Kraton (polymer)

Kraton is the trade name given to a number of high-performance elastomers manufactured by Kraton Polymers, and used as synthetic replacements for rubber. Kraton polymers offer many of the properties of natural rubber, such as flexibility, high traction, and sealing abilities, but with increased resistance to heat, weathering, and chemicals.

Company

The origin of Kraton polymers goes back to the synthetic rubber (GR-S) program funded by the U.S. government during World War II to develop and establish a domestic supply capability for synthetic styrene butadiene rubber (SBR) as an alternative to natural rubber.[1]

Shell Oil Company purchased the Torrance, California facility from the U.S. government that was built to make synthetic styrene butadiene rubber.[2] The company formed Elastomers Division that eventually became Kraton Corporation. Shell Oil Company broadened the product portfolio of elastomers in the 1950s,[3] under the technical leadership of Murray Luftglass and Norman R. Legge.[4]

As part of the divestment program that was announced by Shell in December 1998, the Kraton elastomers business was sold to a private equity firm Ripplewood Holdings in 2000.[5][6] Kraton completed IPO on December 17, 2009, to became a separate publicly traded company.[7] In 2021 Kraton employees won an ASC Innovation Award for "Next Generation of Biobased Tackifiers REvolutionTM".[8] Kraton employees accept an ASC Innovation Award

Properties

Kraton polymers are styrenic block copolymer (SBC) consisting of polystyrene blocks and rubber blocks. The rubber blocks consist of polybutadiene, polyisoprene, or their hydrogenated equivalents. The tri-block with polystyrene blocks at both extremities linked together by a rubber block is the most important polymer structure observed in SBC. If the rubber block consists of polybutadiene, the corresponding triblock structure is: poly(styrene-block-butadiene-block-styrene) usually abbreviated as SBS. Kraton D (SBS and SIS) and their selectively hydrogenated versions Kraton G (SEBS and SEPS) are the major Kraton polymer structures. The microstructure of SBS consists of domains of polystyrene arranged regularly in a matrix of polybutadiene, as shown in the TEM micrograph. The picture was obtained on a thin film of polymer cast onto mercury from solution, and then stained with osmium tetroxide.

The glass transition temperature (Tg) of the polybutadiene blocks is typically −90 °C and Tg of the polystyrene blocks is +100 °C. So, at any temperature between about −90 °C and +100 °C Kraton SBS will act as a physically crosslinked elastomer. If Kraton polymers are heated substantially above the Tg of the styrene-derived blocks, that is, above about 100 °C, like 170 °C the physical cross-links change from rigid glassy regions to flowable melt regions and the entire material flows and therefore can be cast, molded, or extruded into any desired form. On cooling, this new form resumes its elastomeric character. This is the reason such a material is called a thermoplastic elastomer (TPE). The polystyrene blocks form domains of nanometre size in the microstructure, and they stabilize the form of the molded material. Depending on the rubber-to-polystyrene ratio in the material, the polystyrene domains can be spherical or form cylinders or

Applications

Kraton polymers are always used in blends with various other ingredients like paraffinic oils, polyolefins, polystyrene, bitumen, tackifying resins, and fillers to provide a very large range of end-use products ranging from hot melt adhesives to impact-modified transparent polypropylene bins, from medical TPE compounds to modified bitumen roofing felts, or from oil gels to elastic attachments in diapers.[9]

It can make asphalt flexible, which is necessary if the asphalt is to be used to coat a surface that is below grade or for highly demanding paving applications like F1 racing tracks.[10]

Kraton-based compounds are also used in non-slip knife handles.[11][12]

The earliest commercial components using Kraton G (thermoplastic rubber) in the automobile industry were in 1970s.[13]