EP0901505A1

EP0901505A1 - Process for preparing a copolyester

Info

Publication number: EP0901505A1
Application number: EP97920884A
Authority: EP
Inventors: William Alasdair Macdonald; Simon Vernon Mortlock
Original assignee: Imperial Chemical Industries Ltd
Current assignee: EIDP Inc
Priority date: 1996-05-24
Filing date: 1997-05-13
Publication date: 1999-03-17
Also published as: JP2000510900A; CN1269869C; GB9610939D0; WO1997045470A1; JP4032141B2; CN1219943A

Abstract

A process for preparing a copolyester containing 55 to 95 mole % of alkylene terephthalate units and 5 to 45 mole % of alkylene isophthalate units, uses a reaction mixture containing 10 to 150 ppm of titanium atoms as catalyst and 10 to 100 ppm of phosphorus atoms as stabilizer, both based on the weight of the final copolyester. The copolyester has good colour properties and is particularly suitable for forming into a film for use in packaging applications, especially for food and drink.

Description

Process for Preparing a Copolyester

This invention relates to a process for preparing a copolyester and to a film made therefrom.

It is well known to prepare a polyester or copolyester in a two stage process. In the first stage monomer is produced at a temperature in the range from 150 to 265°C generally by reacting a dicarboxylic acid with a glycol (direct esterification) or alternatively by reacting an alkyl ester of a dicarboxylic acid with a glycol (ester interchange). A catalyst such as manganese acetate or zinc acetate is normally employed in the ester interchange reaction, whereas direct esterification can be carried out in the absence of a catalyst. In the second stage of (co)polyester production the monomer molecules are condensed together (polycondensation) by heat in the presence of a catalyst. Glycol is produced during the condensation reaction and removed under reduced pressure. A wide range of polycondensation catalysts have been employed commercially such as antimony tπoxide, germanium dioxide and zinc acetate. Antimony has been the preferred polycondensation catalyst. Unfortunately there are increasing environmental concerns about the use of antimony, and the use thereof is likely to be restricted in the future, including limits on the disposal of antimony-containing wastes. There is thus a need to be able to commercially prepare (co)polyester by means of an antimony-free catalyst system. WO 93/223667 is directed to a method of producing polyethylene terephthalate using an antimony-free polycondensation catalyst system which consists of 10 to 75 ppm of lithium and 15 to 80 ppm of germanium.

US-A-5417908 describes an antimony-free process for making polyethylene terephthalate using manganese, inorganic phosphorus and germanium. Isophthalate/terephthalate copolyester has been used, inter alia, for forming amorphous and oriented films, and as heat-sealable and ink-receptive coating layers, in packaging applications such as packaging food and drink products, for example on a polyethylene terephthalate film substrate. Advantages of isophthalate/terephthalate copolyester over polyethylene terephthalate include a reduced tendency to crystallise and a reduced rate of crystallisation. There is a particular requirement for film used in food and drink contact applications to have good aesthetic properties, ie of high clarity and non-yellow in colour.

GB-A-825549 is directed to a process of producing a polyester film which is derived from ethylene glycol, 65 to 95% by weight of terephthalic acid, and 5 to 35% by weight of isophthalic acid. GB-A-825549 discloses shrinkable films made from the aforementioned copolyester. The polyethylene (terephthalate/isophthalate) copolyester was produced using antimony polycondensation catalyst.

EP-A-0035835 discloses a composite film having an oriented first polyester layer, preferably polyethylene terephthalate, and a heat-sealable amorphous second layer, preferably ethylene terephthalate/ethylene isophthalate copolyester. The copolyester was produced by conventional polymerisation.

Titanium compounds have been used as polyester catalysts. Unfortunately the use of titanium generally results in polyester of poor colour quality, ie yellow. GB-A-1421972 is directed to the use of a titanium compound as a direct esterification catalyst in polyester, preferably polyethylene terephthalate, production.

The titanium compound is deactivated after esterification by the presence of a phosphorous compound, and antimony is used as the polycondensation catalyst. The examples of GB-A-1421972 show that in the absence of antimony, no polymerisation occurred, illustrating the ineffectiveness of the titanium compound as a polycondensation catalyst.

