Benzothiazole Sulfenamide Research Articles

Method Development for Separation and Analysis of Tire and Road Wear Particles from Roadside Soil Samples

A comprehensive understanding of tire and road wear particles (TRWPs) and their detection and quantification in soils is still challenged by the lack of well-set standardized methods, inherent technological inconsistencies, and generalized protocols. Our protocol includes soil sampling, size separation, and organic matter removal by using hydrogen peroxide followed by density separation and analysis. In this context, roadside soil samples from different sites in Kansas and Ohio, USA, were collected and analyzed. Tire cryogrinds analogous to TRWPs were used to evaluate various density separation media, and collected particles more than 1 mm in size were then subjected to infrared spectroscopy (IR), thermogravimetric analysis (TGA), and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) to confirm TRWP presence. Particles smaller than 1 mm were Soxhlet extracted, followed by gas chromatography-mass spectrometry (GC-MS) to validate the presence of tire-related intermediates. SEM-EDX validated the presence of elemental combinations (S + Zn/Na) ± (Al, Ca, Mg, K, Si) attributed to tires. Ketones, carboxylic acids, epoxies, cyclohexane, and benzothiazole sulfenamide (BTS) intermediates were the most probable tire-related intermediates observed in the roadside soil samples. Thus, this simple, widely applicable, cost-effective sample preparation protocol for TRWP analysis can assist TRWP research advancement in terrestrial environments.

Investigation of abiotic degradation of tire cryogrinds

The abundance of microplastics found in the environment is a major cause of concern. Tire tread particles containing additives such as curing accelerators and antioxidants, can be a major source of elastomer pollution in the environment. Such tire particles combined with road pavement particles are referred to as tire and road wear particles, TRWP. The environmental availability from parent elastomers and the release of additives in the process of abiotic degradation were evaluated using freeze-thaw, wet-dry and accelerated UV-weathering experiments. Acceleration factor determination tests were conducted to correlate UV-exposure to the natural aging in the environment. Freeze-thaw testing showed many additives such as diphenyl guanidine (DPG), benzothiazole sulfenamide (BTS) and para-phenylene diamine (6 PPD) as tetrahydrofuran leachates and BTS transformation products. Further, UV exposure equivalent to 1.5 yr., 3 yr. and 5 yr. aging resulted in the formation a combination of ketones and carboxylic acids for styrene butadiene rubber (SBR), natural rubber (NR), and butadiene rubber-based tire cryogrinds. Attenuated total reflectance- Fourier- transform infrared spectroscopy (ATR-FTIR) was used to detect the degradation of the elastomers on UV-exposure while gas chromatography-mass spectroscopy (GC-MS) was used as a nontargeted, suspect screening analysis technique. The degradation intermediates and leachates identified using GC-MS represents useful data for the life cycle analysis of the functional polymers and additives and their possibility of environmental release.

Fragmentation patterns of protonated benzothiazole sulfenamides by atmospheric pressure chemical ionization

Benzothiazole sulfenamides of N-t-butylbenzothiazole-2-sulfenamide (TBBS), N-cyclohexylbenzothiazole-2-sulfenamide (CZ), and N,N'-dicyclohexylbenzothiazole-2-sulfenamide (DZ) are most popular sulfur cure accelerators. In this study, the benzothiazole sulfenamides were ionized by atmospheric pressure chemical ionization (APCI) method, the product ions were directly analyzed using a single quadrupole mass spectrometer, and their fragmentation patterns were characterized. Structures and energies of the fragment ions and the precursor ions were calculated, an appropriate precursor ion was determined, and the fragmentation mechanisms were suggested. Protonated benzothiazole sulfenamides had ammonium structures, and they were rearranged through 5- and 7-membered cyclic structures to produce the fragment ions of m/z 166 and 183, respectively. DZ had the other fragment ion of m/z 265. Relative abundances of the fragment ions for TBBS were much greater than those for CZ and DZ. It was found that the steric hindrance and the number of hydrogen atoms participating in the rearrangements influenced on efficiency of the fragmentations. Relative abundances of the fragment ions increased as the source fragmentor voltage increased. The intensity ratios of the (m/z 166)/(m/z 183) were also increased by increasing the source fragmentor voltage. The (m/z 166)/(m/z 183) ratios of TBBS were smaller than those of CZ and DZ. The benzothiazole sulfenamides could be directly identified by comparing kinds and relative abundance ratios of the fragment ions as well as their protonated molecules. And difference in the fragmentation behaviors according to the benzothiazole sulfenamides might be related to their cure characteristics.

