The chemistry of sulfur curing. Part I. Kinetics of vulcanization of an EPDM elastomer accelerated by zinc dimethyldithiocarbamate

The maximum cure rates of an EPDM rubber were determined by using the Monsanto oscillating disk rheometer and correlated against the initial concentrations of curing ingredients. Unaccelerated and ZnDMDC-accelerated sulfur curing systems were used. The results were interpreted to indicate that both unaccelerated and ZnDMDC-accelerated curing reactions proceed simultaneously in ZnDMDC-accelerated stocks. The maximum cure rates of the ZnDMDC-accelerated crosslinking reactions were found to be first-order in initial ZnDMDC concentration, second-order in initial cure site concentration, variable fractional order in initial sulfur concentration, and approximately zero-order in initial zinc oxide concentration. The maximum cure rates of the ZnDMDC-accelerated reaction were shown to be limited by the low solubility of ZnDMDC in the polymer. The maximum cure rates of the unaccelerated crosslinking reaction were found to be 0.6-order in initial sulfur concentration, 1.2-order in initial cure site concentration, and of a positive variable order in initial zinc oxide concentration.     

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 Fillers and Curing Systems on the Physico-Mechanical Properties and Stability of EPDM Vulcanizates

ABSTRACT

The thermal stability was investigated for ethylene propylene diene terpolymer (EPDM) rubber loaded with different white and black fillers, namely, kaolin, quartz, polyvinylchloride PVC, talc, graphite, medium thermal carbon black MT, semi reinforcing furnace black SRF, and high abrasion carbon black HAF. The fillers were added at a fixed level of 20 phr. Two curing systems were used, tetramethyl thiuram disulfide (TMTD) and sulfur/N-cyclohexyl-2-benzothiazyl sulfenamide (S/CBS). The effects of the curing system and type of filler on the physico-mechanical properties, before and after thermal aging were investigated. It was found that the vulcanizates cured with S/CBS system gave better physico-mechanical properties than the TMTD cured samples. However, TMTD cured EPDM has thermal stability performance superior to the other vulcanizing system.     

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.     

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.     

硫化体系对燕化EPDM中试产品性能的影响

研究了硫黄硫化体系和过氧化物体系对燕化中试产品三元乙丙橡胶(EPDM)性能的影响。实验结果表明,硫黄硫化体系中硫黄用量为1.5份时胶料物理机械性能最好;硫黄用量为2份时,胶料耐热老化性能最好。硫黄硫化体系中,二硫化四基秋兰姆(TMTDM)、2硫醇基苯并噻唑(M)和N-环己基-2-苯并噻唑次磺酸胺(CZ)3种促进剂并用时协同作用较好,胶料硫化速度最快,力学性能最好;促进剂TMTD用量为1.5份时,胶料压缩永久变形最小,耐热性能最好。过氧化物硫化体系中,当DCP用量由2份增加至5份时,胶料硫化速度提高,力学性能下降,压缩永久变形性能有一定改善。三烯丙基异氰脲酸酯(TAIC)用量为5份时,胶料压缩永久变形最小,耐热氧老化性能最好。     

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.     

Effect of fillers and vulcanizing systems on the physicomechanical and electrical properties of EPDM vulcanizates

The physicomechanical and dielectric properties were investigated for ethylene propylene diene terpolymer (EPDM) rubber loaded with different white fillers, namely, kaolin, quartz, polyvinylchloride PVC, and talc. The white fillers were added at a fixed level of 20 phr. Two vulcanizing systems were used, tetramethyl thiuram disulfide (TMTD) and sulfur/N-cyclohexyl-2-benzothiazyl sulfenamide (S/CBS). It was found that the curing systems affect the dielectric properties more than the filler type. The effect of the curing system and type of filler on the physicomechanical properties before and after thermal aging were also investigated, and the data obtained were interpreted.     

