WO2006089885A1

WO2006089885A1 - Device and method for conferring a surface structure on a plastic film

Info

Publication number: WO2006089885A1
Application number: PCT/EP2006/060125
Authority: WO
Inventors: Yannick Gerard; André POTY
Original assignee: Renolit SE
Current assignee: Renolit SE
Priority date: 2005-02-22
Filing date: 2006-02-21
Publication date: 2006-08-31
Anticipated expiration: 2007-08-22
Also published as: FR2882294B1; FR2882294A1

Abstract

Device comprising two engraved rollers to confer a surface structure on both faces of a plastic film by pressing the film between the two rollers, characterized in that at least one of the rollers comprises a central layer (1), an intermediate layer (2) made of synthetic material and an outer layer (3) made of rigid material whose surface is engraved with the negative of the structure to be conferred.

Description

Device and method for conferring a surface structure on a plastic film

The invention relates to a device for conferring a surface structure on a plastic film. It also relates to a method using the said device.

In many applications, plastic films must have a surface structure. "Surface structure" means a surface state that is not smooth but controlled. An example of such a surface structure is graining. Grained plastic films, in addition to their aesthetic qualities and ease or pleasantness of handling, have fewer risks of undesirable sticking ("blocking"), particularly following electrostatic phenomena. Certain surface structures may also confer particular optical qualities on the film. In certain cases, plastic films have a surface structure on one face only.

However, usually, it is desirable for both faces of the film to be structured.

It is known practice to produce in two steps plastic films structured on both of their faces. In these known methods, films structured on only one face are produced separately and they are assembled in a second step, the unstructured faces facing one another. Such a method is evidently quite costly. In addition, when the surface structure is intended to confer particular optical qualities on the film, the surface structures of both faces must be precisely coordinated, something which further complicates production.

The invention aims to provide a device making it possible to confer a surface structure on both faces of a plastic film simply and precisely and makes it possible to obtain optical effects thanks to the coordination of the surface structures of the two faces.

Consequently, the invention relates to a device comprising two engraved rollers to confer a surface structure on both faces of a plastic film by pressing the film between the two rollers, characterized in that at least one of the rollers comprises a central layer (1), an intermediate layer (2) made of synthetic material and an outer layer (3) made of rigid material whose surface is engraved with the negative of the structure to be conferred.

Thanks to this device, it is possible to obtain a quality of contact between the two faces of the film and the rollers that makes it possible for example to grain both faces of the film simultaneously, something which significantly improves and simplifies the known methods. In the invention, "graining" means the act of conferring on a film a surface state having a controlled roughness. This roughness, characterized by the Ra as measured according to the ISO4287 standard, is advantageously between 1 and 100 μ, preferably between 10 and 50 μ. In addition, it has a large degree of reproducibility so that the Ra₁ measured at different locations of a film grained by means of the device according to the invention are such that at least 90 % of the Ra₁ lie between 0.5 advantageously 0.75 and 1.5 advantageously 1.25 Ra_m, Ra_m being the mean Ra₁ value.

The device according to the invention may be incorporated into any method of working plastic materials. It is particularly suited to calendering or extrusion-dip coating methods. In these applications, the rollers of the device typically, but not limitingly, have diameters varying from 100 to 400, preferably from 200 to 300 mm and lengths varying from 50 to 300, preferably from 100 to 250 cm. It is most particularly suited to extrusion-dip coating which comprises the extrusion of a sheet of plastic by means of a flat die, the sheet being drawn at its die outlet and then cooled and rapidly thinned. "Sheet" in this specification means any flat product having a width at least 10, preferably 100 times greater than its thickness. "Film" means a sheet having a thickness of less than 1 mm, advantageously less than 300 μ, preferably 200 μ, still more preferably 100 μ. Such films also advantageously have a width greater than 1 metre.

Plastics may be thermosetting or thermoplastic. They are advantageously thermoplastic. "Thermoplastic plastic" means a material that becomes fluid when it is sufficiently heated, the change of fluidity being reversible when it is cooled. Polyolefins, polyamides, polyvinyl chloride (PVC) are preferred. The device according to the invention is particularly suited to working PVC and polypropylene. Polypropylene is preferred.

