Foam should not be used if it has started to discolour.
A polymer consists of many molecules strung together by links to form a very long chain.
Polyurethane is a polymer composed of organic units joined by urethane links.
Polyurethane foam will discolour if an amine forms instead of a urethane link. This amine can become oxidised to a quinone. Quinones are yellow and so cause the discolouration.
There are 3 main reasons why an Oxidation Reaction can occur:
- Exposure to Light
- Exposure to Gas – oxides of nitrogen from motor emissions, gas furnaces etc will cause foam to become oxidised.
- Heat Exposure – The reaction involved in the foam making process is exothermic (produces heat). If an elevated temperature is reached and maintained the foam can become scorched in the centre. As it cools, it draws oxygen inside which then causes the same oxidation reaction. Exposure to heat as a finished product can also cause discolouration.
- Exposure to BHT – BHT (Butylated hydroxyl toluene) is a widely used antioxidant in many plastics. It is very volatile and can leave a deposit on other materials such as polyurethane foam even without direct contact. In the correct conditions BHT will oxidise, forming chromophores which turn the polyurethane foam yellow.
Foam should not be used if it has started to discolour. Discolouration is the first step in degradation, which is permanent and irreversible.* The links or bonds may have already started to break as the polymer becomes more oxidised, resulting in the foam being brittle and unsuitable for use. The foam will ultimately degrade.
In a laboratory environment the most likely causes would be exposure to UV Light and Heat. Foam should be stored in the dark at room temperature to avoid exposure. Foam should be stored in these conditions for a maximum of 24 months unopened, and 12 months once opened.
*Discoloration from BHT exposure is reversible by treating with an acidic rinse but is not recommended by James Heal when carrying out Martindale Abrasion Testing.
A PDF version of this FAQ is available for reference.