JP-A-6170911 discloses a method of producing a polyester, particularly polyethylene terephthalate, film using a polyester polymerised with 7 to 120 ppm of titanium atoms and adding up to 150 ppm of phosphorous atoms after polymerisation and prior to melt extrusion. There are particular difficulties involved in producing a non-antimony containing isophthalate/terephthalate copolyester which possesses good colour and high IV. Catalyst systems which are effective with polyethylene terephthalate do not necessarily result in an isophthalate/terephthalate copolyester having the required properties. We have now devised a process for preparing an isophthalate/terephthalate copolyester, and a film produced therefrom, which overcomes or reduces at least one of the aforementioned problems.

Accordingly, the present invention provides a process for preparing a copolyester comprising in the range from 55 to 95 mole % of alkylene terephthalate units and 5 to 45 mole % of alkylene isophthalate units, using a reaction mixture comprising 10 to 150 ppr f titanium atoms which act as a catalyst, and 10 to 100 ppm of phosphorus atoms whicn act as a stabiliser, both based on the weight of the final copolyester.

The invention also provides a polyester film which comprises a copolyester comprising in the range from 55 to 95 mole % of alkylene terephthalate units and 5 to

45 mole % of alkylene isophthalate units, produced using a reaction mixture comprising 10 to 150 ppm of titanium atoms which act as a catalyst, and 10 to 100 ppm of phosphorus atoms which act as a stabiliser, both based on the weight of the copolyester.

The invention further provides a composite film which comprises a first polyester layer and a second layer which comprises a copolyester comprising in the range from 55 to 95 mole % of alkylene terephthalate units and 5 to 45 mole % of alkylene isophthalate units, 10 to 150 ppm of titanium atoms and 10 to 100 ppm of phosphorus atoms, both based on the weight of the copolyester.

The copolyester prepared according to the present invention preferably comprises in the range from 60 to 93, more preferably 65 to 90, particularly 70 to 85, and especially 75 to 82 mole % of alkylene terephthalate units, and correspondingly preferably 7 to 40, more preferably 10 to 35, particularly 15 to 30, and especially 18 to 25 mole % of alkylene isophthalate units.

The alkylene component is suitably derived from one or more glycols, particularly aliphatic glycols, eg ethylene glycol, diethylene glycol, triethylene glycol,

1,3-propanediol, 1,3-butane diol, 1 ,4-butane diol, 1,5-pentane diol, 2,2-dimethyl 1 ,3-propane diol, neopentyl glycol and 1 ,4-cyclohexanedimethanol. The copolyester is preferably derived from at least 70, more preferably at least 80, and particularly at least 90 and up to 100 mole % of ethylene glycol, ie ethylene is a preferred alkylene component. A particularly preferred glycol combination is from 95 to 100 mole % ethylene glycol and from 0 to 5 mole % diethylene glycol. Thus, preferred copolyesters comprise ethylene terephthalate and ethylene isophthalate.

The titanium atom or compound acts a catalyst, and suitable compounds include inorganic titanates such as sodium titanate and lithium titanate, and preferably organic titanates, more preferably alkyl titanates, and particularly tetraalkyl titanates, wherein the alkyl group has 1 to 6 carbon atoms. Preferred titanates include tetramethyl titanate, tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate and tetrahexyl titanate. Tetraisopropyl titanate is a particularly preferred titanate. Other suitable titanates include triethaπolamine titanate, and the titanates of ethylene glycol, hexylene glycol and octylene glycol. Two or more of the aforementioned titanates may be used together.

The total metal content of the titanium compound catalyst preferably comprises in the range from 50 to 100%, more preferably 75 to 100%, particularly 90 to 100%, and especially approximately 100% of titanium, ie titanium is preferably the only metal atom present in the titanium compound. The concentration of titanium atoms present in the reaction mixture is preferably in the range from 15 to 120, more preferably 20 to 100, particularly 30 to 90, and especially 40 to 80 ppm based on the weight of the final copolyester.

