In crude terms, creep is one answer to “What does this stuff do when I try to smash it?”. Brittle materials (glass, or ceramics for example) hang tough under stress but can suddenly snap or shatter. Elastomeric materials bend or stretch a lot – but then spring back when the stress is removed (rubber or elastic). Plastic materials react to stress by bending or deforming, but not springing completely back. Most materials have some plasticity, especially polymers, but the amount of plasticity varies greatly.
If you have about five minutes, pull up a chair and I’ll tell you a story about one of Bay Material’s battles with creep, and how it led to a useful discovery and a professional mantra related to the Zendura material that most of your are familiar with today.
Several years ago, Bay Materials was trying to develop a better polymer for dental appliances. It turns out the mouth is a challenging environment for polymers. The polymer must be inert and safe (We’re looking at you BPA). Also, it will be a thin sheet under high stress, and must be transparent, stain resistant, tough, and thermo-formable. On top of all that, the mouth is warm and humid, which makes a lot of materials that might have performed well in a cool dry environment behave badly.
The “high stress” part is particularly significant – a dental material’s job is usually to push or pull against teeth to move them into a new position. If the material creeps under stress, it won’t be pushing or pulling the teeth with the same force anymore.
This meant innovation had to start with testing methods. There are good standards that define how to measure material creep, such as the creatively named ASTM D2990-01, but sticking to the standard defaults would not have told us much about how materials would work in the mouth! So we kicked things up a notch by constructing an environmental chamber around the test area that could run at 37C and 100% relative humidity (Read more at Stress Relaxation in Dental Appliances). That warm, humid, realistic test environment ensured our technicians had smooth, well hydrated hands.. but also let us weed through candidate materials faster. Unfortunately, we soon discovered a new problem.
Annealing is a process of controlled heating and cooling that increases ductility and sometimes strength of a formed part or material. It was a natural next step to develop a higher-performing retainer material. There are lots of textbook approaches to annealing polymers, so we warmed up some of our ovens and gave it a shot…and immediately had things go wrong. If you anneal a fairly hydrophilic polymer (Urethane, for example), and then move it to a humid stress environment, it breaks down like a Kardashian hiking in the jungles of Congo.
Usually, annealing allows a molded part to distribute stress evenly – but as the Urethane re-hydrates from annealing, the alignment isn’t retained! After some head-scratching and more tests we came upon a solution. Like most working solutions, the idea seems obvious in retrospect: anneal the parts in a controlled, humid environment. This unusual step allows a hydrophilic polymer to retain hydrated structure while still redistributing internal stress.
Of course, the idea wasn't obvious to us ahead of time. Bay Material’s habitually realistic testing early in the development process paid off by enabling a better process and product. In fact, the process turned out to be novel, and applicable to many high-performance polymers destined for high-humidity environments. We developed a recently-issued patent from it that we’re quite proud of, but moreover got a reminder that it almost always takes a lot of work to get an “obvious” solution that works.
Take the pain early. Doing work up front to make testing and selection rigorous and close to “real” conditions pays off in terms of faster overall development and higher-quality process afterwards. Admittedly this approach can be discouraging – it makes progress feel slow at first – but it really pays off.
If you have any next material-science projects and want us to help share the pain (early or late in the process), don’t hesitate to contact us.