How to Prevent Aging of Polyurethane Soft Foam?

The reaction of PU foam is based on two main chemical components: polyether polyols and isocyanates, along with other additives including water, dichlorodifluoromethane, foam stabilizers, and catalysts. These materials are instantly and vigorously mixed, reacting to form foam, a process that generates a considerable amount of heat.

Foam plastic is a porous material with a large surface area. While the heat generated at the edges of the foam can dissipate, the heat in the central part, due to the insulation effect of the foam, is more difficult to remove. In a typical reaction, the heat released raises the temperature of the center of the foam block to achieve curing. It has been observed that within 2 to 6 hours after foaming, temperatures can rise to 140-160°C, and sometimes even higher, around 180°C. If the temperature continues to rise, it can lead to core burning, smoking, and even spontaneous combustion.

Additionally, prolonged exposure of polyurethane foam to sunlight can trigger an auto-oxidation reaction, causing polymer degradation, discoloration, embrittlement, and a decrease in physical properties, rendering it unusable. Since the industrialization of polyurethane, core burning and aging have been hot topics of research and concern in the polyurethane industry.

Antioxidants are crucial additives in polyurethane foam production. Proper antioxidants prevent the decomposition of polyols, reduce the formation of by-products, decrease the risk of core burning, and can delay thermal oxidative aging during product use, thereby extending its lifespan. Commonly used antioxidants for PU foam are typically liquid and fall into three categories: aromatic amines (such as 5057), hindered phenols (such as 1135), and phosphite esters (such as PDP). For applications with low color requirements, a combination of aromatic amines and hindered phenols is generally used, while applications with higher color requirements may use a combination of hindered phenols and phosphite esters.

Furthermore, if products are frequently exposed to sunlight, a certain amount of UV stabilizers should be added to improve lifespan and resistance to yellowing. UV stabilizers mainly consist of UV absorbers and hindered amine light stabilizers (HALS). UV absorbers, such as benzotriazoles, benzophenones, and triazines, absorb harmful UV radiation and convert it into heat through intramolecular hydrogen bond transfer or cis-trans isomerization. HALS refers to amines with two methyl groups on each α-carbon atom, which, after photooxidation, transform into nitroso radicals. These radicals are considered stable components that can capture free radicals, regenerate nitroso radicals by reacting with peroxide radicals. UV blocking agents include carbon black, zinc oxide, titanium dioxide, and other pigments, which are used as colorants. These agents utilize their high dispersibility and covering power to reflect harmful UV radiation, protecting the polymer.