This invention relates to a coated polyester film, and in particular to a product made therefrom.

A polyester film is often used with other materials, for example by coating or laminating polyester film to form a laminate or composite which exhibits improved properties, compared to the component materials. It is well known in the art that many coating or adhesive compositions do not adhere well to polyester film. In addition, inks and lacquers do not generally adhere well to polyester film. Consequently, it is desirable to improve such properties by providing an adherent layer of an appropriate coating medium on the surface of polyester film. An adherent layer material is chosen which is generally more receptive to subsequently applied coating layers than polyester film per se. Problems may be encountered in providing adequate adhesion between the adherent layer and a polyester film substrate, particularly a relatively hydrophobic substrate such as a polyethylene terephthalate film. Acrylic coating systems have been routinely used in the art as adherent layers on polyester film. Unfortunately, prior art acrylic adherent layers may suffer from the aforementioned adhesion problem. There is also a need to improve the adhesion of acrylic adherent layers to a range of additionally applied coating layers.

Metal sheet laminated with polyester film, particularly when comprising a heat-sealable layer, can be used, inter alia, for forming cans such as drawn and wall-ironed cans (known as DWI cans) and drawn and redrawn cans (known as DRD cans). In particular, the laminated metal sheet can be used to form can lids for food and drink cans which contain an opening flap, such as the "pull-tab" type, "push-in tab" type, and especially lids with push-in circular openings. GB-A-2197603 describes a method of producing a can end containing push-in openings. During production of the tin lid the opening flap is partially cut free from the lid. In order to create a sufficient seal, particularly for lids of cans containing carbonated beverages, it is generally necessary to seal the opening flap with a plastisol, for example as described in GB-A-2197634. Unfortunately the plastisols used, such as polyvinyl chloride plastisols, exhibit inadequate adhesion to can lids which have been laminated with polyester film.

Adhesion of the plastisol to the surface of the polyester film is particularly poor after the can has been subjected to pasteurisation.

We have now devised a polyester film coated with an adherent layer which reduces or substantially overcomes at least one of the aforementioned problems. Accordingly, the present invention provides a polymeric film comprising a substrate layer of polyester material having on at least one surface thereof an adherent

layer compnsing an acrylic resin and at least one adhesion promoter selected from a citrate ester, an adipate ester and an epoxidised vegetable oil

The invention also provides a method of producing a polymenc film by forming a substrate layer of polyester matenal, and applying to at least one surface thereof an adherent layer compnsing an acrylic resin and at least one adhesion promoter selected from a citrate ester, an adipate ester and an epoxidised vegetable oil

A substrate for use in the production of an adherent layer coated polymenc film according to the invention suitably compnses any polyester matenal capable of forming a self-supporting opaque, or preferably transparent, film or sheet By a "self-supporting film or sheet" is meant a film or sheet capable of independent existence in the absence of a supporting base

The substrate of an adherent layer coated film according to the invention may be formed from any synthetic, film-forming, polyester matenal Suitable thermoplastic, synthetic, matenals include a synthetic linear polyester which may be obtained by condensing one or more dicarboxy c acids or their lower alkyl (up to 6 carbon atoms) diesters, eg terephtha c acid, isophthahc acid, phtha c acid, 2,5-, 2,6- or 2,7- naphthalenedicarboxy c acid, succinic acid, sebacic acid, adipic acid, azelaic acid, 4,4'-dιphenyldιcarboxylιc acid, hexahydro-terephthalic acid or 1 ,2-bιs-p-carboxyphenoxyethane (optionally with a monocarboxylic acid, such as pivalic acid) with one or more glycols, particularly an aliphatic glycol, eg ethylene glycol, 1,3-propanedιol, 1,4-butanedιol, neopentyl glycol and 1 ,4-cyclohexanedιmethanol A polyethylene terephthalate film or a polyethylene naphthalate film is preferred A polyethylene terephthalate film is particularly preferred especially such a film which has been biaxially oriented by sequential stretching in two mutually perpendicular directions, typically at a temperature in the range 70 to 125°C, and preferably heat set, typically at a temperature in the range 150 to 250°C, for example as descnbed in GB-A-838,708

The polyester film substrate for production of a coated film according to the invention may be unonented, or preferably onented, such as uniaxially onented, or more preferably biaxially onented by drawing in two mutually perpendicular directions in the plane of the film to achieve a satisfactory combination of mechanical and physical properties Simultaneous biaxial onentation may be effected by extruding a thermoplastics polymenc tube which is subsequently quenched, reheated and then expanded by internal gas pressure to induce transverse onentation, and withdrawn at a rate which will induce longitudinal orientation Sequential stretching may be effected in a stenter process by extruding the thermoplastics substrate matenal as a flat extrudate

which is subsequently stretched first in one direction and then in the other mutually perpendicular direction. Generally, it is preferred to stretch firstly in the longitudinal direction, ie the forward direction through the film stretching machine, and then in the transverse direction. A stretched polyester substrate film may be, and preferably is, dimensionally stabilised by heat-setting under dimensional restraint at a temperature above the glass transition temperature thereof.