The titanium compound may be added after monomer formation, but is preferably added prior to monomer formation, ie prior to the direct esterification or ester interchange reaction. Thus, the titanium compound is preferably catalysing at least the polycondensation reaction. The titanium compound preferably provides in the range from 60 to 100%, more preferably 75 to 100%, particularly 90 to 100%, and especially approximately 100% of the catalytic activity employed in the polycondensation reaction. Thus, the titanium compound is preferably the sole polycondensation catalyst present in the reaction mixture.

The phosphorus atom or compound acts as a stabiliser, and suitable compounds include phosphorus acids, phosphinic acids, phosphates and phosphites. Particular examples include phosphorous acid, phosphoric acid, phosphonic acid, sodium dihydrogen phosphate, tris nonylphenyl phosphite, trimethyl phosphate, triethyl phosphate, tributyl phosphate, tripheήyl phosphate, diphenyl phosphite, dibutyl phosphite, dimethylphenyl phosphonate and ammonium phosphate. The phosphorus compound is preferably selected from the group consisting of phosphoric acid, phosphorous acid, and tris nonylphenyl phosphite. The concentration of phosphorus atoms present in the reaction mixture is preferably in the range from 20 to 80, more preferably 30 to 70, particularly 40 to 60, and especially 45 to 55 ppm based on the weight of the final copolyester.

The phosphorus compound may be added prior to monomer formation, but is preferably added after monomer formation, ie after the direct esterification or ester interchange reaction, and prior to the polycondensation reaction.

The mole ratio of titanium atoms to phosphorus atoms present in the reaction mixture is preferably in the range from 0.5 to 2.0, more preferably 0.75 to 1.5, particularly 0.9 to 1.2, and especially 1.0 to 1.1.

In a preferred embodiment of the invention the reaction mixture additionally comprises a blue toning material preferably in the range from 6 to 200, more preferably

15 to 140, particularly 30 to 80, and especially 40 to 60 ppm based on the weight of the final copolyester. Cobalt is a preferred blue toning material, preferably added in salt form such as cobalt acetate.

The blue toning material is preferably added after monomer formation, ie after the direct esterification or ester interchange reaction, and prior to the polycondensation reaction. The blue toning material is however, preferably added after the phosphorus compound.

In addition, the reaction mixture preferably comprises a diethylene glycol suppressant, such as sodium hydroxide, preferably in the range from 10 to 200, more preferably 30 to 70 ppm based on the weight of the final copolyester. The diethylene glycol suppressant is preferably added prior to monomer formation, more preferably at the same time as the titanium compound.

Copolyester produced according to the present invention preferably has an intrinsic viscosity (IV) in the range from 0.5 to 0.8, more preferably 0.6 to 0.7, and particularly 0.63 to 0.67. The copolyester surprisingly exhibits a low IV drop on film extrusion, preferably less than 0.04, more preferably in the range from 0.001 to 0.03, particularly 0.005 to 0.02, and especially 0.01 to 0.015.

In a particularly preferred embodiment of the invention, the copolyester exhibits a colour "b" value (ie yellowness value) in the range from -5 to 15, more preferably -3 to 10, particularly 0 to 8, and especially 4 to 6. It is a particularly surprising feature of the present invention that copolyester can be obtained using a titanium catalyst system which possesses a "b" value within the aforementioned preferred ranges.

The polyester film according to one embodiment of the present invention, comprising a copolyester as described herein may, be unoriented, or preferably oriented, such as uniaxially oriented or more preferably biaxially oriented by sequential stretching in two mutually perpendicular directions, typically at a temperature in the range from 70 to 125^βC. Formation of the film may be effected by any process known in the art for producing a polyester film, for example a tubular or a flat film process. In a tubular process simultaneous biaxial orientation may be effected by extruding a thermoplastics polyester tube which is subsequently quenched, reheated and then expanded by internal gas pressure to induce transverse orientation, and withdrawn at a rate which will induce longitudinal orientation.

