polyamine in effect cross links the copolymer through the carboxyl groups in the form of diamine or polyamine salts in a more random and less crystalline form this cross linking is reversible so that the ionomer sheet when heated to the melting point becomes formable and processible and can be used in a laminate or substrate or even used alone without any additional substrate some amount of irreversible cross linking can be accomplished if desired by heating the ionomer resin film to 325 u00b0 400 u00b0 f for varying lengths of time under vacuum to cause the amine salts to convert to amides str6 the conversion to the amides may range from 10 100 conversion depending on the length of time of the heating this high temperature cross linking of the finished article provides some cross linking and heat set and thereby can increase the melting point of the ionomer sheet in the final article if desired in addition to adding toughness to the sheet material the primary aliphatic or alicyclic diamines or polyamines containing the ch 2 nh 2 group can form the strongest bonds with the carboxyl group and therefore form the amino salt bond at the highest possible temperature of all the diamines or polyamines if there is too great a distance between the nh 2 groups and the chain is not rigid too much flexibility occurs and the cross linking effect becomes diminished and the diamine or polyamine may not.