Polymeric films have been used in a variety of applications, including magnetic tape, photographic film, display film and protective film. Polyester films, particularly biaxially oriented films of poly (ethylene terephthalate), are particularly useful since they have good mechanical properties, heat resistance and chemical resistance. The polyester film is, in many instances, used in association with a second layer, for instance a magnetic layer, a photosensitive layer or a hard protective layer. Accordingly, the polyester film must have good adhesion to the subsequently applied layer and for this purpose the polyester substrate is often provided with a primer layer.

For instance, JP-A-61/85436 discloses a polyester substrate onto which is coated an aqueous dispersion of a polyester comprising terephthalic acid at 40-95 mol% and 5- sodiumsulphoisophthalic acid (5-SIPA) at 0.5-5 mol% of the total acid component; and ethylene glycol and an etherglycol, wherein the etherglycol comprises 5-70 mol% of the total glycol component. The primer coating is described as providing good adhesion to magnetic coatings as well as good blocking resistance (windability).

Polyester films have also been used as substrates for printed information, images and other graphic work. In these applications, the polyester film usually has an ink-adherable or primer layer applied thereto. In certain applications, the ink used in the printing is a radiation-curable ink, for instance by ultraviolet radiation or by electron-beam. The use of radiation-curable inks offers advantages since they are generally faster to process, and do not require drying since they contain no solvents. The absence of solvent also means that fewer volatile organic compounds are released in comparison with thermally-cured inks.

For applications requiring multi-colour printing, or layering of multiple images, several separate printing steps may be required. After each printing step, the printed film is cured with radiation. Thus, the final printed multi-colour film has undergone multiple printing steps and multiple radiation-curing steps.

However, a problem with known films is that the adhesive power of the primer layer to anchor a radiation-curable ink to the substrate deteriorates with multiple radiation curing steps, especially when UV radiation is used to cure the ink. While the adhesion of a radiation-cured ink applied to the primed substrate in a first printing curing step is not necessarily adversely affected by subsequent printing and curing steps, the adhesive power of the primed substrate to subsequently applied radiation-cured inks will progressively deteriorate with each subsequent printing and curing cycle. Thus, when several different curable inks are printed and cured to produce an image, the primed film exhibits progressively poorer adhesion to subsequently applied inks as the number of curing steps increases.

Polycarbonate films have previously been used as printable substrates suitable for radiation-curable inks requiring multiple radiation curing steps. However, such films are generally more expensive than polyester substrates, and have poorer resistance to heat, organic solvents and other environmental damage. It would be desirable to provide a more durable and lower cost alternative to polycarbonate films.

It is an object of this invention to provide a film suitable as a substrate for radiation-curable inks, particularly UV-curable inks, which demonstrates improved adhesion after multiple radiation-curing steps. It is a further object of this invention to provide a more durable and lower cost substrate for radiation-curable inks, particularly UV-curable inks, which demonstrates improved adhesion after multiple radiation-curing steps.

According to the present invention, there is provided the use of a composition comprising an aqueous polyester having a glass transition point (Tg) of 20 to 85°C, preferably 30 to 85°C, preferably from 35 to 80°C, as a primer coating layer on a polymeric substrate for the purpose of improving adhesion of radiation-curable ink thereto in a printing process comprising a plurality of radiation-curing steps.

According to a further aspect of the present invention, there is provided a method of improving the adhesion of radiation-curable ink to a polymeric substrate in a printing process comprising a plurality of radiation-curing steps, said method comprising the application of a primer coating layer composition comprising an aqueous polyester having

a glass transition point (Tg) of 20 to 85°C, preferably 30 to 85°C, preferably from 35 to 80°C, to a surface of said substrate. The method of the invention optionally further comprises applying one or more radiation-curable ink (s) according to a process known in the art, and curing the ink (s) in one or more radiation-curing step (s).

The use and method described above are particularly useful wherein a plurality of radiation-curable inks and a plurality of curing steps are used to generate an image or design from said radiation-curable inks, particularly wherein said radiation is UV radiation.

Such an image may require at least 2, 3 or 4 curing steps and often as many as 10 or more curing steps. The coated substrate is therefore one which is particularly suitable as a substrate for an image consisting of a plurality of radiation-cured inks, and which requires a plurality of radiation curing steps, particularly wherein said radiation is UV radiation.

