Polyurethanes have outstanding abrasion resistance, often outwearing metals, plastics, and rubbers by a wide margin of 8 to 1 (or more).  The coefficient of friction generally decreases with increasing compound hardness.

Hardness

Soft Best for uses with particle impingement abrasion. Also great for uses where friction is desired, such as drive wheels.
Medium Last longest in abrasive slurry types of service, like pump impellers.
Hard Best in impact and sliding types of abrasion found in sand, gravel, coal, and ore mining applications.
Very Hard Great for bushing and bearing applications.  Uses include wet or dry environments, often in the presence of sand, grit or mud.

One of the most common arguments for the use of polyurethanes today is that they are abrasion resistant. For decades polyurethanes have been used in highly abrasive environments due to their remarkably abrasion resistant properties when compared to other elastomers, plastics, and metals. Polyurethane components have consistently shown to be up to 10 times more abrasion resistant than other elastomers.

Standard test method for measuring urethane abrasionWhile it’s common knowledge that urethanes are highly abrasion resistant, it’s notably difficult to substantiate these claims with often used ASTM test results. For most applications, abrasive erosion occurs from one of two different mechanisms — sliding abrasion and impingement abrasion.

Sliding abrasion occurs when one surface slides over another surface and tiny tears occur at the interface between localized areas of high strain. For this type of abrasion, it’s usually best to select a harder urethane compound which has a low coefficient of friction and high tear strength.

Impingement abrasion occurs when particles with relatively high velocity impact a surface. If that surface isn’t able to effectively absorb all the energy from the particles, tiny sections will break off due to localized areas of high stress and high strain. For this type of abrasion, it’s usually best to select a softer more resilient material that can absorb the energy from the particle impact and let the particles bounce off with little or no damage to the impacted surface.

Polyurethane Track PadsThe problem that arises when trying to simulate these situations in a laboratory setting is the generation of heat. In the field, these components are usually worn gradually over a relatively long period of time. In order for laboratory testing to be achievable, the rate of wear needs to be accelerated in some way. Typically the methods for accelerating the rate of wear generate excess heat in the polyurethane. Heat has long been the Achilles Heel of polyurethane components, as the compounds lose their superior performance at elevated temperatures.

The compromise we typically come to is to use laboratory testing to formulate a hypothesis on a few different polyurethane compounds we believe will work best for a given abrasion resistant application. Due to our large variety of polyurethane casting equipment, we can easily provide a customer with several compounds for real world abrasion resistant testing. Based on which compound works the best in actual usage, we can finalize the material selection.

For more information on laboratory testing polyurethanes for abrasive contact us to speak with one of our design engineers.