Kuraray SEPTON™ SEEPS-OH HG252 is a terminal-hydroxyl-functionalized hydrogenated styrenic block copolymer in which the midblock is a hydrogenated poly(ethylene/ethylene-propylene) sequence between polystyrene end blocks. The designation SEEPS-OH identifies the hydrogenated styrene-ethylene/ethylene-propylene-styrene backbone and the terminal hydroxyl group. Unlike a conventional polyether diol, the hydroxyl groups are present at the ends of a high-molecular-weight block copolymer, not as a low-viscosity telechelic liquid. The terminal groups enable covalent reaction with isocyanates, epoxides, carboxylic acids, and silanes while the styrenic domains retain a physical network below the polystyrene glass transition. The product is supplied in pellet form for melt processing, solution coating, and reactive compounding.

What Molecular Design Consequences Follow from Terminal Hydroxyl Placement?

The terminal placement of hydroxyl functionality on a SEEPS chain has a direct analytical consequence: the hydroxyl value is low, generally reported by the manufacturer as ≤ 2.0 mg KOH/g by ISO 14900:2017. The corresponding hydroxyl equivalent mass is therefore higher than that of conventional polyether or polyester diols, which restricts volumetric reactivity but reduces shrinkage and moisture sensitivity. The polystyrene block content is reported in the 20 wt% range in Kuraray technical literature, which is sufficient to maintain thermoreversible domain formation but low enough to keep the modulus of uncompounded material in the soft Shore A region.

Thermal analysis of the hydrogenated midblock by ISO 11357-2:2020 typically shows a glass transition near −50 °C; the polystyrene phase exhibits a glass transition or melt endotherm above service temperature depending on annealing. These two transitions define the operating window: the upper bound is controlled by polystyrene domain integrity, while the lower bound is controlled by rubber-phase embrittlement. In the molten state, the product remains processable in the 200–230 °C range, but reactive conversion and residence time must be controlled because chain extension can change melt viscosity rapidly.

Unlike maleic anhydride-grafted SEBS and non-functional SEEPS grades, HG252 places the reactive group at the chain terminus rather than as a pendant graft. Non-functional SEEPS grades in the Kuraray SEPTON line depend on physical entanglement and styrenic domain cohesion; they do not generate covalent interfacial bonds during compounding. Maleic anhydride-grafted SEBS will preferentially form imide or amide bonds with amine-terminated polyamide during reactive extrusion, whereas HG252 will not consume terminal amines; it reacts through hydroxyl pathways. This makes HG252 more suitable for polyurethane and epoxy systems where amine consumption is undesirable. Compared with hydroxyl-terminated liquid polybutadiene, HG252 has much higher melt viscosity and greater cohesive strength after cooling, because the polystyrene blocks recover their physical crosslinks.

Rheological and Thermal Characterisation Using Standard Test Methods

Lot-level characterisation of HG252 is performed using melt, mechanical, and chemical methods. Melt flow rate is measured under a 230 °C, 2.16 kg condition according to ISO 1133-1:2022; this is an incoming feedstock check rather than a measure of reactive conversion. Mechanical test pieces are injection moulded or compression moulded and conditioned under ISO 291:2008 before tensile testing to ISO 37:2017. Hardness testing follows ISO 7619-1:2010 using Shore A or D as appropriate.

Lot-specific data should be obtained from the material certificate because reactive-grade polymers can exhibit wider hydroxyl-value variation than standard SEEPS, and hydroxyl value directly controls stoichiometry in two-component polyurethane reactions. The low hydroxyl value means that formulators must calculate stoichiometry from hydroxyl equivalent mass, not from hydroxyl percentage. HG252 should not be interchanged with polyester diol or polyether diol unless hard-block content, phase separation, and adhesive open time are reoptimized.

In continuous twin-screw reactive processing of HG252, pre-drying of pellets is applied before exposure to isocyanate or silane reagents. Moisture above 200 ppm in the pellet feed can hydrolyse isocyanate groups, generate carbon dioxide, and produce microvoids. Vacuum drying at 70 °C for 4 h or a desiccant dryer with dew point below −40 °C is normally used. Co-rotating twin-screw extruders with L/D 44:1 or greater are preferred for reactive compounding; barrel temperatures are controlled from 180 °C to 230 °C. A flat to slightly forward temperature profile avoids excessive melt temperature near the die. Specific mechanical energy in compounding HG252 into engineering thermoplastics typically falls in the 0.08–0.16 kWh/kg range, though published data for this exact configuration are limited and throughput must be adjusted by melt pressure and post-extrusion residual hydroxyl determination.

