Sulfur Vulcanization Of Natural Rubber Research Articles

Critical Condition of Nanostructured Magnesium Oxide for Accelerated Sulfur Vulcanization of Natural Rubber

Critical Condition of Nanostructured Magnesium Oxide for Accelerated Sulfur Vulcanization of Natural Rubber

Effect of water on the accelerated sulfur vulcanization of natural rubber

Water present in natural rubber was found to play a role in forming carbon-carbon crosslinking junctions before forming carbon-sulfur crosslinking junctions when its effect on the accelerated sulfur vulcanization was investigated by analyzing low–molecular weight products through various instrumental analytical techniques. Samples were prepared by mixing deproteinized natural rubber (DPNR) with various moisture contents with stearic acid, ZnO, sulfur, and N-tert-butylbenzothiazole-2-sulfenamide, and they were vulcanized at 150 °C for various cure times. Low–molecular weight products such as 2,2′-dibenzothiazole disulfide (MBTS), 2-mercaptobenzothiazole (MBT), ZnMBT, Zn2+ and sulfur linked to the rubber were extracted from the vulcanized natural rubbers using benzene and dioxane, and subsequently measured by high-performance liquid chromatography, UV–visible spectroscopy, inductively coupled plasma atomic emission spectroscopy, and combustion analysis. The water present in natural rubber was found to strongly affect the production and formation rate of the low–molecular weight products in the accelerated sulfur vulcanization of natural rubber. As the result it suggests that the water promotes the formation of the C-C linkages, and then it sequentially promotes the formation of the C-S linkages.

Study on Effect of Accelerator on Vulcanization of Natural Rubber through Rubber-state NMR Spectroscopy

Effect of accelerators on the vulcanization of natural rubber was investigated by analyzing structure of the crosslinking junctions through rubber state NMR spectroscopy. The accelerated sulfur vulcanization of natural rubber was carried out with one of the three accelerators, i.e., Zinc diethyldithiocarbamate (ZDEC), Zinc dibutyldithiocarbamate (ZDBC) and Zinc salt of 2-mercaptobenzothiazole (ZMBT), in the presence of sulfur, ZnO and stearic acid. Structure of the crosslinking junctions of the vulcanized natural rubbers was determined through rubber-state NMR spectroscopy with bilevel complete decoupling (BCM), distortionless enhancement by polarization transfer (DEPT) and attached proton test (ATP). The mechanism of the vulcanization of natural rubber was demonstratively analyzed on the basis of the structure of the resulting crosslinking junctions. It was classified into two schemes: that is, radical mechanism and ionic mechanism followed by radical mechanism, depending on the sort of accelerators used in the present study.

The use of diisopropyl xanthogen polysulfide as a potential accelerator in efficient sulfur vulcanization of natural rubber compounds

AbstractWith the emerging concerns on the use of nitrosamine evolving accelerators, safer alternatives are desirable for the rubber industry. In the present study, cure characteristics, physicomechanical properties, crosslink density, aging, dynamic mechanical, and thermal properties of vulcanizates prepared with two nitrosamine safe binary accelerator systems containing diisopropyl xanthogen polysulfide (DIXP) and tetrabenzyl thiuram disulfide (TBzTD), and DIXP and dibenzyl dithiocarbamate (ZBeC) were investigated for efficient sulfur vulcanization of natural rubber (NR) compounds. Synergistic effect in both cure and physicomechanical properties was observed for all the combinations of the two accelerator systems. The composition containing DIXP with a lower loading (0.6 phr) of TBzTD, and ZBeC, demonstrated the highest technological performance. Retention of tensile properties on oxidative aging was highest for the vulcanizate prepared with DIXP/TBzTD (1.4/0.6). Dynamic mechanical and thermogravimetric analysis further confirmed the crosslink density and thermal stability of the network structure of the binary systems. In overall, DIXP shows the potential to be used as a safer accelerator in the manufacture of appropriate dry NR‐based rubber products.

Elucidation of the accelerated sulfur vulcanization of bio oil‐extended natural rubber compounds

AbstractThe elucidation of the role of bio‐oils on the accelerated sulfur vulcanization of natural rubber (NR) compounds is discussed in this study. Two types of bio‐oil, palm oil and soybean oil, were studied in direct comparison with a distillate aromatic extract oil (DAE) as a reference. The scorch and cure times of the bio‐oil‐extended NR compounds were shorter than those containing DAE. The use of bio‐oils gave a higher cure reaction rate constant along with a lower activation energy than the use of DAE. The attenuated total reflectance‐Fourier transform infrared spectroscopy analysis revealed that the fatty acid segment of the bio‐oils can react with zinc oxide to give zinc carboxylate, which is then involved in and promotes the vulcanization reaction. The use of bio‐oils to increase the rate of vulcanization strongly influenced the crosslink density of the obtained NR vulcanizates, yielding NR vulcanizates with a lower crosslinking density. It is proposed here that the bio‐oils might consume the curing agent via the reaction between their own unsaturated fatty acid and sulfur. This was supported by the increased viscosity of the oils after exposure to sulfur at a high temperature. The tensile strength and elongation at break of the bio‐oil‐extended NR compounds were lower and higher, respectively, than the NR extended with DAE oil due to the lower crosslink density of the bio oil‐extended NR vulcanizates.

