2-(4-Morpholinylmercapto)Benzothiazole

Passenger Car Radial Tire Tread — Silica-Loaded Compound Cure Kinetics and Dispersion Thresholds

In passenger car radial (PCR) tread formulations based on solution styrene-butadiene rubber (S-SBR) and high-cis polybutadiene (BR) blends with 80–100 phr precipitated silica, 2-(4-morpholinylmercapto)benzothiazole is typically dosed at 1.2–2.0 phr in combination with a primary sulfenamide such as N-cyclohexyl-2-benzothiazolesulfenamide (CBS) or N-tert-butyl-2-benzothiazolesulfenamide (TBBS) at 0.8–1.4 phr. The morpholinylmercapto moiety affords a slightly longer scorch safety window than TBBS under identical silica coupling agent (bis-triethoxysilylpropyl tetrasulfide, TESPT) conditions, with Mooney scorch t5 at 130 °C routinely extending by 2.5–4.0 min compared to an all-TBBS cure system. Mixing is executed in a tangential Banbury® intermeshing mixer (net chamber volume 270–370 L) with a three-stage, cool-cycle protocol: masterbatch (polymers, silica, TESPT, process oil, antidegradants) dropped at 145–155 °C; repass (silica dispersion refinement) at 140–148 °C; and finalization at 92–98 °C where sulfur (1.6–2.2 phr), ZnO (2.5–3.0 phr), stearic acid (1.5–2.0 phr), and accelerators are added. Exceeding 105 °C in the finalization stage triggers rapid 2-mercaptobenzothiazole (MBT) dissociation from the sulfenamide backbone, initiating sulfur ring-opening and premature crosslink nucleation detectable as a Mooney viscosity uptick of more than 15 MU within 30 s of extended dwell. Vulcanization proceeds in a segmented press cycle: 160–170 °C for 8–14 min, with the cure reversion plateau extending beyond 30 min at 170 °C according to moving-die rheometry per ASTM D5289—an advantage in thick tread sections where thermal lag can induce over-cure in the outer layer while the interior reaches t90. The finished tread exhibits a dynamic storage modulus (E′) at 60 °C below 6.8 MPa when measured by dynamic mechanical analysis (DMA) in tensile mode at 10 Hz and 2% dynamic strain, corresponding to a loss factor (tan δ) at 60 °C of less than 0.105, which correlates with EU tyre label wet grip grade A/B boundaries under regulation (EC) No 1222/2009. Dispersion of the accelerator in silica-filled matrices demands particular attention: if the morpholinylmercapto benzothiazole granular particles exceed 45 μm in median size, undispersed speck staining in the glossy tread surface appears after steam curing, and the localized stoichiometric imbalance yields a coefficient of variation (CV) in Shore A hardness exceeding 4.5% across a single tire build. Pre-drying of the accelerator to 0.1% w/w moisture is enforced where warehouse RH exceeds 65%; hydrolysis liberates morpholine, which acts as a secondary amine and accelerates sulfur crosslinking in an uncontrollable manner, narrowing the processing safety margin by approximately 35%.

Heavy-duty all-steel radial truck and bus tire undertread and base compound layers employ 2-(4-morpholinylmercapto)benzothiazole to satisfy the conflicting requirements of low heat build-up and extended fatigue life under 14–16 ply rated loads. The base compound is predominantly natural rubber (NR, SIR 20/SMR 20), extended with 35–55 phr N330 carbon black and 5–10 phr zinc glycerolate for thermal stability. Because NR already contains naturally occurring amines and lipids that increase cure rate, the addition of a morpholinyl-based sulfenamide with moderate delayed action is preferred over faster TBBS systems. Typical dosage sits at 0.8–1.3 phr alongside 0.3–0.6 phr diphenylguanidine (DPG) as a secondary booster. The rubber is compounded in a four-wing tangential mixer (F370 equivalent) with a masterbatch drop temperature held to 152–158 °C, a single repass, and a finalization stage strictly capped at 96 °C. Cure rheometer data (ASTM D5289, 150 °C, 0.5° arc) for a reference NR-based undertread containing 1.0 phr MBS and 0.4 phr DPG records a minimum torque ML of 2.1–2.4 dNm, maximum torque MH of 16.5–18.0 dNm, ts2 of 4.2–5.6 min, and t90 of 11.8–14.0 min. When TBBS at equal total sulfur donor capacity replaces MBS in the same masterbatch, the ts2 drops to 3.0–3.8 min, an unacceptable shift for factory lines operating multiple tire sizes on a shared curing press where loading, shaping, and bladder inflation require a minimum compound fluidity window of 4.5 min at 130 °C. Finished aramid-nylon cap-base laminates subjected to the Goodrich flexometer test (ASTM D623, method A, 100 °C, 1.0 MPa load, 30 Hz) record a blow-out time exceeding 52 min and a permanent set below 5.0%, confirming that MBS-mediated sulfidic crosslink architecture—predominantly di- and polysulfidic with a controlled monosulfidic fraction—resists reversion and hysteretic heating at the steel breaker interface where operating temperatures can exceed 95 °C during prolonged highway service.

