N-Oxydiethylene-2-Benzothiazole Sulfenamide

N-Oxydiethylene-2-benzothiazole sulfenamide (CAS No. 102-77-2), frequently designated by the trade acronyms MBS, NOBS, or OBS, appears as a pale yellow to white granulate or powder with a faint amine odor. The molecule consists of a 2-mercaptobenzothiazole moiety linked via a disulfide-type sulfur to a morpholine ring, classifying it as a delayed-action sulfenamide accelerator for sulfur vulcanization. Typical commercial models are supplied as free-flowing, dust-suppressed pellets with a nominal active content of 97.0 % (minimum) and a residual oil coating of 1.5–2.5 % when oil-treated. When received, the product’s initial melting point should fall between 78.0 °C and 85.0 °C, with the exact range depending on isomeric purity. The following table summarizes the standard quality control parameters assessed on each lot using established test protocols.

Property	Test Method	Specification Range
Appearance	Visual / ASTM D4570	Pale yellow to white granules
Initial melting point	ASTM D1519 (capillary)	78.0 – 85.0 °C
Loss on drying (70 °C, 2 h)	ASTM D4571	≤ 0.50 %
Ash (800 °C)	ASTM D4574	≤ 0.30 %
Free amine (as morpholine)	Titration (HClO₄)	≤ 0.50 %
Residue on 150 µm sieve	ASTM D4572	≤ 0.10 %

How Does the Morpholine Substituent Alter Vulcanization Kinetics Relative to CBS and TBBS?

The amine fragment of the sulfenamide dictates the rate of accelerator decomposition and thus the scorch safety and cure speed. In a model natural rubber / butadiene rubber (NR/BR 70/30 phr) compound containing 2.0 phr sulfur and 1.5 phr accelerator, the morpholine derivative MBS generates a scorch delay that lies between that of the cyclohexylamine-based CBS and the tert‑butylamine-based TBBS. Mooney scorch data (MS‑t₅ at 121 °C, ASTM D1646) and moving‑die rheometer cure characteristics (ASTM D5289, 150 °C, 1° arc) for all three sulfenamides are compared below. The numeric values represent batch medians from production‑scale internal mixing in a 1.5 L intermeshing tangential mixer (fill factor 0.75, dump temperature 120 °C).

Parameter	MBS	CBS	TBBS
MS‑t₅ (121 °C), min	32.0	25.5	44.0
tₛ₂ (150 °C), min	5.8	4.2	8.1
t₉₀ (150 °C), min	17.5	13.0	22.0
MH-ML, dNm	12.8	13.1	12.3
Tensile strength (ASTM D412), MPa	24.5	25.0	24.0
Modulus at 300 % (M300), MPa	10.2	11.0	9.5

The data confirm that MBS delivers an intermediate cure rate while maintaining a sufficiently long flow time for the injection molding of complex profiles. M300 values trend slightly lower than CBS but higher than TBBS, a consequence of the crosslink density distribution shaped by the amine elimination pathway. However, the principal differentiator is the nature of the liberated amine. MBS releases morpholine, a secondary amine classified as a nitrosatable substance capable of generating N‑nitrosomorpholine (NMOR) under vulcanization conditions. This creates regulatory hurdles not encountered with TBBS, which decomposes into the primary amine tert‑butylamine, yielding less stable nitrosamines. Consequently, where Nitrosamine Directive 93/11/EEC or German TRGS 552 applies—as in rubber articles intended for repeated oral contact—MBS is frequently excluded from the formulation. In production-scale rubber compounding on an intermeshing co-rotating twin-screw extruder (L/D = 48, screw diameter 50 mm), the dispersion threshold of MBS granules with a d₅₀ ≤ 2.0 mm is achieved at a specific mechanical energy input of 0.12–0.18 kWh/kg. Barrel temperatures are profiled with the first three zones held at 60–70 °C to suppress premature softening, the mixing zone at 80–90 °C, and the die head at 90 °C. Under these conditions, the morpholine odor becomes perceptible at the vacuum vent port when the absolute pressure rises above 20 kPa, indicating partial thermal dissociation of the sulfenamide bond. To maintain Mooney consistency of the compound, the free amine content of the raw MBS must be monitored; an increase from 0.3 % to 0.7 % in incoming lots has been correlated with a drop in MS(1+4) Mooney viscosity at 100 °C of 3–5 MU when mixed with N330 carbon black at 50 phr loading. Further, if the granulate has been exposed to ambient relative humidity exceeding 60 % for more than 48 hours, pre‑drying in a vacuum oven at 40 °C for 4 hours is required; otherwise the moisture catalyzes amine release during storage and increases the incidence of porosity in extruded profiles.

