EP1732757A1

EP1732757A1 - Method for the protection of thermoplastic plates

Info

Publication number: EP1732757A1
Application number: EP05751619A
Authority: EP
Inventors: Pierre Gerard; Alberto Luca Stasi
Original assignee: Arkema France SA
Current assignee: Arkema France SA
Priority date: 2004-04-08
Filing date: 2005-04-05
Publication date: 2006-12-20
Also published as: FR2868728B1; US20070110996A1; FR2868728A1; WO2005102692A1

Abstract

A method for the protection of a thermoplastic plate comprising the application of a coextruded simultaneously bi-oriented film consisting of the following layers:a) a central layer made of a propylene homopolymer or copolymer made of propylene and ethylene and/or alpha olefins having 4-8 carbon atoms, with a melt flow index (MFI) (ISO 1133, 230 DEG C/10 min) of at least 1.5 - 10 g/10 min, b) an adhesive outer layer comprising copolymers consisting of ethylene and alpha olefins in C3 - C12, having a specific mass (ASTM 792) of 0.887 g/cm<3> to 0.916 g/cm<3>, or copolymers consisting of ethylene and vinyl acetate (EVA) wherein the vinyl acetate monomer represents 2 % -14 % in terms of mols, said ethylene copolymers having an MFI (ASTM D 1238, 190 DEG C/2.16 kg) of 1 - 10 g/10 min, wherein layer b) is polarized on the surface thereof; the application of the multilayer to the plate is carried out after tensioning of the film by bringing the adhesive layer b) into contact with the plate, and by passing both items through two cylindrical rollers rotating in an opposite direction.

