Method of manufacturing a photographic reflective support

Description

Field of the invention

[0001] The present invention relates to a method of manufacturing a photographic reflective support, more in particular to a method of manufacturing polyester comprising units of ethylene terephthalate as the main repeating units suitable for being melt-extruded, biaxially stretched and annealed according to common means to produce a photographic reflective support.

Background of the invention

[0002] Unlike the so-called transmission-type photographic materials wherein a photographic image is viewed thereon by means of transmitted light, a reflective photographic material means a photographic material wherein an opaque material is used as support and a photographic layer is provided thereon and the photographic image formed in the photographic layer is seen directly by means of reflected light.

[0003] As opaque support material hitherto is widely used a paper support, more in particular a polyethylene (PE)-coated paper wherein a layer of polyethylene in which white pigments are kneaded is provided on a base paper made from pulp.

[0004] Such reflective photographic supports based on PE-coated paper have various disadvantages. During processing developing liquid permeate the base paper through its cut edges resulting in a detrimental influence on the photographic image obtained.

[0005] As a method for overcoming such disadvantages, methods and materials have been proposed which do not employ a base paper but a thermoplastic resin film as support.

[0006] In JP-KOKOKU - No. 114921/1974 an opaque polystyrene type resin film is disclosed; such films however are hard and fragile. From the viewpoint of physical properties such as mechanical strength, polyester resins, and more in particular polyethylene terephthalate (PETP) resins, are preferred for producing films suitable as support for photographic materials.

[0007] In US-P-4,780,402, PETP-films are disclosed wherein barium sulfate is added to the polyester resin as finely divided inorganic particles.

[0008] In EP-A-182253 a reflective photographic material is disclosed comprising a support film of thermoplastic resin essentially comprised of polyester resin and preferably PETP, containing white pigment, made up for 90 % by weight or more of surface-treated titanium oxide meeting a well-defined particle size distribution.

[0009] As polyester and in particular polyethylene terephthalate is an important raw material for the manufacture of i.a. filaments and films, packaging films as well as photographic films, the processes for manufacturing PETP-granulate and converting said PETP-granulate to film are well known to those skilled in the art.

[0010] Film materials are generally obtained by melt-extrusion of the polyester through an extruder, rapidly cooling the melt-extruded sheet on the surface of a cooling drum and then biaxially orienting the solidified material in longitudinal and transverse direction.

[0011] The polyester which is melt extruded can be prepared according to two main methods.

[0012] In a first method there is an ester exchange reaction between dimethyl terephthalate (DMT) and ethylene glycol (EG) to form bis(β-hydroxyethyl-terephthalate (BHET) whereupon polycondensation takes place. Both in the ester exchange reaction and in the polycondensation reaction catalysts are used : e.g. a zinc, magnesium, manganese, or cobalt salt or mixtures thereof in particular manganese acetate as ester exchange catalyst, and antimony compounds and/or germanium compounds (such as antimony and/or germanium dioxide) as polycondensation catalyst, and stabilisers such as e.g. phosphorus compounds.

[0013] In a second method the BHET is obtained by direct esterification of terephthalic acid (PTA) with ethylene glycol (EG). Antimony trioxide and/or germanium dioxide are also added as polycondensation catalyst and a phosphorus compound may be added as stabilizer.

[0014] Particulars about the direct esterification method are described in e.g. EP-A-0105 522 and EP-A-0159 817. Particulars about the first PETP production method (the ester exchange reaction method) may be found e.g. in GB-A-1 221 788, GB-A-1 274 858, GB-A-1 108 096, GB-A-1 185 984 and GB-A-1 091 234.

[0015] Particulars about the film-forming process of PETP starting from PETP-granulate may be found in GB-A-1 269 127, GB-1-1 312 263 and EU-A- 0 022 278.

[0016] One of the most critical factors in the above described PETP-film production process is the extrusion of the PETP-film through the extrusion die onto the moving cooling or quenching surface, constituted by a cooled drum or belt. In this stage the PETP-film should be sufficiently cooled to solidify it, and said cooling should take place sufficiently fast so as to become a substantially amorphous film. A solidified film with too high a degree of crystallinity causes problems during the subsequent longitudinal and transverse stretching stages.