Nitrosamine-safe thiuram disulfide and benzothiazole sulfenamide as a synergistic pair of accelerators for the vulcanization of rubber

Nitrosamine-safe thiuram disulfide and benzothiazole sulfenamide as a synergistic pair of accelerators for the vulcanization of rubber

Influence of Silane Coupling Agent Content on Crosslink Type and Density of Silica-Filled Natural Rubber Vulcanizates

Crosslink type and degree of crosslink density of a rubber vulcanizate determine the physical properties such as modulus, hardness, resilience, elongation at break, heat build-up, and so forth. By increasing the crosslink density, the modulus, hardness, resilience, and abrasion resistance increase, whereas the elongation at break, heat build-up, and stress relaxation decrease. The stress relaxation, tensile strength, and resilience increase in proportion to the content of polysulfides, whereas the fatigue and thermal aging resistance decrease. Silica and carbon black have been used as the main reinforcing agents in rubber compounds but their surface chemistries are very different. Silica has a number of hydroxyl groups on the surface, which results in strong filler-filler interactions and adsorption of polar materials by hydrogen bonds. Since intermolecular hydrogen bonds between hydroxyl groups on the silica surface are very strong, it can aggregate tightly. Its property can cause a poor dispersion of silica in a rubber compound. The adsorption of curatives by silica results in delay of the scorch time and reduction of the delta torque of a silica-filled rubber compound. The polar surface of silica makes hydrogen bonds with polar materials in rubber compounds. Since the silica surface is acidic, especially it forms a strong hydrogen bond with basic materials. N-tert-Butyl-2-benzothiazole sulfenamide (TBBS) and N-cyclohexyl benzothiazole sulfenamide (CBS) are generally used as cure accelerators for rubbers. Since they have basic functional groups such as amide, they are adsorbed well on the acidic silica surface. The adsorption of curatives to silica results in delay of the scorch time and reduction of the crosslink density in silicafilled rubber compounds. In general, silane coupling agent such as bis-(3-(triethoxysilyl)-propyl)-tetrasulfide (TESPT) is used to improve the silica dispersion as well as to prevent adsorption of curatives on the silica surface. The silane coupling agent reacts with silanol on the silica surface and then a siloxane bond is formed. The silane molecule is bound to the silica surface. In the present work, we studied the influence of silane coupling agent content on the crosslink type and level of crosslink density using silica-filled natural rubber (NR) compounds with different TESP contents. Variation of the crosslink density and type after the thermal aging was also investigated. Sulfur linkages are composed of monosulfide, disulfide, and polysulfides. Polysulfides of the total sulfur crosslinks are enhanced by increasing the elemental sulfur content in a rubber compound. Cure accelerators such as TBBS, CBS, 2-mercaptobenzothiazole (MBT), and tetramethyl thiuramdisulfide (TMTD) make the cure rate of a rubber compound faster. Sulfur linkages, especially polysulfides, can be dissociated by heating and this brings about decrease of the crosslink density. Curatives, especially sulfur, remained in a rubber vulcanizate make new crosslinks and this results in increase of the crosslink density. Figure 1 shows the variations of the crosslink densities with the silane coupling agent content. The crosslink densities of the monoand disulfides, polysulfides, and total sulfur linkages were investigated. All the crosslink densities are enhanced by increasing the silane coupling agent

The reaction of benzothiazole sulfenamide with (TMS) 3SiH: An example of degenerate-branched chain process

The reaction of sulfenamide 3 with (TMS) 3SiH initiated by the decomposition of AIBN at 76 °C has been studied in some detail. The reaction is a rare example of a radical chain-branching process. The two main products are dialkylamine 4 and the thiosilane 5. It is also established that 2-mercaptobenzothiazole ( 2) is formed in a substantial yield as one of the by-products. The mechanism of this chain autocatalytic reaction is complex due to a mix of different radical chain reactions and some discussion is provided. The amine obtained in a quantitative yield can arise from two independent routes of attack of (TMS) 3Si radical on sulfenamide 3. The minor route affords thiol 2 that can act as a catalyst for the major route during the reaction course and then gives a salt with secondary amine, which precipitates upon cooling. The origin of autocatalysis is discussed in some detail.

