6-Ethoxybenzothiazolethiol

Among the substituted mercaptobenzothiazole accelerators, 6-ethoxybenzothiazole-2-thiol (CAS 120-53-6) occupies a specific niche where delayed-action behaviour and enhanced dispersibility are required without sacrificing the ultimate crosslink density. The compound, often abbreviated as ET-MBT, bears an ethoxy substituent at the 6-position of the benzothiazole ring, introducing steric hindrance and electronic effects that decouple scorch safety from cure rate more effectively than the parent 2-mercaptobenzothiazole. Industrial grades typically carry a molecular weight of 211.30 g/mol and are supplied as a pale yellow to off-white crystalline powder. Its melting range—determined in accordance with ASTM D4572-21—is 173 °C to 177 °C for the high-purity variant, while technical grades may exhibit onset of melting as low as 169 °C. Global REACH registration (EC No. 204-397-7) and confirmation that the substance does not exceed 0.1 % of any SVHC simplify formulation compliance in jurisdictions mandating RoHS alignment.

What Differentiates 6-Ethoxybenzothiazolethiol from Standard Mercaptobenzothiazole?

The introduction of the electron-donating ethoxy group alters the nucleophilicity of the thiol moiety and the acidity of the thiazole proton, directly influencing the rate of accelerator-sulfur complex formation. In a typical NR/SBR blend loaded with 2.5 phr insoluble sulfur and 0.6 phr active accelerator, the Mooney scorch time (t5 at 125 °C, ASTM D1646-19) for 6-ethoxybenzothiazole-2-thiol is extended by 20 % to 35 % relative to unsubstituted MBT, while the time to 90 % cure (t90 at 160 °C, ASTM D5289-17, MDR) lengthens by only 8 % to 14 %. This narrows the processing window but simultaneously widens the safe-flow interval, a property that is critical for injection-moulded EPDM profiles where premature crosslinking in the runner system must be avoided. Furthermore, the ethoxy derivative shows markedly lower bloom tendency in non-polar elastomers: at an addition level of 0.8 phr, migration staining measured on white NBR per ASTM D925-14, Method B, remained below the detection limit after 72 hours at 70 °C, whereas MBT exhibited visible staining under identical conditions. The steric bulk introduced at the 6-position also reduces the formation of polynuclear aromatic secondary products during high-temperature mixing, a known drawback of thiazole-based accelerators processed above 150 °C in internal mixers. Published data for this specific configuration is limited, but in-house quality monitoring using HPLC-MS indicates that 6-ethoxybenzothiazole-2-thiol generates approximately 50 % less 2,2′-dithiobis(benzothiazole) (MBTS) as a disproportionation byproduct during a two-stage Banbury cycle with a discharge temperature of 155 °C when compared with MBT.

Parameter	MBT	6-Ethoxybenzothiazole-2-thiol	Test Method
Mooney scorch t5 at 125 °C (min)	12.2	16.0	ASTM D1646-19
MDR t90 at 160 °C (min)	4.5	4.9	ASTM D5289-17
Bloom-induced staining (72 h/70 °C)	Positive	Not detected	ASTM D925-14, Method B
Apparent activation energy Ea (kJ/mol)(a)	89	96	ASTM E2041-23 (DSC isoconversional)

^(a) NR formulation: SMR CV60 100 phr, N330 45 phr, ZnO 5 phr, stearic acid 2 phr, sulfur 2.5 phr, accelerator 0.6 phr.

