Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Process of treating scrap or waste product containing solid...
Reexamination Certificate
2001-01-24
2002-06-25
Seidlack, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Process of treating scrap or waste product containing solid...
C524S575500, C528S490000
Reexamination Certificate
active
06410606
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing a natural rubber for a pressure-sensitive adhesive, and a natural rubber-based pressure-sensitive adhesive composition comprising the natural rubber obtained by the process.
BACKGROUND OF THE INVENTION
Natural rubber-based pressure-sensitive adhesives are generally prepared by dissolving a natural rubber in an organic solvent such as toluene, hexane or heptane, and then dissolving therein a compounding additive such as a tackifier resin or a softening agent. However, an unmasticated natural rubber generally has very high molecular weight, and for this reason, it is difficult to directly dissolve the unmasticated natural rubber in an organic solvent. Further, if a natural rubber has too high molecular weight, the natural rubber lacks flexibility when used for a pressure-sensitive adhesive, and as a result, there is the possibility that appropriate pressure-sensitive properties may not be obtained. Therefore, in order to avoid those problems, it is necessary to appropriately decrease a molecular weight of a natural rubber and use such for blending.
A method for decreasing a molecular weight of a natural rubber is generally to previously masticate mechanically with a pressure kneader, a Banbury mixer, an open roll or the like prior to dissolving the same in an organic solvent.
However, this mechanical mastication step to adjust a molecular weight of a natural rubber consumes a huge energy. Further, because a natural rubber has a self-fusion property, the mastication step itself undergoes very large working load. In addition, load is very large particularly in handling or transportation of a rubber after mastication, other than the mastication step itself.
For example, a natural rubber just after mechanical mastication is soft, so that a self-fusion by contact tends to occur. If natural rubber is fused to form a large bulk, this results in great problem on works in the subsequent step. In order to avoid this problem, countermeasures are generally taken to form a natural rubber into predetermined shape and size and spray a powder for self-fusion prevention thereon, or to interpose a release sheet therebetween. However, there are many problems in spraying such a powder or use such a release sheet. For example, where a powder is used, working environment deteriorates by dust, and also a product is contaminated by such a powder in the form of an impurity. Further, where a release sheet is used, it is troublesome to lay many sheets, and also a sheet after use becomes an industrial waste. Furthermore, an unmasticated natural rubber adheres relatively strong to a release sheet, much labor may be required to peel the sheet in the subsequent step.
Therefore, the mastication step involves very disadvantageous elements in production cost, working standpoint and environmental standpoint, and improvement has been demenaded in the mastication step.
Further, as an environmental countermeasure, where low viscosity and high solid content concentration are intended for the purpose of decreasing the amount of an organic solvent used in blending a natural rubber with a pressure-sensitive adhesive, it needs to greatly decrease a molecular weight of a natural rubber. The conventional method involves the disadvantages that the necessity causes to conduct excess mechanical mastication, so that load in the mechanical mastication step further increases than the present.
On the other hand, a method that the degree of mastication is minimized or mastication time is shortened is considered as a method for reducing load of mechanical mastication step as possible. However, those methods have the limitation only that a molecular weight of a natural rubber is decreased to a weigh average molecular weight Mw of about 700,000 to 1,000,000, and a natural rubber having further decreased molecular weight cannot be obtained. Where a natural rubber has a molecular weight of this level, such a natural rubber satisfies properties as a pressure-sensitive adhesive tape. However, where the natural rubber is dissolved in an organic solvent, the resulting solution has considerably high viscosity, and this makes it difficult to uniformly apply a pressure-sensitive adhesive to a substrate in a small thickness in producing a pressure-sensitive adhesive tape. In producing a pressure-sensitive adhesive tape, it is preferable to decrease a molecular weight of a natural rubber to a weight average molecular weight of 700,000 or less in order to suppress a solution viscosity of a pressure-sensitive adhesive low, considering the amount of an organic solvent used.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a process that can efficiently produce a natural rubber for a pressure-sensitive adhesive with a simple operation without passing through complicated steps.
Another object of the present invention is to provide a process that can simply and easily produce a natural rubber for a pressure-sensitive adhesive, having a desired appropriate molecular weight.
Still another object of the present invention is to provide a natural rubber-based pressure-sensitive adhesive composition that can decrease its molecular weight even though having high solid content concentration.
As a result of extensive investigations to achieve the above objects, it has been found that if a specific treatment is applied to an unmasticated natural rubber, the molecular weight of the natural rubber can easily be decreased to the desired molecular weight without passing through a mechanical mastication step, and as a result, a natural rubber solution having low viscosity even though at high solid content concentration can easily be obtained, making it possible to greatly decrease the amount of an organic solvent used. The present invention has be completed based on this finding.
The present invention provides a process for producing a natural rubber for a pressure-sensitive adhesive, which comprises decreasing a molecular weight of an unmasticated natural rubber by the action of a peptizing agent while dissolving the unmasticated natural rubber in an organic solvent.
The present invention also provides a natural rubber-based pressure-sensitive adhesive composition comprising a natural rubber having a weigh average molecular weight Mw of 700,000 or less obtained by the process.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
According to the present invention, the natural rubber for a pressure-sensitive adhesive is obtained by decreasing a molecular weight of an unmasticated natural rubber by the action of a peptizing agent while dissolving the unmasticated natural rubber as is in an organic solvent without controlling the molecular weight in a mechanical mastication step.
Examples of the natural rubber that can be used include standard malaysian rubber (SMR), standard vietnamese rubber (SVR), ribbed smoked sheet (RSS) Nos. 1 to 6 and pale crepe Nos. 1 to 3. However, the natural rubber that can be used is not limited to those so long as it is a general high molecular weight solid type natural rubber.
Examples of the solvent used include aromatic hydrocarbons such as toluene, and aliphatic hydrocarbons such as heptane or hexane. However, the solvent is not limited to those so long as it is an organic solvent that can dissolve a natural rubber.
The peptizing agent used herein means a reagent that can chemically cut a natural rubber molecule in an organic solvent. Preferable examples of the peptizing agent include disulfides such as o,o-dibenzamide diphenyldisulfide, and mercaptans such as 2-benzamide thiophenol zinc salt, 2-thionaphthol, thioxylenol or pentachlorothiophenol. Those compounds having a metal catalyst added thereto may also be used. Further examples of the peptizing agent that can be used include thiazoles such as 2-mercaptobenzothiazole, diacyl peroxides such as benzoyl peroxide, dialkyl peroxides such as dicumyl peroxide, and other organic peroxides.
Compounds containing reactive functional gr
Banba Tomohide
Hirose Isao
Minamizaki Yoshihiro
Umehara Toshiyuki
Asinovsky Olga
Nitto Denko Corporation
Seidlack James J.
Sughrue & Mion, PLLC
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