Accelerated Aging: Is Hotter Better?

Don’t Get Burned

Successful commercialization of a new medical device depends on the outcome of packaging validation efforts, including accelerated aging. Too often, packaging validation takes place in the eleventh hour and accelerated aging becomes the critical path for market launch.  Project managers are pressured to bring in those timelines. Theoretically, one way to accomplish that is to increase the aging temperature thereby further accelerating the process and improving the schedule – right?  After all, cranking up from 55°C to 60°C pulls in one-year real-time equivalent (RTE) from 40 days to 29 days, respectively.  At five years RTE the savings are even more extreme, 199 days is reduced to 141 days.

See details with a look at our Aging and Temperature Chart.

Chart shows: Accelerated Aging Conditioning Times by Conditioning Temperature, Assuming T(RT)=23°C and Q10=2.0

Why Not Crank Up the Heat?

It’s true that accelerated aging is meant to do just that – accelerate the aging process. The goal is to predict how materials would otherwise behave at room temperature in long-term storage.  If the aging temperature becomes so hot that materials melt or burn, it is obviously not representative of “room temperature” in the warehouse or hospital supply room.

The key here is to recognize that in long-term storage, materials will age at room temperature (about 23°C).  The aging  temperature should be set below the temperature at which the materials begin to change their state of matter (i.e. going from solid to molten through melting).  Plastics used in medical device packages can and do melt.  But a key difference compared to, say, ice melting to water, is that plastics have long molecular chains (polymers).  These long chains cause the melting process to be much more drawn out. The result is that a rigid plastic in a solid state (called a “glassy” state) first becomes amorphous (rubbery) before actually melting into a viscous fluid.  The temperature at which the plastic changes from “glassy” to rubbery is called the Glass Transition Temperature, denoted as Tg.

Know Your Packaging Materials

Some plastics have cold glass transition temperatures. This means they are already rubbery at room temperature.  Other plastics have high glass transition temperatures. In this case, they are already brittle and stiff (glassy) at room temperature and become rubbery only when heated.  It is important to research and understand these characteristics in the materials selected for your package construction. At room temperature, is the plastic already beyond its Tg, or is the Tg a hotter temperature?  If the Tg is colder, then you want to make sure and select an accelerated aging temperature that doesn’t exceed the plastic’s melting temperature.  If the Tg is hotter than room temperature, then you don’t want to exceed the plastic’s Tg.  By doing so, you mitigate the risk of unnatural degradation in the aging oven that might otherwise lead to false failures and major schedule delays.

See more with a look at our Glass transition temperature chart

Chart shows: Glass Transition Temperatures of Common Plastics Used in Packaging

While the table gives you an idea, it is key to know the specifications for the exact brand and supplier of materials you select for your package!  Multi-layer structures, polymer blends, additives, processing conditions, and other factors may cause your plastic’s Tg to be different. Ask your supplier for a technical data sheet.

When in doubt, another approach is to conduct accelerated aging with two different temperatures. An example would be to use an “aggressive” high temperature (such as 60C) but run a parallel set of samples at a more conservative 55C.  If there are false failures observed in the 60C group (failures that are due to high heat), then you can fall back on the 55C group that are already in the oven.

 

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