New Thermoplastic Elastomers Offer New Options for Designing Green Products

Synthetic elastomers offer significant promise in addressing global green issues, ranging from water purification to PVC replacement

Doug Smock, Contributing Editor -- Design News, September 2, 2009

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Technology advancements are creating new engineering opportunities for elastomers, offering designers and manufacturers new options for developing green products.

One promising application is in nanofiltration of water sources, where existing technologies are costly or fall short in the face of growing requirements for fresh water in many parts of the globe.

"Current reverse osmosis membranes lack adequate chlorine tolerance due to their dependence on polyamide chemistry; chlorine-tolerant membrane materials would be of great value to the industry," says Donald R. Paul, director, Texas Materials Institute and engineering professor at the University of Texas.

Reverse osmosis is a filtration process in which pressure is used to force a solution through a membrane, retaining salt on one side and allowing water to pass to the other side.

"Sulfonated polymers have been identified as promising materials for membrane applications including use in fuel

cell membranes and reverse osmosis membranes," said Paul in a paper presented at the Annual Technical Conference of the Society of Plastics Engineers this year. Two colleagues at the Texas Materials Institute co-authored the report with Paul.

Among the most interesting materials candidates are selectively midblock sulfonated copolymers, which are said to provide excellent performance in water transport, chemical resistance, selective gas permeability and ion-exchange properties, as well as strong mechanical performance in both wet and dry environments.

New engineering solutions are envisioned for applications in desalination, electrodeionization, electrodialysis, humidification and dehumidification, breathable protective clothing, battery separators, fuel cell membranes, sensors and actuators, reverse osmosis, medical devices, filtration, gas separation, performance outerwear and apparel, energy recovery and antifouling.

Kraton MD9150 and MD9200 were unveiled at the National Plastics Exposition in Chicago last summer at the International Plastics Design Competition, co-sponsored by Design News. The materials were used in bundled arrays of tubes that purify or desalinate water.

"We designed this polymer to compete with high-end membranes," Dr. Lothar Freund, vice president of technology at Kraton, told Design News in an interview. "Many of the existing membranes are not chlorine-resistant. Consequently, in one step, we can desalinate water without removing the chlorine."

Kraton created a new pentablock copolymer architecture, where the polystyrene midblock is modified with a sulfonic acid group. The precursor polymer is a poly (t-butyl styrene-b-(ethylene-r-propylene)-b-styrene-b-(ethylene-rpropylene)-b-(t-butyl styrene) copolymer) or (tBS-EP-S-EP-tBS). The styrene block is selectively sulfonated via acyl sulfate chemistry.

Varying sulfonation levels allow ion exchange capacity of 0.4 to 2.0 meq/g (milli-equivalent per gram). The ion selectivity and unique polymer architecture results in efficient salt rejection.

When cast onto hollow fiber membranes, the sulfonated polymers create a layer which can achieve nanofiltration of water sources. The membranes combine high strength and hydrophilicity, resulting in very low energy consumption. They can be used in a larger water purification or desalinization plant.

Membrane or solution

Engineers should note that the copolymers come in membrane and solution form, providing the opportunity to design unique shapes as well as coatings and laminations. These new sulfonated copolymers offer the customer a greener solution by reducing the processing temperatures as compared to current technology.

One  competing system is perfluorosulfonic acid polymer, a randomly sulfonated copolymer which dates back to the 1960s and is widely used in chlor-alkali cells, fuel cells and batteries. Sulfonation of hydrogenated rubber as end blocks is also widely practiced, but lacks wet strength at sufficient sulfonation levels.
Dr. Freund said that samples of the new Kraton copolymers are being tested by companies that specialize in water filtration.

Dr. Paul and his colleagues confirmed the effectiveness of the new copolymers in research conducted at the Texas Materials Institute. They added: "It is curious that the pure water permeability decreases and the salt permeability

increases upon switching from a batch process to a continuous solution-casting process. Further studies on the

microstructure of these materials are needed to identify the exact cause of this phenomenon."

Other major environmental pushes for highly engineered synthetic elastomers include PVC and bromine replacement. In some cases, TPEs are being made from biobased components.

PolyOne's GLS Thermoplastic Elastomers business developed Versaflex Bio TPEs, which are formulated with up to 70 percent renewable resources. These translucent grades are available in a range of Shore A hardnesses.

"These TPEs break new ground with an exceptionally high level of renewable content, offering designers and manufacturers new options for creating products that reduce environmental impact and appeal to eco-conscious consumers," says Walter Ripple, general manager, GLS Thermoplastic Elastomers.

At this year's National Plastics Exposition, DuPont also exhibited its new renewably sourced TPE, designated Hytrel RS. Hytrel RS is said to provide all the performance characteristics of traditional Hytrel materials. The exact biomass used is a DuPont secret. The TPE is priced at a 10 percent premium. Arkema's Pebax also may include a renewably sourced material, such as caster oil.

A new series of thermoplastic elastomer compounds from Teknor Apex meet UL criteria for flame retardance while providing flexibility and toughness over a broad temperature range.

The four Telcar TL-1934 compounds are styrenic formulations available with Shore A hardnesses from 56 to 88. Teknor Apex recommends them for insulation, jackets, and molded parts for flexible cords, coil cords, and cables in power tools, appliances, industrial robots, welding equipment, and audio and lighting systems.

"Telcar TL-1934 compounds provide excellent flame resistance while meeting RoHS standards by containing no polybrominated diphenyl ether (PBDE) flame retardant," says Andy Claytor, sales director. "In addition, these products deliver excellent performance at temperature extremes and in outdoor environments, exhibit rubber-like flexibility, are oil-resistant, and are available in a broad range of hardnesses."