The inventors have unexpectedly found that the use of the primer coating layer described herein provides an adhesive force to subsequently applied radiation-curable inks which is relatively resistant to repeated exposure to radiation, particularly UV radiation, such that the adhesion of radiation-curable inks to the substrate does not diminish on successive curing steps.

Typically, radiation-curable inks comprise a mixture of monomer (s), oligomer (s), photoinitiator (s) and pigment (s). The present invention is of particular use for radiation- curable inks wherein the oligomer (s) comprise an acrylate resin and/or silicones, particularly acrylate resins selected from the group consisting of acrylic resins, urethane- acrylate resins, epoxy-acrylate resins and polyester-acrylate resins. The monomeric component (s) typically include acrylate monomer (s) and/or acrylated polyols or polyamines having mono-, bi-or tri-functionality. In addition, such inks generally have a low to medium degree of internal cross-linking (typically up to about 25% by weight of a cross-linking agent), and contain pigments, dyes or other colourants. The invention is advantageous in that it improves the adhesion between the substrate and the radiation- curable ink requiring multiple radiation curing steps, as is necessary when several inks are printed and cured sequentially to build up the desired image or graphic, such as in the production of coloured images.

The polymeric substrate layer is a self-supporting film or sheet by which is meant a film or sheet capable of independent existence in the absence of a supporting base. The substrate may be formed from any suitable film-forming polymer, preferably polyester, and particularly a synthetic linear polyester.

The synthetic linear polyesters useful as the substrate may be obtained by condensing one or more dicarboxylic acids or their lower alkyl (up to 6 carbon atoms) diesters, eg terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2, 6- or 2, 7-naphthalenedicarboxylic acid, succinic acid, sebacic acid, adipic acid, azelaic acid, 4,4'-diphenyldicarboxylic acid, hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane (optionally with a monocarboxylic acid, such as pivalic acid) with one or more glycols, particularly an aliphatic or cycloaliphatic glycol, e. g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol and 1,4-cyclohexanedimethanol. An aromatic dicarboxylic acid is preferred. An aliphatic glycol is preferred. Polyesters or copolyesters containing units derived from hydroxycarboxylic acid monomers, such as co-hydroxyalkanoic acids (typically C3-CI2) such as hydroxypropionic acid, hydroxybutyric acid, p-hydroxybenzoic acid, m-hydroxybenzoic acid, or 2-hydroxynaphthalene-6-carboxylic acid, may also be used.

In a preferred embodiment, the polyester is selected from polyethylene terephthalate and polyethylene naphthalate. Polyethylene terephthalate (PET) is particularly preferred.

The substrate may comprise one or more discrete layers of the above film-forming materials. The polymeric materials of the respective layers may be the same or different.

For instance, the substrate may comprise one, two, three, four or five or more layers and typical multi-layer structures may be of the AB, ABA, ABC, ABAB, ABABA or ABCBA type. Preferably, the substrate comprises only one layer.

Formation of the substrate may be effected by conventional techniques well-known in the art. Conveniently, formation of the substrate is effected by extrusion, in accordance with the procedure described below. In general terms the process comprises the steps of extruding a layer of molten polymer, quenching the extrudate and orienting the quenched extrudate in at least one direction.

The substrate may be uniaxially-oriented, but is preferably biaxially-oriented, as noted above. Orientation may be effected by any process known in the art for producing an oriented film, for example a tubular or flat film process. Biaxial orientation is effected by drawing in two mutually perpendicular directions in the plane of the film to achieve a satisfactory combination of mechanical and physical properties.

In a tubular process, simultaneous biaxial orientation may be effected by extruding a thermoplastics polymer 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, the substrate-forming polymer is extruded through a slot die and rapidly quenched upon a chilled casting drum to ensure that the polymer is quenched to the amorphous state. Orientation is then effected by stretching the quenched extrudate in at least one direction 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, i. e. the forward direction through the film stretching machine, and then in the transverse direction.

