Kuraray SEPTON™ SEEPS 4055 is a hydrogenated styrenic block copolymer with hard polystyrene end blocks and a saturated ethylene-ethylene-propylene midblock. The SEEPS designation refers to the midblock sequence created by selective hydrogenation of a polyisoprene precursor; the absence of backbone double bonds separates this family from unhydrogenated SBS and SIS. Supplier technical data list a bound styrene content of 30 wt% and specific gravity of 0.91 when determined according to ASTM D792. Representative compression-moulded properties include tensile strength at break of 24 MPa, elongation at break of 650 %, and Shore A hardness of 70 under ASTM D638 and ASTM D2240 respectively. Melt mass-flow rate is 0.2 g/10 min at 230 °C / 2.16 kg using ASTM D1238. The grade is supplied as low-dust pellets and is used in compounds that require high tensile recovery, low compression set, and polyolefin compatibility.

How Does SEEPS 4055 Differ from SEBS and Unhydrogenated SBS in Recovery, Set, and Service Temperature?

The primary structural distinction is the hydrogenated midblock; SBS and SIS grades retain double bonds that act as initiation sites for thermo-oxidative scission, while SEBS grades contain a saturated poly(ethylene/butylene) midblock with different segmental regularity. SEEPS 4055 has a more regular alternating ethylene-ethylene-propylene sequence that lowers hysteresis at equivalent styrene content. In dynamic mechanical testing, the loss tangent of the saturated midblock appears below −40 °C, whereas the polystyrene endblock transition appears near 100 °C. Compression set at 70 °C for 22 h on injection-moulded compounds measured to ASTM D395 Method B is typically 35–45 % without post-vulcanisation; comparable SBS compounds generally exceed 70 % under the same conditions. Accelerated weathering under ISO 4892-2 cycle 1 produces no visible surface cracking on stabilised black SEEPS 4055 compounds after 1,000 h, whereas SBS compounds require high carbon black loading to reach 250 h without cracking. Compared with lower-molecular-weight SEPTON grades, the 0.2 g/10 min melt flow of SEEPS 4055 is lower, which increases die swell in profile extrusion but improves tensile strength and resistance to cohesive failure in adhesives. Published data for direct substitution into SEBS foam formulations is limited because melt strength and gas permeability differ; pilot-scale evaluation is required.

Injection-Moulding Barrel Profiles, Gate Design, and Post-Mould Shrinkage

On injection-moulding machines with clamp forces above 500 kN, barrel temperatures for SEEPS 4055 compounds are profiled from 170 °C at the feed throat to 210 °C at the nozzle; nozzle temperatures above 220 °C may increase gate stringing. Mould temperatures of 20–45 °C are adequate for fast cycle times, while 60 °C mould temperatures reduce visible flow lines on high-gloss surfaces but extend pack time. Filling speeds of 10–20 cm³/s with packing pressure of 40–60 MPa for 3–5 s control void formation. Post-mould shrinkage on 2 mm plaques under ASTM D955 is 0.8–1.2 % parallel to flow and 0.6–1.0 % perpendicular. The high melt elasticity requires a tab gate of at least 1.5 mm thickness or a valve-gated hot runner; pin gates below 0.8 mm generate melt fracture at the gate. When mould release is required, silicone-free external agents are used because silicone migration interferes with subsequent adhesive bonding or overmoulding. The material does not require post-mould curing, although annealing for 1 h at 100 °C reduces residual stress in thick sections.

Across thin-wall elastic film lines, SEEPS 4055 is normally pre-compounded with polypropylene and paraffinic oil rather than extruded as a neat resin. On a 30:1 L/D single-screw extruder with a barrier screw and mixing pins, a 45-Shore A formulation containing 30 wt% SEEPS 4055, 20 wt% polypropylene homopolymer, and 50 phr paraffinic oil is processed at 200–220 °C. Film thickness below 100 µm requires die-lip temperature at the upper end of this range to suppress surface melt fracture. The saturated midblock provides elastic recovery from the physical network of polystyrene domains; when polypropylene content exceeds 40 wt%, tensile set after 100 % strain increases from 15 % to 35 % when measured immediately after unloading per ASTM D412. This is a hard-phase dilution effect that must be considered in elastic film design. Cast-film lines using chilled rolls at 15–25 °C produce anisotropic tensile strength in machine direction, whereas blown-film operations typically exhibit lower anisotropy but higher die-lip fouling. Published data for tubular blown film at thicknesses below 50 µm is limited; pilot-line validation is required.

