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Thermoplastic elastomer compounds continue upward trend

Applications for thermoplastic elastomers have continued to grow largely at the expense of thermoset rubbers. Innovation in processing and new materials is driving much of this growth Jennifer Markarian reports on some these developments.

Thermoplastic elastomers (TPEs), which combine the processing advantages of thermoplastics with the flexible, low-modulus properties of elastomers, continue to grow in a wide variety of applications. While growth is healthy, TPE compounders globally face a very competitive market. In North America and Western Europe, the TPE supply chain is under severe stress, says Bob Eller, president of Robert Eller Associates LLC, which recently published a multi-client study of the TPE market. He notes that in North America, compounders face a recessive economic environment as well as a severe profitability squeeze between high energy and raw material costs and the difficulty of passing these costs along. The North American automotive market, the largest outlet for olefinic TPVs, is in decline, and other key end-use markets for TPEs, such as consumer goods, are shifting production from North America to Asia. Western Europe is also experiencing a manufacturing shift as TPE applications move to Eastern Europe and China. Multinational compounders have responded to the manufacturing shift by increasing their manufacturing presence in China (see July/August 2008 Plastics Additives & Compounding). However, multinational compounders face competition from local Chinese producers, notes Mr. Eller. In this competitive environment, speciality compounders seek to add value to TPEs by using additive technology, finding the right material combinations from the wide range of available TPEs, providing expertise to help customers that wish to compress their development cycle, and using compounding technology to measure and mix recipes for optimized processing and properties. Additives and new materials aid in the quest for adding value and meeting customer needs.

Types of TPEs Block copolymer TPEs are made up of segmented blocks formed by polymerizing a thermoplastic monomer with an elastomeric comonomer. For example, a styrene block copolymer (SBC or TPE-S or TPS) is styrene-ethylene-butadiene-styrene (SEBS). Engineering TPEs include thermoplastic urethanes (TPE-U or TPU), copolyesters (COPE), and polyether block amides (PEBA or TPE-A). Other types of copolymer TPEs can include the relatively new metallocene-catalyzed polyolefin plastomers (POP) or elastomers (POE) and the brand-new olefinic block copolymers (OBC). Blended TPEs contain an elastomeric phase dispersed in a thermoplastic phase, which is typically polypropylene (PP). In the olefinic TPE category, thermoplastic olefins (TPE-O or TPO) have a non-crosslinked elastomeric phase, while olefinic thermoplastic vulcanates (TPE-V or TPV) have a crosslinked elastomeric phase such as EPDM (ethylene-propylenediene terpolymer). Super-TPVs are blends of an engineering thermoplastic such as polyamide with a high-performance, silicone or acrylate-based elastomer.

Figure 1: Molecular structure of TPE blends (left) and TPE block copolymers (right). Source: Clariant.

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ISSN1464-391X/08 © 2008 Elsevier Ltd. All rights reserved.

TPE compounding

New, improved TPEs compete in higher-value applications

Price pressures and the trend towards commoditization of standard TPEs have led suppliers to develop new materials with enhanced properties that can add value. New TPOs and SBCs are competing with engineering TPEs such as TPU, COPE, and polyamide-based TPEs. For example, some SBCs and TPOs are replacing high-cost COPE in automotive air bag covers, where they can now meet aesthetic requirements and stringent performance requirements at both high and low temperature extremes. TPUs have a wide property envelope, including good abrasion resistance and weatherability, but they are expensive. TPEs are taking pieces of the TPU market where TPEs can meet the property requirements and offer the advantage of a softer material, in applications such as wire and cable and conveyer belts, notes Dr. Sakhalkar. A new class of TPVs can compete with highperformance rubbers like acrylic elastomer (ACM) or ethylene-acrylic elastomers (AEM). Compounder KRAIBURG TPE collaborated with polymer producer Lanxess to develop high-temperature, media-resistant

Figure 2:TPE families. Source: Robert Eller Associates Inc.

