METHOD FOR THE PREPARATION OF FOAMED SHEET AND PRODUCT OBTAINED THEREBY

1,198,407. Cellular articles of monovinyl aromatic polymers. DOW CHEMICAL CO. 20 Sept., 1968 [21 Sept., 1967], No. 44853/68. Headings C3C and C3P. A cellular article, e.g. sheet or tube, is made by heating a mixture of a thermoplastic resinous polymer of a monovinyl aromatic compound, a nucleating agent and a foaming agent consisting of 75-97À5 wt. per cent dichlorodifluoromethane and 25-2À5% of n- and/or iso-pentane. In the examples a mixture of polystyrene and a styrene/maleic anhydride copolymer as nucleating agent is fed to a screw extruder, the foaming agent is fed into the heat plastified gel in the extruder barrel and a tube is extruded through an annular die and is stretched over a mandrel using air lubrication; a minor proportion of the extrudate is ground and recycled.