In recent years, much further development has taken place for the class of thermoplastic elastomers. These are plastics with rubber-like properties which can be processed with thermoplastic processes. A wide range of TPEs is available which differ significantly in their polymer structure and their properties. What they have in common is the basic structure in the form of block polymers, in which the elastomer segments co-polymerise with the base polymer. In many TPE classes, the proportion of elastomer chains in the total polymer can be varied within wide limits, allowing settings from very stiff to almost gel-like. TPEs have largely replaced many plasticised thermoplastics, such as plasticised polyamide. In plasticised thermoplastics, the plasticiser, which is only mixed in but not bonded with the polymer, bleeds out over time, losing its effect and forming a greasy layer on the surface, which prevents adhesion or printing and even dissolves adhesive bonds that originally adhered well. A major plastic whose elastic properties are based on plasticisers is soft PVC which is still frequently used today. TPE does not exhibit these effects. Its elastomer properties do not change over time. Today, the following main TPE groups are differentiated:
- TPE-U: Soft-polyurethane, widely used e.g. in footwear components
- TPE-S: Styrenic block copolymers
- TPE-E: Thermoplastic polyester elastomers, elastomers with good mechanical properties and excellent flexibility at low temperatures
- TPE-O: TPEs on an olefin basis (PP, EPDM)
- TPE-A: also called "PEBA"=polyether block amide. Copolymerisates on the basis of PA6, PA11, PA12. Excellent mechanical properties and flexibility at low temperatures
Most TPEs are non-polar and have a low surface energy. Before bonding or printing, they generally must be mechanically roughened and activated with wet-chemical primers. Plasma activation in an oxygen plasma is an alternative which has many benefits. In one single process step, the surface is cleaned; in particular release agents are removed. A more intensive treatment results in plasma etching of the surface, which makes chemical roughening redundant. A particularly large area of application for TPEs are hard-soft compounds in which TPE is injected onto inserts made of hard thermoplastics, or vice versa. The adhesion of the inserts can be significantly improved by pre-treatment via plasma cleaning and/or plasma activation.