Zinc Bis(1,3-Benzothiazole-2-Thiolate)

Why shift from MBT to its zinc salt in radial tire carcass formulations?

The acute toxicity and the potential for N-nitrosamine generation associated with free 2-mercaptobenzothiazole (MBT) have driven the shift to its zinc salt in large-volume tire manufacturing. Zinc bis(1,3-benzothiazole-2-thiolate) (ZMBT) exhibits a decomposition onset temperature approximately 40–50°C higher than MBT, measured by differential scanning calorimetry at 10°C/min under nitrogen, which translates into a wider processing safety margin during multi-pass internal mixing. In a typical radial tire carcass ply compound based on a 70/30 NR/BR blend, ZMBT is dosed at 0.6–1.0 phr as a secondary accelerator alongside 1.2–1.5 phr N-tert-butyl-2-benzothiazolesulfenamide (TBBS), 2.5–3.0 phr insoluble sulfur (oil-treated, 33% oil), and 0.1–0.2 phr N-(cyclohexylthio)phthalimide (CTP) as a pre-vulcanization inhibitor. The compound is mixed in an intermeshing internal mixer (e.g., Kobe Steel BB-270) with a net chamber volume of 270 L, ram pressure 0.55 MPa, and rotor speed 40 rpm. The masterbatch discharge temperature is limited to 145°C to prevent premature conversion of ZMBT into active intermediate species. After dumping through a dump extruder with a die head set at 110°C, the batch is sheeted and cooled to 35°C on a batch-off unit. The ZMBT and sulfur are introduced on a two-roll mill at a front-roll temperature of 55±5°C and a friction ratio of 1:1.15, with three cross-blend passes and a dwell time not exceeding 6 min. Mooney scorch measured at 135°C per ISO 289-1 typically yields a t5 value above 25 min, while moving die rheometer (MDR) traces at 160°C, 0.5° arc, per ISO 6502-3 show t10 not less than 2.5 min and t90 within 6.5–8.0 min. This balance is critical because the carcass compound must flow and wet the brass-plated steel cord during the initial stages of bladder curing before the viscosity rises.

The structural difference between ZMBT and MBT also impacts adhesion retention at the rubber-to-brass interface. In formulations containing 2.0 phr cobalt boron neodecanoate (Co-BND, 21.5% Co), the replacement of 0.5 phr MBT with 0.8 phr ZMBT improves pull-out force after hot-air aging at 100°C for 72 hours by 9–14%, as determined by the steel cord adhesion test per ASTM D2229-10. The zinc cation reduces the catalytic degradation of the CuZn interphase in the presence of moisture and residual amines. Moreover, the absence of a free thiol group minimizes the formation of zinc mercaptide clusters that can migrate to the cord surface and create a weak boundary layer. The cured tire ply is expected to exceed a fatigue life of 2 million cycles in the belt-edge separation test (BEST) under 50% overload, a condition routinely required in passenger car radial tire validation protocols. However, zinc migration from ZMBT into the brass coating is not zero; surface-sensitive X-ray photoelectron spectroscopy (XPS) depth profiles reveal zinc enrichment of 3–5 at% within the top 10 nm of the sulfide layer when ZMBT loading exceeds 1.0 phr, which can lead to a transition from a ductile Cu₂S/CuS layer to a brittle ZnS-dominated interlayer. Therefore, the upper limit is strictly set at 1.0 phr for cord-skim compounds.

Processing safety on a calendar line for fabric ply coating demands that the compound maintain a Mooney viscosity (ML 1+4 at 100°C) between 55 and 65 units for 8 hours of continuous operation. Any batch exhibiting a Mooney rise greater than 5 units over that period is rejected. ZMBT-related scorch is frequently traced to moisture ingress: the product is hygroscopic, and when stored in opened bags at relative humidity above 60%, the adsorbed water accelerates the dissociation of the zinc-thiolate bond. Pre-drying at 60°C for 2 hours in a desiccant air oven with a dew point of −40°C is mandatory before weighing in bulk bag handling systems operating in coastal or high-humidity sites. Additionally, direct contact with stearic acid during weigh-up should be avoided; ZMBT should be added to the mixer after the fillers and plasticizers to prevent localized acid-catalyzed decomposition that liberates MBT, which then reacts with residual calcium stearate to form a calcium-MBT complex identifiable as a sticky white residue on the dump extruder screw.

Table 1. Variation of MDR cure properties (ISO 6502-3, 160°C, 0.5° arc) and physical properties (ISO 37, ISO 4649) with ZMBT dosage in an NR/BR 70/30 carcass masterbatch containing 1.3 phr TBBS and 2.8 phr sulfur. The optimum processing window in terms of scorch safety and abrasion resistance lies between 0.7 and 1.0 phr.

