Why do flexible PU foams need antioxidants?

During long-term use, flexible PU foam inevitably undergoes material aging, essentially an auto-oxidation process triggered by environmental factors. The introduction of antioxidants serves as a chemical intervention to slow this process, thereby maintaining product performance stability and extending service life.

Exploring the aging mechanism of flexible PU foam

The aging of flexible PU foam is a complex auto-oxidation chain reaction, which can be divided into three key steps:

Chain initiation step: External energy sources (such as UV radiation or heat) cause homolytic cleavage of weak bonds (such as ether bonds) in the molecular chain, generating initial free radicals. These highly reactive radicals are the root cause of oxidative degradation.

Chain propagation cycle: The initial radicals rapidly react with oxygen to form peroxy radicals (ROO·). These peroxy radicals further abstract hydrogen atoms from the polymer chain, generating unstable hydroperoxides (ROOH). These compounds can decompose, catalyzing the formation of more free radicals and exponentially amplifying the reaction.

Chain termination reaction: When the concentration of free radicals reaches a certain level, they terminate via recombination or disproportionation to form inert products, thus ending the chain reaction. However, by this stage, the polymer network is already severely damaged, leading to irreversible performance decline.

Macroscopically, this aging manifests as hardening and embrittlement of the material, loss of dynamic fatigue properties, surface powdering, and color changes (such as yellowing).

Mechanism and synergistic effects of antioxidants

Antioxidants are usually applied in composite systems to provide comprehensive protection by combining the strengths of different additives, achieving synergistic effects. They are mainly classified into two types:

Chain-breaking antioxidants: Also known as free radical scavengers, these additives directly react with highly active radicals, converting them into stable molecules and effectively cutting off the oxidation chain during the propagation stage. Hindered phenols are commonly used in polyurethane; aromatic amines are applied in specific cases.

Preventive antioxidants: Also known as peroxide decomposers, their main role is to decompose hydroperoxides generated during propagation, reducing them into stable alcohols and thereby limiting new radical formation at the source. Typical examples include organic phosphites and thioesters.

When these two types are used in proper combinations, they can produce a “1 + 1 > 2” synergistic effect — the combined efficiency can improve by more than one-third.

Positive effects of antioxidants on product performance

In practical applications, antioxidants extend the aging resistance of flexible PU foam and positively influence key properties:

Mechanical performance: By suppressing polymer network degradation and isomerization, antioxidants help maintain low compression set and favorable stress-relaxation over the long term (e.g., 3–5 years), with retention rates above 80%, ensuring durable support.

Appearance stability: Oxidation causes yellowing due to the formation of quinone-type chromophores and conjugated double bonds. Antioxidants can significantly delay this color change for years while preventing surface peeling caused by chain scission.

Service life: With optimized formulations, antioxidants can substantially extend actual service life. For example, household products (such as mattresses) can approach ~10 years of use, while automotive applications can meet OEM durability warranties of 3–5 years.

Key considerations when using antioxidants

To ensure antioxidants achieve optimal performance, the following factors are critical:

Timing of addition: Antioxidants must be pre-mixed with polyols and other components before polymerization. They cannot be added after foam formation.

Accurate dosage: Typical levels are 0.1%–0.5% (w/w) based on polyether polyol weight; exact amounts should be determined by aging tests (e.g., oven heat aging).

Effectiveness decay: Antioxidants are continuously consumed as they act; their effectiveness declines over time (approximately linearly), but they can significantly delay the onset of critical degradation.

Conclusion

In summary, the value of antioxidants lies in their ability to efficiently interrupt auto-oxidation chain reactions, thereby delaying declines in mechanical properties, appearance, and functional durability of flexible PU foam. While antioxidants do not confer new initial properties, they are indispensable for maintaining long-term stability and are decisive for enhancing competitiveness of mid-to-high-end products.

Given the critical role of antioxidants in material protection, can we infer that future sustainable material research will go beyond merely delaying aging and instead increasingly integrate antioxidants with biodegradable or bio-based materials to balance performance with environmental responsibility?