Tetramethylthiuram disulfide and 2-mercaptobenzothiazole as binary accelerators in sulfur vulcanization. I. Exchange reactions between the accelerators and sulfur in the absence of rubber

Exchange reactions between tetramethylthiuram disulfide, 2-mercaptobenzothizole, and sulfur were studied by heating powdered mixes to vulcanization temperatures at a programmed rate in a DSC. The reaction was stopped at points along the thermal curve and the mixture was analyzed by HPLC. On dissolution, even unheated samples undergo a sulfide exchange reaction leading to a mixed accelerator, while polysulfides of the thiuram and mixed accelerator form in low concentrations. On heating, higher concentrations of these polysulfides are formed, particularly in the presence of elemental sulfur. Dimethyldithiocarbamic acid, formed in the exchange, influences the product spectrum if it remains trapped in the DSC pan. Tetramethylthiourea is formed only at elevated temperatures when dimethylamine, a degradation product of the acid, is trapped in the DSC pan. A series of reactions is proposed to explain the product spectrum obtained under different conditions. © 1995 John Wiley & Sons, Inc.     

Dimethylammonium dimethyldithiocarbamate‐accelerated sulfur vulcanization. I. Thermal stability and reactions with curatives

To investigate its thermal stability and reactivity towards other accelerators dimethylammonium dimethyldithiocarbamate ((dma)dmtc) was heated at programmed heating rates in sealed and open pans in a differential scanning calorimeter and on a thermobalance. (Dma)dmtc was heated on its own, in rubber compounds and in mixes with tetramethylthiuram disulfide, zinc dimethyldithiocarbamate, and ZnO. In open systems (dma)dmtc readily sublimes at temperatures well below its melting point, and it is almost as rapidly lost when compounded on its own with rubber. No decomposition is observed at vulcanization temperatures and (dma)dmtc is unreactive towards tetramethylthiuram disulfide and zinc dimethyldithiocarbamate. Moisture does not promote its decomposition. Reaction with ZnO to give zinc dimethyldithiocarbamate is restricted to the surface of ZnO particles. (Dma)dmtc is soluble in water in which it dissociates. It must be concluded that when its is included in formulations, (dma)dmtc, per se, would act as a vulcanization accelerator; it does not decompose to dimethyldithiocarbamic acid and dimethylamine, both of which are known accelerators. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3067–3073, 2001     

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

The 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.     

A new secondary accelerator for the sulfur vulcanization of natural rubber latex and its effect on the rheological properties

The vulcanization of natural rubber (NR) latex can be effectively carried out at low temperatures by using binary accelerator systems containing thiourea (TU) as a secondary accelerator. It was reported that sulfur‐containing nucleophiles such as thiourea enable the primary accelerator to become effective even at low temperatures, indicating a nucleophilic reaction mechanism in such vulcanization reactions. In the present study, a derivative of thiourea [viz. aminoiminomethyl thiourea (AMT)], which is more nucleophilic than thiourea, is used as a secondary accelerator in the sulfur vulcanization of NR latex. One of the aims of this study was to give conclusive evidence for a nucleophilic reaction mechanism. The synergistic effect of the above thiourea derivative with primary accelerators such as tetramethylthiuram disulfide (TMTD), zinc diethyldithiocarbamate (ZDC), and cyclohexylbenzthiazyl sulfenamide (CBS) was studied at two different temperatures (viz. 100 and 120°C). These binary systems were found to be very effective in reducing the optimum cure time of the different mixes compared to control formulations containing TU. The optimum amount of the secondary accelerator required was also determined. Mechanical properties such as tensile strength and tear strength of the vulcanizates were also evaluated. Chemical characterization of the vulcanizates was carried out by determining the total crosslink density. Values of the cure characteristics evaluated support a nucleophilic reaction mechanism in these vulcanization reactions under review. The effect of this secondary accelerator on the rheological behavior of compounded latex is also studied and was found not to affect adversely. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2781–2789, 2004     

