How Does the Styrene-to-Diblock Balance Control Rheological Behaviour in C5375X?

Calprene® SBS C5375X is a solution-polymerized styrene-butadiene-styrene block copolymer supplied by Dynasol. The polymer is produced as a porous crumb and is stabilized with a non-staining phenolic/phosphite antioxidant package. Manufacturer-published data place the bound styrene content at 31.5 wt% and the diblock content at approximately 10 wt%, with a specific gravity of 0.94 determined under ASTM D792-20. Melt flow index measured at 190 °C with a 5 kg load using ASTM D1238-20 is 15 g/10 min. Compression-moulded plaques yield a Shore A hardness of 65 under ASTM D2240-15 and tensile properties of 16 MPa and 800 % elongation at break under ISO 37:2017. The product is used in hot-melt adhesives, bitumen modification, footwear compounds and rigid polymer blends where low melt viscosity, styrenic hard domains and elastomeric butadiene mid-blocks are required.

The dissolved and molten rheology of C5375X is controlled by immiscible polystyrene end-blocks and the butadiene mid-block. The 31.5 wt% styrene content forms a physically crosslinked network at ambient temperature; the 10 wt% diblock fraction acts as a network diluent because diblock chains contain only one styrene terminus and cannot bridge two styrene domains. This reduces low-shear melt viscosity and increases melt flow index relative to a diblock-free SBS of equivalent peak molecular weight. In a 25 % toluene solution at 25 °C, rotational viscometry under ASTM D2196-20 records a typical Brookfield viscosity of 4,000 mPa·s. The same diblock population reduces tensile strength and increases elongation at break because fewer load-bearing styrenic bridges exist per unit volume.

Transmission electron microscopy of compression-moulded C5375X shows polystyrene domains of 20–30 nm dispersed in a continuous butadiene matrix. Dynamic mechanical analysis under ISO 6721-1:2019 in tensile mode at 1 Hz reveals a butadiene-phase glass transition near −85 °C and a polystyrene-phase glass transition near 95 °C. The plateau storage modulus between these transitions is approximately 10 MPa, consistent with the physical crosslink density expected for an SBS with 31.5 wt% styrene. Published data for this specific configuration is limited; final compound performance therefore requires validation on target production equipment.

In hot-melt adhesive compounding, the grade is dispersed in a jacketed co-rotating twin-screw extruder with an L/D ratio of 40:1 and screw diameter of 25 mm, set to barrel temperatures of 150–170 °C across zones 1–8. Formulations containing 25–35 wt% C5375X, 45–55 wt% C5 aliphatic tackifying resin and 15–25 wt% naphthenic oil produce a Brookfield Thermosel viscosity of 3,000–8,000 mPa·s at 180 °C under ASTM D3236-14. Applied through a slot die at 150–160 °C onto corona-treated PET film, the adhesive is evaluated for peel adhesion under ASTM D1876-08; production batches on a 25 mm extruder show gel particle formation when barrel temperatures exceed 180 °C for more than 20 min residence time. The diblock fraction lowers cohesive strength but reduces stringing and melt fracture during coating.

The Role of the Non-Staining Stabilizer Package in Long-Term Thermal Stability

The as-shipped crumb contains a proprietary non-staining phenolic/phosphite stabilizer system. Oxidative induction time measured on the raw polymer at 200 °C by differential scanning calorimetry under ASTM D3895-19 is typically greater than 20 min. The stabilizer protects the butadiene mid-block during high-shear mixing and storage, but it does not eliminate the need for inert gas blanketing in hot-melt equipment. Extended residence times above 180 °C for more than 6 h can degrade butadiene unsaturation, producing a viscosity increase and burnt-rubber odour. In clear adhesive production, gel particles larger than 100 µm are visible as coating defects; a 25 µm melt filter is therefore fitted upstream of the slot die.