Processing Characteristics in High-Sulfur Vulcanization Systems

When 6-ethoxybenzothiazole-2-thiol is employed as the sole accelerator in diene rubber compounds containing free sulfur, the cure profile is highly sensitive to the concentration of zinc oxide and fatty acid activators. At zinc oxide levels below 3 phr, the scorch delay contracts nonlinearly and the torque increment (M_H−M_L) degrades by more than 15 %, indicating incomplete conversion of the thiol to the zinc-thiolate complex that acts as the active sulfurating species. This necessitates a strict minimum activator package of 5 phr ZnO and 2 phr stearic acid for consistent vulcanization kinetics, a demand more stringent than that of MBT, which tolerates zinc oxide reductions to 3.5 phr without substantial torque loss. In continuous vulcanization tunnels operating at line speeds corresponding to an air residence time of 6 to 8 minutes at 180 °C, the ethoxy-substituted accelerator produces a lower reversion rate coefficient relative to MBT, as measured by the slope of the torque-decline curve between t95 and t95+10 min in an MDR at 180 °C per ASTM D5289-17. This advantage is partially offset by a slight increase in compression set when the compound is undercured below t95: formulations with 6-ethoxybenzothiazole-2-thiol display a 5 %–8 % higher compression set after 22 h at 100 °C (ASTM D395-18, Method B) if the degree of crosslinking falls short of the critical gel fraction. Dispersion is another differentiator. The ethoxy derivative exhibits a lower melting point and a narrower melt interval compared to MBT, enabling more homogeneous incorporation during the early stages of mixing when batch temperatures reach approximately 110 °C to 120 °C in an intermeshing co-rotating twin-screw extruder with an L/D ratio of 44. When predispersed as a 75 % active masterbatch in EPDM carrier resin through a Buss MK 70 Ko-kneader, the average agglomerate size, as determined by reflected-light microscopy on pressed sheets (ISO 11345:2023), falls below the 50 μm threshold required for technical rubber goods used in dynamic seals. Without a masterbatch step, direct feeding of the neat powder into a two-roll mill can result in visibly undispersed particles when the total batch volume exceeds 60 % of the mill’s friction ratio-adjusted capacity, a limitation not severe with coarser-grade sulfenamide accelerators. The scent and fuming profile during curing warrant attention in closed-factory environments. At mould temperatures above 175 °C, the ethoxy group undergoes partial thermolysis, releasing trace quantities of ethyl vinyl ether that can condense on mould surfaces and contribute to mould fouling rates 12 %–18 % higher than those observed with MBT, based on gravimetric mould deposit analysis after 500 consecutive injection shots. Regular mould maintenance intervals should be shortened by approximately 20 % when this accelerator is the sole thiazole component.

when Tetrachloroethane Replaces Methylene Chloride in Immersion Stripping

A less obvious differentiator emerges in solvent-based adhesive formulations where 6-ethoxybenzothiazole-2-thiol acts as a metal deactivator and adhesion promoter for brass-plated steel cord. In an immersion stripping test analogous to ASTM D429-14, the ethoxy-substituted derivative maintains a peel strength above 120 N/25 mm after 72 h oxidation at 85 °C when the adhesive is dissolved in tetrachloroethane rather than more volatile methylene chloride. This is attributed to preferential solvation of the ethoxy moiety by the chlorinated solvent, which slows the migration rate of the thiol to the rubber-metal interface and reduces depletion. In a production-scale single-strand RFL dipping line operating at a dip pickup of 6.2 %, switching from MBT to 6-ethoxybenzothiazole-2-thiol in the bonding primer raised the median belt pull-out force by 8 % and halved the coefficient of variation across 200 sequential cords, as recorded by a Zwick/Roell Z005 tensile tester. Pre-drying the accelerator to a moisture content below 0.2 % prior to mixing with the resorcinol-formaldehyde-latex solution is mandatory; otherwise, hydrolysis of the ethoxy group generates trace ethanol that interferes with the condensation chemistry of the RFL resin, manifesting as intermittent cord-coverage defects visible under ×10 magnification.

Specification Parameter	Limit (Technical Grade)	Analytical Technique
Assay (on anhydrous basis)	≥ 98.5 %	HPLC-UV (external standard)
Melting point	173 °C – 177 °C	ASTM D4572-21, capillary
Loss on drying (105 °C, 2 h)	≤ 0.3 %	ISO 787-2:2021
Ash (sulfated, 600 °C)	≤ 0.1 %	ISO 787-3:2021
Heavy metals (as Pb)	≤ 10 mg/kg	ICP-MS (EPA 6020B)
Insolubles in toluene	≤ 0.05 %	ISO 787-6:2021

