Effect of thiophosphoryl compounds as accelerators and coupling agents on the vulcanization of NR‐XNBR blend

The effect of a number of thiophosphoryl compounds [viz., bis(isobutyl) thiophosphoryl di‐, tri‐, and tetrasulfides and bis(dicyclohexyl) thiophopsphoryl di‐, tri‐, and tetrasulfides] on natural rubber (NR)‐carboxylated nitrile rubber (XNBR) blend was studied. All these thiophosphoryl compounds are capable of forming interrubber links leading to covulcanized blends which exhibit a fair degree of synergism with respect to physical properties, the maximum being obtained at the 25% NR and 75% XNBR blend composition. This is an obvious claim that the blend under investigation is technologically compatible, having some degree of interrubber interaction which is enhanced in case of two‐stage vulcanization. The existence of interrubber interaction is judged by the swelling experiment. The blend morphology assessed by SEM micrographs corroborates the foregoing observations and accounts for the significant improvement in physical properties of the blend vulcanizates, particularly in two‐stage vulcanization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3286‐3299, 2002     

Some investigations on structure-property relationship in XNBR vulcanizates in the presence of bis(diisopropyl thiophosphoryl) disulfide

The study reveals that a new type of sulfidic crosslinks arising from the reaction between carboxylated nitrile rubber (XNBR) and bis(diisopropyl)thiophosphoryl disulfide can improve the mechanical properties of the XNBR vulcanizates. The study further reveals the fact that these sulfidic crosslinks in combination with metallocarboxylate crosslinks produce a significant effect in XNBR vulcanization. Some investigations have been carried out using a methyl iodide probe to elucidate the formation of chemical linkages during cure. Also, attempts have been made to establish the relationship between the structure and physical properties of the XNBR vulcanizates in this respect. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2623–2630, 1997     

Tetramethylthiuram disulfide and 2-mercaptobenzothiazole as binary accelerators in sulfur vulcanization. IV. Vulcanization of polyisoprene in the presence of ZnO

Polyisoprene was vulcanized with binary accelerator systems comprising mixtures of the zinc salts of tetramethylthiuram disulfide (TMTD) and 2-mercaptobenzothiazole (MBT). Samples were heated in a DSC at a programmed rate, the reaction was stopped at points along the thermal curve, and the system was analyzed. Extractable curatives and reaction intermediates were analyzed by HPLC and the crosslink density of samples measured by swelling. DSC curves for the different systems displayed similar characteristics and their similarity to the curve obtained with the zinc salt of TMTD rather than to the curve obtained with the zinc salt of MBT suggested that the reaction was dominated by the former accelerator. This conclusion was supported by HPLC analyses of extractable curatives. A reaction mechanism for the binary system is discussed. © 1996 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.     

过氧化物硫化体系对CM／XNBR共混物性能的影响研究

研究了不同用量过氧化物硫化体系对氯化聚乙烯橡胶（CM）／羧基丁腈橡胶（XNBR）共混硫化胶各相交联密度及热空气老化前后的物理机械性能、耐油性能等的影响规律。结果表明,随硫化体系用量的增加,共混硫化胶两相的交联密度都明显增加,交联密度差值变大;焦烧时间逐渐降低;硬度、拉伸强度、100％定伸应力、体积磨耗量呈上升趋势,永久变形、扯断伸长率明显降低,撕裂强度整体变化不大,常温下体积和质量变化率均很小,高温下体积和质量变化率明显降低后趋于平稳;热空气老化后,硬度、拉伸强度、100％定伸应力、撕裂强度整体上升,扯断伸长率整体下降,体积磨耗量整体变化不大;热油老化后,拉伸强度、100％定伸应力变化不大,扯断伸长率明显降低。     

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.     

Tetramethylthiuram disulfide and 2-mercaptobenzothiazole as binary accelerators in sulfur vulcanization. I. Exchange reactions between the accelerators and sulfur in the absence of rubber

Exchange reactions between tetramethylthiuram disulfide, 2-mercaptobenzothizole, and sulfur were studied by heating powdered mixes to vulcanization temperatures at a programmed rate in a DSC. The reaction was stopped at points along the thermal curve and the mixture was analyzed by HPLC. On dissolution, even unheated samples undergo a sulfide exchange reaction leading to a mixed accelerator, while polysulfides of the thiuram and mixed accelerator form in low concentrations. On heating, higher concentrations of these polysulfides are formed, particularly in the presence of elemental sulfur. Dimethyldithiocarbamic acid, formed in the exchange, influences the product spectrum if it remains trapped in the DSC pan. Tetramethylthiourea is formed only at elevated temperatures when dimethylamine, a degradation product of the acid, is trapped in the DSC pan. A series of reactions is proposed to explain the product spectrum obtained under different conditions. © 1995 John Wiley & Sons, Inc.     