Description

The present invention relates to the field of protection, by masking films, of sheets of thermoplastic materials, more especially of acrylic resin sheets and of polycarbonate sheets. It relates to a method of protecting plates of thermoplastic materials using a multilayer film comprising polypropylene as well as the plate thus coated. Certain thermoplastic polymers, such as acrylic derivatives (in particular polymethyl methacrylate well known under the acronym of PMMA) and polycarbonate are commonly manufactured in the form of rectangular plates of large dimensions and of variable thickness (for example plates of 2 m wide by 3 m long and between 2 mm and 15 mm thick). These plates are obtained by various polymerization and / or shaping processes, in particular by casting or by extrusion of hot granules. They are generally intended for transformers who use them, for example by cutting or thermoforming operations, with a view to making a wide variety of objects, such as baths, domes for the building sector or furniture. With regard in particular to PMMA, well known for its advantageous optical properties and its aesthetic appearance, it is common practice for industrial producers of plates to cover them with protective films, also called masking films or masking films in English. . This is to protect them from possible scratches, scuffs or dust during storage, transport, and more generally during all operations of handling and handling of the plate which occur between the time it is manufactured by the producer and when it is implemented by the transformer. It often happens that the transformer, especially when the plate is intended to be thermoformed, wishes to do this operation on the plate still coated with the masking film, the latter being in contact with the mold wall. This makes it possible to extend the protection of the plate during the thermoforming operation, and up to transport, storage and handling. of the thermoformed object. As an example of such a practice, the case of the manufacture of acrylic resin baths can be cited. There is a known protection process commonly practiced by producers of PMMA plates which comprises, by means of an appropriate device, the application to the plate of a film based on polyethylene, of thickness between 40 and 100 μm. , generally around 60 μm. In such a process, the film and the plate are brought to a temperature between 20 and 100 ° C. Then the film is stretched and brought into contact with the plate, and the assembly is pressed between 2 cylindrical rollers rotating in opposite directions which subject it to a pressure of between 1 and 6 bar and impart to it a translational movement in a horizontal plane. corresponding to a speed between 0.5 and 15 m / min. Following this process, the film thus applied adheres to the plate and protects it during the handling and handling operations which accompany its storage and transport by the manufacturer. It also adheres to the thermoformed plate and to the object thus shaped (for example a bathtub), during the storage and transport operations of the transformer. The film is removed or peeled off at the appropriate time, usually manually. In the above-mentioned device, the film to be applied to the plate is wound on a supply reel placed parallel to one of the sides of the plate to be protected. This reel takes place under the effect of the movement of the film adhering to the plate after its application by the 2 pressure rollers, according to the translational movement previously described. The film is therefore subjected to a certain tension, hereinafter referred to as the “removal tension”. This tension is likely, in turn, to cause a deformation or elongation of the film, so that the latter can be applied to the plate and therefore adhere to it with a certain deformation or elongation compared to its state at rest. . Such a deformation, even a small one, can lead to large rectangular plates (for example 2 m by 3 m) significant elongations in absolute value. So, because of its properties mechanical, the film can resume later, well after its application on the plate (for example during storage of the plate), its initial size by retraction, which can cause a localized or complete detachment of the film, detrimental to the quality of plate protection. Such phenomena are to be avoided. For this, it is necessary that the mechanical properties of the film are such that it has the lowest possible voltage at removal (when it is unwound from the aforementioned supply coil, to be applied to the plate). It is also necessary that the mechanical properties of the film be such that it exhibits the lowest possible deformation for a fixed stress (or tension). The adhesion of the masking film to the plate must, finally, be of good quality and stable over time. Such a property is represented by an adhesion test carried out 24 hours after the application of the film to the plate, and which is described later in the present text. Furthermore, it is also necessary that the film has a suitable tear strength. Such resistance is required on the one hand when the film is applied to the plate, on the other hand when the end user (generally the transformer or the user of the thermoformed object coated with the masking film ) removes the said protective film. In the latter case, the end user exerts a certain force which imposes a constraint on the film: for reasons of convenience, it is preferable that the film does not tear into multiple fragments, but on the contrary that it is separated from the plate while retaining its integrity, therefore in one go. Finally, processors often wish to carry out a quality control relating to the possible presence of imperfections and / or surface defects of the thermoplastic plate before it is shaped. However, polyethylene-based masking films generally weaken the contrast and therefore the visibility of the objects or details they cover. Such an effect, arising from the optical properties of these films, is designated by the English name of "Haze". So it is sometimes necessary to separate the film from the plate in order to carry out a thorough quality control of the thermoplastic plate, then to put it back for the thermoforming operation, which is an awkward operation. When the film is not separated from the plate, and the quality control is carried out by visual inspection through the masking film, the risk exists of not detecting minor defects, which will then appear on the thermoformed object, after have removed the masking film. It is desirable to limit such a risk, by providing a masking film having an improved Haze. An object of the present invention is thus to propose a method of protecting thermoplastic plates by masking film which facilitates the quality control of the plates, due to an improved Haze, while preserving the effectiveness of the protection offered by polyethylene films. Another object of the present invention is to provide a method of protecting thermoplastic sheets which requires a lesser thickness of masking film than polyethylene films, while having suitable mechanical properties (in particular a reduced film tension at deposition). and low strain strain), quality adhesion that is stable over time, and suitable tear resistance. The advantage of a lesser thickness of masking film lies, in addition to an appreciable reduction in the cost of the protection operation, in a reduction in the amount of protective material. This is particularly advantageous when, after removing the masking film, to evacuate it. It has now been found that these objects are achieved in whole or in part by the method which is described below. The subject of the present invention is therefore firstly a method of protecting a plate of a thermoplastic material comprising the application to said plate of a coextruded film, bioriented simultaneously, comprising the following layers: a) a central layer d 'homopolymer of propylene or of copolymer of propylene and ethylene and / or alpha-olefins of 4 to 8 carbon atoms, with an MFI (Melt Flow Index) ISO 1133, 230 ° C / 10 min) of at least 1.5 to 10 g / 10 min, preferably 2 at 4 g / 10 min, b) an adhesive outer layer which comprises copolymers of ethylene and alpha-olefins C ₃ to C ₂ , preferably C ₄ to C ₃ , of a specific mass (ASTM 792 ) from 0.887 g / cm ³ to 0.916 g / cm ³ , or copolymers of ethylene and vinyl acetate (EVA) in which the vinyl acetate monomer represents from 2% to 14% by mole, said copolymers ethylene having an MFI (ASTM D 1238, 190 ° C / 2.16 kg) of 1 to 10 g / 10 min and preferably from 2 to 6, said layer b) being surface polarized; The application of the multilayer film to the plate takes place, after tensioning the film, by bringing the adhesive layer b) into contact with said plate, then passing the assembly between 2 cylindrical rollers rotating in opposite directions. It has in fact been found that the previously described multilayer film offers, due to an improved Haze compared to the polyethylene film, easier inspection of the defects likely to appear on the thermoplastic sheets and, in particular due to lower deposit tension and deformation, better adhesion to the plate it protects. The total thickness of the multilayer film is between 15 μm and 50 μm, preferably between 15 μm and 35 μm and even more preferably between 20 μm and 30 μm. The thickness of the adhesive external layer b) varies between 5 and 30%, preferably between 5 and 20% and more preferably between 5% and 10% of the total thickness of the film. The amount of comonomers in the propylene copolymers used in the central layer a) is between 0.1% and 40% by weight and preferably between 0.1% and 15% by weight. The alpha-olefins of 4 to 8 carbon atoms are for example butene-1, hexene-1, 4-methyl-pentene-1 and octene-1. The polymers used in layer a) are available on the market. By way of example, mention may be made of that known under the brand Moplen®HP522H (Basel). Preferably, as the central layer a), a propylene homopolymer is used. The ethylene copolymers of the adhesive layer b) are preferably copolymers of ethylene with alpha-olefins C? _8l and they are preferably obtained by using in the polymerization catalyst 1 to metallocene. For the latter, reference is made for example to patent WO 91/11488 which describes catalysts based on organometallic coordination compounds, which are derived from the cyclopentadienyl of a metal from group 4b of the periodic table and include mono, di and tricyclopentadienyl and their derivatives with the transition metal. Catalyst 1 is generally used with a co-catalyst 2, preferably chosen from aluminoxanes 2a) which are the products of the reaction of aluminum trialkyl with water. As co-catalyst 2 instead of aluminoxanes, one can use compound 2b) of formula: (Lι-H) ⁺ (A) -

