It seems that TPEs are everywhere. Unfortunately, the term TPE is not terribly descriptive of the details of composition that help us figure out the capabilities of a material. TPE, of course, stands for thermoplastic elastomer. Even in an industry beset by problems developing a common terminology, we can likely all agree on a definition for thermoplastic.
Defining elastomer may be more challenging. Generally, elastomers are understood to be materials that can deformed to a very significant extent and then recover to their original shape and size. An old text book that I had stated with great authority that the criterion for an elastomer was a material that could stretch to an elongation of at least 200 percent (three times its original length) and fully recover.
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But there is more to picking a material than knowing its short-term response to stress. In order to pick the right elastomer, one must understand a variety of properties, such as:
- The range of temperatures over which the material can be used
- tear strength
- chemical resistance
- response to sustained application of a load
Fortunately, some elastomeric materials are categorized in such a way as to tell us something about their composition. Thermoplastic polyurethanes (TPUs), thermoplastic copolyesters (COPEs), and polyether block amides (PEBAs) all tell us something about composition and therefore performance.
But TPE leaves us guessing. Is it based on a styrenic, a polyolefin, or some combination? What is the soft block? This uncertainty can make the selection process difficult and often the shortcomings of a material are not apparent until a mold has been built, parts have been produced, and failures occur during testing due to a particular requirement that the material cannot fulfill.
This can be particularly troublesome for users who are selecting materials for two-shot or multi-shot applications. An important criterion in these applications is adhesion of the soft material to a harder substrate. The ability of two materials to bond without the aid of a mechanical interlock is very much about composition. If there is no information about the make-up of the TPE, it can be difficult to predict whether the material will adhere to the hard portion of the product.
I recall working on an early application in the power tool industry where we attempted to mold a polypropylene-based TPE over a substrate of glass-filled nylon, without much success. Once the manufacturer of the TPE modified the chemistry of their product, the adhesion problem magically disappeared.
Data sheets help only marginally. They give the usual single-point treatment to the material, but they are no substitute for a stress-strain curve. Guidance on an upper end use temperature typically comes in the form of a Vicat softening temperature. Manufacturers who are somewhat more thorough may quote a brittleness point, thus bracketing the upper and lower bounds of useful temperatures.
The only attempt to address longer-term performance comes in the form of a property known as compression set. This can be measured at several different temperatures and over a few different time frames, so comparing materials depends upon finding data generated at equivalent conditions. In general, there is a shortage of data that are a staple in the rubber industry, those crosslinked materials that TPEs aspire to compete against.
The introduction of thermoplastic vulcanizates
In the early 1980s, when the TPE industry was far less developed than it is today, thermoplastic vulcanizates (TPVs) introduced a new wrinkle. These materials took direct aim at crosslinked rubber materials by incorporating a crosslinked ethylene propylene diene monomer (EPDM) phase into a polypropylene matrix.
The methodology for creating such a structure had endless possibilities, but commercially almost every TPV, then and now, is based on this combination. The objective was to mimic the performance of a cross-linked rubber with a thermoplastic that could be injection-molded and recycled. In this respect, TPVs can be thought of as a hybrid of a classical cross-linked rubber and a standard TPE, offering a bridge in performance between these two groups.
As such, TPVs are designed to offer improved long-term mechanical properties such as compression set, as well as superior performance over an extended time at elevated temperatures. And because these materials were targeting the crosslinked rubber market, the manufacturers of these materials published the types of properties that a user of rubber materials was accustomed to seeing. Properties such as retention of tensile strength, elongation, and hardness after prolonged exposure to elevated temperatures or to a hot fluid were published and remain part of some data sheets today.
The early brochures that were published for Santoprene, the first commercial TPV, were full of such information along with dynamic mechanical analysis (DMA) curves for the various grades that spanned a hardness range of 55 Shore A to 50 Shore D. While these materials are still better characterized than most of the more recently introduced TPEs, a lot of the data that was published 35 years ago can be difficult to find today.
TPVs can be more challenging to process than standard TPEs, an example of the almost universal law in plastics that if it processes with greater difficulty it probably has greater utility in application. The presence of the cross-linked (vulcanized) phase gives the materials a high melt viscosity that must be controlled with shear. Even a momentary pause in the flow front can cause a significant increase in viscosity and the associated cosmetic defects.
And while TPVs were intended to replace certain families of crosslinked rubber, it became apparent early on that they could not completely match the properties of the EPDM materials that had been around since the 1960s. The property difference was most evident in compression set testing, where the TPV needed to be approximately twenty points harder to match the performance of the crosslinked material.
Despite these shortcomings, thermoplastic elastomers have significantly extended their reach into most markets and we would be hard pressed to return to a time when all of our elastomers were cross-linked materials. The industry just needs to be a little better at informing us about what we are molding and using.
Further reading:
- Thermoplastic Elastomer (TPE) Application Checklist
- Understanding Polyesters
- A Brief History of Elastomers
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Hello, very interesting topic!
Could souft touch and aesthetic properties be part of the selection criteria? if yes , how to define it and then select properly?
Thanks for taking the time to write this up….I feel there’s always been a need to our new designer/engineers to be updated on those “new” elastomers. I was sold on them back in 1984 when our company decided to mold their own conveyor wheels and rollers rather than buying such from outside. I still have in a ring binder my SANTOPRENE catalogs from Monsanto #TPE-05-02 plus most I got as updates. Same for the GEOLAST that we later used. Have catalogs on that thermoplastic resin dating back to 1985.. Feel free to stop by if ever the need to look through them while you are visiting Penn State.
Stan Smith, PE @ CHEMCUT CORPORATION in State College, PA