Polyester film comprising polymeric phosphonate flame retardant

Abstract

To provide a polyester film which exhibits excellent flame retardancy and excellent optical properties, particularly high clarity and high transparency of the film, for instance as measured by haze and TLT.SOLUTION: A polyester film comprises a polymeric phosphonate flame retardant in an amount of from about 1.0 to about 25.0 mass% relative to the total mass of the film, and further comprises at least one metal cation selected from the group consisting of Group I and Group II metal cations.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a flame-retardant polyester film, as well as a method for manufacturing the film and its use.

Background Art

[0002] The advantageous mechanical properties, dimensional stability and optical properties of polyester films are well known. However, most polymer materials used in building, textile, furniture and electrical applications are flammable. There is a strong desire to reduce the flammability of materials and make them safer to use. The most common approach is to add one or more flame retardant elements to the flammable polymer system, but in that case, these can cause other problems. For example, it has been found to be difficult to achieve a sufficiently high addition level without adversely affecting subsequent processing and / or the physical properties of the polymer composition or polymer film. In particular, it has been difficult to provide a flame-retardant polyester film that exhibits advantageous optical properties (such as high transparency) and / or a low shrinkage rate. Furthermore, it has also been difficult to provide a flame-retardant polyester film without loss of crystallinity that can adversely affect the mechanical properties of the film. The present invention aims to solve one or more of the problems mentioned above, in particular to provide a polyester film with excellent flame retardancy that does not impair the optical properties of the film, in particular without a hard-to-commit reduction in transparency and / or transparency. The present invention has the special purpose of providing a polyester film with excellent flame retardancy and excellent optical properties, in particular high transparency and high clarity of the film as measured by, for example, haze and TLT.

[0003] German Patent Application Publication No. 10043779 describes a biaxially stretched multilayer film suitable for displays and protective glazing, comprising a base layer and a cover layer of a bisbenzene-modified thermoplastic polymer. The cover layer comprises a mixture or blend of two components. The first component may be polyethylene terephthalate (PET), and the second component may be a mixture or blend of a copolymer derived from isophthalic acid, a sulfomonomer, an aliphatic or alicyclic glycol, and optionally an aliphatic dicarboxylic acid. The film may optionally contain a flame retardant such as dimethyl methylphosphonate. German Patent Application Publication No. 10035327 describes an opaque biaxially stretched polyester film comprising at least one layer containing 2 to 60% by mass of a cycloolefin copolymer containing a flame retardant and at least one adhesion promoting surface. The flame retardant may be an organic phosphorus compound such as dimethyl methylphosphonate.

[0004] Japanese Unexamined Patent Application Publication No. 2004131669 describes a biaxially stretched polymer film for a magnetic recording medium, comprising a polyester and a polymer containing a phosphonic acid residue and a divalent phenol residue, and is reported to provide excellent rigidity and toughness. Japanese Unexamined Patent Application Publication No. 2006274112 describes a biaxially stretched polyester film for a magnetic recording medium showing excellent dimensional stability. The film contains an enthalpy relaxation accelerator which may be a metal salt such as a Group II metal salt, and a phosphorous acid compound such as dimethyl phenylphosphonate. Japanese Unexamined Patent Application Publication No. 06200131 describes a PEN-based polyester and a film produced therefrom, showing improved electrical properties and suitable for an electromagnetic tape. The polyester contains antimony, magnesium and a phosphorus element, and the phosphorus element may be selected from dimethyl methylphosphonate. Korean Patent No. 101234613 describes an optical polyester film having high heat resistance, which includes a polyester substrate layer and an adhesive polymer resin undercoat layer on at least one surface of the film. The polyester film contains flame retardant particles selected from carboxyphosphinic acid and dimethyl methylphosphonate.

Summary of the Invention

[0005] According to a first aspect of the present invention, there is provided a polyester film, preferably a stretched polyester film, which contains a polymer phosphonate flame retardant in an amount of about 1.0 to about 25.0% by mass based on the total mass of the film and further contains at least one metal cation selected from the group consisting of Group I and Group II metal cations. Preferably, there is provided a polyester film (preferably a stretched polyester film) which contains a polymer phosphonate flame retardant in an amount of about 1.0 to about 25.0% by mass based on the total mass of the film and further contains at least one metal cation selected from the group consisting of sodium and potassium metal cations. The inventors have surprisingly found that the phosphonate flame retardant in combination with at least one metal cation selected from the group consisting of Group I and Group II metal cations (especially sodium and potassium metal cations) is particularly beneficial in providing a polyester film that exhibits excellent flame retardancy and high transparency and clarity of the film. Surprisingly, the inventors have found that the polyester film exhibits these properties even with a relatively low addition amount of the phosphonate flame retardant.

[0006] The polyester film is a self-supporting film or sheet, which means a film or sheet that can exist independently without a supporting base. The film may be an unstretched (e.g., cast) polyester film. Preferably, the film is uniaxially or biaxially stretched, more preferably biaxially stretched. The polyester constituting the film is preferably a synthetic linear polyester. The polyester constituting the film is preferably crystallizable. The polyester is thermoplastic. Suitable polyesters are obtainable by condensation of one or more dicarboxylic acids or their lower alkyl diesters (up to 6 carbon atoms) with one or more diols. The dicarboxylic acid component contains at least one aromatic dicarboxylic acid, preferably terephthalic acid, isophthalic acid (IPA), phthalic acid, 1,4-, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, preferably terephthalic acid (TA) or 2,6-naphthalenedicarboxylic acid, preferably terephthalic acid. The polyester may optionally contain one or more residues derived from other dicarboxylic acids, which are, for example, 4,4'-diphenyldicarboxylic acid, hexahydroterephthalic acid, 1,10-decanedicarboxylic acid, especially aliphatic dicarboxylic acids of the general formula C n H 2n (COOH)2 (wherein n is from 2 to 8), such as succinic acid, glutaric acid, sebacic acid, adipic acid, azelaic acid, suberic acid or pimelic acid, preferably sebacic acid, adipic acid and azelaic acid, more preferably azelaic acid.

[0007] The diols are preferably aliphatic and cycloaliphatic diols, such as ethylene glycol (EG), 1,3-propanediol, 1,4-butanediol and 1,4-cyclohexanedimethanol (CHDM), preferably ethylene glycol and 1,4-cyclohexanedimethanol (CHDM), preferably ethylene glycol. Polyethylene terephthalate (PET) or polyethylene 2,6-naphthalate (PEN), especially PET is a preferred polyester. Preferred PET and PEN polyesters may optionally contain relatively small amounts of one or more residues derived from the other dicarboxylic acids and / or diols described above. When such small amounts are present, the total amount of the other dicarboxylic acids is preferably less than 10 mol%, preferably less than 5 mol%, preferably less than 1 mol% of the total dicarboxylic acid fraction of the polyester, and / or the total amount of the other diols is preferably less than 15 mol%, preferably less than 10 mol%, preferably less than 5 mol% of the total diol fraction of the polyester.

