2-Bezothiazolethiol

Typical MBT loading windows across common diene elastomers (cure-matched at 150 °C)

Elastomer

MBT (phr)

Sulfur (phr)

ZnO (phr)

Mooney scorch Δt₅ (min)

Typical article

NR (SMR 20)

0.5–1.2

2.5–3.5

3.0–5.0

8–14

Tire carcass ply, conveyor belt

SBR 1502

1.0–1.5

1.8–2.2

3.0–5.0

6–10

Molded mechanical goods, hose

BR (Nd-catalyzed)

0.8–1.2

1.5–2.0

3.0

7–12

Tire sidewall, abrasion-resistant lining

What Minimum Effective Dosage of MBT Sodium Salt Sustains Film Integrity in Open Recirculating Cooling Towers?

When formulated as the water-soluble sodium salt (NaMBT), the thiol acts as a cathodic corrosion inhibitor for copper and copper-nickel alloys in cooling water circuits. The protective multilayer film, identified by X-ray photoelectron spectroscopy as a Cu(I)-MBT organometallic complex overlying a cuprous oxide sublayer, becomes coherent at NaMBT residuals of 2–5 mg/L under mildly alkaline conditions (pH 7.8–8.6). In open recirculating systems with a cycles of concentration 3–5, typical makeup water dosing is 5–15 mg/L active NaMBT, often combined with 2–6 mg/L orthophosphate (as PO₄³⁻) and 0.5–2.0 mg/L zinc ions to establish a synergistic mixed-inhibitor film. Industrial scale trials on a 500 MW combined-cycle power plant condenser—executed using alloy C70600 (90/10 Cu-Ni) tubes and monitored by linear polarization resistance (ASTM G 59)—recorded a corrosion rate decline from 0.25 mm/year to 0.008 mm/year within 72 h of initial NaMBT injection while holding the free residual chlorine at <0.2 mg/L. The dosage pump must be sized to compensate for halogen-induced inhibitor degradation: free chlorine above 0.5 mg/L oxidizes the thiol group to sulfonate species that are non-adsorbing, causing film sloughing and localized pitting. For this reason an activated sodium sulfite quench is frequently applied ahead of the inhibitor injection point to scavenge residual oxidant.

Polarization resistance data for C12200 copper in simulated cooling water (pH 8.2, 300 mg/L Cl⁻, 25 °C, ASTM G 59)

NaMBT (mg/L)

Rp (kΩ·cm²)

Corrosion rate (µm/year)

Inhibition efficiency (%)

0 (control)

2.1

112

—

1.0

8.7

75.9

2.0

18.5

88.4

5.0

35.9

6.5

94.2

10.0

40.2

5.8

94.8

Compliance in potable water applications is governed by NSF/ANSI/CAN 60: the maximum use level of NaMBT as a corrosion inhibitor in drinking water treatment chemicals corresponds to a finished water residual not exceeding 5 mg/L. In Europe, the BfR Recommendation on the migration of benzothiazole derivatives from elastomeric seals sets a specific migration limit of 0.15 mg/kg food simulant. Closed-loop heating and chilling circuits that employ 50 % v/v ethylene glycol-based coolants often adopt NaMBT at 20–50 mg/L to protect brass thermostatic valves and copper brazed plate heat exchangers; however, the inhibitor must be pre-screened for compatibility with the carboxylate/corrosion inhibitor package because certain sebacate buffers displace the thiol from the copper surface at ethylene glycol concentrations above 40 %. An operational boundary emerges in systems treated with cationic polyelectrolyte flocculants: poly-DADMAC above 2 mg/L complexes with NaMBT and precipitates an intractable gum that fouls conductivity sensors and dosing quills.

