Title of Invention

" BIAXIALLY DRAWN COMPOSITE FILM AND A PROCESS FOR THE PREPARATION THEREFOR'

Abstract

A polyester-based, biaxially drawn composite film having a permeability to oxygen, measured at 23°C at 50% relative humidity, of lower than or equal to 3 cm3/m2/24 h, which composite film comprises a polyester base film having a thickness of from 5 /Lm to 50 jLim, coated on at least one of its two faces with a layer of a polyvinyl alcohol which has a number-average degree of polymerization equal to or higher than 350, said layer of polyvinyl alcohol having a thickness of lower than or equal to 0.3 ^m, the mean roughness Rz of the base film being lower than or equal to 0.30 /xm on the face(s) of the film bearing the polyvinyl alcohol layer and that or these said face(s) comprising on average 20 peaks or less of a height equal to or higher than 1 micrometre and 150 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre.

Full Text

The present invention relates to composite films which have good gas-barrier properties and a process for the preparation thereof. Polyester films, especially of polyethylene terephthalate, are widely employed in the packaging field, because of their many advantages, such as their mechanical properties, transparency, nontoxicity, odourlessness and tastelessness. However, their gas-barrier properties may limit their use in applications requiring a high protection of the packaged products against the action of external gases, in particular against the action of atmospheric oxygen or, conversely, may not allow the gas composite inside the packages to be kept constant. In order to overcome this disadvantage it has been proposed in GB-A-1,126,952 to deposit a solution of polyvinyl alcohol on a polymer film in order to create a polyvinyl alcohol layer, since polyvinyl alcohol has good gas-barrier properties. In order to allow a good adhesion of the polyvinyl alcohol layer to films made of cellulose acetate, polycarbonate or polyethylene terephthalate, an intermediate layer of an adhesive made of polyurethane is arranged between the base film and the polyvinyl alcohol layer. EP-A-0,254,468 also describes a composite film making up a base film made of a synthetic thermoplastic polymer such as a polyamide, polyethylene, polypropylene or polyester, having two coatings on the same single face of the base film; the first coating is adjacent to the base film and consists of a urethane primer in a solvent, which in the dry state allows a dispersion of polyvinyl alcohol in aqueous solution to wet the primer, and the second coating is placed over the dried and free surface of the first coating and includes a polyvinyl alcohol-based material which acts as a gas barrier. In the composite films of the prior art the polyvinyl alcohol coating is generally separated from the base film by an adhesive layer made of a polyurethane. Moreover, the polyvinyl alcohol layer is generally thick (for example from 0.15 to 2 micrometres in the claims of EP-A-0,254,468), and this complicates the recyclability of the composite films. The present invention provides polyester films with good gas-barrier properties by virtue of a polyvinyl alcohol coating arranged directly on the surface of the polyester film without the presence of any intermediate adhesive layer. The present invention provides a polyester-based, biaxially drawn composite film having a permeability to oxygen, measured at 23°C at 50% relative humidity, of lower than or equal to 3 cm3/m2/24h, which composite film comprises a polyester base film having a thickness of from 5 jam to 50 um, having a coating on at least one of its two faces with a layer of a polyvinyl alcohol which has a number-average degree of polymerisation equal to or greater than 350, said layer of polyvinyl alcohol having a thickness of lower than or equal to 0.3 um, the mean roughness Rz of the base film being lower than or equal to 0.30 um on the face(s) of the film bearing the polyvinyl alcohol layer and that or these said face(s) comprising on average 20 peaks or less of a height equal to or greater than 1 micrometre and 150 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre. The peak height distribution referred to above to define the surface topography of the polyester base film may especially be determined, in a known manner, by observation in an interferometric microscope allowing the number of interference rings with a light of known wavelength to be counted. The most frequently employed interferometers are the Nomarski, Mirau or Michelson interferometers. The polyester of the base film may be chosen from polyesters which are usually employed for obtaining biaxially