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     

Effect of carboxylic acids on 2‐bisbenzothiazole‐2,2′‐disulfide‐ and tetramethylthiuram disulfide‐accelerated sulfur vulcanization. II. Vulcanization of polyisoprene in the absence of ZnO

Polyisoprene was vulcanized by 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur and tetramethylthiuram disulfide (TMTD)/sulfur in the absence and presence of benzoic and stearic acids. It was found that the crosslink density of MBTS vulcanizates is halved by the addition of carboxylic acids and this can be explained in terms of the attack of the acids on the accelerator polysulfides. TMTD polysulfides are more reactive toward polyisoprene than are MBTS polysulfides, and their addition to the polymer chain occurs before significant attack by the carboxylic acids can reduce the polysulfide concentration. Consequently, the acids have little effect on the crosslink density of TMTD vulcanizates. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1007–1012, 1999     

The role of dimethyldithiocarbamic acid in accelerated sulfur vulcanization

Polyisoprene/tetramethylthiuram disulfide (TMTD)/sulfur compounds were vulcanized under a variety of conditions. TMTD does not decompose to tetramethylthiourea (TMTU) at vulcanization temperatures as has been suggested, neither is it formed as an integral part of the crosslinking process. Instead, it results from the attack of dimethylamine, released on decomposition of dimethyldithiocarbamic acid (Hdmtc), on TMTD. It is demonstrated that the formation of TMTU in vulcanizates may be overlooked, as it is readily lost in the work‐up for HPLC analysis. Hdmtc is shown to play an essential role in the crosslinking process in polyisoprene/TMTD/sulfur formulations, and its removal from the system during vulcanization severely impedes crosslinking. Polysulfidic thiuram‐terminated pendent groups are formed, in part, by the interaction of tetramethylthiuram polysulfides with the polymer chain, but largely by an exchange between Hdmtc and polysulfidic thiol pendent groups. The latter are formed when sulfurated Hdmtc reacts with the polymer chain. Crosslinking of thiuram‐terminated pendent groups is slow, and in the absence of ZnO crosslinking results from reaction between polysulfidic thiuram pendent groups and thiols. Crosslinking is delayed until the bulk of the accelerator is bound to the polymer chain, at which point the concentration of free thiuram groups, in the form of Hdmtc, is low, and exchanges between newly formed thiol pendent groups and Hdmtc is less frequent, permitting crosslinking of thiuram pendent groups with these newly formed thiol pendent groups. Data to support the proposed reaction mechanism is presented. Hdmtc on its own accelerates sulfur vulcanization and acts as a catalyst for the reaction, being regenerated in the crosslinking process. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1371–1379, 1999     

Vulcanization of chlorobutyl rubber. III. Reaction mechanisms in compounds containing combinations of zinc dimethyldithiocarbamate,tetramethylthiuram disulfide,sulfur, and ZnO

Poly(isoprene‐co‐isobutylene) (IIR or butyl) and chlorinated poly(isoprene‐co‐isobutylene) (CIIR or chlorobutyl) compounds containing combinations of zinc dimethyldithiocarbamate [Zn₂(dmtc)₄], tetramethylthiuram disulfide (TMTD), sulfur, and ZnO were vulcanized at 150°C, the reaction was stopped at various points, crosslink densities were determined by swelling, and the concentrations of residual curatives and extractable reaction intermediates and products were determined by high‐performance liquid chromatography and atomic absorption (ZnCl₂). In compounds that did not contain zinc, CIIR crosslinked more slowly than IIR and crosslinking could be explained by the same mechanism as applies to the vulcanization to highly unsaturated rubbers like polyisoprene. In zinc containing compounds, CIIR crosslinked faster because of dehydrohalogenation reactions that led to carbon–carbon crosslinks. As found with ZnO/ZnCl₂ formulations, both ZnCl₂ and conjugated diene butyl are essential precursors to crosslink formation. Zn₂(dmtc)₄ can trap HCl, thus preventing reversion and may also initiate dehydrohalogenation. When the equilibrium crosslink density is reached, 50% of the chlorine originally present in the rubber is extractable as ZnCl₂ and the remainder as dimethylthiocarbamic acid chloride. A mechanism to account for dehydrochlorination and crosslinking in the presence of Zn₂(dmtc)₄ is presented. In compounds with sulfur, crosslinking occurs via accelerated sulfur vulcanization and chlorine abstraction, leading to higher crosslink densities than is achieved with either curative system on its own. Carbon–carbon crosslinks predominate, the slower, accelerated sulfur reaction, making a lesser contribution to the overall reaction. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1309–1316, 2001     

Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

Gum compounds of polyisoprene were vulcanized with a number of different curing systems to give networks with crosslink densities in two different ranges. Stress–strain curves were obtained upon rapid (500 mm/min) and slow (0.5 mm/min) extension. In tetramethylthiuram disulfide (TMTD)/sulfur and zinc dimethyldithiocarbamate/sulfur vulcanizates, which crystallize readily, failure occurred at higher tensile values upon rapid than upon slow extension and this is attributed to a greater contribution to tensile strength by a larger amount of stress‐induced crystallites. X‐ray diffraction showed that 2‐benzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates did not stress‐crystallize and failure occurred at lower tensile values. Furthermore, samples extended rapidly failed at lower tensile values than did slowly extended samples. These differences, compared to TMTD vulcanizates, are attributed to extensive main‐chain modifications (pendent groups), causing delays in the movement of sections of the chain, leading to the load being unequally distributed between chains. The fewer load‐bearing chains ensure earlier failure. The addition of zinc stearate to TMTD/sulfur and MBTS/sulfur formulations increases the ability of vulcanizates of similar crosslink density to crystallize and enhances tensile properties of vulcanizates with similar crosslink densities, outcomes that are attributed to zinc stearate's promoting crosslinking of pendent groups and reducing impediments to crystallization and chain movement. Dicumyl peroxide–cured networks crystallize readily and exhibit a very rapid upturn in the stress–strain curve. However, failure occurs at lower stress values than apply to accelerated sulfur networks and it is suggested that the distribution of subchain lengths between crosslinks may contribute to their inferior properties. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2587–2596, 2001     

Swellability of acrylate methacrylate-cellulose matrix systems and the effect on solute diffusion rates

During the vulcanization of cis-1,4-polyisoprene (IR) with thiruam-related curing systems, dimethyldithiocarbamic acid (DMDCA) is formed as a byproduct, in the formation of either pendent groups or crosslinks. DMDCA is unstable, and decomposes instantly to Me₂NH and CS₂ in the absence of ZnO. The facile reaction of Me₂NH and thiuram-related molecules such as tetramethylthiuram polysulfides, tetramethylthiuram disulfide (TMTD), tetramethylthiuram monosulfide and pendent groups caused (i) increased induction periods, (ii) lower maximum crosslink densities, and (iii) the excessive formation of tetramethylthiourea (TMTU). A most important function of ZnO was to trap the DMDCA via the formation of zinc dimethyldithiocarbamate and water, thereby preventing the detrimental reactions above. The IR/TMTD/ZnO and IR/sulfur/TMTD/ZnO systems were therefore characterized by (i) shorter induction periods, (ii) higher maximum crosslink densities, and (iii) the absence of TMTU.     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. III. The reaction of 2-bisbenzothiazole-2,2′-disulfide with sulfur and ZnO in polyisoprene

The sulfur vulcanization of polyisoprene accelerated by 2-bisbenzothiazole-2,2′-disulfide (MBTS) was investigated. Rubber compounds were heated in a DSC and removed at various temperatures along the DSC thermal curve. The rubber vulcanizate was analyzed for crosslink density and for residual reactants and extractable reaction products. MBTS reacts readily with sulfur, and the polysulfidic accelerator complexes react with the rubber chain to form pendent groups. Crosslinking results from hydrogen abstraction, by the benzothiazole pendent group, from a neighboring chain. 2-Mercaptobenzothiazole, a product of crosslinking, also acts as an accelerator in the later stages of the reaction. MBTS has been shown not to react with ZnO and the higher crosslink densities obtained when ZnO is present are attributed to ZnO aiding the abstraction of the benzothiazole pendent group to give zinc mercaptobenzothiazole. A mechanism for the MBTS acceleration of sulfur vulcanization is proposed. © 1996 John Wiley & Sons, Inc.     

