Solprene® SBS 9618 is an unsaturated styrene-butadiene-styrene block copolymer manufactured by anionic polymerization. The product is supplied as a pelletized solid with a bound styrene content of 31.0 ± 1.0% by mass and a polybutadiene midblock content of 69.0 ± 1.0%. Volatile matter measured according to ASTM D5668 is controlled below 0.5 wt%; total extractables determined by ASTM D5774 are controlled below 1.0 wt%. The stabilizer package is a non-staining phenolic/phosphite combination, and no amine-based antioxidant is present, which avoids premature crosslinking in free-radical curing and minimizes colour shift in white compounds. The two-phase morphology contains polystyrene end-block domains that function as physical crosslinks below the polystyrene glass transition temperature, while the unsaturated butadiene midblock provides elastomeric recovery and room-temperature tack. Bulk density measured by ASTM D1895-17 ordinarily falls between 0.35 g/cm³ and 0.45 g/cm³. Pellets are supplied in 25 kg polyethylene-lined bags or 750 kg bulk containers, with a shelf life of 24 months when stored below 25°C and below 60% relative humidity.

What Distinguishes Solprene® SBS 9618 from Solprene® SBS 411 and Hydrogenated SEBS Grades?

In comparative evaluations, Solprene® SBS 411 is specified for high-viscosity asphalt modification and high-strength compounds, while Solprene® SBS 9618 is positioned in the lower-solution-viscosity range. A 25 wt% toluene solution of Solprene® SBS 9618 typically shows a Brookfield viscosity of 3500–4500 mPa·s at 25°C, whereas the same solution concentration of Solprene® SBS 411 can exceed 8000 mPa·s. This viscosity differential reduces solvent demand in contact adhesives and permits higher solids loading in sprayable formulations. The unsaturated polybutadiene midblock in Solprene® SBS 9618 retains pressure-sensitive tack without hydrogenation, but it imposes a lower continuous service temperature than SEBS materials. SEBS grades resist thermo-oxidative degradation up to 120°C, while SBS 9618 is normally limited to 70–80°C in long-term service. The 9618 grade also contains a controlled diblock fraction below 15 wt%, which preserves cohesive strength and reduces cold flow in adhesive films. Compared with Solprene® SBS 501, grade 9618 is specified with a lower solution viscosity and a higher diblock content, which reduces melt strength but improves tack. Compared with Solprene® SBS 601, it has lower bound styrene and lower Shore A hardness, making it less suitable for high-modulus compounds.

Batch-to-batch reproducibility of solution viscosity on a 25 wt% toluene solution is reported by the producer as ±10%, which is a critical parameter for adhesive coaters because viscosity drift above 5000 mPa·s can exceed the delivery limit of conventional gear pumps. Control of the diblock fraction is also necessary because diblock content above 20 wt% lowers tensile strength by more than 15% and increases blocking of stacked film.

When High-Shear Bitumen Dispersion Passes Through the Polymer Network Threshold

Bitumen modification with Solprene® SBS 9618 is performed in a high-shear mill at 180–190°C with tip speeds of 18–22 m/s. The initial polymer particle size is reduced below 10 μm within 15 min, but full phase development requires 45–60 min for bitumens with a colloidal instability index below 0.36. At polymer contents below 5 wt%, the bitumen remains the continuous phase and the SBS domains act as dispersed stress modifiers. As the concentration increases through 5–7 wt%, a phase inversion to a polymer network occurs. This transition raises the ring-and-ball softening point measured by ASTM D36 by more than 20°C relative to unmodified bitumen. In paraffinic bitumens with a colloidal instability index above 0.40, the polymer phase can separate during hot storage; storage stability is assessed by ASTM D5892 or EN 13399, and a softening point difference above 5°C between top and bottom aliquots indicates incompatibility. Sulfur vulcanization kinetics in the butadiene midblock are sufficiently slow at 180°C to permit 30–45 min of mixing before gel formation, but sulfur addition above 0.15 wt% raises gel content measured by ASTM D2765 from below 1.0 wt% to above 5.0 wt% and can clog the mill.

