En Chuan Taiwan SBS 701 is a linear styrene-butadiene-styrene thermoplastic elastomer produced by En Chuan Chemical Industries Co., Ltd., Taiwan. The grade is supplied as an unfilled triblock copolymer with a bound styrene content of approximately 30 wt%; the balance is a polybutadiene midblock with mixed 1,4-cis and 1,4-trans microstructure. The product does not require vulcanization because the polystyrene end-blocks phase-separate into glassy domains that act as physical crosslinks below the polystyrene glass transition of approximately 95 °C. The standard grade is supplied as porous, non-dusting pellets and contains a non-staining phenolic antioxidant plus a mineral antiblock to reduce silo agglomeration. SBS 701 is distinguished from higher-styrene grades such as SBS 601 by lower Shore A hardness, lower tensile modulus, and higher elongation at break, but also by lower resistance to aromatic and aliphatic oils. It is distinguished from styrene-ethylene-butylene-styrene grades by superior hydrocarbon solubility and lower cost, but by reduced UV and oxidative stability. Compared with lower-viscosity SBS grades, SBS 701 provides higher melt viscosity and greater cohesive strength in bitumen and adhesive networks. Where direct grade-to-grade numerical data are not published for the current production campaign, the comparison is based on class-level linear SBS morphology; the manufacturer’s lot certificate remains the controlling document.

Differential scanning calorimetry per ASTM D3418-21 typically records a polybutadiene midblock glass transition near -85 °C and a polystyrene domain glass transition near 95 °C. The breadth of the polystyrene transition depends on domain size and thermal history. The low glass transition of the midblock is responsible for low-temperature flexibility in modified bitumen and footwear, while the polystyrene domains provide high-temperature stiffness and elastic recovery.

What Changes When SBS 701 Replaces SBS 601 in High-Shear Bitumen Dispersion?

In high-shear bitumen modification, 4–6 wt% SBS 701 is dispersed into 60/70 penetration base bitumen at 180–190 °C using a rotor-stator colloid mill with rotor diameter 250 mm and tip speed above 18 m/s. The polybutadiene midblock swells in the maltene phase by 3–8 times its initial volume, while the polystyrene domains remain associated at temperatures below 200 °C. The resulting network raises the ASTM D36 softening point by 20–30 °C and improves ASTM D6084 elastic recovery at 25 °C to more than 80% when the diblock content is below 12 wt%. Multiple stress creep recovery per ASTM D7405-20 at 0.1 kPa and 3.2 kPa shows non-recoverable creep compliance Jnr below 0.5 kPa⁻¹ at 76 °C for a well-stabilized 5 wt% SBS 701 binder; a Jnr above 1.0 kPa⁻¹ indicates insufficient polymer network or overmixing.

If SBS 701 is substituted for a higher-styrene grade such as SBS 601, the lower bound styrene content reduces high-temperature stiffness at equal polymer loading; operatives typically increase SBS 701 dosage by 0.5–1.0 wt% to match the same softening point while retaining improved low-temperature flexibility. The substitution also reduces low-temperature limiting values in bending beam rheometer tests per ASTM D6648-16, which is beneficial for cold-climate crack resistance.

Storage stability of modified binder is evaluated after 72 h at 163 °C in a sealed tube per EN 13399. Lots with diblock content above 12 wt% or high free polystyrene oligomers develop top-bottom softening point differences greater than 2.5 °C. Production-scale experience in asphalt terminals shows that 7 wt% SBS 701 without extender oil raises Brookfield viscosity at 135 °C per ASTM D4402 above 4,000 mPa·s; this imposes jacketed transfer lines and short residence times in the mixing vessel. Sulfur stabilization at 0.1–0.2 wt% is effective only after the polymer has fully digested; premature sulfur addition during the swelling phase causes gel particles and an irreversible viscosity rise.

High-shear mixer torque analysis shows a peak after 20–30 min of digestion, followed by a plateau as the phase-inverted network forms. A 20 t batch with 6 wt% SBS 701 at 3,000 rpm requires 90–120 min for complete dispersion; lower tip speeds produce polymer-rich skins on the vessel wall and increase the ASTM D4402 viscosity coefficient of variation beyond 10%.

