Thermoplastic Elastomers

Thermoplastic Elastomers

Chapter 4 Thermoplastic Elastomers Thermoplastic elastomers (TPE) are soft thermoplastics with a low E-modulus and high toughness. Also called thermo...

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Chapter 4

Thermoplastic Elastomers Thermoplastic elastomers (TPE) are soft thermoplastics with a low E-modulus and high toughness. Also called thermoplastic rubbers, their toughness is sometimes indicated by Shore A or Shore D to characterize them, as with rubber. Their chemical structure consists of both thermoplastic hard segments and elastic soft segments. The crucial difference to traditional rubber is the lack of, or at least very slight, cross-linking between the molecular chains. Most of the various TPEs offer a cost-effective alternative to rubber in a variety of applications, thanks to its suitability for different processes such as injection molding, extrusion, film, and blow molding. Feature-wise, however, rubber has the advantage of higher elasticity and lower compression under constant load. All the thermoplastic elastomers are ideal for material recycling, although incineration for energy extraction is also an option. TPEs can generally be divided into the following groups: ■■

TPE-O, olefin-based elastomers

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TPE-U, polyurethane-based elastomers

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TPE-S, styrene-based elastomers

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TPE-E, polyester-based elastomers

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TPE-V, olefin-based elastomers with vulcanized rubber particles

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TPE-A, polyamide-based elastomers

4.1 TPE-O TPE-O (or TPO) thermoplastic elastomers, where the “O” stands for “olefin”, are a blend of polypropylene and EPDM uncured rubber particles. Because it has a PP matrix, TPO takes on a semi-crystalline structure. TPO-based elastomers are among the largest and most cost-effective TPEs available. They have been on the market since 1970, and leading manufacturers are Elasto, Elastron, Exxon Mobile, So.F.teR, and Teknor Apex. By mixing the levels of EPDM in PP at concentrations from 10 to 65%, a great range of properties can be achieved. With mixture concentrations below 20% we usually call the materials impact modified PP, while levels above 60% give the more rubber-like properties. The recycling code for TPE-O is > PP + EPDM <.

4.1.1 Properties of TPE-O + Cost-effective substitute for rubber

+ Can be UV stabilized

+ High stretch factor

+ Easy to process

+ Good tear resistance

+ Can be colored

+ Flexible at low temperatures

+ Paintable (primer required)

+ Good surface finish

− Deformation properties (i. e. setting characteristics) not as good as rubber

+ Good chemical resistance

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4.1 TPE-O

Chemical facts: The predominant TPO types are made up of monomers of poly­ propylene and uncross-linked EPDM rubber (ethylene-propylenediene-monomer (M-class)). The properties depend on the monomer units where “n” can be 90–35% and “m” 10–65%.



4.1.2 Application Areas TPO-based elastomers can be used in a variety of applications in the automotive, construction, and engineering industries, also in household products, footwear, and sportswear.

Figure 4.1 The automotive industry is the largest market for TPE-O where it is commonly used in bumpers, spoilers, and interior panels. TPE-O is sufficiently rigid and has good impact resistance even at low temperatures. It can also be painted to the same finish as the car’s sheet metal parts.

Figure 4.2 TPE-O is often used to make sports equipment such as fins, masks, snorkels, and other accessories for scuba diving. TPE-O is also used for sports shoes, ski boots, skates, helmets, protective gear, and the soft grips on rods, rackets, clubs, etc.

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Chapter 4 — Thermoplastic Elastomers

4.2 TPE-S TPE-S (or TPS) thermoplastic elastomers, where “S” is an abbreviation for “styrene block copolymer,” are usually based on SBS or SEBS (see Chemical facts below). SBS probably has the largest market and is used in applications where resistance to chemicals and aging is less critical. SEBS is characterized by substantially better heat resistance, mechanical properties, and UV resistance. TPS elastomers have been on the market since the 1960s, and leading manufacturers are: API, ChiMei, Elasto, Elastron, Enplast, Kraiburg, Radichi, Ravago, So.F.teR, Styrolution, Teknor Apex, and Uteksol. TPE-S can be processed using a variety of methods such as injection molding, extrusion, blow molding, and film blowing. One major advantage is that standard machinery for thermoplastics can be used. The recycling codes are > SBS < or > SEBS <.

4.2.1 Properties of TPE-S + The hardness can be controlled in a wide range

+ Easy to process

+ Good abrasion resistance

+ Good adhesion (for over-molding) with a number of thermoplastic resins such as PP, PS, ABS, and PA

+ Flexible at low temperatures + Can be made transparent + Good gas and moisture permeability

+ Easier to color than TPE-O

− Less chemical resistance than TPE-O

+ Can be UV stabilized

Chemical facts: SBS and SEBS are based on a styrene block copolymer with hard and soft segments. In SBS, the styrene end blocks give the thermoplastic properties, and the center blocks of butadiene give the rubber-like properties. In SEBS, it is the ethylene-co-butane molecules that provide the elastic properties.