in which :

- (A) ^" is a compatible non-coordinating anion, preferably of formula (BQ _q ) ^"

- Li is a neutral Lewis base, - (LH) ⁺ is a Bronsted acid,

- B is an element from groups llla to Via of the periodic table of the elements, with metalloid characteristics, preferably boron, phosphorus or arsenic in the valence state 3 or 5, silicon, and more preferably boron in the valence state 3, - the Qs, which are identical or different from each other, are selected from the following groups: hydrides, halides, alkyls, optionally substituted aryls, for example with halogens, preferably F, alkosides, arylosides, dialkylamido or R ₀ COO ^" where R ₀ has from 1 to 20 carbon atoms, with the condition that Q can only be a halide once,

- q is an integer equal to the valence of B, plus 1. The molar ratio between Al of the aluminoxane component 2a) relative to the amount of the metal of catalyst 1 (metallocene) is between 10,000: 1 and 100: 1 and preferably between 5,000: 1 and 500: 1. In the case of the boron compound of formula 2b), the preferred metallocene / (2b) ratio is between (0.1-

4): 1 and more preferably between (0.5-2.0): 1. The polymerization processes for obtaining the copolymers of ethylene and alpha-olefins from the adhesive outer layer b) are well known in the art, and reference will be made, by way of example, to those indicated in patent WO 91 / 11 488. The comonomers which can be used to prepare the ethylene copolymers of layer b) are, for example, butene-1, hexene-1, 4-methyl-pentene-1 and octene -1. Examples of said copolymers known on the market are those which carry the marks AFFINITY® PL 1880 and

AFFINITY® PL 1850 (DOW). It is preferred to use for layer b) a copolymer of ethylene and vinyl acetate, such as for example GREENFLEX® (POLIMERI

EUROPA). The polarization of the adhesive layer b) can be obtained for example by corona treatment or plasma treatment, preferably by corona treatment. The coextruded film implemented in accordance with the present invention can optionally be supplemented by an external non-adhesive layer c) disposed on the free surface of the central layer a), to improve the uniformity of the thickness of the film. The thickness of the non-adhesive layer c) is between 5% and 30%, preferably between 5% and 20% and more preferably between 5% and

