The Confusing World of TPE

There is an excellent albeit highly technical article in the April 2016 SPE magazine, Plastics Engineering, about the differences between thermoplastic elastomers (TPE) and thermoset rubber. You can read it here.

Because I talk primarily about thermoplastics here at The Weekly Pellet, I thought that I might expand on the article a little bit and talk about the different types of TPE that are commercially available.

As the article in Plastics Engineering mentions, there are six generic classes of TPE that are currently available. I would add polyvinyl chloride (PVC) to this list as well:

1. Styrenic Block Copolymers (SBC)
2. Thermoplastic Polyolefins (TPO)
3. Thermoplastic Vulcanizate (TPV)
4. Thermoplastic Polyurethanes (TPU)
5. Thermoplastic Copolyesters (TPC-ET or COPE)
6. Thermoplastic Polyamides (TPA-ET or PEPA or PEBA)
7. Polyvinyl Chloride (PVC)

The various TPE materials that are on the market today were invented to replace thermoset rubber materials. The manufacturers hope to provide the properties of rubber materials with the ease of processing and recyclability that thermoplastics enjoy.

Advantages of TPE:

1. Easier to process. TPEs can be processed on traditional injection molding machines.
2. Faster cycle times in manufacturing.
3. Scrap made during manufacturing can be easily put back in the process and TPE parts can be recycled at the end of life.
4. Better appearance. TPE materials offer a wide variety of appearance options which is great for things like automotive interior applications.

Advantages of Rubber:

1. Higher heat resistance. Rubber, being a thermosetting material can take a lot of heat.
2. Better chemical resistance
3. Better compression set. Compression set refers to how the material rebounds or bounces back after being compressed.

Let’s break these TPE materials down a little farther.

Styrene Block Copolymers (SBC)

What is it: SBC (styrenic block copolymer) materials are block copolymers and terpolymers (more than two polymers) of styrene and butadiene. This is a broad category of materials that covers a number of different formulations. Material designated as SBS, SEBS, SEPS, SIS and SEP are all styrenic block copolymers. You can look up what all those acronyms stand for if you want but I won’t bore you. Some manufacturers just refer to their material as SBC which means they’re being a bit cagey about the exact make-up of the material. A book could be written about the differences between all of the different SBC formulations but there are more similarities than differences so I will just cover the basics here. SBC materials are not typically injection moldable so most commercial grades are SBC that is compounded with polypropylene or polyethylene.

Here are the highlights:

• Hardness range from 15 Shore A to 50 Shore D
• The softest TPE available
• Good glossy appearance with a smooth surface feel similar to polypropylene
• Highest tensile elongation of any TPE (stretchiest)
• Available in clear
• Less heat resistant than TPV
• More flexible than TPV
• Poor chemical resistance to oil.
• Maximum continuous service temperature 230F
• Common trade names include Kraton, Sunprene, Multiflex
• Applications include gel inserts for shoes, grips for pens and toothbrushes and appliance knobs

Thermoplastic Polyolefin (TPO)

What is it: TPO is a compounded blend of polypropylene and uncrosslinked EPDM rubber and sometimes mineral fillers. These tend to be harder materials ranging from 30 Shore D and up. They tend to be classified by flexural modulus rather than hardness.

Here are the highlights:

• Available in flexural modulus from 90,000 to 350,000 psi.
• Combination of high stiffness and high impact strength
• Excellent appearance
• Some grades are paintable
• Weatherable grades are available
• Maximum continuous service temperature 230F
• Common trade names include Hifax, Mytex, Polyfort
• Applications include automotive bumper fascias, automotive interior panels

Thermoplastic Vulcanizate (TPV)

What is it: TPV is a compounded blend of polypropylene and cross linked EPDM rubber. TPV tends to be very soft and flexible. It is similar in properties to SBC.

Here are the highlights:

• Hardness range from 35 Shore A to 50 Shore D
• Has a matt finish appearance and a very rubber like feel
• Not available in clear. Natural TPV is a light cream color similar to ABS.
• Not as flexible as SBC
• More heat resistant than SBC
• Good chemical resistance
• Runs better if dried but many molders do not dry it
• Maximum continuous service temperature 240 F
• Common trade names include Santoprene, Geolast, Sarlink, Trexprene
• Applications include power tool housings, automotive boots and grommets, automotive cup holder inserts, door bumpers

Thermoplastic Polyurethane (TPU)

What is it: TPU materials are block copolymers formed by the reaction of diisocyanates with short chain diols or long chain diols. They are the best wearing TPE materials and have the highest tensile strength.

Here are the highlights:

• Available in hardnesses from 70 Shore A to 80 Shore D
• The best rebound of any TPE
• The best wear properties of any TPE
• High tensile strength, even for softer grades
• High tear strength
• Must be dried
• Difficult to mold, degrades easily
• Annealing parts improves performance
• Expensive
• Ester based grades are most common and offer the best properties
• Ether based grades offer better hydrolysis resistance
• Heavier than other TPEs with a density around 1.24 g/cm³
• Maximum continuous service temperature 200 F
• Common trade names include Elastollan, Desmopan, Estane, Irogran, Texin
• Applications include in-line skate and skateboard wheels, automotive suspension bushings, running shoe arch supports and medical tubing

Thermoplastic Copolyesters (TPC-ET or COPE)

What is it: TPC-ET materials are copolymers of bifunctional aromatic polyesters. They are the highest heat TPEs available and can take repeated bending forces without breaking.

Here are the highlights:

• Hardness range from 80 Shore A to 80 Shore D
• Higher stiffness than other TPEs
• Can be repeatedly flexed without fracture
• Maximum continuous service temperature 290 F
• Common trade names include Hytrel, Arnitel
• Applications include automotive CV joint boots, coiled air hose tubing

Thermoplastic Polyamides (TPA-ET or PEPA or PEBA)

What is it: TPA-ET materials are copolymers obtained by polycondensation of a carboxylic acid poyamide with alcohol terminated polyether. Think of it as nylon based TPE. It offers properties similar to TPC-ET. This is not a commonly used material that seems to find a home mostly in medical applications.

Here are the highlights:

• Hardness range from 80 Shore A to 70 Shore D
• Can be repeatedly flexed without fracture
• Good oil resistance, even at high temperatures
• Expensive
• Maximum continuous service temperature 290 F
• Common trade names include Vestamid
• Applications include breathable films and medical tubing.

Polyvinyl Chloride (PVC)

What is it: PVC is produced by polymerization of vinyl chloride monomer. Plasticizers, primarily phthalates, are blended into the material to make it soft and flexible. Unlike the other materials here, PVC is available in both a rigid and a flexible form. We will focus on the flexible version here.

Here are the highlights:

• Hardness range from 35 Shore A to 55 Shore D
• Excellent chemical resistance
• Excellent appearance
• Very poor heat resistance
• Poor compression set or rebound
• Heavier than other TPEs, with density around 1.25 g/cm³
• Available in clear
• Poor wear and tear strength properties
• There are concerns about plasticizers causing adverse health effects
• Maximum continuous service temperature 176 F
• Common trade names include Geon, Hy-Vin, Polyvin, Vinika
• Applications include tubing, flooring, automotive interior panel covering

TPE materials, with the exception of PVC, are a relatively new addition to the thermoplastics market having been invented less than 30 years ago. The market for TPE continues to see a large amount of growth outpacing the broader thermoplastics market. The technological advancements that continue in the TPE industry can be a little difficult to keep up with. For instance, in the last 25 years or so, TPO has almost entirely replaced RIM materials for automotive bumper fascias.

I hope this breakdown helps to clarify the differences between these materials and help you in choosing the right TPE for your application.