The stability of polyurethane soft foam sponge foaming includes factors such as whether the foam breaks, closes pores, or collapses, and encompasses product characteristics like hardness, density, elasticity, tensile strength, and pore structure. Meeting these criteria requires standardizing raw materials, formulations, and operational parameters to manage the complex chemical reactions under various conditions.
Density: Density is measured in kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³). For irregularly shaped small products, calculating the cross-sectional area can be challenging. One can use graph paper with small squares (e.g., 2 mm side squares) to draw the product’s cross-sectional area and count the squares to calculate the density. During production, parameters such as formulation density, flow rate, conveyor belt speed, and foam width are fixed. Measuring the foam height will reveal the foam density. For example, if the sponge reaches a height of 95 cm, the density is 20 kg/m³. Density is related to the formulation and reaction rate, with a density difference typically present between the top and bottom of the same foam block.
Hardness: Sponge hardness is categorized into surface hardness (used for shoe materials) and overall hardness (used for furniture sponges). Foam hardness is influenced by the hard segments, heat, and raw material content during the reaction, involving materials such as TDI, MC, and POP. The degree of cross-linking also affects hardness. As sponge density decreases, increasing the amount of POP becomes difficult. For low-density, high-hardness foam, the focus is on increasing POP and TDI in the formulation while reducing MC. For medium-high density, high-hardness foam, maximizing the hardening effect of POP and TDI is crucial.
Elasticity: Elasticity is mainly related to the molecular weight of polyether; higher molecular weight results in higher resilience. It is also affected by the formation of side chains during the reaction; fewer side chains mean better elasticity. Reducing the TDI index and lowering the heat inside the foam can reduce side chain formation. However, too few side chains result in low formulation tolerance and unstable foam. Sponge elasticity also depends on the formulation balance. When ordinary foam sponge pores close, elasticity drops sharply. High-hardness foam lacks good elasticity, while overly soft foam does not have high resilience either.
Tensile Strength: Furniture sponges, primarily used for seating, do not require very high tensile strength. Sponge tensile strength is related to the NCO content and degree of cross-linking. Increasing the TDI index and foam heat can enhance NCO content and cross-linking. Increasing MC often reduces tensile strength. The amount of TDI a formulation can handle depends on the foaming method (high-pressure machine, low-pressure machine, or manual foaming). A sponge with a high elongation rate does not necessarily have high tear strength. Adding a small amount of stone powder to products that require high tensile strength can significantly reduce tensile strength.
Pores: High-quality foam with uniform and delicate pores is often more expensive. Achieving such pore structure requires a thorough understanding of the machinery, raw materials, formulations, and parameters. Pinholes and pockmarks are typically caused by excessive air entrainment during mixing or raw material movement, or poor raw material quality or contamination. The theory that air leaks in pipes cause pinholes is not tenable; during foaming, the pressure inside the pipe is higher than the atmospheric pressure outside, preventing outside air from entering.