SINOPEC Baling SEBS YH-561 is a linear styrene-ethylene-butylene-styrene triblock copolymer supplied as an unextended pellet. Manufacturer data list styrene mass fraction 33 wt%, Shore A hardness 76 determined at 15 s according to ASTM D2240, tensile strength ≥20 MPa according to ISO 37, elongation at break ≥500%, and melt mass-flow rate below 1.0 g/10 min at 230 °C/5 kg according to ISO 1133-1. Volatile matter is stated below 0.5 wt%. The saturated ethylene-butylene midblock provides resistance to ozone, ultraviolet radiation, and thermo-oxidative chain scission that is not available from unsaturated SBS or SIS grades. The grade is used in polyolefin modification, asphalt modification, adhesive compounds, and thermoplastic elastomer sealing profiles. Because the molecular architecture is linear rather than star-shaped, the product generates lower gel formation during high-shear processing than branched SEBS grades of similar styrene content.

Provided that the comparison is made at equivalent hardness, YH-561 differs from oil-extended SEBS grades in that no paraffinic or naphthenic oil is pre-incorporated. This raises melt viscosity and requires more intensive shear during dispersion, but it leaves the compounder free to select an oil or plasticizer without paying for a carrier oil. The saturated midblock also delays embrittlement during long-term heat aging; however, it does not eliminate thermo-oxidative degradation above the manufacturer’s maximum processing temperature. Users in high-temperature applications should verify retention of tensile strength after air-oven aging at 135 °C according to ISO 188 before committing to a formulation.

What Limits Dispersion Quality in Twin-Screw Compounding of YH-561?

In impact modification of polypropylene, YH-561 is typically metered into the main feed throat of a co-rotating twin-screw extruder with an L/D ratio of ≥40:1 and segmented screw elements. Barrel set-point temperatures from 180 °C to 230 °C are used, and melt temperature is maintained below 260 °C. Production-scale compounding has shown that distributive mixing improves when the melt seal is placed before the first kneading block and when the SEBS is pre-blended with polypropylene instead of fed separately at the side port. At addition levels of 10–30 wt%, notched Izod impact strength measured by ASTM D256 improves only when the dispersed elastomer domain size is reduced below approximately 2 µm. Domains above this threshold function as stress concentrators rather than energy absorbers. A screw speed of 300–600 rpm and a specific energy input of 0.18–0.25 kWh/kg are representative for 40–60 wt% YH-561 masterbatch production on lines with 40:1 L/D; formulations with lower SEBS content may require lower shear to avoid excessive melt temperature. The product’s low flow is not a defect in this application because high viscosity contributes to particle break-up under extensional mixing. Filtration is not normally required for unfilled polyolefin modification, but when carbon black or mineral fillers are present a screen pack of 100/200/100 mesh is installed before the die plate to catch agglomerates. The use of side feeding for fillers after the SEBS has melted reduces wear on the primary kneading section and improves filler dispersion.

Pre-drying is usually unnecessary when ambient relative humidity remains below 60%. At higher relative humidity, surface moisture has been observed to generate melt vacuoles and surface roughness in sheet and profile extrusion; a dehumidified-air dryer set at 80 °C for 2 h removes this source. Pellets should not be stored in open bins for more than 24 h in tropical plant conditions. Because YH-561 is unextended, its pellets are harder than oil-extended SEBS and can bridge in bulk feeders with narrow cone angles; feed screws with ≥45° hopper half-angle and vibratory pad activation are used on production lines. Conveying air should be dried to a dew point below −20 °C to prevent condensation in dilute-phase systems.

Rheological Contrast with Oil-Extended SEBS Grades

The table below provides a representative comparison between YH-561 and oil-extended SEBS compounds used in soft-touch grips and seals. Actual oil-extended grades vary with oil type, molecular weight, and styrene content; the ranges are not lot specification values.

