Understanding Antioxidants in Biomedical Polyurethanes

Antioxidants play a crucial role in the stability and longevity of biomedical polyurethanes (PUs). These compounds are categorized into two main groups: chain breaking donor (CB-D) antioxidants and chain breaking acceptor (CB-A) antioxidants. CB-D antioxidants, such as hindered phenols and aromatic amines, work by reducing reactive oxygen species (ROO*) to less harmful hydroperoxides (ROOH). In contrast, CB-A antioxidants, like quinones and stable free radicals, focus on oxidizing alkyl radicals, thus preventing further degradation of the polymer.

Preventive antioxidants operate by interrupting oxidative cycles, primarily through hydroperoxide decomposition using non-radical processes. Key examples include phosphorus-containing antioxidants, such as phosphite esters, and sulfur-containing antioxidants like thiodipropionates. Additionally, metal deactivators and UV absorbers serve as stabilizers that prevent oxidative damage from environmental factors, ensuring the integrity of the PU materials.

The combination of different antioxidants can yield a synergistic effect, enhancing protective capabilities beyond the sum of their individual contributions. For instance, a mixture comprising a peroxide decomposer, a UV absorber, and a radical scavenger can provide robust protection for polyurethanes. However, the effectiveness of such combinations, especially in biomedical applications, demands thorough justification to ensure safety and efficacy.

Understanding the degradation processes in polyurethanes is essential to appreciate the significance of antioxidants. Thermal oxidation is influenced by the composition of the PU, particularly the hard and soft segments. While the urethane unit remains stable, thermal degradation initiates in the urethane segment and continues in the ether part, especially in oxygen-rich environments. Free radical chain reactions drive these processes, which can be exacerbated by metal ion impurities.

Antioxidants can be introduced during the manufacturing or synthesis phases to enhance the stability of PUs. For example, they can help prevent discoloration and degradation caused by autooxidation during the storage and fabrication of polyurethanes. Understanding these mechanisms is vital for optimizing the performance and safety of PUs used in a variety of biomedical applications.