Oil-extended SBR

Oil-extended styrene-butadiene rubber is an emulsion-polymerized SBR in which a defined quantity of paraffinic, naphthenic, or treated distillate aromatic extract oil is introduced into the latex before coagulation. The grade commonly designated ESBR 1712 is a cold-polymerized type with a bound styrene mass fraction of 23.5%, an oil loading of 37.5 phr, and a Mooney viscosity ML(1+4) at 100 °C in the range of 46–56 when tested according to ASTM D1646. ESBR 1721 is a higher-styrene alternative with a bound styrene target near 40% and the same 37.5 phr oil loading; it is selected where higher glass transition temperature and increased wet-skid contribution are required. The oil phase reduces compound Mooney viscosity, lowers cold-flow, permits higher filler loading, and modifies green strength. Evaluation of oil-extended SBR is governed by ISO 2322:2009 and ASTM D3185, which define standard test formulations, mixing schedules, and vulcanization conditions for laboratory comparison.

How Does Oil Extension Alter Rheology and Mixing Behaviour?

The addition of 37.5 phr oil reduces the uncompounded polymer Mooney viscosity by approximately 50% relative to the non-oil-extended precursor, although the reduction depends on oil aniline point, aromatic carbon content, and polymer-oil solubility parameter compatibility. In a 270 L tangential internal mixer, oil-extended ESBR is typically charged with carbon black, zinc oxide, stearic acid, and antioxidant before the first ram lift, with ram pressure maintained between 0.45 MPa and 0.60 MPa. The practical upper boundary for dump temperature lies between 145 °C and 160 °C; exceeding 160 °C increases the probability of accelerator decomposition and scorch. Sulfur and sulfenamide accelerators are therefore added in a second pass after the masterbatch has cooled below 100 °C. Carbon black dispersion improves as oil diffuses into the polymer phase, but excessive free oil can produce mill roll sticking, reduced green tack, and non-uniform filler wetting. Batch-to-batch viscosity variation of ±5 Mu at ML(1+4) 100 °C is observable on production-scale equipment when latex oil addition is not tightly controlled.

For molded goods, the material is commonly vulcanized with sulfur at 1.5–2.0 phr and a sulfenamide accelerator such as N-tert-butyl-2-benzothiazole sulfenamide at 1.0–1.5 phr. Compression molding of pads and grommets is performed at 150 °C to 170 °C under clamp pressures of 10 MPa to 25 MPa, while transfer molding of edge seals requires adequate flow at injection temperatures below the scorch boundary. The cured compounds are tested for tensile strength under ISO 37 or ASTM D412, with typical filled formulations yielding 18–24 MPa tensile strength and elongation at break between 300% and 550% depending on carbon black surface area. Abrasion resistance of conveyor belt covers is evaluated according to ISO 4649; published values for oil-extended SBR cover compounds frequently fall between 120 mm³ and 200 mm³, with higher carbon black structure reducing volume loss at the expense of extrusion viscosity.

When TDAE Oil Replaces Aromatic Oil in Vacuum-Extruded Profiles

Vacuum-calibrated extrusion of sealing profiles and rubber sheets uses pin-barrel cold-feed extruders with screw diameters from 60 mm to 120 mm and typical length-to-diameter ratios of 16:1. When treated distillate aromatic extract oil is substituted for a conventional aromatic extender oil to satisfy European Union polycyclic aromatic limits under Regulation (EC) No 1907/2006, Annex XVII entry 50, the compound may exhibit slightly higher die swell and lower extrusion rate at equivalent screw speed because of changes in polymer-oil interaction. The restriction requires that extender oils placed on the market do not contain more than 1 mg/kg benzo[a]pyrene or more than 10 mg/kg of the sum of eight specified polycyclic aromatic hydrocarbons. TDAE oil retains adequate compatibility with SBR but may require adjustment of the processing aid package to avoid surface tearing at haul-off speeds above 30 m/min. Extrudate porosity becomes a critical defect when melt temperature at the die exceeds 135 °C; vacuum levels above 0.08 MPa gauge are normally applied to remove volatiles and prevent internal void formation. Production extrusion trials with 37.5 phr TDAE-extended ESBR 1712 have shown acceptable dimensional stability when the screw temperature profile is reduced by 5–10 °C relative to aromatic oil grades, though published data for this specific configuration is limited and must be verified on the target line.

