SINOPEC Baling SEBS YH-535 is a linear styrene-ethylene/butylene-styrene triblock thermoplastic elastomer produced by sequential anionic polymerisation and selective hydrogenation of the polybutadiene midblock. Manufacturer-published typical data place the bound styrene content in the range 32–36 wt%, with residual unsaturation below 1 mol%. The grade is supplied in pellet form and is specified for compounding, adhesive bases, sealants, and thermoplastics modification. In compression-moulded specimens tested according to ISO 37, the material exhibits a tensile strength of ≥18 MPa and an elongation at break between 450% and 600%. Hardness measured under ISO 868 is typically 75–80 Shore A. These values position YH-535 above lower-styrene Baling SEBS grades in modulus and tensile strength, but below oil-extended SEBS products in softness. The practical consequence is that YH-535 is selected for technical moulded parts, impact modification, and high-strength compounding rather than ultra-soft gel or cushioning components.

Molecular architecture controls the service performance. The polystyrene endblock glass transition temperature in YH-535 is approximately 95 °C, while the hydrogenated ethylene-butylene midblock exhibits a glass transition near −42 °C. This block sequence creates a physically crosslinked network without chemical vulcanisation; the network reverses when heated above the styrene domain softening point and reforms on cooling. Diblock content is controlled below 1.0 wt% because unbound styrene-ethylene/butylene diblock does not participate in the room-temperature network and depresses tensile strength, creep resistance, and compression set. The hydrogenation process consumes more than 95% of the original butadiene unsaturation, as confirmed by iodine number or infrared spectroscopy. The resulting ethylene-butylene midblock provides resistance to UV, ozone, and heat that is not available in unsaturated SBS grades, and it allows YH-535 to be processed at melt temperatures up to 260 °C for short residence times. Above 280 °C, chain scission and polystyrene domain degradation advance rapidly, especially when the melt is held at low screw speed in a dead spot.

Which Rheological and Mechanical Characteristics Differentiate YH-535 from Adjacent Baling SEBS Grades?

From a rheological standpoint, YH-535 behaves as a strongly pseudoplastic melt with a melt mass-flow rate at 230 °C under a 5 kg load in the range 0.3–1.0 g/10 min when tested according to ISO 1133-1:2022. This places the grade among medium-high-viscosity linear SEBS polymers. The steady-shear viscosity is lower than that of star-branched Baling grades of similar hardness, such as some YH-600 series products, and higher than that of softer linear YH-500 series grades. The practical effect is that YH-535 can be injection moulded with lower pressure than a star-branched SEBS of equivalent Shore A hardness, but it may reduce the melt flow of polypropylene compounds more than a lower-styrene linear SEBS. The melt strength of YH-535 is moderate; in blow moulding and sheet extrusion, sagging is observed below 190 °C unless the formulation includes 10–20 wt% high-melt-strength polypropylene. In tensile deformation, the 300% tensile stress is typically 3.0–5.0 MPa, indicating a well-formed polystyrene domain network. Tensile set after 200% elongation at 23 °C is normally below 25%. Published direct comparative data for every adjacent grade is limited; therefore substitution trials should include rheology screening and moulded-part property verification.

Compounding, Drying, and Thermal Processing Boundaries

Neat YH-535 pellets are only moderately hygroscopic, but surface moisture above 0.05 wt% produces splay and surface defects. Drying in a desiccant dryer at 70 °C for 3 h is required when storage relative humidity has exceeded 60% for more than 24 h. On a co-rotating twin-screw extruder with a 40:1 L/D ratio and segmented screw elements, a typical barrel profile for YH-535 compounds is 160 °C at the feed throat, 190 °C in the melting zone, 220 °C in the mixing zone, and 230 °C at the die. Melt temperature is monitored by infrared probe and kept below 250 °C. Specific energy input of 0.20–0.30 kWh/kg is required to disperse YH-535 in polypropylene; below 0.18 kWh/kg, undispersed elastomer domains larger than 1.0 µm may remain and reduce impact strength. In single-screw profile extrusion, a barrier screw with a compression ratio of 2.5:1–3.0:1 and a 60/80/100 mesh screen pack is used to filter unmelted gels. Screw speed is usually limited to 30–60 min−1 to prevent excessive shear heating. At local shear rates above 1000 s−1, melt fracture and sharkskin can appear, especially when the compound is processed near the lower end of the melt-temperature window. The processing window narrows as filler loading increases; glass-fibre-filled YH-535 compounds should not exceed 240 °C because the combination of high shear and high temperature promotes degradation at the fibre-matrix interface.

