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
1998-10-13
2002-11-26
Sellers, Robert E. L. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S09200D, C525S119000, C525S122000, C525S407000, C525S527000, C525S528000, C525S530000, C528S093000, C528S094000
Reexamination Certificate
active
06486256
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to epoxy resin compositions, particularly, to an epoxy resin composition that when cured exhibits properties useful in structural assembly and, even more particularly, to two-part epoxy adhesive compositions that exhibit one or more improved adhesive properties such as impact, creep and fatigue resistance, as well as durability under service conditions for structural applications.
BACKGROUND OF THE INVENTION
Adhesives have been used in many structural applications. Such structural applications have included vehicles, computer cases, buildings, appliances, etc. For example, structural adhesives have been used in vehicle assembly (e.g., automobile and aircraft assembly) to replace or augment conventional joining techniques such as welds, nuts and bolts, and rivets.
Epoxy compositions are known and have been used for structural adhesive applications. In state-of-the-art epoxy technology today, polymerization catalysts used to achieve higher order oligomers typically are tetraalkyl ammonium or phosphonium salts that do not promote epoxy homopolymerization. Cyclic amidine catalysts, such as imidazoline catalysts and imidazole catalysts, have also been used in adhesives. The adhesives of the prior art are quality adhesives in many applications. Even so, there is a continuing need for higher performance adhesives in order to meet the changing needs of various industries such as, for example, the vehicle assembly industry.
SUMMARY OF THE INVENTION
The present invention is intended, at least in part, to address the ongoing need for higher performance adhesives to meet the needs of various industries, including the vehicle assembly industry (e.g., automobile, aircraft and watercraft industry). Compositions of the invention are believed to be useful in structural adhesive applications either alone or in conjunction with conventional assembly techniques like welding and/or mechanical fastening (e.g., rivets).
We have found a composition useful as a structural adhesive having long term durability under static and/or dynamic loads and substantially improved impact, creep and/or fatigue resistance for use in assembly applications. The composition can include a chain extender, a catalyst, a reactive epoxy resin and one or more polymeric tougheners. At least when mixed together, the present adhesive composition is in a form that can be applied or dispensed (e.g., liquid or paste form). The chain extender, the reactive epoxy resin, the catalyst and the toughener are each in an amount and of a type that are effective, when mixed together, to form a thermally curable adhesive. When the adhesive is cured, at least about 50% by weight of the epoxy resin is chain extended. Preferably, at least 60 wt %, and even more preferably at least 70 wt %, of the epoxy resin is chain extended.
It is preferred that the composition be free, or at least substantially free, of a polyfunctional curing agent (i.e., an agent that chain extends and cross links the epoxy resin). That is, the amount of polyfunctional curing agent should be limited to the point that, when the adhesive is cured, the desired amount of the epoxy resin is chain extended.
The chain extender can comprise an amine, a phenolic compound or a combination thereof. Preferably, the chain extender is all or at least substantially in monomeric form (i.e., the chain extender is not prereacted, prepolymerized or in oligomeric form) prior to being added to the composition. That is, enough of the chain extender is in monomeric form to enable the resulting composition to be applied or dispensed. Preferably, the resulting composition is compatible with state-of-the-art dispensing and rheology (e.g., viscosity) requirements. It is also preferable that the chain extender be dissolvable into the epoxy resin, the catalyst or both, at least at an elevated temperature (e.g., the curing temperature of the composition). It may be desirable for the chain extender to be in solid particulate form and finely dispersed in the epoxy resin and/or the catalyst, where the chain extender dissolves at elevated temperatures.
The phenolic compound preferably includes a dihydric phenol (e.g., a di-hydroxy benzene, such as catechol, resorcinol and/or compounds based thereon), and the amine preferably includes a primary monoamine (e.g., attached to a primary or secondary carbon), a secondary diamine, and compounds based thereon. A polyfunctional or multifunctional amine (e.g., a diamine containing both primary and secondary functionality or multiple primary functionality) will cause chain extending and cross linking (i.e., will function as a curing agent). Even though it will cause cross linking to occur, a polyfunctional amine or other curing agent may be used, but in a limited amount.
The present composition can be a two-part adhesive with the catalyst in a part A and the reactive epoxy resin in a part B. The chain extender is included in at least one of the two parts. When the chain extender of such a two-part adhesive composition includes an amine, the amine is preferably only in the part A. It may be possible to add very small amounts of amine in the epoxy part B, as long as the amount of amine is not enough to adversely affect the part B (e.g., its rheology). When the chain extender of such a two-part composition includes a catechol, the catechol can be in the part A, in the part B or in both. It is preferable that the catechol is in at least the part A. It is surprising that the catechol can be sufficiently stable (i.e., not recrystalize or react) in the epoxy resin to provide a part B with a commercially acceptable shelf life. When the chain extender includes a catechol and resorcinol, at least the part A includes the resorcinol and catechol. The part B can include the catechol without resorcinol. When the chain extender includes another type of phenolic compound, it may also be included in the part A, part B or both.
It can be preferable for at least about 50 wt % of the chain extender to be catechol. When such a chain extender also includes resorcinol, up to about 50 wt % of the chain extender can be resorcinol. It is believed that the adhesive composition can contain in the range of from about 8 wt % to about 30 wt % of such a catechol and resorcinol containing chain extender, based on the amount of chain extender and reactive epoxy.
The catalyst is preferably a base. The catalyst can include a cyclic amidine, a tertiary amine, and substituted analogues thereof The catalyst can comprise one or more of imidazole, imidazoline, a substituted imidazole compound, a substituted imidazoline compound, 1,4,5,6-tetrahydropyrimidine, a substituted 1,4,5,6-tetrahydropyrimidine compound and combinations thereof The chain extender preferably includes catechol. The catalyst can also include one or more substituted pyridines, pyrrolidines and piperidines. The chosen catalyst or catalysts should not contain a group that exhibits an electron withdrawing or stereo chemical effect sufficient to prevent the composition, when mixed together, from forming a thermally curable adhesive suitable for structural bonding. Typically, as the mass of the catalyst increases, the amount of catalyst needed to establish a desired effect also increases, unless any substitution chemistry present has an affect on (i.e., increases or decreases) the effectiveness of the catalyst. The catalyst can comprise two or more different catalysts. We have surprisingly found that a combination of two different amidine catalyst species, in particular cyclic amidine catalysts, can provide enhanced adhesive properties. A preferred combination can include one or more imidazole compounds (substituted or unsubstituted) and one or more imidazoline compounds (substituted or unsubstituted). It is believed that a combination of a 1,4,5,6-tetrahydropyrimidine compound (substituted or unsubstituted) with an imidazoline compound and/or an imidazole compound may also provide enhance adhesive properties.
Preferably, the amount of the catalyst in the adhesive composition is at a level of at le
Robins Janis
Tarbutton Kent S.
Knecht III Harold C.
Sellers Robert E. L.
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