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Presidential Green Chemistry Challenge: 2011 Greener Reaction Conditions Award

Kraton Performance Polymers, Inc.


NEXARTM Polymer Membrane Technology


Innovation and Benefits: Purification of salt water by reverse osmosis is one of the highest-volume uses of membrane filtration. Kraton has developed a family of halogen-free, high-flow, polymer membranes made using less solvent. The biggest benefits are during use: A reverse osmosis plant using NEXARTM membranes can purify hundreds of times more water than one using traditional membranes, save 70 percent in membrane costs, and save 50 percent in energy costs.

Summary of Technology: Polymer membranes are used in a variety of purification processes. Membranes selectively allow some molecules to pass while preventing others from crossing the barrier. Membrane purifications include reverse osmosis water desalination, water ultra-purification, salt recovery, and waste acid recovery. Membrane efficiency is limited by the rate at which water (or another molecule) crosses the membrane, a property called the flux. Increasing the pressure of the "dirty" side of the membrane can increase the flux, but a higher pressure requires a stronger membrane.

Kraton Performance Polymers has developed NEXARTM polymer membrane technology for applications requiring high water or ion flux. Kraton's NEXARTM polymers are block copolymers with separate regions that provide strength (poly(t-butyl styrene)), toughness and flexibility (poly(ethylene–propylene)), and water or ion transport (styrene–sulfonated styrene). These A-B-C-B-A pentablock copolymers exhibit strength and toughness in dry and wet conditions. Kraton's production process for NEXARTM polymers uses up to 50 percent less hydrocarbon solvent and completely eliminates halogenated cosolvents.

The biggest benefits are during use. NEXARTM polymers have an exceptionally high water flux of up to 400 times higher than current reverse osmosis membranes. This could translate into significant reductions in energy and materials use. Modeling shows that a medium-sized reverse osmosis (RO) plant could save, conservatively, over 70 percent of its membrane costs and approximately 50 percent of its energy costs. For applications in electrodialysis reversal (EDR), the higher mechanical strength of NEXARTM polymers makes it possible to use thinner membranes, which reduces material use by up to 50 percent and reduces energy loss due to membrane resistance. More important, NEXARTM polymers eliminate the current use of PVC (poly(vinyl chloride)) in electrodialysis membranes. The outstanding water transport rate of NEXARTM membranes also significantly improves energy recovery ventilation (ERV), by which exhausted indoor air conditions incoming fresh air. For other humidity regulation applications, including high-performance textiles and clothing, NEXARTM polymers offer environmental benefits by completely eliminating halogenated products such as Nafion® polymers and PTFE (poly(tetrafluoroethylene)) that may require hazardous halogenated processing aids.

Kraton introduced NEXARTM polymers in the United States, China, and Germany during 2010. In the third quarter of 2010, Kraton completed its first successful large-scale production of NEXARTM of about 10 metric tons.

Podcast on the technology:

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