Asahi Kasei Tufprene® SBS 126S is a linear styrene-butadiene-styrene block copolymer manufactured by living anionic polymerization. The grade is supplied as a pelletized thermoplastic elastomer in which terminal polystyrene domains provide physical crosslinking; vulcanization is not required for mechanical integrity after cooling. The central polybutadiene block retains olefinic unsaturation, which distinguishes the material from hydrogenated styrenic block copolymers such as SEBS and SEPS. The polymer is identified under CAS 9003-55-8 and is handled in compound houses as a non-hydrogenated SBS feedstock for bitumen modification, hot-melt adhesive compounding, footwear soling, and impact modification of polyolefins and styrenics. Compared with oil-extended SBS grades, 126S is supplied without extender oil, allowing the formulator to select plasticiser type and loading independently.

The nominal composition places bound styrene content at 40 wt% by ASTM D5775. Specific gravity is near 0.95 g/cm³ by ISO 2781:2008, and melt flow rate at 190 °C under 2.16 kg load is typically reported below 1.0 g/10 min by ISO 1133-1:2022. This positions the grade as a low-flow linear SBS relative to high-flow radial SBS variants. Selected typical properties are tabulated below; these values are not specification limits and require confirmation against the current certificate of analysis for lot release.

What Limits the Upper Processing Temperature of a Linear SBS Triblock?

Thermal stability in Tufprene 126S is governed by the unsaturated polybutadiene midblock, not by hydrolysis of the polymer backbone. The polystyrene domains show a glass-transition temperature near 95 °C; melt processing is therefore conducted above 150 °C to achieve domain mobility. Industrial practice on a 40:1 L/D co-rotating twin-screw extruder places zone temperatures between 140 °C and 200 °C, with die head temperature not exceeding 200 °C during continuous operation. Residence time above 220 °C should be treated as a process hazard because allylic hydrogen abstraction on the butadiene segment generates macroradicals that recombine into gel particles. On production lines, gel formation appears as die-pressure fluctuation, optical gels in sheet, and filter-pack plugging at the melt pump. The failure is oxidative/thermal rather than hydrolytic and accelerates in oxygen-rich hoppers or under overmastication at high screw speed.

When ambient relative humidity exceeds 60%, surface moisture may be carried into the feed throat and produce microvoids during extrusion. A desiccant dryer set at 60–80 °C for 2–4 h is used to restore surface dryness before processing. The polymer should not be exposed to strong oxidizers or oxygen-enriched atmospheres in hot-melt mixing, and processing with peroxides should be avoided unless a controlled reactive extrusion step is specifically intended.

At melt-processing shear rates, Tufprene 126S exhibits pseudoplastic flow typical of linear SBS triblocks. Capillary rheometry at 180 °C shows that viscosity at 100 s⁻¹ is controlled mainly by polybutadiene entanglement, while at 1,000 s⁻¹ the styrenic domains are shear-deformed and viscosity decreases. On a single-screw extruder with a 24:1 L/D screw and Maddock mixing section, melt temperature can exceed set point by 10–15 °C at screw speeds above 80 rpm; barrel fans and oil-cooled temperature-control units are used to prevent runaway oxidation. The melt is incompatible with high-molecular-weight polyethylene at high concentration; in polypropylene-rich matrices, co-continuous morphologies are preferred over dispersed spheres to limit delamination at weld lines.

In polymer-modified bitumen production, Tufprene 126S is dispersed into paving-grade or roofing-grade bitumen at loadings commonly ranging from 4 wt% to 7 wt%. A rotor-stator high-shear mixer is operated at 170–190 °C for 60–180 min, depending on base bitumen source and batch size. Under these conditions, maltene fractions swell the polybutadiene midblock and form a continuous elastic network on cooling. Softening-point rise is measured by EN 1427; penetration and elastic recovery are evaluated by EN 1426 and EN 13398, respectively. Field blending data typically show that addition of 5 wt% to a 70/100 pen base bitumen raises the ring-and-ball softening point from 46–54 °C to above 70 °C, while elastic recovery values above 80% are obtained when the dispersion is free of large polymer agglomerates. Storage stability is assessed by hot-tube separation under EN 13399 or ASTM D5976; formulations showing phase separation after 72 h at 180 °C are corrected by adjusting maltene content, adding sulfur at 0.1–0.3 wt%, or increasing shear duration.

