One of our medical device clients contacted us regarding a failure of polyurethane (PU) tubing. One user of this tubing had noticed cracking and discoloration of the PU. We had already conducted testing to establish that the failed tubing had undergone both cross-linking and chain scission, consistent with ultraviolet (UV) exposure. The next step was intentional UV aging of the tubing to see if the same types of failures occurred, which would confirm the mechanism of the failure.
Tubing samples were subjected to UV aging in accordance with ASTM D4329. The aging cycle included 8 hours of UV light at 60°C with 4 hours of condensation at 50°C. Samples were aged for 100, 200, 500, and 1000 hours.
We acquired digital photographs of non-aged and aged samples at each time point. Marked yellowing of the samples occurred in as little as 100 hours of exposure, with the discoloration becoming more pronounced as the exposure proceeded.
Gel Permeation Chromatography
Next we used gel permeation chromatography (GPC), also called size exclusion chromatography (SEC), to analyze these tubing samples. This method provides information about the molecular weight of the sample, which is likely to be changing if brittleness and cracking are observed.
When the samples were dissolved, some insoluble gels were present in all of the aged samples, indicating that they were partially cross-linked. Testing of the soluble portions proceeded using the GPC method. Molecular weights and molecular weight distributions of the samples were calculated using light scattering detection, which provides an absolute molecular weight rather than a value relative to a polymer standard.
The molecular weight averages showed definite differences between samples, with the most notable differences being an increase in polydispersity (Mw/Mn) and increase in the Mz value for the samples exposed to accelerated weathering, as compared to the control sample. Mz is most sensitive to changes in the high molecular weight end of the distribution, so an increase here indicates that some polymer chains are getting longer, which would occur with cross-linking. At the same time, the polydispersity, or breadth of the distribution, is also increasing, indicating that the low molecular weight end of the distribution is also changing with shorter chains apparent. This indicates that some chain scission is also occurring. Note that a decrease in the molecular weight can affect the mechanical properties, leading to failures such as the cracking observed. The differential molecular weight distribution plots of all samples are provided in Figure 2.
The GPC results indicate that the exposure to ultraviolet light during accelerated weathering produced changes in molecular weight distribution that are consistent with the changes observed in the test sample.
So what was our conclusion? UV aging of the samples produced marked yellowing along with significant changes in the molecular weight distribution. It was noted that the GPC results showed that the changes observed after the UV aging were consistent with those observed on the failed tubing, which supports the theory that the tubing failure was indeed caused by UV degradation from sunlight exposure. This information gave our client the information they needed to assess their product’s performance and make future recommendations for use conditions.