Stock material or miscellaneous articles – Structurally defined web or sheet – Continuous and nonuniform or irregular surface on layer or...
Reexamination Certificate
1999-04-15
2001-04-17
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Continuous and nonuniform or irregular surface on layer or...
C428S141000, C428S142000, C428S143000, C428S343000, C428S353000, C428S354000, C428S3550RA, C428S340000, C428S341000, C156S230000, C156S239000, C156S283000, C156S276000
Reexamination Certificate
active
06217981
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an adhesive sheet comprising a flexible substrate and an adhesive layer formed on at least one of the main surfaces of the substrate and containing elastic microspheres. More particularly, the invention relates to an adhesive sheet which has a high peeling adhesion strength in a dry state, a high water-resistant adhesion strength and good bubble escapability.
BACKGROUND OF THE INVENTION
The adhesive layer of an adhesive sheet is generally a single layer if it contains elastic microspheres. Such an adhesive sheet is produced by applying a coating liquid containing elastic microspheres and a tacky polymer directly onto the surface of a substrate and drying the applied coating liquid to form a single layer adhesive film as disclosed in JP-A-6-287525. When such an adhesive sheet is press bonded onto an adherent, raised portions comprising the elastic microspheres and the adhesive layer are in spot contact with the adherent, and passages which can communicate with the atmosphere are formed between the adhesive layer and the adherent. Bubbles which have been trapped between the adhesive layer and the adherent can be easily removed when they are forced to escape by the application of a pressure from the back face of the support towards the adherent in the presence of those passages.
However, the adhesive layer tends to maintain the above spot contact state after the adhesion of the adhesive sheet to the adherent (finish pressing), and therefore the peeling adhesion strength is low. For example, a DINOC™ brand film (commercially available from Sumitomo 3M Co. Ltd. of Tokyo, Japan) which is an example of a decorative sheet having an adhesive layer containing no elastic microspheres has a peel adhesion strength (90 degree peeling strength) of at least about 2 kgf/25 mm (when a bonderized steel plate is used as the adherent), while an adhesive sheet having a single layer adhesive film containing elastic microspheres has the peeling adhesion strength of at most 1.0 to 1.5 kgf/25 mm against the same steel plate.
That is, conventional adhesive sheets containing elastic microspheres are suitable as repositionable sheets, but they are unsatisfactory as decorative sheets which require permanent bonding.
Further, water or vapor (or moisture) penetrates into the interface between the adhesive layer and adherent through the passages and thus an adhesion strength decreases, that is, the water-resistant adhesion strength decreases, if the passages remain after the completion of adhering when the adhesive sheet is used for decorating bath rooms, changing rooms, and so on.
One of the reasons for the continuation of spot contact, that is, the passages communicating with the atmosphere, may be that the tacky polymer layer covering the elastic microspheres is in a single layer state having a small thickness in comparison to the diameter of the microspheres, has small flow during the press adhering of the sheet to the adherent and small flow over time (namely, cold flow), and therefore the contact areas of the adhesive layer do not increase readily.
JP-A-6-322325 discloses an example of adhesive sheets having a dual adhesive layer. The used adhesive layer is prepared by laminating a primer layer containing hollow microparticles and a top adhesive layer on the primer layer. The primer layer functions to compensate surface irregularity of the adherent such as a corrugated board, etc. after the adhesive sheet is adhered to the adherent, and thus good adhesion of the sheet to the irregular surface is achieved. However, the contact between the adhesive sheet and adherent is plane contact since the top surface of the adhesive layer has no projections, and therefore the bubble escapability is not good.
JP-U-5-72946 discloses an adhesive sheet or tape having a dual adhesive layer containing elastic microspheres. The disclosed adhesive tape or sheet comprises a substrate, an adhesive layer containing elastic microspheres, and a layer of viscoelastic materials having a thickness of 20 pm inserted between the substrate and adhesive layer. This structure improves the adhesion of the tape or sheet to adherents having irregular surfaces such as a corrugated board, etc. and achieves repositioning properties of the tape or sheet while preventing spontaneous peeling. However, large cold flow cannot be expected with this structure, and the contact areas of adhesive layer do not readily increase.
SUMMARY OF THE INVENTION
The art needs an adhesive sheet which has improved adhesion properties such as high peeling adhesion strength in a dry state, high water-resistant adhesion strength, and so on while maintaining good bubble escapability which is an advantage of adhesive layers containing elastic microspheres.
To solve this problem in the art, the present invention provides an adhesive sheet comprising a flexible substrate, and an adhesive layer formed on at least one of the main surfaces of said flexible substrate and containing elastic microspheres, wherein said adhesive layer comprises
a) an elastic microsphere layer which is in contact with the main surface of the flexible substrate, comprising said elastic microspheres and a first tacky polymer and having projections containing said elastic microspheres on a surface which is not in contact with said substrate, and
b) a layer of a second tacky polymer which covers said projections containing the elastic microspheres so that raised portions are formed on an adhesion surface.
The presence of raised portions having the specific size on the adhesion surface of the adhesive layer improves the bubble escapability during the adhering step of the adhesive sheet onto the adherent.
The invention also pertains to a method for producing an adhesive sheet, comprising the steps of (a) applying a coating liquid comprising a second tacky polymer on a releasing surface of a release liner, and drying the applied coating liquid to form a layer of tacky polymer, (b) applying a coating liquid comprising elastic microspheres and a tacky polymer on the layer of tacky polymer and drying the applied coating liquid to form an elastic microsphere layer having temporarily raised portions containing the microspheres on its surface, and (c) having a main surface of a flexible substrate in close contact with the surface of said elastic microsphere layer under a specific pressure so that the temporary projections are reversed to form projections having specific sizes on an adhesion surface of an adhesive layer which faces the release liner.
The adhesive layer of the adhesive sheet according to the present invention comprises an elastic microsphere layer and a layer of the second tacky polymer. The elastic microsphere layer is formed so that it has the projections containing the elastic microspheres on the surface which is not in contact with the substrate and it functions as a primer layer for the formation of the raised portions having a specific size. The thickness (or amount) of the tacky polymer which covers the microspheres is relatively small in the elastic microsphere layer which has been formed to provide the effective projections. Therefore, it is difficult for the elastic microsphere layer to improve the adhesion strength, in particular, the peeling adhesion strength (at a peeling angle of 90 degrees) in a dry state and to prevent a large decrease of the peeling adhesion strength (at a peeling angle of 90 degrees) caused by the penetration of water from the atmosphere. Thus, the layer of the second tacky polymer is coated over the projection of the elastic microsphere layer to form the raised portions having a specific size on the adhesion surface. The layer of the second tacky polymer improves the bubble escapability and achieves a high adhesion strength after press adhering the sheet to the adherent.
The invention has the following features and advantages:
The raised portions on the adhesive layer should form passages which communicate with the atmosphere and through which bubbles trapped between the adherent and adhesive layer escape
Abe Hidetoshi
Kanno Mamoru
3M Innovative Properties Company
Bjorkman Dale A.
Watkins III William P.
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