Studies on cure synergism. I. Effect of safe zinc dithiocarbamates on NR vulcanization accelerated by thiazole-based accelerators

Investigations have been carried out on the effect of several zinc dithiocarbamates in the presence of thiazole-based accelerators in the vulcanization of natural rubber (NR), keeping in mind the possibility of introducing safe dithiocarbamates derived from safe amines. Mutual activity is observed in all the binary systems studied, the highest being observed in the zinc dibenzyldithiocarbamate-dibenzothiazyldisulfide (ZBEC–MBTS)-accelerated system. The effect of zinc diisopropyl dithiophosphate, reportedly a safe rubber additive, which is recognized as an age-resistant agent for NR, on cure has also been studied. The study reveals that thiuram disulfide and MBT are always formed from the reaction either between zinc dithiocarbamate (ZDC) and MBTS or between ZDC and N-cyclohexyl-2-benzothiazole sulfenamide (CBS). It has been conclusively proved that 2-mercaptobenzothiazole (MBT) generated from MBTS or CBS reacts with ZDC and produces tetramethyl thinram disulfide (TMTD). The observed mutual activity has been discussed based on the cure and physical data and explained through the results based on HPLC and a reaction mechanism. © 1994 John Wiley & Sons, Inc.     

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.     

Structural characterization of vulcanizates. XII. Efficient vulcanization using a sulfenamide–thiuram disulfide accelerator system

Vulcanizates obtained from the sulfur vulcanization of natural rubber using a combination of the accelerators N-cyclohexylbenzothiazole-2-sulfenamide (CBS) and tetramethylthiuram disulfide (TMTD) have been analyzed in terms of the numbers of poly-, di-, and monosulfide crosslinks, network-bound accelerator residues, and cyclic sulfide chain modifications as a function of cure time. The vulcanization system produced mainly monosulfide crosslinks at optimum cure, although there were differences in detail between this mixed system and a previously reported efficient sulfur vulcanization (EV) system using a high ratio of CBS to sulfur.     

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.     

Studies on the reaction of bis(diisopropyl) thiophosphoryl disulfide with silica in the vulcanization of NR

The reaction between bis(diisopropyl) thiophosphoryl disulfide (DIPDIS) and silica has been investigated. The study reveals that DIPDIS can be used as a coupling agent for silica. The chemical bond between the silanol groups of silica and DIPDIS has been established through the isolation and characterization of isopropyl alcohol that is eliminated from the reaction. The chemical nature of the bond is also supported by the IR analysis of the reaction products of silica and DIPDIS obtained both in the presence and absence of rubber. From the exploratory studies it is indicated that DIPDIS can also react with NR even during mixing of ingredients. When used with 2-mercaptobenzothiazole (MBT) or N-oxydiethylene-2-benzothiazole sulfenamide (OBTS) in the presence of silica, DIPDIS exhibits notable activation of cure and enhances the torque, modulus, tensile strength, and the scorch safety.     

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.     

Bis(N‐benzyl piperazino) thiuram disulfide and dibenzothiazyl disulfide as synergistic safe accelerators in the vulcanization of natural rubber

Some investigation regarding the effect of binary system of accelerators comprising one safe thiuram disulfide, namely bis(N‐benzyl piperazino) thiuram disulfide (BPTD) and dibenzothiazyl disulfide (MBTS), on the vulcanization of NR is carried out. The results are compared with those obtained with conventional tetra methyl thiuram disulfide (TMTD) presently considered as unsafe. The vulcanizates obtained from safe synergistic pair of accelerators (BPTD‐MBTS) possess comparable mechanical properties [modulus, tensile strength, and elongation at break (%)] and exhibits some improvement in heat resistant behavior when compared with those obtained with TMTD‐MBTS system. In the light of mechanical properties, safe BPTD‐MBTS system introduces the safe noncarcinogenic rubber accelerator in the vulcanization of rubber. Same type of synergistic activity may be due to comparable activation energy for both the TDs with the combination of MBTS. Although rate constant values are low for BPTD‐MBTS (6 : 3) compared to TMTD‐MBTS (6 : 3) in the high temperature vulcanization, the crosslinking efficiency of former is very large compared to later. This may be responsible for improvement in heat resistance behavior of the novel accelerator in the combination with MBTS. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Studies on the reactions between thiocarbamyl sulfenamide and 2-(iminodithio)benzothiazole accelerator system in the early stage of vulcanization of NR

