Stock material or miscellaneous articles – Layer or component removable to expose adhesive – Polymer derived only from ethylenically unsaturated monomer
Reexamination Certificate
2001-09-26
2004-02-10
Robertson, Jeffrey B. (Department: 1712)
Stock material or miscellaneous articles
Layer or component removable to expose adhesive
Polymer derived only from ethylenically unsaturated monomer
C428S041800, C428S447000, C427S508000, C427S515000, C427S516000, C427S519000
Reexamination Certificate
active
06689442
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a releasing sheet and a process for producing the same. More particularly, it is concerned with a releasing sheet in which a cured layer of a silicone has favorable adhesiveness to a substrate and non-migration property; which has excellent releasability and high flatness never can be realized by any of conventional sheets; and which is used in labels and pressure sensitive adhesive tapes and is useful as a casting film for producing a ceramic green sheet. Furthermore, the present invention pertains to a process for efficiently producing the above-mentioned releasing sheet.
BACKGROUND ART
A releasing sheet comprises a substrate such as paper, a plastics film and a polyethylene laminated paper sheet, and a release treated layer which is placed thereon and is formed by coating at least one side of the substrate with a silicone as a releasing agent, followed by curing the resin. Such a releasing sheet is used as a tacky label, a carrier sheet at the time of producing a resin sheet, a protective sheet therefor, a casting film for forming a film of a ceramic green sheet, etc.
The performances required of the above-mentioned releasing sheet which is subjected to such purpose of use include for instance, moderate releasability, non-migration property of silicone, stability with the lapse of time, high flatness, moderate surface roughness, coatability at the time of film forming from a resin, pressure sensitive adhesive, ceramic slurry, etc., and the like performances.
Examples of release treatment agents include a releasing agent composed of a silicone of heat curable addition reaction type, heat curable condensation type, room temperature curable type, ultraviolet ray curable type, electron beam curable type, and heat-and-ultraviolet ray curable type and the like, of which is mainly used a releasing agent composed of a heat curable addition reaction type silicone. However, in order to assure a stable cured film, the releasing agent composed of a heat curable addition reaction type silicone is obliged to be cured at a high temperature of usually 140° C. or higher. As a result, such problem is brought about that thermal shrinkage or wrinkle is more prone to be caused on a substrate, especially a substrate which is inferior in heat resistance, since heat is applied to the substrate in the release treatment step, thereby making it impossible to form a releasing sheet having high flatness.
Under such circumstances, in the case of conducting release treatment processing using a heat curable addition reaction type silicone for a film substrate or a paper sheet laminated with polyethylene on double sides each being inferior in heat resistance, an attempt is made to retard processing rate under a heating condition of a low temperature (110 to 130° C.) in order to suppress thermal shrinkage or wrinkle and polyethylene foaming to the utmost. However, such processing treatment brings about inevitably poor productivity, likelihood of insufficient curing and besides, the troublesome problems in non-migration property of silicone, release stability and the like.
In addition, a releasing agent composed of a silicone of any of heat curable condensation type and room temperature curable type is problematic in low rate of curing reaction and besides, inferior non-migration property of silicone and the like.
As compared with the foregoing, a releasing agent composed of a silicone of any of ultraviolet ray curable type and heat-and-ultraviolet ray curable type is useful as a releasing agent capable of solving the above-mentioned problems, and enables curing at a low temperature as well as a high processing rate. Nevertheless in the case of producing a releasing sheet by using any of the aforesaid releasing agent, further enhancement of productivity is desired. In order to further enhance the productivity in a releasing sheet using a conventional releasing agent composed of a silicone of heat-and-ultraviolet ray curable type, the release treatment processing must be carried out at a further high rate. For the sake of such a high rate, it is necessary to enhance the curability of the above-mentioned releasing agent composed of a silicone, whereby importance should be attached to the selection of a photosensitizer to be used therein.
DISCLOSURE OF THE INVENTION
In such circumstances, an object of the present invention is to provide a releasing sheet in which a cured layer of a silicone has favorable adhesiveness to a substrate and non-migration property; which has excellent releasability and high flatness never can be realized by any of conventional sheets; and which is extremely enhanced in productivity.
Other objects of the present invention will be obvious from the text of this specification hereinafter disclosed.
As a result of intensive extensive research and investigation accumulated by the present inventors in order to develop a a releasing sheet which has the aforestated excellent functions and is enhanced in productivity, the following findings and information were obtained.
A conventional releasing agent composed of a silicone of heat-and-ultraviolet ray curable type is produced by adding a photosensitizer to a releasing agent composed of a heat curable addition reaction type silicone resin, and curing the agent by heating treatment and ultraviolet ray irradiation. Therein, use is made of, as a photosensitizer, acetophenone, benzophenone, trimethylsilylbenzophenone, propiophenone, 3-methylacetophenone, 4-methylacetophenone, benzoin ethyl ether, a compound in which benzoin is bonded by ether linkage to both ends of dimethylpoly-siloxane, 4-allylacetophenone, 3-pentyl-acetophenone, 4-methoxy-benzophenone, &agr;-aminoketone, triphenylamine, azobis (isobutyro-nitrile) and the like. It is known that by adding any of the above-exemplified photosensitizer, the curability is enhanced as compared with a conventional releasing agent composed of a heat curable addition reaction type silicone. However, further research and investigation were accumulated on photosensitizers by the present inventors, paying attention to the necessity of using a photosensitizer which is more useful for curing the silicone than the above-exemplified photosensitizer in order to carry out further low temperature and higher rate curing. As a result, it has been found that &agr;-hydroxy ketones and &agr;-diketone dialkyl acetals are effective in particular. The present invention has been accomplished by the above-mentioned findings and information.
That is to say, the present invention provides:
(1) a releasing sheet which comprises a sheet like substrate and a cured layer placed thereon of an addition reaction type silicone composition containing a photosensitizer, characterized in that said cured layer is formed by heat treating a layer of an addition reaction type silicone composition containing at least one photosensitizer selected from the group consisting of &agr;-hydroxy ketones and &agr;-diketone dialkyl acetals, followed by a treatment with ultraviolet ray irradiation.
(2) the releasing sheet as set forth in the preceding item (1), wherein the photosensitizer is at least one species selected from the group of &agr;-hydroxy ketones consisting of 2-hydroxy-1-(4-isopropylphenyl)-2-methylpropane-1-one, 2-hydroxy-2-methyl-1-phenylpropane-1-one, 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methylpropane-1-one and 1-hydroxycyclohexyphenyl ketone, and &agr;-diketone dialkyl acetals consisting of benzyldimethyl acetal and benzyldiethyl acetal.
(3) the releasing sheet as set forth in the preceding item (1) or (2), wherein the addition reaction type silicone composition comprises 0.01 to 10 parts by weight of the photosensitizer based on 100 parts by weight of the total amount of an addition reaction type silicone and a crosslinking agent.
(4) the releasing sheet as set forth in the preceding item (1) or (2), wherein the addition reaction type silicone composition comprises polydimethylsiloxane having a vinyl group as a functional group.
(5) a process for the production of a rel
Nakamura Toru
Shibano Tomishi
Lintec Corporation
Robertson Jeffrey B.
LandOfFree
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