LCY Chemical Globalprene® SEBS 7550 is a linear styrene–ethylene/butylene–styrene triblock copolymer produced by selective hydrogenation of a styrene–butadiene precursor. The ethylene/butylene midblock is saturated, which removes the main-chain unsaturation and changes the oxidative ageing pathway from the high-sensitivity autoxidation of polybutadiene to a hydrogenated polyolefin-like response. Manufacturer technical literature for 7550 reports a bound styrene content of approximately 30 wt%, a specific gravity of 0.90–0.91 when tested per ASTM D792, and a melt flow rate in the 3.5–5.0 g/10 min range at 230 °C under 2.16 kg load per ISO 1133-1. The product is supplied as porous pellets and is used primarily in oil-extended thermoplastic elastomer compounds, polyolefin modification, and applications requiring lower lot-to-lot unsaturation than unsaturated styrenic block copolymers.

The styrenic end-blocks aggregate into rigid domains with an effective glass transition above 90 °C; these domains act as thermally reversible crosslinks and govern tensile strength, whereas the ethylene/butylene matrix provides elastomeric recovery at service temperatures down to approximately −40 °C. This morphology is absent in melt processing above the polystyrene glass transition; the domains dissolve, viscosity is governed by block molecular weight and chain entanglement, and solidification requires cooling below the domain ordering temperature. For SEBS 7550, the domain ordering temperature is not reported for all lot histories; rheological transitions observed by dynamic mechanical analysis should be used to define the lower processing limit. No chemical crosslink is required to develop useful mechanical properties, but creep resistance at elevated temperature is inferior to vulcanised or electron-beam-crosslinked systems.

The values in the table are representative supplier technical data, not contractual lot-release limits. Independent replication on conditioned test specimens is required because SEBS tensile values vary with moulding temperature, cooling rate, and residual diblock content.

What Limits the Service Temperature of the Saturated Ethylene/Butylene Midblock in Hot-Air Ageing?

In unsaturated SBS, the dominant failure mechanism under hot-air ageing is autoxidation at the polybutadiene double bond; the reaction sequence produces allylic hydroperoxides, carbonyl species, crosslinking in the early stage, and embrittlement as chain scission becomes controlling. The hydrogenated midblock of 7550 is reported to have low residual unsaturation by iodine value, shifting the main ageing pathway toward oxidative attack at the styrenic end-block and at residual diene defects when temperatures exceed 90 °C. In compound ageing studies performed at 100 °C for 168 h per ISO 188, typical 7550-based formulations containing 0.1–0.3 phr of a hindered phenolic antioxidant retain measurable elongation, while equivalent unsaturated SBS formulations may lose more than 50% of initial elongation under the same protocol. The styrenic end-blocks soften above the polystyrene glass transition, commonly reported in the 90–100 °C range; continuous load-bearing service above this threshold is therefore not recommended unless the compound is radiation-crosslinked, peroxide-cured, or reinforced with a higher-softening aromatic resin. The saturated midblock also reduces yellowing and melt viscosity drift during repeated reprocessing, an effect quantified by lower yellowness index increase per ASTM E313.

Twin-Screw Compounding Parameters, Oil Partitioning, and Shear Heating

On a 40:1 L/D co-rotating twin-screw extruder used for 50–70 Shore A SEBS/oil/PP compounds, 7550 is fed in the main gravimetric feeder at barrel 1. Paraffinic mineral oil with kinematic viscosity of 40–100 mm²/s at 40 °C per ASTM D445 is injected through a heated lance into barrel 5 or 6, after the polymer has passed the initial plastification zone. Barrel set points normally range from 180 °C in the feed throat cooling zone to 200–220 °C in the mixing zones and 230 °C at the die; the measured melt temperature in the die adaptor must remain below 240 °C to avoid chain scission and oil degradation. Screw speeds in the 250–400 rpm range are common, but shear heating can increase local melt temperature by 10–25 °C in narrow-disc or reverse-flighted zones. Downstream vacuum venting of at least −0.08 MPa gauge removes residual moisture and volatile degradation products. When strand pelletizing, water temperature below 40 °C is maintained to prevent pellet agglomeration; free oil on pellet surfaces after drying indicates either excess oil loading above the equilibrium uptake of the ethylene/butylene phase or inadequate distributive mixing.

