Oil-resistant sole stock formulated from NBR 3305E and a high styrene-butadiene copolymer resin is used for occupational footwear outsoles where swelling in ASTM D471 reference oils and ISO 1817 test liquids must remain below specification limits while Shore A hardness is maintained above 60. The base polymer NBR 3305E has nominal bound acrylonitrile content of 33%, a Mooney viscosity ML 1+4 at 100 °C of approximately 50 MU, and a specific gravity of approximately 0.98 to 1.00; published data for this exact grade in binary high-styrene-resin blends is limited, so formulation boundaries are derived from general emulsion NBR compounding practice and verified by rheometer, oil immersion, abrasion, and tear testing. High styrene resin is incorporated to increase hardness, reduce abrasion loss, improve dimensional stability in injection-molded cup soles, and replace process oil or carbon black on a cost-performance basis, but its influence on oil swelling, tear, low-temperature flex fatigue, and cure rheology is not linear and must be evaluated at addition levels of 10 phr, 20 phr, 30 phr, and 40 phr.

Which high styrene resin grades enter the NBR 3305E soling compound without phase inversion?

Compatibility between NBR 3305E and a high styrene resin is governed by the bound styrene content, melt viscosity, gel level, and residual unsaturation of the resin. High styrene-butadiene copolymers with bound styrene from 60% to 75% and gel content below 2% are preferred for fine dispersion in NBR because they retain some butadiene miscibility with the NBR matrix. Grades above 80% bound styrene raise compound hardness efficiently but create styrene-rich domains visible by phase contrast microscopy when mixed above 25 phr; this can reduce tear strength and increase compression set. The glass transition temperature of the high styrene resin is typically between 30 °C and 60 °C depending on styrene content and butadiene microstructure, while the NBR phase has a glass transition near −25 °C; this creates a broad tan δ damping plateau useful for energy absorption but also increases low-temperature stiffness. Solubility parameter mismatch should remain below 1.5 MPa^0.5 for fine domain dispersion; resins with high 1,2-vinyl content can shift the resin solubility parameter closer to NBR and reduce phase separation. The selected resin should be supplied as powder or free-flowing pastilles, with melt mass-flow rate measured according to ISO 1133-1 at 190 °C and 5 kg unless the supplier specifies alternative conditions, and residual styrene monomer should be controlled according to REACH and workplace exposure limits.

Two-stage mixing in an intermeshing internal mixer with NBR 3305E masticated alone for 60 s to 90 s before carbon black and high styrene resin addition produces better resin dispersion than simultaneous loading. Fill factor is maintained at 0.70 to 0.80, rotor speed is increased from 30 min⁻¹ to 40 min⁻¹ after black incorporation, and the masterbatch is dropped at 120 °C to 135 °C to avoid sulfur prevulcanization in the second stage. The high styrene resin is added with the first half of carbon black because the resin melts between 80 °C and 115 °C and functions as a dispersing aid for carbon black in the NBR phase. N330 carbon black according to ASTM D1765 at 50–60 phr is the primary filler, and a precipitated silica with BET surface area 170–190 m²/g may be added at 10–15 phr with a silane coupling agent to raise tear resistance. If silica is used, the compound must be pre-dried when ambient relative humidity exceeds 60% because moisture lowers silane coupling efficiency and increases injection-mold fouling. Continuous mixing in a twin-screw extruder with L/D 48:1 is possible for high-throughput sole compound production if barrel zones are held at 70 °C to 120 °C and screw speed is reduced to limit shear heating; however, the internal mixer remains the preferred route because of higher filler tolerance and better lot-to-lot torque control.

Oil Absorption, Extraction, and Volume Swell Under ASTM D471 and ISO 1817

Volume swell measurements are performed according to ASTM D471 and ISO 1817 using reference oils IRM 901 and IRM 903 at 100 °C for 72 h unless end-use requirements specify 23 °C for 168 h. NBR with 33% ACN provides an intermediate barrier to aliphatic oils; expected volume swell in IRM 903 is lower than an NBR grade with 28% ACN and higher than a grade with 45% ACN. High styrene resin reduces overall volume swell in aliphatic oils because the aromatic styrene blocks are thermodynamically less favourable to aliphatic oil penetration than polybutadiene; however, the resin also dilutes the nitrile content of the compound, so net oil resistance is controlled by the ratio of high styrene resin to NBR. In compounds where high styrene resin replaces carbon black or process oil, volume swell may remain stable or decrease; when the resin replaces NBR at constant filler loading, oil resistance may decline at loadings above 30 phr because the nitrile mass per cubic centimetre of vulcanizate is reduced. Extraction of unbound resin, process oil, and low-molecular-weight NBR fractions is measured by ISO 1407 or the extraction section of ASTM D297; high styrene resin with a gel content below 2% can be partially extracted by hot toluene, which falsifies oil resistance if extraction is performed after oil immersion. The formulation should use a low-migration plasticizer such as trimellitate or chlorinated paraffin when oil extraction limits are below 10%; dioctyl phthalate is not recommended where EN ISO 20345 and ISO 20344 outsole oil resistance is evaluated because phthalate extraction elevates mass loss and can affect hardness retention.