10% of the total thickness of the film. The total thickness of the film according to this invention, which also includes layer c), is situated within the limits defined above. The non-adhesive layer c) consists of a propylene homopolymer or a copolymer with ethylene and / or an alpha-olefin which has 4 to 8 carbon atoms, with an MFI (ISO 1133, 230 ° C / 10 min) from 2 to 10 g / 10 min. The copolymers of propylene and ethylene and / or alpha-olefins of 4 to 8 carbon atoms used in c) have a comonomer content of 0.1% to 40% by weight and preferably 0.1% by weight to 15% by weight. The polymers used in layer c) are available on the market. By way of example, mention may be made of that known under the name Stamylan®P 15E10FB (DSM). The central layer a) may consist of a single layer or of several layers using the polymers and copolymers indicated in a). It is possible to add to the multilayer film used in accordance with the present invention conventional additives used in plastics, for example slip agents ("slip agents"), anti-blocking agents ("anti-blocking agents"), antistatic agents, dyes, UV protection agents. The coextruded and bioriented films simultaneously applied in accordance with the present invention can be obtained with a process which can be carried out on a line which comprises: - an extruder for the central layer a), for example of the type with two screws or two extruders successive to a screw, said extruder being connected to a vacuum pump to extract the air and to a supply pump to stabilize the force at the outlet, - one-screw or two-screw coextruders for the layers b) and optionally c), - a multilayer extrusion head (die) to obtain a composite sheet formed by layers a), b) and optionally c), - a quenching roll of the sheet, what the for example, by immersing the roller in a water bath, - a sheet preheating system, preferably by infrared rays, - a simultaneous sheet stretching oven, with the following sections, in the direction of advancement of the plate: preheating section, section of simultaneous stretching, stabilization section and cooling section, a device for polarizing layer b) of the multilayer film obtained, for example a device for corona treatment or for plasma treatment. The process for obtaining coextruded and bioriented films simultaneously according to the present invention comprises the following steps: - preparation of a composite sheet, preferably with a thickness of between approximately 1.5 mm and 3.3 mm, by coextrusion of the layers a), b) and optionally c), preferably on a flat head and preferably using temperatures between 160 ° C and 270 ° C; and then quenching ("quenching") of the plate, preferably carried out on a roller immersed in water, preferably at a temperature of 10 ° C to 60 ° C, - preheating of the sheet at a temperature preferably between about 100 ° and about 120 ° C, preferably by infrared rays, - simultaneous stretching, both in the direction of the MD machine and in the transverse direction TD of the sheet, using the apparatus described in the basic patent US 4,853,602 which is incorporated here in full for reference and in the following patents which describe this technology, and which comprises the following steps: - simultaneous stretching of the sheet and obtaining of the film by clamping the edges of the sheet by a series clamps or jaws which slide on two rails and which are independently guided by synchronous linear induction motors, each clamp or jaw sliding on a rail and being driven by an ai permanent mant or by a pair of permanent magnets, pushed by the magnetic wave created by the polar extensions of the linear motor; each section of the drawing device having a series of linear synchronous induction motors operating in continuous mode, supplied with alternating current with phase and frequency modulation, for varying in a manner continues the speed of the jaws and the relative speed between them and therefore the longitudinal stretching ratios of the film; the transverse stretching ratios being regulated by acting on the divergence of the rails on which the clamps or jaws slide, the stretching device comprising one or more sections located inside an oven whose temperature is between approximately 150 ° and 190 ° C, and one or more sections located in an oven whose temperature is between about 130 ° and 140 ° C, in general, the longitudinal stretch ratios being between about 4: 1 and about 9: 1 and preferably between 5.5: 1 and 8.5: 1, and the transverse stretching ratios being between approximately 3: 1 and approximately 8.5: 1 and preferably between 5.5: 1 and 8.5 : 1, and - polarization of the layer b) of the film thus obtained. The temperatures in the parts of the apparatus described above are selected so as to optimize the biaxial orientation of the polymer films made up of the polymers used; as a first approximation, the longitudinal stretching ratio can be considered to be equal to the ratio between the speed of exit of the film from the stretching device (which is related to the frequency of the current which feeds the linear motors of the last section of the device stretching) and the speed of entry of the film into the stretching device (which is related to the frequency of the current which feeds the linear motors of the first section of the stretching device); as a first approximation, it can be considered that the transverse stretching ratio is equal to the ratio between the width of the film at the outlet of the stretching device and the width of the film at the inlet of the stretching device. The "corona" treatment of the adhesive layer b) is preferably carried out by high frequency electrical discharges, preferably with an intensity of 40 to 60 W / cm. As indicated above, the multilayer film which has just been described is used for the protection of plates of thermoplastic materials. It is therefore applied, after it has been tensioned, by bringing the adhesive layer b) into contact with the plate, then passing the assembly between 2 rollers. cylindrical turning in opposite directions. The processing temperature of the film is between 20 ° C and 100 ° C. The film is placed in a reel having the same width as the surface of the plate to be covered, so that the direction of unwinding of the reel coincides with the direction of advancement of the plate. The film adheres to the surface of the plate thanks to its passage between the 2 rollers which exert a pressure generally between 1 and 6 bars, the speed of the line being generally between 0.2 and 15 m / min. The thermoplastic materials to which the protective protection of the plates apply, according to the invention, are chosen in particular from acrylic resins, polycarbonate, poly (ethylene terephthalate), poly (ethylene glycol terephthalate). Among these thermoplastic materials, however, acrylic resins, and very particularly polymethyl methacrylate (PMMA), are preferred. The present invention also relates to the thermoplastic plate coated with the coextruded film as defined above, the adhesive external layer b) of said film being in contact with the plate. Finally, it relates to the object capable of being obtained by thermoforming of said plate.