oriented semicrystalline films. These are film-forming linear polyesters capable of being crystallized by orientation and usually obtained from one or a number of dicarboxylic aromatic acids or their derivatives (for example esters of lower aliphatic alcohols or halides) and from one or a number of aliphatic glycols. Phthalic, terephthalic, isophthalic, 2,5-naphthalenedicarboxylic and 2,6-naphthalenedicarboxylic acids may be mentioned as examples of aromatic acids. These acids may be used in combination with a minor quantity of one or a number of aliphatic or cycloaliphatic dicarboxylic acids such as adipic, azelaic or hexahydroterephthalic acids. Ethylene glycol, 1,3-propanediol and 1,4-butanediol may be mentioned as nonlimiting examples of aliphatic diols. These diols may be used in combination with a minor quantity of one or a number of aliphatic diols of higher carbon condensation (for example neopentyl glycol) or cycloaliphatic diols (for example cyclohexanedimethanol). The crystallizable film-forming polyesters are preferably alkylenediol polyterephthalates or polynaphthalenedicarboxylates and, in particular, the polyterephthalate of ethylene glycol (PET) or of 1,4-butanediol, or copolyesters containing at least 80 mol% of ethylene glycol terephthalate units. The polyester is advantageously a polyethylene terephthalate whose intrinsic viscosity measured at 25°C in ortho-chlorophenol is from 0.6 to 0.75 dl/g. The polyester of the base film must be chosen such that its initial melting temperature is higher than the temperature to which the biaxially drawn film comprising the polyvinyl alcohol layer is heated during its preparation. The mean roughness Rz of the base film (as defined in DIN Standard 4768) is lower than or equal to 0.30 jum and preferably lower than 0.25 /j.m on the face(s) of the film bearing the polyvinyl alcohol layer. It is preferable that, on average, said face(s) should comprise 800 peaks or less of a height lower than 0.4 micrometres, per square millimetre. For industrial applications of the processes for the preparation of the films of the invention, in which the machine speed to which the film is subjected is generally higher than 100 metres per minute, it is more particularly preferred that the face(s) of the base film bearing the polyvinyl alcohol layer should on average comprise 20 peaks or less of a height equal to or greater than 1 micrometre and 100 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre. Finally, it is still more preferable, in this industrial context, that additionally the face(s) of the base film bearing the polyvinyl alcohol layer should on average comprise 500 peaks or less of a height lower than 0.4 micrometres, per square millimetre. It is obvious, however, that when operating at lower speeds, especially in tests in a pilot or laboratory plant, this preferential limitation is not necessary. The face which does not bear a polyvinyl alcohol layer, called the back face, should have sufficient slip properties to permit easy handling of the film, especially its winding over the various guide rolls during drawing operations or its reeling onto itself. This roughness may be brought about in various ways. One of the most common procedures consists in incorporating inert solid fillers into the polyester before the conversion into film. These fillers are generally inorganic fillers such as, for example, silica, titanium dioxide, zirconium dioxide, alumina, silica/alumina mixtures, silicates, calcium carbonate and barium sulphate. These fillers may also consist of polymer particles. The volume-median diameter of the fillers is generally from 1 to 10 micrometres, preferably from 1 to 5 micrometres. The filler content of the film is usually from 0.02 % to 1 % by weight relative to the weight of the polyester. An advantageous form of the invention consists in having different mean roughnesses Rz on the two faces of the base polyester film, for example one equal to or higher than 0.15 micrometres on the back face of the film and one lower than or equal to 0.30 micrometres, and preferably lower than or equal to 0.25 micrometres, on the face of the base film bearing the polyvinyl alcohol coating, the face of the base film which has the polyvinyl alcohol coating preferably comprising on average 20 peaks or less of a height equal to or greater than 1 micrometre and 100 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre and, still more preferably, 500 peaks or less of a height lower than 0.4 micrometres, per square millimetre. The polyester base film may thus consist of two layers exhibiting different surface properties, especially roughnesses. The production of