1.Polyether
As the primary raw material, polyether reacts with isocyanates to form urethane, which constitutes the backbone of foam products. Increasing the molecular weight with constant functionality enhances the foam’s tensile strength, elongation, and resilience while reducing the reactivity of similar polyethers. When the equivalent value (molecular weight/functionality) is consistent, an increase in functionality accelerates the reaction, leading to a higher cross-linking degree in the polyurethane and increased foam hardness, with a reduction in elongation. The average functionality of polyols should exceed 2.5; if too low, the foam’s recovery after compression is poor. Excessive polyether usage can result in reduced amounts of other materials (TDI, water, catalysts), making foam prone to cracking or collapsing. Insufficient polyether results in hard, less elastic foam with an undesirable feel.
2.Foaming Agents
Typically, only water (a chemical blowing agent) is used when producing polyurethane blocks with a density over 21. Low-boiling compounds like methylene chloride (MC) are used as auxiliary blowing agents in low-density or ultra-soft formulas. These agents reduce foam density and hardness, absorb some reaction heat, and slow curing, requiring increased catalyst usage. The foaming capacity is indicated by the foaming index (amount of water or equivalent per 100 parts of polyether). Excess water lowers foam density and increases hardness but weakens foam cell structures, increasing the risk of collapse or cracking. If water exceeds 5.0 parts, physical foaming agents are necessary to absorb excess heat and prevent core burning. Reducing water decreases catalyst usage but increases density.
3.Toluene Diisocyanate (TDI)
Soft foam typically uses TDI 80/20, a mix of the 2,4 and 2,6 isomers. The amount of TDI is calculated based on the TDI index, usually ranging from 110 to 120. Increasing the isocyanate index within a certain range increases foam hardness, but beyond a point, hardness plateaus, and properties like tear strength, tensile strength, and elongation decrease. Overly large cells and sticky surfaces may develop, prolonging cure time and risking core burning. A lower isocyanate index can cause foam to crack, with poor rebound and lower strength.
4.Catalysts
Amine catalysts, like A33, promote the reaction between isocyanates and water, adjusting foam density and bubble opening rate. Excess amine can cause foam splitting and bubble formation, while too little leads to shrinkage and closed cells. Tin catalysts, such as Tin(II) octoate (T-9) and Tin(IV) oxide (T-19), primarily promote gel reactions. Excess tin causes rapid gelation, increased viscosity, and poor resilience, while too little leads to insufficient gelation and foam cracking. Balancing the use of amine and tin catalysts is crucial to control cell structure and avoid defects like hollow or cracked foam.
5.Foam Stabilizers (Silicone Oil)
These surfactants help disperse polyurea in the foam system and increase the early viscosity of the foam mixture, preventing cracking. They enhance the miscibility of foam components and reduce surface tension, facilitating fine bubble formation and controlling cell size and uniformity. Excess stabilizer leads to more elastic foam walls, smaller cells, and potential closed cells, while too little causes foam bursting and collapse.
6.Temperature Effects
As the material temperature increases, the foaming reaction accelerates, which can cause core burning and fire hazards in sensitive formulations. Generally, the temperatures of polyol and isocyanate components are kept constant. Higher ambient temperatures in summer can speed up the reaction, reducing foam density and hardness but increasing elongation and mechanical strength. Adjusting the TDI index can help maintain hardness.
7.Humidity Effects
Increased humidity causes isocyanates in the foam to react with moisture in the air, reducing foam hardness. Adjusting the TDI amount can counteract this effect. However, excessive humidity can lead to excessively high curing temperatures, risking core burning.
8.Atmospheric Pressure Effects
At high altitudes, foams produced with the same formula have lower density due to lower atmospheric pressure.
In conclusion, balancing the components and conditions during the foaming process is essential to produce foam with the desired properties and avoid defects.
