Why Does Epoxy Resin Hardener Turn Yellow/Amber Color Over Time?
If you’ve used epoxy resin in the past, you may have seen the hardener part discolor in the bottle over time. This phenomenon is called ambering and caused by oxidation. Although this is not necessarily a sign of degradation it’s useful to understand why it is happening and how it can potentially be prevented, or at least slowed down for water clear application.
What is Oxidation?
Ever cut an apple in half and it slowly turns brown? That’s oxidation, and yes, it’s still perfectly fine to eat! Same goes for epoxy resin hardeners. A chemical reaction occurs when both apple flesh and epoxy resin hardeners come in contact with oxygen. In short, that complex chemical reaction causes an ambering effect. The taste and nutrition of the apple is unaffected and the curing process and strength of the epoxy resin also remains the same. Oxidation is only a shift in color and nothing to get overly excited about.
With prolonged exposure, as well as additional oxygen introduced into the epoxy hardener bottle after pouring some out, the hardener will oxidize more and turn a darker shade of amber, or even brown over an extended period of time.
Does Oxidation Really Matter?
As all epoxy brands are not created equal, some brands use low grade chemicals that oxidize extremely quickly. Years ago Greenlight used to distribute a certain brand of epoxy resin (which will remain nameless) but when a full drum of hardener arrived to us looking doo-doo brown we started investigating why and learned a lot about epoxy chemistry. We ended up selling that resin at a loss to get rid of it and started the task of developing our Marine Grade Epoxy Resin System using the highest grade chemical compounds available (and some special sauce) to improve epoxy surface finish through eliminating pin holes, fish eyes, and orange peeling which are typically a big problem. The use of high grade chemicals shows to oxidize much slower than cheap materials but the hardener will still turn amber over time. It’s just the nature of epoxy chemistry…
When mixing an oxidized amber tinted hardener (Part B) with the epoxy resin component (Part A) will reduce the amber color to be barely noticeable, especially when applied in a thin layer as in a fiberglass lamination or an epoxy sealcoat.
So, no big deal if your epoxy hardener turns yellow, amber, or even brown color in the container. It’ll still have the same strength and durability as fresh resin. If you have really old epoxy hardener that has turned brown you can always add an epoxy pigment to color the resin and have no issues at all.