[0008] The polyester preferably contains only one dicarboxylic acid, preferably an aromatic dicarboxylic acid, preferably terephthalic acid or 2,6-naphthalenedicarboxylic acid, preferably terephthalic acid. The polyester preferably contains only one diol, preferably an aliphatic diol, preferably ethylene glycol. Preferably, the polyester contains one aromatic dicarboxylic acid and one aliphatic diol. The polyester resin forming the film is the main component of the film and constitutes at least 50%, preferably at least 60%, preferably at least 65%, preferably at least 70%, preferably at least 75%, preferably at least 80%, more typically at least 85%, more typically at least 90% of the total mass of a given layer.

[0009] The intrinsic viscosity of the polyester from which the film is made is preferably at least about 0.60, preferably at least about 0.61, preferably at least 0.62, preferably at least 0.63, preferably at least 0.64, preferably at least 0.65, preferably at least about 0.70, preferably at least about 0.75, preferably at least about 0.80. Preferably, the intrinsic viscosity of the polyester is 0.85 or less, preferably 0.83 or less. The use of a polyester with too high a viscosity can cause difficulties during film production and / or require special, more robust film-forming equipment. For example, raising the viscosity too much may mean that in order to achieve stable film production, the output has to be reduced (i.e., the amount of polyester extruded per unit time is decreased, causing a less economical process), or the extrusion temperature has to be raised to lower the viscosity of the melt, which can cause thermal decomposition of the polymer and loss of related properties. The formation of the polyester is advantageously accomplished in a known manner by condensation or transesterification at a temperature up to about 295°C. In a preferred embodiment, solid-phase polymerization may be used to raise the intrinsic viscosity of the polyester to the target value using conventional techniques well known in the art, such as using a fluidized bed, such as a nitrogen fluidized bed or a vacuum fluidized bed, for example using a rotary vacuum dryer.

[0010] The polyester film contains the phosphonate flame retardant in an amount of about 1.0% to about 25.0% of the total mass of a given layer of the film, preferably at least about 5%, preferably at least about 10%, preferably at least about 12%, preferably about 20% or less, preferably about 18% or less, preferably about 15%. The phosphonate flame retardant is preferably a phosphonate flame retardant of formula I. [Chemical formula] (I) (wherein, Ar is an aromatic group, and -O-Ar-O- is derived from a compound having one or more optionally substituted aryl rings such as, for example, resorcinol, hydroquinone, and bisphenols such as bisphenol A, bisphenol F, and 4,4'-biphenol, phenolphthalein, 4,4'-thiodiphenol, 4,4'-sulfonyldiphenol, or combinations thereof, but is not limited thereto; X is C 1-20 alkyl, C 2-20 alkene, C 2-20 alkyne, C 5-20 cycloalkyl or C 6-20 aryl; n is an integer from 1 to about 100, preferably from 1 to about 75, more preferably from 2 to about 50, or any integer between these ranges)

[0011] The group X is preferably C 1-20 alkyl, preferably C 1-10 alkyl, preferably C 1-5 alkyl, preferably selected from methyl. The group -O-Ar-O- is preferably derived from bisphenol A, bisphenol F, and 4,4'-biphenol, preferably bisphenol A. In a preferred embodiment, the phosphonate flame retardant is, or comprises, a phosphonate flame retardant of formula II.

Chemical formula

[0012] Further embodiments of suitable phosphonate flame retardant compounds are described in U.S. Patent Application Publication No. 2016 / 0168760, the disclosure of which is incorporated herein by reference. The weight average molecular weight (Mw) of the phosphonate flame retardant is preferably in the range of about 10,000 to about 120,000, preferably about 50,000 to about 120,000, preferably about 80,000 to about 120,000 g / mol. The molecular weight (Mw) is appropriately determined by gel permeation chromatography (GPC) and is measured, for example, against a polystyrene (PS) standard using the equipment described below. The phosphonate flame retardant preferably has a phosphorus content of at least about 5% by weight, preferably at least about 8% by weight, preferably at least about 10% by weight, preferably about 20% by weight or less, preferably about 15% by weight or less, preferably about 12% by weight or less, preferably about 11% by weight, based on the weight of the phosphonate flame retardant. The glass transition temperature (Tg) of the phosphonate flame retardant is preferably in the range of about 95 to about 120 °C, preferably in the range of about 110 to about 110 °C.

[0013] After the polyester is produced, preferably the phosphonate flame retardant is introduced into the polyester composition. Preferably, the phosphonate flame retardant is added to the polyester before or during the introduction of the polyester into the extruder used in the film production process. When the phosphonate flame retardant is added to the extruder used in the film production process, it is preferably added at the starting point of the extruder barrel. The phosphonate flame retardant may be introduced into the extruder by a gravimetric feeder to provide the desired amount of the phosphonate flame retardant in the final film. Masterbatch technology may also be used. Thus, the phosphonate flame retardant is preferably blended with the polyester. In a preferred embodiment, the phosphonate flame retardant is the only phosphorus-containing flame retardant, preferably the only flame retardant additive, present in the drawn polyester film. The phosphonate flame retardant may be at least partially copolymerized with the polyester to preferably form a block copolymer.

[0014] The polyester film contains at least one metal cation selected from the group consisting of Group I and Group II metal cations. Preferably, the amount of metal cation present in a given layer is at least 10 ppm by mass, preferably at least 15 ppm, at least 40 ppm, preferably at least 45 ppm, preferably at least 65 ppm, preferably at least 80 ppm, preferably at least 100 ppm, based on the amount of polyester in the given layer. Preferably, the amount of metal cation is about 1000 ppm or less by mass, preferably about 500 ppm or less, preferably about 275 ppm or less, typically about 200 ppm or less, and in one embodiment, about 150 ppm or less, based on the amount of polyester in the given layer. Preferably, the amount of metal cation is in the range of 45 ppm to 500 ppm by mass, more preferably 65 ppm to 275 ppm, more preferably 100 ppm to 200 ppm, based on the amount of polyester in the given layer. As used herein, the terms "Group I" and "Group II" have their conventional chemical meanings and refer to the corresponding groups in the periodic table. In the present invention, the metal cation is preferably selected from Group I metal cations, preferably sodium and potassium, and most preferably sodium.