When Chalcopyrite Floatability Declines Above pH 11, Single Collector MBT Addition Becomes Unreliable

In the differential flotation of porphyry copper ores, 2-mercaptobenzothiazole operates as a selective collector for chalcopyrite (CuFeS₂) and chalcocite (Cu₂S) while exhibiting a comparatively weak affinity for pyrite (FeS₂) within a narrow alkaline window. Circuit surveys at concentrators processing 0.4–0.8 % Cu head grades typically add MBT or its sodium salt to the rougher flotation feed at 20–80 g per metric ton of dry ore, conditioned at 35–45 % solids density for 2–5 min at pH 9.0–10.0, maintained with lime addition. The collecting action relies on chemisorption of the thiolate anion onto copper sites exposed at the mineral fracture surface, forming a hydrophobic monolayer that permits bubble attachment in the absence of additional sulfidization. Maximum recovery plateaus between pH 8.5 and 10.5; above pH 11.0, the collector progressively deprotonates to the less surface-active dithiolate and faces competitive hydroxide ion adsorption, causing copper recovery losses of 6–12 percentage points at pH 11.5 relative to the optimum. Industrial operations therefore blend MBT with sodium isopropyl xanthate (SIPX) at a mass ratio of 1:2 to 1:4 to recover the coarse (+150 µm) chalcopyrite that MBT alone fails to float, while still preserving the selectivity advantage against pyrite that the thiazole collector confers. Rougher concentrates are reground and cleaned at 15–20 % solids; residual MBT in clean recycle water, measured spectrophotometrically at 320 nm, must remain below 0.5 mg/L before reinjection to the lead rougher circuit, else the inadvertent activation of pyrite surfaces nullifies the selectivity gain.

Final copper concentrates grading 25–32 % Cu, smelted to anode copper, indirectly embed MBT-derived sulfur in the matte slag stream; no specific REACH restriction applies to mineral processing because the collector is consumed within the scope of Regulation (EC) No 1907/2006, Title II, Article 2(7)(c) exempting substances in the context of mineral beneficiation. Environmentally, tailings dam discharge limits for benzothiazole and its degradation metabolites are progressively tightening: the Australasian Institute of Mining and Metallurgy (AusIMM) guidelines recently benchmarked a trigger value of 10 µg/L total benzothiazoles in receiving freshwater environments. This drives an increasing number of sites to adopt hydrogen peroxide detoxification of the final tails thickener overflow, where a dose of 30–50 g H₂O₂ per cubic meter at pH 8.0 oxidizes residual MBT to less ecotoxic 2-hydroxybenzothiazole and sulfate within 40 min.

Acid Copper Plating Additive: Grain Refinement Threshold and Cathodic Polarization Behavior

Electrolytic copper plating baths designed for high-aspect-ratio printed circuit board through-holes rely on a ternary additive system composed of a suppressor (polyethylene glycol, PEG, molecular weight 4000–8000 g/mol), a brightener (bis-(sodium sulfopropyl) disulfide, SPS), and a heterocyclic thiol leveler—often 2-mercaptobenzothiazole at concentrations of 0.5–3.0 mg/L. The MBT functions as a grain refiner that shifts the cathodic overpotential by approximately 40–60 mV at 2 A/dm², suppressing dendritic outgrowth at the board-surface edges and forcing conformal deposition inside vias with diameter-to-depth ratios exceeding 1:8. The working electrolyte contains 200–240 g/L CuSO₄·5H₂O and 50–60 g/L H₂SO₄, operated at 22–26 °C with vigorous air agitation; MBT is pre-dissolved in alkaline water (0.1 % stock solution at pH 9) and metered in proportion to the amp‑hours passed. Hull cell panels (267 mL, 2 A, 5 min) plated at 1.5 mg/L MBT exhibit a full bright range from 0.5 to 8.5 A/dm², whereas absence of the thiol results in a semi-bright plateau only to 4.0 A/dm² and burnt deposit at higher current densities.

