Chimei KIBITON® SBS PB-5301 is a linear styrene-butadiene-styrene block copolymer supplied as a neat, oil-free thermoplastic elastomer. The grade is built from styrene end-blocks of approximately 30 wt% and a butadiene midblock with predominant 1,4-addition; the resultant two-phase morphology gives a Shore A hardness of 70–75 by ASTM D2240. The material is specified where elastomeric recovery, tensile strength, and nonpolar substrate compatibility are required without the melt viscosity penalty of higher-molecular-weight SBS grades. The nominal melt flow rate is 4.0–6.0 g/10 min at 190 °C under 5.0 kg load by ASTM D1238, and specific gravity is 0.93–0.95 g/cm³ by ASTM D792. The supplied pellet morphology is porous, which enables rapid oil absorption during compounding but also increases surface area for moisture pickup; storage at 10–30 °C and RH < 60% is therefore specified.

Which Molecular Architecture Parameters Govern Tensile Set and Melt Viscosity?

The tensile set behaviour of PB-5301 is governed by phase separation between the polystyrene end-blocks and the polybutadiene midblock. Styrene content fixed at approximately 30 wt% produces a physical network in which polystyrene domains act as crosslinks below 100 °C, giving a tensile strength at break of 18–22 MPa and elongation at break of 700–900% by ASTM D638-14. Free styrene-butadiene diblock content is held below 1.0 wt% on the certificate of analysis because diblock chains reduce effective network junction density and increase tensile set after 200% elongation. The unsaturated 1,4-butadiene midblock also sets the thermal boundary: at compounding melt temperatures above 200 °C, oxidative gel formation begins, raising apparent viscosity by more than 15% after 10 min residence time. The practical processing window is therefore narrower than for hydrogenated styrenic block copolymers.

On a corotating twin-screw extruder with 40:1 L/D ratio and segmented screws, PB-5301 is typically fed into the main throat at 150–170 °C barrel set temperatures. Because the melt viscosity is shear-rate dependent, screw speeds below 300 rpm avoid excessive viscous heating in unfilled systems; filled compounds containing 20–40 phr calcium carbonate or silica may require barrel temperatures of 170–190 °C and screw speeds of 350–450 rpm to disperse fillers without exceeding a 200 °C melt temperature. A strand pelletising line with water-bath temperature not lower than 30 °C prevents stress whitening and pellet fracture. For injection moulding, barrel profile 160/170/175/180 °C, nozzle 180 °C, and mould temperature 20–40 °C are used for footwear components with wall thickness 2–6 mm; injection speed is adjusted to fill the cavity in 0.8–1.5 s to avoid jetting and flow marks. Pre-drying at 70 °C for 2 h in a desiccant dryer is required when pellet moisture exceeds 0.5 wt%, typically under ambient RH > 60%.

Observed production-scale failure modes associated with PB-5301 concentrate at the die face and post-extrusion cooling units. At melt temperatures above 200 °C in a twin-screw extruder with 40:1 L/D ratio and die-hole diameter 3.0 mm, degraded polybutadiene forms gel particles that bridge land regions of the die plate; the resulting strand diameter variation exceeds ±0.2 mm and causes inconsistent pellet length. Batch-to-batch melt flow rate for this grade is controlled within ±0.5 g/10 min; a shift from 4.5 g/10 min to 5.5 g/10 min lowers injection pressure demand by 8–12% but also reduces melt strength, increasing strand breakage during underwater pelletising. Compounding with filler loadings above 40 phr raises specific energy input above 0.25 kWh/kg, which can push melt temperature beyond the 200 °C oxidation threshold unless screw speed is reduced or barrel temperatures are lowered. These process boundaries are more restrictive than for hydrogenated SEBS because the unsaturated polybutadiene midblock reacts with oxygen at rates that double with every 10 °C increase above 180 °C.

Three application domains account for most PB-5301 consumption. In footwear, the grade is blended with 10–30 phr naphthenic oil and 5–15 phr crystal polystyrene in an internal mixer at 120–140 °C; the resulting compound typically exhibits density below 1.0 g/cm³ and abrasion loss below 250 mm³ by DIN 53516. Injection-moulded unit soles made from such compounds show flex crack resistance beyond 100,000 cycles at 23 °C when tested by ASTM D1052. In hot-melt pressure-sensitive adhesives, PB-5301 is combined with C5 or C9 tackifier resins at 60–100 phr; 180° peel adhesion to stainless steel is a function of coating weight and open time, with values above 12 N/25 mm reported for 30–50 g/m² coat weights. In bitumen modification, PB-5301 is added at 4–7 wt% of binder and dispersed with a high-shear mixer at 170–180 °C for 60–90 min; phase inversion occurs near 5–6 wt%, increasing the ring-and-ball softening point by 15–25 °C by ASTM D36 and retaining ductility above 50 cm at 25 °C by ASTM D113. Published data for the specific PB-5301 formulation in each of these uses is limited; the ranges cited are field-typical for linear SBS with 30 wt% styrene and must be revalidated on production-scale equipment.

