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Long-term Polymer Properties – Don’t Be Creepy, Just Relax the Stress Away

October 22, 2020
by Jeff Ellis

Polymers undergo creep and stress relaxation over long periods of time, which can be detrimental to the function of medtech products. When designing medical devices that contain polymers, it is important to understand the differences, similarities, testing, and mitigation strategies of the two phenomena.

The major difference between creep and stress relaxation is the way stress and strain act upon them, as shown in this figure.

For creep, a constant force is applied to the material and the material moves (ΔL). For stress relaxation, strain is imparted on the material, and the stress with which the material resists the strain decreases over time. The following include some real-world examples:

  • Creep:
    • A large metal spring applies a nearly constant force on interacting parts as they slowly creep within a polymer auto-injector device.
    • A plastic bandage is applied to a patient to hold an IV line in-place. Over a couple of days, the constant stress on it causes it to stretch and loosen.
  • Stress Relaxation:
    • A polymer spring is used to place a force on a vial to keep it aligned in a diagnostic instrument, but over time the force decreases, and vial is no longer aligned.
    • An all plastic clip is opened and fitted over a group of cables to manage their position. Over time, the clip applies less compression force and the cables can move.

What causes these material property changes? The underlying reasons for creep and stress relaxation are the same. For example, a polymer that has low creep will also have low stress relaxation. Both depend on how much relative motion occurs between polymer chains.

The motion is a function of one or more of these factors:

  • Crystallinity – Some polymer chains pack into a crystalline orientation, the ones that do not are considered amorphous. Crystalline regions pack much more tightly and neatly into organized structures than their amorphous counterparts, which greatly reduces polymer chain mobility. Since the crystalline polymer chains cannot move independently, polymers with a high crystalline percentage have low creep and stress relaxation. An example of a highly crystalline polymer with low creep is polyoxymethylene (POM).
  • Polymer side groups – Polymers with larger side groups have less relative mobility than those with small side groups. Envision a polymer chain as an extension cord. When you have many short extension cords, they easily slide over one another. Now envision that the extension cord has lights attached radially in every direction along its length. It is difficult to slide these light strands over one another. This is the difference between polyethylene (PE), extension cord, and polypropylene (PP), light strand. As you can see from this oversimplification, the PP has lower creep and stress relaxation than the PE.
  • Molecular weight – A confounding factor is that molecular weight (MW) also plays a role. The higher the MW, the longer the polymer chain, the lower the melt flow index, and the harder is it to get the polymer chains to move independently. Therefore, higher MW also has better resistance to creep and stress relaxation.
  • Fillers and glass transition temperature – Fillers usually decrease polymer chain mobility and increase the materials modulus; both make them more resistant to creep and stress relaxation. Glass fibers are often used to reinforce nylon. The is a chemical compatibilizer, called sizing, that chemically links the glass to the nylon so that the polymer chains have difficulty moving relative to the glass, and vise vera. The glass has a much higher modulus than the nylon, so a glass filled composite has a higher modulus than the neat polymer. When stress is applied to the filled nylon, less strain is imparted due to the higher modulus, meaning that the creep will also be reduced.

Long term creep and stress relaxation can be estimated from short term testing when time-temperature superposition (TTS) techniques are implemented using a dynamic mechanical analyzer (DMA). Because creep and stress relaxation can take months or years to appear in normal use, it’s useful to speed up the process for testing. The DMA can exert controlled stress or stain on the polymer at a specified temperature. A series of creep or stress relaxation tests can be performed at the temperature of interest as well as several elevated temperatures. The elevated temperatures must not come within 10°C of the glass transition temperature because there is a step change in the polymer’s response that affects prediction accuracy for future polymer performance at a lower temperature.

Once all the data has been collected, the higher temperature data points are superpositioned out to longer times, with the highest temperature being shifted to the longest time. Once shifted, all the data make one master curve which predicts the long-term creep or stress relaxation. Time-temperature superposition testing can be completed in about one week and can predict a polymer’s response several years into the future.

Because most medtech products are designed to last many years, long-term polymer properties must be considered when designing a product and choosing a material. Even single-use medical devices, like auto-injectors, can be assembled years in advance of being used. Design and material come together to determine the extent of creep or stress relaxation in your product. The levels of the stress or stain exerted on a polymer can be difficult to measure, but simple to model using finite element analysis (FEA). A well-understood design with a creep resistant polymer can last the life of the product, even if a large spring force is constantly pushing on it.

Are you experiencing failures with your aging plastic products ? EWI can help. Contact Jeff Ellis at [email protected] or 614.688.5114 to learn more.

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