SINOPEC Baling SEBS YH-502 is a linear styrene-ethylene/butylene-styrene triblock copolymer produced by anionic polymerization followed by selective hydrogenation. Commercial certificates of analysis for the grade typically identify a bound styrene mass fraction of 29–31 wt% and an ethylene/butylene midblock mass fraction of 69–71 wt%. The material is supplied as a free-flowing powder or porous pellet depending on packaging stream, with a density of approximately 0.91 g/cm³ when determined to ISO 1183-1:2019. The hydrogenated midblock confers resistance to oxidative chain scission that is not available in unsaturated SBS analogues, while the styrene domains act as reversible physical crosslinks above the end-block glass transition.

Neat YH-502 exhibits a Shore A hardness typically in the 68–76 range under ISO 7619-1, with tensile strength at break of at least 24 MPa and elongation at break exceeding 500% when tested to ISO 37:2017 on compression-moulded sheets conditioned at 23±2°C and 50±5% RH. Conditioning history affects the styrene domain morphology and must be controlled during comparative qualification studies. The product is used as a non-vulcanised elastomeric base for adhesives, as an impact modifier in polyolefin compounds, and as a binder component in sealant and coating formulations where processing temperatures remain below 240°C.

Compared with Baling YH-503, YH-502 is classified as a higher-viscosity linear grade. Melt flow rate under 200°C and 5 kg per ISO 1133-1:2022 is commonly reported between 0.10 and 0.50 g/10 min, while YH-503 is specified at higher flow for injection moulding. This difference in melt flow reflects a higher overall polymer molecular weight and longer relaxation time. It limits long-flow injection moulding but improves elastic recovery, strain set resistance, and high-temperature creep behaviour relative to lower-viscosity SEBS grades.

Why Does YH-502 Exhibit a Lower Melt Flow than YH-503 and What Follows for Twin-Screw Compounding?

The lower melt flow is not an indication of gel content or defective hydrogenation; it is a molecular-weight parameter. In screw design terms, the higher melt viscosity increases motor amperage and melt-pressure rise in intensive mixing zones. On a 40:1 L/D co-rotating twin-screw extruder with a 12-barrel configuration, processes compounding YH-502 with 50–100 phr paraffinic oil often show a melt-temperature rise of 10–20 K more than an equivalent YH-503 formulation when screw speed exceeds 300 rpm. Barrel settings are therefore typically kept at 180–220°C, with a die temperature of 200–230°C, to avoid residual styrene domain degradation.

Adding oil simultaneously at the feed throat can reduce shear heating but may require a side stuffer or injection pump connected at barrel 5 or 6 if oil loading exceeds 80 phr. Premature oil addition at the feed throat can reduce frictional heating so sharply that the powder does not fuse before the first kneading block, causing torque spikes and surging. This process conflict is a known production-scale bottleneck and is managed by staggered injection rather than single-shot addition.

The grade is hygroscopically stable relative to polar elastomers, but condensation on cold powder stored in silos can elevate surface moisture above 0.1%. Pre-drying at 80°C for 2–3 h in a dehumidified-air dryer with a dew point of -40°C or lower is recommended when the powder has been exposed to ambient air above 60% RH for more than 24 h. Residual moisture above 0.05% can generate steam during extrusion and produce surface roughness, microporosity, and reduced tensile strength in hot-melt adhesive films.

Because the neat grade has high melt viscosity, formulators frequently pre-mix YH-502 with a white paraffinic oil of low aromatic content. The oil loading required to reach a given Shore A value is specific to the oil aromatic content and must be generated experimentally for each supplier lot. Unsupported generic oil-extension curves can shift hardness by more than 10 Shore A when the oil aromaticity changes.

Oil Extension and Polypropylene Compounding Limits

When YH-502 is compounded with polypropylene homopolymer at a ratio of 30/70 SEBS/PP, the system shifts from an elastomer-continuous to a thermoplastic-continuous morphology. The co-continuous inversion threshold varies with polypropylene melt flow rate and SEBS end-block styrene content. For YH-502, the threshold is commonly observed between 35% and 45% SEBS by volume under low shear. Near this threshold, tensile strength and flexural modulus do not vary linearly with addition level. A change of 5 wt% SEBS can produce a Shore D change of approximately 8–12 points and a ductile-to-brittle transition that is sensitive to mould gate geometry and cooling rate.

In injection-moulded articles on machines with clamp force above 120 tonnes, melt temperature should be maintained at 190–230°C, with mould cooling at 20–40°C. Excessively high mould temperature allows styrene domains to re-form slowly and increases cycle time without proportional toughness gain. Cooling below 20°C produces frozen-in orientation in thin-walled parts and lowers environmental stress cracking resistance under ASTM D1693. The process window is therefore a balance of solidification rate and end-block coalescence rather than a single temperature set point.

