Greener Reaction Conditions Award: Kraton Performance Polymers, Houston

Houston-based Kraton Performance Polymers landed the award for Greener Reaction Conditions for developing the NEXAR family of halogen-free, high-flow copolymer membranes for water and other purification applications. The copolymer’s durable multiblock design enables faster flow rates than current membranes, leading to significant savings in membrane costs and energy use.

About 40% of the world’s people live in water-stressed areas, and roughly 3.5 billion people are expected to live in such areas by 2025. The growing demand for clean drinking water is driving the rapid growth of the water-purification and -desalination market. But today’s dominant purification technology, reverse osmosis, has a number of environmental strikes against it, notes Lothar Freund, Kraton’s vice president of technology.

Thin, nonporous permeable membranes on the order of 50 µm thick selectively allow some molecules to pass through while preventing others from crossing the barrier, Freund explains. Membrane efficiency is limited by the rate at which water or ions cross the membrane—a property called flux—which depends on polymer morphology. Standard membranes are made of cross-linked aromatic polyamides, which have relatively low water flux rates, Freund says.

Increasing pressure can increase the flux, he says, but will require more energy to power water pumps. Energy cost for a reverse-osmosis plant can be as high as 40% of the total cost of the facility, he notes. Higher pressure also means that a stronger membrane is required and that the membranes wear out more quickly, thereby increasing material consumption and costs, he adds.

Kraton was the original inventor of styrenic block copolymer chemistry in the 1960s, Freund notes. The company has continued to be an innovator in this area, he says, and every five to 10 years it creates new applications for styrenic block copolymers and opens up space for new markets.

“NEXAR is our latest advance,” Freund says. “But it’s not the typical styrenic block copolymer.”

The innovation with NEXAR polymers lies with the unique pentablock copolymer design, Freund explains. “The base chemistry is still styrene, but then we add a sulfonation step that adds novel functionality to the polymer.”

The central block is poly(syrene-co-styrene sulfonate), which modulates water or ion flux, he says. The amount of sulfonation in this “ionomer” block can be adjusted from 10% to 100%, depending on the end-use application, Freund adds.

Sitting on either side of the ionomer is a poly(ethylene-co-propylene) block, which provides toughness and flexibility. Next comes a poly(tert-butylstyrene) block on each side, which provides mechanical strength.

This design optimizes the copolymer for use as a water-purification membrane. “The biggest benefit of this copolymer is that reverse-osmosis membranes can be made thinner but maintain high strength, enabling up to 400 times higher flux than other membrane materials you will find on the market,” Freund says. NEXAR membranes also tend to not get fouled up with filtered salts as quickly as polyamide-based membranes, he notes.

An additional benefit of NEXAR technology, he says, is that polymer production needs up to 50% less hydrocarbon solvent and does not require halogenated cosolvents. The solvent that is used is captured and recycled, and spent membrane material can be recycled, just as postconsumer plastics are recycled.

In a modeling study comparing a medium-sized traditional reverse-osmosis plant and a NEXAR plant, Kraton scientists found that when producing 20 million gal of purified water per day, the traditional plant needs some 2,500 standard membrane cartridges of 400 sq ft each. With NEXAR, only 653 of these cartridges would be needed, reducing membrane cost by more than 70% and energy cost by more than 50%. The model was based on conservative assumptions regarding flux rate and membrane thickness, Freund notes, so actual savings likely will be even greater.

Besides reverse osmosis, other potential applicatins for NEXAR include electrodialysis, a method used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential. This technology is used for desalination of brackish water, salt recovery, waste acid recovery, and generation of ultrapure water.

Kraton’s polymers offer much better mechanical strength than traditional ionomer-based electrodialysis membranes, Freund says, while maintaining the same permeability. The higher strength makes it possible to use thinner membranes to reduce material consumption and avoid the use of polyvinyl chloride to bind ionomers in molded sheets, he notes.

NEXAR membranes also improve energy-recovery ventilation (ERV), Freund says, which is the process of recovering the heat energy contained in exhaust air and using it to treat the incoming air in heating, ventilation, and air conditioning systems. ERV helps reduce energy consumption for cooling and heating and improves indoor air quality.

In other humidity-regulation applications, halogenated materials are often used, Freund adds. For example, tubes made from DuPont’s Nafion sulfonated fluoropolymer are used for air-drying applications, and microporous polytetrafluoroethylene fibers are used for water-wicking textile fibers found in sports apparel and clothing for firefighters. NEXAR polymers could replace those halogenated materials, he says, which are of concern for their environmental persistence.

The initial development and demonstration of the first NEXAR sulfonated pentablock copolymer was completed in 2007. The first pilot production of polymers occurred in 2009, and the first successful large-scale production of about 10 metric tons was completed in late 2010.

“When it comes to these membranes, they are all expensive,” Freund observes. “So it really comes down to their functionality and performance. And right now with NEXAR we have a much better water flux rate that is allowing us to excel in developing these new applications with our business partners.”