Among linear styrene-butadiene-styrene block copolymers used in thermoplastic elastomer compounding, Chimei KIBITON® SBS PB-5308 is a medium-styrene, non-oil-extended triblock copolymer supplied as porous pellets. The manufacturer-reported composition is approximately 30 wt% bound styrene, with the balance butadiene, and no extender oil is specified in the standard grade. Technical data for the grade list a specific gravity of 0.94 g/cm³ according to ASTM D792-20, a Shore A hardness of 68 according to ASTM D2240-15, a tensile strength of 18 MPa according to ASTM D638-14, and an elongation at break of 550% by the same method. Melt mass-flow rate is 8 g/10 min under a 5 kg load at 190 °C according to ISO 1133-1:2022. Ash content is specified below 0.2 wt% and volatile matter below 0.5 wt% by ASTM D5667 and ASTM D5668, respectively. The grade is supplied with documentation that supports conformity to REACH registration obligations and RoHS Directive 2011/65/EU annex II heavy-metal restrictions. No food-contact statement is assigned by the manufacturer; therefore, use in food-contact rubber articles requires independent migration testing under FDA 21 CFR 177.2600. Pre-drying at 70–80 °C for 2 h is required when storage humidity exceeds 60% RH to avoid steam-popping in hot-melt or bituminous processes.

What Are the High-Shear Mixing Limits for Asphalt and Bituminous Membrane Applications?

Production-scale bitumen modification with PB-5308 begins with dry feeding into a rotor-stator high-shear mill at a tip speed of 18–25 m/s and a jacketed vessel temperature of 170–185 °C. In dense-graded wearing courses the modifier dosage is typically 3–5 wt%, while waterproofing membranes requiring low-temperature flexibility may use 6–8 wt%. The lower processing boundary of 170 °C is viscosity-limited; below this temperature, base bitumen restricts dispersion and the polymer phase can remain as agglomerates above 50 µm. The upper boundary of 185 °C is degradation-limited: a production-scale mixer operated at 195–200 °C for 90 min has produced an irreversible softening point drop and an increase in separation index measured by ASTM D7173. Batch-to-batch melt flow rate variation of ±1 g/10 min may require adjustment of the high-shear interval or addition of 0.5–1.0 wt% of low-viscosity naphthenic oil to maintain equivalent particle size reduction. The modified binder is characterized by a softening point increase of 15–25 °C over the base bitumen and by a storage-stable morphology when the top-to-bottom softening point difference in a cigar-tube separation test according to EN 13399 remains below 2.5 °C. For performance grading under AASHTO M320-23, a 4 wt% dosage commonly shifts the high-temperature grade upward by one grade interval of 6 °C; however, published data for PB-5308 across all base binder sources is limited because response depends on asphaltene content and maltene phase compatibility. Compared with higher-styrene linear SBS grades, PB-5308 shows lower mixing viscosity and less melt-pressure fluctuation during pellet feed. Compared with radial SBS grades, its linear architecture reduces melt elasticity and residual gel after prolonged high-shear mastication, which lowers screen-pack plugging in continuous membrane lines but may require a slightly higher dosage to reach the same softening point target.

When PB-5308 is compounded into polypropylene and mineral-filled thermoplastic elastomer formulations, the linear triblock structure imposes a narrower shear window than oil-extended grades. Compounding trials on a co-rotating twin-screw extruder with an L/D ratio of 40:1 and screw diameter of 75 mm use barrel temperatures of 160–190 °C and screw speed of 300–500 rpm. The grade is generally added at 20–40 wt% to a polypropylene homopolymer having a melt mass-flow rate of 12 g/10 min measured by ISO 1133-1:2022. Under these conditions, the polypropylene phase remains continuous and PB-5308 forms discrete domains with a number-average domain size below 2 µm; tensile elongation of the compound increases from 120% to 420% when PB-5308 content rises from 20 wt% to 40 wt%, as determined by ISO 527-2 on injection-molded type 1A specimens. Injection molding uses nozzle temperature of 200 °C, injection pressure of 120–160 MPa, and mold temperature of 40 °C. Mold shrinkage according to ISO 294-4 is typically 1.2–1.6% in the flow direction, which is higher than that of talc-filled polypropylene homopolymer and must be compensated in tool design. The grade does not require vulcanization; instead, polystyrene endblocks create reversible physical crosslinks. This distinction is operationally significant: continuous extrusion lines can be restarted without the scorch constraints common to sulfur-cured SBR compounds, but the absence of covalent crosslinks also limits resistance to hot aromatic solvents and imposes a practical upper service temperature near 95–100 °C.

When Hot-Melt Adhesive Tackifier Loadings Exceed 40 wt%, Phase Clarity Is Not Guaranteed

In hot-melt pressure-sensitive adhesive compounding, PB-5308 is mixed with C-5 hydrocarbon tackifier resins and naphthenic or paraffinic oils in a vertical sigma-blade mixer at 140–170 °C for 45–90 min. At tackifier loadings of 30–40 wt%, the ternary blend presents a single glass transition temperature centered between −45 °C and −30 °C and a plateau modulus consistent with peel adhesion in the range of 3–6 N/25 mm when laminated to stainless steel and tested at a 180° peel angle according to ASTM D3330/D3330M-04(2018). Above 40 wt% tackifier, dynamic mechanical analysis shows a secondary tan δ peak between −20 °C and 0 °C, indicating phase separation and loss of cohesive strength. For solvent-borne contact adhesives, PB-5308 is dissolved at 20–30 wt% solids in a toluene/ethyl acetate blend; solution viscosity at 25 °C increases from 3,000 to 8,000 mPa·s depending on solvent composition and may be determined by ASTM D1084. Amine-based adhesion promoters should be avoided because residual amines accelerate thermal yellowing and can promote base-catalyzed oxidation of the butadiene midblock. Compared with higher-styrene SBS grades, PB-5308 dissolves more rapidly and provides lower cohesive strength but improved low-temperature tack. Compared with oil-extended SBS grades, the absence of predispersed extender oil gives the formulator independent control of oil addition, typically 5–10 phr in hot-melt adhesives.

Repeated Extension and Recovery Behavior in Vulcanizate-Free Compounds

Because PB-5308 does not require vulcanization, applications such as footwear soles, elastic films, and cable jacketing rely on phase-separated polystyrene endblocks for elastic recovery. After ten cycles of extension to 200% strain at ambient temperature, an unfilled PB-5308 compound retains approximately 85% of initial stress at 100% strain, as measured by a tensile retraction protocol adapted from ASTM D412. Compression set of an unfilled compound measured by ISO 815-1 after 22 h at 70 °C is typically 35–45%, which is higher than sulfur-vulcanized EPDM and imposes a design limit for sealing applications. The operational temperature range in air is bounded by polystyrene segment relaxation at 95–100 °C at the upper end and by polybutadiene segment mobility at approximately −70 °C at the lower end. Prolonged exposure to ozone or ultraviolet light requires antiozonant addition at 2–3 phr because the unsaturated polybutadiene midblock is inherently susceptible to oxidative attack. Published data for long-term cyclic fatigue of PB-5308 in footwear outsole geometries is limited; laboratory tensile fatigue tests should be completed before production qualification. Differences from oil-extended and high-styrene grades appear most clearly in compounding mass balance: PB-5308 contains no predispersed extender oil, so footwear recipes must add process oil separately, typically 10–15 phr, while higher-styrene linear grades provide higher tensile strength but require higher processing temperatures and show slower solvent dissolution.