DETAILED DESCRIPTION OF THE INVENTION More particularly according to the present invention, the subject films preferably comprise kaolinite and titanium oxide particulates having a mean particle size less than or equal to 0.
7 microns.
The polyester which is used to prepare the films according to the invention can be any known polyester as obtained from a dicarboxylic acid or its lower alkyl esters, such as terephthalic acid, isophthalic acid, orthophthalic acid, naphthalene-2, 5-dicarboxylic acid, naphthalene-2, 6-dicarboxylic acid, naphthalene-2, 7-dicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, diphenyldicarboxylic acid and hexahydroterephthalic acid, and from one or more diols such as ethylene glycol, propane-1, 3-diol, butane-1, 4-diol, neopenthlglycol and cyclohexane-1, 4-dimethanol.
In general, the polyester used is a homopolymer or a copolymer containing essentially alkylene terephthalate units.
Preferably, the said polyester contains at least 80% by weight of ethylene glycol terephthalate recurring units and, even more preferably, at least 90% by weight of such recurring units.
The polyester can also be a mixture of several homopolymers of which 80% by weight, and preferably 90% by weight, comprises polyethylene glycol terephthalate.
The preparation of polyesters of this type is in itself well known to this art.
Typically, such preparation comprises a stage involving interexchange (or transesterification) between a lower alkyl ester of terephthalic acid (such as dimethyl terephthalate) and ethylene glycol, in the presence of a catalyst such as, for example, manganese acetate, calcium acetate or magnesium acetate.
Next follows the stage involving polycondensation of the ethylene glycol terephthalate in the presence of a polycondensation catalyst such as antimony oxide or a germanium compound and, most commonly, in the presence of a phosphorus compound such as phosphorous acid, phosphoric acid, or esters thereof.
The fillers are preferably introduced and incorporated during the polycondensation stage in the form of a suspension of the particles in the diol used
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Nonmetallic Processes 
