Kuraray SEPTON™ SEEPS 4033 is a selectively hydrogenated styrenic block copolymer categorized as styrene-ethylene/ethylene-propylene-styrene (SEEPS). The grade is supplied as pelletized thermoplastic elastomer with a styrene mass fraction of 30 wt% and a density of 0.91 g/cm³ under ISO 1183-1. Manufacturer data list Shore A hardness at 76 per ISO 868, tensile strength at 26 MPa per ISO 37, and elongation at break at 480% for compression-molded specimens. The saturated ethylene/ethylene-propylene midblock distinguishes SEEPS 4033 from unhydrogenated SBS and SIS triblock elastomers in oxidative and ultraviolet ageing environments. The low melt flow rate restricts neat injection molding; the product is primarily specified as a compounding base for soft-touch, sealing, film, and damping compounds requiring a balance of polyolefin adhesion and elastomeric recovery.

The polymer contains polystyrene end-blocks that phase-separate into discrete domains with a glass transition above ambient service temperature, while the saturated midblock remains elastomeric below the styrenic domain softening point. Residual midblock unsaturation after selective hydrogenation is typically reduced to less than 1.0 mol%, which lowers the rate of chain scission under heat and UV exposure compared with polydiene-based block copolymers. This structural saturation also shifts the compatibility envelope away from polar ester plasticizers and toward paraffinic and naphthenic process oils. Because the midblock is fully saturated, conventional sulfur vulcanization is not applicable; crosslinking, when required, is achieved via peroxide/coagent systems or silane grafting.

Why Does Midblock Hydrogenation Shift Service Temperature and Oil Retention?

Selective hydrogenation converts the isoprene-derived sequences into an ethylene/ethylene-propylene copolymer without destroying the styrenic domains that provide physical crosslinks. In unhydrogenated SBS, the butadiene midblock remains vulnerable to ozone attack, autoxidation at temperatures above 70°C, and UV-induced embrittlement unless high antioxidant loadings are used. SEPTON™ SEEPS 4033 avoids this liability and can be formulated for continuous air exposure at 100°C or intermittent exposure to 120°C, provided that primary antioxidants are included to protect the styrenic domains and process oil against long-term oxidation. The reduced polarity of the saturated midblock increases the absorption capacity for paraffinic oil; compounds with 100 phr of high-viscosity paraffinic oil can retain Shore A hardness below 30 without immediate exudation, although storage at 70°C for 168 h should be used to confirm retention. Compared with SEBS grades of similar styrene content, the ethylene-propylene character shifts the dynamic mechanical loss peak and can reduce compression set in compounds where the styrenic domain network remains continuous. Direct property translation from SEBS formulations is not automatic because the melt viscosity of SEEPS 4033 is higher and the oil distribution between phases changes.

Direct comparison with lower-styrene SEPS and SEBS grades is formulation-specific. The 30 wt% styrene content of SEPTON™ SEEPS 4033 increases the density of the styrenic domain network; in tensile testing under ISO 37, this generally shifts the stress-strain curve toward higher stress at a given elongation compared with grades below 20 wt% styrene, but the magnitude depends on sample preparation and oil loading. In compounds where Shore A hardness below 10 is required, a lower-hardness grade or higher oil loading is typically evaluated. The saturated midblock also makes the polymer non-vulcanizable by sulfur systems, unlike SBS/SIS, so it should not be specified where conventional sulfur cure is required unless used as a thermoplastic phase in a dynamic vulcanizate.

The representative values below are drawn from compression-molded, unfilled specimens and should be confirmed against the current certificate of analysis for the specific lot. Conditioning and sample preparation follow ISO 23529 unless otherwise noted.

High-Shear Processing Boundaries in Twin-Screw Compounding

Compounding of SEPTON™ SEEPS 4033 on production-scale equipment is performed in co-rotating twin-screw extruders with L/D 40 or greater. Barrel temperature profiles are set between 190°C and 210°C in the melt zone. Oil injection is positioned downstream of the polymer melting zone because early injection reduces the frictional heat required for plastication and can cause gross phase inversion in formulations above 120 phr paraffinic oil. The high molecular weight of the grade increases torque at screw speeds above 250 min⁻¹; processors operating above this threshold often reduce feed rate to keep specific energy input within the line’s drive limit. Localized shear heating above 230°C degrades the styrenic end-block network and reduces ultimate tensile strength under ISO 37. Production line observations also link inadequate devolatilization of residual water or oil predispersion solvent to surface splay and internal voids in extruded profiles; vacuum venting at gauge pressures near -0.08 MPa is typical.