such unsymmetrical films may be carried out using the technique of coextrusion of two polyesters comprising different filler contents and, if appropriate, different fillers. The polyester employed is conveniently the same for the two coextruded layers and the layer which is to receive the polyvinyl alcohol layer is not filled. The relative thicknesses of the two polyester layers of which the base polyester film consists may vary widely. The unfilled (or less highly filled) layer bearing the polyvinyl alcohol generally has a thickness greater than or equal to 0.5 jim, preferably greater than or equal to 1.0 /urn. Within the scope of the invention it is not ruled out to obtain a polyester base film exhibiting different surface properties on both faces, using other known means of the prior art. It is thus possible, according to the teaching of EP-A-0,378,954, to impart a good slip to the back face of the base film by depositing on the said back face a modified polymer obtained by aqueous phase radical polymerization of at least one acrylic monomer and of a water-dispersible polyester derived from at least one dicarboxylic aromatic acid and from at least one aliphatic diol and comprising a plurality of sulphonyloxy groups, especially sodium sulphonate. As indicated above, the polyvinyl alcohol layer has a thickness equal to or lower than 0.3 jum. If desired, this thickness may be lower than or equal to 0.20 /Ltm or even lower than 0.10 yum, in order to improve further the recyclability of the film. It is rare to go down to thicknesses lower than 0.05 jitm in practice. The invention also relates to composite films such as those described above, which additionally -(Q - comprise on one of their faces a printing or a printing primer layer, or else a heat-sealable layer on the face free from the polyvinyl alcohol layer. The printing layer may be deposited by known printing methods such as, for example, photogravure, flexography or silkscreen printing. Inks in nonaqueous solution or dispersion are preferably employed. The heat-sealable layer is preferably of a polyolefin type (especially polyethylenes, polypropylenes and ethylene-vinyl acetate). These composite films may be obtained by adhesive backing with the aid of a single- or two-component adhesive or by extrusion coating. In this latter case the face free from the polyvinyl alcohol layer is precoated in a known manner with a bonding primer. The composite films thus produced may be used for the manufacture of packaging such as sachets, small tub closures and overwraps, intended to contain oxidation-sensitive products. They are very particularly suited for packaging under a modified atmosphere. As indicated above, one of the advantages of film of this type is that it can be easily recycled, by virtue of the low thickness of the polyvinyl alcohol layer. The present invention also relates to a process for the preparation of the films described above. More precisely the present invention provides a process for the preparation of a film as defined above wherein the polyester base film is coated on at least one of its faces, which has a mean roughness Rz of lower than or equal to 0.30 /xm and comprises on average not more than 20 peaks of a height equal to or greater than 1 micrometre and not more than 150 peaks of a height of between 0.4 and 1 micrometre, per square millimetre, with an aqueous solution of a polyvinyl alcohol containing at least 95 % vinyl alcohol units, said polyvinyl alcohol exhibiting in aqueous solution at a concentration of 4 % and at 20°C a viscosity of higher than or equal to 4 mPa s, and the coated film is heat-treated at a temperature of equal to or higher than 170°C. It is preferable that the face of a polyester film on which the polyvinyl alcohol coating is carried out additionally comprises on average 800 peaks or less of a height lower than 0.4 micrometres, per square millimetre. As indicated above, within the scope of an industrial application of the process according to the invention at a high machine speed (generally more than 100 metres per minute), it is advantageous that the coating with the aqueous solution of polyvinyl alcohol should be carried out on at least one face of a polyester film comprising on average 20 peaks or less of a height equal to or greater than 1 micrometre and 100 peaks or less of a height between 0.4 and 1 micrometre, per square millimetre. Finally, still with the prospect of an industrial application, it is more particularly preferred that the coating with the aqueous solution of polyvinyl alcohol should be carried out on at least one face of a polyester film additionally comprising on average 500 peaks or