Innovation drives TPE growth

TPE growth over the past several years has come largely from replacing thermoset rubbers and from growth of `soft-touch' overmoulding in a wide range of automotive, consumer, and other applications. Continuing innovation ­ such as multishot moulding and new materials that push the property envelope ­ is driving growth in both current and new applications, says Paul Killian, technical market manager for TPEs at RTP Company. "New things are still happening in overmoulding," says Joe Kutka, technology launch manager at PolyOne's GLS, noting that overmoulding TPEs are providing more than just bonding and feel by improving other properties like oxygen and moisture barrier. New compounds with improved bonding properties, wider processing windows, and ability to bond to multiple resin types meet processors' demand for ease-of-use, adds Killian. Automotive continues to be the largest market for TPEs, with soft-touch overmoulding common in automotive interiors. TPOs and TPVs continue to grow in automotive interiors because they are less expensive than TPU and save processing costs when used in two-shot moulding, note experts. High-gloss, moulded-in-colour TPOs are beginning to penetrate weatherable, exterior

automotive applications such as body-side mouldings, says Bill Greer, sales and marketing manager at speciality compounder Vi-Chem Corporation, whose compounds are in commercial trials. "Matching bodypaint colour and scratch-and-mar resistance are key, along with low temperature impact and 5 to 10 year weatherability," notes Mr. Greer. New super-TPVs are expected to penetrate under-the-hood (bonnet) automotive applications, where their improved heat and oil resistance allows them to compete with thermoset rubbers. A new growth area is the medical market, where increasing requirements for design flexibility, new regulatory restrictions of additives such as phthalates, and the potential for replacing PVC are opening up opportunities for TPEs, comments Dr. Sachin Sakhalkar, new business development manager for the Thermoplastic Elastomer Divison of Teknor Apex. GLS's Kutka adds that healthcare and medical part producers are concerned about the `green' aspects of product disposal as well as product safety-in-use. Part producers are looking for plasticizer-free products that are clear and able to be sterilized. RTP's Killian notes that TPEs add value in medical devices, such as in a softtouch, ergonomic grip that can make a tool easier to use. He adds that disposable, home-care products are another growing segment for TPEs.

Stoppers made from TPEs manufactured by PolyOne GLS.

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TPE compounding

TPVs that can be used in under-the-hood (bonnet) applications. "Previously, TPVs have been limited to use-temperatures less than 140°C and have been resistant to water-based solvents but not to fuels," explains Dr. Jörg Mertinkat, deputy research and development director at KRAIBURG. The new materials use a polar elastomer that is resistant to nonpolar media like fuels and can withstand temperatures up to 170°C. They are currently under trial at processors and are scheduled for commercial introduction this autumn. "The driving force for use will be the processing cost-savings offered by thermoplastics compared to thermosets. Recyclability is another benefit," says Dr. Mertinkat. Dow recently introduced a new class of allolefinic TPE called InfuseTM olefinic block copolymer (OBC), which compounders are using to create value-adding properties. The all-olefinic TPE has properties similar to a styrenic TPE, which gives it improved costperformance. Compounds using OBC offer a non-tacky, dry feel and diminished dirt and lint pick-up. "OBC offers a feel and a rheology that we can't get with other chemistries," says Kutka at GLS, whose new Dynalloy OBC alloys based on Infuse OBC include injection moulding grades and new blowmouldable grades. Teknor Apex's new Telcar OBC TPE, a blend of a rigid polyolefin with Infuse OBC, has improved properties and processing compared to styrenic TPEs with similar Shore hardness, standard TPOs, and TPVs, says the company. While standard TPEs contain mineral oils to improve cost and flowability, Telcar OBC has the same benefits without oil, and is also plasticizer free. Absence of these migratory components is important in medical, food, and potable water applications with strict extractable requirements, explains Dr. Sakhalkar. New TPE materials can be used to enhance material flow at the moulding or extrusion step, says Bryan Kazmer, general manager at Vi-Chem. He explains: "Beyond just reducing cycle time, new compounds give processors the ability to make cosmetic parts without flow-induced blemishes while maintaining acceptable cycle times. This is especially important in the niche of high-gloss, superior finish moulding, where final cosmetics come from the mould rather than the paint booth." Compounders are now considering environmental sustainability of TPEs. "All-olefinic TPEs have a lower carbon footprint than other TPE types. Also, efforts are underway to get renewable content through use of bio-based TPEs and bio-based colours," notes Dr. Sakhalkar. Arkema's Ken Rittenhouse, industrial manager for Pebax, describes the use of bio-based TPEs as a revolution that is just beginning. He comments: "Bio-based materials have been looked at for many years, but are now actually being used. We see lots