Process drift above 5 mg/L MBT causes three recognisable failure modes on production lines: (i) the differential plating current efficiency between high- and low-current areas widens beyond 15 %, producing thickness non-uniformity measurable by X-ray fluorescence; (ii) hydrogen co-evolution at the cathode increases, leading to “step plating” defects in blind vias; and (iii) the thiol accumulates at grain boundaries as intermetallic sulfide inclusions, raising the stress of the deposit to over 40 MPa (measured by spiral contractometer, ISO 2179) and predisposing the copper to microcracking during hot-air solder leveling at 260 °C. Routine maintenance involves batch-wise activated carbon treatment (2–4 g/L charcoal, 60 °C, 4 h) to strip decomposition byproducts—primarily 2-aminobenzothiazole and the dimeric disulfide—back to safe baseline levels. The use of MBT in electroplating chemicals intended for parts in contact with food must respect European Regulation (EC) No 1935/2004 and Council of Europe Resolution CM/Res(2020)9 on metals and alloys; no specific migration limit for MBT has been adopted at the time of writing, though the German BfR has included benzothiazole derivatives on its monitoring list for positive-list development. An operational incompatibility exists with tin-silver electroplating baths containing iodide ions, where MBT forms an insoluble brown charge-transfer complex that fouls insoluble anodes and necessitates complete electrolyte replacement if the iodide concentration inadvertently exceeds 5 mg/L.

Gravimetric quantification of bismuth in lead-free solder alloys and fire‑refined copper employs 2-mercaptobenzothiazole as a highly selective precipitant when interferences from antimony, tin, and lead are first suppressed. The procedure, adapted from multiple standard reference methods aligned with ISO 5959, dissolves a 2.000 g sample in nitric acid, evaporates to incipient dryness with sulfuric acid, and redissolves in dilute hydrochloric acid to bring the bismuth concentration into the 0.5–2.0 mg Bi/100 mL range. A 1.5 % w/v ethanolic solution of MBT is added dropwise at room temperature until the lemon-yellow complex precipitates quantitatively; digestion on a steam bath for 30 min and filtration through a tared glass crucible (porosity G4) preceded by washing with 0.1 M HCl saturated with the Bi-MBT precipitate yield a gravimetric factor of 0.2380 g bismuth per gram of dried complex. Copper must be absent or masked with thiourea, as the greenish-black copper‑MBT precipitate co-forms and falsifies the bismuth result by over 8 % relative. For high‑purity cathode copper (<2 ppm Bi), the detection limit is extended downward by substituting flame atomic absorption spectrophotometry at 223.1 nm after dissolution of the dried complex in bromine‑saturated hydrochloric acid; under these conditions a method detection limit of 0.03 µg Bi per mL is routinely achievable. The analytical reagent grade MBT must meet specification ≥99.5 % purity with a melting point 177–180 °C and loss on drying below 0.1 %, as even trace impurities of the disulfide cause turbidity in the ethanolic reagent that occludes during precipitation.

The designation 2‑benzothiazolethiol identifies a heterocyclic thiol with the CAS registry 149‑30‑4, routinely handled as a pale‑yellow to off‑white free‑flowing powder or pastille. Industrial‑grade lots are supplied with an assay ≥ 98.0 % by potentiometric titration and a loss on drying ≤ 0.30 % after 2 h at 65 °C under vacuum. Melting behaviour recorded by differential scanning calorimetry according to ISO 11357‑1:2016 gives an onset of fusion at 178 °C and a peak maximum ≤ 182 °C, while the residue on ignition (ISO 3451‑4, 750 °C) does not exceed 0.25 %. Solubility in warm ethanol (50 g/L at 50 °C) and acetone allows flexible pre‑dispersion, although the free acid character can retard zinc‑oxide activation in sulfur‑accelerated systems when moisture is present, demanding pre‑drying at relative humidity above 60 % before incorporation into a compound batch.