The examples which follow are given purely by way of illustration of the process according to the invention, and should not be used in any way to limit its scope. In these examples, the masking film is characterized in the following manner. The deformation of the film is measured for an applied stress (or tension) of 10 MPa by means of a tensile test carried out according to standard ASTM D882, and is called hereinafter “deformation coefficient”. The Haze of the film is measured according to the ASTM D 1003 standard and is all the better as it corresponds to a lower value. Example 1 (comparative): Application of a polyethylene film on a PMMA plate: A coextruded non-oriented polyethylene film 60 μm thick is used. This film is representative of a film used industrially as a masking film for PMMA plates. It has the following composition: - central layer: formed of LDPE (low density polyethylene) which can be mixed with LLDPE (linear low density polyethylene) or MDPE (medium density polyethylene), - the surface adhesive layer consists m-VLDPE (very low density polyethylene obtained with metallocene). The deformation coefficient of this film is 5%. Its Haze is 9.88. A laboratory device is used as an application device comprising: - a reel for supplying masking film, - rollers on which the film passes, before its application to the plate and one of which is provided with '' a sensor measuring the tension it supports, - a set of rollers bringing the film on the one hand and the plate on the other hand in the vicinity of each other, just upstream of 2 cylindrical pressure rollers of diameter 9 cm and whose axes are located in the same vertical plane separated by a distance of about 40 cm. One of these rollers is integral with a motor, - a roller device for evacuating the plate covered with film on an output table. The coils and rollers mentioned above are cylindrical in shape and have a generator length of 30 cm. The polyethylene film is pre-wound on the supply reel (placed in the appropriate place on the device) at a thickness of approximately 3 cm. This reel is unwound manually so as to bring the film between the 2 pressure rollers. A rectangular plate of PMMA, dimensions 47 cm by 23 cm and thickness 3 mm, is cut. This plate is introduced into the device, and the motor is activated in order to apply the film by means of the 2 pressure rollers. The film tension measured by means of the roller fitted with the appropriate sensor is approximately 2 kg. The quality of the adhesion of the film applied to the plate is evaluated by a peeling test carried out 24 hours after application of the film to the plate and storage of the assembly at 25 ° C. and 50% humidity. relative. The purpose of this test is to measure - under specific conditions - the force required to peel off the film. According to this test, a rectangular test tube 5 cm wide and 15 cm long is cut from the plate covered with the film. This test piece is fixed vertically in the fixed jaw of a dynamometer (or traction machine). The film is peeled off at one end over a length of about 10 cm, and this end is fixed to the movable jaw of the machine. The movable jaw moves vertically upwards at a constant speed (fixed at 30 mm / min) by peeling off the free end of the film strip at an angle of 180 ° relative to the test piece. A force sensor integrated into said mobile jaw measures the force corresponding to this displacement, which is expressed in grams and is called "adhesion value". This test is adapted from the 4001 "Peel Adhesion of Adhesive Tape on Stainless Steel" test method of ARMFA (European Association for the Self Adhesive Tape Industry). The adhesion value of the film on the plate measured by this test is 42 g.

