EP0704766B1 - Transparent elements for an electrostatic photocopy - Google Patents

Abstract

Transparent elements for use in electrostatic copiers comprise: (1) a composite polyester support (S) comprising: (a) a relatively thick layer (A) of semi-crystalline polyester; and (b) on face(s) of the layer (A), a thin layer (B) of a polyester that is the same or different to the polyester constituting layer (A); and (2) a primer coating (P) less than or equal to 0.3 mu thick on the face(s) of the support to promote adhesion of the toner to the support, comprising an acrylic polymer having a glass transition temp. Tg of 10-50 degrees C., a content of free-COOH functions of less than 50 millimoles per 100 g acrylic polymer.

Description

The present invention relates to transparent elements comprising a polyester support for electrostatic photocopying.

We know that photocopies that can be projected on a screen by means of a overhead projector are easily obtained from a source document by photocopying electrostatic on a transparent support. According to this process, a charge is imparted uniform electrostatic, positive or negative, at a photoconductive surface retaining this charge when kept away from light and then forming on said surface the image of a subject to be copied (text, drawing, etc.) by means of a optical system and a light source. In areas where the loaded surface is struck by light, the electric charge is dissipated by grounding, the others zones then constituting the electrostatic image of the source document; particles an electrostatic powder ink (hereafter toner) with a charge opposite to that of the electrostatic surface are then deposited on the latter by attraction electrostatic then the image thus obtained is brought into contact with a support, by transparent example, which is given an opposite electric charge to transfer toner from the electrostatic surface on the support. The image thus obtained is fixed on the support by heat treatment and / or by pressure treatment.

The transparent supports used to make photocopies electrostatic sprayers must meet various requirements to allow obtaining excellent quality projected images, especially sharp images. They must have in particular transparency, dimensional stability and high sliding power, poor ability to accumulate static electricity charges and good adhesion to the image printing material. We know that these supports must more particularly present a transparency such as the percentage of light dispersed by the passage of a ray of light through their thickness, or turbidity, which is less than or equal to 7%. We also know that these supports, when they are obtained from a bi-oriented film, in particular a polyester, must have a dimensional stability such as their shrinkage at 150-170 ° C, that is to say in the temperature conditions necessary for fixing the toner, i.e. less than 1% within stretching directions.

Due to their excellent mechanical, physical and chemicals, polyester films are a material of choice for making transparent supports for projectable electrostatic photocopies. However, their low sliding ability, their great power of static electricity accumulation by friction or induction and their relative chemical inertia, which results in low adhesion of the toner to the support, are the source of problems which required solutions complicating obtaining transparent supports and making it more expensive. So the problem of slippage of polyester films cannot be solved by creating roughness surface by means of particles of a filler dispersed in the mass of the polyester. Indeed, due to the thickness of the films intended to serve as a photocopy medium electrostatic between 50 and 200 µm, the presence of charge throughout their thickness gives them a high turbidity incompatible with such use. The transparency can only be obtained at the cost of a reduction in the filler content which compromises the sliding of the supports and consequently the ability of the supports to slide on top of each other in the oars used in photocopying machines or to slide on the metal surfaces of said machines. The low affinity of films polyester for toner results in easy removal of toner during manipulation of photocopies and progressive deterioration of the image. Of his side, the ability to accumulate static electricity from the polyester support both disrupts the regular deposit of toner on the support during photocopying, which affects the quality of the image, and the sheet feed of the supports from a ream.

To solve the problems posed by the adhesion of the toner to the support, the transparency, sliding and the ability to accumulate charges, it has been proposed to deposit by coating on at least one face with a transparent polyester film free of charge, a coating consisting of a polymeric binder having at least good adhesion to the toner, containing fillers creating on the support a roughness sufficient for it impart the necessary slip and, if necessary, an antistatic agent; according to one variant of this solution, a coating is deposited on one side of the polyester film charged adhesion primer and on the other side an antistatic coating.

Thus, in American patent US 4,526,847, it has been proposed to deposit on a polyester film free of filler a coating of a composition constituted by a nitrocellulose solution in an organic solvent (esters, ketone) containing a plasticizer, a filler with a particle size of 0.3 to 10 µm (colloidal silica for example) and an antistatic agent.

In European patent application EP-A-332 183, there have been described transparent elements for electrostatic photocopying constituted by a support filler-free polyester with a layer of an acrylic binder coating containing a filler and an antistatic agent. It is however necessary to interpose between the acrylic coating and the polyester a primer facilitating adhesion to the polyester of the layer intended to receive the toner.

In European patent application EP-A-104 074, it was further suggested to deposit on one side of a polyester support film an acrylic coating containing a filler and providing adhesion to the toner and on the other side of the polyester support, a coating consisting of an electrically conductive polymer; the establishment of a primary coating between the polyester backing and the toner receiving layer is still recommended.

EP-A-240.147 and EP-A-442.567 describe transparent elements comprising a support polyester and a layer for receiving an image comprising an acrylic resin. JP-A-4060644 describes a polyester film for making a microfilm, a film for an overhead projector, consisting of a sheet composed of two layers having a specific thickness ratio.

The solutions proposed in the aforementioned prior art are only partially satisfactory. The use of solvent-based compositions organic poses safety and hygiene problems. Some of the solutions retained involve multiplying the layers of coatings: a) installation of a primer with good adhesion to the polyester support and to the receiving layer of the toner; b) deposition of a receiving layer providing adhesion to the toner, a good slipperiness and, where appropriate, good electroconductivity; and c) possibly deposit, on the side of the support film opposite to that receiving the toner, with an antistatic coating. We also noted that the introduction of an antistatic agent in the layer of adhesion intended to receive the toner may, depending on the nature of said agent, decrease the effectiveness of the adhesion of the toner to the receiving layer. The presence of charges in the disadvantage of the toner receiving layer is that it requires a thickness of said layer which goes beyond what is necessary from the sole point of view of improving the adhesion of the toner to the support.

