2-Mercaptobenzothiazole Zinc

CAS No. 155-04-4 describes zinc bis(2-mercaptobenzothiazole), a coordination complex of zinc(II) with two deprotonated 2-mercaptobenzothiazole ligands. The compound is supplied as a pale yellow powder with a faint thiazole odor, molecular formula C₁₄H₈N₂S₄Zn and molecular weight 397.86 g/mol. Unlike an ad hoc combination of 2-mercaptobenzothiazole (MBT) and zinc oxide, pre-formed ZnMBT delivers a stoichiometrically fixed zinc-to-thiol ratio of 1:2, eliminating dispersion-dependent activation variability in sulfur-vulcanized elastomers. Beyond its primary role as a delayed-action rubber accelerator, the substance functions as a film-forming corrosion inhibitor for yellow metals in lubricating greases, engine coolants and metalworking fluids, and as a non‑leaching biocide in polymer articles.

What Distinguishes Industrial Zinc MBT Grades from Accelerator Combinations Using MBT and Zinc Oxide?

Commercial ZnMBT is available in multiple physical grades including a 96–98 % active fine powder, a dust-suppressed oil-coated variant containing 1.5–2.0 % naphthenic process oil, and a pre-dispersion in an elastomeric binder at 75 % active content. Specification parameters are controlled by high‑performance liquid chromatography (purity ≥ 95.0 %), complexometric zinc titration (Zn content 15.5–16.5 %), Karl Fischer moisture determination per ASTM D1533 (moisture ≤ 0.5 %), and wet sieve residue on a 45 µm screen (≤ 0.5 %). The material decomposes above 300 °C without a defined melting point. The operational distinction between pre-formed ZnMBT and a masterbatch generating zinc mercaptide in situ from MBT and ZnO is rooted in process capability. During open mill or internal mixer compounding of a natural rubber/carbon black system, ZnO dispersion determines the local stoichiometry of the accelerator complex. Batch‑to‑batch Mooney scorch time (MS at 121 °C, ASTM D1646) fluctuations of ±15 % are common with MBT/ZnO because the zinc mercaptide formation is rate‑limited by zinc oxide particle size and agglomerate breakdown. In a nitrile rubber (NBR) formulation containing 60 phr N550 carbon black, replacing a 2.0 phr MBT/5.0 phr ZnO pair with 1.2 phr ZnMBT reduced the scorch time range across seven consecutive production batches from 7.2 ± 0.9 min to 8.5 ± 0.3 min measured on an oscillating disc rheometer at 135 °C (ISO 6502-3). The tighter distribution reflects the elimination of diffusion‑controlled zinc‑accelerator complex formation. In a silica‑reinforced solution S‑SBR passenger tire tread, ZnMBT serves as a secondary accelerator alongside sulfenamide primaries (CBS or TBBS), providing a plateau cure that extends reversion resistance. The table below compares cure metrics obtained from a moving die rheometer at 160 °C according to ASTM D5289 for three accelerator systems at equimolar thiazole content.

Table 1 — Cure Characteristics of Silica-Filled S-SBR/BR Tread Compound at 160 °C (ASTM D5289)

Property	ZnMBT 1.3 phr + CBS 1.7 phr	MBT 1.0 phr / ZnO 3.0 phr + CBS 1.7 phr	CBS 2.5 phr (sulfenamide‑only)
Minimum torque ML (dNm)	1.8	1.9	1.6
Maximum torque MH (dNm)	14.2	13.8	14.8
Scorch time ts2 (min)	4.8	3.9	5.5
Cure time t90 (min)	11.2	10.5	9.8
Tensile strength (MPa) — ASTM D412	18.9	17.7	19.2
Elongation at break (%)	480	465	455

The ZnMBT‑boosted compound displays a processing safety margin (ts2) intermediate between the sulfenamide‑only reference and the MBT/ZnO system, while the delta torque (MH − ML) remains comparable. The plateau effect manifests as a broad optimum cure range; over‑curing from t90 to 30 min reduces tensile strength by less than 5 % in the ZnMBT variant, versus a 12 % drop for the MBT/ZnO combination, attributed to slower post‑vulcanization crosslink network degradation. In tire manufacturer extrusion lines, this translates to a wider processing window on a pin‑barrel cold‑feed extruder operating at die head temperatures between 105 °C and 120 °C.

