2-Mercaptobenzothiazole Disulfide

Introduced to rubber compounding as the disulfide oxidation product of 2-mercaptobenzothiazole, 2-Mercaptobenzothiazole Disulfide (CAS 120-78-5) functions as a delayed-action primary accelerator for sulfur-curable diene elastomers. Commercial supply encompasses multiple physical forms: free-flowing powder (technical grade, purity ≥98%), oil-coated powder to suppress dust, and pre-dispersed masterbatches—most commonly 75% or 80% active content bound in an EPDM or EVA polymeric carrier, designated as MBTS-75 or MBTS-80. The neat powder exhibits a melting range of 167–175 °C (ASTM E324) with a relative density of approximately 1.50 at 25 °C. Loss on drying, measured at 70 °C under vacuum, is specified at ≤0.5% for standard grades, while ash content remains below 0.3%. A critical quality parameter is free 2-mercaptobenzothiazole (MBT) content, typically controlled to ≤2.0%—exceeding this threshold accelerates scorch tendencies and depresses the scorch delay the disulfide is intended to provide.

Field experience on a 270 L intermeshing internal mixer (Werner & Pfleiderer GK 270E, 4-wing rotors, fill factor 0.72) processing 70 phr N330 carbon black in NR-based truck tread compounds confirms that predispersion formats reduce incorporation time by 18–22 seconds compared to neat powder when dump temperature is held at 145 °C. However, predispersion introduced at ≤0.8 phr carries an invisible processing penalty: the carrier polymer (10–20% by weight of the predispersion) can alter phase viscosity enough to reduce the shear heating rate by 1.5–2.0 °C/min on a 120 °C starting temperature profile, potentially shifting the curative dispersion threshold toward the batch-to-batch variance band if rotor speed is not adjusted upward by 2–3 rpm.

During sulfur vulcanization, the disulfide undergoes homolytic cleavage to generate 2-mercaptobenzothiazole radicals, which then participate in the formation of the active sulfurating complex. This mechanism endows 2-mercaptobenzothiazole disulfide with a scorch safety advantage over its parent MBT: in a standard ASTM D5289 moving-die rheometer curve at 160 °C, a typical 1.0 phr MBTS in high-cis 1,4-polyisoprene with 2.5 phr sulfur yields a t_s2 (scorch time) of 4.8–5.5 minutes, roughly 2.0–2.5 minutes longer than an equimolar amount of MBT under identical conditions. Differential scanning calorimetry kinetic analysis per ASTM E2041 places the apparent activation energy of MBTS-accelerated vulcanization at 92–98 kJ/mol for NR/SBR blends, moderately higher than the 82–88 kJ/mol observed for MBT, implying tighter process-control windows when cure temperature deviates more than ±4 °C from the target.

Why Does Free MBT Content Influence Compound Viscosity Before Vulcanization?

Residual free MBT acts as a peptizer in natural rubber matrices during mastication. When free MBT exceeds 2.5%, a 0.8–1.2 Mooney unit drop in ML(1+4) at 100 °C becomes measurable after 24 hours of maturation at 23 °C and 50% RH. On a 60 mm single-screw extruder (L/D 20:1) processing a 40 Shore A NR compound for weatherstrip profiles, this viscosity loss translates to a die swell fluctuation of ±3.2% with a corresponding cross-sectional dimension inconsistency exceeding ±0.15 mm on a 6 mm section. Production records from a continuous microwave-curing line (UHF power 12 kW, line speed 18 m/min) correlate out-of-spec scrap rates above 2.1% with MBTS lots carrying free MBT above 2.3%. For this reason, high-performance automotive sealing profiles often source MBTS-80 predispersion with free MBT guaranteed below 1.2%, effectively shifting the process window away from the viscosity-critical edge.

A separate constraint emerges in formulations where MBTS is paired with amine-generating accelerators such as diphenylguanidine (DPG) or di-ortho-tolylguanidine (DOTG). Secondary amine release during vulcanization can interact with free MBT degradation products to promote nitrosamine formation, which falls under the purview of EU Directive 93/11/EEC and Germany’s TRGS 552. Consequently, MBTS pre-dispersions for the European automotive weatherstrip sector are often manufactured exclusively with a non-staining phenolic antioxidant-stabilized EPDM binder that carries 0.0% free amine comonomer, a characteristic not required for general industrial goods where nitrosamine content is not regulated at finished-article level.

When to Select MBTS Over MBT in High-Temperature Injection Molding of NR Bushes

Injection-molded natural rubber bushings intended for heavy-duty truck suspension modules are processed on horizontal rubber injection presses with clamp forces of 400–650 tons and injection pressures reaching 180 MPa. Mold temperatures are typically held at 165–175 °C, leaving a narrow safety margin before onset of reversion. In this regime, the longer scorch delay of MBTS allows complete cavity filling without premature crosslinking at flow front boundaries, a failure mode that manifests as concentric delamination at the injection gate interface during cyclic fatigue testing per ISO 132. When MBT was substituted for MBTS at equimolar concentration in a commercial NR/BR bushing formulation, the scrap rate due to incomplete knitting rose from 0.7% to 4.3% over a 6-month production period on a 500-ton REP horizontal machine.