A DSC study of curative interactions. IV. The interaction of tetramethylthiuram disulfide with ZnO,sulfur, and stearic acid

The interaction of combinations of sulfur, tetramethylthiuram disulfide (TMTD), ZnO, and stearic acid were investigated by differential scanning calorimetry in the absence of rubber. TMTD decomposed partially to tetramethylthiuram monosulphide on liquefaction. Sulfur and TMTD reacted at vulcanization temperatures, and although the exact composition of all the products was not established, several features involving DSC and HPLC analysis were interpreted in terms of the formation of tetramethylthiuram polysulfides (TMTP). TMTD decomposed much faster to volatile products such as Me₂NH, CS₂, and CS when heated in the presence of stearic acid. Contrary to literature reports on the facile reaction of TMTD and ZnO to yield zinc perthiomercaptides (or zinc dimethyldithiocarbamate), the TMTD/ZnO reaction was found to be extremely sluggish under a variety of conditions. In the presence of sulfur, too, the TMTD/ZnO reaction was of negligible importance. It was inferred that several reactions occurred concurrently on heating a TMTD/stearic acid/ZnO system. These reactions were not observed for the sulfur/TMTD/stearic acid/ZnO mixture per se, but, instead, the stearic acid/ZnO reaction was very prominent. The formation of zinc stearate occurred at temperatures as low as 77°C in the quadruple system. TMTD and zinc stearate were virtually unreactive at vulcanization temperatures. None of the reactions involving ZnO could be attributed to the formation of a zinc perthiomercaptide, generally accepted to be a precursor in thiuram vulcanization.     

A benzoxazole sulfenamide accelerator: Synthesis,structure, property,and implication in rubber vulcanization mechanism

A benzoxazole sulfenamide and its related zinc compound were synthesized and characterized, which are shown to be useful accelerators for sulfur vulcanization. In comparison with the benzothiazole accelerator, the benzoxazole sulfenamide revealed nearly no reversion, an improved feature that is desirable for tire industry. Through the synthesis of the zinc complex 9 , which was assumed to be an accelerator‐activator for vulcanization, the study aimed to shed some light on the vulcanization mechanism. The crystal structure of the zinc complex 9 showed that two benzoxazole‐2‐thiol ligands were attached to the zinc center in different isomeric forms, with one in the thio form (linked via sulfur atom) while the other in thio–keto form (linked via nitrogen atom). Lack of the vulcanization accelerator activity from 9 led to the assumption that the sulfurating species might be zinc complexes containing only one benzoxazole‐2‐thiol ligand. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39699.     

The thermal decomposition of sulfenamide accelerators

The 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.     

Studies on novel binary accelerator system in sulfur vulcanization of natural rubber

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Thiourea and its derivatives are important secondary accelerators in this context. It is suggested that thiourea containing binary systems of rubber vulcanization proceed by a nucleophilic reaction mechanism. Amidinothioureas (ATUs), which are derivatives of thiourea, have been investigated extensively as secondary accelerators. One of the aims of this study was to get further proof with regard to the theory of the nucleophilic reaction mechanism in such binary systems. In the present study phenyl substituted ATU was used as a secondary accelerator along with mercaptobenzothizyl disulfide, tetramethyl thiuram disulfide, or cyclohexyl benzthiazyl sulfenamide in the sulfur vulcanization of natural rubber. The results showed an appreciable reduction in the cure time for the mixes containing the ATU compared to the reference mixes. These results are indicative of a nucleophilic reaction mechanism in the vulcanization reaction under review. These vulcanizates also showed comparatively better tensile properties and good retention of these properties after aging. The optimum dosages of the secondary accelerator required for these vulcanization reactions were also derived. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1–8, 2001     

A new synergistic combination of safe accelerators for the improvement of physical properties in gum and filled vulcanizates

