Accelerated aging is a vital part of most research and development. In many cases, an off-the-shelf environmental chamber is sufficient for accelerated aging tests. That’s even the case when testing materials (like building materials) must withstand a wide range of (or massive swings in) temperature, pressure, and humidity. For example, daily temperature swings of 40ºF to 50ºF are not abnormal in the American West. In the high plains of North Dakota, Chinook winds have been known to cause 40ºF temperature swings in minutes. 

But there are some applications that require more. This is especially the case when testing under ASTM standards (like ASTM F1980) to determine how medical devices (often composed of a variety of metal and synthetic components of different compositions) will perform as they age.

With synthetics, we expect changes in color. Some materials to become brittle over time (as they lose plasticisers), while others become soft and sticky (as their sorption continues). 

But there can be unforeseen consequences or premature failures when components of different compositions age at different rates or have very different changes in dimensions, flexural properties, strength, or conductance over time. 

In these situations, exposing the entire item to extended high-heat exposure will simulate aging by:

• continuing to relieve molding stresses
• continuing shrinkage
• advancing polymerisation and/or curing of thermoset plastics
• driving out moisture, solvents, or plasticisers
• accelerating sorption of plasticisers
• breaking down the polymer itself

But getting meaningful and reliable results with high-heat exposure-based accelerated aging tests hinges on knowing that you are able to reach those high temperatures, completely penetrate the item with heat, and verifiably maintain those temperatures for the days, weeks, or months required by your accelerated aging test protocol.

Autoclaves for Improved Accelerated Aging Testing

Tony Collins, Managing Director of Priorclave Ltd., has experience both with environmental testing chambers and autoclaves and believes a good autoclave offers several advantages in running accelerated aging tests.

“A lot of times these tests are done with an environmental test chamber,” he explains. “Stick it in a chamber for two or three weeks at 98ºC and 98%RH [relative humidity].” 

But that can be very time consuming, especially when doing accelerated aging tests for items like artificial joints, which you need to confirm will last for decades. Under those conditions, it could take months or even years to see how the plastics used in a new artificial hip might age.

“You can actually do that [same test] in an autoclave much more quickly,” according to Collins. “The steam is a very effective way of transferring heat under pressure.” This is why steam autoclaves are preferred for critical sterilisation tasks. “That holds true for accelerated aging as well. You can get a lot more heat in, more quickly and more effectively, than you would in an oven” or chamber with high relative humidity (RH) but standard atmospheric pressure.

This reduction in testing time—often around 75% or more—is important. It isn’t just that it reduces the development or production timeline; it also results in better accelerated aging tests. As ASTM International notes in “ASTM D3045: Standard Practice for Heat Aging of Plastics Without Load” (emphasis added): “Effects of [high-heat] exposure may be quite variable, especially when specimens are exposed for long intervals of time.”   

This complicates reproducibility and limits what can be reasonably deduced from any given run of tests. By shrinking the testing time, you implicitly increase the reliability of the test.  

Testing Works Best with Priorclaves

But that doesn’t mean that every autoclave is good for accelerated aging tests (even though all autoclaves will do them faster than other options).  

ASTM D3045 goes on to explain:

“Factors that affect the reproducibility of data are the degree of temperature control of the enclosure, humidity of the oven, air velocity over the specimen, and period of exposure. Errors in exposure are cumulative with time.”

Priorclaves are designed to allow users to track and control these variables, thus reducing errors and unknowns. According to Tony Collins, “There are many options built into our controls that aren’t normally used for lab sterilisation processes, but which are extremely useful for research and development or accelerated aging. There’s an awful lot we can do quite easily that just isn’t possible with other autoclaves.” 

These features include:

• precise and accurate temperature/pressure controls and logging
• customisable pressure and temperature ramps
• automatic cycle repeat (with or without a pause between cycles)
• air ballasting
• flexible programmable controls