A study of curative interactions in cis-1,4-polyisoprene. X. The cis-1,4-polyisoprene/sulfur/zinc dimethyldithiocarbamate and cis-1,4-polyisoprene/sulfur/zinc dimethyldithiocarbamate/ZnO systems

Aspects of the mechanism of zinc dimethyldithiocarbamate (ZDMC)-accelerated sulfur vulcanization were discussed. The trends in the efficiency parameter E, confirmed that crosslinking is preceded by the formation of pendent groups RS_xSX [R = polyisoprenyl, X = Me₂NC(S)] in ZDMC-based systems. The index x in RS_xX was calculated as 5.82 in the cis-1,4-polyisoprene (IR)/sulfur/ZDMC/ZnO compound at the initial stages of curing, compared to 3.23 in the absence of ZnO. The high value of x supports the postulation that elemental sulfur and ZDMC react at the early stages of vulcanization, to form the active sulphurating agent XS_xSZnSSX. Crosslinks form by either a disproportionation reaction between two α-methylic or α-methylenic pendent groups RS_xX, or a reaction between a pendent group RS_xX and the rubber chain—these routes are the same as that suggested for the IR/tetramethylthiuram disulfide (TMTD)/ZnO compound. The beneficial role of ZnO and zinc stearate is shown, as in the case of ZnO in the IR/TMTD/ZnO system, to be related to their ability to trap dimethyldithiocarbamic acid, which formed in the generation of pendent groups and crosslinks. ZnS is inactive in this regard. The formation of ZnS is characteristic of natural rubber/sulfur/ZDMC/ZnO systems, as opposed to IR/TMTD/ZnO mixtures where little ZnS forms.     

Study of curative interactions in cis-1,4-polyisoprene. XII. The cis-1,4-polyisoprene–sulfur–tetramethylthiuram disulphide–ZnO–stearic acid vulcanization system

Several aspects on the mechanism of vulcanization in the synthetic cis-1,4-polyisoprene (IR)-sulfur-tetramethylthiuram disulphide (TMTD)–ZnO system were harmonized. The differential scanning calorimetry (DSC) thermograms showed that the vulcanization processes became better resolved on increasing the curative loading in the compound. Two major crosslinking reactions occurred consecutively in the IR (100)–sulfur (9.46)–TMTD (8.86)–ZnO (3.00) mixture, viz the IR–sulfur–TMTD–ZnO and IR–sulfur–zinc dimethyldithiocarbamate (ZDMC) (or IR–sulfur–ZDMC–ZnO) reactions. In the first process poly-and disulfidic pendent groups RS_xSX (R = polyisoprenyl, X = Me₂NC (S), x ≥ 1) formed via the IR–XSS_xSX reaction, and in the second via the IR–XSS_xZnSSX reaction. Thermogravimetric analysis (TGA) and high-pressure liquid chromatography (HPLC) data showed that dimethyldithiocarbamic acid liberated during the IR–sulfur–TMTD–ZnO reaction was trapped by ZnO to yield ZDMC. Hence ZDMC was a product, and not precursor, of this crosslinking process. A comparison of reactions in IR–sulfur–TMTD–ZnO and poly(ethylene-co-propylene)–sulfur–TMTD–ZnO mixtures showed that the participation of IR molecules was essential for ZDMC formation. The ZDMC concentration remained constant at ～ 38.4 mol % during the later stages of cure, showing that it did not participate in the desulfuration reactions of polysulfidic links. In the presence of stearic acid the stearic acid–ZnO reaction occurred at 87°C as was manifested by an intense crystallization peak of zinc stearate. The vulcanization processes were the same both in the presence and absence of stearic acid.     

Study of curative interactions in cis-1,4-polyisoprene (IR). V. General experimental procedure

The tetramethylthiuram disulfide (TMTD) and zinc dimethyldithiocarbamate (ZDMC) related vulcanization of cis-1,4-polyisoprene (IR) were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and electron spin resonance (ESR). The progress of the reactions in a given compound was monitored by analyzing the vulcanizate at selected points along the DSC curing curve. Full details of the analysis procedures are given. Analysis were mainly concerned with measurement of the crosslink densities, percentage of polysulfidic crosslinks, and the types and quantities of extractable compounds by thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC).     

