Abstract: A process is provided for preparing a thermoplastic polymer foam, comprising providing a foamable composition comprising a thermoplastic polymer and a blowing agent, wherein the blowing agent comprises from 10% to 60% by weight Z-1,1,1,4,4,4-hexafluoro-2-butene (Z-HFO-1336mzz) and from 40% to 90% by weight 1,1-difluoroethane (HFC-152a), and wherein said thermoplastic polymer comprises polystyrene homopolymer, a polystyrene copolymer, styrene-acrylonitrile copolymer, or blends thereof expanding said foamable composition to produce a closed cell, smooth skin polymer foam, having a density of less than 42 kg/m2.