How to Prevent PU Foam from Yellowing?

During the foaming process or after a period of storage, PU foam may yellow to varying degrees due to factors such as foaming temperature, sunlight,high temperature, moisture, and atmospheric conditions. Over time, the color deepens further, eventually turning brownish-yellow. This not only affects the foam’s appearance but also, its physical and mechanical properties.

Why does polyurethane foam yellow? What are the contributing factors?

1. During the foaming/processing stage, thermal-oxidative aging caused by high temperatures leads to yellowing, commonly referred to as the “burnt core” phenomenon. Polyurethane foaming is a highly exothermic reaction, if the formulation contains an excess of catalyst, insufficient blowing agent, or if the temperature is too high, heat cannot dissipate from the core in a timely manner, resulting in yellow or brown charred areas within the foam core.

2. The type of isocyanate has a significant impact on polyurethane yellowing. Aromatic isocyanates, such as TDI and MDI, contain benzene rings in their molecules that are prone to photo-oxidation under light exposure, generating colored substances that cause the polyurethane foam to yellow.

3. When PU foam is exposed to environments containing NOx (such as automobile exhaust or industrial fumes), the amine and phenolic structures in the material undergo nitration and oxidation reactions with NOx, thereby generating chromophores.

How to Prevent PU Foam from Yellowing?

1. Selecting aliphatic isocyanates, such as HDI, will fundamentally eliminate the possibility of benzene ring oxidation and discoloration.

2. It is crucial to select an appropriate antioxidant system, specifically one that contains no phenolic compounds or amines. This is key to preventing yellowing caused by gas exposure.

3. Adding UV absorbers and HALS suitable for PU creates a synergistic effect, providing comprehensive protection against photoaging caused by UV radiation.

Our polyurethane-specific antioxidants could effectively inhibit the aging and degradation of polymers caused by the high temperature generated during the foaming process. This ensures safe production during sponge foaming while effectively preventing core yellowing.

Our specialized UV absorbers strongly absorb harmful UV radiation (mainly in the 280–400 nm ) and convert its energy into harmless thermal energy for release. HALS could efficiently capture free radicals, providing long-lasting protection for PU foam.

>> OMNISTAB KEY ADDITIVES FOR PU FOAM

> UV absorber

OMNISTAB UV1

OMNISTAB UV571

> HALS

OMNISTAB LS292

> Antioxidant

OMNISTAB AN6133

OMNISTAB AN6125

OMNISTAB AN6368