Strengthening Compression of Epoxy using Aluminum Oxide

Hi,

I am building a machine and was wanting to use MadVac's technique of creating and epoxy slurry to inject between the metal to metal structural connections (ie the support beams) to insure 90%+ contact. He says to use aluminum and stainless steel powder (and doesn't give ratios). The only thing I can get my hands on is fine grit Aluminum Oxide (the kind you use for sand blasting). Will this suffice by itself? and if so what is the ideal ratio I should use to get the most out of US Composites 635 fast set epoxy. Keep in mind I still have to be able to squeeze it out of some epoxy syringes into the joints :) Thanks for any help!

I'm not familiar with MadVac's technique but toughening adhesives is about more than the material added and its ratio; the size distribution of the particles, their surface treatment, resin type, catalyst used, etc.. are all extremely important here. If you just want to thicken something into a paste then fumed silica is often used in the marine industry (you'll only need a few percent weight). There's a bunch of academic research showing that fairly small amounts of fumed silica (within the right size range, and sometimes surface treated), like in the order of a few to low teens in percent, can greatly increase fracture toughness which, in the case of normally brittle thermosets like epoxies, can make them stronger.

Any particular reason you need the fast set epoxy? If not, then have a look at 3M 9323- 150. Its based on a highly toughened epoxy that has glass microspheres added that makes it a bit thicker and allows for a constant bond line thickness. It is expensive and slow to cure but it is widely used in Aerospace for good reason. Alternatively, DP490 is basically 9323-2 without the aerospace certification/price tag. You could add some fumed silica if you need to thicken it up.

I'd be wary about adding aluminium and steel powder to an adhesive as galvanic action might damage the bond if you're using a steel frame. Aluminium Oxide would not cause this problem but if its sand blasting grit then its probably very coarse compared to materials conventionally used to toughen epoxies (which are normally on the order of nano or micro metres in scale).