High temperature vulcanization of elastomers: 2. Network structures in conventional sulphenamide-sulphur natural rubber vulcanizates

A natural rubber (NR) gum mix with a conventional N-cyclohexyl-2-benzothiazylsulphenamide (CBS) accelerated sulphur system (0.5:2.5 CBS/S) was vulcanized at temperatures from 140°C to 200°C. The influence of cure temperature on (a) the chemical crosslink density, (b) the distribution of crosslink types, (c) the extent of sulphidic main-chain modifications, and (d) the zinc sulphide formation was investigated. Results show that elevated cure temperatures produce a network with a lower crosslink density, in particular a lower polysulphidic crosslink density. The formation of intramolecular sulphidic groups and zinc sulphide increases with increasing temperatures. The possibility of chain scission during vulcanization was examined by a quantitative analysis of the sol-gel data. Less than 1 site of scission per 100 crosslinked isoprene units was established in the temperature range of 140–200°C. The network results can be satisfactorily correlated with the physical properties of a tyre tread mix of NR as reported in Part 1. Mechanistic interpretations are made to account for the network results.     

Effect of curing systems and temperatures on the activity of retarder in natural rubber mixes

The effect of vulcanization temperature (150° and 180°C) on the structure and technical properties of gum natural rubber vulcanizates with four different 2-(morpholinodithio)-benzothiazole (MDB) — sulphur ratios in presence of N-(cyclohexyl thio)-phthalimide (CTP, 0.5 phr) has been determined at the respective optimum cure times. The influence of curing system, temperature and retarder on (a) the chemical crosslink density, (b) the distribution of crosslink types, (c) the extent of sulphidic main chain modification and (d) the zinc sulphidic formation have been investigated. The network results have been correlated with the technical properties. Increasing the cure temperature by 30°C (from 150° to 180°C) does not alter the activity of CTP. It is effective in all curing systems, but effectiveness was maximum in the conventional system. At 0.5 phr level, CTP caused reduction in most technical properties.     

High temperature vulcanization of elastomers: 3. Network structure of efficiently vulcanized natural rubber mixes

The effect of vulcanization temperature (140–200°C) and time on the structures of pure gum natural rubber vulcanizates with two different N-cyclohexyl-2-benzothiazylsulphenamide (CBS): sulphur ratios (A, 3·5:1·5; B, 6·0:0·4 CBS/S) has been determined. Analyses of vulcanizates were carried out as reported in Part 2. Results show that both mixes are efficient in crosslinking, resulting in mainly monosulphidic crosslinks and relatively few modifications of the rubber chains. Raising the cure temperature from 140°C reduces the density of chemical crosslinks, particularly those of monosulphidic crosslinks, obtainable in the vulcanizates. This decrease in crosslink density has been shown to be irreversible with respect to cure temperature. The formation of intramolecular sulphidic groups and zinc sulphide increases with rising cure temperature, but this increase is small compared with that reported for the conventional CBS-accelerated system. The main difference between mixes A and B is that mix A yields a higher level of crosslinks and a major proportion of cyclic sulphides as main-chain modification. Negligible chain scission occurs during vulcanization at 140–200°C. These network results are interpreted mechanistically, and essential network features for obtaining good physical properties in high temperature vulcanizates are deduced.     

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

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

Analysis of the variation of molecular parameters of NR during vulcanization in the frame of the conformational tube model

The network structure of natural rubber (NR) achieved during vulcanization was analyzed using the model of rubber elasticity based on the tube concept, applied to the treatment of the stress-strain measurements. This theory allows the separation of constraint and crosslink contributions. Also, network parameters can be calculated. The crosslink level of the material was changed using different times and temperatures of cure. The change in the molecular parameters of the model with the degree of cure was estimated. The present research is useful for the precise determination of the chemical crosslink density of the NR network. The values were compared with those obtained by means of equilibrium volume swelling measurement. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1085–1092, 1997     

Effect of mineral filler sepiolite on sulphur-free thiuram vulcanization of natural rubber

The effect of sepiolite on the sulphurless vulcanization of NR by tetramethylthiuram disulphide, with and without activator (triethanolamine) was studied. The influence on: a) kinetic vulcanization parameters, b) crosslinking density, c) crosslinking types, d) efficiency factor E and e) zinc sulphide formation was considered. The vulcanization rate increases with increasing sepiolite level but crosslinking density is reduced. Based on these results a vulcanization mechanism is proposed and discussed.     