According to the invention, the intermediate layer that is approximately cylindrical in shape is made of synthetic material. The material will be chosen to obtain an optimum deformability under the conditions of use. Usually, silicone- based rubbers or elastomers give good results. The use of a synthetic material with a Shore A hardness of less than 80, preferably less than 70, is recommended. It is preferable for the hardness to be greater than 20, if possible 30, values lying between 40 and 60 being most preferred.

The outer layer of the roller according to the invention must make it possible to obtain a good wear resistance of the roller surface and transmit the deformability of the intermediate synthetic layer. It must therefore also be able to deform easily, which requires a maximum thickness, depending on the material used. This material must be a good conductor of heat. Advantageously it has a rigidity, measured by the modulus of elasticity, at least ten times greater than that of the intermediate layer, preferably greater than 20 GPa. Any material combining these conditions may be used, such as for example certain high- performance polymers. However, it is recommended that the outer layer be made of metal, such as for example copper, steel or nickel.

In one advantageous embodiment of the roller according to the invention, the outer layer consists of chromium-plated nickel and has a thickness of between 0.1 and 1 mm.

The central layer of the roller according to the invention is intended to give it good rolling properties. It comprises means of rotatably fastening to a frame. These means comprise a shaft or segments of shafts or else means of fastening to a shaft or segments of a shaft. The central layer is advantageously made at least in part from metals having a good wear resistance such as the customary steels.

Usually, it is recommended that there be a good contact between the intermediate and outer layers. Given the thinness of the outer layer, it is recommended that the intensity of this contact along a generatrix of the roller, that is to say the local pressure exerted by the intermediate layer on the inner face of the outer layer, be correctly controlled. Specifically, any local pressure difference will be manifested by a variation, however minimal, in the same location, in the outer diameter of the roller, which could have a detrimental effect on the treatment of the film.

To be able to control this pressure locally, in one advantageous embodiment of the device according to the invention, the intermediate layer consists of the juxtaposition of annular sections, each section surrounding a main rigid ring, having a lesser thickness than that of the section, at least one optional secondary rigid ring being affixed to a main ring. In this embodiment, the length of the layer corresponds to the sum of the thicknesses of the sections. Furthermore, the thickness of the secondary rings corresponds to the difference in thickness between the sections and the main rings.

In this embodiment, all the juxtaposed sections and the rings are squeezed together. When a secondary ring is affixed to a main ring, the main ring compensating for the difference in thickness between the section and the main ring, the corresponding section is not placed in compression during squeezing. On the other hand, the sections surrounding the rings with no secondary rings - A -

will be compressed and will tend to expand in the radial direction. The rings are made of a rigid material. This means a material at least twice, preferably five times, more preferably ten times more rigid than the synthetic material constituting the sections. As a result, during squeezing, the rings hardly deform at all. They can advantageously be made of metal. When the user desires to increase the pressure of the intermediate layer on the outer layer in a certain location, the secondary ring that is closest thereto is removed. As a result, when the assembly is squeezed again, the section from which the secondary ring has been removed is compressed, which increases the pressure that it exerts on the outer layer.

In a variant of the roller according to the invention, it is recommended to cool the outer layer in a known manner by contact with a fluid circulating on the outside of the roller. In particular, an air jet or a water spray combined with a means of drying or spinning off excess water or any other known conditioning device may be used.

In another advantageous embodiment of the roller according to the invention, at least one duct is made in the roll, advantageously in the intermediate layer, substantially parallel to the axis of the roller, at least part of the external surface of the duct being in contact with the intermediate layer, for the circulation of a pressurized conditioning fluid, and this roller comprises fluid inlet and outlet means allowing the duct to be connected to an external unit for conditioning the fluid as to temperature and pressure. The ducts are advantageously made by inserting flexible tube segments, for example made of silicone elastomer, in cylindrical orifices made in the intermediate layer. In this embodiment, the intermediate layer must have a thickness sufficient for at least one duct to be able to be made therein. Preferably it has a thickness of at least 5, advantageously 10 mm. The maximum thickness depends on the diameter of the roller. For rollers having an external diameter lying between 20 and 100 cm, it is recommended that the thickness of the central layer does not exceed a quarter of the external diameter of the roller.

