apparatus for applying a reactant gas to an extrudable polymer , as , for example , a carbonate resin having the phenolic hydrogen end atoms replaced by stress corrodant preventive groups which are halohydrocarbon groups or a silane groups or the hydrogen end atoms replaced by a capping group having fluorescent properties to provide ultraviolet light and stress corrosive protection . the apparatus includes a heatable extruder barrel having a receiving end communicating with said polymer supply containing means and a discharge end for communication with an extrusion die , an extruder screw rotatably mounted in the barrel for moving a supply of non - fused polymer from the receiving end through the barrel for heating to fusion temperature therein and to the discharge end thereof for passage therefrom through the extrusion die , an annular reactant gas inlet communicating with the interior periphery of the barrel at a position between the receiving and discharge ends thereof upstream from the position therein wherein the polymer reaches its fusion temperature , and an annular reactant by - product gas vent communicating with the interior periphery of the barrel at a position downstream of the reactant gas inlet .