WO2012066217A1

WO2012066217A1 - Use of a temporary protective film to protect a metal surface during laser cutting; temporary protective film

Info

Publication number: WO2012066217A1
Application number: PCT/FR2011/052608
Authority: WO
Inventors: Pascal Lutz
Original assignee: Pascal Lutz
Priority date: 2010-11-15
Filing date: 2011-11-09
Publication date: 2012-05-24
Also published as: FR2967365A1

Abstract

The present application describes the use of a temporary protective film (6) to protect a planar surface of a metal sheet from mechanical damage, notably during a laser cutting or laser piercing method, said temporary protective film (6) comprising: a backing film (1); on one of its faces, known as the "underside face", which is in adhesive contact with said at least one planar surface of said sheet, an adhesive layer (4); on the other of its faces, known as the "top surface", a non-stick layer (5); in which, in said temporary protective film: the adhesive layer (4) on the underside face is either based on synthetic or natural rubber and has been applied in solution in an appropriate solvent, or based on an acrylic adhesive and has been applied, in both instances, by coating onto the underside face of the backing film; the non-stick layer (5) is based on polymer silicones which have been modified using acrylate functional groups.

Description

USE OF A TEMPORARY PROTECTIVE FILM FOR PROTECTING A METAL SURFACE WHEN CUTTING BY LASER BEAM;

TEMPORARY PROTECTION FILM

The invention relates to the technical field of coatings for the temporary protection of metal sheet surfaces, and more specifically to plastic films for the protection of surfaces during the processing, cutting or drilling of sheets.

In many situations, it is desirable to apply a temporary protective coating to a smooth surface, in particular to prevent staining or damage to surfaces, especially scratches or tool marks, during their manufacture, cutting, handling, transportation or installation. Self-adhesive film protective coatings are used for this purpose. These coatings are temporary coatings and are not intended to provide protection against moisture, corrosion or chemicals. They must remain in place as long as necessary, and in particular during cutting and drilling operations, and they must be able to be removed at the latest before final use of the sheet without leaving any trace detrimental to the end use. They are usually flexible plastic films, transparent or colored, especially polyethylene, which are coated on one side with a layer of adhesive. The other side, which can be printed, has a non-stick coating to facilitate the unwinding of the temporary protection film supplied in reel; if an impression is present, this non-stick coating must also protect the impression against tearing by the adhesive layer. The adhesive strength of the adhesive layer must be adapted to the material of the surface on which the film is to be applied. For example, a film to be applied to an aluminum surface will advantageously have a lower adhesive strength than a film for protecting a stainless steel surface.

To protect stainless steel sheets, for example, self-adhesive films consisting of a plastic support (generally made of polyethylene with a thickness typically between 50 μηι and 100 μ) and an adhesive material based on rubber are used. natural. These films can be used when cutting and laser drilling metal sheets, but they can lead to some problems. Indeed, the lasers used for cutting and drilling, generally C0 ₂ lasers, marketed for example by equipment manufacturers such as the companies Amada or Trumpf, make it possible to achieve a high speed of cutting and drilling; this speed increases with the energy density of the beam. The protective film must have a neutral behavior when cutting or drilling; in particular, it must not decompose, take off, form bubbles or show a significant withdrawal in order to continue to properly exercise its protective function. However, for commercially available films, it can be seen that the cutting or drilling speed that can be exploited industrially is currently limited by the behavior of the film during said cutting or drilling operation. It is observed in particular that the film is peeled locally in an area close to the cut line, or forms bubbles. The fact of being able to remedy this limitation would make it possible to exploit the known cutting or laser drilling processes with a higher energy density, and therefore with a greater speed of cutting or piercing: this would immediately increase the productivity. certain types of cutting or drilling machines. Here is the first problem that the present invention seeks to solve.

Moreover, whatever the substrate on which these films are applied, the applicant has found that the unwinding of film reels requires a large unwinding force. This force must be greater than the cohesive force between the turns of the coil, and is therefore directly related to the adhesive force and the width of the roll. For the temporary protection films according to the state of the art, suitable for the protection of stainless steel sheets, this force is generally greater than 150 cN per centimeter of width. It is known that the unwinding of the film reels generates noise; this noise level increases with the unwinding force and with the unwinding speed. This noise level, measured at a distance of approximately 1 to 2 meters, exceeds 75 decibels and can easily exceed 1 10 decibels, which imposes special protection measures. It is therefore desirable to have temporary protection films whose unwinding, at high unwinding speeds, is not accompanied by such a high level of noise emission. This represents the second problem that the present invention seeks to solve.

