Denka Asaflex SBC 815 is a styrene-butadiene block copolymer supplied by Denka Company Limited as a transparent, high-styrene-content thermoplastic. The material is produced by sequential anionic polymerization, resulting in a phase-separated morphology in which polystyrene end-blocks form discrete glassy domains that act as reversible physical crosslinks, while the butadiene midblock provides impact-energy dissipation and low-temperature ductility. The product is typically supplied in pellet form for injection moulding, sheet extrusion, and blow moulding. Because the grade is formulated for optical clarity and rigidity rather than soft-elastomer behaviour, its property profile occupies a different quadrant from oil-extended SBS thermoplastic elastomers. Published numerical data for this specific Denka grade are limited; the property envelope in Table 1 is therefore compiled from supplier-typical transparent SBC values and must be verified against the current certificate of analysis for the purchased lot.

Sequential anionic polymerization controls the block molecular weights and the styrene-to-butadiene ratio. Gel-permeation chromatography with polystyrene calibration is used to measure number-average molecular weight and dispersity of the soluble fraction; for commercial SBC resins of this class, number-average molecular weight is typically in the range of 60 000–150 000 g/mol, with dispersity of 1.05–1.20. The butadiene midblock contains 1,4-cis, 1,4-trans, and 1,2-vinyl microstructures; the 1,2-vinyl content influences the glass transition temperature of the polybutadiene phase. In transparent SBC grades, vinyl content is often controlled between 7–12 mol% to suppress crystallinity and maintain optical clarity. The polystyrene domain glass transition temperature is near 95–105 °C, while the polybutadiene phase glass transition temperature is near −80 to −60 °C. These two transitions produce the low-temperature impact resistance observed in notched impact testing at −30 °C.

Transmission electron microscopy of ruthenium-tetroxide-stained ultrathin sections shows the hard polystyrene domains as dark, dispersed rods or lamellae depending on styrene content. In high-styrene SBCs such as the 815 grade, the morphology is typically a co-continuous or lamellar arrangement rather than discrete spherical domains. Interdomain spacing is in the tens-of-nanometre range, below the wavelength of visible light, which preserves transparency. Stress-whitening occurs only when local tensile dilation voids the polybutadiene phase; higher styrene content reduces overall stress-whitening compared with lower-hardness SBC grades, but weld lines remain susceptible because of unfavourable domain orientation.

Thermomechanical Property Envelope for the 815 Grade

The following values define the expected performance envelope for transparent styrene-butadiene block copolymers of this type when conditioned at 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291:2008. The ranges are suitable for preliminary material selection and mould-filling simulation but are not batch-release limits. For acceptance sampling, the supplier’s certificate of analysis and the specified test method versions govern.

The hardness and flexural modulus values place the 815 grade at the rigid end of the transparent SBC spectrum. Users should not use the lower bound of the elongation range for safety-critical snap-fit design; ISO 527-2 tensile bars cut from injection-moulded plaques may overestimate ductility relative to moulded parts with weld lines or gate blush. For tensile property comparison, specimen geometry is critical. ISO 527-2 type 1A bars machined from annealed plaques may yield higher elongation at break than ASTM D638-14 type I bars moulded with end-gated tools because of orientation differences. Notched Izod data on 3.2 mm specimens should not be used alone to predict drop-impact performance on thin-wall containers; instrumented puncture testing at 2.0 m/s impact speed gives more relevant failure-energy data.

How Does Asaflex SBC 815 Differ from General-Purpose Polystyrene and Soft SBS Elastomers?

General-purpose polystyrene typically exhibits a notched Izod impact of 12–20 J/m under ASTM D256-23 and fails through craze propagation. Transparent styrene-butadiene block copolymers of this class shift the failure mode toward shear yielding because the polybutadiene domains cavitate and terminate crazes before catastrophic crack growth. Asaflex SBC 815 is therefore positioned as a transparent, ductile replacement for general-purpose polystyrene in thin-wall parts that must survive drop impacts. Soft SBS thermoplastic elastomers, by contrast, contain lower styrene content and are supplied with Shore A hardness values below 90; they are oil-extended or compounded for elastic recovery. The 815 grade is not an elastomer but a rigid thermoplastic with Shore D hardness in the 60–68 range, giving it closer affinity to styrene-butadiene copolymers and methyl methacrylate-butadiene-styrene grades. Direct numerical comparison with lower-numbered Asaflex grades is limited; selection between the 800-series grades should be made on the basis of side-by-side moulding trials using ISO 294-1:2023 multi-cavity tools and the current supplier datasheet.

Compared with bisphenol-A polycarbonate, Asaflex SBC 815 has lower density, lower continuous-use temperature, and lower notched impact strength but provides easier thin-wall flow and lower drying burden. Polycarbonate typical density is 1.20 g/cm³ with drying at 120 °C for 4 h; SBC parts of similar thickness are approximately 15–20 % lighter. Continuous-use temperature under load is lower for SBC because polystyrene domain softening begins near 70–90 °C, while polycarbonate can tolerate 120 °C. This positions the 815 grade for non-thermal packaging and consumer goods rather than hot-fill or autoclave applications. Within the Denka Asaflex range, the numerical suffix denotes differences in melt flow and hardness. The 815 grade is typically selected where higher stiffness and surface hardness are required relative to softer grades; published quantitative differences between the 800-series grades are limited and should be confirmed by side-by-side injection moulding with the same tool and cycle.