In an advantageous variant of this embodiment, the intermediate layer is covered with a protective layer made of synthetic material having a hardness greater than that of the intermediate layer. This protective layer advantageously has a Shore A hardness greater than 50, preferably 60. It is recommended that its hardness does not exceed 100, preferably 90, the values lying between 60 and 80 being the most preferred. This protective layer makes it possible to correct the surface unevennesses that may be created by the presence of at least one duct in the intermediate layer.

In the embodiment of the invention in which the intermediate layer results from the juxtaposition of annular sections, a portion of the duct or ducts is made in each section, the totality of the duct resulting from the juxtaposition of the portions of duct.

It is recommended that the conditioning fluid inlet and outlet means comprise an access to the means of fastening the roller to the frame supporting the roller via the central layer. This layer therefore advantageously comprises a portion of the conditioning fluid inlet and outlet means, so as to transport the latter from the duct or ducts to the outside of the roller via the means of rotatably fastening the cylinder to the frame.

In many cases, it is preferable to make several ducts advantageously distributed evenly over the circumference of the intermediate layer. It is recommended that the number of ducts be at least 10.

The fluid is advantageously a liquid, such as for example water, where necessary supplemented by additives making it possible to cool it below O⁰C without freezing. The fluid circulating in the duct or ducts has a temperature and a pressure that can be controlled continuously by adjusting the external unit. The temperature of the fluid is advantageously less than the surface temperature that it is desired to maintain on the surface of the roller during the working of the plastic. The circulation of such a fluid makes it possible to control the temperature of the synthetic material, while preventing it from overheating during working. It has been observed that such control improves the adhesion between the intermediate synthetic layer and the outer layer on the one hand and between the intermediate synthetic layer and the central layer of the roller on the other hand. In addition, controlling the temperature of the synthetic material reduces the risks of expansion due to overheating, which uncontrollably modifies the external diameter of the roller. When the device according to the invention is used for working plastics, it is possible in this way to obtain a greater and better controlled contact surface between the film and the roller.

The ducts made in the intermediate layer must be suitable for the circulation of a fluid pressurized by an external unit. "Pressurized fluid" means a fluid that has as it enters the duct or ducts a pressure at least sufficient to overcome the pressure loss created by its circulation in the duct. The external unit, which is typically a refrigeration unit, is therefore furnished with a pressurization device such as for example a circulation pump. The circulation flow must be sufficient to provide a good thermal conditioning of the intermediate layer. It is recommended that the flow be such that the temperature difference of the conditioning fluid between its entering and its leaving the roller is less than 1O⁰C, advantageously 5⁰C, preferably 2⁰C, more preferably I⁰C. However, the conditioning fluid pressure at the roller inlet is advantageously greater than 200 kPa. It is recommended not to exceed 3000 kPa, the preferred values being 300 to 2000 kPa. Because of the considerable pressurization of the fluid, markedly greater than that necessary to circulate the fluid, the duct or ducts expand and compress the intermediate layer, something which ensures that the latter has good contact with the outer and central layers. If the fluid is sufficiently pressurized, it is even possible for the compression of the intermediate layer alone to be responsible for the contact between the intermediate layer and the outer layer in the sense that, when the fluid is not pressurized, it is easily possible to slide the outer layer off the intermediate layer. It is then easy to change the outer layer.

The device according to the invention makes it possible to confer a surface structure simultaneously on both faces of a plastic film.

Consequently, the invention also relates to a method for conferring a surface structure on both faces of a plastic film according to which the film is pressed by means of a device according to the invention.

