Filled polyester film

Abstract

Provided herein is a biaxially oriented polyester film comprising a first inorganic particle P1 and a second inorganic particle P2. The film has a total thickness of 1.5 to 8.0 [mu] m. The first inorganic particles P1 have an average particle size of 0.1 to 0.5 [mu] m and are present in the film at a concentration of 100 to 3000 ppm, and the second inorganic particles P2 have an average particle size of 1.50 to 3.50 [mu] m and are present in the film at a concentration of 100 to 2000 ppm. The film exhibits a haze of no more than 5.0%.

Description

Technical Field

[0001] The present invention relates to a filled polyester film having low haze and low thickness and a method for producing the same. The present invention also relates to the use of said film, in particular as a functional layer in laminated glass. Background Art

[0002] Laminated glass generally comprises two or more glass sheets, with one or more layers of adhesive resin, such as polyvinyl butyral (PVB) or ethylene vinyl acetate (EVA), on either side of an intermediate layer, where the resin layer conventionally has a thickness of 0.2 to 1 mm. The glass layers are usually inorganic glass, but can also be made of a transparent rigid organic polymer layer (such as polycarbonate). The purpose of the adhesive resin layer and the laminated structure is to impart additional or improved properties to the structure, such as improved strength or shatter resistance. Such laminated structures are particularly useful in automotive applications (especially windshields) and in the construction industry.

[0003] It is also known to incorporate one or more functional layers comprising a thermoplastic polymer into laminated glass in order to impart some additional desired properties to the structure, such as light filtering, for example selective removal of certain wavelengths or reduction of solar heat transmission. A variety of thermoplastic polymers have been proposed for this purpose, including polyethylene terephthalate (PET). The functional layer can comprise a polymer substrate and one or more coatings or laminated layers capable of imparting the desired function (such as conductivity). Such conductive layers can provide conventional and known functions, such as electrical heating, and thus the polymer substrate should not only exhibit the desired optical properties (i.e., optical performance and / or optical activity), but also exhibit good dielectric strength and good thermal dimensional stability. The functional layer is usually embedded between two or more layers of adhesive resin (such as PVB or EVA), since such adhesive layers generally adhere better to the outer glass layers than the thermoplastic polymer functional layer.

[0004] In the construction industry, functional laminated glass is sometimes referred to as "architectural glass", and new functions for such materials are desired. For example, the relatively high radio frequencies of 5G transmission signals (and future 6G and subsequent generations) mean that they are attenuated by modern laminated glass. In order to avoid installing expensive in-building networks, it would be desirable to provide laminated glass with improved 5G signal penetration in order to enhance 5G signal propagation within buildings. In addition, higher frequency 5G signals are less able (relative to 4G signals) to propagate long distances or penetrate solid objects, thus requiring the expensive installation of a larger number (usually smaller) of line-of-sight base stations in order to propagate the signal, especially in urban environments. Therefore, another desired function is to provide optically transparent glass that can provide passive reflection of 5G signals to cover dead-zones without the need to be connected to a power source.

[0005] Accordingly, there is a need for laminated glass that exhibits new functions and the optical properties of conventional laminated glass. The difficulty in using a functional layer in laminated glass lies in providing a thermoplastic polymer film as a substrate, which has sufficiently high optical properties, including high transparency, low haze, and low optical defects, while maintaining ease of manufacture and good handling characteristics (especially rollability without adhesion or jamming). Optical defects can be particularly problematic in films with very smooth surfaces, which are more susceptible to winding-induced defects. There is a particular need for laminated glass having novel or improved electroactive characteristics that can be provided by an electroactive functional layer, and thus the polymer substrate should also exhibit high dielectric strength. Summary of the Invention

[0006] An object of the present invention is to provide a functional film and its polymer substrate suitable for use in laminated glass to solve one or more of the above problems. A particular object of the present invention is to provide a functional film and its polymer substrate suitable for use in functional laminated glass, wherein the film and the substrate exhibit high transparency, low haze, low optical defects, and high dielectric strength, especially wherein the functional film is electroactive and preferably also exhibits high thermal dimensional stability.

[0007] According to a first aspect of the present invention, there is provided a biaxially oriented polyester film comprising a first inorganic particle P1 and a second inorganic particle P2, wherein:

[0008] (i) the film has a total thickness of 1.5 to 8.0 μm;

[0009] (ii) the first inorganic particle P1 has an average particle size of 0.10 to 0.50 μm;

[0010] (iii) the first inorganic particle P1 is present in the film at a concentration of 100 to 3000 ppm;

[0011] (iv) the second inorganic particle P2 has an average particle size of 1.50 to 3.50 μm;

[0012] (v) the second inorganic particle P2 is present in the film at a concentration of 100 to 2000 ppm; and

[0013] (vi) the film exhibits a haze of no more than 5.0%.

[0014] The film of the present invention exhibits a surprising and advantageous combination of low haze, high transparency, low optical defects, ease of manufacture, good handling characteristics, and high dielectric strength.

[0015] The polyester film described herein is a self-supporting film or sheet, which means a film or sheet that can exist independently in the absence of a supporting substrate.

[0016] One or more polyesters forming the film are preferably crystalline polyesters. The polyester is a properly synthesized linear polyester. Such polyesters can be obtained by condensing one or more dicarboxylic acids or their lower alkyl (up to 6 carbon atoms) diesters with one or more diols. The dicarboxylic acid component contains at least one aromatic dicarboxylic acid, and the at least one aromatic dicarboxylic acid is preferably terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 2,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid or 2,7-naphthalenedicarboxylic acid, and is preferably terephthalic acid (TA) or 2,6-naphthalenedicarboxylic acid, and is preferably terephthalic acid. The polyester may also contain one or more residues derived from other dicarboxylic acids, such as 4,4'-diphenyldicarboxylic acid, hexahydro-terephthalic acid, 1,10-decanedicarboxylic acid, aliphatic dicarboxylic acids, including those having the general formula C n H 2n (COOH)2, where 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). The diol is preferably selected from aliphatic and cycloaliphatic diols, such as ethylene glycol (EG), 1,3-propanediol, 1,4-butanediol and 1,4-cyclohexanedimethanol (CHDM), and is preferably selected from ethylene glycol (EG). Preferably, the polyester contains only one dicarboxylic acid, preferably the aromatic dicarboxylic acid, and preferably terephthalic acid. Preferably, the polyester contains only one diol, preferably an aliphatic diol, preferably ethylene glycol. Preferably, the polyester contains an aromatic dicarboxylic acid and an aliphatic diol. Polyethylene terephthalate (PET) or polyethylene 2,6-naphthalate (PEN), especially PET, is a preferred polyester. The polyester may optionally contain a relatively small amount of one or more residues derived from the above other dicarboxylic acids and / or diols, and when such a small amount is present, the total amount of the one or more other dicarboxylic acids is preferably less than 10 mol%, preferably less than 5 mol%, preferably less than 1 mol% of the total dicarboxylic acid portion of the polyester of a given layer, and / or the total amount of the one or more other diols is preferably less than 15 mol%, preferably less than 10 mol%, preferably less than 5 mol% of the total diol portion of the polyester of a given layer.

