Kuraray SEPTON™ SEBS 8851 is a hydrogenated styrenic block copolymer of the polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene type. The polymer is supplied as dense, free-flowing pellets and is compounded by molders and polymer modification firms for applications in which high tensile elongation, low density, and resistance to thermal-oxidative ageing are specified. The midblock is produced by selective hydrogenation of a butadiene midblock, leaving a saturated ethylene-co-butylene chain. The absence of residual olefinic unsaturation in the midblock differentiates the product from conventional SBS and SIS elastomers in the same supplier portfolio. The styrene content is typically in the 30 wt% to 35 wt% range, as determined by supplier Fourier-transform infrared or nuclear magnetic resonance methods; the glass transition temperature of the saturated midblock is below −40 °C, while the polystyrene domains soften above 95 °C. These two thermal transitions establish the application window: low-temperature flexibility is retained down to −50 °C in many compounds, while continuous service at 90 °C to 100 °C is limited by styrene-domain softening.

The product is used in polypropylene-based compounds, soft-touch grips, automotive interior skins, gaskets, cable sheathing, and elastic films. In these applications, SEPTON 8851 is not used neat; it is typically extended with paraffinic or naphthenic oil and reinforced with polypropylene or other polyolefins to balance hardness, tensile strength, and processability. The product’s saturated midblock offers resistance to ozone, ultraviolet light, and hot-air ageing compared with SBS, but it does not provide inherent oil resistance and must not be selected for components immersed in aromatic fuels or polar solvents. Published product-specific data in public literature is limited; accordingly, the property bands used in this document are supplier-typical values for SEBS grades and should be verified against the lot certificate of analysis from Kuraray.

What Distinguishes SEPTON 8851 from Lower-Molecular-Weight SEBS and Unsaturated SBS Grades?

The primary difference is structural. SBS grades contain butadiene midblocks with residual double bonds; during melt processing above 180 °C, those double bonds can undergo chain scission, crosslinking, and colour formation. SEPTON 8851 contains a fully hydrogenated poly(ethylene-co-butylene) midblock, which reduces oxidation rate and allows higher processing temperatures without the same level of viscosity drift. Compared with lower-molecular-weight SEBS grades in the same family, 8851 is positioned for higher dilute-solution viscosity and higher tensile stress retention at 300% elongation. The practical consequence is a higher elastic recovery, lower compression set, and better retention of tensile strength after oil extension. Compounders often choose 8851 when an injection-molded part must survive repeated flexing at 70 °C and still show less than 60% compression set after 22 h under ASTM D395-18 Method B. In comparison to SBS, the product may require slightly higher processing torques because of the saturated midblock and higher entanglement density.

On production-scale melt-compounding lines, the grade is typically processed on a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1 to 48:1. Barrel temperatures are set from 180 °C in the feed zone to 230 °C in the mixing section, with the die and melt pumps held below 210 °C to prevent melt fracture. Although SEBS has low equilibrium moisture uptake compared with polyamides and TPU, storage at relative humidity above 60% can generate surface moisture that appears as vacuum-port flooding or pellet porosity. A desiccant dryer at 60 °C to 80 °C for 2 h to 4 h is sufficient when surface moisture is suspected. Paraffinic oil is preferably injected after the first high-shear kneading block using a heated gear pump, with distributive mixing elements downstream to incorporate the oil into the poly(ethylene-co-butylene) midblock. The vacuum devolatilization zone is commonly set to below −0.09 MPa gauge pressure on barrels 8 and 9. Screw speed and feed rate must be matched to avoid oil split-out and die-face pelletizing difficulties.

When Formulating with Paraffinic Oil and Polypropylene, What Rheological Boundaries Apply?

The critical formulation boundary is oil absorption capacity, not melt temperature. At oil loadings above 150 phr relative to the neat SEBS, the compound can exhibit oil bleed, reduced tensile strength, and plate-out on pelletizing and injection-molding feed throats. Oil uptake is influenced by the styrene content and molecular weight of the SEBS; high-styrene domains prevent unlimited midblock swelling by maintaining a physical crosslink network. A co-continuous polypropylene phase in the 10 wt% to 30 wt% range can reduce oil bleed and improve hardness stability. On a 25 mm to 40 mm co-rotating twin-screw extruder operated at 300 rpm to 600 rpm, specific energy input for a 50 Shore A soft-touch compound is generally between 0.15 kWh/kg and 0.35 kWh/kg. If local melt temperature measured by infrared thermocouple exceeds 240 °C, oxidative degradation and styrene-domain disruption may occur even though the midblock is hydrogenated. In that range, gel-like particles and gloss reduction in molded parts are early indicators of thermal damage.

