Comparison of the effects of x-ray and gamma irradiation on engineering thermoplastics
When radiation is used to sterilize medical devices, it is generally by gamma irradiation. Gamma is an irradiation technology based on radioactive cobalt-60 as source. Although very reliable and widely adopted by the industry, the cobalt presents challenges in terms of supply, transport, waste disposal and security (Dethier, 2016). For this reason, installing new facilities is restricted in certain geographies.
Electron beam treatment is a commercial alternative to gamma technology, but due to its different characteristics with respect to dose rate and penetration, it cannot replace gamma irradiation under all circumstances and requires another logistic workflow with regard to product packaging and irradiation.
X-ray is less established today as a sterilization technique but approaches gamma much more closely in terms of dose rate and penetration depth allowing a similar workflow. The main advantage of x-ray technology is that it does not require radioactive material. Instead, its radiation is generated through electric power. Switching products from gamma to x-ray is technically straightforward, but regulations require revalidation of the effectiveness which is a costly exercise. In addition, x-ray facilities are currently less widespread than those offering gamma. These factors prevent rapid industry adoption and growth. However, we expect to see a shift towards x-ray sterilization in the future (Gamma Industry Processing Alliance, 2017).
There have been extensive studies looking at the effects of ionizing radiation on thermoplastic materials. Older studies focus on the effects of gamma radiation. Krasnansky et al. (1961) rated the stability of plastics towards gamma radiation based on the amount of evolved gases. Polyesters and polystyrene (PS) are rated as very stable, followed by polycarbonate (PC) and polyamide (PA), whereas polyethylene (PE) and polypropylene (PP) were considered less stable. The chemical changes that occur in polyolefins have stimulated characterization and development efforts around additive stabilization packages (Alariqi, 2017; Kremser, 2020; Selcan Turker et al., 2018). Mechanical performance of engineering resins such as PC are only mildly affected by gamma irradiation according to De Melo et al. (2007). Araújo et al. (1998) came to the same conclusions. Color changes are eminent, though. Stabilizers for PC have been evaluated by Ferreira et al. (2009) and additional work has been done in industry (e.g. Avakian, 1985; Nelson et al., 1989; de Brouwer et al., 2012).
Comparisons of radiation modes are of more recent origin. Croonenborghs et al. (2007) have compared the effects of gamma and x-ray radiation on PS, polyacrylonitrile-co-butadiene-co-styrene (ABS), PE, PP and plasticized polyvinylchloride (PVC). They noted that PS and ABS are stable against radiation in the tested range up to 120 kGy. PE and PVC showed some crosslinking which was made visible in impact and tensile testing. PP showed chain scission leading to reduced tensile yield strength. In no case, however, differences between the effects caused by gamma or x-ray were observed.
Burgstaller et al. (2021) looked at different radiation modes including gamma, e-beam and x-ray at dosages up to 100 kGy. They showed equivalence for most of the properties measured on a variety of plastics such as PET, PC, ABS, HDPE and PMMA and stable performance of most materials except for PP where degradation took place.
Recent publications from Fifield et al. (2021a, 2021b) also make the comparison between gamma and x-ray radiation effects on medical devices and their materials. They conclude that the two different radiation modes have largely the same effects on the components and their materials. They investigated low density polyethylene, chlorobutylrubber, polyolefin elastomers and PP. Where they did see differences, they were small and not necessarily affecting the final product performance.
The aim of this study is to compare the effects of gamma and x-ray radiation on several engineering resins which are frequently used in medical devices and which may be subjected to radiation sterilization. In addition to the comparison, also the effects of such radiation on the mechanical and optical properties of these materials will be investigated. Furthermore a radiation stabilized medical grade polycarbonate will be compared to an unstabilized medical grade polycarbonate.
