Asahi Kasei Tuftec™ H1521 is supplied as a neat, non-oil-extended styrene-ethylene-butylene-styrene block copolymer with a styrene-to-hydrogenated midblock ratio near 20/80 wt%. The polymer is produced by anionic polymerisation of styrene and butadiene, followed by selective hydrogenation of the polybutadiene midblock to an ethylene-butylene sequence. This architecture creates a two-phase morphology in which polystyrene domains with a glass-transition temperature near 95 °C act as physical crosslinks in the solid state. Representative values published in supplier technical literature include a specific gravity of 0.91 when tested according to ISO 1183-1, Shore A hardness of 50–55 when tested according to ISO 868, tensile strength of 4.5–5.5 MPa and elongation at break of 600–700% when tested according to ISO 37, and melt flow rate of 4.5–5.5 g/10 min at 230 °C and 2.16 kg when tested according to ISO 1133-1:2022. Specimens are typically compression-moulded at 200 °C and conditioned at 23 °C and 50 % relative humidity for 24 h before mechanical testing. These ranges reflect lot-to-lot variation and are not specification limits.

The grade is positioned for flexible compounds where low hardness, high elongation, low density, and melt reprocessability are required. Because the terminal polystyrene block content is relatively low, room-temperature modulus and tensile strength are lower than those of higher-styrene Tuftec grades, while elongation at break and low-temperature flexibility are enhanced. The material also has lower density than many thermoplastic polyurethane and plasticised polyvinyl chloride systems, and it does not require vulcanisation.

What Distinguishes H1521 from Higher-Styrene SEBS and Non-SEBS Elastomers?

Against Tuftec H1517, the principal difference is styrene content. H1517 is supplied with a styrene fraction near 33 wt%, which increases the volume fraction of glassy polystyrene domains and raises Shore A hardness into the 80–85 range. H1521, at approximately 20 wt% styrene, remains in the 50–55 Shore A range and gives lower tensile strength but higher elongation at break. The lower polystyrene block content also reduces the strength of the polystyrene–polypropylene interfacial interaction in polypropylene blends; this can be advantageous when a softer overmoulded layer is required but may require higher addition levels in impact modification. Published data for this specific configuration is limited and should be confirmed by pilot compounding.

Compared with ethylene-propylene-diene monomer rubber, H1521 requires no vulcanisation, is melt-reprocessable, and exhibits lower density; however, upper continuous service temperature under load is lower and compression set at elevated temperature is generally higher. Compared with SBS block copolymers, the hydrogenated ethylene-butylene midblock provides improved resistance to thermo-oxidative degradation and ultraviolet-induced chain scission. Compared with flexible PVC, H1521-based compounds do not require high-molecular-weight external plasticisers to achieve low hardness, although paraffinic process oils are commonly added as extenders.

For low-hardness gel compounds, a practical starting-point formulation comprises 100 phr H1521, 200–250 phr paraffinic white oil with aromatic content below 0.1 wt%, and 20–30 phr fumed silica or calcium carbonate when a yield stress is required. The oil is injected after the first kneading section of a co-rotating twin-screw extruder to prevent screw slip and to maintain torque stability. Oil uptake is relatively high because the low terminal-block content reduces the mechanical resistance of the polystyrene network to midblock swelling. Gel hardness is commonly checked after ageing at 25 °C for 24 h because the final hardness of oil-extended SEBS develops slowly as the paraffinic oil partitions into the ethylene-butylene phase.

When H1521 Is Run on a 40:1 Co-Rotating Twin-Screw Compounding Line

On production-scale lines using a co-rotating twin-screw extruder with a 40:1 length-to-diameter ratio and 12 barrel sections, a typical barrel profile is 180 °C in the feed zone, 190–210 °C through kneading and mixing zones, and 200 °C at the die. H1521 pellets are fed into barrel section 1 by gravimetric feeder. Paraffinic white oil with kinematic viscosity of 40–100 cSt at 40 °C is injected into barrel section 5 downstream of the first kneading block. Oil injection before a complete melt seal produces screw slip, torque fluctuation, and melt-pressure oscillation at the die greater than 0.5 MPa. Fillers are side-fed at barrel section 7 to minimise attrition and to avoid excessive torque rise in the early melting zone.