Batch-to-batch variation in base bitumen often has a larger effect on finished softening point than minor lot-to-lot variation in SBS melt-flow. Plants compensate by adjusting polymer loading within 0.2–0.5 wt% and by monitoring rotor-stator amperage as an indirect indicator of dispersion quality. Sulfur-vulcanised systems crosslink the unsaturated midblock at 180 °C, improving storage stability, but sulfur addition above 0.3 wt% may cause premature gelation and viscosity rise on the mixing vessel.

When Tufprene 126S Is Substituted for Radial SBS in Hot-Melt Adhesive Compounding

In pressure-sensitive and assembly adhesives, the choice between linear and radial SBS changes melt viscosity, tack, and cohesive-strength balance. Tufprene 126S is a linear triblock; at equivalent styrene content, linear architecture tends to produce a higher low-shear viscosity and less shear-thinning response than a star-branched SBS. Substitution of 126S for a radial SBS therefore generally raises low-shear application viscosity and peel strength while reducing loop tack at high plasticiser loading. The formulator compensates by lowering polymer content from 30 wt% to 25 wt%, increasing midblock-compatible C5 resin loading, or raising mixing temperature within the 160–180 °C window. Brookfield Thermosel viscosity at 160 °C with spindle 27 is used as a batch-release test; formulations are adjusted to maintain a target range, commonly 3,000–8,000 mPa·s, depending on coating line speed and die lip gap. Peel adhesion is measured by ASTM D903 or PSTC-101, and shear adhesion failure temperature is evaluated under ASTM D4498. Because the grade retains unsaturation, it is compatible with aromatic-modified C9 and styrenated tackifier systems; hot-melt processing with strong peroxides or oxygen-enriched atmospheres is avoided because these conditions induce premature crosslinking and char formation.

In footwear soling and sheet compounds, Tufprene 126S is dry-blended with polystyrene or polypropylene, plasticising oil, and inorganic filler before twin-screw compounding. Typical weight ratios are 100 phr 126S, 30–60 phr naphthenic or paraffinic oil, 20–40 phr general-purpose polystyrene, and 10–30 phr calcium carbonate. The compounded material is pelletized and later injection-molded at melt temperatures of 180–200 °C and mold temperatures of 25–50 °C. Hardness, tensile set, and abrasion resistance are evaluated by ISO 868, ISO 2285, and ISO 4649. Because of the linear structure, mold shrinkage is more anisotropic than with radial SBS or hydrogenated SEBS; gate freeze-off is controlled by maintaining injection speed and pack pressure. In polypropylene impact modification, the grade is incorporated at 10–20 wt% with a peroxide-free, co-continuous morphology. Published data for this specific formulation configuration is limited; compounders therefore verify Charpy impact by ISO 179-1 and tensile properties by ISO 527-2 before release.

Compared with lower-molecular-weight high-flow SBS grades, Tufprene 126S has a lower melt flow rate and requires higher processing temperatures or lower screw speeds to avoid melt fracture at high output. Compared with hydrogenated Tuftec SEBS, the product has inferior thermo-oxidative stability and UV resistance but better compatibility with bitumen and aromatic substrates. These are operational distinctions rather than performance deficiencies: the non-hydrogenated midblock permits sulfur vulcanization and reactive grafting in rubberized asphalt and chem-cured systems. For outdoor applications, the unsaturated midblock requires stabilization with carbon black, hindered amine light stabilizers, or UV absorbers. Weathering resistance is inferior to hydrogenated SEBS and is evaluated under ISO 4892-3 before field exposure.

For end-use compliance, converters should evaluate the formulated compound under the relevant food-contact or construction-products regulation. The base polymer may be assessed under 21 CFR 177.1810 for styrene block polymers in repeat-use food-contact applications, provided the finished article meets extraction limits and end-use conditions. The polymer is supplied under the manufacturer’s REACH registration and must be included in the article’s RoHS assessment only if the formulated compound contains restricted additives; the polymer itself is outside the homogeneous-material bans in RoHS 2011/65/EU. No statement of suitability for medical implants is inferred from these general regulatory references.