[0015] Preferably, the Group I or Group II cation is present in the form of a salt with a suitable counteranion preferably selected from hydroxide ion, polyacrylate ion, bicarbonate ion, carboxylate ion, chloride ion, acetate ion, formate ion and nitrate ion. In a preferred embodiment, the anion is selected from hydroxide ion or polyacrylate ion, preferably polyacrylate ion. Suitable polyacrylates include those having a mass average molecular weight (M W ) of about 1,000 to about 10,000 g / mol. The molecular weight determination here is carried out using two GPC Ultrastyragel columns, 10 3 and 10 4It can be carried out on a Hewlett-Packard 1050 Series HPLC system equipped with Å (5 μm mixing, 300 mm x 19 mm, Waters Millipore Corporation, Milford, MA, USA) and THF as the mobile phase. The molecular weight is calculated by comparing with the retention time of polystyrene standards. It has been found that the incorporation of such metal cation-containing species results in a film having excellent flame retardancy and high transparency and clarity. The metal cation (preferably in the form of its salt) may be added to the polyester or its monomer before polymerization. In a preferred embodiment, the metal cation (preferably in the form of its salt) is added at the start of the polymerization reaction to prepare the polyester.

[0016] The polyester film may further contain any other additives conventionally used in the production of polyester films. Thus, reagents such as particulate fillers, hydrolysis stabilizers, antioxidants, UV stabilizers, crosslinking agents, dyes, lubricants, radical scavengers, heat stabilizers, surfactants, gloss enhancers, decomposition accelerators, viscosity modifiers and dispersion stabilizers can be incorporated as appropriate. Particulate fillers, hydrolysis stabilizers (preferably glycidyl esters of branched monocarboxylic acids) and antioxidants (preferably hindered phenols, secondary aromatic amines and hindered amines) have particular utility in the present invention, and suitable additives in this regard are disclosed in WO 2012 / 120260, the disclosure of which is incorporated herein by reference. UV stabilizers are also particularly useful. The particulate filler can improve the handleability and winding property during production and / or adjust the optical properties, as is well known in the art. The particulate filler is typically a particulate inorganic filler (e.g., metals or metalloid oxides such as alumina, titania, talc and silica (especially precipitated or diatomaceous silica and silica gel), calcined china clay and alkali metal salts such as carbonates and sulfates of calcium and barium). The particulate inorganic filler is preferably finely divided, and its volume median particle size (the equivalent spherical diameter corresponding to 50% of the total particle volume, read on the cumulative distribution curve associating the volume% with the particle diameter, often referred to as the "D(v,0.5)" value) is preferably in the range of 0.01 to 5 μm, more preferably 0.05 to 1.5 μm, particularly 0.15 to 1.2 μm. Preferably, at least 90% by volume, more preferably at least 95% by volume of the inorganic filler particles are within the range of the volume median particle size ±0.8 μm, particularly ±0.5 μm. The particle size of the filler particles can be measured by electron microscopy, Coulter counter, sedimentation analysis and static or dynamic light scattering. Techniques based on laser light diffraction are preferred. The aforementioned conventionally used additives may be introduced into the polymer in a conventional manner. For example, the components can be mixed with the polymer by mixing with the monomer reactants from which the film-forming polymer is derived, or by tumble blending or dry blending, or by compounding in an extruder followed by cooling and usually grinding into granules or chips. Masterbatch technology can also be used.

[0017] The formation of the polyester film can be achieved by conventional extrusion techniques well known in the art. Generally, the process includes extruding a layer of molten polymer at a temperature in the range of about 275 to about 300 °C, preferably about 290 to 295 °C, quenching the extrudate, and stretching the quenched extrudate. The stretching can be achieved by any process known in the art for the manufacture of stretched films, such as the tubular method or the flat film method. Biaxial stretching is brought about by pulling in directions perpendicular to each other within the plane of the film, so that a satisfactory combination of mechanical and physical properties is achieved. In the tubular method, a thermoplastic polyester tube is extruded, which is subsequently quenched, reheated, and then inflated by the pressure of an inert gas to cause transverse stretching, and further drawn at a rate that causes longitudinal stretching, thereby achieving simultaneous biaxial stretching. In a preferred flat film method, the film-forming polyester is extruded through a slot die and rapidly quenched on a cooled casting drum so that the polyester is reliably quenched into an amorphous state. Then, the stretching is achieved by stretching the quenched extrudate at least in one direction at a temperature higher than the glass transition temperature of the polyester. The quenched flat extrudate can be first stretched in one direction, usually the longitudinal direction, i.e., the forward direction of the film stretcher, and then in the transverse direction to achieve continuous stretching. The stretching in the forward direction of the extrudate is conveniently carried out on a set of rotating rolls or between two pairs of nip rolls, and then the transverse stretching is achieved by a tenter device. The stretching is generally carried out such that the dimensions of the stretched film are 2 to 5 times, more preferably 2.5 to 4.5 times, the original dimensions in the direction or directions of stretching. More preferably, the stretching is carried out such that the dimensions of the stretched film are 3.0 to 3.3 times the original dimensions of the longitudinal draw and 3.3 to 3.9 times the original dimensions of the transverse draw. If stretching in only one direction is required, a larger draw ratio (e.g., up to about 8 times) may be used. Typically, the stretching is carried out at a temperature higher than the T g of the polyester, preferably at a temperature about 15 °C higher than the T g of the polyester. It is not necessary to stretch uniformly in the longitudinal and transverse directions, but it is preferred to do so if balanced properties are desired.

[0018] To promote the desired crystallization of the polyester, it is possible and preferred to dimensionally stabilize the drawn film by heat setting while dimensionally holding at a temperature above the glass transition temperature but below the melting temperature of the polyester. During heat setting, a small amount of dimensional relaxation may be carried out in the transverse direction (TD) by a procedure known as "towing". Towing can include a dimensional relaxation of about 2 to 4%. As is known in the art, dimensional relaxation in the process or machine direction (MD) is also possible. The actual heat setting temperature and time vary depending on the composition of the film and its desired final heat shrinkage, but selections should not be made that substantially reduce toughness such as the film's tear resistance. Within the scope of these constraints, a heat setting temperature of generally about 180 to 245 °C is desirable. In one embodiment, the heat setting temperature is in the range of about 200 to about 225 °C, which gives an improvement in hydrolysis stability. After heat setting, the film is typically quenched rapidly at a high speed to cause the desired degree of crystallization of the polyester.