The present invention proposes precisely to solve in a simple way the problem of obtaining transparent elements for photocopying electrostatic said elements having excellent transparency, good slippery, good adhesion of the toner to the support and good electroconductivity and free from the disadvantages of previous transparent elements.

a) une couche (A) épaisse en polyester semi-cristallin ;

b) sur au moins une des faces de la couche épaisse (A), une couche mince (B) en polyester identique ou différent de celui constituant la couche (A)

et en ce que le revêtement primaire (P) comprend un polymère acrylique ayant une température de transition vitreuse comprise entre 10°C et 50°C, un taux de fonctions carboxyliques libres -COOH inférieur à 50 millimoles pour 100 grammes dudit polymère acrylique et en ce que ledit revêtement primaire (P) a une épaisseur égale ou inférieure à 0,3 µm.More specifically, the subject of the present invention is transparent elements for electrostatic photocopying comprising a transparent polyester support (S) comprising on at least one of its faces a primary coating (P) for adhesion of the toner to the polyester support, characterized in that the polyester support (S) is a composite consisting of:

a) a thick layer (A) of semi-crystalline polyester;

b) on at least one of the faces of the thick layer (A), a thin layer (B) of polyester identical or different from that constituting the layer (A)

and in that the primary coating (P) comprises an acrylic polymer having a glass transition temperature of between 10 ° C and 50 ° C, a rate of free carboxylic functions -COOH of less than 50 millimoles per 100 grams of said acrylic polymer and that said primary coating (P) has a thickness of 0.3 µm or less.

For the purposes of the present invention, the expression "elements" transparencies for electrostatic photocopying "of elements usable directly to make projectable photocopies on a screen and taken in the form of a film continuous or sheets of suitable format obtained by cutting the films.

The free carboxylic functions are the free carboxylic functions total of the acrylic polymer.

The polyesters constituting the layers (A) and (B) of the support film (S) can be the same or different, although it's easier to use the same polyester to the two types of layers. As such, we can, for layer (A), call on the polyesters usually used to obtain bi-oriented semi-crystalline films. he these are film-forming linear polyesters, crystallizable by orientation and obtained from usually from one or more dicarboxylic aromatic acids or their derivatives (esters of lower aliphatic alcohols, halides for example) and one or more several aliphatic glycols. As an example of aromatic diacids, mention may be made of phthalic, terephthalic, isophthalic, naphthalenedicarboxylic acid-2,5; naphthalenedicarboxylic-2,6. These acids can be associated with a minor amount one or more aliphatic dicarboxylic acids such as adipic acids, azelaic, hexahydroterephthalic. As non-limiting examples of diols aliphatic, there may be mentioned ethylene glycol; propanediol-1,3; 1,4-butanediol. These diols can be combined with a minor amount of one or more aliphatic diols more carbon condensed (neopentylglycol for example) or cycloaliphatic (cyclohexanedimethanol). Preferably, the crystallizable film-forming polyesters are polyterephthalates or polynaphthalenedicarboxylates of alkylenediols and, in in particular, polyethylene terephthalate (PET) or 1,4-butanediol or copolyesters comprising at least 80 mol% of terephthalate or naphthalene-dicarboxylate units of alkylene glycols. Advantageously, the polyester is a polyterephthalate ethylene glycol whose viscosity index, measured in a 50/50 mixture by weight of phenol and 1,2-dichloro benzene according to ISO 1628-5, is between 55 ml / g and 75 ml / g.

The thin layer (B) can be made of the same polyester crystallizable as layer (A) or by a non-crystallizable polyester or less crystallizable as the polyester constituting the layer (A). We then call on polyesters containing greater or lesser amounts of amorphous units such as those derived from isophthalic acid, neopentylglycol or cyclohexanedimethanol. It would not be departing from the scope of the present invention to use a blend of a crystallizable polyester and a polyester with amorphous patterns for make the thin charged layer (B). So we could use mixtures comprising from 20 to 80% by weight of a crystallizable polyester and from 80 to 20% by weight polyester with amorphous patterns. Layer (B) could also be made up with a copolyester having a plurality of sulfonic groups or their salts alkali, alkaline earth or ammonium metals (hereinafter referred to as sulfonated copolyester), or by mixtures of copolyesters of this type with one or more polyesters do not containing no sulfonic groups such as semi-crystallizable polyesters or polyesters with amorphous patterns.

The acrylic polymer comprises units chosen from the units derived acrylic acid, methacrylic acid, alkyl acrylates, methacrylates alkyl, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, N-methylolacrylamide, N-methoxymethacrylamide, styrene, butadiene, esters vinyl, at least part of these units coming from an alkyl acrylate and / or a alkyl methacrylate.

Preferably, the acrylic polymer used in the invention contains at least units derived from alkyl acrylates chosen from acrylate methyl, ethyl acrylate, propyl acrylates and butyl acrylates and / or units derived from alkyl methacrylates chosen from methyl methacrylate, ethyl methacrylate, propyl methacrylates and butyl methacrylates. The acrylic copolymers comprising units derived from methyl acrylate and / or ethyl and methyl and / or ethyl methacrylate are particularly suitable for constitute the primary coating (P) of the transparent elements of the invention. The acrylic polymer can also include acrylic acid and / or acid units methacrylic, insofar as the rate of free carboxylic acid functions is less than 50 millimoles per 100 grams of said acrylic polymer and preferably remains less than or equal to 30 millimoles per 100g.

The acrylic polymer used in the composition of the primary coating (P) elements of the invention advantageously have a temperature of glass transition between 15 ° C and 30 ° C.

The anti-static character of the elements for electrostatic photocopying is an important parameter.

To improve this antistatic character, the acrylic polymer constituting the primary (P) elements for photocopying may contain up to 25% by weight of a compound (monomer or polymer) with quaternary ammonium groups.

This compound containing a quaternary ammonium group may be present in mixture with the acrylic polymer described above or may constitute a part patterns of said acrylic polymer. In other words, we can implement a mixing the acrylic polymer with a compound containing ammonium groups quaternary or a copolymer comprising units described above for the acrylic polymer and patterns with quaternary ammonium groups.