The oil absorption capacity of PB-5301 is controlled by its porous pellet morphology. In a ribbon blender at 25–40 °C, paraffinic oil loadings up to 20 phr are absorbed within 30 min; above 20 phr, absorption is incomplete after 2 h, and a high-speed mixer at 70–80 °C is required to prevent oil bleeding from stored dry blends. This behaviour differs from dense SEBS pellets, which absorb oil more slowly and often require an additional ageing step of 24 h.

Comparative positioning against oil-extended, hydrogenated, and SIS triblock grades

Relative to oil-extended SBS grades, PB-5301 contains no extender oil as supplied; the compounder controls oil type and loading, avoiding storage migration and batch-to-batch oil-content drift observed when pre-extended pellets are warehoused above 35 °C. Compared with hydrogenated styrenic block copolymers, PB-5301 has a polybutadiene midblock with residual unsaturation; continuous service in air is bounded at 70–80 °C, and compounded articles exposed to UV or ozone require addition of 0.3–0.8 phr of a p-phenylenediamine antiozonant and 0.5–1.0 phr of a benzotriazole UV absorber. Against SIS triblocks, PB-5301 provides higher tensile strength and better abrasion resistance but lower room-temperature tack; pressure-sensitive adhesive formulations based on PB-5301 typically require tackifier resins at the upper end of the 60–100 phr loading range to match SIS tack. The melt flow of PB-5301 is lower than that of high-flow SBS grades designed for thin-wall injection moulding; when cycle time below 10 s is required, a higher-flow grade should be evaluated because PB-5301 may freeze off at mould temperatures below 20 °C in parts thinner than 1.5 mm.

Extruded profile and sheet applications are less common for PB-5301 because the melt strength at typical profile extrusion temperatures 170–190 °C is lower than that of higher-molecular-weight SBS grades. When profile extrusion is attempted, a die draw ratio below 3:1 and a cooling water bath at 30–40 °C are specified; higher draw ratios cause neck-in above 10% and surface haze. Coextrusion with polypropylene at 190–200 °C requires a tie-layer based on styrene-ethylene-butylene-styrene adhesive, because direct bonding to polypropylene fails below 2 N/25 mm peel strength without surface activation.

When the Bitumen Phase Inversion Boundary Falls Below 4 wt%

In paving-grade bitumens with saturates above 20 wt% and asphaltenes below 10 wt%, the transition from bitumen-continuous to polymer-rich network can occur at PB-5301 additions below 4 wt%; this early inversion is accompanied by a storage-stable morphology but a lower softening point increase of 8–12 °C. Conversely, in oxidised bitumens with asphaltenes above 15 wt%, phase inversion may require 6–7 wt% polymer, and mixing below 170 °C produces dispersed polymer globules with diameter 10–30 µm rather than a continuous network. On a high-shear rotor-stator mixer operating at 4,000 rpm, a mixing time of 60 min at 180 °C is often sufficient; stopping at 30 min leaves residual elastic recovery below 70% by ASTM D6084, while extending mixing to 120 min can reduce viscosity by 10–15% through midblock chain scission. The operational boundary is a function of binder composition as much as polymer loading; laboratory rheological screening with a dynamic shear rheometer at 60 °C and 1 rad/s is required before plant-scale batching.

Compliance documentation for PB-5301 includes a REACH Regulation (EC) 1907/2006 statement confirming no Substances of Very High Concern above 0.1 wt% in the supplied pellet, and a RoHS Directive 2011/65/EU declaration covering the restricted substances including the four phthalates through Delegated Directive (EU) 2015/863. For food-contact applications, the raw polymer is not a finished food-contact material; the formulated article must be assessed under 21 CFR 177.2600 or the applicable EU national migration standard. Processors should note that PB-5301 is incompatible with strong oxidising acids and should not be combined with amine-based additives at elevated temperatures, because amines accelerate hydroperoxide decomposition in the unsaturated midblock and can generate gel particles larger than 100 µm in extruded profile. The product should be stored in unopened packaging at 10–30 °C; exposure to direct sunlight or UV lamps degrades the polybutadiene segment within 4–6 weeks, increasing yellowness index and reducing elongation at break by more than 20%.