Study of curative interactions in cis-1,4-polyisoprene. VI. Interaction of some combinations of sulfur,tetramethylthiuram disulfide,and ZnO

The interaction of curatives in the systems cis-1,4-polyisoprene (IR)–sulfur, IR–sulfur–ZnO, IR–tetramethylthiuram disulphide (TMTD), and IR–sulfur–TMTD were studied. Thermal events observed in the differential scanning calorimetry curing curves characteristic of these systems were explained in terms of the melting/liquefaction of compounds, the evaporation of gases, and the vulcanization process itself. The similarity of the IR–sulfur and IR–sulfur–ZnO curing curves suggested that sulfur and ZnO were unreactive during vulcanization. On heating the IR–TMTD and IR–sulfur–TMTD systems, gases such as Me₂NH and CS₂ formed easily. Although the maximum crosslink densities in the latter systems were low, the crosslink formation was found to be strongly exothermic. The sulfur efficiency parameter E was estimated for the IR–sulfur–TMTD system and decreased steeply from 37.5 (at 143.2°C) to 16.6 (at 151.0°C). This was taken as evidence that much of the bound sulfur was initially combined in pendent groups. Then E increased dramatically toward the advanced stages of cure, emphasizing the extraordinary inefficient manner in which sulfur was utilized to form crosslinks.     

Thiuram-accelerated sulfur vulcanization. I. The formation of the active sulfurating agent

Mixtures of tetramethylthiuram disulfide (TMTD)/sulfur/ZnO were heated in a DSC to various temperatures. Zinc dimethyldithiocarbamate (Zn₂(dmtc)₄ formed only in undried TMTD/ZnO mixes, the reaction being catalyzed by water on the ZnO surface. The presence of ZnO delays the decomposition of TMTD by adsorbing thiuram sulfenyl radicals, which are needed to initiate tetramethylthiuram monosulfide (TMTM) and tetramethylthiuram polysulfide (TMTP) formation. Increased amounts of TMTM are formed in mixes where ZnO is present, and TMTP are detected prior to TMTM formation. Zn₂(dmtc)₄ does not react with sulfur under conditions where labile hydrogen atoms are not available. © 1996 John Wiley & Sons, Inc.     

Study of curative interactions in cis-1,4-polyisoprene (IR). VII. The cis-1,4-polyisoprene–tetramethylthiuram disulfide–ZnO system

A detailed account of the mechanism of crosslinking in the cis-1,4-polyisoprene (IR)-tetramethylthluram disulfide (TMTD)–ZnO system is given. Many experimental observations were harmonized in terms of a radical mechanism, rather than an ionic mechanism. Electron spin resonance (ESR) spectra on the IR–TMTD–ZnO system, recorded at 120°C, inter alia revealed resonance lines in the vicinity of g = 2.02. These were related to the rapid formation of thiuram persulphenyl radicals XS, on the homolytic splitting of tetramethylthiuram polysulfides. The Moore–Trego efficiency E dropped from 11.5 (at 140.0°C) to 3.5 (at 146.9°C), indicating that a substantial part of the sulfur atoms was initially to be associated with pendent groups. The formation of these pendent groups could be viewed as an irreversible, concerted reaction without the formation of a true alkenyl radical intermediate. Crosslinks would form by either a disproportionation reaction between two α-methylic or α-methylenic pendent groups RS_xX or a reaction between a pendent groups RS_xX (R = polyisoprenyl, x ≥ 2, X = Me₂NC(S)) and the unsaturated polymer chain. The latter crosslink formation reactions were regarded as rate determining in the vulcanization sequence. A mechanism is proposed that does not require the participation of ZnO in the formation of the active sulfurating agent.     

