PURPOSE: To provide the titled composition giving a highly expanded formed material having excellent appearance, closed cell structure, large cross-section, and excellent heat-insulation and crushioning properties, by compounding a polyolefin with a volatile organic foaming agent and a specific foam modifier.

CONSTITUTION: The objective composition can be prepared by mixing (A) a polyolefin with (B) a volatile organic foaming agent (preferably a halogenated hydrocarbon composition containing ≥30wt% of dichlorodifluoroethane) and (C) preferably 0.1W10pts.wt., based on 100pts.wt. of the component (A), of a foam modifier comprising the compound of formula (R1 is 6W10C hydrocarbon group or 6W20C aromatic hydrocarbon group; R2 is 1W6C hydrocarbon group; n is 1W3) (e.g. hydroxyethyl octyl ether), preferably by a master batch process, etc.

USE: Manufacture of a thick-walled board, etc.