.sub.2 CF.sub.2 O] and [CF.sub.2 CF(CF.sub.3)O];
• or, when z is 0, an aromatic diradical;
• so as to produce a foam having uniformity of cell size, small cell size, and high void content.