Oil-extended SBR is also used in tire retread compounds, shoe soling, floor mats, automotive underlayment, and vibration damping pads. In vibration damping pads, the glass transition temperature of ESBR 1712 near -50 °C provides mechanical loss control in the -20 °C to 10 °C service range; damping performance is measured as loss factor according to ISO 4664-1. The product is not recommended for continuous exposure to mineral oil, brake fluid, or strong oxidative environments without protective co-curing with a higher-acrylonitrile or higher-EPDM phase.

Representative Grade Specifications and Cure System Compatibility

The following table summarises representative public supplier datasheet ranges for three commercial oil-extended ESBR grades. These values are target ranges and differ by production site, oil batch, and polymerization controls.

Sulfur cure compatibility is governed by the unsaturated butadiene backbone, and conventional cure rates are achieved with sulfur levels from 1.4 phr to 2.2 phr. Oil-extended grades do not fundamentally alter the vulcanization chemistry, but the oil dilutes the reactive diene concentration; therefore crosslink density at equal sulfur loading is lower than in non-oil-extended SBR. Cure curves obtained with an oscillating disc rheometer under ISO 6502 or ASTM D2084 typically show a minimum torque reduction of 15–25% relative to a non-oil-extended SBR of equivalent bound styrene. Scorch safety measured as tS2 at 160 °C is commonly 4–8 min for a sulfenamide-accelerated system, but this window narrows when acidic aromatic oil impurities accelerate the decomposition of sulfenamide accelerators. The use of naphthenic oil grades such as ESBR 1778 improves low-temperature flexibility and reduces color formation in light-filled formulations, though it can reduce tensile modulus at equal filler loading because of lower polymer-oil interaction energy.

Oil-Extended ESBR Grades Exhibit Predictable Differences from Solution SBR and EPDM

Compared with solution-polymerized SBR, oil-extended ESBR contains a broader molecular weight distribution and higher branching, which contributes to reduced extrusion swell and improved mill stability but also higher heat build-up in dynamic applications. Solution SBR with the same bound styrene content and lower vinyl content generally exhibits lower hysteresis and better abrasion resistance in tire treads; however, solution SBR is typically supplied without extender oil, so compound viscosity must be adjusted by adding free oil during mixing. Oil-extended ESBR therefore offers a direct route to high oil loading without the additional mixing time required for oil absorption in dry solution SBR. Compared with EPDM, oil-extended ESBR has a branched unsaturated hydrocarbon backbone that provides higher mechanical strength at room temperature but inferior ozone resistance, heat aging, and phosphate-ester fluid resistance. EPDM requires peroxide or sulfur-donor cure systems for saturated-backbone stability, whereas oil-extended ESBR crosslinks readily with conventional sulfur accelerators.

The comparative table reflects typical filled compound properties rather than neat polymer values; tensile strength is determined according to ISO 37 or ASTM D412, and glass transition temperature by differential scanning calorimetry under ASTM D7426. The lower tensile strength of EPDM at equal filler loading is offset by superior retention after hot-air aging at 125 °C for 168 h, where oil-extended ESBR typically loses 30–50% of original elongation due to oxidative crosslinking and chain scission. Solution SBR can be functionalized at the chain end to improve filler interaction and reduce rolling resistance of passenger tire treads; oil-extended ESBR lacks this chain-end functionalization and therefore shows higher hysteresis in tire applications unless compensated with low-surface-area carbon black or reduced oil level.

In tire tread compounding, ESBR 1712 is combined with 60–80 phr of N220 or N330 carbon black, 2–5 phr antioxidant, 1.5–2.0 phr sulfur, and 1.0–1.5 phr sulfenamide accelerator. The compound is processed through a three-pass mixing sequence to avoid destructive temperature rise. Passenger tread vulcanization at 150 °C to 165 °C produces a storage modulus dominated by filler networking and crosslink density. The product provides acceptable abrasion resistance and wet skid performance for passenger and light truck treads, but it does not match the rolling resistance potential of functionalized solution SBR grades. In applications requiring continuous exposure to ozone above 50 pphm at 40 °C, antiozonant loading must exceed 3 phr and surface cracking resistance must still be confirmed under ISO 1431-1. The operational boundary for oil-extended ESBR is therefore defined by a low-temperature brittleness limit near -50 °C and a hot-air aging threshold near 100 °C for unprotected compounds.