Chimei KIBITON® SBS PB-5502 is a linear styrene-butadiene-styrene block copolymer supplied in porous pellet form. The grade is produced by sequential anionic polymerization; the resulting triblock architecture contains polystyrene endblocks that vitrify below approximately 95 °C and a polybutadiene midblock that remains elastomeric between the glass transition of the polybutadiene phase near −85 °C and the service limit set by styrene-domain softening. The nominal bound styrene content is 31.5 wt%, and diblock content is controlled at ≤10 wt%. Specific gravity is 0.94 when measured according to ASTM D792-20. Melt flow rate is 3.5 g/10 min at 190 °C/5 kg per ASTM D1238-20. Compression-molded plaques aged for 24 h at 23 °C and 50% RH exhibit tensile strength of 18.0 MPa, elongation at break of 700%, and Shore A hardness of 70 when tested per ASTM D638-14 and ASTM D2240-15e1. Volatile matter is specified at ≤0.5 wt%, and ash at ≤0.3 wt%. These characteristics make the grade suitable for pressure-sensitive adhesives, bitumen modification, and blends with polyolefins or polystyrene. The absence of extender oil distinguishes PB-5502 from oil-extended SBS grades and allows independent adjustment of compound hardness and oil dosage.

Thermal degradation, shear thinning, and processing-boundary behaviour in molten PB-5502

The melt rheology of PB-5502 is pseudoplastic. Capillary rheometry at 180 °C shows apparent shear viscosity decreasing from approximately 1,200 Pa·s at 10 s⁻¹ to 120 Pa·s at 10³ s⁻¹; this response supports high-speed extrusion without the excessive pressure drop encountered with higher-molecular-weight radial grades. At 200 °C, the zero-shear viscosity is low enough for solvent-free hot-melt coating; however, residence times above 20 min at 200 °C produce measurable gel formation unless a hindered phenol antioxidant and phosphite stabilizer are present. Thermal degradation onset of the polybutadiene midblock is observed at 300 °C by thermogravimetric analysis under nitrogen, but oxidative degradation begins below 180 °C in air. In twin-screw compounding, barrel zones are therefore set to 150 °C/165 °C/175 °C/180 °C from feed to die, with screw speed from 200 rpm to 400 rpm on a 40:1 L/D co-rotating machine. Melt pressure at the die is typically 2–5 MPa depending on throughput. Pre-drying at 70 °C for 2–4 h is required only if sacks have been opened above 60% RH; otherwise, pellets may be processed without drying. Because PB-5502 contains no oil extender, it produces less fuming and die deposit than oil-extended SBS under identical conditions.

Solvent-borne pressure-sensitive adhesive systems based on PB-5502 use toluene or toluene/ethyl acetate mixtures at 25–35 wt% solids. The solution viscosity of a 25 wt% toluene solution is 400–600 mPa·s per ASTM D2196-20, a range that supports reverse-roll and comma coating at line speeds of 80–120 m/min. Tackification with hydrogenated glycerol rosin ester at 60–100 phr increases the tan δ peak of the polybutadiene phase from −80 °C to approximately −30 °C; loop tack measured by ASTM D6195-03(2011) rises from 4 N/25 mm to 12–18 N/25 mm. Peel adhesion on stainless steel per ASTM D3330/D3330M-04(2018) is reported at 12–20 N/25 mm for a 25 µm dry film. Cohesive failure is controlled by crosslinking with 0.5–1.0 phr cumene hydroperoxide at 150 °C for 5 min or by electron-beam irradiation at 30–40 kGy; after crosslinking, static shear adhesion at 1 kg per ASTM D3654/D3654M-06(2019) exceeds 24 h. PB-5502 maintains lower solution viscosity than PB-5702 at equal solids, which reduces coating defects at high line speed, but it yields slightly lower cohesive strength than PB-5702 after crosslinking. Hot-melt adhesives can be formulated by mixing with 100–200 phr C5 tackifier and 20–40 phr naphthenic oil in a sigma-blade mixer at 160–170 °C; the resulting adhesive exhibits open time of 30–60 s and bond strength on polypropylene of 2–4 MPa.