The synergistic activity of bis(diisopropyl) thiophosphoryl disulfide (DIPDIS) in the presence of dithio bis(N-β-OH ethyl piperazine) (DTEP), a safe amine-based accelerator, has been studied both in gum and filled stock vulcanization of NR. This study, concerning the effect of variation of sulfur concentration on the gum vulcanization of NR, reveals that, although modulus values of the vulcanizates increase with the level of sulfur, the tensile strength values attain a maximum at 1.5 phr sulfur for an equimolar (4.5:4.5) concentration of DIPDIS and DTEP. The concentration of accelertors was varied from 6 mmol to 18 mmol phr, keeping the molar ratio at 1:1. It was noticed that, although the torque and modulus values increased, the tensile strength values decreased after attaining a maximum at 12 mmolar concentration of DIPDIS and DTEP accelerators, when the sulfur concentration was fixed at 1.5 phr. The synergistic activity of DIPDIS-DTEP accelerated mixes is considered to arise from the generation of some intermediate accelerators that are governed by the proportion of accelerators, sulfur, filler, etc., present in the mix. Some investigations were also carried out to optimize the amount of carbon black at these fixed levels of accelerators and sulfur. A regarding tensile strength, it increased up to a loading of 50 phr carbon black, but beyond this level a gradual fall in the value was noticed. However, modulus, torque, and hardness increased with the concentration of carbon black. The optimum dispersion of carbon black for this system was obtained following Lee's method. © 1995 John Wiley & Sons, Inc.     

Comparison of tetraethyl‐ and tetramethylthiuram disulfide vulcanization. I. Reactions in the absence of rubber

The reactions of tetraethylthiuram di‐ and monosulfide (TETD and TETM, respectively) were investigated in the absence of rubber and compared with those reported previously for tetramethylthiuram compounds. The reactions of TETM, TETD, and zinc diethyldithiocarbamic acid with zinc oxide and sulfur were investigated by differential scanning calorimetry, and the reaction products analyzed by high performance liquid chromatography. TETM was shown to be more stable at vulcanization temperatures (±150°C), but also less reactive with sulfur than tetramethylthiuram disulfide (TMTD). The reactions of TETD are very similar to those of TMTD, the TETD reacting slower than the TMTD to form analogous products. In the presence of zinc oxide, the formation of the zinc compound of TETD, zinc diethyldithiocarbamic acid, occurred readily. TMTD does not react readily with zinc oxide. The reaction of TETD with sulfur lead to the formation of polysulfidic accelerator species, although the concentrations formed in the absence of rubber were considerably less than that formed by the corresponding TMTD system. These differences in reactivity would affect the vulcanization reactions that take place in the rubber. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2292–2299, 2001     

Synthesis and effect of surface modified nano ZnO in natural rubber vulcanization

It is well known that surface of ZnO acts both as a reactant and a catalytic reaction template in rubber vulcanization by activating and bringing together reactants. The particles of accelerators, fatty acid, and sulfur diffuse through the polymer matrix and get adsorbed on the surface of ZnO, forming intermediate complexes. Hence dispersion of ZnO in the elastomer matrix is a determining parameter. Capping is one of the novel techniques for increasing ZnO‐stearic acid/accelerator interaction, thereby enhancing their activities. During the sol–gel precipitation of nano ZnO, if a suitable capping agent is added, agglomeration of ZnO particles gets reduced, leading to the formation of nano particles. Since only very few studies are found reported on synthesis of accelerator‐capped ZnO and its application in rubber vulcanization, attempts have been made in this study to synthesize our novel accelerator N‐benzylimine aminothioformamide (BIAT)‐capped‐stearic acid‐coated nano ZnO (ZOBS), and BIAT‐capped ZnO (ZOB) to investigate their effects in natural rubber (NR) vulcanization. Efforts have also been made to synthesize stearic acid‐coated nano zinc phosphate (ZPS) with an aim to find an alternative to conventional ZnO in vulcanization. Mechanical properties like tensile strength, tear resistance, abrasion resistance, and compression set were found out. Swelling values of the vulcanizates as a measure of crosslink densities were also determined. Optimum dosage of ZOBS and a combination of ZOB and ZPS were also derived and found that capped ZnO is superior in NR vulcanization to conventional ZnO in improving cure properties including scorch safety and mechanical properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Influence of sulfenamide accelerators on cure kinetics and properties of natural rubber foam