Enhancing the Performance of Rubber with Nano ZnO as Activators

The vulcanization of rubber is a chemical process to improve the mechanical properties by cross-linking unsaturated polymer chains. Zinc oxide (ZnO) acts as an activator, boosting the rubbers' sulfur vulcanization. Maintaining the level of ZnO content in the rubber compounds as low as possible is desirable, not only for economic reasons but also to reduce the environmental footprint of the process. In this contribution, octylamine (OA) capped ZnO nanoparticles (5 nm diameter), prepared through a thermal decomposition method, were demonstrated to be efficient activators for the sulfur vulcanization of natural rubber, enabling the reduction of the required amount of ZnO as compared to commercial systems. The effect of different ZnO activators (OA capped ZnO/commercial indirect process ZnO) on the curing characteristics, cross-linking densities, and mechanical performance, as well as the thermal behavior of rubber compounds, were investigated. Compared to the commercial indirect process ZnO, OA capped ZnO nanoparticles not only effectively enhanced the curing efficiency of natural rubber but also improved the mechanical performance of the composites after vulcanization. This was interpreted as, by applying the OA capped ZnO nanoparticles, the ZnO levels in rubber compounding were significantly reduced under the industrial vulcanization condition (151 °C, 30 min).

Effect of nitrosamine safe diisopropyl xanthogen polysulfide accelerator on cure and static mechanical properties of natural rubber compounds

Sulfur vulcanization of natural rubber (NR) is mainly carried out with the use of accelerators in order to achieve desired network structure and consequently the specific material properties. Many traditional organic accelerators are made from secondary amines and are capable of nitrosation during and after vulcanization in the presence of nitrosating agents. Therefore, the use of most of the nitrosamine generating traditional accelerators in rubber formulations are restricted by stringent rules and regulations imposed by world environmental and health-related government agencies. Diisopropyl Xanthogen Polysulfide (DIXP) has been identified as a nitrosamine safe novel accelerator and literature on use of this accelerator in natural rubber compounds is limited. The aim of this study was to evaluate the influence of DIXP on cure and static mechanical properties of NR compounds. A series of NR compounds was prepared with DIXP by varying the accelerator dosage from 0.2 phr to 2.2 phr at 0.4 phr intervals. Cure and static mechanical properties of the compounds were evaluated and compared with those of the compounds prepared with the commonly used nitrosamine safe N-tert-butyl-2-benzothiazole sulfenamide (TBBS) accelerator. Results showed that most of the properties of NR compounds prepared with DIXP are inferior in comparison to those of the compounds prepared with TBBS. Nevertheless, NR compounds prepared with DIXP possessed satisfactory results in regard to optimum cure time and elongation at break.

Preparation of zinc‐oxide‐free natural rubber nanocomposites using nanostructured magnesium oxide as cure activator

ABSTRACTThe effect of sol–gel synthesized magnesium oxide (MgO) nanoparticles as cure activator is studied for the first time in the vulcanization of natural rubber (NR) and compared with conventional zinc oxide (ZnO) in terms of cure, mechanical, and thermal properties. The NR vulcanizate with 1 phr (Parts per hundred parts of rubber) nano MgO shows an excellent improvement in the curing characteristics and the value of cure rate index is about 400% greater for NR vulcanizate containing 1 phr nano MgO in comparison to the NR vulcanizate with 5 phr conventional ZnO. Both mechanical and thermal properties of NR vulcanizate are found to be satisfactory in the presence of 1 phr nano MgO as cure activator in comparison to conventional NR vulcanizate. This study shows that only 1 phr nano MgO can successfully replace 5 phr conventional ZnO with better resulting properties in the sulfur vulcanization of NR. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42705.

Crosslinking via sulfur vulcanization of natural rubber and cellulose nanofibers incorporating unsaturated fatty acids

The reinforcement of sulfur-vulcanized natural rubber using cellulose nanofibers (CNFs) was investigated.