The substrate is suitably of a thickness from 6 to 300, particularly from 10 to 150, and especially from 12 to 50 μm.

By "acrylic resin" is meant a resin which comprises at least one acrylic and/or methacrylic component.

The acrylic resin component of the adherent layer of a film according to the invention is preferably thermoset and preferably comprises at least one monomer derived from an ester of acrylic acid and/or an ester of methacrylic acid, and/or derivatives thereof. In a preferred embodiment of the invention, the acrylic resin comprises greater than 50 mole %, preferably less than 98 mole %, more preferably from 60 to 97 mole %, especially from 70 to 96 mole %, and particularly from 80 to 94 mole % of at least one monomer derived from an ester of acrylic acid and/or an ester of methacrylic acid, and/or derivatives thereof. A preferred acrylic resin for use in the present invention preferably comprises an alkyl ester of acrylic and/or methacrylic acid where the alkyl group contains up to ten carbon atoms such as methyl, ethyl, n-propyl, isopropyl, π-butyl, isobutyl, terbutyl, hexyl, 2-ethylhexyl, heptyl, and n-octyl. Polymers derived from an alkyl acrylate, for example ethyl acrylate and butyl acrylate, together with an alkyl methacrylate are preferred. Polymers comprising ethyl acrylate and methyl methacrylate are particularly preferred. The acrylate monomer is preferably present in a proportion in the range 30 to 65 mole %, and the methacrylate monomer is preferably present in a proportion in the range of 20 to 60 mole %.

Other monomers which are suitable for use in the preparation of the acrylic resin of the adherent layer, which may be preferably copolymerised as optional additional monomers together with esters of acrylic acid and/or methacrylic acid, and/or derivatives thereof, include acrylonitrile, methacrylonitrile, halo-substituted acrylonitrile, halo-substituted methacrylonitrile, acrylamide, methacrylamide, N-methylol acrylamide, N-ethanol acrylamide, N-propanol acrylamide, N-methacrylamide, N-ethanol methacrylamide, N-methyl acrylamide, N-tertiary butyl acrylamide, hydroxyethyl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylamino ethyl methacrylate, itaconic acid, itaconic anhydride and half esters of itaconic acid.

Other optional monomers of the acrylic resin adherent layer polymer include vinyl esters such as vinyl acetate, vinyl chloroacetate and vinyl benzoate, vinyl pyridine, vinyl chloride, vinylidene chloride, maleic acid, maleic anhydride, styrene and derivatives of styrene such as chloro styrene, hydroxy styrene and alkylated styrenes, wherein the alkyl group contains from one to ten carbon atoms.

A preferred acrylic resin, derived from 3 monomers comprises 35 to 60 mole % of ethyl acrylate/30 to 55 mole % of methyl methacrylate/2 to 20 mole % of methacrylamide, and especially comprising approximate molar proportions 46/46/8% respectively of ethyl acrylate/methyl methacrylate/acrylamide or methacrylamide, the latter polymer being particularly effective when thermoset, for example in the presence of about 25 weight % of a methylated melamine-formaldehyde resin.

A preferred acrylic resin, derived from 4 monomers comprises a copolymer comprising comonomers (a) 35 to 40 mole % alkyl acrylate, (b) 35 to 40 mole % alkyl methacrylate, (c) 10 to 15 mole % of a monomer containing a free carboxyl group, and (d) 15 to 20 mole % of a monomer containing a sulphonic acid group and/or a salt thereof. Ethyl acrylate is a particulariy preferred monomer (a) and methyl methacrylate is a particularly preferred monomer (b). Monomer (c) containing a free carboxyl group ie a carboxyl group other than those involved in the polymerisation reaction by which the copolymer is formed, suitably comprises a copolymerisable unsatu rated carboxylic acid, and is preferably selected from acrylic acid, methacrylic acid, maleic acid, and/or itaconic acid; with acrylic acid and itaconic acid being particularly preferred. The sulphonic acid group monomer (d) may be present as the free acid and/or a salt thereof, for example as the ammonium, substituted ammonium, or an alkali metal, such as lithium, sodium or potassium, salt. The sulphonate group does not participate in the polymerisation reaction by which the adherent copolymer resin is formed. The sulphonic acid group monomer is preferably aromatic, and more preferably is p-styrene sulphonic acid and/or a salt thereof.

The weight average molecular weight of the acrylic resin can vary over a wide range but is preferably within the range 10,000 to 10,000,000, and more preferably within the range 50,000 to 200,000.