[0017] The polyester is the main component of the film and accounts for at least 50% by weight, preferably at least 65% by weight, preferably at least 80% by weight, more preferably at least 85% by weight, and typically at least 95% by weight of the total weight of the layer.

[0018] The intrinsic viscosity of the polyester film is generally in the range of about 0.5 to about 0.8, preferably about 0.55 to 0.7 and more preferably about 0.55 to 0.65.

[0019] The formation of the polyester is generally conveniently achieved by condensation or transesterification in a known manner at a temperature of up to about 295 °C. In a preferred embodiment, solid-state polymerization can be used to increase the intrinsic viscosity of the crystallizable polyester to a desired 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 using a rotary vacuum dryer.

[0020] The formation of the film can be achieved by conventional melt-extrusion techniques well known in the art. Generally speaking, the method comprises the steps of extruding a polymer layer at a temperature within the range suitable for the melting temperature, for example within the range of about 250 to about 300 °C (or generally not more than about 10 °C higher than the crystalline melting point of the polymer), quenching the extrudate and orienting the quenched extrudate.

[0021] Orientation can be achieved by any method known in the art for producing an oriented film, such as the tubular or flat film method. Biaxial orientation is achieved by stretching in two mutually perpendicular directions in the plane of the film, thereby achieving a satisfactory combination of mechanical and physical properties. Biaxial orientation can be achieved by simultaneous orientation or by sequential orientation. Simultaneous orientation is preferably achieved.

[0022] Simultaneous biaxial orientation can be achieved, for example, in the tubular method by extruding a thermoplastic polyester tube, subsequently quenching the thermoplastic polyester tube, reheating it, and then expanding it by internal gas pressure to induce transverse orientation and withdrawing it at a rate that will induce longitudinal orientation. Particularly suitable methods of simultaneous biaxial orientation are disclosed in EP-2108673-A and US-2009 / 0117362-A1, the disclosures of which are incorporated herein by reference.

[0023] Another preferred technique is the flat film method, in which the film-forming polyester is extruded through a slot die and rapidly quenched on a chilled casting drum to ensure that the polyester is quenched to an amorphous state. Then, orientation is achieved by stretching the quenched extrudate in two mutually perpendicular directions at a temperature above one or more glass transition temperatures of the polyester. Sequential orientation can be achieved by first stretching in one direction, usually the longitudinal (or machine) direction (MD) (i.e., the direction forward through the film stretcher), and then stretching the flat quenched extrudate in the transverse direction (TD). The forward stretching of the extrudate is conveniently achieved on a set of rotating shaft rolls or between two pairs of feed rolls, and then the transverse stretching is achieved in a tenter frame device.

[0024] Typically, stretching is carried out such that the dimensions of the oriented film are 2.0 to 5.0 times, preferably not more than 4.0 times, preferably not more than 3.7 times, preferably in the range of 2.0 to 4.0 times, more preferably 2.5 to 4.0 times, more preferably 3.0 to 4.0 times, more preferably 3.3 to 3.7 times its original dimensions in each stretching direction. The stretching is carried out at a temperature higher than the T g of the polyester composition, preferably at least about 5 °C higher than T g , preferably at least about 15 °C higher, and preferably in the range of about T g + 5 °C to about T g + 30 °C. Generally, the stretching is carried out at a temperature in the range of about 5 to about 155 °C, preferably about 5 to about 110 °C. If balanced film properties are desired, the film is stretched equally in the machine direction and the transverse direction.

[0025] Preferably, a simultaneous biaxial stretching method is used, which is particularly advantageous for preparing the films of the present invention.

[0026] The stretched film can and preferably is heat-set at a temperature above one or more glass transition temperatures of the polyester but below its melting temperature (T M ) while being dimensionally supported to induce the desired crystallinity of the polyester. During heat-setting, a small amount of dimensional relaxation can be carried out in the transverse direction (TD) and / or the machine direction (MD). The dimensional relaxation is preferably not more than 5.%, and preferably 1.0 to 3.0%. The dimensional relaxation can be achieved by conventional techniques in the art. In a sequential orientation process, the dimensional relaxation in the MD is relatively more complex than that in the TD because a reduced line tension or speed is required. For this reason, in cases where MD relaxation is needed, a simultaneous orientation process is preferably used, and in this embodiment, simultaneous relaxation in the MD and TD is generally achieved. The actual heat-setting temperature and time will vary depending on the composition of the film and its desired final heat shrinkage, but should not be chosen such that the toughness properties of the film, such as tear resistance, are significantly reduced. Within these limitations, the heat-setting temperature is generally about 80 °C lower than the melting temperature of the film (i.e., T M - 80 °C) to about 10 °C lower than T M (i.e., T M - 10 °C), more typically about T M - 70 °C to about T M - 20 °C, and preferably in the range of 150 to 245 °C, more preferably not more than 225 °C, most preferably not more than 200 °C. Preferably, the heat-setting temperature is in the range of 180 to 225 °C, more preferably 180 to 200 °C. After heat-setting, the film is usually rapidly quenched to induce the desired crystallinity of the polyester.

[0027] Advantageously, the film can and preferably is made in air, i.e. where the film is not made under an inert gas (such as nitrogen or a noble gas such as argon) atmosphere (including extrusion, casting and stretching steps). Thus, the polyester compositions and films described herein are thermally stable and do not require any special processing conditions, in particular an inert atmosphere, during manufacture or storage.

[0028] The final thickness of the polyester film is in the range of 1.5 to 8.0 μm, preferably 2.0 to 8.0 μm, preferably 2.5 to 8.0 μm, preferably 2.5 to 6.0 μm, preferably 2.5 to 5.0 μm, preferably 3.0 to 5.0 μm, preferably 3.0 to 4.0 μm.