The application of 2-(4-morpholinylmercapto)benzothiazole in ethylene-propylene-diene monomer (EPDM) automotive coolant hose inner liners diverges sharply from diene rubber practice because EPDM’s low unsaturation and peroxide cure preference require accelerator-swollen granules to be staged entirely differently. In sulfur-donor-cured EPDM liners chosen for organic acid technology (OAT) coolant resistance, MBS is restricted to 0.5–1.0 phr in a synergistic triad with zinc dibenzyldithiocarbamate (ZBEC) at 0.8–1.2 phr and tetramethylthiuram disulfide (TMTD) at 0.2–0.4 phr. The correct loading order is critical: MBS must be pre-dispersed in carbon black N550 (65–90 phr) and paraffinic oil (35–50 phr) during the upside-down mixing phase in an intermeshing internal mixer before ZBEC and TMTD are introduced at the flip; if MBS is charged together with the ultrafine thiuram, a premature exothermic decomposition occurs at 78–82 °C owing to the catalytic effect of the dithiocarbamate metal center on the sulfenamide bridge, evidenced by a temperature spike of 6–8 °C within 20 s of ram lowering. Extrusion of the peroxide-free sulfur-vulcanized EPDM tube compound on a cold-feed pin barrel extruder (L/D 16:1, screw diameter 90 mm) at a head pressure of 8–12 MPa and screw speed of 28–34 rpm yields a die swell below 12% and a surface roughness (Ra) under 1.8 μm when the MBS-mediated scorch time exceeds 8 min at 120 °C (MDR). After mandrel-supported saturated steam vulcanization at 175 °C for 12–16 min, the liner demonstrates volume swell below 3.5% after 168 h in ASTM D471 reference fuel C + 15% ethanol at 70 °C, and no visible blistering after 1,000 h aging in G12++ coolant at 130 °C. Multilayer hose constructions using a braided aramid reinforcement and an EPDM outer cover often adopt a graded accelerator profile: the cover layer uses 0.2–0.4 phr less MBS than the liner to retard covulcanization and avoid fiber-adhesive bond degradation at the tube-cover interface, maximizing burst resistance above 4.5 MPa at 125 °C.

How Does the Morpholinylmercapto Benzothiazole Cure System Impact Edge-Flaw Tolerance in Multi-V-Ribbed Belts?

For ethylene-vinyl acetate (EVM) and chloroprene (CR) multi-V-ribbed belt body compounds, 2-(4-morpholinylmercapto)benzothiazole is introduced as a secondary accelerator at 0.6–1.0 phr to retard the onset of cure during the transfer molding stage while preserving an efficient crosslink density at temperatures as low as 155 °C. The rib profile geometry imposes a severe processing constraint: uncured compound must flow into trapezoidal grooves of 0.8–1.2 mm depth without trapping air or experiencing scorch-induced porosity at the rib peaks before full mold closure. In a typical CR belt carcass formulation (Neoprene WRT type, 100 phr; N774 carbon black 45 phr; dioctyl sebacate 12 phr; MgO 4 phr; ZnO 5 phr; ETU-free cure with 0.8 phr ZDBC and 0.7 phr MBS), the MBS extends the Mooney scorch time at 121 °C to 12–15 min, providing an ample flow window for the pre-form blank to fill the mold cavities under a clamping force of 800–1,200 tons. Once the press closes and the temperature ramps to 165 °C, the cure acceleration kicks in sharply: the rheometer curve (ASTM D5289, 0.5° amplitude) demonstrates a t10 of 1.6–1.9 min and a t90 of 6.0–7.2 min, compatible with a 7–9 min total cycle time. Belts cured with MBS exhibit edge-crack initiation life exceeding 1.2 × 10⁶ cycles at 3,200 rpm on a four-point bending endurance rig (ISO 1813, 80 °C ambient), whereas specimens accelerated solely with thiazole/dithiocarbamate pairs fail at 0.7–0.9 × 10⁶ cycles due to inferior crosslink stability at the rib root radius where strain amplitude under reverse bending reaches 12–15%.