Investigations on the interactions of the synergistic pair of accelerators comprising of thiocarbamyl sulfenamide and 2-(iminodithio)benzothiazole have been carried out through hplc. N-cyclopentamethylene thiocarbamyl-N'-oxydiethylene sulfenamide, N-oxydiethylenethiocarbamyl-N'-cyclopentamethylene sulfenamide (OTCS), 2-morpholinodithiobenzothiazole, and 2-piperidinodithiobenzothiazole (PDB) have been selected as the reactants which, when suitably combined, provide most effective accelerator systems capable of generating heat and age resistant vulcanizates. The OTCS–PDB system has been found to yield vulcanizates which exhibit the greatest crosslink density, scorch safety, and highest modulus and tensile strength. An attempt has been made to explain the higher activity of the binary systems studied as compared to that obtained from comparable thiocarbamyl sulfenamide plus dibenzothiazyl disulfide system. Zin–dithiocarbamates have been found to influence the curing reactions depending upon the composition of the mixed accelerators. Cyclohexyl thiophthalimide, a versatile cure retarder, reacts with the accelerators to yield cyclohexyl thiomorpholine and cyclohexyl thiopiperidine, as the case may, which also have a profound effect upon cure retardation as noticed in the present investigation.     

Retarding and antioxidant activity of 1-cyclohexylthio-2-mercaptobenzimidazole in the vulcanization of NR accelerated by thiocarbamyl sulfenamide

The effect of 1-cyclohexylthio-2-mercaptobenzimidazole (CMB) on the vulcanization of NR accelerated by N-oxydiethylene thiocarbamyl-N′-oxydiethylene sulfenamide (OTOS) has been studied. It is found that CMB delays the onset of cure and generates 2-mercaptobenzimidazole during vulcanization. The results indicate the retarding as well as antioxidant activity of CMB.     

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

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Binary accelerator systems are being widely used in industry and are becoming increasingly popular because of the fact that such mixed systems can effectively prevent prevulcanization, permit the vulcanization to be carried out at a lower temperature in a shorter time, and produce a vulcanizate with superior mechanical properties compared to those of a stock cured with a single accelerator. Thiourea and its derivatives are important secondary accelerators in this context. It is suggested that thiourea containing binary accelerator systems cause rubber vulcanization to proceed by a nucleophilic reaction mechanism. In the present study 1‐phenyl‐5‐ortho, ‐meta, and ‐para‐tolyl derivatives of 2,4‐dithiobiurets, which are more nucleophilic than thiourea and vary in their nucleophilic reactivity, are used as secondary accelerators along with 2‐morpholinothiobenzothiazole in the vulcanization of natural rubber. The results show an appreciable reduction in the cure time for the mixes containing the dithiobiurets compared to the reference mix. These results are indicative of a nucleophilic reaction mechanism in the vulcanization reaction under consideration. These vulcanizates also demonstrate comparatively better tensile properties and good retention of these properties after aging. An attempt is also made to correlate the variation in physical properties to chemical crosslink formation in the various vulcanizates. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3173–3182, 2003     

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

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

Influence of hydrofuramide on network structure and kinetics of thiuram vulcanization

The influence of hydrofuramide (a reaction product of furfural and ammonia) on the network structure and overall kinetics of thiuram vulcanization of natural rubber is reported. The nature of the network structure of the thiuram vulcanizates in the presence and absence of hydrofuramide was determined using triphenyl phosphine (Ph₃P) as a chemical probe. The presence of hydrofuramide produces structural complexity of the vulcanizate. The overall kinetics of thiuram vulcanization follows a first-order-rate law in the presence as well as in the absence of hydrofuramide. The rate constant increases while the energy of activation decreases in the presence of hydrofuramide.     

促进剂M-La的合成及对天然橡胶的硫化促进作用

采用沉淀法用对氨基苯甲酸、氢氧化钠和硝酸镧在乙醇环境下合成了促进剂对氨基苯甲酸镧(M-La),研究了其对天然橡胶硫化的促进作用。通过红外光谱分析、热分析和元素分析等确定所合成促进剂M-La的分子式为La(C_7 H_6 NO_2)_3·2 H_2O。硫化性能测试结果表明,随着促进剂M-La用量的增加,天然橡胶的硫化速率指数由139增加至227,焦烧时间逐渐延长而正硫化时间基本维持不变,硫化胶的加工安全性不断增强;硫化胶交联密度由最初的1.95×10~(-4) mol/g逐渐增大至2.41×10~(-4) mol/g。促进剂M-La对天然橡胶硫化的促进作用比较明显。     

