Kraton™ D SBS D1155 is a linear styrene-butadiene-styrene triblock copolymer supplied as clear pellets. The grade contains approximately 31 wt% bound styrene and has a specific gravity of 0.94 per the manufacturer technical bulletin. Because the molecular architecture is linear rather than radial or diblock-rich, D1155 provides lower melt viscosity and lower toluene solution viscosity than high-molecular-weight linear types such as Kraton D1101, while retaining the phase-separated morphology responsible for elastomeric recovery. The polystyrene end-blocks form glassy domains with a glass transition near 95°C; the polybutadiene midblock contributes a low-temperature transition near -85°C. This two-phase structure allows room-temperature stiffness and elastic recovery without vulcanization. Typical uses include hot-melt and solvent-borne adhesives, bitumen modification, and thermoplastic elastomer compounds for footwear and molded goods. Differences from other Kraton D SBS grades arise primarily from molecular weight distribution, diblock content, melt flow, and the resulting balance between cohesive strength and processability. In unfilled compression-molded films, the manufacturer reports tensile strength and elongation at break values consistent with general-purpose SBS rather than high-styrene or high-hardness specialties. The grade is therefore selected when lower processing viscosity is required and maximum cohesive strength is not the sole design variable.

How Does the Linear Triblock Architecture Govern Melt Rheology?

The linear structure of D1155 reduces entanglement density relative to radial SBS grades and lowers zero-shear viscosity at a given bound styrene content. Single-point melt flow rate is typically measured at 200°C with a 5 kg load using ISO 1133-1:2022 or ASTM D1238, but the value is not sufficient for setting die pressure because the melt is strongly non-Newtonian. Capillary rheometry between 170°C and 190°C is required to fit the power-law index and to predict pressure drop through strand dies. On a co-rotating twin-screw extruder with L/D 40:1 and a screw diameter of 40 mm, stable compounding with hydrocarbon oil and polystyrene is obtained using barrel setpoints from 150°C in the feed zone to 180°C in the mixing zones. Melt temperatures above 200°C at the die are not recommended; local shear heating in kneading blocks can initiate gel formation and produce surface pitting in the extrudate. A die land length-to-diameter ratio of at least 10:1 is typically used to reduce die swell and stabilize strand pelletizing.

The processing window for this grade is narrow when high-shear kneading blocks are used. Because the polybutadiene midblock is unsaturated, oxidative chain scission and crosslinking compete at elevated melt temperatures. A production-scale observation is that moving the final two barrel zones from 190°C to 180°C reduces die-lip gel accumulation without significantly increasing torque. Batch-to-batch variability in diblock content also influences melt elasticity; lots with higher diblock content show lower cohesive strength and may require a reduction in screw speed to maintain strand integrity. Incoming resin should therefore be characterized by gel permeation chromatography against a certified reference before large-scale compounding, particularly when the compound is used in thin-gauge film or adhesive coating.

In hot-melt pressure-sensitive adhesive formulations, D1155 is combined with C5 aliphatic tackifiers and naphthenic or paraffinic oils. A starting formulation typically contains 25–40 phr tackifier and 5–15 phr oil per 100 phr polymer; the exact ratio is adjusted to balance loop tack and shear adhesion failure temperature. Coating viscosity is controlled by oil level and by the molecular weight of the SBS. The lower solution viscosity of D1155 relative to D1101 permits higher solids content in toluene-based coating lines without exceeding slot-die pressure limits; Brookfield viscosity at 25°C and 30 wt% solids is lower by a factor that depends on solvent quality and aging. Adhesive performance is tested by ASTM D6195 for loop tack, ASTM D4498 for heat-fail temperature in shear, and ASTM D3330 for peel adhesion of pressure-sensitive tape constructions. Aromatic oils above 30 phr are avoided because they plasticize the polystyrene domains and reduce the upper service temperature of the adhesive. Pellets should be dried at 60°C for 2 h when ambient relative humidity exceeds 60%; although the polybutadiene midblock is hydrophobic, surface condensation on cold pellets can generate microbubbles in extruded films.

Slot-die coating of solvent-borne D1155 adhesives requires control of solution temperature and filtration. Coating solutions are typically filtered through 20–50 μm screens to remove gel particles that form during storage. Viscosity drift during aging can occur if the solvent evaporates unevenly from open containers; sealed storage under nitrogen is recommended. In solvent-free hot-melt coating, melt viscosity at application temperature is the critical variable. The melt must remain below 200°C for extended pot life; heated hose setpoints are generally maintained at 175–185°C to avoid degradation while delivering sufficient flow through slot-die manifolds.