When KIBITON® PB-5502 is compounded into bitumen at elevated temperatures

Polymer-modified bitumen is prepared by dispersing 3–5 wt% PB-5502 into 100 parts of penetration-grade bitumen using a high-shear rotor-stator mixer at 2,000–4,000 rpm. Mixing temperature is maintained at 170–180 °C for 60–90 min; below 160 °C swelling of the polystyrene domains is incomplete, while above 190 °C the polybutadiene segments undergo chain scission. The SBS phase absorbs maltenes and forms a continuous interpenetrating network when base-bitumen compatibility is sufficient. Ring-and-ball softening point rises from 48 °C to 68–75 °C per ASTM D36/D36M-14(2020). Penetration at 25 °C decreases from 80–100 dmm to 50–65 dmm per ASTM D5/D5M-20. Elastic recovery at 25 °C exceeds 70% when tested according to EN 13398. Low-temperature flexibility, measured as Fraass breaking point per EN 12593, remains below −15 °C at the 5 wt% dosage. Storage stability by the tube test according to EN 13399 shows a softening point difference of ≤2.5 °C after 72 h at 180 °C for compatible base bitumens; aromaticity index and asphaltene content of the base bitumen must be controlled. The lower diblock content of PB-5502 relative to PB-5302 reduces the fraction of unattached SBS molecules that can migrate to the surface during hot storage, which improves stability but raises minimum mixing torque. Published data for specific bitumen-source compatibility is limited; laboratory-scale tube tests are required for each base bitumen.

Injection-molded TPE compounds are produced by blending PB-5502 with polypropylene and, in some cases, 10–20 wt% general-purpose polystyrene or polyphenylene oxide. For a 70:30 PB-5502:polypropylene blend, hardness increases to 85 Shore A, compression set at 70 °C after 22 h is 35% per ASTM D395-18 Method B, and tensile strength is 15–20 MPa per ASTM D638-14. The compound is processed on a reciprocating screw injection machine with a compression screw of 20:1 L/D, shutoff nozzle, and mold clamp force of 100–150 t. Barrel temperatures are set at 160 °C, 175 °C, 185 °C, and 190 °C; injection pressure is 50–70 MPa; hold pressure is 30–40 MPa for 5–10 s. Mold temperature is kept at 20–35 °C to accelerate solidification of the polystyrene domains and prevent flash. Thin-wall sections down to 1.0 mm can be filled when the gate design avoids long flow lengths; weld-line tensile strength is reduced by 15–25% relative to the bulk and should be accounted for in ribbed sole patterns. Compared with PB-5702, PB-5502 has a lower melt viscosity and shorter cycle time, but the molded article has lower hardness and lower tensile modulus.

What differentiates PB-5502 from adjacent KIBITON grades and from radial SBS?

Within the KIBITON linear SBS series, PB-5502 is an intermediate-styrene grade. PB-5302 is typically supplied with a lower bound styrene content in the 30 wt% region and a higher diblock fraction; this reduces melt viscosity and increases pressure-sensitive tack, but lowers tensile strength and increases hot-melt stringing. PB-5702 is a higher-styrene grade, often near 40 wt%, which raises Shore A hardness and cohesive strength at the expense of solution viscosity and low-temperature flexibility. PB-5502 at 31.5 wt% bound styrene and ≤10 wt% diblock content provides a balance for adhesives and bitumen modification. In comparison with oil-extended SBS, PB-5502 is supplied without extender oil, so formulators control oil type and loading independently; this is relevant for low-VOC and low-migration applications. Compared with radial SBS architectures, the linear structure of PB-5502 gives lower die swell and lower melt viscosity at equivalent molecular weight. Radial SBS grades can develop higher tensile strength and lower compression set because of branched network connectivity, but they may require higher processing torque and shear heating. The differences should be evaluated with the supplier’s certificate of analysis, because published data for adjacent grades is limited and lot-to-lot variation in melt flow and diblock content occurs within specified control limits.

Regulatory status is application-specific. The base polymer is supplied with a REACH registration under Regulation (EC) No 1907/2006; downstream formulators are responsible for assessment of the final article. RoHS Directive 2011/65/EU screening for lead, cadmium, mercury, hexavalent chromium, PBB, and PBDE is reported at the raw-polymer level. For adhesive applications involving indirect food contact, compliance with 21 CFR 175.105 must be demonstrated on the finished adhesive; PB-5502 is not cleared as a direct food-contact substance. The grade should not be used in medical-device elastomers without biocompatibility testing according to ISO 10993-1:2018. Storage in sealed containers below 30 °C is recommended; prolonged ultraviolet exposure degrades the polybutadiene midblock and raises gel content. The polymer is incompatible with strong oxidizing acids and chlorinated solvents at elevated temperatures; prolonged contact with paraffinic oils above 50 phr may reduce tensile strength because of selective migration into the polybutadiene phase.