The acrylic resin preferably comprises at least 30% by weight of the layer and, more preferably, between 40% and 99%, particularly between 50% and 80%, and especially between 70% and 80% by weight relative to the total weight of the adherent layer. The acrylic resin is generally water-insoluble. The coating composition including the water-insoluble acrylic resin may nevertheless be applied to the polyester film substrate as an aqueous dispersion.

The at least one adhesion promoter present in the adherent layer is selected from a citrate ester, an adipate ester and an epoxidised vegetable oil. An epoxidised vegetable oil is particularly preferred.

A citrate ester adhesion promoter preferably has the general structure

C— O— R'

CH ₂ O

X— O-C— C— O— R ²

CH,

C— O— R ³

wherein at least one of R ¹ , R ² and R ³ , which may be the same or different, represent an alkyl group, optionally substituted, a cyclohexyl group, a benzyl group and/or a phenyl group. X represents hydrogen or Y-C(O)-, wherein Y represents hydrogen or an alkyl group preferably containing from 1 to 5, more preferably from 1 to 3 carbon atoms. X preferably represents a butyryl, and more preferably an acetyl group. R ¹ and/or R ² and/or R ³ preferably do not contain an aromatic group, and more preferably represent an alkyl group. The alkyl group preferably contains from 1 to 20, more preferably from

1 to 10, particularly from 2 to 8, and especially from 2 to 4 carbon atoms. The alkyl group is preferably a straight chain. If the alkyl group is branched, the group preferably contains one branch. R ¹ and/or R ² and/or R ³ may comprise more than one alkyl group linked by one or more ether groups, for example ethoxyethyl or butoxyethyl groups. In a preferred embodiment of the invention R ¹ and R ² and R ³ represent the same alkyl group, suitably an ethyl, butyl or hexyl group, and particulariy a butyl group.

Preferred citrate esters include triethyl citrate, tri-n-butyl citrate, tri-iso-butyl citrate, tri-2-ethylhexyl citrate, acetyl triethyl citrate, acetyl tri-n-butyl citrate, acetyl tri-iso-butyl citrate, acetyl tri-n-hexyl citrate, acetyl tri-2-ethylhexyl citrate, n-butyryl tri-n-hexyl citrate, mono-, di- and tri stearyl citrate and mono-iso-propyl citrate. A particularly preferred citrate ester is acetyl tributyl citrate, especially the n-butyl moiety.

The molecular weight of the citrate ester is preferably less than 1000, more preferably in the range from 250 to 800, particularly from 300 to 550, and especially from 350 to 450.

An adipate ester adhesion promoter preferably has the general structure

R' O C CH- CH ₂ CH ₂ CH ₂ C O R ²

wherein at least one of R ¹ and R ² , which may be the same or different, represent an alkyl group, optionally substituted, a cyclohexyl group, a benzyl group and/or a phenyl group. R ¹ and/or R ² preferably do not contain an aromatic group, and more preferably represent an alkyl group. The alkyl group preferably contains from 1 to 20, more preferably from 1 to 10, particularly from 2 to 8, and especially from 2 to 6 carbon atoms. The alkyl group may be a straight chain, or preferably branched, more preferably containing one branch. R ¹ and/or R ² may comprise more than one alkyl group linked by one or more ether groups, for example ethoxyethyl or butoxyethyl groups. In a preferred embodiment of the invention R ¹ and R ² represent the same alkyl group, suitably an ethyl, butyl or hexyl group, and particularly a 2-ethylhexyl group.

Preferred adipate esters include diethyl adipate, di-n-butyl adipate, di-iso-butyl adipate, di-n-hexyl adipate, di-iso-hexyl adipate, di-2-ethylhexyl adipate, di-n-octyl adipate, di-iso-octyl adipate, di-n-nonyl adipate, di-iso-nonyl adipate, dicapryl adipate, heptyl nonyl adipate, n-octyl n-decyl adipate, iso-octyl iso-decyl adipate, di-iso-octyl adipate, di-iso-decyl adipate, benzyl octyl adipate, dibutoxyethyl adipate and dibenzyi adipate. Di-2-ethylhexyl adipate is a particularly preferred adipate ester.

The molecular weight of the adipate ester is preferably less than 1000, more preferably in the range from 170 to 700, particulariy from 250 to 500, and especially from 300 to 400.

An epoxidised vegetable oil adhesion promoter is preferably formed by epoxidising naturally occurring vegetable oils, which are triglycerides, ie Westers of giyceroi and mixed unsaturated fatty acids. Preferred epoxidised vegetable oils include epoxidised linseed oil, epoxidised safflower oil, epoxidised soybean oil, epoxidised castor oil, epoxidised tall oil, epoxidised tung oil, epoxidised com oil, epoxidised cotton seed oil, epoxidised rape seed oil and epoxidised peanut oil. Epoxidised soybean oil is a particularly preferred

epoxidised vegetable oil. Epoxidised soybean oil is commercially available, for example ADK Cizer 0-130P, supplied by Asahi Denka, Japan.

