Materials Monday: Characteristics of Copolymers
Move over rubber. Here comes thermoplastic elastomer, the squishy, stretchy, and superbly strong class of polymers that’s good for everything from weather stripping to sleep apnea masks to handles on power tools. Thermoplastics and, more broadly, polymers play an important role in our everyday lives. See our previous blog post, Materials Monday: Polymers on Parade, for our introduction to this ubiquitous material. Imagine a world without Legos (made of acrylonitrile butadiene styrene, a.k.a. ABS), Wiffle Balls (polyurethane), or Flexible Flyer sleds (high-density polyethylene). What would we do to stay entertained on weekends? We’ve also talked about the two main types of thermoplastics with our blogs on amorphous and semicrystalline polymers (See those blogs at Materials Monday: All About Amorphous and Materials Monday: A Semi-crystalline Synopsis).
And yet, there’s another important type of thermoplastic out there; it’s called thermoplastic elastomer (TPE). TPE has characteristics of both rubber and plastics, combining the flexibility of rubber with the processing advantages of plastics. The Shore durometer test is used to grade the hardness of plastics and rubbers. Shore hardness numbers range from 0 to 100, with the higher number meaning harder material. There are three popular Shore scales, Shore 00, Shore A, and Shore D, which cover materials ranging from very soft gels and rubbers to semi-rigid plastics, hard rubbers and hard plastics. Materials with a very soft texture like rubber and TPE should be tested with Shore A. For injection molding, it’s common to specify the hardness using the Shore A scale between 50 and 90.
Six classes of TPE exist, which we’ll discuss in a moment, but for now, know that TPE is what chemistry geeks call a copolymer—a polymer made of two or more monomers, just like ABS, polyvinyl chloride (PVC), and good old polytetrafluoroethylene, better known as Teflon. Unlike those other copolymers, however, TPE has the upper hand in several areas, providing a best of all worlds combination of flexibility, mechanical strength, absence of creep under load (unlike Teflon), and excellent resistance to wear, solvents, and heat. It’s also recyclable and simpler to process than the alternative, vulcanized rubber.
It’s these last two factors, in fact, that help explain TPE’s rising popularity in the automotive, consumer electronics, industrial machinery, and medical industries (some TPEs are also biocompatible). Because of this, TPE and its brethren are found in everything from weather stripping to sleep apnea masks to handles on power tools. TPE rocks.
Back up the TPE truck
But hold on: what’s wrong with vulcanized rubber? For that matter, what does “vulcanize” even mean? Vulcanization is a type of thermosetting. It’s the process of curing any type of rubber—natural or synthetic—by exposing it to heat and sulfur. Yuck. But doing so makes the rubber harder and more elastic, which is why you’ll often see vulcanized rubber used in radiator hoses, shoe soles, car and truck tires, and hockey pucks.
Rubber has a slight edge over TPE in high heat applications—depending on the type, TPE begins to melt at 340°F or so, while natural rubber holds the line up to 356°F and silicone rubber (which is not actually rubber at all but rather a synthetic elastomer) takes the thermal resistance prize with an 842°F melting point. Vulcanized rubber, on the other hand, doesn’t melt at all—it catches fire instead, an alarming event that is outright dangerous and therefore prevents its use in many applications.
Rubber also has a price advantage, so it’s unlikely you’ll ever see TPE tires on your car, fire risk notwithstanding. Where you will see TPE used is in any plastic injection molding application calling for extreme performance, recyclability, and versatility, and where the faster processing speeds of TPE over thermosetting rubber outweigh its higher material cost. One of the most exciting design uses of TPE is for overmolding. The soft, squishy handle on your power drill is most likely made of overmolded TPE, as is that of your toothbrush, disposable razor, and practically any other plastic injection molded product requiring an ergonomic, slip-proof grip. TPE can also be used with fused filament fabrication (a popular type of 3D printing) as well as extrusion, blow molding, and thermoforming.
Here are the three most popular of the six classes of TPE, along with some of their many uses:
Thermoplastic polyurethane (TPU) can be thought of as the switch-hitter of the TPE family. Highly customizable, TPU can be made hard like rigid plastics or as soft and squishy as a latex mattress. It’s UV resistant, making it suitable for outdoor applications, is breathable and abrasion resistant (think tennis shoes and other sporting goods), and has flame-retardant properties, so it is often used for jacketing of specialty cables. And thanks to its soft touch and ergonomic feel, TPU is also a favorite for overmolded handles on screwdrivers and power tools.
- Thermoplastic Vulcanizate (TPV) edges into Vulcanized rubber territory, albeit without any of that stinky sulfur. TPV employs “dynamic vulcanization” during processing and includes tiny bits of cured EPDM (ethylene propylene diene monomer) rubber encased in a polypropylene matrix. Many in the industry know it as Santoprene, a popular brand of TPV owned by the Celanese Corporation that is used widely for plastic injection molding. Whatever you call it, TPV offers excellent durability and dimensional stability, is lightweight, and finds its way into seals and gaskets, knobs and buttons, plugs, grommets, tubes, and hoses.
- Thermoplastic polyamides (TPA) are a great example of the “copolymer” term introduced earlier. That’s because their molecular structure is composed of alternating blocks of rigid polyamide interspersed with soft polyether blocks. Manipulating the placement and ratio of these blocks gives scientists great latitude to create materials that range from very hard and rigid to very soft and flexible. They can be made tough enough for gears and similar mechanical components, stiff yet forgiving enough for cold weather gear like ski boots, and supple enough for catheters and balloons. As with the rest of the TPE family, TPA is good stuff.
Pick three more
Those were the three most well-known TPEs, and this quick look at the three less well-known options rounds out our overview:
- Thermoplastic copolyester (TPC) boasts high mechanical strength and chemical resistance and is paintable and glossy in appearance.
- The mouthful thermoplastic polyolefin elastomer (TPO) is a blend of polypropylene or polyethylene with some rubber and a filler such as glass or carbon fiber filler thrown in. It carries many of the same properties as other members of the TPE family but sits at the harder end of the durometer scale, so is most suitable for high-impact uses—automobile dashboards, for instance, or even roofing material.
- Last but not least is thermoplastic styrenic elastomer (TPS), said to be the most rubberlike of all TPEs. Here again, though, the copolymer term comes into play, and a host of TPS variations based on different “soft segments” provides a broad range of properties and potential applications.
In closing, you can think of TPE as the “new rubber” (even though it’s been around longer than the automobile), but one that’s recyclable and much easier to fabricate into a practically infinite variety of products. As noted, TPE can be a bit more expensive than rubber, but given the fact that it’s faster and simpler to process, any delta in material cost is offset by its greater manufacturability. TPE can be hard like a hockey puck or possess nearly gel-like flexibility, is suitable for temperatures from the Antarctic to the Saharan desert, and resists tearing, chemical attack, oils and electricity. Feel like chatting more about TPE and all its variants? You know where to find us.
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