LCY Chemical Globalprene® SEBS 7551 is a linear styrene-ethylene/butylene-styrene block copolymer produced by anionic polymerisation of styrene and butadiene, followed by selective hydrogenation of the polybutadiene midblock. The product is supplied in pellet form for compounding, impact modification, soft-touch overmoulding, wire-and-cable insulation, and adhesive/sealant formulations. The material is identified by the CAS Registry Number 66070-58-4 and carries the trade designation 7551 within the Globalprene SEBS series. The hydrogenated ethylene/butylene midblock has a glass transition temperature below −40 °C, while the styrene domains exhibit a glass transition near 95 °C. This thermally reversible phase structure forms physical crosslinks that permit melt reprocessing without sulfur or peroxide cure. Because the midblock is saturated, the grade is positioned for applications where unsaturated styrenic block copolymers are limited by thermo-oxidative or ultraviolet degradation.

Typical Physical and Mechanical Data for Compression-Moulded SEBS 7551

These values are representative of compression-moulded neat-resin specimens and are not lot-release specifications. Lot certification should be requested for exact styrene content, solution viscosity, and melt mass-flow rate, because SEBS 7551 shipments can vary within manufacturer control limits. Mechanical values respond to specimen preparation: ISO 37 tensile bars conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for 24 h provide lower data scatter than unconditioned specimens. For injection-moulded articles, anisotropic flow-induced orientation raises tensile strength parallel to the mould-fill direction and reduces elongation perpendicular to flow; ASTM D638-14 and ISO 527-1/2 tests on Type 1A or Type 5 test pieces should therefore be matched to the end-use stress orientation.

What Limits Melt Temperature and Shear Rate During Extrusion?

Mechanical work during compounding raises melt temperature because SEBS 7551 is pseudoplastic and shear thinning. At low shear rates below 10 s^-1, apparent viscosity remains high, but above 100 s^-1 viscosity falls by roughly an order of magnitude. Melt temperature should be kept below 230 °C at the die because thermo-oxidative chain scission accelerates above 240 °C; residence time above 230 °C should not exceed 10 min in a single pass. These boundaries define the practical operating window on a co-rotating twin-screw extruder with an L/D ratio of 32:1 to 40:1.

In compounding, barrel temperatures are set from 160 °C in the feed zone to 200 °C before the die, with screw speeds between 250 rpm and 400 rpm depending on torque. A vacuum vent at −0.06 to −0.08 MPa gauge removes residual volatiles and moisture. Pelletising is normally performed by an underwater or strand pelletiser with water temperature below 40 °C to prevent pellet blocking. Production-scale twin-screw compounding experience shows that lot-to-lot variation in solution viscosity can shift melt pressure at the die by 10 % to 20 % if feed rate is held constant; pressure-control rather than speed-control operation is therefore used for consistent pellet dimensions.

Injection moulding of SEBS 7551 requires melt temperatures of 180 °C to 220 °C and mould temperatures of 20 °C to 50 °C. Lower mould temperatures shorten cycle time by accelerating styrene-domain solidification, but excessively cold mould surfaces can generate flow lines in thick sections. Injection pressure is typically 40 to 80 MPa, with back pressure between 3 and 5 MPa and low to moderate screw speed. Because the resin is not strongly hygroscopic, drying is often unnecessary in controlled environments below 60 % relative humidity; however, moisture above 0.05 wt% as measured by Karl Fischer titration, ISO 15512, should be removed in a desiccant dryer at 80 °C for 2–3 h before melt processing.

Thermo-oxidative degradation of SEBS proceeds by free-radical attack at the tertiary hydrogen of the butylene unit in the ethylene/butylene midblock, causing chain scission and a measurable reduction in melt viscosity. Stabilisation for compounding typically combines a hindered phenolic antioxidant at 0.1 to 0.3 wt% with a phosphite process stabiliser at 0.1 to 0.2 wt%. The stabiliser package should be selected for low interaction with polypropylene or paraffinic oil when the grade is used in oil-extended compounds.

When SEBS 7551 Replaces SBS or EPDM in Weather-Resistant Compounds

Unsaturated SBS grades contain main-chain double bonds in the butadiene midblock that act as initiation sites for ozone and ultraviolet attack. The hydrogenated ethylene/butylene midblock in SEBS 7551 removes this weak point; ozone resistance can be evaluated according to ISO 1431-1. In outdoor sealing, cable sheathing, or overmoulding applications, this difference allows compounds based on SEBS 7551 to retain tensile and elongation properties after exposure where SBS compounds typically lose surface integrity. Published data for this specific configuration in open literature is limited, but the stabilisation mechanism is well established for hydrogenated styrenic block copolymers.

