Polymer composition for forming release films

Abstract: The invention provides a polymer composition for forming release films for contact with surfaces containing pressure-sensitive adhesives, characterized in that it is formed of a reactive polyolefin containing functional groups and a reactive release substance, which have been caused to react by compounding in a molten state.

Description: The invention relates to a polymer composition from which release films can be prepared for contact with surfaces containing pressure-sensitive adhesives. Release covers are used in labels, floor slabs, wallpapers, plasters, stickers, etc. Release covers are traditionally made of kraft paper, which is coated with silicone. A silicone polymer and a catalyst are spread on the base material and the final polymerization occurs by heating and/or irradiation. The silicone coating is to be cross-linked as a uniform surface and anchored to the subweb. A low tensile strength, a varying thickness and an uneven surface place limitations, which have become important as the processing speeds and automatic label applications have increased. Precoated release papers, which are papers coated with polyolefins on one or both sides, have solved part of the above-mentioned problems. In this case, the paper is first coated with a polyolefin and thereafter with silicone, i.e. two separate coating steps are needed, as described in the US publication 3 403 045. In addition, the silicone treatment requires a catalyst. In the release films are used also polyester films, most often polyethylene terephthalate, which are coated with a silicone polymer. According to the US publication 453 366, the film is treated by irradiating it in an inert atmosphere before spreading the silicone. In any case, polyester films can also be coated without irradiation (FI 893868), when a certain polydiorganosiloxane is used. In the US patent 4 859 511 is described a release film, the base of which can be a paper, nonwoven, a plastic film or an extrusion coated paper and which is precoated before spreading the release substance. The precoating material is polar, e.g. polybutadiene or polyisoprene. The silicone coating is spread after the precoating and dried or hardened depending on the properties of the silicone. In the EP publication 252 483 is described a fluorosilica polymer, which can be used as a release substance on various base materials, such as paper, various plastic films and metals. The release substance is spread on the base material and hardened. JP-A-60-094485 discloses a release coating composition derived from reaction of a polysiloxane with a polyisocyanate and a hydroxyalkyl acrylate. EP-A-0254050 discloses a release coating made of a polyethylene/vinylsiloxane mixture. EP-A-0334051 discloses a release coating made by co-extrusion of a polypropylene and a difunctional silicone. Also polyolefin films can thus be used in release films. However, the siliconization has to be performed as a separate processing step and irradiation and/or heating is needed for fixing the silicone. The temperature is relatively high, generally about 150°C, and the properties of the polyolefin deteriorate. The present invention solves the problems related to known release films in a very simple manner. Instead of using a separate base material, which is coated with a release substance, such as silicone, the invention provides a polymer composition which as such acts as a release substance and whose treatment properties correspond to the treatment properties of LDPE, and it can thus be formed directly as a release film by means of a conventional film preparation technique. According to the invention there is provided a polymer for forming release films for contact with surfaces containing pressure-sensitive adhesives, characterized in that it is formed of a polyolefin to which compounds containing functional groups have been joined by copolymerizing, or grafting in another way than by means of said functional groups, said functional groups being acid or acid anhydride, silane, hydroxy, epoxy, amino, ketene or isocyanate groups, and a reactive release substance that is silicone, fatty acid compound or fluoropolymer containing functional groups, which polyolefin and release substance have been compounded in a molten state. The polymer composition developed for the preparation of a release film is comprised of a reactive polyolefin and a reactive silicone or some other reactive release substance, and these are caused to react with each other by compounding in a molten state. In this way, it is possible to make the silicone or some other release substance chain join to the polyolefine chain. The reactive polyolefine can be any polyolefine, to which a reactive organic compound is joined by copolymerization, gafting or some other means by joining to the chain. Organic compounds suitable for this purpose may be such compounds, which contain acid or acid anhydride, silane, hydroxyl, amino, epoxy, cetene, or any other