TPE for refrigerator door seals from Teknor Apex. (Photo: Dow Chemical Company) of interest, particularly in Europe," Arkema's Pebax Rnew, introduced at K 2007, uses a castor-oil based polyamide monomer (Arkema's well-established Rilsan11) with a polyether comonomer. Arkema offers a range of Rnew grades, with renewable carbon contents of 25-95 per cent depending on the formulation. Commercial trials started in 2008, with initial applications in sporting goods and other consumer goods. Applications in medical and electronics markets are expected to follow. Rnew has properties and processing equivalent to Pebax grades based on oil-based polyamides, says Basker Lalgudi, Arkema's marketing development manager for Pebax and Rilsan. Other new bio-based TPEs include EMS-Grivory's new biosourced PA-12 Grilflex PEBA products and Merquinsa's Pearlthane® ECO TPU.

Additives aid push for improved properties and processing

In addition to new resins, additives are helping drive improved properties in TPEs. Clariant's amorphous Licocene® performance polymers are used to improve melt flow and release properties of TPEs. Crystalline, maleic-anhydride grafted Licocene products are used to enhance adhesion of TPEs to other resins or to other surfaces like metal or glass, say the company.

PolyOne GLS supplies TPE compounds for a number of end-use applications.

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TPE compounding

Teknor Apex used Dow-developed technology in creating new TPEs for applications including soft toys and soft grip tools. (Photos: Dow Chemical Company)

Improving scratch and mar resistance involves balancing the use of fillers, which affect scratch appearance to varying degrees depending on their geometry and loading, and additives that soften the matrix or lubricate the surface to improve scratch resistance. Ciba's Erasure® SR100 helps to meet demanding requirements for automotive interiors, and is also gaining interest in consumer products, notes Dr. Marie Lauer Beret, Ciba's marketing manager for automotive in Europe, Africa, and the Middle East. Recent studies at Ciba show that, compared to traditional slip agents used for improving scratch and mar, Erasure SR100 provides better scratch and mar resistance together with less potential for negative interactions in TPE compounds.

Use of talc as a reinforcing filler is expected to increase, driven by the desire to replace a portion of the increasingly expensive resin as well as the desire for improved properties. Talc improves flexural modulus, heat deflection temperature, creep resistance, and dimensional stability, and can be used in TPEs to help meet increasing property requirements, says Gilles Mali, rubber development manager at Rio Tinto Minerals. He predicts that talc will take market share from calcium carbonate, which is widely used as a cheap filler in SEBS but does not improve strength properties. Rio Tinto Minerals' Mistron® talc has a microcrystalline morphology, with round particles that create a high surface area, yielding higher reinforcement in SEBS than talcs with a macrocrystalline, lamellar morphology. Mistron talcs are also used in

Play mat made of Teknor Apex TPE. (Photo: Dow Chemical Company)

TPVs to improve dispersion of the rubber phase and as nucleating agents in foamed TPVs. In TPOs, where talc acts to reinforce the thermoplastic phase, talcs with highly lamellar structures provide the highest performance. For example, the high aspect ratio, or lamellarity, of Rio Tinto Minerals' HAR® talcs results in high stiffness and strength properties because PP crystallizes along the broad surface of the flat, lamellar particles, explains Mr. Meli. Rio Tinto Minerals' Jetfine® talcs combine a lamellar structure with ultrafine particle size, resulting in an improved balance of stiffness and low temperature impact strength. TPEs use conductive additives for electrostatic dissipative (ESD) applications, but adding conductive fillers typically increases modulus and reduces flexibility, says Mr. Killian. He notes that RTP recently addressed this shortcoming, introducing a new series of conductive TPV compounds that maintains flexibility and elastomeric feel so that the compounds can compete with conductive EPDM rubber. Initial target uses include grounding applications in electrical, power distribution and automotive markets. RTP is also actively developing carbon nanotube modified TPEs that will provide a new level of highly flexible, conductive TPEs. Focus on environmental effects, health and safety concerns, and regulatory issues affect many additives used in TPEs. "We see customers re-examining their product portfolio in light of the green movement and the desire to use chemistries that are both considered safe now and should still