The limiting value of ZDMC formation: new insight into the reaction of ZNO and TMTD

The reaction of ZnO and tetramethylthiuram disulfide (TMTD) was reinvestigated in detail. Under conditions where evaporation of volatiles is possible, TMTD and an excess of ZnO are found to produce bis(dimethyldithiocarbamato)zinc(II) (ZDMC) in limiting amounts close to 60 mol %, irrespective of the ratio between ZnO and TMTD. This result points to the operation of more than one route toward ZDMC. When ZnO and TMTD are reacted in closed vessels in inert atmosphere, a nucleophilic reaction of ZnO with TMTD was confirmed by GC–mass spectroscopy (MS) detection of COS and NMR observation of tetramethylthiourea (TMTU). This route is found to account for about 70 mol % of the total amount of ZDMC formed. A previously unrecognized redox reaction between ZnO, sulfur, and TMTD, furnishing ZnSO₄ and ZDMC, is responsible for approximately 15 mol % of the amount of ZDMC. Other products that were detected are CO₂, CS₂, and tetramethylurea, whereas ZnSO₃, ZnS, and dioxygen were absent. Based on the latter observation, the operation of a mechanism constituting radical reduction of water by TMTD, yielding dioxygen, was excluded. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1247–1257, 1999     

A DSC study of curative interactions. III. The interaction of TMTM with ZnO,sulfur, and stearic acid

The interaction among various combinations of sulfur, tetramethylthiuram monosulphide (TMTM), ZnO, and stearic acid were studied by differential scanning calorimetry in the absence of rubber. Sulfur and TMTM reacted to form tetramethylthiuram disulphide, and the ternary eutectic mixture melted at about 80°C. The absence of the S_γ → S_μ transition in sulfur/TMTM mixes was related to a sulfur/TMTM interaction, wherein the eightmembered sulfur rings were opened at temperatures well below 170°C. The interaction of stearic acid with TMTM led to the decomposition of TMTM, but the reaction was largely suppressed when both ZnO and sulfur were present in the mixture. No evidence was found for the formation of a zinc-accelerator complex of the type normally attributed a role in the accelerated sulfur vulcanization.     

Covulcanisation of natural rubber and chlorobutyl rubber. Effect of curative diffusion between phases and ZnCl2 addition

Abstract

Natural rubber (NR) and chlorobutyl rubber (CIIR) were compounded with various formulations containing tetramethylthiuram disulphide (TMTD), sulphur, and ZnO, and masterbatches of these compounds were blended in a 70 : 30 NR/CIIR ratio and vulcanised in a press at 150°C. Crosslink densities of vulcanisates were determined by swelling and tensile properties measured. In formulations in which the concentration gradient permitted the diffusion of TMTD and sulphur to a CIIR phase containing ZnO, tensile strengths were slightly better than 70% of the values of NR compounds of similar crosslink densities. In formulations in which TMTD and sulphur diffused to the faster curing NR phase, blend properties were also better than 70% of those of NR compounds, but at higher crosslink densities, tensile strengths and elongation at break decreased in parallel. This was attributed to failure in a layer of more highly crosslinked material formed within the NR phase close to the interface. Although ZnCl₂ can crosslink CIIR to NR, thus ensuring good interfacial bonding, the addition of ZnCl₂ to the CIIR masterbatch led to attack on the TMTD accelerator and a significant reduction in crosslink density and tensile properties of blends.     

Swellability of acrylate methacrylate-cellulose matrix systems and the effect on solute diffusion rates

During the vulcanization of cis-1,4-polyisoprene (IR) with thiruam-related curing systems, dimethyldithiocarbamic acid (DMDCA) is formed as a byproduct, in the formation of either pendent groups or crosslinks. DMDCA is unstable, and decomposes instantly to Me₂NH and CS₂ in the absence of ZnO. The facile reaction of Me₂NH and thiuram-related molecules such as tetramethylthiuram polysulfides, tetramethylthiuram disulfide (TMTD), tetramethylthiuram monosulfide and pendent groups caused (i) increased induction periods, (ii) lower maximum crosslink densities, and (iii) the excessive formation of tetramethylthiourea (TMTU). A most important function of ZnO was to trap the DMDCA via the formation of zinc dimethyldithiocarbamate and water, thereby preventing the detrimental reactions above. The IR/TMTD/ZnO and IR/sulfur/TMTD/ZnO systems were therefore characterized by (i) shorter induction periods, (ii) higher maximum crosslink densities, and (iii) the absence of TMTU.     

