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 of vulcanization with tetramethylthiuram disulfide and sulfur vulcanization accelerated by tetramethylthiuram disulfide of styrene–butadiene rubber in presence and absence of dicumyl peroxide

The salient features of nonelemental sulfur vulcanization by tetramethylthiuram disulfide (TMTD) and elemental sulfur vulcanization promoted by TMTD both in presence and absence of ZnO and stearic acid have been studied. In stock containing TMTD, a higher rate constant value for dicumyl peroxide (DCP) decomposition was observed. TMTD decreases the crosslinking density due to DCP depending on its concentration. An entirely radical mechanism has been advanced in the absence of ZnO. ZnO or ZnO–stearic acid seems to alter the entire course of reaction. The rate of crosslinking increases in the presence of ZnO or ZnO–stearic acid. Moreover, crosslinks are formed additively (further supported from the activation energy data), and mixed crosslink formation has been confirmed by the methyl iodide test of the vulcanizates. Stearic acid has no effect on crosslink formation. An ionic chain mechanism has been postulated in the presence of ZnO, as suggested by British authors.     

Crosslinking behavior and enhanced mechanical properties of acrylonitrile‐butadiene rubber composites by incorporating aluminum ammonium sulfate particles

The crosslinked structure formed by the metal coordination bonding provides excellent and new properties for rubber materials. Herein, the crosslinking of acrylonitrile‐butadiene rubber (NBR) is induced by introducing aluminum ammonium sulfate (NH₄Al(SO₄)₂·12H₂O) particles. The crosslinking behavior, morphology, mechanical properties, and the Akron abrasion resistance of NBR/NH₄Al(SO₄)₂·12H₂O composites were fully explored. The results show that the three‐dimensional crosslinking structure is held together by metal–ligand coordination bonds between the nitrile group and AI(III). The coordination crosslink density exhibits a considerable increase with the addition of NH₄Al(SO₄)₂·12H₂O. Thus, the mechanical properties and abrasion resistance of the obtained composites are better than that of NBR/sulfur system. Interestingly, the elongation at break for NBR/NH₄Al(SO₄)₂·12H₂O composites is over 2000% due to the nature of coordination bonds. The abrasion volume loss decreases to 0.4 cm³ for NBR/NH₄Al(SO₄)₂·12H₂O composites with 20 phr NH₄Al(SO₄)₂·12H₂O particles as compared to 0.75 cm³ for NBR/sulfur system. The obtained NBR composites with facile preparation and excellent mechanical properties make the composites based on metal coordination bonding attractive for practical use. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 879–886     

Kinetics and Mechanism of low-Temperature Oxidation of H2S with Oxygen in the gas Phase

The possibility of the formation of polysulfides during oxidation of H₂ S with oxygen on oxide catalysts has been checked, and the sequence of the reaction stages at temperatures below the sulfur dew point determined. The amount of polysulfides formed during H₂ S oxidation has been found to exceed significantly that obtained in the reaction of sulfur with H₂ S. Polysulfides are concluded to be intermediates in H₂ S oxidation to sulfur. The rate of formation of SO₂ from sulfur vapor is shown to be negligibly low at 100-200°C. A reaction scheme involving the formation of sulfur from polysulfides and the formation of sulfur dioxide by direct oxidation of H₂ S is suggested.     

Microbial removal of sulfur dioxide from a gas stream with net oxidation to sulfate

A study has been conducted of the feasibility of utilizing the sulfate reducing bacteriumDesulfovibrio desulfuricans and the chemoautotrophThiobacillus denitrificans as a basis of a microbial process for the removal of sulfur dioxide from a gas with net oxidation to sulfate. In reactors-in-series, SO₂ was reduced to H₂S in the first stage by D.desulfuricans. The H₂S was then stripped with nitrogen and sent to a second stage where it was oxidized to sulfate by T.denitrificans. A sulfur balance demonstrated complete reduction of SO₂ to H₂S in the first stage and complete oxidation of H₂S to sulfate in the second stage.     

Nonstationary Phenomena in the Low-Temperature Gas-Phase Catalytic Oxidation of H2S with Oxygen

The conditions of the existence were determined and reasons were revealed for the appearance of periodic oscillations of the SO₂concentration in the reaction products and warming temperature in the catalyst bed in the oxidation of H₂S with oxygen at temperatures below the dew point of sulfur on the V–Al–Ti oxide catalyst. The formation of polysulfides during the reaction was experimentally found. It was proposed that the adsorption of sulfur and polysulfides on the catalyst surface is responsible for the observed oscillatory processes.     

