cost-effective for mass-production since it is easy to process by injection moulding or extrusionplus it is recyclable and RoHS-compliant. 33-100siemens/cm thanks to a matrix structure that holds the conductive filler of either silver-coated glass or nickel-coated graphite in place more evenly than in earlier products.Thermoplastic elastomers are remarkably versatileeither used on their own or overmoulded to provide tactile or functional features on components moulded from other polymers. Material properties can vary considerably from one grade to the nextwith the difference being down to the chemistry and the presence of filler materials or additives.One of the major advantages of thermoplastic elastomer (TPE) materials today is thatunlike their vulcanised rubber counterpartsthey can be recycled and reused. Modern TPEs are relatively easy to process by mouldingextruding and blow mouldingand the availability of clear grades means that they are found in a wide variety of applications.Furthermorenew grades are still being developed at a healthy paceso there is a continual stream of innovative materials from which designers can choose to enhance existing products or create entirely new concepts.Towards the end of 2007Arkema launched an interesting range of TPEs that are described as having a 20–90percent renewable carbon content. For 60years Arkema has been using Amino11 chemistrybased on castor oilto produce Rilsan PA11. The latest Pebax Rnew range of TPEs is also based on Amino11 chemistry and is therefore claimed to reduce fossil energy consumption as well as emissions of carbon dioxidethe main greenhouse gas (Fig.1).Pebax Rnew elastomers are available with hardnesses ranging from 25D to 72D and offer the same properties as the existing Pebax gradesincluding light weightflex fatigue resistance and good spring-back. Moreoverthese properties are retained over a wide temperature range. This ‘green’ TPE will be of interest to designers creating products for environmentally aware consumers in the electronicssportsautomotive and other markets.Regulatory complianceIn some applications the choice of material is restricted by the need for regulatory compliance. The healthcare industry is an obvious exampleand GLS has just introduced a new grade of clear TPE that is free from phthalates and plasticisers. VersaflexCLE95TPE enables manufacturers to comply with recent regulations – such as EU Directive 2005/84/EC – that limit phthalates in toys and childcare articlesas well as market demands for low-extractable TPEs (Fig.2). This material is also ‘water-clear’ and can be safely sterilised in an autoclave or by gamma radiation. It is suitable for extruded tubing and films as well as blow-moulded bagscontainersinfant care items and toys. VersaflexCLE95TPE has a hardness of 95Shore A and a 575percent elongation at breakthough GLS says it can tailor the compound to give different hardnessescolours and flow properties.The same company has also recently launched a family of TPEs for injection moulding and blow moulding based on The Dow Chemical Company’s Infuse olefin block copolymers (OBCs). Known as the Dynalloy OBC injection-moulding and blow-moulding gradesthese materials enable GLS to offer customers a broad array of standard and customised TPE alloys for a wide range of applications. Dynalloy OBCs benefit from a silky soft-touch feelexcellent colourabilityisotropic shrinkage and flow characteristics that enable their use in complex moulds with long flow pathsboth for standalone and polypropylene overmoulding applications (Fig.3).Five standard injection-moulding grades of Dynalloy OBC are offeredincluding opaque and translucent materials with hardness levels ranging from 60 Shore A down to a gel-like 5 Shore A. They feature a non-tackysoft-touch feel that avoids dust pick-up. Other performance highlights include improved compression set and creep resistance. In additionGLS utilised the rheology characteristics of OBCs to formulate a soft-feel and flexible blow-moulding grade with a 50Shore A hardness that can be customised for higher or lower softness to meet specific market needs. Target markets include consumer and office productshardwarefood packaging and medical; applications range from comfort grips for writing instruments and cosmetic itemsto prosthetics and overmoulded fabrics.In high-volume applicationsTPEs have traditionally been burdened by relatively low melt flow rateswhich makes them unsuitable for long flow paths and short cycle times. Howeversuppliers are working to improve thisas illustrated by Solvay Engineered Polymers’ series of vulcanised thermoplastic elastomers (TPVs) that