Thermoplastic elastomer

Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) which consist of materials with both thermoplastic and elastomeric properties. While most elastomers are thermosets, thermoplastics are in contrast relatively easy to use in manufacturing, for example, by injection molding. Thermoplastic elastomers show both advantages typical of rubbery materials and plastic materials. The principal difference between thermoset elastomers and thermoplastic elastomers is the type of crosslinking bond in their structures. In fact, crosslinking is a critical structural factor which contributes to impart high elastic properties. The crosslink in thermoset polymers is a covalent bond created during the vulcanization process. On the other hand the crosslink in thermoplastic elastomer polymers is a weaker dipole or hydrogen bond or takes place in only in one of the phases of the material(see TPE-V or TPV) . TPE is commonly used to make suspension bushings for automotive performance applications because of its greater resistance to deformation when compared to regular rubber bushings. There are six generic classes of TPEs generally considered to exist commercially. They are styrenic block copolymers, polyoefin blends, elastomeric alloys, thermoplstic polyurethanes, thermoplstic copolyester and thermoplastic polyamides. The examples of TPE products that come from block copolymers group are Pellethane (Dow chemical), Arnitel (DSM), Hytrel (Du pont) and more. While there are three main commercial products of elastomer alloy: Santoprene (Monsanto), Geolast (Monsanto) and Alcryn (DU Pont).

In order to qualify as a thermoplastic elastomer, a material must have these three essential characteristics:

1. The ability to be stretched to moderate elongations and, upon the removal of stress, return to something close to its original shape.

2. Processable as a melt at elevated temperature.

3. Absence of significant creep.

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Background

It was not until the 1950s, when thermoplastic polyurethane polymers became available, that TPE became commercially reality. During the 1960s styrene block copolymer became available, and in the 1970s a wide range of TPEs came on the scene. The worldwide usage of TPEs (680,000 tons/year in 1990) is growing at about 9% per year.

Advantages of thermoplastic elastomer

TPE materials have the potential to be recyclable since they can be molded, extruded and reused like plastics, but they have typical elastic properties of rubbers which are not recyclable owing to their thermosetting characteristics. TPE also require little or no compounding, with no need to add reinforcing agents, stabilizers, cure systems. Hence, batch-to-batch variations in weighting and metering components are absent leading to improved consistency in both raw materials and fabricated articles. TPEs can be easily colored by most types of dyes. Besides that, it consumes less energy and closer and more economical control of product quality.

Disadvantages of thermoplastic elastomer

The disadvantages of TPEs relative to conventional rubber or thermoset are relatively high cost of raw materials, general inability to load TPEs with low cost fillers such as carbon black therefore prevents TPEs from being used in automobiles tires, poor chemical and temperature resistance, and its compression set is high and low thermal stability.

Processing

The two most important fabrication methods with TPEs are extrusion and injection molding. Compression molding is seldom, if ever, used. Fabrication via injection molding is extremely rapid and highly economical. Both the equipment and methods normally used for the extrusion or injection molding of a conventional thermoplastic are generally suitable for TPEs. TPEs can also be processed by blowing, thermoforming and heat welding.

References

• Modern Plastic Mid-October Encyclopedia Issue, Introduction to TPEs, page:109-110

• Thermoplastic Elastomer Journal by Mustafa Caykoylu