TWO COMPONENT CURABLE EPOXY RESIN COMPOSITION HAVING A LONG POT LIFE

Abstract: A two-part curable composition, the first part comprising (a) a resin mixture comprising 60-95% of compounds having epoxy function groups and 5-40% of compounds having acry­ late or methacrylate ester groups; and (b) .5-5% based on resin mixture weight of a free radical photoinitiator, and the second part comprising an amine component selected from polyoxyalkylene polyamines, tertiary amines and mixtures thereof. The compositions which are characterized by UV sur­ face cures in less than a minute, and long pot life, have part­ icular utility in electronics applications.

Description: In U.S. Patents 4,051,195 and 4,181,645 there are described compositions of diamines or polyamines, poly(meth)acrylic esters and polyepoxide resins. These compositions rely on rapid Michael addition between the amine and the acrylic esters to cure the acrylic portion of the composition. In U.S. 4,410,680 there are described heat curable resins comprising a blend of: (a) 100 parts by weight of a polyepoxide, particularly a normally liquid polyepoxide and more particularly a glycidyl polyether of a polyhydric phenol, (b) From about 10 to about 200 parts per weight of at least one ethylenically unsaturated monomer, especially an aromatic monomer, and preferably styrene, (c) A curing amount of an aliphatic or cycloaliphatic polyfunctional amine or polyamide, and (d) An effective amount of an amine - compatible peroxide free - radical initiator. This reference states further that the selection of the amine and the peroxy compounds should preferably be one wherein the particular amine tends to start the cure of the epoxy groups before the peroxy initiates the vinyl polymerization at the temperatures employed. The system is a single stage cure system and there is no discussion of competitive acrylic cure rates between Michael addition of amine to the acrylic groups and peroxy initiated free radical cure of those groups. The present invention, like the prior art references above, is a composition of epoxy, amine, and acrylic functional compounds. However, the invention differs from the prior art in that the composition further comprises a free radical photoinitiator and the amine portion of the composition is limited to theose for which Michael addition to the acrylic resin is relatively slow at ambient temperatures giving a product in which the amine-epoxy reaction is the predominant amine reaction. Examples of the amine portion include polyoxyalkylene polyamines having a molecular weight of about 200 or above, preferably polyoxypropylene polyamines having molecular weights of about 200-6, 500, and/or tertiary amines which catalyze epoxy homopolymerization such as 2,4,6-tri-(dimethylaminomethyl)phenol. Accordingly the present invention provides a two-part curable composition, characterised by first part comprising (a) a resin mixture comprising 60-95% of compounds having epoxy function groups and 5-40% of compounds having acrylate or methacrylate ester groups; and (b) 0.5-5% based on resin mixture weight of a free radical photoinitiator, and a second part comprising an amine component the amine being selected from those in which the amine-epoxy reaction is predominant amine reaction. The compositions are characterised by UV surface cures in less than a minute, typically 10-20 seconds with commercial UV curing equipment; a long pot life, at least 1-2 hours, typically 4-8 hours; a shadow cure at ambient temperature where UV light does not penetrate; little or no exotherm; and use of amines which have relatively low handling hazard. A further aspect of the invention comprises applying a composition as described above as a coating or potting layer on a substrate having shadow areas inaccessible to UV irradiation, irradiating the composition on the substrate with UV light to immobilize the composition and then permitting the composition to completely cure under ambient conditions. The compositions and method have particular utility in electronics applications. The compositions are particularly suited for use as environmentally protected coatings for electronic circuit boards and as potting materials for electronic components such as potentiometers and the like which utilize thermal plastic housing materials such as polybutylene terephthalate. The composition of the invention is maintained prior to use as a two-part composition. The first part comprises a resin mixture comprising 60-95% by weight of compounds having epoxy functional groups, preferably 70-85%, and 5-40% of compounds having plural (meth)acrylic ester functional groups, preferably 15-30%. The first part further comprises 0.5-5%, preferably 1-3% based on the weight of the resin mixture, of a free radical photoinitiator. The second part comprises the amine resin, preferably 75-100% of polyoxyalkylene diamine and 0-25% of tertiary amine. The epoxide resins useful in this invention comprise those compounds having an average of more than 1 vicinal epoxy or oxirane group per molecule. The polyepoxides may be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic and may be substituted if desired with noninterferring substituents such as halogen atoms, hydroxyl groups, ether radicals, and the like. They may also be monomeric or polymeric. Suitable polyepoxide compounds are described in the aforementioned U.S. Patents 4,051,195 and 4,410,680. Particularly preferred epoxy resins are glycidyl polyethers of polyhydric phenols, particularly the glycidyl polyether of Bisphenol A. The term "(meth)acrylate ester" as used herein includes esters of both acrylic and methacrylic acids. Suitable poly(meth)acrylate ester compounds are also described in U.S. 4,051,195 and include (meth)acrylic esters of alkylene glycols, alkoxylene glycols and various other polyhydric alcohols, and reaction products of epoxide resins and acrylic or methacrylic acid. Urethane acrylic resins may also be used. Preferred are acrylic eaters of polyhydric alcohols such as ethoxylated trimethylolpropane triacrylate. The amines are selected from those for which Michael addition to the acrylic resin is relatively slow at ambient temperatures giving a product in which the amine-epoxy reaction is the predominant amine reaction and has a pot life after mixing of at least 1 hour. This extended pot life permits the composition to be mixed in bulk, applied to substrates and irradiated to polymerize the acrylic portion of the composition under practical industrial conditions. Preferred amines include polyoxypropylene diamines of molecular weight 200-about 6,500, such as Jeffamine(R) D230, D400, D2000 and 5000 sold by Texaco. A suitable tertiary amine is 2,4,6-tri-(dimethylaminomethyl)phenol (DMP-30). Other polyfunctional tertiary amines known in the epoxy catalyst art may also be usefully employed. Either amine may be used alone or in combination. Generally, a small amount of the tertiary amine is used as an accelerator for the Jeffamine(R). Excessive amounts of tertiary amine are not recommended since they tend to produce brittle cured products. It is recommended that the polyoxyalkylene polyamine be used in approximately equivalent ratios to the epoxide, i.e. about 0.6-about 1.0 amine hydrogen atom per epoxide group. By photocuring the inventive compositions in the UV inaccessible areas of an appropriate substrate, faster processing times and improved coating properties are obtained over compositions which rely exclusively on Michael addition or on a peroxy heat cure to polymerize the acrylic groups. Further, by using amines for which the Michael addition reaction is slow, a convenient two stage curing, long pot life product is obtained. The invention is illustrated by the following nonlimiting examples. EXAMPLES Composition 1 was prepared as a two-part epoxy formulation. Mixing 4.8 parts by weight A to 1.0 part B gave a material with a pot life of 1 hour that was surface cured by exposure to high intensity UV light (100 mw/cm² for 20 seconds). Secondary cure via the amine after 24 hours at room temperature gave a product with the properties shown in the Tables. Composition 2 was prepared as a two-part epoxy formulation. Mixing 2 parts by volume A to 1 part B gave a material with a pot life of 1 hour that was surface cured by exposure to high intensity UV light (100 mw/cm² for 20 seconds). Secondary cure via the amine after 24 hours at room temperature gave a product with the properties shown in the Tables. The above compositions were tested against three leading prior art compositions. Composition 3 contains a polyester polyisocyanate formulated with an acrylate monomer. It was cured by exposure to high intensity UV light (100 mw/cm² for 20 seconds) followed by secondary moisture cure from exposure to air at room temperature overnight. Composition 4 contains an acrylated urethane prepolymer which was cured with a primer. Composition 5 contains an acrylated urethane prepolymer formulated with an acrylate monomer. It was cured by exposure to high intensity UV light (100 mw/cm² for 20 seconds). A series of poly(butyleneterephthalate) (Valox 420 SEO) lap shears were bonded to glass plates with compositions 1, 2, 3 and 4. Table 1 shows the generally superior adhesion and moisture resistance of the instant compositions 1 and 2 compared to the prior art compositions 3 and 4. All specimens were fully cured before exposure to the environments indicated. The specimens were allowed to cool to room temperature before testing at room temperature. A series of carbon-filled Valox (R) 420 SEO lap shears were bonded to glass plates as above. Table 2 shows that the superior performance of the instant compositions extends to carbon-filled Valox (R) . Table 3 shows the results of various tests involving Composition 2, 3 and 5. A group of trimming potentiometers were potted with the compositions and cured as described above. Only instant Composition 2 passed all the tests.

