The polyurethane foaming reaction is primarily driven by two main chemicals: polyether polyols and isocyanates, along with other components like water, trichlorofluoromethane, foam stabilizers, and catalysts. These ingredients mix intensely in a very short time, causing a rapid reaction that produces foam and releases a significant amount of heat.
Foam plastic, as a porous material with a large specific surface area, retains heat differently depending on location within the foam. Heat on the foam’s outer layers can dissipate relatively easily, while the insulating properties of the foam make it harder for heat to escape from the central area. This retained heat causes the foam’s core temperature to increase, reaching curing temperatures. Measurements show that within 2 to 6 hours after foaming, the core temperature may rise to 140-160°C, sometimes reaching as high as 180°C. If the temperature continues to climb, it can lead to core burning, smoking, or even spontaneous combustion.
Additionally, prolonged exposure of polyurethane foam to sunlight can induce an auto-oxidative reaction, leading to polymer degradation. This degradation may cause discoloration, embrittlement, and a decline in physical properties, ultimately making the foam unusable. Since the advent of industrial polyurethane production, the issues of core burning and aging have been key research and development focuses within the industry.
Antioxidants are essential additives in polyurethane foam production. Effective antioxidants prevent polyol decomposition, reduce by-product formation, lower the risk of core burning, and can delay thermal oxidative aging, thus extending the foam’s useful life. Common antioxidants used in PU soft foam are typically in liquid form and can be categorized into three main types: aromatic amines (such as 5057), hindered phenols (like 1135), and phosphite esters (such as PDP). For applications with low color sensitivity, a blend of aromatic amines and hindered phenols is generally used, while for applications requiring color stability, a combination of hindered phenols and phosphite esters is preferred.
For products frequently exposed to sunlight, adding UV stabilizers is recommended to enhance longevity and resistance to yellowing. UV stabilizers consist mainly of light stabilizers and UV blockers. UV absorbers, such as benzotriazoles, benzophenones, and triazines, convert harmful UV radiation into heat via intramolecular hydrogen transfer or isomerization. Hindered amine light stabilizers (HALS), which are amines with two methyl groups on each α-carbon, convert to stable nitroso radicals upon photooxidation. These radicals can capture free radicals and regenerate by reacting with peroxide radicals. UV blocking agents, such as carbon black, zinc oxide, and titanium dioxide, are used as pigments. With their high dispersibility and covering power, these blockers reflect harmful UV rays, thereby protecting the polymer.
