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
1996-08-20
2004-04-20
Sellers, II, Robert E. (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...
C525S065000, C525S113000, C525S119000, C525S423000, C525S438000, C525S481000, C525S486000, C525S524000
Reexamination Certificate
active
06723803
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to compositions used as adhesives. More particularly, the present invention relates to flexible epoxy compositions for use as adhesives, which are designed to be stable at room temperature.
BACKGROUND ART
The flexible polymer market offers a wide variety of adhesive polymers, including such compounds as polyurethanes, polysulfides, silicones, and epoxy compounds. In particular, epoxy compounds demonstrate the ability to strongly adhere to a variety of materials, including metal, glass, plastic, wood, and fiber, and consequently are often used to bond dissimilar materials. However, various disadvantages long associated with epoxy compounds have dampened industry enthusiasm for using epoxy compounds as adhesives in automated bonding processes.
One-component epoxy-based adhesives are available for industrial application in two basic forms: rigid epoxy adhesives and frozen pre-mix, flexible epoxy adhesives. Rigid epoxy adhesives include such compounds as Bisphenol-A epoxy adhesives and novolacs. These rigid epoxy adhesives exhibit strong adhesion for many materials and may be conveniently stored at room temperature. However, these adhesives form brittle bonds that are often insufficiently pliant for bonding dissimilar materials. For example, a brittle bond between dissimilar materials with different thermal expansion rates may be unable to withstand the stresses caused by the thermal mismatch, so that both the bond and its adherends may be susceptible to failure.
Frozen pre-mix, flexible epoxy adhesives are also employed by industry, although the usage of such adhesives is far surpassed by the usage of rigid epoxy adhesives. A description of frozen pre-mix, flexible epoxy adhesives is found in U.S. Pat. No. 4,866,108, assigned to the present assignee, which discloses and claims the composition behind Flexipoxy 100 Adhesive, a frozen flexible epoxy adhesive developed for spacecraft electronic applications. In comparison to rigid epoxy adhesives, flexible epoxy adhesives form more pliable bonds that are capable of successfully adapting to stresses between dissimilar materials caused by differing rates of expansion. However, in contrast to rigid epoxy adhesives, frozen pre-mix flexible epoxy adhesives must be stored in a frozen state and must be thawed prior to use. Moreover, frozen adhesives offer a limited working life of only about 2 to 8 hours once thawed, whereas at least one week of working life is realistically required for general automated bonding operations. Therefore, frozen pre-mix, flexible epoxy adhesives are widely considered impractical for use in high volume automated processing given the scheduling difficulties wrought by both the need to thaw the adhesives as well as the limited working life available after thawing.
Thus, a need remains for a one-component epoxy adhesive that offers the convenience of room temperature storage while also providing pliable bonds capable of withstanding the rigors of varying expansion rates-between bonded materials. This need is particularly acute for high volume automated bonding operations, which can tolerate neither frequent periods of downtime for thawing nor the expense of discarding adhesives that quickly become ineffective. This need must be achieved without sacrificing good adhesion characteristics.
DISCLOSURE OF INVENTION
In accordance with the present invention, epoxy adhesive compositions are provided which are storable at room temperature, produce non-brittle, flexible bonds, exhibit strong adhesion, and have good processing characteristics. These compositions possess most, if not all, of the advantages of the above prior art compositions while overcoming their above-mentioned significant disadvantages.
The room-temperature stable, one-component, flexible epoxy-based adhesive compositions of the present invention comprise:
(a) at least one polyepoxide resin having a hardness not exceeding a durometer Shore D reading of 45 when cured with a stoichiometric amount of diethylene triamine (“DETA”); and
(b) a substantially stoichiometric amount of at least one latent epoxy resin curing agent,
wherein the cured combination of the polyepoxide resin and the latent curing agent exhibits a durometer Shore A of less than 95.
The polyepoxide resin component of the present compositions is a flexible epoxy resin. Flexible epoxy resins are defined herein to be those epoxy resins having a durometer Shore D measurement of 45 or less when cured with DETA. Polyepoxide resins suitably employed in the practice of the invention include, but are not limited to, diepoxide resins, triepoxide resins, and poly(oxypropylene)epoxide resins. Optionally, the epoxy resin component may also incorporate a semi-flexible resin such as the adduct of two moles of bisphenol A with one mole of the diglycidyl ester of linoleic dimer acid, the trifunctional novolac epoxy of cardanol, and the difunctional epoxy of cardanol. Semi-flexible epoxy resins are defined to be those epoxy resins having a durometer Shore-D value ranging from about 45 to 75 when cured with DETA. In comparison, rigid epoxy resins are defined as epoxy resins having a Shore-D value exceeding about 75 when cured with DETA.
The epoxy resin curing agent employed to cure the epoxy resin is a latent curative. As such, there is no reaction between the curing agent and the epoxy resin component at room temperature. Rather, the epoxy resin component is cured in the presence of the curing agent upon exposure to elevated temperatures. The present combination of epoxy resin and the latent epoxy resin curing agent remains uncured and Theologically stable at room temperature for months or even years, thus providing a long storage life conducive to automated bonding operations.
Aside from epoxy resin and latent curing agent components, other components that may be optionally added to the present adhesive compositions include fillers, thixotropic agents, and flexibilizers.
The present adhesive compositions are prepared by first mixing the following components to form a thoroughly wetted mass: (1) liquid components comprising the flexible polyepoxide component, and optionally, at least one component selected from the group consisting of flexibilizers and semi-flexible resins; and (2) solid components comprising at least one latent epoxy resin curing agent, and optionally, at least one component selected from the group consisting of fillers and thixotropic agents. The thoroughly wetted mass is then reacted at a temperature ranging from about 100° C. to 175° C. to form the present cured flexible epoxy adhesives, although the composition is designed to cure at a relatively low temperature ranging from about 100° C. to 125° C. in less than about two hours.
In sum, the adhesive of the present invention is novel in providing an epoxy-based adhesive composition that is storable for weeks as a single component mixture at room temperature, curable at relatively low temperatures in less than two hours, and flexible upon curing to temperatures as low as −50° C. Thus, the present adhesives offer the best features of the prior art adhesives. Like the frozen, pre-mix, flexible epoxy adhesives, the present adhesives form pliant bonds that withstand the stresses of thermal mismatch. Like the rigid epoxy adhesives, the present compositions may be conveniently stored at room temperature and easily processed. The ability of the present adhesives to provide strong flexible bonds without wreaking havoc on production schedules effectively promotes epoxy-based adhesives to use in automated bonding processes. In short, these adhesives enable industry to avail itself of the excellent adhesive qualities associated with epoxy compounds without sacrificing the advantages of automation. Importantly, the compositions of the present invention offer these benefits without employing solvent components, so that environmental integrity is maintained.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The adhesive compositions of the present invention have been developed to provide indu
Hermansen Ralph D.
Lau Steven E.
Lenzen, Jr. Glenn H.
Raufer Colin
Raytheon Company
Sellers II Robert E.
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