Effect of Bromination on Epdm Curing Behavior and its Tensile Properties

Ethylene propylene diene rubber (EPDM) was brominated. The curing behavior and tensile properties of the brominated EPDM (BEPDM) were investigated using a typical sulfur curing formulation. The brominated EPDM was observed to cure with a shorter cure induction time than the unbrominated one. The activation energies (E _a) of curing for EPDM and brominated EPDM were found to be approximately 145 kJ/mol and 58 kJ/mol, respectively. Analysis of curing behavior with individual curatives and stress-strain measurements indicates that these significant improvements could be attributed to the role of bromine as a more efficient crosslinking site, forming C—O—C crosslinks. In addition, the bromine also seems to increase the solubility of sulfur in the rubber, promoting sulfur crosslinks which are apparently not observed with the unbrominated EPDM. Consequently, the BEPDM displayed significantly higher tensile strength than the unbrominated one.     

A study of the rate of formation of polysulfides of tetramethylthiuram disulfide

The rate of formation of tetramethylthiuram polysulfides (TMTP), that play an important role in vulcanization, was studied. After a short induction period (<30 s), tetramethylthiuram disulfide (TMTD) and TMTD-sulfur mixes, heated to 130–150°C in the absence of rubber, rapidly form a series of TMTPs. The concentrations of TMTPs of lower sulfur rank increase most rapidly, indicating that sulfur atoms are added to the accelerator sequentially. The incorporation of sulfur molecules to give TMTPs, which subsequently desulfurate, does not occur. Equilibrium concentrations of the various TMTPs are achieved in about 2 min. Little tetramethylthiourea is formed below 200°C. Tetramethylthiuram monosulfide (TMTM) is stable, but TMTM-sulfur mixes form TMTPs. A mechanism is proposed to account for the large amount of TMTM formed on heating TMTD in the absence of sulfur and the correspondingly higher TMTP concentrations in the presence of sulfur. © 1995 John Wiley Sons, Inc.     

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.     

Development of EPDM nanocomposites in presence of compatibiliser

Abstract

The most significant factor, which prescribes the enhancement of properties in rubber by the incorporation of nanoclay, is its distribution in the rubber matrix. The study deals with the utilisation of epoxidised natural rubber (ENR) as a polar compatibiliser to achieve better dispersion of nanoclay in non‐polar polymer matrix. Epoxidised natural rubber–nanoclay composites (EC) were prepared by solution mixing. The obtained nanocomposites were incorporated in ethylene propylene diene terpolymer (EPDM) with sulphur as a curing agent. The morphological studies proved the intercalation of nanoclay in ENR, and further incorporation of EC in EPDM matrix leads to exfoliation of nanoclay. Curing study demonstrated faster scorch time, cure time and increase in maximum torque for the nanoclay loaded EPDM compounds compared to pure one. Dynamic mechanical thermal analysis showed increase in storage modulus and lesser damping characteristics for the compounds containing nanoclay loading in EPDM matrix followed by substantial improvement in the overall mechanical properties.     

Curing characteristics and kinetics of EPDM and EOC compounds in co-vulcanization as blend

Possibility of co-vulcanization of ethylene octene copolymer (EOC) and ethylene propylene diene terpolymer (EPDM) molecules was studied by assessing the curing characteristic and crosslinking kinetics of EOC and EPDM compounds. Regarding curing, the curing characteristics, cure rate index, and the torque difference (M_H − M_L) of the EPDM compound are quite similar to those of the EOC compounds at the curing temperature of 180 °C, especially when the EOC octene comonomer content is 5.9 or 9.7 mol %. The kinetic parameters E _a and k were analyzed. The study showed that EOC with 9.7 mol % octene comonomer content is very suitable for blending with EPDM, as it has crosslinking kinetics similar to the EPDM. This observation confirms the possibility of chemical co-crosslinking at EPDM–EOC interfaces, especially at the curing temperature of 180 °C. Differential scanning calorimetry and dynamic mechanical analysis were also used to assess interfacial crosslinking. The lowest activation energy of vulcanization is found for the EPDM/EOC blend with 9.7 mol % octene comonomer contents. Furthermore, chemical co-crosslinking in combination with chain flexibility in the EPDM/EOC blend with 9.7 mol % octene comonomer contents give lower tan δ at room temperature than for the blends with octene comonomer contents of 5.9 and 16.9 mol %. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47613.     