The molecular weight of the epoxidised vegetable oil is preferably less than 10,000, more preferably in the range from 250 to 2000, particularly from 500 to 1500, and especially from 700 to 1300.

The adhesion promoter should preferably be present in the adherent layer in an amount of less than 10%, more preferably in the range from 0.05% to 8%, particularly from 0.1% to 5%, and especially from 0.5% to 2.0% by weight relative to the total weight of the adherent layer. The invention is not limited to the addition of a single adhesion promoter, and, if desired, two or more different adhesion promoters may be present in the adherent layer.

If desired, the adherent layer coating composition may also contain a cross-linking agent which functions to cross-link the layer thereby improving adhesion to the polyester film substrate. Additionally, the cross-linking agent should preferably be capable of internal cross-linking in order to provide protection against solvent penetration. Suitable cross-linking agents may comprise epoxy resins, alkyd resins, amine derivatives such as hexamethoxymethyl melamine, and/or condensation products of an amine, eg melamine, diazine, urea, cyclic ethylene urea, cyclic propylene urea, thiourea, cyclic ethylene thiourea, alkyl melamines, aryl melamines, benzo guanamines, guanamines, alkyl guanamines and aryl guanamiπes, with an aldehyde, eg formaldehyde. A useful condensation product is that of melamine with formaldehyde. The condensation product may optionally be alkoxylated. The cross-linking agent may suitably be used in amounts of up to 60%, preferably up to 50%, more preferably in the range from 20% to 45% by weight relative to the total weight of the adherent layer. A catalyst is also preferably employed to facilitate cross-linking action of the cross-linking agent. Preferred catalysts for cross-linking melamine formaldehyde include para toluene sulphonic acid, maleic acid stabilised by reaction with a base, morpholinium paratoluene sulphonate, and ammonium nitrate. The adherent layer coating composition may be applied before, during or after the stretching operation in the production of an oriented film. The adherent layer coating composition is preferably applied to the film substrate between the two stages (longitudinal and transverse) of a thermoplastics polyester film biaxial stretching operation. Such a sequence of stretching and coating is suitable for the production of an adherent layer coated linear polyester film, particularly polyethylene terephthalate film, substrate, which is preferably firstly stretched in the longitudinal direction over a

senes of rotating rollers, coated, and then stretched transversely in a stenter oven, preferably followed by heat setting

An adherent layer coated polyester, especially polyethylene terephthalate, substrate is suitably heated up to 240°C, preferably up to 220°C, in order to dry the aqueous medium, or the solvent in the case of solvent-applied compositions, and also to assist in coalescing and forming the coating into a continuous and uniform layer The cross-linking of cross-linkable coating compositions is also achieved at such temperatures

The adherent layer coating composition is preferably applied to the polyester film substrate by any suitable conventional technique such as dip coating, bead coating, reverse roller coating or slot coating

The ratio of substrate to adherent layer thickness may vary within a wide range, although the thickness of the adherent layer suitably should not be less than 0 001% nor greater than 10% of that of the substrate, and is preferably in the range from 0 002% to 5%, more preferably 0 004% to 0 5% of the substrate

The adherent layer is preferably applied to the polyester substrate at a dry coat weight in the range from 0 05 to 5 mgdm ² , especially 0 1 to 2 0 mgdm ² The thickness of the dry adherent layer is preferably less than 1 5 μm, more preferably in the range from 0 01 to 1 0 μm, and particularly 0 02 to 0 5 μm For films coated on both surfaces, each adherent layer preferably has an adherent coat weight and adherent layer thickness within the preferred ranges

Pnor to deposition of the adherent layer onto the polyester substrate, 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 adherent layer A preferred treatment, because of its simplicity and effectiveness, is to subject the exposed surface of the substrate to a high voltage electπcal stress accompanied by corona discharge Alternatively, the substrate may be pretreated with an agent known in the art to have a solvent or swelling action on the polyester substrate, for example a halogenated phenol dissolved in a common organic solvent eg a solution of p-chloro-m-cresol, 2,4-dιchlorophenol, 2,4,5- or 2,4 6- tπchlorophenol or 4-chlororesorcιnol in acetone or methanol

In a particularly preferred embodiment of the invention, an additional heat-sealable layer is present on the surface of the substrate layer, remote from the adherent layer The heat-sealable layer suitably compnses a polyester resin, particularly a copolyester resin derived from one or more dibasic aromatic carboxyhc acids, such as terephthalic acid, isophthalic acid and hexahydroterephthalic acid, and

one or more glycols, such as ethylene glycol, diethyiene glycol, triethylene glycol and neopentyl glycol. Typical copolyesters which provide satisfactory heat-sealable properties are those of ethylene terephthalate and ethylene isophthalate, especially in the molar ratios of from 50 to 90 mole % ethylene terephthalate and correspondingly from 50 to 10 mole % ethylene isophthalate. Preferred copolyesters comprise from 65 to 85 mole % ethylene terephthalate and from 35 to 15 mole % ethylene isophthalate, and especially a copolyester of about 82 mole % ethylene terephthalate and about 18 mole % ethylene isophthalate.