Effect of vulcanizing system on the crosslink density of nitrile rubber compounds

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Binary accelerator systems are being widely used in industry and are becoming increasingly popular because of the fact that such mixed systems can produce a vulcanizate with superior mechanical properties compared to those of stock cured with a single accelerator. The authors have studied the performance of a binary accelerator system based on cyclohexyl benzothiazole sulfenamide (CBS), tetramethyl thiuram disulphide (TMTD) in the sulfur vulcanization of nitrile rubber. The amount of sulfur and accelerator was varied to change the network crosslink density of vulcanizates. The observed mutual activity has been discussed based on the mechanical properties and crosslink density. The physical crosslink density of the various nitrile rubber mixes was estimated using the Kinetic Theory of Elasticity. The mechanical properties of the various rubber compounds were related to the corresponding crosslink density estimated for each compound. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2440–2445, 2005

Cure modification of natural rubber containing benzothiazole accelerators by some modified thiocarbamyl sulfenamides

The effects of N-cyclopentamethylene thiocarbamyl-N-(cyclohexyl,thiocyclohexyl)sulfenamide (CPCTS) and N-oxydiethylene thiocarbamyl-N-(cyclohexyl,thiocyclohexyl)sulfenamide (ODCTS) as cure modifiers on the vulcanization of natural rubber (NR) containing benzothiazole accelerators were studied. CPCTS and ODCTS were used separately with 2-mercaptobenzothiazole and N-oxydiethylene benzothiazole sulfenamide to determine their effects on these accelerators. CPCTS and ODCTS retarded the accelerators. Enhanced activity was found with respect to the torque, scorch, modulus, and tensile strength and was believed to be due to the formation of different intermediate components during the early stage of the vulcanization of NR. Reactions likely responsible for acceleration and retardation were explored. CPCTS and ODCTS provided better cure retardation than N-cyclohexyl thiophthalimide. The chemical characterization of the NR vulcanizates correlated with the physical properties obtained at 140°C. Stocks containing CPCTS or ODCTS produced mostly monosulfide linkages, which were affected when N-cyclohexyl thiophthalimide was used. This provided information concerning the heat-resistance properties of the vulcanizates.

Binary Cure Systems of 1,6-Bis(N,N'-dibenzylthiocarbamoyldithio)-hexane and Benzothiazole Sulfenamides in Carbon Black-filled Natural Rubber Compounds

Binary cure system is composed of different two cure accelerators, which can cause a synergy effect to delay the scorch time and to increase the cure rate. In this study, binary cure systems between 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)-hexane (DBTH) and benzothiazole sulfenamides were investigated using carbon black-filled natural rubber compounds. N-Cyclohexyl-2-benzothiazole sulfenamide (CBS), N-tert-butyl-2-benzothiazole sulfenamide (TBBS), and 2-(morpholinothio) benzothiazole (MOR) were employed as benzothiazole sulfenamides. The binary cure systems show scorch safty at high temperature. The binary cure systems have faster cure rate and better reversion resistance than the single cure system of the benzothiazole sulfenamides. DBTH is found to be more effective to decrease the viscosity of a compound than the benzothiazole sulfenamides. Physical properties of the vulcanizates with the binary cure system are better than those of the vulcanizates with the single one.

Solid-state13C-NMR and equilibrium-swelling studies of filled, TBBS-accelerated sulfur vulcanization of natural rubber

The effective network chain density (νe) and the density of the chains between sulfurizations (2[S]chem) for natural rubber vulcanized with N-t-butyl benzothiazole sulfenamide (TBBS) in different levels of HAF–HS (N347) carbon black were determined by equilibrium swelling and solid-state 13C-NMR measurements, respectively. The proportion of the intermolecular crosslinks over a whole range of sulfurized products (Ec) for the gum stock was estimated by the comparison of the chemical network chain density (νchem) calculated from νe using Mullins's approach with 2[S]chem. Ec increases with cure, reaches a maximum at an optimum cure state (trpar;, and decreases during an overcuring. It was observed from the NMR results that the total amount of chemical sulfurization reactions occurring in the rubber is almost independent of the level of carbon black loading. Based on the assumption that the amount of the intermolecular crosslinking reactions is also constant with the carbon black content, the amount of physical crosslinks (vent) in filled formulations was determined by subtracting the estimated νchem from the νe. The linear relationship obtained from the plot of the vent at the optimum cure state (t90) as a function of carbon black content postulates that the physical adsorption plays a major role in the polymer–filler interactions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1391–1399, 1998