[0029] The concentration and size of the inorganic particles in the film are important factors for providing the desired properties of the film. The concentration and size of the particles in the film and the film thickness can be adjusted to achieve the desired combination of the optical properties and handling characteristics of the film. The concentration and size of the first inorganic particles P1 are particularly important for achieving the desired optical properties, while the concentration and size of the second inorganic particles P2 are particularly important for achieving the desired handling characteristics.

[0030] The first inorganic particles P1 are present in the film at a concentration of 100 to 3000 ppm, preferably 100 to 1500 ppm, preferably 500 to 1000 ppm by weight of the film. The second inorganic particles P2 are present in the film at a concentration of 100 to 2000 ppm, preferably 200 to 600 ppm, preferably 400 to 600 ppm by weight of the film.

[0031] The first inorganic particles P1 have an average particle size in the range of 0.10 to 0.50 μm, preferably 0.25 to 0.35 μm.

[0032] The second inorganic particles P2 have an average particle size in the range of 1.50 to 3.50 μm, preferably 1.8 to 3.1, preferably 1.8 to 2.3 μm.

[0033] The second inorganic particles P2 preferably have an average particle size smaller than the film thickness.

[0034] As used herein, the term "average particle size" refers to the median particle size of the volume distribution (the equivalent spherical diameter corresponding to 50% of the volume of all particles, read on the cumulative distribution curve relating volume % to particle diameter, commonly referred to as the "Dv50" or "D50" value).

[0035] The inorganic particles are preferably selected from metalloid oxides (such as alumina, titanium oxide, zirconium oxide, zinc oxide, talc, and silica (especially precipitated silica or diatomaceous earth and silica gel)), silicone, calcined clay, and alkali metal salts (such as carbonates and sulfates of calcium and barium). Preferably, the inorganic particles are selected from silica and silicone. Preferably, the silica is amorphous silica. Preferably, the silicone is methylsilsesquioxane. Preferably, the first inorganic particle P1 is selected from silica particles, amorphous silica particles. Preferably, the second inorganic particle P2 is selected from silicone particles, preferably spherical silicone particles. Preferably, the silicone is methylsilsesquioxane.

[0036] The polyester films described herein exhibit a haze of not more than 5.0%, preferably not more than 3.0%, preferably not more than 2.5%.

[0037] Preferably, the polyester films exhibit a total light transmittance (TLT) of at least 85.0%, preferably at least 88.0%, preferably at least 89.0% in the wavelength range of 300 - 800 nm.

[0038] The polyester films described herein preferably have a surface that is smooth enough to allow subsequent deposition of a conductive layer or pattern, thereby ensuring its integrity and / or uniformity (i.e., without breaks or pinholes or other discontinuities). However, the film surface preferably also exhibits a certain degree of surface roughness to allow for easy fabrication and handling while providing low haze.

[0039] Preferably, the films described herein exhibit a surface roughness (Ra) of not more than 150 nm, preferably not more than 120 nm, preferably not more than 110 nm, preferably not more than 100 nm, preferably in the range of 20 nm to 100 nm, preferably 20 nm to 50 nm.

[0040] The polyester films preferably exhibit a surface roughness (Rt) of not more than 1800 nm, preferably not more than 1500 nm, preferably not more than 1250 nm, usually at least 250 nm and more usually at least 500 nm and preferably in the range of 500 nm to 1250 nm.

[0041] The polyester films preferably exhibit a surface roughness (Rz) of not more than 400 nm, preferably not more than 350 nm, preferably not more than 300 nm, preferably at least 100 nm and more usually at least 150 nm, more usually at least 200 nm and preferably in the range of 100 to 400 nm.

[0042] When a polyester film is used as the substrate of a conductive layer as described herein, its dielectric properties are important. Preferably, the film exhibits a thickness-normalized breakdown voltage of at least 350, preferably 350 - 500, and typically in the range of 300 - 450 V / μm. Preferably, the breakdown voltage of the film is in the range of 1000 - 2000 V, preferably 1200 - 2000 V.

[0043] For defects smaller than 500 μm in size, the polyester films described herein preferably exhibit an optical defect count of no more than 20, preferably no more than 15, preferably no more than 10 per square meter, and further preferably, for defects of at least 2.0 mm in size, the optical defects per square meter are zero.

[0044] The polyester film preferably exhibits isotropic shrinkage, preferably where the ratio of MD / TD shrinkage is in the range of 1.1 to 1.7, preferably 1.2 to 1.5, preferably where the shrinkage in the machine direction of the film does not exceed 4.0% (preferably in the range of 2.0 to 4.0%), and the shrinkage in the transverse direction of the film does not exceed 3.0% (preferably in the range of 1.0 to 3.0%), where the shrinkage is measured in air at 150 °C for 30 minutes. This isotropic shrinkage can be achieved by an asymmetric draw ratio and / or by asymmetric dimensional relaxation in the machine and transverse directions, but is preferably achieved by asymmetric dimensional relaxation, preferably where the film is symmetrically drawn. Isotropic shrinkage is particularly advantageous for manufacturing curved laminated glass (such as windshields), which can be curved in a first direction and be substantially linear in a second orthogonal direction, or be curved to different radii of curvature in orthogonal directions. The manufacture of curved glass plates is generally known in the art and can be achieved, for example, by the method disclosed in US-2007 / 0029026-A.

[0045] In an alternative embodiment, the polyester film exhibits balanced shrinkage, where the MD and TD shrinkage values are substantially the same (i.e., the MD and TD shrinkage values are within less than 5% of each other), preferably where the shrinkage in each direction does not exceed 4.0%, preferably does not exceed 3.0%, and is preferably 1.0 to 4.0%, preferably 1.0 to 3.0%, where the shrinkage is measured in air at 150 °C for 30 minutes.

[0046] As described above, the polyester film can be used as the substrate of a conductive layer that is suitable for providing an electroheating function to laminated glass, particularly in the automotive industry, and as is well known in the art, and in this case, the conductive layer typically takes the form of a conductive wire pattern. The polyester film having the above-described thermodimensional stability characteristics is particularly useful in such applications.

[0047] The components of the polyester film can be introduced into the polyester composition in a conventional manner. For example, one or more additives can be introduced by mixing with the monomer reactants from which the film-forming polyester composition is derived, or one or more additives can be mixed with the polyester composition by tumbling or dry blending or by compounding in an extruder, followed by cooling and typically comminuting into pellets or flakes. The masterbatch mixing technique can also be used.