Claims: A two-part curable composition, characterised by first part comprising (a) a resin mixture comprising 60-95% of compounds having epoxy function groups and 5-40% of compounds having acrylate or methacrylate ester groups; and (b) 0.5-5% based on resin mixture weight of a free radical photoinitiator, and a second part comprising an amine component the amine being selected from those in which the amine-epoxy reaction is the predominant amine reaction. A composition as in claim 1 characterised in that the amine component comprises 75-100% of polyoxyalkylene diamine and 0-25% of a tertiary amine. A composition as in claim 2 characterised in that the polyoxyalkylene polyamine is selected from polyoxypropylene polyamines having average molecular weights between about 200 and 6500. A composition as in claim 2 characterised by having a tertiary amine component where the tertiary amine component is 2,4,6-tri(dimethylaminomethyl)phenol. A composition as in any of claims 1 to 4 characterised by one or more of the following features: (a) the (meth)-acrylate ester component comprises a (meth)-acrylate ester of a polyhydric alcohol; (b) the methacrylate ester component is present in the range of 15-30% by weight of the resin mixture; (c) the photoinitiator component is present in the range of 1-3% of the weight of the resin mixture; and (d) the epoxy functional compound is present in the range of 70-85% by weight in the resin mixture. A composition as in any of claims 1 to 5 characterized in that the epoxy compound is a glycidyl polyether of a polyhydric phenol, preferably a glycidyl polyether of Bisphenol A. A method of providing a cured coating or potting material on a substrate having shadow areas inaccessible to UV irradiation characterized by applying a mixture of the two parts of a composition as claimed in any of claims 1 to 6 to the substrate, including the shadow areas thereof, irradiating the composition of the substrate with UV light for sufficient time to immobilize the composition and then permitting the composition to completely cure under ambient conditions. A method as in claim 7 characterised in that the substrate comprises an electronic circuit board or component. A substrate characterised by being covered at least in part with the cured composition, the cured composition being applied by the method as claimed in claim 7. A substrate as in claim 9 characterised by comprising an electronic circuit board or component.

Inventors: KROPP, PHILIP L., NATIVI, LARRY A.
Applicants: LOCTITE CORPORATION
Application year: 1986
Main classification: C08G59
IPC codes: H05K3, C08G59, H01L23, G03F7, C08F283, C08L63