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. Alternatively, the cast film may be stretched simultaneously in both the forward and transverse directions in a biaxial stenter. Stretching is effected to an extent determined by the nature of the polymer, for example polyethylene terephthalate is usually stretched so that the dimension of the oriented film is from 2 to 5, more preferably 2.5 to 4.5 times its original dimension in the or each direction of stretching. Typically, stretching is effected at temperatures in the range of 70 to 125°C. Greater draw ratios (for example, up to about 8 times) may be used if orientation in only one direction is required. It is not necessary to stretch equally in the machine and transverse directions although this is preferred if balanced properties are desired.

A 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

polyester but below the melting temperature thereof, to induce crystallisation of the polyester. The actual heat-set temperature and time will vary depending on the composition of the film but should not be selected so as to substantially degrade the mechanical properties of the film. Within these constraints, a heat-set temperature of about 135° to 250°C is generally desirable, as described in GB-A-838708.

The film may be further heat-stabilised by heating it under low tension (i. e. with the minimum possible dimensional restraint) at a temperature above the glass transition temperature of the polyester but below the melting point thereof, in order to allow the majority of the inherent shrinkage in the film to occur (relax out) and thereby produce a film with very low residual shrinkage and consequently high dimensional stability. The tension experienced by the film during this heat-stabilisation step is typically less than 5 kg/m, preferably less than 3.5 kg/m, more preferably in the range of from 1 to about 2.5 kg/m, and typically in the range of 1.5 to 2 kg/m of film width. The temperature to be used for the heat stabilisation step can vary depending on the desired combination of properties from the final film, with a higher temperature giving better, i. e. lower, residual shrinkage properties. The duration of heating will depend on the temperature used but is typically in the range of 10 to 40 sec, with a duration of 20 to 30 sees being preferred. This heat stabilisation process can be carried out by a variety of methods, including flat and vertical configurations and either"off-line"as a separate process step or"in-line"as a continuation of the film manufacturing process.

Where the substrate comprises more than one layer, preparation of the substrate is conveniently effected by coextrusion, 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 multi-layer polymeric film, which may be oriented and heat-set as hereinbefore described. Formation of a multi-layer substrate may also be effected by conventional lamination techniques, for example by laminating together a preformed first layer and a preformed second layer, or by casting, for example, the first layer onto a preformed second layer.

The substrate layer is suitably of a thickness between about 5 and 350pm, preferably from 50 to about 300 um and particularly from about 75 to about 250, um.

The primer coating layer is a composition comprising an aqueous polyester having a glass transition point (Tg) of 20 to 85°C, preferably 30 to 85°C, preferably from 35 to 80°C. In one embodiment, the aqueous polyester has a Tg of from 40 to 85°C, and particularly from 45 to 80°C.

The aqueous polyester is a water-soluble or water-dispersible polyester. The aqueous polyester is a polyester produced from a polycarboxylic acid component such as terephthalic acid, isophthalic acid, phthalic acid, 2,6-naphthalenedicarboxylic acid, hexahydroterephthalic acid, 4,4'-diphenyldicarboxylic acid, phenylindanedicarboxylic acid, adipic acid, sebacic acid, sodium 5-sulfoisophthalic acid (5-SIPA), potassium 5- sulfoisophthalic acid, trimellitic acid or the like and a polyhydroxy compound component such as ethylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, 1,6- hexanediol, 1,4-cyclohexanedimethanol, glycerine, trimethylolpropane or an addition product of bisphenol A with an alkylen oxide (particularly ethylene oxide). In one embodiment, the aqueous polyester comprises a polyether component. In a further embodiment, the aqueous polyester comprises a component containing an S03 group (particularly an S03Na group) or a COO group (particularly a COONa group).

In a preferred embodiment, the primer composition contains an aqueous polyester which is water-dispersible, hereinafter referred to as AP1, wherein the polycarboxylic acid component of the aqueous polyester comprises terephthalic acid at 40-99.5 mol% and 5- sodiumsulphoisophthalic acid (5-SIPA) at 0.5-5 mol% of the total acid component; and the polyhydroxy component of the aqueous polyester comprises ethylene glycol and an etherglycol, wherein the etherglycol comprises 5-70 mol% of the total glycol component, and wherein the etherglycol has formula (I), H (OCnH2n) m-0-A-0- (CnH2nO), H (I) wherein A is a bivalent aromatic hydrocarbon having 6-20 carbon atoms;

n is an integer 2-4; and 1 and m, which can be the same or different, are 0,1 or higher wherein 2 < (m+l) < 10.