A DSC study of curative interactions. IV. The interaction of tetramethylthiuram disulfide with ZnO,sulfur, and stearic acid

The interaction of combinations of sulfur, tetramethylthiuram disulfide (TMTD), ZnO, and stearic acid were investigated by differential scanning calorimetry in the absence of rubber. TMTD decomposed partially to tetramethylthiuram monosulphide on liquefaction. Sulfur and TMTD reacted at vulcanization temperatures, and although the exact composition of all the products was not established, several features involving DSC and HPLC analysis were interpreted in terms of the formation of tetramethylthiuram polysulfides (TMTP). TMTD decomposed much faster to volatile products such as Me₂NH, CS₂, and CS when heated in the presence of stearic acid. Contrary to literature reports on the facile reaction of TMTD and ZnO to yield zinc perthiomercaptides (or zinc dimethyldithiocarbamate), the TMTD/ZnO reaction was found to be extremely sluggish under a variety of conditions. In the presence of sulfur, too, the TMTD/ZnO reaction was of negligible importance. It was inferred that several reactions occurred concurrently on heating a TMTD/stearic acid/ZnO system. These reactions were not observed for the sulfur/TMTD/stearic acid/ZnO mixture per se, but, instead, the stearic acid/ZnO reaction was very prominent. The formation of zinc stearate occurred at temperatures as low as 77°C in the quadruple system. TMTD and zinc stearate were virtually unreactive at vulcanization temperatures. None of the reactions involving ZnO could be attributed to the formation of a zinc perthiomercaptide, generally accepted to be a precursor in thiuram vulcanization.     

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.     

A study of the rate of formation of polysulfides of tetramethylthiuram disulfide

The rate of formation of tetramethylthiuram polysulfides (TMTP), that play an important role in vulcanization, was studied. After a short induction period (<30 s), tetramethylthiuram disulfide (TMTD) and TMTD-sulfur mixes, heated to 130–150°C in the absence of rubber, rapidly form a series of TMTPs. The concentrations of TMTPs of lower sulfur rank increase most rapidly, indicating that sulfur atoms are added to the accelerator sequentially. The incorporation of sulfur molecules to give TMTPs, which subsequently desulfurate, does not occur. Equilibrium concentrations of the various TMTPs are achieved in about 2 min. Little tetramethylthiourea is formed below 200°C. Tetramethylthiuram monosulfide (TMTM) is stable, but TMTM-sulfur mixes form TMTPs. A mechanism is proposed to account for the large amount of TMTM formed on heating TMTD in the absence of sulfur and the correspondingly higher TMTP concentrations in the presence of sulfur. © 1995 John Wiley Sons, Inc.     

Comparison of tetraethyl‐ and tetramethylthiuram disulfide vulcanization. I. Reactions in the absence of rubber

The reactions of tetraethylthiuram di‐ and monosulfide (TETD and TETM, respectively) were investigated in the absence of rubber and compared with those reported previously for tetramethylthiuram compounds. The reactions of TETM, TETD, and zinc diethyldithiocarbamic acid with zinc oxide and sulfur were investigated by differential scanning calorimetry, and the reaction products analyzed by high performance liquid chromatography. TETM was shown to be more stable at vulcanization temperatures (±150°C), but also less reactive with sulfur than tetramethylthiuram disulfide (TMTD). The reactions of TETD are very similar to those of TMTD, the TETD reacting slower than the TMTD to form analogous products. In the presence of zinc oxide, the formation of the zinc compound of TETD, zinc diethyldithiocarbamic acid, occurred readily. TMTD does not react readily with zinc oxide. The reaction of TETD with sulfur lead to the formation of polysulfidic accelerator species, although the concentrations formed in the absence of rubber were considerably less than that formed by the corresponding TMTD system. These differences in reactivity would affect the vulcanization reactions that take place in the rubber. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2292–2299, 2001     

