Process for making foam articles having good low temperature toughness from high melt strength polypropene materials

A foam article is made by (1) irradiating a propylene polymer material selected from the group consisting of (a) a terpolymer of propylene, ethylene, and butene-1, (b) a polyolefin composition that includes (i) about 31% to about 39 % of a copolymer of propylene and ethylene, and (ii) about 58% to about 72% of a terpolymer of propylene, ethylene, and butene-1, and (c) a polyolefin composition that includes (i) about 30% to about 65% of a copolymer of propylene and butene-1, and (ii) about 35% to about 70% of a copolymer of propylene and ethylene, and (2) extruding the irradiated propylene polymer material through a die in the presence of a physical expanding agent and a cell nucleating agent, whereby a structure is formed whose density is at least 10X less than the initial density of the propylene polymer material. The foam articles exhibit improved flexibility and low temperature toughness compared to conventional propylene polymer materials.