N-Cyclohexyl-2-Benzothiazole Sulfenamide

A passenger tyre tread compound based on natural rubber (NR) and solution-polymerised styrene–butadiene rubber (SSBR) with high-dispersion silica relies on a delayed-action sulfenamide to reconcile processing safety with rapid vulcanisation. N‑Cyclohexyl‑2‑benzothiazole sulfenamide (CBS) is typically dosed at 1.2–1.8 phr in combination with 1.8–2.2 phr elemental sulfur and 0.3–0.5 phr N,N′‑diphenylguanidine (DPG) as secondary accelerator for silica‑filled systems. Production‑scale mixing in an intermeshing‑rotor internal mixer (net chamber volume 270 L, fill factor 0.70) follows a two‑stage protocol: the non‑productive stage incorporates NR, SSBR, precipitated silica, organosilane coupling agent (bis‑triethoxysilylpropyl tetrasulfide), zinc oxide (3.0 phr), stearic acid (2.0 phr), antidegradants (N‑1,3‑dimethylbutyl‑N′‑phenyl‑p‑phenylenediamine and 2,2,4‑trimethyl‑1,2‑dihydroquinoline polymer), and treated distillate aromatic extract oil, with a dump temperature controlled below 150 °C to prevent silanisation reversal. After cooling to ambient temperature on a batch‑off cooler, the masterbatch is charged into a second internal mixer for the productive stage, where CBS, sulfur, and DPG are added at a ram‑over‑rotor start temperature of 50–60 °C; the dump temperature is strictly limited to 95–105 °C to avoid premature cross‑link formation (scorch). On a two‑roll mill with a nip gap of 4 mm and a front‑to‑rear friction ratio of 1:1.15, the compound is sheeted off and cooled. The Mooney viscosity ML(1+4) at 100 °C typically falls between 50 MU and 70 MU, and the Mooney scorch t5 at 135 °C exceeds 18 min, providing ample processing latitude for extrusion of tread profiles. Cure kinetics measured on a moving‑die rheometer (MDR) per ASTM D5289‑21 at 160 °C, 0.5° arc and 1.67 Hz reveal a scorch time ts2 of approximately 2.5–3.2 min and a cure time t90 of 6.0–7.5 min—a narrow, well‑defined cure curve that yields a rapid cross‑linking stage after the safe induction period. Tensile properties tested on cured sheets according to ISO 37:2017 (type 2 dumbbells) attain tensile strength above 18 MPa, elongation at break 450–550 %, and modulus at 300 % elongation (M300) of 8–10 MPa. DIN abrasion loss (ISO 4649:2021, Method A) remains below 120 mm³, meeting passenger car tread abrasion targets. Residual polycyclic aromatic hydrocarbons (PAHs) are monitored under Commission Regulation (EU) No 1272/2013 amending REACH Annex XVII Entry 50; CBS‑cured silica tread compounds routinely yield individual PAH levels below the 1 mg kg⁻¹ threshold provided the CBS purity exceeds 97 % and low‑PCA extender oils are used. The processing window narrows sharply above 110 °C: CBS exhibits a steep drop in scorch safety, and hot‑spots in an extruder barrel operating beyond this temperature can generate scorched particles visible as surface defects in the extruded tread.

Industrial conveyor belt cover compounds formulated with NR and high‑abrasion‑resistant carbon black (N220 or N330) demand an accelerator that combines moderate scorch delay with high ultimate cross‑link density to withstand severe mechanical wear. CBS is introduced at 1.0–1.5 phr together with sulfur at 2.0–2.5 phr in a Banbury‑type mixer (net volume 160 L). The single‑stage mixing cycle (ram pressure 0.6 MPa) adds CBS and curatives after the black incorporation and oil addition phases, ensuring a final dump temperature no higher than 105 °C. On a cooling mill, the stock is cross‑blended and sheeted at 2.5 mm thickness. Rheometer data (ISO 6502‑2:2018, 150 °C) indicate ts2 above 5 min and t90 below 12 min to permit efficient press vulcanisation of belt sections up to 25 mm gauge. Tensile strength according to ISO 37 exceeds 20 MPa, and tear resistance (trouser tear, ISO 34‑1:2022, Method B) surpasses 80 N mm⁻¹. Abrasion loss measured under ISO 4649 is held below 120 mm³, and the cured cover compound meets the tear‑propagation resistance required for EN ISO 14890:2013 heavy‑duty belting. Because the belt carcass is typically bonded with a resorcinol‑formaldehyde‑latex (RFL) adhesive system, the compound must be free of amine‑based anti‑reversion agents that can poison the adhesion interphase; CBS does not liberate secondary amines that interfere with the RFL dip. Post‑cure, the free CBS residue is undetectable below 0.05 wt % in dichloromethane extracts, confirming compliance with EU REACH persistant‑organic‑pollutant screening without additional washing stages.