[0048] The polyester film described herein preferably has a conductive layer disposed on its surface. Preferably, the conductive layer is disposed on the first surface of the film. For the primary end uses described herein, the conductive layer is a transparent conductive layer well known in the art. Suitable transparent conductive layers can be made of any conductive material, particularly metals (such as gold and silver), metal alloys, and metal oxides (particularly doped or mixed metal oxides) and metal nitrides (such as titanium nitride). Suitable materials include doped or mixed zinc oxide (such as fluorine-doped zinc oxide (FTO), or zinc oxide doped with tin, indium, boron, gallium, or aluminum (such as AZO)), doped or mixed tin oxide (such as fluorine-doped tin oxide, or indium tin oxide (ITO)), and cadmium oxide (such as cadmium stannate). ITO is particularly preferred. The conductive layer is composed of a conductive material, which may comprise conductive particles in a binder. The conductive material can be a conductive ink.

[0049] The conductive layer can include a pattern or network of the conductive material. Such patterns and networks are nanoscale and provide various functions. For example, improved 5G transmission in architectural glass can be achieved by a transparent and conductive nanoscale metal network. Improved 5G reflectivity can be achieved by a nanoscale patterned transparent conductive layer.

[0050] The conductive material can be disposed by any suitable technique, such as printing, sputtering, vacuum deposition, etc., particularly by sputtering. Any suitable printing technique can be used to coat the conductive ink onto the polyester film substrate, and suitable printing techniques can include offset printing, gravure printing, screen printing, flexographic printing, thermal transfer printing (TTP), or laser transfer printing (LTP). Other suitable techniques include chemical vapor deposition (CVD), plasma CVD, inductively coupled plasma CVD, capacitively coupled CVD, atomic layer deposition (ALD), or reactive thermal or electron beam evaporation.

[0051] A preferred method of coating a conductive layer (especially a silver, zinc boride or ITO layer, and especially an ITO layer) is carried out at a temperature below 150 °C, preferably below 120 °C, preferably not exceeding 100 °C and more preferably not exceeding 70 °C, and preferably there is no further step of heating the substrate before depositing the conductive material (i.e. where the method is carried out at room temperature (20 °C)), and as is more commonly known in the art at a temperature of at least 40 °C. Such methods reduce the need for the polyester film in terms of its dimensional stability requirements, and this therefore allows the film to be manufactured at a lower heat setting temperature (preferably not exceeding 200 °C), which in turn advantageously reduces the haze component in the film associated with increased crystallinity, thus allowing the manufacture of a thinner film with enhanced optical transparency.

[0052] A particularly preferred method of preparing a conductive layer (especially ITO) involves sputtering a conductive material especially at a temperature below 150 °C, preferably below 120 °C, preferably not exceeding 100 °C.

[0053] The thickness of the conductive layer is preferably in the range of about 1 nm to about 1000 nm, preferably at least 3 nm, preferably at least about 5 nm, preferably at least about 10 nm, preferably at least about 25 nm, and preferably not exceeding about 300 nm, preferably not exceeding about 150 nm, preferably not exceeding about 100 nm, preferably not exceeding about 50 nm and preferably about 3 nm to about 300 nm, preferably about 10 nm to about 150 nm, preferably about 25 to 50 nm.

[0054] As used herein, in the context of a conductive layer, the term "transparent" means transparent to visible light (as measured herein, suitably in the range of 300 - 800 nm), such that the assembly of the polyester film and the conductive layer described herein also meets the above thresholds for total light transmittance and haze specified for the polyester film itself.

[0055] The polyester film described above may optionally have one or more optically active layers provided thereon, especially layers that at least partially filter or reflect radiation solar energy at selected wavelengths within one or more of the visible light, UV or other wavelength regions, including dyed or colored layers to impart color and / or UV absorption or reflection layers. Such layers are known in the art and comprise materials that absorb or reflect wavelengths in the desired region of the electromagnetic spectrum.

[0056] Advantageously, the polyester film described herein can be used as a functional layer in laminated glass, especially where the functional layer is an electroactive layer.

[0057] According to a second aspect of the present invention, there is provided a method of preparing a film as described herein, the method comprising the following steps:

[0058] (i) Extruding a molten polyester layer;

[0059] (ii) The molten polyester layer is preferably biaxially stretched in two mutually perpendicular directions, preferably by simultaneous biaxial stretching.

[0060] (iii) The film is preferably heat-set under tension at a temperature in the range of not more than 225 °C, preferably not more than 200 °C, preferably in the range of 180 to 225 °C, more preferably 180 to 200 °C.

[0061] (iv) At a temperature in the range of 180 to 200 °C, the film is sized to relax by 1.0 to 3.0% in the machine direction of the film, and / or at a temperature in the range of 180 to 200 °C, the film is sized to relax by 1.0 to 3.0% in the transverse direction of the film, and preferably the sizing relaxation is carried out in both directions; and

[0062] (i) Optionally, the conductive layer is preferably disposed on the surface of the film by sputtering.

[0063] According to a third aspect of the present invention, there is provided a multi-layer assembly comprising a polyester film and one or more glass layers, wherein the polyester film preferably has a conductive layer disposed on its surface as described above with respect to the first aspect of the present invention, and preferably wherein the polyester film is sandwiched between two glass layers.

[0064] In the third aspect, the polyester film can be directly disposed on the surface of the glass layer. However, preferably, the assembly further comprises an adhesive resin layer between the polyester film and the glass layer.

[0065] In a preferred embodiment of the third aspect (hereinafter referred to as Embodiment 3A), the multi-layer assembly comprises the polyester film (preferably having a conductive layer disposed on its surface), and further comprises a first and a second glass layer and a first and a second adhesive resin layer, such that the layer sequence is: first glass layer / first adhesive resin layer / polyester film / second adhesive resin layer / second glass layer. In this embodiment, the multi-layer assembly is preferably curved in at least a part thereof such that the assembly can be curved in a first direction and be substantially linear in a second orthogonal direction, or can be curved to different degrees of curvature in the orthogonal directions. Such a curved laminated glass plate is particularly suitable as a windshield or a rear window in automotive applications. Such an assembly in which there is a conductive layer is particularly suitable as a laminated glass plate having an electric heating function, especially in the automotive industry as described above. The polyester film having the above-mentioned thermal dimensional stability characteristics, especially the isotropic shrinkage characteristics, is particularly useful in such applications.

[0066] Preferably, the adhesive resin layer is selected from PVB or EVA, preferably PVB.

