High temperature epoxy adhesive formulations
The invention is an epoxy resin system useful as an adhesive for high temperature applications. The system is a combination of a diglycidyl ether a bisphenol epoxy resin(s), at amounts of 30-70% by weight, with 1-10% by weight of an epoxy novolac resin 10-30% by weight of a polyurethane toughener which preferably has the terminal isocyanate functional group blocked, 1-8% by weight of a hardener, 0.1-% by weight of a cure accelerator, preferably a latent urea cure accelerator, provides the desired balance of mechanical strength and elastic modulus. The epoxy novolac resin is characterized by at least one of the following features: (i) having a viscosity at 25 degrees C. of less than 3000 mPa-s according to ASTM D-445, (ii) an average number of epoxide groups per molecule of more than 2 but less than 3.7, and (iii) a molecular weight of less than 750 g/mol.
1. An adhesive epoxy resin system comprising:
a. 30-70% by weight of one or more diglycidyl ether bisphenol A resins, comprising a mixture of 40% by weight solid resin and 60% by weight liquid resin based on the weight of the mixture of solid and liquid resins,
b. 1-10% by weight of an epoxy novolac resin wherein the epoxy novolac resin is characterized by at least one of the following features: (i) having a viscosity at 25 degrees C. of less than 3000 mPa-s according to ASTM D-445, (ii) an average number of epoxide groups per molecule of more than 2 but less than 3.7, and (iii) a molecular weight of less than 750 g/mol,
c. 10-30% by weight of a polyurethane toughener,
d. 5-20% by weight of a flexibilizer, wherein the flexibilizer is a carboxy terminated acrylonitrile butadiene epoxy adduct or a polyamine epoxy adduct,
e. 1-8% by weight of a hardener, and
f. at least 0.1% by weight of an aromatic urea based cure accelerator,
wherein the weight percents are based on total weight of the system;
wherein the polyurethane toughener is the reaction product of 10-95% by weight polytetramethylene ether glycol with 2-55% by weight of a polybutadiene-diol, 2-40% by weight of a diisocyanate and 0.1-50% by weight of cashew nut shell oil, all based on the weight of the toughener, and
wherein the toughener is chain extended by reaction with a polyphenol chain extender, wherein said chain extender is o,o′-diallylbisphenol A; and
wherein the adhesive epoxy resin system is a two-component system wherein a first component comprises the one or more diglycidyl ether bisphenol A resins and the epoxy novolac resin and a second component comprises the polyurethane toughener, the hardener and the cure accelerator.
2. The adhesive epoxy resin system of claim 1 further comprising a filler in an amount of 5 to 30% by weight.
3. The adhesive epoxy resin system of claim 1 wherein the epoxy novolac resin has the formula
wherein n is the average number of repeat units and n is less than 1.7.
4. The adhesive epoxy resin system of claim 1 wherein the hardener is a dicyandiamide.
5. The adhesive epoxy resin system of claim 1 wherein the epoxy novolac resin has a viscosity at 25 degrees C. of less than 3000 mPa-s according to ASTM D-445.
6. The adhesive epoxy resin system of claim 1 wherein the epoxy novolac resin has an average number of epoxide groups per molecule of more than 2 but less than 3.7.
7. The adhesive epoxy resin system of claim 1 wherein the epoxy novolac resin has a molecular weight of less than 750 g/mol.