Preferably, the compound with quaternary ammonium groups represents from 2% to 15% by weight relative to the weight of the polymer assembly acrylic / compound with quaternary ammonium groups.

dans laquelle :

• R₁ représente un radical acyloxyalkyle ou acylaminoalkyle comprenant un groupement acyle saturé ou comportant une double liaison éthylénique carbone-carbone, un radical alcoxyalkyle, un radical aryloxyalkyle, un radical alkylaryloxyalkyle, un radical alcényloxyalkyle, un radical alcénylaryloxyalkyle,
• R₂, R₃ et R₄, identiques ou différents, représentent un radical alkyle ayant de 1 à 6 atomes de carbone, un radical polyoxyéthylène -(CH₂-CH₂-O-)_n-H ou -(CH₂-CH₂-O-)_n-CH₃ avec n représentant un nombre de 1 à 12,
• X représente un anion choisi dans le groupe comprenant les halogénures, en particulier le chlorure, le sulfate, le sulfonate, les alkylsulfonates tels que méthyl-sulfonate, les arylsulfonates, les arylalkylsulfonates, le carbonate, les alkyl-carbonates tels que méthyl-carbonate, le nitrate, le phosphate, les alkyl-phosphates ou des mélanges de ces anions.

The compounds with quaternary ammonium groups, which are used or which are at the origin of polymers with quaternary ammonium groups themselves used, correspond to the following general formula (I):

in which :

• R ₁ represents an acyloxyalkyl or acylaminoalkyl radical comprising a saturated acyl group or comprising an ethylene carbon-carbon double bond, an alkoxyalkyl radical, an aryloxyalkyl radical, an alkylaryloxyalkyl radical, an alkenyloxyalkyl radical, an alkenylaryloxyalkyl radical,
• R ₂ , R ₃ and R ₄ , identical or different, represent an alkyl radical having from 1 to 6 carbon atoms, a polyoxyethylene radical - (CH ₂ -CH ₂ -O-) _n -H or - (CH ₂ -CH ₂ -O-) _n -CH ₃ with n representing a number from 1 to 12,
• X represents an anion chosen from the group comprising the halides, in particular the chloride, the sulfate, the sulfonate, the alkyl sulfonates such as methyl sulfonate, the aryl sulfonates, the arylalkyl sulfonates, the carbonate, the alkyl carbonates such as methyl carbonate, nitrate, phosphate, alkyl phosphates or mixtures of these anions.

Depending on whether the radical R ₁ comprises an ethylenic double bond or not, the compound containing quaternary ammonium groups of formula (I) will be copolymerizable with the acrylic polymer or will be used in admixture with said acrylic polymer.

Among the compounds of formula (I), which cannot be polymerized, mention may be made of stearamidopropyl dimethyl β-hydroxyethyl ammonium nitrate.

   dans lesquelles :

• R₅, R₆ et R₇, identiques ou différents, représentent un atome d'hydrogène,
  un radical méthyle ou un radical éthyle,
• R₈, R₉ et R₁₀, identique ou différents, représentent un radical alkyle ayant 1 à 4 atomes de carbone, un radical polyoxyéthylène -(CH₂-CH₂-O-)_m-H ou -(CH₂-CH₂-O-)_m-CH₃ avec m représentant un nombre de 1 à 8,
• R₁₁ représente un radical divalent tel que polyéthylène ou hydroxyalkylène ayant 1 à 8 atomes de carbone,
• X représente un anion choisi dans le groupe comprenant les halogénures, en particulier le chlorure, le sulfate, le sulfonate, les alkylsulfonates tels que méthyl-sulfonate, les arylsulfonates, les arylalkylsulfonates, le carbonate, les alkyl-carbonates tels que méthyl-carbonate, le nitrate, le phosphate, les alkyl-phosphates ou des mélanges de ces anions.

Among the polymerizable compounds of formula (I), there may be mentioned more particularly the monomers of general formulas (II) or (III):

in which :

• R ₅ , R ₆ and R ₇ , identical or different, represent a hydrogen atom,
  a methyl radical or an ethyl radical,
• R ₈ , R ₉ and R ₁₀ , identical or different, represent an alkyl radical having 1 to 4 carbon atoms, a polyoxyethylene radical - (CH ₂ -CH ₂ -O-) _m -H or - (CH ₂ -CH ₂ -O-) _m -CH ₃ with m representing a number from 1 to 8,
• R ₁₁ represents a divalent radical such as polyethylene or hydroxyalkylene having 1 to 8 carbon atoms,
• X represents an anion chosen from the group comprising the halides, in particular the chloride, the sulfate, the sulfonate, the alkyl sulfonates such as methyl sulfonate, the aryl sulfonates, the arylalkyl sulfonates, the carbonate, the alkyl carbonates such as methyl carbonate, nitrate, phosphate, alkyl phosphates or mixtures of these anions.

When using a compound with quaternary ammonium groups, the acrylic polymer does not have a free carboxylic acid function.

The antistatic device is evaluated from the measurement of a half-discharge time. The film surface is loaded with corona treatment at a potential of 500 V. The corona charging device is stopped and the decrease in the temperature is observed. surface potential. The measurement records the time to reach a surface potential 250 V. The shorter the time, the more the film is antistatic. We consider generally a satisfactory level of antistatism is reached as soon as this time of half discharge is less than or equal to 20 seconds and preferably less than or equal at 10 seconds.

The thickness of the primary coating (P) is preferably equal or less than 0.2 µm.

It is generally preferable that there are no charge particles in the thick layer (A) and that said fillers are present in the thin layers (B), so that the support films have as high a transparency as possible, that is to say a turbidity (or haze) less than or equal to 7%.

The nature of the charges present in the layer (B) is not critical and we can use the charges usually used to communicate to movies polyester with sufficient roughness to ensure good machinability. We do preferably use mineral fillers such as oxides and salts of elements of groups II, III and IV of the periodic table. As an illustration, we can cite metal salts such as calcium carbonate or barium sulfate; of oxides such as silica, alumina, zirconia, mixtures of oxides, silicates or aluminosilicates. These charges may have undergone a treatment intended to limit or prevent agglomeration of the particles which compose them and / or limit or prevent decohesion at the polyester / particle interface.

The particle concentration is chosen so as to ensure the support film both a turbidity less than or equal to 7% and a sufficient roughness (Rz lower or equal to 0.6 µm). In general, the concentration and the particle size of the charges are chosen according to the thickness of the layer (B) and the melting point of the (or) polymer (s) constituting it and can be determined by a person skilled in the art for give the elements the desired transparency and roughness. We can do anything particularly refer to the teaching of patent EP-A-0 260 258 which describes films which may very well be suitable as transparent polyester support for the elements transparencies for electrostatic photocopying of the invention.

