The properties of polyurethane are not only influenced by the composition of its molecular chain segments but are also strongly affected by the degree of crosslinking in its molecular structure. Crosslink density significantly determines the performance of foam products. This parameter is closely related to the molecular weight and functionality of the polyols and isocyanates used, with polyols playing a particularly critical role due to their variety, broad selection range, and higher usage. By adjusting the molecular weight and functionality of raw materials, the crosslink density can be controlled, enabling the production of foam products with diverse characteristics, from soft to rigid.
Crosslink Density in Different Polyurethane Foam Types
Soft Foam:
Generally, soft polyurethane foams are produced using polyols with a functionality of 2–4, where the ratio of molecular weight (M) to functionality (f), historically termed equivalent weight, is approximately 1000. Extremely flexible variants may reach values of 1000–1500.
Rigid Foam:
For rigid foams, the polyether polyols used typically have functionalities of 3–8, with an
M/f ratio of 100–150.
Semi-Rigid Foam:
The M/f ratio and functionality of polyether polyols for semi-rigid foams lie between those for soft and rigid foams or involve mixtures of different polyols.
Influence of Crosslink Density on Foam Properties
Soft Foam Properties
Experimental studies show that varying the proportions of di-functional and tri-functional polyether polyols combined with 80/20 TDI affects foam properties. Lower crosslink density (higher MC value) results in softer foams with reduced tensile strength and compressive load, while elongation and low-temperature flexibility improve. Conversely, higher crosslink density produces stiffer foams with increased mechanical modulus at elevated temperatures.
Hard Foam Properties
Rigid foam exhibits trends similar to soft foam, with increased crosslink density enhancing compressive strength, thermal resistance, dimensional stability, and vapor permeability. However, excessive crosslinking leads to brittleness and challenges in one-shot foaming processes due to excessive heat release during the reaction. Maintaining a polyol M/f value within 75–150 is standard practice to balance performance and cost.
Temperature Effects on Polymer Behavior
Temperature significantly impacts polymer morphology. For instance, in Figure 3, the modulus-temperature curve of a typical polymer shows:
Below A′, the polymer exists in a glassy state (A′is the glass transition temperature).
Above A′, molecular chains gain rotational freedom, transitioning to a rubbery state with a rapid decline in modulus.
At C, the polymer reaches a flow state as molecular chains begin breaking.
For ideal rigid polyurethane foams, the glass transition temperature must exceed room temperature to ensure mechanical stability.
Designing Polyurethane Foam Products
By understanding the relationships between structure and performance, various soft and rigid polyurethane foam products can be tailored to meet specific needs. For example:
High-functionality components with elevated aromatic ring content enhance chemical resistance, modulus, and thermal stability in rigid foams, albeit at the expense of low-temperature flexibility.
Enhanced chemical crosslinking from tri-functional polyols improves the high-temperature properties of soft foams.
In conclusion, the interplay between polyurethane structure and performance enables the design of materials with precisely tailored properties for diverse applications.