In the preferred flat film process a film-forming copolyester is extruded through a slot die and rapidly quenched upon a chilled casting drum to ensure that the copolyester is quenched to the amorphous state. Orientation is then effected by stretching the quenched extrudate at a temperature above the glass transition temperature of the polyester. Sequential orientation may be effected by stretching a flat, quenched extrudate firstly in one direction, usually the longitudinal direction, ie the forward direction through the film stretching machine, known as the machine direction

(MD), and then in the transverse direction (TD). Forward stretching of the extrudate is conveniently effected over a set of rotating rolls or between two pairs of nip rolls, transverse stretching then being effected in a stenter apparatus. The polyester film is preferably stretched so that the dimension of the oriented film is from 2.0 to 4.0, more preferably 2.5 to 3.5, and particularly 2.7 to 3.1 times in the MD, and from 2.5 to 4.5, more preferably 3.0 to 4.0, and particularly 3.6 to 4.0 times in the TD.

The stretched film may be, and preferably is, dimensionally stabilised by heat-setting under dimensional restraint at a temperature above the glass transition temperature of the film-forming copolyester but below the melting temperature thereof, to induce crystallisation of the copolyester. The polyester film is suitably heat-set at a relatively low temperature, preferably from 60 to 150°C, more preferably 70 to 120'C, particularly 75 to 100°C, and especially 80 to 90°C.

The polyester film is preferably suitable for heat-sealing to itself by using conventional heat-sealing apparatus and conditions, whereby the seal is formed by heating two or more layers of the polyester film to a temperature at which the polyester is softened or melted, resulting in a heat-seal bond being formed. The polyester film preferably exhibits a heat-seal strength, measured by sealing the film to itself, in the range from 200 to 1500, more preferably 300 to 1200, and particularly 400 to 1000 Nnτ\

The polyester film preferably has a degree of crystallinity in the range from 2% to 20%, more preferably 4% to 15%, and particularly 6% to 10%.

In one embodiment of the invention the polyester film is transparent, exhibiting high optical clarity and low haze, preferably having a wide angle haze, being measured according to the standard ASTM D 1003-61, of <8%, more preferably <6%, particularly <5%, and especially <3%, preferably for a 40 μm thick film. The aforementioned optical characteristics can be suitably achieved by having little or no paniculate additive present in the film. The polyester film may contain relatively small quantities of filler material in order to provide handleability, for example in the range from 5 to 3000 ppm, preferably 50 to 2000 ppm, and more preferably 100 to 1000 ppm. Suitable fillers include inorganic materials such as silica, china clay, calcium carbonate, and organic materials such as silicone resin particles. Spherical monodisperse fillers may be employed.

However, in an alternative embodiment of the invention the polyester film is opaque, which is defined as a film exhibiting a Transmission Optical Density (Sakura Densitometer; type PDA 65; transmission mode) of from 0.75 to 1.75, and particularly of from 1.2 to 1.5, preferably for a 40 μm thick film. The polyester film is conveniently rendered opaque by incorporating into the polyester, an effective amount of an opacifying agent. Suitable opacifying agents include an incompatible resin filler, a particulate inorganic filler or a mixture of two or more such fillers.

By an "incompatible resin" is meant a resin which either does not melt, or which is substantially immiscible with the copolyester, at the highest temperature encountered during extrusion and fabrication of the film. The presence of an incompatible resin usually results in a voided film, by which is meant comprises a cellular structure containing at least a proportion of discrete, closed cells. Suitable incompatible resins include polyamides and olefin polymers, particularly a homo- or co-polymer of a mono-alpha-olefin containing up to 6 carbon atoms in its molecule, for incorporation into the copolyester. Preferred materials include a low or high density olefin homopolymer, particularly polyethylene, polypropylene or poly-4-methylpentene-1, an olefin copolymer, particularly an ethylene-propylene copolymer, or a mixture of two or more thereof. Random, block or graft copolymers may be employed. The amount of incompatible resin filler present in the polyester film is preferably in the range from 2% to 30%, more preferably 3% to 20%, particularly 4% to 15%, and especially 5% to 10% by weight, based on the weight of the copolyester.