In extruded dense rubber profiles for curtain wall glazing and expansion joint sealing, where interlocking geometric complexity demands both shape fidelity and consistent physical properties at a line speed of 15–25 m/min, the substitution of commodity thiazole-only cure systems with 2-(4-morpholinylmercapto)benzothiazole at 1.0–1.8 phr in a EPDM/butyl blend (70/30 phr) addresses the pervasive issue of die-plate scorch accumulation. The production extrusion line features a vented pin-type extruder (60 mm screw, 14:1 L/D) feeding a microwave-hot air combination continuous vulcanization unit. The head pressure fluctuates between 10 MPa and 16 MPa during gasket profile runs; compounds accelerated with a fast TMTD/ZDBC combination develop cured agglomerates at the breaker plate perforations within 4–6 h of uninterrupted operation, necessitating a daily tear-down. MBS-delayed systems, by shifting the onset of crosslinking to a die-zone temperature of 118–122 °C (versus 95–100 °C for thiuram-only), extend the continuous extrusion window to beyond 18 h before screen-pack replacement is required. The microwave unit’s absorbed power density is calibrated at 2.2–2.8 kW ·h/kg compound; post-cure tensile strength (ISO 37, type 2 dumbbell) reaches 11.5–13.0 MPa with elongation at break of 380–420%, sufficient to meet ASTM C864 compression set requirements (≤ 25% after 22 h at 100 °C) for dense elastomeric seals. Because the morpholinyl substituent imparts a slight polarity to the accelerator molecule, migration of the unreacted residue into the glazing sealant contact zone is suppressed relative to CBS, confirmed by Fourier-transform infrared (FTIR) microscopy mapping showing an interfacial sulfenamide concentration below 0.08 wt% after accelerated aging equivalent to 5 years of field exposure according to EN 1279-2.

Mining and aggregate conveyor belt top-cover compounds handling abrasive ore undergo a constant trade-off between cure latency during the continuous Rotocure press cycle and ultimate gouge resistance. A typical NR/BR (80/20 phr) cover loaded with 50 phr N220 carbon black and 8 phr reinforcing silica uses MBS at 1.3–1.7 phr alongside 0.4–0.7 phr tetrabenzylthiuram disulfide (TBzTD) to generate a plateaus in the cure curve at 150–165 °C for broad pre-press gap tolerances. The steel-cord-reinforced belt is assembled in a multi-layer pre-build that passes through a two-drum continuous Rotocure with platen heating zones segmented into 165 °C, 175 °C, and 175 °C sections under a specific contact pressure of 0.4–0.6 MPa. At a linear speed of 0.6–1.2 m/min, the cover compound must resist scorch for the 3–5 min transit from the laying head to the final pressure roll. MBS delivers ts2 at 135 °C of 9.2–11.0 min, an increment of 2.5–3.0 min over TBBS-based covers. The cured cover exhibits a DIN abrasion loss (ISO 4649, method A) of 62–72 mm³, tensile strength of 22–25 MPa, and tear resistance (ISO 34-1, trouser) exceeding 60 N/mm. Heavy-haul belts conveying copper ore at 7–9 m/s and 3,200 t/h capacity exhibit a cover wear life exceeding 18 months before replacement; MBS-mediated networks resist the oxidative hardening at crack tips under repeated impact of 200–300 mm rocks by maintaining a sulfidic crosslink density of 1.05–1.20 × 10⁻⁴ mol/cm³, as determined by equilibrium swelling in toluene, versus networks from ultra-fast accelerators that densify to 1.80–2.00 × 10⁻⁴ mol/cm³ within 2,000 h of service, causing premature fatigue spalling.

Wire and Cable Compound Screening Matrix — International Standards and Accelerator Constraints

