Kuraray SEPTON™ SEBS TU-S5265 is a hydrogenated styrenic triblock copolymer composed of hard polystyrene end-blocks and a saturated ethylene-butylene midblock. The material is supplied as a dense pellet and is identified under the general SEBS CAS registry number 66070-58-4. In melt-state evaluations, the grade is characterised according to ISO 1133-1:2022 for melt mass-flow rate, ISO 1183-1:2019 for density, ISO 868:2003 for Shore hardness, ISO 37:2017 for tensile stress-strain properties, ISO 815-1:2014 for compression set, and ISO 36:2020 for tear strength. Regulatory documentation for food-contact or medical applications must be confirmed against the current Kuraray technical data sheet; published secondary sources do not state lot-specific compliance status for FDA 21 CFR 177.1810, EU Regulation No 10/2011, or REACH SVHC.

The principal technical distinction between TU-S5265 and general-purpose SEBS grades appears in the melt-flow range and in the resulting injection-pressure requirement for thin-wall moulding. The grade is therefore assigned to high-flow moulding and compounding applications where a moderate hardness, low compression set, and polyolefin compatibility are specified without the use of migratory plasticisers.

When TU-S5265 Replaces Unmodified SEBS in Thin-Wall Injection Moulding

On a reciprocating-screw injection moulding machine with a screw L/D ratio between 20:1 and 24:1 and a compression ratio of 2.5:1 to 3.0:1, replacement of an unmodified low-flow SEBS with TU-S5265 shifts the feasible fill-pressure window downward for cavity wall sections below 1.0 mm. The shift is quantified by melt mass-flow rate under ISO 1133-1:2022 at 230 °C with a 2.16 kg load; supplier literature for TU-S5265 reports a value in the 5 g/10 min to 15 g/10 min range, while many unmodified SEBS grades of similar hardness remain below 5 g/10 min under the same conditions. The numerical value should be rechecked against the current technical data sheet because lab-to-production shifts of 10 % are common when regrind or colour masterbatch is introduced.

The barrel temperature profile is normally set from 180 °C at the rear zone to 220 °C at the nozzle, with a maximum melt temperature of 240 °C. Holding the melt above 250 °C for more than 10 minutes can initiate chain scission; the first observable effect is a decrease in tensile elongation at break when tested according to ISO 37:2017. Pre-drying is required when equilibrium moisture content exceeds 0.1 wt%. A desiccant hopper dryer with a dew point ≤ −30 °C and a bed temperature of 70 °C to 80 °C for 2 h to 4 h is sufficient for pellets stored in sealed containers. Re-drying is advised if the pellets are exposed to relative humidity above 60 % RH for more than 24 h. Moisture does not hydrolyse the saturated midblock, but volatile expansion at the nozzle produces splay and surface defects in fast-cycle parts.

Because SEBS is shear-sensitive, the screw speed should be limited to a peripheral speed of 0.2 m/s to 0.5 m/s. High-shear mixing raises melt temperature through viscous dissipation and can produce local viscosity non-uniformity. The non-return valve should be of a ball type rather than a sliding-sleeve type when high back-pressure is used; sliding-sleeve valves can trap degraded material and cause intermittent black specks. Back pressure is typically set between 0.5 MPa and 1.0 MPa. Injection velocity should be staged to avoid jetting: 30 mm/s to 60 mm/s for gate penetration, then 60 mm/s to 120 mm/s for cavity filling. Published data for the exact pressure reduction achievable with TU-S5265 on a specific clamp force machine is limited; however, processing audits on high-flow SEBS grades of this class have recorded peak injection pressure reductions of 10 MPa to 20 MPa on 1000 kN machines when filling 1.5 mm wall parts.

The grade should not be combined with amine-based antiozonants or amine-based adhesion promoters, because residual amine species can accelerate oxidative degradation during heat aging. A phenolic/phosphite stabiliser system is preferred. In oil-extended formulations, only paraffinic or naphthenic white oils with a kinematic viscosity between 40 mm²/s and 500 mm²/s at 40 °C should be used; aromatic oils plasticise the polystyrene end-blocks and lower service temperature. Oil loading above 120 phr can produce exudation under storage at 70 °C for 24 h.

What Limits Use in Soft-Touch Overmoulding onto Polyolefins?

Overmoulding of TU-S5265 onto polypropylene or polyethylene relies on melt fusion and interdiffusion at the interface rather than on polar chemical bonding. The substrate surface should reach 60 °C to 90 °C before the second shot; below 50 °C the interdiffusion depth is insufficient and peel strength measured by ISO 36:2020 or a 90° peel jig may fall below 2 N/mm. The grade is not recommended for direct overmoulding onto nylon or polycarbonate without an adhesive tie layer because the ethylene-butylene midblock has a low polar solubility parameter. When a tie layer is used, the polar surface energy of the substrate should be raised above 40 mN/m by corona or plasma treatment; the treated surface remains active for 15 minutes to 30 minutes depending on ambient humidity.