It is particularly advantageous to use loads having a relatively narrow particle size distribution, i.e. at loads monodisperse.

The shape of the particles of the charges introduced into the layer (s) (B) is not critical and we can use loads of various forms, spherical or not.

When a layer (B) is placed on each of the faces of the layer (A), the two layers (B) can be distinguished from each other by their thickness, nature, concentration or particle diameter of the filler. For some obvious reasons of simplicity, the two layers (B) are preferably identical.

Although the composite polyester support film can be obtained by all known processes for obtaining composite films, use is preferably made of composite films obtained by coextrusion which exhibit excellent cohesion to the interface of the layer (A) and of the layer (s) (B).

To this end, a flow (A) of polyester which can be crystallized and, simultaneously, using a second extruder, a flow of polyester (B) intended to form the layer (s) (B). The two extruders are connected to a coextrusion box in which the flow (B) can be, if necessary, divided into two streams (B). The streams of molten polymers are transformed into a film amorphous multilayer by passing through a flat die and the amorphous film thus obtained is subjected to the usual filming operations: quenching, stretching, heat setting and winding.

The filming conditions are those usually used industrially for obtaining oriented semi-crystalline polyester films. Coming out of the die, the amorphous composite film is cooled to a temperature between 10 ° C. and 45 ° C on a casting drum.

The conditions for stretching the extruded composite film are those usually used in the manufacture of semi-crystalline polyester films. Thus, it is possible to carry out a mono-stretching or a bi-stretching carried out successively or simultaneously in two directions in general orthogonal or in sequences at least 3 stretches where the stretching direction is changed in each sequence. Through moreover, each mono-directional stretch can itself be carried out in several steps. We can thus associate stretching sequences such as two successive bi-stretch treatments, each stretch can be carried out in several phases.

Preferably, the composite film is subjected to bi-stretching in two perpendicular directions. You can, for example, first stretch in the direction of movement of the film (longitudinal stretching), then to a stretching according to a perpendicular direction (transverse stretching) or vice versa. In general, stretching longitudinal is carried out at a rate of 3 to 5 (i.e. the length of the stretched film represents 3 to 5 times the length of the amorphous film) and at a temperature of 80 to 135 ° C and the transverse stretching is carried out with a rate of 3 to 5 at a temperature of 90 to 135 ° C and preferably between 100 and 125 ° C.

After stretching, the composite film is subjected to a heat treatment at a temperature between 160 and 240 ° C.

Stretching can also be carried out simultaneously, i.e. both in the longitudinal direction and in the transverse direction, for example with a rate stretching from 3 to 5 and at a temperature of 80 to 120 ° C.

The speed of the extruders depends on the thicknesses desired for the layers (A) and (B) after stretching.

The thickness eA of the layer (A) can vary within wide limits; in generally, it is between approximately 50 μm and approximately 150 μm. Although the thickness Layer eB (B) is not critical, it must be chosen so that the support film retains excellent transparency and in particular lower turbidity equal to 5% and preferably less than or equal to 4%. The thickness allowing achieving this goal depends, to some extent, on the focus and particle size of the filler present in the layer (B); in a way general, it is preferable that the thickness of the layer (B) is equal to or less than 3 µm. Preferably, eB is between 0.5 μm and 1.5 μm.

Due to the temperatures to which the elements are subjected transparencies during electrostatic photocopying operations, the polyester support must have excellent dimensional stability at 150 ° C. More specifically, the support must have a shrinkage rate at 150 ° C in both directions of stretching, less than or equal to 1% and preferably less than or equal to 0.7%. This objective is achieved by subjecting the bi-stretched and heat-fixed film to a relaxation treatment in the direction transverse and in the longitudinal direction according to the usual well-known methods of one skilled in the art. Preferably, the withdrawal rates in the longitudinal direction and in the transverse directions are chosen so that they do not have a difference of value too large, so as to avoid the formation of micro-waves of the elements transparencies after passing through the photocopying machines. It is better that the difference in shrinkage values in both directions is less than or equal to 0.3%.

In the transparent elements for electrostatic photocopying according to the invention, the polyester composite support (S) preferably has a turbidity less than or equal to 7%, a withdrawal rate in the longitudinal directions and transverse stretch at 150 ° C less than or equal to 1% and a total roughness Rz less than or equal to 0.6 µm.

The coating (P) can be deposited on the composite polyester film by the various techniques known to those skilled in the art. So a dispersion or an aqueous solution of the acrylic polymer chosen can be deposited by gravity from a slot casting machine, or by passing the film through the emulsion or solution or by means of transfer rollers. The thickness of the layer is controlled by everything appropriate means. The coating can be deposited either before stretching the film (line coating), either after stretching before or after heat setting (coating reprise). However, it is preferred to carry out the coating of the polyester film before stretching or between two draws.

Preferably, a latex of the acrylic polymer, prepared, will be used. by emulsion, microemulsion or, where appropriate, by polymerization polymerization in an organic medium. These techniques familiar to a person skilled in the art do not will not be recalled here.

So acrylic polymers, used in the context of this invention are preferably used in the form of stable dispersions, or latex, in water or a hydro-organic medium. When the polymer does not contain hydrophyl group making it possible to easily obtain a latex, it can be associated with one or several ionic or nonionic surfactants, such as those usually used for obtaining aqueous dispersions well known to those skilled in the art. For obtaining the particles, the polymer content of the latexes is not critical and may vary within wide limits. In general, latexes containing from 1 to 50% by weight of polymer are well suited; preferably, latexes containing from 5 to 30% by weight of polymer. The particle size of the polymer constituting the latex is chosen in such a way that the final thickness of the coating (P) does not erase the roughness of the underlying loaded layer (B); in general, latex is used whose diameter of the polymer particles is between approximately 0.01 μm and 0.3 µm and preferably between 0.05 µm and 0.15 µm.

The amount of aqueous coating composition deposited on the film depends on the one hand on its dry extract content and on the other hand on the desired thickness for coating the finished film, i.e. after stretching and heat setting when coating takes place online. This amount also depends on the time of coating; we must obviously take into account the variation in thickness of the front coating and after stretching, when the coating is carried out before stretching.