Particulate inorganic fillers suitable for generating an opaque polyester film include conventional inorganic pigments and fillers, and particularly metal or metalloid oxides, such as alumina, silica and titania, and alkaline metal salts, such as the carbonates and sulphates of calcium and barium. The particulate inorganic fillers may be of the voiding or non-voiding type. Suitable particulate inorganic fillers may be homogeneous and consist essentially of a single filler material or compound, such as titanium dioxide or barium sulphate alone. Alternatively, at least a proportion of the filler may be heterogeneous, the primary filler material being associated with an additional modifying component. For example, the primary filler particle may be treated with a surface modifier, such as a pigment, soap, surfactant coupling agent or other modifier to promote or alter the degree to which the filler is compatible with the polyester. The volume distributed median particle diameter (equivalent spherical diameter corresponding to 50% of the volume of all the particles, read on the cumulative distribution curve relating volume % to the diameter of the particles - often referred to as the "D(v,0.5)" value) of the inorganic filler, preferably titanium dioxide, is preferably in the range from 0.2 to 5 μm, more preferably 0.4 to 1.5 μm, and particulariy 0.8 to 1.2 μm. The amount of inorganic filler, particularly of titanium dioxide, incorporated into the polyester film should be not less than 1% nor exceed 30% by weight, based on the weight of the copolyester. Particularly satisfactory levels of opacity are achieved when the concentration of filler is in the range from about 5% to 20%, preferably 10% to 15%, and more preferably 12% to 13% by weight, based on the weight of the copolyester. The preferred titanium dioxide particles may be of anatase or rutile crystal form.

Particle size of the filler particles may be measured by electron microscope, coulter counter, sedimentation analysis and static or dynamic light scattering. Techniques based on laser light diffraction are preferred. The median particle size may be determined by plotting a cumulative distribution curve representing the percentage of particle volume below chosen particle sizes and measuring the 50th percentile.

The thickness of a polyester film according to the invention is preferably in the range from 5 to 150 μm, more preferably 15 to 100 μm, particularly 20 to 70 μm, and especially 30 to 50 μm.

A polyester film according to the invention may be coated on one or both surfaces with one or more additional coating, ink, lacquer and/or metal layers, for example to form a laminate or composite which exhibits improved properties, such as handleability, antistatic, adhesion promoting or release, compared with the component materials. Suitable coating materials include film-forming polymeric resins such as acrylic resins, copolyesters, styrene copolymers, acrylic copolymers, functionalised polyolefins, polyvinyl alcohol, cellulosic materials such as nitrocellulose, ethylcellulose and hydroxyethylcellulose. Blends or mixtures of any of the aforementioned polymeric resins may be employed. Prior to the deposition of a coating medium onto the polyester film, the exposed surface thereof may, if desired, be subjected to a chemical or physical surface-modifying treatment to improve the bond between that surface and the subsequently applied coating layer. A preferred treatment is corona discharge. Alternatively, the surface of the polyester film may be pre-treated with an agent known in the art to have a solvent or swelling action thereon, such as a halogenated phenol dissolved in a common organic solvent eg a solution of p-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or 2,4,6-trichlorophenol or 4-chlororesorciπol in acetone or methanol.

The coating medium may be applied to an already oriented polyester film surface, but application of the coating medium is preferably effected before or during the stretching operation. In particular, it is preferred that the coating medium should be applied to the polyester film surface between the two stages (longitudinal and transverse) of a biaxial stretching operation.

The composite film according to one embodiment of the present invention comprises a first polyester layer and a second layer comprising a copolyester as described herein. The first layer preferably comprises a polyester comprising ethylene terephthalate as the main repeating unit, preferably in an amount of up to 100 mole %, more preferably in the range from 96 to 100 mole %, particularly 98 to 100 mole %, and especially 100 mole %, ie polyethylene terephthalate homopolymer. When the polyester contains less than 100 mole % of ethylene terephthalate repeating units, the minor repeating units is preferably ethylene isophthalate. The preferred polyethylene terephthalate first layer is preferably biaxially oriented.

