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.     

Study of curative interactions in cis-1,4-polyisoprene (IR). VII. The cis-1,4-polyisoprene–tetramethylthiuram disulfide–ZnO system

A detailed account of the mechanism of crosslinking in the cis-1,4-polyisoprene (IR)-tetramethylthluram disulfide (TMTD)–ZnO system is given. Many experimental observations were harmonized in terms of a radical mechanism, rather than an ionic mechanism. Electron spin resonance (ESR) spectra on the IR–TMTD–ZnO system, recorded at 120°C, inter alia revealed resonance lines in the vicinity of g = 2.02. These were related to the rapid formation of thiuram persulphenyl radicals XS, on the homolytic splitting of tetramethylthiuram polysulfides. The Moore–Trego efficiency E dropped from 11.5 (at 140.0°C) to 3.5 (at 146.9°C), indicating that a substantial part of the sulfur atoms was initially to be associated with pendent groups. The formation of these pendent groups could be viewed as an irreversible, concerted reaction without the formation of a true alkenyl radical intermediate. Crosslinks would form by either a disproportionation reaction between two α-methylic or α-methylenic pendent groups RS_xX or a reaction between a pendent groups RS_xX (R = polyisoprenyl, x ≥ 2, X = Me₂NC(S)) and the unsaturated polymer chain. The latter crosslink formation reactions were regarded as rate determining in the vulcanization sequence. A mechanism is proposed that does not require the participation of ZnO in the formation of the active sulfurating agent.     

Study of curative interactions in cis-1,4-polyisoprene. VI. Interaction of some combinations of sulfur,tetramethylthiuram disulfide,and ZnO

The interaction of curatives in the systems cis-1,4-polyisoprene (IR)–sulfur, IR–sulfur–ZnO, IR–tetramethylthiuram disulphide (TMTD), and IR–sulfur–TMTD were studied. Thermal events observed in the differential scanning calorimetry curing curves characteristic of these systems were explained in terms of the melting/liquefaction of compounds, the evaporation of gases, and the vulcanization process itself. The similarity of the IR–sulfur and IR–sulfur–ZnO curing curves suggested that sulfur and ZnO were unreactive during vulcanization. On heating the IR–TMTD and IR–sulfur–TMTD systems, gases such as Me₂NH and CS₂ formed easily. Although the maximum crosslink densities in the latter systems were low, the crosslink formation was found to be strongly exothermic. The sulfur efficiency parameter E was estimated for the IR–sulfur–TMTD system and decreased steeply from 37.5 (at 143.2°C) to 16.6 (at 151.0°C). This was taken as evidence that much of the bound sulfur was initially combined in pendent groups. Then E increased dramatically toward the advanced stages of cure, emphasizing the extraordinary inefficient manner in which sulfur was utilized to form crosslinks.     

A study of curative interactions in cis-1,4-polyisoprene. VIII. Network maturing reactions in the cis-1,4-polyisoprene/tetramethylthiuram disulfide/ZnO system

Several aspects concerning the network maturing reactions of the cis-1,4-polyisoprene (IR)/tetramethylthiuram disulfide (TMTD)/ZnO curing system, were evaluated with reference to previously published literature. The crosslink density increased progressively from 1.42 × 10^?5 mol cm^?3 rubber at 140.0°C, to 8.30 × 10^?5 mol cm^?3 at the higher temperature of 190.0°C, which tied in with the fact that natural rubber (NR) or IR/TMTD/ZnO systems show negligible reversion provided sufficient ZnO is present. The increase in the crosslink density value was accompanied by a systematic buildup of monosulfidic crosslinks, and a substantial decrease in the concentration of tetramethylthiuram monosulfide (TMTM). Calculations showed, in addition, that the increase in the concentration of zinc dimethyldithiocarbamate (ZDMC) and the decrease in the TMTM concentration, were interdependent. It is shown that TMTM, rather than ZDMC, was involved in the crosslink shortening reactions.     

Study of curative interactions in cis-1,4-polyisoprene (IR). V. General experimental procedure

The tetramethylthiuram disulfide (TMTD) and zinc dimethyldithiocarbamate (ZDMC) related vulcanization of cis-1,4-polyisoprene (IR) were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and electron spin resonance (ESR). The progress of the reactions in a given compound was monitored by analyzing the vulcanizate at selected points along the DSC curing curve. Full details of the analysis procedures are given. Analysis were mainly concerned with measurement of the crosslink densities, percentage of polysulfidic crosslinks, and the types and quantities of extractable compounds by thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC).     

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.     

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.     

