Studies of accelerated sulfur vulcanization of styrene–butadiene rubber by N-cyclohexylbenzothiazol-2-sulfenamide in the presence and absence of dicumyl peroxide

This study embodies the results and discussion of a comprehensive and systematic investigation of the mechanism of sulfuration of styrene–butadiene rubber accelerated by N-cyclohexylbenzothiazole-2-sulfenamide (CBS) with and without activators. Dicumyl peroxide (DCP) has been taken as a chemical aid to distinguish between free-radical and polar mechanisms of sulfuration. The rate constant for DCP decomposition in presence of CBS and the reduction in crosslink density by CBS have also been studied. With a constant amount of DCP and sulfur the crosslink density increases with increasing CBS concentration. In the presence of ZnO and stearic acid, crosslinking proceeds faster than in a similar system without these ingredients, and with DCP the crosslinks are found to be formed nearly additively as confirmed by methyl iodide treatment of the vulcanizates. In the absence of DCP, the crosslinking is characterized by an induction period, even in presence of ZnO and stearic acid. In the presence of sulfur, the 2-mercaptobenzothiazole (MSH) or amine or amine salt form crosslinkins by ionic reaction.     

Studies on diphenylguanidine–accelerated sulfur vulcanization of styrene butadiene rubber in the presence and absence of dicumylperoxide

Diphenylguanidine (DPG) raises the rate of decomposition of dicumylperoxide (DCP) and the crosslinking maxima due to DCP is lowered to some extent by DPG. When the molar proportion of DPG–S is approximately 1:1, no additive results of crosslink formation (as reported for NR) are observed. Zinc oxide and stearic acid increase the rate of crosslinking as well as the crosslinking maxima. In the present study it appears most probable that DPG-accelerated sulfuration of SBR is an ionic process. At a constant level of DCP and sulfur crosslink density increases when the amount of DPG is increased; a constant level of DCP and DPG crosslinking increases with rising sulfur concentration. An increase in the concentration of DPG or sulfur leads to greater formation of the complex as well as a change in its composition. The effect of DPG is more pronounced, for by reducing the number of sulfur atoms more sulfur is available for crosslinking. Sulfur absorbs little SO₂ or H₂S; if it is already saturated, there is no perceptible effect. DPG does absorb H₂S or SO₂ and the rate and maxima of crosslinks is increased. The effect of SO₂ is higher because of the higher K_a values of H₂SO₃ and consequently higher concentration of HSO₃^?. Reversion is a free-radical process inhibited by free DPG present in the system. In the presence of zinc oxide and stearic acid the reaction follows a polar mechanism as well as a radical mechanism.     

Homolytic dissociation of the vulcanization accelerator tetramethylthiuram disulfide (TMTD) and structures and stabilities of the related radicals Me2NCSn* (n = 1-4)

The homolytic dissociation of the important vulcanization accelerator tetramethylthiuram disulfide (TMTD) has been studied by ab initio calculations according to the G3X(MP2) and G3X(MP2)-RAD theories. Homolytic cleavage of the SS bond requires a low enthalpy of 150.0 kJ mol-1, whereas 268.0 kJ mol-1 is needed for the dissociation of one of the C-S single bonds. To cleave one of the SS bonds of the corresponding trisulfide (TMTT) requires 191.1 kJ mol-1. Me2NCS2* is a particularly stable sulfur radical as reflected in the low S-H bond dissociation enthalpy of the corresponding acid Me2NC(=S)SH (301.7 kJ mol-1). Me2NCS2* (2B2) is a sigma radical characterized by the unpaired spin density shared equally between the two sulfur atoms and by a 4-center (NCS2) delocalized pi system. The ESR g-tensors of the radicals Me2NCSn* (n = 1-3) have been calculated. Both TMTD and the mentioned radicals form stable chelate complexes with a Li+ cation, which here serves as a model for the zinc ions used in accelerated rubber vulcanization. Although the binding energy of the complex [Li(TMTD)]+ is larger than that of the isomeric species [Li(S2CNMe2)2]+ (12), the dissociation enthalpy of TMTD as a ligand is smaller (125.5 kJ mol-1) than that of free TMTD. In other words, the homolytic dissociation of the SS bonds of TMTD is facilitated by the presence of Li+ ions. The sulfurization of TMTD in the presence of Li+ to give the paramagnetic complex [Li(S3CNMe2)2]+ is strongly exothermic. These results suggest that TMTD reacts with naked zinc ions as well as with the surface atoms of solid zinc oxide particles in an analogous manner producing highly reactive complexes, which probably initiate the crosslinking process during vulcanization reactions of natural or synthetic rubber accelerated by TMTD/ZnO.     