Solution preparation of HG252 differs from bulk melt handling. The polymer dissolves in aromatic solvents and certain ketone/ester blends; dissolution is slower than SEBS of equivalent molecular weight because of the polar end groups. A high-shear disperser or enclosed solvent blender is used to reach resin solids of 20–40 wt%. Once dissolved, the terminal hydroxyl groups can be titrated directly by ISO 14900:2017 on a dried solution sample. In moisture-cure silyl-terminated formulations, the terminal hydroxyl group is first reacted with an isocyanato-alkyl-trialkoxysilane; residual isocyanate is monitored by titration under ISO 14896:2009.

When Hydroxyl End Groups Enter Reactive Extrusion and Adhesive Formulations

The hydroxyl terminus of HG252 is used most effectively in one-step or two-step reactive formulation. In polyurethane reactive hot melts, the polymer is combined with a polyester or polyether diol, an isocyanate-terminated prepolymer, and a non-interfering tackifier. The terminal hydroxyl groups act as a chain extension site; they increase molecular weight before moisture cure and contribute hydrogenated midblock elasticity. Mixing is carried out in a heated planetary mixer or a continuous reactor at 120–140 °C under vacuum. The dispensed melt is applied with a gear pump and slot die; final moisture cure is evaluated by ISO 11339:2010 T-peel and ISO 4587:2003 lap shear. Published data for HG252-specific formulations are more limited than for conventional polyurethane systems, so joint conditioning and full factorial screening are required.

In epoxy and engineering thermoplastic toughening, terminal hydroxyl groups react with anhydride curatives or carboxylic acid end groups. Addition of HG252 at 5–15 wt% to a brittle epoxy anhydride network can shift the glass transition downward and absorb fracture energy through particle cavitation or shear yielding; the exact transition depends on whether the hydroxyl group is fully converted. For polyamide blends containing terminal carboxyl groups, reactive extrusion under vacuum can form ester linkages between HG252 and the matrix. The improvement in notched impact strength measured by ISO 179-1:2010 is observed only when the reactive residence time exceeds the esterification half-life and when melt temperature does not exceed the degradation threshold.

For structural adhesives, HG252-containing films can be benchmarked against epoxy-rubber systems. The hydrogenated midblock offers resistance to UV and thermal oxidation but has lower intrinsic adhesion to bare aluminium than functionalised polybutadiene; surface preparation with silane primers or chromic acid anodising may be necessary. Peel and shear values are formulation-specific. Published data for this specific configuration are limited, so comparative data should be generated with a reference hydroxyl-terminated polybutadiene or carboxyl-terminated butadiene-acrylonitrile copolymer.

Thermal Oxidation Resistance Narrows the Upper Service Boundary

The hydrogenated midblock gives HG252 better resistance to thermal oxidation than unsaturated styrenic block copolymers, but continuous exposure in air above 100 °C requires antioxidant protection. Stabilisation is typically achieved with a hindered phenolic primary antioxidant and a phosphite secondary antioxidant during melt compounding; long-term heat ageing follows ISO 188:2023 with retained tensile properties evaluated by ISO 37:2017. Solvent and fluid resistance is not a universal property: the material swells in aromatic and chlorinated solvents, and compatibility with synthetic ester lubricants or biofuels must be evaluated by ISO 1817:2015. At low temperatures, the hydrogenated midblock approaches its glass transition near −50 °C, so impact resistance falls as the application approaches −45 °C unless plasticiser is introduced. Service above 120 °C under load can allow polystyrene hard-domain flow if the article is not crosslinked through the terminal hydroxyl groups.

Storage stability is improved by keeping the product sealed in a dry environment; prolonged exposure to high humidity increases adsorbed moisture, which can interfere with isocyanate stoichiometry and cause foaming. A shelf-life check by melt flow rate and hydroxyl value is recommended before use in high-precision polyurethane metering lines.

Regulatory verification for HG252-containing compounds follows the declaration requirements of REACH under EC 1907/2006. Articles sold in the EU require checking of candidate list substances above the 0.1 wt% threshold under Article 33. For electrical and electronic equipment, RoHS compliance under Directive 2011/65/EU requires restricted-substance testing according to the IEC 62321 series; total concentration limits apply to lead, cadmium, mercury, hexavalent chromium, PBB, and PBDE. Food-contact status is not conferred by the raw pellet; migration testing under EU 10/2011 or compliance with 21 CFR 177.2600 must be demonstrated on the final formed article.