Method for increasing the surface tension during biaxial stretching of a film

Abstract

Method for increasing the surface tension during the biaxial stretching of a film made of at least one thermoplastic polymer, wherein the film is first stretched in the longitudinal direction, then subjected to a first plasma treatment, then stretched in the transverse direction and then subjected to a second plasma treatment, characterized in that both plasma treatments take place in an atmosphere containing a maximum of 5 vol.% oxygen, the second plasma treatment takes place at a pressure reduced compared to atmospheric pressure of a maximum of 100 hPa, and the film is wound up after the stretching in the transverse direction and unwound again before the second plasma treatment.

Description

[0001] The invention relates to a method for increasing the surface tension during the biaxial stretching of a film made of at least one thermoplastic polymer according to one of the preambles of claims 1 or 2.

[0002] It is known to pretreat the surfaces of plastics, particularly the surfaces of plastic films, before printing with ink. Such surface pretreatment is necessary to increase the very low surface tension after extrusion of the plastic film and thus improve the adhesion of inks or - in the case of films to be metallized - metals. For this purpose, the film is usually subjected to a plasma treatment, which can be a treatment with a plasma torch or within a plasma-containing environment. Alternatively, the plasma can be generated by a dielectrically impeded discharge, which is known as a corona discharge. The ions in the plasma transfer their charge and energy to the surface of the film, causing fragmentation, splitting, or chain breaks in the polymer surface, which increases the surface tension of the film surface.

[0003] The treatment can be performed at atmospheric pressure or reduced pressure. Plasma or corona treatments can be performed immediately after film extrusion and directly before winding, or immediately before the printing or metallization step.

[0004] The increase in surface tension caused by plasma or corona treatment is measured in mN / m. For an untreated, biaxially stretched polypropylene film, the surface tension is a maximum of 30 mN / m, whereas corona treatment can achieve a surface tension of up to 45 mN / m. The effect of the surface treatment, i.e., the surface tension of the treated film, decreases over time and can be refreshed under certain conditions by further corona treatment.

[0005] From US 2005 / 0096431 A1 a method for producing a biaxially stretched polymer film is known, wherein a plasma or corona treatment is provided before or after a respective stretching stage, wherein the surface tension of the polymer film is increased by the plasma or corona treatment and thus the treated polymer film has improved adhesion properties to adhesive or bonding layers.

[0006] From JP 2007-211320 A, a method for plasma-treating a biaxially stretched polymer film and subsequently applying a metal layer by means of PVD (physical vapor deposition) to the plasma-treated biaxially stretched polymer film within a metallization system is known, wherein the surface tension of the polymer film is increased by means of the plasma treatment, thereby achieving improved adhesion between the applied metal layer and the previously plasma-treated biaxially stretched polymer film.

[0007] DE 10 2005 052 441 C5 describes a process for increasing the surface tension of a biaxially oriented film made of thermoplastic polymers. The film is first stretched longitudinally and then transversely. After longitudinal stretching and before transverse stretching, the film is treated with atmospheric pressure plasma on one surface. After transverse stretching and before winding, the film is additionally subjected to a second treatment using corona or flame, this second treatment being applied to the same surface. This is intended to ensure that the films exhibit good adhesion properties in further processing processes, even after typical storage times of more than six months.

[0008] However, it turned out that with this process, the adhesion of UV ink was very low after the second corona treatment. Coating was not possible. Surface tension measurements were taken, which ranged between 38 and 46 mN / m and higher. Nevertheless, the UV ink could not adhere to the surface, and thus many coatings could not be applied. For example, reliably adhering metallic coatings cannot be applied to such a film, even though it has a high surface tension.

[0009] The task is to further develop a generic process in such a way that optimal adhesion of UV ink and other coatings, in particular metallic coatings, can take place after the second treatment.

[0010] This object is achieved by the characterizing features of claims 1 and 2, respectively. Advantageous embodiments can be found in subclaims 3 to 9.

[0011] An embodiment of the invention is described in more detail below.

[0012] A film consisting of a thermoplastic polymer, produced either by extrusion or coextrusion, which may consist of one or more polymer layers and has a thickness of between 20 and 30 µm, is stretched at a temperature of between 80 and 120° in a longitudinal stretching ratio of 1:4 in a manner known per se.