Kinetics of thiuram–accelerated sulfur vulcanization

On the basis of continuous measurements in a Vuremo curemeter at 145°C, kinetics of tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of natural rubber has been investigated. It was found that the cure rates increase with increasing TMTD concentration, the sulfur content being kept constant, up to a TMTD:S weight ratio of 2:1. Beyond this value, the cure rates again decrease. This TMTD:S ratio corresponds to 3.8 gram atoms of sulfur per mole TMTD, and it is in good agreement with findings that in TMTD-accelerated sulfur vulcanization systems the peak value of zinc dimethyldithiocarbamate (ZnDMDC) formation reaches an endvalue when the stocks contain 4 gram atoms of sulfur per mole TMTD. These facts lead us to suppose that ZnDMDC is the actual accelerator in TMTD-accelerated sulfur systems. Support for this view derives from our experiments with model curing systems as well as from the generally known practical experience that dithiocarbamates are faster accelerators than thiuram disulfides. For the reasons described above and for the finding that the dependences of the ultimate extent of vulcanization (network chain density) on the concentration of TMTD in the absence and in the presence of elemental sulfur are analogous, the mechanism of thiuram-accelerated sulfur vulcanization is very probably similar to that of sulfur-free thiuram vulcanization.     

Thiuram vulcanization of natural rubber in the presence of hydrofuramide and ethanolamines

Influence of hydrofuramide and ethanolamines on efficient vulcanization (EV), semi-EV, and conventional sulfur vulcanization accelerated by tetramethyl thiuram disulfide is reported. In the case of thiuram vulcanization, the amines increase the cure rate but reduce the crosslink density. The interaction of tetramethyl thiuram disulfide with amines has been studied. The amines interact with thiuram disulfide (TMTD) giving rise to the formation of gaseous products. Thermogravimetric analysis shows weight loss of about 50% in case of TMTD–ethanolamine system and 25% in case of TMTD–hydrofuramide system. Gas chromatographic studies of the gaseous products indicate that it consists mainly of H₂S. The other constituents in the gaseous product are CO₂ and a thiol. A possible reaction based on these observations to account for decrease in crosslink density in thiuram vulcanization caused by amines is reported.     

A new secondary accelerator for the sulfur vulcanization of natural rubber latex and its effect on the rheological properties

The vulcanization of natural rubber (NR) latex can be effectively carried out at low temperatures by using binary accelerator systems containing thiourea (TU) as a secondary accelerator. It was reported that sulfur‐containing nucleophiles such as thiourea enable the primary accelerator to become effective even at low temperatures, indicating a nucleophilic reaction mechanism in such vulcanization reactions. In the present study, a derivative of thiourea [viz. aminoiminomethyl thiourea (AMT)], which is more nucleophilic than thiourea, is used as a secondary accelerator in the sulfur vulcanization of NR latex. One of the aims of this study was to give conclusive evidence for a nucleophilic reaction mechanism. The synergistic effect of the above thiourea derivative with primary accelerators such as tetramethylthiuram disulfide (TMTD), zinc diethyldithiocarbamate (ZDC), and cyclohexylbenzthiazyl sulfenamide (CBS) was studied at two different temperatures (viz. 100 and 120°C). These binary systems were found to be very effective in reducing the optimum cure time of the different mixes compared to control formulations containing TU. The optimum amount of the secondary accelerator required was also determined. Mechanical properties such as tensile strength and tear strength of the vulcanizates were also evaluated. Chemical characterization of the vulcanizates was carried out by determining the total crosslink density. Values of the cure characteristics evaluated support a nucleophilic reaction mechanism in these vulcanization reactions under review. The effect of this secondary accelerator on the rheological behavior of compounded latex is also studied and was found not to affect adversely. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2781–2789, 2004     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. I. Comparison of sulfur and 2-mercaptobenzothiazole accelerated reactions

