Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From sulfur-containing reactant
Reexamination Certificate
1999-10-23
2001-10-30
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From sulfur-containing reactant
C528S376000, C528S387000
Reexamination Certificate
active
06310175
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Technical Field
The present invention relates to a new star-shaped block copolymer and a production process therefor, wherein the star-shaped block copolymer exhibits high heat resistance despite having a low viscosity.
B. Background Art
There is a known star-shaped block copolymer that has a structure in which at least three polymer chains are radially extended from a central polyvalent mercaptan segment. This star-shaped block copolymer generally has more excellent heat resistance than simple chain polymers. Herein, the heat resistance means the cohesion or strength of polymers at high temperature, for example, the holding power at high temperature in the field of pressure sensitive adhesives. The increase of the molecular weight is an effective method to enhance the heat resistance. In the case of the simple chain polymers, however, the simple increase of the molecular weight leads to great increase of viscosity and therefore causes problems of workability. In comparison, because of the above structure, the star-shaped block copolymer can have its heat resistance enhanced without so much increase of its viscosity. Thus, the star-shaped block copolymer is favorably used for purposes to which the heat resistance is demanded, such as hot-melt pressure sensitive adhesives.
The star-shaped block copolymer is generally synthesized by a process comprising the multistep radical polymerization which involves the use of a polyvalent mercaptan having a valence of 2 to 6 as an initiator wherein the elementary steps of the multistep radical polymerization differ in polymerizable monomers as used therein (JP-A-07-179538):
Also as to the star-shaped block copolymer, in the case where higher heat resistance is demanded thereto, the above method involving the increase of the molecular weight might enhance the heat resistance in some degree, but the increase of the viscosity would be still so great that problems of workability would occur.
SUMMARY OF THE INVENTION
A. Object of the Invention
An object of the present invention is to provide a new star-shaped block copolymer and a production process therefor, wherein the new star-shaped block copolymer has high heat resistance despite having a low viscosity.
B. Disclosure of the Invention
To solve the above problems, the present inventors diligently studied and variously devised how the star-shaped structure of the star-shaped block copolymer should be. As a result, the inventors led to a thought that if polymers having star-shaped structures got bonded to each other, it might be possible to make them exhibit high heat resistance even without increasing their molecular weights. And the inventors further established a production process for a new star-shaped block copolymer having such a structure and confirmed that the above bonding structure could provide desired results. Thus, the present invention has been completed.
A new star-shaped block copolymer, according to the present invention, comprises a plurality of star-shaped structures in which at least three chain polymer segments are radially extended from a central polyvalent mercaptan segment, wherein the star-shaped structures are bonded to each other at their chain polymer segments.
Although not especially limited, this star-shaped block copolymer preferably has a holding power of not less than 100 minutes, more preferably not less than 1,000 minutes, wherein the holding power is determined by a process including the steps of: preparing a sample by melt-coating a hot-melt pressure sensitive adhesive in a thickness of 25 &mgr;m onto a 38-&mgr;m-thick PET film, wherein the hot-melt pressure sensitive adhesive includes the star-shaped block copolymer; and then attaching the sample onto a stainless steel sheet such that the adhesion area would be 1.27 cm×1.27 cm; and then pressing the sample onto the stainless steel sheet by running a 2-kg-heavy roller thereon back and forth once; and then adjusting the temperature of the product to 60° C. for 30 minutes; and then applying a load of 1 kg to the sample; and then measuring how long the sample has not fallen off from the stainless steel sheet since the application of the load.
A production process for a new star-shaped block copolymer, according to the present invention, comprises the multistep radical polymerization in the presence of a polyvalent mercaptan wherein the elementary steps of the multistep radical polymerization differ in polymerizable monomers as used therein, with the production process being characterized in that at least one of the elementary steps of the multistep radical polymerization involves the joint use of a polyfunctional monomer with the polymerizable monomer.
In this process, although not especially limited, it is preferable that the weight ratio of the polyfunctional monomer to the polyvalent mercaptan (weight of polyfunctional monomer/weight of polyvalent mercaptan) is less than 2, or that the weight ratio of the polyfunctional monomer to the total of the polymerizable monomers (weight of polyfunctional monomer/total weight of polymerizable monomers) is less than 0.05, or that the joint use of the polyfunctional monomer with the polymerizable monomer is involved only in the first radical polymerization step among the elementary steps of the multistep radical polymerization, because these conditions easily prevent the gelation from occurring when producing the new star-shaped block copolymer.
Furthermore, in the above process according to the present invention, although not especially limited, it is preferable that the second radical polymerization step among the elementary steps of the multistep radical polymerization includes the steps of: all at once mixing a polymer solution, resultant from the first radical polymerization step, with a polymerizable monomer which is to be used in the second radical polymerization step; and then polymerizing the resultant mixture in the presence of the polyfunctional monomer, or that the second radical polymerization step among the elementary steps of the multistep radical polymerization includes the step of gradually adding a polymer solution, resultant from the first radical polymerization step, and a polymerizable monomer, which is to be used in the second radical polymerization step, to a vessel to mix them, while polymerizing the resultant mixture in the presence of the polyfunctional monomer. Such a process can easily and surely give the new star-shaped block copolymer that exhibits excellent holding power as specifically defined above.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.
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Kobayashi Nobuhiro
Yoshida Masatoshi
Asinovsky Olga
Nippon Shokubai Co. , Ltd.
Seidleck James J.
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