1. Microstructure of SEP and SEPS
Full English name of SEPS:Styrene-ethylene/propylene-styrene block copolymer, i.e. styrene-ethylene/propylene-styrene block copolymer, which is made from styrene-isoprene polymer by selective hydrogenation. , selective hydrogenation refers to saturation of double bond in conjugated diene block or chain unit in polymer, while aromatic ring of styrene chain unit is not hydrogenated, which is called S-structure. The double bonds of isoprene block or chain link are saturated to form an alternating ethylene-propylene structure called EP structure. Therefore, hydrogenated styrene-isoprene polymer is commonly referred to as SEPS.
Figure 1. Chemical structure of SEPS
Where m=50～400 n=50～5000
SEPS is a block structure, and if it is a full diblock structure, then it is a styrene-ethylene/propylene-styrene-ethylene/propylene block copolymer, which we often refer to as SEP. The nature of diblock is completely different from triblock, so application is also different. Diblock does not have elasticity and strength, and is usually used to increase viscosity of an ointment liquid.
Assumption: Due to diblock structure of AB, it is difficult for styrene to become a physical crosslinking point, resulting in very poor high temperature resistance of SEP. So if you want to use it as a modified elastomer, you need to use SEP/SEPS together.
Figure 2. Chemical structure of SEP
Where m=50～400 n=100～200
2. The essence and application of SEP and SEPS
SEP/SEPS is a two-phase microscopic incompatible material separation system that uses styrene as a plastic segment and is dispersed in a polyethylene-propylene rubber segment. Before processing, blocks of polystyrene are combined into solid domains. When exposed to heat and shear forces during processing, polystyrene blocks begin to soften and become fluid. When cooled, polystyrene blocks rearrange and harden. Blocking rubber mesh. This physical phenomenon provides high strength and elasticity of SEPS. If a diblock styrene-isoprene polymer (SI) is hydrogenated to form an HSE, dispersed phase of HSE is still polystyrene, but there is no rigid polystyrene segment at beginning or end of molecular chain. Microdomains, so rubber soft segment can be stretched relatively easily during flow or stretching of polymer, giving polymer some special performance properties.
The product of hydrogenated polyisoprene, unlike hydrogenated polybutadiene, has a shorter number of methyl units and therefore does not crystallize. There is no crystallization in EP rubber segment, so SEPS is softer than partially crystallized SEBS, and elasticity and hysteresis of SEPS is better than that of corresponding SEBS; when oil is used as a rubber plasticizer, segment based on SEPS rubber The flexibility of thermoplastic elastomers is further enhanced. The inherent properties of hydrogenated polyisoprene polymers should allow SEPS-based styrenic thermoplastic elastomers to have such excellent flexibility and high elasticity.
Source: Xiamen Sanjiang, compiled by Renke team
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