The effect of types of sulfenamide accelerator, i.e., 2‐morpholinothiobenzotiazole (MBS), N‐t‐butylbenzothiazole‐2‐sulfenamide (TBBS), and N‐cyclohexyl benzothiazole‐2‐sulfenamide (CBS) on the cure kinetics and properties of natural rubber foam was studied. It has been found that the natural rubber compound with CBS accelerator shows the fastest sulfur vulcanization rate and the lowest activation energy (E_a) because CBS accelerator produces higher level of basicity of amine species than other sulfenamide accelerators, further forming a complex structure with zinc ion as ligand in sulfur vulcanization. Because of the fastest cure rate of CBS accelerator, natural rubber foam with CBS accelerator shows the smallest bubble size and narrowest bubble size distribution. Moreover, it exhibits the lowest cell density, thermal conductivity and thermal expansion coefficient, as well as the highest compression set as a result of fast crosslink reaction. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44822.     

N,N′‐Pentamethylenethiuram disulfide‐ and N,N′‐pentamethylenethiuram hexasulfide‐accelerated sulfur vulcanization. I. Interaction of curatives in the absence of rubber

N,N′‐pentamethylenethiuram disulfide (CPTD), CPTD/sulfur, and N,N′‐pentamethylenethiuram hexasulfide (CPTP6) were heated in a DSC at a programmed heating rate and isothermally at 140°C. Residual reactants and reaction products were analyzed by HPLC at various temperatures or reaction times. CPTD rapidly formed N,N′‐pentamethylenethiuram monosulfide (CPTM) and N,N′‐pentamethylenethiuram polysulfides (CPTP) of different sulfur rank, CPTP of higher sulfur rank forming sequentially, as reported earlier for tetramethylthiuram disulfide (TMTD). As with TMTD, the high concentration of the accelerator monosulfide that develops is attributed to an exchange between CPTD and sulfenyl radicals, produced on homolysis of CPTD. However, a different mechanism for CPTP formation to that suggested for TMTD is proposed. It is suggested that disulfenyl radicals, resulting from CPTM formation, exchange with CPTD and/or CPTP already formed, to give CPTP of higher sulfur rank. CPTD/sulfur and CPTP6 very rapidly form a similar product spectrum with CPTP of sulfur rank 1–14 being detectable. Unlike with TMTD/sulfur, polysulfides of high sulfur rank did not form sequentially when sulfur was present, CPTP of all sulfur rank being detected after 30 s. It is proposed that sulfur adds directly to thiuram sulfenyl radicals. Recombination with sulfenyl radicals, which would be the most plentiful in the system, would result in highly sulfurated unstable CPTP. CPTP of higher sulfur rank are less stable than are disulfides as persulfenyl radicals are stabilized by cyclization, and the rapid random dissociation of the highly sulfurated CPTP, followed by the rapid random recombination of the radicals, would result in the observed product spectrum. CPTP is thermally less stable than is TMTD and at 140°C decomposed rapidly to N,N′‐pentamethylenethiourea (CPTU), sulfur, and CS₂. At 120°C, little degradation was observed. The zinc complex, zinc bis(pentamethylenedithiocarbamate), did not form at vulcanization temperatures, although limited formation was observed above 170°C. ZnO inhibits degradation of CPTD to CPTU. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2718–2731, 2000     

Studies on a new binary accelerator system containing TMTD and amidinothiourea in sulfur vulcanization of natural rubber

