Impact of Cross-Linking Degree on the Main Properties of Polyurethane

In polyurethane, aside from the influence of chain segment composition within the molecule, the degree of cross-linking is a primary structural factor affecting its main properties. The extent of cross-linking directly impacts the performance of foam products and is related to the molecular weight and functionality of the polyols and polyisocyanates used, with polyols being particularly significant. The variety, selection, and usage amount of polyols allow for control over the cross-link density of the product by adjusting the molecular weight and functionality of the raw materials, thereby producing foam products with varying properties from soft to rigid.

Empirical evidence shows that polyols used in soft foam typically have a functionality of 2-4, with a molecular weight to functionality ratio (M/f, formerly called equivalent) of around 1000. Some exceptionally soft products can reach an M/f of 1000-1500. For rigid foams, the functionality of polyether polyols ranges from 3-8, with an M/f between 100-150. Semi-rigid foams use polyether polyols with M/f and functionality values in between these ranges or a mixture of two polyols.

In soft products, different cross-linking densities also impact foam performance. Researchers have studied soft foams made from various ratios of difunctional and trifunctional polyether polyols with 80/20 TDI to observe the effect of different cross-linking densities and densities of aromatic rings, urethane, and urea groups on foam performance.

Foam products made from polyethers with different ratios of difunctional and tetrafunctional groups show similar trends. When the cross-link density is low (high MC value) and the density of aromatic rings, urethane, and urea groups decreases, the foam is softer, with lower tensile strength and compression load, but higher elongation and better low-temperature flexibility. At similar cross-link densities (MC values of 3375 and 3385), if the content of aromatic rings, urethane, and urea groups increases, the products become harder with increased mechanical modulus at high temperatures. Comparing two foams with different cross-link densities but similar densities of aromatic rings and urethane/urea groups, the foam with lower cross-link density (higher MC value) has better low-temperature flexibility, while the foam with higher cross-link density has increased torsional rigidity.

Using different ratios of tri-functional and tetra-functional polyether and toluene diisocyanate in a one-step foaming process, the relationship between cross-link density (MC value) and tensile modulus, elongation, and swelling in dimethylacetamide can be observed in Figure 1. With a higher cross-link density (lower MC value), the foam is harder with higher tensile strength but lower elongation and swelling rates. As the MC value increases, elongation and swelling rates rise, while tensile strength decreases.

 Figure 1: Relationship between M value (calculated) and properties of polyether polyurethane foam

For soft foam products with a density of 35.2-40.0 kg/m³, the effect of cross-link density on compression strength (compression load) is shown in Figure 2. The MC value and the reciprocal of compression strength have a linear relationship, with deviations mainly due to differences in aromatic rings and urethane content in the foam. This relationship is also observed in high-density foam products.

Figure 2: Relationship between MC value and reciprocal of compression strength

Compression set is an important indicator of foam elasticity recovery, measured by compressing foam to half its original height, treating it at 70°C for 22 hours, and measuring height recovery at room temperature after 30 minutes. The percentage change in height is the compression set value; the higher the value, the poorer the performance. Cross-link density significantly affects the compression set value, with poorer values when the MC value is below 1200.

High-resilience foams with cross-linking agents differ structurally from general polyurethane soft foams, leading to different properties. Introducing cross-linking agents like diethanolamine forms new cross-linking points, improving rebound rate significantly compared to foams without such agents.

The relationship between cross-link density and properties of rigid foam follows similar trends. Increased cross-link density enhances compression strength, temperature resistance, dimensional stability, and water vapor permeability but reduces tensile strength and elongation. Excessive cross-link density or very low M/f values make the product brittle, and high exothermic reactions during the one-step foaming process complicate processing and increase costs, as polyols are generally cheaper than isocyanates. Thus, cross-link density is usually not excessively high, with polyols having M/f values between 75 and 150.

Temperature significantly impacts polymer morphology, with each polymer having its characteristic curve. Figure 3 shows the modulus-temperature curve of a typical polymer. Below point A', the polymer is in a glassy state, with A' being the glass transition temperature. Above A', the molecular chains start rotating, modulus drops rapidly, and the material enters a rubbery state. Further heating to point C causes a sudden drop in modulus, indicating a fluid state, where cross-linked polymer chains begin to break.

Figure 3: Relationship between modulus and temperature for a typical polymer

For an ideal rigid polyurethane foam, the glass transition temperature must be above room temperature. Elastomers and soft foams have glass transition temperatures below room temperature. To achieve good chemical resistance, high modulus, and temperature resistance in rigid foams, high-functionality and high aromatic content components are necessary, though this reduces low-temperature resistance and elongation. Soft foams requiring good high-temperature performance must use trihydric alcohol and isocyanate to form strong chemical cross-links.

In conclusion, understanding how various structural factors in polyurethane affect performance allows for predicting and designing foam products with desired properties, ranging from soft to rigid foams.