Polyisoprene compounds with sulfur and with sulfur and 2-mercaptobenzothiazole (MBT) were vulcanized by heating in a differential scanning calorimeter (DSC) at a programmed rate. The reaction was stopped at various temperatures along the thermogram and the product analyzed by determining the crosslink density and crosslink type, and by determining the amount of extractable curatives and soluble reaction products by high-performance liquid chromatography. DSC cure curves and plots of crosslink density and extractable sulfur vs. temperature for the unaccelerated and MBT accelerated compounds can be made to coincide by shifting them along the temperature axis. It is suggested that MBT accelerated sulfur vulcanization occurs by the same mechanism as unaccelerated sulfur vulcanization, with SH⁺ ions from MBT adding across the carbon–carbon double bond, thus increasing the rate at which the reaction is initiated. © 1995 John Wiley & Sons, Inc.     

A comparison of tetraethylthiuram and tetramethylthiuram disulfide vulcanization. II. Reactions in rubber

The reactions of tetraethylthiuram disulfide (TETD) with polyisoprene were investigated under vulcanization conditions. Samples of polyisoprene compounded with various combinations of TETD, sulfur, and ZnO were heated in a differential scanning calorimeter to various degrees of vulcanization. The crosslink density of the compounds was determined by swelling, and the extractable residual curatives and reaction products were analyzed with high‐performance liquid chromatography. TETD caused crosslinking to occur in the absence of added sulfur, as did tetramethylthiuram disulfide (TMTD), both sulfur donors. In the presence of sulfur, the formation of TETD polysulfides occurred immediately before the crosslinking reaction started. The TETD polysulfides were the initial crosslinking agents. The ready reaction between TETD and zinc oxide to form zinc diethyldithiocarbamic acid resulted in considerably higher crosslink densities. This greater reactivity between TETD and zinc oxide, compared with that between TMTD and zinc oxide, did not lead to any noticeable differences in the vulcanizate. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1119–1127, 2002     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. III. The reaction of 2-bisbenzothiazole-2,2′-disulfide with sulfur and ZnO in polyisoprene

The sulfur vulcanization of polyisoprene accelerated by 2-bisbenzothiazole-2,2′-disulfide (MBTS) was investigated. Rubber compounds were heated in a DSC and removed at various temperatures along the DSC thermal curve. The rubber vulcanizate was analyzed for crosslink density and for residual reactants and extractable reaction products. MBTS reacts readily with sulfur, and the polysulfidic accelerator complexes react with the rubber chain to form pendent groups. Crosslinking results from hydrogen abstraction, by the benzothiazole pendent group, from a neighboring chain. 2-Mercaptobenzothiazole, a product of crosslinking, also acts as an accelerator in the later stages of the reaction. MBTS has been shown not to react with ZnO and the higher crosslink densities obtained when ZnO is present are attributed to ZnO aiding the abstraction of the benzothiazole pendent group to give zinc mercaptobenzothiazole. A mechanism for the MBTS acceleration of sulfur vulcanization is proposed. © 1996 John Wiley & Sons, Inc.     

Effect of aerosil on the course of thiuram accelerated sulfur vulcanization

Tetramethylthiuram disulfide (TMTD)-accelerated sulfur vulcanization of natural rubber has been investigated at temperatures from 100°C to 145°C. Continuous measurements in a Vuremo curemeter were used to estimate the extent of crosslinking, which was plotted against cure time. The cure curves as well as their linearized forms (dependences of the logarithm of the extent of vulcanization on the cure time) clearly show that at lower cure temperatures the course of the vulcanization differs significantly from the first-order rate law. These digressions have been removed by the addition of a highly dispersed silica gel, Aerosil, which simultaneously speeds up the course of the vulcanization up to the value corresponding to the rate of zinc dimethyldithiocarbamate (ZnDMDC)-accelerated sulfur vulcanization. These results are in accordance with our recent theory supposing that ZnDMDC is the actual accelerator in TMTD-accelerated sulfur systems. In the presence of Aerosil, the formation of ZnDMDC from TMTD is catalyzed via dispersed silica gel. Support for this view derives from the temperature dependences of vulcanization reactions. The activation energies of TMTD-accelerated sulfur vulcanizations in the absence (31 kcal/mole) and in the presence of Aerosil (23.5 kcal/mole) correspond exactly to the values calculated from the rate constants of the thiuram decrease in TMTD-accelerated vulcanization (30 kcal/mole) and from the rate constants of crosslinking in the dithiocarbamate-accelerated sulfur vulcanization (23 kcal/mole), respectively.     