It is well known that the use of binary accelerator systems in rubber vulcanization provide better physical and chemical properties to the vulcanizates. The present work reports the results obtained on the study of amidinothiourea as a secondary accelerator along with tetramethyl thiuram disulfide (TMTD) in the sulfur vulcanization of natural rubber. Vulcanizates containing amidinothiourea have an appreciable increase in mechanical properties and better retention of these properties after aging compared to the reference mixes used. These mixes containing amidinothiourea showed appreciable reduction in optimum cure time as well. This suggests a nucleophilic reaction mechanism in the vulcanization reactions under review. Mixes with varying concentrations of amidinothiourea were tried; and the optimum level of amidinothiourea required has been derived based on the cure characteristics of the mixes and physical properties of the vulcanizates. © 1994 John Wiley & Sons, Inc.     

A comparison of tetraethylthiuram and tetramethylthiuram disulfide vulcanization. II. Reactions in rubber

The reactions of tetraethylthiuram disulfide (TETD) with polyisoprene were investigated under vulcanization conditions. Samples of polyisoprene compounded with various combinations of TETD, sulfur, and ZnO were heated in a differential scanning calorimeter to various degrees of vulcanization. The crosslink density of the compounds was determined by swelling, and the extractable residual curatives and reaction products were analyzed with high‐performance liquid chromatography. TETD caused crosslinking to occur in the absence of added sulfur, as did tetramethylthiuram disulfide (TMTD), both sulfur donors. In the presence of sulfur, the formation of TETD polysulfides occurred immediately before the crosslinking reaction started. The TETD polysulfides were the initial crosslinking agents. The ready reaction between TETD and zinc oxide to form zinc diethyldithiocarbamic acid resulted in considerably higher crosslink densities. This greater reactivity between TETD and zinc oxide, compared with that between TMTD and zinc oxide, did not lead to any noticeable differences in the vulcanizate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1119–1127, 2002     

The reactions of 2‐(4‐morpholinothio)‐ and 2‐(4‐morpholinodithio) benzothiazole in the absence of polyisoprene

2‐(4‐morpholinothio)benzothiazole (MOR) and 2‐(4‐morpholinodithio)benzothiazole (MDB) were heated with sulfur and ZnO in a DSC. The products formed at various temperatures were identified and analyzed by HPLC. At temperatures below 200°C, decomposition of the accelerator in the absence of other curatives was slow, degradation products being mainly 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS) and 2‐mercaptobenzothiazole (MBT). A rapid exothermic decomposition above 200°C resulted in the formation of MBT (or its amine salt) and 2‐(4‐morpholino)benzothiazole (MB). MOR and MDB reacted with sulfur to form higher polysulfides. MDB was shown to react more readily with sulfur than MOR and the delayed action of MOR in rubber can therefore not be ascribed to a stable polysulfide as suggested by other authors. Neither MOR nor MDB was found to react with ZnO. A limited reaction between MBT and ZnO was observed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1235–1240, 1999     

A new binary accelerator system for the sulfur vulcanization of natural rubber latex

Vulcanization of latex products are usually carried out at lower temperatures compared to dry rubber products. It has been suggested that, in latex vulcanization systems where thiourea is used as a secondary accelerator, it acts as a nucleophilic reagent facilitating the cleavage of the sulfur bonds in the primary accelerator like TMTD or CBS at lower temperature. But no conclusive proof is given to such a postulate. In the present study 1-phenyl-2,4-thiobiuret (DTB II) and 1,5-diphenyl-2,4-dithiobiuret (DTB III), which are more nucleophilic than thiourea and which vary in their nucleophilic reactivity, were studied as secondary accelerators along with tetramethyl thiuram disulphide (TMTD) and N-cyclohexylbenzothiazyl sulphenamide (CBS) in the vulcanization of natural rubber latex. These binary systems were found to be very effective in reducing the optimum vulcanization time. Also it was noted that 1-phenyl-2,4-dithiobiuret, which is more nucleophilic, is more reactive (as observed from the reduction in optimum cure time) as a secondary accelerator, indicating a nucleophilic reaction mechanism in the vulcanization reactions under review. The optimum dosages of the secondary accelerators required were derived. Physical properties like tensile strength, 300% modulus, and elongation at break of the latex vulcanizates were also studied. There is a definite advantage with respect to many of these properties for dithiobiuret systems compared to the systems containing TMTD alone or TMTD/thiourea. DTB III gives higher values in many of these physical properties than DTB II. Chemical characterization of the vulcanizates was also carried out to correlate the physical properties with the type of chemical crosslinks formed. © 1993 John Wiley & Sons, Inc.     

