Dynasol Calprene® SBS 501 is a linear styrene-butadiene-styrene block copolymer produced by anionic polymerisation. The product is non-oil-extended and is characterised by a nominal bound styrene content of 31 wt% and a diblock content reported as less than 1 wt% in manufacturer technical literature. The molecular architecture consists of polystyrene end-blocks that associate into glassy domains below approximately 95 °C. These domains serve as thermoreversible physical crosslinks within a continuous polybutadiene midblock phase. Because the grade does not require vulcanisation, it can be reprocessed through conventional thermoplastic equipment after initial compounding. The low diblock content is the primary structural factor distinguishing Calprene 501 from high-diblock SBS products intended for soft-touch or high-tack applications.

The values in Table 1 are representative batch-release data and do not constitute contractual specification limits. Variations of ±5% may occur between production campaigns. Incoming inspection should include melt mass-flow rate and tensile strength on standard compression-moulded sheets because these two properties are sensitive to changes in molecular weight and diblock content.

What Processing Limits Arise From the Low-Diblock Architecture?

Because diblock content is below 1 wt%, the melt exhibits higher elasticity and slower stress relaxation than high-diblock SBS grades of equivalent molecular weight. On a co-rotating twin-screw extruder with an L/D ratio of 40:1, a practical barrel profile begins at 120 °C in the feed zone, increases to 150–170 °C in the plasticating and mixing zones, and reaches 180 °C at the die. Operation above 190 °C accelerates thermo-oxidative degradation of the polybutadiene segment. Operation below 145 °C raises melt viscosity above 2000 Pa·s and degrades filler dispersion. The stable processing window is therefore approximately 170 °C ± 5 °C for sustained compounding runs. Pre-drying at 60 °C for 2 h in a desiccant dryer is required when ambient relative humidity exceeds 60%. Residual moisture above 0.1 wt% produces surface defects and viscosity reduction through chain scission. During profile extrusion, the high elasticity of the low-diblock architecture produces die swell values typically between 1.5 and 2.0 at a shear rate of 100 s⁻¹. Downstream haul-off calibration must compensate for this swell. Published data for this specific configuration is limited to general processing guides, and line-specific trials are required to confirm die geometry and barrel set points.

Modification of paving-grade bitumen can be performed in a high-shear rotor-stator mill. Calprene 501 is dosed at 3–5 wt% of the binder and dispersed at 170–180 °C for 45–60 min at rotor-tip shear rates above 3000 s⁻¹. During mixing, the polymer absorbs light maltene fractions and swells. Above the critical polymer content, the polymer-rich phase becomes continuous. Below 2 wt% polymer addition, phase inversion is incomplete and the modified binder retains a dispersed-elastomer morphology. Softening point of a conventional 50/70 penetration-grade bitumen can rise from approximately 46 °C to 70–85 °C at 4 wt% loading, measured according to ASTM D36/D36M-14(2020). Storage stability is evaluated under EN 14023:2010 by comparing the softening point of the upper and lower thirds of an oven-aged tube after 72 h at 180 °C. A difference greater than 5 °C indicates phase separation attributable to insufficient aromatic content, inadequate mixing time, or excessive polymer loading. Base binders with low aromatic content may produce gel particles. In such cases an aromatic-rich flux oil is required before polymer addition.

Adhesive Formulation, Tackification, and Solvent Release

In solvent-borne adhesive systems, Calprene 501 is dissolved in toluene or cyclohexane at solids contents of 20–30 wt%. The viscosity of a 25 wt% solution in toluene at 25 °C is typically 4000–8000 mPa·s when measured with a Brookfield viscometer in accordance with ASTM D2196-20. In hot-melt pressure-sensitive adhesives, the polymer is compounded with C5 aliphatic tackifier resins at 160–170 °C. Tackifier loading between 60 phr and 100 phr is used to balance wet-out and cohesive strength. Loadings below 60 phr reduce tack on low-energy substrates such as polyethylene. Loadings above 100 phr decrease shear adhesion failure temperature. Because the diblock content is low, the grade provides higher cohesive strength and lower cold flow than high-diblock SBS adhesives, but it also requires higher processing temperatures and longer solvent-evaporation cycles. Tack retention after solvent casting is improved by adding hindered phenolic antioxidants at 0.5–1.0 phr. Amine-based antioxidants are not recommended for this polymer because they can interfere with stabiliser performance and promote discolouration.

