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."
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