There are many types of temporary protection films in the state of the art, these films being used in many technical fields, often very specific. We limit ourselves here to two examples:

Patent Application FR 2,644,194 (Groupe Titel) describes a protective film intended for the temporary protection of rough surfaces, usable to protect building surfaces during sandblasting or application of paints. This low density polyethylene film having a thickness of between 50 μηι and 80 μηι has on one of its faces a layer of acrylic permanent adhesive with a thickness of between 20 μηι and 80 μηι, and on the other side a very thin layer of an anti-adhesion product (silicone oil).

Patent Application EP 1 918 344 A1 (Nitto Denko Corp.) discloses a temporary polypropylene protective film for optical surfaces such as prisms, which can be unwound with a low unwinding force. This film comprises on one side a layer of rubber-based adhesive and on the other side an anti-adhesion coating consisting of a copolymer comprising acrylic polymer chains onto which silicone skeletons are grafted (approximately 15 to 40% mass) this graft copolymer may be mixed with a fluoropolymer and / or with a crosslinking agent. The release coating is applied in the liquid phase and dried. Such a coating process may have two disadvantages: on the one hand, drying takes time, and this may limit the speed of travel in a continuous process and / or cause contamination of the product by the modified silicone. On the other hand, the presence of a crosslinking agent in the mixture limits the life of this mixture, which poses problems of production management. These two aspects related to the method of application of the release coating represent the third problem that the present invention seeks to solve or avoid.

Objects of the Invention Surprisingly, the inventor has found a unique solution to the three problems indicated. Thus, a first object of the invention is the use of a temporary protective film to protect at least one flat surface of a metal sheet against mechanical damage, in particular during a cutting or drilling process. by laser beam, said temporary protection film comprising:

a support film, transparent or colored, single-layer or multi-layer, preferably of polyethylene or polypropylene or a mixture of the two;

- On one of its faces, called "lower face", which is in adhesive contact with said at least one flat surface of said sheet, an adhesive layer;

- on the other of its faces, called "upper face", which can be printed, a non-stick layer; characterized in that in said temporary protection film:

the adhesive layer on the underside is either based on synthetic or natural rubber and has been applied in solution in a suitable solvent, or based on an acrylic adhesive, and has been applied, in both cases, by coating on the lower face of the support film;

the release layer is based on polymeric silicones modified with acrylate functional groups crosslinked by a radical-type reaction initiated preferably by UV radiation in the presence of a photoinitiator. Said metal sheet may be a steel sheet and more particularly a stainless steel sheet, or a sheet of aluminum or aluminum alloy. Its surface to be protected may be bare or have a permanent surface coating, such as a lacquer or varnish. A second object of the invention is a laser beam cutting or drilling method of a metal sheet of which at least one flat surface of said sheet is protected during said cutting or piercing by a temporary protective film (6). , said temporary protection film comprising:

a support film, transparent or colored, single-layer or multi-layer, preferably polyethylene or polypropylene or a mixture of the two;

- on the other of its faces, called "upper face", which can be printed, a non-stick layer;

said method being characterized in that in said temporary protection film:

the adhesive layer on the underside is either based on synthetic or natural rubber and has been applied in solution in a suitable solvent, or based on acrylic glue, and has been applied, in both cases, by coating on the underside support film;

the release layer is based on polymeric silicones modified with acrylate functional groups.

In this method, it is possible to use a steel sheet, and more particularly a stainless steel sheet, or a sheet of aluminum or aluminum alloy, or any other metal material that can be cut by laser beam. Its surface protected by said temporary protective film may comprise a permanent surface coating, such as a lacquer or varnish. In this method, it is preferred to use a beam generated by a CO ₂ laser.

Another object of the invention is a method of protecting a flat surface of a metal sheet against mechanical damage, comprising the following steps: (a) supplying a metal sheet; (b) unwinding the protective film according to the invention, (c) applying said protective film to the surface (and preferably over the entire surface) of said metal sheet.