feature the company’s proprietary cure technology. Known as Nexprene9500SHF (super high flow) materialsthese TPVs exhibit viscosities that are up to 60percent lower than those of conventional TPEs. Nexprene9500SHF series grades range in hardness from 55 to 85 Shore A. Each has been developed to deliver higher melt-flow ratesshorter fill timeslonger flow paths and lower injection pressures.Hamid Tavakolithe product manager for Nexprene9500SHFcomments: "The viscosity levels exhibited by these materials are quite remarkable. The new SHF grades flow so much better that injection moulding cycles can be as much as 15percent shorter than with traditional vulcanised thermoplastic elastomers."Initial evaluations of moulded parts indicate that surface aesthetics attainable using Nexprene9500SHF materials are superior to those achieved using conventional TPEswith gloss levels controlled and consistentand weld lines much less apparent.Nexprene9500SHF series materials are very stable and they can be processed at higher temperatures than standard TPVs. Processing at elevated temperatures accelerates and improves bonding to other TPVs in applications such as corner mouldings for automotive glazing. Among the applications for which the new series was developed is the encapsulation of window glasswhich typically involves lower temperatures (between 190 and 245¢ªC) and lower moulding pressures to avoid glass breakage. At such temperaturesthe Nexprene9500 series materials exhibit viscosities that remain 40 to 60percent below those of traditional TPVswhich can contribute to lower scrap rates and lower costs (Fig.4).In soft-touch or flexible injection moulded partsNexprene SHF materials are suitable for hard-to fill mouldssuch as for parts with very thin walls or those involving long or complex flow paths. The excellent surface aesthetics produced by these elastomers is particularly important to designers of automotive interiors.Insulating and conductingPolymers are usually classified as insulators – both thermally and electrically – but there are occasions where conductivity is beneficial. Premix is a company that has addressed this needwith its Pretherm thermally conductive TPEs and Preseal electrically conductive TPEs.Pretherm TPE compounds are designed for heat dissipation in small electronics devicesutilising the filler boron nitride to give good thermal conductivity of up to 5.3.W/mK without influencing the material's electrical insulating properties. This material is said to be The trend in the electronics industry is towards smaller devices with more features. Howeverthe smaller the device isthe greater the need to control excess heat. Pretherm compounds are useful in electronics applications such as gasketscasings and gap filling. Traditionally silicon-based compounds and glues have been used as an intermediate heat conductor between the circuit board and metal plate. Premix claims that using Pretherm TPE compounds enable the production process to be simplified and accelerated radicallyas Pretherm TPEs can be co-moulded with the metal component without the need for primers. Because the compounds use the thermally conductive but electrically insulating filler boron nitridethey can be placed in direct contact with sensitive electronics without any risk of short-circuits.One application where there is a specific requirement for electrical conductivity is shielding against EMI (electromagnetic interference). Premix has developed PresealTPE5020 and PresealTPE6080 for EMI shieldingwith a uniform conductivity of Other advantages of the Preseal materials include good adhesion to a wide range of polymersincluding polyamides and polycarbonatesand no need for a primer when used on aluminium and stainless steel. The compounds can be processed by injection moulding and extrudingflowing characteristics are described as excellentcycle times are short and there is no need for vulcanisation. Process waste and end products can be recycled with no degradation of mechanical or electrical properties.While this article has looked at a selection of TPEs with specialist propertiesthere are numerous other grades available for general standaloneovermoulding and co-moulding applications. Around 10 years agowhen toothbrushes first started to be marketed with soft overmoulded TPE gripsthe author was told that the process added around 5 per cent to the production costyet the toothbrush could be sold for a 90percent premium. With product differentiation becoming ever more importantTPEs should not be viewed as relatively expensive alternatives to other thermoplasticsbut as a cost-effective way to add perceived value.