Yet MBTS alone cannot develop a tear strength above 45 kN/m at 23 °C (ISO 34-1, trouser method) in high-crosslink-density NR; a secondary ultrarapid accelerator such as tetramethylthiuram disulfide (TMTD) at 0.15–0.25 phr is co-fed to push tear resistance into the 55–60 kN/m range. The binary system introduces a processing-tradeoff: the TMTD component sharpens the onset of the vulcanization exotherm, narrowing the safe molding window to ±3 °C at 170 °C. Feedback-controlled mold temperature control with response time below 1.2 seconds (Band 6A, AMS 2750-compliant thermocouples) becomes mandatory to hold dimensional variation under ±0.05 mm on the bushing inner diameter.

Pre-drying of MBTS powder is mandated when the storage environment exceeds 60% relative humidity. Equilibrium moisture absorption under these conditions can raise water content to 0.8–1.0% within 72 hours, introducing porosity during injection molding if the bulk molding compound reaches 120 °C before venting. A vacuum-assisted venting cycle (−0.95 bar) on the mold partially mitigates this, but the preferred factory practice is hot-air tray drying at 80 °C for 2 hours until moisture content falls below 0.3% measured by Karl Fischer titration (ISO 760).

Interaction with Sulfenamide CBS in Asymmetric Curing Systems

In SBR/NR blends used for tire undertread layers, 2-mercaptobenzothiazole disulfide is often used in combination with N-cyclohexyl-2-benzothiazolesulfenamide (CBS) to modulate cure reversion. At a ratio of 1:1.2 MBTS to CBS, the torque-time profile recorded on a moving-die rheometer at 170 °C displays a plateau-like maximum that extends for 3.2–4.0 minutes beyond t_max, delaying the 5% reversion point by nearly 40% compared to straight CBS. This synergy is exploited on 180 °C continuous tandem salt-bath curing lines, where residence time must be held within ±1.8 seconds to maintain blow-out resistance per FMVSS 109. Published data from a pilot-scale Berstorff ZE 62 twin-screw extruder (co-rotating, 48:1 L/D) processing an 80/20 SBR/NR base demonstrates that substituting MBTS for half the CBS reduces the radial temperature gradient across the extrudate surface from 4.2 °C to 2.6 °C, attributable to the slower initiation kinetic profile.

The disulfide structure also reduces blooming behavior relative to its monosulfide analog. Thin-film chromatography conducted on cured NR sheets aged 14 days at 40 °C per ISO 11346 shows surface exudation levels for MBTS alone at 1.5 phr are less than 0.07 wt% of total additive, while the equivalent MBT formulation shows 0.18 wt%, necessitating a post-cure surface wipe for adhesion-critical components such as air-spring bellows. This quantitative migration difference directly influences specification selection for dynamic applications where surface contamination reduces vulcanized splice adhesion below the 4.5 N/mm threshold required by EN 283 for curtain-side tensioning springs.

In continuous co-extrusion of EPDM sponge sealing profiles, the addition of 1.2 phr MBTS-75 alongside 0.8 phr zinc dibutyldithiocarbamate (ZDBC) requires precise temperature profiling across the extruder barrel: the first two zones must remain below 80 °C to prevent premature decomposition of the ZDBC component, while the die head can tolerate 100–105 °C because MBTS delays the onset of crosslinking long enough to permit pressure-driven cell expansion following the microwave pre-cure. Deviation above 108 °C at the die head, however, collapses the cell structure within 1.5 seconds, converting the target 0.48 g/cm³ density sponge into a hard elastomer exceeding 0.95 g/cm³.

When compounding in a laboratory-scale 1.6 L Banbury mixer (Brabender Plasticorder) with NR/SMR CV60, the incorporation sequence must be strictly controlled: adding MBTS simultaneously with carbon black and oil before sulfur results in a 12–15% reduction in dispersion index compared to adding it after two-thirds of the carbon black has been incorporated. This sequence sensitivity is attributed to preferential adsorption of the accelerator onto carbon black aggregates, reducing the effective concentration available for crosslinking and producing a low modulus (M₁₀₀ drop of 0.2–0.3 MPa) that mimics under-cure. The batch-to-batch modulus scatter on a 3-month production log for a 60 Shore A NR mat compound narrowed from ±0.42 MPa to ±0.18 MPa after written work instructions mandated a two-stage mixing protocol with MBTS added exclusively in the second stage at 95 °C maximum.