Solid‐state 13C‐NMR and equilibrium‐swelling studies of filled, TBBS‐accelerated sulfur vulcanization of natural rubber

The effective network chain density (νe) and the density of the chains between sulfurizations (2[S]chem) for natural rubber vulcanized with N-t-butyl benzothiazole sulfenamide (TBBS) in different levels of HAF–HS (N347) carbon black were determined by equilibrium swelling and solid-state 13C-NMR measurements, respectively. The proportion of the intermolecular crosslinks over a whole range of sulfurized products (Ec) for the gum stock was estimated by the comparison of the chemical network chain density (νchem) calculated from νe using Mullins's approach with 2[S]chem. Ec increases with cure, reaches a maximum at an optimum cure state (trpar;, and decreases during an overcuring. It was observed from the NMR results that the total amount of chemical sulfurization reactions occurring in the rubber is almost independent of the level of carbon black loading. Based on the assumption that the amount of the intermolecular crosslinking reactions is also constant with the carbon black content, the amount of physical crosslinks (vent) in filled formulations was determined by subtracting the estimated νchem from the νe. The linear relationship obtained from the plot of the vent at the optimum cure state (t90) as a function of carbon black content postulates that the physical adsorption plays a major role in the polymer–filler interactions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1391–1399, 1998

Reactions of 2-(4-morpholinothio)- and 2-(4-morpholinodithio)benzothiazole in the presence of polyisoprene

2-(4-Morpholinothio)benzothiazole (MOR) and 2-(4-morpholinodithio)-benzothiazole (MDB) were reacted, in combination with sulfur and ZnO, in the presence of polyisoprene (IR). Samples were heated in a DSC at 2.5°C/min and characterized by swelling experiments. The products formed at various temperatures were analyzed by HPLC. Crosslinking only occurred once all the benzothiazole sulfenamide had been consumed, the onset of vulcanization characterized by a considerable increase in 2-mercaptobenzothiazole (MBT) concentration. Crosslinking occurred earlier in all corresponding MDB formulations. Higher crosslink densities were recorded with addition of ZnO. The delayed action experienced in MOR systems was attributed to an exchange reaction between benzothiazole-terminated pendent groups and MOR and not due to the stability of the disulfide, MDB. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1093–1099, 1997

Tetraisobutylthiuram Monosulfide: How Can It Be Both a Cure Retarder and Accelerator with Benzothiazole Sulfenamides?

Abstract N, N, N′, N′-Tetraisobutylthiuram monosulfide (TiBTM) performs both as a retarder and kicker when used with 2-benzothiazole sulfenamide accelerators. Model reactions were run and followed by HPLC to identify the intermediates that form between TiBTM (and its various analogs) and the primary accelerators. Some of these intermediates were independently synthesized and evaluated as retarders/kickers. The isobutyl groups are responsible for imparting the scorch delay characteristics to one of these intermediates. One other intermediate is most likely responsible for the kicker effect.

Novel Sulfur Vulcanization Accelerators Based on Mercapto-Pyridine, -Pyrazine, and -Pyrimidine

Abstract Sulfur vulcanization accelerators derived from 2-mercaptobenzothiazole (MBT) have been a staple for the rubber processing industry for over 65 years. Most noteworthy are the sulfenamide derivatives, which provide various combinations of scorch delay and cure rate, depending upon the basicity and steric nature of the sulfenamide N-substituent(s). While the effect of the amine moiety on benzothiazole sulfenamide accelerator performance has been well documented, the effect of the S-linked heterocyclic moiety of the sulfenamide has been investigated to a much lesser extent. Thus, to gain a better understanding of the role of the heterocycle, a variety of novel disulfides and sulfenamides based on pyridine have been synthesized and tested. The performance of the pyridine based accelerators have been compared to each other and to the various benzothiazole analogues. Until recently, the corresponding sulfenimides (compounds containing two divalent thiol moieties attached to an amine nitrogen ) seem to have received little attention in the rubber industry. Sulfenimides based on pyridine, pyrazine and pyrimidine have therefore also been synthesized and tested. The differences in preparation and vulcanization activity for the benzothiazole sulfenimides and sulfenamides are described, as well as some rudimentary differences which have appeared in the early stages of mechanistic work. It has been discovered in the course of this work that other heterocyclic thiols may be substituted for 2-mercaptobenzothiazole in the basic sulfenimide structure to gain advantages both in faster cure rate and extended scorch delay. The behavior of these novel sulfenimides in common rubber cure systems is presented together with representative physical properties of the rubber vulcanizates.