[0067] In another preferred embodiment of the third aspect (hereinafter referred to as Embodiment 3B), the multilayer assembly comprises a conductive layer formed by a pattern of conductive material or a conductive mesh, particularly a conductive layer that is functionally adapted to interact with radio frequency wavelengths within the scope of 5G (or subsequent generations) mobile communication technologies. Thus, the conductive layer is preferably adapted to functionally interact with one or more of the ranges within 600 MHz to 70 GHz and preferably selected from 600 - 900 MHz (so-called "low band"), 1.7 - 4.7 GHz (particularly 3.3 - 4.2 GHz) (so-called "mid band"), and 24 - 54 GHz (particularly 24 - 47 GHz, and typically 24 - 30 GHz) (so-called "high band") and particularly within the mid band and high band ranges of radio frequency wavelengths. This embodiment of the invention can be particularly used for propagating mobile communication (particularly 5G) transmission signals within the high band range, especially in metropolitan or urban areas. This embodiment of the invention can also be applicable to mobile communication signals of subsequent generations, such as 6G (95 GHz to 3 THz). Such patterns and meshes are nanoscale (i.e., less than 1000 nm, typically not exceeding 100 nm). Such multilayer assemblies are particularly useful as architectural glass in building construction, especially in metropolitan or urban areas. As mentioned above, improved 5G transmission in architectural glass can be achieved by a transparent and conductive nanoscale metal mesh. Improved 5G reflectivity can be achieved by a nanoscale patterned transparent conductive layer.

[0068] Particularly useful in the third aspect is an assembly comprising one or more of the above-mentioned optically active layers, particularly a layer that at least partially filters or reflects radiant solar energy of selected wavelengths within one or more of the visible light, UV, or other wavelength ranges, including a dyed or colored layer to impart color and / or a UV absorption or reflection layer. The optically active layer is preferably disposed between the polyester film and the glass layer, and in the presence of an adhesive resin layer, the one or more optically active layers are disposed between the polyester film and the adhesive resin layer. Thus, in Embodiment 3A, the optically active layer is preferably disposed together with the polyester film between the first and second adhesive resin layers.

[0069] The glass in the glass layer is typically inorganic glass, but may also be made of a transparent rigid organic polymer layer (such as polycarbonate). Preferably, the glass is inorganic glass (i.e., glass based on silicate or silica).

[0070] The multilayer assembly of the third aspect of the invention is preferably a glass plate, preferably a window, preferably a window in a motor vehicle (preferably a windshield or a rear window) or an architectural window in a building.

[0071] In a fourth aspect of the present invention, there is provided an electrically heatable laminated glass sheet, said electrically heatable laminated glass sheet comprising the multi-layer assembly of the third aspect, said multi-layer assembly comprising said polyester film having a conductive layer disposed on its surface, and wherein said multi-layer assembly further comprises a first and a second glass layer and a first and a second adhesive resin layer such that the layer sequence is: first glass layer / first adhesive resin layer / polyester film / second adhesive resin layer / second glass layer. In a preferred embodiment of the fourth aspect, the glass sheet is curved and the polyester film exhibits isotropic shrinkage, preferably such that the ratio of MD / TD shrinkage is in the range of 1.1 to 1.7, and wherein the polyester film exhibits a shrinkage of not more than 4.0% in the machine direction of the film and a shrinkage of not more than 3.0% in the transverse direction of the film, wherein the shrinkage is as defined above.

[0072] In a fifth aspect of the present invention, there is provided the use of the multi-layer assembly of the third aspect, particularly as an electrically heatable laminated glass in automotive applications, said multi-layer assembly comprising said polyester film having a conductive layer disposed on its surface, and further comprising a first and a second glass layer and a first and a second adhesive resin layer. In a preferred embodiment of the fifth aspect, the multi-layer assembly is curved and the polyester film exhibits isotropic shrinkage, preferably such that the ratio of MD / TD shrinkage is in the range of 1.1 to 1.7, and wherein the shrinkage in the machine direction of the film is not more than 4.0% and the shrinkage in the transverse direction of the film is not more than 3.0%, wherein the shrinkage is as defined above.

[0073] In a sixth aspect of the present invention, there is provided a laminated glass sheet, said laminated glass sheet comprising the multi-layer assembly of the third aspect (particularly embodiment 3B), wherein said polyester film has said conductive layer and optionally said one or more optically active layers disposed thereon, wherein said conductive layer consists of a pattern or a grid of conductive material, said conductive layer being adapted to interact functionally with those radio frequency wavelengths of mobile communication technology preferably selected from one or more of the ranges of 600 - 900 MHz, 1.7 - 4.7 GHz and 24 - 54 GHz.

[0074] In a seventh aspect of the present invention, there is provided the use of the multi-layer assembly of the third aspect (particularly embodiment 3B) as a laminated glass sheet in building construction, wherein said polyester film has said conductive layer and optionally said one or more optically active layers disposed thereon, wherein said conductive layer consists of a pattern or a grid of conductive material, said conductive layer being adapted to interact functionally with those radio frequency wavelengths of mobile communication technology preferably selected from one or more of the ranges of 600 - 900 MHz, 1.7 - 4.7 GHz and 24 - 54 GHz.

[0075] It should be understood that the preferences and descriptions of the polyester film as described above for the first aspect of the present invention apply equally to each of the second and subsequent aspects.

[0076] Characteristic Measurement

[0077] The films described herein are characterized using the following analyses:

[0078] (i) The optical transparency of the film is evaluated by measuring the total luminous transmittance (TLT) and haze (percentage of transmitted visible light scattered) through the total thickness of the film using a BYK Gardner haze-gard dual in accordance with the standard test method ASTM D1003. Unless otherwise stated, TLT values and ranges are reported herein in the wavelength range of 300 - 800 nm.

[0079] (ii) The inherent viscosity (in dL / g) of the polyester and polyester film is measured on a ViscotekTM Y - 501C relative viscometer at 25 °C using a 0.5 wt% solution of polyester in orthochlorophenol in accordance with ASTM D5225 - 98(2003) (see, e.g., Hitchcock, Hammons, and Yau, American Laboratory (August 1994) “The dual - capillary method for modern - day viscometry”), and the inherent viscosity is calculated using the Billmeyer single - point method:

[0080] η = 0.25η 比浓 + 0.75(lnη 相对 ) / c

[0081] Where:

[0082] η = inherent viscosity (dL / g),

[0083] η 相对 = relative viscosity,

[0084] c = concentration (g / dL), and

[0085] η 比浓 = reduced viscosity (dL / g), reduced viscosity is equal to (η 相对 - 1) / c (also denoted as η sp / c, where η sp is the specific viscosity).

