N-Oxydienthylene-2-Benzothiazole Sulfenamide

Tire Tread Compounds and the Critical Balance of Scorch Safety with Optimum Crosslink Density

In passenger and truck tire treads formulated with a solution-polymerized styrene‑butadiene rubber (S‑SBR)/butadiene rubber (BR) blend, the accelerator must provide a processing window broad enough to survive multiple high‑shear mixing stages and downstream extrusion without premature vulcanization. Addition of 0.8–1.5 phr N‑oxydiethylene‑2‑benzothiazole sulfenamide (NOBS) to a silica‑rich tread recipe — typically containing 70–90 phr highly dispersible silica and a bifunctional organosilane such as bis‑(triethoxysilylpropyl) tetrasulfide (TESPT) — delays the scorch onset at 130 °C by approximately 8–12 minutes compared to an equimolar loading of N‑cyclohexyl‑2‑benzothiazole sulfenamide (CBS). This delay is essential because the silanization reaction between silica and TESPT proceeds efficiently only above 140 °C, yet the compound temperature in an intermeshing co‑rotating twin‑screw extruder (L/D 48:1) can easily overshoot to 155–160 °C during the final masterbatch stage. If scorch time (t_s2 at 135 °C, per ASTM D5289‑21) falls below 6 minutes, the resulting micro‑gel particles create visible surface defects in the extruded tread profile and reduce dynamic fatigue resistance. NOBS maintains a Mooney scorch (ML 1+4 at 121 °C) of 28–34 minutes even after a 3‑pass silica dispersion cycle, while CBS‑accelerated compounds often drop below 15 minutes under identical thermal history. The delayed action is attributed to the steric hindrance of the oxydiethylene substituent, which slows the decomposition of the mercaptobenzothiazole (MBT)‑amine complex before the active sulfurating agent forms.

Cure kinetics acquired by moving‑die rheometry (ASTM D5289‑21, arc 0.5°) reveal a characteristic induction period followed by a steep torque increase. With 1.2 phr NOBS and 1.8 phr sulfur, the t₉₀ at 160 °C is typically 12–14 minutes for a 70/30 S‑SBR/BR matrix containing 50 phr carbon black N234 as a secondary filler. The resulting crosslink density, estimated by equilibrium swelling in toluene (ISO 1817:2022), yields a Flory–Rehner network chain density (ν_e) of (1.35–1.55) × 10⁻⁴ mol/cm³. This translates to a 300% modulus (ISO 37:2017) of 10.5–12.0 MPa and a tensile strength not lower than 21 MPa. Crucially, the reversion resistance — the torque decay beyond t₁₀₀ at 180 °C — is 12–15% over 30 minutes, significantly better than the 20–25% drop observed with TBBS‑accelerated systems, making NOBS particularly suitable for truck treads operating under sustained high internal heat generation (service temperatures exceeding 90 °C in the shoulder region).

Why Does Steel Cord Skim Compound Choose a Delayed‑Action Sulfenamide Over Thiazoles?

Adhesion between brass‑coated steel wire and the surrounding skim rubber in radial tires depends on the controlled formation of a copper sulfide (Cu_xS) interfacial layer during vulcanization. The adhesion promoter, typically cobalt stearate or cobalt neodecanoate at 0.8–1.5 phr as cobalt metal, requires a sulfurating species that is generated after the compound has flowed and fully wetted the cord surface. If active sulfur is released too early, before the rubber penetrates the cord filaments, a thick, non‑stoichiometric Cu_xS layer forms, leading to brittle interfacial fracture and pull‑out forces well below the target of 400 N/25 mm (ASTM D2229‑21). NOBS, dosed at 2.0–2.5 phr with 4.0–5.0 phr insoluble sulfur (IS 7020), retards MBT release sufficiently that the compound maintains a minimum Mooney viscosity (ML 1+4 at 100 °C) of 50–55 MU for at least 8 minutes during the calendar application step. This allows the low‑molecular‑weight cobalt salt to migrate to the brass surface and form a uniform, ductile sulfide film. Wire adhesion force after cure (30 min at 150 °C, platen press) is consistently 420–460 N/25 mm with 95‑100% rubber coverage, confirmed by visual rating according to ASTM D2229 Annex A.