Formation of a polymeric film comprising heat-sealable and substrate layers may be effected by conventional techniques, for example by laminating together a preformed substrate and a preformed heat-sealable layer, or by casting the heat-sealable layer onto a preformed substrate. Conveniently, however, formation is effected by coextrusion of the substrate and heat-sealable layers, either by simultaneous coextrusion of the respective film-forming layers through independent orifices of a multi-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 coextruded polymeric film. The coextruded polymeric film is stretched to effect molecular orientation of the substrate, and preferably heat-set. Generally, the conditions applied for stretching the polymeric film will induce partial crystallisation of the heat-sealable layer polymer and it is therefore preferred to heat set under dimensional restraint at a temperature selected to develop the desired morphology of the heat-sealable layer. Thus, by effecting heat-setting at a temperature below the crystalline melting temperature of the heat-sealable polymer and permitting or causing the polymeric film to cool, the heat-sealable polymer will remain essentially crystalline. However, by heat-setting at a temperature greater than the crystalline melting temperature of the heat-sealing polymer, the latter will be rendered essentially amorphous. Heat-setting of a polymeric film comprising a polyester substrate and a copolyester heat-sealable layer is conveniently effected at a temperature within a range of from 175 to 200°C to yield a substantially crystalline heat-sealable layer, or from 200 to 250°C to yield an essentially amorphous heat-sealable layer. An essentially amorphous heat-sealable layer is preferred. The heat-sealable layer is preferably capable of forming a heat-seal bond to a metal sheet, by heating to soften the polymeric material of the heat-sealable layer and

applying pressure without softening or melting the polyester material of the substrate layer.

The metal sheet to which the polymeric film according to the invention can be laminated to, is suitably aluminium or steel or alloys thereof. An aluminium sheet is preferably of 0.02 to 0.4 mm thickness, and a steel sheet is preferably of 0.05 to 0.4 mm thickness. The steel may be nickel, zinc or tin plate, black plate, phosphated black plate or electrically chromium coated with both chromium metal and chromium oxide. A laminated metal sheet preferably comprises, in order, (i) a metal sheet, (ii) a polyester substrate layer, and (iii) an adherent layer comprising an acrylic resin and a citrate ester.

The metal sheet can be laminated to the polymeric film according to the present invention by any of the standard laminating techniques known in the art. EP-A-0312302, the disclosure of which is incoφorated herein by reference, describes a suitable laminating process. In a preferred process, the metal sheet is preheated to a temperature in the range from 140 to 350°C, a polymeric film is positioned on at least one side of the metal sheet, and the assembly is passed through nip rolls. The resultant laminate is reheated to a temperature in the range from 260 to 300 ^β C and rapidly quenched in a water bath.

Metal sheet laminated with polymeric film according to the invention, particularly when comprising a heat-sealable layer, can be used, inter alia, for forming cans, especially can lids containing an opening flap, and particulariy can lids containing a partially cut opening flap which has been sealed with a plastisol, preferably a polyvinyl chloride plastisol, as hereinbefore described. The structure of the can lid preferably comprises, in order, (i) a metal sheet, (ii) a heat-sealable layer, (iii) a polyester substrate layer, (iv) an acrylic resin and citrate ester adherent layer, and (v) a plastisol layer. The plastisol does not normally form a complete layer covering the surface of the adherent layer, but instead is applied in a line approximately 2 to 3 mm wide covering the partial cut in the opening flap. Thus, for tin lids with push-in circular openings, the plastisol is applied as a ring over the partial circular cut. Tin lids with push-in circular openings usually have two such openings, one for releasing the pressure, and the other for pouring out the contents.

One or more of the layers of a polymeric film according to the invention, ie substrate, adherent, heat-sealable 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 incoφorated in the substrate and/or adherent and/or heat-sealable layer(s), as appropriate, in particular, a polyester substrate may comprise a particulate filler, such as silica, of small particle size. Desirably, a filler, if employed in a substrate layer, should be present in a small amount, not exceeding 0.5%, preferably less than 0.2%, by weight of the substrate. In one embodiment of the invention a filler is present in the adherent layer, preferably at a concentration in the range from 0.01% to 2%, more preferably 0.03% to 1%. Silica is a suitable filler, preferably having an average particle size in the range from 0.005 μm to 2.0 μm, more preferably in the range from 0.3 μm to 0.7μm.