A final subject of the invention is a temporary protective film for metal surfaces, in particular for stainless steel surfaces, comprising:

characterized in that in said temporary protection film:

the non-stick layer is based on silicones of the type

If (X, Y, Z) ¹ - [SiO (R) ₂ ] _m - [SiO (R) R 1 -O-CO- (CH = CH ₂ )] _n - [SiO (R) ₂ ] _P - Si ( X, Y, Z) ² wherein Χ, Υ, Ζ are atoms or groups having atoms selected from 0, C, H, Si;

R = linear alkyl and especially CH ₃ , C ₂ H ₅ , C ₃ H ₇ ,

R1 = linear alkylene, preferably derived from R,

and especially CH ₂ , C ₂ H _4, C ₃ H ₆ ,

m, n and p are integers, wherein m and p can be 0, and Si (X, Y, Z) ¹ is preferably SiO (R) ₃ and / or Si (X, Y, Z) ² is preferably Si (R) _3.

Said anti-adhesive layer can be obtained by UV-induced polymerization of silicone type

SiO (R) ₃ - [SiO (R) ₂ ] _m - [SiO (R) R 1 -CO- (CH = CH ₂ )] _n - [SiO (R) ₂ ] _p - Si (R) ₃ where R = linear alkyl and especially CH ₃ , C ₂ H ₅ , C ₃ H ₇ ,

R1 = linear alkylene, preferably derived from R,

and especially CH ₂ , C ₂ H _4, C ₃ H ₆ , m, n and p are integers, knowing that m and p can be 0.

In a general manner, in the context of the invention (whatever the object under consideration):

The thickness of the temporary protective film is advantageously between 15 μηι and 200 μηι and preferably between 45 μηι and 100 μηι;

The support film preferably comprises one, two or three layers;

The release layer is advantageously applied at a rate of about 0.01 to 2 g / m ² and preferably between 0.10 to 0.80 g / m ² ;

The adhesive layer is advantageously applied at the rate of approximately 1 to 40 g / m ² and preferably 3 to 15 g / m ² of dry extract per m ²

The release layer is preferably polymerized by UV light in a controlled atmosphere with reduced oxygen content (less than 10 ppm and preferably less than 50 ppm).

figures

Figure 1 shows schematically the structure of a temporary protection film according to the invention.

Figure 2 shows the photograph of a temporary protection film according to the state of the art in use situation, that is to say, stuck on a stainless steel sheet, after laser cutting. detailed description

The structure of a temporary protection film 6 according to the invention is shown in FIG. 1. According to the invention, to protect the surface of the metal sheet, which may be a steel sheet, and in particular stainless steel, when its transformation, in particular by cutting or drilling with a laser beam, is selected a temporary protective film 6 of particular structure. This temporary protection film 6 according to the invention, intended to protect metal surfaces, in particular stainless steel surfaces, during the operations of transformation of these sheets, in particular by laser cutting processes, comprises

a support film 1, transparent or colored, made of polyethylene or polypropylene or a mixture of the two, on one of its faces, called "lower face", an adhesive layer 4 based on synthetic or natural rubber,

- On the other of its faces, called "upper face", a release layer 5. The upper face of the support film may be printed by the deposition of an ink layer 3, by any suitable technique, in particular by flexography. The two faces of the support film 1, before the deposition of the adhesive layer 4 and the release layer 5, and before the possible printing 3, must receive a surface treatment, typically by corona discharge.

The support film 1 is advantageously produced by a method of hot coextrusion of polymers, or by bonding / lamination of at least two films, possibly printed, using a suitable adhesive, or by a combination of the two processes. All these methods are known as such. To produce the temporary protection film 6 according to the invention, it is possible to use a support film 1 previously produced with the indicated methods, or to implement all of these technologies (including surface treatment and coating) for realize it in one step.

(a) Support film 1

It is a plastic film generally produced from polymers derived from the families of polyethylene, polypropylene, polyester, polystyrene, polycarbonate as well as the polymers of these families possibly modified with comonomers and / or polymers of organic and / or organometallic type. These polymers are used individually or in a mixture of two or more of these polymers to form the support film 1. Films which are not likely to release dangerous fumes during welding or laser beam drilling are preferred and this is avoided. title PVC. Polyethylene films are preferred, as are polypropylene films. The support film 1 may contain organic, inorganic, organometallic or metallic additives intended to color these films and / or modify some of their mechanical or physico-chemical characteristics. Said additives may comprise all the mineral, organic or organometallic fillers that can usually be incorporated into these plastics. These additives are used as a single component or as a mixture of different compounds without limitation in quantity and in number according to the desired properties. (b) Surface treatment on the faces of the support film 1

The surface of the support film 1 may optionally be treated to improve these adhesion capacities by a modification of its surface tension. More specifically, it is desired to have sufficient surface tension to ensure the wettability necessary for good adhesion of the printing 3, the adhesive layer 4 and the release layer 5. This modification of the surface tension can be carried out by known techniques, for example by a corona treatment, a plasma treatment or the application of a primer. Corona treatment is preferred, a simple and effective method.