Bitumen Modification: Storage Stability and High-Temperature Rutting Resistance

Addition of D1155 to paving-grade bitumen at 3–7 wt% raises the softening point and increases elastic recovery after cyclic loading. High-shear dispersion at 180–190°C is required to reduce the polymer-rich phase to droplets below 10 μm; low-shear mixing swells the polymer but does not produce a stable network. Performance is assessed by EN 1427 for softening point, EN 1426 for needle penetration at 25°C, and EN 13398 for elastic recovery. Storage stability is quantified by maintaining a sample at 180°C for 72 h and measuring the difference in softening point between the top and bottom thirds of the container. The linear architecture of D1155 disperses more rapidly than radial SBS grades of similar styrene content, but it may require a slightly higher polymer addition to meet the same high-temperature grading as a radial grade. In roofing felt and waterproofing membrane compounds, the grade is also used to improve low-temperature flexibility and fatigue resistance of oxidized bitumen.

High-shear mixing equipment for bitumen modification should be sized to generate rotor tip speeds above 15 m/s; lower tip speeds produce coarse polymer domains and poor storage stability. A typical batch protocol includes pre-swelling of pellets in hot bitumen for 30–45 min under low agitation, followed by high-shear dispersion for 60–90 min at 180–190°C. Prolonged exposure to air at these temperatures can harden the bitumen by oxidation; nitrogen blanketing of the mixing vessel is used when extended storage is required. The final modified binder should be sampled for softening point and elastic recovery before discharge to storage tanks.

Compounding of D1155 for injection-molded footwear soles requires a co-rotating twin-screw extruder with sufficient distributive mixing. A typical formulation uses 100 phr D1155, 30–60 phr general-purpose polystyrene, 20–40 phr hydrocarbon oil, and 10–25 phr silica filler; the compound is processed at melt temperatures of 175–190°C. On a 1500 kN hydraulic injection-molding press with a 50 mm screw, injection pressures of 80–100 MPa and hold pressure maintained until gate freeze are typical for thin-wall soles. Abrasion resistance is measured by DIN 53516; tear strength is measured by ASTM D624; Shore hardness is measured by ASTM D2240. Oil levels above 50 phr are not recommended because abrasion resistance and tensile strength decline sharply. Filler dispersion should be verified by scanning electron microscopy on cryofractured specimens; agglomerates greater than 20 μm can act as crack initiation sites during flex fatigue.

When D1155 Replaces D1101 in High-Shear Compounding Operations

A direct substitution of D1155 for D1101 is not appropriate in every formulation. D1101 has a higher molecular weight and higher solution viscosity; it provides greater tensile strength, elongation, and cohesive strength in unfilled or lightly filled systems. D1155 is selected when the higher melt viscosity of D1101 creates excessive torque, poor wet-out of fillers, or unstable strand pelletizing. In production trials, lower torque and lower melt pressure are observed with D1155 on the same screw configuration, but the magnitude depends on oil level, filler type, and screw design. If a compound based on D1101 passes a tensile or tear specification, replacement with D1155 may require increasing the polystyrene content or reducing the oil level to compensate for the shorter chain length. Capillary rheometry and mechanical testing according to ASTM D638-14 and ASTM D624 are required before lot qualification. Published data for direct drop-in performance in highly filled systems is limited; qualification trials on the specific production line are therefore necessary.

The qualification protocol should include a melt-flow comparison, tensile stress-strain testing, tear resistance, Shore hardness, and compression set if the compound is used in footwear or seals. In addition, the melt strength of D1155 is lower than that of D1101; blown-film or cast-film lines running D1101 may require adjustment of melt temperature or air ring settings to maintain bubble stability when D1155 is used. If the final application requires high-temperature creep resistance, D1155 alone may not match the performance of higher-molecular-weight or crosslinkable SBS grades.

Compliance is anchored to the following standard designations

Storage below 40°C in a dry area protected from ultraviolet light is required. Prolonged exposure to direct sunlight can crosslink the polybutadiene midblock and produce gels. The neat polymer should not be processed above 200°C for extended periods; oxygen at high temperature degrades the unsaturated midblock. Strong oxidizing acids and aromatic hydrocarbon solvents at elevated temperatures can attack the styrenic domains. Additives with strong basic character, including certain amines, should be evaluated before use because they can accelerate degradation of the unsaturated polybutadiene segment. The product is not intended for continuous service above 70°C in unvulcanized compounds because the polystyrene domains soften and creep under load. For applications requiring sustained heat resistance above 90°C, a vulcanized or crosslinked system based on a saturated midblock is required.