Gives the requirements for the following polyimide films with or without heat sealable fluoroethylene-propylene (FEP) coatings; based on poly(N,N'-p,p'-oxydiphenylene pyromellitimide)(sheet 4); based on poly(N,N'-p-phenylene biphenyl tetra carboxylimide)(sheet 5); based on poly(N,N'-p,p'-oxydiphenylene biphenyl-tetracarboxylimide)(sheet 6).