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 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 1960s.

Properties

Kraton is a block copolymer consisting of polystyrene and polybutadiene regions. The microstructure 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 -70°C and Tg of the polystyrene blocks is +100°C. So, at any temperature between about -70°C and +100°C Kraton will act as an elastomer. If Kraton is heated substantially above the Tg of the styrene-derived blocks, that is, above about 100°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 from. On cooling, this new form resumes its elastomeric character. This is the reason such a material is called a thermoplastic elastomer. The polystyrene blocks form domains of nanometre size in the microstructure, and they stabilise the form of the moulded material. Depending on the precise composition of polystyrene in the material, the domains can be spherical or form lamellae. Similar structures have been found in block polymers made from polyisoprene and polystyrene.

Applications

Kraton has a broad range of applications that spans from bumpers to bellows, vibrational dampers, couplers, and grommets. Also used throughout the automotive, major and small appliances, and aerospace industries. American Motors (AMC) used this polymer plastic on the AMC Eagle for the wheel arch flares that flowed into rocker panel extensions. Some grades of Kraton can also be dissolved into hydrocarbon oils to create "shear thinning" grease type products that are used in the manufacture of telecommunications cables containing optical fibres. Another use is the creation of "gel candles", which can be formed by dissolving a tri-block Kraton into a white-oil (liquid paraffin) and adding colours if required.

Kraton costs in the range of US$1 per kilogram and is also used in rubber footwear and shoe soles, as well as for hot melt adhesives. It can make asphalt flexible, which is necessary if the asphalt is to be used to coat a surface that is below grade. Kraton is also popular for use in non-slip knife handles.

References

*Lewis, PR and Price, C, "Electron Microscopy of sym-SBS block polymers", Polymer (1972), 13, 20-26.
* [https://secure.microplastics.com/MaterialData.aspx?mat=THERMOPLASTIC%20RUBBER%20KRATON%207705.htm Micro Plastics Material Data sheet for Kraton G polymers based on SEBS (styrene ethylene butylene styrene)] , retrieved on 2008-06-01.