This rheological contrast has direct consequences in injection molding. YH-561 demands higher cavity pressure and faster injection speed than oil-extended grades. Gates should be sized for apparent viscosity at the relevant shear rate; at 1000 s⁻¹, unextended SEBS can exhibit viscosity an order of magnitude above that of oil-extended compounds. Pack pressure must be held until gate freeze, and screws with low compression ratios of 2:1 to 2.5:1 are preferred to avoid shear heating. Mold temperatures of 30–50 °C are adequate for demolding, but thick sections may require cooling times longer than those used for SBS because the low melt mass-flow rate retains heat. Surface defects on molded parts are reduced when nozzle and hot-runner temperatures are kept within 10 °C of the front zone set-point. Venting is critical because residual volatiles and surface moisture generate burn marks at vent depths below 0.02–0.05 mm. Cold runner systems with full-round runner diameters of 6–10 mm are used for large parts, while hot runners require insulated gates to prevent stringing.

For hot-melt adhesive formulations based on YH-561, the saturated midblock permits use in outdoor adhesive films where SIS or SBS would require higher antioxidant levels. Solution viscosity is substantially higher than that of oil-extended SEBS at equivalent solids. In toluene at 25 wt% solids, rotational viscometer readings are commonly in the range of 2,000–8,000 mPa·s at 25 °C; the value is formulation-specific and should be generated with a Brookfield RVT or equivalent. Tackifier selection is limited empirically by cloud point and loop tack. Hydrogenated C9 hydrocarbon resins with Ring-and-Ball softening point 95–115 °C per ASTM E28 are preferred over highly aromatic resins to maintain color and oxidative resistance. Monomeric ester plasticizers reduce viscosity but can migrate; if low fogging is required, migration tests such as DIN 75201 or SAE J1756 should be used instead of visual haze ratings. Reactive isocyanate or silane systems are not required for adhesion to nonpolar substrates; surface treatment may be required for bonding to metals.

When YH-561 Replaces SBS in Hot-Mix Asphalt Modification

In asphalt modification, YH-561 is not a drop-in replacement for SBS at identical mixing conditions. The saturated midblock requires higher shear and longer dispersion. Production mixers commonly maintain rotor tip speed at 18–25 m/s and bitumen temperature at 180–190 °C for SEBS addition levels of 3–7 wt%. SBS typically disperses at 170–180 °C and 9–15 m/s; YH-561 falls outside this window because its unextended melt viscosity is higher. The benefit appears in storage stability and oxidation resistance. Unsaturated SBS chains undergo thermal and oxidative crosslinking in the asphalt binder; the EB midblock of YH-561 retards this mechanism, but phase separation under static storage remains a limiting factor unless a compatibilizer or high-shear recirculation is used. Published data for performance grading of YH-561-modified asphalt in PG 76-22 or PG 82-22 specifications is limited; elastic recovery per ASTM D6084 and multiple stress creep recovery per AASHTO T 350 should be measured on the finished binder rather than inferred from SBS data.

Within the Baling SEBS product family, YH-561 is positioned as an unextended medium-styrene linear grade. Lower-styrene linear SEBS grades in the same portfolio provide lower hardness and greater elastic recovery; higher-styrene or oil-extended grades provide lower melt viscosity or lower hardness. YH-561 is not a maleic anhydride–grafted grade and contains no reactive functional group, so it will not form covalent bonds with polyamide or polyester. In polar polymer blends, adhesion must be achieved through compatibilizers rather than by relying on the base SEBS. This distinction is critical when evaluating SEBS products for overmolding onto engineering thermoplastics.

In compounds containing flame-retardant fillers, YH-561 should not be combined with amine-based stabilizers or residual alkaline impurities above 0.2 wt% when long-term air-oven aging at 135 °C is specified. Secondary amine antioxidants can interact with halogenated flame retardants and generate acidic species that attack the EB midblock. If a halogen-free system is used, a stabilizer package based on hindered phenols and phosphites is preferred; long-term heat aging according to ISO 188 at 125 °C should be used to confirm property retention. For food-contact or medical device applications, no blanket compliance should be assumed for YH-561. Styrene block copolymers may fall under 21 CFR 177.1810 as rubber articles intended for repeated use, but the final article must be tested for extractables and migration under the intended use conditions. The manufacturer does not list USP Class VI for this specific grade. Processing aids should be limited to 0.1–0.3 wt% because excessive lubricant exudes to the surface and reduces adhesive bond strength in overmolded assemblies.