Cure characteristics of ethylene propylene diene rubber-polypropylene blends. I. Calculation of state of cure in blends containing conventional sulfur curing system under variable time-temperature conditions

Various blends of polypropylene and ethylene propylene diene terpolymer were prepared. In these blends, rubber phases were compounded with an optimum sulfur curing system. The equivalent cure time and state of cure of the mixes were studied in the temperature range from 120 to 190°C by a rheometer. The temperature-independent parameters (E₁, E₂, S₁, and S₂) and specification of the temperature dependence curing rate constant (k) and induction period (t₀) were determined. The activation energies of the crosslinking reaction are almost similar for the virgin EPDM and EPDM/PP mixes. The value 9 Kcal/mol was considered sufficiently accurate for these mixes. A discussion of temperature-conversion factor of the mixes is also provided. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1231–1236, 1997     

Structure and properties of dynamically cured EPDM/PP blends

The structure and properties of polyolefin blends of ethylene–propylene–diene terpolymer (EPDM) and polypropylene were studied. Blends were prepared in a laboratory internal mixer where EPDM was cured with PP under shear with dicumyl peroxide (DCP) at different shear conditions (blend–cure). Blends were also prepared for comparison from EPDM which were dynamically cured in the absence of PP and blended later (cure–blend). The effect of DCP concentration, intensity of the shear mixing, and rubber/plastic composition were studied. In blend–cure, the melt viscosity increased with increasing DCP concentration in blends of 75% EPDM and 25% PP, but it decreased with increasing DCP concentration in blends of 75% PP and 25% EPDM. In cure–blend, however, the melt viscosity increased with increasing DCP concentration for all compositions. The melt viscosity decreased with increasing intensity of the shear mixing presumably due to the formation of the smaller segregated microdomain of the crosslinked EPDM gels in both blend–cure and cure–blend materials. The crystallization rate was higher in EPDM/PP blends than in PP homopolymer. The crystallization rates for various blending conditions were also compared.     

用校准曲线法测定EPDM硫化胶的交联密度及交联键结构

根据“每消耗3mol二硫化四甲基秋兰姆（TMTD）分子产生1mol交联键”理论,建立了EPDM纯胶和炭黑N326（50phr)增强EPDM体系的化学交联密度－溶胀因子校准曲线,以此测定了硫黄硫化EPDM的交联密度和交联键类型,TMTD硫化EPDM的交联键类型,用溶胀法测定了EPDM／TMTD的纯胶体系和炭黑增强体系的总交联密度,结果表明,溶胀法测得的交联密度要比由TMTD质量摩尔浓度计算得到的值小。     

1,2-PBR和BR对EPDM硫化和力学性能的影响

考察了过氧化物硫化体系和硫黄硫化体系ＥＰＤＭ／１,２－聚丁二烯橡胶（１,２－ＰＢＲ）和ＥＰＤＭ／ＢＲ并用胶的硫化特性、交联密度和物理性质。结果表明：采用过氧化物硫化体系时,ＥＰＤＭ与１,２－ＰＢＲ和ＢＲ并用后,硫化胶的交联密度、１００％定伸应力、邵尔Ａ型硬度和撕裂强度增大,且前的增幅较大;采用硫黄硫化体系时,１,２－ＰＢＲ和ＢＲ均可提高ＥＰＤＭ的硫化速率,且随着１,２－ＰＢＲ用量的增大,硫化胶的     

Vulcanization of Nitrile Rubber. Part I. Effect of Some Accelerators on the Physicomechanical Properties of Vulcanized Nitrile Rubber for Resistance to Fuel and General Use

