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     

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.     

The effect of carbon black on tetramethylthiuram disulfide accelerator sulfur vulcanization of polyisoprene

Polyisoprene was vulcanized with the tetramethylthiuram disulfide/sulfur/ZnO system in the presence and absence of N330 carbon black. Crosslinking was carried out in a DSC at a programmed heating rate, the reaction stopped at points along the thermal curve, and the system analyzed. Residual curatives and reaction intermediates were determined by HPLC and crosslink densities by swelling in benzene. Combinations of the powdered curatives were also heated with and without carbon black and analyzed. It is shown that the step in the vulcanization sequence, influenced by carbon black, is the formation of tetramethylthiuram polysulfides that act as the active sulfurating agent in vulcanization. Carbon black catalyzes their formation, and to a lesser extent, the formation of accelerator terminated polysulfidic pendent groups on the chain. © 1995 John Wiley & Sons, Inc.     

Benzothiazole‐accelerated sulfur vulcanization. I. 2‐Mercaptobenzothiazole as accelerator for 2,3‐dimethyl‐2‐butene

2,3‐Dimethyl‐2‐butene (TME) was used as a model compound for polyisoprene in a study of 2‐mercaptobenzothiazole (MBT)‐accelerated sulfur vulcanization. Mixes that contained curatives only were heated in a DSC to various temperatures, while those that also contained TME were heated isothermally at 150°C in evacuated, sealed glass ampules. Heated mixtures were analyzed for residual curatives, intermediates, and reaction products by HPLC. It is proposed that MBT forms polysulfidic species (BtS_xH) in the presence of sulfur and that these react with TME via a concerted, substitutive reaction pathway to form polysulfidic hydrogen‐terminated pendent groups of varying sulfur rank (TME–S_xH). MBT is released as a by‐product of this reaction. Crosslinking occurs slowly as a result of the interaction of polythiol pendent groups, the rate being dependent on the pendent group concentration. H₂S is released on crosslinking. 2,3‐Dimethyl‐2‐butene–1‐thiol was synthesized and reacted in the presence of sulfur to confirm the formation of crosslinked products (TME–S_x–TME). Benzothiazole‐terminated pendent groups (TME–S_xBt) were not observed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1377–1385, 2000     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. IV. The reaction of polyisoprene with N-cyclohexylbenzothiazole sulfenamide,sulfur, and zinc oxide

Polyisoprene was vulcanized with N-cyclohexylbenzothiazole sulfenamide (CBS), sulfur, and zinc oxide by heating in a Differential Scanning Calorimeter (DSC) at a programmed rate to given temperatures. The reaction was quenched and the product analyzed. Soluble curatives and reaction intermediates were analyzed by high-performance liquid chomatography (HPLC) and the crosslink density of the network determined by swelling. The delayed action of the CBS accelerator is explained in terms of an exchange reaction between benzothiazole terminated polysulfidic pendent groups on the polymer chain and CBS to yield unreactive amine terminated pendent groups and 2-bisbenzothiazole-2,2′-disulfide (MBTS). MBTS reacts with sulfur to form 2-bisbenzothiazole-2,2′-polysulfides (MBTPs), which also form pendent groups. Crosslinking does not commence until all of the CBS has been consumed and pendent groups are no longer deactivated. 2-Mercaptobenzothiazole (MBT) is released only on crosslinking. When MBT is present in the formulation at the outset of the reaction it traps cyclohexylamine released when CBS adds to the chain as a pendent group. The MBT-amine salt participates in a reaction that regenerates MBTS, which is, thus, not consumed in the vulcanization process. ZnO does not react with CBS, and its role in increasing the crosslink density is attributed to its promoting crosslinking reactions between pendent groups and neighboring chains rather than intramolecular reactions, which lead to cyclization. © 1996 John Wiley & Sons, Inc.     

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.     

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     

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.     