[0019] Preferably, the film is further stabilized by the use of an in-line relaxation stage. Alternatively, the relaxation treatment can be carried out offline. In this additional step, the film is heated at a temperature lower than the temperature of the heat-setting stage while significantly reducing the tension in the MD and TD directions. The tension the film is subjected to is a small tension, usually less than 5 kg / m of the film width, preferably less than 3.5 kg / m, preferably less than 2.5 kg / m, and usually in the range of 1.0 - 2.0 kg / m. In the relaxation process that controls the film speed, the reduction in film speed (and thus the relaxation of strain) is usually in the range of 0 - 2.5%, preferably 0.5 - 2.0%. During the heat stabilization step, there is no increase in the lateral dimension of the film. The temperature used in the heat stabilization step can vary depending on the combination of properties desired for the final film, but higher temperatures result in better, i.e., less residual shrinkage. Temperatures in the range of 135 - 250 °C are generally desirable, 150 - 230 °C are preferred, and 170 - 200 °C are more preferred. The heating time depends on the temperature used but is usually in the range of 10 - 40 seconds, and a time of 20 - 30 seconds is preferred. This heat stabilization process can be carried out in various ways, including horizontal and vertical configurations, "offline" as a separate process step, or "in-line" as a continuation of the film manufacturing process. The film thus processed exhibits less heat shrinkage than that manufactured without such relaxation after heat setting.

[0020] The polyester film may be either a single layer or a composite structure including a plurality of polyester layers. As described below, one layer of the multilayer polyester film may be a heat-sealable layer. In a preferred embodiment, the polyester film is a single layer. The formation of the composite structure is achieved either by coextruding the individual film-forming layers simultaneously through separate orifices of a multi-orifice die and then fusing the still-molten layers together, or preferably by single-pass coextrusion (wherein the molten streams of the individual polymers are first combined in a flow path leading to the die manifold and then extruded together from the die orifices without mixing under streamline flow conditions to produce a multilayer polymer film), preferably by coextrusion, and this multilayer polymer film can be stretched and heat-set as described above. The polyester layer of the composite film is selected from the above-mentioned polyesters, preferably PET or PET-based polyester. Any or each layer in the composite film may contain any of the additives mentioned above, and the additives in a given layer may be the same as or different from those in other layers, and the amounts may be the same or different. Preferably, the phosphonate flame retardant is in at least the outer layer of the multilayer film, preferably in each layer of the multilayer film. In one embodiment, the polyester film preferably comprises two or three layers having a layer structure of AB or BAB. The outer layer of the multilayer polyester film appropriately contains a particulate filler to improve handleability and winding properties during production as is conventional in the art, and such particulate filler is generally appropriately present in an amount not exceeding 2.5% by weight of the layer, preferably not exceeding 2.0% by weight, preferably not exceeding 1.1% by weight, preferably not exceeding 0.6% by weight, preferably not exceeding 0.5% by weight, preferably not exceeding 0.3% by weight.

[0021] The film of the present invention may be heat-sealable. The heat-sealing ability is preferably provided by disposing a heat-sealable layer on the polyester layer. Alternatively, the polyester of the single-layer film can be heat-sealed by itself. However, preferably, the heat-sealing ability is provided by disposing a heat-sealable layer on a polyester layer that cannot be heat-sealed by itself.

[0022] The heat-sealable polyester is preferably a copolyester selected from copolyesters derived from one or more diols and one or more dicarboxylic acids, the copolyester containing at least three different types of monomer repeating units, and preferably the aliphatic diol and dicarboxylic acid are selected from the above-mentioned dicarboxylic acids and diols. The copolyester of the heat-sealable copolyester layer is the main component of the layer and constitutes at least 50% by mass, preferably at least 65% by mass, preferably at least 80% by mass, preferably at least 90% by mass, more typically at least 95% by mass of the total mass of the layer. The heat-sealable copolyester is preferably selected from the following: (i) A copolyester containing, preferably consisting of, a first aromatic dicarboxylic acid (preferably TA), a second aromatic dicarboxylic acid, and an aliphatic diol, preferably a copolyester derived from repeating units consisting of TA, IPA, and EG. In this embodiment, the second aromatic dicarboxylic acid (preferably IPA) is present in an amount of preferably about 5 to about 30 mol%, preferably about 10 to about 25 mol%, preferably about 10 to about 20 mol%, preferably about 15 to about 20 mol% of the acid fraction of the copolyester. (ii) A copolyester containing, preferably consisting of, a first aromatic dicarboxylic acid (preferably TA), an aliphatic diol (preferably EG), and an alicyclic diol (preferably CHDM). Preferably, the copolyester contains only one kind of aromatic dicarboxylic acid (preferably TA). Preferably, the copolyester contains only one kind of aliphatic diol (preferably EG) glycol. Preferably, the copolyester contains only one kind of alicyclic diol (preferably CHDM). The preferred copolyester is derived from TA, EG, and CHDM. The preferred molar ratio of the alicyclic diol to the aliphatic diol is in the range of 10:90 to 70:30, preferably in the range of 10:90 to 60:40, preferably 20:80 to 40:60, more preferably 30:70 to 35:65. Accordingly, the glycol fraction of the copolyester preferably contains 10 to 70 mol%, preferably 10 to 60 mol%, preferably 20 to 40 mol%, preferably 30 to 35 mol% of the alicyclic diol, and 30 to 90 mol%, preferably 40 to 90 mol%, preferably 60 to 80 mol%, preferably 65 to 70 mol% of the aliphatic diol. In a preferred embodiment, this copolyester is a copolyester of terephthalic acid containing about 33 mol% of 1,4-cyclohexanedimethanol and about 67 mol% of ethylene glycol in terms of the glycol fraction. (iii) A copolyester containing and preferably consisting of an aromatic dicarboxylic acid, an aliphatic dicarboxylic acid and an aliphatic diol. A preferred aromatic dicarboxylic acid is terephthalic acid. Preferred aliphatic dicarboxylic acids are selected from sebacic acid, adipic acid and azelaic acid. The concentration of the aromatic dicarboxylic acid present in the copolyester is preferably in the range of 45 to 80, more preferably 50 to 70, particularly 55 to 65 mol% based on the dicarboxylic acid component of the copolyester. The concentration of the aliphatic dicarboxylic acid present in the copolyester is preferably in the range of 20 to 55, more preferably 30 to 50, particularly 35 to 45 mol% based on the dicarboxylic acid component of the copolyester. Particularly preferred examples of such copolyesters are: (a) copolyesters of azelaic acid and terephthalic acid with an aliphatic glycol, preferably ethylene glycol; (b) copolyesters of adipic acid and terephthalic acid with an aliphatic glycol, preferably ethylene glycol; and (c) copolyesters of sebacic acid and terephthalic acid with an aliphatic glycol, preferably butylene glycol.

