Chimei KIBITON® SBS PB-585 is a linear styrene-butadiene-styrene triblock copolymer produced by anionic polymerisation. The grade is supplied as a porous pellet with a nominal bound styrene content of 32 wt% and a specific gravity of 0.94 measured under ISO 2781. The polybutadiene mid-block contains mixed 1,4-cis, 1,4-trans, and 1,2-vinyl isomer units; the 1,2-vinyl content is relevant to thermal stability and compatibility with polar resins. The polystyrene end-blocks form glassy domains with a glass transition temperature near 95 °C in annealed samples and provide reversible physical crosslinking. PB-585 is specified for polymer-modified bitumen, hot-melt and solvent-borne adhesives, footwear compounds, and impact modification of styrenic resins. The melt flow rate of the neat polymer is determined at 190 °C with a 5 kg load, placing PB-585 in the medium-flow region of the KIBITON SBS portfolio.

What Limits the Melt Processing Window of PB-585?

PB-585 processes as a thermoplastic in conventional extrusion and injection equipment, but the practical melt temperature window is 150–180 °C. At temperatures below 150 °C, the polystyrene domains are not sufficiently softened for homogeneous dispersion; single-screw extruders operating below this threshold can generate unmelted feed particles and high screw torque. Above 180 °C, the unsaturated polybutadiene mid-block undergoes thermo-oxidative chain scission when residence time exceeds 20 min. This degradation is observed as a melt viscosity decrease of 15–30% and a reduction in tensile elongation from approximately 700% to below 500% on plaques tested under ISO 37. The product should be pre-dried when surface moisture exceeds 0.1 wt%: 60–70 °C for 2 h in a desiccant dryer with a dew point of ≤ -40 °C is sufficient. Solvent systems with high aromatic content can dissolve the polystyrene domains; ketone-rich solvents are not recommended for solution blending because of phase separation. For long-term storage, the supplier specifies a maximum warehouse temperature of 40 °C and avoidance of direct ultraviolet exposure because the butadiene segment is susceptible to UV-induced crosslinking and discolouration.

Specification Matrix for KIBITON PB-585 Under Laboratory Test Methods

Table 1 summarises the typical property envelope for PB-585. The values originate from producer-controlled laboratory testing and are quoted as reference values, not as batch release limits. The lot-specific certificate of analysis should be used for production decisions.

Mechanistic Role of 1,2-Vinyl Isomers in Thermal Stability and Asphalt Compatibility

The 1,2-vinyl content of the polybutadiene mid-block exerts a measurable effect on processing and end-use properties. Higher 1,2-vinyl content increases the glass transition temperature of the rubber phase and reduces the rate of strain-induced crystallisation; in PB-585, the vinyl microstructure is balanced to maintain high elastomeric recovery without excessive melt elasticity. During asphalt modification, the pendant vinyl group provides steric hindrance that retards the collapse of the swollen polymer network at elevated storage temperatures. This contributes to the separation resistance measured under ASTM D5976. During hot-melt adhesive processing, the same unsaturation participates in slow radical-mediated crosslinking when oxygen is not excluded; gel content determined by Soxhlet extraction increases from 0.2 wt% to 1.5 wt% after 2 h of open mixing at 180 °C. The use of inert gas and staged temperature profiles is therefore mandatory for continuous adhesive lines.

In polymer-modified bitumen, PB-585 is dispersed at 3–5 wt% into penetration-grade bitumen using a Silverson high-shear mixer or equivalent rotor-stator device operating at 3,000–5,000 rpm for 45–60 min at 170–180 °C. The polybutadiene mid-block absorbs maltene and aromatic fractions, expanding its volume by roughly 4–10 times depending on the base bitumen saturate/aromatic ratio, while the polystyrene end-blocks remain associated as physical crosslinks. The resulting polymer network raises the ring-and-ball softening point from 45–48 °C to 65–85 °C when measured under ASTM D36. Storage stability is evaluated by ASTM D5976 or EN 13399; a difference in softening point between the top and bottom sections of a 72 h oven tube at 180 °C of less than 2.5 °C is generally required for homogeneous PMB. Processing at 170 °C or lower often produces visible gel particles and poor separation resistance, while prolonged mixing above 190 °C causes viscosity degradation and loss of elastic recovery. Rotational viscosity at 135 °C using a Brookfield DV-II+ thermosel with spindle 27 at 20 rpm typically increases from 800 mPa·s for the unmodified bitumen to 1,800–3,000 mPa·s at 4 wt% PB-585. Published data for higher addition levels in specific bitumens is limited; plant trials with addition above 7 wt% have shown pumping difficulties and require short-loop recirculation equipment with heated jacketed lines.