(c) Adhesive layer 4

It is made from natural and / or synthetic rubber. More particularly, it is made from natural rubber optionally modified with acrylic copolymers, synthetic rubber such as a block copolymer styrene or their hydrogenated derivatives, a styrene-butadiene-styrene copolymer or their hydrogenated derivatives, or a polyisobutylene. These products are used individually or in a mixture of two or more of these different products whose Mooney viscosity may possibly be modified by a mechanical kneading process, usually used in the rubber processing industry, particularly with the aid of a grille or an internal mixer.

This adhesive mass is generally formulated with additives: tackifying resin, crosslinking agent, vulcanizing agent, silane coupling initiator, organic, inorganic, organometallic or metallic dyes, plasticizing agent, antioxidant, light stabilizing agent, UV absorber agent, lubricating agent, polymerization inhibiting agent and more generally all additives commonly used in the formulation of rubbers. These additives are incorporated as a single component or a mixture of two or more of these additives without limitations in quantity and in number depending on the desired performance.

Tackifying resin refers to hydrocarbon petrochemical resins, rosin and derived resins, terpene and derivative resins and other resins used in the rubber processing industry.

One of the essential technical characteristics of the temporary protection film according to the invention is the fact that for its preparation, the adhesive layer 4 is applied to the support film 1 by coating with a solution of the synthetic rubber-based glue. or natural in a suitable solvent. This makes it possible to obtain a film with a glue that is evenly distributed and homogeneous, particularly cohesive, unlike glues based on natural or synthetic rubber applied without solvent (hot-melt type glues, ie applied hot) that are not suitable for this purpose. temporary protection of the surface, precisely because of insufficient cohesion. These films coated with hot-melt adhesives are used in large quantities, especially as permanent adhesive films, particularly in the packaging or label sector.

Alternatively, an acrylic adhesive can be used for the preparation of the adhesive layer which will be intended for surfaces other than stainless steel such as aluminum or surfaces with a permanent surface coating (prelacquered or varnished surfaces).

According to the invention, in an advantageous embodiment of the temporary protection film, all the components, including in particular at least one synthetic or natural rubber and at least one suitable solvent, are mixed in a reactor provided with a system of protection. stirring to achieve a homogeneous adhesive mass. The coating system used to deposit on the underside of the support film a thin and homogeneous layer of the adhesive mass is generally of the "roll over roll" type, i.e. with two parallel rollers. Thus, two cylinders with parallel axes in rotation having a suitable spacing are fed continuously with the adhesive mass which is deposited on the underside of the support film. Then, this layer is dried in order to evaporate the solvent; this step is typically performed in a hot air oven.

Typically, between 1 g / m ² and 40 g / m ² of glue, expressed as solids, is deposited on the underside of the support film.

(d) Non-stick layer 5

The choice of the release layer is also critical for the invention. According to the invention, this layer comprises polymeric silicones modified with acrylate, vinyl and / or ethylene functional groups. Modified polydialkylsiloxane silicones, and more particularly silicone type silicones, are preferred.

R1 = linear alkylene, preferably derived from R,

and especially CH ₂ , C ₂ H _4, C ₃ H ₆ , m, n and p are integers, knowing that m and p can be 0, knowing that the terminal groups SiO (R) ₃ and / or Si (R) ₃ may be replaced by other groups, here called "Si (X) (Y) (Z)", which contain a silicon atom and in which which Χ, Υ, Ζ are atoms or groups having atoms selected from 0, C, H, Si.

Such a release layer 5 can be obtained by UV-induced polymerization.