Example 2: Application of a coextruded film, simultaneously bioriented, comprising polypropylene, on a PMMA plate: 1- Preparation of the coextruded film: A coextruded film is prepared which has the composition indicated below, it is bioriented simultaneously using the simultaneous biaxial stretching process described above, working under the following conditions: - layer a): temperature the extruder from 250 ° C to 270 ° C; rotation speed of the extruder 160 rpm; - layer c): temperature of the extruder from 250 ° C to 270 ° C; rotational speed of the extruder 19 rpm; - layer b): temperature of the extruder from 210 ° C to 250 ° C; speed of rotation of the extruder 38 rpm; - head temperature: 250 ° C; - temperature of the roller and quenching bath: 30 ° C; - temperature of the IR preheating unit from 100 ° C to 320 ° C; - temperature of the drawing oven from 160 ° C to 170 ° C; - power of the corona treatment device: 50 W / cm. On the line, the following stretching parameters are used: longitudinal stretch ratio of 4.5 to 6; transverse stretch ratio of 6.0 to 9. The coextruded film has the following composition: outer layer c): polypropylene homopolymer with an MFI = 3, known on the market under the brand Stamylan®P 15E10FB (DSM), thickness 0.8 μm; central layer a): polypropylene homopolymer with an MFI = 2, known on the market under the brand Moplen®HP522H; adhesive layer b): EVA, percentage of vinyl acetate of 2.5 mol%, MFI = 2.2, thickness 0.8 μm. The surface of layer b) of the film is treated by high frequency electrical discharges ("corona" treatment) at an intensity of 50 W / cm. The total thickness of the film is 30 μm. Its strain coefficient is 1%, and its Haze is 2.06. 2- Application of the film thus prepared on the PMMA plate:

Example 1 is repeated, replacing the polyethylene film with the film prepared in accordance with paragraph 1. A film tension of 0 is measured, apart from the measurement uncertainties. The adhesion value of the film is 27 g. It thus appears that the film of Example 2 has in the machine which applies it to the plate a tension lower than that of the film of Example 1, and is therefore less likely to deform, all the more so as it has a much lower coefficient of deformation. These properties make it particularly suitable as a masking film, in particular due to the low risk of shrinking. In addition, its adhesion value is comparable to that of the polyethylene film. Finally, due to its Haze value almost 5 times lower than that of the film of Example 2, the film of Example 2 is particularly suitable for the visual inspection of defects likely to affect the surface of the thermoplastic plates. , without removing the masking film.

Claims

1. A method of protecting a plate of a thermoplastic material comprising the application to said plate of a coextruded film, biorient simultaneously, comprising the following layers: a) a central layer of propylene homopolymer or propylene copolymer and ethylene and / or alpha-olefins of 4 to 8 carbon atoms, with an MFI ("Melt Flow Index") ISO 1133, 230 ° C / 10 min) from at least 1.5 to 10 g / 10 min, preferably from 2 to 4 g / 10 min, b) an adhesive external layer which comprises copolymers of ethylene and of C ₃ alpha-olefins C ₁₂ , preferably C ₈ to C ₈ , with a specific gravity (ASTM 792) of 0.887 g / cm ³ to 0.916 g / cm ³ , or copolymers of ethylene and vinyl acetate (EVA) in which the vinyl acetate monomer represents from 2% to 14 mol%, said ethylene copolymers having an MFI (ASTM D 1238, 190 ° C / 2.16 kg) from 1 to 10 g / 10 min and preferably of 2 6, said layer b) being surface polarized; the application of the multilayer film to the plate is made, after tensioning the film, by bringing the adhesive layer b) into contact with said plate, then passage of the assembly between 2 cylindrical rollers rotating in opposite directions.

2. Method according to claim 1, characterized in that the total thickness of the multilayer film is between 15 μm and 50 μm, preferably between 15 μm and 35 μm.

3. Method according to one of claims 1 or 2, characterized in that the total thickness of the multilayer film is between 20 microns and 30 microns.

4. Method according to one of claims 1 to 3, characterized in that the thickness of the adhesive external layer b) varies between 5 and 30%, preferably between 5 and 20% of the total thickness of the film.

5. Method according to one of claims 1 to 4, characterized in that one uses for the central layer a), a propylene homopolymer.

6. Method according to one of claims 1 to 5, characterized in that one uses for layer b) a copolymer of ethylene and vinyl acetate.

7. Method according to one of claims 1 to 6, characterized in that the coextruded film is completed by an external non-adhesive layer c) disposed on the free surface of the central layer a).

8. Method according to one of claims 1 to 7, characterized in that the polarization of the adhesive layer b) is obtained by corona treatment.

9. Method according to one of claims 1 to 8, characterized in that the thermoplastic material of the plate is chosen from acrylic resins, polycarbonate, poly (ethylene terephthalate), poly (ethylene glycol terephthalate ).

10. Method according to one of claims 1 to 9, characterized in that the thermoplastic material of the plate is PMMA.

11. Thermoplastic plate coated with the coextruded film as defined in claim 1, the adhesive external layer b) of said film being in contact with the plate.

12. Object capable of being obtained by thermoforming of the plate as defined in claim 11.