Adherent layer coated polyester films of the present invention can be used to form various types of composite structures by coating or laminating additional materials onto the adherent layer coated substrate film. For example the adherent layer coated polyester films may exhibit good adhesion when laminated with metal foils such as copper, aluminium and nickel, which can be used to form circuit boards. Vacuum bag lamination, press lamination, roll lamination or other standard lamination techniques can be utilised to form the aforementioned laminates.

Deposition of a metallic layer onto the, or each, adherent layer may be effected by conventional metallising techniques, for example by deposition from a suspension of finely-divided metallic particles in a suitable liquid vehicle, or, preferably, by a vacuum deposition process in which a metal is evaporated onto the adherent resin surface in a chamber maintained under conditions of high vacuum. Suitable metals include palladium, nickel, copper (and alloys thereof, such as bronze), silver, gold, cobalt and zinc, but aluminium is to be preferred for reasons both of economy and ease of bonding to the resin layer.

Metallising may be effected over the entire exposed surface of the adherent layer or over only selected portions thereof, as desired.

Metallised films may be prepared in a range of thicknesses governed primarily by the ultimate application for which a particular film is to be employed. A lacquer layer may be applied over the adherent layer to produce a film suitable for use as a drafting film. The lacquer layer preferably comprises one or more polyvinyl alcohol and/or polyvinyl acetal resins. Polyvinyl acetal resins can be suitably prepared by reacting polyvinyl alcohols with aldehydes. Commercially available polyvinyl alcohols are generally prepared by hydrolysing polyvinyl acetate. Polyvinyl alcohols are usually classified as partially hydrolysed (comprising 15 to 30% polyvinyl acetate groups) and completely hydrolysed (comprising 0 to 5% polyvinyl acetate

groups). Both types of polyvinyl alcohols, in a range of molecular weights, are used in producing commercially available polyvinyl acetal resins. The conditions of the acetal reaction and the concentration of the particular aldehyde and polyvinyl alcohol used will determine the proportions of hydroxyl groups, acetate groups and acetal groups present in the polyvinyl acetal resin. The hydroxyl, acetate and acetal groups are generally randomly distributed in the molecule. Suitable polyvinyl acetal resins include polyvinyl butyral, and preferably polyvinyl formal.

The lacquer layer preferably additionally comprises finely divided particulate material. When the polymeric film is to be used as a drafting material, the particulate material employed should impart a surface roughness to the film surface which can be marked and will retain the impressions of writing implements such as pencils, crayons and ink.

The finely divided particulate material may be selected from silica, silicates, ground glass, chalk, talc, diatomaceous earth, magnesium caΦonate, zinc oxide, zircon ia, calcium caΦonate and titanium dioxide. Finely divided silica is the preferred material for the production of drafting materials, together with which smaller quantities of the other materials may be incoφorated, to obtain the required degree of translucency and to increase the toughness and mark resistance of the coating.

Desirably, a filler, if employed in a lacquer layer, should be present in an amount of not exceeding 50% by weight of polymeric material, and the average particle size thereof should not exceed 15 μm, preferably less than 10 μm, and especially from 0.1 to 5 μm. The adherent layer coated films can be coated with a range of other organic and/or aqueous solvent based inks and lacquers, for example additional acrylic coatings, cellulose acetate butyrate lacquer, and diazonium coatings for drawing office applications. A variety of UV and solvent based inks can be adhered to polymeric films according to the invention, for example for use in the manufacture of a membrane touch switch.

Inks adhere on to the surface of the adherent layer of a polymeric film according to the invention, without any significant absoφtion of ink into, ie below a depth of a few Angstroms from the surface of, the adherent layer. Thus, an adherent layer as described herein is not suitable for use as an ink-absorbent layer in ink jet printing. Ink jet printing utilises a relatively thick, generally porous ink-absoΦent layer whereby drops of ink are absoΦed into substantially the entire thickness of the layer in order to build up a sufficient depth of colour in the final image. A membrane touch switch is usually constructed from three or four layers of polymeric film. A typical switch will have a graphics layer, beneath which are two

membranes or circuitry layers, each screen printed with conducting ink circuitry, separated by a spacer film with die cut holes. Contact between the circuits is achieved by finger pressure over the die cut holes. The graphics layer is usually reverse printed, and a wide variety of UV- cured inks and lacquers, and solvent inks and lacquers are used. There is therefore a need for the graphics layer to have good adhesion to a wide spectrum of inks and lacquers. Adherent layer coated films according to the invention are particulariy suitable for use as a graphics layer, or alternatively as a circuitry layer, in a membrane touch switch.