Diimide reduction of cis-1,4-polyisoprene with p-toluenesulphonylhydrazide

The hydrogenation of cis-1,4-polyisoprene with diimide generated in situ from p-toluenesulphonylhydrazide (TSH), was investigated under various conditions. In aromatic solvents at 100–140°C, the rate of hydrogenation was increased with increase in concentration of polyisoprene and of TSH. Part of the polymer was depolymerized and cyclized during the reaction. Increasing the hydrogenation tended to decrease the rate of sulphur vulcanization, of the compounded rubber and the physical properties of vulcanizates were poor. The reaction of polyisoprene rubber with TSH, was also carried out in a solid state at 140°C for 20–60 min. It was found that by using a large amount of TSH hydrogenation and cyclization of rubber occurred. The quantity of TSH used as a blowing agent, for rubber in the manufacture of sponge rubber, i.e. 5–10 phr, did not cause hydrogenation.     

Vulcanization of Nitrile Rubber. Part I. Effect of Some Accelerators on the Physicomechanical Properties of Vulcanized Nitrile Rubber for Resistance to Fuel and General Use

Compounding mixtures were prepared based on acrylonitrile-butadiene rubber (Krynac 803) to which increasing parts per hundred rubber (pphr) of the following accelerates were added: tetramethyl thiuram disulfur (TMTD in series A₁), tetramethyl thiuram monosulfur (TMTM in series A₂), 2-mercaptobenzothiazole (MBT in series A₃), and 2,2′-dithiobisbenzothiazole (MBTS in series A₄). Effects of the quantity and type of accelerator on the vulcanization characteristics were studied. Physicomechanical investigations and swelling resistance were determined for samples immersed in benzene:benzol mixture at normal temperature. It was found that according to the type and quantity of accelerator, the optimum cure time decreased with increasing the quantity of accelerator. Relatively higher values of scorch time (8 min, at 1.0–3.0 pphr), which seems to ensure safe processing during production was obtained for TMTM (series A₂). TMTD (in quantity of 2.0 pphr) led to the highest value of rate of cure (14.0 min^?1). The best values of tensile strength (210 kg/cm², and 208 kg/cm²) were reached in TMTD (series A₁) and TMTM (series A₂), whereas MBT (series A₃) and MBTS (series A₄) resulted in tensile strength values of 140 and 155 kg/cm², respectively. The highest value of modulus (86 kg/cm²) was obtained in TMTD (series A₁ at 2.0 pphr). Whereas TMTD (series A₁) and TMTM (series A₂) led to relatively lower values of elongation at break (180% and 190%), it was found that MBT (series A₃) and MBTS (series A₄) led to higher values (280% and 260%). Small effects on the elasticity were observed when changing the type and quantity of accelerator. Relatively higher values of hardness were reached for series A₁ and A₂ as compared with series A₃ and A₄. The lowest values of percent volume increase after immersion in benzene:benzol mixture (8.3%) was obtained for A₁ (at 1.5 pphr of TMTD), as compared with the other three accelerators.     

Crosslinking of natural and polybutadiene rubbers with thiuram sulfur donors in the presence of a thiuram monosulfide

The influence of tetramethylthiuram monosulfide (TMTM) in the vulcanization process of natural rubber and butadiene rubber with dipentamethylene thiuram tetrasulfide (DPTT) was investigated. Vulcanization parameters as reaction rate and reversion varied when TMTM was added in NR‐DPTT formulations. Crosslinking density and proportion of monosulfidic crosslinks increased with TMTM content increase. However, a drop in physical properties was observed at high TMTM content and at long times of curing well over t₉₇. Formation of densely crosslinked areas due to polymerization reaction was postulated as the origin of these variations. In BR‐DPTT compounds, torque and monosulfidic crosslink proportions increased when TMTM was added. Polymerization reaction takes place in BR systems too. However, in BR vulcanizates, C—C bonds decreased with TMTM proportion, whereas in NR vulcanizates increased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1756–1761, 2004     

