In paving-grade polymer-modified bitumen (PMB) production, KTR® SBS 655 is introduced into straight-run bitumen preheated to 160–180 °C at a dosage of 3.0–7.5 wt% before high-shear dispersion. The linear SBS triblock architecture governs the modification mechanism: polystyrene end-blocks phase-separate into glassy domains with a service temperature below their glass transition, while the polybutadiene midblock forms a continuous elastomeric network that absorbs thermal and mechanical strain. Without sufficient shear, the initial pellet or crumb morphology persists as coarse agglomerates exceeding 50 μm, and the resulting PMB can fail hot storage stability within 72 h. A rotor-stator high-shear mill or Siefer colloidal mill operating at 3,000–5,000 rpm is used until the dispersed SBS particle size falls below 10–20 μm, after which low-shear agitation continues for 60–120 min to permit progressive swelling of the rubber-rich phase. Barrel and mixing head jacket temperatures are controlled at 175–190 °C; sustained exposure above 200 °C for more than 2 h accelerates oxidative scission of the polybutadiene midblock and can reduce elastic recovery by up to 15 percentage points in laboratory ageing trials.

Routine PMB evaluation follows EN 14023:2010 and ASTM D5976-22 for selection of base materials and verification of final properties. Ring-and-ball softening point according to ASTM D36/D36M typically rises from 48–52 °C for the unmodified 70/100 penetration-grade bitumen to 78–85 °C at 6.0 wt% SBS. Elastic recovery at 25 °C measured by ASTM D6084/D6084M-21 commonly exceeds 80% when the SBS network is fully dispersed; values below 60% indicate incomplete phase inversion. Penetration at 25 °C by ASTM D5/D5M-20 decreases from 65–75 dmm to 35–45 dmm depending on SBS loading, and Fraass breaking point by EN 12593 can shift to −18 °C or lower with a suitable low-wax flux. Storage stability is evaluated in a vertical aluminium tube at 180 °C for 72 h under EN 13399; the difference in softening point between top and bottom sections should remain below 5 °C. Published data for KTR® SBS 655 specifically in high-modulus 7.5 wt% PMB with polymer-modified microsurfacing is limited; pilot-mill validation is recommended because equipment shear history, base bitumen asphaltene content, and additive sulphur crosslinking alter the final storage modulus.

What Limits Low-Temperature Flexibility in High-SBS Roofing Membrane Compounds?

Low-temperature flexibility in SBS-modified bituminous roofing membranes is governed by the continuity of the styrene-rich physical crosslink network and by the plasticiser partition between bitumen and rubber phase. In calender-grade formulations, KTR® SBS 655 is used at 8–14 wt% together with 55–65 wt% oxidised or distilled bitumen, 20–25 wt% mineral filler such as talc or calcium carbonate, and 1–3 wt% antioxidant. The mixing sequence is performed in a heated double-sigma blade mixer at 180–190 °C; the SBS is added after the bitumen and filler have reached a homogeneous viscosity, then shear is maintained at 40–70 rpm until a uniform leather-like compound is obtained. Calendering onto a polyester or glass-fibre carrier at 0.8–1.5 m/min line speed produces a 3–5 mm thick membrane, and the finished sheet is cooled through the glass transition of the polystyrene domains at 15–40 °C before rolling.

Performance is tested under EN 13707:2013 for reinforced bitumen sheets for roofing. Cold bending at −20 °C using EN 1109 requires no visible cracking; formulations with high filler loading above 25 wt% may fail this criterion because filler particles create stress concentration points that nucleate fracture through the polybutadiene midblock. Dimensional stability at 80 °C for 6 h must remain within ±0.5%, and tensile strength per ASTM D5147/D5147M-18 is typically 4.0–6.0 MPa in the machine direction. For exposed roofing membranes, external fire performance is classified under EN 13501-5 with mineral granules, and formulations must be adjusted so that the asphalt-rich surface layer retains sufficient cohesion to anchor the granules without being displaced during the heat of combustion. Addition of paraffinic process oils must be limited to 5 wt% or less; above this limit the oil migrates into the styrene domains over time, lowers the effective glass transition, and reduces the upper service temperature before indentation failure occurs.

Injection-grade midsole compounds containing KTR® SBS 655 are typically compounded on a corotating twin-screw extruder with an L/D ratio of 40:1 and screw speed 250–400 rpm. The formulation contains 100 phr SBS, 20–40 phr general-purpose polystyrene, 5–20 phr naphthenic or paraffinic processing oil, 0.5–1.5 phr azodicarbonamide blowing agent, and 0.1–0.4 phr dicumyl peroxide crosslinking agent. Barrel temperature zones are set between 145 °C and 175 °C; the melt temperature at the die must remain below 190 °C to prevent premature decomposition of the blowing agent and to limit thermal gelation in the polybutadiene segments. The resulting pellets are injection moulded into microcellular sole units at clamp pressures of 80–120 t depending on cavity area, with mould temperature held at 30–50 °C and back pressure 5–10 bar during plastication to avoid gas phase segregation. Finished microcellular parts reach a density of 0.30–0.45 g/cm³, Shore A hardness 55–70, tensile strength per ASTM D412-16 of 8–12 MPa, elongation 500–700%, and compression set after 24 h at 70 °C of 20–35% according to ASTM D395-18 Method B.

