Kraton

Kraton is the trade name given to a number of high performance elastomers manufactured by Kraton Polymers (NYSE: KRA), and used as synthetic replacements for rubber. Kraton polymers offers many of the properties of natural rubber, such as flexibility, high traction, and sealing abilities, but with increased resistance to heat, weathering, and chemicals. It was first made by the chemical division of the Shell Oil Company in the 1950s, under the technical leadership of Murray Luftglass and Norman R. Legge. Shell sold its Kraton polymers business to private equity firm Ripplewood Holdings in March 2001.

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 consist 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 stabilise the form of the moulded material. Depending on the rubber to polystyrene ratio in the material, the polystyrene domains can be spherical or form cylinders or lamellae. The hydrogenated Kraton polymers named Kraton G exhibit improved resistance to temperature (processing at 200–230 °C is common), to oxidation and to UV. SEBS and SEPS due to their polyolefinic rubber nature present excellent compatibility with polyolefins and paraffinic oils.