A process for the production of a polymeric film comprising a copolyester
having an acid component and a diol component, said acid component
comprising a dicarboxylic acid and a sulfomonomer containing a sulfonate
group attached to the aromatic nucleus of an aromatic dicarboxylic acid,
said process comprising the steps of: (i) melt-extruding a layer of said
copolyester; (ii) stretching the extrudate in at least one direction;
(iii) heat-setting the film by raising the temperature of the stretched
film to a temperature T.sub.1 in a first heating zone such that
(T.sub.M-T.sub.1) is in the range of from 5 to 30.degree. C., and then
raising the temperature of the film to a temperature T.sub.2 in a second
heating zone such that (T.sub.M-T.sub.2 is in the range of from 0 to
10.degree. C., wherein T.sub.M is the peak melting temperature of the
polymeric film; wherein T.sub.2 is greater than T.sub.1; and wherein the
times which a transverse section of the film spends in the first and
second heating zones are defined by t.sub.1 and t.sub.2, respectively,
such that the ratio of t.sub.1 to t.sub.2 is at least 2:1; and a
polymeric film obtainable thereby having an ultimate tensile strength at
destruction in the range of 2 to 15 kgf/mm.sup.2 in the machine direction
and 2.5 to 17 kgf/mm.sup.2 in the transverse direction.