Footwear compounding with SBS 701 is carried out on intermeshing co-rotating twin-screw extruders with L/D ratios of 34:1 to 44:1 and screw speeds of 250–300 rpm. Barrel temperatures are set at 160–190 °C from feed throat to die; at melt temperatures above 210 °C the unsaturated butadiene midblock undergoes oxidative chain scission and crosslinking, producing melt-pressure variation greater than ±5% and surface defects in injection-molded outsoles. On injection molding machines with clamp force 120–180 metric tons, nozzle temperature 170–190 °C, and injection pressure 60–90 MPa, 8 mm wall sections fill in cycle times of 35–60 s. Formulations containing 10–30 wt% general-purpose polystyrene and 5–15 wt% naphthenic oil yield hardness values from 55 to 70 Shore A; pre-drying at 80 °C for 2 h is required when silo moisture exceeds 0.1 wt% because surface moisture creates voids. Microcellular sole formulations with azodicarbonamide blowing agent at 0.5–2.0 wt% require shut-off nozzles and holding pressure below 15 MPa to prevent densification. Amine-based antiozonants should be avoided in UV-exposed footwear because they can accelerate oxidative attack of the butadiene midblock.

Solvent-Borne Adhesive Viscosity and Polymer Concentration Limits

For solvent-borne adhesive operations, SBS 701 is dissolved in toluene, methyl ethyl ketone, or ethyl acetate at 20–25 wt% solids for sprayable assembly adhesives. The linear architecture produces lower solution viscosity than a radial SBS of equivalent molecular weight; at 25 wt% solids in toluene a Brookfield RV spindle 7 at 20 rpm and 25 °C typically reads 3,000–6,000 mPa·s. At 40 wt% solids the viscosity typically exceeds 15,000 mPa·s, limiting airless spray application; HVLP systems require dilution to 20 wt% solids or lower. Addition of 5–10 wt% rosin ester tackifier on polymer solids increases loop tack per ASTM D6195 and reduces room-temperature film shrinkage, but reduces heat resistance. 180° peel adhesion on stainless steel per ASTM D903 is lower for low-styrene SBS films than for 40 wt% styrene grades, although adhesion to low-density polyethylene improves because the softer film wets the substrate surface. Coatings deposited at 200 g/m² dry weight and dried at 25 °C and 50% RH exhibit open times of 15–30 min. Contact adhesives should be stored above 5 °C; cold storage promotes phase separation of the polystyrene domains and requires re-mixing at 40 °C for 2 h. Free isocyanate crosslinkers require moisture-free solvents and immediate coating because moisture-cure viscosity rise becomes significant within 30–60 min.

When Extruder Residence Time Exceeds 180 Seconds at Melt Temperatures Above 200 °C

At melt temperatures above 200 °C, thermal degradation of the butadiene midblock proceeds through free-radical chain scission and crosslinking. Residence time above 180 s in a production extruder increases gel content and generates black specks in transparent compounds. A melt-pressure transducer ahead of the screen pack should remain below 140 bar; a pressure increase of 10–15% over the stable baseline indicates gel accumulation on screens with 100–150 µm mesh. Before shutdown, a polystyrene-rich purge compound should be run at 180–200 °C; PVC or chlorinated compounds must not remain in the barrel because thermal degradation releases hydrogen chloride and attacks the unsaturated midblock.

Rotational rheometry at 180 °C in oscillatory shear per ISO 6721-10 shows a storage modulus plateau at angular frequencies below 0.1 rad/s. A low-diblock triblock exhibits tan δ below 1.0 at 0.1 rad/s; an increase indicates a diblock-rich or low-molecular-weight lot and correlates with lower bitumen elastic recovery. Batch-to-batch melt mass-flow rate variation greater than ±15% under ASTM D1238 should trigger barrel temperature adjustment of ±5 °C and screw back pressure adjustment of ±5 bar to maintain consistent shot weight.

Quality control laboratories receiving SBS 701 verify melt mass-flow rate per ASTM D1238 at 200 °C with 5 kg, moisture per ASTM D5668 at 160 °C to constant weight, and hardness on compression-molded plaques per ASTM D2240. A plaque molded at 180 °C for 5 min and conditioned at 23 °C and 50% RH for 24 h should give Shore A readings of 68–75; readings below 65 suggest oil contamination, high diblock content, or insufficient mold pressure. Published data for this specific configuration is limited; the incoming lot data must be compared with the producer’s certificate of analysis before production adjustments.