Hydrogenated styrene-ethylene/butylene-styrene triblock copolymers in the Asahi Kasei S.O.E.™ product line are produced through selective hydrogenation of styrene-butadiene-styrene precursors. S1609 is an uncompounded, non-oil-extended SEBS grade supplied as pellets with a nominal styrene content of 20 mass %. The polystyrene end blocks phase-separate into glassy domains with a glass transition temperature near 95 °C; the saturated ethylene/butylene midblock displays a glass transition temperature near -50 °C. This domain structure functions as a thermo-reversible physical crosslink network and does not require sulfur or peroxide cure. Supplier-published typical values for S1609 include density of 0.91 g/cm³ by ISO 1183-1:2019, Shore A hardness of 72 measured after 15 s by ISO 48-4:2018, tensile strength at break of 25 MPa and elongation at break of 680 % by ISO 37:2017, and melt flow rate of 2.0 g/10 min at 230 °C under 2.16 kg by ISO 1133-1:2022. Published molecular weight distribution and solution viscosity data are often absent from standard grade summaries; when gel-permeation chromatography data or branching parameters are critical, technical service information should be requested. Compared with unsaturated SBS, S1609 has largely saturated midblock unsaturation, which reduces thermo-oxidative chain scission and ozone-induced surface cracking. Compared with oil-extended SEBS grades such as S1611, S1609 contains no process oil, so the formulator controls plasticizer type, loading, and additive partitioning.

What Processing Differences Separate S1609 from Lower-Viscosity S.O.E.™ Grades?

The melt flow rate differential between S1609 and lower-viscosity S.O.E.™ grades such as S1605 controls equipment selection. S1609 at 2.0 g/10 min under 230 °C/2.16 kg is intended for extrusion, coextrusion, blow molding, and compounding where melt strength and drawdown resistance are required. S1605 at approximately 20 g/10 min is used in thin-wall injection molding and high-flow polyolefin compounds. On a 40:1 L/D co-rotating twin-screw extruder, S1609 produces higher motor torque and higher head pressure than S1605 under identical screw speed because of its higher molecular weight. Replacement of S1605 with S1609 in the same screw configuration can increase die pressure by 20–35 % and reduce screw speed by 10–20 % to avoid torque limits. In single-screw extrusion, S1609 requires a barrier screw or a screw with dispersive mixing elements; a general-purpose three-zone screw can produce transverse melt temperature variation above 5 °C and gel-like streaks in film. Fractional melt flow values make thin-wall mold filling below 1.0 mm difficult unless the grade is blended with polypropylene or process oil. The higher molecular weight also reduces the onset of melt fracture in annular dies but raises the practical lower melt temperature to approximately 170 °C in lab-scale extruders. Published shear viscosity curves for S1609 are limited in public literature; capillary rheometry at 200 °C, 220 °C, and 240 °C is recommended before screw design or die lip adjustment.

Prior to compounding, S1609 pellets should be dried to a residual moisture level below 0.05 mass %. A desiccant dryer delivering air at -30 °C dew point and 70 °C for 2–3 h is a common starting point. Un-dried resin at relative humidity above 60 % can generate surface splay, melt viscosity fluctuation, and porosity in extruded profiles. The saturated midblock is thermally stable up to approximately 250 °C in normal processing; above 260 °C, degradation of the polystyrene block segments accelerates and gel particles may form. Purging with linear low-density polyethylene or a dedicated SEBS purge compound is recommended when transitioning from polyvinyl chloride, acetal, or amine-containing masterbatches because residues can cause localized decomposition. In production-scale twin-screw compounding, oil injection before the first kneading section can flood the vent because unabsorbed oil destroys the melt seal; oil injection downstream of the first melt seal is used to maintain stable vent operation. Melt filtration with screen packs of 250–500 µm aperture is often placed before pelletizing to remove gels and foreign matter.

Compounding Response to Paraffinic Oil, Polypropylene Addition, and Filler Loading

Because S1609 is supplied without extender oil, final compound hardness can be adjusted from approximately 30 Shore A to 70 Shore A by adding paraffinic white oil with kinematic viscosity of 20–70 cSt at 40 °C. Oil absorption in the ethylene/butylene midblock lowers melt viscosity and tensile strength while increasing elongation at break and lowering hardness. At oil loadings above 20 phr, tensile strength decreases nonlinearly, and oil bloom may occur if the oil aromatic content or molecular weight is incompatible with the EB midblock. Polypropylene addition in the range of 20–50 mass % raises high-temperature compression set resistance and lowers creep, but hardness rises and elastic recovery declines. Addition of 10–20 mass % polypropylene is typical in soft-touch overmolding formulations to improve adhesion to PP substrates and to improve processability in injection molding. Filler loading with calcium carbonate beyond 30 mass % reduces tensile strength and should be paired with a coupling agent if strength retention is required. The saturated midblock tolerates dry blending with polyolefins, but pre-dispersed masterbatches are preferred for tackifier levels above 5 mass % to avoid phase separation and screw slippage. Aromatic process oils are not recommended because they plasticize the polystyrene end blocks and reduce upper service temperature.