[0023] In an alternative embodiment, the heat-sealable functionality may be provided by coating a non-polyester heat-sealing polymer on the polyester film described herein. Suitable heat-sealing layers preferably contain and preferably consist of ethylene vinyl acetate (EVA). Suitable EVA polymers can be obtained from DuPont as Elvax™ resins. Typically, these resins have a vinyl acetate content in the range of 9% to 40%, typically 15% to 30%. The formation of the heat-sealable layer can be achieved by conventional techniques, and the method of formation depends on the properties of the heat-sealable layer. The heat-sealable layer containing copolyesters (i) and (ii) is suitably prepared by coextrusion. The heat-sealable layer containing copolyester (iii) and EVA is suitably prepared by coating a polymer heat-sealable to the above polyester layer. The coating can be achieved using any suitable coating technique, for example, as described below. The thickness of the heat-sealable layer is preferably 40% or less, more preferably 30% or less, still more preferably 25% or less, and preferably at least 2.5%, more preferably at least 5%, preferably about 10% to about 20% of the total thickness of the film.

[0024] In the most preferred embodiment, the polyester film is optically transparent. As used herein, the term "optically transparent" refers to a layer that gives a percentage of the total visual light transmittance (TLT) for scattered light in the visible wavelength region of 8% or less, preferably 5% or less, preferably 4% or less, preferably 3% or less, preferably 2% or less, preferably 1% or less, and / or for light in the visible region (400 nm to 700 nm) of at least 80%, preferably at least 88%, more preferably at least about 90%, still more preferably at least about 92%. Preferably, the optically transparent film meets both of these criteria. In this embodiment, any filler in the film is typically present in only a small amount, generally not exceeding 2.5%, preferably not exceeding 2.0%, preferably not exceeding 1.1%, preferably not exceeding 0.6%, preferably not exceeding 0.3% of the mass of the layer, and preferably the filler is silica. In this embodiment, the winding property of the film (i.e., no blocking or sticking when the film is wound onto a roll) is improved, and there is no unacceptable decrease in haze or other optical properties.

[0025] In an alternative embodiment, the polyester film is opaque. The opaque film preferably exhibits a transmission optical density (TOD) of at least 0.4, preferably at least 0.5, preferably at least 0.6, preferably at least 0.7, preferably at least 1.0, preferably at least 1.5, and in one embodiment, preferably at least 2.0, preferably at least 3.0, preferably at least 4.0. Suitable opacifying agents include carbon black or metal fillers such as aluminum powder, which are known in the art. The opaque film may be colored if desired, for example, white, gray or black. Suitable whitening agents include the particulate inorganic fillers mentioned above, particularly barium sulfate and titanium dioxide, especially titanium dioxide. The white film preferably has a whiteness index of at least 60, preferably at least 85, preferably at least 90, preferably at least 95, typically about 120 or less, typically about 105 units or less, and / or preferably exceeds 85, preferably exceeds 90, preferably exceeds 92.00, typically in the range of 90.00 to 100.00, more typically in the range of 92.00 to 95.00 for the L* value.

[0026] The intrinsic viscosity of the polyester film is preferably at least about 0.70, preferably at least about 0.75, preferably at least 0.80, and preferably about 0.85 or less, preferably about 0.83 or less. The polyester film of the present invention exhibits surprisingly good flame retardancy. In particular, the polyester film exhibits the VTM0 class (as defined by the UL test method described below), which has long been the goal of polyester film manufacturers. Conventional polyester films typically exhibit only a combustion classification of VTM2 class. However, for many applications, the polymer component, when exhibiting the VTM0 class, would seem to be only acceptable for use as part of an apparatus or fixture with respect to flammability, which has limited the range of applications where polyester films can be used. The film of the present invention is semi-crystalline. As used herein, the term "semi-crystalline" refers to a film having a degree of crystallinity of at least 5%, preferably at least 10%, preferably at least 15%, preferably at least 20%, preferably at least 25%, and typically 50% or less, or 45% or less, or 40% or less. The polyester film of the present invention exhibits a low shrinkage rate of preferably less than 10%, preferably less than 5%, preferably less than 3%, preferably less than 2%, preferably less than 1% at 150 °C for 30 minutes, particularly in the machine direction (longitudinal dimension) of the film. Preferably, such shrinkage values are shown in both dimensions (i.e., longitudinal and transverse dimensions) of the film.

[0027] The film of the present invention preferably has a tensile strength at break (UTS) of at least about 15, preferably at least about 18, preferably at least about 19, preferably at least about 20, preferably at least about 21 kg / mm 2 in each of the longitudinal and transverse directions of the film. The multilayer film of the present invention preferably has an elongation at break (ETB) of at least 130%, preferably at least 150%, preferably at least 160%, preferably at least 170%, preferably at least 180%, preferably at least 190%, preferably at least 200% in each of the longitudinal and transverse directions of the film. It will be understood that the terms "longitudinal direction" and "transverse direction" of the film refer to the direction in which the film is stretched during its production. The term "machine direction" is also used herein to refer to the longitudinal direction. The polyester film of the present invention may optionally be coated with an acrylic resin layer. Such an acrylic acid layer may be used as an adhesion promoting layer on the surface of the polyester film, for example, as an ink receiving layer that improves adhesion of inks, dyes and / or lacquers.

[0028] As used herein, the term "acrylic resin" refers to a resin containing at least one acrylic acid and / or methacrylic acid component. The acrylic resin of the ink receiving layer is preferably thermosetting. The acrylic resin of the ink receiving layer preferably contains at least one monomer derived from an ester of acrylic acid and / or an ester of methacrylic acid, and / or a derivative thereof. Preferably, the acrylic resin contains more than 50 mol%, preferably less than 98 mol%, more preferably in the range of 60 - 97 mol%, particularly 70 - 96 mol%, especially 80 - 94 mol% of at least one monomer derived from an ester of acrylic acid and / or an ester of methacrylic acid, and / or a derivative thereof. Preferred acrylic resins contain alkyl esters of acrylic acid and / or methacrylic acid having alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl (terbutyl), hexyl, 2-ethylhexyl, heptyl and n-octyl, containing up to 10 carbon atoms. Preferably, the acrylic resin contains an alkyl acrylate (preferably ethyl acrylate and / or butyl acrylate) and an alkyl methacrylate (preferably methyl methacrylate), and preferably the acrylic resin contains ethyl acrylate and methyl methacrylate. The acrylate monomer is preferably present in a ratio in the range of 20 - 80 mol% (preferably 30 - 65 mol%), and the methacrylate monomer is preferably present in a ratio in the range of 20 - 80 mol% (preferably 20 - 60 mol%).

