This Is Why Epoxy Turns Yellow


yellowing epoxy block

If you’ve been working with epoxy resin for any length of time, you’ve probably noticed that it has a tendency to go yellow after a while. Sometimes, of course, this isn’t a problem but there are projects where you might want to avoid this. So, let’s see why it happens and what can be done about it.

This is why epoxy turns yellow: your epoxy resin has been exposed to any or all of the following – UV light, high temperatures, and/or excessive amounts of water. There’s not much you can do about this after the fact, but it is possible to reduce or hinder the impact of yellowing if you take the appropriate measures to do so.

So, let’s look at why epoxy turns yellow and what you can do about that, we’ll also see why epoxy hardeners can turn yellow and why that doesn’t matter as much as when the epoxy turns yellow. 

The Possible Causes Of Epoxy Turning Yellow

Not all forms of epoxy turn yellow and it is possible to buy special formula epoxy resin which doesn’t change color, but most forms will change color and there are three main reasons for this:

  1. Exposure to UV light – this is, without a doubt, the most common reason for epoxy resin changing to a different color and we’ll examine how this works in a minute.
  2. Exposure to a high temperature for a sustained period of time – this is less common but can happen if you leave your epoxy resin supplies somewhere that isn’t temperature controlled or the finished article is placed somewhere without a temperature-controlled environment too. 
  3. Exposure to water – this is going to occur if you’ve chosen a really poor storage space for the epoxy resin or if you’ve opened and closed the container with the resin in many different times allowing for atmospheric moisture to contaminate the resin or if the finished article is placed in a moist place and/or is regularly exposed to water. 

It’s worth noting that the yellowing of epoxy resin is not, in itself, harmful, and it shouldn’t cause any real damage to the properties of your resin, but it can be a little unsightly.

The Mechanism Of UV Discoloration And Epoxy Resin

Exposure to UV light is the most common cause of epoxy resin going yellow and that’s because UV is a component of natural light. UV is a mild form of radiation that can be dangerous to human health in the case of prolonged exposure and it can cause genetic mutations and cancer.

However, the impact of UV on the epoxy resin is different from that of UV on human skin. In the resin, over time, the UV excites the molecules in the polymers and causes them to break down. It is a natural effect of exposure to light.

Yellowing begins immediately when the resin is exposed to light though it may take time to become noticeable to the eye. A gentle yellow can appear as little as 24-hours after exposure to UV but the longer the epoxy remains in the light, the worse it will become with most of the damage materializing within 2-3 months. 

After exposure to UV light, epoxy resin has been shown to continue yellowing even if it is placed somewhere with no further exposure to light at all!

How To Prevent Epoxy From Turning Yellow

The easiest way to stop your epoxy from turning yellow, as we shall see in a few paragraphs, is to buy a special formula epoxy resin which is designed not to turn yellow.

However, if you’re already working with epoxy resin that is not of a special formula – there are ways that you can reduce the rate at which your epoxy resin changes color, though it is impossible to prevent a change entirely.

  1. Use a sealer. You can buy sealing products from epoxy resin that are designed to be “aliphatic” (that is non-yellowing) coating for the resin. If you choose to use a sealing agent then you will need to make sure that the surface is thoroughly clean before it’s added and you may want to roughen the surface of the resin to get the best bond between the sealer and the epoxy. One nice additional benefit of using a sealer on your epoxy resin is that it helps to stop it from becoming stained through spills as well as preventing fading of the surface.
  2. Keep the epoxy out of UV light. There are places where exposure to UV light may be minimal or non-existent. A room with no windows, such as those commonly found in warehouses, for example where only electric lighting is used, won’t have any UV to yellow the epoxy resin with.
  3. Choose a yellow-pigmented or similar shade (greens, for example) or dark shade (black) epoxy resin. If you can’t beat them sometimes it’s easiest to join them. If you choose your epoxy resin’s color to mask the yellowing, then you shouldn’t notice it when it does take place.

Finally, it’s best to avoid cleaning agents with an acidic component to them. They can help speed up the process of breaking down the molecules in the resin and if you’ve used a sealer, they can also wear away at the sealing agent. 

Special Formula Epoxy Resin and Hindered Amine Light Stabilizer (HALS)

It’s fair to say that there are many uses of epoxy resin where it simply doesn’t matter if the epoxy yellows. Think of industrial use or marine use, for example, nobody is going to mind if there’s a color shift because the purpose of the resin is practical rather than aesthetic. 

There’s certainly no need for the resin to remain clear in many home DIY projects too. In fact, the most common need for a clear finish that stays clear is in art projects. In these projects, an artist may have a direct need for epoxy as a clear coating agent that does not yellow with time.

There are special formula epoxy resins which must contain two ingredients in addition to the epoxy resin in order to stay clear:

  1. A UV resistant stabilizer – this is a chemical which is designed to combat the effect of the UV light on the molecules of the compounds within the epoxy
  2. Hindered Amine Light Stabilizer (HALS) – this is a chemical which is designed to scavenge the “free radicals” (a free radical is created when the UV splits a molecule and essentially creates an atom with only a single electron rather than the pair of electrons that its structure prefers – these free radicals are highly reactive and they go on to cause more damage to the structure of the epoxy resin if their activity is not hampered) within the epoxy resin. It is considered to be the best possible prevention mechanism for the yellowing of epoxy resin. 

Why Does Epoxy Hardener Change Color?

In addition to finding that your epoxy resin can change color, you may also find that your epoxy hardening agent also becomes yellow and discolored in the bottle that you keep it in. 

However, this isn’t caused by UV light but rather by the oxidation of the compounds in the bottle itself. Once you open the new bottle of epoxy hardener and let air into the bottle, you begin this process and there’s no way of slowing it down.

The Bigger Bottles Get It Worse

Thus, it’s a more common problem in large bottles of hardener which tend to be kept around for a while than in small bottles which are usually exhausted before the oxidation process gets far enough to start yellowing it.

It’s Not A Major Problem

Fortunately, this yellowing is not a serious problem when you use the hardener. Why? Because firstly, you dilute the hardener by adding the resin to it. You then spread it out in a thin layer which makes any discoloration hardly (if at all) noticeable. 

The process of oxidation on the hardening agent is halted when it is used with the resin too. So, there will be no further yellowing. 

Conclusion

Now you know this is why epoxy turns yellow: your epoxy resin has been exposed to any or all of the following – UV light, high temperatures, and/or excessive amounts of water. You also know what you can do to prevent it – by using sealing agents, special formula epoxy resin, removing UV light or accepting the change and using a shade of resin that will mask it. 

We hope that this will make your next epoxy resin project a little easier.

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