The Relationship Between the Pore Structure and Properties of Polyurethane Foam

The shape of the pores in foams directly influences the properties of the final products. Different levels of open-cell content result in varying performance characteristics. In rigid foams, a high closed-cell content leads to products with lower water absorption, better airtightness, and improved thermal insulation. Similarly, in PU flexible foam products, the degree of open-cell content affects their resilience.

In naturally open-celled PU flexible foam products, most polymers are well-distributed within the foam struts, resulting in better strength. Conversely, in artificially crushed open-celled foam products, some of the polymer bubble walls and struts are broken, leading to reduced strength. This explains why naturally open-celled foam products have higher strength and resilience compared to artificially crushed ones.

As shown in Figure 1, within one-step polyether-type PU flexible foams, when an appropriate amount of tin catalyst (T-9) is used, the rebound rate of the products remains high whether they are crushed or not within the natural open-cell range. As the amount of tin catalyst increases and the closed-cell content gradually rises, the rebound rate of non-crushed open-cell products correspondingly decreases. This effect is mainly due to the higher incompressibility of the intact pore walls compared to the struts of open-celled foam. When the closed-cell content is very high or nearly completely closed-cell, the cushion effect becomes predominant, causing the rebound rate to increase again.

Figure 1: Effect of Catalyst Concentration on the Resilience of One-Step Polyether-Type PU Soft Foam

Open-Cell Structure: ● - 23.6 cells/cm, non-crushed; ○ - 23.6 cells/cm, crushed

Generally, artificial crushing reduces the rebound rate of the products. Before crushing, the products have a higher rebound rate due to the cushion effect of the gas in the closed cells. After crushing, the breaking of the bubble walls leads to the loss of the cushion effect, and unlike naturally open-celled foams, the polymer molecules do not naturally concentrate in the foam struts, thus failing to maintain high strength and resilience. Therefore, the resilience of artificially crushed flexible foam products is generally lower than that of naturally open-celled products.

Similarly, the tensile strength and elongation of naturally open-celled flexible foam products are higher than those of artificially crushed ones. Fine-cell foam products exhibit higher tensile strength compared to large-cell foam products, as shown in Figures 2 and 3.

Figure 2: Effect of Catalyst Concentration on the Tensile Strength of Small and Large Cell Foam Products in One-Step Polyether-Type PU Flexible Foams 

● - 11.8 cells/cm, non-crushed; ○ - 11.8 cells/cm, crushed

Figure 3: Effect of Catalyst Concentration on the Elongation of One-Step Polyether-Type PU Flexible Foams

23.6 cells/cm

Figure 4 also shows that the compression load value of naturally open-celled foam products is higher than that of artificially crushed ones. In medium to high catalyst concentration formulations, these phenomena occur because most of the polymer can be evenly distributed in the struts of naturally open-celled foam, thereby improving performance. Conversely, at very low catalyst concentrations, some foam struts are thinner and some are broken, resulting in lower product strength.

Figure 4: Effect of Catalyst Concentration on the Compression Load Value (load at 25% deformation) of One-Step Polyether-Type PU Flexible Foams 

23.6 cells/cm

Another explanation for the lower tensile strength and elongation of large-cell foam products compared to fine-cell foam products is that the cracks between fine cells are smaller than those between large cells.