N-Tert-Butyl-2-Benzothiazolesulphenamide

In the formulation of radial passenger car tire treads based on natural rubber (NR) and butadiene rubber (BR) blends—typically a 70/30 to 80/20 ratio—N-tert-butyl-2-benzothiazolesulphenamide (TBBS) is metered at 0.8–1.2 phr alongside insoluble sulfur (2.0–2.5 phr), a ZnO/stearic acid activator system (3.0–5.0 phr ZnO, 1.0–2.0 phr stearic acid), and carbon black N234 at 45–55 phr. Compounding takes place in a two-stage internal mixer; masterbatch mixing in a 270 L intermeshing Banbury operating at 45–50 rpm rotor speed reaches a drop temperature of 155–165°C, after which the stock is sheeted on a twin-roll mill with a front-roll temperature set to 55–60°C and a friction ratio of 1:1.08. The second stage, where TBBS and sulfur are introduced, must rigidly cap dump temperature at 105°C; excursions above 120°C trigger decomposition of the sulfenamide into 2-mercaptobenzothiazole (MBT) and tert-butylamine, detected as a pungent amine odor and causing a drastic reduction in scorch safety that leads to micro-gel formation in the extruder head of pin-barrel cold-feed extruders used for tread profiling. Extrusion is performed through a 200 mm diameter vented extruder with an L/D ratio of 16:1, screw speed 20–30 rpm, and barrel zones controlled at 70–85°C; the head pressure must not exceed 15 MPa to avoid local overheating. Vulcanization kinetics, recorded on a moving-die rheometer (MDR) per ASTM D5289-17 at 160°C, yield a minimum torque M_L of 1.8–2.5 dN·m, maximum torque M_H of 14–18 dN·m, scorch time t_s2 of 3.5–4.5 min, and a t₉₀ of 9–12 min. Curing is executed in double-cavity segmented mold presses under 20–24 MPa clamp force at 150–160°C for 12–15 min. Finished treads, destined for P-metric and LT-metric PCR tires, exhibit tensile strength above 20 MPa and elongation at break exceeding 450% when tested per ASTM D412-16 die C, and DIN abrasion loss below 110 mm³ per ISO 4649:2017. Regulatory conformance requires the cured compound to meet EU 1907/2006 (REACH) Annex XVII entry 50 restrictions on eight polycyclic aromatic hydrocarbons (PAHs); the sum of benzo(a)pyrene, benzo(e)pyrene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, and dibenzo(a,h)anthracene must remain below 1 mg/kg as determined by GC-MS following extraction per AfPS GS 2019:01 PAK. Moreover, N-nitrosamine migration into artificial saliva measured according to EN 12868:1999 shall not exceed 10 μg/kg for materials accessible to children, a critical boundary that limits the maximum TBBS concentration in certain compound variants.

What Mechanism Underpins TBBS Selection in Steel Cord Skim Compounds?

Adhesion development between brass-coated steel cord and a high-modulus NR skim compound in radial tire belts relies on a delayed crosslinking onset that permits the rubber to flow and encapsulate each filament before gelation restricts conformability. TBBS at 1.0–1.5 phr is co-formulated with a resorcinol-formaldehyde donor (e.g., hexamethoxymethylmelamine, HMMM) at 4–6 phr, precipitated silica at 10–15 phr, cobalt naphthenate (0.5–1.0 phr Co content), and a sulfur loading of 4–5 phr to achieve the required crosslink density for belt reinforcement. Masterbatch mixing in an interlocking-rotor internal mixer with a rotor speed ceiling of 35 rpm limits the dump temperature to 145°C; the finalizing pass incorporating TBBS and sulfur drops at 95–100°C. Calendering onto brass-coated wire cord is performed on a 4-roll Z-calender at a nip gap of 0.35–0.55 mm, with roll temperatures balanced at 80°C top, 85°C middle, and 75°C bottom to minimize cord tension variation that would otherwise manifest as wire displacement in the cured belt. The cord is preheated to 105–115°C immediately upstream of the calender to eliminate moisture that promotes dezincification of the brass coating; residual moisture above 0.1% measurably reduces initial adhesion force. Unaged pull-out adhesion, tested according to ISO 5600:2017 on 3×0.20+6×0.35HT construction cord, returns values of 350–500 N; after humid aging at 85°C and 95% RH for 14 days, rubber coverage on the extracted cord must remain above 80%. The delayed-action profile of TBBS, giving a Mooney scorch t₅ at 127°C of 18–25 min, is critical: it extends the processing window sufficiently to allow the RF–silica network to migrate and form covalent bonds with copper in the brass before vitrification of the polymer matrix. From a compliance standpoint, skim compounds fall under the same REACH Annex XVII PAH limitations as tread compounds, and additionally material intended for the North American market must satisfy the FMVSS 139 endurance test criteria that indirectly validate belt durability over 34-hour wheel testing.

