Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
1998-11-16
2001-04-10
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S3550EP
Reexamination Certificate
active
06214460
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to screen-printable adhesives and heat-curable adhesive films.
BACKGROUND OF THE INVENTION
Screen printing of adhesives is known in the art and is used advantageously to apply adhesives to selected areas on a substrate. The adhesive printed or coated areas can subsequently be used to adhere to a second substrate. Typical screen-printable adhesives are pressure-sensitive adhesives which are tacky at room temperature, or heat-activatable adhesives, which are not tacky at room temperature, but become tacky when heated. Examples of screen-printable adhesives include (meth)acrylic polymers and copolymers dispersed in an organic solvent or water.
Acrylic adhesives, both pressure-sensitive and heat-activatable types, are widely used in industry because they are stable over time, and they can be formulated to adhere to a wide variety of different surfaces. Typical acrylic adhesives are prepared as taught in U.S. Pat. No. 24,906 RE (Ulrich). With the advent of more stringent environmental controls, the technology in adhesives in general has evolved from solvent-based materials to water-based materials, and to a degree, solvent-free materials. Solvent-free acrylate adhesives are known and fall in various categories of processing such as heat-activatable coating and radiation curing which includes E-beam curing, ultraviolet light processing, and gamma radiation processing. Solvent-free crosslinked compositions are known in the art, but they would provide little utility for adhesively bonding to other substrates since they are highly crosslinked and do not flow or become tacky on heating.
Ultraviolet light processed adhesives are described in U.S. Pat. No. 4,181,752 (Martens et al.). While known adhesives processed by ultraviolet light have their own utility and advantages, they do not screen print well because they tend to become stringy during screen printing. Thus, an ongoing need exists for pressure-sensitive and heat-activatable screen printable adhesives that are solvent-free, can be screen printed without the use of a solvent, and provide good shear strength and peel strength.
An adhesive that has the ability to establish multiple discreet electrical connections, often in extremely close proximity, between two substrates is known as an “anisotropically conductive adhesive.” Typically, these adhesives are in the form of transfer tapes or free standing films where an insulating adhesive matrix contains sufficient conductive particles to allow electrical conduction through the thickness of the film (the z-axis) while providing no conductivity in the plane of the film. Such film types are known as “z-axis adhesive films” or “ZAF.” A typical use for this adhesive is to provide connection between a flexible printed circuit and a rigid circuit such as a flat panel display or epoxy-glass laminate printed circuit board.
Several ZAF materials are described in the literature. Some of these ZAF materials use non-reactive hot-melt type adhesive compositions such as styrene/butadiene/styrene block copolymers. They provide a long shelf life and short bond times at low temperatures. However, they show poor resistance to elevated temperature and humidity aging. Other ZAF materials use thermoset resins that crosslink, usually with the aid of curatives or catalysts, at the bonding temperatures. However, these ZAF materials typically require high bond temperatures, such as 170° C. or higher, and are difficult to use on temperature sensitive substrates.
Additionally, known ZAF materials are manufactured using solvent casting. Solvents usually must be captured or destroyed, and solvents can lead to damage of substrates and components. Additionally, the use of catalysts which are effective at lower temperatures typically leads to reduced shelf life of the ZAF. The use of photoactivated curatives in ZAF materials is also known. However, these adhesives need to be protected from light to avoid premature photoactivation. The methods of the present invention utilize heat curable adhesive films that are capable of rapidly bonding at low temperature, have a long shelf-life at ambient temperature, and provide stable electrical/and or thermal connections over a prolonged period of time.
SUMMARY OF THE INVENTION
The present invention provides a screen-printable adhesive composition capable of being applied to a substrate at room temperature comprising the following components:
(a) 25 to 100 parts by weight of at least one alkyl acrylate monomer;
(b) 0 to 75 parts by weight of at least one reinforcing comonomer;
(c) from 25 to 150 parts polyepoxide resin per 100 parts acrylate monomers; and
(d) an effective amount of a heat-activatable polyepoxide resin curing agent,
wherein said composition and components are substantially solvent free and said composition has a yield point of greater than 3 Pascals and a viscosity of less than 6000 centipoise at 25° C.
In another aspect, the present invention provides a screen-printable adhesive composition capable of being applied to a substrate at room temperature comprising the following components:
(a) 25 to 100 parts by weight of at least one alkyl acrylate monomer;
(b) 0 to 75 parts by weight of at least one reinforcing comonomer; and
(c) an effective amount of a core-shell polymer or a semi-crystalline polymer to provide a screen-printable composition,
wherein said composition and components are substantially solvent free and said composition has a yield point of greater than 3 Pascals and a viscosity of less than 6000 centipoise at 25° C.
In another aspect, the present invention provides a method of providing an electrical interconnection comprising the steps of:
a) applying a heat-curable electrically conductive adhesive film to an electrically conductive substrate, said adhesive film comprising an acrylate polymer, a polyepoxide resin, an effective amount of a heat-activatable modified aliphatic amine polyepoxide resin curing agent, and an effective amount of an electrically conductive material, said acrylate polymer comprising the polymerization reaction product of:
(i) an acrylate monomer; and
(ii) a crosslinking agent having an acrylate moiety,
wherein said composition and components (i) and (ii) are substantially solvent free; and
b) curing said polyepoxide resin in said adhesive film by heating said adhesive film to a temperature of between 90 to 180° C. for from 15 seconds to 5 minutes.
In another aspect, the present invention provides a method of providing heat transfer comprising the steps of:
a) applying a heat-curable thermally conductive adhesive film to a substrate, said adhesive film comprising an acrylate polymer, a polyepoxide resin, an effective amount of a modified aliphatic amine polyepoxide resin curing agent, and an effective amount of a thermally conductive material, said acrylate polymer comprising the polymerization reaction product of:
(i) an acrylate monomer; and
(ii) a crosslinking agent having an acrylate moiety,
wherein said composition and components (i) and (ii) are substantially solvent free; and
b) curing said polyepoxide resin in said adhesive film by heating said adhesive film to a temperature of between 90 to 180° C. for from 15 seconds to 5 minutes.
The present invention also provides tapes using the above adhesive compositions.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the methods and articles particularly pointed out in the written description and claims hereof.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to screen printable adhesive compositions and heat curable electrically and/or thermally conductive
Bluem Gregory L.
Haak Christopher A.
McCormick, Jr. Fred B.
Tead Stanley F.
3M Innovative Properties Company
Copenheaver Blaine
Pratt Christopher C.
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