[0086] (iii) Evaluate the thermal shrinkage of a film sample with dimensions of 254 mm x 254 mm at a specific predetermined temperature. The film sample is cut along a specific direction with respect to the machine direction and the transverse direction of the film and is marked for visual inspection. After heating the specimen to the predetermined temperature (by placing it in a heating oven at that temperature) and maintaining it for a predetermined time interval, it is cooled to room temperature and its dimensions are manually re-measured. Calculate the thermal shrinkage and express it as a percentage of the original length.

[0087] (iv) The film thickness is measured using a Hildebrand Thickness Gauge series HTG-B according to the standard test method ISO 4593:1993. Take the average of 9 measurements.

[0088] (v) The average particle size is measured by laser diffraction (preferably Fraunhofer diffraction). The particle size mentioned herein is measured using a particle size analyzer (Mastersizer) (such as 3000) available from Malvern. The median particle size is determined by plotting a cumulative distribution curve representing the percentage of the volume of particles below the selected particle size and measuring the 50th percentile.

[0089] (vi) The surface roughness is measured according to ISO 21920-3:2021 using a Form i60 induction system (Taylor-Hobson Precision) by the mechanical contact method. The induction system has a 450 mm motorized column, a 60 mm horizontal traverse unit, and a 1 mm vertical range, and operates Ultra 6.1.12.1 software. Calculate the following surface roughness parameters:

[0090] Ra is defined as the arithmetic mean peak height of the measured surface; the peak height is the absolute value of the deviation of the surface profile from the average of the center line.

[0091] Rt is defined as the total height of the roughness profile and is the difference between the height of the highest peak and the depth of the deepest valley of the measured surface.

[0092] Rz is defined as the average roughness depth of five Rzi values from five sampling lengths of the surface, where each Rzi (the maximum height of the roughness profile) is the sum of the height of the highest peak and the depth of the deepest valley relative to the average line within the sampling length.

[0093] Thus, as is conventional in the art, surface roughness parameters are calculated relative to the mean center line of the surface. The mean center line is also referred to as the "average line", and the average line is the average level of the sample surface. It should be understood that the surface of the polymer film may not be completely flat and may have gentle undulations on its surface. The average line is a straight line that passes through the center of the undulations and surface height deviations, separating the profile such that the areas above and below the average line are equal.

[0094] (vii) The thickness-normalized breakdown voltage (in V / μm) is defined herein as the breakdown voltage (V) per micron of film thickness. The breakdown voltage (V) is measured by placing a polyester film (25 cm 2 sample) between a pair of aluminum foil electrodes and increasing the voltage (usually from 0 to 3000 V) until film breakdown is observed. The film breakdown voltage is defined as the voltage at which a current of 10 microamps is detected passing through the film thickness. The breakdown voltage is reported as the average of 10 measurements.

[0095] (viii) The optical defect count is a measure of the optical quality of the film and is measured by the number of defects visible to the naked eye per square meter on the surface or in the bulk material. Measurement is carried out using the Dr. Schenk Web Inspection System WFF-1320, which consists of 6 cameras (camera type: DC-13, 4k; 24V power supply; air-cooled) and 6 beam line illumination units CL40 (LED type Luxeon Star LXHL-LD3C: wavelength 627 nm; optical power 690 mW; beam divergence 130°), and is installed every 2400 mm of film width above the film to be inspected within the film rewinding device. The distance between each camera is 400 mm, providing a visual inspection range of 420.5 mm (on the film surface), with an overlap of 20.5 mm between adjacent cameras and a total inspection range of 2420.5 mm. The inclination of the illumination and cameras with respect to the film surface is 90°. Each illumination unit illuminates the scanning range of one camera unit. The web material is inspected during transport. Each camera objective has a focal length of 105 mm. The pixel size is 10x10 μm, the pixel count is 4096, providing a line length of 40.96 mm and an optical resolution of 103 μm / pixel. The pixel clock is 50 MHz and the scanning rate is 11.95 kHz. In this study, measurements are made along the film length during the film rewinding operation, and the web speed is 10 m / min. Defects are preferably classified into different sizes, typically 110 - 220 μm, 220 - 500 μm, and >500 μm and ≥2.0 mm, where the term "size" refers to the longest dimension of the defect, and the defect count is reported as the number of each category size detected per square meter. An optical defect is defined as an irregularity in the film that causes a change in the amount of light transmitted through the film relative to one or more adjacent regions of the film. This appears as a change in the brightness of the transmitted light signal relative to a baseline (defined as the average brightness on the film web), which is typically referred to as a dark or bright event. Optical defects in polyester films are typically classified into streaks, gels, catalyst residues, and additive aggregations. Detailed Description of the Invention

[0096] The present invention is further illustrated by reference to the following examples and drawings. The examples are not intended to limit the scope of the present invention as described above.

[0097] Embodiment

[0098] Embodiments G1 to G10

[0099] The PET compositions were prepared using conventional techniques and contain various different amounts and identities of inorganic particles. Two initial series of films were prepared by extruding these compositions on a laboratory-scale film stretching apparatus, yielding amorphous cast extrudates with a thickness of 75 or 110 μm (i.e., a first "thin" series and a second "thick" series). The cast extrudates were simultaneously stretched in the MD and TD, with a draw ratio of approximately 3.5 in each direction and a temperature of 100 °C. The films were then heat-set under dimensional constraints at an average temperature of approximately 190 °C. The following particles were used:

[0100] Particle P1: Amorphous silica particles with a density of 1.32 g / cm 3 were used in all Examples G1 to G10.

[0101] Particle P2: Spherical methylsilsesquioxane particles with a density of 1.43 g / cm 3 were selected from one type of particle in Examples G1 to G3, a different type of particle in Examples G4 to G6, and a mixture of these particles in Examples G7 to G10.

[0102] Table 1 below reports the average size and concentration of the particles in these films. The density of PET is 1.35 g / cm 3 .

[0103] Table 1

[0104]

[0105] The properties of the films were measured by the test methods disclosed herein. The final thickness of the "thin films" was approximately 6 μm (i.e., within the scope of the present invention), and the final thickness of the "thick films" was approximately 9 μm (i.e., outside the scope of the present invention). The TLT of the films was at least 89.0%.