[0017] Such problems are particularly difficult to solve in case PETP polymer incorporating a substantial amount of finely divided inert inorganic filler such as barium sulfate, silica or titanium dioxide, has to be melt-extruded and biaxially stretched so as to obtain a PETP-film sheet suitable as support for a reflective photographic material.

[0018] Indeed the numerous finely divided inert inorganic filler material act as crystallisation nuclei in the PETP-polymer and hence it is very difficult to melt-extrude such PETP-polymer through a slot-like orifice so as to obtain a substantially amorphous film on the moving quenching member.

[0019] When the degree of crystallinity of the melt-extruded film is too high it is quasi-impossible to perform the subsequent biaxial stretching processes without frequent ruptures of the film. The latter phenomenon has been confirmed by experiments and is also explicitly recognised in US-P-4,699,744. In the latter specification it is noted that sheet materials which are made by melting, extruding and cooling of the thermoplastic resin containing a large amount of white inorganic pigment are too brittle to be submitted to a steady stretching process without frequent ruptures, to obtain a final film with a sufficient toughness. Further it is described in said specification that the biaxial stretching process comprising stretching the PETP-film material lengthwise and then widthwise or in the converse order, has a serious drawback in that the material is ruptured particularly in the second, transverse stretching step. Some techniques of simultaneously biaxial stretching also have a similar drawback of rupture.

[0020] In said specification a solution is proposed being a particular method of manufacturing a photographic reflective support overcoming the above-mentioned drawbacks of rupture, said method comprising :
heat-treating chips and/or resin scraps composed of a thermoplastic resin mainly comprising a polyester resin and containing a white inorganic pigment in an amount of not less than 10 % by weight, at a temperature of from 180°C to 245°C for a period of not shorter than 3 hours in a vacuum of not more than 20 mm Hg;
feeding said heat-treated chips and/or resin scraps into an extruder to be melted and then extruded in the form of a sheet out of the die of the extruder;
cooling and solidifying the resulting sheet on a cooled casting drum;
stretching said cooled and solidified sheet 6 to 16 times in terms of area, lengthwise and then widthwise or vice versa, or biaxially at the same time, at a temperature within the range of from a temperature not lower than the glass transition point of said thermoplastic resin to a temperature not higher than 150°C;
thermosetting said stretched sheet at a temperature within the range of from a temperature not lower than 170°C to a temperature of the melting point of said thermoplastic resin;
and making the resulting photographic reflecting support be from 40 to 300 microns in thickness and not higher than 20 % in visible wavelength range transmittance.

[0021] For the application of the above described method most of the process parameters of the PETP-polymer melt-extrusion apparatus should be particularly set in view of the manufacture of PETP-film suitable as support for reflective photographic material.

[0022] However the larger part of PETP-films used as photographic support are transparant, clear film materials and the demand for opaque photographic supports is substantially lower. So when on a film extrusion apparatus regularly producing clear transparent PETP-supports, opaque PETP-supports should (temporarily) be manufactured according to the above described process, most if not all of the process parameters must be changed, the latter giving rise to substantial material losses during the switch from clear to opaque PETP support-manufacture, and later during the switch again from opaque to clear PETP support-manufacture.

[0023] Apart from this drawback, the problems of rupture during lengthwise and/or widthwise stretching of opaque PETP-films are not satisfactorily solved by the above process.

Object of the invention

[0024] It is therefore an object of this invention to provide a steady process for the manufacture of opaque PETP-film suitable as support for reflective photographic materials.

[0025] More in particular it is an object of this invention to provide a process for the manufacture of PETP-chips and the conversion of these PETP-chips to PETP-film suitable as support for reflective photographic materials whereby the problems of frequent rupture during film biaxial stretching do not occur. Further objects of our invention will become apparent from the description hereinafter.

Summary of the invention

[0026] We have found that the above objects can be met by providing a method of manufacturing an opaque polyester copolymer support for reflective photographic materials comprising an inorganic inert pigment preferably in an amount of at least 10%, said polyester comprising units of ethylene terephthalate as the main repeating units, said method comprising the following steps in the order given :

• either supplying terephthalic acid and ethylene glycol to bis(betahydroxyethyl)terephthalate or its oligomer so as to carry out the esterification step, or supplying dimethylterephthalate and ethylene glycol so as to carry out transesterification,
• polycondensing under conditions of increasingly reduced vacuum,
• either granulating the obtained polymer, followed by drying the granules and feeding them to an extruder, or directly feeding the obtained copolymer in melt-form to an extruder,
• extruding the polymer in the form of a sheet,
• quenching and solidifying the resulting sheet on a quenching member,
• biaxially stretching the sheet, and
• annealing the biaxially stretched film,

characterised in that terephthalic acid or dimethylterephthalate respectively is replaced by another aromatic dicarboxylic acid, or by the dimethylester of another aromatic dicarboxylic acid respectively, and/or ethylene glycol is replaced by another diol , said replacement(s) amounting to maximum 20 mol percent with respect to ethylene glycol and terephthalic acid, or dimethylterephthalate respectively.