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     

Thiuram vulcanization of natural rubber in the presence of hydrofuramide and ethanolamines

Influence of hydrofuramide and ethanolamines on efficient vulcanization (EV), semi-EV, and conventional sulfur vulcanization accelerated by tetramethyl thiuram disulfide is reported. In the case of thiuram vulcanization, the amines increase the cure rate but reduce the crosslink density. The interaction of tetramethyl thiuram disulfide with amines has been studied. The amines interact with thiuram disulfide (TMTD) giving rise to the formation of gaseous products. Thermogravimetric analysis shows weight loss of about 50% in case of TMTD–ethanolamine system and 25% in case of TMTD–hydrofuramide system. Gas chromatographic studies of the gaseous products indicate that it consists mainly of H₂S. The other constituents in the gaseous product are CO₂ and a thiol. A possible reaction based on these observations to account for decrease in crosslink density in thiuram vulcanization caused by amines is reported.     

Structural characterization of vulcanizates. Part VI. The 2-mercaptobenzothiazole-accelerated natural rubber–sulfur system

The crosslinking efficiency of sulfur in the vulcanization of natural rubber in the presence of 2-mercaptobenzothiazole, zinc oxide, and lauric acid has been determined as a function of cure time, cure temperature, and lauric acid concentration. With a low concentration of lauric acid structurally complex networks are formed, which contain 11–19 combined sulfur atoms per chemical crosslink present. The complexity increases with time of vulcanization. With a high lauric acid concentration much simpler networks are formed, which become progressively more simple as reaction proceeds (6 network-combined sulfur atoms per chemical crosslink, decreasing to 2.4 with time). Increasing the cure temperature from 100 to 140°C. reduces the efficiency of crosslinking in both cases. The changes in efficiency are attributed to the influences of the reaction variables (in particular, the concentration of rubber-soluble complexes of the zinc laurate with zinc benzothiazolyl mercaptide) on the structure and subsequent reactions of initially formed polysulfidic crosslinks.     

N,N′‐Pentamethylenethiuram disulfide and N,N′‐pentamethylenethiuram hexasulfide accelerated sulfur vulcanization. III. Vulcanization of polyisoprene and 2,3‐dimethyl‐2‐butene in the absence of ZnO

Polyisoprene and model compound, 2,3‐dimethyl‐2‐butene, were vulcanized with N,N′‐dipentamethylenethiuram disulfide (CPTD), CPTD/sulfur and N,N′‐dipentamethylenethiuram hexasulfide (CPTP6) in the absence of ZnO and residual extractable curatives and reaction intermediates analyzed by HPLC at various stages of the reaction. Accelerator polysulfides, required for the formation of accelerator‐terminated polysulfidic pendent groups, form rapidly, or are present from the outset in the case of CPTP6. Model compounds confirm the formation of thiuram‐terminated polysulfidic pendent groups as intermediates in the vulcanization process. Removal of pentamethylenedithiocarbamic acid (Hpmtc) from the system during heating delays the onset of vulcanization and leads to very low crosslink densities. Rubbers heated under vacuum can subsequently be crosslinked by the addition of zinc stearate, pointing to the presence in the compound of thiuram‐terminated pendent groups. However, such pendent groups do not readily crosslink on their own, and hydrogen‐terminated polysulfidic pendent groups, formed by the reaction of sulfurated Hpmtc with the polymer, are suggested to be involved in the crosslink formation. N,N′‐Pentamethylenethiurea forms in the vulcanizate, but is not as product of crosslinking reactions, rather of CPTD degradation. The data are discussed with respect to mechanisms proposed in the literature for crosslinking, and it is concluded that the data support recently formulated mechanisms in which crosslinking involves reaction between thiuram and thiol‐terminated pendent groups, with Hmptc playing and essential role in the overall process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1100–1111, 2000     

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.     