In an advantageous embodiment of the method according to the invention, a device is used in which at least one duct (4) is made in the intermediate layer (2), substantially parallel to the axis of the roller, for the circulation of a pressurized conditioning fluid, and this roller comprises fluid inlet and outlet means (5) allowing the duct to be connected to an external unit for conditioning the fluid as to temperature and pressure.

According to a recommended variant of this embodiment, the conditioning fluid is pressurized to a pressure of 300 to 2000 kPa. In this embodiment of the method according to the invention, a particularly well controlled contact surface between the film and the rollers is obtained. The quality of this contact surface makes it possible to work, particularly to laminate, thin films without the appearance of rippling, in particular of films having a thickness of less than 300 μ, preferably 200 μ, more preferably 100 μ. By using known methods and devices, a greater local pressure needs to be exerted on the films to thin them by lamination, which causes the appearance of rippling, that it to say an accumulation of rotating melted plastic upstream of the roller. The presence of such rippling excludes the working of multi-layer films. Consequently, in an advantageous variant of the method according to the invention, the film is a multi-layer film. Such a film may be made by surface application or coextrusion.

Finally, the excellent control of the external diameter of the roll used in the method according to the invention makes it possible to work films having particular optical effects, by imposing slightly different peripheral speeds to the two rollers between which the film is pressed. Particular features and details of the invention will emerge from the appended figures.

Figure 1 illustrates an advantageous embodiment of the invention. It shows the intermediate layer (2) made of synthetic material, the protective layer (2'), the outer layer (3) and the ducts (4). The conditioning fluid circulating in the ducts is cooled by a refrigeration unit (10) and pressurized by means of a pump (9). The rollers are driven by motors Ml and M2. Figures 2 and 3 illustrate the zone of contact between the rollers.

Figure 4 shows in greater detail a roller of the device according to the invention. It also shows the central layer (1), the intermediate layer (2) made of synthetic material, the protective layer (2'), the outer layer (3), the ducts (4) and the conditioning fluid inlet-outlet means (5).

Figure 5 illustrates a particular embodiment of the intermediate layer. It illustrates the sections (6), the main rings (7) and the secondary rings (8).

Claims

C L A I M S

1. Device comprising two engraved rollers to confer a surface structure on both faces of a plastic film by pressing the film between the two rollers, characterized in that at least one of the rollers comprises a central layer (1), an intermediate layer (2) made of synthetic material and an outer layer (3) made of rigid material whose surface is engraved with the negative of the structure to be conferred.

2. Device according to the preceding claim, characterized in that the synthetic material has a Shore A hardness of between 40 and 60.

3. Device according to either one of the preceding claims, characterized in that the outer layer has a thickness of between 0.1 and 1 mm.

4. Device according to any one of the preceding claims, characterized in that the intermediate layer (2) consists of the juxtaposition of annular sections (6), each section surrounding a main rigid ring (7), having a thickness less than that of the section, at least one optional secondary rigid ring (8) being affixed to a main ring.

5. Device according to any one of the preceding claims, characterized in that at least one duct (4) is made in the roll, preferentially in the intermediate layer (2), substantially parallel to the axis of the roller, at least part of the external surface of the duct being in contact with the intermediate layer, for the circulation of a pressurized conditioning fluid, and in that this roller comprises fluid inlet and outlet means (5) allowing the duct to be connected to an external unit for conditioning the fluid as to temperature and pressure.

6. Device according to the preceding claim, characterized in that the intermediate layer (2) comprises at least one duct (4) and is covered with a protective layer (2') made of synthetic material having a hardness greater than that of the intermediate layer.

7. Method for conferring a surface structure on both faces of a plastic film, according to which the film is pressed by means of a device conforming to any one of Claims 1 to 6.

8. Method for conferring a surface structure on both faces of a plastic film, according to which the film is pressed by means of a device conforming to one of Claims 5 or 6 in which a pressurized conditioning fluid circulates.

9. Method according to the preceding claim, in which the conditioning fluid is pressurized at a pressure of 300 to 2000 kPa.

10. Method according to any one of Claims 7 to 9, in which the two rollers rotate at a different peripheral speed, in order to confer optical effects on the film.