Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2002-01-31
2004-09-14
Aftergut, Jeff H. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S275700, C156S330000
Reexamination Certificate
active
06790310
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sheet-form, curable pressure-sensitive adhesive, and more particularly to a sheet-form, curable pressure-sensitive adhesive which is sufficiently tacky to be adhered to an adherend and also has the ability to cure upon application of an activation energy thereto, either before or after the adherend is combined with another adherend therethrough.
2. Background Art
Acrylic pressure-sensitive adhesives have been widely used in a variety of pressure-sensitive adhesive-carrying products including pressure-sensitive adhesive tapes due to their excellent properties such as resistance to heat, environment and oils. The general design of such acrylic pressure-sensitive adhesives controls their elastic modulus to a reduced level to develop pressure-sensitive adhesive strength thereof. Accordingly, acrylic pressure-sensitive adhesives, in turn, are difficult to develop their adhesive strength to a higher level as regular adhesives exhibit. This prevents their use in applications requiring a high joint strength, as for joining steels for vehicles, housing or building structural members, or casing members.
Under such circumstances, a curable pressure-sensitive adhesive has been proposed which provides a good operational characteristics and is safe due to absence of volatile contents while capable of providing high levels of joint strength and film toughness as comparable to those of regular adhesives. For example, Japanese Patent Laying-open No. Hei 2-272076 discloses an adhesive tape which utilizes a curable pressure-sensitive adhesive having a composition containing an acrylate monomer and an epoxy resin. Among the photopolymerizable compositions, the acrylate monomer alone is polymerized to provide the pressure-sensitive adhesive tape. For the purpose of enhancing its adhesion strength, the epoxy resin is heat cured after the pressure-sensitive adhesive tape is adhered to an adherend.
However, the method disclosed in the Japanese Patent Laying-open No. Hei 2-272076 utilizes heat for curing the epoxy resin. This prevents the use of materials having a poor heat resistance as the adherend, resulting in imposing limitations upon material selection of the adherend.
Japanese Patent Kohyo No. Hei 5-506465 discloses a pressure-sensitive adhesive which has a composition including a radically photopolymerizable component such as an acrylate monomer, cationically photopolymerizable component such as an epoxy compound and an organometallic complex salt polymerization initiator. This pressure-sensitive adhesive has been proposed in an attempt to enhance tackiness thereof. Both of radical and cationic photopolymerizations proceed during the stages of manufacturing said pressure-sensitive adhesive and complete by the time when the pressure-sensitive adhesive is formed into a sheet form. That is, an adequate strength has been given to the thus obtained pressure-sensitive adhesive before it is adhered to an adherend. Accordingly, while the pressure-sensitive adhesive is sufficiently tacky to adhere to the adherend, the application of an external stimulation such as heat or radiation thereto contributes no further improvement in adhesion strength.
On the other hand, epoxy resin adhesives exhibit excellent physical and chemical properties including creep resistance, light resistance, water resistance, heat resistance and chemical resistance. In addition, they provide high adhesion strength and have the ability to adhere to a wide range of materials such as metals, plastics and glasses. All of the above have led to a wide-spread use of the epoxy resin adhesives when joining various material members each other.
However, the general use of those epoxy resin adhesives in liquid form disadvantageously results in uneven coating and edge flowing derived from excessive application thereof to provide a poor appearance at joint interface edges, and prevents repeated application thereof onto once applied surface. Also, the typical epoxy resin adhesives are of two-part type so that a mixing ratio of a base resin to a curing agent is limited to a narrow range. Accordingly, care must be taken in order to mix them in appropriate proportions. The failure to do so is likely to prevent the prepared adhesives from exhibiting a purposed adhesive strength.
In view of the above, a proposal has been made to provide a sheet-form epoxy resin adhesive wherein the epoxy resin adhesive is formed into a sheet or film (Japanese Patent Laying-open No. Sho 60-173076). A sheet-form epoxy resin adhesive however has a high elastic modulus in its ordinary state to exhibit a low initial tackiness. This causes problems that the sheet-form epoxy resin adhesive is unable to provide adequate tackiness and satisfactory operational characteristics when joining adherends. Furthermore, the insufficient adherence thereof to an adherend is inevitably followed by severe conditions such as an elevated temperature and/or pressing under high-pressure to insure complete adhesion to the adherend. Such severe processing conditions prohibit the application of the disclosed adhesive to otherwise tolerable adherends.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a sheet-form, curable pressure-sensitive adhesive which has an adequate initial tackiness, and is capable of being cured upon application of an activation energy thereto and also of being used for joining members having insufficient heat resistances while eliminating the necessities of additional severe conditions such as an elevated temperature and/or pressing under high pressure when applied to an adherend.
Another object of the present invention is to provide a method for joining members which utilizes the above sheet-form, curable pressure-sensitive adhesive.
In accordance with the present invention, a sheet-form, curable pressure-sensitive adhesive is provided which comprises a composition including (A) a high molecular weight polymer, (B) a compound containing an epoxy group and (C) a polymerization initiator which, when an activation energy is applied thereto, initiates the compound (B) to undergo a ring-opening polymerization. In the sheet-form, curable pressure-sensitive adhesive, the high molecular weight polymer (A) has an adequate initial cohesive strength and is sufficiently tacky to readily adhere to an adherend. Also, the application of the activation energy activates the polymerization initiator (C) which causes the ring-opening polymerization of the compound (B) to take place. As a result, the sheet-form adhesive is cured to provide a satisfactory adhesion strength.
Furthermore, since the curing caused by application of the energy does not progress rapidly, the adequate tackiness of the sheet-form, curable pressure-sensitive adhesive enables an easy adherence thereof to the adherend even after the energy application.
The present invention is explained in more detail below.
The High Molecular Weight Polymer (A)
The high molecular weight polymer (A) used in the present invention is added to provide the sheet-form, curable pressure-sensitive adhesive with tackiness in its ordinary state. Any high molecular weight polymers may be used as the polymer (A), as long as they have the ability to impart tackiness and are sufficiently compatible with the compound (B) and the polymerization initiator (C).
Examples of the high molecular weight polymer (A) include acrylic polymers, polyesters, polyurethanes, silicones, polyethers, polycarbonates, poly(vinyl ether)s, poly(vinyl chloride)s, poly(vinyl acetate)s, ethylene-vinyl acetate copolymers, polyisobutylene. Of the above, acrylic polymers are preferably used since they have the ability to utilize a radical photopolymerization reaction as described hereinafter, the excellent compatibility with the compound (B) containing an epoxy group, the excellent tack properties and the others.
Although any particular limitations are not imposed upon a molecular weight of the above high molecular weight polymer, it preferably has a relatively hig
Fukui Hiroji
Nakasuga Akira
Aftergut Jeff H.
Armstrong Kratz Quintos Hanson & Brooks, LLP
Haran John T.
Sekisui Kagaku Kogyo Kabushiki Kaisha
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