Formation of the second copolyester layer on the first polyester layer may be effected by conventional techniques, for example by casting the copolyester onto a preformed first layer. Conveniently, however, formation of a composite film is effected by coextrusion, either by simultaneous coextrusion of the respective film-forming layers through independent orifices of a muiti-orifice die, and thereafter uniting the still molten layers, or, preferably, by single-channel coextrusion in which molten streams of the respective polymers are first united within a channel leading to a die manifold, and thereafter extruded together from the die orifice under conditions of streamline flow without intermixing thereby to produce a composite film.

A coextruded film is preferably stretched to effect molecular orientation of at least the first layer, and preferably heat-set. Generally, the conditions applied for stretching the first layer will induce partial crystallisation of the copolyester of the second layer and it is therefore preferred to heat set under dimensional restraint at a temperature selected to develop the desired moφhology of the second layer. Thus, by effecting heat-setting at a temperature below the crystalline melting temperature of the copolyester of the second layer and permitting or causing the composite to cool, the copolyester will remain essentially crystalline. However, by heat-setting at a temperature greater than the crystalline melting temperature of the copolyester, the latter will be rendered essentially amoφhous. Heat-setting of a composite film comprising a polyethylene terephthalate first layer and a copolyester second layer is conveniently effected at a temperature within a range of from 175 to 200°C to yield a substantially crystalline copolyester second layer, or from 200 to 250°C to yield an essentially amoφhous copolyester second layer. An essentially amoφhous copolyester second layer is preferred. Copolyester second layers may be disposed on one or both sides of the first polyester layer. The composite film may have a total thickness in the range from 5 to 500 μm, suitably 5 to 200 μm, preferably 7 to 100 μm, more preferably 9 to 50 μm, particularly 11 to 20 μm, and especially 12 to 15 μm. The or each copolyester second layer(s) preferably constitute from 1 to 50%, more preferably 2 to 40%, particularly 5 to

30%, and especially 10 to 20% of the total composite film thickness. The copolyester layers preferably have a thickness of up to 30 μm, more preferably in the range from 0.5 to 20 μm, particulariy 1 to 10 μm, and especially 2 to 5 μm.

In one embodiment of the invention the first layer polyester, preferably polyethylene terephthalate, is formed using non-titanium catalysis, more preferably by using a standard antimony catalyst system. The first layer polyester is suitably produced by using a reaction mixture comprising 100 to 500, more preferably 150 to 350, and particularly 225 to 275 ppm of antimony based on the weight of the final polyester. The polyester film, composite film, or additional coating layer(s), may conveniently contain any of the additives conventionally employed in the manufacture of polymeric films. Thus, agents such as dyes, pigments, voiding agents, lubricants, anti-oxidants, anti-blocking agents, surface active agents, slip aids, gloss-improvers, prodegradants, ultra-violet light stabilisers, viscosity modifiers and dispersion stabilisers may be incorporated into the film-forming (co)polyester and/or coating layer medium.

The polyester film and composite film according to the present invention, are particularly suitable for use as a packaging film for example by heat-sealing the film to itself, particularly for packaging food and drink products.

The invention is illustrated by reference to the following examples. The following test procedures were used.

1 ) Colour "b" value of the copolyester was measured on the Hunter scale by using a Colourgard 2000.

2) Intrinsic viscosity (IV) of the copolyester was measured by solution viscosity of an 8% solution of copolyester in orthochlorophenol at 25°C. Example 1

The copolyester was made by using a batch terephthalic acid based route. The esterification vessel was charged with 1810 Kg of terephthalic acid (80.5 mole %), 440 Kg of isophthalic acid (19.5 mole %), 1050 litres of ethylene glycol, 50 ppm of sodium hydroxide and 500 ppm of tetraisopropyl titanate (TIPT). The mixture was heated up to 265°C until all the water had been distilled off. 165 ppm of phosphoric acid stabiliser was then added, and the reaction mixture transferred to an autoclave. 300 ppm of cobalt acetate tetrahydrate was added, the reaction mixture was heated to 295°C and polymerisation under vacuum occurred. The final copolyester product had an IV of 0.67 and a colour "b" value of 5.