What Restricts Mixing Temperatures in CBS‑Accelerated NBR Hose Liners?

Nitrile rubber (NBR) compounds for hydraulic and fuel hose inner liners require controlled‑scorch processing combined with resistance to aggressive fluids. CBS is dosed between 0.8 phr and 1.2 phr in NBR grades with 33–45 % acrylonitrile content; sulfur levels are set at 1.5–1.8 phr. Compounding is performed on a tangential‑rotor internal mixer (chamber volume 75 L, rotor speed 40 rpm). The critical limitation is the onset of CBS‑accelerated scorch at the low‑shear zones of the mixer body where wall temperatures can exceed 110 °C. Below 100 °C stock temperature, CBS gives a Mooney scorch t5 at 125 °C of at least 12 min; when the compound mass temperature drifts to 112 °C due to friction heat from carbon‑black incorporation (N550), the scorch safety collapses to below 6 min. Therefore, jacket cooling water must be maintained at 8–12 °C inlet temperature, and the mixing fill factor is restricted to 0.65 to avoid heat build‑up caused by rotor‑wall shear. After dumping, the stock is passed three times through a tight‑nip two‑roll mill (gap 2 mm) to dissipate thermal energy rapidly before sheet‑off. Vulcanisate properties after press cure at 160 °C for t90 + 2 min exhibit hardness Shore A (ISO 48‑4:2018) of 68–72, tensile strength 14–16 MPa, and volume swell in ASTM IRM 901 oil (ISO 1817:2015, 70 h at 100 °C) limited to 15 %. The low nitrosamine‑forming potential of CBS is advantageous for articles intended for drinking‑water contact when subsequent post‑cure washing is limited; n‑nitrosamine content in aqueous extracts (migration test per EN 12873‑1:2014) remains below the 0.01 µg L⁻¹ detection threshold. The hose liner formulation cannot be substituted with thiuram‑type accelerators due to the risk of copper‑catalysed degradation in fuel blends containing ethanol; CBS does not generate dithiocarbamate residues that accelerate copper corrosion (DIN 50961:2012, weight loss < 0.2 mg after 24 h).

When EPDM Dense Profiles Demand Low Compression Set After Long‑Term Heat Aging

Extruded EPDM dense profiles for automotive glass‑run channels and boot seals are frequently sulfur‑cured with CBS as the primary accelerator to achieve a flat cure plateau and minimal property drift during continuous‑use temperatures up to 120 °C. The base formulation employs an EPDM grade with an ethylene content of 55–65 wt % and ethylidene norbornene (ENB) content of 5–8 wt %, providing a sufficient diene level for sulfur cross‑linking. CBS is added at 1.0 phr, accompanied by 1.2 phr sulfur, 0.4 phr tetramethylthiuram disulfide (TMTD) as ultra‑accelerator for surface bloom control, and a mixed metal‑oxide activator system (5 phr ZnO, 1 phr stearic acid). Mixing is executed in an intermeshing mixer with a rotor speed of 30 rpm and a maximum batch temperature of 90 °C during curative addition. The compound is then extruded through a pin‑barrel extruder (L/D 16) equipped with a profile die and a microwave‑hot‑air vulcanisation line; the extrudate surface finish requires a scorch time t5 at 130 °C not shorter than 8 min—achieved with CBS—while the t90 at 190 °C in the microwave unit is held to 3.5 min to ensure complete cure before the end of the cooling unit. Compression set tested under constant deflection (ISO 815‑1:2019, 25 % compression, 24 h at 150 °C) measures below 30 %, and the change in tensile strength after ageing in a hot‑air oven (ISO 188:2017, 70 h at 150 °C) does not exceed −10 %. Emissions and fogging behaviour required by VDA 278:2020 are satisfied: volatile organic compound (VOC) value remains under 100 µg g⁻¹ and the fogging condensate (FOG) value under 500 µg g⁻¹, owing to the high molecular weight of CBS and its low vapour pressure; the thiuram co‑accelerator is limited to prevent excessive carbon disulfide emissions. The system is incompatible with peroxide co‑agent packages that rely on alkaline coagulation, because the amine structure of CBS would deactivate the acidic coagulant residues and yield unpredictable scorch times.