The modified silicones used in the context of the present invention may be crosslinked by UV radiation; for this purpose it must include a photo-initiator. The photoinitiator is preferably an aromatic ketone, and especially a 2-hydroxy-2-methyl-1-phenyl-1-propanone, the phenyl group may comprise functional groups. The product marketed under the trademark Tego ® A17 may be suitable for this purpose. The modified silicones used in the context of the present invention may be based on commercial products; for example, those marketed by the company Evonik under the trade name Tego®, and more especially a mixture between Tego® products RC 71 1 and Tego® RC 902, or the product Tego® RC 902 alone. The release layer is generally applied with a coating system implementing the combination of several cylinders engraved or smooth in rotation in order to deposit the sufficient amount of silicone varnish to obtain the desired properties. Then the strip passes under a UV lamp under controlled atmosphere, in particular with oxygen content as low as possible, to form by polymerization the release layer. It is then ready to be wound for storage, waiting to be used,

e) Application on the surface of a sheet

The inventor has found that the protective film according to the invention can be unrolled very easily, to be applied to the surface of a metal sheet. Its progress is regular, without jerks or clashes. Its flow is silent, even at high speed, whereas the films commonly used to protect metal sheets during laser cutting lead to a noise level well above 75 decibels, measured under the regulatory conditions. f) Laser cutting or drilling process

This cutting or piercing can be performed with any laser usually used for this purpose, and in particular with a C0 ₂ laser. Any face of the sheet, or its faces, can be protected by the temporary protective film. The inventor has discovered that the use of the temporary protection film described above makes it possible to increase the energy density during cutting and laser beam drilling, without the temporary protection film forming bubbles or off. g) Advantage of the process according to the invention

The cutting or drilling method according to the invention has many advantages. Without being bound by this theoretical explanation, the inventor believes that the reduction of the unwinding forces of the temporary protection film prevents the film from being applied to the metal sheet in a stretched state. Indeed, it is known that the unwinding force can cause traction of the film in its long direction, which leads to its elongation. The inventor has observed that with the temporary protective films commonly used to protect metal sheets, this elongation can reach or exceed 2%, and that these films are applied to the metal sheets in a state of elastic elongation, knowing that their elastic return is not immediate (hysteresis phenomenon): once stuck on the surface of the sheet, the film can not return to its original state and remains in a state of elastic elongation. Therefore, it retains the propensity to return elastically to its original state as soon as it has been removed from the sheet. It is also known that the unwinding force increases with the adhesive power of the film; Furthermore, to protect steel sheets is used protective films whose adhesive side has a greater adhesive strength than in the case of aluminum sheets for example.

In practical terms, this phenomenon can be visualized and measured on a sample of sheet protected by a protective film by a "peeling and repositioning" measure of the protective film: The protective film is removed from a sample of sheet metal protected by the latter, a length of 1 meter by 2.5 cm wide, for example, depending on the laying direction of the film, then waiting for at least a minute and repositioning the film thus taken off on the surface it protected initially: a shrinkage of the film is observed with respect to the initially protected surface.

This measurement only makes sense if there is a sticking force of the protective film on the lower sheet to the film yield stress (CSE) of the film support 1. Indeed, if this peel force is higher than the CSE, the peel force of the protective film is no longer applied in the elastic zone of the support film 1 which then undergoes a final deformation which can lead to the rupture film when removing the sheet.

It would therefore be desirable to be able to apply a temporary protective film to the surface to be protected, preferably without deformation or stress, and to be able to remove it with a release force preferably lower than the CSE of the support film 1.

If during laser cutting the adhesive bond between the underside of the film and the surface of the sheet is reduced by excessive local heating, or if the support film undergoes a chemical or physico-chemical modification by excessive local heating or mechanical damage, the elastic recovery of the film can be done locally and in an uncontrolled manner, which leads to the formation of bubbles and other defects. Furthermore, the protective film glued against the surface of the sheet develops a shearing force parallel to the surface of the sheet which is likely to reduce the adhesion between the protective film and the sheet.

The process according to the invention, based on the use of a special temporary protective film, prevents said film from being applied in a state of elastic elongation. This has a favorable effect on the adhesive bond between the sheet and the film, because there is no shear parallel to the surface. This also prevents the elastic return of the film, accompanied by bubbling, where the adhesive bond is weakened, for example by local heating due to the laser beam.