Complexation of the vulcanization accelerator tetramethylthiuram disulfide and related molecules with zinc compounds including zinc oxide clusters (Zn4O4)

Zinc chemicals are used as activators in the vulcanization of organic polymers with sulfur to produce elastic rubbers. In this work, the reactions of Zn(2+), ZnMe(2), Zn(OMe)(2), Zn(OOCMe)(2), and the heterocubane cluster Zn(4)O(4) with the vulcanization accelerator tetramethylthiuram disulfide (TMTD) and with the related radicals and anions Me(2)NCS(2)(*), Me(2)NCS(3)(*), Me(2)NCS(2)(-), and Me(2)NCS(3)(-) have been studied by quantum chemical methods at the MP2/6-31+G(2df,p)//B3LYP/6-31+G* level of theory. More than 35 zinc complexes have been structurally characterized and the energies of formation from their components calculated for the first time. The binding energy of TMTD as a bidendate ligand increases in the order ZnMe(2)相似文献   

Effects of cellulose nanocrystals on the vulcanization of natural rubber/cellulose nanocrystals nanocomposite and corresponding regulating strategies

Vulcanization is a vital process in rubber processing, it endows rubber with valuable physical and mechanical properties, making rubber a widely used engineering material. In addition to vulcanization agent, reinforcing fillers play a non-ignorable influence on the vulcanization of rubber nanocomposites. Herein, the effects of cellulose nanocrystals (CNCs) on the vulcanization of natural rubber (NR)/CNCs nanocomposite was studied. It was found that even though the addition of CNCs can effectively improve the dispersion of ZnO in NR matrix, the vulcanization of NR was inhibited. This may be attributed to the CNCs' adsorption of vulcanizing agents (DM, ZnO) and the acidic chemical environment on the surface of CNCs. In order to improve the vulcanization properties of NR/CNCs nanocomposite, tetramethyldithiochloram (TMTD) and triethanolamine (TEOA) were used as a combination accelerator and curing activator, respectively, and polyethylene glycol (PEG) was introduced to screen hydroxyl groups on the surface of CNCs to prohibit the CNCs' adsorption of vulcanizing agents. The results indicate that TMTD and TEOA effectively improved the vulcanization rate of NR/CNCs nanocomposite and increased the crosslink density by an order of magnitude. Subsequently, the tensile strength, tear strength, and so forth. of NR/CNCs nanocomposite were significantly improved. However, PEG hardly help to improve the vulcanization properties of NR/CNCs nanocomposite. In addition, the control samples without CNCs were prepared and characterized, the comparation between NR and NR/CNCs nanocomposite shows that the synergistic effect of crosslink density and CNCs' reinforcement more effectively improve mechanical properties of NR. This work not only elucidates the inhibiting mechanisms of CNCs on the vulcanization of NR, but also provides practical strategies for improving the vulcanization and properties of NR/CNCs nanocomposite. It may accelerate the application of CNCs as rubber reinforcing filler.     

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

The 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.     

Crosslink network evolution of BIIR/EPDM blends during peroxide vulcanization

Crosslink network evolution of brominated butyl rubber (BIIR)/ethylene–propylene–diene-monomer rubber (EPDM) blends during peroxide vulcanization is studied at a meso-scale level. In this work, EPDM is added as a co-agent to increase the crosslink density of BIIR vulcanization. With increasing EPDM content from 0 to 20 phr, the maximum torque of BIIR/EPDM compounds during vulcanization increases by 73%, reaching to 3.40 dNm. Vulcanization kinetic study shows that addition of EPDM favors to the crosslinking of BIIR compound. Meanwhile, the addition of 20 phr EPDM contributes to an increase in the crosslink density of BIIR/EPDM(80/20) vulcanizate, avoiding downward trend at post-cure period in comparison with BIIR only. Crosslink network evolution of BIIR/EPDM blends is divided into three periods during peroxide vulcanization at 150 °C. The role of EPDM in the crosslink network evolution is studied by proton nuclear magnetic resonance, and a “network patching” mechanism is proposed in which EPDM is implied to work as patch on damaged crosslink network resulted from the degradation nature of BIIR.     

