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
1994-12-22
2004-05-04
Gulakowski, Randy (Department: 1746)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S088000, C525S089000, C525S098000, C525S185000, C525S326500, C525S342000
Reexamination Certificate
active
06730737
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a releasable pressure sensitive adhesive composition. More specifically, the present invention relates to a releasable, pressure sensitive adhesive composition comprising a crosslinked block copolymer based on a conjugated diene and a vinyl aromatic hydrocarbon.
BACKGROUND OF THE INVENTION
Releasable pressure sensitive adhesives are used in numerous applications wherein tackiness and removability without leaving traces of adhesives on the substrate (even after repeated use) are required. Specific examples include self-adhering memo labels, removable tapes, and reusable closure systems, such as closures of packages containing tissues. Several adhesive compositions meeting the above requirements have already been proposed in the past. For instance, U.S. Pat. No. 4,680,333 discloses a removable pressure sensitive hot melt adhesive composition comprising an ABA type block copolymer, with A being polystyrene and B polyisoprene, and a substantial amount (up to 50% by weight of the composition) of aliphatic hydrocarbon resin having a softening point of less than 30° C. Such hydrocarbon resins mainly include liquid tackifying resins. In addition to these components, other components, such as high softening point aliphatic hydrocarbon resins, i.e. solid tackifying resins, mineral oil or petroleum, metallic salts of fatty acids and antioxidants, may be present too.
WO 88/01631 and WO 88/01636 both relate to a readhering and releasable gelled solid adhesive for coating substrates, particularly paper, comprising a rubber component, a water-soluble or water-dispersible tackifying agent, and a gelling agent. A friction reducing agent, comprising a polyhydric aliphatic alcohol may be present as well. Natural rubber latex is stated to be the preferred rubber component. The preferred gelling agent is sodium stearate.
EP 0,443,263 discloses a repositionable adhesive coated substrate whereby the adhesive layer is comprised of 20 to 80% by weight of an elastomeric styrenetbutadiene, styrenelisoprene, or styrenelethylene-butylene block copolymer with the balance up to 100% by weight being a tackifier material. This tackifier material is either a tackifying resin (liquid and/or solid) or a blend of a tackifying resin and a liquid plasticizer oil. The purpose of the tackifier material is to modify the composite midblock glass transition temperature (CMTg) which is taught to be an important parameter for the properties of the adhesive composition.
Although the releasable and readherable adhesive compositions disclosed in the documents discussed hereinbefore perform satisfactorily, there is still interest in developing alternatives. One of the reasons for that is the presence of relatively large amounts of plasticizer and/or liquid tackifying resins in the prior art compositions. EP 0,443,263 teaches that these components are necessary for obtaining a composition having a low CMTg which is said to be essential for good releasability. One of the objects of the present invention is to provide a releasable pressure sensitive adhesive composition containing a relatively small amount of plasticizer and/or liquid tackifying resin which still exhibits excellent releasability in combination with sufficient tackiness, even after having been used repeatedly.
Adhesive compositions containing a relatively small amount of plasticizer and/or liquid tackifying resin are known from EP 0,519,086. This European patent application discloses an adhesive tape wherein both sides are coated with an adhesive layer. One adhesive layer is made of an acrylic or rubber-based pressure sensitive adhesive composition and a relatively small amount of silicone polymer, while the other adhesive layer is solely constituted of an acrylic or rubber-based pressure sensitive adhesive composition. Due to the different composition of both layers, they can be easily released and separated from each other at their interface, thus enabling the adhesive tape to be stored or sold as a roll. The easy separation of both layers at their interface is attributed to the presence of the silicone polymer in one of the layers which bleeds to the surface of the adhesive layer where it forms a silicone film. Due to its visoelasticity, this silicone film exhibits adhesive properties and hence can serve as an adhesive layer. Since the silicone polymers disclosed do not contain any reactive group or atom which might interact with the acrylic or rubber polymer molecules, it is clear that the silicone polymer does not act as a crosslinking agent for connecting the various acrylic or rubber polymer molecules present in the pressure sensitive adhesive composition.
U.S. Pat. No. 4,831,080 discloses a composition comprising a pressure sensitive adhesive and a liquid organohydrogenpolysiloxane containing on the average at least two silicon-bonded hydrogen atoms per molecule. When this composition is combined with a composition comprising the same pressure sensitive adhesive and a curing agent for said liquid organohydrogenpolysiloxane, a crosslinkable pressure sensitive adhesive composition is obtained. However, the adhesive properties are such that it cannot be used as a releasable adhesive. Hence, the use of the composition as a releasable adhesive is evidently not intended. The pressure sensitive adhesive may be based on styrene-butadiene random copolymers as well as on acrylic or butyl polymers. Styrene-butadiene block copolymer-based adhesives are not explicitly disclosed. Among the large group of suitable curing agents disclosed, there are mentioned platinum compounds, such as platinum acetylacetonate and chloroplatinic acid. As becomes apparent from the working examples of the patent, particularly examples 34 and 35, such platinum compounds must be used in conjunction with a curing accelerator, such as methylvinylsiloxane, in order to achieve an acceptable level of crosslinking in styrene-butadiene polymer-based pressure sensitive adhesives.
The present invention aims to provide a releasable pressure sensitive adhesive composition which adheres very well to various substrates including paper and plastics, can be removed from the substrate without leaving any traces behind, and can be used repeatedly without loss of tackiness. It is a further object of the present invention to provide a releasable pressure sensitive adhesive composition which contains a relatively small amount of plasticizing oil and/or liquid tackifying resin. More specifically, the present invention aims to provide a releasable pressure sensitive adhesive composition based on a monovinyl aromatic-conjugated diene block copolymer wherein the block copolymer molecules are crosslinked via a crosslinking reaction with an organohydrogenpolysiloxane cross-linking agent without the use of any curing accelerator. It is also an object of the present invention to provide a releasable pressure sensitive adhesive composition wherein the monovinyl aromatic-conjugated diene block copolymer is crosslinked through vinyl groups present in the poly(conjugated diene) blocks.
Surprisingly, it has now been found that releasable pressure sensitive adhesive compositions based on crosslinked block copolymers can be obtained without using any curing accelerator and that such compositions need contain only relatively small amounts of plasticizer, i.e. no more than 50 parts by weight per 100 parts by weight of block copolymer.
SUMMARY OF THE INVENTION
Accordingly, the present invention relates to a releasable pressure sensitive adhesive composition comprising:
(a) a block copolymer comprising at least one poly(monovinyl aromatic hydrocarbon) block and at least one poly(conjugated diene) block and having a vinyl content of at least 3%,
(b) a solid tackifying resin, and
(c) a plasticizer,
wherein the block copolymer has been crosslinked through its vinyl groups by means of a reaction with an organohydrogenpolysiloxane crosslinking agent containing at least two hydrogen atoms which are directly bonded to a silicon atom. The crosslinking takes place in the presence o
De Keyzer Noel Raymond Maurice
Vermunicht Geert Emiel Anna
Gulakowski Randy
Kraton Polymers U.S. LLC
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