After coating, the polyester film is heat treated to remove water contained in the coating and, if necessary, to cause crosslinking of the polymer. In the case of online coating, it is generally not necessary to heat treatment; drying and possibly crosslinking are made during stretching and heat setting. We would not, however, leave the framework of the present invention by proceeding, in this case, prior to stretching and thermofixation, to a heat treatment sufficient to cause the drying of the coated layer.

The thickness eP of the coating layer (P) is such that it does not erase the roughness of the underlying loaded layer (B). The use of online coating and choice of acrylic polymer allow this objective to be achieved, without compromising toner adhesion. In general, eP is between 0.02 and 0.2 µm and preferably between 0.02 and 0.15 µm.

The use of a support (S) consisting of a thick uncharged layer (A) and at least one thin layer (B) preferably containing a filler, allows easy access, after depositing the adhesion layer (P) to elements transparencies for electrostatic photocopying which have all the properties transparency, processability and toner adhesion required. It is, finally, not necessary to multiply the deposition of the adhesion layers on the support (S), by primary example of adhesion then toner receiving layer, and to introduce into this last of the charges which may be subject to abrasion during the use of transparent. Furthermore, the use of a coextruded composite support (S) makes it possible to easily achieve a good compromise between transparency and machinability.

The following examples illustrate the invention.

Example 1

• eau   853 g
• laurylsulfate de sodium   1,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 853 g
• sodium lauryl sulfate 1.5 g

The solution is brought to 80 ° C.

1.05 g of ammonium persulfate dissolved in 18.95 g of water are added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   175,5 g
• méthacrylate de méthyle   117,9 g
• acide méthacrylique   4,5 g
• diméthacrylate d'éthylèneglycol   2,1 g

b) pendant 8 h la solution suivante :

• persulfate d'ammonium   0,84 g
• bicarbonate de sodium   1,23 g
• laurylsulfate de sodium   1,5 g
• eau   46,4 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 175.5 g
• methyl methacrylate 117.9 g
• methacrylic acid 4.5 g
• ethylene glycol dimethacrylate 2.1 g

b) for 8 h the following solution:

• ammonium persulfate 0.84 g
• sodium bicarbonate 1.23 g
• sodium lauryl sulfate 1.5 g
• water 46.4 g

The reaction mixture is maintained for 1 hour at 80 ° C., then is cooled.

A latex A with 25% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 20 ° C and a rate of free carboxylic functions of 17 millimoles per 100 g of polymer.

Example 2

• eau   730 g
• émulsifiant à fonction amine quatemisée par des groupes hydroxyéthyle (Ethoquad C12®)   1,5 g
• acrylate d'éthyle   13,8 g
• méthacrylate de méthyle   10,2 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 730 g
• emulsifier with amine function quaternized by hydroxyethyl groups (Ethoquad C12®) 1.5 g
• ethyl acrylate 13.8 g
• methyl methacrylate 10.2 g

The solution is brought to 75 ° C.

0.30 g of water-soluble initiator of the diazo compound type is added to quaternary ammonium group (V50) dissolved in 9.7 g of water.

a) pendant 7 h les mélanges suivants :

• acrylate d'éthyle   124,2 g
• méthacrylate de méthyle   91,8 g

et

• chlorure d'acryloxyéthyl triméthyl ammonium   75 g
• eau   45 g

b) pendant 8 h la solution suivante :

• initiateur V50   3,6 g
• émulsifiant Ethoquad C12®   1,5 g
• eau   50 g

We then continuously introduce:

a) for 7 hours the following mixtures:

• ethyl acrylate 124.2 g
• methyl methacrylate 91.8 g

and

• 75 g acryloxyethyl trimethyl ammonium chloride
• water 45 g

b) for 8 h the following solution:

• initiator V50 3.6 g
• Ethoquad C12® emulsifier 1.5 g
• water 50 g

The reaction mixture is maintained for 1 hour at 75 ° C., then is cooled.

A latex B with 26% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 21 ° C and a rate of free carboxylic functions of 0 millimole per 100 g of polymer.

Example 3

• eau   760 g
• émulsifiant Ethoquad C12®   1,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 760 g
• Ethoquad C12® emulsifier 1.5 g

The solution is brought to 60 ° C.

0.30 g of initiator V50 dissolved in 9.7 g of water is added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   172,8 g
• méthacrylate de méthyle   127,2 g

b) pendant 8 h la solution suivante :

• initiateur V50   3,6 g
• émulsifiant Ethoquad C12®   1,5 g
• eau   50 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 172.8 g
• methyl methacrylate 127.2 g

b) for 8 h the following solution:

• initiator V50 3.6 g
• Ethoquad C12® emulsifier 1.5 g
• water 50 g

The reaction mixture is kept for 1 hour at 60 ° C., then is cooled.

A latex C with 27% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 20 ° C and a rate of free carboxylic functions of 0 millimole per 100 g of polymer.

Two mixtures are prepared from this latex, respectively containing 1.6% (C1) and 3.6% (C2) by weight of stearamidopropyl dimethyl nitrate p-hydroxyethyl ammonium relative to the total weight of the latex (to have after dilution 17% of total dry extract respectively 1% and 2% of the ammonium compound quaternary).

Example 4

• eau   730 g
• émulsifiant Ethoquad C12®   1,5 g
• acrylate d'éthyle   16,5 g
• méthacrylate de méthyle   12 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 730 g
• Ethoquad C12® emulsifier 1.5 g
• ethyl acrylate 16.5 g
• methyl methacrylate 12 g

The solution is brought to 75 ° C.

0.30 g of water-soluble initiator V50 dissolved in 9.7 g of water is added.

a) pendant 7 h les mélanges suivants :

• acrylate d'éthyle   148,5 g
• méthacrylate de méthyle   108 g

et

• chlorure d'acryloxyéthyl triméthyl ammonium   19 g
• eau   30 g

b) pendant 8 h la solution suivante :

• initiateur V50   3 g
• émulsifiant Ethoquad C12®   1,5 g
• eau   50 g

We then continuously introduce:

a) for 7 hours the following mixtures:

• ethyl acrylate 148.5 g
• methyl methacrylate 108 g

and

• acryloxyethyl trimethyl ammonium chloride 19 g
• water 30 g

b) for 8 h the following solution:

• initiator V50 3 g
• Ethoquad C12® emulsifier 1.5 g
• water 50 g

The reaction mixture is maintained for 1 hour at 75 ° C., then is cooled.