Synergistic effect of 4,4′‐bis(maleimido) diphenylmethane and zinc oxide on the vulcanization behavior and thermo‐mechanical properties of chlorinated isobutylene–isoprene rubber

It is well known that the vulcanization of chlorinated isobutylene–isoprene rubber (CIIR) with ZnO alone is very slow and the resulting vulcanizate shows low crosslinking density as well as poor mechanical and thermal properties. For these problems to be addressed, 4,4′‐bis(maleimido) diphenylmethane (BMDM) has been applied along with ZnO to cure CIIR. The curing behavior was investigated using oscillating disk rheometer and differential scanning calorimetry analysis. The crosslinking densities, mechanical, dynamic mechanical, and thermal properties of the vulcanizates were also evaluated as a function of BMDM content. From the rheometer and differential scanning calorimetry analysis, it has been confirmed that the rate of curing, as well as the extent of curing between CIIR and ZnO, has remarkably increased in the presence of BMDM. The Diels–Alder reaction between the in situ formed conjugated diene butyl from CIIR/ZnO with the maleic moieties of BMDM has been identified as a primary reason for the higher rate and state of cure. The crosslinking densities of the vulcanizate CIIR/ZnO have significantly increased with increase in the content of BMDM. As a result, the compression set of the vulcanizate was remarkably decreased at elevated temperature. Moreover, the vulcanizates contain BMDM that has exhibited high thermal degradation temperature and good retention of the mechanical properties even after 7 days of thermal ageing. Copyright © 2016 John Wiley & Sons, Ltd.     

Curing and mechanical behavior of carboxylated NBR containing hygrothermally decomposed polyurethane

Hygrothermally decomposed polyester-urethane (HD-PUR) has been added as modifier (up to 20 phr) to sulfur crosslinked carboxylated nitrile rubber (XNBR). The curing and mechanical characteristics of the XNBR have been investigated as a function of the HD-PUR loading in presence and absence of carbon black (CB). The addition of HD-PUR increased the cure rate of both unfilled and CB filled XNBR but resulted in compounds of lower crosslink density and thus of lower stiffness and strength. Based on infrared spectroscopic results it was speculated that the amine functionality of HD-PUR affected the formation of the ionic clusters formed by the reaction between the -COOH groups of XNBR and ZnO. This occurred likely via a coordination complex. Evidence was also found for the formation of -CONH- linkages. Both coordination complexing and chemical reaction between -COOH and -NH₂ resulted in a lower overall “crosslinking degree” and as a consequence HD-PUR acted as plasticizer in XNBR.     

Extraction and spectrophotometric determination of platinum(IV) using tetramethylthiuram disulfide

The extraction and photometric determination of platinum(IV) with tetramethylthiuram disulphide has been studied. Addition of an excess of stannous chloride and maintaining acidity to 2–6 M with HCl is necessary for quantitative extraction. With 10 ml of TMTD in toluene and equilibration for 30 sec, platinum is extracted as a pale yellow complex in the organic phase. It has λ_max at 348 nm and obeys Beer's law in the concentration range of 0.2 to 9.0 μg of Pt per ml. The complex is stable for 3 hr. The ratio of Pt:TMTD in the extracted species is found to be 1:1. Most of the anions and cations are tolerated in large ratio. The method can be satisfactorily applied for the determination of platinum in the presence of four- and eightfold excesses of ruthenium and osmium, respectively.     

Role of hydrogen disulfide in the vulcanization process

Summary It was shown that the breakdown of hydrogen disulfide in a-olefins results in the exclusive formation of addition products (mono-, di-, and tetra-sulfides) without formation of hydrogen sulfide and free sulfur.Similar results were obtained when sulfur reacted with monoethanolamine (vulcanization accelerator) in -olefin solution.The reaction products were isolated and characterized.     