[0029] Suitable other monomers for use in the preparation of acrylic resins, which are preferably copolymerized as optional additional monomers together with said esters of acrylic acid and / or methacrylic acid and / or their derivatives, include acrylonitrile, methacrylonitrile, halo-substituted acrylonitrile, halo-substituted methacrylonitrile, acrylamide, methacrylamide, N-methylolacrylamide, N-ethanolacrylamide, N-propanolacrylamide, N-methacrylamide, N-ethanolmethacrylamide, N-methylacrylamide, N-tert-butylacrylamide, hydroxyethyl methacrylate, glycidyl acrylate, glycidyl methacrylate, dimethylaminoethyl methacrylate, itaconic acid, itaconic anhydride and half esters of itaconic acid. Other optional monomers 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 chlorostyrene, hydroxystyrene and alkylated styrene, where the alkyl group contains from 1 to 10 carbon atoms.

[0030] Preferred acrylic resins are derived from three monomers and contain 35 to 60 mol% (preferably 40 to 50 mol%) of ethyl acrylate, 30 to 55 mol% (preferably 40 to 50 mol%) of methyl methacrylate, and 2 to 20 mol% (preferably 5 to 10 mol%) of acrylamide or methacrylamide, preferably containing approximately molar ratios of 46 / 46 / 8% of ethyl acrylate / methyl methacrylate / acrylamide or methacrylamide, respectively. Preferably, the polymer is thermosetting, for example, in the presence of about 25% by weight of a methylated melamine-formaldehyde resin. Further preferred acrylic resins are derived from four monomers and comprise a copolymer containing (a) 35 to 40 mol% of an alkyl acrylate, (b) 35 to 40 mol% of an alkyl methacrylate, (c) 10 to 15 mol% of a monomer containing a free carboxyl group, and (d) 15 to 20 mol% of a monomer containing a sulfonic acid group and / or a salt thereof. Ethyl acrylate is a particularly preferred monomer (a), and methyl methacrylate is a particularly preferred monomer (b). The monomer (c) containing a free carboxyl group (i.e., a carboxyl group other than those involved in the polymerization reaction for forming the copolymer) suitably contains a copolymerizable unsaturated carboxylic acid and is preferably selected from acrylic acid, methacrylic acid, maleic acid, and / or itaconic acid (preferably from acrylic acid and itaconic acid). The sulfonic acid group monomer (d) may be present as the free acid and / or a salt thereof, such as an ammonium, substituted ammonium, or alkali metal salt such as lithium, sodium, or potassium salt. The sulfonate group is not involved in the polymerization reaction for forming the copolymer resin. The sulfonic acid group monomer is preferably aromatic and more preferably p-styrene sulfonic acid and / or a salt thereof.

[0031] The mass average molecular weight (M W ; measured as described herein) of the acrylic resin can vary over a wide range but is preferably in the range of 10,000 to 1,000,000, more preferably in the range of 50,000 to 200,000. The acrylic resin component of the ink receiving layer preferably comprises at least 30% by mass, more preferably 40 to 99% by mass, particularly 50 to 85% by mass, especially 70 to 80% by mass, based on the total mass of the ink receiving layer. The acrylic resin of the ink receiving layer is preferably the main component of the layer.

[0032] The composition from which the ink-receiving layer of the second embodiment is derived suitably also contains a crosslinking agent. In particular, the ink-receiving layer is an acrylic resin-containing layer. The crosslinking agent functions to improve the adhesion of the polyester base layer. The crosslinking agent should also function to internally crosslink with the ink-receiving layer to impart solvent resistance. Suitable crosslinking agents include epoxy resins, alkyd resins, amine derivatives such as hexamethoxymethylmelamine and / or amines such as melamine, diazine, urea, cyclic ethylene urea, cyclic propylene urea, thiourea, cyclic ethylene thiourea, alkylmelamine, arylmelamine, benzoguanamine, guanamine, alkylguanamine and condensation products of arylguanamine with aldehydes such as formaldehyde. A useful condensate is the condensate of melamine with formaldehyde. The condensate may be alkoxylated. The crosslinking agent may suitably be used in an amount in the range of up to 70% by mass, preferably 1 to 60% by mass, more preferably 15 to 50% by mass, particularly 20 to 30% by mass, based on the total mass of the ink-receiving layer. A catalyst is preferably used to facilitate the crosslinking action of the crosslinking agent. Preferred catalysts for crosslinking melamine formaldehyde include p-toluenesulfonic acid, maleic acid stabilized by reaction with a base, morpholinium p-toluenesulfonate and ammonium nitrate. The composition from which the ink-receiving layer of the second embodiment is derived optionally contains a plasticizer to assist in film formation and handling. Any suitable plasticizer can be used, such as phthalic esters such as alkylbenzyl phthalate, dialkyl adipate and m.p-cresol propoxylate.

[0033] Acrylic resins are generally insoluble in water. Typically, the acrylic resin is applied to the polyester base layer as a coating composition in the form of an aqueous dispersion. The ink-receiving layer containing an acrylic resin can typically be applied in the form of a coating composition before, during, or after the stretching operation in the production of the stretched film. The coating composition is preferably applied between the two stages (longitudinal and transverse) of the biaxial stretching operation on the polyester substrate. The polyester substrate coated with the acrylic resin is heated to drive out the diluent of the composition (usually water, although an organic solvent can be used additionally or as an alternative), assist in fusing the coating to form a continuous and homogeneous layer, and facilitate the crosslinking of the crosslinkable coating composition (typically up to 240 °C, preferably up to 220 °C). Any suitable conventional coating technique such as dip coating, bead coating, reverse roll coating, or slot coating may be used. The coating composition is preferably applied to the polyester substrate with a dry coating mass in the range of about 0.05 - 5 mg / dm 2 , particularly 0.1 - 2.0 mg / dm 2 .

[0034] The thickness of the ink-receiving layer (preferably an ink-receiving layer containing an acrylic resin) in the second embodiment is preferably in the range of 1.5 μm or less, more preferably 0.01 - 1.0 μm, particularly 0.02 - 0.5 μm. The total thickness of the film of the present invention is preferably 500 μm or less, preferably 350 μm or less, preferably 250 μm or less, preferably 150 μm or less, preferably 100 μm or less, preferably 75 μm or less, preferably 50 μm or less, preferably at least about 5 μm, typically at least about 10 μm. The film of the present invention is suitable for any environment or end use where flame retardancy is desired or advantageous, for example, in the production of screens, shades, or wall coverings, in the production of electrical components and circuits such as capacitors and flexible printed circuits, thin-film touch switches, and in the production of components used in the construction and transportation industries. The film is particularly useful in the production of assembled exterior materials. The film is also particularly suitable, such as for use inside aircraft.