less of a height lower than 0.4 micrometres, per square millimetre. The coating of the polyester base film with a solution of polyvinyl alcohol may be carried out in line or by reprocessing. In-line coating is preferably carried out. Still more preferably the coating will be carried out in line on a monoaxially drawn polyester base film which will be drawn again after coating at least in the direction opposite to that of the first drawing. Before the coating of the base film the surface of the base film is generally subjected to a physical treatment (such as, for example, corona, flame or plasma) to ensure good spreading of the polyvinyl alcohol layer over said base film. This treatment enables the surface tension of the monoaxially drawn film to be brought to a value that is higher than that of the polyvinyl alcohol coating, and preferably to equal to or higher than 54 mN m. The aqueous solution of polyvinyl alcohol which is used generally has a concentration of 1 % to 20 % by weight, preferably from 5 % to 15 % by weight, of the polyvinyl alcohol with respect to the total weight of the solution. The solution is carefully prepared, firstly cold with simple stirring and then by heating to a temperature not exceeding 95°C; it is filtered after cooling. This solution must be free from gels. The absence of gels may be checked by measuring turbidity, solids content and refractive index. The polyvinyl alcohol employed in the process of the invention is a commercially available compound. It may be employed as it is or may be prepared especially by hydrolysis of vinyl carboxylates, more particularly polyvinyl acetates or their copolymers which are rich in vinyl acetate units, such as vinyl acetate/ethylene (or EVA) copolymers. As stated above, the polyvinyl alcohol employed comprises at least 95 % of vinyl alcohol units (degree of hydrolysis of at least 95 %). It preferably comprises at least 97 %, still more preferably at least 98% of vinyl alcohol units. The polyvinyl alcohol of which the coating of the composite film of the invention consists preferably has a viscosity in aqueous solution at a concentration of 4 % and at 20°C, measured in an apparatus of Brookfield type, which is equal to or higher than 4 centipoises (or 4 mPa s), which corresponds, within the experimental errors associated with the measurements, to a number-average degree of polymerization equal to or higher than 350. The choice of the concentration of polyvinyl alcohol in the solution and of the apparatus used for the coating is determined especially by the desired thickness of the final polyvinyl alcohol layer. Without allowing the process of the invention to be limited to this embodiment, the in-line coating is commonly carried out with the aid of rolls which are photoengraved according to the so-called "reverse gravure" technique. Although it is not limiting either, the polyester base film is in most cases drawn lengthwise (that is to say in the machine direction) before the in-line coating with the aid of the aqueous polyvinyl alcohol solution. This drawing may be performed in one or more stages, as also can the drawing following the in-line coating. The temperature at which the coated film is heat-treated preferably is from 180°C to 240°C and still more preferably from 200°C to 230°C. The polyester base may be prepared by extrusion of a polyester as defined above, comprising one or a number of inert fillers, in particular to give it a sufficient slip. It may also be prepared by coextrusion, on the one hand, of an unfilled or more slightly filled polyester and, on the other hand, of a filled polyester. In this second alternative form the polyester base film comprises a rough filled layer forming the so-called back face and an unfilled, or more slightly filled, layer forming the so-called front face which will receive the coating of the polyvinyl alcohol solution. The filled polyester layer generally has a mean roughness Rz which is higher than or equal to 0,15 micrometres and preferably higher than or equal to 0,30 micrometres, whereas the unfilled or more slightly filled front layer has a total roughness lower than or equal to 0.30 micrometres and preferably lower than or equal to 0.25 micrometres; as indicated above, the face of the base film on which the coating with the polyvinyl alcohol solution is carried out comprises on average 20 peaks or less of a height equal to or greater than 1 micrometre and 100 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre, and still more preferably 500 peaks or less of a height lower than 0.4 micrometres, per square millimetre. The composite films of the invention have an excellent resistance to