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 pyromelitimide); based on poly(N,N'-p-phenylene biphenyl tetra carboxymide); based on poly(N,N'-p,p'oxydiphenylene biphenyl-tetracarbocylimide).