The adherent layer coated films can also be coated with photographic emulsions, such as light-sensitive gelatinous silver halide emulsions, especially conventional X-ray emulsions. The light-sensitive emulsion may optionally include any of the conventional additives normally used therein.

The adherent layer coated films can also be used to form magnetic cards, such as credit cards, and in particular a "pre-paid card", eg a telephone card, and an "intelligent card" capable, for example, of storing information relating to a number of financial transactions. A magnetic card preferably comprises, in order, (i) a magnetic layer, (ii) an adherent layer comprising an acrylic resin and a citrate ester, (iii) a polyester substrate layer, (iv) an adherent layer as defined in (ii) above, and (v) a graphics layer. A range of conventional binders may be used for the magnetic coating materials, which are well known to the man skilled in the art. A wide range of inks and lacquers may be used in the graphics layer, such as aqueous and organic solvent-based materials, particularly electron beam- and other UV-curable inks.

The adherent layer coated films can be coated with a vinyl chloride polymer layer to form a polyester/vinyl chloride polymer laminate. The vinyl chloride polymer may be a homopolymer or a copolymer containing at least 50 mole %, preferably at least 80 mole %, of polymerised vinyl chloride. The layer of vinyl chloride polymer may be formed on the adherent layer coated polyester film by any suitable technique. For example, a plastisol or an organosoi derived from a vinyl chloride polymer can be coated onto the adherent layer coated polyester film, and heated to gel the plastisol or the organosoi (after removal of the diluent) into a coherent layer. Pressure may also be applied, or alternatively the plastisol may be embossed. The plastisol may be foamable, so that a foamed vinyl chloride polymer layer is formed.

In an alternative method, a preformed vinyl chloride polymer film and an adherent layer coated polyester film are fed through hot rollers in order to bond the films together to form a laminate.

The polyester/vinyl chloride polymer laminate can be used as cushioned flooπng, solid (ie unfoamed) wail and floor tiles, identification cards, cladding, and adhesive tapes in packaging and electπcal applications

In a preferred embodiment of the invention, the polyester/vinyl chloπde polymer laminate can be used in making a floor coveπng, such as a floor tile The floor tile preferably compnses, in order, (i) a vinyl chloride polymer film, (u) an ink layer, (in) an acrylic resin and citrate ester adherent layer, and (iv) a polyester substrate layer The adherent layer, as defined herein, provides suφπsingly improved adhesion between the ink layer and the polyester substrate The ink layer provides the decorative pattern on the floor tile The ink layer preferably compnses a vinyl chloπde/acryhc ink

The invention is illustrated by reference to the accompanying drawings in which Figure 1 is a schematic sectional elevation, not to scale, of a polyester film having a substrate and adherent layer

Figure 2 is a similar schematic elevation of a coated film with an additional heat-sealable layer on the remote surface of the substrate

Figure 3 is a similar schematic elevation of a film as shown in Figure 2, with an additional metal sheet on the remote surface of the heat-sealable layer

Figure 4 is a similar schematic elevation of a film/metal sheet structure as shown in Figure 3, with an additional plastisol layer on the remote surface of the adherent layer

Figure 5 is a plan view of the surface of the structure shown in Figure 4 from the direction of view of the arrow

Referπng to Figure 1 of the drawings, the film comprises a polyester substrate layer (1) having an adherent layer (2) bonded to one surface (3) thereof The film of Figure 2 further compnses an additional heat-sealable layer (4), bonded to the second surface (5) of the substrate (1)

The film of Figure 3 further compnses an additional metal sheet (6), bonded to the remote surface (7) of the heat-sealable layer (4)

The film/metal sheet structure of Figure 4 further compnses an additional plastisol layer (8), bonded to the remote surface (9) of the adherent layer (2)

The plan view of Figure 5 illustrates the plastisol layer (8) in the form of a πng coveπng the partially cut circular flap (10), shown as a dotted line, in the metal sheet The invention is further illustrated by reference to the following examples Example 1 A polyethylene terephthalate film was melt extruded, cast onto a cooled rotating drum and stretched in the direction of extrusion to approximately 3 5 times its oπginal

dimensions. The monoaxially oriented polyethylene terephthalate substrate film was coated on one side with an adherent layer coating composition comprising the following ingredients:

Acrylic resin 163 ml (46% w/w aqueous latex of methyl methacrylate/ethyl acrylate/methacrylamide : 46/46/8 mole %, with 25% by weight methoxylated melamine-formaldehyde)

Citroflex A-4 37.5 ml (*2% w/w aqueous dispersion)

Cymel 385 234 ml

(10% w/w aqueous solution of melamine formaldehyde)

Ammonium nitrate 6 ml (10% w/w aqueous solution)

Demineralised water to 2.5 litres

* prepared by adding 189 ml of Citroflex A-4 (acetyl tri-n-butyl citrate, supplied by Morflex Inc., USA) to 756 ml of Synperonic NP 10 (nonyl phenol ethoxylate, supplied by ICI), stirring to form a white paste, adding 8.5 litres of demineralised water and stirring in a high speed mixer.