Screw speed is typically limited to 300–500 rpm. Higher speeds can raise melt temperature above 240 °C and promote gel formation and midblock chain scission. Pelletising is conducted by underwater die-face cutting with water temperature of 30–50 °C. The melt should be kept below 240 °C during normal operation; residence time above 260 °C should be minimised because hydrogenated midblock degradation can generate hard particles and reduce elongation at break.

Pre-drying of H1521 pellets is not generally required because the polymer is non-hygroscopic. However, compounds containing hygroscopic fillers such as calcium carbonate with high surface moisture or aluminium trihydrate require pre-drying at 80–90 °C until residual moisture is below 0.05 wt%. Aromatic process oils should be avoided at levels above approximately 5 phr because they partition into the polystyrene domains, reduce the effective glass-transition temperature of the physical crosslinks, and lower upper service temperature. Chlorinated solvents and strong oxidising acids are incompatible in service.

Employing H1521 in hot-melt pressure-sensitive adhesive systems requires selection of a hydrogenated hydrocarbon tackifier that associates with the ethylene-butylene midblock. A typical starting formulation contains 100 parts H1521, 200–280 parts hydrogenated hydrocarbon tackifier with softening point of 90–110 °C, and 60–100 parts paraffinic oil. Coating viscosity at 180 °C is controlled by the oil-to-tackifier ratio and is often maintained below 10 Pa·s for slot-die coating. Thermal stability during adhesive production is improved relative to unsaturated SIS or SBS systems because the saturated midblock resists chain scission under hot-melt tank conditions.

In polypropylene impact modification, the use of H1521 at 10–20 wt% dispersed as 0.5–1.5 µm particles in a continuous polypropylene matrix can shift the ductile-brittle transition temperature to lower temperatures. The lower styrene content reduces the modulus penalty at low temperatures but also provides less interfacial adhesion than a styrene-rich SEBS. Twin-screw dispersion is carried out using a paddle-to-kneading block sequence that imposes elongational mixing after the polymer has reached the molten state; particle-size distribution is commonly evaluated by scanning electron microscopy after selective extraction of the polypropylene phase. Published data for this specific grade in high-melt-strength polypropylene is limited, and pilot-scale validation is required for target impact values.

Regulatory Documentation, Lot-to-Lot Variability, and Operational Boundaries

Documentation for H1521 should identify the material as SEBS under ISO 1043-1. Melt flow rate is reported according to ISO 1133-1:2022 at 230 °C and 2.16 kg. Hardness is determined according to ISO 868 with a 15 s reading on a compression-moulded plaque. Tensile properties are determined according to ISO 37 using type 1 dumbbell specimens at a test speed of 500 mm/min. These test designations allow comparison against other styrenic elastomers, but the values are not specification limits. For critical applications, the user should obtain a lot-specific certificate of analysis from the supplier.

Operational boundaries for H1521 are defined primarily by melt temperature, oil aromaticity, and service contact with strongly polar or chlorinated solvents. The melt-processing window is narrower than that of some high-flow polyolefins; barrel setpoints should be kept below 220 °C for extended residence times, and start-up after a line stoppage should include a purge sequence to remove degraded hold-up material. Compression set values should be evaluated according to ISO 815-1 when the compound is intended for sealing applications because the low styrene content can increase compression set at elevated temperature relative to higher-styrene SEBS grades. The use of amine-based heat stabilisers is not required and is not recommended unless compatibility with the compound is confirmed; incompatible stabiliser packages can bloom to the surface and reduce adhesive bond strength in overmoulded parts.