1. Field of the Invention
The present invention relates to a polymer composition which contains a mixture of uncoupled polymers and radial coupled polymers, wherein the rubbery mid block is an isoprene/butadiene block. The invention also relates to adhesive compositions which contain the particular polymer composition.
2. Background of the Art
Adhesive compositions based on styrenic block copolymers as thermoplastic elastomeric components are well known in the art. These compositions are used for instance as PSA (pressure sensitive adhesive) for industrial tapes, packaging tapes and labels, and in multipurpose hot-melt adhesive compositions which may be used to bond or construct articles in the manufacture of disposable soft goods, such as diapers, feminine care articles, surgical drapes and the like.
Styrene-isoprene-styrene block copolymers (S-I-S) and styrene-butadiene-styrene block copolymers (S-B-S) are widely used in these adhesive compositions. Both classes of block copolymers give the adhesive compositions specific properties related to the respective inherent characteristics of these block copolymers. For example, the softness of S-I-S makes this polymer the material of choice for pressure sensitive applications in tapes and labels. Alternatively, the elevated cohesion of S-B-S makes this material attractive for construction adhesives for disposable soft goods.
When compounded into hot melt adhesives S-I-S polymers degrade by a chain scission mechanism; molecular weight is reduced and the cohesive strength of the adhesive is lowered. S-B-S polymers on the other hand tend to degrade by further chemical cross-linking increasing the cohesive strength of the adhesive, but also increasing the elastic modulus, forming a too hard and non-tacky adhesive. The thermal decomposition of both S-I-S and S-B-S based adhesives can ruin the utility of the adhesive product. It would be an advantage if polymers with less tendency to either fall apart (scission) or cross-link would be developed.
In WO 02/057386 A2, an adhesive composition is described comprising (i) one or more styrenic block copolymers, (ii) a tackifier resin, and (iii) one or more plasticizers, wherein the styrenic block copolymers is of the general structureA-C-A  (1),or(A-C)n—X  (2),wherein each A independently is a polymer block of an aromatic vinyl compound, and C is a mixed polymer block (B/I) of butadiene (B) and isoprene (I) in a weight ratio B:I in the range of 30:70 to 70:30, and said polymer block C has a glass transition temperature (Tg) of at most −50° C. (determined according to ASTM E-1356-98), n is an integer equal to or greater than 2, and X is the residue of a coupling agent, and wherein the tackifier resin is an aromatic hydrocarbon resin.
While the adhesive compositions of the '386 published patent application are an improvement over prior compositions, there still exists a need for a mixed mid-block co-polymer that has a radial structure yet still features low melt viscosity and good shear properties. We have found that an unexpectedly good balance of viscosity and shear can be realized in radial compositions that contain high amounts of diblock.