Poly(styrene-block-butadiene-block-styrene)

Poly(styrene-block-ethylene-co-butane-block-styrene) 

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4.3 TPE-V

4.2.2 Application Areas TPS-based elastomers are used in a variety of applications across a broad range of industries: automotive, consumer products, construction, healthcare, footwear, sportswear, electrical cable, and engineering. In some of these products TPE-O can also be an option.

Figure 4.3 “Crocs,” slippers, flip-flops, and rubber boots are often made of SEBS. Other footwear industry components, such as soles, insoles, and heels, are also often in SBS or SEBS.

Figure 4.4 Soft handles with high friction for tools, pens, knives, and other grips are often made of TPE-S. Many TPS qualities have good adhesion to other thermoplastics and are therefore suitable for multi-component injection molding and coextrusion.

4.3 TPE-V TPE-V (or TPV) thermoplastic elastomers, where the “V” stands for “vulcanized,” are blends of polypropylene and dynamically vulcanized (cross-linked) EPDM rubber particles. If the rubber particles in the mixture are uncross-linked, the thermoplastic elastomer is TPE-O or TPO (where the “O” stands for “olefin”). Since the TPV elastomer consists of a PP matrix, it takes on a semi-crystalline structure. The technology behind TPE-V was patented in 1962 but was developed further in the 1970s and 1980s. The leading manufacturers of TPE-V are Elasto, Elastron, Enplast, ExxonMobil Chemicals, So.F.te.R., Teknor Apex, and Zeon Chemicals. Compared with TPE-O, TPE-V has better mechanical properties, chemical resistance, and higher service temperature. The recycling code is > PP + EPDM <.

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Chapter 4 — Thermoplastic Elastomers

4.3.1 Properties of TPE-V + Available in a hardness range of 20 Shore A to 65 Shore D

+ Excellent ozone, UV, and weathering resistance

+ Good abrasion resistance

+ Easy to process. Good adhesion to other thermoplastics

+ Good tear resistance + Wide service temperature range (–50 °C to +125 °C) + Good chemical resistance

+ Can be colored and painted (with primer) + Better fatigue resistance than TPO and TPS, although inferior to rubber

Chemical facts: TPV materials consist of monomers of polypropylene and dynamically vulcanized EPDM rubber (ethylene-propylene-diene monomer (M-class). The hardness depends on the mixing ratio and may vary between 20 Shore A and 65 Shore D. For formulas see TPE-O! 

4.3.2 Application Areas TPE-V is widely used in the automotive industry for sealing strips in doors, bellows, air ducts, and in the electrical and electronics industry for fittings to outdoor cables, connectors, and solar panels. In engineering, construction, and appliance industries, TPE-V is used for all sorts of seals, e. g. in refrigerator and washing machine doors.

Figure 4.5 Door and window seals in cars are often made of TPE-V thanks to the material’s superior resistance to wear and chemicals, its outdoor durability, and excellent sealing capacity at a wide range of ambient temperatures.

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4.4 TPE-U

4.4 TPE-U Thermoplastic polyurethane, known as TPE-U or TPU, is partially crystalline. It occurs as two different variants, the first based on polyester and the second on polyether. Polyurethane was first launched by Bayer in 1940 in textile fibers known as Perlon U. Today, Covestro markets the material under the name Desmopan. Other leading producers are BASF Elastollan and Merquinsa with Pearlthane and Pearlcoat. TPU based on polyester has the best mechanical properties and the best resistance to heat and mineral oils, while the polyether type has the best low-temperature flexibility and resistance to hydrolysis and microbiological attack. In contrast to thermoset polyurethane PUR, TPE-U is ideal for recycling. The recycling code is > TPU < or > TPE-U <.

4.4.1 Properties of TPE-U + Can be produced from renewable raw materials

+ High transparency

+ Excellent abrasion resistance

+ Good hydrolysis resistance

+ Excellent performance at low temperatures + High shear strength

+ Good oil and grease resistance – Slightly more difficult to process than other types of TPE

+ High elasticity

Chemical facts: The monomer in TPU is very large, as illustrated here:



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Chapter 4 — Thermoplastic Elastomers

4.4.2 Application Areas TPE-U is widely used in shoe soles and other footwear components because of its excellent adhesion to other materials. It is also used for the treads on wheels and for mechanical components due to its good abrasion resistance and damping capacity. The material is also used for extruded tubing to replace rubber, e. g. in fire hoses.

Figure 4.6 TPU is the predominant material in compact treads for castors and can be injection molded directly onto other plastics or metal wheels.

