Asahi Kasei S.O.E.™ SEBS S1613 is a hydrogenated styrene–ethylene/butylene–styrene block copolymer classified under ISO 18064:2022 as TPE-S. The saturated midblock removes olefinic unsaturation from the butadiene precursor, which reduces ozone-induced chain scission and ultraviolet embrittlement while retaining thermoplastic processability. S1613 is supplied as pellets and is typically used in compounded form rather than as a neat polymer; polyolefin modification, soft-touch overmoulding, profiles, grips, and gaskets are common application fields. The grade is positioned in the higher-hardness segment of the S.O.E. series because its styrene content is approximately 30 wt%, yielding Shore A hardness values in the 60–68 range and a melt flow rate near 3.5 g/10 min under ASTM D1238-23 at 230 °C/2.16 kg. Unlike unsaturated styrene-butadiene-styrene, S1613 does not require vulcanization, and scrap can be reground into thermoplastic recycle streams within the limits described below.

Typical Property Envelope and Test Standards

The values above are representative compact-moulded values determined after 24 h conditioning at 23 °C and 50 % RH; they are not release limits. Melt flow and tensile properties shift with oil extension, polyolefin addition, and moulding conditions. Because S1613 is often compounded with paraffinic oil at 50–120 phr, mechanical properties in final articles may be lower than neat-polymer values. Published data for oil-extended S1613 in specific overmoulding applications is limited; full qualification on production tooling is required.

Why Is S1613 Positioned as a Higher-Hardness S.O.E. Grade?

The 30 wt% polystyrene end-block fraction controls the thermal and mechanical response of S1613. Under ASTM D638-22, tensile modulus and yield behaviour are rate-dependent: deformation above 50 mm/min promotes shear yielding of the polystyrene domains, while slow deformation below 5 mm/min allows greater viscous flow and higher elongation. The saturated poly(ethylene/butylene) midblock provides a sub-ambient glass transition, typically near −50 °C, which retains flexibility in low-temperature impact-modification applications. In polypropylene blends, addition of 10–30 wt% S1613 can improve impact resistance, but the exact low-temperature impact improvement must be verified by ASTM D3763-18 or ISO 6603-2:2000 on injection-moulded plaques because particle size and domain dispersion depend on screw design. Tensile set under ASTM D412-16 is higher than vulcanised rubber, which is a limitation in long-term cyclic loading.

Within the Asahi Kasei S.O.E. SEBS line, S1613 is differentiated from lower-styrene grades by its higher polystyrene end-block content, which raises hardness, tensile strength, and compression resistance. The harder grades in the S1600 series show less room-temperature creep than low-hardness gel grades, but they require higher melt temperatures and more shear to achieve uniform dispersion in polyolefin matrices. Compared with unsaturated SBS, S1613 retains its midblock oxidative stability because the ethylene/butylene segment has no allylic hydrogen; this allows outdoor weathering performance to be evaluated by ASTM D1149-18 ozone testing or ISO 1431-1:2022 accelerated ozone exposure. The trade-off is that the fully saturated midblock is less polar than nitrile or urethane elastomers, so solvent resistance remains limited to nonpolar and mildly polar environments.

The comparison above is based on typical S.O.E. series data for unfilled, non-oil-extended grades; compounded formulations can shift these relative positions. S1613 is therefore not a direct drop-in for S1605 in soft gel applications unless hardness and oil acceptance are rebalanced. No single grade substitution should be made without re-measuring tensile set under ASTM D412-16 and compression set under ASTM D395-18, because molecular weight and domain spacing produce lot- and grade-dependent recovery kinetics. S1613 differs from Asahi Kasei SEPS grades because the ethylene/butylene midblock has a longer run length between chain folds than hydrogenated isoprene; this affects low-temperature flexibility and compression set. Published data for direct property comparison between S1613 and SEPS in the same hardness range is limited, so selection should be based on prototype testing under ASTM D395-18 and ASTM D624-20.

Melt rheology of S1613 follows a shear-thinning pseudo-plastic response typical of styrenic block copolymers. At 200 °C, apparent viscosity under low shear can exceed 1,000 Pa·s, but at injection-moulding shear rates of 10,000–50,000 s⁻¹, viscosity falls sufficiently to fill thin sections. The pressure-temperature window is less severe than for reactive TPVs; no ±5 °C cliff-edge exists. However, oil phase compatibility and PS domain stability impose a practical melt-temperature upper limit of 250 °C. Residence times above 10 min at melt temperatures above 230 °C can produce gel particles and an increase in yellowness index. Use of nitrogen purge and low-shear screw elements is recommended for optical-critical parts. Capillary rheometry per ISO 11443:2021 should be used to generate viscosity curves for mould-filling simulation; single-point melt flow rate is inadequate for thin-wall tool design.

Compounding of S1613 on a corotating twin-screw extruder with an L/D ratio of 32:1 to 44:1 typically uses zone setpoints of 160–210 °C and a die temperature of 200–230 °C. Paraffinic white oil with kinematic viscosity between 40 mm²/s and 100 mm²/s at 40 °C is added downstream of the melt seal to control phase inversion and avoid excessive shear heating. Screw speeds of 250–350 rpm are common for 25 mm laboratory lines; production-scale 50–75 mm machines require lower rotational speeds and specific energy monitoring because viscous dissipation can exceed 0.25 kWh/kg and produce local melt temperatures above 240 °C. At melt temperatures above 250 °C, the polystyrene end blocks begin to degrade by chain scission; the saturated midblock is more thermally stable, but gel particles and viscosity drift can occur. S1613 should be pre-dried if surface moisture exceeds 0.05 wt%, typically at 80 °C for 2 h in a desiccant dryer. In injection moulding, melt temperatures of 190–230 °C and mould temperatures of 20–50 °C are used; clamp force requirements follow standard polyolefin calculations of approximately 3–5 tonnes per square inch of projected area, but thin-wall parts with flow-length-to-thickness ratios above 150:1 require independent pressure-drop simulation. Published data for S1613 on specific production-scale extruder configurations is limited, so start-up should include a melt-temperature survey and residence-time distribution measurement.

