Substrate provided with removable adhesive of polyepoxide,...

Stock material or miscellaneous articles – Layer or component removable to expose adhesive – Release layer

Reexamination Certificate

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C428S300700, C428S413000, C428S414000, C523S412000, C525S109000, C525S113000, C525S114000, C525S118000, C525S119000

Reexamination Certificate

active

06579588

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to thermosettable adhesive compositions.
Thermosettable adhesive compositions have been used in a variety of applications where a semi-structural bond between two substrates is required. The semi-structural bond is necessary to ensure that the substrates are inseparable. In most applications, the bond is designed to be permanent. There are applications, however, in which it would be preferable for the adhesive composition to exhibit high performance bond properties during use (i.e., the period and environmental conditions, e.g., temperature range, over which the adhesive composition performs as a semi-structural adhesive), yet be removable after use. A tension exists between these opposing performance criteria. In the aerospace industry, for example, decorative sheets are often adhered to the interior walls of aircraft cabins using thermosettable adhesives. Over time the decorative sheets become marred (e.g., soiled, cut or torn) and styles change. It would be preferable if these decorative sheets could be removed and replaced with new sheets. Following cure, however, substrates bonded together by thermoset adhesive compositions are substantially inseparable. As a result, efforts to separate the substrates are often unsuccessful and result in substrate damage. In addition, the cured adhesive composition exhibits unpredictable cohesive and adhesive failure at either substrate.
A variety of thermosettable adhesive compositions are used to form semi-structural bonds to substrates. Thermosettable polyurethane adhesive compositions are often used to bond substrates together. Single package solvent-borne thermosettable polyurethane adhesive compositions rely on atmospheric moisture for curing. Water-borne thermosettable polyurethane adhesive compositions are cured by the addition of water dispersible isocyanate groups to the adhesive composition. The isocyanate groups react with the urea, amino-hydrogen and hydroxyl groups present in the water-borne prepolymer to crosslink the composition.
U.S. Pat. No. 3,765,972 (Wesp) describes a pressure-sensitive adhesive composition for use wherever a strong permanent bond is desired. The adhesive composition includes a latex and a transient tackifier that includes an epoxy resin and a curing agent. The latex portion of the adhesive composition provides the film-forming capability of the composition.
U.S. Pat. No. 5,464,902 (Recker) describes incorporating minor quantities of functionalized, partially crosslinked, elastomeric particles having a glass transition temperature of less than 10° C. into epoxy resin systems to toughen the epoxy resin systems against impact-induced damage. The toughened matrix resin systems may be utilized as neat films in structural adhesives or may be scrim supported.
U.S. Pat. No. 4,049,483 (Loder et al) describes a hot melt adhesive system of hot melt adhesive and inherently tacky elastomeric copolymer microspheres. The hot melt system has pressure sensitive adhesive characteristics at room temperature. The patent further describes adding a tackifying agent to the hot melt adhesive system to enhance the room temperature adhesion of the adhesive surface. Once the hot-melt adhesive system has been heat-activated, the adhesive is capable of forming a substantially permanent high strength hot melt bond. The basic properties of the hot melt matrix are unaffected by the inclusion therein of the microspherical adhesive.
SUMMARY OF THE INVENTION
In a first aspect, the invention features a thermosettable adhesive composition that includes a polyepoxide resin, a curing agent, and a plurality of microspheres. The microspheres,the polyepoxide resin, and the curing agent, and the relative amounts thereof, are selected such that upon cure (i.e., a sufficient degree of crosslinking to achieve a semi-structural bond to a substrate) the composition is capable of forming a semi-structural bond to a substrate and is cleanly thermally removable from the substrate. The adhesive composition can also include up to about 20% by weight flame retardant.
The cured adhesive compositions preferably exhibit a peel adhesion strength of at least 3.5 N/cm (2 pounds per inch width (“piw”)), more preferably at least 10.5 N/cm (6 piw), when measured on an abraded phenolic resin impregnated fiberglass substrate or a polycarbonate substrate at room temperature (about 20° C. to 25° C.). In addition, preferred adhesive compositions exhibit no greater than about 35% retention (more preferably no greater than about 20% retention) of initial peel adhesion strength at a temperature greater than the upper use temperature. The adhesive compositions preferably exhibit a peel adhesion strength of at least 3.5 N/cm (2 piw) measured at room temperature and no greater than about 35% retention of initial peel adhesion strength at a temperature greater than the upper use temperature.
In one preferred embodiment, the cured composition exhibits no greater than about 35% retention of initial peel adhesion strength at a temperature greater than about 50° C., more preferably the composition exhibits no greater than about 35% retention of initial peel adhesion strength at a temperature of about 15° C. greater than the upper use temperature.
Preferred adhesive compositions have a ratio of weight of polyepoxide resin to weight of microspheres of between about70:30 and about 35:65. The adhesive composition is preferably dispersed in water. The adhesive composition may be tacky or tack-free prior to cure.
The adhesive composition preferably cures at a temperature greater than room temperature, more preferably between about room temperature and about 200° C.
Preferred microspheres are characterized as having a surface that is essentially free of functional groups capable of reacting with the polyepoxide resin. The microspheres can be tacky or tack-free, solid or yhollow microspheres. Preferred microspheres include tacky, solid microspheres. The microspheres preferably have an average diameter between about 1 micrometer and about 20 micrometers. The microspheres preferably include the reaction product of isooctyl acrylate, acrylic acid and poly(ethyleneoxide)acrylate.
The curing agents preferably include heat-activated curing agents or photolytically-activated curing agents. The curing agents can include a blend of an epoxy homopolymerizationcatalyst (e.g., tertiary amines, imidazoles, substituted derivatives of imidazoles and combinations thereof) and an addition curing agent (e.g., primary and secondary amines).
In another embodiment, the invention features an article that includes a substrate having a surface, at least a portion of which is provided with the above-described thermosettable adhesive composition. The substrate can be a rigid substrate or a flexible substrate, e.g., a film. The article may further include a second substrate in contact with the adhesive composition. The adhesive composition of the article, upon cure, preferably exhibits a peel adhesion of at least about 3.5 N/cm measured on the substrate at room temperature, and no greater than about 35% retention of initial peel adhesion strength at a temperature greater than the upper use temperature.
In another aspect, the invention features a method for making an article that includes providing the above-described adhesive composition on at least a portion of a substrate (e.g., a more rigid substrate). The method can further include contacting the adhesive composition with a second substrate (e.g., a more flexible substrate). The method preferably also includes curing the adhesive composition.
In one preferred embodiment, the invention features a method for making an adhesive article that includes contacting a surface with an article that includes the above-described thermosettable adhesive composition and heating the composition to a temperature sufficient to cure the composition. The method may further include contacting a substrate with the adhesive composition before heating the composition.
In other embodiments, the invention features a method for removin

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