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
1997-06-12
2001-01-09
Szekely, Peter A. (Department: 1714)
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...
C524S271000, C524S274000, C524S474000, C524S484000, C524S485000, C524S486000, C524S499000, C525S098000, C525S099000
Reexamination Certificate
active
06172156
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a hot melt pressure sensitive adhesive comprising a block copolymer having a high diblock content, a low melt point styrene domain associating resin and a plasticizer. Preferably, the block copolymer is a fully saturated linear block copolymer having styrene endblocks and ethylene/butylene or ethylene/propylene midblocks. Specifically, this invention relates to a hot melt adhesive which fails cohesively during use.
BACKGROUND OF THE INVENTION
Hot melt pressure sensitive compositions are used widely in the adhesive industry due to their ability to adhere to a wide variety of substrates. Block copolymers are a preferred base polymer because of their elastomeric properties, their ability to be tackified, their compatibility with a wide range of tackifiers and plasticizers, and their high cohesive strength. Block copolymers are also popular because of the wide variety of grades available with varying molecular weights, styrene contents, coupling efficiencies and types of midblocks. The block copolymers most widely used have styrene endblocks, but the amount of styrene being present in varying amounts. Representative examples of midblocks most often encountered include isoprene, butadiene, ethylene/butylene and ethylene/propylene.
A primary property of a hot melt pressure sensitive composition is that it remains tacky at room temperature and can be adhered to substrates after it has completely cooled from its molten state. Block copolymers by themselves are inherently nontacky and it is necessary to use tackifying resins and plasticizers to develop the tack necessary for pressure sensitive adhesion.
Pressure sensitive adhesives are utilized for various applications including, but not limited to, labels, tapes, and packaging.
Block copolymers generally exhibit high cohesive strength resulting in adhesives that have high cohesive strength. While high cohesive strength is desirable for most applications such as for tapes and labels, this may not always be the requirement. Cohesive failure is a desirable property for such applications as seal/reseal in the packaging industry. This type of package is peelable and reclosable indicating that it's easy for a consumer to open initially, and it may be resealed later either through use of mechanical means or through an adhesive for instance. This type of package may be utilized for perishable goods.
The films utilized for manufacturing seal/reseal type packages are generally polymeric in nature and may comprise polyethylene, polypropylene, polyvinyl chloride, polyester, and acrylonitrile-methyl acrylate copolymer polymerized and/or mixed with butadiene as a terpolymer. The problem encountered with block copolymer based products for seal/reseal applications of this nature is that the adhesive generally remains on one side of the container or the other due to the fact that the internal strength of the adhesive is as high or slightly higher than the bond strength or peel adhesion to the substrate. This results in stringing of the adhesive as it has a tendency to remain on the package, but also to stay associated with itself. The adhesive therefore does not remain adhered to the substrate as intended, but strings in an attempt to remain with both resulting in what is known in the art as “adhesive confusion.”
There is prior art that describes the use of a higher cohesive strength adhesive that remains associated preferentially with one side of the package and does not fail cohesively. This is an attempt to alleviate the problem of stringing of the adhesive but it does not, however, solve the problem of “adhesive confusion,” as the internal strength of the adhesive is only slightly higher than the peel adhesion to the substrate.
U.S. Pat. No. 4,096,203 to St. Clair teaches a low cohesive strength hot melt adhesive employing either a styrene-butadiene-styrene(SBS) block copolymer or a styrene-isoprene-styrene(SIS) block copolymer. SBS and SIS block copolymer based adhesives are known to exhibit poorer thermal stability, and also exhibit an odor that is not as desirable for the packaging of perishables such as food.
The current inventors have found a solution to the aforementioned problems by employing hot melt pressure sensitive adhesives based on block copolymers having a high diblock content which fails cohesively during use.
SUMMARY OF THE INVENTION
The present invention relates to a hot melt pressure sensitive adhesive comprising a) from about 15% to about 40% by weight of a block copolymer having a coupling efficiency of less than about 75%; b) from about 20% to about 50% by weight of a tackifying resin selected from the group consisting of those tackifying resins having a softening point of less than about 100° C.; c) from about 30% to about 50% by weight of a plasticizing oil; and d) up to about 15% of a compatible polymer wherein the total polymer content does not exceed about 40% by weight of the adhesive. This hot melt pressure sensitive adhesive fails cohesively during use.
Preferably, the block copolymers employed are fully saturated linear block copolymers having styrene endblocks and ethylene/butylene or ethylene/propylene midblocks. The block copolymers comprise from about 15% to about 45% styrene by weight of the block copolymer, preferably from about 15% to about 35% styrene by weight of the block copolymer and more preferably from about 15% to about 25% styrene by weight of the block copolymer.
These adhesives are characterized by viscosities of less than about 5000 cPs at 175° C., preferably less than about 5000 cPs at 1 50° C. and more preferably viscosities of less than about 2500 cPs at 150° C., T-peel values of less than about 5 lbs/inch or 1 kg/cm when measured using an Instron® and softening points of between about 65° C. and 85° C. The adhesives are further characterized by having excellent thermal stability and low odor.
These adhesives are useful where a seal/reseal or peelable/reclosable type of closure is needed. The packaging industry uses seal/reseal containers for perishables such as cheese and meat. Such containers are often polymeric and are made from polyethylene, polypropylene, polyester, polyvinyl chloride, and acrylonitrile-methyl acrylate copolymer polymerized and/or mixed with butadiene for instance. The adhesives exhibit excellent adhesion to these difficult to bond substrates.
The low viscosities also allow for low application temperatures for heat sensitive substrates. The temperature of application for the adhesives is preferably between about 120° C. and about 165° C.
The low cohesive strength combined with excellent adhesion allows for a good seal prior to opening, but a low opening force, and a smooth peel with a lack of stringing or legging exhibited upon opening the package. The adhesives are inherently tacky and allow for a reseal of the package so that perishables may stay fresher, and offer further benefit due to the low odor. A further advantage is that these adhesives may be made for tamper evident packaging if it is desired.
DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
The block copolymers useful herein include linear triblock copolymers having the general configuration A-B-A and linear diblock copolymers having the general configuration A-B. The A block is styrene and the B block may be isoprene, butadiene, ethylene/butylene, ethylene propylene and mixtures thereof. Preferably, the B block is ethylene/butylene, ethylene/propylene and mixtures thereof. The coupling efficiency is less than about 75% and preferably less than about 65%. The coupling efficiency refers to the amount of triblock present as compared to the amount of diblock present in the polymer. The styrene content of the block copolymers is from about 15% to about 45% by weight of the block copolymer, preferably from about 15% to about 35% by weight and more preferably from about 15% to about 25% by weight and the melt index is commonly less than about 100 g/10 minutes and more commonly less than about 75 g/10 minutes. In contrast to adhesive compositions having low diblo
Lindquist Jeffrey S.
Ryan Lisa L.
Fischer Carolyn A.
H. B. Fuller Licensing & Financing Inc.
Szekely Peter A.
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