mechanical attacks such as, in particular, creasing. This means that their barrier properties, already referred to, are not, or not greatly, decreased after such a mechanical attack. Therefore, the present invention provides for a biaxially drawn composite film having a permeability to oxygen, measured at 23°C at 50% relative humidity, of lower than or equal to 3 cm3/m2/24h, which composite film comprises a polyester base film having a thickness of from 5 (Jm to 50 pm, having a coating on at least one of its two faces with a layer of polyvinyl alcohol which has a number-average degree of polymerisation equal to or greater than 350, said layer of polyvinyl alcohol having a thickness of lower than or equal to 0.3 jjm, the mean roughness Rz of the base film being lower than or equal to 0.30 urn on the face(s) of the film bearing the polyvinyl alcohol layer and that of these said face(s) comprising on average 20 peaks or less of a height equal to or higher than 1 micrometre and 150 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre. The present invention also provides for a process for the preparation of a biaxially drawn composite film having a permeability to oxygen, measured at 23°C at 50% relative humidity, of lower than or equal to 3cm3/m2/24h as claimed in any one of the preceding claims wherein the polyester base film is coated by any conventional means on at least one of its faces, which has a mean roughness Rz of lower than or equal to 0.30 jam and comprises on average not more than 20 peaks of a height equal to or greater than 1 |4m and not more than 150 peaks of a height between 0.4 and 1 um, per square millimetre, with an aqueous solution of a polyvinyl alcohol containing at least 95% vinyl alcohol units, said polyvinyl alcohol exhibiting in aqueous solution at a concentration of 4% and at 20°C a viscosity of higher than or equal to 4 mPa s, and the coated film is heat-treated at a temperature of equal to or higher than 170°C, wherein the polyester base film has optionally been prepared by coextrusion of an unfilled or more slightly filled polyester or of a filled polyester, such that the base film comprises a filled layer which is rough on the back face and an unfilled or more slightly filled layer on the front face which receives the coating of the polyvinyl alcohol solution. The Examples which follow further illustrate the invention. EXAMPLES GENERAL PROCEDURE A composite film of polyethylene terephthalate (PET) is prepared by coextrusion, on the one hand, of a PET containing 0.0800 % of silica obtained by precipitation of gels and exhibiting a mean diameter of 3.3 micrometres (measurement on a laser particle size analyzer of Sympatec (trademark) and Helos type), which forms the base film and, on the other hand, of an unfilled PET whose external face will receive the polyvinyl alcohol (PVA) coating. The polyester film has a thickness of 12 micrometres in the various Examples or Comparative Tests, unless mentioned otherwise, and a mean roughness Rz of 0.45 micrometres on the back face. It includes an unfilled layer (receiving the PVA coating) which has a thickness of 0.5 to 2.0 micrometres according to the Examples, in order to vary the distribution of peak heights per square millimetre of the surface intended to receive the PVA coating. The coextruded film is first of all drawn lengthwise with a draw ratio of 3.4; it is then subjected to a corona treatment which brings its surface tension to 58 mN m. The face of the film comprising the unfilled layer is then coated with the aid of a photogravure coating system with an engraved roll. The machine speed during the coating is 200 m/min. The coating is performed by starting with an aqueous PVA solution at a concentration of 10 %, carefully prepared and free from gels. Unless mentioned otherwise, the PVA employed contains 98-99 % of vinyl alcohol units and has a viscosity of 5.5 mPa s (measured at 20°C in an aqueous solution at a concentration of 4 % with the aid of a Brookfield LV (trademark) viscometer). The coated film is then subject to a transverse drawing with a ratio of 4 and is then heat-treated at a temperature of 230°C (unless mentioned otherwise). The thickness of the PVA layer of the biaxially drawn film is 0.10 micrometres (unless mentioned otherwise). Examples 1 to 3 and Comparative Tests a, b and c. These various tests are carried out with composite films whose faces that are to receive the PVA coating exhibit different peak height distributions. Test a is carried out without a PVA coating. Permeability to oxygen was measured at 23°C at 50 % relative humidity for each of the films; it is expressed in cm3/m2/24h (measurements carried