Rigidity in Thermoplastic Materials

Customers turn to thermoplastic materials to deliver many different performance properties. For example, when you’re talking about relative flexibility or rigidity—a common first line property that can be important to both product performance as well as installation ease—you can’t say “give me the hardest thing you got.” Other properties might come into play, such as a need for low compression set or chemical resistance. A material might be rigid enough for, let’s say, a particular marine application, but if it doesn’t also offer outstanding resistance to UV rays, it would not likely be a good choice. 

Shore A and Shore D

The relative hardness of materials like thermoplastics are generally measured using the Shore durometer test. Referred to now as ASTM D2240, this test measures the depth of an indentation in the material created by a given, standardized force generated by a presser foot.  It uses two scales, Shore A and Shore D, each with 100 points, with the former used for generally soft materials and the latter for harder ones. A lower Shore A rating, for example, might describe a material that is soft, flexible and bendable. One that stretches well and can bend around corners and returns immediately to state upon any flexing. A high Shore D rating might describe a hard, rigid, unyielding surface; a hard hat material like polyethylene would likely demonstrate a high Shore D rating.

It’s important to note that the values are used for comparison of materials within that same scale —they are not meant to be a continuum. For example, a Shore 50A has no relation to a Shore 50D. Sometimes one might see a material overlapping with a high Shore A and a low Shore D, and those numbers can be used for comparison with other materials on the same scale, but comparing the A score to the D score doesn’t mean too much. 

A Wide Range of Thermoplastics in our Portfolio 

As materials experts, Cooper Standard ISG draws upon a wide range of potential thermoplastic materials when specifying the optimum material to meet the needs of a particular application, and the relative flexibility/rigidity is a common starting point. Of note, the rigidity of a particular family of materials can vary depending on how they are made.  For example, a thermoplastic co-polyester (TPC) material can run nearly the gamut on the Shore D scale, while displaying other valuable properties such as differing levels of chemical and abrasion resistance, so the right balance can be found.

Where good working rigidity is prized, Thermoplastic Vulcanizates (TPVs), which can achieve a mid-point on the Shore D scale and provide a good cost/performance ratio, are suitable for many applications and are often a good starting point to consider. Thermoplastic Polyolefins (TPOs) are generally harder on the Shore D scale than TPVs, but one might give up some temperature resistance, fuel and chemical resistance, and compression set, so this would need to be considered , and may or may not be an issue depending on the application.  

Moving up to even greater rigidity, to a Shore 75D or higher, polypropylene can be a good choice, and polyurethanes can often be tailored to be even more rigid on the Shore D scale—again, both materials come with their own mix of properties to consider.

Bottom line: there is a whole hierarchy of thermoplastic materials on the flexibility/rigidity scale, many of them perhaps less familiar to you than others. However, when selecting a material, the whole mix of other properties required needs to also be taken into account as well.  

Still, there is an optimum material for every application, and there might even be better material than what is currently being used on your application. In any case, ask us for an expert materials opinion for any industrial sealing application, new or existing. We are here to help.