The soft-touch effect is obtained by oil extension. A hardness below 50 Shore A requires addition of a paraffinic or naphthenic white oil; the oil should be injected into the melt rather than dry-blended at the feed throat when the addition level exceeds 20 phr. Migration kinetics in SEBS matrices are concentration-dependent, and exudation is first observed at oil loadings above 120 phr after storage at 70 °C for 24 h. For automotive interior parts, long-term fogging behaviour is evaluated by ISO 6452 gravimetric or reflectometric methods; oil-selected grades should produce a fogging condensate below 2 mg in the 100 °C / 16 h gravimetric procedure. Published data for this specific configuration is limited; the current technical data sheet may report a different fogging value depending on oil type.

Compression set is a limiting factor in sealing applications. Under ISO 815-1:2014, type A specimen, 70 °C for 22 h, TU-S5265-type materials typically exhibit compression set in the 25 % to 40 % range, which is lower than many general-purpose SEBS grades that exceed 35 % to 60 % under identical conditions. This difference is attributed to styrene domain morphology and the absence of unsaturated midblock crosslinking. Continuous service temperature should not exceed 110 °C in unstressed air aging. Above 120 °C, tensile strength retention after 168 h according to ISO 188:2011 may fall below 80 % unless an antioxidant system is added. The grade is not recommended for direct immersion in hot water above 80 °C for extended periods because water absorption and interfacial plasticisation can reduce peel adhesion to polyolefin substrates.

Hardness, Tensile Set, and MFR Comparative Data

The following table summarises the test method designations and the numerical ranges most often reproduced in supplier literature for TU-S5265 and for an unmodified SEBS reference class of similar Shore A hardness. The TU-S5265 column should be interpreted as the published commercial range for this grade; the unmodified SEBS column is a general reference class and does not identify a specific competitor grade. Where a cell is not disclosed, the reader must consult the current Kuraray technical data sheet rather than rely on secondary compilations.

For regulatory screening, the following compliance matrix is used by raw-material manufacturers for SEBS compounds. The status column is based on the general hydrogenated styrenic block copolymer class; lot-specific TU-S5265 certification may differ and must be requested from the supplier.

Compounding of TU-S5265 with polypropylene, fillers, and stabilisers is performed on co-rotating twin-screw extruders with L/D ratios between 40:1 and 52:1. Side-feeding of calcium carbonate or talc after the polymer melting zone prevents premature screw wear; filler loadings above 30 wt% increase melt pressure and require barrel temperatures in later zones to be reduced by 5 °C to 10 °C relative to the unfilled compound to avoid chain scission. For oil-extended formulations, injection of paraffinic oil at 40 °C to 60 °C into the melt at zone 5 or zone 6 of a 12-zone barrel improves dispersion and prevents feed-throat blockage. When oil is added directly to the main feed, pellet clumping is observed at addition levels above 20 phr. The specific mechanical energy of the extruder should not exceed 0.25 kWh/kg for unfilled TU-S5265 compounds, because excessive shear energy reduces molecular weight and lowers the tear strength measured by ISO 36:2020. The extruder vacuum vent should be set to −0.08 MPa to −0.09 MPa to remove volatile oil fractions and moisture; vent flooding indicates either screw speed above 500 min⁻¹ or a barrel temperature profile that is too low in the mixing zones.

The selection of TU-S5265 over higher-styrene SEBS grades or over maleic anhydride-grafted SEBS is based on flow length, hardness, and adhesion requirements. Higher-styrene SEBS grades have greater tensile modulus but require higher melt temperature and show lower flowability; maleic anhydride-grafted SEBS grades provide polar adhesion to nylon or metal but are more hygroscopic and must be dried to below 0.05 wt% moisture. In contrast, TU-S5265 is usually selected for thin-wall polyolefin-compatible parts where the processing window is narrow and a low-to-medium hardness is acceptable. Published data comparing the peel strength of TU-S5265 to that of a maleic anhydride-grafted grade on nylon is limited; the difference in adhesion mechanism should be verified by a 90° peel test according to ISO 36:2020 or a customer-specific method before tool construction.

In injection moulding trials on 800 kN to 1200 kN machines, surface defects such as jetting, weld-line cracking, and splay have been traced to insufficient nozzle temperature, wet feedstock, or excessive back pressure. Weld-line strength in TU-S5265 is dependent on melt temperature at the flow front; when the flow front temperature drops below 200 °C, the tensile strength at the weld line measured by ISO 37:2017 can be 30 % to 50 % lower than the bulk tensile strength. Multi-gate tooling should be arranged so that melt fronts meet at the highest possible cavity temperature; use of a conformal cooling circuit with water temperature above 40 °C reduces visible weld-line depth. Published data for this specific configuration is limited; the above range is representative of SEBS compounds of this hardness class.

TU-S5265 differs from styrene-butadiene-styrene block copolymers in that the hydrogenated midblock is saturated, which provides resistance to ozone cracking. Under ISO 1431-1:2004 ozone resistance testing, unmodified SBS grades show surface cracking at 50 pphm ozone concentration within 24 h, whereas SEBS grades of this class typically withstand 200 pphm for more than 72 h without visible crack formation. This distinction is a primary reason for selecting TU-S5265 in exterior automotive or construction profiles where long-term weathering is required.