线形低密度聚乙烯／废胶粉热塑弹性体动态硫化性能研究

利用动态硫化法制备了线形低密度聚乙烯(LLDPE)/废胶粉(GTR)热塑弹性体。重点研究了两种交联剂:硫和过氧化二异丙苯(DCP)对共混物性能的影响。加入一定量的苯乙烯-丁二烯-苯乙烯(SBS)共聚物作为增容剂。结果表明,经过DCP动态硫化后的共混物的力学性能比简单共混的共混物有明显的提高,而加入硫磺体系对共混物力学性能影响不大甚至有所下降。通过红外光谱、热分析(DSC)和扫描电镜(SEM)对共混物的热行为和表面形态研究表明,加入DCP交联剂使LLDPE、SBS和胶粉之间发生了交联反应,从而增加了胶粉颗粒与LLDPE间的界面相容性,使其热塑性弹性体的力学性能得以提高。     

Identification of volatile products produced during the peroxide vulcanization of poly(vinyl alkyl ethers)

Volatile products are produced during vulcanization of the polymers of vinyl methyl, ethyl, isopropyl and n-butyl ethers with dicumyl peroxide, both in the presence and, absence of added sulfur. They were identified and estimated by gas-liquid chroma tography and mass spectrometry. The principal products formed during the vulcanization of poly(viny1 ethyl ether) with dicumyl peroxide were ethane and acetaldehyde and to a lesser extent methane. The addition of sulfur in the curing recipe resulted in a sharp increase in the proportion of ethyl alcohol, a large increase in methane, and a large decrease in ethane. The formation of these volatile products and the observed changes due to the presence of sulfur in the curing recipe can be accounted for on the basis of side-chain scission of the polymer radicals produced during vulcanization. The length and/or shape of the alkyl group in poly(viny1 alkyl ether) determine the composition of the volatile products. The data are in harmony with the postulated chemistry of vulcanization of these polymers.     

Thermoanalytical methods in vulcanizete analysis. Differential scanning calorimetry. Observations on the vulcanization process

Differential scanning calorimetry was used to follow the unaccelerated and accelerated sulfur vulcanization process in natural rubber and polybutadiene compounds. It was established that in both hard and soft rubber vulcanization, the heat of vulcanization (ΔH_v) depends only on the sulfur concentration provided other ingredients (carbon black, zinc oxide, stearic acid) and the elastomer blend ratio remain constant. Organic accelerators alter the temperature dependence of the exotherm but have no effect on ΔH_v over a considerable concentration range.The DSC exotherm was used to determine variations in sulfur/accelerator concentrations in production compounds. Analysis time is short—approx. 5 min—and a routine quality control method is suggested.     

Influence of temperature on curative interactions with the participation of metal carboxylates as adhesive promoters: the interactions of Cu(II) hexadecanoate with stearic acid,N,N′-dicyclohexylbenzothiazole-2-sulfenamide,sulfur and zinc oxide

The influence of temperature on the interactions between Cu(II) hexadecanoate (CuC₁₆) and N,N′-dicyclohexylbenzothiazole-2-sulfenamide (DCBS), stearic acid, sulfur and zinc oxide (ZnO) were studied by differential scanning calorimetry, infrared spectroscopy, electron paramagnetic resonance spectroscopy and X-ray analysis in the absence of rubber. Comparison of the results reveals that in most cases physical processes (dissolution, melting) occurred in the studied systems.However, the most favorable thermal reaction appears to be the formation of Cu(II) complexes with DCBS (or DCBS fragments) and the formation of zinc stearate in the presence of stearic acid and ZnO on the action of heat.     

Studies of conventional sulfur vulcanization of SBR rubber: Analysing the reaction products from thermal degradation of the accelerator by means of MCC-IMS technique

A combination of knowledge on curing process of rubber mixes with novel methods of chemical analysis, a new unconventional approach to analysis of rubber vulcanization is presented in this study. Six SBR samples containing various N-tert-butyl-2-benzothiazole sulfenamide (TBBS) accelerator: sulfur ratios (within) the range of conventional (CV) sulfur vulcanization system were studied using multi-capillary column ion mobility spectrometry (MCC-IMS) technique. For these analysis, calibration/dilution curves were established. Moreover, data from MCC-IMS were correlated with other parameters of the rubber vulcanizates – their crosslink density and structure as well as their tensile strength and modulus at elongation. For such comparison, one of the reaction products from thermal decomposition of TBBS, benzothiazole was selected. Furthermore, the concentration of benzothiazole released during the vulcanization process corresponded well with the crosslink density of the rubber vulcanizates studied. It was even possible to calculate the crosslink density from the concentration of benzothiazole determined by MCC-IMS, using Boltzmann fitting curve. The presented results could be an important contribution in understanding the mechanisms occurring during rubber vulcanization, demonstrating a new approach to testing and evaluation of the process.     

