One-component toughened epoxy adhesives containing a mixture of latent curing agents
A one-component toughened epoxy-modified polyurethane and/or urea adhesive includes a mixture of dicyandiamide and a dihdyrazide as curing agents. The mixture of curing agents permits the adhesive to be cured at lower temperatures while developing good adhesive and mechanical properties in the cured adhesive.
1. A method for bonding two substrates, comprising forming a layer of an adhesive comprising in admixture A) at least one non-rubber-modified epoxy resin, B) at least one toughening agent, C) at least one epoxy curing catalyst and D) a curing agent mixture that includes dicyandiamide and one or more dihydrazide compounds, the dicyandiamide and one or more dihydrazide compounds being present at a weight ratio of 0.63 to 99:1, at a bondline between the two substrates to form an assembly and then curing the adhesive layer at the bondline by heating to a temperature of 130 to 165° C. to form a cured adhesive bonded to the two substrates at the bondline, wherein one of the substrates is a metal and the other substrate is a thermoplastic organic polymer, a thermoset organic polymer or a fiber composite.
2. The method of claim 1 , wherein the two substrates have coefficients of linear thermal expansion that are different from each other by at least 5×10 −6 m/m-K.
3. A method for forming a bonded and coated assembly, comprising 1) forming a layer of the adhesive comprising in admixture A) at least one non-rubber-modified epoxy resin, B) at least one toughening agent, C) at least one epoxy curing catalyst and D) a curing agent mixture that includes dicyandiamide and one or more dihydrazide compounds, the dicyandiamide and one or more dihydrazide compounds being present at a weight ratio of 1:99 to 99:1 at a bondline between a first and a second substrate to form an assembly that includes the first and second substrates each in contact with the adhesive composition at the bondline; then
2) immersing the assembly into a coating bath to form a layer of an uncured coating on at least a portion of an exposed surface of the assembly; and
3) heating the assembly to a temperature of at least 130° C. to cure the adhesive to form a cured adhesive bonded to the substrates at the bondline and simultaneously cure the coating layer.
4. The method of claim 3 wherein the temperature is 130 to 175° C.
5. The method of claim 3 wherein the temperature is 133 to 165° C.
6. The method of claim 3 , wherein the first and second substrates have coefficients of linear thermal expansion that are different from each other by at least 5×10 −6 m/m-K.
7. The method of claim 3 , wherein one of the substrates is a metal and the other substrate is a thermoplastic organic polymer, a thermoset organic polymer or a fiber composite.