4.5 TPE-E The abbreviation for polyester-based thermoplastic elastomers is usually TPE-E or TPC-ET, but sometimes TPE-ET and TEEE are also used. They can all be classified as semi-crystalline and have the following characteristics: excellent toughness and elasticity, high resistance to creep, impact and flex-fatigue, and flexibility at low temperatures. They also keep their mechanical properties at elevated temperatures. TPE-E was launched in 1972 by DuPont under the trade name Hytrel. DuPont was also the first to make TPE-E from renewable raw materials, the so-called Hytrel RS, where RS stands for “Renewable Source.” Other manufacturers of TPE-E include Celanese with Riteflex, DSM with Arnitel, and LG with Keyflex. The recycling code is > TPE-E < or > TPC-ET <.

4.5.1 Properties of TPE-E + Can be produced from renewable raw material

+ Good sound and vibration damping

+ High impact strength at low temperatures

+ Easy to process, even in complex geometries

+ Excellent fatigue resistance + Flexible at low temperatures (i.e. service temperature range of −40 °C to +120 °C) + Varying temperature has a negligible effect on stiffness

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+ Excellent oil and solvent resistance

+ Easy to process thanks to very good thermal stability

4.5 TPE-E

Chemical facts: TPE-E or TCP-ET are thermoplastic polyether-ester block copolymers consisting of a hard (crystalline) segment based on polybutylene terephthalate and a soft (amorphous) segment based on long chained polyether glycols. The material properties are determined by the ratio between the soft and hard segments.

The structure of TPC-ET is:



4.5.2 Application Areas TPE-E is widely used by the automotive industry for bellows, air ducts, and air-bag covers. The electrical and electronics industry use it for cables and connectors, and it is also used in sports equipment and ski boots.

Figure 4.7 Ski boots are often manufactured in TPE-E. There are also bio-based grades. These materials can consist of almost a third renewable raw materials, but still have the same characteristics as if they were made from petroleum-based raw materials.

Figure 4.8 Airbag covers can be manufactured in TPE-E. This material is characterized by outstanding flexibility and high impact resistance at both low (−40 °C) and high (110 °C) temperatures. The TPE material is co-molded as a topcoat on a stiffer material that has slits to break more easily.

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Chapter 4 — Thermoplastic Elastomers

4.6 TPE-A Amide-based thermoplastic elastomer is abbreviated to TPE-A or PEBA (where PEBA stands for polyether block amide). The material is similar in structure to the polyester-based TPE-E, with hard and soft segments in the polymer chain (see Chemical facts below). Compared to other advanced thermoplastic elastomers, TPE-A has a lower density, better mechanical properties, higher service temperatures, and better chemical resistance. The material can be made both transparent and with permanent antistatic properties. Due to its high service temperature (> 135 °C), TPE-A sometimes replaces silicone rubber and fluoroelastomers. TPE-A was launched on the market in 1981 by Atochem under the trade name Pebax. Today, Pebax is manufactured by Arkema, who also has launched the first biobased grade under the trade name Pebax Rnew. Other manufacturers are Evonik with Vestamid E and EMS with Grilamid. The recycling code is > TPE-A < or > PEBA <.

4.6.1 Properties of TPE-A + Can be produced from renewable raw materials

+ Very high damping capacity

+ Can be made transparent

+ High service temperature

+ Good chemical resistance + Excellent strength and toughness

+ Excellent at low temperature − Cost of material

+ Good elastic recovery

Chemical facts: TPE-A-materials are composed of an amorphous polyether alternately coupled to a crystalline amide segment. The polyether segments can be based on polyethylene glycol (PEG), polypropylene glycol (PPG), or polytetramethylene glycol (PTGM). Polyamide segments can be based on PA6, PA66, PA11, or PA12. The ratio of polyether and polyethylene amide segments, which can vary from 80/20 to 20/80, controls the elastomer’s hardness–from soft (75 Shore A) to hard (65 Shore D). This formula shows the construction of a typical TPE-A, where: X = polyamide, Y = polyether



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4.6 TPE-A

4.6.2 Application Areas The products manufactured in TPE-A include sport shoes, ski boots, ski goggles, flexible hoses, bellows, “breathable” film, shock-absorbing products, “silent gear wheels,” conveyor belts, and medical products such as catheters.

Figure 4.9 Soft keys with a “rubber feeling” are produced in TPE-A, often in bright colors. The material is excellent to color via a number of methods, e. g. masterbatch, powder pigments, and liquid coloring. You can also print it in a variety of ways, including laser printing and “inmold decoration.”

Figure 4.10 TPE-A is often used in medical applications. There are biocompatible grades and it can be sterilized. The figure shows a catheter made from low-friction grade TPE-A. The material is ideal for extrusion into tubing that can then be welded using a variety of methods such as ultrasonic welding, mirror welding, induction welding, and high-frequency welding.

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