Lubricant Additive Packages and Copper Corrosion Benchmarks

ZnMBT is incorporated into industrial gear oils, lithium‑complex greases, and heavy‑duty engine oil additive packages because it chemisorbs onto copper and brass surfaces, forming a passivating film that prevents both catalytic oxidation of the base oil and corrosive attack from active sulfur carriers. In a ISO VG 220 mineral oil containing a sulfurized isobutylene extreme‑pressure agent at 2.0 wt%, addition of 0.25 wt% ZnMBT improves the copper strip corrosion rating from 3a to 1b when tested at 100 °C for 3 h per ASTM D130. The effect is sustained even after 96 h of thermal stressing at 135 °C in the presence of 0.5 % water, as measured by the modified turbine oil rust test (ASTM D665 procedures A and B), where ZnMBT synergizes with zinc dialkyldithiophosphates to maintain a rust‑free steel coupon. Wear protection is assessed on a four‑ball extreme‑pressure tester. In an NLGI Grade 2 lithium‑complex grease, incorporating 0.5 wt% ZnMBT reduces the wear scar diameter from 0.52 mm to 0.38 mm under 40 kg load, 1200 rpm, 75 °C for 60 min (ASTM D2266). The concentration range typically employed spans 0.1–1.0 wt%; above 1.2 % in hydrocarbon oils, filter plugging has been observed in full‑flow lubricating systems due to micro‑crystalline precipitation at cold start‑up temperatures below −10 °C, although 1‑2 % solubility improvers (e.g., alkyl methacrylate copolymers) mitigate this. In automotive coolant formulations, ZnMBT protects brass radiators, copper heater cores and solder joints against galvanically induced copper leaching. A standard glassware corrosion test following ASTM D1384 with a mixed‑metal bundle (copper, solder, brass, steel, cast iron, cast aluminum) shows that 0.2 wt% ZnMBT, when blended into a monobasic alkali metal carboxylate‑based OAT coolant, limits copper weight loss to 2.1 mg per coupon after 336 h at 88 °C, compared to 10.4 mg for the uninhibited fluid. Unlike water‑soluble sodium mercaptobenzothiazole, ZnMBT has an aqueous solubility below 0.01 g/100 mL at 25 °C, so it does not deplete the corrosion‑inhibiting zinc surface layer on galvanized components, a failure mode reported for NaMBT‑containing coolants in long‑duration fleet trials. The coolant stability test ASTM D2570 confirms that ZnMBT films remain intact after 1000 h of recirculation without promoting silicate gelation, a recognized incompatibility of MBT‑based inhibitors with conventional silicated formulations.

When ZnMBT Incorporation in Polyolefin Waste Piping Exceeds 0.2 % for Anti‑fungal Function

ZnMBT is registered as an active substance for biocidal products under the EU Biocidal Products Regulation (BPR) for incorporation in plastics, coatings and synthetic fibers. In flexible PVC flooring and EPDM roofing membranes, loadings of 0.1–0.5 % provide fungistatic performance meeting ISO 846:2019 Method A (no growth rating 0) against Aspergillus niger and Penicillium funiculosum after 28‑day incubation at 29 °C and 95 % relative humidity. Polyurethane foam treated with 0.2 % ZnMBT shows zero fungal defacement when evaluated according to ASTM G21‑15. The biocide operates as a contact‑active, non‑leaching agent because the zinc‑thiolate complex remains tightly bound in the polymer matrix. Migration into deionized water after 28 days at 50 °C is typically < 0.02 % of the incorporated mass, as determined by HPLC‑UV analysis of the eluate. Nevertheless, exposure to alkaline cleaning solutions (pH > 9.5) or prolonged UV‑A radiation (340 nm) can destabilize the complex, releasing free MBT and causing surface yellowing. Therefore, ZnMBT‑modified articles are not recommended for transparent neutral‑colored polycarbonate applications requiring long‑term outdoor weathering without UV stabilizer packages.

Benchmarking Zinc MBT’s Delayed‑Action Profile Against Thiuram and Dithiocarbamate Accelerators

The table below positions ZnMBT relative to common thiazole, thiuram and dithiocarbamate accelerators on key vulcanization performance parameters. Scorch safety data are indicative Mooney t5 values at 120 °C in an unfilled NR masterbatch (ASTM D1646).