[0027] According to a preferred embodiment of said method the polyester copolymer is the polycondensation reaction product of ethylene glycol and either terephthalic acid, the latter being replaced by isophthalic acid for up to a maximum of 20 mol percent, or dimethyl terephthalate, replaced by dimethyl isophthalate, for up to a maximum of 20 mol percent.

[0028] Further preferred embodiments will become clear from the description hereinafter.

Detailed description of the invention

[0029] Opaque thermoplastic film sheets for use as support of reflective photographic materials, made up of copolymers according to our invention, are produced according to the per se known polyester chips production methods and polyester film extrusion process, except the composition of the raw materials for the polyester chips production and certain precautions preferably taken in the polyester film extrusion process.

[0030] According to our invention, in the polyester chips production process according to the direct esterification method up to 20 mol percent of the ethylene glycol is replaced by another diol such as 1,3-propanediol 1,4-butanediol , neopenthylglycol or 1,4-cyclohexanedimethanol , and/or up to 20 mol percent of the terephthalic acid is replaced by another aromatic dicarboxylic acid such as isophthalic, phthalic, 2,5-, 2,6- and 2,7-napthalene dicarboxylic acid, diphenyl dicarboxylic acid or hexahydroterephthalic acid. In principle, the terephthalic acid could also be replaced up to a maximum of 20 mol percent by another dicarboxylic aliphatic acid such as succinic acid, sebacic acid, adipic acid, azelaic acid or the like. However, the substitution of the terephthalic acid by another dicarboxylic aromatic acid is preferred in view of the mechanical strength comprising i.a. dimensional stability, of the resulting thermoplastic film support.

[0031] According to an alternative embodiment of our invention, when the so-called ester-exchange or transesterification reaction method is used for the production of polyester, in particular ethylene terephthalate, up to 20 mol percent of the ethylene glycol is replaced by one of the diols above mentioned, and/or up to 20 mol percent of the dimethylterephthalate is replaced by the dimethylester of one of the aromatic dicarboxylic acids above mentioned.

[0032] According to a preferred embodiment of our invention, when the direct esterification and polycondensation process is used for the production of PETP, up to maximum 20 mol % of terephthalic acid is replaced by isophthalic acid, and when the ester exchange or transesterification and polycondensation process is used, up to maximum 20 mol % of dimethylterephthalate is replaced by dimethylisophthalate.

[0033] After direct esterification, resp. transesterification, and polycondensation, the copolyester resin has an intrinsic viscosity of 0.5-0.6 which is determined according to the procedure set forth hereinafter.

[0034] The finely divided inert inorganic pigment used in the thermoplastic opaque support of our invention, may be selected from titanium oxide, barium sulfate, calcium carbonate, silica, talc, zinc sulfide and clay as well as certain combinations hereof. The white inorganic pigment is prefereably not larger than 20 um, especially not larger than 10 um in its average particle size. The amount of the white inorganic pigment dispersed into the thermoplastic opaque support of our invention depends to some extent on the type of pigment, but is essentially at least not less than 10 parts by weight and preferably not more than 30 parts by weight per 100 parts by weight of pigment plus resin. Chips composed of the copolyester resin as aforementioned containing more than 10 weight % of the dispersed white inorganic pigment can be manufactured with varied methods including the method in which the pigment is added to and dispersed into a glycolic compound such as ethylene glycol to form a slurry which is in turn polymerized, and then dried and chipped; and the method in which the pigment is mixed and dispersed, with the use of a Banbury mixer or a twin screw extruder, into the copolyester resin as aforementioned which is then chipped.