[0035] According to a second aspect of the present invention, there is provided the use of a polymeric phosphonate flame retardant and at least one metal cation selected from the group consisting of Group I and Group II metal cations (particularly sodium and potassium, especially sodium) for imparting flame retardancy to a polyester film (especially a stretched film) or improving the flame retardancy of a polyester film, wherein the polyester film contains the polymeric phosphonate flame retardant in an amount of about 1.0 to about 25.0% by mass based on the total mass of the film. According to a third aspect of the present invention, there is provided a method for imparting flame retardancy to a polyester film (especially a stretched film) or improving the flame retardancy of a polyester film, the method comprising the step of supplying into the film a combination of a polymeric phosphonate flame retardant in an amount of about 1.0 to about 25.0% by mass based on the total mass of the film and at least one metal cation selected from the group consisting of Group I and Group II metal cations (particularly sodium and potassium, especially sodium). It will be understood that the selections and elements described with respect to the first aspect of the present invention apply equally to the second and third aspects of the present invention. As used herein, the term "improving the flame retardancy" is understood to mean that the flame retardancy of the polyester film is improved as compared to the polyester film in the absence of the combination of the polymeric phosphonate flame retardant and the at least one metal cation.

[0036] In particular, the method for imparting flame retardancy to the polyester film or improving the flame retardancy of the polyester film is preferably a use or method that imparts to the polyester film the VTM-0 rating of the UL94 test method. In a preferred embodiment, the use or method is a use or method that imparts to the polyester film both high transparency and transparency together with the flame retardant properties as defined herein. Characteristic measurement

[0037] The following analysis was used to characterize the films described in this specification. (i) Optical transparency was evaluated by measuring the haze (percentage of visible light scattered and transmitted) through the total thickness of the film using an M57D sphere haze meter (Diffusion Systems) in accordance with the total light transmittance (TLT) and the standard test method ASTM D1003. (ii) Transmission optical density (TOD) was measured in transmission mode using a Macbeth Densitometer TR 927 (obtained from Dent and Woods Ltd, Basingstoke, UK). (iii) L*, a* and b* color coordinate values (CIE (1976)), whiteness index and yellowness index were measured using a standard colorimeter conforming to the principle of ASTM D 313, such as a Konica Minolta CM3600a. (iv) The flame retardancy is evaluated by the UL94 Vertical Flame Test for Thin Materials (VTM) method. Test specimens of 200 mm x 50 mm were cut from each film sample and wrapped around a rod to form a "cone" shape. Five test specimens from each film sample were used for the test. The specimens were not conditioned. A line was drawn 75 mm from the upper end of the rod. Before applying the flame, the upper end of each rod was held closed by a spring clamp, and the test specimen was held vertically using a stand. The flame of the burner was adjusted to produce a blue flame of 20 ± 1 mm. In order to achieve the exact height, it was necessary to continuously adjust the propane supply and the air inlet. The film sample was held vertically, and the flame was applied at a 45° angle to the bottom of the tube 10 mm below the film. The flame application was for 3 seconds, and a distance of 10 mm was maintained by moving the burner upwards as the film shrank thereafter. After 3 seconds, the flame was removed from the specimen, and the combustion duration (seconds) after the first flame application was noted (A). The 3-second flame application was repeated, and in addition to the combustion duration (seconds) after the second flame application, the glowing duration (seconds) was noted (B). The total of the combustion durations (seconds) for all five specimens was noted (C). Observation D was yes if the test piece burned up to the 125 mm mark. Observation E was yes if droplets of melted polymer fell from the specimen during flame application. Observation F was yes if the holding clamp burned during flame application. The classification for this test method is given as follows:

[0038]

Table 1

Examples

[0039] Using conventional synthetic procedures, a series of PET polyesters (P1 - P7) containing various additives shown in Table 1 were prepared. Unless otherwise specified, the amounts in Table 1 are given by mass relative to the final mass of the produced polymer. Polyester P1, P2 and P7 were subjected to solid - state polymerization to increase their IV.

[0040]

Table 2

[0041] (Comparative Examples 1 - 4 and Examples 1 - 3) A first series of polyester films based on polyesters P1, P6, and P7 was produced. The draw ratio in the forward direction was from about 3.0 to 3.3; the draw ratio in the transverse direction was from about 3.3 to 3.5; the heat setting was carried out at a temperature of about 190 to about 225 °C, preferably in an initial zone having a temperature of about 225 °C and a final zone having a temperature of about 190 °C. The phosphonate flame retardant of Formula I was added to the polyester during film production at the starting point of the extruder barrel. In all cases, the phosphonate flame retardant was FRX Nofia® HM1100 (available from FRX Polymers, Inc.) having the structure shown below as Formula (1).

[0042] [Chemical formula] (1) Table 2 shows the amount of phosphonate flame retardant present in the film. It also shows the haze and TLT (measured as described herein) of the final film.

[0043] [Table 3] The results in Table 2 demonstrate that the addition of sodium ions significantly improves the optical properties of polyester films containing FRX Nofia® HM1100. Comparative Examples 2 and 4, and Example 3 were tested for flame retardancy. Surprisingly, only Example 3 achieved a VTM-0 rating despite having the same amount of flame retardant as Comparative Examples 2 and 4, indicating that the sodium salt is important not only for flame retardancy but also for optical properties.

[0044] (Examples 4 - 16) A second series of polyester films based on PET polyesters P1 - P7 was manufactured. In all cases, these contained 15% by mass of FRX Nofia® HM1100 and had a final film thickness of 50 μm. In Examples 5 - 16, one side was coated with an acrylic resin adhesion - promoting layer (thickness <0.5 μm). The film compositions are described in Table 3 along with the properties measured as described herein.

[0045] The results demonstrate that the polyester films of the present invention unexpectedly exhibit excellent flame - retardancy of the VTM - 0 grade according to the UL94 test method, as well as excellent optical properties of low haze and high total visual light transmittance in the visible region.

Table 4

[0046] A third series of sodium - salt - containing films based on polyester P2 was manufactured. These contain various amounts of the FRX Nofia® HM1100 flame - retardant as shown in Table 4.

