Sulfenamide Accelerators Research Articles

The thermal decomposition of sulfenamide accelerators

AbstractThe thermal decomposition of three sulfenamide accelerators N‐cyclohexylbenzothiazole sulfenamide (CBS), 2‐(4‐morpholinothio) benzothiazole (MOR) and 2‐t‐butylaminobenzothiazole sulfenamide (TBBS) were investigated by differential scanning calorimetry. The sulfenamides decompose rapidly at 210–220°C, yielding a number of products, including reactive polysulfidic complexes. Thus, CBS gives N‐cyclohexylamino‐2‐benzothiazole polysulfides (CBP), 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS), 2‐bisbenzothiazole‐2,2′‐polysulfides (MBTP), 2‐bisbenzothiazole‐2,2′‐monosulfide (MBTM), 2‐mercaptobenzothiazole (MBT), and 2‐N‐cyclohexylaminobenzothiazole (CB). The polysulfides are unstable, and on prolonged heating, only MBT and CB remain. The amine fragment of the accelerator is present as the amine salt of MBT. At lower temperatures, the sulfenamides are relatively stable. MOR forms an analogous product spectrum. The decomposition of TBBS is endothermic, in contrast to the exothermic reaction observed with CBS and MOR, and the concentrations of the various polysulfides do not decrease on prolonged heating. Reaction mechanisms are proposed to account for the formation of the products. © 1994 John Wiley & Sons, Inc.

The interaction of sulfenamide accelerators with sulfur, ZNO, and stearic acid in the absence of rubber

AbstractThe interaction of sulfur, ZnO, stearic acid, and the three sulfenamide accelerators N‐cyclohexylbenzothiazole sulfenamide (CBS), 2‐(4‐morpholinothio) benzothiazole (MOR), and 2‐t‐butylaminobenzothiazole sulfenamide (TBBS) were investigated by differential scanning calorimetry in the absence of rubber. In the presence of sulfur, the same product spectrum is formed as in its absence, but at lower temperatures. Thus, CBS gives N‐cyclohexylamino‐2‐benzothiazole polysulfides (CBP), 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS), 2‐bisbenzothiazole‐2,2′‐polysulfides (MBTP), and 2‐bisbenzothiazole‐2,2′‐monosulfide (MBTP), 2‐mercaptobenzothiazole (MBT), and 2‐N‐cyclohexylaminobenzothiazole (CB). In the presence of sulfur, the amount of polysulfides formed initially is higher but the polysulfides are unstable, and on prolonged heating, only MBT and CB remain. MOR and TBBS form analogous product spectra. The sulfenamides do not react with ZnO or zinc stearate. The MBT–amine complex prevents MBT, formed on decomposition, from reacting to give zinc mercaptobenzothiazole (ZHBT). Reaction mechanisms are proposed to account for the formation of the products. © 1994 John Wiley & Sons, Inc.

N-t-Butyl Benzothiazole Sulfenimide a New Long-Delay, Slow-Curing Primary Amine Based Accelerator.

TBSI is a new accelerator which provides: Long scorch delay and slow cure rate High modulus development Improved reversion resistance Excellent storage stabilityTBSI can be used for: Replacement of secondary amine-based sulfenamide accelerators Scorchy or difficult to process compounds Compounds exposed to prolonged cure cyclesMaintaining adhesion of rubber to brassed steel10)

Effect of Humidity and Water Content on the Cure Behavior of a Natural-Rubber Accelerated Sulfur Compound

Abstract Certain accelerators are susceptible to chemical attack by water. Consequently, the cure behavior of compounds containing these accelerators is likely to be affected by the water content of the compound and the humidity conditions of processing and storage. A quantitative investigation into the influence of humidity and water content on cure behavior was carried out using a natural-rubber compound with MBS (N-morpholylbenzothiozole-2-sulfenamide) as the principal accelerator. Samples of the compound were treated at different humidities, their water contents determined, and their cure curves measured. Scorch time was inversely related to treatment humidity and water content of the compound, whereas cure rate was directly related to humidity and water content. A study of the mechanism by which water affects the cure behavior of the compound revealed that hydrolysis of the accelerator, MBS, would inhibit its scorch-delay action and could produce MBT (2-mercaptobenzothiazole), which would increase the cure rate. It is concluded that compounds containing other benzothiazole sulfenamide accelerators are likely to exhibit similar behavior. The adverse effect of moisture on the cure behavior of compounds can be minimized by carrying out storage and processing at as low a humidity and temperature and for as short a time as possible.