[0035] The copolymer resin comprising the white inorganic inert pigment can be produced according to a so-called batch or discontinuous production process or according to a continuous process. In either process the copolymer resin of the thermoplastic opaque support of our invention may be produced according to the ester-exchange or transesterification method or according to the direct esterification method, in both instances followed by polycondensation and chips cutting. The discontinuous process for the production of polyester as well as suitable apparatus for the performance of said process, are disclosed in US-P-4,008,048.

[0036] The continuous process for the production of polyester as well as suitable apparatus for the performance hereof are disclosed i a. in "HITACHI Continuous Polyester Process", Hitachi Review Vol . 27 (1978) No. 1, pages 13-16 and "HITACHI Continuous Plant", Hitachi Review Vol. 28 (1979), No. 2, pages 83-88.

[0037] Apart from the basic raw materials comprising ethylene glycol and terephthalic acid or its dimethyl ester, the substitute diol and/or substitute dicarboxylic acid or dimethylester thereof, and the white finely divided inorganic pigment such as preferably BaSO₄ and TiO₂, various other additives are required or preferred for producing chips suitable for being melt-extruded to the opaque thermoplastic film support of our invention.

[0038] Such additives include e.g. transesterification and polycondensation catalysts, stabilisers, electro-conductivity enhancing additives, as well as optical brighteners and dyestuffs.

[0039] In case the ester-exchange or transesterification production process is employed, one of the commonly employed transesterification catalysts should be used. Such compounds include e.g. a zinc, magnesium, manganese, or cobalt salt or mixtures thereof, in particular manganese acetate. These compounds are usually added as a solution in ethylene glycol well before the start of the esterification reaction.

[0040] In case the direct esterification production process is employed, no catalyst is required. However in order to enhance the electroconductivity of the produced resin, in view of a good electrostatic adherence of the extruded film to the quenching member during melt-extrusion, as described hereinafter, electroconductivity enhancing additives are usually provided to the oligomer reaction mixture obtained after the performance of the direct esterification reaction. Such additives are e.g. the transesterification catalysts cited above.

[0041] Further one of the known polycondensation catalysts, such as e.g. GeO₂ and/or Sb₂O₃, should be added as a solution in ethylene glycol before the start of the polycondensation reaction.

[0042] Further the polyester resin preferably contains a stabiliser selected from a.o. a phosphorous compound, e.g. phosphoric acid, phosphorous acid, phosphonic acid and/or esters of these acids, in particular e.g. triphenyl or trimethylphosphate, dimethyl phosphite.

[0043] Optical brighteners may be included as solutions in ethylene glycol added to the oligomer reaction mixture, or subsequently, prior to the film extrusion operation, e.g. by injection during extrusion. These compounds should be present in amounts up to 1500, preferably 500 ppm on the polyester. A suitable example is the brightener commercially available undere the trade name "Leucopur", marketed by Sandoz N.V., Haachtsesteenweg 226-234, B - 1030 Brussels, Belgium.

[0044] Further dyestuff may be incorporated in the polyester so as to slightly modify its color. A suitable example is Ceresblau, marketed by Bayer AG.

[0045] The copolyester resin so obtained may then be further melt-extruded according to per se conventional means to the thermoplastic opaque support of our invention except for certain precautions to be preferably taken during the cooling of the melt-extruded resin on the moving quenching member, usually constituted by a rotating drum.

[0046] The conventional process of extruding polyester granulate to film comprises, as set forth supra, drying the polyester granulate, followed by melt extrusion upon a quenching drum, stretching the polyester film first in the longitudinal direction, subsequently in transverse direction, finally followed by heat setting the biaxially stretched film.

[0047] More in particular, in the extrusion process of making polymer film, the molten polymer is extruded through a slot-like orifice and the extruded polymer is received on a quenching drum or drums on which the temperature of the extruded film is lowered sufficiently rapidly through the softening range of the polymer to obtain a substantially amorphous film. The cooling capacity of the quenching drum, along with the cooling of the molten polyester film by air-blowing is a critical factor in obtaining a substantially amorphous film.

