cs ag hg and cu suitable divalent metal ions are be 2 mg 2 ca 2 sr 2 ba 2 cu 2 cd 2 hg 2 sn 2 pb 2 fe 2 co 2 ni 2 and zn 2 suitable trivalent metal ions are al 3 sc 3 fe 3 and y 3 the preferred metal ions regardless of the nature of the copolymer are na and zn 2 these metals are preferred because they result in ionic copolymers having the best combination of copolymer solid state properties with retention of melt fabricability it is not essential that only one metal ion be employed in the formation of the ionomers and more than one metal ion may be preferred in certain applications ionomers of the type described above are well known in the art and are available from e i du pont de nemours and company under the trademark surlyn u00ae while nonionomeric copolymers those not cross linked by metal ions are operable in the process of the present invention it usually will be more advantageous to use ionomers especially those in which at least 10 of the carboxylic groups are ionized because films of such polymers have better physical properties this invention is now illustrated.