out in an apparatus of Oxtran (trademark) type 300H, from the company Modern Control line.) . Table 1 below shows the peak height distributions per square millimetre and the mean roughness of the film face receiving the PVA coating, and the values of permeability to oxygen. (Table Removed) Table 1 Example 4 and Comparative Test d Example 1 is repeated but the heat-treatment of the PET film is performed at a different temperature. Table 2 below shows the measured permeabilities to oxygen, according to the treatment temperature adopted (Example 1 is repeated by way of comparison). (Table Removed) Table 2 Comparative Tests e and f These tests were performed with the same film as in Example 1 but using for the coating a polyvinyl alcohol which had a percentage of vinyl alcohol units (or degree of hydrolysis) lower than 95 %. The permeability obtained with the films thus prepared is shown in Table 3 below (Example 1 is repeated by way of comparison). (Table Removed) Table 3 Example 5 and Comparative Test q These tests were performed with the same film as in Example 1, but using for the coating a polyvinyl alcohol which had a viscosity (measured at 20°C in an aqueous solution at a concentration of 4 % with the aid of a Brookfield LV (trademark) viscometer) other than 5.5 mPa s (the corresponding number-average degree of polymerization DPn, given by the producer of the PVA, is also recorded). The permeability obtained with the films thus prepared is shown in Table 4 below (Example 1 is repeated by way of comparison). (Table Removed) Table 4 Example 6 Example 1 is repeated, with the same procedure, the film having a total thickness of 40 jum and an unfilled layer receiving the polyvinyl alcohol coating having a thickness of 2 jum. The permeability to oxygen of the film coated with a 0.1 jum layer is 1.5 cm3/m2/24h. WE CLAIM: 1. A biaxially drawn composite film having a permeability to oxygen, measured at 23°C at 50% relative humidity, of lower than or equal to 3 cm3/m2/24h, which composite film comprises a polyester base film having a thickness of from 5 jam to 50 jum, having a coating on at least one of its two faces with a layer of polyvinyl alcohol which has a number-average degree of polymerisation equal to or greater than 350, said layer of polyvinyl alcohol having a thickness of lower than or equal to 0.3 jam, the mean roughness Rz of the base film being lower than or equal to 0.30 jam on the face(s) of the film bearing the polyvinyl alcohol layer and that of these said face(s) comprising on average 20 peaks or less of a height equal to or higher than 1 micrometre and 150 peaks or less of a height of between 0.4 and 1 micrometre, per square millimetre. 2. A film as claimed in claim 1, wherein the polyester is an alkylenediol polyterephthalate or polynaphthalenedicarboxylate, or a copolyester containing at least 80 mol% of ethylene glycol terephthalate units. 3. A film as claimed in claim 2, wherein the polyester is an ethylene glycol polyterephthalate or 1,4-butanediol polyterephthalate. 4. A film as claimed in any one of the preceding claims, wherein the face bearing the polyvinyl alcohol layer on average comprises 800 peaks or less of a height lower than 0.4 micrometres, per square millimetre. 5. A film as claimed in any one of the preceding claims, wherein the face of the base film bearing the polyvinyl alcohol layer on average comprises 20 peaks or less of a height equal to or higher than 1 micrometre and 100 peaks or less of a height between 0.4 and 1 micrometre, per square millimetre. 6. A film as claimed in any one of the preceding claims, wherein the face of the base film bearing the polyvinyl alcohol layer on average comprises 500 peaks or less of a height lower than 0.4 micrometres, per square millimetre. 7. A film as claimed in any one of the preceding claims, wherein the two faces of the base film have different mean roughnesses Rz, equal to or greater than 0.15 micrometres on the back face of the film and lower than or equal to 0.30 micrometres on the base film face bearing the polyvinyl alcohol coating. 8. A film as claimed in claim 7, wherein the face of the base film bearing the polyvinyl alcohol coating has a mean roughness Rz of lower than or equal to 0.25 micrometres. 9. A film as claimed in claim 7 or 8, wherein the face of the base film bearing the polyvinyl alcohol coating comprises on average 20 peaks or less of a height equal to or higher than 1 micrometre and 100 peaks or less of a height between 0.4 and 1 micrometre, per square millimetre. 10. A film as claimed in claim 9, wherein the face of the base film bearing the polyvinyl alcohol coating comprises on average 500 peaks or less of a height lower than 0.4 micrometres, per square millimetre. 