Fatigue Life Optimisation in NR Engine Mount Formulations

Natural rubber engine mounts and suspension bushings demand high dynamic crack growth resistance and minimal heat build‑up under cyclic loading. CBS is employed at 0.8–1.0 phr in a conventional high‑sulfur (2.5 phr) NR compound filled with N330 carbon black (45 phr). The productive mixing stage in a tangential‑rotor internal mixer (fill factor 0.72) maintains a final batch temperature under 95 °C to preserve the long scorch delay (t5 > 25 min at 120 °C) needed for complex‑shape injection‑moulding with multi‑cavity tools. Injection moulding machine screw temperature is set at 80 °C, barrel at 70 °C, and mould temperature at 160 °C; cure time is programmed as t90 + 3 min determined from an MDR at 160 °C. Vulcanisate properties after demoulding show hardness 55–60 Shore A, tensile strength 22–26 MPa, and elongation at break 500–550 %. Dynamic characterisation on a servo‑hydraulic fatigue tester (MTS 831, 10 Hz, constant‑strain control, 0.5 mm displacement) gives a crack initiation life exceeding 10⁶ cycles under zero‑to‑tension loading, significantly longer than equivalent TBBS‑cured mounts because CBS‑derived cross‑links are predominantly di‑ and polysulfidic in the early stages, which reorganise under dynamic loading and retard crack growth. Compression set (ISO 815‑1, 24 h at 70 °C) is controlled to < 20 %. The mount compound must also comply with the European Union End‑of‑Life Vehicle Directive (2000/53/EC) regarding heavy metals: cadmium, lead, mercury, and hexavalent chromium are absent from the raw CBS and from all compound ingredients, confirmed by X‑ray fluorescence screening per ASTM F2617‑15. A documented limitation is the need for pre‑drying the compound at 50 °C for 2 h when relative humidity in the production hall exceeds 60 %, otherwise condensed moisture on cold NR bales promotes hydrolysis of the sulfenamide group during storage and reduces the active CBS content.

Rubber Sole Compounds for Vulcanised Footwear Shells

Vulcanised rubber soles for safety and casual footwear made from NR/BR/SBR blends (e.g., 40/30/30 phr) profit from the delayed‑action characteristic of CBS to accommodate multistage moulding of deep‑lug profiles. CBS is incorporated at 1.0–1.4 phr along with 2.0–2.3 phr sulfur and 0.2 phr of a retarder (N‑(cyclohexylthio)phthalimide) when processing thick‑section soles exceeding 12 mm wall thickness. The compounds are mixed on a roll mill (diameter 550 mm, friction ratio 1:1.2) because the small batch sizes in footwear manufacturing do not justify internal‑mixer amortisation; mill temperature is held at 60–70 °C by chilled‑water circulation. After sheeting, the stock is cut into blanks and compression‑moulded in multi‑cavity aluminium moulds at 150 °C under 15 MPa pressure. The cure time is determined by a rheometer (ISO 6502‑2) as t90 plus an additional 2 min to ensure through‑cure. Physical property requirements under SATRA TM404 (footwear‑specific tensile method) demand tensile strength above 12 MPa and elongation at break exceeding 350 %, both easily met with CBS acceleration. DIN abrasion loss is kept below 150 mm³, and the flex‑crack resistance after 100 kilocycles in a De Mattia tester (ISO 132:2017) shows a cut growth not exceeding 3 mm. Chemical compliance must address Europe’s REACH Annex XVII Entry 50 (PAH restrictions) and the PAH‑free certification for shoes sold on the German market (GS‑Mark, AfPS GS 2019:01 PAK); the CBS supplier’s certificate of analysis routinely confirms benzo[a]pyrene < 0.2 mg kg⁻¹ and sum of the eight listed PAHs below 0.5 mg kg⁻¹. In low‑density blown‑sole compounds activated with azodicarbonamide blowing agent, CBS’s steady cure curve avoids the premature skin cure that causes trapped gas and blister defects during foaming; the mould must be vented during the first 15 s of the cycle to allow gas escape while the CBS‑induced cure lag keeps the compound in a low‑viscosity state. The absence of amine‑type nitrosamine precursors in CBS simplifies the declaration for shoes intended for children, when tested according to EN 71‑3:2019+A1:2021 for migration of certain elements.