Batch-to-batch melt flow variation within the 3.5–5.0 g/10 min range changes the wetting rate of PP domains. A lot at the lower MFR end may require an increase in oil injection temperature to 60–70 °C and a 5–10% increase in screw speed to maintain visual surface uniformity. A lot at the higher MFR end may require a reduction in barrel temperature in zones 6–8 to avoid oil exudation and pellet porosity. These adjustments are made against torque, melt pressure before the die, and strand swell data rather than temperature alone.

Dynamic mechanical analysis of oil-extended 7550 compounds shows that the ethylene/butylene phase glass transition, reported near −42 °C for the neat midblock, shifts toward −60 °C as paraffinic oil loading increases to 50 phr; the polystyrene end-block storage modulus transition remains above 90 °C unless aromatic oil is present. This selective softening mechanism is exploited in low-hardness compounds, but oil saturation beyond the equilibrium uptake of the EB phase results in phase separation and surface bloom. Oil exudation is measured gravimetrically after 7 days at 60 °C; values above 0.5 wt% indicate an over-oiled or poorly stabilised compound.

On production-scale twin-screw lines, common failure modes include feed throat bridging when the porous pellets are wet, oil splash back through the main feed when oil injection is positioned too early, and die-lip buildup from oxidative gel formation at melt temperatures above 240 °C. Die-lip buildup is monitored by die pressure fluctuation; an increase above 0.5 MPa in pressure variability over 30 min is an early indicator of gel accumulation. Periodic purge with polypropylene homopolymer at 220 °C is used to remove residual SEBS before shutdown; purging compounds containing abrasive fillers should not be used because they accelerate screw and barrel wear.

Pre-drying of 7550 is not required when moisture content is below 0.05%, but porous pellets stored at relative humidity above 60% can absorb surface moisture and should be dried at 80 °C for 2 h using a desiccant dryer. Karl Fischer titration per ISO 15512 is used to determine water content; values above 0.1 wt% require drying at 80 °C for 2–4 h in a dehumidifying hopper. Drying at temperatures above 90 °C can fuse pellets in the hopper and should be avoided. Injection moulding of pre-compounded 7550 materials uses a feed throat maintained at 25–40 °C, metering zone temperatures of 200–220 °C, and compression ratios of 2.0–2.5:1. The packing pressure is usually limited to 50–70% of peak injection pressure; higher packing increases density but can create anisotropic shrinkage and surface sink. Mould release agents containing silicone or wax should be avoided because they can migrate to the bonded surface in overmoulded parts and reduce weld-line strength.

When 7550 replaces SBS in overmoulding compounds, the plasticiser partition coefficient is altered

Direct one-to-one substitution of 7550 into low-hardness injection moulding formulations designed for unsaturated SBS is not usually possible because the melt viscosity is higher and the saturated midblock absorbs paraffinic oil differently. In a 25–35 Shore A overmoulding compound, the formulator may need to increase paraffinic oil content by 5–15 phr to reach the same hardness while maintaining flow length. If oil is increased without adjusting polypropylene homopolymer content, tensile strength per ISO 527-2 may fall below 5 MPa, and tear strength per ISO 34-1 may drop by more than 20%. Adding 10–20 wt% of isotactic polypropylene with a melt flow rate of 10–25 g/10 min per ISO 1133-1 reinforces the continuous phase and restricts swelling of the styrenic domains. Aromatic process oils above 10% of the total oil phase partition into the polystyrene domains, plasticising the end-blocks and reducing the upper service temperature; such formulations often show surface tack after 72 h at 70 °C per ISO 188. Hydrogenated tackifying resins, rather than rosin esters, are preferred when adhesion to low-energy substrates is required.

Published data for specific adhesion values on polar substrates are limited; compound-level testing per ASTM D6862 or ASTM D903 is required because raw polymer cohesive strength does not predict bond strength.