[0048] Also in order to obtain a rapid quenching of the film, it is important that the heat transfer from the film to the quenching drum surface be high. Good heat transfer may be obtained when the film is securely adhered to the quenching drum surface. This is conventionally effected by depositing electrostatic charges to the upper surface of the molten film from a corona-discharge station, prior to the point where the film first contacts with its lower surface the quenching surface which is electrically grounded. Although this process is conventionally applied when melt-extruding polyester films, the industrial execution of this process is hampered by various phenomenons, which are inherent to ionization discharging. Therefore, for producing the opaque thermoplastic resin film according to our invention suitable as support for the reflective photographic material, the process and apparatus as described in US-P-4,310,294 is preferably employed.
According to said specification, the process for melt-extruding a polyester film comprises extruding the polyester in the form of a continuous film from the extrusion die onto a moving quenching drum, maintaining an electrical potential difference between said quenching drum and the extrusion die, the magnitude of such potential difference being sufficient to improve the adherence of said film to said quenching drum, and finally withdrawing the solidified film from the quenching drum, for further treatment, including longitudinal and transverse stretching. Further particulars about this preferred method of electrostatic adherence are described in said specification.

[0049] By stretching the quenched film longitudinally and transversely while the polymer is at the lower end of the softening range. above the second order transition temperature of the polymer, the film can be subjected to molecular orientation leading to an improvement in various physical properties of the film, notably the tensile strength.

[0050] The longitudinal stretching of the film is usually achieved by passing the film first around a series of slowly rotating rollers and then around a series of rollers which are rotated more rapidly, and by heating the film between the two series of rollers to a temperature such that the film undergoes plastic elongation under the traction forces imposed on it due to the different speeds of the two series of rollers.

[0051] The heating of the film to the desired stretching temperature occurs in two stages in the conventional methods. In the first stage the film is heated to a temperature that is somewhat higher than the second order transition temperature (hereinafter called Tg) of the film. When stretching the polymer film for which Tg = 69°C, said first stage heating usually heats the film to a temperature between 78° and 80°C. This first stage heating is effected by heating the first series of rollers. The first stage heating temperature should not exceed 82°C because at temperatures higher than 82°C the film starts to stick to the roller surfaces. In the second heating stage the film is heated to the stretching temperature by means of IR radiation. A common stretching temperature of the film is within the range 85° to 95°C.

[0052] The cooling of the film after stretching is effected by cooling the more rapidly rotating rollers to a temperature well below the Tg of the film but above the dew point of the atmosphere in order to avoid condensation effects. The described steps of heating, cooling and stretching a film are disclosed in US-A-4093695 to a process for making polymeric film.

[0053] This conventional process of longitudinally stretching the PETP-film shows however some disadvantages.

[0054] Therefore, the following process of longitudinally stretching the PETP-film is preferentially applied for the application of the present invention. This process comprises longitudinally stretching a substantially amorphous polyethylene terephthalate film during its longitudinal conveyance by exerting longitudinal stretching forces on the film by first and second traction means located at spaced positions along the path of conveyance of the film, while heating the film between said first and second traction means by means of infrared radiation to a temperature such that the film undergoes plastic elongation under said stretching forces, and cooling the stretched film. In this proces the temperature of the first traction means is not higher than 65°C, the said heating is achieved by directly and symmetrically exposing both sides of the film first to diffused IR-radiation which causes the film temperature to increase but insufficiently for plastic elongation to occur thereby preheating the film, and then to concentrated infrared radiation which heats the film to a temperature between 100° and 120°C whereby rapid plastic elongation of the film occurs under said stretching forces, and whereby the film is rapidly cooled to below its second order transition temperature, before it reaches the second traction means. Then the film is immersed in a bath of cooling liquid which is continuously renewed at the region where the film enters the bath, and wherein the liquid surface at that region is kept quiescent.

[0055] More particulars about this process of longitudinally stretching are described in EU-A-0022278, cited supra.

[0056] After the film has left the cooling liquid bath, drying of the liquid-cooled film may occur by means of squeezing means and heaters.

[0057] A typical stretch ratio of the PETP-film in longitudinal direction is 3.3:1, while the film is kept at a temperature of approx. 100°C. The typical stretch ratio of the PETP-film in transverse direction is also 3.3:1. The biaxially stretched film should finally be heat-set at a temperature of approx. 205°C.

