What do “Crack Resistance” and “Stain Resistance” Mean?

by Aaron Stewart October 31, 2018

Previously we’ve talked about how using a proprietary polyurethane resin gives Zendura an exceptionally strong polymer backbone and why it’s an ideal choice compared to commonly used resins such as PETG (polyester), PP and commodity others.

Some people have asked for specifics about how materials are rated for properties like “crack resistance” and “stain resistance,” so here's some hard data. Read on to see how Zendura underwent some very rigorous tests to ensure it can take a bite out of the competition (but patients with Zendura aligners and retainers can't take a bite out of it)!

Impact Strength (Crack Resistance)

Impact Strength is defined as “the ability of a plastic to absorb mechanical energy in the process of deformation and fracture under impact loading.” Or more colloquially, “how hard can I whack this thing without it fracturing, deforming or breaking?” 

To scientifically measure the impact strength, we use the ASTM D 5420 (GC) test method using a Gardner Impact Tester, equipped with a 0.64 inch die and an 0.625 inch punch. Falling Weight Impact, also known as Gardner Impact, is a common method for evaluating the impact strength or toughness of a plastic. A weight is dropped from a measured height and the impact energy needed to cause failure via crack initiation and crack propagation is determined. The higher the weight is lifted, the higher the impact energy transferred to the material under test.

Zendura was evaluated for its “impact strength alongside with other orthodontic materials:



Stain Resistance

Dental appliances are frequently exposed to severe staining agents when worn by patients (e.g., highly staining liquids like strong coffees and teas and mouthwash), causing the plastic to yellow or even structurally deteriorate.

One test we performed to determine resistance to staining was to immerse the parts in mustard (a strong staining agent) and placed in an oven at 37C for 24 hours. Raising the temperature during soaking accelerates almost all processes governed by molecular kinetics – in other words diffusion or staining will happen much faster at 37C than at room temperature.

To accurately measure the color of the part itself both before and after the staining procedure, we used a colorimeter. Specifically The CIELAB Color model allowed us to accurately measure and quantify changes in “yellow-ness” (b*) that resulted from a 24 hour mustard soak. The color change was measured using a Byk-Gardner Color-guide and yellow index (b*) and plotted before and after the mustard immersion.

At the end of the mustard soak, Zendura showed little change in level of yellow: 


Test results like these make us confident that Zendura is the best material available for many kinds of dental appliances, and we hope it helps make orthodontists/dentists and labs confident that it's worth their time in trying it!

In short, Zendura is harder to break under applied stress, and more chemically resistant to common staining agents such as coffee, tea, oil, toothpaste, mustard, etc. than other materials.

Aaron Stewart
Aaron Stewart