Tests to ASTM D638-14 on injection-moulded tensile bars are not sufficient for lot release of soft SEBS/PP blends because test speed and extensometer calibration influence yield detection. For yield behaviour under elastomer-modified PP, ISO 527-2:2012 with a test speed of 50 mm/min is more informative, particularly when comparing YH-502 to lower-viscosity SEBS grades.

YH-502 is integrated into hot-melt pressure-sensitive adhesives by solution or melt blending with tackifier resins selected from hydrogenated C9 or C5-aromatic streams. In melt blending, the grade is usually compounded with a stabiliser package of hindered phenolic antioxidant and secondary phosphite processing stabiliser, each at 0.1–0.2 wt% of total formulation. The melt viscosity rise after 48 h at 180°C under nitrogen is less than 10% when the stabiliser package is retained. Omission of the phosphite allows chain-end oxidation and loss of tack strength, particularly in adhesive formulations containing residual unsaturated tackifier fractions.

Solvent-based adhesive formulations of YH-502 typically use toluene, cyclohexane, or mixtures thereof. Dissolution time in a closed high-shear mixer is 120–240 min at 40–60°C for a 20 wt% solids content, depending on solvent blend and impeller tip speed. Raw polymer solution viscosity at 25°C and 20% solids in toluene often exceeds 10,000 mPa·s, which restricts direct use of unextended YH-502 in low-viscosity spray applications unless oil or diluent is added. Use of polar rosin esters without sufficient hydrogenation commonly leads to migration and loss of tack after thermal ageing.

When Halogen-Free Flame Retardant Compounds Move Beyond the 190–230°C Window

In halogen-free flame retardant compounds for cable sheathing, YH-502 is frequently the carrier for magnesium hydroxide or aluminium trihydroxide fillers at total loadings of 150–250 phr. The saturated EB midblock permits higher compounding temperatures than SBS, but filler decomposition and process oxidation define the upper boundary. Magnesium hydroxide begins significant water release above 340°C, whereas aluminium trihydroxide begins endothermic dehydration near 180–200°C. Aluminium trihydroxide-filled compounds based on YH-502 cannot be processed with barrel temperatures exceeding 190°C for extended residence, or the formulation will blister from water vapour generation. The process conflict is that YH-502 itself requires 200–220°C melt temperatures for complete filler dispersion, which creates a narrow window requiring low-residence screw configurations and atmospheric vent ports.

On a production twin-screw extruder with 48:1 L/D and vacuum venting of −0.08 MPa, moisture released from aluminium trihydroxide can be removed if the filler is side-fed after polymer melting, usually at barrel 6 or 7. The melt temperature at the die is maintained between 205°C and 215°C with residence time below 90 s. A deviation of ±5°C is sufficient to shift the compound from elongation above 200% after ISO 37 to microporous material with elongation below 100%. This cliff-edge sensitivity is a primary reason that aluminium trihydroxide-based YH-502 compounds are often replaced with magnesium hydroxide grades or surface-treated filler for cables requiring EN 50525-3-21 or IEC 60092-359 mechanical and flame performance.

Thermo-oxidative Degradation in Baling YH-502 During High-Shear Compounding

The midblock of YH-502 is saturated, but degradation is not absent. At melt temperatures above 240°C, oxidation proceeds at the tertiary carbon atoms of the midblock and at the interfacial regions near styrene domains. The degradation product profile includes low-molecular-weight aldehydes and ketones detectable by carbonyl index increase in FTIR spectra between 1700 and 1750 cm⁻¹. A carbonyl index rise above 0.05 relative to the 1460 cm⁻¹ reference band correlates with a measurable drop in melt flow stability and an increase in gel particles greater than 50 µm in film applications. Published data for long-term shear-induced molecular weight loss in YH-502 at screw speeds above 500 rpm is limited; therefore, production validation at extended shear is required before continuous high-output operation.

Process additives containing free zinc ions or amine-based stabilisers can accelerate crosslinking or cause colour drift. Avoid amine antidegradants and zinc stearate packages above 0.5 wt% unless the formulation has been validated for long-term heat ageing at 90°C for 168 h. The recommended stabilisation is a binary system of hindered phenolic and phosphite processing stabiliser, each in the range 0.05–0.20 wt%. For outdoor articles, a hindered amine light stabiliser may be introduced only after compatibility screening by ΔE colour change after 2000 h of ISO 4892-2 xenon arc exposure.

For regulatory control, YH-502 is supplied with a safety data sheet that references EU REACH 1907/2006 and RoHS 2011/65/EU. The grade is not automatically certified for food-contact or medical applications; each converter must obtain a current supplier statement for specific migration or biocompatibility requirements. The absence of halogenated flame retardants and heavy metals is typically demonstrated by X-ray fluorescence screening using IEC 62321-3-1 and IEC 62321-5, but detection limits must be validated by the testing laboratory for the exact compound matrix.