In the embodiment of the invention wherein the primer composition comprises aqueous polyester AP1, the content of terephthalic acid in the aqueous polyester is preferably greater than 60 mol%, more preferably greater than 80 mol% and most preferably greater than 85 mol%. Preferably, the content of the 5-SIPA component in the aqueous polyester is 1 to 5 mol% and more preferably 1 to 3 mol%. The aqueous polyester contains terephthalic acid and 5-SIPA in the stated ranges but can also contain other polycarboxylic acids such as those mentioned above, including aromatic dicarboxylic acids such as isophthalic acid, phthalic acid and diphenyldicarboxylic acid, and/or aliphatic dicarboxylic acids such as adipic acid, azeleic acid and sebacic acid, and/or alicyclic dicarboxylic acids such as 1,3- cyclohexanedicarboxylic acid and 1, 4- cyclohexanedicarboxylic acid. Of these, aromatic dicarboxylic acids are preferred, preferably isophthalic acid. The other dicarboxylic acids are preferably present at levels of no more than 50 mol%, preferably, no more than 30 mol%, preferably no more than 15 mol% and preferably no more than 10 mol% of the total acid component of the aqueous polyester.

In the embodiment of the invention wherein the primer composition comprises aqueous polyester AP 1, the preferred content of the etherglycol is 10-60 mol%. The group A in the etherglycol of formula (I) is preferably selected from [1,4-phenyl], [4, 4'-biphenyl], [4,4'- diphenylether], [4, 4'-diphenylsulphone] and a [bisphenol A] residue, preferably a [bisphenol A] residue. Thus, A is preferably selected from:

In the etherglycols of formula (1), it is preferred that n = 2 and that 2 < (m+l) < 8. The ethylene glycol component may be replaced by up to 20 mol% or up to 10 mol% by other aliphatic glycols such as the glycols mentioned above, including 1,4-butanediol and 1,4- cyclohexanedimethanol.

In the embodiment of the invention wherein the primer composition comprises aqueous polyester API, the IV of the copolyester is preferably 0.2-0. 8. The copolyester API is essentially insoluble in water, meaning that if 3-5 mm chips are placed in hot water at 90°C for 3 hours, the weight loss is less than 1 wt%, preferably less than 0. 5wt% and most preferably less than 0.1 wt%.

Suitable aqueous polyesters for use in the present invention include: (a) a copolyester comprising terephthalic acid (90 mol %), isophthalic acid (6 mol %), potassium 5-sulfoisophthalate (4 mol %), ethylene glycol (95 mol %) and neopentyl glycol (5 mol %) (Tg=68°C) ; (b) a copolyester comprising 2,6-naphthalenedicarboxylic acid (50 mol %), terephthalic acid (46 mol %), sodium 5-sulfoisophthalate (4 mol %), ethylene glycol (70 mol %), and addition product of bisphenol A with ethylene oxide (30 mol %) (Tg=80°C) ; (c) a copolyester comprising terephthalic acid (85 mol %), isophthalic acid (15 mol %), ethylene glycol (57 mol %), 1,4-butanediol (40 mol %), diethylene glycol (2 mol %), and polyethylene glycol (1 mol %) (Tg=47°C) ; (d) a copolyester comprising terephthalic acid (70 mol %), isophthalic acid (28 mol %), sodium 5-sulfoisophthalate (2 mol %), ethylene glycol (70 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (30 mol %) (Tg=30°C) ; (e) a copolyester comprising 2,6-naphthalenedicarboxylic acid (71 mol %), isophthalic acid (15 mol %), sodium 5-sulfoisophthalate (14 mol %), ethylene glycol (70 mol %), and addition product of 1 molar bisphenol A with 2 molar ethylene oxide (30 mol %) (Tg=90°C) ; (f) a copolyester comprising terephthalic acid (50 mol %), isophthalic acid (48 mol %), sodium 5-sulfoisophthalate (2 mol %), ethylene glycol (50 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (50 mol %) (IV = 0.55) ;

(g) a copolyester comprising terephthalic acid (85 mol %), isophthalic acid (13 mol %), sodium 5-sulfoisophthalate (2 mol %), ethylene glycol (50 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (50 mol %) (IV = 0.61) ; (h) a copolyester comprising terephthalic acid (85 mol %), isophthalic acid (13 mol %), sodium 5-sulfoisophthalate (2 mol %), ethylene glycol (70 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (30 mol %) (IV = 0.65) ; and (i) a copolyester comprising terephthalic acid (85 mol %), isophthalic acid (11 mol %), sodium 5-sulfoisophthalate (4 mol %), ethylene glycol (70 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (30 mol %) (IV = 0.58).