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     

Effect of vulcanization temperature and synergism of accelerators on the network and technical properties of efficiently vulcanized natural rubber mixes

The effect of an increase in vulcanization temperature from 150° to 180°C on the network structure and technical properties of gum natural rubber vulcanizates has been studied using six different binary combinations of tetramethyl thiuram disulphide (TMTD) and 2-(morpholinodithio)benzothiazole (MDB) at fixed sulphur level (0.6 parts per hundred rubber). On the basis of processing safety, reversion resistance, technical properties, retention of properties after aging and percentage losses in properties on increasing the vulcanization temperature by 30°C, if was found that the combination with MDB (1.2 phr) TMTD (0.6 phr) showed the best synergism of the two accelerators. Results of chemical characterization of vulcanizate networks have been correlated with their technical properties. At higher vulcanization temperature, lower degree of crosslink density and with increasing extent of sulphidic main chain modification, a reduction in strength, modulus, hardness and resilience resulted and brought about a higher compression set and a greater heat build-up during Goodrich flexometer tests. The better cut growth and flex properties of vulcanizates cured at 180°C have been explained on the basis of the distribution of different types of crosslinks and stress relaxation caused by lower crosslink density.     

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.     

Tetramethylthiuram disulfide and 2-mercaptobenzothiazole as binary accelerators in sulfur vulcanization. II. Exchange reactions between the accelerators and their zinc salts in the absence of rubber

Exchange reactions between tetramethylthiuram disulfide, 2-mercaptobenzothiazole, and sulfur in the presence of ZnO were studied by heating powdered mixes to vulcanization temperatures at a preprogrammed rate in a DSC. The reaction was stopped at points along the thermal curve and the mixture was analyzed. Sulfide exchange reactions between the accelerators leads to a mixed accelerator and dimethyldithiocarbamic acid that is trapped by ZnO to give the zinc accelerator complex bis(dimethyldithiocarbamato)zinc (II). Exchange also occurs between the accelerators and ligands on both the thiuram and benzothiazole zinc accelerator complexes. Zinc complexes containing ligands of both accelerators were synthesized. These showed little interaction on being heated with sulfur, but on dissolution yielded a spectrum of products similar to that obtained in the other system containing zinc. Reactions to account for changes in the spectrum of products on heating different mixes of these curatives to different temperatures are discussed. © 1995 John Wiley & Sons, Inc.     

Tetramethylthiuram disulfide and 2-mercaptobenzothiazole as binary accelerators in sulfur vulcanization. III. Vulcanization of polyisoprene in the absence of ZnO

Polyisoprene was vulcanized with the binary accelerator system tetramethylthiuram disulfide–2-mercaptobenzothiazole (TMTD–MBT) in the absence of ZnO. Samples were heated in a DSC at a programmed rate, the reaction was stopped at points along the thermal curve, and the system was analyzed. Extractable curatives and reaction intermediates were analyzed by HPLC and the crosslink density of samples measured by swelling. Two cross-linking reaction sequences were identified, the first being initiated by polysulfides of the mixed accelerator N,N-dimethyldithiocarbamylbenzothiazole disulfide, and the second by MBT. All the TMTD is consumed in the first reaction sequence. Synergism of the reaction is discussed in terms of recent work detailing a reaction mechanism for TMTD-accelerated vulcanization. © 1996 John Wiley & Sons, Inc.     