Mechanical data for S1609 are usually generated on 2 mm compression-molded plaques conditioned for 24 h at 23 °C and 50 % relative humidity. Tensile tests to ISO 37:2017 use Type 1 dumbbells at 500 mm/min. The reported Shore A value is measured after 15 s; instantaneous and delayed readings for thermoplastic elastomers may differ by 2–4 points. Compression set after 22 h at 70 °C for a compression-molded plaque may fall in the range of 25–35 %, depending on molding conditions and annealing. Because the physical crosslink network is thermo-reversible, compression set increases with test temperature; use at 100 °C generally requires a polypropylene or crosslinked system.

Soft-touch overmolding compounds based on S1609 are processed on multi-shot injection molding machines with clamp force and shot capacity matched to the polypropylene carrier. Melt temperature from 200 °C to 230 °C and mold temperature from 30 °C to 60 °C are typical starting conditions. Adhesion to PP is generally achieved without primer; 90° peel adhesion after 24 h conditioning at 23 °C may be measured to ASTM D6862-11 or an equivalent internal method. Peel failure in screening trials often occurs at the interface when the mold temperature is below 20 °C or when the PP substrate contains substantial talc or mineral filler. S1609-containing elastic films are produced by cast film extrusion with a chill roll temperature of 15–25 °C and low air velocity at the frost line; the high melt strength supports web tension without sag. Published data for film tear strength, moisture vapor transmission rate, and elastic recovery for this specific grade remain limited; application-specific testing is required before product approval.

Although the saturated midblock improves oxidation resistance relative to SBS, stabilizer packages are still required. A hindered phenolic primary antioxidant at 0.1–0.3 phr and a phosphite secondary antioxidant at 0.1–0.2 phr are typical. For UV-stabilized outdoor compounds, a hindered amine light stabilizer system at 0.2–0.5 phr and a benzotriazole or triazine UV absorber at 0.1–0.3 phr are used. Weathering tests to ASTM D4329-13 may reveal gloss loss and microcracking after 500–1000 h in unpigmented formulations; titanium dioxide loading above 2 phr delays surface degradation. Published weathering curves specific to S1609 are limited, so end-use validation is required.

When S1609 Is Evaluated for Low-Extractable Sealing and Food-Contact Components

The base resin may be referenced under FDA 21 CFR 177.1810 as a styrene block copolymer intended for repeated-use food-contact articles, but the final compound must meet extraction limits for total nonvolatile extractives and specific migration of any added plasticizer, tackifier, or antioxidant. In the European Union, a complete migration assessment under EU 10/2011 is required on the finished article because additives and process oils can alter overall migration and specific migration limits. Low-extractable formulations exclude aromatic process oils and low-molecular-weight tackifiers; extractives in n-hexane or 50 % ethanol can increase when oil loading exceeds 20 phr. For medical tubing or gaskets, compliance to ISO 10993-5 and USP <87> must be demonstrated on the sterilized final device. Gamma irradiation at 25–50 kGy can induce crosslinking or chain scission in the presence of certain antioxidants; steam autoclaving at 121 °C for 30 min is generally tolerated when the compound is stabilized. Published data for this specific grade under all sterilization modalities are limited, and material certification must be verified on the final formulated article.

S1609 does not require vulcanization or sulfur cure, unlike EPDM compounds, and can therefore be reprocessed and reground. However, standard S1609 compounds generally show lower compression set and hot-oil resistance than PP-based thermoplastic vulcanizates or EPDM systems. The continuous service temperature under load is lower than PP/EPDM TPVs; in high-temperature automotive seals, S1609 compounds should not be used above 80–90 °C unless crosslinked or PP-modified. Compared with olefinic TPVs, S1609 provides lower hardness and softer surface feel without cure-system complexity but lower long-term elastic recovery at elevated temperatures.

S1609 is not suitable for immersion in aromatic hydrocarbon solvents, chlorinated solvents, strong oxidizing acids, or continuous service above 70–80 °C under load without a supporting polypropylene or thermoplastic vulcanizate phase. In adhesive systems, high loadings of rosin ester tackifiers can plasticize the polystyrene domains and reduce upper service temperature; aliphatic hydrocarbon tackifiers are generally preferred. The grade should not be combined with additives that generate free radicals or strong acids during extrusion because preferential degradation of the ethylene/butylene midblock reduces elongation and increases gel count. Storage is recommended below 40 °C and away from ultraviolet exposure to prevent surface oxidation of unstabilized pellet surfaces.