Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2001-03-07
2004-10-05
Buttner, David J. (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C525S525000
Reexamination Certificate
active
06800371
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to adhesive compositions and adhesives containing thioether groups and bonded substrates using such adhesives.
BACKGROUND OF THE INVENTION
Epoxy structural adhesives, although widely used for a variety of applications, do have some deficiencies. One limitation of epoxy structural adhesives is their tendency to swell and in some cases degrade when exposed to water and/or solvents. For example, some of the known structural adhesives currently used in ink-jet cartridges absorb up to 10 percent by weight of the liquid ink components. This swelling will result in a drastic reduction in the glass transition temperature of the adhesive, converting the adhesive from a strong, structural adhesive into a weak elastomer that is unable to maintain bond strength. The water and/or solvent penetration into adhesives can also lead to chemical degradation because of reactions with water and/or solvent. The swelling and degradation of the structural adhesive eventually leads to interfacial failure of the adhesive-substrate bond. A highly swollen or degraded adhesive network provides a pathway for other components to migrate to and degrade an adhesive substrate interfacial bond. This mechanism can lead to corrosion of the substrate if corrosive ions migrate through the swollen adhesive.
A conventional method of reducing the sensitivity of an adhesive to swelling by water or solvents is to drive the network forming reaction to very high levels of crosslink density. High crosslink density generally leads to high glass transition temperatures and requires high cure temperatures. These factors can lead to high levels of interfacial stress between the adhesive and substrate if the substrate is a metal, glass or ceramic because the thermal expansion of amorphous organic resins such as epoxy resins is much higher than most metals, inorganic glasses or ceramics. For example, the coefficient of thermal expansion (CTE) for an epoxy resin below its glass transition is generally about 60 ppm/° C. The CTE for silicon is 2.6-2.8 ppm/° C. If a high modulus epoxy resin is bonded to a silicon die at a high temperature, a significant amount of interfacial stress is built up between the adhesive and silicon die. If the silicon die is large and thin, the result can be undesirable deformation or bowing of the die. If the die is rigid enough not to deform, the residual stress from the CTE mismatch is stored at the interface, weakening the bond.
Two factors that affect the interfacial stress in an adhesive-substrate bond are the CTE mismatch and the modulus of the adhesive. The problems associated with CTE mismatch can be alleviated through addition of a “flexibilizer” which lowers the modulus of the epoxy structural adhesive. The flexibilizer reacts, at least to some extent, with the epoxy resin during cure to provide flexibility in the polymer backbone. Typically, such flexibilizers are used to reduce the glass transition temperature, reduce the viscosity, and improve the ductility of the epoxy adhesive. The addition of a flexibilizer reduces the interfacial stress by increasing the rubbery or low modulus range of the adhesive and reducing the glassy or high modulus range of the adhesive. During much of the cooling cycle after the cure, the flexible adhesive will be in the rubbery/low modulus state, where interfacial stress is minimized. However, conventional flexibilizers such as polyalkylene oxide amines and epoxy terminated ethers render such adhesives particularly sensitive to water and/or solvent swelling and degradation.
Epoxy terminated polysulfides are another type of epoxy flexibilizers. While polysulfide polymers are known to have good fuel and moisture resistance, they typically contain chemically and thermally unstable disulfide and formal linkages, which make them unsuitable for many applications.
Another deficiency of epoxy resins is their limited adhesion to certain substrates. Epoxy resins are known to have excellent adhesion to some metals and good adhesion to some plastics. In general, epoxy-based adhesives do not provide good adhesion to non-polar plastics such as acrylonitrile-butadiene-styrene (ABS) polymers. The use of epoxy-terminated liquid polysulfide polymers in epoxy-based adhesives is known to improve adhesion to some types of substrates such as steel. However, epoxy-terminated liquid polysulfide polymers have not been demonstrated to improve adhesion to plastics such as ABS polymers, poly(methyl methacrylate) (PMMA) polycarbonate, polyimide, or silicon.
Thus, there remains a need for epoxy-based adhesives having improved water and solvent resistance and/or improved adhesion to difficult to bond substrates such as those made from ABS, poly(methyl methacrylate) (PMMA), polycarbonate, polyimide, silicon dioxide, and silicon die.
SUMMARY OF THE INVENTION
The invention provides epoxy resin adhesives that contain thioether segments incorporated into the crosslinked network. A “thioether segment” is defined as a divalent sulfur atom bonded to two carbon atoms. A segment may contain a sequence of two or more thioether groups bonded together. The thioether segments are incorporated into an epoxy resin adhesive by adding a thioether-containing flexibilizer to the adhesive composition. Generally, the more thioether segments incorporated into the flexibilizer and the fewer oxygen ether groups, the more the adhesives are resistant to swelling or absorption due to water and/or solvent exposure.
In one aspect, the invention provides a curable composition for making an adhesive containing thioether segments comprising a mixture of epoxy resin, catalyst and/or curative, and epoxy reactive thioether-containing compound. Preferred epoxy reactive thioether-containing compounds are the thioether di-epoxides. Preferred thioether di-epoxides include 2-{[3-({2-[(2-{[3-(2-oxiranylmethoxy)propyl]sulfanyl }ethyl)sulfanyl]ethyl}sulfanyl)propoxy]methyl }oxirane; 2({3-[(6-{[3-(2-oxiranylmethoxy)propyl]sulfanyl }hexyl)sulfanyl]propoxy}methyl)oxirane; and 2-({3-[(2-{[3-(2-oxiranylmethoxy)propyl]sulfanyl}ethoxyethoxyethyl)sulfanyl]propoxy}methyl)oxirane.
The epoxy reactive thioether-containing compounds described herein can be used to increase flexibility and reduce interfacial stress of the resulting adhesive but have a minimal negative impact on the water and chemical resistance of the adhesive.
Another aspect of the invention is a method of bonding a substrate comprising the steps of contacting the substrate with an adhesive composition comprising epoxy resin, epoxy reactive thioether-containing compound, and catalyst and/or curative, and curing the adhesive composition. The adhesive compositions of the invention provide adhesives having improved adhesion to substrates including those made from ABS polymers, polyimide, polycarbonate, poly(methylmethacrylate), silicon dioxide, and silicon die.
Another aspect of the invention provides an adhesive comprising the reaction product of epoxy resin, catalyst and/or curative, and epoxy reactive thioether-containing compound. The adhesives of the invention are cured adhesive compositions of the invention.
Other aspects of the invention are articles comprising adhesives comprising epoxy resin, epoxy reactive thioether-containing compound, and a curative bonded to substrates comprising silicon, plastic, metal, or combinations thereof.
Adhesion levels of the adhesives of the invention to plastics such as ABS and polycarbonate are in excess of those seen from epoxy-based adhesives that do not contain sulfur atoms or that contain epoxy-terminated polysulfide polymers. In addition, formulations containing epoxy reactive thioether-containing compounds have improved adhesion to polyimide and silicon dioxide passivated silicon wafer die than adhesives that contain polyalkylene oxide amines such as bis-3-aminopropylpolytetramethyleneoxide. Adhesives of the invention may be low stress and are water and s
Gross Kathleen B.
Noe Susan C.
Pocius Alphonsus V.
Schultz William J.
Thompson Wendy L.
3M Innovative Properties Company
Bardell Scott A.
Buttner David J.
Keehan Christopher
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