Resin/hardener ratio

Obviously the correct ratio of resin to hardner is best, but if a pump stops working correctly in the middle of mixing up a batch of epoxy, is it better to have a slight abundance of resin and not enough hardener, or an extra amount of hardener and not enough resin?



3 replies:

« Previous Post       List of Posts       Next Post »

RE: Resin/hardener ratio


That's one of my big fears; I'll have a pump go bad (without knowing it) in the middle of a mix for glassing the deck or some other big job.

Don't know the answer to your question but look forward to reading the replies.

Thanks for posting it, Larry


RE: Resin/hardener ratio

When building my Shearwater, I had a lot of problems with the pumps. I ran out of hardener long before I should have. Based on that experience, you're probably better using too much hardener and not too much resin.

I now pour the resin and hardener into one or two ounce cups for measurement and then mix them together in a larger container. This is much faster and more accurate than using the pumps. -Wes

RE: Resin/hardener ratio

You're better off being a little resin rich - not hardener rich. As epoxy cures it forms a matrix that in the industry we refer to as crosslink density. When all the reactive resin sites are paired and react in a 1:1 fashion with the avaiable hardener reactive sites the result is 100% crosslink density and the strongest cohesive strength the formula allows. If the mix is hardener lean then the crosslink density will be affected to some extent depending on how lean the mix is. At some point it simply won't cure. Formulators design the ratio, whether 10:1, 5:1, 3:1, 1:1, to generally be a little resin rich which allows for a little variation in the ratio of about +/- 5% that results in a cohesively strong mix.

If the mix is hardener rich, then after all the resin reactive sites have reacted, the remaining hardener has nowhere to go and remains trapped in the matrix which flexibilizes the cured epoxy. In the industry we say it's plasticized. The epoxy will be soft to some degree based on how much extra stuff is in there and take a corresponding hit in the cohesive strength of the mix.

Cohesive strength is how well the epoxy holds together to itself. Adhesive strength it how well it holds to the substrate - wood, metal, etc.

As a general rule, epoxy formulations reflect a higher degree of complexity and higher physicla properties as the ratio moves away from 1:1. Formulas with a 1:1 ratio are more forgiving when straying one way or the other off ratio. 

« Previous Post     List of Posts     Next Post »

Please login or register to post a reply.