How can ageing studies be done in both real time and accelerated?
Pierre Moulinié and Paul Nowatzki, Covestro, discuss how ageing studies with polymers are done, both in real time and accelerated.
Manufacturers of medical devices are expected to provide guidance on the service life of their products. Furthermore, devices are often stored for many months prior to use, and knowledge about a device’s shelf life is also necessary. How a material’s properties change over several years is therefore an important aspect of whether it is suitable to use in a medical device. For plastics, factors such as elevated usage temperatures, exposure to UV radiation or being under constant mechanical load can cause significant physical property changes over time. A more subtle, yet still relevant change that can occur at ambient conditions is termed physical ageing.
Data for a medical grade polycarbonate collected in Covestro laboratories serve as an example of estimating the ageing factor Q10.
What are ageing and accelerated ageing?
Rigid plastics, i.e., those below their glass-transition temperatures (Tg), have polymer chains in a configuration that was frozen-in during processing. Despite being below Tg and “frozen”, the polymer chains still undergo a slow densification process. Over time, the decrease in free volume between polymer chains can slightly alter polymer properties. Since storing samples for several years is not always feasible, accelerated ageing studies apply the time-temperature superposition principle to use data from test samples stored at elevated temperatures for relatively short periods to predict changes at ambient conditions after several years. In the case of polycarbonate, although the decrease in ductility from ageing is sometimes measurable, the polymer remains overall ductile and tough. So-called Q10 factors estimate how the rate of ageing changes with an increase in temperature of 10°C and are used to design accelerated ageing experiments.
The Q10 factors for accelerated ageing
The simplicity of the Q10 approach makes its use for ageing studies of materials in medical equipment fairly common. Some researchers claim that Q10 factors are oversimplified and have proposed models that may more accurately reflect changes over time.
The following equations capture how Q10 factors are used:
Where TAA is the temperature for accelerated ageing, and AAF, the accelerated ageing factor, reflects the ratio of ageing rates between TAA and ambient temperature (TRT). It follows that the accelerated ageing time can be estimated by:
Complications in accelerated ageing studies at high temperature can arise from chemical or structural changes in the polymer. Degradation, oxidation, crystallisation which can also occur in plastics, especially at elevated temperatures, or creep from prolonged mechanical stress can complicate the analysis and hamper extrapolations to ambient temperature. If the actual application entails conditions that may prompt chemical or structural changes, then real-time ageing studies may be unavoidable.
Q10 derivation example: polycarbonate
For polycarbonate, the tensile stress at yield is a sensitive indicator of ageing and is useful to construct the equations to estimate Q10. In the example that follows, a medical grade polycarbonate (Makrolon 2458 550115, from Covestro) was injection-moulded into ASTM tensile bars according to recommended procedures. Accelerated ageing was done by subjecting tensile bars to temperatures between 60°C and 100°C for various lengths of time, up to 100 hours. Using the time-temperature superposition principle, the tensile strengths at each temperature were shifted to form a master curve. The shift factors were plotted versus inverse absolute temperature to construct an Arrhenius plot. The slope of this plot represents the activation energy that describes how ageing rate depends on temperature. The ratios of Arrhenius reaction rates at the TAA and TRT gives AAF, which is used with equation (1) to find Q10. For the polycarbonate studied in this article, a Q10 of roughly 7.6 was obtained in the 25-55°C range which is a common range for accelerated ageing studies.
Models vs. real-time ageing
Tensile bars of an analogous polycarbonate grade were stored at ambient conditions for 3.6 years. The tensile yield strength was measured immediately after moulding and again 3.6 years later and was found to have increased by 0.7 MPa. To simulate years at ambient temperature, tensile bars of the same grade were aged at 55°C for intervals up to 168 h. A Q10 of 7.6 predicts that 3.6 years at ambient conditions (25°C) is like 72 hours at 55°C, and indeed the tensile yield strength increased from 0.6 ± 0.2 MPa over this interval. The predicted and actual results were deemed to be in excellent agreement.