Compared with EPDM elastomers, SEBS 7551 does not require sulfur or peroxide vulcanisation. EPDM compounds develop a covalent network during curing, which prevents melt recycling and causes scrap to be discarded or devulcanised. SEBS 7551 forms physical crosslinks from styrene domains that dissociate above the styrene glass transition and reform on cooling; this permits regrind of sprues, runners, and off-specification parts at addition rates up to 20 wt% without automatic loss of tensile strength, provided the regrind is dry and not degraded. Melt-flow stability across repeated extrusion cycles can be used to verify regrind suitability; an increase in MFR of more than 25 % after three cycles, measured according to ISO 1133-1:2022 at 230 °C/5 kg, indicates thermal damage.

Within the Globalprene SEBS series, grades with lower styrene content exhibit lower hardness and modulus but also lower tensile strength and higher blocking tendency. Grades with higher styrene content show higher hardness, higher modulus, and often higher melt viscosity. SEBS 7551 is therefore selected for balanced mechanical integrity and processability when dry-blended with polypropylene, paraffinic oil, fillers, and stabilisers. When compression-set resistance at 70 °C is critical, a higher-performance SEPS or SEEPS grade may be required; SEBS 7551 is not the highest-performing grade for long-term heat-ageing compression set. ISO 815-1 compression set after 22 h at 70 °C for neat SEBS 7551 is typically in the range of 45 % to 60 %, which is higher than many SEEPS grades used for automotive sealing applications.

Compounds prepared from SEBS 7551 are used in automotive interior skins, gear-shift boots, airbag covers, soft-touch tool handles, cable insulation, and medical article components when final compliance is established. In polymer modification, additions of 5 to 20 wt% SEBS 7551 to polypropylene homopolymer improve impact strength as measured by notched Izod impact, ASTM D256-10, while reducing flexural modulus. For a 10 wt% addition, the impact modification effect is strongly influenced by the copolymer-to-polypropylene viscosity ratio; phase morphology and impact-stress transfer require an SEBS domain size below 5 µm in injection-moulded test plaques. Scanning electron microscopy of cryofractured specimens is used to verify dispersion, because impact performance drops when the domain size exceeds 10 µm.

Formulation Boundaries for Oil Extension and Polypropylene Blends

Paraffinic process oils are used to reduce compound hardness and melt viscosity. Oil absorption is not unlimited; at oil loadings above 80 phr, oil exudation and tensile strength loss are common in polar or humid environments. The oil should be a low-viscosity, high-paraffinic oil with an aniline point above 100 °C and low aromatic content to avoid solvation of the styrene domains. Naphthenic oils may be tolerated at lower loadings but can reduce tensile strength more than paraffinic oils at equal hardness; the choice should be validated by measuring tensile strength retention after 168 h of heat ageing at 70 °C according to ISO 188.

Polypropylene homopolymer or random copolymer is added to raise hardness, modulus, and upper service temperature. Blends containing 20 to 50 wt% polypropylene shift the Shore A hardness upward and eventually enter the Shore D range. The addition of 20 wt% polypropylene raises the Vicat softening temperature and reduces melt flow; processing on a single-screw extruder with an L/D of 24:1 or higher and a Maddock mixing section improves dispersion. Incompatibility with amine-based additives should be evaluated: primary amines can react with trace oxidation products or functionalised stabiliser by-products and may cause discolouration. This is particularly relevant in sealant formulations that include amine-based epoxy curatives.

Compliance statements for SEBS 7551 are application-dependent. The following matrix records standard designations commonly referenced for final article qualification.

The product is not supplied as a medical-grade or food-contact-certified material unless explicitly specified in the lot certificate. Final article compliance depends on all additives, processing aids, pigment systems, and service-environment interactions. Operational boundaries include avoidance of melt temperatures above 240 °C, storage below 40 °C away from direct sunlight, and segregation from strong oxidising agents. When pellets are stored for more than 12 months, solution viscosity and MFR should be retested before use because molecular weight distribution can shift during prolonged warehouse storage. The resin should not be blended with certain amine-based additives or heavy-metal salts that catalyse thermo-oxidative degradation unless the stabiliser package is adjusted and validated.