[0058] Heat-setting or heat-relaxing biaxially oriented polyester film involves holding the film at a reduced longitudinal tension while it is heated. Such heat-relaxed or annealed biaxially oriented polymer film exhibits a high degree of dimensional stability and resistance to shrinkage at elevated temperatures up to the heat-relaxing temperature. The principle of heat-relaxing and heat-relaxing devices are described in e.g. US-2,779,684, US-4,160,799 and US-3,632,726. During heat-setting a polymer film, the film should be kept at a temperature between the glass transition temperature and the melt temperature of the polymer, while the film is prevented from shrinking, so as to increase the crystallinity of the film. Such an enhanced crystallinity offers the heat-relaxed film its higher mechanical strength, inclusive of its improved dimensional stability. In view hereof, the amount of diol substituting the ethylene glycol or the amount of dicarboxylic aromatic acid substituting the terephthalic acid or the amount of dimethylester of such aromatic acid substituting the dimethyl terephthalate in the polyethylene terephthalate chain should be limited to maximum 20 and preferably 15 % mol percent. When higher substitution percentages are applied, the increased crystallinity of the biaxially oriented PETP-film cannot be achieved by heat-setting and the resulting film shows insufficient dimensional stability.

[0059] After the heat-setting the beaded edges of the film are trimmed, the margins of the film are knurled and the final film is wound up.

[0060] The invention will further be illustrated by means of examples.

Comparative Example 1

[0061] Polyethylene terephthalate was produced in a laboratory reactor according to the discontinuous transesterification and polycondensation reaction process, more in particular according to the process described in the already cited US-P-4,008,048.

[0062] The production capacity of one batch amounted to 20 kg of PETP-polymer, starting from the following raw materials :

dimethylterephthalate	19.4 kg
ethylene glycol	12.4 kg
Mn(OAc)₂	7.95 g
DeO₂,	1.05 g
Sb₂O₃ and	2.92 g
diethylphosphite	4.14 g

[0063] The manganese acetate was used as transesterification catalyst. This catalyst was added as a solution of Mn(OAc)₂ in ethylene glycol , said solution being added at the beginning of the transesterification reaction.

[0064] The mixture of GeO₂/Sb₂O₃ was used as polycondensation catalyst. This catalyst mixture was added as a solution of GeO₂/Sb₂O₃ in ethylene glycol, said solution being added at the end of the transesterification reaction.

[0065] The diethylphosphite compound was added also as a solution in ethylene glycol , acting as stabiliser for the PETP obtained.

[0066] The intrinsic viscosity of the PETP produced amounted to 0,65 dl/g and its crystalline melting point was 264°C.

[0067] The intrinsic viscosity was measured on a solution of 0.5 g of PETP in 100 ml of a mixture of 40 parts by weight of phenol and 60 parts by weight of 1,2-dichloorbenzene, in the way as described in ISO 1628/1 and ISO 1628/5 standards.

[0068] Further a suspension in ethylene glycol containing 3,88 g of BaSO₄, a small amount of TiO₂, Leucopur and Ceresblau as above mentioned was added to the oligomer mixture at the end of the transesterification reaction so as to become an opaque white PETP-polymer.

Example 1

[0069] The process for producing PETP-polymer as described under Comparative Example 1 was repeated with the difference however that apart from dimethylterephthalate dimethylisophthalate was added as raw material in such amount that its concentration in the copolymer produced amounted to 2 % by weight.

Example 2

[0070] The process for producing PETP-polymer as described under Comparative Example 1 was repeated with the difference however that apart from dimethylterephthalate dimethylisophthalate was added as raw material in such amount that its concentration in the copolymer produced amounted to 5 % by weight.

Example 3

[0071] Polyethylene terephthalate was produced in a commercial scale reactor system according to the discontinuous transesterification and polycondensation reaction process, as described in the already cited US-P-4,068,048. The production capacity of one batch amounted to 1820 kg of polymer, starting from the following raw materials :

dimethylterephthalate	1467 kg
dimethylisophthalate	33 kg
ethylene glycol	869 kg
Mn(OAc)₂	568 g
GeO₂	81 g
Sb₂O₃	225 g
diethylphosphite	427 g
BaSO₄	293 kg
TiO₂	32.6 kg
Leucopur	350 g
Ceresblau	0.35 g

[0072] The manganese acetate was used as esterification catalyst; this catalyst was added as a solution of Mn(OAc)₂ in ethylene glycol , said solution being added at the begin of the transesterification reaction. The mixture of GeO₂/Sb₂O₃ was used as polycondensation catalyst. This catalyst mixture was added as a solution of GeO₂/Sb₂O₃ in ethylene glycol , said solution being added at the end of the transesterification reaction.

[0073] The BaSO₄, TiO₂, Leucopur and Ceresblau compounds were added as a suspension in 1048 kg of ethylene glycol to the oligomer mixture at the end of the transesterification reaction so as to become an opaque white copolymer.