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     

Effect of water and hydrogen sulfide in zinc dimethyldithiocarbamate accelerated sulfur vulcanization

Rubber and the model compound 2,3‐dimethyl‐2‐butene (TME) are vulcanized with zinc dimethyldithiocarbamate [Zn₂(dmtc)₄] accelerated sulfur formulations. When heated in dry nitrogen, Zn₂(dmtc)₄ is stable at vulcanization temperatures. However, it shows a mass increase when heated in moist nitrogen, indicating strong coordination with water; in a nitrogen/H₂S atmosphere rapid degradation to dimethyldithiocarbamic acid (Hdmtc) and ZnS occurs. Model compound studies show that crosslinked sulfides are essentially bis(alkenyl) and confirm the absence of accelerator terminated pendant groups in the vulcanizates, while the ease with which rubber vulcanizates crystallize on cooling in a density column also suggests that pendant groups are largely absent. However, the rates of crystallization, measured as the time for the crystallization process to go to 50% completion, are slower in lightly crosslinked gels than in peroxide cures of similar crosslink density, particularly in the vulcanizates cured in a vacuum; this is interpreted as an indication that some residual pendant groups are present in Zn₂(dmtc)₄ vulcanizates. Water promotes the rate of crosslink formation in both rubber and TME systems, and it is suggested that the strong coordination of water with zinc in Zn₂(dmtc)₄ promotes its reactivity. The H₂S liberated in the vulcanization process promotes decomposition of Zn₂(dmtc)₄ to Hdmtc, and this reaction makes an important contribution to the amount of Hdmtc that is formed in situ. The importance of Hdmtc as an accelerator and its role in providing alternative routes to crosslink formation in Zn₂(dmtc)₄ accelerated sulfur vulcanization are discussed. It is suggested that water, which is liberated when Hdmtc reacts with ZnO to form Zn₂(dmtc)₄, activates newly formed Zn₂(dmtc)₄ molecules; and this accounts for the beneficial influence of ZnO in Zn₂(dmtc)₄ formulations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1516–1531, 2002     

The limiting value of ZDMC formation: new insight into the reaction of ZNO and TMTD

The reaction of ZnO and tetramethylthiuram disulfide (TMTD) was reinvestigated in detail. Under conditions where evaporation of volatiles is possible, TMTD and an excess of ZnO are found to produce bis(dimethyldithiocarbamato)zinc(II) (ZDMC) in limiting amounts close to 60 mol %, irrespective of the ratio between ZnO and TMTD. This result points to the operation of more than one route toward ZDMC. When ZnO and TMTD are reacted in closed vessels in inert atmosphere, a nucleophilic reaction of ZnO with TMTD was confirmed by GC–mass spectroscopy (MS) detection of COS and NMR observation of tetramethylthiourea (TMTU). This route is found to account for about 70 mol % of the total amount of ZDMC formed. A previously unrecognized redox reaction between ZnO, sulfur, and TMTD, furnishing ZnSO₄ and ZDMC, is responsible for approximately 15 mol % of the amount of ZDMC. Other products that were detected are CO₂, CS₂, and tetramethylurea, whereas ZnSO₃, ZnS, and dioxygen were absent. Based on the latter observation, the operation of a mechanism constituting radical reduction of water by TMTD, yielding dioxygen, was excluded. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1247–1257, 1999     

Synthesis of Zn/Mg oxide nanoparticles and its influence on sulfur vulcanization

To reduce the ZnO levels in rubber compounds, mixed metal oxide nanoparticles of zinc and magnesium (Zn_1−xMg_xO) have been synthesized and used as activator. The aim is to obtain better curing properties due to its nanosize and to take advantage of the behavior of both ZnO and MgO in sulfur vulcanization. The model compound vulcanization approach with squalene as a model molecule for NR and CBS as accelerator has been used to study the role of the mixed metal oxide along the reaction. The results found show that with Zn_1–xMg_xO nanoparticles the reaction of CBS becomes faster, higher amounts of MBT are formed at shorter reaction times, and the consumption of sulfur occurs faster in comparison with standard ZnO. Furthermore and more important, an increased crosslink degree calculated as the total amount of crosslinked squalene is obtained. All these findings indicate that Zn_1−xMg_xO is a promising candidate to reduce the ZnO levels in rubber compounds. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Evidence of ionic components in vulcanization mixtures by means of electric current measurements: Investigations with natural rubber/sulfur/zinc bis(dimethyldithiocarbamate) and comparison mixtures