N‐benzoyl‐N′N′‐disubstituted thioureas—A new binary accelerator system and its effect of nucleophilicity in sulfur vulcanization of natural rubber

The effect of several new binary accelerator systems were studied with special emphasis to the relationship of nucleophilicity to their rheological and mechanical properties. In this study, dialkyl/azacycloalkyl substituted benzoylthioureas (BTU 1‐5) were used as secondary accelerators (SA) along with three different primary accelerators (PA) viz., Cyclohexylbenzthiazylsulfonamide (CBS), Mercaptobenzothiazole (MBT) or Zincdithiocarbamate(ZDC) in the sulfur vulcanization of natural rubber. The effect of these secondary accelerators on the rheological and mechanical properties was found to be improved when compared to the reference mixes. It is noted that the N‐benzoyl‐N′N′‐piperidinylthiourea (BTU 4) is more effective as a secondary accelerator due to the higher nucleophilicity of the same when compared with the test compounds. The ZDC‐BTU gives the best result as binary accelerator system in natural rubber. Chemical characterization was carried out by determining the total crosslink density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

A DSC study of curative interactions. II. The interaction of 2,2′-dibenzothiazole with ZnO,sulfur, and stearic acid

The interaction of combinations of sulfur, 2,2′-dibenzothiazole (MBTS), ZnO, and stearic acid were studied by differential scanning calorimetry. A MBTS/stearic acid interaction was indicated as evidenced by the effect the MBTS/stearic acid combination had on the melting of sulfur, the S_α → S_β transition being suppressed in favor of a S_α → S_γ transition. The dissolution/interaction of MBTS in molten sulfur was also delayed by the MBTS/stearic acid interaction, which, it is proposed, involved protonation of the N atom in MBTS by stearic acid. MBTS did not affect the formation of zinc stearate from ZnO and stearic acid, but when sulfure was added to the mixture, the ZnO/stearic acid reaction did not go to completion. No direct evidence for the formation of 2,2′-dibenzothiazole polysulphides was found, but the absence of the S_γ → S_μ transition in sulfur/MBTS mixes was interpreted as indirect evidence of a reaction between these curatives. There was no evidence for the formation of a sulfur/MBTS/ZnO compound of the type generally attributed the role of an active sulfurating agent in accelerated sulfur vulcanization.     

A DSC study of curative interactions. III. The interaction of TMTM with ZnO,sulfur, and stearic acid

The interaction among various combinations of sulfur, tetramethylthiuram monosulphide (TMTM), ZnO, and stearic acid were studied by differential scanning calorimetry in the absence of rubber. Sulfur and TMTM reacted to form tetramethylthiuram disulphide, and the ternary eutectic mixture melted at about 80°C. The absence of the S_γ → S_μ transition in sulfur/TMTM mixes was related to a sulfur/TMTM interaction, wherein the eightmembered sulfur rings were opened at temperatures well below 170°C. The interaction of stearic acid with TMTM led to the decomposition of TMTM, but the reaction was largely suppressed when both ZnO and sulfur were present in the mixture. No evidence was found for the formation of a zinc-accelerator complex of the type normally attributed a role in the accelerated sulfur vulcanization.