Vulcanization kinetics are measured on a moving die rheometer according to ISO 6502 at 160 °C. A sulfur-accelerated system based on 1.8–2.2 phr sulfur, 1.2–1.8 phr N-cyclohexylbenzothiazole-2-sulphenamide, 0.3–0.6 phr diphenylguanidine, 4–5 phr zinc oxide, and 1–2 phr stearic acid is preferred for sole stock. The high styrene resin consumes part of the cure system through its residual butadiene unsaturation, and amino-type antioxidants or amine-based accelerators should be avoided in peroxide-cured versions because they deplete peroxide radicals and reduce crosslink density; in sulfur-cured compounds, certain amine antidegradants can accelerate prevulcanization when high styrene resin is present. The optimum cure time at 160 °C is typically between 90 s and 150 s for injection-molded soles, but the exact t_c90 must be measured for each resin loading because the high styrene phase can act as a heat sink and delay thermal equilibrium at the centre of a 2 mm outsole. Scorch safety t_s2 should exceed 2 min at 120 °C for injection molding with hot-runner systems; otherwise the compound will prevulcanize in the runner and produce surface flow lines and knit-line weaknesses.

When high styrene resin loading exceeds 30 phr in injection-molded outsoles

At loadings above 30 phr, process behavior changes from a softened NBR compound to a stiff, shear-sensitive blend that can exhibit higher barrel torque, increased backpressure, and shorter scorch time at the same melt temperature. Injection molding machines with clamp force from 150 t to 250 t are used for multi-cavity cup soles; barrel zones are set at 50–80 °C in the feed, 70–90 °C in the compression zone, and 80–100 °C at the nozzle, with mold temperature held at 150–165 °C. If high styrene resin loading exceeds 30 phr, melt viscosity at the nozzle may deviate by more than 20% relative to the control, requiring a reduction in screw speed and an adjustment of injection pressure; published data for this specific configuration is limited, and processing windows must be determined by mold-fill studies using ISO 1133-1 and injection mold flow simulation. The resin-rich phase can cause anisotropic shrinkage of 0.8% to 1.4% in the longitudinal direction compared with 0.5% to 0.8% across flow; this warpage can be reduced by increasing mold residence time and reducing gate injection speed. The high styrene resin should not be exposed to melt temperatures above 200 °C because thermal degradation can generate styrene monomer and increase volatile organic compound emissions; at production scale, nozzle and hot-runner thermocouple readings should not exceed 120 °C in sulfur-cured compounds. Batch-to-batch variation in high styrene resin melt flow index is a frequent source of injection molding instability observed on production lines; incoming resin lots should be checked for styrene content by Fourier transform infrared spectroscopy and for melt mass-flow rate by ISO 1133-1 at 190 °C and 5 kg.

Failure Mode Mapping in DIN Abrasion and Trouser Tear

DIN abrasion loss is measured according to ISO 4649 with a rotating drum method, and trouser tear resistance is measured according to ISO 34-1. High styrene resin up to 25 phr generally improves abrasion resistance by increasing hardness and delaying microcutting wear, but above this concentration the tear strength can drop because the styrene-rich domains act as stress concentrators. Failure modes observed in tensile-fracture surfaces include brittle domains at high styrene resin loadings and phase separation boundaries visible after osmium tetroxide staining in scanning electron microscopy; a dispersed domain size below 1 µm is associated with acceptable tensile retention, whereas domains above 5 µm indicate incomplete mixing. The compound should be evaluated according to ASTM D624 die C for tear resistance and ISO 4649 for abrasion loss; if the abrasion loss exceeds 150 mm³ and the tear strength falls below 30 kN/m, the resin loading or mixing protocol must be adjusted. High styrene resin also affects flex fatigue: compounds with more than 30 phr high styrene resin often show earlier crack initiation in repeated flexing at −10 °C compared with unfilled or carbon-black-only NBR. The selection of a high-styrene resin with medium bound styrene from 60% to 70% and low gel content can maintain tear and flex performance better than a high-styrene grade with bound styrene above 80%.

Production-scale quality control includes incoming resin melt mass-flow rate, Fourier transform infrared styrene content, moisture, and particle size distribution; NBR 3305E lot Mooney viscosity is logged against mixer torque and drop temperature to detect batch drift. The final compound is tested before release by ISO 6502 rheometer curves, ASTM D2240 hardness after 15 s, tensile strength and elongation at break by ISO 37, ISO 4649 abrasion loss, and ISO 1817 oil swell in IRM 903 after 72 h at 100 °C. If oil swell exceeds the limit, the high styrene resin content is reduced or the NBR replaced with a higher-ACN grade such as 35% ACN; however, this raises glass transition temperature and reduces low-temperature flexibility. The compound is not released if hardness varies more than ±3 Shore A within a batch, if t_s2 falls below 1.5 min at 120 °C, or if DIN abrasion loss exceeds the internal specification for the sole stock.