Cure characterization of triglycidyl epoxy/aromatic amine systems

The curing of triglycidyl para-aminophenol (TGPAP) epoxy resin with three aromatic amine hardeners, diaminodiphenye sulphone (DDS), pyridinediamine (PDA), and toluenediamine (TDA), has been investigated. A series of iosthermal cures was conducted and analyzed by Fourier transform infrared spectrometry (FTIR) and differential scanning calorimetry (DSC). The chemical reactions occurring during cure were monitored at different temperatures by qualitative and quantitative estimation of different groups in the IR spectra, and the ratio of rate constants (k₂/k₁) were evaluated. Dynamic DSC analysis of TGPAP/TDA resulted in two exothermal peaks, indicating cure kinetics different from those of TGPAP/DDS and TGPAP/PDA systems, which gave a single exothermal peak. Various kinetic parameters such as total heat of reaction. ΔH′, activation energy E_a, Frequency factor z, and order of reaction n were evaluated for all the three systems. From the initial kick-off temperatures and activation energy values it was concluded that the rate of curing followed the order TDA > PDA > DDS. The reaction conversions during cure, evaluated from IR analysis, were exactly the same as those obtained from DSC Borchardt–Daniels kinetics. Using this model, the plots of time vs. temperature for different conversions were constructed for all the three systems; on the basis of these, the cure cycles can be fixed. © 1995 John Wiley & Sons, Inc.     

Cure index and activation energy of vulcanization of natural rubber and epoxidized natural rubber vulcanized in the presence of antioxidants

The cure index and apparent activation energy of vulcanization of one grade of natural rubber (SMR L) and two grades of epoxidized natural rubbers (ENR 25 and ENR 50) were studied in the presence of three types of antioxidants [viz., 2,2′methylene‐bis(4‐methyl‐6‐tertbutylphenol) (AO 2246), poly‐2,2,4‐trimethyl‐1,2‐dihydroquinoline (TMQ), and N‐isopropyl‐N′‐phenyl‐p‐phenylenediamine (IPPD)] in the temperature range of 120–180°C by using a Monsanto automatic Mooney viscometer. Accelerated sulfur vulcanization system and up to 5 phr of antioxidant concentration was used throughout the investigation. Results indicate that both cure index and apparent activation energy of vulcanization are dependent on the type and concentration of the antioxidant used. AO 2246 (a phenol‐based antioxidant) would retard vulcanization as reflected by the higher cure index and activation energy, an observation which is attributed to the solvation and steric hindrance effects of the antioxidant. On the contrary, both TMQ and IPPD (amine‐based antioxidants) exhibit reverse behavior due to the catalytic effect of the antioxidants in generating more active sulfurating agents for vulcanization. In all cases, SMR L gives higher cure index and apparent activation energy than the corresponding ENR, a phenomenon which is associated with the activation of the adjacent double bond by epoxide group in the latter. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3234–3238, 2000     

Comparative study among TMTD,TBzTD, and ZBEC accelerators in isobutylene-isoprene elastomer vulcanization

Accelerators such as tetramethylthiuram disulfide (TMTD) are responsible for releasing nitrosamines, considered carcinogenic by international organizations. Tetrabenzylthiuram disulfide (TBzTD) and zinc dibenzyldithiocarbamate (ZBEC) are indicated as substituents of TMTD for not releasing noxious nitrosamines. Thus, the objective of this work was to compare the use of TMTD, TBzTD, and ZBEC accelerators in the production of isobutylene-isoprene rubber compounds. It was evaluated the effect of TBzTD/mercaptobenzothiazole and ZBEC/N-cyclohexyl-2-benzothiazolesulfenamide pairs in the rheological properties and the first pair achieved better results. Subsequently, it was evaluated compounds containing TBzTD and ZBEC accelerators separately. The experimental design data of the obtained maximum torque were correlated by an interpolation method (thin-plate spline interpolation method). Physical–mechanical and thermodynamic measurements showed a decrease in the mechanical properties of TBzTD and ZBEC compounds. Thermogravimetric analysis did not indicate thermal differences between the compounds, Fourier transform infrared and swelling degree analyses corroborated the difference found in the crosslink densities of the compounds. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48965.     

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.     

N,N’-间亚苯基双马来酰亚胺作EPDM与过氧化物共硫化剂的硫化机理

以N,N’－间亚苯基双马来酰亚胺（PBM）为三元乙丙橡胶（EPDM）与过氧化二异丙苯（DCP）交联的共硫化剂,利用平衡溶胀法和红外光谱等方法,探讨了EPDM／DCP／PBM硫化体系的硫化机理。结果表明,PBM在用量小于2mmol／g时,对EPDM／DCP硫化体系有较弱的抑制作用,当用量大于4mmol／g时,能显著改善EPDM／DCP硫化体系的交联程度。     

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