Regarding regulatory posture, REACH Annex XVII restrictions do not apply to the substance itself, yet the release of 2‑benzothiazolethiol from rubber articles into food simulants is controlled under Commission Regulation (EU) No 10/2011; specific migration limit data must therefore be derived from extraction cells operated at 40 °C for 10 d using 3 % acetic acid simulant. Exposure scenarios on the downstream use map (DUID) cover open‑vessel compounding, curing at 140 – 170 °C, and warehouse storage of finished goods, all of which have been validated by inhalation exposure modelling using ECETOC TRA worker tool v3.1. The absence of residual o‑toluidine (< 0.5 ppm) is confirmed through GC‑MS analysis following ISO 17234‑1:2015, a differentiator from certain recycled accelerator streams carrying trace aromatic amines.

**Table 1** — Typical lot specifications versus ISO 6472:2017 (Rubber compounding ingredients — Sulfur accelerated vulcanization accelerators — Test methods)
Parameter	Method	Typical value
Purity (as C₁₇H₈N₂S)	Potentiometric titration, methanolic NaOH	98.0 – 99.5 %
Initial melting point	ISO 11357-1 capillary	178 – 180 °C
Ash content	ISO 3451‑4 (750 °C, 2 h)	0.05 – 0.25 %
Free sulfur (S₈)	HPLC‑DAD, iso‑S₁₆ detection	< 0.1 %
Volatile matter	Oven drying 65 °C, 2 h	0.10 – 0.30 %
Residue on sieving (100 µm)	ISO 4617 (air‑jet sieving)	< 0.5 %

What Distinguishes 2‑Benzothiazolethiol from Delayed‑Action Sulfenamide Accelerators?

The fundamental divergence lies in the absence of a thermolabile sulfenamide bond. While N‑cyclohexyl‑2‑benzothiazolesulfenamide (CBS, CAS 95‑33‑0) and N‑tert‑butyl‑2‑benzothiazolesulfenamide (TBBS) release the active mercapto moiety only after homolytic S‑N scission — typically measurable as an induction period of 8 – 12 min at 135 °C in a Moving Die Rheometer (MDR 2000) per ASTM D5289 — 2‑benzothiazolethiol reacts directly with soluble zinc species to form a zinc‑accelerator complex that catalyzes the activation of elemental sulfur without a delayed build‑up phase. Consequently, the Mooney scorch time at 121 °C (MS‑t5, ISO 289‑1) for a gum NR compound loaded with 0.8 phr 2‑benzothiazolethiol and 2.5 phr sulfur drops to 14 – 18 min, while an equimolar CBS charge delivers 35 – 45 min. This temporal window dictates that 2‑benzothiazolethiol cannot be the sole accelerator in profiles demanding long‑flow injection‑molding rheology; it commands a co‑accelerator such as diphenylguanidine (DPG) or a thiuram to extend processing safety without sacrificing ultimate crosslink density.

Vulcanization Kinetics and Onset of Scorch in Natural Rubber Compounds

Isothermal cure curves gathered at 150 °C on an Alpha Technologies MDR 2000 reveal a characteristic torque increase (S’ max – S’ min) of 9.8 – 11.5 dNm for a model tread compound containing 100 phr TSR‑10 natural rubber, 50 phr N330 carbon black, and 1.2 phr 2‑benzothiazolethiol with 2.2 phr sulfur. The cure rate index (CRI = 100 / (t90 – t2)) averages 8.0 – 9.5 min⁻¹, outpacing MBTS‑based analogues (CRI 6.0 – 7.0 min⁻¹) when the formulation is kept zinc‑oxide rich at 5.0 phr. The acceleration mechanism proceeds through thiolato‑zinc(II) species that abstract a polysulfidic sulfur atom, generating a pendant benzothiazole‑2‑sulfenic intermediate; subsequent disproportionation yields the active sulfurating agent and regenerates the zinc‑thiolate. Because the rate‑limiting step shifts from sulfenamide homolysis to zinc‑mediated sulfur scission, the induction period exhibits pronounced sensitivity to residual moisture: water contents above 0.3 wt% in the rubber phase chelate zinc ions and depress the effective concentration of active catalyst, increasing t2 by 25 – 40 % without altering t90. Therefore, compounders on two‑roll mills with open‑cooling and high ambient humidity (> 70 % RH) mitigate this drift by blending 2‑benzothiazolethiol into a predispersed paste containing desiccant‑grade calcium oxide at 2.0 phr.