[0106] It was observed that haze was mainly related to the concentration of the larger second inorganic particle P2. The "thin film" samples of Examples G1, G4, G5, and G6 and especially Example G1 advantageously gave low haze values, and these formulations were therefore the subject of further study.

[0107] The surface roughness of the films was such that R a was in the range of 40 to 110 nm. It was observed that for the same concentration of particles, each film in the "thin" series exhibited a higher surface roughness than the corresponding formulation in the "thick" series.

[0108] Embodiments M1 to M3

[0109] A particulate PET composition corresponding to the second series of Formulation G1 was prepared using conventional techniques with different film thicknesses, i.e., where the polyester composition contained:

[0110] (i) Amorphous silica particles P1 with an average particle size of 0.3 μm at 780 ppm; and

[0111] (ii) Spherical methylsilsesquioxane particles P2 with an average particle size of 2.05 μm at 540 ppm.

[0112] The film is prepared as follows on a film production line at manufacturing scale: These compositions are extruded through a film-forming die at a temperature of 275 °C onto a water-cooled rotating quench drum maintained at a temperature of 25 °C to obtain an amorphous cast extrudate, which is then heated to a temperature of 100 °C and then simultaneously stretched in the MD and TD at a draw ratio of 3.45 in each direction. The film is then thermally set under dimensional restraint at a temperature of about 190 °C and then simultaneously undergoes 2% dimensional relaxation in each of the machine and transverse directions at a temperature of 190 °C. The optically transparent film is then characterized using the test methods mentioned herein. Table 2 below reports the identity and some properties of these films (n / m = not measured), confirming that the films of the present invention have a favorable combination of low thickness, low haze, and low surface roughness, as well as acceptable resistivity properties. Despite the very smooth surface, the films can be easily fabricated and processed, and no negative handling or winding characteristics are observed.

[0113] The inventors have observed that the dimensional relaxation of the film allows for a reduction in the thermally setting temperature, which advantageously results in a reduction in the haze of the final film without significantly compromising the shrinkage properties at 150 °C.

[0114] A conductive transparent ITO layer is deposited on the film surface by sputtering at a temperature not exceeding 100 °C to provide a transparent conductive functional composite film, which enables the production of functional laminated glass therefrom.

[0115] Table 2

[0116]

Claims

1. A biaxially oriented polyester film, the biaxially oriented polyester film comprising a first inorganic particle P1 and a second inorganic particle P2, wherein: (i) The film has a total thickness of 1.5 to 8.0 μm; (ii) The first inorganic particle P1 has an average particle size of 0.1 to 0.5 μm; (iii) The first inorganic particle P1 is present in the film at a concentration of 100 to 3000 ppm; (iv) The second inorganic particle P2 has an average particle size of 1.50 to 3.50 μm; (v) The second inorganic particle P2 is present in the film at a concentration of 100 to 2000 ppm; and (vi) The film exhibits a haze of not more than 5.0%.

2. The polyester film according to claim 1, wherein the total thickness is in the range of 2.0 to 8.0 μm, preferably 2.5 to 8.0 μm, preferably 2.5 to 6.0 μm, preferably 2.5 to 5.0 μm, preferably 3.0 to 5.0 μm, preferably 3.0 to 4.0 μm.

3. The polyester film according to any of the preceding claims, wherein the first inorganic particle P1 is present in the film at a concentration of 100 to 1500 ppm, preferably 500 to 1000 ppm, based on the weight of the film.

4. The polyester film according to any of the preceding claims, wherein the second inorganic particle P2 is present in the film at a concentration of 200 to 600 ppm, preferably 400 to 600 ppm, based on the weight of the film.

5. The polyester film according to any of the preceding claims, wherein the first inorganic particle P1 has an average particle size in the range of 0.25 to 0.35 μm and / or wherein the second inorganic particle P2 has an average particle size in the range of 1.8 to 3.1 μm, preferably 1.8 to 2.3 μm.

6. The polyester film according to any of the preceding claims, wherein the first inorganic particle P1 is selected from amorphous silica particles.

7. The polyester film according to any of the preceding claims, wherein the second inorganic particle P2 is selected from silicone particles, preferably spherical silicone particles, preferably wherein the silicone is methylsilsesquioxane.

8. The polyester film according to any of the preceding claims, wherein the polyester is polyethylene terephthalate.

9. The polyester film according to any of the preceding claims, wherein the second inorganic particle P2 has an average particle size smaller than the film thickness.

10. The polyester film according to any of the preceding claims, the polyester film exhibits a surface roughness (Ra) of not more than 150 nm, preferably not more than 120 nm, preferably not more than 110 nm, preferably not more than 100 nm, preferably in the range of 20 nm to 100 nm, preferably 20 nm to 50 nm.

11. The polyester film according to any of the preceding claims, wherein the polyester film exhibits a surface roughness (Rt) of not more than 1800 nm, preferably not more than 1500 nm, preferably not more than 1250 nm and preferably in the range of 500 nm to 1250 nm, and / or a surface roughness (Rz) of not more than 400 nm, preferably not more than 350 nm, preferably not more than 300 nm, preferably at least 100 nm and more typically at least 150 nm, more typically at least 200 nm and preferably in the range of 100 to 400 nm.

12. The polyester film according to any of the preceding claims, wherein the polyester film exhibits isotropic shrinkage, preferably wherein the ratio of MD / TD shrinkage is in the range of 1.1 to 1.7, preferably 1.2 to 1.5, and preferably wherein the shrinkage in the machine direction of the film does not exceed 4.0% (preferably in the range of 2.0 to 4.0%) and the shrinkage in the transverse direction of the film does not exceed 3.0% (preferably in the range of 1.0 to 3.0%), where the shrinkage is measured in air at 150 °C for 30 min.

13. The polyester film according to any one of claims 1 to 11, wherein the polyester film exhibits balanced shrinkage, preferably wherein the shrinkage in each of the MD and TD directions does not exceed 4.0%, preferably does not exceed 3.0%, and preferably 1.0 to 4.0%, preferably 1.0 to 3.0%, where the shrinkage is measured in air at 150 °C for 30 min.

14. The polyester film according to any of the preceding claims, wherein the film exhibits a thickness-normalized breakdown voltage of 350 - 500 V / μm and / or a breakdown voltage of 1000 - 2000 V, preferably 1200 - 2000 V.