In medium-voltage ethylene-propylene rubber (EPR) insulation shielding and low-voltage EPDM jacketing compounds, 2-(4-morpholinylmercapto)benzothiazole usage is restricted to 0.5–1.2 phr because any accelerator residues that survive the continuous vulcanization tube and migrate to the conductor polymer interface contribute to long-term ion diffusion and electrical treeing under wet conditions. The compound is prepared in a cold tangential mixer with all raw materials pre-dried in a hopper (80 °C, dew point −30 °C), and MBS is added together with the dicumyl peroxide crosslinking agent (if a co-agent sulfur donor is desired) or slotted into a pure sulfur-cure EPDM package where it partners with ZDBC at 0.6–0.8 phr. The extrusion line for insulation combines a 90 mm hot-feed extruder with a catenary continuous vulcanization (CCV) tube operated at 2.0–2.4 MPa nitrogen pressure and five temperature zones ramping from 200 °C to 280 °C. The extremely rapid heat transfer in CCV demands that the compound’s t2 at 200 °C exceed 0.8 min to prevent skin-cure before the melt is fully degassed; MBS chemistries maintain t2 near 1.1–1.3 min, in contrast to TMTD-only systems that t2 falls below 0.5 min. The CCV exit compound must meet the hot-set test requirements of IEC 60811-501: elongation under 0.2 MPa load at 200 °C shall not exceed 175%, and the permanent set after cooling shall be less than 15%. Splicing and jointing compounds, applied as ambient-temperature tape wraps, use MBS at 0.3–0.5 phr in a self-amalgamating EPDM formulation vulcanizing under the action of a peroxide initiated at 150 °C by an on-site heating blanket; the low accelerator loading prevents premature cure during storage at 35 °C shelf life verification per IEC 60400-2.

When Low-Profile Automotive Weatherseals Demand MBS as a Cure-Rate Modifier in Complex Coextruded Sponge-Dense Profiles

Automotive body weatherseals manufactured by coextrusion of a microcellular sponge EPDM core and a dense EPDM skin layer rely on gradations of accelerator activity to achieve simultaneous foaming and cure coincidence. The sponge layer, blown with azodicarbonamide (2.5–4.0 phr) at a decomposition temperature of 195–205 °C, requires delayed sulfur crosslinking to permit gas cell expansion independent of matrix stiffening. Here, 2-(4-morpholinylmercapto)benzothiazole at 1.5–2.2 phr in combination with ZDBC (0.4–0.7 phr) and sulfur (1.2–1.8 phr) provides a wide processing plateau: the MDR vulcanization curve at 180 °C shows a t10 of 2.5–3.2 min and t90 of 8.2–9.8 min, while the gas release initiates uniformly at 1.8–2.2 min into the microwave cavity. The dense skin compound for the metal-insert gripping lip uses a reduced MBS level of 0.9–1.3 phr and a higher carbon black loading to achieve Shore A hardness of 72 ± 3 and a stabilised compression set (ISO 815, 72 h at 85 °C) of 18–22%. The two streams converge in a multiple-ram coextrusion die head with mandrel temperature controlled at 70–75 °C; any difference in scorch rate between the sponge and dense layers exceeding 15% (as measured by t5 deviation) results in convex warping of the seal profile after hot-air vulcanization at 220–250 °C for 2.5–3.5 min. The finished seal is submitted to a dynamic compression cycling test per BMW GS 97017, measuring permanent deformation after 500,000 strokes: profiles manufactured with MBS balancing retain a sealing force above 4.0 N/cm², while accelerated networks that cause over-cure shrinkage in the sponge core drop below 3.2 N/cm² by mid-test.

Footwear vulcanized EVA/sponge rubber laminates for athletic and safety shoes incorporate MBS at 0.6–1.0 phr in the solid rubber outsole layer to adjust crosslink synchronization with the EVA foam midsole during a single-shot in-mold bonding cycle. The outsole compound is based on NR/SBR (60/40 phr) filled with 35–45 phr N330 carbon black and 10–15 phr precipitated silica, processed in a two-roll mill (friction ratio 1.2:1) at a front roll temperature of 60–70 °C. The MBS, together with ZnO (4 phr), stearic acid (1.5 phr), sulfur (1.8 phr), and a secondary guanidine (0.5 phr DPG), is split-added to avoid early scorch at the mill nip. Expansion of the EVA midsole, triggered at mold temperatures of 155–165 °C, releases acetic acid that can degrade metal-polymer bonding; MBS-stabilised outsole cure onset is delayed by about 90 s relative to the foam expansion peak, ensuring the outsole reaches adequate green strength only after the midsole foam cells have completed nucleation and expansion, avoiding migration voids at the bond line. After curing for 6–8 min at 160 °C, the shoe bottom demonstrates a peel strength (SATRA TM404, method C) exceeding 3.5 kN/m between the outsole and midsole, a flex crack growth resistance (DIN 53543) after 30,000 cycles of less than 2.5 mm, and a slip resistance (ASTM F2913, wet ceramic tile) coefficient of friction above 0.45. Where production shifts to direct vulcanization-on-canvas footwear, the lower heat capacity of the cotton duck upper necessitates a further reduction in MBS to 0.5–0.7 phr to prevent scorch staining on the fabric at the welt and toe cap due to local temperature overshoot at the aluminum last surface.