N-benzoyl-N′N′-disubstituted thioureas-A new binary accelerator system and its effect of nucleophilicity in sulfur vulcanization of natural rubber

The effect of several new binary accelerator systems were studied with special emphasis to the relationship of nucleophilicity to their rheological and mechanical properties. In this study, dialkyl/azacycloalkyl substituted benzoylthioureas (BTU 1-5) were used as secondary accelerators (SA) along with three different primary accelerators (PA) viz., Cyclohexylbenzthiazylsulfonamide (CBS), Mercaptobenzothiazole (MBT) or Zincdithiocarbamate(ZDC) in the sulfur vulcanization of natural rubber. The effect of these secondary accelerators on the rheological and mechanical properties was found to be improved when compared to the reference mixes. It is noted that the N-benzoyl-N′N′-piperidinylthiourea (BTU 4) is more effective as a secondary accelerator due to the higher nucleophilicity of the same when compared with the test compounds. The ZDC-BTU gives the best result as binary accelerator system in natural rubber. Chemical characterization was carried out by determining the total crosslink density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Investigation on synergic activity of N‐benzylimine aminothioformamide binary accelerator system in sulfur vulcanization of natural rubber

AbstractThis study explored the possibility of preparing a new thio accelerator, N‐benzylimine aminothioformamide to investigate its synergic accelerator activity in three conventional binary accelerator systems, N‐cyclohexyl benzothiazyl sulfenamide, mercapto benzothiazyl disulfide, and tetramethylthiuram disulfide in the sulfur vulcanization of natural rubber. Each system was found effective in reduction of cure time as a further proof of the nucleophilic mechanism suggested in the earlier reports. The vulcanizates of the mixes of the three systems were experimented for evaluating their physicomechanical properties. Most of the properties were found better than those of the reference mixes. Crosslinks were also evaluated for correlating the properties. Based on the evaluated properties, optimum dosage of the new binary systems were derived. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Amidino Thiourea as a Secondary Accelerator in the Sulphur Vulcanization of Natural Rubber Containing Fillers

As a part of our study on the mechanism of vulcanization we investigated a number of thiourea derivatives as secondary accelerators in the sulphur vulcanization of natural rubber. In the present study, the accelerator activity of a new binary accelerator system containing unsubstituted amidinothiourea viz. aminoiminomethyl thiourea (ATU) along with different primary accelerators has been examined. Different fillers like carbon black, silica and china clay are incorporated in these mixes under review. Various compositions of ATU are examined for the above vulcanization study. The optimum concentration of this secondary accelerator in different systems has also been derived. The mixes are examined for different cure characteristics and the vulcanizates are evaluated for their physical properties. In order to study the effect of the secondary accelerator on the temperature of vulcanization, these mixes are cured and evaluated at two different temperatures, viz. 120o and at 150oC. The correlation between physical properties and chemical cross links in the different vulcanizates is also examined by determining the cross link density values of the vulcanizates. These binary systems are found to be effective in the filled systems studied. The optimum cure time has been found to be reduced considerably by this secondary accelerator. At the lower temperature i.e., at 120°C, the reduction in cure time is found to be more significant with increase in the concentration of ATU especially when compared with systems containing thiourea. ATU being more nucleophilic than thiourea this points to a nucleophilic reaction mechanism in these vulcanization reactions. The tensile properties of the vulcanizates from these systems are better/comparable to those of the reference mixes. There is favourable increase in many of the physical properties like hardness, compression set, abrasion loss, tear strength etc. The crosslink density values evaluated also support the above conclusions. The non-toxic nature of ATU, which is used in pharmaceutical applications, is an added advantage in this context.

Studies on the effects of cyclohexylbenzthiazylsulfenamide accelerated sulfur vulcanization of natural rubber. Part-I. Kinetics of vulcanization

Cyclohexylbenzthiazylsulfenamide (CBS) is used as delayed action accelerator for the sulfur vulcanization of elastomers. It is now finding increasing application with ultra accelerators like tetramethyl thiuram disulfide (TMTD) and zinc diethyl dithiocarbamate (ZDC) in the vulcanization of natural rubber (NR). The evaluation of kinetic studies on accelerator blends (CBS/ TMTD and CBS/ZDC) is described. The kinetics of crosslink formation by blend systems vary markedly with formulations and the type of ultra accelerator being used. The crosslinking rate of NR gum stock in the absence of ultra accelerator is first order. However, the blend system gives crosslinking rate of dual nature. The rate shows first order at the initial part (low temperature near about 120) and higher order at the later part of vulcanization (high temperature near about 140). The activity of ultra accelerator fragments alters the amine moiety of sulfenamide system according to their chemical structure.