The coated film was passed into a stenter oven, where the film was stretched in the sideways direction to approximately 3.5 times its original dimensions. The coated biaxially stretched film was heat set at a temperature of about 220 ^β C by conventional means. Final film thickness was 30 μm. The dry coat weight of the adherent layer was approximately 0.4 mgdm ² and the thickness of the coating layer was approximately

0.04 μm.

The strength of adhesion of the adherent layer to the substrate was measured using the adhesion test described below.

The adherent layer of an A4 sheet of adherent layer coated film produced above was coated with Acheson 451 SS ink (a dielectric UV cured ink) using a No 3 Meyer bar. 25 mm strips of the same adherent layer coated film were placed, adherent layer down, on top of the ink to form a symmetrical laminate. The laminate was pressed by rolling thereon a 19 Kg cylindrical weight. The laminate was passed through a Primark

UV curing machine. The upper 25 mm strip was then manually peeled away from the lower A4 sheet. The peel was difficult to perform and resulted in the initial peel surfaces being white due to substrate delamination, ie cohesive failure of the polyethylene terephthalate. This result indicates very good adhesion at both the ink/adherent layer interface and the adherent layer/substrate interface.

Example 2

This is a comparative example not according to the invention. The procedure of Example 1 was repeated except that the coating composition did not contain any Citroflex A-4. In the adhesion test, the peel was easy to perform and resulted in the ink layer cleanly peeling away from one of the substrate layers, ie interfacial failure had occurred at the ink/adherent layer interface and/or the adherent layer/substrate interface. This result indicates poor adhesion. Example 3

The procedure of Example 1 was repeated except that instead of melt extruding a single polyethylene terephthalate polymer, separate streams of a substrate polymer of polyethylene terephthalate, and a heat-sealable polymer comprising a copolyester of 82 mole % ethylene terephthalate and 18 mole % of ethylene isophthalate were supplied from separate extruders to a single channel coextrusion assembly, and extruded through a film-forming die onto a cooled rotating drum. The adherent layer coating composition was applied to the surface of the polyethylene terephthalate substrate, as described in Example 1. Final film thickness of the substrate was 24 μm, and the heat-sealable layer was 6 μm thick.

The adherent layer of an A4 sheet of adherent layer coated film was coated with a 0.5 mm thick layer of a standard vinyl chloride polymer plastisol, using a No 500 Meyer Bar. A second adherent layer coated film was then placed on top of the plastisol layer. After drying in an oven at 180°C for 4 minutes, the assembly was allowed to cool to room temperature, cut into 20 mm wide strips and the peel strength thereof determined using an Instron operating at a speed of 500 mm min '. The mean peel strength obtained was 1400 g/20 mm indicating very good adhesion.

Example 4

This is a comparative example not according to the invention. The procedure of Example 3 was repeated except that the coating composition did not contain any Citroflex A-4. The mean peel strength obtained was 750 g/20 mm indicating only moderate adhesion.

Example 5

The procedure of Example 1 was repeated except that Edenol D81 (epoxidised soybean oil, supplied by Henkel Performance Chemicals, UK) was used in the adherent layer coating composition instead of Citroflex A-4. The strength of adhesion of the adherent layer to the substrate was measured using the adhesion test described in Example 1. The result indicated very good adhesion at both the ink/adherent layer interface and the adherent layer/substrate interface.

Example 6 The procedure of Example 3 was repeated except that Edenol D81 (epoxidised soybean oil, supplied by Henkel Performance Chemicals, UK) was used in the adherent layer coating composition instead of Citroflex A-4. The mean peel strength obtained in the vinyl chloride polymer plastisol adhesion test was 2050 g/20 mm indicating excellent adhesion. Example 7

The procedure of Example 1 was repeated except that di-2-ethylhexyl adipate was used in the adherent layer coating composition instead of Citroflex A-4.

The strength of adhesion of the adherent layer to the substrate was measured using the adhesion test described in Example 1. The result indicated very good adhesion at both the ink/adherent layer interface and the adherent layer/substrate interface.

Example 8

The procedure of Example 3 was repeated except that di-2-ethylhexyl adipate was used in the adherent layer coating composition instead of Citroflex A-4. The mean peel strength obtained in the vinyl chloride polymer plastisol adhesion test was 1500 g/20 mm indicating very good adhesion.

The above examples illustrate the improved properties of adherent layer coated films of the present invention.