Thermoplastic elastomer moulding

ISO 23711 2003 Elastomeric seals - Requirements for materials for pipe joint seals used in water and drainage applications - Thermoplastic elastomers ISO 14910-1 1997 Plastics - Thermoplastic polyester/ester and polyether/ester elastomers for moulding and extrusion - Part 1 Designation system and basis for specifications ISO 14910-2 1997 Plastics - Thermoplastic polyester/ester and polyether/ester elastomers for moulding and extrusion - Part 2 Preparation of test specimens and determination of properties... 

Whilst thermoplastic elastomers are being included in most test method standards, we do not have a procedure for their preparation and moulding. 

Thermo set as well as thermoplastic and elastomer binders are used in compression, injection and calendaring process respectively. Extrusion moulding uses elastomers as well as thermoplastic materials such as PVC. Compression and injection moulding processes make only rigid magnets, calendaring makes flexible magnets and extrusion can lead to both states. 

If the diol chain extender is used in exact molar proportion to the unreacted isocyanate, then a linear polyurethane elastomer is obtained. The resulting thermoplastic elastomer may be extruded or injection-moulded, and the properties arise from the ability of the hard and soft segments to form semi-crystalline domains that act as virtual crosslinks in the polymer and give it elastomeric properties, as shown in Figure 1.17. 

Figure 3.18 A large full-automated production plant for truck floor-covering mats using the Foam Film technology. The parts are made of thermoformed PVC (or thermoplastic elastomer) foam-backed with PU, which is kept separate from the mould with a film (adhesive type).

The main advantage of such triblock copolymers is that they can be moulded and recycled simply by heating the material above the glass transition temperature of polystyrene, unlike classical vulcanised rubbers, which cannot be reused without degradation as they are chemically cross-linked. Such SBS-based materials are called thermoplastic elastomers. However, for various reasons, including cost, the commercial use of such polymers is rather limited compared with the use of natural and classical synthetic rubbers. 

During processing block copolymers are subjected to flow. Eor example thermoplastic elastomers formed by po/y(styrene-butadiene-styrene) (PS-PB-PS) triblock copolymers, are moulded by extrusion. This leads to alignment... 

Resinous eyeUzed NR is a thermoplastic elastomer and finds applications as an adhesive, in paints and in printing inks. Its tensile properties and modulus also make it suitable for use as shoe soles. It is used as a toughening agent and reinforcing resin in, for example, hard mouldings and industrial rollers. 

Thermoplastic elastomers are materials which possess, at normal temperatures, the characteristic resilience and recovery from extension of cross-linked elastomers but which exhibit plastic flow at elevated temperatures and can be fabricated by the usual techniques applied to thermoplastics, e.g., injection moulding and extrusion. These effects are associated with linear polymers containing segments which give rise to inter-chain secondary valency forces of polar attraction. At normal temperatures, these interactions have the effects of conventional covalent cross-links but at elevated temperatures the secondary bonds dissociate and the polymer exhibits thermoplastic behaviour. 

Block copolymers are useful in many applications where a number of different polymers are connected together to yield a material with hybrid properties. For example, thermoplastic elastomers are block copolymers containing a rubbery matrix (polybutadiene or polyisoprene) containing glassy hard domains (often polystyrene). The block copolymer, a kind of polymer alloy, behaves as a rubber at ambient conditions, but can be moulded at high temperatures due to the presence of the glassy domains that act as physical crosslinks. In solution, attachment of a water soluble polymer to an insoluble polymer leads to the formation of micelles in amphiphilic block copolymers. The presence of micelles leads to structural and flow characteristics of the polymer in solution that differ from either parent polymer. 

Multi-shot moulding techniques are well established, their growth being pushed by the development of thermoplastic elastomer (TPE) materials, enabling rigid and flexible material combinations such as those described previously to be employed. These can also be seen in a variety of other applications from automotive seals to bras. 

This section will introduce material selection issues for multi-shot moulding as well as providing some introductory information on two classes of materials which may not be familiar to the non-specialist moulder namely thermoplastic elastomers (TPE) and liquid silicon rubbers (LSR). This section will begin, however, with consideration of adhesion. 

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