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TPE compounding

be considered safe ten years from now," says Mr. Kutka. For example, changes to the European Union's RoHS that further limit use of brominated flame retardants have led suppliers to introduce additional halogenfree compounds. Teknor Apex introduced new RoHS-compliant halogenated TPEs and a series of halogen-free V-0 TPEs. "While non-halogenated compounds are highly filled and therefore generally more rigid and brittle, our new compounds are formulated to overcome this, with hardnesses down to 50 Shore A," says Dr. Sakhalkar. Clariant recently added a non-halogenated flame retardant for polyester-based TPEs to its range of Exolit® OP. The phosphorusbased flame retardants effectively balance mechanical properties and flame retardancy in TPEs, says the company. in vulcanization, because phenolics can interfere with colorants. "Peroxide vulcanization can be more difficult, but a TSE in connection with side-fed EPDM offers good temperature control, which results in a stable reaction," explains Mr. Dahl. One of the difficulties in TSE compounding of TPVs is feeding the rubber component (often oil-extended EPDM) that is supplied in bales. The rubber bales can be ground into pellets or crumbs with talc added to prevent agglomeration, but these are free-flowing for only a few hours before they again become sticky and difficult to feed, notes Mr. Dahl. Krauss-Maffei Berstorff's solution to this problem is a patented TPV compounding system that continuously feeds strips of baled EPDM into a single-screw rubber extruder where they are melted and fed through a rubber gear pump into the co-rotating twinscrew extruder. A pressure-based control system adjusts the single-screw extruder speed so that the downstream gear pump is always completely filled and can act as a precise volumetric feeder. The idea for continuous EPDM feeding began in 2004, but was refined and introduced in mid-2008 as a TPV compounding system. Several commercial lines are now running, notes Mr. Dahl. "TPVs are growing ­ they are the TPE of the future," says Mr. Dahl, who notes that Krauss-Maffei is working on improving machines to compound new TPV blends that substitute PP or EPDM with other materials to improve properties. Contacts: Arkema Tel: +1 800 225 7788 Website: www.arkema.com Ciba Specialty Chemicals Tel: +1 302 992 5600 Website: www.ciba.com/plastics Clariant Tel: +49 821 479 2693 Website: www.clariant.com KraussMaffei Berstorff Tel: +49 89 88990 Website: www.kraussmaffei.com KRAIBURG TPE Tel: +49 8638 9810 278 Website: www.kraiburgtpe.com PolyOne (GLS) Tel: +1 440 930 1000 Website: www.polyone.com Rio Tinto Minerals Tel: +33 561 50 20 20 / +1 800 325 0299 (U.S.) Website: www.riotintominerals.com Robert Eller Associates, Inc. Tel: +1 330 670 9566 Website: www.robertellerassoc.com RTP Company Tel: +1 507 454 6900 Website: www.rtpcompany.com Teknor Apex Tel: +1 401 725 8000 Website: www.teknorapex.com Vi-Chem Corporation Tel: +1 616 247 8501 Website: www.vichem.com

New TPV compounding equipment

TPVs can be compounded in internal batch mixers, Buss Kneaders, and twinscrew extruders (TSE). TPV compounding includes both mixing the thermoplastic and elastomeric phases and crosslinking the elastomeric phase. Ralf J. Dahl, vice-president for twin screw extruders at Krauss-Maffei Berstorff, sees a trend towards using continuous TSE compounding rather than batch mixing of TPVs. He also sees growing use of peroxide accelerators to replace phenolics

Machine combination for TPE-V production. (Diagram: KraussMaffei)

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