Basically, polymer materials are used for constructing products. In other words creating a product with a certain shape, dimensions, strength, weight and color. Hundreds of different polymer materials are available nowadays. Construction or engineering materials can be divided in four main categories: thermoplasts, thermosets, elastomers and composites.
Thermoplasts are the most popular polymer materials. A strong asset of thermoplasts is their ease of processing and shaping. They can be processed in the melt and subsequently easily shaped into almost any desired product design.
Thermoplasts range from bulk plastics to specialty materials. This variety is depicted in the triangle below; in general, the higher the technical performance the more expensive the material.
Drawbacks of thermoplasts are a limited dimensional stability, chemical resistance and hardness. If these criteria are important, so-called thermosets are preferred. Thermosets are heavily cross-linked polymer materials that yield very tough and stable structures. However, once shaped the process is irreversible (in contrast to thermoplasts), but techniques that are used for shaping wood and metal can be used perfectly on thermosets.
Elastomers are flexible and elastic materials that when deformed can recontract to their original shape upon releasing the applied tension. They are either chemically cross-linked (rubbers) or physically cross-linked (thermoplastic elastomers or TPE's).
The first rubber was obtained from a natural source, the tree Hevea Brasiliensis, by extracting it in the form of an aqueous emulsion (latex). This very valuable rubber is extensively used, but for certain applications new synthetic rubbers are developed with more specific and tunable properties. Some widely used synthetic rubbers are listed in the table below.
|Isoprene Rubber (IR)|
Synthetic version of natural rubber
Low hysteresis losses
Good flow properties
Injection molded articles
|Butadiene rubber (BR)|
|Styrene-butadiene rubber (SBR)|
|Butyl Rubber (IIR)|
|Chloroprene Rubber (CR)|
Good chemical resistance
The degree of cross-linking is an important parameter, it largely determines the properties of a rubber. Rubbers are chemically cross-linked (vulcanized) whereas TPE's are physically cross-linked. Physical cross-linking is induced by crystallization or domain formation. In contrast to rubbers, TPE's can be easily processed in the melt since the physical cross-linking is reversible. There are several sorts of TPE's:
TPE-S, triblock-copolymers based on polystyrene (SBS, SIS)
TPE-U, polyurethane based
TPE-E, polyester based
TPE-A, polyamide based
TPE-O, unvulcanized, based on polypropylene (PP/EPDM)
TPE-V, partly vulcanized, based on polypropylene
TPE's are used in all kinds of industries, such as the automotive industry and shoes and clothing manufacturing. They are also suitable to modify the properties of other materials, such as bitumen. Furthermore, TPE's are widely used in adhesives, coatings and as climate membrane in sportswear and rainproof clothing (e.g. Sympatex®).
Composites are polymer materials that combine two or more different materials. Most popular are fiber composites and filler composites. For very demanding constructions for which weight and strength are crucial factors, such as airplanes and cars, fiber composites are increasingly used. The incorporation of fibers strongly alters the mechanical properties of a polymer material; the stiffness and tensile strength can be increased for instance, which broadens the application range of the material. Also the use of inorganic fillers such as glass beads generates new application opportunities. Fillers can also be applied to reduce costs, carbonates are mainly used for that purpose. But a filler like, for instance, aluminum trihydrate (ATH) adds value to the material as it improves the fire resistance of a polymer and increases the stiffness at the same time.