In polypropylene homopolymer impact modification, 10–25 wt% of 7550 is compounded with a 10–25 g/10 min MFR polypropylene. The SEBS domains act as stress concentrators and stop crack propagation; notched Charpy impact strength per ISO 179-1 increases relative to the neat homopolymer, while flexural modulus per ISO 178 decreases. Because the saturated midblock is thermally stable, impact retention after repeated extrusion is generally better than with unsaturated SBS modifiers. However, at addition levels above 25 wt%, the co-continuous morphology can produce a drop in tensile yield stress and an increase in solvent swelling; the optimum addition level should be determined by scanning electron microscopy and Gardner impact testing per ASTM D5420.

In 30:1 L/D or 40:1 L/D co-rotating twin-screw compounding of 7550-modified PP impact copolymers, the dispersion of SEBS domains depends on the rheological viscosity ratio and the screw shear history. At barrel temperatures below 200 °C, the viscosity ratio between 7550 and PP homopolymer is unfavorable; unmelts or large SEBS agglomerates can persist into the underwater pelletizer. A side-fed PP/SEBS split feed is sometimes used when the PP melt flow rate is below 10 g/10 min. Scanning electron microscopy of etched cryofractures shows SEBS domain sizes from 0.5 μm to 3 μm in compounded binary blends; domain size increases when the screw speed is below 200 rpm or when the ratio of SEBS to PP exceeds 30:70. Residual diblock in 7550 acts as an interfacial compatibiliser; lots with low diblock content can show reduced impact improvement and larger dispersed domains. Published diblock levels for this specific grade are limited and should be confirmed by GPC with UV/RI dual detection.

For cable sheathing and soft-grip formulations, 7550 is compounded with 20–35 wt% polypropylene and 30–60 phr paraffinic oil to achieve 50–70 Shore A. Flame-retardant versions may use magnesium hydroxide or aluminium trihydrate; because these fillers increase viscosity and moisture uptake, a counter-rotating twin-screw or internal mixer is often preferred for filler dispersion. The compounder should avoid uncoated magnesium hydroxide at loadings above 60 phr because the resulting higher torque and residence time can exceed the thermal stability window of the stabiliser package.

For low-extractable medical and sealant compounds, 7550 is selected because the hydrogenated midblock contributes lower oxidised fragment formation than unsaturated SBS. However, raw polymer compliance alone is not sufficient; finished device testing per ISO 10993-5 and ISO 10993-10 is required for cytotoxicity and skin sensitisation, and process oils and stabilisers must also be biocompatible. In hot-melt sealant formulations, the high-styrene block structure of 7550 raises the softening point relative to many unsaturated SBS grades and reduces cold-flow; hydrogenated hydrocarbon tackifying resins with ring-and-ball softening points of 95–115 °C per ASTM E28 are used. Rosin ester and aromatic tackifiers are generally avoided because they plasticise the styrenic domains, reduce cohesive strength, and can create low-molar-mass extractable fractions.

Hot-melt adhesive formulations based on 7550 are processed in heated sigma-blade mixers or twin-screw extruders at 180–200 °C. The high styrenic domain strength reduces cold-flow and provides cohesive strength, but wetting on polar substrates is limited in the absence of a hydrogenated tackifier or a functionalised polyolefin. Peel testing per ASTM D6862 on stainless steel and polypropylene is required to establish adhesion; published data for this specific grade are limited and values are strongly influenced by tackifier loading.

Operational boundaries include incompatibility with high loadings of polar fillers without a compatibiliser; calcium carbonate above 30 wt% can reduce tensile strength and increase water uptake. The product should not be compounded with amine-based heat stabilisers that promote acid-catalysed dehydrochlorination in PVC blends; for PVC/SEBS alloys, mixed metal stabilisers are preferred. Continuous exposure to strong mineral acids, aromatic solvents, and chlorinated hydrocarbons causes swelling or extraction of the styrenic phase; published data for these specific configurations are limited and compound-level immersion testing per ISO 175 is required.