For bitumen modification, C5375X is introduced into penetration grade 60/70 bitumen at 180–190 °C using a high-shear rotor-stator mixer operating at 3,500–5,000 rpm for 4–6 h. The recommended polymer loading is 3–7 wt%; below 3 wt% the elastic network is incomplete and the modified binder behaves as a viscous liquid. At 5 wt% C5375X, the ring-and-ball softening point under ASTM D36/D36M-14 increases from 48 °C to approximately 80 °C, and the elastic recovery at 25 °C under ASTM D6084-18 exceeds 60 %. Storage stability of the hot blend is evaluated by the 48 h oven test at 163 °C described in ASTM D5976-96; top-bottom softening point differences above 2.5 °C indicate phase separation. Addition of 0.1–0.3 wt% sulfur as a vulcanizing agent after SBS dispersion reduces phase separation but narrows the processing window to ±5 °C because excessive crosslinking produces gelation in the storage tank.

High-shear mixing initially disperses the crumb; the SBS particles absorb maltenes from the bitumen, swell to several times their original volume and form a continuous polymer-rich phase. Dispersion is considered complete when fluorescence microscopy shows no polymer particles larger than 10 µm. In a 5 wt% blend, the complex modulus at 60 °C measured by dynamic shear rheometry under ASTM D7175-15 reaches 1–5 kPa at 10 rad/s. Failure to maintain shear above 3,500 rpm yields a coarse dispersion with softening point below 70 °C and poor low-temperature flexibility.

Compositional Specifications and Standard Test Methods

Tensile specimens are die-cut from compression-moulded sheets cured under 150 °C and 10 MPa for 10 min, following the conditioning requirements of ISO 23529:2016. Hardness values are measured after a 15 s dwell under ASTM D2240-15.

In solvent-borne adhesive manufacturing, C5375X is dissolved in toluene or cyclohexane at 25–35 wt% solids using a slow-speed turbine mixer at 80–100 rpm. The solution viscosity at 25 °C follows a shear-thinning profile; low-shear Brookfield viscosity is approximately 20,000 mPa·s at 30 wt% solids. Coating is performed on a knife-over-roll line at 5–15 m/min with a wet-film gap of 200 µm. Residual solvent is removed in a three-zone forced-air oven at 60 °C, 80 °C and 110 °C. Solvent-borne systems are selected when heat-sensitive substrates require processing below 120 °C.

When C5375X Is Selected Instead of Higher-Styrene or Coupled SBS Grades

Comparative selection is based on four measurable properties: styrene content, diblock content, oil extension and coupling architecture. A higher-styrene SBS with 40 wt% styrene produces a harder compound and better resistance to aromatic oils, but requires higher processing temperatures and yields a stiffer low-temperature response. A low-diblock coupled grade with a melt flow index near 2 g/10 min provides higher tensile strength and lower cold flow, but demands longer mixing cycles and higher torque in twin-screw extrusion. C5375X occupies the low-viscosity, moderate-hardness segment: the 15 g/10 min melt flow index permits barrel temperature reductions of 10–20 °C relative to low-flow coupled grades in hot-melt equipment. Compared with oil-extended SBS containing 35 phr naphthenic oil, C5375X delivers a higher neat tensile strength because the butadiene network is not pre-plasticized; however, formulation viscosity must be controlled by external oil addition, and cold flow is greater than that of the oil-extended grade at equal compound hardness.

Storage and handling boundaries apply. The crumb should be stored in sealed packaging at 25 °C and protected from direct sunlight. Shelf life from the date of certification is 24 months. If the crumb is exposed to relative humidity above 60 % for more than 48 h, moisture absorption can introduce surface defects in clear hot-melt coatings; dehumidified-air drying at 50 °C for 2 h is then recommended. The polymer is incompatible with strong oxidizing agents and with high-aromatic process oils that plasticize the polystyrene domains and reduce tensile strength.

In compounding for footwear and mechanical rubber goods, the grade is charged into an internal mixer with a fill factor of 0.75 at 50 rpm and a jacket temperature of 120–140 °C. The single-stage cycle is limited to 8 min to prevent thermo-oxidative chain scission; the compound is discharged at 150 °C and sheeted on a two-roll mill.