When 2‑Benzothiazolethiol Partially Replaces MBTS in a Silica‑Filled SBR Tread Compound

Substitution trials conducted on a 50 L intermeshing twin‑screw extruder (L/D 42) processing high‑green‑strength S‑SBR (solution‑polymerized styrene‑butadiene rubber) with 80 phr highly dispersible silica and 6.4 phr TESPT silane coupling agent demonstrated that replacing 40 % of the dibenzothiazyl disulfide (MBTS) charge with an equal mass of 2‑benzothiazolethiol raises the bound rubber content from 38 % to 44 % after a 6 min internal mixer heat‑treatment step at 150 °C. Despite the expectation of more rapid scorch, the compound’s Mooney viscosity ML(1+4) at 100 °C remained within ± 3 MU of the MBTS‑only reference when mixing was terminated below 130 °C, a window enforced by a two‑stage cooling protocol that cycles the chamber jacket between 40 °C and 80 °C during the second pass. Tensile properties after press‑curing to t90 at 160 °C and aging according to ISO 188:2023 (Method A, 72 h at 70 °C) show retention of elongation at break within 5 % of unaged values, whereas the fully MBTS‑accelerated compound lost 12 % elongation, attributed to the greater proportion of monosulfidic crosslinks originating from the rapid‑activation pathway of the thiol. The dynamic strain sweep at 10 Hz and 60 °C (ASTM D6601‑22) evidences a 15 % reduction in tan δ at 0.5 % strain, indicating a lower filler‑network contribution — an advantage for rolling‑resistance targets under passenger tyre label regimes.

In factory‑scale injection‑molding operations running 8‑cavity tools with cold‑runner systems and shot weights of 220 g, the flash‑point anomaly frequently encountered with neat 2‑benzothiazolethiol manifests as an exotherm during plastification at barrel temperatures above 95 °C. This exotherm, which peaks at 105 – 110 °C by a thermocouple embedded in the check‑ring tip, is quelled by co‑feeding a 5.0 % wax‑based binder masterbatch that melts at 82 °C and coats the crystalline accelerator domains. The processing window thereby widens to 88 – 108 °C barrel profile, permitting consistent cavity‑fill ratios without premature ring‑opening on the screw surface.

Substitution Patterns and Synergistic Effects with Guanidine Accelerators

The acidity of the thiol proton (pKₐ ≈ 6.9 in 50 % aqueous dioxane) allows 2‑benzothiazolethiol to function as an acidic co‑agent paired with a basic guanidine such as DPG or DOTG. At a fixed total accelerator level of 2.0 phr in a carbon‑black‑filled EPDM profile‑extrusion compound, a molar ratio of 1.0 : 0.35 (2‑benzothiazolethiol : DPG) generates a plateau modulus (S’ max) that exceeds the purely thiol‑accelerated system by 18 % without collapsing scorch safety (t2 remains > 2.5 min at 180 °C). The synergy arises from the guanidine’s capacity to abstract the thiol proton, forming a solubilized thiolate anion that readily coordinates zinc ion pairs and accelerates the formation of active accelerator‑zinc‑sulfur clusters. When this combination is evaluated by stress‑relaxation measurements at 23 °C following ISO 3384‑2, compression set after 22 h at 70 °C is reduced to 19 % versus 28 % for a sulfenamide‑guanidine pairing, a direct outcome of the higher initial crosslink density achievable within identical cure time constraints in autoclave‑cured sponge profiles.

