Kuraray SEPTON™ SEEPS 4044 is a hydrogenated styrenic block copolymer consisting of polystyrene end-blocks and a saturated ethylene-ethylene/propylene midblock. The polymer is supplied as a pelletized, oil-free feedstock for thermoplastic elastomer compounding. In the uncompounded state, typical values reported in Kuraray technical documentation include 30 wt% styrene, 0.91 g/cm³ specific gravity, 55 Shore A hardness, 25 g/10 min melt flow rate at 230 °C and 2.16 kg, 22 MPa tensile strength, and 450% elongation at break. These are nominal values obtained on compression-molded plaques; they are not sales specifications and will vary with molding history, cooling rate, and test preparation.

Melt flow rate is determined according to ASTM D1238 or ISO 1133-1:2022. The intermediate flow of SEEPS 4044 fills a processing window between higher-toughness, lower-flow SEEPS grades and higher-flow, lower-strength SEEPS grades. Unlike unsaturated SIS or SBS block copolymers, the hydrogenated midblock removes most main-chain carbon-carbon double bonds, so sulfur vulcanization is not available. The physical crosslink network is instead formed by the polystyrene end-blocks below the polystyrene glass transition.

Below the styrene glass transition, the polystyrene domains create a physical network that resists cold flow. Above the glass transition, the domains soften and permit reprocessing by injection molding, extrusion, or compression molding. This thermal reversibility differentiates SEEPS 4044 from thermoset rubber and from dynamically vulcanized TPVs, which contain a crosslinked rubber phase.

Mechanical and Rheological Profile of Uncompounded SEEPS 4044

The following table compiles typical uncompounded properties. Test method designations are retained because the numerical values are method dependent and should not be compared across alternative methods.

Because the as-supplied grade is not oil extended, the uncompounded hardness of 55 Shore A is higher than that of many finished TPE compounds. Compound hardness is adjusted with paraffinic or naphthenic process oils and polypropylene. At oil loadings between 50 phr and 150 phr with 10 wt% to 30 wt% polypropylene, compound hardness typically falls between 20 Shore A and 70 Shore A. Published data for this specific grade at every incremental loading is limited, and pilot compounding is required to establish the hardness-oil-loading curve.

The saturated midblock provides a measurable reduction in compression set relative to SEBS grades of equivalent styrene content and hardness. Under ASTM D395 Method B, compression set after 22 h at 70 °C is commonly used as a screening parameter. Exact values for SEEPS 4044 depend on oil type, filler content, and the degree of polypropylene crystalline network formation; they should not be inferred from uncompounded tensile data alone.

Dynamic mechanical behavior in similar SEEPS architectures shows two distinct loss tangent peaks associated with the rubber midblock and the styrene domains. The saturated rubber phase retains elastomeric character below the polystyrene glass transition, while the styrene domains provide high-temperature mechanical integrity. Published dynamic mechanical thermal analysis curves for this exact 4044 grade are limited in open literature, so compound screening should include dynamic mechanical analysis across the intended use temperature range.

What Limits Oil Uptake, Compression Set, and Heat Aging in SEEPS 4044 Compounds?

Oil uptake is controlled by the solubility parameter and molecular weight distribution of the hydrogenated midblock. The ethylene-ethylene/propylene structure absorbs paraffinic and naphthenic oils preferentially over aromatic oils. In co-rotating twin-screw compounding, liquid oil injection is commonly moved downstream to zone 3 or zone 4 after the polymer has formed a melt seal. Early oil injection at the feed throat can produce screw slip, torque oscillation, and poor dispersive mixing. Barrel temperatures from 160 °C to 220 °C are used for initial melting, and the melt temperature should not remain above 250 °C for extended residence time because thermal degradation at the block junction leads to viscosity loss and reduced elastic recovery.

Twin-screw extruders with L/D ratios of 40:1 to 52:1 are commonly used for high oil-loading SEEPS compounds because they provide sufficient residence time for oil dispersion without requiring excessive barrel temperature. Production-scale behavior in co-rotating twin-screw extrusion shows that torque fluctuations can be reduced by pre-blending polypropylene powder with a portion of the oil or by using split oil injection. If all oil is added in a single port at the feed zone, undissolved oil can lubricate the screw and reduce back-pressure, producing inconsistent pellet porosity and later melt flow drift.

