Hydrogenated Styrenic Block Copolymers: SEP vs. SEPS vs. SEEPS

Hydrogenated Styrenic Block Copolymers (HSBCs) have revolutionized the world of Thermoplastic Elastomers (TPEs). Offering the rubber-like elasticity of thermoset rubbers combined with the easy processability of thermoplastics, polymers like SEP, SEPS, and SEEPS are foundational to modern high-performance materials.

If you are a formulator, plastics engineer, or materials researcher, choosing the right grade can make or break your product design. In this article, we break down the molecular structures, performance nuances, and primary applications of SEP, SEPS, and SEEPS.

1. Molecular Structural Characteristics

The performance of styrenic block copolymers relies heavily on their architecture—specifically the ratio and arrangement of hard polystyrene (PS) end-blocks and soft hydrogenated polydiene mid-blocks.

• SEP (Styrene-Ethylene/Propylene Diblock Copolymer):

  • Structure: A-B type, consisting of one hard polystyrene segment (A) and one soft ethylene-propylene segment (B).

  • Characteristics: Because it is a diblock structure, it lacks the physical network crosslinking found in triblock polymers unless blended. It features good melt flowability, excellent optical transparency, and low solution viscosity.

• SEPS (Styrene-Ethylene/Propylene Triblock Copolymer):

  • Structure: A-B-A type, featuring two hard polystyrene end-blocks flanking a soft ethylene-propylene mid-block (derived from hydrogenated polyisoprene).

  • Characteristics: The dual polystyrene end-blocks form physical crosslinks below their glass transition temperature (Tg), delivering a stable, highly elastic network without requiring conventional chemical vulcanization. It does not readily crystallize, contributing to superior elongation and low-temperature flexibility.

• SEEPS (Styrene-Ethylene-Ethylene/Propylene-Styrene Triblock Copolymer):

  • Structure: An A-B-A triblock copolymer consisting of two polystyrene (PS) hard end-blocks and a hydrogenated soft mid-block derived from isoprene-butadiene copolymers. 

  • Characteristics: SEEPS is known for its good oil absorption and retention, as well as its compatibility with certain polyolefins, particularly polyethylene (PE). Commercial grades are available in different molecular weights, offering flexibility in balancing tensile strength, elongation, and processing characteristics.