Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-12-17
2001-08-28
Gallagher, John J. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S331400, C525S453000, C528S059000
Reexamination Certificate
active
06280561
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to extrudable moisture cure hot melt urethane adhesives comprising a urethane prepolymer.
BACKGROUND OF THE INVENTION
Hot melt adhesives and moisture cure urethane adhesives are well known generic adhesive classes. Hot melt adhesives generally comprise thermoplastic materials which can be conveniently applied by extruding the adhesive composition at elevated temperatures onto a workpiece to which it is desired to fix another workpiece. The adhesive bond increases in integrity as the composition cools. Hot melt adhesives generally have good green strength, i.e., initial strength before complete setting of the bond, and they have the additional benefits of being easy to handle and free of solvents which must be evacuated during the setting or curing process of the adhesive. However, hot melt adhesives have the drawback that after cooling to form a bond line, the thermoplastic compositions can be temperature sensitive. In other words, the bonding mass can lose bond strength as the temperature of the workpiece and the bond line increase. Further, hot melt adhesives tend to be physically unstable in the presence of hydrocarbon solvents and some organic compositions.
In contrast, many curable urethane adhesives have little green strength. Thus, after the application of a curable urethane adhesive, the structure to be bonded must be externally supported until the urethane can cure to a strong, resilient, highly crosslinked bond line. Additionally, solvents may be used in these adhesives which may need to be evaporated during cure. However, once cured, these urethane adhesives have high tensile strength and have little or no temperature sensitivity. They also offer good resistance to many solvents after cure.
Clearly, a single adhesive composition displaying both curing and hot melt properties is a desirable goal, since the resulting adhesive, in theory, could possess quick, high strength green bonding and strong, crosslinked cured adhesive bonding.
U.S. Pat. No. 4,623,709 discloses polyurethane prepolymers in which polyhydric alcohols are initially reacted with the faster reacting isocyanate group of an asymmetrical diisocyanate with the slowly reacting group remaining intact, after which the reaction products are combined with a symmetrical diisocyanate of which the equally reactive isocyanate groups again react more quickly than the slowly reacting groups of the first polyfunctional isocyanate compound. The products obtained by this process are allegedly distinguished by a low residual monomer content.
U.S. Pat. No. 5,441,808 discloses hot melt adhesive compositions combining the characteristics of thermoplastic hot melt adhesives and reactive adhesives and are prepared by blending a polyester polyether thermoplastic elastomer with a polyisocyanate prepolymer.
U.S. Pat. No. 3,931,077 discloses reactive, high viscosity hot melt adhesive compositions comprising a specific high viscosity reactive urethane prepolymer, a specific ethylene-vinyl acetate thermoplastic polymer and a phenolic or abietic acid-type tackifying resin.
U.S. Pat. No. 4,585,819 discloses the combination of an isocyanate prepolymer, a thermoplastic polyurethane or polyester and a synthetic resin which may be a ketone resin, a hydrogenation product of acetophenone condensation resins, and mixtures thereof.
U.S. Pat. No. 4,775,719 discloses a thermally stable, hot melt moisture cure polyurethane adhesive composition comprising an ethylene-vinyl monomer having an ethylene content of 55 wt % or less, a polyisocyanate prepolymer, and an aromatic tackifying resin.
U.S. Pat. No. 4,808,255 discloses a thermally stable, moisture cure urethane hot melt adhesive comprising a thermoplastic polymer, a compatible, curing urethane polyalkylene polyol prepolymer, and a tackifying agent.
U.S. Pat. No. 4,820,368 discloses a thermally stable, moisture cure urethane hot melt adhesive comprising a thermoplastic polymer, a compatible, curing urethane polyester polyol prepolymer, and a tackifying agent.
SUMMARY OF THE INVENTION
The present invention provides an improved moisture curable urethane hot melt adhesive composition having low melt viscosities and good green strength. Also provided is a method for adhesively joining or sealing two substrates using such adhesive composition. The method generally comprises
(1) applying onto a substrate a solvent-free, moisture curable urethane hot melt adhesive composition comprising the polyisocyanate prepolymer reaction product of (a) a compound containing at least two active hydrogen atoms according to the Zerewittenoff test, preferably a polyol, and (b) a polyisocyanate, the adhesive composition being applied at an elevated temperature sufficient to melt extrude the adhesive composition;
(2) contacting the melt extruded adhesive composition disposed on the substrate to a second substrate such that a green bond is formed; and
(3) exposing the adhesive bond to moisture causing a crosslinking reaction within the adhesive, thereby forming a moisture-cured bond.
According to the present invention, the prepolymer used in the moisture curable hot melt adhesive composition is prepared by reacting a polyisocyanate having an NCO functionality of at least 2 with a compound containing X active hydrogen atoms according to the Zerewittenoff test (where X is at least two), preferably a polyol having an OH functionality of at least 2, the prepolymer containing free NCO ranging from 0.2 to 8 wt % and less than 2 wt % unreacted isocyanate monomer. At least 90 wt %, preferably 95 wt %, of the prepolymer reaction product obtained by the reaction of the polyisocyanate with the active hydrogen-containing compound should comprise a prepolymer of X moles polyisocyanate per mole of such active hydrogen-containing compound, i.e., a “perfect” prepolymer. In the preferred embodiment, at least 90 wt %, preferably 95 wt %, of the prepolymer reaction product obtained by the reaction of toluenediisocyanate (TDI) with a long chain polyol having three OH groups comprises a “perfect” prepolymer of 3 moles TDI per mole long chain polyol.
Advantageously, the adhesive composition comprising such prepolymer is a low viscosity liquid at the application temperature and forms a solid upon cooling which serves to adhere the two substrates brought into contact with it. Subsequently, the prepolymer reacts with ambient moisture to form a crosslinked polyurethane-urea with improved strength and durability.
Compared to the conventional urethane prepolymers of the prior art, the present prepolymers exhibit lower viscosity at application temperature which results in better wetting and adhesion to substrates. In addition, the lower viscosity also allows the applicator to use lower processing temperatures which expands the number and type of substrates that can be adhered. The significantly lower monomer content results in reduced potential for worker exposure. A more regular resin structure in the prepolymer will result in more rapid crystallization and “green strength” development.
Moreover, the lower oligomer/monomer content upon crosslinking results in a more uniform network structure with improved properties relative to conventional materials.
DETAILED DESCRIPTION OF THE INVENTION
The moisture curable urethane hot melt adhesive composition used in the present inventive method comprises an NCO-terminated urethane prepolymer (also called a polyisocyanate prepolymer) prepared by reacting a polyisocyanate having an NCO functionality of at least 2 with a compound containing at least two active hydrogen atoms according to the Zerewittenoff test, preferably performing the reaction in an NCO to active hydrogen equivalent ratio of ≧4:1, preferably, 6-8:1. The unreacted polyisocyanate monomer in the prepolymer reaction product is removed by distillation or other treatment to a concentration of less than 2%, preferably, less than 0.1%. The prepolymer reaction product should contain free isocyanate, i.e., free NCO functionality, ranging from 0.2 to 8 wt %, preferably, 0.5 to 3 wt %.
Su
McInnis Edwin Lee
Quay Jeffrey Raymond
Santosusso Thomas Michael
Air Products and Chemicals Inc.
Gallagher John J.
Leach Michael
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