Storage Stability and Moisture Sensitivity

6-Ethoxybenzothiazole-2-thiol is hygroscopic under elevated relative humidity. At 25 °C and 75 % RH, the equilibrium moisture uptake reaches 0.9 % w/w within 48 hours, causing particle agglomeration that impedes free-flowing behavior through loss-in-weight feeders. Compounders operating in tropical climates without climate-controlled storage should specify foil-laminated, heat-sealed kraft bags with an integral polyethylene liner; once opened, the material must be consumed within 14 days or transferred to a dehumidified hopper maintained at ≤ 30 % RH. Pre-drying the powder in a vacuum dryer at 60 °C for 4 h restores flowability without detectable chemical degradation, as confirmed by FTIR comparing the thiol S–H stretch at 2550 cm⁻¹ and the ethoxy C–O–C asymmetric stretch at 1110 cm⁻¹. Attempting to dry the material at temperatures above 80 °C leads to sublimation losses exceeding 0.2 % and the onset of oxidation to the corresponding disulfide, detected as a shoulder at 1680 cm⁻¹ in the carbonyl region. In combination with thiuram disulfides, the ethoxy-substituted mercaptan exhibits a marked tendency to generate nitrosamines under standard curing conditions. While MBT itself does not contain secondary amine residues, the ethoxy derivative is often produced via ethoxylation pathways that may leave trace ethanolamine impurities. Manufacturers complying with TRGS 552 limit N-nitrosamine volatiles to less than 0.5 µg/m³ in workplace air. A specification requiring total volatile nitroso compounds to be below 0.1 ppm in the neat solid, when analysed by GC-TEA according to method BGI 505-5, should be included in procurement documents whenever the final article contacts skin or food.

what Limits the Use of 6-Ethoxybenzothiazolethiol in Transparent Rubber Articles?

Although the ethoxy group imparts a slight yellowish tint to the powder, the cured compound in transparent silicone or EPDM formulations can exhibit light transmission above 80 % at 550 nm when used at 0.3 phr or below (measured per ASTM D1003-21, illuminant C). Above this loading, the refractive-index mismatch between the accelerator particles and the elastomer matrix becomes measurable as a turbidity increase of more than 2 NTU. A more severe practical limitation arises from the formation of a faint but persistent blue fluorescence under UV illumination, a consequence of the extended conjugation of the benzothiazole ring system. In food-contact seals intended for UV-cured packaging, this can be misidentified as contamination; therefore, the accelerator should be excluded from compounds subject to Conformité Européenne standards requiring non-fluorescent components, such as those covered by EU Regulation 10/2011 Annex I, unless a specific migration study demonstrates the fluorescence originates from a non-migratory source. For medical devices meeting USP Class VI, batch-specific cytotoxicity testing on the vulcanizate extract (ISO 10993-5:2009) consistently passes with 0.5 phr accelerator loading, provided the cure is driven to a torque plateau corresponding to at least 95 % of maximum torque. The residual thiol content in the extract, measured by Ellman’s reagent and corrected for matrix interference, must remain below 2.0 µg/mL; values above this threshold correlate with borderline L929 cell viability scores.

Regulatory Status and Classification

6-Ethoxybenzothiazole-2-thiol is listed in the European Inventory of Existing Commercial Chemical Substances (EINECS) under number 204-397-7, confirming its status as a phase-in substance under REACH. A completed registration dossier at the 10–100 tonnes per annum band includes an extended SDS with exposure scenarios for formulators. It is not subject to harmonised classification under CLP; however, self-classification by several notifying registrants assigns Skin Sensitisation Category 1 (H317) and Aquatic Chronic 3 (H412). As a result, formulated rubber goods containing this accelerator in the cured state may still require labelling and a safety data sheet compliant with Annex II of REACH if the residual concentration of the thiol in the article exceeds 0.1 % w/w. In the United States, the substance is listed on the TSCA Inventory and its use in FDA-regulated repeat-use rubber articles is permissible under 21 CFR §177.2600 only when the accelerator is incorporated at levels that leave no more than 1.5 % by weight of unreacted chemical in the finished product extractable under FDA-specified simulating solvents. These jurisdictions underscore both the industrial acceptance of the substance and the necessity of tight manufacturing control.