Structural characterization of vulcanizates. Part VII. The N-cyclohexylbenzothiazole-2-sulfenamide-accelerated natural rubber–sulfur system

The crosslinking efficiency of sulfur in the vulcanization system comprising (in parts by weight) natural rubber (100), sulfur (1.5), N-cyclohexylbenzothiazole-2-sulfenamide (2.37), zinc oxide (5), and lauric acid (1–10) is relatively insensitive to the lauric acid concentration and to the temperature of vulcanization (between 100 and 140°C.). The networks formed contain initially 8–11 combined sulfur atoms per chemical crosslink present, but this number falls progressively to about 4 as the reaction proceeds. The results are consistent with the intermediate formation of a rubber-soluble complex of cyclohexylamine with zinc benzothiazolyl mercaptide. This complex is believed to be responsible also for the further slow crosslinking which the vulcanizates undergo on standing at room temperature.     

Study of curative interactions in cis-1,4-polyisoprene. XII. The cis-1,4-polyisoprene–sulfur–tetramethylthiuram disulphide–ZnO–stearic acid vulcanization system

Several aspects on the mechanism of vulcanization in the synthetic cis-1,4-polyisoprene (IR)-sulfur-tetramethylthiuram disulphide (TMTD)–ZnO system were harmonized. The differential scanning calorimetry (DSC) thermograms showed that the vulcanization processes became better resolved on increasing the curative loading in the compound. Two major crosslinking reactions occurred consecutively in the IR (100)–sulfur (9.46)–TMTD (8.86)–ZnO (3.00) mixture, viz the IR–sulfur–TMTD–ZnO and IR–sulfur–zinc dimethyldithiocarbamate (ZDMC) (or IR–sulfur–ZDMC–ZnO) reactions. In the first process poly-and disulfidic pendent groups RS_xSX (R = polyisoprenyl, X = Me₂NC (S), x ≥ 1) formed via the IR–XSS_xSX reaction, and in the second via the IR–XSS_xZnSSX reaction. Thermogravimetric analysis (TGA) and high-pressure liquid chromatography (HPLC) data showed that dimethyldithiocarbamic acid liberated during the IR–sulfur–TMTD–ZnO reaction was trapped by ZnO to yield ZDMC. Hence ZDMC was a product, and not precursor, of this crosslinking process. A comparison of reactions in IR–sulfur–TMTD–ZnO and poly(ethylene-co-propylene)–sulfur–TMTD–ZnO mixtures showed that the participation of IR molecules was essential for ZDMC formation. The ZDMC concentration remained constant at ～ 38.4 mol % during the later stages of cure, showing that it did not participate in the desulfuration reactions of polysulfidic links. In the presence of stearic acid the stearic acid–ZnO reaction occurred at 87°C as was manifested by an intense crystallization peak of zinc stearate. The vulcanization processes were the same both in the presence and absence of stearic acid.     

Sulfur vulcanization of natural rubber accelerated with 2-mercaptobenzothiazole plus tetramethylthiuram disulfide