In hot-melt pressure-sensitive adhesive compounding, Solprene® SBS 9618 is mixed with C5 tackifying resins and naphthenic process oils in a sigma-blade mixer at 160–170°C for 25–35 min. The addition sequence is critical: polymer is masticated first with 20–30% of the total tackifier to shear the polystyrene domains, and the remaining tackifier and oil are added only after a clear melt is obtained. If the mixer temperature exceeds 180°C, the butadiene midblock begins thermo-oxidative chain scission, which is detected as a reduction in melt viscosity of more than 15% and a loss of loop tack measured by ASTM D6195. Oil loading is limited to 25 phr because higher levels cause oil migration to the carrier surface within 72 h and reduce the 180° peel adhesion on stainless steel below 2.0 N/cm when measured by ASTM D3330. At 20 phr oil, the time to visible surface exudation at 40°C exceeds 500 h, but at 30 phr it is shorter than 72 h. The migration rate follows a concentration-dependent diffusion process through the polybutadiene-rich phase.

Extruder Torque and Die Pressure Signatures in Twin-Screw Compounding

On a 40-mm co-rotating twin-screw extruder with L/D 32:1, Solprene® SBS 9618 is processed in a temperature profile from 150°C in the feed zone to 180°C at the die. Screw speed is maintained between 180 rpm and 250 rpm because lower speeds produce incomplete dispersion of the styrene domains, while higher speeds generate shear heating above the degradation threshold. At a feed rate of 350–400 kg/h, the die pressure typically remains between 45 bar and 60 bar. A rise in torque of 10–15% without a corresponding change in feed rate indicates that fused styrene domains have formed and that localized melt temperature has exceeded 200°C. The same signal appears when the polymer is fed without pre-blended oil because the elastomeric pellets deform under compression in the feed throat and reduce feed intake. The recommended pre-blending step uses a low-shear tumble blender at 70°C for 20 min to allow oil absorption before the extruder. The specific energy input under these conditions is approximately 0.20–0.25 kWh/kg, and the melt temperature at the die is held between 185°C and 195°C.

In injection molding of thermoplastic elastomer compounds based on Solprene® SBS 9618, a clamp force adequate for multi-cavity tools is 4–6 kN/cm² of projected part area, and the melt temperature at the nozzle is kept below 200°C. The low melt viscosity allows filling of thin sections down to 0.8 mm at injection speeds of 30–60 mm/s. Holding pressure is set at 40–60% of injection pressure to avoid flash at the parting line. Mold temperature is controlled at 25–40°C to accelerate solidification of the styrene domains; demolding is possible when the part surface temperature is below 50°C. In polypropylene blends containing 30–70 wt% Solprene® SBS 9618, the morphology shifts from dispersed SBS islands below 40 wt% to a co-continuous phase between 40 wt% and 60 wt%. The co-continuous region raises notched Izod impact resistance at -30°C when measured by ASTM D256, but it reduces flexural modulus by more than 25% relative to the neat polypropylene matrix.

Thermal Degradation Pathways Set the Upper Processing Limit at 200°C

Thermo-oxidative degradation of the unsaturated butadiene midblock follows free-radical chain scission and crosslinking pathways. At melt temperatures above 200°C, gel content measured by ASTM D2765 rises rapidly, and the melt flow rate declines by more than 30% from the initial value. At temperatures below 170°C, the styrene domains do not flow sufficiently to allow dispersion in bitumen or adhesive resins. The effective processing window is therefore 170–200°C, and the narrowest tolerance applies to hot-melt mixing at 180°C because the butadiene degradation rate doubles for every 10°C increase above 180°C. The polymer should be stored at relative humidity below 60%; if surface moisture exceeds 0.2 wt%, pre-drying at 80°C for 2 h is required. Drying above 90°C should be avoided because pellet surface tack increases and the granules may bridge in the hopper throat. Combination with amine-based additives or strong oxidizing agents is incompatible because these species accelerate midblock crosslinking and generate gel specks. Free sulfur at concentrations above 0.15 wt% can initiate premature vulcanization during melt processing and is avoided unless a controlled vulcanization step is specified.

Solvent-borne contact adhesives using Solprene® SBS 9618 are formulated at solids contents of 30–40 wt% in toluene or cyclohexane. The solution viscosity at 25 wt% is low enough to allow roller-coat application without excessive stringing. After solvent evaporation, the dry film exhibits a 180° peel strength on stainless steel of 3.0–4.5 N/cm when measured by ASTM D3330. The styrene content of 31 wt% provides room-temperature shear resistance, and the polybutadiene midblock contributes tack at 10–40°C. At temperatures below 0°C, tack declines because the polybutadiene glass transition is approximately -70°C, but polar substrates with surface energy above 38 mN/m retain adhesion. The polymer is not recommended for food-contact adhesive layers unless migration testing on the finished article demonstrates compliance with FDA 21 CFR 177.1810 extraction limits.

Regulatory documentation for Solprene® SBS 9618 should be verified against the finished-article application. The following table lists the principal standards relevant to downstream qualification.