Polymerization of olefin oxides and of olefin sulfides

Two new catalyst systems, sulfur–diethylzinc and 98% hydrogen peroxide–diethylzinc, have been investigated for polymerizing propylene oxide. The sulfur–diethylzinc catalyst system has a broad range of sulfur/zinc atomic ratio for polymerizing propylene oxide heterogeneously to high molecular weight materials in high yields. The highest polymer yield is obtained at the sulfur/zinc atomic ratio of 3–3.5. Like the water–diethylzinc system, the hydrogen peroxide–diethylzinc system has a narrow range of hydrogen peroxide/diethylzinc molar ratio in the vicinity of 0.57 for optimum polymer yield. Crystallinity measurements by x-ray diffraction of a few polymers prepared with these three catalyst systems showed that they are fairly similar in the extent of their crystallinity. A plot of the per cent of polymer insoluble in acetone against inherent viscosity of the original polymer also showed that the polymers prepared with sulfur–diethylzinc and hydrogen peroxide–diethylzinc catalyst systems have similar amounts of crystallinity. Data are given for the polymerizability of ethylene oxide, 1,2-butene oxide, styrene oxide, propylene sulfide, 1,2-butene sulfide, and a vulcanizable copolymer of propylene oxide and allyl glycidyl ether with the sulfur–diethylzinc catalyst system. The polymers from the olefin sulfides had lower inherent viscosities than the polymers from the corresponding olefin oxides. Aging of the sulfur–diethylzinc catalyst (S/Zn atomic ratio = 3.5) improved the yield of poly(propylene oxide). The yield was essentially unchanged when propylene oxide was polymerized in six different solvents. The formation of C₂H₅S_xZnSC₂H₅ and C₂H₅S_xZnS_yC₂H₅ (x and y are integers between 2 and 8) and possibly C₂H₅S_xZnC₂H₅ as the catalytically active species is postulated during the reaction of sulfur and diethylzinc.     

硫化CoMo/Al2O3催化剂上H2同时催化还原SO2和NO(Ⅱ)——催化反应活性相及机理探讨

主要通过XPS表征、热力学计算以及一系列设计的评价实验等方法,对硫化CoMo/Al2O3催化剂上H2同时催化还原SO2和NO反应的活性相、吸附活性位以及反应机理进行了研究.结果表明,金属硫化物相是SO,和NO转化的主要活性相,并与载体Al2O3共同承担H2S转化为单质硫的作用.此外,反应过程中产生的品格空位也对NO转化起着重要作用.催化剂表面的阴离子空位是SO2和NO共同的吸附活性位,SO2对NO的吸附有抑制作用,而催化剂表面的L碱佗也是SO2的吸附活性位,NO可促进SO2的氧化吸附.最后,本文从反应分子的吸附与活化、NO的转化及品格硫的流失、SO2还原到H2S、H2S的转化、晶格硫的补充等5个方面提出了反应机理.     

Swelling studies of crosslinked poly(p-phenylene terephthalamide) copolymers in sulfuric acid

In an attempt to improve the mechanical properties of extended chain polymers such as poly(p-phenylene terephthalamide) (PPTA), a crosslinkable terephthalic acid derivative (XTA) has been developed which can be incorporated into copolymers in various concentrations and activated after polymerization. The crosslinking of PPTA-co-XTA copolymer particles was investigated through a series of swelling experiments in concentrated H₂SO₄. The data show a systematic decrease in equilibrium swelling with increasing XTA content, indicating the XTA units are in fact acting as crosslink sites. Values for crosslink density were calculated from the Flory-Rehner theory of polymer swelling and compared with previous findings on crosslinked rigid polymer network systems. The effective number of crosslinks per XTA unit (efficiency) predicted by the Flory-Rehner theory increases and then decreases with % XTA. The decrease in crosslinking efficiency at high XTA concentrations is consistent with differential scanning calorimetry data which show the enthalpy of XTA reaction decreasing slightly with % XTA. The deviations at low % XTA may represent a failure of the Flory-Rehner theory to properly describe the rubbery elasticity of extended chain polymers. © 1994 John Wiley & Sons, Inc.     