The sulfur vulcanization of NR by a MBT-TMTD mixed accelerator system both in the presence and absence of ZnO and stearic acid with or without DCP has been studied. The rate of decomposition of DCP in the presence of both MBT and TMTD is quite similar to that in the MBT system alone. The reduction in crosslinking due to DCP is dependent mainly on MBT. The decomposition product of TMTD may contribute to it only at the later stage of vulcanization. Though TMTD has no influence on the decomposition rate, it reacts with MBT at least in the initial stage of vulcanization and suppresses the retardation caused by MBT on DCP vulcanization in accordance with the free sulfur decrease, the nature of crosslinking formation both in the presence and absence of DCP, and the methyl iodide treatment of the vulcanizates. The vulcanization process of the MBT-TMTD-S-NR system has been interpreted in terms of both free radical and polar mechanisms. The domination of either of these depends on the dominant amount of either MBT or TMTD in the acclerator ratio. According to the initial high rate of crosslink formation, free sulfur decrease and also the initial additiveness of crosslinking in stocks containing DCP, the vulcanization process of MBT-TMTD-ZnO-St. acid-S-NR system has been explained in terms of an ionic mechanism. The pronounced synergistic nature of such systems has been interpreted by the enhanced activation of MBT-S-ZnO-St. acid complex due to the dithiodicarbamate ion formed in the initial stages of vulcanization, and also by the activation of TMTD accelerated vulcanization due to the mercaptobenzothiazylion.     

Effect of zinc oxide concentration on the course of thiuram-accelerated sulfur vulcanization

Tetramethylthiuram disulfide-accelerated sulfur vulcanization of natural rubber has been investigated. Continuous measurements in a Vuremo curemeter at 145°C were used to estimate the effects of zinc oxide concentration on the induction periods, on the first-order rate constants, and on the ultimate extents of crosslinking, on the extents of degradation reaction (reversion), and on the extents of relaxation of vulcanizates at the cure temperature. The concentration of zinc oxide has practically no influence on the rate of thiuram-accelerated sulfur cure. The values of the ultimate extents of crosslinking increase with increasing the zinc oxide content in the rubber compound up to a certain limit corresponding to the theoretical amount of zinc oxide which is necessary for the formation of zinc dimethyldithiocarbamate from tetramethylthiuram disulfide and zinc oxide during the vulcanization reaction. From the point of view of the reversion, however, this limit value of zinc oxide concentration is not sufficient. The relaxation measurements provide the same results. On the basis of these, for thiuram-accelerated sulfur vulcanizations, the optimum zinc oxide content in the rubber mix of 2.5 phr has been calculated. This value is in very good agreement with the optimum value of zinc oxide concentration found for both sulfenamides and thiazoles-accelerated sulfur cures.     

Effect of aerosil on the course of thiuram accelerated sulfur vulcanization

Tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of natural rubber has been investigated at temperatures from 100°C to 145°C. Continuous measurements in a Vuremo curemeter were used to estimate the extent of crosslinking, which was plotted against cure time. The cure curves as well as their linearized forms (dependences of the logarithm of the extent of vulcanization on the cure time) clearly show that at lower cure temperatures the course of the vulcanization differs significantly from the first-order rate law. These digressions have been removed by the addition of a highly dispersed silica gel, Aerosil, which simultaneously speeds up the course of the vulcanization up to the value corresponding to the rate of zinc dimethyldithiocarbamate (ZnDMDC)-accelerated sulfur vulcanization. These results are in accordance with our recent theory supposing that ZnDMDC is the actual accelerator in TMTD-accelerated sulfur systems. In the presence of Aerosil, the formation of ZnDMDC from TMTD is catalyzed via dispersed silica gel. Support for this view derives from the temperature dependences of vulcanization reactions. The activation energies of TMTD-accelerated sulfur vulcanizations in the absence (31 kcal/mole) and in the presence of Aerosil (23.5 kcal/mole) correspond exactly to the values calculated from the rate constants of the thiuram decrease in TMTD-accelerated vulcanization (30 kcal/mole) and from the rate constants of crosslinking in the dithiocarbamate-accelerated sulfur vulcanization (23 kcal/mole), respectively.     

A comparison of the tetramethylthiuram disulphide accelerated sulfur vulcanization of polybutadiene and polyisoprene

A detailed study of the vulcanization of polybutadiene with various combinations of sulphur, tetramethylthiuram disuphide, ZnO, stearic acid, and zinc dimethyldithiocarbamate is reported. Vulcanization was conducted by heating samples at 2.5°C/min in a differential scanning calorimeter. The reaction was stopped at various points along the vulcanization exotherm, the soluble reaction products and residual curatives were extracted and analyzed, and the crosslink density and percentage polysulphidic crosslinks were determined. The overall reaction mechanism was found to be similar to that for polyisoprene, but the reaction, once initiated, was faster than in polyisoprene, the crosslink density was higher, and the percentage polysulphidic crosslinks was lower. These differences are discussed. © 1993 John Wiley & Sons, Inc.     