And finally, the choice of materials for the support film, the adhesive layer and the release layer prevents the formation of black lines on the sheet. The laser cutting is always accompanied by a decomposition of the polymers present on the cutting line, but the materials used in the context of the present invention do not lead to the formation of hazardous emanations or deposition of material on the metal sheet under - jacente. Examples

The following examples illustrate the invention and represent concrete embodiments, but do not limit the scope of the invention. Example 1

A temporary protection film according to the invention was made comprising a polyethylene support film 90 μηι thick produced by coextrusion of three layers (colorless, white, black). Corona treatment was applied on both sides of the support film. On the outside, about 0.30 g / m ² of an acrylate-modified silicone composition, namely a 50/50 mixture by weight of Tego ® RC 902 and Tego ® RC 71 1 products, was applied. To this mixture was added 2% by mass of Tego ® A17 photoinitiator. The crosslinking was carried out using a UV lamp (approximately 200 W / cm) under a controlled nitrogen atmosphere with an oxygen threshold of less than 50 ppm.

On the underside was then applied an adhesive solution (solids content 23%) comprising natural rubber in a suitable solvent, deposited at a rate of about 10 g of dry extract per m ² on the underside.

This temporary protection film shows rolling forces reduced by a factor of 4 compared with existing films for this use (for example the product sold under the name 4224RE from Novacel) and a sound level of less than 80 decibels during the course of unfolding. at low or high speed (from 5 m / min to 300 m / min).

This film according to the invention is applied to one of the faces of a 1 mm thick stainless steel sheet. The "peeling / repositioning" test described above did not reveal a measurable shrinkage. This sheet was placed on a standard laser cutting bench, with the film towards the laser, and sheet metal parts of various more or less complex shapes were cut with the aid of a standard model C0 ₂ laser. . Even at unusually high cutting speeds (and therefore light intensities), no bubbling or peeling of the film was observed during these operations. Neither has there been any significant shrinkage of the film on the sheet in the vicinity of cutting or puncturing lines. After cutting and / or perforation of the sheet metal, the film is easily removed without tearing and without residual traces on the protected surfaces.

Example 2

A black / white coextruded polyethylene film (thickness: 90 μηι) was treated by corona treatment on both sides, then a UV silicone varnish (Tego ® RC 902 with 2% by mass of the photoinitiator was applied to the outer surface). Tego ® A 17), with the addition of a small amount of acrylate, at a rate of 0.5 g (+/- 0.1 g) per m ² (coil 2) or with a carbamate varnish according to the state of the art (coil 1). Then, for each coil, the underside of the film was coated with a natural rubber-based glue in a suitable solvent, deposited at a rate of about 12 g / m ² . The width of the film was 10 cm.

The unwinding force was measured on a standard-type laboratory unwinder with unwinding speeds of between 10 m / min and 100 m / min; this speed range is representative of the conditions of use provided for this type of film. The results are collated in the table.

It can be seen that the film of the coil 1 requires an unwinding force approximately 17 to 18 times greater than that of the film of the coil 1.

Moreover, it is found that under the conditions indicated, the unwinding of the film of the coil 2, with a noise level of less than 50 decibels, is quieter than the unwinding of the film of the coil 1, which generated a noise of 97 decibels, measured under the same conditions.

Example 3

From the same polyethylene film as that described in Example 2, and using the treatments and coatings described in Example Two, two coils with a width of 1500 mm were prepared, one ("Coil 2"). ) corresponding to the invention, the other ("coil 1) in the state of the art.

With the aid of an industrial unwinding device, two stainless steel sheets having a width of 1500 mm and a length of 2500 mm were covered, one of the film of the coil 2 (according to the invention ), the other of the film of the coil 2 (according to the state of the art). In each sheet was then cut and peeled a strip of film size 2 cm x 38 cm; these samples being taken on the two sheets at the same place, depending on the laying direction of the film. After a relaxation time of about one minute, each strip was repositioned on the sheet where it had been peeled off.

For the film of the coil 1 (state of the art), a shrinkage of about 4.5 mm is observed, ie 1.18%. The film of the coil 2 (according to the invention) showed no shrinkage. On each of the two sheets, laser cutting and drilling were then carried out under identical conditions. With the film of the coil 1, there is a detachment and removal of the film areas with high cutting concentration. FIG. 2 represents a photograph of the film stuck on its stainless steel sheet after cutting and drilling with a laser beam. Many defects, including peeling and shrinkage, are readily seen near the areas worked by the laser beam, ie around the holes and near the cut edges. Some of these defects are indicated by an arrow. The film of the coil 2 does not show these defects.