Synergism between Thiocarbamyl and 2-Benzothiazyl Sulfenamide Accelerators

Synergism between Thiocarbamyl and 2-Benzothiazyl Sulfenamide Accelerators

Sulfur Vulcanization with Two Plateaus—Formation of an Inhibitor during Vulcanization?

Abstract Efficient sulfur vulcanizing systems based on thiazolesulfenamide accelerators derived from secondary amines show unusual and complex vulcanization kinetics in NR. This feature does not appear to have been reported previously. Depending on the nature and purity of the accelerator, the ratio of accelerator to sulfur, the cure temperature, the type of rubber being used, and the presence of other additives, two distinct periods of crosslinking may be evident, separated by a more or less well-defined plateau. This plateau is longer and more distinct if the accelerator is pure, its ratio to sulfur is high, the cure temperature is low, and there are no other additives present. Sulfenamide accelerators derived from both primary and secondary amines may give rise to this behavior in rubbers other than NR. The observed behavior has been shown to be due to the formation, during the first cure period, of an extractable crosslinking inhibitor. The structure of this inhibitor is still in doubt, but the present balance of evidence favors it being an aminodithiobenzothiazole [Structure (1)] or a dithiobisdialkylamine [Structure (2)] or a polythio variant of those structures.

Stabilized Curative Blends for Rubber

Abstract Blending of curatives offers several advantages. These include a sharply reduced number of weighings of curative ingredients, reduced weighing costs, reduced dusting and improved cleanliness. These advantages might not be realized with some curative blends. For example, diphenyl guanidine (DPG) could not be blended with sulfenamide accelerators because it reacted chemically with the sulfenamides. Later work assessed the effect of 2-mercaptobenzothiazole (MBT), sulfur (S), or free amine on the hydrolytic stability of N-cyclohexyl-2-benzothiazyl-sulfenamide (CBS) that was highly purified. Results showed that free amine or sulfur accelerate the hydrolysis, but to a lesser extent than MBT. The present work shows that aging of curative blends (like S, CBS, and DPG) sharply reduces the scorch resistance of rubber compounds containing the aged blends. Selected additives will stabilize the blends.

Curing SBR with Thiocarbamyl Sulfenamide Accelerators

Abstract We have found that the physical properties of conventional SBR/BR tire tread vulcanizates obtained with benzothiazole sulfenamide accelerators can more efficiently be obtained using a thiocarbamyl sulfenamide accelerator. Since thiocarbamyl sulfenamides utilize sulfur more efficiently than benzothiazole sulfenamide accelerators, they should be used at 15 to 20% higher sulfur levels compared to those used with the benzothiazole sulfenamides. The combination of greater efficiency and higher sulfur level used with the thiocarbamyl sulfenamides allows the accelerator level to be reduced by about 40%. Vulcanizate properties as well as the crosslink networks of these two systems are the same when used in this manner.

A Curing System for Rubber Bonded to Brass-Plated Tire Cord

Abstract Devising a suitable vulcanizing system for steel cord carcass mixes presents unusual problems. A relatively high proportion of sulfur needs to be present to react with the brass plating, but this is not generally regarded as good compounding practice, particularly from the point of view of heat-aging resistance. If an attempt is made to improve matters by increasing the ratio of sulfenamide accelerator to sulfur, adhesion suffers. To help resolve these problems, it is suggested that the accelerator N-oxydiethylenebenzothiazole-2-sulfenamide is used in conjunction with dimorpholinyl disulfide. Good adhesion can be obtained with no more than 3.5 parts phr of sulfur and only a moderate amount of vulcanization activator, such as zinc stearate, is needed for maximum state of vulcanization. Insufficient work has been carried out to reach a position where a complete formulation can be recommended, but it is believed that the following vulcanizing system could be used as the basis for achieving a high standard of adhesive bond between a natural rubber tire carcass mix and brass-plated steel tire cord: (see PDF for table)

Model Compound Vulcanization—Part IV. Comparative Assessment of Sulfenamide and Triazine Accelerators

Abstract The application of model compound vulcanization (MCV) to the rating of the relative efficiency of CBS (N-Cyclohexylbenzothiazole-2-sulfenamide) and an experimental triazine-based accelerator is demonstrated. The sulfur to accelerator ratios were established at which both accelerators lead to the same total sulfidic product yield. At these ratios, selected initial properties of natural rubber vulcanizates and their reversion resistance are similar. Hence, the increased reversion resistance of triazine-accelerated natural rubber vulcanizates is essentially due to their greater efficiency in converting sulfur into sulfidic crosslinks. A higher level of the less efficient CBS leads to similar results. This analysis shows that MCV simplifies the evaluation of accelerators.

