The styrene-butadiene-styrene block copolymer Dynasol Calprene® SBS 7318 is a linear thermoplastic elastomer supplied as compacted porous pellets. Its polymer architecture consists of polystyrene end blocks separated by a polybutadiene midblock; the bound styrene content is reported by the manufacturer as 31 wt% based on the polymer fraction, with a specific gravity of 0.94 g/cm³ when determined under ASTM D792-20. The CAS registry number is 9003-55-8. The material is classified for bitumen modification, adhesive compounding, footwear soling, and thermoplastic impact modification. The pelletized form is designed for gravimetric feeding and dry-blend oil absorption; pellet bulk density is typically in the range of 0.45 g/cm³ to 0.55 g/cm³ for material handling purposes. Residual volatile matter is specified below 0.5 wt% and ash below 0.5 wt%; these limits are operationally significant because volatile residues in a closed barrel disrupt melt-seal stability in twin-screw extrusion, while ash contributes to screen-pack pressure rise in hot-melt adhesive filtration.

Which Rheological and Thermal Boundaries Define 7318 Processing?

The melt mass-flow rate is commonly reported as 6 g/10 min at 190 °C under a 5 kg load when tested to ASTM D1238-20 or ISO 1133-1:2022. This melt-flow position places the grade between low-viscosity adhesive SBS types and high-molecular-weight radial grades used where hot mixing torque is tolerated. For die design, capillary rheometry is required because the single-point melt-flow test does not define the full shear-viscosity curve. The grade exhibits shear-thinning behaviour, which permits high-shear processing at moderate head pressures, but low-shear zones near screen packs and adapters can retain material long enough to form gel specks. Co-rotating twin-screw extruders with L/D ratios of 40:1 or higher are used for filled compounds; the barrel profile from feed to die is typically maintained between 150 °C and 180 °C. Die temperatures above 200 °C are avoided because thermal degradation of the polybutadiene block accelerates above 220 °C, producing gel particles and increasing melt viscosity unpredictably. A production-scale failure mode observed in high-shear lines is the formation of crosslinked particulates in stagnation zones when screw speed exceeds 350 rpm and specific mechanical energy input is held above 0.25 kWh/kg for extended residence times; this is an equipment-dependent boundary, not a product specification limit, and requires screw-profile optimization and zone-temperature recalibration. Pre-drying at 70 °C for 2 h in desiccant dryers is applied when storage relative humidity exceeds 60 %, because surface moisture on cold pellets can produce porosity in extruded profiles and interfere with silane-based coupling agents in filled formulations.

In solvent-borne adhesives, the grade is dissolved in blends of aliphatic and aromatic hydrocarbon solvents; solution viscosities at solids contents between 15 wt% and 25 wt% are controlled by styrene end-block association, and the linear backbone reduces the incidence of microgel agglomerates compared with radial SBS architectures. Hot-melt formulations are compounded in sigma-blade mixers or continuous compounders at 160 °C to 180 °C; tackifier compatibility is strongest with C5 aliphatic resins and aromatic-modified C5 resins having softening points from 95 °C to 110 °C. The unfilled polymer is characterized under ASTM D412-16 by a tensile strength of approximately 18 MPa and an elongation at break near 800 %. Peel adhesion on flexible PVC and stainless steel is evaluated by ASTM D903-98(2024), while loop tack is measured under ASTM D6195-03(2022); numerical peel values vary with tackifier loading and substrate surface energy, and published data for this specific grade in a fixed adhesive matrix is limited.

