Kumho Petrochemical KTR® SBS 104 is a neat, non-oil-extended styrene-butadiene-styrene block copolymer belonging to the KTR thermoplastic elastomer series. The molecular architecture comprises polystyrene terminal blocks and a polybutadiene mid-block; below the polystyrene glass transition the terminal blocks phase-separate into glassy domains that behave as physical crosslinks, and above that transition the material flows as a melt. The grade designation 104 identifies a controlled balance of melt flow and styrene content within the 100-series portfolio, but grade-specific numerical data must be obtained from the manufacturer’s certificate of analysis because secondary sources provide limited disclosure for this configuration. The product is handled as a low-dust porous pellet for dry blending, melt compounding, and solventborne or bituminous processing. Residual diblock content, bound styrene content, melt flow rate, and volatile matter are batch-controlled; residual diblock above 1 wt% typically reduces tensile strength and increases compression set in SBS systems.

The two-phase morphology is produced by thermodynamic incompatibility of polystyrene and polybutadiene; at commercial molecular weights, phase separation yields polystyrene domains that act as multifunctional physical crosslinks. Transmission electron microscopy after osmium tetroxide staining is used to check phase continuity and domain size. When evaluated by differential scanning calorimetry according to ISO 11357-2, the polybutadiene phase of SBS typically shows a glass transition near -85°C, while the polystyrene phase shows a transition near 95°C. This thermal span permits low-temperature flexibility down to approximately -70°C and a practical upper service limit near 80°C for continuous deformation. In capillary rheometry at 200°C, linear SBS grades with melt flow rates between 1 g/10 min and 5 g/10 min exhibit pronounced shear thinning across apparent shear rates of 10¹ s⁻¹ to 10³ s⁻¹. Viscosity is controlled by molecular weight and polystyrene content; higher styrene levels raise low-shear viscosity and hardness while reducing low-temperature elasticity.

How Does Thermo-Oxidative Degradation Limit the Extrusion Operating Window?

Thermal degradation of unsaturated SBS proceeds through two competing routes: oxidative chain scission in the polybutadiene block and radical recombination leading to gel formation. On a corotating twin-screw extruder with 40:1 L/D and segmented screw elements, a barrel temperature profile from 150°C to 180°C is used for unfilled linear SBS. Melt temperatures above 200°C reduce the tolerable residence time to less than 90 s before visible gel specks appear. Production lines show increasing pressure drop across breaker plates and screen packs as gel accumulates; screen pack change intervals can shorten from 48 h to 12 h when melt temperature exceeds 210°C. Screw speed is typically maintained between 200 rpm and 300 rpm; higher shear contributes to localized temperature rise and melt fracture. For unfilled SBS processed at 180°C, temperature variation greater than ±5°C can produce inconsistent die swell and surface roughness. Injection molding of KTR® SBS 104 compounds uses melt temperatures of 170–190°C, back pressure below 2 MPa, and mold temperatures of 20–40°C so that the polystyrene domains solidify rapidly.

Batch-to-batch variance in porous pellet bulk density, typically 0.25–0.45 g/cm³, can alter gravimetric feeder calibration; loss-in-weight feeders are therefore calibrated for each lot. Feed throat temperature is kept below 40°C to prevent pellet blocking and screw slip. For adhesive solution processes, the polymer is charged slowly into solvent under high-shear agitation because rapid addition of pellets can form solvent-swollen gel clumps. Solution viscosity at 25 wt% solids in toluene for linear SBS grades can exceed 5,000 mPa·s; the actual value depends on molecular weight and diblock content.

Surface moisture on porous SBS pellets is removed before melt processing to prevent microvoids and melt viscosity instability. A dehumidifying hopper with a dew point below -20°C or a static air oven at 60–70°C for 2–4 h is used when ambient relative humidity exceeds 60%. Overfeeding damp SBS into a twin-screw extruder produces irregular melt pressure at the die and reduces elongation at break by localized bubble defects. Drying above 80°C is avoided because porous pellets can agglomerate and bridge in the hopper. For adhesive and sealant solvent processes, solids loadings above 30 wt% in toluene or cyclohexane become viscosity-limited; solvent stripping in devolatilizing equipment is maintained below 150°C to avoid dried-film degradation.

Mechanical property response is measured through standardized dumbbell and hardness protocols.

