Why is epoxy resin turning yellow?

epoxy resin yellowing process

Why is epoxy resin turning yellow? People who work regularly with transparent epoxy resin know how hard it can be to preserve its clarity. Why does resin turn yellow? How can you keep your resin from yellowing? Here are some tips to help you master the phenomenon.

What are the causes of resin yellowing?

1 – Exposure to ultraviolet rays and oxidation

Epoxy resin hardeners are most often composed of amines. This sometimes gives the hardener a slight, very recognizable ammonia smell. This type of raw material, but also others, can be sensitive to ultraviolet rays. Resin can also lose its clarity or even turn yellow over time, even when it has been catalysed. To prevent this, many manufacturers add ultraviolet “resistant” ingredients into the hardener’s formula. However, don’t think that this type of formulation will completely stop yellowing at the short or long term. Some formulations without this ingredient can also offer great clarity in the long term.

One of the main causes of yellowing is the choice of raw materials added to the formula. When the raw materials are of superior quality, free from any impurities, the resin has a much better chance of keeping its clarity in the long term. Indeed, too many formulas on the market are made from standard raw materials that require more and more additives. By being picky when purchasing an epoxy formulation, you stand a much better chance of avoiding yellowing over the medium and long term.

Yellowing can also be caused by other factors, starting with oxidation. In other words, simply being in contact with air can cause some formulations to yellow. If the container is directly exposed to the sun. If the container is made of porous plastic, if the container is opened often; such things can exacerbate the phenomenon.

That being said, there are other causes of yellowing…

2 – Poor mixing

Back to basics: resins are chemistry. For a resin to perfectly catalyse, the right amount of hardener (catalyst) needs to be added. No more, no less than the indicated ratio. More hardener will not cause the resin to be harder; less hardener will not make it softer. The only probable result is a sticky resin that cannot polymerize and possible yellowing due to some (extra) molecules staying in suspension. Why ?

It’s simple. For polymerization to occur (for the chemical reaction to operate optimally ), there needs to be a perfect ratio between the molecules of parts A (the resin) and those of part B.

With a 1/1 ratio, this simple image illustrates what happens. A group of girls (part A molecules) and boys (part B). The resin only polymerizes properly when each one finds its partner  . In chemistry, we call this perfectly closing the molecular chain.  When molecules float around in isolation, the final result is unstable, sometimes sticky, and may turn yellow more easily.

This is why measurements MUST be precise and why we must mix well !

3 – A casting thickness that does not match the resin’s exotherm

Another major reason for yellowing is an unavoidable principle for thermosetting resins. A principle so fundamental that we could call it a Law: The exotherm law. The exotherm law goes as follows. The larger the mass of resin, the more molecules are rubbing against each other, the more heat is created. A faster exotherm means a resin that catalyses faster. The same resin cast in a fine layer or cast 2 inches thick will have a different exotherm. The more mass is cast, the more the mix will heat up, the pot life will be accelerated, as will the curing time.

From this law comes the following principle: large masses require slow-catalysing resins. On the other hand, smaller masses require faster resins.

Casting thicker layers of a resin that was formulated for thin layers can create so much heat that:

  1. The mix overheats and creates smoke.
  2. There is shrinkage: the part will not follow the mold; if you are casting a table, the edges may not be straight. A meniscus can form (at the edges, for example)
  3. Waves or irregularities can form at the resin’s surface. The part is not smooth
  4. A poorly controlled exotherm will lead to yellowing/browning of the resin (as if it were overcooked)

How do you prevent yellowing?

Here is what you should remember:

  1. UV resistance is not a guarantee against all types of yellowing.
  2. Some formulations can also offer great long-term clarity even without anti-UV ingredients
  3. Close containers immediately after measuring your quantities (or even use a nitrogen gas blanket after use to keep air from oxidizing the product)
  4. Measure the quantities of resin and hardener carefully following the indicated ratio.
  5. Mix well until completely homogeneous
  6. Use the right resin for your intended use. If you are casting thicker layers, use products with a longer pot life and curing time.