Compression set performance in the compound is physical, not chemical. The saturated midblock cannot be sulfur vulcanized; it depends on the polystyrene domains and on oil immiscibility pressure. In stepwise heat aging, compounds are evaluated under ASTM D395 Method B at 70 °C, 100 °C, and 120 °C for 22 h. SEEPS 4044 compounds generally show lower compression set than SEBS compounds of similar hardness, but the advantage is reduced if high loadings of polar fillers above 30 wt% are present. High filler loading disrupts the elastic recovery pathway and reduces elongation at break, diminishing the practical benefit of the SEEPS midblock.

The comparison is qualitative because exact compound hardness and melt flow depend on formulation. The positioned differences are derived from nominal uncompounded values reported in supplier literature and should not be used as direct substitution ratios without compounding trials. Relative to SEBS grades such as SEPTON 8004, the SEEPS midblock gives lower compression set at equivalent hardness under ASTM D395 Method B and better property retention after hot-air aging.

When SEEPS 4044 Is Injection Molded onto Polypropylene Substrates

Overmolding onto polypropylene is a primary application for compounds based on SEEPS 4044. The melt temperature of the compound is normally set between 190 °C and 230 °C, with tool temperature between 30 °C and 50 °C. Under these conditions, a primerless melt bond is formed when the polypropylene surface is clean and sufficiently hot. Injection molding machines with clamp force between 80 t and 120 t have been used for thin-wall parts below 1 mm; the intermediate melt flow of 4044 permits filling at lower injection pressure than formulations based on SEPTON 4033 of the same compound hardness.

Delamination failure in overmolded parts is observed when the polypropylene substrate cools below 100 °C before the TPE melt is injected, when mold release agents are transferred to the substrate surface, or when the TPE compound contains excessive internal lubricants that bloom to the interface. Preheating the substrate, cleaning the surface, and verifying the actual melt temperature with a needle pyrometer are process controls used to maintain adhesion. Textured surfaces and mechanical interlocks further stabilize the bond in applications with repeated flexing.

For automotive cowl seals, soft-touch grips, and consumer electronic overmolding, compounds based on SEEPS 4044 are typically formulated with polypropylene and white mineral oil. Pigmented compounds can be evaluated for weathering under ISO 4892-2; the base polymer is not a complete weathering system, and the UV-stabilizer package must be selected for the intended exposure. In wire and cable jacketing, flame-retarded compounds require additional evaluation under IEC 60332 or locally applicable cable standards. Published data for this specific 4044 grade in flame-retarded wire and cable formulations is limited, so pilot-scale runs are necessary before production approval.

Medical device components such as syringe plunger tips, IV bag ports, and tubing grips are candidate applications because the saturated midblock does not contain labile double bonds that contribute to oxidation. However, biocompatibility must be established on the finished compound. Extractables and leachables testing under ISO 10993-18 may be required for prolonged patient contact. High-purity paraffinic oils and polypropylene copolymers are necessary to meet low extractables limits; published data for this specific 4044 grade in those exact formulations is limited.

Hot-melt adhesives and sealants based on SEEPS 4044 benefit from the high oil absorption and low-temperature flexibility. In adhesive formulations, tackifiers and oils are added; loop tack and 180° peel strengths are assessed by ASTM D6195 and ASTM D3330. Published data for this specific polymer in adhesive performance is limited, and formulation-specific adhesion measurements are required for substrate compatibility.

Food-contact status is not conferred by the base polymer alone. In the United States, compounds can be formulated to meet 21 CFR 177.1810 for styrene block polymers when all additives, oils, and use conditions comply with the regulation. In the European Union, finished food-contact articles are assessed under Regulation 10/2011 for overall migration and specific migration limits. For medical device components, the finished compound must be evaluated under ISO 10993-1 and ISO 10993-5; USP Class VI status of the base polymer should not be assumed. REACH and RoHS obligations apply to the compounded article and require lot-specific documentation from each raw-material supplier. These regulatory limits, rather than the base SEEPS polymer, determine allowable end-use compliance.