Technology advancements are creating new engineeringopportunities for elastomers, offering designers and manufacturers new optionsfor developing green products.

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

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

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

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

cell membranes and reverse osmosis membranes," said Paul ina paper presented at the Annual Technical Conference of the Society of PlasticsEngineers this year. Two colleagues at the Texas Materials Institute co-authoredthe report with Paul.

Among the most interesting materials candidates areselectively midblock sulfonated copolymers, which are said to provide excellentperformance in water transport, chemical resistance, selective gas permeabilityand ion-exchange properties, as well as strong mechanical performance in bothwet and dry environments.

New engineering solutions are envisioned for applications indesalination, electrodeionization, electrodialysis, humidification anddehumidification, breathable protective clothing, battery separators, fuel cellmembranes, sensors and actuators, reverse osmosis, medical devices, filtration,gas separation, performance outerwear and apparel, energy recovery andantifouling.

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

"We designed this polymer to compete with high-endmembranes," Dr. Lothar Freund, vice president of technology at Kraton, told Design News in an interview. "Many ofthe 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. Theprecursor polymer is a poly (t-butylstyrene-b-(ethylene-r-propylene)-b-styrene-b-(ethylene-rpropylene)-b-(t-butylstyrene) copolymer) or (tBS-EP-S-EP-tBS). The styrene block is selectivelysulfonated via acyl sulfate chemistry.

Varying sulfonation levels allow ion exchange capacity of0.4 to 2.0 meq/g (milli-equivalent per gram). The ion selectivity and uniquepolymer architecture results in efficient salt rejection.

When cast onto hollow fiber membranes, the sulfonatedpolymers create a layer which can achieve nanofiltration of water sources. Themembranes combine high strength and hydrophilicity, resulting in very lowenergy consumption. They can be used in a larger water purification ordesalinization plant.

Membrane or solution

Engineers should note that the copolymers come in membraneand solution form, providing the opportunity to design unique shapes as well ascoatings and laminations. These new sulfonated copolymers offer the customer agreener solution by reducing the processing temperatures as compared to currenttechnology.

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

Dr. Paul and his colleagues confirmed the effectiveness ofthe new copolymers in research conducted at the Texas Materials Institute. Theyadded: "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 engineeredsynthetic elastomers include PVC and bromine replacement. In some cases, TPEsare being made from biobased components.

PolyOne's GLS Thermoplastic Elastomers businessdeveloped Versaflex Bio TPEs, which areformulated with up to 70 percent renewable resources. These translucent grades areavailable in a range of Shore A hardnesses.

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

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

A new series of thermoplastic elastomer compounds from Teknor Apex meet ULcriteria for flame retardance while providing flexibility and toughness over abroad temperature range.

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

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