and chemically plasticizes the foam so that it is more flexible an essential element of the masterbatch is the alpha methyl styrene all commercially available styrene polymers can be used as the polystyrene compound however it is essential that the vicat softening temperature of the chosen styrene polymer be between 55 and 65 at 50 u00b0 c hr one commercially known is amoco 39 s resin 18 240 which has a vicat softening temperature of 60 5 at 50 u00b0 c hr and 62 0 at 120 u00b0 c hr another essential element of the masterbatch is the styrene ethylene butylene styrene block copolymer one commercially known is shell 39 s kratron g 1650 the blowing agent preferably used is that grade of carbon dioxide or nitrogen which is normally commercially available for the purposes of this specification such carbon dioxide or nitrogen is referred to as 100 percent carbon dioxide or nitrogen even though they may contain minor impurities or amounts of other compounds the blowing agent can be added to a polymer melt in either a gaseous or liquid form or combinations thereof the use of an extrusion process for the manufacture of polystyrene foam is typical but is not required such a process includes a primary extruder a blowing agent addition system a secondary extruder an annular die a sheet cutter or slitter and a sheet gathering device however the use of this exact equipment set up is not required in the process of this invention polystyrene foam is formed in a continuous.