Changes in sol fraction during peroxide crosslinking of linear low-density polyethylenes with homogeneous distribution of short chain branching

FTIR measurements of sol fractions were carried out in order to assess side reactions during dicumyl peroxide (DCP) crosslinking of linear low density polyethylenes (LLDPEs) with homogeneous distribution of short chain branching. The changes of methyl and olefinic unsaturation concentration were measured as a function of degree of branching and DCP content. The changes of methyl and vinylidene concentration indicate that the scission probability has greater significance for the samples with initial branch content above 31 CH₃/1000 C and increases with increasing DCP concentration. The analysis of trans-vinylene concentration indicates that the probability of disproportionation also increases with increasing DCP content. The changes of methyl concentration in the sol fraction of the sample containing initially ca. 23 CH₃/1000 C with increasing DCP content could be interpreted using the dependence of crosslinking efficiency on branch content found previously for some range of branch concentration. © 1995 John Wiley & Sons, Inc.     

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.     

Reaction mechanism and modeling study for the oxidation by SO2 of o‐xylene and p‐xylene in Claus process

Claus process, comprising of a furnace and a catalytic unit, is used to produce sulfur from H₂S. The aromatic contaminants (benzene, toluene, and xylenes) in H₂S feed form soot, and clog and deactivate the catalysts. Xylenes are known to be the most damaging ones. Therefore, there is a need to oxidize them in the furnace to enhance catalyst life. This article presents a kinetics study on the oxidation of o‐ and p‐xylene radicals by SO₂ (an oxidant that is already present in the furnace) using density functional theory and a composite method. The mechanism begins with H‐abstraction from xylenes to form xylyl radicals, followed by exothermic addition of SO₂ to them. The breakage of O S bond in the xylyl‐SO₂ adducts leads to the loss of SO molecule, while the remaining O atom on them helps in their oxidation. The isomerization study shows that less‐stable dimethylphenyl radicals have a high tendency to isomerize to resonantly stabilized methylbenzyl radicals. However, methylbenzyl radicals have lower reactivity toward SO₂ than dimethylphenyl radicals. The reaction rate constants were found using transition state theory. The reactor simulations reveal that p‐xylene has lower reactivity toward SO₂ than o‐xylene, and CO, SO, and CHO are the main by‐products of oxidation.     

On the emitters of sulfuric acid sonoluminescence

A comparative study of the sonoluminescence spectra of water and argon-saturated aqueous H₂SO₄ solutions was carried out. At an H₂SO₄ concentration of 18 mol L⁻¹, the sulfuric acid sonoluminescence is fifty times more intense than water sonoluminescence. The sulfuric acid luminescence spectrum differs from the water sonoluminescence spectrum caused by the emission of excited water molecules and OH radicals from the gas phase of cavitation bubbles. The sulfuric acid sonoluminescence spectrum exhibits maxima at 330, 420, 500, and 630 nm. Emitters of sonoluminescence of sulfuric acid are the singlet (330–340 nm) and triplet (∼420 nm) excited SO₂ molecules formed by sonolysis of H₂SO₄ molecules. Another product of sonolysis of H₂SO₄, atomic oxygen, is assumed to be responsible for the luminescence at λ = 630 nm. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1742–1745, August, 2005.     

A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel‐cell electrodes, generally comes from S‐containing species such as hydrogen sulfide (H₂S). The S‐containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O₂ into gaseous SO₂. According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO₂ are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO₂ formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO₂ desorption at either room temperature or high temperatures.     

Decomposition of Gaseous Sulfide Compounds in Air by Pulsed Corona Discharge

The effectiveness of applying a pulsed corona discharge to the destruction of olfactory pollution in air was investigated. This paper presents a comparative study of the decomposition of three representative sulfide compounds in diluted concentrations: hydrogen sulfide (H₂S), dimethyl sulfide (DMS), and ethanethiol (C₂H₅SH), which could be completely removed when a sufficient but reasonable energy density was deposited in the gas. DMS showed the lowest energy cost (around 30 eV/molecules); C₂H₅SH and H₂S had an EC of respectively 45 eV and 115 eV. The efficiency of the non-thermal plasma process increased with decreasing the initial concentration of sulfide compounds, while the energy yield remained almost unchanged. SO₂ was the only identified byproduct of H₂S decomposition, but the sulfur balance suggests the formation of undetected SO₃. The byproducts analyzed during the degradation of DMS and C₂H₅SH enabled to propose a reaction mechanism, starting with radical attack and breaking of C–S bonds.     

Microbial reduction of sulfur dioxide with pretreated sewage sludge and elemental hydrogen as electron donors

It has been demonstrated that heat- and alkali-pretreated sewage sludge may serve as an electron donor and carbon source for SO₂ reduction byDesulfovibrio desulfuricans. A continuousD. desulfuricans culture was operated for 6 mo with complete reduction of SO₂ to H₂S. The culture required only minor amounts of mineral nutrients in addition to pretreated sewage sludge. It has also been shown that the sulfate-reducing bacteriumDesulfotomaculum orientis can be grown on H₂ as an energy source, CO₂ as a carbon source, and SO₂ as a terminal electron acceptor. Complete reduction of SO₂ to H₂S was observed.     