In injection-moulding trials on polished P20 tool steel cavities, Asaflex SBC 815 produces low-haze parts only when melt temperature, mould temperature, and injection speed are controlled within narrow limits. Predrying is mandatory when pellet moisture exceeds 0.05 wt% by ISO 15512:2019 Karl Fischer titration; typical desiccant-dryer settings are 60–70 °C for 2–4 h. At relative humidity above 60 %, hopper drying with a closed-loop desiccant system and a nitrogen purge prevents surface splay and haze. These defects appear as fan-shaped silver streaks radiating from the gate and are caused by vaporisation of water absorbed on the pellet surface during plasticating.

The melt-processing window for this grade is bounded at the low end by incomplete plasticating and at the high end by thermal degradation. Barrel settings typically follow a rising profile: rear 150–170 °C, centre 170–190 °C, front 180–200 °C, and nozzle 190–210 °C. Melt temperatures above 220 °C initiate polybutadiene oxidation and gel formation, while melt temperatures below 170 °C increase apparent viscosity to the point that thin-wall sections below 1 mm may exhibit short shots. Residence time at the upper melt temperature should not exceed 5–10 min; longer residence in the barrel leads to yellowing and black specks from degraded material at the screw root and check ring.

Melt Rheology, Screw Design, and Residence-Time Boundaries

The melt is strongly shear-thinning. For transparent SBC grades with similar styrene content, capillary rheometry per ISO 11443:2021 gives apparent viscosity in the range of 200–500 Pa·s at 200 °C and 100 s⁻¹, falling to below 100 Pa·s at 1000 s⁻¹. Mould-filling simulation should use cross-WLF or Bird-Carreau viscosity fits generated from three-temperature capillary data, not single-point melt-flow-rate values. Screw geometry on reciprocating injection machines should be a general-purpose polyolefin-type screw with L/D 20:1–25:1, compression ratio 2.5:1–3.0:1, and a check-ring radial clearance no greater than 0.05–0.10 mm. A shallow metering depth reduces overheating in the transition zone, where shear heating can locally exceed the barrel setpoint by 10–20 °C.

Injection speeds should be moderate to high, with fill times of 0.8–2.0 s for parts of 1–3 mm wall thickness; excessive shear rates above 10 000 s⁻¹ at the gate can surface-melt the polystyrene domains and create flow marks. Holding pressure is typically 20–50 MPa hydraulic pressure, depending on machine intensification ratio and part geometry. Mould temperatures of 20–40 °C are recommended; polished mould surfaces below Ra 0.1 µm are required for low haze. Gate dimensions should avoid abrupt transitions: a fan gate of 0.8–1.2 mm thickness reduces jetting and improves optical uniformity.

When Optical Clarity and Food-Contact Compliance Constrain Additive Selection

Additive packages in Asaflex SBC 815 must be chosen to avoid haze, migration, and regulatory non-compliance. Colour concentrates based on polystyrene or SBC carriers are added at 1–3 wt% and require predrying before blending. Slip and antiblock additives based on high-purity erucamide or silica can reduce blocking in extruded sheet at loadings of 500–2000 ppm, but excess migratory additives increase surface haze and may alter food-contact migration behaviour. For food-contact applications, the raw resin is typically supported by a supplier statement referencing FDA 21 CFR 177.1810 for styrene block polymers and Regulation (EU) No 10/2011 with specific migration limits; the converter must verify the final article under the applicable migration testing protocol. For medical device housings and fluid-contact parts, biocompatibility screening is governed by ISO 10993-5:2009 for cytotoxicity and ISO 10993-10:2021 for skin sensitisation and irritation; these tests are performed on the finished device, not on the raw pellet.

In transparent packaging and consumer-goods applications, Asaflex SBC 815 is processed into display cartons, cosmetic containers, and parts requiring a combination of clarity and drop-impact resistance. Sheet extrusion lines with 30:1–36:1 L/D barrier screws and polished chrome-plated chill rolls at 15–30 °C are used to produce sheet with thickness 0.3–2.0 mm; the sheet is subsequently thermoformed at 120–150 °C surface temperature. In injection moulding, the grade is used for transparent caps, lids, and housings where weld-line strength and gate blush are controlled through sequential valve gating and hot-runner manifolds with 220–240 °C nozzle setpoints. For medical diagnostic components, the material is often specified because its density of approximately 1.00–1.02 g/cm³ allows lightweight devices; however, sterilisation by ethylene oxide or gamma radiation must be validated for the specific grade and part geometry. Published data for gamma-sterilised Asaflex 815 is limited; converters should request radiation-stability data or conduct ISO 11137-1:2006 dose-setting studies.

Gelation and Oxidation Occur Above the 220 °C Melt-Temperature Ceiling

The main operational boundary for Asaflex SBC 815 is thermal-oxidative degradation of the butadiene midblock. At melt temperatures above 220 °C, gel particles form through crosslinking of polybutadiene unsaturation; these gels appear as fisheyes in extruded sheet and as black specks in injection-moulded parts. The degradation is autocatalytic and is accelerated by dissolved oxygen, residual peroxide from colour concentrates, and stagnant material in hot-runner dead spots. The material should not be purged with cellulose-based purging compounds above 230 °C, and regrind content should be limited to 20–40 % by weight unless optical and mechanical property data for the specific regrind fraction are available. Contact with amines, strong bases, and transition-metal stearates should be avoided because these compounds can catalyse oxidative chain scission or discolouration. The grade is not recommended for continuous service above 60–70 °C under load, because the polystyrene domains soften near the Vicat softening temperature and creep resistance decreases. For outdoor applications, the butadiene segment requires UV stabilisation; unpigmented or transparent natural parts exposed to QUV-A 340 nm testing per ASTM D4329-21 may yellow before 500 h unless compounded with a suitable UV absorber and hindered amine stabiliser at loadings specified by the stabiliser supplier.