A latex D with 27% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 23 ° C and a rate of free carboxylic functions of 0 millimole per 100 g of polymer.

Example 5

• eau   665 g
• émulsifiant Ethoquad C12®   1,5 g
• acrylate d'éthyle   10,5 g
• méthacrylate de méthyle   7,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 665 g
• Ethoquad C12® emulsifier 1.5 g
• ethyl acrylate 10.5 g
• methyl methacrylate 7.5 g

The solution is brought to 75 ° C.

0.30 g of water-soluble initiator V50 dissolved in 9.7 g of water is added.

a) pendant 7 h les mélanges suivants :

• acrylate d'éthyle   94,5g
• méthacrylate de méthyle   67,5 g

et

• chlorure d'acryloxyéthyl triméthyl ammonium   150 g
• eau   60 g

b) pendant 8 h la solution suivante :

• initiateur V50   3,6 g
• émulsifiant Ethoquad C12®   1,5 g
• eau   50 g

We then continuously introduce:

a) for 7 hours the following mixtures:

• ethyl acrylate 94.5g
• methyl methacrylate 67.5 g

and

• acryloxyethyl trimethyl ammonium chloride 150 g
• water 60 g

b) for 8 h the following solution:

• initiator V50 3.6 g
• Ethoquad C12® emulsifier 1.5 g
• water 50 g

The reaction mixture is maintained for 1 hour at 75 ° C., then is cooled.

A latex E with 27% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 17 ° C and a rate of free carboxylic functions of 0 millimole per 100 g of polymer.

Comparative test 1

• eau   848,5 g
• laurylsulfate de sodium   1,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 848.5 g
• sodium lauryl sulfate 1.5 g

The solution is brought to 80 ° C.

1.05 g of ammonium persulfate dissolved in 18.95 g of water are added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   162 g
• méthacrylate de méthyle   99 g
• acide méthacrylique   39 g

b) pendant 8 h la solution suivante :

• persulfate d'ammonium   0,84 g
• bicarbonate de sodium   1,23 g
• laurylsulfate de sodium   1,5 g
• eau   46,4 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 162 g
• methyl methacrylate 99 g
• methacrylic acid 39 g

b) for 8 h the following solution:

• ammonium persulfate 0.84 g
• sodium bicarbonate 1.23 g
• sodium lauryl sulfate 1.5 g
• water 46.4 g

The reaction mixture is maintained for 1 hour at 80 ° C., then is cooled.

A latex 1 at 25% of dry extract is obtained.

The acrylic polymer has a glass transition temperature of 33 ° C and a rate of free carboxylic functions of 150 millimoles per 100 g of polymer.

Comparative test 2

• eau   751 g
• laurylsulfate de sodium   1,5 g
• polyester hydrodispersable   93 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 751 g
• sodium lauryl sulfate 1.5 g
• water-dispersible polyester 93 g

The water-dispersible polyester used is a patterned copolyester derived from terephthalic acid, isophthalic acid, Na isophthalic acid-5-sulfonate and ethylene glycol, sold under the brand name Gérol PS20.

The solution is brought to 80 ° C.

0.72 g of ammonium persulfate dissolved in 19.28 g of water are added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   58,5
• méthacrylate de méthyle   120 g
• acide méthacrylique   26,4 g
• diméthacrylate d'éthylèneglycol   2,1 g

b) pendant 8 h la solution suivante :

• persulfate d'ammonium   0,58 g
• bicarbonate de sodium   0,85 g
• laurylsulfate de sodium   1,05 g
• eau   47,52 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 58.5
• methyl methacrylate 120 g
• methacrylic acid 26.4 g
• ethylene glycol dimethacrylate 2.1 g

b) for 8 h the following solution:

• ammonium persulfate 0.58 g
• sodium bicarbonate 0.85 g
• sodium lauryl sulfate 1.05 g
• water 47.52 g

The reaction mixture is maintained for 1 hour at 80 ° C., then is cooled.

A latex J with 27% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 30 ° C and a rate of free carboxylic functions of 102 millimoles per 100 g of polymer.

Comparative test 3

• eau   848,5 g
• laurylsulfate de sodium   1,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 848.5 g
• sodium lauryl sulfate 1.5 g

The solution is brought to 80 ° C.

1.05 g of ammonium persulfate dissolved in 18.95 g of water are added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   105,6 g
• méthacrylate de méthyle   181,5 g
• acide méthacrylique   12,9 g

b) pendant 8 h la solution suivante :

• persulfate d'ammonium   0,84 g
• bicarbonate de sodium   1,23 g
• laurylsulfate de sodium   1,5 g
• eau   46,4 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 105.6 g
• methyl methacrylate 181.5 g
• methacrylic acid 12.9 g

b) for 8 h the following solution:

• ammonium persulfate 0.84 g
• sodium bicarbonate 1.23 g
• sodium lauryl sulfate 1.5 g
• water 46.4 g

The reaction mixture is maintained for 1 hour at 80 ° C., then is cooled.

A latex K with 25% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 56 ° C and a rate of free carboxylic functions of 50 millimoles per 100 g of polymer.

Comparative test 4

• eau   848,5 g
• laurylsulfate de sodium   1,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 848.5 g
• sodium lauryl sulfate 1.5 g

The solution is brought to 80 ° C.

1.05 g of ammonium persulfate dissolved in 18.95 g of water are added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   111 g
• méthacrylate de méthyle   163 g
• acide méthacrylique   26 g

b) pendant 8 h la solution suivante :

• persulfate d'ammonium   0,84 g
• bicarbonate de sodium   1,23 g
• laurylsulfate de sodium   1,5 g
• eau   46,4 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 111 g
• methyl methacrylate 163 g
• methacrylic acid 26 g

b) for 8 h the following solution:

• ammonium persulfate 0.84 g
• sodium bicarbonate 1.23 g
• sodium lauryl sulfate 1.5 g
• water 46.4 g

The reaction mixture is maintained for 1 hour at 80 ° C., then is cooled.

A latex L with 25% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 58 ° C and a rate of free carboxylic functions of 100 millimoles per 100 g of polymer.