A study of curative interactions in cis-1,4-polyisoprene. X. The cis-1,4-polyisoprene/sulfur/zinc dimethyldithiocarbamate and cis-1,4-polyisoprene/sulfur/zinc dimethyldithiocarbamate/ZnO systems

Aspects of the mechanism of zinc dimethyldithiocarbamate (ZDMC)-accelerated sulfur vulcanization were discussed. The trends in the efficiency parameter E, confirmed that crosslinking is preceded by the formation of pendent groups RS_xSX [R = polyisoprenyl, X = Me₂NC(S)] in ZDMC-based systems. The index x in RS_xX was calculated as 5.82 in the cis-1,4-polyisoprene (IR)/sulfur/ZDMC/ZnO compound at the initial stages of curing, compared to 3.23 in the absence of ZnO. The high value of x supports the postulation that elemental sulfur and ZDMC react at the early stages of vulcanization, to form the active sulphurating agent XS_xSZnSSX. Crosslinks form by either a disproportionation reaction between two α-methylic or α-methylenic pendent groups RS_xX, or a reaction between a pendent group RS_xX and the rubber chain—these routes are the same as that suggested for the IR/tetramethylthiuram disulfide (TMTD)/ZnO compound. The beneficial role of ZnO and zinc stearate is shown, as in the case of ZnO in the IR/TMTD/ZnO system, to be related to their ability to trap dimethyldithiocarbamic acid, which formed in the generation of pendent groups and crosslinks. ZnS is inactive in this regard. The formation of ZnS is characteristic of natural rubber/sulfur/ZDMC/ZnO systems, as opposed to IR/TMTD/ZnO mixtures where little ZnS forms.     

Crystallization of vulcanizates. II. Low‐temperature crystallization as a function of extent of cure for polybutadiene vulcanized with dicumyl peroxide,tetramethylthiuram disulfide, 2‐bisbenzothiazole‐2,2′‐disulfide,and zinc–accelerator complexes

Polybutadiene compounds, vulcanized to various degrees of cure, were crystallized in a density column at ?16°C. The percentage crystallinity of vulcanizates was also determined by differential scanning calorimetry where samples, precooled at a programmed rate, were reheated. Curing with peroxides has little effect on either the rate or the extent of crystallization, except at very high crosslink densities, although the induction period prior to crystallization increases progressively with increased crosslink density. Tetramethylthiuram disulfide (TMTD)/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur vulcanizates, cured for progressively longer periods, were found to have lower densities, a result attributed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period for crystallization increases and both the rate and the extent of crystallization decrease with extent of cure. These changes are more marked for MBTS vulcanizates that do not crystallize once a gel has formed. Formulations with zinc stearate develop higher crosslink densities and crystallize to a greater extent on cooling, showing the effect of zinc stearate in the crosslinking of pendent groups. The densities of both zinc dimethyldithiocarbamate [Zn₂(dmtc)₄]– and zinc mercaptobenzothiazole [Zn(mbt)₂]–accelerated sulfur vulcanizates increase with cure time, a result attributed to the formation of ZnS in the compounds. Zn₂(dmtc)₄ compounds crystallize extensively on cooling, pointing to limited main‐chain modification. It is suggested that main‐chain modification in these vulcanizates may comprise cyclic sulfide formation. Zn(mbt)₂ compounds crystallize less readily than Zn₂(dmtc)₄ compounds, but to a greater extent than MBTS/sulfur compounds. The crystallization of the vulcanizates is discussed in terms of vulcanization reactions that give rise to crosslinking with the different formulations used. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2573–2586, 2001