Aging Plastics

People age. All living creatures age. Polymers age, too, even though they’re not alive. When they age, they — like us — become more fragile and can break. And this can be a matter of life and death.

Measuring, evaluating or simulating aging of plastic materials can be important for a number of long-term plastic applications, such as in the construction industry. We expect plastic pipes and plastic insulation to be stable and functional for a long time. On the other hand, we want biodegradable medical polymers to break down at a desired rate. Polymer aging is directly connected to stability, and often can be predicted and programmed, if needed, for specific applications.

What Causes Aging?

What makes plastics age? Usually a combination of things, such as temperature, and exposure to ultraviolet light, visible light, atmospheric components, humidity, or liquids. The polymer aging processes can be further separated into physical aging, chemical aging, thermal aging, etc. We have to remember that most polymers are amorphous and undergo physical relaxation and other structural changes with time. Generally speaking, the softer a polymer is, the more flexible its polymer chains will be, and the more prone it will be to time-related changes. The chemical composition of the polymer, the presence and concentration of oxidation-prone groups, and the amount of plasticizers or additives can also have a strong effect on the process.

Since we can’t ask polymers how they feel, measuring plastic aging as it happens is important to prevent accidents. There now is an AgeAlert sensor, developed by Georgia Tech and Polymer Aging Concepts, Inc., that can measure and transmit information about plastic degradation:

AgeAlert is a low-cost conductive composite sensor, which precisely measures the degradation state of virtually any degradable product […] As the product ages, the polymeric components of the product shrink very slightly. Minute changes in the product’s shrinkage during aging provide large changes in the AgeAlert sensor’s resistance output.

It is important that the miniature sensor can be embedded in the plastic and work in real time. However, it does not require batteries until it is “interrogated.”

Currently used to monitor the state of electrical insulation, “AgeAlert sensors help predict failure in advance and reduce unplanned malfunctions of motors, generators, transformers, solid propellants, industrial rubber and other polymeric materials.”

Determining Useful Life

Another useful feature is not only to record the aging process, but also to predict when the aging becomes detrimental to the product functioning. The smart sensor can use modeling to predict remaining “useful life” of insulation just using the resistance data and avoid the failures.

While tracking plastic aging is important, how do we predict the future aging behavior of a new polymer? With novel materials and applications entering the market daily, the ability to estimate how stable will the product be can be vital. Think about medical devices, especially medical implants, with materials having a specific lifespan. Some, like breast implants, are not lifetime devices, while others are so stable they can sometimes outlive the implant recipient. Prediction of aging behavior is performed by simulating the environment for which the product is destined, whether conditions inside the body or the outside weather. And when we need data fast there are accelerated aging methods. More than 2,000 accelerated aging testing protocols have been accepted by the American Society for Testing and Materials (ASTM), depending on product applications.