(j) a copolyester comprising terephthalic acid (97 mol %), isophthalic acid (1 mol %), sodium 5-sulphoisophthalate (2 mol %), ethylene glycol (60 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (40 mol %) (IV = 0.58) ; (k) a copolyester comprising naphthalene dicarboxylic acid (60 mol%), isophthalic acid (36 mol %), sodium 5-sulphoisophthalic acid (4 mol %), ethylene glycol (60 mol %), and addition product of 1 mole bisphenol A with 2 moles propylene oxide (40 mol %).

The values of mol% for the components of the copolyesters (a) to (k) above represent the relative proportions of that component in terms of the total acidic component, or in terms of the total glycol component, as appropriate.

The primer coating layer is preferably applied in the form of a composition in which the aqueous polyester is present in an amount of 0.3 to 20% by weight, preferably 2 to 8% by weight, based on the total weight of the coating composition.

The preparation and further exemplification of suitable aqueous polyesters are disclosed in JP-A-61/85436 and US-5910356, the disclosures of which is incorporated herein by reference.

The primer layer may be applied to one or both surfaces of the substrate.

The primer coating composition may be applied to an already oriented substrate.

However, application of the primer coating composition is preferably effected before or during the stretching operation (s). For instance, the primer coating may be applied to the

film substrate between the two stages (longitudinal and transverse) of a biaxial stretching operation. Thus, the film substrate may be stretched firstly in the longitudinal direction over a series of rotating rollers, coated with the primer coating composition, and then stretched transversely in a stenter oven, and preferably then heat-set. Alternatively, however, the primer coating composition is applied before any stretching of the cast film.

Prior to deposition of the primer coating composition onto the 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 coating.

Physical surface-modifying treatments include flame treatment, ion bombardment, electron beam treatment, ultra-violet light treatment and corona discharge. A preferred treatment, because of its simplicity and effectiveness is to subject the exposed surface of the substrate to a high voltage electrical stress accompanied by corona discharge. Corona discharge may be effected in air at atmospheric pressure with conventional equipment using a high frequency, high voltage generator, preferably having a power output of from 1 to 20 kw at a potential of 1 to 100 kv. Discharge is conventionally accomplished by passing the film over a dielectric support roller at the discharge station at a linear speed preferably of 1.0 to 500 m per minute. The discharge electrodes may be positioned 0.1 to 10.0 mm from the moving film surface.

Alternatively, or in addition to a physical surface-modifying treatment, the primer coating composition may be applied in combination with a surfactant which is chemically unreactive with the primer. This surfactant promotes wetting of the substrate with the aqueous coating solution in order to improve adhesion, and may be selected from anionic and nonionic surfactants such as polyoxyethylene alkylphenyl ether, polyoxyethylene-fatty acid ester, sorbitan fatty acid ester, glycerine fatty acid ester, fatty acid metal soap, alkyl sulfate, alkyl sulfonate and alkyl sulfosuccinate. The surfactant may be used in a proportion of 0 to 10% by weight, preferably 0 to 8% by weight, preferably 0 to 4% by weight based on the composition forming the primer coating layer.

The primer coating composition may be applied to the polyester film substrate as an aqueous solution, an aqueous dispersion or emulsion by any suitable conventional coating

technique such a gravure roll coating, reverse roll coating, dip coating, bead coating, slot coating or electrostatic spray coating. Preferably, the application of the primer coating is effected to provide a dry coat weight within the range 0.1 to 10 mg/dm2, preferably 0.2 to 5 mg/dm2, and typically within the range 0.2 to 4 mg/dm2.

In order to provide an effective bond to the radiation-curable inks, the primer coating layer preferably has a thickness in the range of about 0.01 to 1. 0 um, preferably in the range of about 0.02 to about 0.25 urn. If the primer coating is not thick enough, the adhesive force will be insufficient, while if coating is too thick, the haze and/or handleability of the film may be adversely affected. The ratio of substrate thickness to the thickness of the primer coating layer may vary within a wide range, although the thickness of the primer coating layer preferably should not be less than 0.004% nor greater than about 10% of that of the substrate.