Performance Level of Accelerator cum Antioxidants in Different Rubbers

1-( N-oxydiethylene thiocarbamyl)-2-(N-oxydiethylene thio) ben zimidazole (MBSPT) has been found to act as a dual function accelerator cum antioxidant in natural and styrene butadiene rubber. Its dual functional activ ity has been evaluated in different diene rubbers viz. NR, SBR, CR, BR, IIR, NBR and EPDM. The accelerating function of MBSPT in these diene rubbers was compared with a standard commercial accelerator, N-cyclohexyl-2- benzothiazole sulfenamide (CBS), and the antioxidant function of MBSPT with two different commercial antioxidants viz. N-isopropyl-N'-phenyl-p- phenylenediamine (IPPD) and 2-mercapto benzimidazole (MBI). The results of this study show better dual performance of MBSPT in NBR, SBR, CR, NR and IIR rubbers in accelerating and antiaging characteristics. MBSPT, therefore, can be prescribed for technical rubber products instead of two monofunctional additives.

A Study of Thiocarbamyl Sulfenamide/Zinc Oxide and TMTD/Zinc Oxide Sulfurless Cure Systems

Abstract In sulfurless cures of NR, soluble thiocarbamyl sulfenamides are found to produce the same state of cure as thiuram disulfides when compared on an equivalent dithiocarbamyl group basis. The addition of MBTS or benzothiazole sulfenamides to thiocarbamyl sulfenamides, or to thiurams, increases the modulus in proportion to the amount added. Adding sufficient benzothiazole sulfenamide appears to double the amount of sulfur donated by these thiocarbamyl accelerators and simultaneously increases monosulfide crosslink formation. The addition of zinc dibutyldithiocarbamate to thiocarbamyl sulfenamides significantly increases the modulus but has no effect on TMTD. Thiuram disulfides are generally reported to donate one sulfur per mole for crosslink formation. However, critical analysis of the literature and the above data indicates that two moles of TMTD are required to form one crosslink, i.e. donate one sulfur.

2-(Isopropylsulfinyl)-Benzothiazole as a Delayed Action Thiazole Accelerator

Abstract At vulcanization temperatures, 2-(isopropylsulfinyl)-benzothiazole(ISB) generates 2-benzothiazole sulfenic acid as a reactive intermediate. In this respect (i.e., formation of a sulfenyl species), ISB is analogous to a benzothiazole sulfenamide, but without the built-in amine activation. Thus, ISB alone is slow, but responds to a variety of activators. The potential to independently select the activator can offer a wider latitude in meeting today's compounding challenges. For example, ISB activated by tetrabenzylthiuram disulfide represents a system devoid of precursors of volatile introsamines ISB with thiophosphoryl sulfides can provide effective, amine-free cure packages. Where nitrosamines are not a factor, tetramethylthiuram monosulfide (TMTM) remains the best general purpose activator. Examples of the effects of ISB/activator systems on cure characteristics and physical properties in a variety of recipes are presented. A unique, practical synthesis of ISB in two steps from MBT, propylene and bleach is outlined, and a more generally adaptable laboratory procedure is described in detail. Diagnostic reactions of ISB and substituent effects of related derivatives, designed to support the postulated acceleration mechanism, are described.

Solid State C-13 NMR Studies of Vulcanized Elastomers. XII. Accelerated Sulfur-Vulcanized High-Vinyl Polybutadiene

Abstract Solid state 13C NMR measurements were made on accelerated sulfur-vulcanized polybutadiene with a high vinyl content (70%). The systems studied included conventional, semi-efficient and efficient formulated with N-t-butyl benzothiazole sulfenamide (TBHS) as the accelerator. The kinetics of the three vulcanization systems were different, but the final levels of cure were similar. The resonances due to TBBS disappear as the vulcanization proceeds. The ratio of monosulfidic to polysulfidic crosslinks as a function of accelerator to sulfur ratio initially decreases rapidly and then increases in favor of the monosulfidic component. A variety of sulfur-bonded accelerator fragments was found in the system. This reinforces the polar-radical mechanism.

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.

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.