What Is Stress Relaxation?
Stress relaxation describes what happens when you take your shoes off and rub your toes on the soft carpet of the Hawaiian resort you're staying at. Congratulations, all of us at Bay Materials hope you enjoy your trip!
Wait, we're talking about the material science property “stress relaxation”. In that case, stress relaxation describes how a material behaves when subjected to prolonged constant strain over a period of time. In other words, if you press (or pull) on a material long enough and hard enough, what happens? Does it slowly give way? Crack? Warp? A change like bending, cracking or creeping is the material's way of releasing stress.
Stress relaxation test data is often presented in the form of a stress vs. time plot. (See example below)
If the stress in the material stays high, it is hanging tough and resisting the load applied to it. If the material is warping or bending in response to the applied strain, its stress will drop. Just like yours would if you had that vacation earlier – we are sorry if you are not at the Hawaiian resort but instead learning about material science here.
Why is Stress Relaxation Important in a Dental Appliance?
Low stress relaxation is an important performance criterion of an aligner. An aligner's job is to apply specific stress to teeth that guides them into ideal position over time. This means the teeth are applying strain right back to the retainer! So a retainer with lower stress relaxation will better ensure that it is the teeth that move instead of the retainer material deforming.
When an aligner is applied to a patient’s teeth, it is typically loaded (or stressed) to a pre-determined stress-strain level. From that moment the aligner starts to move the teeth, but at the same time it is slowly losing some of the applied stress or relaxing. Gradually the aligner loses its ability to apply the necessary force to continue moving teeth, requiring replacement with a fresh set of appliances.
Therefore, it is important to determine and understand the stress relaxation properties of various plastics to be able to design effective aligners capable of moving teeth to a certain distance in a short period of time.
What Type of Material is Ideal for Aligners?
A fully elastic and non-relaxing polymer or elastomer such as a rubber band sounds like an ideal material for use in an aligner. However, such elastomers are typically too soft and lack the strength and modulus properties needed for moving teeth. Metals have very low stress relaxation, but are ruled out because of problems like machining, safety, comfort, cost and that people don't want to look like Jaws from Moonraker.
Cosmetically, aligners are desired to be transparent or water clear and non-yellowing. Practically, aligners need to be chemically resistant, i.e. can withstand all kinds of foods and liquids absorbed, such as oils, mustard, coffee or wine, to name a few, without losing properties, staining or cracking.
So realistically, the category of usable materials for retainers is plastics. Among plastic materials, various requirements like bio-compatibility and regulatory constraints leave thermoplastic polyurethanes (PU) as the best performing material currently available. Other materials commonly used such as PETG or PP do not achieve the same balance of low stress relaxation, rigidity and flexibility that a properly processed PU material has.
Does Stress Relaxation alone Play all the Tricks?
Although stress relaxation is an important performance factor of an aligner, several other key properties of a material also play critical roles in the overall effectiveness of the aligner treatment. These properties include material toughness, impact strength, chemical resistance and optical transmission (clarity).
A material can be processed and made to have really low stress relaxation property, however not tough enough to resist cracking during trimming or when ground by teeth. Therefore to develop a material that will achieve an optimum balance of all key properties is non-trivial and a work out of years of experience, many cycles of trials and error as well as deep material science expertise.
How Stress Relaxation is Tested?
Evaluating aligners for stress relaxation properties is complicated by the uneven shape of the appliance as well as the actual performance environment inside the mouth (saliva, temperature, humidity and food chemicals). To avoid such complications, Zendura material properties are determined in sheet form (not thermoformed) using simple geometry test parts. Zendura is evaluated under recognized test methods such as the American Standard Test Methods (ASTM).
Given that aligners are worn inside the mouth of a patient for long hours each day Zendura is tested for Stress Relaxation in controlled environments, such as constant temperature 37C, and in 95-100% relative humidity (RH), for pre-determined time periods, around 24 hours. Testing in 100% RH is identified with a part fully immersed in water.
Stress relaxation tests are performed under various strain levels, typically in the range 1-5%. Results from such accelerated or harsh tests are then used to make inferences to thermoformed aligners’ performance properties. This controlled accelerated testing is common and acceptable practice in polymers and in various applications.
At Bay Materials, new and advanced materials or processes are typically subjected to such rigorous tests for accurate determination of any observed performance benefits over an existing product. A graphical representation of stress relaxation at constant strain e is displayed.
Our material scientists at Bay Materials have dedicated their effort to the development of advanced orthodontic material for over a decade and demonstrated their profound knowledge in polymer science and excellent capability in achieving the superior material performance based on our clients’ specific needs.