Comparative test 5

• eau   848,5 g
• laurylsulfate de sodium   1,5 g

In a 1.5 liter reactor fitted with an anchor stirrer, the following are introduced:

• water 848.5 g
• sodium lauryl sulfate 1.5 g

The solution is brought to 80 ° C.

1.05 g of ammonium persulfate dissolved in 18.95 g of water are added.

a) pendant 7 h le mélange suivant :

• acrylate d'éthyle   85,5 g
• méthacrylate de méthyle   175,8 g
• acide méthacrylique   38,7 g

b) pendant 8 h la solution suivante :

• persulfate d'ammonium   0,84 g
• bicarbonate de sodium   1,23 g
• laurylsulfate de sodium   1,5 g
• eau   46,4 g

We then continuously introduce:

a) for 7 h the following mixture:

• ethyl acrylate 85.5 g
• methyl methacrylate 175.8 g
• methacrylic acid 38.7 g

b) for 8 h the following solution:

• ammonium persulfate 0.84 g
• sodium bicarbonate 1.23 g
• sodium lauryl sulfate 1.5 g
• water 46.4 g

The reaction mixture is maintained for 1 hour at 80 ° C., then is cooled.

A latex M with 25% dry extract is obtained.

The acrylic polymer has a glass transition temperature of 61 ° C and a rate of free carboxylic functions of 150 millimoles per 100 g of polymer.

Examples 6 to 11 and comparative tests 6 to 10. Preparation of transparent elements for photocopying.

The support film of the following examples is a polyterephthalate film bi-stretched ethylene glycol (PET), having a total thickness of 100 µm, composed of central layer A of 98.4 μm of unfilled PET film and on each of the faces of the layer A of a layer of 0.8 μm of PET comprising 0.3000% of a mineral filler with an average diameter of 3.5 µm (Sylobloc® type silica), the particle size having been measured with a Sympathec® brand laser granulometer and of the Helos type.

The preparation is carried out as follows. The coextruded amorphous film is first of all mono-stretched in the longitudinal direction with a rate of 3.8, then coated on one of its faces with a latex as prepared in the examples and comparative tests described above (after dilution to present a dry extract of 17% by weight), at a rate of 1.4 g / m ² (in the wet state).

The coating is dried, then the film is stretched in the transverse directions with a rate of 3.8. The final layer of acrylic polymer is 0.06 µm.

The bi-stretched film is thermofixed at 235 ° C.

The film thus obtained has a haze of 4.5, a total roughness R _z of 0.45 μm and shrinkages at 150 ° C. of 0.6% in the longitudinal and transverse directions.

On each of the transparent elements for photocopying thus obtained, we measures toner adhesion and half-discharge time (representative of antistatism).

• 0 correspond au toner totalement arraché (très mauvais)
• 10 correspond au toner pas du tout arraché (excellent)
• des valeurs de 8 à 10 sont considérées comme satisfaisantes.

Toner adhesion is measured by photocopying a test pattern using a Toshiba ^® BD 2810 photocopier. It is noted from 0 to 10, based on the evaluation of the amount of toner torn from the transparent element , after folding and scraping:

• 0 corresponds to toner completely removed (very bad)
• 10 corresponds to toner not removed at all (excellent)
• values from 8 to 10 are considered satisfactory.

The table below brings together the main characteristics of the latexes used for coating films as well as the results of toner adhesion and half-charge time measurements. Examples and Comparative Tests Latex used Latex coch Tg ° C of latex Toner adhesion half discharge time Example 6 AT 17 20 9 > 100 s Example 7 B 0 21 9 1.5 s Example 8 C1 0 20 9 15 s Example 9 C2 0 20 9 4s Example 10 D 0 23 9 > 100 s Example 11 E 0 17 9 <1 s Trial 6 I 150 33 3 > 100 s Trial 7 J 102 30 4 > 100 s Trial 8 K 50 56 7 > 100 s Trial 9 L 100 58 6 > 100 s Trial 10 M 150 61 3 > 100 s

Claims (21)

1. Transparent elements for electrostatic photocopying, including a transparent polyester base (S) comprising on at least one of its faces a primer coating (P) for adhesion of the toner to the polyester base, which are characterised in that the polyester base (S) is a composite consisting of :

   (a) a thick layer (A) of semicrystalline polyester;

   (b) on at least one of the faces of the thick layer (A), a thin layer (B) of a polyester which is identical with or different from that forming the layer (A);

   and in that the primer coating (P) comprises an acrylic polymer which has a glass transition temperature ranging from 10°C to 50°C, a free -COOH carboxylic functional group content lower than 50 millimoles per 100 grams of the said acrylic polymer, and in that the said primer coating (P) has a thickness equal to or smaller than 0.3 µm.
2. Elements according to claim 1, characterised in that the polyesters forming the layers (A) and (B) of the base film (S) are preferably crystallisable film-forming polyesters chosen from the polyterephthalates and polynaphthalenedicarboxylates of alkylenediols, in particular the polyterephthalate of ethylene glycol or of 1,4-butane-diol, and the copolyesters containing at least 80 mol % of alkylene glycol terephthalate or naphtalene-dicarboxylate units.
3. Elements according to either of claims 1 and 2, characterised in that the acrylic polymer contains units chosen from units derived from acrylic acid, methacrylic acid, alkyl acrylates, alkyl methacrylates, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, N-methylolacrylamide, N-methoxymethacrylamide, styrene, butadiene, vinyl esters, at least some of these units originating from an alkyl acrylate and/or an alkyl methacrylate.
4. Elements according to anyone of claims 1 to 3, characterised in that the acrylic polymer contains at least units derived from alkyl acrylates chosen from methyl acrylate, ethyl acrylate, propyl acrylates and butyl acrylates and/or units derived from alkyl methacrylates chosen from methyl methacrylate, ethyl methacrylate, propyl methacrylates and butyl methacrylates and preferably contains units derived from methyl and/or ethyl acrylate and from methyl and/or ethyl methacrylate.
5. Elements according to anyone of claims 1 to 4, characterised in that the molar quantity of free -COOH carboxylic functional groups in the acrylic polymer is equal to or lower than 30 millimoles per 100 grams of the said acrylic polymer.
6. Elements according to anyone of claims 1 to 5, characterised in that the acrylic polymer has a glass transition temperature ranging from 15°C to 30°C.
7. Elements according to anyone of claims 1 to 6, characterised in that the acrylic polymer forming the primer (P) of the elements for photocopying contains up to 25 % by weight of a compound (monomer or polymer) containing quaternary ammonium groups.
8. Elements according to claim 7, characterised in that the compounds containing quaternary ammonium groups, which are used or which are the source of polymers containing quaternary ammonium groups which are themselves used, correspond to the following general formula (I) :

