Epoxy adhesive resistant to open bead humidity exposure
Epoxy adhesives are made using core-shell rubbers and small amounts if any of other elastomeric materials. The epoxy adhesives contain a mixture of latent curing agents and/or high levels of calcium oxide. The adhesives exhibit excellent bonding properties even when an open or closed bead is exposed to humid air for prolonged periods before the epoxy adhesive is cured.
1. A one-component, heat-curable epoxy adhesive comprising in admixture:
A) 45 to 75 weight percent, based on the weight of the epoxy adhesive, of a mixture of non-rubber-modified epoxy resins containing at least 80% by weight of a polyphenol polyglycidyl ether or a mixture of two or more polyphenol polyglycidyl ethers, wherein the mixture of non-rubber-modified epoxy resins is a liquid at 23° C. and has an epoxy equivalent weight of 175 to 250;
B) 12 to 25 weight percent, based on the weight of the epoxy adhesive, of core-shell rubber particles;
C) 5 to 25 weight percent, based on the weight of the epoxy adhesive, of one or more particulate inorganic fillers that include calcium oxide, wherein the calcium oxide constitutes at least 5 weight percent of the weight of the epoxy adhesive; and
D) two or more latent epoxy curing agents including a first latent epoxy curing agent that becomes activated when heated to a temperature within the range of 60 to 120° C. and a second epoxy curing agent that becomes activated when heated to a temperature of at least 140° C., the two or more epoxy curing agents together being present in an amount sufficient to cure the epoxy adhesive,
wherein the epoxy adhesive contains no more than 2 weight percent, based on the weight of the mixture of non-rubber-modified epoxy resins, of elastomeric compounds terminated in epoxy or epoxy-reactive groups and/or which contain urethane and/or urea groups.
2. The one-component, heat curable epoxy adhesive of claim 1 , further comprising a latent catalyst.
3. The one-component, heat-curable epoxy adhesive of claim 2 wherein the second latent epoxy curing agent that becomes activated when heated to a temperature of at least 140° C. includes one or more of dicyandiamide, methyl guanidine, dimethyl guanidine, trimethyl guanidine, tetramethyl guanidine, methylisobiguanidine, dimethylisobiguanidine, tetramethylisbiguandidine, heptamethylisobiguanidine, hexamethylisobiguanidine, acetoguanamine and benzoguanamine.
4. The one-component, heat-curable epoxy adhesive of claim 3 wherein the first latent epoxy curing agent that becomes activated when heated to a temperature within the range of 60 to 120° C. includes at least one of ethylene diamine, diethylene triamine, triethylene tetraamine and tetraethylene pentaamine blocked with a blocking group selected from one or more of phenol, an alkyl-substituted phenol, benzyl alcohol, an alkyl-substituted benzyl alcohol, or an aliphatic carboxylic acid having 2 to 20 carbon atoms.
5. The one-component, heat curable epoxy adhesive of claim 2 wherein the mixture of non-rubber-modified epoxy resins includes at least one liquid diglycidyl ether of bisphenol A, at least one solid diglycidyl ether of bisphenol A and at least one liquid or solid diglycidyl ether of bisphenol F, and up to 5% monohydrolyzed species that are present as impurities in one or more of the non-rubber-modified epoxy resins.
6. The one-component, heat curable epoxy adhesive of claim 2 wherein component (C) includes fumed silica.
7. The one-component, heat curable epoxy adhesive of claim 2 wherein the mixture of non-rubber-modified epoxy resins, the core-shell rubber particles, the particulate inorganic fillers, the latent curing agents and latent curing catalyst together constitute at least 93% by weight of the epoxy adhesive.
8. The one-component, heat curable epoxy adhesive of claim 1 wherein the mixture of non-rubber-modified epoxy resins, the core-shell rubber particles, the particulate inorganic fillers and the two or more latent curing agents together constitute at least 93% by weight of the epoxy adhesive.
9. The one-component, heat curable epoxy adhesive of claim 1 containing no more than 1 weight percent, based on the weight of the epoxy adhesive, of elastomeric compounds terminated in epoxy or epoxy-reactive groups and/or which contain urethane and/or urea groups.
10. The one-component, heat curable epoxy adhesive of claim 1 containing no more than 0.25 weight percent, based on the weight of the epoxy adhesive, of elastomeric compounds terminated in epoxy or epoxy-reactive groups and/or which contain urethane and/or urea groups.
11. A method comprising forming a layer of an epoxy adhesive of claim 1 at a bondline between two substrates, and curing the layer to form an adhesive bond between the two substrates.
12. The method of claim 11 including the steps of applying the epoxy adhesive onto one of the substrates in the form of a bead having a cross-sectional area of 20 mm 2 or less, exposing the bead to air containing at least 5 g of water vapor per cubic meter for a period of at least 48 hours, then applying the second substrate to the adhesive bead to form an adhesive layer at a bondline and curing the adhesive layer at the bondline to form the adhesive bond.