Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-12-22
2003-02-11
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S925000, C156S598000
Reexamination Certificate
active
06518355
ABSTRACT:
TECHNICAL FIELD
The present invention relates to rubber-based pressure-sensitive adhesives and a pressure-sensitive adhesive sheet using the same. More particularly, the invention relates to natural-rubber-based pressure-sensitive adhesives and a pressure-sensitive adhesive sheet using the same.
BACKGROUND ART
In most processes for producing a natural-rubber-based pressure-sensitive adhesive, a mastication treatment for reducing the molecular weight of natural rubber has hitherto been conducted in order to improve the plasticity of the natural rubber and impart satisfactory pressure-sensitive adhesive properties to the pressure-sensitive adhesive. A natural rubber having low plasticity gives a pressure-sensitive adhesive composition solution which has a high solution viscosity and reduced applicability to substrates for pressure-sensitive adhesive tapes and which therefore is undesirable from the standpoints of environmental sanitation and safety such as the prevention of disasters because a large amount of an organic solvent is necessary for lowering the rubber concentration. Furthermore, a natural rubber having too high a molecular weight has reduced compatibility with tackifier resins and this inhibits impartation of pressure-sensitive adhesive properties or results in cases where the pressure-sensitive adhesive layer is too elastic to be suitable for use as a pressure-sensitive adhesive tape.
Although the mastication step is an indispensable step in the production of a natural-rubber-based pressure-sensitive adhesive as described above, it has been pointed out from long ago that mastication is accompanied by the wasteful use of time and energy whatever methods are used for mastication. For example, the mechanical mastication in which natural rubber is kneaded by applying mechanical force thereto with a two-roll mill, Banbury mixer, or the like necessitates a large amount of energy. On the other hand, the chemical mastication which is conducted by adding a peroxide or the like to a solution or latex containing natural rubber necessitates much time. In these mastications, a compound called a peptizing agent, represented by a thiuram type, mercaptan type, or disulfide type compound, a zinc compound thereof, or the like, or a phthalocyanine/iron catalyst or the like as a catalyst for mastication is generally added. Such additives not only cause the rubber to emit an odor and discolor but also may foul the adherend when the rubber is used as a pressure-sensitive adhesive tape. Because of this, the rubber has limited uses as pressure-sensitive adhesive tapes. Consequently, if the addition of a peptizing agent or the like becomes unnecessary, the rubber is expected to be used in a wider range of applications.
As described above, to omit or shorten the mastication step in the production of a natural-rubber-based pressure-sensitive adhesive and a pressure-sensitive adhesive sheet using the same has been a major subject which should be investigated for a drastic improvement for establishing a more efficient production process.
Accordingly, an object of the invention is to provide a rubber-based pressure-sensitive adhesive which has excellent pressure-sensitive adhesive properties, does not substantially necessitate a mastication step, and can be easily produced with a small amount of energy in a short time, and to provide a pressure-sensitive adhesive sheet using the same.
DISCLOSURE OF THE INVENTION
The present inventors made intensive investigations in order to accomplish the object. Through these investigations, they have found that fine particles having a particle diameter of 1 &mgr;m or smaller are present in the serum obtained in condensing and purifying a raw natural rubber latex with a continuous centrifugal separator, that the rubber (skim rubber) constituting these fine particles is a linear polymer having a low molecular weight, and that when this rubber is used as the rubber ingredient of a rubber-based pressure-sensitive adhesive, not only the mastication step can be substantially omitted but excellent pressure-sensitive adhesive properties are obtained. The invention has thus been completed.
Namely, the invention provides a rubber-based pressure-sensitive adhesive which contains as the main rubber component a natural rubber comprising, as the main component, fine particles having a particle diameter of 1 &mgr;m or smaller contained in a raw natural rubber latex.
The invention further provides a rubber-based pressure-sensitive adhesive which contains as the main rubber component a linear natural rubber polymer contained in a raw natural rubber latex.
The invention furthermore provides a rubber-based pressure-sensitive adhesive which contains as the main rubber component a natural rubber having a weight-average molecular weight of from 100,000 to 1,000,000 contained in a raw natural rubber latex.
The invention still further provides a rubber-based pressure-sensitive adhesive which contains as the main rubber component a natural rubber having a rupture strength of 0.5 MPa or lower in terms of green strength contained in a raw natural rubber latex.
The invention still further provides a rubber-based pressure-sensitive adhesive which contains as the main rubber component a natural rubber contained in the serum obtained by centrifuging a raw natural rubber latex.
The pressure-sensitive adhesive sheet of the invention has a pressure-sensitive adhesive layer constituted of any of the rubber-based pressure-sensitive adhesives described above on at least one side of a substrate.
BEST MODE FOR CARRYING OUT THE INVENTION
The feature of a rubber-based pressure-sensitive adhesive of the invention resides in that it contains as the main rubber component a natural rubber comprising, as the main component, fine particles having a particle diameter of 1 &mgr;m or smaller contained in a raw natural rubber latex.
The particle diameters of the fine particles are, for example, about from 0.001 to 1 &mgr;m, preferably about from 0.05 to 0.8 &mgr;m. The proportion of the natural rubber constituting the fine particles in the main rubber ingredient of the rubber-based pressure-sensitive adhesive is, for example, 50% by weight or higher, preferably 70% by weight or higher, more preferably 80% by weight or higher, most preferably 90% by weight or higher, based on the weight of the main rubber ingredient. Consequently, a natural rubber latex or the like having a particle diameter exceeding 1 &mgr;m can be incorporated into the main rubber ingredient.
Methods for preparing the fine particles are not particularly limited. However, the most general methods include a technique in which particles having a large particle diameter are removed from a raw natural rubber latex by the sedimentation method to obtain fine particles having a desired particle diameter. The sedimentation method includes, for example, the technique in which a centrifugal separator is used to utilize interaction between particle diameter and centrifugal force.
The natural rubber comprising, as the main component, fine particles having a particle diameter of 1 &mgr;m or smaller has a lower molecular weight than the natural rubber conventionally used as a raw material for rubber-based pressure-sensitive adhesives and, in addition, is characterized by consisting mainly of a linear polymer unlike the natural rubber which has been conventionally used. More specifically, the natural rubber which has been conventionally used has a weight-average molecular weight as determined by GPC of about 2,000,000, whereas the natural rubber constituting the fine particles has a weight-average molecular weight of about from 100,000 to 1,000,000, preferably from 200,000 to 1,000,000, more preferably about from 500,000 to 800,000. It is presumed that these fine particles correspond to a product yielded at the initial stage of the biosynthesis of high-molecular rubber in natural rubber trees and, hence, have a low molecular weight and a linear structure in which the amount of abnormal groups contained in the polymer chain is small. Because this natural
Shibata Kazuhiko
Tanaka Yasuyuki
Egwim Kelechi C.
Nitto Denko Corporation
Wu David W.
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