Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-09-02
2002-05-07
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
active
06384138
ABSTRACT:
FIELD OF INVENTION
This invention is related to a novel hot melt pressure sensitive adhesive composition for use with oriented polypropylene films. Specifically, the invention relates to a composition comprising a blend of SIS and SBS block copolymers combined with a blend of hydrocarbon resins which result in a low cost adhesive with properties similar to those made with SIS alone.
BACKGROUND OF THE INVENTION
Packaging tape is one of the largest volume applications for pressure sensitive adhesives. While paper tapes and vinyl film tapes have been traditionally used in packaging, there has been increased interest in the use of oriented polypropylene films, particularly biaxially oriented polypropylene films.
Of the different adhesives used with oriented polypropylene films, block copolymer based hot melt adhesives are the most popular. These adhesives are generally based on styrene-isoprene block copolymers because of their superior performance over time. One draw back of such adhesive compositions is that they are relatively expensive when compared to adhesives based on other block copolymers such as styrene-butadiene block copolymers.
Styrene-butadiene block copolymer based adhesive compositions have been tried with oriented polypropylene tapes without success. While the initial adhesive properties are acceptable, the aged properties decline rapidly when compared to SIS based adhesive compositions. Furthermore, the required adhesion to paperboard was not achieved.
It would be useful; therefore, to develop an adhesive composition for use with oriented polypropylene tapes having the properties of a SIS based adhesive at a lower cost. The invention described herein accomplishes this by blending SIS block copolymers with SBS block copolymers. The hydrocarbon tackifier resin used must also consist of a blend of a SIS compatible resin and a SBS compatible resin. The resulting adhesive has the properties of an adhesive based on SIS alone but at a significantly lower cost.
U.S. Pat. No. 5,523,343 discloses a pressure sensitive hot melt adhesive composition comprising a blend of a radial SBS copolymer and a SIS block copolymer. The composition also includes a tackifier resin and plasticizer oils. The patent is silent concerning the use of a blended tackifier resin
SUMMARY OF THE INVENTION
The adhesive composition of the present invention comprises an elastomeric component and a tackifying resin component. The elastomeric component in turn comprises a blend of Styrene-Isoprene (SIS) block copolymers and Styrene-Butadiene (SBS) block copolymers. The SIS copolymer comprises from 20 to 80 wt % of the elastomeric component while the SBS copolymer comprises from 80 to 20 wt % of the elastomeric component. The preferred ranges of the SIS copolymer are from 30 to 80 wt %, more preferable 50 to 80 wt %. The preferred ranges of the SBS copolymer are from 70 to 20 wt %, more preferable 50 to 20 wt %.
The tackifier component comprises a blend of at least two hydrocarbon tackifier resins, the first comprising a resin compatible with SIS block copolymers and the second comprises a resin compatible with SBS copolymers. In the preferred embodiment, the relative ratios of the two resins should be similar to that for the block copolymers in the elastomeric component.
In the adhesive composition, the elastomeric component comprises from 40 to 60 wt %, with 45 to 55 wt % being preferred and 45 to 50 wt. % most preferred. The tackifying resin component comprises from 40 to 60 wt %, with 45 to 55 wt % preferred and 50 to 55 wt % most preferred. In the practice of the invention, the total amount of tackifier component in the final adhesive composition may exceed that of the elastomeric component. Thus, in practice, the tackifier component may exceed 100 wt. % based on the total elastomer component.
Typical performance characteristics achieved by the adhesive compositions of the present invention when applied to BOPP films at the coating weight of 20 grams/cm
2
include (i) shear on cardboard at 40° C., minimum 10 hours average and (ii) Ball tack, maximum 4 cm.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to a novel hot melt pressure sensitive adhesive useful in the manufacture of packaging tapes manufactured with oriented polypropylene films. Specifically, the adhesive comprises an elastomeric component, which in turn comprises a blend of SIS and SBS block copolymers, and a tackifier component, which comprises a blend of a SIS compatible hydrocarbon resin and a SBS compatible hydrocarbon resin. The resulting adhesive exhibits the same attributes as adhesives produced from SIS alone at reduced manufacturing costs.
The Elastomeric Component
The elastomeric component comprises a blend of SIS and SBS block copolymers. The SIS block copolymer ranges from 20 to 80 wt %, preferably 30 to 80 wt % and more preferably 50 to 80 wt % of the Elastomeric Component and the SBS block copolymer comprises from 80 to 20 wt %, preferably 70 to 20 wt % and more preferably 50 to 20 wt % of the Elastomeric component.
The SIS block copolymer used in the Elastomeric Component of the adhesive is a thermoplastic elastomer having the structure (S-I)
n−1
S wherein S is substantially a polystyrene block, I is substantially a polyisoprene block and n is an integer of from 2 to 10 wherein the content of the polystyrene in the thermoplastic elastomer ranges from 10 to about 30 wt % and wherein the number average molecular weight (Mn) of the thermoplastic elastomer ranges from about 50,000 to about 500,000. The preferred SIS block copolymer used in the invention is a triblock copolymer of the formula above wherein n=2, i.e., a linear polymer of the formula S-I-S wherein S is substantially a polystyrene block and I is substantially a polyisoprene block. These block copolymers may be prepared by well known anionic solution polymerization techniques using lithium-type initiators such as disclosed in U.S. Pat. Nos. 3,251,905 and 3,239,478, the complete disclosure of which is hereby incorporated by reference for purposes of United States practice.
Preferred block copolymers have a number average molecular weight (determined by GPC) in the range of from about 50,000 to 500,000, more preferably from about 90,000 to about 250,000, even more preferably 90,000 to 175,000 and most preferably 90,000 to 135,000. These block copolymers contain from about 10 to 30 wt % polymerized styrene, more preferably from about 15 to 25 wt % polymerized styrene and most preferably from about 16 to 20 wt % polymerized styrene.
The SIS block copolymer may also comprise a blend of two different SIS copolymers, one having a polystyrene block content of 10 to 20 wt % and the other having a different styrene content of from 15 to 35 wt %, blended to a ratio in the range of from 10:1 to 1:10 parts by weight. The use of two different SIS block copolymers offers the advantage of improved cohesive strength and more precisely tailoring polystyrene content in the preferred target range of from 15 to 25 wt %, or 16 to 20 wt % respectively.
Although pure triblock SIS copolymer is preferred (one having less than 0.1 wt % of diblock polymer) the SIS copolymer may also contain from about 0.1 to about 65 wt %, preferably less than or equal to 25 wt %, most preferably less than 20 wt % of the triblock copolymer of diblock copolymer having the structure S-I and containing from about 10 to 30 wt % polystyrene block. This material may be present as an impurity in the manufacture of the triblock copolymer or may be separately blended with the triblock as a further technique for achieving target polystyrene content or modifying the cohesive properties of the composition. Preferred number average molecular weights of the diblock SI copolymers range from about 100,000 to about 250,000.
While linear triblock SIS copolymers are preferred in the practice of this invention, radial SIS block copolymers may also be used. The radial SIS copolymers should have the same styrene levels as the linear copolymers discussed above, e.g., from about 10 to 30 wt % polymerized sty
Jacob Lutz E.
Swiggers Eddy
Alexander David J.
Boykin Terressa M.
ExxonMobil Chemical Patents Inc.
Runyan, Jr. Charles Edwin
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