Heat-Resistant Conveyor Belt Cover Compounds

Cover compounds for fabric-reinforced conveyor belts handling hot clinker or sintered ores at continuous service temperatures up to 150°C are formulated on EPDM or a EPDM/NR blend, with TBBS contributing a controlled rate of cure that combats reversion in the bulk. Dosage levels reside in the 0.6–1.0 phr band, supplemented by sulfur at 0.8–1.2 phr and a secondary ultra-accelerator (e.g., tetrabenzylthiuram disulfide, TBzTD) at 0.2–0.4 phr to flatten the cure plateau and retain physical properties after extended post-cure thermal aging at 150°C for 168 hours, evaluated per ISO 188:2011. A major processing hazard is bloom of free TBBS on the uncured sheet surface, especially at ambient temperatures below 15°C when solubility in EPDM drops exponentially; mill operators mitigate this by splitting the TBBS addition into 60% in the internal mixer and 40% on a cooling batch-off mill maintained at 45°C. The cover stock is calendered to a thickness of 5–8 mm on a 4-roll inverted L calender and subsequently laminated onto a carcass comprising EP/NR skim compounds. Continuous vulcanization is performed on a Rotocure drum press with a contact pressure of 0.5–0.7 MPa and a drum temperature profile rising from 160°C at the inlet to 180°C at the midpoint, giving a residence time of 25–35 min. The finished Class 2 and Class 3 belts must conform to ISO 14890:2013, which specifies minimum tensile strength (15 MPa), elongation at break (350%), and abrasion resistance (120 mm³) before and after thermal aging. Additionally, covers intended for the European mining sector are subject to the EN 14973:2015 fire resistance classification, requiring a surface resistivity below 3×10⁸ Ω and a propane burner test according to EN 12881-1:2014. TBBS-containing EPDM covers achieve a tear strength of 35–45 N/mm by the trouser method of ISO 34-1:2015, though a documented drop of 10–15% occurs after immersion in ASTM No. 3 oil for 70 hours at 100°C, constraining the compound’s use to dry-heat applications only.

Within continuous vulcanization lines for EPDM sponge and solid automotive weatherstrip profiles, TBBS functions as a secondary accelerator at 0.4–0.7 phr in conjunction with zinc dibutyldithiocarbamate (ZDBC) or zinc dimethyldithiocarbamate (ZDMC) primary accelerators to balance the blowing agent decomposition kinetics with the crosslinking rate. The compound, containing paraffinic process oil (40–60 phr), carbon black N550/N774 blends, and azodicarbonamide blowing agent at 3–5 phr, is extruded through a 90 mm pin-barrel cold-feed extruder with a vented vacuum section maintaining −0.08 MPa to eliminate porosity defects. The profile exits into a microwave-hot air hybrid curing oven: the microwave cavities operating at 2.45 GHz with adjustable power up to 12 kW raise the extrudate core temperature to 140–160°C within 120 seconds, followed by hot air modules at 220–240°C for a total residence time of 5–8 min. A persistent failure mode on line is the formation of “elephant skin” surface roughness when TBBS concentration drops below 0.4 phr, which delays melt strength build-up too much and allows the expanding gas cells to rupture the surface skin before crosslinking stabilizes the cellular structure. Compression set testing following ISO 815-1:2019 (method A, 25% compression, 70 hours at 100°C) must return values below 50% for solid profiles and below 25% for sponge profiles. Finished weatherstrip assemblies are tested for fogging according to DIN 75201:2011, with a condensable mass limit of 2 mg at 100°C for 16 hours; TBBS, having a vapor pressure of 3.2×10⁻⁴ Pa at 25°C, contributes minimally to volatile condensate provided cure conversion exceeds 95%. The material must also satisfy VDA 278:2011 VOC and FOG emission thresholds, which dictate a total VOC below 100 μg/g for the sample. Formulators are alerted that combinations of TBBS with sulfenamide accelerators containing amine moieties (e.g., N-cyclohexyl-2-benzothiazolesulphenamide, CBS) can cause synergistic nitrosamine generation under the acidic conditions of artificial sweat testing (EN 1811:2011), a restriction important for door seal surfaces routinely grasped by occupants.