Oil extension of YH-535 with a white mineral oil having a kinematic viscosity of 40–100 mm²/s at 40 °C is a standard route to lower hardness. In a high-speed mixer heated to 90 °C, the porous pellets absorb oil within 20 min; at ambient temperature, the same oil may require up to 24 h. The practical oil-absorption limit before surface exudation is approximately 120 phr for neat YH-535 and 80 phr when 20 wt% isotactic polypropylene is present. At oil loadings above 100 phr, tensile strength falls below 5 MPa and compression set at 70 °C tested according to ASTM D395-18 exceeds 60%. The addition of 10 wt% polypropylene raises the Vicat softening temperature by 15–20 °C under ISO 306, but reduces elongation at break by 25–35%. For adhesive and sealant formulations, YH-535 is compounded with C5 hydrocarbon tackifier at 100–200 phr and a naphthenic or paraffinic oil at 50–100 phr; the ring-and-ball softening point is then adjusted to 85–110 °C following ASTM E28. C9 aromatic tackifiers at high loadings can depress the polystyrene domain strength and are generally limited to 20 phr unless the formulation is specifically designed for high-tack pressure-sensitive adhesives.

When YH-535 Replaces Unsaturated SBS or Lower-Styrene SEBS in Impact-Modified Polypropylene

Substitution of YH-535 into an impact-modified polypropylene formulation originally based on a lower-styrene linear SEBS alters the balance between stiffness and toughness. The flexural modulus of the compound, measured on 4 mm injection-moulded bars according to ISO 178, typically increases by 10–25% at a constant elastomer content of 20 wt%. This increase is attributed to the higher polystyrene endblock fraction, which forms stiffer domains in the polypropylene matrix. Notched Izod impact strength at 23 °C according to ISO 180/A can remain above 15 kJ/m² if the dispersed elastomer domain size is maintained below 1.0 µm; that requires a screw configuration with at least two kneading zones and a specific energy input of 0.20–0.30 kWh/kg. The trade-off is that the compound melt flow rate decreases by 0.5–2.0 g/10 min under 230 °C/2.16 kg; therefore a barrel temperature increase of 10–15 °C is often required. In low-temperature impact tests at −30 °C, brittle failure can occur if the elastomer domains are larger than 1.5 µm, so moulding conditions that create high orientation and coarse morphology should be avoided. Compared with unsaturated SBS, YH-535 gives improved retention of tensile strength after weatherometer exposure for 1000 h; the loss is typically below 10%, whereas a similar SBS compound may lose more than 30%. Published data for this specific configuration is limited; therefore the weatherometer comparison should be verified with the target colour masterbatch and stabiliser package.

On a production-scale injection moulding line with a clamp force of 1200 kN and a 40 mm screw diameter, YH-535 compounds are processed with a melt temperature of 220–240 °C and an injection speed of 40–80 mm/s. The gate diameter is maintained above 1.5 mm for unfilled compounds and above 2.0 mm for glass-fibre-filled compounds because narrow gates produce high shear and surface defects. Mould temperatures of 20–50 °C are used for high-gloss surfaces; 50–70 °C is preferred for complex housings where knit-line strength is critical. Shrinkage after 24 h at 23 °C is anisotropic: in-flow shrinkage is typically 1.5–2.0% and cross-flow shrinkage is 0.8–1.2% when measured according to ISO 294-4. In profile extrusion, a vacuum calibration tank with water at 15–25 °C is used to set dimensions before the surface develops tack. Sections thicker than 4 mm require additional cooling time of 10–20 s/mm because the low thermal conductivity of YH-535 slows heat removal and can cause sink marks. The polymer is not recommended for continuous service above 100 °C in load-bearing applications, because the polystyrene domains soften and creep accelerates. It is also not recommended for contact with strong oxidising acids, chlorinated solvents above 40 °C, or high-aromatic process oils above 50 phr because these conditions compromise the phase-separated network.