Contribution to the mechanism of the thiuram vulcanization. I. Polymerization of styrene in the presence of tetramethylthiuram disulfide and natural rubber

The influence of the concentration of tetramethylthiuram disulfide (TMTD) on grafting of natural rubber by styrene at 80°, 95°, 115°, and 130°C and constant molar ratio of rubber and styrene was studied. It was found that the dependence R_p = f([TMTD]^½) at all followed temperatures goes through a maximum and that TMTD substantially decreases the amount of bound rubber in the graft copolymer. The analysis of the kinetic data and the results of separation of polymer mixtures showed the significant role in the process of the termination reactions of the growing polymer and the rubber radicals with the RS radicals. The derived kinetic relation is in good agreement with the experimental, results and allows calculation of the transfer rate constants of RS radical on rubber.     

Effect of mineral filler sepiolite on sulphur-free thiuram vulcanization of natural rubber

The effect of sepiolite on the sulphurless vulcanization of NR by tetramethylthiuram disulphide, with and without activator (triethanolamine) was studied. The influence on: a) kinetic vulcanization parameters, b) crosslinking density, c) crosslinking types, d) efficiency factor E and e) zinc sulphide formation was considered. The vulcanization rate increases with increasing sepiolite level but crosslinking density is reduced. Based on these results a vulcanization mechanism is proposed and discussed.     

硫化促进剂TMTD含量的测定

本文提出了TMTD含量的测定方法是将试样用氯仿溶解，然后采用分光光度法进行测定，该方法简便、快速、准确、省时，值得在实际生产中推广。     

Studies on xanthate/dithiocarbamate accelerator combination in NR/BR blends

Zinc butyl xanthate [Zn(bxt)₂] was prepared in the laboratory. The effect of this xanthate with zinc diethyl dithiocarbamate (ZDC) on the vulcanization of natural rubber (NR), polybutadiene rubber (BR), and NR/BR blend has been studied at different temperatures. The amounts of Zn (bxt)₂ and ZDC in the compounds were optimized by varying the amount of ZDC from 0.75 to 1.5 phr and Zn (bxt)₂ from 0.75 to 1.5 phr. The cure characteristics were also studied. HAF filled NR, BR, and NR/BR blend compounds were cured at different temperatures from 60 to 150°C. The sheets were molded and properties such as tensile strength, tear strength, crosslink density and elongation at break, compression set, abrasion resistance, etc. were evaluated. The results show that the mechanical properties of 80NR/20BR blends are closer to that of NR vulcanizates, properties of 60NR/40BR blends are closer to BR vulcanizates, while the 70NR/30BR blends show an intermediate property. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3516–3520, 2007     

无废水法生产橡胶硫化促进剂2-巯基苯并噻唑

以苯胺作为溶剂,对M(2-巯基苯并噻唑)进行结晶精制,可将粗M中的大部分胶体去除掉。设计了苯胺单程循环和多程循环2种流程,并分别给出结果数据。实验结果表明,精制后M的熔点可达到180℃以上,产品纯度高,无废水、废渣产生。该方法基本上能达到与酸碱处理相同的结果。     

RPA分析天然橡胶不同硫化体系的动态性能

用RPA分析不同硫化体系对NR动态性能的影响进行研究。研究发现：频率增加,混炼胶和硫化胶的G’增加,混炼胶的Tanδ减小,硫化胶的Tanδ增加;应变增加,混炼胶和硫化胶的G’减小,混炼胶的Tanδ在应变大于20%迅速增加,硫化胶的Tanδ在应变2%出现峰值;温度增加,混炼胶的G’减小、Tanδ增加,硫化胶的G’增加、Tanδ减小。不同硫化体系三种扫描下,硫化胶的G’顺序：SEV>EC>CV>EV,主要是单硫键、双硫键与多硫键保持一定的比例才能获得较高的G’。     

促进剂种类对NR硫化胶动态性能的影响

探讨促进剂NS、CZ、DZ对天然橡胶(NR)硫化胶动态性能的影响。结果表明,由不同的促进剂得到的硫化胶的交联密度不同,硫化胶的动态性能对频率、温度和应变的依赖性也不完全相同。由NS和CZ得到的硫化胶的交联密度较高且相近,其储能模量(G′)随频率和温度的升高而增加的变化趋势几乎重合,但在较低的应变下硫化胶的G′随应变振幅的提高而降低;而由促进剂DZ得到的硫化胶,由于交联密度较低,其G′随温度的升高而略有增加。