A competitive risk arises when free amine content in the accelerator or external amine‑based antioxidants (e.g., diphenylamine derivatives) accelerates the MBT‑zinc complex dissociation. In cobalt‑containing stocks, excessive free amine accelerates the corrosion of the brass coating before vulcanization, producing a green‑colored compound and adhesion loss of up to 30‑40%. NOBS, with a low free amine specification (<0.3 wt% morpholine as per GB/T 8829‑2013 or supplier certificates), mitigates this degradation pathway. Furthermore, post‑vulcanization aging in steam (90 °C, 95% RH for 7 days) shows adhesion retention of 82‑86% with NOBS compared to 68‑73% for CBS‑based skims, as measured by wire pull‑out force. The lower hygroscopic tendency of the oxydiethylene fragment also minimizes moisture uptake during ambient storage of the accelerator itself; if moisture content exceeds 0.5 wt%, hydrolysis can generate free MBT, disturbing the scorch delay and lowering the effective accelerator activity by 5‑8%.

Conveyor Belt Carcass Skim: Managing Covulcanization Between Natural Rubber and SBR in Thick‑Section Plies

Textile‑reinforced multi‑ply conveyor belts, where carcass layers consist of alternating NR‑rich skim compound and polyester‑nylon (EP) fabric, demand a cure system that synchronizes the vulcanization rates of the high‑NR skim and the adjacent cover compounds often containing higher BR or SBR levels. A 70/30 blend of NR (RSS1) and SBR 1502 used as carcass skim is typically accelerated with 0.6–0.9 phr NOBS combined with 0.15–0.25 phr tetramethylthiuram disulfide (TMTD) as a secondary accelerator. The combination achieves a t₉₀ of 20–22 minutes at 147 °C in a 15 mm thick section, which closely matches the cure profile of a 60/40 NR/BR cover compound accelerated with 0.8 phr NOBS only. Mismatched cure states in the boundary layer — where the skim under‑cures relative to the cover — create a weak interphase that initiates ply separation under repeated flexing (belt troughing and reverse bends). The crosslink distribution uniformity, quantified by differential scanning calorimetry (DSC) deconvolution of the vulcanization exotherm, shows a single peak with NOBS/TMTD, indicating a homogeneous network formation rather than separate NR‑SBR domains.

Processing the skim on a four‑roll calender at a nip temperature of 80–90 °C requires a compound with low nerve and high tack. NOBS contributes to this aim by not activating before the fabric dip (resorcinol‑formaldehyde‑latex, RFL) curing, which occurs partially in the hot calendering zone. The compound’s green strength, measured as the stress at 50% elongation (ISO 9026:2021), must exceed 0.4 MPa to prevent fabric distortion during winding onto the building drum. Over‑acceleration with a fast primary accelerator like MBTS would reduce green strength below this threshold. After press cure at 4.5 MPa platen pressure, the adhesion between skim and fabric (ISO 252:2023, strip method) should be 8–12 N/mm for a polyester warp yarn. The NOBS/TMTD system consistently delivers 9.5–11.5 N/mm with cohesive rubber failure, while a MBTS‑accelerated system often gives only 5–7 N/mm and a mixed cohesive/adhesive fracture pattern, indicating insufficient polymer inter‑diffusion before the onset of vulcanization.

Compression Set Reduction in EPDM Automotive Weatherseals and Coolant Hoses

Ethylene‑propylene‑diene monomer (EPDM) compounds used in automotive door seals, trunk seals, and coolant hoses are frequently vulcanized with a combination of sulfenamide and thiuram or dithiocarbamate ultra‑accelerators to balance rapid hot‑air continuous vulcanization (HAV) line speeds (5‑15 m/min) and low compression set after thermal aging. A typical microwave‑HAV line cures extruded sponge profiles in a 250‑300 °C hot air tunnel with a residence time of 2–4 minutes. The compound must maintain a scorch‑free processing window at 90–100 °C inside the extruder head (pin‑and‑die crosshead) and then vulcanize completely within the short tunnel. NOBS at 1.0–1.8 phr with 0.8–1.2 phr zinc dibutyldithiocarbamate (ZDBC) and 0.4–0.8 phr sulfur donor (e.g., dithiodimorpholine, DTDM) extends the t_s2 at 121 °C to 23–28 minutes while delivering a t₉₅ at 180 °C of only 1.5–2.0 minutes, as measured by a rotorless curemeter (ISO 6502‑3:2023). This steep cure characteristic is essential because the profile surface must develop sufficient modulus to resist deformation on the cooling conveyor.

