Estimating the Shelf Life of Medical Plastics by Accelerated Ageing

All materials degrade over time, but the effects of ageing on polymers can be particularly dramatic. Just have a look at PVC guttering or black PP car bumpers that have been exposed to the elements for several years, leading to severe embrittlement and bleaching. While ageing occurs more rapidly when polymers are exposed to weathering, ageing is also observed in medical plastic products stored in protective opaque packaging. Several mechanisms are responsible for the typical ageing effects seen in unpigmented polymers of yellowing and a loss in flexibility. This degradation is chiefly caused by UV exposure, oxidative damage and changes to the polymers crystalline structure referred to as physical ageing. Ageing can reduce molecular weight by chain scission, induce crosslinking and modify crytallinity, all of which alter mechanical performance giving rise to the risk of product failure. Polymer ageing can be monitored using a range of analytical methods including thermal analysis, spectroscopy, gel permeation chromatography, density measurements and mechanical tests.