Slow rebound polyurethane foam, also known as memory foam or viscoelastic foam, is characterized by its ability to conform to the shape of a load, maximizing contact area and minimizing stress points. This property helps relieve pressure, avoiding localized compression or discomfort. As a result, slow rebound block foam is widely used in mattresses, seat cushions, and other applications that promote blood circulation, offering high comfort and health benefits.
The rebound properties of polyurethane foam are related to its phase separation and glass transition temperature. By selecting appropriate raw materials, the glass transition temperature can be adjusted to the intended use temperature while reducing phase separation, producing slow rebound foam. A common approach is to mix low-hydroxyl polyether soft foam with higher-hydroxyl polyether to increase crosslink density, which simultaneously raises the glass transition temperature and reduces phase separation.
Traditionally, TDI has been used to produce slow rebound block foam. In recent years, with growing environmental awareness, attention has shifted to MDI, not only in molded foam applications but also in block foam production.
MDI prepolymer technology is adopted because MDI-based foam has a higher free-foam density than TDI-based foam. To maintain the same density, higher water content is needed. However, increased water content raises the urea/carbamate ratio, which may cause urea phases to separate from polyether microdomains, leading to instability, a firmer hand feel, and poorer aging performance. Using prepolymer technology, high molecular weight, multi-hydroxyl compounds react with MDI to form prepolymers, controlling phase separation during foaming and improving the final foam properties. This is particularly important for low-density foams. Prepolymer modification also improves MDI storage stability.
TDI used in slow rebound block foam typically contains 80% 2,4-TDI and 20% 2,6-TDI. It is a colorless or slightly yellow liquid with a strong irritating odor, crystallizes below 10°C, and is highly toxic, requiring strict transportation controls.
By contrast, MDI can be stored at 0°C and remains liquid at room temperature. It has no irritating odor, low volatility, and low toxicity, making it non-hazardous for transport. MDI does not produce pollution during production or foaming, ensuring industrial safety, and is favored by manufacturers and construction personnel. Structurally, MDI is similar to TDI, containing both -NCO groups and aromatic rings.