Specialist thermoplastic elastomer grades
Thermoplastic elastomers are remarkably versatile, either 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 next, with 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 that, unlike their vulcanised rubber counterparts, they can be recycled and reused. Modern TPEs are relatively easy to process by moulding, extruding and blow moulding, and the availability of clear grades means that they are found in a wide variety of applications.
Furthermore, new grades are still being developed at a healthy pace, so 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 2007, Arkema launched an interesting range of TPEs that are described as having a 20–90percent renewable carbon content. For 60years Arkema has been using Amino11 chemistry, based on castor oil, to 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 dioxide, the 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 grades, including light weight, flex fatigue resistance and good spring-back. Moreover, these 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 electronics, sports, automotive and other markets.
Regulatory compliance
In some applications the choice of material is restricted by the need for regulatory compliance. The healthcare industry is an obvious example, and 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 articles, as 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 bags, containers, infant care items and toys. VersaflexCLE95TPE has a hardness of 95Shore A and a 575percent elongation at break, though GLS says it can tailor the compound to give different hardnesses, colours 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 grades, these 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 feel, excellent colourability, isotropic shrinkage and flow characteristics that enable their use in complex moulds with long flow paths, both for standalone and polypropylene overmoulding applications (Fig.3).
Five standard injection-moulding grades of Dynalloy OBC are offered, including opaque and translucent materials with hardness levels ranging from 60 Shore A down to a gel-like 5 Shore A. They feature a non-tacky, soft-touch feel that avoids dust pick-up. Other performance highlights include improved compression set and creep resistance. In addition, GLS 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 products, hardware, food packaging and medical; applications range from comfort grips for writing instruments and cosmetic items, to prosthetics and overmoulded fabrics.
In high-volume applications, TPEs have traditionally been burdened by relatively low melt flow rates, which makes them unsuitable for long flow paths and short cycle times. However, suppliers are working to improve this, as 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) materials, these 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 rates, shorter fill times, longer flow paths and lower injection pressures.
Hamid Tavakoli, the product manager for Nexprene9500SHF, comments: "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 TPEs, with gloss levels controlled and consistent, and 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 glass, which typically involves lower temperatures (between 190 and 245¢ªC) and lower moulding pressures to avoid glass breakage. At such temperatures, the Nexprene9500 series materials exhibit viscosities that remain 40 to 60percent below those of traditional TPVs, which can contribute to lower scrap rates and lower costs (Fig.4).
In soft-touch or flexible injection moulded parts, Nexprene SHF materials are suitable for hard-to fill moulds, such 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 conducting
Polymers 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 need, with its Pretherm thermally conductive TPEs and Preseal electrically conductive TPEs.
Pretherm TPE compounds are designed for heat dissipation in small electronics devices, utilising 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
cost-effective for mass-production since it is easy to process by injection moulding or extrusion, plus it is recyclable and RoHS-compliant.
The trend in the electronics industry is towards smaller devices with more features. However, the smaller the device is, the greater the need to control excess heat. Pretherm compounds are useful in electronics applications such as gaskets, casings 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 radically, as 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 nitride, they 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 shielding, with a uniform conductivity of
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.
Other advantages of the Preseal materials include good adhesion to a wide range of polymers, including polyamides and polycarbonates, and no need for a primer when used on aluminium and stainless steel. The compounds can be processed by injection moulding and extruding, flowing characteristics are described as excellent, cycle 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 properties, there are numerous other grades available for general standalone, overmoulding and co-moulding applications. Around 10 years ago, when toothbrushes first started to be marketed with soft overmoulded TPE grips, the author was told that the process added around 5 per cent to the production cost, yet the toothbrush could be sold for a 90percent premium. With product differentiation becoming ever more important, TPEs should not be viewed as relatively expensive alternatives to other thermoplastics, but as a cost-effective way to add perceived value.