[0074] The concentration of BaSO₄, TiO₂ and isophthalate in the copolymer produced amounted to 16.7, resp. 1.8, resp. 1.7, expressed in weight percent of the total copolymer weight.

[0075] The intrinsic viscosity of the copolymer produced measured as set forth above amounted to 0,65 dl/g, its crystalline melting point was 260°C.

Example 4

[0076] The process described under example 3 was repeated with the following differences however

dimethylisophthalate	31 kg (instead of 33 kg)
diethylphosphite	640 g (instead of 427 g)
BaSO₄	304 kg (instead of 293 kg)
TiO₂	none

[0077] The BaSO₄, Leucopur and Ceresblau compounds were added as a suspension in 1216 kg ethylene glycol

[0078] The concentration of BaSO₄ and isophthalate in the copolymer produced amounted to 16,9, resp.1,7, expressed in weight percent of the total copolymer weight.

[0079] The intrinsic viscosity of the copolymer produced measured as set forth above amounted to 0,65 dl/g, its crystalline melting point was 260°C.

Evaluation

[0080] The white inorganic copolyester polymers produced according to the Examples 1 to 4 were steadily converted by a conventional film-extrusion process and apparatus to opaque film sheets suitable as support for white photographic materials.

[0081] The white PETP-polymer produced in accordance with the procedure described under Comparative Example 1 on the contrary when melt-extruded according to conventional means posed numerous problems, in particular ruptures, during the longitudinal and transverse stretching operations, due to its too high degree of crystallinity after being melt-extruded to sheets.

Claims

1. A method of manufacturing an opaque polyester copolymer support for reflective photographic materials comprising an inorganic inert pigment, said polyester comprising units of ethylene terephthalate as the main repeating units, said method comprising the following steps in the order given :

- either supplying terephthalic acid and ethylene glycol to bis(betahydroxyethyl )terephthalate or its oligomer so as to carry out the esterification step, or supplying dimethylterephthalate and ethylene glycol so as to carry out transesterification,

- polycondensing under conditions of increasingly reduced vacuum,

- either granulating the obtained polymer, followed by drying the granules and feeding them to an extruder, or directly feeding the obtained copolymer in melt-form to an extruder,

- extruding the polymer in the form of a sheet,

- quenching and solidifying the resulting sheet on a quenching member,

- biaxially stretching the sheet,

- annealing the biaxially stretched film,

characterised in that terephthalic acid or dimethylterephthalate respectively is replaced by another aromatic dicarboxylic acid or by the dimethylester of another aromatic dicarboxylic acid respectively, and/or ethylene glycol is replaced by another diol , said replacement(s) amounting to maximum 20 mol percent with respect to ethylene glycol and terephthalic acid or dimethylterephthalate respectively.

2. The method of claim 1, wherein the polyester copolymer is the polycondensation reaction product of ethylene glycol and either terephthalic acid, replaced for an amount up to a maximum of 20 mol percent by isophthalic acid or dimethylterephthalate, replaced for an amount up to a maximum of 20 mol percent by dimethyl isophthalate

3. The method of claim 2, wherein the molar ratio of either terephthalic/isophthalic acid, or dimethylterephthalate/dimethylisophthalate is comprised between 85:15 and 99:1.

4. The method of claim 3, wherein said molar ratio is comprised between 93:7 and 99:1.

5. The method of any of the preceding claims wherein the electrostatic adherence of the extruded polyester sheet to the quenching member is enhanced by maintaining an electrical potential difference between the quenching member and the extrusion die.

6. The method of any of the preceding claims wherein the amount of inorganic inert pigment of the polyester copolymer support is at least 10%.

7. An opaque photographic reflective support comprising at least 10% of an inorganic inert pigment characterised in that said support is made of a copolymer comprising ethylene terephthalate as the main repeating unit, being the polycondensation reaction product of ethylene glycol and terephthalic acid, resp. dimethylterephthalate, ethylene glycol being substituted by another diol and/or terephthalic acid resp. dimethylterephthalate being substituted by another aromatic dicarboxylic acid, resp. dimethylester of another aromatic dicarboxylic acid, either substitution amounting to maximum 20 mol percent with respect to ethylene glycol and/or terephthalic acid, resp. dimethyl terephthalate.