Adhesion of kevlar aramid cords to rubber

Adhesion development to Kevlar aramid yarn is discussed in relation to polymer characteristics and parameters of molecular structure that determine specific or entropic interactions with adhesives. The effects of rubber compounding variables on adhesion of Kevlar tire cords treated with epoxy/RFL adhesives were determined using the single-cord pull-out and 2-ply peel adhesion tests, which differ in sensitivity to physical properties (stiffness) of the rubber compound. Kevlar adhesion was unaffected by changes in the curing system (e.g., sulfur level, type of sulfenamide accelerator) used in a high-modulus tire belt compound. Methylene donor/acceptor-type bonding agents in the compound improved the adhesion initially and after exposure of dipped cords to ozone. RFL modifications (e.g., use of preformed versus in situ RF resins) improved adhesion. Effects of rubber compounds and adhesive on adhesion are discussed in relation to their chemical and physical characteristics. Lastly, two types of adhesive systems were developed for bonding Kevlar to high-modulus nitrile and neoprene compounds used in hoses. The simple system involves curing an aqueous epoxy dip and then applying an air-drying, solvent-based cement (polyisocyanate). The other consists of curing two aqueous dips (epoxy/RFL with latex modification).

Studies of Heat Generation in Polyester Cord Tires

Abstract Measurements of loss factor in polyester cord and cured rubber stock were made. From the results, the equilibrium cord temperatures in a rotating tire were estimated. These calculated values agree with measured values reported in the literature. It is shown that, for truck tires, cord temperatures can easily rise to levels where cord degradation in the presence of sulfenamide accelerators is known to occur. Measurements on cords which had been degraded by aminolysis indicate that loss factors increase as a result of the degradation and cause significant increases in local steady-state temperatures. This effect will tend to accelerate the rate of degradation at those locations. The increases in loss factor agree with observations on loss factors at increasing levels of crystallinity in polyester fibers in the literature.

The Analysis of Rubber Vulcanizates—Identification of Amine Residues from Sulfenamide Accelerators by Thin Layer Chromatography

Abstract The amine residues from sulfenamide accelerators can be identified in rubber vulcanizates by treatment of the latter with 4-chloro-7-nitrobenzo-2,1,3-oxadiazole (NBD-Cl) and examination of the products by thin-layer chromatography.

Rubber Stocks for Improved Performance of Polyester Cord Tires

Abstract Carcass stocks were developed with acceptable properties and good adhesion to polyester tire cords after severe heat aging in laboratory tests. These stocks contained vulcanization accelerators that did not liberate amines and were based either on synthetic poly (isoprene) (IR)/butadiene/styrene (SBR) elastomer blends, as recommended in previous work, or natural rubber/SBR compounds containing an additive to inhibit the deleterious effects of amines present as impurities in NR. Standard SBR/poly (butadiene) (BR) tread stocks containing amine-free accelerators were developed to match the exceptionally long scorch time and good physical properties attained with sulfenamide accelerators normally used in such stocks. Large batches (˜400 pounds) of amine-free tread stock were processed without any scorch problems on a commercial Banbury mixer and tread extruder. With these tread stocks, containing amine-free accelerators, it is possible to build completely amine-free polyester cord tires. In a variety of wheel tests designed to assess durability under extreme conditions, 2-ply tires with cords of Dacron polyester containing amine-free tread and carcass stocks showed highly improved durability (180-440 per cent) compared to control tires made with stocks containing amines. In a wheel test simulating the running of a tire underinflated for an extended period (low pressure endurance test), tire durability increased with decreasing amine content of the rubber stocks used. In a special high temperature tire fatigue test to produce break-above-bead failures, tires combining an experimental Dacron polyester with low carboxyl content and in amine-free stocks ran 350 per cent longer than tires of standard T-68 Dacron in amine containing stocks. These tests clearly illustrated the additive benefits of combining an improved polyester with stocks having reduced amine content.

Low Sulfur-Sulfenamide Accelerator Combinations for Injection Molding of Cis-1,4-Poly(Isoprene)

Abstract Earlier work with cis-l,4-poly(isoprene) suggested that low levels of sulfur and high levels of sulfenamide accelerator with high levels of secondary accelerator was an excellent method for curing injection molded stocks. However, subsequent experience showed that despite the benefits of reduced sulfur concentrations on reversion, increased sulfur, increased sulfenamide, and reduced secondary accelerator concentrations were necessary for adequate cure and safety. From that experience we explored further and found that, if the sulfenamide concentration is held constant at a higher level and the secondary accelerator concentration is increased as sulfur concentration is decreased, nearly equivalent hardness and modulus can be maintained throughout most cure conditions with no loss in safety and cure rate and much improvement in reversion resistance. Studies were made in a carbon black filled stock with N-cyclohexyl-2-benzothiazole sulfenamide (CBS), N-oxodiethylene-2-benzothiazole sulfenamide (DBS), and N-morpholinyl-2-benzothiazolyl disulfide (MBD) as primary accelerators and tetramethylthiuram monosulfide (TMTM) and tetramethylthiuram disulfide (TMTD) as secondary accelerators.