Adsorption of water on sulfur dioxide pre‐exposed Zircaloy‐4 surfaces

We investigate the interaction of water (H₂O) with sulfur dioxide (SO₂) pre‐exposed Zircaloy‐4 (Zry‐4) surfaces. Adsorption of SO₂ shifts the Zr(MNN) Auger electron feature by 3.0 eV, whereas subsequent water adsorption attenuates the sulfur Auger signal and results in the development of a zirconium oxide, Zr(MNV)_o, feature. No further shift in the Zr(MNN) transition is observed with increasing H₂O exposures. Following higher H₂O exposures on SO₂‐saturated Zry‐4 surfaces, linear heating results in water desorption near 500 K. This temperature is more than 200 K lower than the desorption temperature of water from clean Zry‐4 surfaces. Copyright © 2006 John Wiley & Sons, Ltd.     

Novel Copper(II) Thiodibenzoic Acid Coordination Polymers by in situ Extrusion of Sulfur from 2,2′‐Dithiodibenzoic Acid and the Unique Oxidation of Disulfide to Sulfate

Slow diffusion reaction of 2,2′‐dithiodibenzoic acid (dtdb) with CuCl₂ in the presence of N‐donor ligands results in the formation of different coordination polymers where both S–S and C–S scission and oxidation of S is observed. X‐ray diffraction analysis of [Cu(tdb)(phen)(H₂O)]₂ · 2H₂O.2DMF] ( 1 ), [Cu(tdb)(py)₂(H₂O)]₂ ( 3 ), and [Cu(tdb)(bipy)(H₂O)]₂ · 0.5H₂O ( 4 ) (tdb = thiodibenzoic acid, phen = phenanthroline, py = pyridine, bipy = 2,2′‐bipyridine) show that the metal ions are coordinated to the carboxylate oxygen atoms of the in situ generated tdb ligand in a monodenate fashion. In [Cu(phen)(SO₄)₂(H₂O)₂]_n ( 2 ) and [Cu(bipy)(SO₄)₂(H₂O)₂]_n ( 5 ), the sulfur is oxidized to sulfate ions prior to coordination with the metal. Complex 1 has a dimeric structure with π–π interactions between the phen ligands, whereas 3 and 4 form 1D polymeric chains.     

Fluorsulfonylmethylidene sulfur difluoride oxide,FSO2?CH  SF2  O

SO₃ adds across the CS double bond of H₂C  SF₄ with formation of 2-tetrafluoro-4-dioxo-1,2,4-oxadithietane, which rearranges to fluorsulfonylmethylidene sulfur difluoride oxide, F SO₂ CH  SF₂  O in the presence of CsF. © John Wiley & Sons, Inc.     

Dynamic mechanical behavior of acrylonitrile butadiene rubber/poly(ethylene‐co‐vinyl acetate) blends

The dynamic mechanical behavior of uncrosslinked (thermoplastic) and crosslinked (thermosetting) acrylonitrile butadiene rubber/poly(ethylene‐co‐vinyl acetate) (NBR/EVA) blends was studied with reference to the effect of blend ratio, crosslinking systems, frequency, and temperature. Different crosslinked systems were prepared using peroxide (DCP), sulfur, and mixed crosslink systems. The glass‐transition behavior of the blends was affected by the blend ratio, the nature of crosslinking, and frequency. sThe damping properties of the blends increased with NBR content. The variations in tan δ_max were in accordance with morphology changes in the blends. From tan δ values of peroxide‐cured NBR, EVA, and blends the crosslinking effect of DCP was more predominant in NBR. The morphology of the uncrosslinked blends was examined using scanning electron and optical microscopes. Cocontinuous morphology was observed between 40 and 60 wt % of NBR. The particle size distribution curve of the blends was also drawn. The Arrhenius relationship was used to calculate the activation energy for the glass transition of the blends, and it decreased with an increase in the NBR content. Various theoretical models were used to predict the modulus of the blends. From wide‐angle X‐ray scattering studies, the degree of crystallinity of the blends decreased with an increasing NBR content. The thermal behavior of the uncrosslinked and crosslinked systems of NBR/EVA blends was analyzed using a differential scanning calorimeter. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1556–1570, 2002