Process conflicts arise when the compound is blended with high-cis polybutadiene rubber above 20 phr; the difference in solubility parameter creates macroscopic phase separation at the skin of the moulded part, visible as surface haze and tear strength loss. High melt temperatures above 200 °C during injection moulding create gas blister defects at the part surface because the blowing gas nucleates inhomogeneously at degraded rubber domains. Oil selection is critical: aromatic content above 5 wt% in the extrusion formula softens the styrene-rich phase and reduces dimensional stability of the expanded sole after 48 h annealing at 50 °C. Published data for KTR® SBS 655 in microcellular injection foam is limited to standard formulations; rheological characterisation by capillary rheometry at 180 °C is required before transfer to high-pressure foam lines.

Slot-Die Coating Viscosity and Tackifier Compatibility in SBS 655-Based Hot-Melt PSAs

Hot-melt pressure-sensitive adhesives based on KTR® SBS 655 are formulated with 20–40 wt% hydrogenated hydrocarbon tackifier, 5–15 wt% naphthenic oil, and 0.5–1.5 wt% stabiliser. Mixing is carried out under a nitrogen blanket in a horizontal sigma-blade mixer at 160–180 °C until the tackifier is fully dissolved and the viscosity stabilises. For slot-die coating at 120–180 °C, the melt viscosity should be held between 5,000 mPa·s and 25,000 mPa·s at 180 °C; below 5,000 mPa·s the coating weight varies beyond ±2 g/m², and above 25,000 mPa·s the die pressure rises above 40 bar and generates transverse streak defects. The C5/C9 resin choice must be matched to the polybutadiene midblock; high softening point resins above 115 °C reduce tack but raise the shear adhesion failure temperature to 70–85 °C.

Pressure-sensitive performance is measured by FINAT FTM 1 loop tack and ASTM D1876 180° peel on stainless steel; typical values are 8–14 N/25 mm loop tack and 6–10 N/25 mm peel depending on coating thickness of 25–35 μm. Shear resistance under FINAT FTM 8 at 40 °C with a 1 kg load often exceeds 24 h. At tackifier loadings above 45 wt%, the rubbery plateau shortens and the adhesive loses cohesive strength, leading to adhesive transfer and ghost marks on release liner. Ageing at 70 °C for 7 days in a forced-air oven may reduce peel by up to 15% unless the unsaturated midblock is adequately stabilised. The suitability window is narrower than for low-diblock SIS grades if the certificate of analysis confirms higher diblock content; use at −25 °C requires validation by low-temperature peel and mandrel flex tests before specifying KTR® SBS 655 for freezer-grade label stock.

When KTR® SBS 655 Is Melt-Blended with General-Purpose Polystyrene for Rigid Sheeting

Melt blending of KTR® SBS 655 with general-purpose polystyrene (GPPS) is performed on a co-rotating twin-screw extruder at 190–210 °C with an L/D ratio of 36:1 or larger. Addition of 10–30 wt% SBS creates a two-phase morphology in which the polybutadiene midblock forms discrete rubber particles with diameters of 0.5–2.0 μm, while the polystyrene end-blocks and GPPS matrix form a continuous rigid phase. The resulting high-impact sheet exhibits notched Izod impact strength of 8–15 kJ/m² according to ISO 180, compared with 1.5–2.5 kJ/m² for unmodified GPPS. Tensile strength per ISO 527-2:2021 decreases from 45 MPa for GPPS to 28–35 MPa at 30 wt% SBS, and flexural modulus per ISO 178 falls from 3.0 GPa to 1.5–1.8 GPa. Melt mass-flow rate according to ISO 1133-1:2022 at 200 °C under 5 kg drops by 30–60% relative to unmodified GPPS, depending on the SBS loading and screw shear history.

Thermo-oxidative degradation of the polybutadiene midblock is the main process risk: when melt temperatures exceed 220 °C or residence time exceeds 3 min, gel particles form and progressively blind screen packs of 80 mesh or finer. Processors should meter SBS pellets downstream of the first feed zone to limit thermal history and use a vacuum devolatilisation port at −0.08 MPa to remove moisture and low-molecular-weight volatiles. The blend should be purged with GPPS after run completion; shutdown without purging leaves SBS-rich residue that degrades on restart and causes black specks in the next production cycle. For thin-gauge sheet below 1 mm, chill-roll temperature should be held at 60–80 °C to minimise frozen-in orientation and to prevent the sheet from cracking during die cutting.

Extruded anti-fatigue mats and structural vibration-damping sheets use KTR® SBS 655 as the base elastomer because its styrenic physical crosslinks provide recoverable energy dissipation under cyclic compression. A typical compound contains 100 phr SBS, 30–50 phr calcium carbonate or barium sulphate, 10–20 phr paraffinic oil, and 1–2 phr antioxidant, processed on a single-screw extruder with a flat die at 165–185 °C. The extrudate is calendered and embossed to produce a sheet thickness of 2–8 mm; dynamic mechanical analysis under ISO 6721-1:2019 at 25 °C and 10 Hz yields a loss factor tan δ of 0.25–0.45. Tensile set after 100% elongation is 15–25% according to ISO 2285, indicating recovery from short-term deformation but insufficient creep resistance for sustained compressive loads above 0.05 MPa at temperatures above 50 °C. Shore A hardness of the filled sheet is typically 60–75. When barium sulphate is used at 50 phr to increase density for acoustic damping, the increase in compound viscosity raises melt temperature by 5–10 °C and reduces line speed by 10–20%; screw cooling is required to avoid scorch. Published data for KTR® SBS 655 in high-frequency acoustic damping above 500 Hz is limited; material selection should be confirmed by DMA master curve generation over the target temperature and frequency range.