groups, which react with reactive silicones or other release substances. Suitable polyolefines, which can be applied to the inventive reactive polymer, include copolymers of ethylene and carboxyl acids or carboxyl acid derivatives, such as ethylene-acrylic acid and ethylene-metacrylic acid copolymers, ethylene-butyl acrylate, ethylene-ethyl acrylate, ethylene-methyl acrylate, ethylene-hydroxyethyl metacrylate and ethylene-glycidyl metacrylate. Suitable are also copolymers of ethylene and vinyl alcohol as well as copolymers of ethylene and hydrolyzable silanes, such as ethylene-vinyl trimetoxy silane copolymers. The ethylene copolymers may also be grafted with acids or acid anhydride, e.g. carboxyl acids. Examples of these include ethylene-butyl acrylate copolymers grafted with carboxyl acids. Reactive release substances are in turn such silicones or other substances, which contain acid or acid anhydride, ketene, epoxy, hydroxy (silanol or carbinol), amino or hydrogen groups, which are bound thereto, or any reactive groups, which can react with reactive polyolefines. These groups can be located at the ends of the chains (primary groups) or as side groups (secondary groups). Also unsaturated groups are possible, since they can be grafted to polyolefines, or silicones or other release substances can be caused to remain in the polyolefine by means of long paraffinic groups without a chemical reaction. By admixing said reactive polyolefines and reactive release substances in a molten state, a release polymer is obtained, which is a reactive compound and which can be delivered to film manufacturers in a granulated form. The compounding can be performed with a Bambury mixer, a continuos mixer, a two-screw compounding device or any compounding device, whose temperature and delay time are sufficient for causing the reactive components to fully react with each other. In certain cases, the reactive compounding can also be performed in connection with the film preparation. A release film is a film (monofilm, co-extruded film, laminated film or (co)extrusion coated web), whose surface layer is a release compound. The film can also be prepared by means of a blown-film, cast-film or extrusion coating technique, whereby the subweb can be paper, aluminium, plastic or any other subweb suitable for extrusion coating. In the inventive reactive polymer, the quantity of the reactive release substance can vary within the range 0.1-20% by weight, although the suitable quantity can generally be selected within the range 1-10% by weight. In connection with higher release- substance contents, the reaction compound becomes too cross-linked. The tape, with which said release film is used, can be rubber- or acrylate-based or any tape, which is used for technical applications (e.g. as a backing film for emery paper, in which case the tape has to adhere to steel), for packing purposes (e.g. as a backing film for labels, in which case the tape has to adhere to glass, PVC, PET, etc.) and for hospital purposes (e.g. as a backing film for EKG-electrodes, in which case the tape has to adhere to skin). Various non-limiting examples have been given below for illustrating the invention, which, with respect to release/adhesion properties, have been tested as follows: The release compounds were made with a Bambury mixer (200°C, 10 min) and pressed into sheets with a thickness of 2 mm. Rubber-based standard testing tape (Beyersdorf's TESA4154) was pressed onto these sheets such that the pressure was 70 g/cm², time 20 h and temperature 23°C. The adhesion (release force) between the release-compound sheets and the tape was tested with the Instron pulling device, when the pulling speed was 300 mm/min (180° peel test) and the release value was given in N/cm. For PVC and PET in the same way as above. The readhesion was measured in the same way as the release value. The following reactive polyolefine grades were used: NCPE0414 Acid-grafted 17% ethylene-butyl acrylate NCPE6664 Ethylene-vinyl trimetoxy silane (2%) copolymer NCPE0465, Neste Ethylene-hydroxyethyl metacrylate (8%) copolymer Primacor 3340, Dow Ethylene-acrylic acid (9%) copolymer The following reactive release substances were used: L-9300, Union Carbide Silicone containing glycidyl groups DC-109, Dow Corning Silicone containing hydroxyl groups (primary silanol) MH-15, Bayer Polymethyl hydrogen siloxane based silicone Reoplast 39, Ciba-Geigy Epoxidized soybean oil AKD, Raisio Alkyl cetene dimer SLM 52-173, Wacker Chemie Silicone containing silanol groups FD 350, Wacker Chemie Silicone containing silanol groups FD 80, Wacker Chemie Silicone containing silanol groups Example 1 Various release substances were melt-mixed with a reactive ethy-lene-silane copolymer (NCPE6664). The results are given in Table 1. Release substance Adhesion of compound to tape (N/cm) Adhesion of tape to glass 3% L9300 0.63 1.9 3% DC-109 0.72 1.7 3% DC-109 + 0.1% DBTDL 0.53 2.0 3% MH-15 0.62 1.5 None 1.4 1.9 Table 1 shows that by adding reactive release substances the tape