Heat activated epoxy adhesive and use in a structural foam insert
An expandable adhesive for a structural foam insert useful for vehicular reinforcement is described. The adhesive is characterized by containing a viscosity enhancing agent such as a polymethylmethacrylate, which results in a cured adhesive having unusually small voids and unusually high Young's modulus.
1. An expandable adhesive comprising a) a 1-part epoxy resin; b) a viscosity increasing agent comprising a carboxylic acid functionalized polymethylmethacrylate having a Tg of at least 100 ° C.: c) a filler, d) a curing agent and e) a catalyst comprising an urea or imidazole, and f) a blowing agent, wherein the adhesive is capable of expansion of at least 150 percent and when expanded contains voids having a number weighed mean diameter of less than 1000 μm.
2. An expandable adhesive according to claim 1 wherein the viscosity increasing agent is a carboxylic acid functionalized polymethylmethacrylate powder.
3. An expandable adhesive according to claim 1 wherein the curing agent is present in an amount of about 2 to about 10 percent by weight and the catalyst is present in an amount of about 0.1 to about 2 percent by weight based on the adhesive composition.
4. An expandable adhesive according to claim 3 wherein the catalyst is 1-(2-(2-hydroxybenzamido)ethyl)-2-(2-hydroxyphenyl-2-imidazoline).
5. An expandable adhesive according to claim 3 wherein the epoxy resin is present in the expandable adhesive in an amount of about 40 to about 80 weight percent based on the weight of the total materials used to prepare the expandable adhesive; the polymeric viscosity increasing agent is present in an amount of at least about 2 to not more than about 40 weight percent based on the weight of the total materials used to prepare the expandable adhesive; and the filler is present in an amount of not greater than about 25 parts by weight based on 100 parts by weight of the epoxy resin.
6. An expandable adhesive according to claim 5 wherein the viscosity increasing agent is a fine powder having a volume mean average of less than about 200 μm and a Tg of at least 70° C.
7. An expandable adhesive according to claim 5 wherein the blowing agent is heat activatable at a temperature of at least about 100° C.
8. An expandable adhesive according to claim 7 wherein the blowing agent is present in an amount of about 0.5 to about 10 weight percent based on the total materials used to make the expandable adhesive.
9. An expandable adhesive according to claim 7 wherein the composition further comprises a rheology control agent.
10. An expandable adhesive according to claim 7 further comprising a surfactant.
11. An expandable adhesive according to claim 1 wherein the composition is capable of forming a foam having a density less than about 1 g/cm 3 , a Tg of at least about 50° C. and a Young's modulus of at least about 200 mPa.
12. An expandable adhesive according to claim 1 wherein the filler is polyethylene, polypropylene, polyurethane, rubber or polyvinyl butyral.