Polymerization of styrene with tetramethylthiuram disulfide as an initiator in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy

The bulk polymerization of styrene was investigated with tetramethylthiuram disulfide (TMTD) as an initiator in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) at 123 °C. The polymerization proceeded in a controlled/living way; that is, the polymerization rate was first‐order with respect to the monomer concentration, and the molecular weight increased linearly with conversion. The molecular weights of the polymers obtained were close to the theoretical values, and the molecular weight distributions were relatively low (weight‐average molecular weight/number‐average molecular weight = 1.1–1.3). The rate of polymerization with TMTD as an initiator was faster than that with benzoyl peroxide, and the rate was independent of the initial concentration of TMTD in the presence of TEMPO. The obtained polystyrene was functionalized with ultraviolet‐light‐sensitive ? SC(S)N(CH₃)₂ groups, which was characterized with ¹H NMR spectroscopy. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 543–551, 2005     

Insight into vulcanization mechanism of novel binary accelerators for natural rubber

A novel TU derivative, N-phenyl-N′-(у-triethoxysilane)-propyl thiourea(STU), is prepared and its binary accelerator system is investigated in detail. Compared to the control references, the optimum curing time of NR compounds with STU is the shortest, indicating a more nucleophilic reaction occurs. The Py-GC/MS results present that the phenyl isothiocyanate fragment still remains in the NR/STU compounds with or without extracting treatment, but no silane segment can be found in the vulcanizate with extracting treatment. Vibrations of C=S, NH and aromatic ring in FTIR experiments and a new methyne carbon peak, as well as the peaks of phenyl group of STU, in the solid state 13C-NMR experiments are found in the NR/STU vulcanizate with extracting treatment. Moreover, the crosslinking density of vulcanizates with STU evolves to lower level, indicating the sulfur atom of STU does not contribute to the sulfur crosslinking. Therefore, a new vulcanization kinetic mechanism of STU is propounded that the thiourea groups can graft to the rubber main chains as pendant groups by chemical bonds during the vulcanization process, which is in accordance with the experimental observations quite well.     

Reclaiming of ground rubber tire (GRT) by a novel reclaiming agent

The present paper describes the mechanical reclaiming of ground rubber tire (GRT) by tetra methyl thiuram disulfide (TMTD), a multi-functional reclaiming agent. The versatility of the proposed agent is that it acts as a reclaiming agent during reclaiming and as a curing agent during revulcanization of the reclaimed sample. Reclaiming of GRT was carried out on an open two roll mixing mill at various time intervals and different concentrations of the reclaiming agent (TMTD). The degree of reclaiming was evaluated by measurement of the gel content, inherent viscosity of sol rubber, Mooney viscosity of the reclaimed rubber, crosslink density, swelling ratio and molecular weight between two crosslink bonds as a function of milling time. Also, the influence of the gel content on crosslink density at various time intervals on the open two roll mixing mill was determined. A unique correlation between gel fraction and crosslink density obtained at various time intervals and concentrations of reclaiming agent indicated that an optimization of the concentration of TMTD and milling time has a positive influence on improving the efficiency of reclaiming. The reclaiming conditions have been optimized in view of the mechanical properties of the revulcanized GRT and the aging resistance properties of the revulcanized reclaim. The influence of the concentration of sulfur on the mechanical properties in the revulcanized reclaim was also studied. Scanning electron microscopy (SEM) studies further indicate the coherency and homogeneity in the revulcanized reclaim rubber when reclamation is carried out by optimum concentration of TMTD after maximum time of reclaiming.     

The effect of dynamic vulcanization on the properties of polymer-elastomer blends containing crumb rubber

The influence of dynamic vulcanization on the amount of the sol fraction, the crosslink density, the melt flow index, and the mechanical properties of ternary (isotactic polypropylene-rubber-crumb rubber) and binary (rubber-crumb rubber) blends was studied. Two types of ethylene-propylene-diene terpolymer (elastomer) were used as the rubber component, the oil-free elastomer and the elastomer extended with paraffin oil during its synthesis. The blends were vulcanized in the presence of a sulfur accelerating system. It was shown that blends with crumb rubber having a particle size of less than 0.1 mm exhibited the best mechanical and rheological characteristics. The introduction of crumb rubber into thermoplastic elastomers that contain the oil-free ethylene-propylene-diene terpolymer leads, at a certain ratio of the components, to a rise in the melt flow index, regardless of the crumb-rubber particle size and of whether the rubber component was vulcanized.