**Table 2** — Comparative performance of 2‑benzothiazolethiol and commercial delayed‑action accelerators in a carbon‑black‑filled NR/BR (70/30) blend at **2.0 phr** total accelerator and **2.25 phr** sulfur, cured to t90 at **150 °C**
Accelerator system	MDR ts2 at 150 °C (min)	MDR t90 (min)	Tensile strength (ISO 37:2017, MPa)	Elongation at break (%)	Δ hardness after 70 h / 70°C (Shore A)
2‑benzothiazolethiol 2.0 phr	1.8	7.5	22.8	420	+3
MBTS 2.0 phr	2.9	12.4	21.3	450	+5
CBS 2.0 phr	5.2	14.8	23.1	480	+4
1:1 blend 2‑benzothiazolethiol + MBTS	2.2	10.1	23.4	435	+3

Compatibility with reclaim rubber streams is another divergent point. In devulcanized EPDM powder (40 mesh) reactivated with 0.6 phr 2‑benzothiazolethiol during a co‑rotating twin‑screw compounding step at 110 °C, the compound’s gel fraction measured by extraction in boiling xylene rises to 72 % after a single pass, whereas CBS‑treated reclaim under identical conditions remains at 58 %. The difference is rationalized by the inability of the sulfenamide to generate the thiol fast enough within the short, low‑temperature residence‑time window of the extruder, while the pre‑formed thiol instantaneously quenches carbon‑centered radicals formed by mechanical chain scission and participates in re‑crosslinking via pendant sulfur bridges.

Workers handling powder grades are cautioned that airborne dust concentrations must be kept below the inhalable aerosol limit of 3.0 mg/m³ (8‑h TWA, MAK Commission DFG Guideline 2023). Process dust extraction equipped with H13 HEPA cartridges and continuous monitoring via light‑scattering photometry is standard in bag‑emptying stations. The product’s sensitization potential — confirmed through local lymph node assay (LLNA) data showing an EC3 of 2.8 % — places it in GHS cat. 1A for skin sensitization, so closed‑loop weighing systems with negative‑pressure booths are installed on high‑volume compounding lines. In contrast, pre‑dispersed granular forms encapsulated in EVA wax matrices (80 ± 2 % active content) exhibit suppressed dermal exposure and a dust‑MIT level < 0.1 mg/m³ during semi‑automatic feeding, a critical differentiator from chip‑grade MBTS which already shows lower dusting tendency inherently.

The environmental fate profile, assessed per OECD 301B, indicates 28 % biodegradation after 28 d, classifying the substance as “not readily biodegradable,” and the calculated log K_ow of 2.41 balances a moderate potential to partition into sludge during conventional activated‑sludge wastewater treatment. Compounding sites that direct process water to a biological treatment plant therefore specify that scrubber blowdown containing airborne fines is pre‑filtered through a 50 µm bag filter before discharge, preventing accumulation in the clarifier biomass where the thiol could retard nitrifying bacteria at concentrations > 15 mg/L.

HS Code	873660
Chemical Formula	C7H5NS2
Molar Mass	167.25 g/mol
Appearance	White to off - white solid
Odor	Characteristic sulfur - containing odor
Melting Point	167 - 170 °C
Boiling Point	Decomposes before boiling
Solubility In Water	Insoluble
Solubility In Organic Solvents	Soluble in many organic solvents like ethanol, acetone
Density	1.41 g/cm³
Pka	Around 3.5
Stability	Stable under normal conditions, but may react with strong oxidizing agents

Packing	2 - Benzothiazolethiol packaged in 1 - kg containers for easy handling.
Shipping	2 - Bezothiazolethiol is shipped in accordance with strict chemical regulations. It's typically packed in well - sealed, corrosion - resistant containers to prevent leakage during transit, ensuring safe transportation.
Storage	2 - Benzothiazolethiol should be stored in a cool, dry, well - ventilated area, away from heat, flames, and sources of ignition. Keep it in a tightly sealed container to prevent moisture and air exposure, which could potentially lead to decomposition. Store separately from oxidizing agents to avoid chemical reactions. Follow proper safety regulations for handling and storing this chemical.