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
2001-06-15
2003-09-02
Niland, Patrick D. (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...
C524S490000, C524S507000, C524S590000, C525S123000, C525S455000
Reexamination Certificate
active
06613836
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to hot melt adhesives, in particular reactive hot melt adhesives having improved green strength.
BACKGROUND OF THE INVENTION
Hot melt adhesives are solid at room temperature but, upon application of heat, melt to a liquid or fluid state in which form they are applied to a substrate. On cooling, the adhesive regains its solid form. The hard phase(s) formed upon cooling the adhesive imparts all of the cohesion (strength, toughness, creep and heat resistance) to the final adhesive. Curable hot melt adhesives, which are also applied in molten form, cool to solidify and subsequently cure by a chemical crosslinking reaction. An advantage of hot melt curable adhesives over traditional liquid curing adhesives is (1) their ability to provide “green strength” upon cooling prior to cure and (2) provide adhesives of very low crosslinking density and thus high levels of flexibility and toughness.
The majority of reactive hot melts are moisture-curing urethane adhesives. These adhesives consist primarily of isocyanate terminated polyurethane prepolymers that react with surface or ambient moisture in order to chain-extend, forming a new polyurethane polymer. Polyurethane prepolymers are conventionally obtained by reacting diols with diisocyanates. Pure diols are favored for use, instead of polyols with higher functionality, to avoid excessive branching that can lead to poor pot stability. Methylene bisphenyl diisocyanate (MDI) is favored over lower molecular weight isocyanates to minimize volatility. Cure is obtained through the diffusion of moisture from the atmosphere or the substrates into the adhesive, and subsequent reaction. The reaction of moisture with residual isocyanate forms carbamic acid. This acid is unstable, decomposing into an amine and carbon dioxide. The amine reacts rapidly with isocyanate to form a urea. The final adhesive product is a lightly crosslinked material held together primarily through hydrogen bonding, urea groups and urethane groups.
The prior art discloses that that the performance of reactive hot melt adhesives for most applications may be substantially improved by the incorporation of acrylic polymers into conventional polyurethane adhesives, in particular reactive hydroxy-containing and non-reactive acrylic copolymers. Improvement in green strength may be obtained by adding higher molecular weight polymers (reactive or not) and/or incorporating crystalline diols, most commonly polyester diols.
These prior art adhesives are extremely tough, with outstanding low temperature flexibility, heat and chemical resistance, and specific adhesion to polar substrates. Adhesion to a wide range of other substrates may be obtained through the addition of adhesion promoters such as silane coupling agents. Despite these advances in the art, there remains a need for improvements in reactive hot melt technology to expand the application of such adhesives and their effectiveness in such applications. The present invention addresses this need.
SUMMARY OF THE INVENTION
The invention provides a moisture curable reactive hot melt adhesive having improved green strength.
One aspect of the invention is directed to a polyurethane reactive hot melt composition containing a polyester polyol, a polyether polyol, and/or a hydroxyl-functional acrylic polymer and an effective amount of an aliphatic C
5
-C
10
oligomer. A preferred aliphatic C
5
-C
10
oligomer is a terpene oligomer. The composition may also include a non-functional acrylic polymer.
Another aspect of the invention is directed to a method of improving the green strength of a polyurethane adhesive comprising adding an effective amount of an aliphatic C
5
-C
10
oligomer to a reactive hot melt adhesive formulation, wherein the adhesive comprises an acrylic resin and/or an acrylic polyol.
Still another aspect of the invention is directed to a method for bonding materials together which comprises applying the reactive hot melt adhesive composition of the invention in a liquid form to a first substrate, bringing a second substrate in contact with the composition applied to the first substrate, and subjecting the applied composition to conditions which will allow the composition to cool and cure to an irreversible solid form, said conditions comprising moisture.
Yet another aspect of the invention is directed to an article of manufacture comprising the adhesive of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The disclosures of all documents cited herein are incorporated in their entireties by reference.
It has now been discovered that the addition of an aliphatic C
5
-C
10
terpene oligomer to a hot melt polyurethane containing acrylic resins, in particular reactive hydroxy-containing and/or non-reactive acrylic copolymers results in a reduction in viscosity with little or no loss in green strength. Viscosity can, if desired, be adjusted to produce an adhesive with similar viscosity and improved green strength. A preferred oligomer for use in the practice of the invention is an alpha-pinene oligomer such as Sylvares TR-A25, commercially available from Arizona Chemical.
The moisture curable, hot melt polyurethane adhesives of the invention may be prepared through the reaction of a mixture of polyols with an isocyanate-containing compound at a temperature of from about 250° F. to about 275° F. The preferred isocyanate-containing compound is MDI. The polyols used include polyether, polyester and/or hydroxy-functional polymers. Non-functional acrylic can advantageously be added, and is required if no functional acrylic is included in the formulation. To this mixture the terpene oligomer is added in an amount of from about 2 to about 25%, more preferably from about 10% to about 20%, and most preferably from about 10% to about 15%.
The urethane prepolymers which can be used to prepare the adhesives of the invention are those conventionally used in the production of polyurethane hot melt adhesive compositions. Any suitable compound which contains two or more isocyanate groups may be used for preparing the urethane prepolymers. Typically from about 5 to about 25 parts by weight of an isocyanate is used.
Organic polyisocyanate which may be used to practice the invention include alkylene diisocyanates, cycloalkylene diisocyanates, aromatic diisocyanates and aliphatic-aromatic diisocyanates. Specific examples of suitable isocyanate-containing compounds; include, but are not limited to, ethylene diisocyanate, ethylidene diisocyanate, propylene diisocyanate, butylene diisocyanate, trimethylene diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, cyclopentylene-1,3-diisocyanate, cyclo-hexylene-1,4-diisocyanate cyclohexylene-1,2-diisocyanate, 4,4′-diphenylmethane diisocyanate, 2,2-diphenylpropane-4,4′-diisocyanate, xylylene diisocyanate, 1,4-naphthylene diisocyanate, 1,5-naphthylene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, diphenyl-4,4′-diisocyanate, azobenzene-4,4′-diisocyanate, diphenylsulphone-4,4′-diisocyanate, 2,4-tolylene diisocyanate, dichlorohexa-methylene diisocyanate, furfurylidene diisocyanate, 1-chlorobenzene-2,4-diisocyanate, 4,4′,4″-triisocyanatotriphenylmethane, 1,3,5-triisocyanato-benzene, 2,4,6-triisocyanato-toluene, 4,4′-dimethyldiphenyl-methane-2,2′,5,5-tetratetraisocyanate, and the like. While such compounds are commercially available, methods for synthesizing such compounds are well known in the art. Preferred isocyanate-containing compounds are methylenebisphenyldiisocyanate (MDI), isophoronecliisocyanate (IPDI) and toluene diisocyanate (TDI).
Most commonly, the prepolymer is prepared by the condensation polymerization of a polyisocyanate with a polyol, most preferably the polymerization of a diisocyanate with a diol. The polyols used include polyhydroxy ethers (substituted or unsubstituted polyaikylene ether glycols or polyhydroxy polyalkylene ethers), polyhydroxy polyesters, the ethylene or propylene oxide adducts of polyols and the monosubstituted esters of glycerol, as well as
Almer Charles W.
National Starch and Chemical Investment Holding Corporation
Niland Patrick D.
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