15. The polyester film according to any of the preceding claims, wherein for defects with a size less than 500 μm, the film exhibits an optical defect count per square meter of not more than 20, preferably not more than 15, preferably not more than 10, and also preferably for defects with a size of at least 2.0 mm, the optical defects per square meter are zero.

16. The polyester film according to any of the preceding claims, wherein the film has a first surface and a second surface, wherein a conductive layer is disposed on the first surface, preferably selected from metals, metal alloys, and metal oxides (especially doped or mixed metal oxides) and metal nitrides, such as doped or mixed zinc oxide (especially fluorine-doped zinc oxide, or zinc oxide doped with tin, indium, boron, gallium, or aluminum), doped or mixed tin oxide (especially fluorine-doped tin oxide, or indium tin oxide), and cadmium oxide (especially cadmium stannate), and preferably wherein the conductive layer is an indium tin oxide layer, preferably wherein the thickness of the conductive layer is in the range of 1 nm to 1000 nm, preferably 3 nm to 300 nm, preferably 10 nm to 150 nm, preferably 25 to 50 nm.

17. The polyester film according to any of the preceding claims, wherein the polyester film exhibits a haze of not more than 3.0%, preferably not more than 2.5%.

18. The polyester film according to any of the preceding claims, wherein the polyester film exhibits a total light transmittance (TLT) of at least 85.0%, preferably at least 88.0%, preferably at least 89.0% in the wavelength range of 300 - 800 nm.

19. The polyester film according to any of the preceding claims, wherein one or more optically active layers have been disposed thereon, and the one or more optically active layers at least partially filter or reflect radiant solar energy of selected wavelengths within one or more of the visible light, UV, or other wavelength ranges.

20. A multi - layer assembly comprising the polyester film according to any of the preceding claims and one or more glass layers, preferably wherein the polyester film is sandwiched between two glass layers.

21. The multi - layer assembly according to claim 20, wherein the multi - layer assembly comprises the polyester film, a first and a second glass layer, and a first and a second adhesive resin layer, such that the layer sequence is: first glass layer / first adhesive resin layer / polyester film / second adhesive resin layer / second glass layer.

22. The multi - layer assembly according to claim 21, wherein the adhesive resin layer is selected from polyvinyl butyral (PVB) or ethylene - vinyl acetate (EVA), preferably PVB.

23. The multi - layer assembly according to any one of claims 20 to 22, wherein a conductive layer and / or one or more optically active layers have been disposed on the polyester film, and the one or more optically active layers at least partially filter or reflect radiant solar energy of selected wavelengths within one or more of the visible light, ultraviolet light, or other wavelength ranges.

24. The multi - layer assembly according to any one of claims 20 to 23, wherein the multi - layer assembly is curved.

25. The multi - layer assembly according to claim 24, wherein the polyester film exhibits isotropic shrinkage, preferably wherein the ratio of MD / TD shrinkage is in the range of 1.1 to 1.7, preferably 1.2 to 1.5, and preferably wherein the shrinkage in the machine direction of the film does not exceed 4.0% (preferably in the range of 2.0 to 4.0%) and the shrinkage in the transverse direction of the film does not exceed 3.0% (preferably in the range of 1.0 to 3.0%), where the shrinkage is measured in air at 150 °C for 30 min.

26. The multi - layer assembly according to any one of claims 20 to 25, wherein the multi - layer assembly is a glass sheet.

27. An electrically heatable laminated glass sheet comprising the multi - layer assembly according to any one of claims 23 to 25, wherein the conductive layer and optionally the one or more optically active layers have been disposed on the polyester film.

28. Use of the multi - layer assembly as defined in any one of claims 23 to 25 as an electrically heatable laminated glass in automotive applications, wherein the conductive layer and optionally the one or more optically active layers have been disposed on the polyester film.

29. The multilayer assembly according to any one of claims 20 to 25, the multilayer assembly comprising the polyester film and a conductive layer formed of a pattern of conductive material or a conductive mesh, the conductive layer being adapted to interact functionally with those radio frequency wavelengths of mobile communication technology preferably selected from one or more of the ranges of 600 - 900 MHz, 1.7 - 4.7 GHz, and 24 - 54 GHz.

30. A laminated glass sheet, the laminated glass sheet comprising the multilayer assembly according to any one of claims 20 to 25, wherein the conductive layer and optionally the one or more optically active layers have been disposed on the polyester film, wherein the conductive layer is formed of a pattern of conductive material or a conductive mesh, the conductive layer being adapted to interact functionally with those radio frequency wavelengths of mobile communication technology preferably selected from one or more of the ranges of 600 - 900 MHz, 1.7 - 4.7 GHz, and 24 - 54 GHz.

31. Use of the multilayer assembly as defined in any one of claims 20 to 25 in a building construction as a laminated glass sheet, wherein the conductive layer and optionally the one or more optically active layers have been disposed on the polyester film, wherein the conductive layer is formed of a pattern of conductive material or a conductive mesh, the conductive layer being adapted to interact functionally with those radio frequency wavelengths of mobile communication technology preferably selected from one or more of the ranges of 600 - 900 MHz, 1.7 - 4.7 GHz, and 24 - 54 GHz.

32. A method for preparing a biaxially oriented polyester film as defined in any one of claims 1 to 19, the method comprising the following steps: (i) Extruding a molten polyester layer; (ii) Biaxially stretching the molten polyester layer in two mutually perpendicular directions, preferably by simultaneous biaxial stretching; (iii) Thermally setting the film under tension at a temperature not exceeding 225 °C; (iv) Relaxing the film size by 1.0 to 3.0% in the machine direction of the film and / or relaxing the film size by 1.0 to 3.0% in the transverse direction of the film at a temperature in the range of 180 to 200 °C; and (v) Optionally disposing a conductive layer on the first surface of the film, preferably by sputtering.

33. The method according to claim 32, wherein the biaxial stretching ratio in each direction does not exceed 4.0, preferably does not exceed 3.7, and is preferably in the range of 2.5 to  4.0, preferably in the range of 3.0 to 4.0, and preferably in the range of 3.3 to 3.

7.

34. The method according to claim 32 or 33, wherein the film is symmetrically stretched, and optionally wherein the size relaxation is asymmetric.

35. The method according to any one of claims 32 to 34, wherein the thermal setting is carried out at a temperature not exceeding 200 °C, preferably 180 to 200 °C.

36. The method according to any one of claims 32 to 35, wherein the conductive layer is disposed on the first surface of the film at a temperature below 150 °C, preferably below 120 °C, more preferably not exceeding 100 °C.

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