The primed film may be subjected to a further physical surface-modifying treatment to improve the bond between that surface and a subsequently applied coating.

One or more of the layers of the coated film described herein 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, radical scavengers, UV absorbers, fire retardants, thermal stabilisers, anti-blocking agents, surface active agents, slip aids, optical brighteners, gloss improvers, prodegradents, viscosity modifiers and dispersion stabilisers may be incorporated in the substrate and/or coating layer (s) as appropriate. In particular the substrate and/or primer layer, may comprise a particulate filler. The filler may, for example, be a particulate inorganic filler or an incompatible resin 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 layer polymer, at the highest temperature encountered during extrusion and fabrication of the layer. The presence of an incompatible resin usually results in a voided layer, by which is meant that the layer 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. 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 polymeric resin may be cross-linked.

Particulate inorganic fillers include conventional inorganic fillers, and particularly metal or metalloid oxides, such as alumina, silica (especially precipitated or diatomaceous silica and silica gels) and titania, calcined china clay 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 substrate layer polymer.

The inorganic filler, if used, is preferably finely-divided, and 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) thereof is preferably in the range from 0.01 to 5 um, more preferably 0.05 to 1. 5 um, and particularly 0.15 to 1. 2 um.

The size distribution of inorganic filler particles is also an important parameter, for example the presence of excessively large particles can result in the film exhibiting unsightly'speckle', i. e. where the presence of individual filler particles in the film can be discerned with the naked eye. It is preferred that none of the inorganic filler particles should have an actual particle size exceeding 30 um. Particles exceeding such a size may be removed by sieving processes which are known in the art. However, sieving operations are not always totally successful in eliminating all particles greater than a chosen size. In practice, therefore, the size of 99.9% by number of the inorganic filler particles should not

exceed 30 um, preferably should not exceed 20 um, and more preferably should not exceed 15 um. Preferably at least 90%, more preferably at least 95% by volume of the inorganic filler particles are within the range of the volume distributed median particle diameter : 1 : 0. 8 um, and particularly 0. 5 urn.

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.

If employed in a coating layer, the filler particles, such as AerosilTM OX50 or SeahostarTM KEP30 or KEP50, may be present in an amount of from about 0 to about 5%, and more preferably 0.1 to 2.5% by weight relative to the weight of the polymer of the coating layer.

The components of the composition of a layer may be mixed together in a conventional manner. For example, by mixing with the monomeric reactants from which the layer polymer is derived, or the components may be mixed with the polymer by tumble or dry blending or by compounding in an extruder, followed by cooling and, usually, comminution into granules or chips. Masterbatching technology may also be employed.

In a preferred embodiment, the substrate layer is optically clear, preferably having a % of scattered visible light (haze) of <6%, more preferably <3. 5 % and particularly <2%, measured according to the standard ASTM D 1003. In this embodiment, the substrate layer is unfilled or filler is typically present in only small amounts, generally not exceeding 0.5% and preferably less than 0.2% by weight of the substrate polymer.

In an alternative embodiment of the invention, the substrate layer is opaque and highly filled, preferably exhibiting a Transmission Optical Density (TOD) (Sakura Densitometer ; type PDA 65 ; transmission mode) in the range from 0.1 to 2.0, more preferably 0.2 to 1. 5, more preferably from 0.25 to 1.25, more preferably from 0.35 to 0.75 and particularly 0.45 to 0.65. The substrate layer is conveniently rendered opaque by incorporation into the polyester blend of 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, as hereinbefore described. The amount of filler present in an opaque substrate layer is preferably in the range from 1% to 30%, more preferably 3% to 20%, particularly 4% to 15%, and especially 5% to 10% by weight, based on the weight of the substrate layer polymer.