Oscillatory shear rheometry at 200°C is used to establish the zero-shear viscosity plateau for lot release. The grade exhibits pronounced shear thinning above 10 s⁻¹; however, published data for this specific configuration is limited, and end users should run capillary rheometry at 190°C, 210°C, and 230°C to generate a temperature-dependent viscosity model for mold-filling simulation. The Cox-Merz rule is not assumed valid for this multiphase system, so dynamic and capillary data are correlated empirically. Pre-drying is required after storage at relative humidity above 60%; desiccant drying at 80°C for 2 h to 3 h is sufficient for surface moisture removal. Avoid amine-based processing stabilizers because certain primary amines can disrupt the styrenic domain network and lower hardness after ageing.

When Compression Set and Tensile Retention Compete at High Plasticizer Loadings

Seal and gasket compounds based on SEPTON™ SEEPS 4033 generally use high-viscosity paraffinic oil to reduce hardness below 30 Shore A, but the plasticizer also dilutes the styrenic network and increases compression set. Testing under ISO 815-1 with type B specimens after 22 h at 70°C typically produces compression set values of 25% to 45% for oil-extended compounds, with the higher end associated with oil loadings above 180 phr. At oil loadings above 150 phr, tensile strength under ISO 37 decays non-linearly, and the risk of oil bloom increases during storage below 10°C. Substitution of a portion of the paraffinic oil with a low-molecular-weight propylene-ethylene copolymer reduces migration but raises Shore A hardness unless the copolymer addition is offset by additional oil. Volatility and fogging are evaluated under ISO 1817 or OEM-specific thermogravimetric methods; paraffinic oils with kinematic viscosity below 40 mm²/s at 40°C are generally avoided for interior automotive applications.

In soft-touch overmolding onto polypropylene, SEPTON™ SEEPS 4033 is typically compounded with 10–20 wt% crystalline PP homopolymer and 30–60 phr paraffinic oil. Adhesion to PP substrates is usually evaluated by ISO 8510-1 or automotive peel tests; the target failure mode is cohesive tearing within the elastomer layer rather than adhesive delamination. Melt temperature at the overmolding interface should exceed 190°C to allow chain interdiffusion. For UV-exposed interior components, compounds are evaluated under ISO 105-A02 or SAE J1885 depending on the OEM specification. The saturated midblock reduces the loading of hindered amine light stabilizers required relative to SBS, but carbon black or inorganic pigments remain necessary for long-term color retention. Published data for this specific application configuration is limited to compound-specific studies and should not be extrapolated without trial.

Low Melt Flow Restricts Neat Injection Molding and Enables Blown-Film Stability

The low melt flow rate of SEPTON™ SEEPS 4033 makes neat injection molding impractical in thin-wall tools because incomplete cavity filling and high injection pressure limit the feasible flow length. When injection molding is required, the grade is diluted with 20–30 wt% polyolefin and 50–100 phr oil, and the tool is designed with gates larger than 1.5 mm and emphasis on avoiding jetting. In blown film, the same high melt strength improves bubble stability at blow-up ratios of 1.5:1 to 2.5:1. Barrel temperatures are reduced to 180°C–200°C to maintain viscosity, and die gaps are widened to 1.0–1.5 mm to avoid sharkskin surface defects. Stretching in oriented film is possible at draw ratios above 3.0 at temperatures between 60°C and 80°C. Draw ratios above 4.0 or film temperatures above 90°C promote tear propagation and reduce machine-direction tensile strength under ISO 37.

SEPTON™ SEEPS 4033 is incompatible with strong acids, chlorinated solvents, and polar ketones, which swell or dissolve the hydrocarbon midblock. Storage at high humidity above 60% requires pre-drying because surface moisture generates voids during extrusion. In flame-retardant compounds, halogen-free systems require compatibility testing; some ammonium polyphosphate grades can induce acid-catalyzed hydrolysis under humid ageing. Use in food-contact or medical applications requires confirmation of the finished article against the applicable regulatory framework such as FDA 21 CFR 177.1810, EU Regulation 10/2011, or USP <87> biological reactivity; the polymer supplier’s compliance statement applies only to the raw polymer and not to the compounded article.