Stretching the limits

Continued growth has pushed thermoplastic elastomers beyond toothbrush handles into more sophisticated applications. While TPE applications have expanded rapidly over the last five years, the market has yet to plateau. Designers and processors continue to find innovative ways for this large, diverse family of materials (See Table 1) to add value to products in a host of markets.

Overmolding still growing

“We’re seeing a lot of requests to overmold TPEs on engineering resins such as polycarbonate, nylon and ABS,” says Walt Ripple, director of marketing GLS Corporation, a company that formulates and compounds all material classes of TPEs, with the exception of TPOs. “This market for overmolding is far from mature.”

The growth in overmolding on engineering resins is a result of exploding applications in the consumer electronics, appliance and hardware markets. Ripple says designers are eager to incorporate soft-touch elements on these types of high-volume consumer products because of a number of aesthetic and practical benefits. From an ergonomic standpoint, TPEs dampen vibration and provide a high-friction surface for improved gripping. TPEs can also create design elements that are the basis for immediate product differentiation — for example, unique color, feel and texture. Manufacturers are refreshing the colors of their product lines more frequently, Ripple reports. He reports another growing trend is the use of scented TPEs, for example mint- or fruit-scented materials in toothbrushes and toys.

Overmolding on an engineering resin is more technically challenging than overmolding on a commodity plastic, such as polypropylene. Many types of generic TPEs bond strongly to a commodity plastic substrate by a simple melt bond formed during injection molding. However, melt bonding between engineering resins and TPEs is inhibited by the difference in polarities of the materials, so designers have to rely on other methods, such as mechanical interlocks, to get them to stick. Ripple says GLS has developed TPE grades that chemically bond to a specific engineering resin.

“Customers typically need a lot of support for overmolding on engineering resins,” says Ripple. “This is where our experience as a custom compounder is valuable. We work with a wide range of materials and can develop a specific TPE to bond to the customer’s exact grade of resin.”

Teknor Apex has recently launched a new grade of TPE materials said to have stronger adhesion to nylon than TPEs traditionally used. Tekbond 6700 Series compounds bond well to glass-filled nylon and unfilled nylons even when the substrate is at room temperature. This performance characteristic of the materials opens up the option of using less expensive insert molding for overmolding, rather than two-shot molding. Most styrenic TPE alloys and thermoplastic vulcanizates (TPVs) traditionally specified for nylon overmolding require heated substrates to bond, so insert molding is not an option.

“Tekbond 6700 compounds enable molders to take on short-run jobs without the big investment entailed by two-shot molding,” says Edwin Tam, Teknor Apex senior market manager for consumer products, noting that the materials can also be used in two-shot molding machines. The 6700 Series compounds includes Shore A hardness grades of 45, 55 and 65.

Automotive/industrial applications break new ground

The market for automotive glass-run channels and window seals is increasingly moving away from EPDM and thermoset materials and toward TPEs, as designers seek to capture the benefits of thermoplastic materials. In addition to being recyclable, TPEs are easier to color and provide faster cycle times in comparison to EPDM. TPEs have traditionally been more costly than EPDM, but with the development of new TPE grades that better mimic the stress-relaxation properties of rubber, more OEMs and processors have gradually come around to the view that TPEs offer a net gain in these applications.

According to a source at machinery manufacturing firm, a number of Canadian-based companies, including a Magna division facility and Cooper-Standard (Stratford, ON) are injection molding TPE end-cap or corner moldings used on weatherstrip seals.

DuPont Dow Elastomers (DDE) has introduced a number of new grades of its metallocene-catalyzed polyolefin elastomer, Engage, with improved properties for automotive and other applications. The new grades are primarily based on ethylene butene. According to Mike Waller, advance manager DDE Canada, the ethylene butene grades offer improved low temperature performance and better processability.

“Low temperature performance is critical in a lot of automotive applications,” says Waller, noting that the new materials are a good fit for large-part or thin-wall applications such as bumper fascias, claddings and IP skins. Waller also reports that the grades are well suited for blow molded parts, and that DDE is working with companies to develop the first commercial blow molding applications.