11. A film as claimed in any one of the preceding claims, wherein the polyester base film consists of two layers exhibiting different surface properties. 12. A film as claimed in claim 11, wherein the two layers of the base films have different surface roughnesses. 13. A film as claimed in claim 11 or 12, wherein the unsymmetrical polyester base film has been produced by the coextrusion of two polyesters with different filler contents and, if appropriate, different fillers. 14. A film as claimed in claim 13, wherein the polyester employed is the same for the two coextruded layers, the layer which is to receive the polyvinyl alcohol layer is not filled. 15. A film as claimed in any one of claims 11 to 14, wherein the unfilled, or more slightly filled, layer receiving the polyvinyl alcohol coating has a thickness of greater than or equal to 0.5 jam. 16. A film as claimed in claim 15, wherein the unfilled, or more slightly filled, layer has a thickness of greater than or equal to 1.0 17. A film as claimed in any one of the preceding claims, wherein the polyvinyl alcohol layer has a thickness of equal to or lower than 0.20 18. A film as claimed in any one of the preceding claims, which additionally comprises on one of its faces a printing or printing primer layer or, on the face free from the polyvinyl alcohol layer, a heat- sealable layer. 19. A process for the preparation of a biaxially drawn composite film having a permeability to oxygen, measured at 23°C at 50% relative humidity, of lower than or equal to 3cm3/m2/24h as claimed in any one of the preceding claims wherein the polyester base film is coated by any conventional means on at least one of its faces, which has a mean roughness Rz of lower than or equal to 0.30 jam and comprises on average not more than 20 peaks of a height equal to or greater than 1 jam and not more than 150 peaks of a height between 0.4 and 1 ^m, per square millimetre, with an aqueous solution of a polyvinyl alcohol containing at least 95% vinyl alcohol units, said polyvinyl alcohol exhibiting in aqueous solution at a concentration of 4% and at 20°C a viscosity of higher than or equal to 4 mPa s, and the coated film is heat-treated at a temperature of equal to or higher than 170°C, wherein the polyester base film has optionally been prepared by coextrusion of an unfilled or more slightly filled polyester or of a filled polyester, such that the base film comprises a filled layer which is rough on the back face and an unfilled or more slightly filled layer on the front face which receives the coating of the polyvinyl alcohol solution. 20. A process as claimed in claim 19, wherein the coating of the polyester base film with an aqueous solution of polyvinyl alcohol is carried out in line or by reprocessing. 21. A process as claimed in claim 20, wherein the coating is carried out in line on a monoaxially drawn polyester base film which is drawn on again after coating at least in the direction opposite to that of the first drawing. 22. A process as claimed in any one of claims 19 to 21, wherein the aqueous solution of polyvinyl alcohol comprises 1% to 20% by weight of the polyvinyl alcohol with respect to the total weight of the solution. 23. A process as claimed in claim 22, wherein the aqueous solution of polyvinyl alcohol comprises 5% to 15% by weight of the polyvinyl alcohol with respect to the total weight of the solution. 24. A process as claimed in any one of claims 19 to 23, wherein the polyvinyl alcohol contains at least 97% of vinyl alcohol units. 25. A process as claimed in claim 24, wherein the polyvinyl alcohol contains at least 98% of vinyl alcohol units. 26. A process as claimed in any one of claims 19 to 25, wherein the temperature at which the coated film is heat-treated is from 180°C to 240°C. 27. A process as claimed in claim 26, wherein the temperature at which the coated film is heat-treated is from 200°C to 230°C. 28. A biaxially drawn composite film substantially as herein described with reference to the foregoing examples. 29. A process for the preparation of a biaxially drawn composite film substantially as herein described with reference to the foregoing examples.

Documents:

364-del-1995-abstract.pdf

364-del-1995-claims.pdf

364-del-1995-correspondence-others.pdf

364-del-1995-correspondence-po.pdf

364-del-1995-description (complete).pdf

364-del-1995-form-1.pdf

364-del-1995-form-13.pdf

364-del-1995-form-2.pdf

364-del-1995-form-29.pdf

364-del-1995-form-3.pdf

364-del-1995-form-4.pdf

364-del-1995-form-9.pdf

364-del-1995-gpa.pdf

364-del-1995-petition-138.pdf

364-del-1995-petition-others.pdf