A molten web of the copolyester prepared above was extruded in a conventional manner from a slot die on to the polished surface of a cooled rotating drum upon which the web was quenched to below the glass transition temperature of the polyester to provide an amorphous film. The quenched film was then reheated and drawn about 2.9 times its original length in the MD, passed into a stenter oven and the sheet stretched in the TD to approximately 3.8 times its original dimensions, followed by heat setting. Final film thickness was 40 μm.

The copolyester film had an IV of 0.66, ie the IV drop on film extrusion was 0.01.

Example 2

Separate streams of a first layer polymer of polyethylene terephthalate, prepared using 50 ppm of sodium hydroxide, 300 ppm antimony trioxide, 250 ppm of phosphoric acid and 200 ppm of cobalt acetate tetrahydrate, and a second layer copolyester prepared as described in Example 1 , were supplied from separate extruders to a single channel coextrusion assembly. The polymer layers were extruded through a film-forming die onto a water cooled rotating, quenching drum to yield an amorphous cast composite extrudate. The cast extrudate was heated to a temperature of about 80°C and then stretched longitudinally at a forward draw ratio of 3.2:1. The composite sheet was passed into a stenter oven, where the sheet was dried and stretched in the sideways direction to approximately 3.4 times its original dimensions. The biaxially stretched composite sheet was heat set at a temperature of about 225"C. Final film thickness of the composite sheet was 25 μm. The first polyethylene terephthalate layer was 20 μm thick, the second copolyester layer was 5 μm thick.

Example 3

This is a comparative example not according to the invention.

The procedure of Example 2 was repeated except that the second layer copolyester was prepared in a similar manner to the first layer polyethylene terephthalate, ie using 50 ppm of sodium hydroxide, 300 ppm antimony trioxide, 250 ppm of phosphoric acid and 200 ppm of cobalt acetate tetrahydrate, ie not using titanium catalysis.

The composite films produced in Examples 2 and 3 were subjected to an antimony extraction test by refluxing separate film samples for two hours in both water and 15% ethanol. The film of Example 2 showed a suφrising 5 fold decrease in water, and a 8.3 fold decrease in 15% ethanol, in the amounts of antimony extracted compared to the film of Example 3.

Claims

1. A process for preparing a copolyester comprising in the range from 55 to 95 mole % of alkylene terephthalate units and 5 to 45 mole % of alkylene isophthalate units, using a reaction mixture comprising 10 to 150 ppm of titanium atoms which act as a catalyst, and 10 to 100 ppm of phosphorus atoms which act as a stabiliser, both based on the weight of the final copolyester.

2. A process according to claim 1 wherein the reaction mixture additionally comprises a blue toning material.

3. A process according to either one of claims 1 and 2 wherein the reaction mixture additionally comprises a diethylene glycol suppressant.

4. A process according to any one of the preceding claims wherein the reaction mixture is antimony free.

5. A process according to any one of the preceding claims wherein the copolyester has a colour "b" value in the range from -5 to 15.

6. A polyester film which comprises a copolyester comprising in the range from

55 to 95 mole % of alkylene terephthalate units and 5 to 45 mole % of alkylene isophthalate units, produced using a reaction mixture comprising 10 to 150 ppm of titanium atoms which act as a catalyst, and 10 to 100 ppm of phosphorus atoms which act as a stabiliser, both based on the weight of the copolyester.

7. A polyester film according to claim 6 wherein the copolyester has an intrinsic viscosity drop on film extrusion in the range from 0.001 to 0.03.

8. A composite film which comprises a first polyester layer and a second layer which comprises a copolyester comprising in the range from 55 to 95 mole % of alkylene terephthalate units and 5 to 45 mole % of alkylene isophthalate units, 10 to 150 ppm of titanium atoms and 10 to 100 ppm of phosphorus atoms, both based on the weight of the copolyester.

9. A composite film according to claim 8 wherein the polyester of the first layer is produced using a reaction mixture comprising antimony.

10. A composite film according to either one of claims 8 and 9 wherein the polyester of the first layer comprises polyethylene terephthalate.