Table 5

Claims

1. 1. A polyester film comprising a polymeric phosphonate flame retardant in an amount of about 1 to about 25 weight percent, based on the total weight of the film, and further comprising at least one metal cation selected from the group consisting of sodium and potassium metal cations.

2. 2. The polyester film according to claim 1, which is oriented, preferably biaxially oriented.

3. 3. The polyester film of claim 1 or 2, wherein the intrinsic viscosity of the film is at least about 0.

70.

4. 4. The polyester film according to claim 1, comprising said phosphonate flame retardant in an amount of at least about 5% by weight, preferably at least about 10% by weight, preferably at least about 12% by weight, preferably not more than about 20% by weight, preferably not more than about 18% by weight, based on the total weight of the film.

5. 5. The polyester film of claim 1, wherein the phosphonate flame retardant is a phosphonate flame retardant of Formula I. 【Chemistry 1】 (I) (In the formula, Ar is an aromatic group and -O-Ar-O- is derived from a compound having one or more optionally substituted aryl rings, such as, but not limited to, resorcinol, hydroquinone, and bisphenols, such as bisphenol A, bisphenol F, and 4,4'-biphenol, phenolphthalein, 4,4'-thiodiphenol, 4,4'-sulfonyldiphenol, or combinations thereof; X is C 1-20 Alkyl, C 2-20 Alkenes, C 2-20 Alkyne, C 5-20 Cycloalkyl or C 6-20 is aryl; n is an integer from 1 to about 100, preferably from 1 to about 75, preferably from 2 to about 50, or any integer between these ranges.

6. X is C 1-20 Alkyl, preferably C 1-10 Alkyl, preferably C 1-5 6. The polyester film according to claim 5, wherein the alkyl radicals are selected from alkyl, preferably methyl.

7. The polyester film according to claim 5 or 6, wherein -O-Ar-O- is derived from bisphenol A, bisphenol F and 4,4'-biphenol, preferably bisphenol A.

8. The polyester film of any one of claims 1 to 7, wherein the phosphonate flame retardant has the formula II: 【Chemistry 2】 (II)

9. The polyester film according to any one of claims 1 to 8, wherein the weight average molecular weight (Mw) of the phosphonate flame retardant is in the range of about 10,000 to about 120,000, preferably about 50,000 to about 120,000, preferably about 80,000 to about 120,000 g / mol.

10. 10. The polyester film according to any one of claims 1 to 9, wherein the phosphonate flame retardant is characterized by a phosphorus content of at least about 5 wt.%, preferably at least about 8 wt.%, preferably at least about 10 wt.%, preferably not more than about 20 wt.%, preferably not more than about 15 wt.%, preferably not more than about 12 wt.%, preferably about 11 wt.%, based on the weight of the phosphonate flame retardant.

11. The polyester film according to any one of claims 1 to 10, wherein the glass transition temperature (Tg) of the phosphonate flame retardant is in the range of about 95 to about 120°C, preferably in the range of about 110 to about 110°C.

12. 12. The polyester film according to claim 1, wherein the amount of said metal cations present in the film is at least 10 ppm, preferably up to about 1000 ppm, relative to the amount of polyester, and / or said metal cations are present in the form of a salt with a counter anion selected from hydroxide, polyacrylate, bicarbonate, carboxylate, chloride, acetate, formate and nitrate.

13. The polyester film according to any one of claims 1 to 12, which exhibits a VTM-0 rating in UL94 testing.

14. The polyester film according to any one of claims 1 to 13, which is a biaxially oriented film and / or the polyester is polyethylene terephthalate.

15. The polyester film of any one of claims 1 to 14, wherein the polyester from which the film is made has an intrinsic viscosity of at least 0.65 and no greater than 0.

85.

16. 16. The polyester film according to any one of claims 1 to 15, having a film thickness of about 250 μm or less, preferably about 150 μm or less, preferably about 100 μm or less, preferably 75 μm or less, preferably 50 μm or less, and preferably at least about 5 μm, preferably at least about 10 μm.

17. 17. The polyester film according to any one of claims 1 to 16, which is a transparent film, wherein the haze of said film is about 8% or less, preferably about 5% or less, preferably about 4% or less, preferably about 3% or less, preferably about 2% or less, preferably about 1% or less, and / or the total light transmittance of said film in the visible region is at least about 85%, preferably at least about 88%, preferably at least about 90%, preferably at least about 92%.

18. 17. The polyester film according to claim 1, wherein the film has a whiteness index of at least 85, and / or an L* value of at least 90, and / or a transmission optical density of at least about 0.

5.

19. The polyester film according to any one of claims 1 to 18, which is a single-layer polyester film.

20. The polyester film according to any one of the preceding claims, wherein the film is coated with an adhesion promoting layer, preferably an acrylic resin.

21. The polyester film according to any one of claims 1 to 20, wherein the phosphonate flame retardant is at least partially copolymerized with the polyester, preferably in the form of a block copolymer.

22. The polyester film according to any one of claims 1 to 21, which is a semi-crystalline film exhibiting a crystallinity ranging from 20% to 50%.

23. 23. The polyester film according to any one of claims 1 to 22, exhibiting less than 5% shrinkage at 150°C for 30 minutes in both the longitudinal and transverse dimensions of the film.

24. The film has a resistance of at least 15 kg / mm ​​in each of the machine and cross directions. 2 The polyester film according to any one of claims 1 to 23, exhibiting an ultimate tensile strength (UTS) of at least 1.0% and / or an elongation at break (ETB) of at least 130% in each of the machine and transverse directions of the film.

25. 1. Use of a polymeric phosphonate flame retardant and at least one metal cation selected from the group consisting of Group I and Group II metal cations in a polyester film to impart flame retardancy to or improve the flame retardancy of the polyester film, wherein the polyester film comprises the polymeric phosphonate flame retardant in an amount of about 1.0 to about 25.0 weight percent, based on the total weight of the film.

26. 1. A method for imparting flame retardancy to or improving the flame retardancy of a polyester film, comprising providing in said film a combination of a polymeric phosphonate flame retardant in an amount of from about 1.0 to about 25.0 weight percent, based on the total weight of said film, and at least one metal cation selected from the group consisting of Group I and Group II metal cations.

27. 27. The use according to claim 25 or the method according to claim 26, wherein the use or method for imparting flame retardancy or improving the flame retardancy of the polyester film is a use or method for imparting a VTM-0 rating to the polyester film according to UL 94 test method.

28. Use or method according to claim 25, 26 or 27, wherein said polymeric phosphonate flame retardant and / or said at least one metal cation and / or said polyester film are as described in any of claims 1 to 24.

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