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  Crosslinkage pressure-sensitive adhesive substances by means of electron beamsRelated Patent Categories: Adhesive Bonding And Miscellaneous Chemical Manufacture, Methods, Surface Bonding And/or Assembly ThereforThe Patent Description & Claims data below is from USPTO Patent Application 20060054264. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is a division of U.S. application Ser. No. 10/220,582, filed Sep. 3, 2002, now allowed, which is a 371 of PCT/EP01/02792, filed Mar. 13, 2001, which claims foreign priority benefit under 35 U.S.C. .sctn. 119 of the German Patent Application No. 100 14 563.9 filed Mar. 23, 2000. [0002] The invention relates to a process for the electron beam crosslinking of pressure-sensitive adhesive tapes while avoiding damage to the release layer on the reverse. [0003] The service properties of adhesive tapes are determined quite essentially by the properties of the pressure sensitive adhesive (PSA). Its adhesive is composed of at least one high molecular mass elastomer plus where appropriate one or more tack-defining resins and also further additions for modifying the properties, improving the stability, etc. For further information on the composition, preparation, and properties of adhesive tapes, refer to the article "Tapes, Adhesive" in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 2000 Electronic Release, Wiley-VCH, Weinheim 2000. [0004] To produce the adhesive tapes, the adhesives are applied in one or more thin films to suitable backing or carrier materials such as films, papers, nonwovens, wovens, etc. For this purpose they must be converted into a fluid state of appropriate viscosity, which can be done by dissolving them in solvents, dispersing them in liquids or, in the case of those known as hotmelts, by melting them at elevated temperatures. These coatings are subsequently solidified by evaporating the solvent or dispersion medium or by cooling the hotmelts to give the pressure-sensitive adhesive. The hotmelts in particular, but also the other pressure-sensitive adhesives, lose their strength again by heating, thereby limiting the scope of use. This can be prevented by crosslinking, thereby expanding the scope for use of the adhesive tapes thus produced. Crosslinking comprehends all reactions which lead to a chemical bond between the polymer chains of the elastomer. This can be achieved by means of chemically initiated or radiation-chemically initiated reactions. The latter may in turn be initiated by UV radiation or by high-energy radiation such as accelerated electrons (EBC). Preference is given to employing radiation crosslinking (UV and EBC technology), since these reactions take place very quickly and do not require thermally labile groups. A further process for preparing pressure-sensitive adhesives starts from reactive liquid monomers and oligomers, which are cured by a radiation to form a cohesive pressure-sensitive adhesive. An overview has been given by Karmann and Zollner in "Radiation Curing of PSA--An Overview", RadTech Europe, Maastricht 1995. Since there are no fundamental differences between the above-described crosslinking and curing, both are referred to below as "crosslinking". [0005] The UV technology is relatively inexpensive in terms of apparatus. Owing to the photoinitiators required and the absorption of light by the adhesives, however, crosslinking cannot be carried out unrestrictedly. For example, transparent acrylic PSA tapes can be crosslinked efficiently up to a maximum of about 100 g/m.sub.2. In the case of adhesives comprising resins or fillers, the limits are much lower. For natural-rubber adhesives, UV crosslinking is even less favorable. The EBC technology is significantly better suited to 35 this purpose. Fortunately, a majority of the polymers used for PSAs such as the polyacrylates, natural rubber and other polydiene rubbers, ethylene-vinyl acetate, polydimethylsiloxanes, are polymers which crosslink predominantly under the influence of electron beams (see, for example, J. E. Wilson, "Radiation Chemistry of Monomers, Polymers, and Plastics" Marcel Dekker, Inc., New York, 1974). Given a sufficiently high electron accelerating voltage, even PSAs with film thicknesses more than several 100 g/m.sup.2 are penetrated almost completely, and crosslinked, irrespective of their composition. Further advantages here are the rapidity of these processes, the minimal requirements imposed on the structure and composition of the components to be crosslinked (UV-active groups need not be present), and the ability to regulate the operation precisely. Nevertheless, this technology too has disadvantages. In order to achieve uniform crosslinking of the adhesive, the adhesive tape must be transirradiated with sufficient uniformity, so that even the backing films are subjected to more or less high radiation doses. In this process, some polymers become degraded, and so become brittle and lose mechanical load-bearing properties. Other polymers undergo discoloration during and after irradiation. Particularly serious damage occurs to release layers, which are frequently present on the reverse of the backing films in order to obtain a desired adjustment of the unwind force after the tape has been wound up: following exposure to radiation, they lose activity, as a result of which the unwind forces of the adhesive tapes rise unacceptably. This applies in particular to silicone layers following storage at elevated temperatures. [0006] This damage can be reduced if the adhesive tape runs 30 with its reverse over a thermal conditioning plate under the electron accelerator or is on a metallic thermal conditioning roller, both of which act as radiation traps. Nevertheless, the effect is inadequate at radiation doses above about 10 kGy. [0007] Patent application DE 198 46 902 A1 describes how, by choosing the accelerating voltage, it is possible to keep the radiation dose absorbed by the release layer on the reverse of a release liner low and to crosslink the adhesive film with a substantially higher radiation dose. In the case of unfavorable thickness ratios between the adhesive film and the