When EPDM-Based Radiator Hoses Require Cure Efficiency Above 150°C

Coolant hose constructions with an EPDM inner tube, aramid or polyester reinforcement, and an EPDM cover demand a fast cure system that withstands post-vulcanization heat soak without reverting; TBBS is employed at 0.8–1.2 phr together with tetramethylthiuram monosulfide (TMTM) at 0.3–0.5 phr and sulfur at 1.5–2.0 phr to raise the modulus plateau and reduce compression set after prolonged exposure to a 50/50 water–glycol mixture at 130°C. The inner tube compound is extruded via a 60 mm vented cold-feed extruder with pin-conversion technology, achieving a dimensional tolerance of ±0.15 mm on a 20 mm outer diameter, after which the tube is braided on a 24-carrier Maypole braider under 45–55 N of yarn tension to prevent tube collapse. The cover is crosshead-extruded in tandem and the entire assembly is cured on a mandrel in a saturated-steam autoclave at 165°C for 25–30 min. Under these conditions, a Mooney scorch t₅ at 125°C of 12–15 min is necessary to avoid scorch in the head of the crosshead die where residence time can reach 8 min during line stoppages. Adhesion between tube and reinforcement, measured by a peel test per ISO 36:2020, must exceed 2.5 N/mm before and after immersion in the coolant mixture for 1000 hours at 120°C; TBBS-based cure systems without excessive free MBT show superior adhesion retention because MBT migration to the rubber–fiber interface catalyzes thermo-oxidative degradation of the aramid finish. The final hose assemblies are certified to SAE J20:2005 Class D-3, which mandates an impulse test of 150,000 cycles at 125°C and a burst pressure greater than 2.4 MPa. A practical limitation observed on production curing presses is mold-fouling at TBBS levels above 1.3 phr after approximately 40 cure cycles, requiring ultrasonic cleaning with a 40 kHz bath, a cost factor that forces compounders to stay within the lower half of the recommended dosage range.

Microcellular rubber soling sheets for athletic footwear, constructed from SBR/BR/NR ternary blends with a specific gravity target of 0.55–0.70 g/cm³, incorporate TBBS at 0.6–1.0 phr as the primary delayed-action accelerator, paired with a benzoquinone dioxime–red lead blowing system that decomposes coincidently with the onset of crosslinking at 150–155°C. Compounding is performed in a kneader that discharges at 90–100°C, avoiding the 130°C threshold where premature blowing agent activation would create closed-cell porosity in the pre-formed slab. The stock is calendered into 4–6 mm sheets, die-cut, and compression-molded in multi-cavity hydraulic presses under 15–18 MPa clamp pressure with a cure cycle of 6–8 min. Aesthetics-driven constraints are paramount: unreacted TBBS and its cleavage product MBT contribute to pink-to-brown discoloration under UV exposure and perspiration; when white or pastel-color solings are specified, a non-staining sulfenamide such as N-cyclohexylbis(2-benzothiazolesulfen)amide (CBBS) is substituted, though at a sacrifice of 15–20% in tensile strength of the cellular network. TBBS remains the cost-performance optimum for black and dark-colored solings, where a typical compound achieves a tensile strength of 6.5–8.0 MPa and an elongation of 380–450% determined on microcellular specimens per ASTM D3574-17 Test B. The dimensional stability during processing, measured as mold shrinkage, registers 1.8–2.3% on an average 270 mm sole length, a magnitude that must be compensated in the die design. Compliance obligations include California Proposition 65 for N-nitrosamines and the CPSIA lead content limit of 90 mg/kg in accessible substrate components; extraction testing via EPA Method 3050B and subsequent ICP-AES verifies compliance at the batch level.