The addition of PB-585 also modifies low-temperature cracking resistance. Force ductility tests under EN 13589 or ASTM D6084 are used to measure the ductility of PMB at 4 °C; PB-585-modified binders typically retain a higher low-temperature elongation than unmodified bitumen. The unsaturated mid-block provides sites for shear-induced chain orientation during mixing, but it also creates a sensitivity to oxidative ageing. Ageing tests under EN 12607-1 or ASTM D2872 should be performed before plant scale-up; a retention of elastic recovery above 60% after rolling thin-film oven ageing is a common specification for PMB supplied under EN 14023.

When PB-585 Is Compounded with Rosin Ester Tackifiers in Hot-Melt Adhesives

Hot-melt adhesive formulations containing PB-585 are blended with rosin ester tackifiers and naphthenic or paraffinic oil at 160–180 °C. A representative starting formulation contains 25 wt% PB-585, 50 wt% rosin ester with a softening point of 100 °C, and 25 wt% naphthenic oil. Viscosity at 180 °C measured by ASTM D3236 using a Brookfield RVDV-II+ with spindle 27 typically falls between 3,000–8,000 mPa·s. Loop tack measured by ASTM D6195 on stainless steel and 180° peel measured by ASTM D903 can be used to monitor adhesion; PB-585 contributes high cohesive strength due to its styrenic domain network, while the rosin ester improves wettability. Compounding must be performed under nitrogen to reduce polybutadiene oxidation. Mixing time above 170 °C should be limited to 30 min; longer residence may cause darkening, viscosity drift, and loss of tack. The use of phenolic antioxidants at 0.1–0.5 phr is standard; amine-based additives should be avoided because they can promote yellowing and may interfere with rosin ester compatibility. If the adhesive is applied by slot-die coating, the melt should be filtered through a 100–200 µm screen pack to remove gel specks formed during prolonged hold times.

Footwear soling compounds based on PB-585 are formulated with crystal polystyrene, mineral oil, silica, and calcium carbonate in an internal mixer or co-rotating twin-screw extruder. A typical twin-screw compounding line uses an L/D ratio of 36:1–44:1 with zone temperatures of 150 °C, 160 °C, 165 °C, 170 °C, 175 °C, 175 °C, and 170 °C from feed to die. Screw speed is maintained at 250–350 rpm to disperse the high-viscosity polystyrene domains without excessive shear heating. The resulting TPR compound has a Shore A hardness in the range 55–75 depending on plasticiser and polystyrene loading, and injection moulding is performed with barrel temperatures of 150–180 °C and mould temperatures of 20–40 °C. Clamp force requirements follow standard thermoplastics calculations, typically 4–6 kN/cm² of projected mould area. Failure to vent volatiles during twin-screw compounding may cause surface roughness and reduced tear strength, evaluated by ISO 34-1. When regrind is reused, the proportion should not exceed 30 wt% without mechanical testing, because repeated heat history lowers elongation at break and increases gel content.

Comparative Differentiation Against SEBS, SIS, and Emulsion SBR

The principal differences between PB-585 and other styrenic elastomers arise from mid-block saturation, monomer identity, and molecular architecture. Unlike SEBS, which is produced by selective hydrogenation of the polybutadiene segment, PB-585 retains diene unsaturation that improves compatibility with bitumen and rosin esters but reduces oxidative stability at continuous melt temperatures above 180 °C. In comparison with SIS, PB-585 has a higher service-temperature range because of the higher glass transition of the polystyrene domains, and a higher tensile strength at equivalent styrene content, but lower initial tack. Compared to emulsion SBR, PB-585 is melt-processable and exhibits reversible physical crosslinking rather than irreversible sulphur crosslinks; this allows reprocessing of production scrap in thermoplastics lines. Within the KIBITON SBS range, PB-585 is differentiated from oil-extended or high-styrene grades by its balance of flow and end-use mechanical properties; direct lot-specific comparison is documented in the producer’s certificate of analysis.

In styrenic resin modification, PB-585 is used at 5–15 wt% in general-purpose polystyrene or high-impact polystyrene to increase impact strength without complete loss of modulus. Dispersion is performed in a co-rotating twin-screw extruder at temperatures below 200 °C. The SBS domains induce shear yielding and crazing, increasing notched Izod impact strength under ISO 180/A from 2–3 kJ/m² to 10–25 kJ/m² depending on SBS content and phase morphology. Excessive screw shear should be avoided because it can reduce the dispersed domains below the critical size for effective craze stabilisation; optical and scanning electron microscopy are used to monitor dispersed domain diameter in the range 0.5–1.0 µm. Published data for this specific configuration is limited for PB-585; compound-specific impact and haze measurements must be generated by the converter because the final properties depend on screw speed, temperature profile, and dilution ratio.