Influence of Chemical Structure on the Dynamic Properties of Sulfur-Vulcanized, Carbon Black-Reinforced Natural Rubber

Abstract A comparison is made of the composition and properties of the different rubber vulcanizate networks obtained by varying the ratio of sulfur to sulfenamide accelerator and by the thermal aging of vulcanizates containing predominantly polysulfide crosslinks. It is concluded that the changes in network structure which can take place, for example, during the service life of natural rubber tires are not the direct cause of failures of the type associated with rubber fatigue at high temperatures. However, a reduction in the total number of crosslinks can accelerate failure by increasing the amount of heat generated during flexing. More stable networks giving improved resistance to fatigue at high operating temperatures are obtained by the use of higher ratios of accelerator to sulfur than are conventionally employed.

Rubber Chemicals from Cyclic Amines. V. Thiocarbamyl Sulfenamide Accelerators Derived from 3-Azabicyclo(3.2.2)Nonane

Abstract Evaluation of thiocarbamyl sulfenamides (I) prepared from 3-azabicyclo-(3.2.2) nonane and other amines indicates that the curing properties of stocks accelerated by (I) are influenced by the amino groups which constitute the thiocarbamyl and sulfenamide moieties, R and R′, respectively, and also vary with the formulation of the rubber stocks. The fastest cures are obtained when R is pyrrolidinyl or dimethyl and slowest cure when R′ is N-tert-butyl or N-morpholino. The lowest maximum crosslink density is obtained with morpholino- or piperidinothiocarbonyl sulfenamide. In a non-extended SBR black stock the crosslinking reaction generally proceeds as a second order reaction. The major part of the crosslinking reaction of the slower curing stocks proceeds by a fast first order rate followed by a slower first order rate.

The influence of chemical structure on the dynamic properties of sulfur‐vulcanized, carbon black‐reinforced natural rubber

AbstractA comparison is made of the composition and properties of the different rubber vulcanizate networks obtained by varying the ratio of sulfur to sulfenamide accelerator and by the thermal aging of vulcanizates containing predominantly polysulfide crosslinks. It is concluded that the changes in network structure which can take place, for example, during the service life of natural rubber tires are not the direct cause of failures of the type associated with rubber fatigue at high temperatures. However, a reduction in the total number of crosslinks can accelerate failure by increasing the amount of heat generated during flexing. More stable networks giving improved resistance to fatigue at high operating temperatures are obtained by the use of higher ratios of accelerator to sulfur than are conventionally employed.

Migration of Antioxidants and Accelerators in Natural and Synthetic Rubber II. Sulfenamide System Studies and Comparison of Age Resister Migration under Inert and Practical Conditions

Abstract Radiochemical techniques developed for studying extender oil migration in various elastomers have been adapted for use in investigating the migration of age resisters and curatives. This paper contains basic diffusion coefficient data for nine compounds including both staining and nonstaining antioxidants and members of the sulfenamide, thiazole and thiuram accelerator series. Techniques for synthesizing these age resisters and curatives in the carbon-14 and sulfur-35 labelled form are described. Migration was studied in natural rubber, SBR, cis-polybutadiene and EPT vulcanizates. Computer techniques were used to calculate diffusion coefficients from radiochemical count data. Results from preliminary migration studies under practical curing and end-use conditions are also presented. In the case of phenyl-2-naphthylamine migration in cis-polybutadiene, a pronounced decrease in diffusivity with increasing surface area of the carbon black filler was observed.

Thermostability of Sulfenamide Accelerators in Tire Compounds

Abstract The laboratory-type decomposition isotherms plotted by the authors have shown that in all the investigated commercial-type sulfenamide accelerators, dissociation already occurs at 120° C. Within the range of the required mixing times, 140° C can be regarded almost generally as the critical temperature. As shown by technological investigations, at 140° C and higher, there is definitely a marked effect on processing properties, and on the vulcanization behavior of the compounds. The results of the investigation clearly demonstrate, once again, that when compounding temperatures are going to exceed 140° C, it is necessary to mix in the sulfenamide accelerators in a second stage at lower temperatures, preferably below 100° C. Even when the accelerators are incorporated on a mill, immediately after the internal mixer is discharged, a dissociation of the sulfenamide accelerator must be reckoned with if the compound cannot be sufficiently cooled.