Conveyor belt cover stocks operating in mining environments demand a curing package that simultaneously maximizes crosslink density and abrasion resistance. When ZMBT is paired with N-cyclohexyl-2-benzothiazolesulfenamide (CBS) in an NR/BR 60/40 blend, the cure rate index (CRI = 100/(t90 − ts2)) increases by approximately 15% compared to an MBT-CBS system at equal sulfur levels of 2.2 phr, while the reversion resistance after overcure at 160°C for 60 min shows a 9% smaller drop in tensile strength. This is critical for belts exposed to continuous dynamic loading and bulk material temperatures that can locally elevate the cover surface to 110°C. A production compound for a DIN Y-grade belt cover incorporates 1.0–1.3 phr ZMBT, 0.7–0.9 phr CBS, 2.2 phr rhombic sulfur, 1.5 phr N-isopropyl-N′-phenyl-p-phenylenediamine (IPPD), and 3 phr highly aromatic extender oil. The entire fill factor of the Intermix E-type mixer is held at 0.72, with a rotor speed of 35 rpm and ram pressure 0.6 MPa. Discharge occurs precisely when the stock temperature probe reads 135°C; any excursion above 142°C results in a measurable increase in crosslink precursor formation, evidenced by a rise in the compound Mooney viscosity (ML 1+4 at 100°C) from a target of 72 to above 85 within 30 min of cooling. The hot masterbatch is passed through a strainer-extruder equipped with 120-mesh screens before batch-off cooling. ZMBT and sulfur are introduced on a mill at 55°C front-roll temperature. Belt sections are cured in a multi-daylight hydraulic press at 150°C and 18 MPa for 35 min. Cover properties must satisfy DIN ISO 4649 abrasion loss below 110 mm³, tensile strength per ISO 37 exceeding 18 MPa, and tear strength per ISO 34-1 (trouser tear) exceeding 40 kN/m. If a flame-resistant designation such as MSHA 30 CFR Part 14 is required, ZMBT usage is not restricted but the compound must contain substantial additions of chlorinated paraffin (15–20 phr) and antimony trioxide (8–12 phr). In such heavily loaded systems, ZMBT solubility in the polymer matrix approaches its limit, and surface blooming may occur after prolonged static storage. A washing step with a methyl ethyl ketone/isopropanol mixture is often introduced in the finishing line to restore adhesion for cold-bond splicing procedures.

Low-compression-set EPDM coolant hose formulations

EPDM compounds used in extruded automotive coolant hoses must withstand prolonged contact with glycol-based coolants at peak temperatures of 125°C while maintaining compression set values below 35% after aging per ISO 815-1:2014, method B, 125°C/22 h. Achieving such performance with a sulfur-donor cure system rather than a more expensive peroxide system requires precise kinetic control. In a typical formulation based on EPDM with 4.5% ethylidene norbornene (ENB) and a Mooney viscosity (ML 1+4 at 125°C) of 65, ZMBT is applied at 1.8–2.2 phr in combination with 1.2 phr dithiomorpholino disulfide (DTDM), 0.4 phr tellurium diethyldithiocarbamate (TDEC), and 0.3 phr elemental sulfur. The quasi-EV (efficient vulcanization) cure profile generates short crosslinks with high thermal stability; moving-die rheometer data at 180°C indicate a delta torque (MH − ML) of 14.2 dNm and a t90 of 3.8 min. The extrusion is performed on a cold-feed pin-barrel extruder with a 90 mm screw diameter and an L/D of 16:1. Screw temperature is maintained at 75°C, barrel zone temperatures at 82°C, and the die head at 90°C. High die swell is compensated by a 12% draw-down ratio between the die land cross-section and the vulcanization line haul-off speed. Continuous vulcanization proceeds in a microwave/hot-air tunnel (UHF power 6 kW, air temperature 220°C) followed by a steam tube at 0.8 MPa. Compression set measured on 12.5 mm cylindrical buttons reaches 29%, meeting the OEM specification. Zinc ion leaching from the ZMBT-cured hose wall into the coolant is monitored by inductively coupled plasma atomic emission spectroscopy (ICP-AES) after 1,000 h recirculation at 100°C; values exceeding 0.8 mg/L are associated with pitting corrosion on aluminum heater cores. Therefore, a post-cure extraction step (water immersion at 80°C for 4 h) is introduced to lower extractable zinc to < 50 ppm of rubber. The final hose assembly must comply with SAE J20 R4-class requirements for dimensional stability and burst pressure. A specific incompatibility exists with amine-functionalized antioxidants: when 0.5 phr polymerized 2,2,4-trimethyl-1,2-dihydroquinoline (TMQ) is replaced by 4,4′-methylenebis(2,6-di-tert-butylphenol) as the antioxidant, the induction time before crosslinking in the rheometer increases by 40%, indicating that TMQ participates in the activation of the ZMBT-DTDM system. Any substitution must be validated with full factorial experimental design across the cure system.