Continuous low‐level current (CLLC) measurements for detecting ionic species in the course of vulcanization reactions were applied to investigate the vulcanization of a mixture of natural rubber (NR), sulfur (S), and zinc bis(dimethyldithiocarbamate) (ZnDMTC). A dc voltage was applied to the reaction mixture in a special vulcanization mold and the current (e.g., in the range of 10⁻⁹ A) was measured. Temperature‐dependent current maxima were found after reaction times t_max. The simplest explanation is that transitory ionic species occur during vulcanization. An activation energy (E_a ) = 116.4 kJ/mol, similar to that obtained in previous chemical investigations, was determined from the decrease of t_max with increasing temperature. The maxima corresponded to reaction times where a strong increase of polymer crosslinking was observed, as measured using vulcametry. For comparison, dc measurements were carried out with the corresponding mixture without elemental sulfur (NR/ZnDMTC) and mixtures containing zinc stearate (ZnST) instead of zinc bis(dimethyldithiocarbamate) (NR/S/ZnST and NR/ZnST). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2206–2212, 2000     

Bis(diisopropyl) thiophosphoryl disulfide in cis-1,4-polyisoprene vulcanization reactions. I. As a sulfur donor

Bis(diisopropyl)thiophosphoryl disulfide (DIPDIS) is used as a sulfur donor vulcanizing system for cis-1,4-polyisoprene. It is shown that the network structure consists of poly- and disulfidic crosslinks at early stages of cure, simplifying at optimum cure to monosulfidic crosslinks. It is thought that pendent accelerator groups are bound to the rubber molecule at early stages of cure, but are subsequently replaced by cyclic sulfidic groups. The good thermal and thermal oxidative aging behaviour of the vulcanizate is due to the formation of zinc diisopropyldithiophosphate (ZDP) in situ.     

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.     

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     

Kinetics of thiuram–accelerated sulfur vulcanization

On the basis of continuous measurements in a Vuremo curemeter at 145°C, kinetics of tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of natural rubber has been investigated. It was found that the cure rates increase with increasing TMTD concentration, the sulfur content being kept constant, up to a TMTD:S weight ratio of 2:1. Beyond this value, the cure rates again decrease. This TMTD:S ratio corresponds to 3.8 gram atoms of sulfur per mole TMTD, and it is in good agreement with findings that in TMTD-accelerated sulfur vulcanization systems the peak value of zinc dimethyldithiocarbamate (ZnDMDC) formation reaches an endvalue when the stocks contain 4 gram atoms of sulfur per mole TMTD. These facts lead us to suppose that ZnDMDC is the actual accelerator in TMTD-accelerated sulfur systems. Support for this view derives from our experiments with model curing systems as well as from the generally known practical experience that dithiocarbamates are faster accelerators than thiuram disulfides. For the reasons described above and for the finding that the dependences of the ultimate extent of vulcanization (network chain density) on the concentration of TMTD in the absence and in the presence of elemental sulfur are analogous, the mechanism of thiuram-accelerated sulfur vulcanization is very probably similar to that of sulfur-free thiuram vulcanization.     

Influence of zinc oxide during different stages of sulfur vulcanization. Elucidated by model compound studies

The addition of zinc oxide (ZnO) as an activator for the sulfur vulcanization of rubbers enhances the vulcanization efficiency and vulcanizate properties and reduces the vulcanization time. The first part of this article deals with the reduction and optimization of the amount of ZnO. Two different rubbers, solution‐styrene‐butadiene rubber and ethylene–propylene–diene rubber, have been selected for this study. The results demonstrate that the curing and physical properties can be retained when the level of ZnO (Red Seal) is reduced to 1 or 2 phr, respectively. Of particular interest is nano‐ZnO, characterized by a nanoscale particle distribution. The cure characteristics indicate that with nano‐ZnO, a reduction of zinc by a factor of 10 can be obtained. In the second part, model compound vulcanization is introduced to investigate the effects of ZnO during the different stages of vulcanization. Experiments are described with two models, squalene and 2,3‐dimethyl‐2‐butene, both with benzothiazolesulfenamide‐accelerated vulcanization systems. The results demonstrate the influence of ZnO during the different stages of the vulcanization. With ZnO present, a marked decrease can be observed in the sulfur concentration during an early stage of vulcanization, along with a slight delay in the disappearance of the crosslink precursor. The crosslinked product distribution is influenced as well. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1388–1404, 2005     

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-accelerated sulfur vulcanization. I. The formation of the active sulfurating agent

Mixtures of tetramethylthiuram disulfide (TMTD)/sulfur/ZnO were heated in a DSC to various temperatures. Zinc dimethyldithiocarbamate (Zn₂(dmtc)₄ formed only in undried TMTD/ZnO mixes, the reaction being catalyzed by water on the ZnO surface. The presence of ZnO delays the decomposition of TMTD by adsorbing thiuram sulfenyl radicals, which are needed to initiate tetramethylthiuram monosulfide (TMTM) and tetramethylthiuram polysulfide (TMTP) formation. Increased amounts of TMTM are formed in mixes where ZnO is present, and TMTP are detected prior to TMTM formation. Zn₂(dmtc)₄ does not react with sulfur under conditions where labile hydrogen atoms are not available. © 1996 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