Bitumen modification response, elastic recovery, and phase separation control

In polymer-modified bitumen, Calprene SBS 7318 is typically dispersed at 3 wt% to 6 wt% based on bitumen mass in a high-shear rotor-stator mill at 180 °C to 190 °C. The polymer swells by absorbing maltenes; the resulting polymer-rich phase increases ring-and-ball softening point and elastic recovery. Viscosity is measured by ASTM D4402-19 using a Brookfield thermosel; elastic recovery is measured by EN 13398:2017; storage stability is assayed by EN 13399:2017 or ASTM D7173-20. A processing conflict arises when milling temperature is raised above 200 °C to reduce blend viscosity: the viscosity decrease is partly irreversible because thermal chain scission lowers the polymer’s contribution to elastic recovery. For high-viscosity bitumens, temperature is therefore kept at the lower end of the window and rotor speed is increased instead. In vertical storage tanks, phase separation is conventionally controlled by keeping the difference between top and bottom softening points below 5 °C after 72 h at 160 °C. In heavily waxy bitumens, paraffinic waxes compete with the polybutadiene midblock for maltene absorption, which suppresses swelling; this is mitigated by adding aromatic-rich flux oils at 0.5 wt% to 2 wt%, not by increasing polymer loading beyond 6 wt%, which raises blend viscosity and cold-temperature cracking tendency. Inline viscosity at 135 °C after modification is typically controlled in the range of 2.0 Pa·s to 4.0 Pa·s for paving-grade binders when measured by EN 13302; actual values depend on base bitumen source and polymer dosage.

When 7318 replaces higher-styrene or SEBS grades in thermoplastic compounding

Compared with Calprene 540, which carries a bound styrene content near 40 wt%, Calprene SBS 7318 has a smaller glassy styrene domain volume; the result is lower Shore A hardness and better low-temperature flexibility, but reduced tensile strength and reduced resistance to aromatic and mineral oils. In polypropylene modification at addition levels from 5 wt% to 20 wt%, the grade contributes impact resistance by forming dispersed elastomer domains. Domain size is controlled by screw design, back pressure, and melt residence time. A production-scale observation is that back pressure above 15 bar in injection molding can increase shear heating and reduce dispersed elastomer domain size below the optimum 0.5 μm to 2 μm range, reducing notched Charpy impact performance when tested to ISO 179-1:2023. Compared with hydrogenated styrenic block copolymer SEBS grades such as Calprene H6120, the unsaturated polybutadiene midblock of 7318 provides higher tack and adhesion to low-surface-energy substrates, but also lower thermo-oxidative stability; continuous service above 80 °C in air requires antioxidant packages, and the compound should not be combined with transition-metal stearates that catalyze oxidation. The grade is incompatible with highly polar polymers such as polyamide and polycarbonate; without a functionalized compatibilizer, interfacial delamination occurs under load. In comparison with Calprene 501, 7318 exhibits a slightly higher melt-flow index under identical test conditions, which reduces fill pressure in thin-wall parts; however, published data for this specific part geometry is limited.

Footwear soling compounds made from this SBS are compounded with polystyrene, naphthenic or paraffinic oil, and fillers in internal mixers or twin-screw extruders. Injection molding is performed at melt temperatures of 160 °C to 190 °C and mold temperatures of 30 °C to 50 °C; melt temperatures above 200 °C reduce viscosity but generate splay and dimensional shrinkage when parts are ejected hot. The density of unfilled soling compounds is lower than PVC or TPV alternatives, typically in the range of 0.98 g/cm³ to 1.05 g/cm³ depending on filler content; density is measured by ASTM D792-20 or ISO 1183-1:2019. Abrasion resistance depends on filler type and dispersion quality; poor dispersion of silica or calcium carbonate at filler loadings above 20 wt% creates hard spots that initiate flex cracks. Batch-to-batch variance in melt-flow index at the lower specification limit can alter injection molding cushion position and packing pressure, particularly in multi-cavity molds with runner diameters below 3 mm.

Operational boundaries include pre-drying at 70 °C when pellet surface moisture is visible or storage relative humidity exceeds 60 %. In adhesive formulations, amine-based additives are avoided because they interfere with tackifier resin compatibility and can accelerate color development. Release agents containing silicone or fluorochemicals are kept out of closed-loop regrind streams because they lower bond strength and paint adhesion. Data for highly filled flame-retardant compounds using brominated systems beyond 30 wt% loading for this specific grade are limited.