Post‑cure compression set (ISO 815‑1:2021, 25% compression, 150 °C for 24 h) target for a dense EPDM coolant hose is < 30%. An NOBS‑cured compound with an ethylene content of 55 wt% and ENB diene 4.5 wt% typically reaches 18–22%, whereas a CBS‑accelerated analogue often exhibits 28–34% due to a lower effective network chain density and greater chain scission during aging. The difference is traced to the slower, more uniform crosslinking with NOBS, which reduces the concentration of labile sulfur bridges. However, a known limitation: when the coolant hose compound contains amine‑based antioxidants (e.g., polymerized 2,2,4‑trimethyl‑1,2‑dihydroquinoline, TMQ) at levels beyond 1.0 phr, the synergy between the amine’s basicity and NOBS’s decomposition can shorten t_s2 unpredictably by 15–20%. Batch‑to‑batch Mooney scorch testing at 121 °C is therefore mandatory, and deviations greater than ±3 minutes from the target value of 26 minutes necessitate adjustment of the ZDBC addition level.

Footwear Outsoles: Translucent and High‑Abrasion Rubber Compounds for Athletic Shoes

Injection‑molded translucent outsoles based on high‑cis butadiene rubber (BR) with a minor fraction of isoprene rubber (IR) for improved green tack require an accelerator that does not discolor the compound or cause bloom, while yielding a high abrasion resistance index. NOBS, at a reduced loading of 0.4–0.7 phr with 1.8–2.2 phr sulfur and 0.8–1.2 phr tetraethylthiuram disulfide (TETD), produces a light‑colored vulcanizate with a DIN abrasion loss (ISO 4649:2021) of 35–45 mm³. This falls well within the acceptance criterion for premium athletic soles (< 50 mm³). The absence of a cyclohexylamine‑type residue, which is characteristic of CBS decomposition and can cause yellowish discoloration under UV exposure, is a distinct advantage. Accelerated UV aging in a Xenon‑arc chamber (ISO 105‑B02:2014, cycle 4) for 48 h results in a color change (ΔE*) of < 3.5 for the NOBS‑cured compound, compared to 6–8 for CBS‑cured analogues.

Processing on a reciprocating screw injection molding machine with a clamp force of 2500 kN requires a compound with low viscosity and rapid cavity filling at 80–90 °C melt temperature, followed by instantaneous cure at mold temperature 170–175 °C. The delay in scorch onset allows an injection pressure of 80–100 MPa to be maintained without localized scorch in the sprue and runner system. Cure time per cycle is 90–110 seconds for a 4‑cavity mold producing 8 mm thick outsoles. The T_g of the cured compound, measured by differential scanning calorimetry (ISO 11357‑2:2020), is ‑95 ± 2 °C, ensuring flexibility at sub‑zero temperatures. An operational boundary: if the compound is stored for more than 3 days at relative humidity exceeding 70%, absorbed moisture reacts with the accelerator during injection, generating gaseous decomposition products that cause visible porosity in the transparent matrix. Pre‑drying in a hopper dryer at 60 °C for 2‑3 hours restores scorch safety.

How Do Industrial Roll Coverings Exploit the Cure‑Rate Gradients Created by a Sulfenamide‑Thiazole System?

Large‑diameter paper mill rolls and textile squeeze rolls are manufactured by spirally wrapping calendered rubber sheets onto a steel core, followed by autoclave curing in a steam‑pressurized vessel at 130–140 °C for 3–6 hours. A gradient cure system is deliberately engineered using 0.6 phr NOBS and 1.4 phr dibenzothiazyl disulfide (MBTS) in a 60/40 NR/BR blend. MBTS acts as a scorch‑inducing component that initiates crosslinking in the early stages of the slow heat‑up ramp (0.5 °C/min), while NOBS delays the bulk of the crosslinking until the entire mass reaches near‑uniform temperature, typically after 90–120 minutes. This prevents the outer wrap layers from overcuring and developing a stiff, brittle skin before the innermost layer adjacent to the steel core achieves 80% cure state. The shore A hardness gradient across a 25 mm thick roll covering is maintained within ±2 points, verified by micro‑indentation profile.

The external surface of the roll covering must be ground to a final diameter with a concentricity tolerance of ±0.05 mm. Over‑cured surface layers cause grinding wheel loading and burn marks; under‑cured zones smear. The NOBS/MBTS pair yields a surface tensile strength (ISO 37:2017) of 22–24 MPa and elongation at break of 480–520%, which are uniform through the grinding depth of 1.5 mm. An additional constraint arises from the bonding system — a conventional isocyanate‑based adhesive (Chemlok 205/220) applied to the sand‑blasted steel core. The amine‑type byproducts from CBS decomposition can neutralize the isocyanate functionality, reducing the adhesive peel strength from a required 12 N/mm to below 6 N/mm. NOBS, releasing morpholine derivatives rather than free cyclohexylamine, shows no such reduction, and peel values remain at 11.5–13.5 N/mm when tested according to ISO 813:2019.