TPE-S

Block copolymers based on styrene and butadiene (TPE-S or SBS, SEBS) have been first introduced as alternatives to thermoset rubbers. They are produced by anionic polymerization. Styrene monomer is first introduced in a reactor and polymerized using for example butyl lithium as catalyst building a low molecular weight block. When the styrene has been reacted, butadiene is added and the polymerization continues and builds a high molecular weight block of polybutadiene. A coupling agent is then added and the SBS block copolymer is created.

The butadiene block can be hydrogenated to give another soft block with a structure that is close to ethylene butylenes random copolymers. The so formed block copolymer is called SEBS. It has a better thermal stability than SBS thanks to the removal of the double bonds of the rubber block.

Polystyrene and polybutadiene or polyethylene being incompatible, the block copolymers are two phases systems. The polystyrene is the dispersed phase that ties the polybutadiene or the polyethylene together and behaves like a crosslinking.

As such SBS and SEBS block copolymers are difficult to process. They are compounded with oil, filler and polyolefins to make them easy to process and to adjust their properties to the applications.

SBS compounds are mainly used in the shoe industry. For other market segments, they are used for low demanding applications where the product lifetime is short and where long-term UV and thermal stability are not needed mainly in injection molding.

Because of their better thermal stability, SEBS compounds are used for more demanding applications in all market segments. One key application is grips where the soft touch, anti slip properties, good resistance to oil and grease are needed. Standard TPE-S bond to PP, PE or PS; special grades that bond to engineering resins (PA, ABS, PC) are available. In childcare, they are used as an alternative to soft PVC in, for example, baby bottle nipples.

Till recently, SEBS compounds were limited in applications where compression set was important at high temperatures (above 70°C). New grades are now available that have addressed this issue and now compete with thermoset rubbers or TPE-V.