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

AbstractThe 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

Sulfur Vulcanization of Natural Rubber for Benzothiazole Accelerated Formulations: From Reaction Mechanisms to a Rational Kinetic Model

Abstract The chemistry of accelerated sulfur vulcanization is reviewed and a fundamental kinetic model for the vulcanization process is developed. The vulcanization of natural rubber by the benzothiazolesulfenamide class of accelerators is studied, where 2-(morpholinothio) benzothiazole (MBS) has been chosen as the representative accelerator. The reaction mechanisms that have been proposed for the different steps in vulcanization chemistry are critically evaluated with the objective of developing a holistic description of the governing chemistry, where the mechanisms are consistent for all reaction steps in the vulcanization process. A fundamental kinetic model has been developed for accelerated sulfur vulcanization, using population balance methods that explicitly acknowledge the polysulfidic nature of the crosslinks and various reactive intermediates. The kinetic model can accurately describe the complete cure response including the scorch delay, curing and the reversion for a wide range of compositions, using a single set of rate constants. In addition, the concentration profiles of all the reaction intermediates as a function of polysulfidic lengths are predicted. This detailed information obtained from the population balance model is used to critically examine various mechanisms that have been proposed to describe accelerated sulfur vulcanization. The population balance model provides a quantitative framework for explicitly incorporating mechanistically reasonable chemistry of the vulcanization process.

Studies on accelerated sulphur vulcanization of natural rubber using 1-phenyl-2, 4-dithiobiuret/tertiary butyl benzothiazole sulphenamide

The use of binary accelerators has gained tremendous importance since it increases the production rate of the article made of that elastomer. The authors have analyzed the performance of a novel binary accelerator system in the sulphur vulcanization of natural rubber (NR). The vulcanizates resulting from the binary accelerated process obtained good mechanical properties, ageing and swelling resistance. Network characterization of the mixes was done using swelling measurements, stress–strain analysis etc. The chain entanglement density was measured using dynamic mechanical analysis. However the performance is found to be dependent on the relative proportion of mono, di and polysulphidic linkages in the material. The result of the study points out that the proposed system can be active in NR regardless of the vulcanization recipe and temperature. The performance of the new binary system in filled vulcanizates is also studied. Based on the processing, mechanical and chemical characterization an optimum concentration is suggested for the new system.

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

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

Spectroscopic Characterization of the Molecular Structure of Elastomeric Networks

Abstract In vulcanization, chemical crosslinks are formed across elastomeric polymer chains improving both the strength and elastic properties of the rubber. An understanding of the formation, structure, and stability of vulcanizates is therefore very important. Solid-state NMR and NMR imaging have been effective methods to study many different aspects of vulcanization. In solid-state NMR, several peaks appear in the C-13 spectrum of vulcanized rubber. Through model studies, NMR analysis, and chemical shift additivity calculations, these peaks were assigned to their respective vulcanizate structures. Once this assignment was made, the concentration of each vulcanizate structure formed could be followed with time under a variety of different conditions. In unaccelerated sulfur vulcanization of natural rubber (NR) and polybutadiene rubber (BR), many inefficient (cyclic or intramolecular) structures were formed as compared to intermolecular crosslinks. In accelerated NR and BR sulfur vulcanization, NMR was used to study vulcanizate concentration dependence on (a) type of formulation (efficient, semi-efficient, or conventional), (b) type of accelerator, (c) extent of cure, and (d) different concentration of ingredients (sulfur, activator, etc.). Solid-state NMR was also used to study different parameters in butyl rubber and to identify elastomers in binary blends of chloroprene rubber (CR) and NR, CR and chlorosulfonated polyethylene (CSM), NR and CSM, and styrene—butadiene rubber (SBR) and acrylonitrile—butadiene rubber (NBR) as well as the tertiary blend of NR/SBR/BR. In several studies, the effect of filler (carbon black or silica) on vulcanization was studied. Additionally, the thermo-oxidative degradation of sulfur vulcanizates in NR with heating time and temperature was observed using NMR. NMR imaging has been useful in the determination of internal inhomogeneities arising from inadequate mixing, gradients in crosslinking chemistry, filler distribution, blends, and coagents.

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

Solid-state13C-NMR and equilibrium-swelling studies of filled, TBBS-accelerated sulfur vulcanization of natural rubber

The effective network chain density (νe) and the density of the chains between sulfurizations (2[S]chem) for natural rubber vulcanized with N-t-butyl benzothiazole sulfenamide (TBBS) in different levels of HAF–HS (N347) carbon black were determined by equilibrium swelling and solid-state 13C-NMR measurements, respectively. The proportion of the intermolecular crosslinks over a whole range of sulfurized products (Ec) for the gum stock was estimated by the comparison of the chemical network chain density (νchem) calculated from νe using Mullins's approach with 2[S]chem. Ec increases with cure, reaches a maximum at an optimum cure state (trpar;, and decreases during an overcuring. It was observed from the NMR results that the total amount of chemical sulfurization reactions occurring in the rubber is almost independent of the level of carbon black loading. Based on the assumption that the amount of the intermolecular crosslinking reactions is also constant with the carbon black content, the amount of physical crosslinks (vent) in filled formulations was determined by subtracting the estimated νchem from the νe. The linear relationship obtained from the plot of the vent at the optimum cure state (t90) as a function of carbon black content postulates that the physical adsorption plays a major role in the polymer–filler interactions. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1391–1399, 1998