For a typical 50 Shore A compound, S1613 is often blended with a polypropylene homopolymer and paraffinic oil. The oil-to-SEBS ratio controls hardness and migration. If the oil loading exceeds the absorption capacity of the saturated midblock, bleed-out occurs within weeks at ambient temperature; this failure mode is observed on production lines as surface tack and dust pickup. Gravimetric aging at 70 °C for 168 h followed by visual inspection and mass loss measurement is used to screen formulations, but no single ASTM method captures all migration modes. Polyolefin addition above 30 wt% can reduce compression set but increases tension set and surface hardness. The final formulation should be stabilised with an antioxidant package suitable for food or medical use if those markets are intended.

When Chlorinated Solvents, Aromatic Oils, or Strong Oxidizers Contact S1613

S1613 is not a solvent-resistant elastomer. The polystyrene end blocks are swollen or dissolved by aromatic hydrocarbons, chlorinated solvents, ketones, and esters; contact with these fluids causes hardness loss, dimensional change, and extractables migration. Paraffinic and naphthenic oils are commonly used as extenders, but the aromatic fraction in the plasticiser should be kept below approximately 5 wt% of the oil because aromatic components partition into the polystyrene domains and reduce service temperature. Strong oxidizers, including concentrated hydrogen peroxide and hypochlorite solutions, attack the saturated midblock at elevated temperature; long-term immersion above 50 °C is not recommended without aging trials. The polymer withstands water, dilute acids, and dilute bases at room temperature, but continuous hot water above 80 °C can reduce physical crosslink integrity rather than cause hydrolysis. In overmoulding onto polypropylene or copolyester substrates, premature delamination can occur if substrate surface temperature falls below the PS block softening range. On production lines, substrate preheating to 60–80 °C and use of a texture depth above 15 µm improve mechanical interlock. Published data for this specific S1613 configuration is limited; destructive peel testing per ASTM D6862-11 is necessary to establish bond strength.

In wire and cable jacketing, S1613 can be used in blends with polypropylene or low-density polyethylene to achieve flexibility and heat resistance. The saturated midblock provides good electrical resistivity; volume resistivity may fall in the 10¹⁴–10¹⁶ Ω·cm range for unfilled compounds, but final values depend on carbon black or flame-retardant additives. Combustion performance must be formulated with halogen-free flame retardants such as magnesium hydroxide or aluminium trihydrate; these fillers reduce tensile strength and require coupling-agent optimization. A limiting factor in cable applications is the high loading of flame retardant needed to pass vertical flame tests, which can reduce extrudability and increase melt pressure. Production-scale single-screw extruders with 24:1 to 30:1 L/D and low-compression screws are used, but published data for S1613 in this specific configuration is limited.

For medical tubing or stopper applications, the chosen compound must be formulated without extractable processing oils above migration limits. Neat S1613 has lower extractables than oil-extended compounds, but the moulder should verify cytotoxicity and hemolysis under ISO 10993-5:2009 and ISO 10993-4:2017 on the final device. Sterilisation by gamma irradiation at 25 kGy or ethylene oxide can alter tensile strength and colour; dose mapping and post-sterilisation compression set testing are required. Published data for S1613 in radiation-sterilised medical devices is limited; users should request supplier ageing studies rather than assume stability.

Field failures with SEBS overmoulded parts often involve gloss change, tackiness, or loss of adhesion after cleaning agents. These are not inherent to S1613 alone but to the compound. For cleaning-agent resistance, the finished assembly should be exposed to the actual solvent blend for 72 h at 40 °C under ISO 2812-1:2017 and then tested for hardness retention and visual change. Chlorinated and aromatic cleaning solvents are particularly aggressive; alcohol-based cleaners are typically less damaging but can extract low-molecular-weight stabilisers.

Regulatory and food-contact status for S1613 must be established on the finished article, not assumed from the neat grade. The supplier certificate should be reviewed for compliance with REACH Article 33 obligations, RoHS 2011/65/EU Annex II thresholds for cadmium, lead, mercury, hexavalent chromium, PBB, and PBDE, and any applicable FDA 21 CFR sections such as 177.1810 or 177.2600 if repeated food-contact conditions are claimed. The neat polymer is not biodegradable and should not be incinerated in uncontrolled facilities; emissions during thermal decomposition include carbon monoxide and low-molecular-weight hydrocarbons. S1613 has a shelf life typically limited to 24 months when stored below 30 °C and away from direct ultraviolet exposure; long-term storage above 40 °C may cause pellet blocking and antioxidant depletion. For medical device applications, ISO 10993-1:2018 biocompatibility assessment is required on the sterilised finished device, and ethylene oxide or gamma irradiation must be validated for physical property retention. If the compound contains oil or polyolefin modifiers, the regulatory status of the final blend can differ from the neat S1613 grade.