The incorporation of a particulate material into a coating layer can assist in the handling of the film, for instance to improve windability and minimise or prevent"blocking". In the embodiments described herein wherein the film is only primed and coated on one side of the substrate, the reverse side of the substrate may optionally be coated with a"slip coating"to improve the handleability of the film. Suitable slip coatings may comprise potassium silicate, such as that disclosed in, for example, US Patent Nos. 5925428 and 5882798, the disclosures of which is incorporated herein by reference. Alternatively, a slip coating may comprise a discontinuous layer of an acrylic and/or methacrylic polymeric resin optionally further comprising a cross-linking agent, as disclosed in, for example, EP- A-0408197, the disclosure of which is incorporated herein by reference.

Alternatively, the reverse side of the substrate may optionally be coated with a protective coating, or a"hard-coat", in order to improve the durability and/or scratch-resistance of the film. Suitable hard-coatings include radiation-cured acrylics (which are generally highly cross-linked), such as disclosed in US-6,265, 133 and US-5, 998,013 ; radiation-cured urethane-acrylics, such as disclosed in US-6, 110, 988 ; hybrid organic/inorganic coatings, such as disclosed in US-6,072, 018 and US-5,665, 814; electron beam-cured compositions, such as disclosed in US-6,017, 974; and thermally-cured compositions, such as methylolmelamine compositions (for instance as disclosed in US-4,442, 177), organopolysiloxanes compositions (for instance as disclosed in US-3,707, 397), polyester- melamine or acrylic-melamine compositions (for instance as disclosed in US-3,843, 390), and allyl resins (for instance as disclosed in US-2,332, 461). Other hard coatings are disclosed in US-3,968, 305, US-3,968, 309, US-4,198, 465, US-4,319, 811. The disclosures of these documents are incorporated herein by reference.

In one embodiment, the reverse side of the substrate is first coated with the primer coating described herein prior to application of a hard-coat protective layer, particularly a

radiation-cured acrylic hard-coat protective layer, on the reverse side of the substrate. The primer coating has good solvent-resistance to the solvents typically used in the application of a hard-coat, thereby improving adhesion of the hard-coat to the substrate.

The provision of a printing film which has a symmetric cross-section, i. e. a film comprising a substrate having the same coating composition on both sides thereof, provides particular advantages in that the manufacturing process thereof becomes more efficient and cost-effective. The symmetric film also provides advantages of ease of use and handling in applications wherein a subsequent coating is applied onto only one side of the film, such as a subsequently applied hard coating, or in applications wherein opposite sides of the film are coated with different compositions.

The printed films obtainable using the invention may be used in a variety of applications, such as the graphics layer in membrane touch switches (including panels on electronic equipment such as microwave ovens etc). A membrane touch switch is usually constructed from three, four or more layers of polymeric film. A typical switch will have a graphics layer, beneath which are two membranes or circuitry layers, each of which is 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 using a wide variety of radiation-cured inks and lacquers, as well as thermally-cured solvent inks and lacquers. The circuitry layers may also be printed using radiation-and/or thermally-cured dielectric and conductive inks. The graphics layer is generally transparent so the image can be viewed when reverse printed, while the circuit layers are usually hazy. The primed films described herein are particularly suitable for use as a graphics layer in a membrane touch switch, but may also be used as the circuitry layer therein. The coated film of the present invention may also be of use in other applications requiring radiation-resistant adhesion, particularly UV-resistant adhesion.

The following test methods may be used to determine certain properties of the polymeric film: (i) Wide angle haze is measured using a Hazegard System XL-211, according to ASTM D 1003-61.

(ii) Adhesion of the coated film to UV-curable inks was measured using the following inks: Dense Black 071-009 (Sericol Inc.) Opaque White MSK-1046 (NorCote International Inc.) The UV curable inks were applied to the coated film using a 390 mesh screen printing rig. This allowed a uniform amount of coating to be applied to the film in a reproducible manner. The inks were cured using a UV curer (at 46 feet/min using a Type H bulb at 263 mJ/cm2). Separate samples of the coated film were passed through the UV curer 9 times before the inks were applied. The ink curing stage was thus the tenth curing iteration. A control sample was also tested without the initial 9 UV passes. The inked surfaces were then scored with a cross hatch tool.

The tool consists of six blades with blade protrusion approximately 0.006 inch. The film is scored four times at a 45 degree angle to the machine direction of the film.

A second set of four score marks is made at right angles to the first. This gives four areas of 25 scored squares. To the scored area, a length of adhesive tape (3M 600 Tape) approximately 75 mm long is applied and smoothed out to exclude any trapped air. The tape is then pulled away, keeping the tape at an acute angle to the film surface. The number of squares of ink that are removed by the tape give a percentage failure for ink adhesion.