When MBS Replaces CBS in Silica-Filled Tread Formulations

Substituting CBS with MBS in a precipitated-silica passenger‑tire tread compound (silica 80 phr, TESPT coupling agent 6.4 phr) shifts the silanization equilibrium and filler‑filler interaction. The morpholine liberated early in vulcanization competes with the silane for acidic silanol groups on the silica surface, retarding the formation of covalent filler‑rubber bonds. Dynamic mechanical analysis (DMA, 10 Hz, 0.1 % strain, ASTM D5992) reveals that the tan δ at 60 °C rises by 0.015–0.025 units when MBS is used in place of CBS at the same molar loading, pointing to higher rolling resistance. To compensate, processors either increase the silane dosage by 8–12 % or pre‑coat the silica with a mono‑functional hydrophobizing agent. On the production floor, this translates to an extended silanization mixing window; the drop temperature of the silanization stage must be pushed from 145–150 °C to 152–155 °C to achieve equivalent coupling efficiency, but thermal exposure beyond 155 °C risks auto‑combustion of the morpholine vapor, a safety constraint that limits throughput. Published data for a direct CBS‑to‑MBS swap in a passenger‑tire tread with a twin‑screw sheeter‑die system remains limited, but pilot‑scale trials on a 1.5 km extrusion line indicate a 2‑3 % reduction in extrusion speed to maintain dimensional tolerance owing to reduced green‑strength of the silica‑rich stock. Regulatory restrictions on secondary amines in vulcanization fumes have reshaped selection criteria across the molded goods sector. Under EU CLP Regulation 1272/2008, MBS is classified as Skin Sens. 1 (H317) and is listed with an aquatic chronic toxicity hazard. More significantly, rubber articles containing MBS that come into contact with food simulants must comply with Commission Regulation (EU) No 10/2011 on plastic materials; the specific migration limit for morpholine is not explicitly listed, but as a secondary amine it falls under the scrutiny of national enforcing authorities. In practice, many industrial hose and gasket manufacturers have migrated to TBBS‑based cure systems or to dithiocarbamate boosters when full compliance with the German BfR Recommendation XXI for food contact is required. The morpholine‑derived accelerator thus remains principally confined to technical rubber goods—conveyor belt covers, vibration dampers, and engine mounts—where nitrosamine accumulation in confided air spaces is managed through local exhaust ventilation and controlled post‑cure annealing cycles that purge residual amine.

Storage Stability and the Impact of Relative Humidity on Free Amine Content

Accelerated aging tests performed in accordance with ASTM D4572 (storage at 50 °C, 90 % relative humidity) demonstrate that sealed, desiccant‑protected pails of MBS maintain a free amine content below 0.50 % for 12 months. Once a container is opened, the humidity uptake follows a near‑linear rate of 0.05 % moisture per exposure hour under 25 °C and 65 % RH. Free amine rises above 1.0 % after approximately 72 hours of open‑air storage under these conditions, at which point Mooney scorch times of subsequently mixed compounds decrease by 8–12 %. Blooming of free sulfenamide on the surface of calendered sheets appears as a chalky deposit when the accelerator’s free amine exceeds 1.2 % and the sheet is cooled rapidly from calender temperatures. Therefore, the recommended storage environment is a well‑sealed original package kept at ≤ 25 °C and < 60 % RH. In-line gravimetric loss‑on‑drying checks (70 °C, 2 h) before day‑tank loading are the minimum preventive measure for installations running low‑scorch‑margin formulations on multi‑cavity injection presses with clamp forces above 200 t.