The production of uniform closed-cell foam in athletic shoe midsoles depends on matching the decomposition rate of the blowing agent with the crosslinking rate of the polymer matrix. Microcellular EVA/NR blends (typically 60/40 ratio) processed by injection molding at melt temperatures of 85–95°C require a delayed-action curing package that prevents premature gas loss. Here, ZMBT at 0.8–1.0 phr is combined with 1.0 phr CBS, 1.5 phr azodicarbonamide (blowing agent ADC, decomposition onset 158°C), and 0.7 phr urea-based kicker to align gas evolution with cure progression. The crosslinking peak measured by oscillating disk rheometer at 165°C is centered at 7.5 min, coinciding with the 90% gas evolution time of the ADC-kicker system. Molding is carried out in aluminum molds with a triple-deck shuttle press at 165°C and 15 MPa clamp force. The foam exhibits a density of 0.28 g/cm³ and a split-tear strength of 4.2 kN/m. ZMBT performs the additional function of stabilizing the cell walls against post-expansion collapse: zinc ions react with residual stearic acid to form zinc stearate in situ, which diffuses to the cell-gas interface and reduces internal air permeability. However, ZMBT loading above 1.2 phr causes an increase in closed-cell size from a mean of 85 μm to over 140 μm, visible as a loss of surface smoothness and a reduction in compression resistance at 50% deflection from 270 kPa to 210 kPa. The crosslinking agent must be fed as a pre-dispersed powder: direct addition of neat ZMBT to the EVA melt leads to agglomerates that act as nucleation sites for gross gas pockets during mold expansion. A masterbatch approach using 40% ZMBT dispersed in EVA wax is employed to ensure a particle size distribution below 10 μm D90.

Moisture-resistant insulation compounds for underground cables

Moisture-induced dielectric failure in underground power cables imposes strict limits on ionic impurity content in insulation compounds. Ethylene-propylene rubber (EPR) and EPDM insulations for medium-voltage cables up to 36 kV employ ZMBT in ultra-low-sulfur formulations (0.3–0.5 phr sulfur) to minimize the concentration of hygroscopic sulfate byproducts that can form treeing channels. A benchmark insulation recipe contains EPDM 100 phr, calcined kaolin clay 120 phr, paraffinic process oil 10 phr, zinc oxide 5 phr, ZMBT 0.8 phr, zinc dibutyldithiocarbamate (ZDBC) 0.6 phr, and polymerized TMQ 1.5 phr. Mixing is performed in a 160 L tangential mixer with optimized turn-down cooling to keep discharge temperature below 130°C. The compound is pelletized and then fed into a continuous vulcanization (CV) line consisting of a single-screw extruder feeding a catenary tube. The vulcanizing tube operates at 1.6 MPa saturated steam pressure, corresponding to a temperature of 200°C; line speed is adjusted to produce a residence time of 2.5 min, which is sufficient to reach a rheometer t98 of 2.2 min as verified by a cure simulator in-line. After cooling under nitrogen atmosphere, the insulated core is subjected to a water immersion test per IEC 60502-2: insulation resistance is monitored at 90°C in deionized water for 14 days. Volume resistivity must remain above 10^14 Ω·cm. ZMBT-based compounds achieve this threshold with a margin of 1.5 orders of magnitude compared to conventional MBT-accelerated systems, because the zinc mercaptide crosslink residues are less prone to hydrolysis than free thiol-terminated fragments. A vital process checkpoint is the moisture content of the calcined clay: it must be pre-dried to < 0.1% before weighing, as water vapor released in the extruder generates micro-voids that act as initiators for water tree propagation. The use of ZMBT also necessitates the complete exclusion of lead-based stabilizers historically used in CV lines, since the formation of lead mercaptide complexes depletes the active accelerator and creates dark specks in the insulation wall detectable by a 1.2 kV/mm partial discharge test per IEC 60885-3.

Pre-vulcanized natural rubber latex formulations destined for thin-film surgical and examination gloves have historically relied on dithiocarbamate accelerators, but the demand for nitrosamine-free products has elevated the role of ZMBT. A typical prevulcanization process employs a 60% centrifuged latex concentrate with 0.5 phr ammonia preserved. The accelerator dispersion, prepared by ball-milling ZMBT (50% active paste with anionic surfactant) together with zinc oxide (0.5 phr dry), elemental sulfur (1.2 phr), and a styrenated phenol antioxidant, is stirred into the latex and heated to 60°C for 4 hours until the chloroform coagulation index reaches a stage 3–4 semi-vulcanized state. The compounded latex is dipped into coagulant-coated formers at 50°C, leached in warm water at 70°C for 2 min, and then cured in a hot-air oven at 120°C for 20 min. The ZMBT dosage is tightly controlled at 0.8–1.0 phr dry rubber content; exceeding 1.0 phr increases the risk of Type IV contact dermatitis because residual unbound zinc mercaptobenzothiazole can be extracted by sweat simulant (per EN 455-3:2015). Under the standard artificial sweat test, the extracted MBT concentration (measured by HPLC-UV after diazomethane derivatization) must remain below 0.08 μg/cm². This is achieved by an additional post-cure chlorination step (300 ppm available chlorine, 5 min) that oxidizes surface-bound ZMBT into inert zinc sulfate species. The glove tensile properties after aging per EN 455-2 (accelerated aging at 70°C for 7 days) must retain at least 75% of the original elongation at break: ZMBT-vulcanized films typically show 18 MPa tensile strength and 720% elongation before aging, falling to 14.5 MPa and 580% after aging, meeting the 75% retention threshold. A practical limitation on production scale arises from the foaming tendency of ZMBT dispersions when mixed with latex containing high residual ammonia; defoaming with silicone-based antifoam at 0.02% is required to prevent pinholes in the dipped film. Furthermore, the use of ZMBT in a latex compound that will later be co-vulcanized with a carboxylated nitrile over-dip is discouraged, because zinc ions migrate into the nitrile layer and create ionic clusters that cause stiffness increase and cracking upon flex.

Table 2. Regulatory compliance matrix for ZMBT-accelerated rubber articles. All migration limits refer to the potentially extractable fraction of 2-mercaptobenzothiazole or total zinc. The listed values are derived from the referenced standards and must be verified by an accredited laboratory under ISO 17025 conditions.

When replacing dithiocarbamates with ZMBT in EPDM sponge profiles, what happens to cell morphology?

Closed-cell EPDM weatherstrip sponges manufactured for automotive body seals conventionally use ultra-accelerator packages based on zinc dimethyl- or zinc diethyldithiocarbamate. Substitution of these accelerators with ZMBT is driven by the need to eliminate volatile N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) in the work environment, but the replacement alters the kinetic match with the blowing agent. In a microwave-cured profile containing EPDM (60 phr ethylene, 8% ENB), carbon black N550 (40 phr), calcium carbonate (50 phr), paraffinic oil (80 phr), and azodicarbonamide (4 phr) activated by zinc oxide and urea, the ZMBT dose is set at 1.5–2.0 phr alongside 2.0 phr TBBS and 1.5 phr sulfur. The MDR cure trace at 170°C shows a t10 of 0.9 min and a t90 of 4.2 min, which is 30% slower in the early stage than a ZDMC/TBBS system, shifting the onset of gas expansion to a slightly lower crosslink density. As a result, the expansion ratio decreases from 2.8 to 2.2, but the cell size distribution narrows from a coefficient of variation of 35% to 18% as measured by scanning electron microscopy at 50× magnification. This uniformity improves compression load deflection (CLD) at 30% compression to 72 kPa with a hysteresis loss below 22%.

The processing window in a continuous microwave-hot air line (UHF 6 kW, hot air 230°C) is limited: below 155°C the ADC decomposition is incomplete and the sponge density exceeds 0.45 g/cm³, while above 185°C the EPDM phase begins to degrade and cell walls collapse, dropping CLD below 45 kPa. A thermal scanning rheometer attachable to the UHF exit confirms a stable plateau between 160°C and 180°C. One recurring plant-floor failure is mold fouling on the sizing calibrators following the curing tunnel; free MBT from ZMBT decomposition deposits on stainless steel surfaces and builds up a brown varnish that alters the die gap by 0.2 mm over 8-hour continuous runs. The raw ZMBT specification must therefore limit free MBT to less than 0.3% as determined by UV-visible spectroscopy at 320 nm after extraction with acetonitrile. In-line cleaning with a 5% sodium carbonate solution at 80°C every 4 hours is necessary to maintain dimensional tolerance. The final profile is tested for water absorption according to ISO 2896 (immersion 24 h at 23°C) and must not exceed 5% by volume. ZMBT-cured sponges consistently show 3.8–4.2% absorption, slightly above dithiocarbamate equivalents, because the polar zinc mercaptide linkages create a marginally more hydrophilic network. If the weatherstrip requires a 10-year UV/ozone resistance warranty (per ISO 1431-1, 200 pphm ozone, 20% strain, 40°C, 48 h), no cracking is observed when the formulation includes 4 phr microcrystalline paraffin wax and 1.5 phr of a polymeric hindered amine light stabilizer.