(iii) Intrinsic Viscosity is measured on a 1% (weight/volume) solution of the polymer in o-chlorophenol at a temperature of 25°C in accordance with standard methods.

The invention is further illustrated by the following examples. It will be appreciated that the examples are for illustrative purposes only and are not intended to limit the invention as described above. Modification of detail may be made without departing from the scope of the invention.

EXAMPLES Example 1 A molten web of polyethylene terephthalate (PET) 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 polymer to provide an

amorphous film. The film was passed through a corona discharge treater at about 1.8 to 2.0 kW (about 1.3 J/cm2) each side, and then coated on both sides with a primer coating composition comprising the following ingredients: 1. A copolyester comprising terephthalic acid (97 mol %), isophthalic acid (1 mol %), sodium 5-sulphoisophthalate (2 mol %), ethylene glycol (60 mol %), and addition product of 1 molar bisphenol A with 4 molar ethylene oxide (40 mol %) (IV = 0. 58) ; in the form of a 21 % w/w aq. dispersion: 5.6 litres.

The dispersion may be prepared by heating the copolyester in a mixture of water and dioxan containing a surfactant, removing the dioxan by distillation. Alternatively, the surfactant is added after removal of the dioxan. The surfactant was nonylphenylethoxylate (at approximately 15% by weight (as solids)) although other surfactants (including alkyl ethoxylates, and particularly alkyl ethoxylate propylates (alkyl-EOPOs) ) may also be used.

2. Seahostar KEP30 (powder supplied by Nippon Shokubai; incorporated into the coating layer to improve the slip properties of the film; used as a 15% w/w dispersion in water with 0. 035% dodecylsulphate dispersion aid): 150 mls ; and 3. Distilled Water: 19.3 litres The coated film was dried in a conventional web drier and then reheated and drawn about 3.5 times its original length in the longitudinal direction at a temperature of about 80°C.

The monoaxially oriented PET film was then 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 215°C by conventional means and then passed through a corona discharge treater at about 2.5 to 3.0 kW (approximately 0.15 J/cm2). The final film thickness was 175 Fm. The dry coat weight of the coating layer was approximately 0. 50mgdm, and the thickness was approximately 0. 050 um.

Example 2 This is a comparative example not according to the invention. The procedure of example 1 was repeated using an acrylic coating composition comprising the following ingredients: 1. Acrysol WS-50 (38% w/w aq. Dispersion supplied by Rohm & Haas): 5.8 litres.

2. Distilled Water: 119 litres.

The composition (prepared at 4% solids) was applied off-line to the biaxially stretched, heat-set and corona-treated substrate, using a No. 0 Meyer bar and dried at 120°C for 90 seconds. The dry coat weight was about 1.40 mgdm~2.

Example 3 This is a comparative example not according to the invention. The procedure of Example 2 was repeated using a crosslinking agent (modified melamine formaldehyde resin (Cymel (g) 301; Cytec Industries) ) at about 12% of the resin solids.

Example 4 The procedure of example 1 was repeated except that the primer coating composition had the following composition: 1. An aqueous copolyester of naphthalene dicarboxylic acid (60 mol%), isophthalic acid (36 mol%), sodium 5-sulphoisophthalic acid (4 mol%), ethylene glycol (60 mol%), and addition product of 1 mole bisphenol A with 2 moles propylene oxide (40 mol %).

(14% w/w aq. dispersion): 4965 grams. The dispersion may be made as described for Example 1. In this case, the surfactant used was alkyl-EOPO (5. 3% as solid).

2. Aerosil OX-50 (silica powder supplied by Degussa-Huls AG; incorporated into the primer coating to improve the slip properties of the film ; used as a 10% w/w dispersion in water): 375 ml; 3. Emulgator K30 (10% in water): 40 ml; 4. Distilled water: 9.6 litres The films of the above Examples were analysed using the adhesion test described herein and the results are given in Table 1.

Table 1 Example Percentage ink removed Black 1 pass Black 10 passes White 1 pass White 10 passes 1 0 0 2 0 2 100 100 90 100 3 30 90 10 90 4-2-10