In order to boost the performance of a gas cap it produces for Briggs & Stratton Corp., Thermotech (Hopkins, Minnesota) made a number of material changes, including switching from melt-processable rubber to Ticona’s Riteflex 655 thermoplastic polyester elastomer in the production of a gasket. The material, with a Shore A hardness of 55, limits swelling resulting from repeated exposure to gasoline. Thermotech molds the cap and the gasket in an eight-cavity mold. The gasket is inserted into the cap in post-mold assembly and heat-staking operation.

A Japanese car company recently replaced the PVC in the molded skin of an arm rest with a Kraton G block co-polymer TPE. The car company wanted to replace PVC because of health and environmental concerns. The Kraton G polymer, Multi-Flex TPE, was developed by Multibase, a division of Dow Corning Company specializing in compounding of engineering plastics and elastomers.

Multi-Flex TPE can be used in existing, standard injection molding machines. The material has good UV stability to reduce cracking, as well as improved color retention. When compared with PVC containing plasticizers, tests show Multi-Flex released fewer of the volatile fumes which can cause windows to fog.

Autac Inc. (North Branford, CT) has increasingly turned to TPEs to replace PVC and Neoprene in the manufacture of retractile cords. The cords are made by extruding TPE over a copper core conductor. The cords are used in a variety of food, industrial and medical applications, for instance as power cords for dental instruments.

Marie-Louise Burkle, president and CEO of Autac, says TPEs provide a number of advantages, including enhanced retractile properties and better resistance to oil and chemicals. Burkle says the company is trying to expand applications for TPE cords in the automotive and truck after-markets

“Cords used on tractor trailers to attach hydraulic and power lines are generally made of PVC, which is not the best material to use in an oil-gas environment,” Burkle says.

Softer still

One newer trend in the TPE market is the increasing use of ultra-soft TPEs and moldable gels in a variety of recreation, furniture, appliance and personal care applications, says GLS Corporation’s Walt Ripple. Ripple says there is growing interest in using these materials to improve ergonomics and comfort of everyday products and devices.

GLS has introduced what it claims is the softest injection moldable TPU in the industry. Versollan RU 2203 has a Shore A hardness of 45 and surface finish with a soft, rubbery feel and look. The material is a new addition to the Versollan line of fast-cycle time TPU elastomers launched by the company last year. The materials have exceptional oil resistance and excellent adhesion to a number of engineering resins.

“The key to these ultra-soft materials is to make them soft without losing performance properties,” Ripple says.

GLS has also introduced a new gel-like grade of TPE, Versaflex CL 2003, with a hardness/softness rating of 30 Shore 00. The Shore 00 scale provides a more accurate rating of super-
soft materials than the Shore A scale. The material is water-clear and can either be injection molded or extruded. Target applications include gel inserts for bicycle seats, shoe sole inserts, furniture arm rests and keyboard wrist pads.

A grade of NexPrene thermoplastic vulcanizate from Solvay Engineered Polymers plays a key role in the function and aesthetics of an innovative bicycle rack built by Stromberg Carlson Products, Inc. in Traverse City, MI. The All X-cess bike rack is made to hold either two or four bicycles. The rack’s key design feature is a pair of trays onto which a bicycle’s wheels are placed to support the weight of the bike. Engineer’s specified NexPrene 1064A TPV for the injection molded pads on the clamping device that locks the bicycle wheels in the trays. The UV-stabilized TPV is a fully crosslinked EPDM/PP compound designed to deliver the performance of thermoset rubber combined with the ease of thermoplastic processing.

A set of ribbed wheels injection molded with Santoprene TPV is one of the keys in the design the Pressure PoolCleaner. The ribbed wheels help maintain the ideal amount of water flow and suction under the machine to enable the cleaner to climb steep walls and prevent it from getting stuck. The wheels are injection molded in-house by Poolvergnuegen, the Santa Rosa, CA-based manufacturer of the PoolCleaner. The company specified a UV-stable, colorable grade of Advanced Elastomer System’s Santoprene TPV to provide protection against bright sunlight. The grade of Santoprene also has superior resistance to chlorine and other pool chemicals. The wheels are currently molded in electric blue.

It is this versatility to perform and please the senses that makes TPEs a hit above and below the water.