Claims

1. Use of a temporary protective film (6) to protect at least one flat surface of a metal sheet against mechanical damage, during a cutting process or laser beam drilling,

said temporary protection film (6) comprising:

a support film (1), transparent or colored, monolayer or multi-layer, preferably polyethylene or polypropylene or a mixture of the two;

- On one of its faces, called "lower face", which is in adhesive contact with said at least one flat surface of said sheet, an adhesive layer (4);

- On the other of its faces, called "upper face", which can be printed, a release layer (5);

characterized in that in said temporary protection film:

the adhesive layer (4) on the underside is either based on synthetic or natural rubber and has been applied in solution in a suitable solvent, or based on an acrylic adhesive, and has been applied in both cases by coating on the underside of the support film;

the release layer (5) is based on polymeric silicones modified with acrylate functional groups.

2. Use according to claim 1, characterized in that said release layer (5) has been obtained by UV-induced polymerization.

3. Use according to claim 1 or 2, characterized in that said metal sheet is a steel sheet and more particularly a stainless steel sheet, or a sheet of aluminum or aluminum alloy.

4. Use according to any one of claims 1 to 3, characterized in that said surface of said metal sheet to be protected comprises a permanent surface coating, such as a lacquer or varnish.

5. Use according to any one of claims 1 to 4, characterized in that said release layer (5) is applied at a rate of about 0.01 to 2 g / m ² , and preferably about 0 , 10 to 0.80 g / m ² .

6. A method for cutting or drilling by laser beam of a metal sheet at least one flat surface of said sheet is protected during said cutting or piercing by a temporary protective film (6), said temporary protection film ( 6) comprising:

a support film (1), transparent or colored, monolayer or multilayer, preferably of polyethylene or polypropylene or a mixture of the two;

said method being characterized in that in said temporary protection film:

the adhesive layer (4) on the underside is either based on synthetic or natural rubber and has been applied in solution in a suitable solvent, or based on acrylic emulsion, and has been applied, in both cases, by coating on the underside of the support film (1);

7. The method of cutting or drilling according to claim 6, characterized in that said sheet is a steel sheet, and more particularly a stainless steel sheet, or a sheet of aluminum or aluminum alloy.

8. A method of cutting or drilling according to claim 6 or 7, characterized in that the surface of said sheet protected by said temporary protective film (6) comprises a permanent surface coating, such as a lacquer or varnish.

9. Cutting method according to any one of claims 6 to 8, characterized in that said laser beam is generated by a C0 ₂ laser.

10. Temporary protective film (6) for metal surfaces, particularly for stainless steel surfaces, comprising:

- On one of its faces, called "lower face", which is in adhesive contact with said at least one flat surface of said sheet, an adhesive layer (4); - On the other of its faces, called "upper face", which can be printed, a release layer (5);

characterized in that in said temporary protection film (6):

the adhesive layer (4) on the underside is either based on synthetic or natural rubber and has been applied in solution in a suitable solvent, or based on an acrylic adhesive, and has been applied in both cases by coating on the underside of the support film (1);

the non-stick layer (5) is based on silicones of the type

If (X, Y, Z) ¹ - [SiO (R) ₂ ] _m - [SiO (R) R 1 -O-CO- (CH = CH ₂ )] _n - [SiO (R) ₂ ] _P - Si ( X, Y, Z) ² wherein X, Y, Z are atoms or groups having atoms selected from 0, C, H, Si;

R = linear alkyl and especially CH ₃ , C ₂ H ₅ , C ₃ H ₇ ,

R1 = linear alkylene, preferably derived from R,

and especially CH ₂ , C ₂ H _4, C ₃ H ₆ ,

1 1. Protective film (6) according to claim 10, characterized in that said release layer (5) is applied at a rate of about 0.01 to 2 g / m ² , and preferably about 0.10 to 0.80 g / m ² .

12. Protective film (6) according to claim 10 or 1 1 characterized in that the adhesive layer is applied at a rate of about 1 to 40 g and preferably about 3 to 15 g of dry extract per m ² .

13. Protective film (6) according to any one of claims 10 to 12, characterized in that said release layer (5) has been obtained by polymerization induced by UV rays.

14. Protective film (6) according to claim 13, characterized in that said release layer has been polymerized in a controlled atmosphere with reduced oxygen content, preferably less than 10 ppm and even more preferably less than 50 ppm.

A method of protecting a flat surface of a metal sheet against mechanical damage, comprising the steps of:

(a) supplying a metal sheet;

(b) unwinding the protective film (6) according to one of claims 9 or 10; (c) applying said protective film (6) to the surface of said metal sheet, and preferably over the entire surface.