can be easily loosened and that also relatively good adhesion results to glass are obtained. An advantageous combination is obtained when using DC-109, and the results further improve when the condensation catalyst DBTDL of silanols (dibutyl tin dilaureate) is used. Example 2 Ethylene-hydroxyethyl metacrylate copolymer-based (test grade NTR-354) mixtures and these deviate from the above in that the quantity of reactive groups is very large, owing to which reactive release substances can be added considerably more and possibly diluted subsequently e.g. with HDPE. Release substance Compound adhesion to tape Adhesion of tape to base (N/cm) Steel Glass PVC PET 6% L-9300 0.31 1.6 1.2 1.6 1.1 6% MH-15 0.42 1.2 1.7 1.3 1.6 6% Reoplast 39 0.41 1.5 1.9 1.5 1.3 6% AKD 0.20 1.8 2.0 1.6 1.4 6% AKD + 50:50 dilution with HDPE 0.21 2.6 2.3 2.0 1.8 None 1.6 1.7 1.8 1.1 1.0 Table 2 shows that large quantities of reactive release substances can be easily joined to the hydroxy group without that the adhesion properties of the tape deteriorate, and in addition, these mixtures can be diluted maintaining the release properties. Example 3 The effect of the viscosity of the release subtance on the release force was examined. In all three silicones used, the reactive part was silanol. However, the viscosities deviated considerably from each other. SLM 52-173, which has the highest polymerization degree (viscosity), produces the best release properties. The release properties improve as the viscosity of the release substance increases. Table 3 also shows that the release properties can be improved further by means of the water treatment. Example 4 Finally, a 3-layer film was made with the blown-film technique, the structure of which film was: 15 µm EBA/10 µm HDPE/15 µm release mixture. EBA was Neste's grade NCPE6417 containing 17% of butyl acrylate, and HDPE was NCPE7007. The idea was that the layer thickness of the release mixture would be as small as possible so that the product would be cheap and the migration would be less detrimental. EBA was used as the second outer layer to make the unreacted release substance absorb into it. The HDPE middle layer in turn gives the product stiffness, which improves the release properties. Table 4 shows the results of two 3-layer films. Polymer + release substance Adhesion to tape (N/cm) Adhesion to base (N/cm) Steel Glass PVC PET NCPE6664 + 6% DC-109 0.68 0.74 1.7 1.1 0.62 Primacor 3440 + 6% DC-109 1.0 0.56 1.7 1.4 0.33 The results show that also coextruded films have release properties and after the loosening the tape adheres well especially to glass.

Claims: A polymer composition for forming release films for contact with surfaces containing pressure-sensitive adhesives, characterized in that it is formed of a polyolefin, to which compounds containing functional groups have been joined by copolymerizing, or grafting in another way than by means of said functional groups, said functional groups being acid or acid anhydride, silane, hydroxy, epoxy, amino, ketene or isocyanate groups, and a reactive release substance that is silicone, fatty acid compound or fluoropolymer containing functional groups, which polyolefin and release substance have been compounded in a molten state. A polymer composition according claim 1, characterized in that said polyolefin is a copolymer or graft polymer of ethylene. A polymer composition according to claim 1, characterized in that the functional groups of the release substance are silanol, carbinol, alkoxysilane, acid, amino, epoxy, hydrogen silane, ketene or isocyanate groups. A polymer composition according to claim 1, characterized in that the reaction has been achieved by condensation of silanol groups by means of water and a silanol condensation catalyst. A polymer composition to claim 4, characterized in that the polyolefin is an ethylenealkoxysilane co- or terpolymer and the release substance is a silicone containing silanol groups. A polymer composition according to claim 4, characterized in that the polyolefin is an ethylenehydroxyalkyl-(meth)acrylate co- or terpolymer and the release substance is a release substance containing ketene groups. A release film for contact with surfaces containing pressure-sensitive adhesives, characterized in that it contains a polymer composition according to claims 1-6 in the form of a film. A release film according to claim 7, characterized in that it comprises a mixture of said polymer composition and an unmodified polyolefin. A release film according to claim 7, characterized in that it comprises a polymer composition according to claims 1-6 coextruded on the surface of a film material not containing said polymer composition.

Inventors: BERGSTROM CHRISTER, JAFS AKE, JAKOBSSON STEFAN, MARKKULA MARJA-LEENA, SEPPA INARI, BERGSTROM, CHRISTER, JAFS, AKE, JAKOBSSON, STEFAN, MARKKULA, MARJA-LEENA, SEPPA, INARI
Applicants: KWH PLAST AB OY, NESTE OY, OY KWH PLAST AB
Application year: 1991
Main classification: C09J7
IPC codes: C09J123, C08F293, C09J7, C08F297, C08F8