Effect of carboxylic acids on 2‐bisbenzothiazole‐2,2′‐disulfide‐ and tetramethylthiuram disulfide‐accelerated sulfur vulcanization. II. Vulcanization of polyisoprene in the absence of ZnO

Polyisoprene was vulcanized by 2‐bisbenzothiazole‐2,2′‐disulfide (MBTS)/sulfur and tetramethylthiuram disulfide (TMTD)/sulfur in the absence and presence of benzoic and stearic acids. It was found that the crosslink density of MBTS vulcanizates is halved by the addition of carboxylic acids and this can be explained in terms of the attack of the acids on the accelerator polysulfides. TMTD polysulfides are more reactive toward polyisoprene than are MBTS polysulfides, and their addition to the polymer chain occurs before significant attack by the carboxylic acids can reduce the polysulfide concentration. Consequently, the acids have little effect on the crosslink density of TMTD vulcanizates. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1007–1012, 1999     

Intermediate temperature electrical properties in 4 mol% ZnO-doped Zr0.92Y0.08O2-α/NaCl-KCl composite electrolyte

In this paper, 4?mol% ZnO-doped Zr_0.92Y_0.08O_2-α (8YSZ) and its 8YSZ+4ZnO/NaCl-KCl composite electrolyte were synthesized by a solid-state reaction. The X–ray diffraction (XRD) analysis indicates that 8YSZ+4ZnO and inorganic chlorides phases can coexist. The inorganic chlorides decrease the synthesis temperature of 8YSZ+4ZnO. The highest conductivities of 8YSZ+4ZnO and 8YSZ+4ZnO-NK are 7.0?×?10^?3 S?cm^?1 and 7.7?×?10^?2 S?cm^?1 at 700?°C, respectively. The oxygen concentration discharge cell shows that 8YSZ+4ZnO and 8YSZ+4ZnO-NK are good oxide ionic conductors under an oxygen-containing atmosphere. Finally, an H₂/O₂ fuel cell based on the 8YSZ+4ZnO-NK electrolyte reached the maximum power density (P_max) of 315.5?mW?cm^?2 at 700?°C.     

Effect of stearic acid concentration on the reversion behavior of epoxidized natural rubber

The effect of stearic acid concentration on the reversion behavior of epoxidized natural rubber (ENR 25 and ENR 50) was carried out in the temperature range of 150–180°C and 0.5–14.5 phr of stearic acid loading. Three common accelerators; namely, zinc dimethyldithiocarbamate (ZDMC), tetramethylthiuram disulfide (TMTD), and N-morpholinylbenzothiazole-2-sulfenamide (MBS) and conventional sulfur vulcanization system was used throughout the study. A Monsanto oscillating-disk rheometer was used to determine the reversion behavior of the rubber. Results indicate that for all the accelerators used, reversion decreases with increasing stearic acid concentration. The rate of decrease is more gradual up to about 6.5 phr of stearic acid, after which a rapid drop of reversion is observed for both ENR 25 and ENR 50. This observation is attributed to the increasing amount of mono- and disulfidic crosslinks as a result of desulfuration of polysulfidic crosslinks. The higher the stearic acid loading, the greater is the retardation effect on vulcanization; that is, more time is available for desulfuration. Increasing temperature would increase the reversion of the rubber vulcanizate because of the increase in thermal energy to decompose more crosslinks, including mono- and ether crosslinks for temperature higher than 160°C. ZDMC (an ultrafast accelerator) gives a higher reversion than MBS (a delay-action accelerator), because desulfuration occurs more slowly in the former system. The observed decrease in reversion as stearic acid concentration increases is technologically importance, because the aging property of the rubber vulcanizate is significantly improved. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1165–1169, 1999