Compounds spanning 30–90 Shore A are generated by adjusting oil and polypropylene concentration. At 30 Shore A, a formulation containing 100 phr SEEPS 4055 and 80 phr paraffinic oil retains tensile strength above 8 MPa, but the elastic network becomes sensitive to oil migration at 60 °C. At 90 Shore A, addition of 50 wt% polypropylene raises flexural modulus while reducing recovery after 100 % strain. The oil is absorbed in two stages on a heated high-speed mixer at 80–100 °C; free oil should remain below 0.5 wt% before extrusion to prevent screw slip. On a co-rotating twin-screw extruder, downstream oil injection is preferred when total plasticiser exceeds 40 phr. The grade can therefore be used across soft-touch grips, rigid impact modifiers, and intermediate tubing compounds without changing to a different SEEPS type.

Repeated Gamma and Steam Exposure Leaves the Saturated Midblock Oxidation-Resistant but Not Crosslinked

Gamma irradiation at 25–40 kGy in air produces a measurable 10–20 % loss of elongation at break in stabilised SEEPS 4055 compounds, while unhydrogenated SBS grades undergo severe embrittlement at the same dose. The loss is associated with chain scission in the polystyrene domains rather than midblock oxidation. Ethylene oxide sterilisation at 55 °C for 3 h does not change Shore A hardness when specimens are conditioned to 23 °C / 50 % RH for 48 h. Steam autoclave exposure at 121 °C for 15 min is limited to approximately 50 cycles for a 50 Shore A medical compound; compression set increases by 15 percentage points after 50 cycles, and the polystyrene endblock glass transition begins to permit dimensional recovery loss. Extractables testing for USP Class VI and ISO 10993-5 cytotoxicity is required on the finished device because the saturated midblock lowers leachable oligomer content relative to SBS but does not eliminate all low-molecular-weight fractions. Antioxidant package selection must avoid primary aromatic amines because these can migrate and interfere with cell-culture assays.

When Hot-Melt Adhesive Formulations Replace SEBS with SEEPS 4055, Mix Viscosity and Cohesive Strength Shift Together

In hot-melt adhesive formulations, SEEPS 4055 is used at 15–25 wt% loading with a tackifier resin, paraffinic or naphthenic oil, and polypropylene stabiliser. At 180 °C, Brookfield viscosity of a formulation containing 20 wt% SEEPS 4055 and 60 wt% C5 tackifier is typically 2,000–4,000 mPa·s; replacing an SEBS of equivalent styrene content often raises viscosity by 20–40 %, requiring application temperature increase of 5–10 °C. Cohesive strength improves at 50 °C; T-peel assemblies on untreated polypropylene show cohesive failure rather than adhesive delamination when conditioned for 24 h at 23 °C / 50 % RH. Open time on aluminium substrates is 20–30 s at 180 °C application temperature. The saturated midblock reduces odour and oxidative yellowing during storage at 60 °C for 4 weeks compared with SBS-based hot melts. Low shear rate viscosity rises sharply when tackifier loading exceeds 60 wt%, which can cause nozzle clogging in gear-pump melt delivery systems.

Because the saturated midblock is non-polar, SEEPS 4055 has limited resistance to polar solvents, aromatic hydrocarbons, and chlorinated solvents; immersion in ASTM Reference Fuel C causes swelling above 100 %. Compounds should not be combined with amine-based additives that promote premature discolouration, or zinc stearate levels above 0.5 phr if optical clarity is required. Continuous service above 120 °C is not recommended for load-bearing parts because the polystyrene endblock domains soften; intermittent service to 150 °C is possible only when the part is supported and stress is below 0.1 MPa. Processing must avoid prolonged exposure above 260 °C and open flame. Extended outdoor exposure requires carbon black, UV stabiliser, or co-extrusion with a UV-opaque layer; otherwise chalking develops after 36 months under ISO 4892-2-equivalent conditions.