Compounding mixtures were prepared based on acrylonitrile-butadiene rubber (Krynac 803) to which increasing parts per hundred rubber (pphr) of the following accelerates were added: tetramethyl thiuram disulfur (TMTD in series A₁), tetramethyl thiuram monosulfur (TMTM in series A₂), 2-mercaptobenzothiazole (MBT in series A₃), and 2,2′-dithiobisbenzothiazole (MBTS in series A₄). Effects of the quantity and type of accelerator on the vulcanization characteristics were studied. Physicomechanical investigations and swelling resistance were determined for samples immersed in benzene:benzol mixture at normal temperature. It was found that according to the type and quantity of accelerator, the optimum cure time decreased with increasing the quantity of accelerator. Relatively higher values of scorch time (8 min, at 1.0–3.0 pphr), which seems to ensure safe processing during production was obtained for TMTM (series A₂). TMTD (in quantity of 2.0 pphr) led to the highest value of rate of cure (14.0 min^?1). The best values of tensile strength (210 kg/cm², and 208 kg/cm²) were reached in TMTD (series A₁) and TMTM (series A₂), whereas MBT (series A₃) and MBTS (series A₄) resulted in tensile strength values of 140 and 155 kg/cm², respectively. The highest value of modulus (86 kg/cm²) was obtained in TMTD (series A₁ at 2.0 pphr). Whereas TMTD (series A₁) and TMTM (series A₂) led to relatively lower values of elongation at break (180% and 190%), it was found that MBT (series A₃) and MBTS (series A₄) led to higher values (280% and 260%). Small effects on the elasticity were observed when changing the type and quantity of accelerator. Relatively higher values of hardness were reached for series A₁ and A₂ as compared with series A₃ and A₄. The lowest values of percent volume increase after immersion in benzene:benzol mixture (8.3%) was obtained for A₁ (at 1.5 pphr of TMTD), as compared with the other three accelerators.     

Characterization of ethylene–propylene rubber and ethylene–propylene–diene rubber networks

The stress–strain (S/S) and the swelling equilibrium behavior in a series of ethylene propylene rubber (EPR) and ethylene propylene diene monomer (EPDM) networks were investigated and the results were employed to evaluate the effects of varying the cure conditions on the crosslinking efficiency in these networks. The S/S curve of completely swollen vulcanizates is in agreement with the predictions of rubber elasticity theory, while that of dry or partially swollen vulcanizates is fully described by the Mooney-Rivlin equation. ? values determined in benzene were found to vary linearly with v_r (v_r = equilibrium volume fraction of rubber in swollen sample). Crosslinking efficiency, moles of crosslinks produced per moles of crosslinking agent used, ranges from 3.7 in peroxide-cured EPDM (55% wt ethylene and 2.6% unsaturation) to 0.15 in similarly cured EPR (43% ethylene). Efficiency in the latter system improves to 0.6 by addition of a coagent (sulfur) to the cure formula. Crosslinking efficiency in EPDM (55% ethylene) was found to increase in the order: peroxide- > resin- > sulfur-cured. In the EPDM sulfur vulcanizates, changing the terpolymer in the cure formula resulted in significant changes in the crosslinking efficiency.     

交联硫化体系对PP／POE／EPDM热塑性弹性体性能的影响

采用动态硫化法制备聚丙烯／聚烯烃弹性体／三元乙丙橡胶（PP/POE／EPDM）共混型热塑性弹性体,研究了交联前后不同POE／EPDM并用对比体系力学性能的影响。采用交联剂过氧化二异丙苯（DCP）及DCP／S硫化体系对PP／POE／EPDM体系进行硫化,研究了力学性能的变化。结果表明,EPDM可有效降低材料的硬度和断裂永久变形。助交联剂硫黄（S）对PP／POE／EPDM体系有较好的硫化作用,固定DCP用量为3份,S用量为0．4份时,体系力学性能最佳,交联对体系硬度影响很小。