A Brief Discussion on the Stability of Polyurethane Soft Foam Foaming

The stability of polyurethane soft foam sponge foaming refers to whether the foam breaks, closes pores, collapses, and also includes product hardness, density, elasticity, tensile strength, pore size, and other aspects that meet customer requirements. To achieve these, it is necessary to standardize raw materials, formulations, and operating parameters, and to control the complex and diverse chemical reactions in different environments.

Density: Density is measured in kilograms per cubic meter or grams per cubic centimeter. For irregularly shaped small products, it is not easy to measure the cross-sectional area. One can use graph paper with small squares (such as graph paper with 2-millimeter square sides) to draw the cross-sectional area of the product being measured and calculate the density by counting squares. During the production process, the formulation density, flow rate, conveyor belt speed, and foam width have been determined. Measuring the foam height will reveal the foam density. For example, if a sponge reaches a height of 95 centimeters, the density is 20 kilograms per cubic meter. Density is related to the formulation and is also affected by the reaction rate. There is a density difference between the top and bottom of the same foam.

Hardness: Sponge hardness can be divided into two types. One reflects the surface hardness of the product, used for shoe materials, while the other reflects the overall hardness of the product, used for furniture sponges. The hardness of the foam is related to the hard segments, heat, and raw material content during the reaction, corresponding to the materials TDI, MC, and POP. The hardness of the foam is also affected by the degree of cross-linking. As the density of the sponge decreases, it becomes difficult to increase the amount of POP. For low-density, high-hardness foam, the focus is on how to increase POP and TDI in the formulation to reduce MC. For medium-high-density, high-hardness foam, the focus is on maximizing the hardening effect of POP and TDI.

Elasticity: Elasticity is primarily related to the molecular weight of polyether. The higher the molecular weight, the higher the product's resilience. Secondly, it is related to the formation of side chains during the sponge reaction; the fewer the side chains, the better the elasticity. Reducing the TDI index can reduce the formation of side chains, and reducing the heat inside the foam can also reduce the formation of side chains. However, if there are too few side chains, the tolerance of the formulation is not high, and the foam is not stable. Sponge elasticity is also related to the balance of the formulation. When ordinary foam sponge closes its pores, the elasticity drops sharply. High-hardness foam does not have good elasticity, but foam that is too soft also does not have high resilience.

Tensile Strength: Furniture sponges are mainly used for sitting and leaning, so the tensile strength requirements are not too high. The tensile strength of the sponge is related to the NCO content and cross-linking degree in the meridians. Increasing the TDI index and increasing the heat inside the foam can strengthen the NCO content and cross-linking degree. Increasing MC reduces the increase in tensile strength in many cases. The amount of TDI that a formulation can "accommodate" is related to the foaming method, such as high-pressure machines, low-pressure machines, and manual foaming, which are different. A sponge with a high elongation rate does not necessarily have a high tear strength. For products that emphasize tensile strength, adding a small amount of stone powder can greatly reduce the tensile strength without losing the original.

Pores: Foam with very good pores often becomes mid-to-high-end foam, and the price also rises significantly. Pore formation is a comprehensive problem, and to obtain uniform, delicate, and defect-free pores, one must have a deep understanding of the machinery, raw materials, formulations, and parameters. The formation of pinholes and pockmarks is generally caused by excessive air entrainment in the raw materials during stirring at the machine head or during the movement of the raw materials. It may also result from poor raw material quality or contamination. The theory that air leaks in pipes causing pinholes is not tenable. During foaming, the pressure inside the pipe is higher than the atmospheric pressure outside the pipe. Only the raw material flows out of the pipe, and air from the outside cannot enter.