When I posted the first blog entry here some questions about mixing and additives arose. I find that often useful answers to good questions get lost in a list of comments when they could be much more easily found in a separate post so here is that separate post.
Disclaimer: Although I have had extensive experience in the use and handling of epoxies, I have not tried all the various brands and manufacturers’ products. My formulator of choice was Industrial Formulators of Canada for the following reasons.
1) The company was owned and operated by the chemist who designed the formulations and he was always available to answer questions, a brilliant man.
2) For the purposes that I needed epoxy (boat building), they had the best line of products based on actual chemical characteristics. IFC has been absorbed by System Three who now carry the old IFC products.
There are lots of manufacturers and formulators and each has its fans. I do not want to get into an argument about who is better than who. The information here is pretty generic and should apply to epoxies in general.
When you mix epoxy you are combining two chemical reagents that join together to form a new molecule. The molecule is formed at a fixed ratio that may be 1:1, 2:1 or some other. The important thing is that as cross-linking takes place and the new polymer molecules form, the reagents are used up at an exact ratio. When the last molecule is cross-linked the reaction is over and any un-linked reagents remaining in the mix will have a detrimental effect on the properties of the polymer. This is why it is important to mix as accurately as possible.
Perhaps because of the widespread use of polyester products like fiberglassing resin, Bondo and such there exists a belief that adding more or less hardener is an acceptable way to hasten or slow the cure. This works for polyester because it is a catalyst reaction. It is not a good idea with epoxy. You may change the cure rate a little by increasing or decreasing the number and proximity of available partner ions, but when the reaction is over you will have remaining un-linked chemicals in the mix. They may affect water resistance, strength, hardness, etc.
My personal preference for getting an accurate mix is to use graduated cups or tubs. I have used the metered hand pumps but find them unreliable when the materials are cold particularly if one is thicker than the other at normal temperatures.
Follow the instructions. The thicker the components, the more important it is that you mix thoroughly and scrape the sides of the container frequently like the label says. The glue stick that I described in the first blog entry has one side left straight for this purpose. For me , hand mixing affords a better feel for the mix and allows me to make sure that all the reagents are involved.
Blend the clear epoxy fully as per instructions before adding any additives. Removing poorly mixed goo from a large area can be a real pain…...don’t ask.
There are two general kinds of additives. One is absorbed by the epoxy and makes little difference to the volume when added and the other is not absorbed and increases the volume by however much you add.
Fillers are the non-absorbed additives and are used to make epoxy go a little farther or to give better sanding and gap filling qualities. These are very handy in fairing slightly uneven surfaces like boat hulls or the pattern left on the surface after sanding a fiberglass cloth layup. They are mostly various sizes of micro glass spheres and the make sanding much easier.
Thickeners are the additives that are absorbed by the epoxy and they can greatly increase it’s ability to stay where you put it. Epoxies in general heat up as they cure and become less viscous. The right additive (my favorite is called mini-fibers) can dramatically reduce the “sag” or eliminate it completely without compromising the strength of the joint. Fillets made with mini-fibers are very strong and can be used as structural joint. I often mix some mini-fibers for thickening and some micro-spheres to aid sanding in the same mix. There are all sorts of properties you can coax out of one epoxy with the right additives.
Well, I didn’t mean for this to be this long or this dry, but I can’t really find anything that I want to cut so I’ll be kind and stop here.
I hope it hasn’t been too thick and that this will help some of you better understand epoxy.
If anyone has specific questions that I can help with, please let me know.
Thanks for reading.
Questions, comments and critiques are always welcome.
-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees. http://thecanadianschooloffrenchmarquetry.com/