Influence of different irradiation modalities on post-irradiation behavior and properties of highly-crystalline PP and its implications for radiation sterilization of medical devices

Concerns regarding the dangers of EtO to health, security, and environmental risks are driving single-use (SU) medical device manufacturers more and more toward radiation technologies. Sterilization can be successfully applied for most biomaterials and disposable medical devices using any of the three ionizing radiation modalities (gamma, X-ray, and e-beam (EB)). However, it is well known that ionizing radiation can significantly alter their structure and properties. Undesired structural changes and property deterioration can occur long after irradiation, i.e., during storage and implementation. Semicrystalline thermoplastic polymers such as isotactic polypropylene (iPP) are particularly sensitive to these effects due to the structural peculiarities and presence of long-lived free radicals in the crystalline core.

The evolution of long-lived free radicals in the highly crystalline PP structure and their impact on changes in thermal properties, crystallinity, and microstructure after irradiation, i.e., during storage, was investigated. Two different PP homopolymers with a high degree of isotacticity were prepared by slow cooling after compression molding to obtain structures with the highest possible crystallinity. Subsequently, the samples were irradiated by electron beam and gamma radiation, focusing on the maximum sterilization dose of 50 kGy. The presence and evolution of free radicals were followed using electron spin resonance (ESR) spectroscopy for up to 6 months. Additional characterization was conducted by optical microscopy (OM), scanning electron microscopy (SEM), wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC).