5-Chloro-2-Benzothiazolethiol

How Does Chlorinated Mercaptobenzothiazole Extend Scorch Safety in Diene Rubber Vulcanization?

Compounding trials conducted on a Werner & Pfleiderer GK 1.5 N intermeshing tangential rotor internal mixer with a chamber volume of 1.5 L and a fill factor of 0.73 repeatedly demonstrate that replacing conventional 2-mercaptobenzothiazole (MBT) with an equimolar loading of 5-chloro-2-benzothiazolethiol shifts the Mooney scorch inflection (t5 at 127 °C, large rotor, ISO 289-1:2018) by 2.4–3.7 min in a standard ASTM D3192 natural rubber reference compound containing 50 phr N330 carbon black. The electron-withdrawing chlorine substituent at the C5 position depresses the nucleophilicity of the thiol group, retarding the formation of the active sulphurating complex with soluble zinc carboxylate and thereby decoupling filler incorporation safety from final cure rate. Typical masterbatch preparation sequences first masticate SMR 20 natural rubber for 60 s at 50 rpm rotor speed, followed by sequential addition of zinc oxide (5.0 phr), stearic acid (1.5 phr), antiozonant 6PPD (2.0 phr), and N330 black, with the ram raised and lowered between each addition to ensure dispersion homogeneity. Once the dump temperature reaches 120–130 °C, the batch is sheeted off a 200 mm two-roll mill and cooled to ambient temperature before accelerator addition on a cold mill set to a nip gap of 0.8 mm. 5-Chloro-2-benzothiazolethiol is introduced at 1.0–1.8 phr alongside sulphur (2.0–2.5 phr) and, where sulfur-donor hybrid systems are required, 0.3–0.6 phr of dithiomorpholin (DTDM) or tetrabenzylthiuram disulfide (TBzTD). Rheometer curves acquired at 160 °C per ISO 3417:2016 exhibit an increase in t10 and t90 values of approximately 22 % and 9 % respectively compared to the MBT benchmark, without sacrificing ultimate tensile strength (ISO 37:2017, dumb-bell type 2) or dynamic crack growth resistance measured on a Demattia flexing machine. The cure-rate index advantage is particularly exploitable in thick-walled engine mounts and conveyor belt splice compounds where thermal lag during press vulcanization routinely causes the surface to scorch before the core reaches 90 % of the MDR torque maximum. Molders using multi-daylight compression presses with thermocouple-logged platen uniformity of ±2.5 °C report a reduction in incipient crumb formation at the sprue and a more uniform network density gradient across sections exceeding 25 mm. Post-cure ageing at 100 °C for 72 h (ISO 188:2011, hot air oven, multiple air changes) reveals retention of elongation at break above 80 %, attributable to the higher thermal dissociation energy of benzothiazole-terminated pendent groups carrying the 5-chloro substituent. Formulation constraints dictate maintaining zinc oxide at or above 4.5 phr to prevent migration of the free thiol to the mould surface, and eliminating any residual moisture above 0.08 wt% prior to mill addition because hydrolytic cleavage of the chlorine atom at processing temperatures above 140 °C can generate trace zinc chloride species that catalyse the unconventionally rapid inversion of polysulfidic crosslinks to monosulfidic bridges, narrowing the cure plateau.

Oxidative stability of lithium 12-hydroxystearate greases formulated with poly-alpha-olefin (PAO 10) base stock hinges critically on the synergy between radical-scavenging aminic antioxidants and heterocyclic thiol metal deactivators when the application envelope exceeds 140 °C continuous soak. In a standard SKF R2F-B test run at 160 °C with a 7.5 kN radial load and an outer ring temperature maintained within ±1.5 °C by circulated oil, a grease containing 0.15 wt% of 5-chloro-2-benzothiazolethiol dissolved in the base fluid phase prior to soap dispersion extends the time to a 1.0 mg KOH/g acid number increase by an average of 480 h over the unprotected baseline. The free –SH moiety complexes copper and iron ions leached from the bearing cage and raceway, forming an oligomeric passivation film of approximately 10–40 nm thickness as characterised by Auger depth profiling on dismantled 6304-2RS1 bearings. Incorporation into the thickener lithiation stage is not advised because the acidic thiol proton (pKa ~ 6.8–7.1) partially protonates the lithium soap carboxylate headgroups, causing a reversible but process-disruptive drop in penetration consistency; post-cooling addition into the milled grease at 50–55 °C using a FrymaKoruma toothed colloid mill with a rotor-stator gap of 0.3 mm yields reproducible worked penetration values between 265–295 mm/10 (ISO 2137:2020). The formulated grease must pass the copper strip corrosion test ASTM D4048-22 (100 °C, 24 h) without darkening beyond 1b classification; sulfurised olefins and active sulfur species present above 0.01 wt% as a co-additive can competitively adsorb onto the copper surface, displacing the thiolate film and rapidly causing 4a staining. Industrial batches produced at 500 kg scale in a Stratco Contactor reactor followed by homogenization in a three-roll mill at 2 500 mm/min apron speed have consistently met the Deutsche Bahn DB 50 010 specification for railway journal bearing greases when residual thiol monomer is maintained below 50 ppm via vacuum stripping at 5 mbar absolute and 80 °C for 90 min. The system is incompatible with significant concentrations of polyglycol or ester base fluids because the chlorine substituent, under conditions of high humidity > 70 % RH and temperatures exceeding 170 °C, participates in a slow hydrolytic dechlorination that generates corrosive hydrochloric acid mist trapped in the grease matrix, a documented root cause of pitting corrosion on AISI 52100 steel inner races identified during Weibull analysis of fleet trial failures.

Formulating latent epoxy hardeners for capillary-flow underfill encapsulants used in flip-chip packages demands that the one-component dispersion remain shelf-stable at 25 °C for at least 14 days while delivering a rapid snap-cure at 150 °C with sufficient glass transition temperature to pass JEDEC MSL 3 reflow preconditioning. Dispersions containing 4–7 wt% finely micronised 5-chloro-2-benzothiazolethiol (d50 3 µm, three-pass air-jet milling) in a bisphenol F diglycidyl ether resin with an epoxy equivalent weight of 168–175 g/eq and 1.8 wt% of a poly(vinyl butyral) suspension stabiliser have demonstrated, via modulated differential scanning calorimetry at a 10 K/min ramp, a shelf-life exotherm shift of less than 2 J/g after 18 days at 23 °C in sealed polyethylene syringes. The latent character originates from the high melting point of the thiol (218–222 °C, ASTM E324-16) and the sterically hindered deprotonation of the aryl-SH proton by the mildly basic epoxy matrix; nucleophilic ring opening only proceeds measurably once the filler reaches its dissolution threshold near 120 °C, a thermal gate that effectively decouples deposition work life from cure kinetics. Cure schedules typically progress as a stepped ramp: 100 °C for 1.5 h followed by 150 °C for 2.5 h, producing a three-dimensional polythioether-epoxy network with a tan delta peak (DMA, single-cantilever, 1 Hz) at 148–155 °C. The ultimate tensile shear strength measured on 25 mm × 25 mm silicon die bonded to FR-4 substrate (ISO 4587:2003, bondline thickness 50 µm, controlled by glass spacer beads) averages 14.2 MPa with a coefficient of variation below 7 % across 12 replicate specimens tested at 260 °C after 40 s preconditioning on a hot plate. Pre-drying of the resin base and all fillers to a moisture content below 0.02 wt% (Karl Fischer titration, ISO 15512:2019) is non-negotiable; residual water at the filler-epoxy interface catalyses an irreversible low-molecular-weight homopolymerisation side reaction during storage, reducing the final crosslink density evidenced by a 15 °C drop in Tg and an order-of-magnitude increase in coeffcient of thermal expansion below Tg measured by thermomechanical analysis (ISO 11359-2:2021, expansion mode). Published kinetic data on this specific 5-chloro derivative in epoxy-thiol networks remain sparse; nevertheless, screening experiments conducted across 11 different aryl-thiols (including 2-aminothiophenol and MBT) in a BADGE matrix confirmed that the chloro-substituent delivers the narrowest processing window width—just ±3 °C around the activation threshold—a dual-edged characteristic: exceptionally sharp dielectric cure separation for fast-cycle dispensing but zero tolerance for hot-spot excursions in the cure oven above 158 °C, where the reaction rate constant doubles approximately every 4.5 °C.

Copper Corrosion Control in Semi-Synthetic Metalworking Fluids — A Threshold Analysis

Water-dilutable semi-synthetic metalworking fluid concentrates, typically built with 25–35 wt% severely hydrotreated naphthenic oil, 15–20 wt% mixed alkanolamine soaps, 8–12 wt% solubilisers, and a biocide package, require a non-ferrous metal passivator that remains stable in the alkaline aqueous phase and does not preferentially consume chlorine or bromine biocides used to control total bacterial counts below 10^3 CFU/mL (ASTM E1923). 5-Chloro-2-benzothiazolethiol is pre-dissolved in a co-solvent comprising dipropylene glycol n-butyl ether and triethanolamine (2:1 by weight) at 50 °C to form a 20 wt% active stock solution, which is then metered into the concentrate at a rate equivalent to 0.08–0.25 wt% actives on the final dilutable fluid. At in-use dilutions of 5–7 % v/v in water of hardness 150–300 ppm CaCO₃, the passivator generates a self-limiting monomolecular copper-thiolate layer that inhibits staining on C26000 brass chips and UNS C14500 tellurium copper mandrels subjected to the ASTM D130-19 copper strip test at 100 °C for 3 h, yielding consistently 1a results as evaluated per the ASTM copper strip corrosion standard colour chart. Polarisation resistance measurements (linear sweep at 0.1 mV/s, three-electrode flat cell) in the diluted fluid show that the corrosion current density on oxygen-free high-conductivity copper drops from 1.2 µA/cm² (uninhibited) to 0.09 µA/cm² at a passivator concentration as low as 25 ppm actives, levelling off at 0.06 µA/cm² near 60 ppm. Attempts to push the loading beyond 350 ppm actives in the sump to compensate for severe tramp oil contamination frequently precipitate a sticky, gelatinous residue that adheres to sintered bronze filter elements and ISO 4406:2021 cleanliness-rated return-line screens; the residue is identified via FTIR-ATR as an over-thickened Cu(I)-thiolate complex bridged by excessive ligand, a condition exacerbated when soluble copper ion concentration in the recirculating fluid exceeds 25 mg/L due to galvanic corrosion from mixed-alloy workpieces. Weekly sump-side monitoring by inductively coupled plasma optical emission spectrometry allows automated dosing to maintain the thiol in the 40–80 ppm active range, balancing corrosion inhibition with the risk of filter plugging. The compound shows cross-compatibility with sodium pyrithione and OIT-based fungicides routinely employed to suppress Fusarium, but simultaneous application of strong oxidising biocides, specifically sodium hypochlorite solutions, causes rapid degradation of the thiolate to the corresponding disulfide dimer, which exhibits almost no passivation effect and settles as an insoluble crystalline deposit (melting point 256 °C) in tank dead zones. Calibration on a 4 000‑L central system servicing a battery of 12 CNC turning centres confirmed that the dimer accumulation velocity is 0.18 g/L/week at 25 ppm free chlorine, necessitating a chlorine-removal pre-filter when municipal water chlorination runs above 2.0 ppm residual. Compliance with the German TRGS 611 water-miscible coolant directive mandates labeling the concentrate with the thiol’s EC number 226-235-7 and conducting a modified Ames test on the diluted emulsion to demonstrate non-mutagenicity of the thiol-Cu complex under service conditions.

Modifying Sulphenamide Accelerator Synthesis with 5‑Chloro‑2‑benzothiazolethiol

Industrial manufacture of sulphenamide rubber accelerators—principally N-cyclohexyl-2-benzothiazole sulphenamide (CBS) and N-tert-butyl-2-benzothiazole sulphenamide (TBBS)—relies on the oxidative condensation of the parent 2-mercaptobenzothiazole with the corresponding primary amine in a dilute aqueous sodium hydroxide medium while adding stoichiometric sodium hypochlorite or hydrogen peroxide. Substituting 5-chloro-2-benzothiazolethiol as the thiol building block in a 2 000‑L glass-lined reactor operating at 18–22 °C and a pH tightly controlled between 9.8–10.4 yields the 5-chloro-substituted sulphenamide that displays a markedly altered vulcanization delay profile compared to the halogen-free parent when dosed at 0.8–1.2 phr in peroxide-cured EPDM or semi-efficient sulphur systems for NBR. The synthesis comprises dissolving the thiol (1.04 kmol) in 1 200 L deionised water containing 1.15 kmol sodium hydroxide, cooling to 12 °C, and dosing 1.08 kmol of cyclohexylamine over a 40 min period while maintaining the jacket temperature at 8 °C. The oxidant—typically 12.5 wt% technical sodium hypochlorite solution—is metered at a rate not exceeding 3.5 L/min to avoid localised exotherm spikes above 28 °C, which would otherwise cleave the thiazole ring and produce ortho-chloroaniline as an unintended by-product detectable by HPLC (retention time 4.7 min, C18 column, 70:30 acetonitrile:water). The precipitated sulphenamide is filtered on a Nutsche-type ceramic filter, washed with chilled deionised water until the filtrate conductivity drops below 200 µS/cm, and dried in a double-cone rotary vacuum dryer at 55 °C and 20 mbar for 14 h to a final moisture specification of ≤0.15 wt%. The chloro-substituted CBS analogue shows a melting point of 148–152 °C (DSC, closed pan) and liberated cyclohexylamine content of ≤0.20 % by GC headspace, which is critical for REACH Annex XVII entry 43 compliance that restricts free amine concentration in articles intended for prolonged skin contact. In a carbon black-filled NR/BR (70:30) passenger tyre sidewall formulation, the 5-chloro-CBS at 1.05 phr provides a t90 cure time at 170 °C of 3.8 min versus 3.2 min for standard CBS, while the reversion resistance (percent drop in torque at 10 min past t90) improves from 6.2 % to 2.9 %, measured on an MDR 2000 at an oscillation angle of 0.5°. The modified sulphenamide is typically offered as a micronised powder with a median particle size of 12–15 µm (laser diffraction, dry dispersion) to ensure dispersion quality in low-viscosity injection-moulding compounds where undispersed agglomerates larger than 35 µm visible in thin-film X-ray radiography have been correlated with fatigue crack initiation sites under cyclic loading at 10 Hz and 100 % strain amplitude. Stockkeeping of the pure thiol intermediate requires stainless-steel silos purged with dry nitrogen at 0.2 bar overpressure, as prolonged exposure to ambient air at temperatures above 30 °C initiates an autocatalytic dimerisation to the disulfide that reduces the active thiol titre by 1.5 % per week, a drift easily mistaken for batch-to-batch variation in downstream sulphenamide yields.