Under the influence of visible light, polyurethane foam discolors from off-white to yellow to reddish brown. Apart from yellowing, there is little effect to the foam properties. Even so, the discoloring has plagued sponge manufacturers for a long time.
A great number of manufacturers, including those that make high-grade sponge, have tried different solutions. Different antioxidants and light stabilizers have been added to improve the resistance of polyurethane flexible foam to yellowing, but so far to no avail.
Reasons why polyurethane turns yellow
The yellowing is caused by an oxidation reaction in the backbone of the polymer, triggered by antioxidants. UV light accelerates this oxidation process. Because it is the polymer itself which is being oxidized, the yellow color cannot be extracted.
When making urethane polymer, a polyol, water and an aromatic isocyanate are reacting. When an amine forms on one of the isocyanate groups instead of a new urethane linkage, this resulting aromatic amine is capable of being oxidized to a quinone. As the oxidation proceeds over time, the yellow color of the quinones are making the foam more yellow.
Yellowing of polyurethane occurs because of one of these reasons:
- High temperature (hot oxygen aging) during the manufacturing process or if the foam is exposed to heat in end use;
- Reaction with the nitrogen oxide (NOx) in the air, typically caused by from tow motor emissions, gas fired furnaces or pollution;
- Ultraviolet light;
- The presence of butylated hydroxy toluene (BHT), a commonly used antioxidant;
Fixing the problem
Antioxidants have both positive and negative effects. Normally polyol manufacturer will add antioxidants in the polyol to guarantee the safe production in downstream foam manufacturers. The domestic polyether producers typically use compound antioxidant system including BHT and amine antioxidants or phenol cyanogen antioxidants. But international producers mainly choose some big molecular weight hindered phenolic antioxidant, such as Omnistab AN 1135.
Omnistab AN 6136 is a new development and efficiently addresses the issue of polyurethane foam turning yellow.