In footwear soling compounds, Calprene 501 is typically mixed with general-purpose polystyrene, naphthenic oil, and calcium carbonate in an internal mixer or on a two-roll mill at 150 °C. Addition of 20 phr calcium carbonate reduces compound cost but lowers tensile strength. Measured on compression-moulded sheets according to ISO 37:2017, tensile strength remains above 15 MPa at this filler loading. Abrasion resistance of the finished soling formulation is assessed by DIN 53516:2018. Typical volume loss values fall between 150 mm³ and 250 mm³, with the lower end corresponding to filler-free or low-filler compounds. Injection moulding of soling units is carried out with nozzle temperatures of 170–185 °C and mould temperatures of 25–40 °C. Clamp force requirements are estimated at 0.3–0.5 kN/cm² of projected area. Mould shrinkage is anisotropic. Longitudinal shrinkage is typically 1.2–1.8% and transverse shrinkage is 0.8–1.2%. Prototype tooling should be adjusted after measuring shrinkage on 10 consecutive mouldings because the orientation of the low-diblock structure amplifies flow-induced anisotropy.

Polymer modification with Calprene 501 is performed in twin-screw compounding with polypropylene or polystyrene. At addition levels of 5–15 wt%, the SBS phase improves notched Izod impact strength measured under ISO 180:2023. The styrene end-blocks partially compatibilise with polystyrene, while the polybutadiene midblock provides energy absorption. At levels above 20 wt%, melt viscosity and tensile modulus decline, and the material transitions to a thermoplastic elastomer blend rather than a toughened thermoplastic. Processing requires distributive mixing sections because the high molecular weight of the elastomer leads to coarse dispersion if only shear mixing is used. Extruder torque is typically 15–25% higher than with high-diblock SBS at the same throughput.

When Selecting Between Calprene 501 and Low-Viscosity SBS Grades

Grade selection is determined by the balance between melt flow, hardness, cohesive strength, and tack. Calprene 501 has a melt mass-flow rate of 2.0 g/10 min at 190 °C and 5 kg in accordance with ISO 1133-1:2022. High-diblock SBS grades with similar styrene content commonly exhibit melt flow rates of 10–25 g/10 min under the same test conditions. These grades flow more readily but have lower tensile strength and higher tack because the diblock chains act as boundary lubricants and reduce physical crosslink density. Oil-extended SBS grades contain 30–50 phr process oil and are softer. Shore A hardness may be 50–60, whereas Calprene 501 is typically 85 under ASTM D2240-15. The selection of Calprene 501 is appropriate when a formulation requires high cohesive strength, resistance to cold flow, and high hardness without incorporating extender oil. It is not appropriate when the process requires melt flow above 10 g/10 min or when the final article must retain high tack and low hardness.

The comparative data in Table 2 are typical values for product types and do not replace grade-specific datasheets. High-diblock products provide higher tack and easier melt flow but suffer from lower tensile strength and greater creep. Oil-extended grades reduce mixing torque and hardness but introduce extender-oil migration at service temperatures above 70 °C. Calprene 501 avoids extender migration because it is supplied without process oil. The low diblock content also contributes to improved tensile set and lower hysteresis at elevated service temperatures, but it narrows the melt-processing window and increases die swell.

Regulatory status must be verified for the intended application. Styrene-butadiene-styrene block copolymers are covered by FDA 21 CFR 177.1810 for certain food-contact uses, but end-use extraction testing is required. The product should not be assumed compliant with food-contact or medical requirements without application-specific migration data. Under EU Regulation (EC) No 1907/2006 (REACH), the manufacturer’s registration status and substance volume band must be confirmed from the safety data sheet. RoHS compliance is assessed under Directive 2011/65/EU. Calprene SBS 501 contains no intentionally added lead, mercury, cadmium, hexavalent chromium, PBB, or PBDE above the directive limits. Storage stability is improved by keeping the material below 25 °C and away from direct sunlight. Prolonged exposure to ozone or ultraviolet radiation causes surface crazing and loss of tensile strength. The grade is not recommended for continuous service in contact with strong oxidising acids, chlorinated solvents above 50 °C, or transition-metal stearates that catalyse oxidative chain scission.