Table 2 — Comparative Vulcanization Accelerator Profile in Natural Rubber

Parameter	MBT	ZnMBT	NaMBT (30 % aq.)	MBTS	ZDEC	TMTD
Classification	Thiazole, slow	Thiazole zinc salt, medium	Water‑soluble thiazole	Thiazole disulfide, medium	Dithiocarbamate, ultrafast	Thiuram, ultrafast
Activation window (°C)	110–130	115–135	100–120	115–135	95–110	100–115
Mooney t5 at 120 °C (min)	14.0	24.8	10.2	19.6	5.8	3.9
Solubility in NR compound	Moderate	Good	Low (aqueous phase)	Moderate	High	High
Typical dosage in NR (phr)	1.0–2.0	0.8–2.5	0.5–1.5 (as dry)	0.6–2.0	0.2–0.8	0.1–0.5
Sulfur‑donor character	No	No	No	Yes, slow	No	Yes, potent
Nitrosamine potential	Negligible	Negligible	Negligible	Negligible	High (NDMA)	High (NDMA)
Primary application field	Industrial rubber goods (secondary acc.)	Tires, profiles, hoses, anti‑corrosion	Latex foams, water‑based adhesives	General purpose (replacement for MBT)	EPDM profiles, fast‑cure molded goods	Cable sheathing, heat‑resistant articles

ZnMBT’s position as a moderately fast, non‑nitrosamine, delayed‑action booster is evident when compared to ZDEC and TMTD, which provide curing onset below 110 °C and contribute to N‑nitrosodimethylamine formation in compounds containing secondary amine contaminants. Substituting TMTD with ZnMBT in a sulfur‑cured EPDM extrusion formulation extends the scorch safety margin at 110 °C by approximately 6 min, allowing safe incorporation of a high‑structure carbon black fraction. The water insolubility of ZnMBT (< 0.01 g/100 mL) recommends it over NaMBT for solid rubber processing, whereas aqueous NaMBT dominates latex dipping where rapid zinc ion equivalence is achieved through external zinc oxide.

Why Pre‑drying ZnMBT at Ambient Humidity Above 60 % Relative Humidity Is Recommended?

The powder exhibits hygroscopic character at relative humidity exceeding 60 % at 23 °C, with equilibrium moisture uptake reaching 0.8 wt% after 48 h exposure. Retained moisture in the accelerator prior to compounding generates micro‑voids during high‑temperature vulcanization, leading to a 0.5–1.2 dNm reduction in maximum rheometer torque and a sporadic surface crazing pattern on molded parts. Drying in a forced‑air oven at 60 °C for 2 h (bed depth ≤ 5 cm) restores moisture content to below 0.3 %. On a production‑scale silo storage system, the dew point of the pneumatic conveying air should be maintained below −20 °C. From a compounding compatibility perspective, ZnMBT reacts with strongly alkaline filler coatings, particularly sodium‑neutralized silicas with pH > 9.0. Under such conditions, the zinc cation is displaced, liberating free MBT, which drastically reduces scorch safety and can cause premature crosslinking in the feed throat of a twin‑screw extruder with an L/D ratio of 28:1 running at 35 rpm. Neutral to slightly acidic compound pH (6.5–7.5) is optimal. The substance also shows antagonism with primary aliphatic amine antioxidants (e.g., di‑beta‑naphthyl‑p‑phenylenediamine) at elevated processing temperatures; a 10 % loss in accelerator activity is documented when the compound is mill‑mixed at 115 °C for over 8 min in their presence, necessitating a late‑stage addition downstream in a tandem mixing line. The substance is listed on the TSCA Inventory, registered under REACH, and the migration limit for zinc from toy materials specified in EN 71‑3:2019+A1:2021 (46 000 mg/kg) is not exceeded at typical rubber‑part loadings, though dedicated extraction testing per EN 71‑10 is required for finished articles intended for children under three years. The zinc complex does not release free mercaptobenzothiazole under neutral leaching conditions, distinguishing it from some MBT‑based powder blends that can elevate extractable thiazole levels above sensory thresholds in potable water contact items evaluated under BS 6920 or AS/NZS 4020.