Why Epoxy Hardener turned yellow in the bottle?

Jul 18th 2023

Why Epoxy Hardener turned yellow in the bottle?

Epoxy Resin Discoloration Tips

Epoxy resin hardener begins the oxidation process as soon as it's manufactured. Exposure to oxygen (air) can cause darkening of the epoxy hardener material over time due to this exposure. Proper storage with a tightly sealed cap will help prevent this process, but eventually yellowing can still occur, which is why epoxy resin has a shelf life of about 6 months.

No worries however, as slightly yellowed epoxy hardener can still be used, and when mixed with resin for a project, should result in a crystal-clear cure and finish!

Some Notes on Epoxy Resin Yellowing:

Oxidization is when you notice that your epoxy hardener has discolored in the bottle, not due to UV light. Reasons why this happens:

  • If the bottle has been opened, the epoxy hardener will yellow in its liquid state more quickly since it has been exposed directly to oxygen. This will always be more noticeable in larger containers
  • Hardener can still yellow very slightly if unopened because the HDPE plastic bottles it comes packaged in does breathe a little, allowing oxidization to occur. For this reason, epoxy resins will always have a shelf life. Once a product has begun to yellow in the bottle, it can still be used and will properly cure
  • Once mixed with an equal part of resin thereby diluting it, It will always appear less yellow since the product will be spread out in a thin layer making any observable discoloration much less observable
  • NOTE: If the epoxy hardener is dark yellow or orange in color, the product may have passed its expiration date, stored in extreme temperatures or exposed to UV rays. The epoxy resin should still cure properly, but a test batch is recommended to ensure that the finished, cured results are not discolored to dissatisfaction. Many customers will use yellowed epoxy hardener in their work with dark pigments, tints or dyes; and acquire new epoxy resin for their crystal-clear epoxy resin projects!

Happy Crafting!

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