Tensile stress, elongation at break, and modulus for SBS thermoplastic elastomers are determined on compression-molded sheets cut into ISO 37:2017 Type 3 or ASTM D412-16 Die C dumbbells. Hardness is measured by ASTM D2240-15e1 or ISO 48-4:2018 on specimen stacks of controlled thickness after a specified relaxation interval. Compression-molded sheets are prepared at 160–170°C for 5–10 min followed by cooling to 23°C under pressure. Conditioning at 23°C and 50% relative humidity for 24 h before testing reduces day-to-day variability. The stress-strain curve of SBS typically shows an initial steep elastic region, a yield-like plateau, and a large extension at break; tensile set measured after 200% elongation indicates network recovery. For linear SBS grades with bound styrene contents between 30 wt% and 40 wt%, published class-level tensile strengths commonly range from 20 MPa to 33 MPa, with elongation at break exceeding 700% and Shore A hardness from 70 A to 85 A. These values are class-level behavior and must not be read as KTR® SBS 104 batch specifications.

In footwear and rigid-polymer modification, KTR® SBS 104 is dry-blended with polystyrene, oils, fillers, and stabilizers and then melt-compounded in a corotating twin-screw extruder at 150–180°C. For impact modification of polystyrene, addition levels from 5 wt% to 20 wt% are evaluated by notched Izod impact according to ASTM D256-10e1; below 5 wt% toughness modification is usually negligible, and above 20 wt% melt viscosity increases and transparency is lost. In injection-molded footwear soles, the compound is processed with melt temperatures of 170–190°C and mold temperatures of 20–40°C; clamp force is set according to the projected area at 3–5 kN/cm². The physical crosslinks of polystyrene domains allow repeated deformation recovery, but continuous load-bearing service above 70–80°C produces creep and set.

Bitumen modification requires high-shear milling because SBS and bitumen are immiscible at the molecular scale. Typical addition levels of 3–7 wt% SBS in paving bitumen are processed in a rotor-stator high-shear mixer at 175–190°C for 1–3 h. A sulfur donor or other free-radical stabilizer may be used below 0.2 wt% to suppress phase separation. Dynamic shear rheometer measurements according to AASHTO M320 or EN 14023:2010 show increased complex modulus at elevated pavement temperatures, and elastic recovery by ASTM D6084-21 can exceed 60% after complete dispersion. Storage stability is assessed by the difference in softening point between top and bottom specimens after 72 h at 163°C; a difference below 2.5°C is considered acceptable for many polymer-modified bitumen specifications. Low-temperature thermal cracking is characterized by bending beam rheometer tests according to AASHTO T313; the polybutadiene mid-block reduces stiffness and increases creep rate at temperatures below -18°C.

When KTR® SBS 104 Replaces Radial, Oil-Extended, or Hydrogenated Block Copolymers

Substitution should account for architecture, extender oil, and hydrogenation. Radial SBS grades are coupled into multi-arm structures and display higher elasticity and melt tension but lower melt flow and greater shear sensitivity than linear SBS. Oil-extended SBS contains process oil at 30–50 phr, which lowers hardness and increases melt flow but introduces oil migration that can interfere with adhesive bond strength and coating adhesion. KTR® SBS 104 is supplied as a non-oil-extended grade; direct one-to-one replacement of an oil-extended grade without compensating plasticizer results in higher compound hardness and mixing torque. Hydrogenated SEBS grades have a saturated mid-block that tolerates continuous service above 100°C and outdoor UV exposure more effectively than unsaturated SBS, but SEBS requires higher melt temperatures and may not match the low-temperature elasticity of SBS at similar hardness.

Regulatory documentation for KTR® SBS 104 is confirmed through the supplier’s product stewardship summary and certificate of analysis. In the European Union, the product is subject to REACH Regulation (EC) 1907/2006; as a polymer, registration obligations may be reduced under Article 2(9), but communication of SVHC content above 0.1 wt% is required. For articles marketed in the EU, RoHS Directive 2011/65/EU Annex II restrictions for lead, mercury, cadmium, hexavalent chromium, PBB, and PBDE apply at the homogeneous material level. Food-contact use is application-specific and requires verification against FDA 21 CFR 177.1810 or 21 CFR 177.2600; a supplier letter of assurance should be obtained before use. The material should not be processed for extended periods at melt temperatures above 200°C, and it should not be combined with strong oxidizing agents or exposed to ozone-generating equipment because unsaturation in the polybutadiene segment accelerates chain scission and surface cracking. Uncontrolled peroxide addition in free-radical modification can crosslink the unsaturated mid-block and produce an irreversible viscosity rise; such modifications require closed-loop melt temperature control and torque monitoring. At the plant level, exposure controls are based on dust generation from pellets; local exhaust ventilation is specified for areas where pellets are ground or conveyed at high velocity.