backing, however, the possibilities are limited. [0008] It is an object of the invention, therefore, to provide a process for electron beam crosslinking of pressuresensitive adhesives which makes it possible to prevent or at least very substantially reduce the damage for the reverse of the release layers without having to restrict the radiation dose needed for the crosslinking of the adhesive film, and to do so even when the thickness proportions between adhesive layer and backing are unfavorable. [0009] Surprisingly, and unforeseeable for the skilled worker, it has been found that an advantageous procedure of this kind is possible by introducing a suitable contact medium between the reverse of the adhesive tape and the roller, especially a thermal conditioning roller, during the irradiation process. [0010] The invention accordingly provides a process for electron beam crosslinking of a coating of pressuresensitive adhesives on a backing material, characterized in that [0011] the pressure-sensitive adhesive film located on the backing material is crosslinked by electron beams by means of an irradiation means, [0012] the backing material coated with the pressure-sensitive adhesive system to be crosslinked runs over a roller during irradiation, [0013] there is a contact medium between the roller and the backing during irradiation. BRIEF DESCRIPTION OF THE DRAWING [0014] The invention will now be described in greater detail with referene to the drawing, wherein: [0015] FIG. 1 is a depiction of a setup for carrying out a process embodiment according to the present invention. [0016] The contact medium is preferably removed in whole or in part from the backing material following irradiation. [0017] Contact between the backing material and the roller must be ensured over the entire irradiation area of the backing material provided with the pressure-sensitive adhesive system. [0018] Preferably in accordance with the invention, this 5 process is carried out in a unit composed of an electron accelerator, a roller, preferably a thermal conditioning roller, a fluid applicator, and, if desired, a drying unit. [0019] In a very advantageous procedure, the roller is thermally conditioned. In the text below, in all cases where reference is made only to the roller, the thermal conditioning roller is expressly included as one of the preferred embodiments. The thermal conditioning of the roller is situated preferably within a range from -10.degree. C. to 200.degree. C., with very particular preference from 5.degree. C. to 150.degree. C. [0020] The roller used is commonly an earthed metal roller which absorbs the incident electrons and the x-rays which form in this case. It is preferably equipped with an effective thermal conditioning system in order to ensure dissipation of energy, particularly in the form of heat energy. In order to prevent corrosion it is normally covered with a protective coat. This coat is preferably selected so as to be wetted effectively by the contact medium. In general, the surface is conductive. It may also be more favorable, however, to coat it with one or more coats of insulating or semiconducting material. [0021] Rollers which can be used advantageously for the inventive process are steel rollers, especially those which carry a coating for improving the corrosion resistance and/or wetability, these coatings preferably being of appropriate metals (chromium, for example), metal oxides or ceramic. [0022] In the inventive process the contact medium is applied advantageously either to the reverse of the backing or to the described roller; it is also possible, however, for it to be applied contactlessly, by spray application, for example. During electron beam irradiation, the contact medium is located between the roller and the backing material. [0023] The contact medium of the invention that is used is a material which is capable of producing a contact between the backing material and the roller's surface, particularly a material which fills the cavities between backing material and roller surface (for example, unevennesses in the roller's surface, bubbles). [0024] Appropriate for this purpose are fluid materials which may be present within a wide viscosity range. Accordingly, the contact medium may be composed, for example, of a pressure-sensitive adhesive or of another material which flows onto the backing material and so displaces the air between backing and roller. Moreover, soft, "conforming" materials may be used as the contact medium. On the one hand, it is possible preferentially to use pliable materials such as, for example, soft rubber, plasticized PVC, other plasticized polymers, and similar materials. If they are are firmly connected to the roller, they must exhibit sufficient radiation resistance and also, preferably, sufficient thermal and electrical conductivity. [0025] It is particularly advantageous in accordance with the invention, especially when using contact media lacking sufficient radiation resistance, not to leave the contactmedium permanently on the roller but instead to apply it to the roller before the irradiation operation and to remove it from the roller again after the irradiation operation. The latter principle may be realized, for example, in an excellent way in accordance with the invention by introducing it into the process with the backing material: for example, in the form of a film which runs onto the roller with the backing material. In a further advantageous embodiment, the contact medium is in the form of a replaceable covering on the roller. The contact medium may be replaced during the irradiation operation (continuous replacement) or between the individual irradiation operations (discontinuous replacement). The continual replacements prevent it being so severely impaired by the ongoing irradiation that it loses its function. [0026] In particular in the case of sufficient radiation 10 resistance or sufficiently low radiation load, however, it may likewise be advantageous, for corresponding variants of the inventive process, for the contact medium to remain permanently on the roller. Continue reading... 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