[0013] The film is then subjected to a plasma treatment at atmospheric pressure. The film is guided over a dielectrically coated roller in a controlled atmosphere. A high-frequency high voltage is applied between the metallic interior of the roller and a counter electrode opposite the roller, causing a dielectrically impeded discharge. Such a discharge at atmospheric pressure in air is referred to as a corona discharge, and a corresponding treatment of a film is referred to as a corona treatment. The treatment atmosphere contains at least 95 vol.% of a gas mixture containing essentially nitrogen and a maximum of 5 vol.% oxygen. In a preferred embodiment, the atmosphere contains only nitrogen, possibly carbon dioxide, and a maximum of 5 vol.% oxygen, although it is also possible to work exclusively with nitrogen and a maximum of 5 vol.% oxygen.Furthermore, layer-forming precursor gases, in particular acetylene, or other additional gases may be present in the atmosphere as an admixture.

[0014] The film is then stretched transversely at a transverse stretching ratio of 1:8 in a conventional manner, followed by a further high-energy treatment, which also takes place in the form of a plasma or corona treatment in the same atmosphere as described above.

[0015] Both treatments of the film take place across its entire width, with the second treatment taking place at a pressure reduced to a maximum of 100 hPa compared to atmospheric pressure.

[0016] In a preferred embodiment, the second plasma treatment is integrated within a metallization system and takes place immediately before a metallic layer is applied to the film in order to increase the adhesion of this metallic layer to the film surface.

[0017] The residence time of the polymer film in the first plasma is preferably between 0.01 and 4 seconds while the energy input is between 0.6 and 30 kJ / m 2 lies.

[0018] In a modified embodiment, the film can also be wound up after transverse stretching, stored temporarily, and then unwound again at a later time and subjected to a second plasma treatment. The plasma treatments can take place on both sides or just one side of the film.

[0019] The film treated according to the invention has the advantage over films treated according to the prior art that UV inks and thus other coatings, especially metallizations, also exhibit high adhesion. A surface tension of 58 mN / m is achieved after the second plasma treatment.

[0020] Although a comparably high surface tension is measured using known methods, the adhesion of coatings, especially metallizations, is significantly improved using the present method. This is apparently due to the fact that other chemical parameters play a role that cannot be captured by surface tension measurements.

[0021] When implementing the invention, it is important that the atmosphere, i.e. the composition of the treatment gases, is controlled during the first and second plasma treatment, and in particular that as little oxygen as possible is present.

[0022] It is particularly advantageous if the maximum oxygen concentration is kept as low as possible. For example, an oxygen concentration of less than 1 vol.%, preferably 0.2 vol.% oxygen content, can be used. During operation, the oxygen content generally increases anyway due to the conversion of carbon dioxide to oxygen, so treatment is ideally initiated with a low oxygen content.

Claims

[1] A process for increasing the surface tension during the biaxial stretching of a film made of at least one thermoplastic polymer, wherein the film is first stretched in the longitudinal direction, then subjected to a first plasma treatment, then stretched in the transverse direction and then subjected to a second plasma treatment, characterized by that both plasma treatments take place in an atmosphere containing a maximum of 5 vol.% oxygen, the second plasma treatment is carried out at a pressure reduced compared to atmospheric pressure of a maximum of 100 hPa, and the film is wound up in the transverse direction after stretching and unwound again before the second plasma treatment. [2] A method for increasing the surface tension during the biaxial stretching of a film made of at least one thermoplastic polymer, wherein the stretching is carried out simultaneously in the longitudinal direction and in the transverse direction, wherein the film is subjected to a first plasma treatment before the simultaneous stretching and to a second plasma treatment after the simultaneous stretching, characterized by that both plasma treatments take place in an atmosphere containing a maximum of 5 vol.% oxygen, the second plasma treatment takes place at a pressure reduced compared to atmospheric pressure of a maximum of 100 hPa, and the film is wound up after stretching and unwound again before the second plasma treatment. [3] Method according to one of the preceding claims, characterized bythat the second plasma treatment takes place within a metallization system and the film is provided with a metallic layer immediately after this treatment. [4] Method according to one of the preceding claims, characterized by that the film is a single-layer or multi-layer film made of at least one polyolefin. [5] Method according to one of the preceding claims, characterized by that both plasma treatments take place on one side of the film. [6] Method according to one of claims 1-4, characterized by that the two plasma treatments take place on both sides of the film. [7] Method according to one of the preceding claims, characterized by that the residence time of the film in the first plasma treatment is between 0.01 and 4 seconds. [8] Method according to one of the preceding claims, characterized bythat the energy input during the first plasma treatment of the film is between 0.6 and 30 kJ / m 2 lies. [9] Method according to one of the preceding claims, characterized by that at least one of the two plasma treatments takes place in an atmosphere which contains exclusively nitrogen and carbon dioxide, preferably exclusively nitrogen.