The role of thiol intermediates in 2‐mercaptobenzothiazole accelerated sulfur vulcanization of rubber model compounds

The model compound, 2,3‐dimethyl‐2‐butene (TME), was vulcanized using 2‐mercaptobenzothiazole (MBT) and sulfur. MBT was not consumed during the vulcanization reaction. The resultant crosslink products were bis(alkenyl) in nature. 2,3‐Dimethyl‐2‐buten‐1‐thiol (TME‐SH) was identified as being present in the vulcanization mixture by a postcolumn derivatization technique. The appearance of thiol was coincident with crosslinking. Polysulfanes (H₂S_n) were formed on crosslinking. Studies of the reaction of TME‐SH and sulfur indicated a rapid reaction to form crosslink products and polysulfanes. No monosulfidic crosslink species were formed in these reactions. Closer investigation revealed the presence of small quantities of what appeared to be highly reactive polysulfidic thiols. This is the first time that such species have been identified in vulcanization systems. Consequently, MBT‐accelerated vulcanization of TME is proposed to occur via the reaction of MBT and S₈ to form polysulfidic MBT, which then reacts with TME to form polysulfidic thiols. These thiols then rapidly react via a metathesis reaction pathway to provide crosslink products and polysulfanes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 47–54, 2003     

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.     

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.     

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     

Crystallization of vulcanizates. I. Low‐temperature crystallization as a function of extent of cure for polyisoprene vulcanized with tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide/sulfur

The crystallization of polyisoprene, vulcanized to various degrees of cure with tetramethylthiuram disulfide/sulfur and 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur formulations, was studied in a density column at ?25°C. The densities of vulcanizates before crystallization decrease progressively with cure time, which is ascribed to an increase in free volume occasioned by the formation of accelerator‐terminated pendent groups on the polymer chain. The induction period before the onset of crystallization increases and both the rate of and the degree of crystallization decrease with extent of cure. This is attributed primarily to the presence of residual pendent groups on the polymer chain and secondly to crosslink formation. The changes are more marked with MBTS formulations where pendent groups are more bulky. MBTS compounds fail to crystallize once vulcanized to the point where a gel has formed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2565–2572, 2001     

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.     

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.     

Dimethylammonium dimethyldithiocarbamate‐accelerated sulfur vulcanization. II. Vulcanization of rubbers and model compound 2,3‐dimethyl‐2‐butene

Rubber and model compound 2,3‐dimethyl‐2‐butene were vulcanized for various times with dimethylammonium dimethyldithiocarbamate [(dma)dmtc]‐accelerated sulfur formulations in the absence of ZnO. Model compound systems were analyzed by HPLC, and no reaction intermediates containing pendent groups were found. Crosslinked sulfides, characterized by ¹H‐NMR, were found to be essentially bis(alkenyl). Residual curatives were extracted from rubber compounds vulcanized for various times and analyzed by HPLC. Compounds, cured to various crosslink densities, were found to crystallize readily in a density column at subambient temperatures. This supports evidence from model compound systems that pendent groups are largely absent from vulcanizates. It is suggested that a reaction mechanism, similar to that applicable to zinc dimethyldithiocarbamate‐accelerated sulfur vulcanization, may be applicable with (dma)dmtc accelerated formulations. Very limited crosslinking occurred on heating compounds under vacuum, and this can be attributed largely to the rapid loss of (dma)dmtc from rubber at elevated temperatures. However, the slower rate of crystallization on cooling of the gels, compared to the rate in press‐cured vulcanizates of similar crosslink density, was interpreted as evidence that some pendent groups did form during heating with (dma)dmtc/sulfur. Crosslinking of such pendent groups may be inhibited by the loss of (dma)dmtc, that, like zinc dimethyldithiocarbamate, may catalyze their crosslinking, and/or to the loss under vacuum of dimethyldithiocarbamic acid that would form thiol pendent groups that would rapidly crosslink with thiuram pendent groups. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3074–3083, 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     

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.     

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.