Accelerated aging protocols are designed to rapidly determine the effect of time and the environment on the integrity of packages and the physical properties of the component packaging materials.
Accelerated aging studies are based upon the assumption that the chemical reaction involved in the deterioration of materials follow the Arrhenius reaction rate function. This function states that a 10°C increase or decrease in temperature results in approximately, a two times or one-half time change in the rate of a chemical reaction (Q10).
The equation used to determine the appropriate accelerated aging time is:
- tAA = accelerated aging time
- tRT = desired real-time aging
- TAA = accelerated aging Temperature (°C)
- TRT = ambient Temperature (°C)
- Q10 = reaction rate coefficient
Typically Q10, the reaction rate coefficient, is typically set equal to 2 for polymeric systems.
As the accelerated aging temperature increases, the time needed to conduct the test decreases. However, the increases in efficiency must be balanced against the risks associated with assuming that high temperature aging properties correspond to room temperature properties.
Choosing a Temperature
Rollprint recommends that when conducting accelerated aging studies with flexible packaging materials, the accelerated aging temperature be limited to 50°C (122°F). At temperatures above 50°C, we often see non-linear changes in the barrier properties, crystallinity, and for peelable products, seal strength. In addition, factors such as the pressure differential between the package and atmosphere, outgassing of the product, and the atmospheric conditions at which the product was packaged become much more critical and can lead to false failures. If the product contains volatile components (e.g. alcohol), the activity of volatiles often significantly increases above 50°C. This can cause non-linear package-product interactions as well as cause extreme pressure build-up within the package.
If as a result of time constraints, it is desired to perform the accelerated aging tests at a temperature greater that 50°C on a package that does not contain volatile components, Rollprint recommends that accelerated aging tests be run in parallel – one at the higher temperature and one at 50°C. Unless extensive testing has shown that a higher temperature is appropriate, accelerated aging temperatures should never exceed 60°C (140°F). If volatile components are involved, the temperature should never exceed 50°C (122°F).
Humidity and Accelerated Aging
As the properties of some material are impacted by humidity, it may be desirable to consider the effects of humidity when conducting accelerated aging tests. Care should be taken to choose a humidity level that reflects conditions that can exist in the real world.
Relative humidity is a measure of the amount of water suspended in the air relative to its absolute capacity. Since airs capacity for holding water increases as the temperature of the air increases, matching the relative humidity for an accelerated aging test with the relative humidity expected at ambient conditions can create unrealistic conditions. This may cause unnatural physical changes to materials (e.g. delamination of extrusion coated materials using water-based primers).
Rollprint recommends maintaining the same water concentration (i.e. absolute humidity) that may be seen at ambient conditions at the accelerated aging temperature. For example, 90% relative humidity at 25°C ( 77°F) equates to 28,000 ppm water in the air. If the accelerated aging test were to be conducted at 50°C (122°F), the relative humidity should be 23% in order to maintain the water concentration at 28,000 ppm. Psychometric calculators are available on the Internet for calculating the equivalent water concentration at various temperatures and relative humidities. The table below shows the concentration of water in air as a function of temperature and relative humidity.
If additional information regarding accelerate aging or the development of protocols is desired, please reference ASTM F1980 Standard Guide for Accelerated Aging of Sterile Medical Device Packages, AAMI TIR 17-1997 Radiation Sterilization – Material Qualification or contact your Rollprint Account Manager.