Applications requiring repeated flexural deformation, low plasticizer migration, and absence of vulcanization scrap include automotive interior skins, instrument-panel trim, gear-shift sleeves, power-tool grips, and medical tube compounds. For automotive interior parts, fogging is assessed by ISO 6452:2021; for medical tubing, biocompatibility is evaluated under the ISO 10993 series with the final compound and sterilization plan. The product is not a drop-in replacement under all conditions. In soft-touch grips, it may be combined with SEBS grades of lower solution viscosity to improve flow length in thin-wall injection molds; in medical film, it may be blended with SEPS or a polyolefin to adjust moisture-vapour transmission. The choice of antimicrobial additives, radiopaque fillers, or high-molecular-weight silicone oil must be validated for each compound because these additives can alter the triblock network and release profile.

Compression Set and Chemical Resistance Thresholds at Elevated Service Temperatures

The continuous upper service temperature of unfilled SEBS compounds is governed by the polystyrene domain glass transition near 95 °C. At 70 °C, compression set for unfilled SEPTON 8851 typically falls between 40% and 60% after 22 h under ASTM D395-18 Method B, depending on the oil level and the presence of polypropylene or SEPS. At 100 °C, compression set rises sharply because the styrene domains soften and lose physical crosslink strength. The saturated midblock offers excellent resistance to ozone and ultraviolet ageing compared with unsaturated SBS; however, the polymer is not resistant to aromatic hydrocarbons, ketones, esters, chlorinated solvents, or high-aromatic gasoline. These fluids swell the poly(ethylene-co-butylene) midblock and reduce tensile strength. Chemical resistance is evaluated by immersion testing according to ASTM D543-21 with the specific process fluid, temperature, and duration. For hydraulic oils and food-contact cleaning agents, the final compound must be tested because additives such as oil, fillers, and nucleating agents can accelerate extraction.

Regulatory compliance is evaluated at the formulated compound level, not for the neat polymer alone. For food-contact applications, the United States regulation is 21 CFR 177.1810, which sets extraction limits for styrene block copolymers in food simulants. In the European Union, migration testing follows EN 1186 and the overall migration limits in EU 10/2011. Under REACH EC 1907/2006, the neat grade is not expected to contain substances of very high concern above 0.1 wt%; under RoHS 2011/65/EU Annex II, the restricted heavy metals and flame retardants are not expected to be present in the neat pellets above threshold values. These statements are supplier declarations, not end-product certifications. Pigment masterbatches, mineral fillers, and processing aids must be individually assessed for food-contact, medical, and electrical applications.

Thermal Property Bands that Govern Regrind and Reuse in Polyolefin Compounds

Regrind utilization is bounded by the thermal history of the styrene domains and by oxidative induction time. In production trials, a 20 wt% to 30 wt% regrind fraction can be tolerated in a polypropylene-rich soft-touch compound when the melt temperature is held below 220 °C and the residence time is less than 120 s. Above that fraction, injection-molded parts often show surface roughness, reduced strain at break, and higher gel content. The relevant tensile evaluation is ASTM D638-22 on specimens prepared by ISO 294-1, with tensile strength and elongation tracked against virgin compound controls. For medical or automotive interior parts, regrind use is often limited by customer specification rather than by polymer chemistry; those specifications may prohibit regrind entirely in skin-contact or crash-relevant components. The saturated midblock permits re-extrusion with less viscosity drift than SBS, but repeated cycles above 230 °C will still reduce molecular weight and undermine the triblock network.

Where a high-hardness compound or lower melt viscosity is required, SEPTON 8851 may be co-blended with higher-styrene SEBS or SEPS grades. The blend ratio is controlled by hardness measured under ASTM D2240 and by dynamic mechanical analysis using ISO 6721-1 to detect separate loss modulus peaks. Published data for this specific blend configuration is limited; production validation must include injection-molding trials and chemical-resistance immersion tests.