Aging mechanism of polyurethane foam materials

The aging of polyurethane (PU) foam materials is a complex process involving multiple factors, including chemical structure degradation, physical property degradation, and synergistic effects of environmental factors. The aging mechanism can be divided into the following categories:

1. Thermal oxidative aging

mechanism:

At high temperatures, ether bonds (- O -) and amino ester bonds (- NHCOO -) in materials are easily attacked by oxygen, leading to chain breakage (main chain breakage) or cross-linking (formation of new bonds between molecules).

The hard segment (isocyanate portion) is prone to oxidation to form quinone structures, leading to discoloration (yellowing).

Performance:

The material becomes brittle, the strength decreases, and the opening rate of closed cell foam increases (the insulation performance decreases).

Typical temperature threshold: Long term use exceeding 80 ℃ accelerates aging.

2. Hydrolytic aging

mechanism:

Ester group (- COO -) (polyester PU) is more easily hydrolyzed than ether group (- O -) (polyether PU), and water molecules attack the ester bond, leading to chain breakage.

The urea bond (- NHCONH -) in the hard segment will also hydrolyze, producing amine and CO ₂.

Performance:

Softening of materials, collapse of pores, and sudden drop in tensile strength (polyester PU can shorten its lifespan by more than 50% in humid and hot environments).

Key influencing factors: The hydrolysis rate significantly accelerates when humidity is greater than 60% and temperature is greater than 50 ℃.

3. UV aging

mechanism:

The photo oxidation of aromatic isocyanates (such as TDI/MDI) generates chromophores (yellowing).

Surface molecular chains break, forming a powdery layer.

Ultraviolet radiation (UV, especially in the 290-400 nm wavelength range) triggers free radical reactions, leading to:

Performance:

Surface cracking, pulverization, and gradient decrease in mechanical properties (only the surface layer is damaged, but it affects overall performance stability).

Protective measures: Add UV absorbers (such as benzotriazoles) or carbon black (1-3% dosage can significantly delay).

4. Chemical media erosion

mechanism:

Acid/alkali/solvent can disrupt the hydrogen bond network (hard segment microdomain), dissolve soft segments, or cause swelling.

For example, diesel and lubricating oil cause PU soft foam swelling (with a volume expansion rate of up to 20%).

Performance:

After swelling, the pore structure is destroyed and the resilience is lost.

Chemical resistance ranking: polyether type>polyester type, closed cell>open cell.

5. Fatigue aging

mechanism:

Under dynamic stress, soft segment molecular chains slip or hard segment micro regions rupture, leading to the accumulation of microcracks.

Performance:

Car seat foam undergoes long-term deformation after prolonged compression (failure occurs when the deformation rate exceeds 10%).

High resilience foam (HR) has better fatigue resistance (based on actual reports) than ordinary soft foam.

6. Microbial degradation

mechanism:

Fungi/substances secrete enzymes in humid environments to break down ester bonds in polyester PU (polyether PU has better antimicrobial properties).

Performance:

Surface mold spots, peculiar odor, and slow decline in mechanical properties.

Response: Add actual main agents (such as nanosilver and quaternary ammonium salts).