   

   in which:

   R₁ denotes an acyloxyalkyl or acylaminoalkyl radical containing a saturated acyl group or containing a carbon-carbon ethylenic double bond, an alkoxyalkyl radical, an aryloxyalkyl radical, an alkylaryloxyalkyl radical, an alkenyloxyalkyl radical, an alkenylaryloxyalkyl radical,

   R₂, R₃ and R₄, which are identical or different, denote an alkyl radical containing from 1 to 6 carbon atoms, a polyoxyethylene radical -(CH₂-CH₂-O-)_n-H or -(CH₂-CH₂-O-)_n-CH₃ with n denoting a number ranging from 1 to 12,

   X is an anion chosen from the group including halides, in particular chloride, sulphate, sulphonate, alkyl sulphonates such as methyl sulphonate, arylsulphonates, arylalkyl sulphonates, carbonate, alkyl carbonates such as methyl carbonate, nitrate, phosphate, alkyl phosphates or mixtures of these anions.
9. Elements according to claim 8, characterised in that the compounds of formula (I) are chosen from non-polymerisable compounds such as stearamidopropyldimethyl-β-hydroxyethyl-ammonium nitrate.
10. Elements according to claim 8, characterised in that the compounds of formula (I) are chosen from the polymerisable monomers of formulae (II) or (III) :

    

    

    in which :

    R₅, R₆ and R₇, which are identical or different, denote a hydrogen atom, a methyl radical or an ethyl radical,

    R₈, R₉ and R₁₀, which are identical or different, denote an alkyl radical containing from 1 to 4 carbon atoms, a polyoxyethylene radical -(CH₂-CH₂-O-)_m-H or -(CH₂-CH₂-O-)_m-CH₃ with m denoting a number ranging from 1 to 8,

    R₁₁ denotes a divalent radical such as polyethylene or hydroxyalkylene containing from 1 to 8 carbon atoms,

    X denotes an anion chosen from the group including halides, in particular chloride, sulphate, sulphonate, alkyl sulphonates such as methyl sulphonate, arylsulphonates, arylalkylsulphonates, carbonate, alkyl carbonates such as methyl carbonate, nitrate, phosphate, alkyl phosphates or mixtures of these anions.
11. Elements according to anyone of claims 7 to 10, characterised in that the compound containing quaternary ammonium groups represents from 2 % to 15 % by weight relative to the weight of the combination of acrylic polymer and compound containing quaternary ammonium groups and in that the acrylic polymer does not contain any free carboxylic acid functional group.
12. Elements according to any one of claims 1 to 11, characterised in that the thickness eP of the primer coating (P) ranges from 0.02 µm to 0.2 µm and preferably from 0.02 µm to 0.15 µm.
13. Elements according to anyone of claims 1 to 12, characterised in that there are no filler particles in the thick layer (A) and in that the said fillers are present in the thin layers (B) in order that the films have a haze (or cloudiness) lower than or equal to 7 %.
14. Elements according to anyone of claims 1 to 13, characterised in that the thickness eA of the layer (A) ranges from 50 µm to 150 µm and in that the thickness eB of the layer (B) is equal to or smaller than 3 µm and preferably from 0.5 µm to 1.5 µm.
15. Elements according to anyone of claims 1 to 14, characterised in that the composite polyester base (S) has a haze lower than or equal to 7 %, a shrinkage ratio in the lengthwise and transverse drawing directions at 150°C which is lower than or equal to 1 % and a total roughness Rz lower than or equal to 0.6 µm.
16. Process for the preparation of elements according to anyone of claims 1 to 15, characterised in that the composite polyester base film is obtained by a coextrusion process in which a flow (A) of crystallisable polyester is extruded with the aid of a first extruder and, simultaneously, a flow of the polyester (B) intended to form the layer(s) (B) is extruded with the aid of a second extruder, the two extruders being connected to a coextrusion adapter in which the flow (B) may, if appropriate, be divided into two flows (B), the polymer melt streams being converted into a amorphous multilayer film by passing through a flat die and the amorphous film thus obtained being subjected to the usual film manufacturing operations such as quenching, drawing, heat-setting and reeling and in that a coating (P) is deposited on at least one face of the polyester film.
17. Process according to claim 16, characterised in that the extruded amorphous composite film is subjected to biaxial drawing in two perpendicular directions, firstly to a lengthwise drawing and then to a transverse drawing, or vice versa, the lengthwise drawing being performed with a ratio of 3 to 5 and at a temperature of 80 to 135°C and the transverse drawing being performed with a ratio of 3 to 5 and at a temperature of 90 to 135°C and preferably of 100 to 125°C, the composite film being subjected after drawing to a heat treatment at a temperature ranging from 160 to 240°C.
18. Process according to either of claims 16 and 17, characterised in that the deposition of the coating (P) on the composite polyester film is carried out using various known techniques such as the deposition of an aqueous dispersion or solution of the acrylic polymer, by gravity from a slot caster or by passing the composite polyester film through the emulsion or the solution or else by means of transfer rolls, the thickness of the layer being controlled by any suitable means and it being possible for the deposition of the coating to take place either before any drawing of the film when the coating takes place in-line or after drawing, before or after heat-setting when the coating takes place out-of-line coating, and preferably before the drawing operation or between two drawings.
19. Process according to claim 18, characterised in that a stable dispersion, or latex, in water or in a hydroorganic medium, of the acrylic polymer is used to carry out the deposition of the coating (P).
20. Process according to claim 19, characterised in that the latex used contains from 1 to 50 % by weight of acrylic polymer and preferably from 5 to 30 % by weight of acrylic polymer.
21. Process according to either of claims 19 and 20, characterised in that the diameter of the particles of the acrylic polymer forming the latex is from 0.01 µm to 0.3 µm and preferably from 0.05 µm to 0.15 µm.