橡胶微观结构对NBR硫化加工特性的影响

研究了丁腈橡胶(NBR)微观结构对NBR及NBR/炭黑混炼胶硫化加工性能的影响.研究表明:丙烯腈含量增加,分子极性增加,物理交联密度增加,致使焦烧时间缩短,正硫化时间先增长后缩短,提高了胶料的加工性能,硫化速度先增加后减小.加入炭黑后,对低丙烯腈含量的NBR1846的硫化转矩影响最大,减小了因丙烯腈含量造成的流变性能的变化,缩短了硫化时间,提高了硫化转矩和硫化程度,但炭黑对NBR的硫化速度影响不明显.     

Kinetics of accelerated vulcanization. Sulfur vulcanization accelerated with 2-mercaptobenzothiazole and its zinc salt in presence of zinc oxide and zinc oxide and stearic acid

Sulfur vulcanization of natural rubber (NR) and styrene-butadiene rubber (SBR) accelerated with 2-mercaptobenzothiazole and zinc oxide with or without stearic acid has been studied and the results compared with those obtained by replacing the thiazole with its zinc salt. The order of the rate of vulcanization as measured from free sulfur decrease or crosslink formation is observed to be unity with respect to time and less than one with respect to accelerator concentration. In absence of stearic acid, the zinc salt is much less efficient than the thiazole in promoting vulcanization, and the efficiency of the latter shows a noticeable improvement over what is attained in absence of zinc oxide. Addition of stearic acid not only raises the efficiency of both the systems to the same level but also leads to a faster rate of vulcanization, a much higher degree of crosslinking, avoidance of reversion, and production of vulcanizates with considerably improved mechanical properties. The formation of zinc sulfide also reveals interesting variation. Attempts have been made to interprete these results in terms of complex formation between zinc salt of thiazole and zinc stearate.     

A comparison of the tetramethylthiuram disulphide accelerated sulfur vulcanization of polybutadiene and polyisoprene

A detailed study of the vulcanization of polybutadiene with various combinations of sulphur, tetramethylthiuram disuphide, ZnO, stearic acid, and zinc dimethyldithiocarbamate is reported. Vulcanization was conducted by heating samples at 2.5°C/min in a differential scanning calorimeter. The reaction was stopped at various points along the vulcanization exotherm, the soluble reaction products and residual curatives were extracted and analyzed, and the crosslink density and percentage polysulphidic crosslinks were determined. The overall reaction mechanism was found to be similar to that for polyisoprene, but the reaction, once initiated, was faster than in polyisoprene, the crosslink density was higher, and the percentage polysulphidic crosslinks was lower. These differences are discussed. © 1993 John Wiley & Sons, Inc.     

Network evolution based on general-purpose diene rubbers/sulfur/TBBS system during vulcanization (I)

In order to investigate the rule of network evolvement during vulcanization of diene rubber, generally used diene rubbers, including natural rubber (NR), polybutadiene rubber (BR), and styrene butadiene rubber (SBR), were cured with the same curing agents but at different time. The cured rubbers were then analyzed with Rubber Process Analyzer (RPA2000), dissolution/swell, and Magnetic Resonance Crosslink Density Spectrometer (MR-CDS 3500). Through data analysis on torques obtained from RPA2000, gel contents from dissolution/swell experiment and crosslink densities obtained from MR-CDS 3500, a new concept about the formation of rubber network during induction period of vulcanization was proposed. The experiments showed that even in scorch delay period, crosslink density, gel content and torque of the three rubbers increased as curing time went. The increase of these parameters indicated the formation of primary crosslink among several macromolecules which could be called local primary network-the first stage of network development. After this stage, both crosslink density and gel content of these three rubbers increased abruptly. The change percent of torque, crosslink density and gel content increased from 5% to 15%, 13%-33% and 2.5%-20% respectively which meant that almost one third of the vulcanization had been carried out during this period. Corresponding photographs of dissolution/swell experiments showed that whole gels which only swell but didn't solve in their solvents were formed. So the mutation point can be taken as the second stage of network development - a fundamental network had been formed. For NR, BR and SBR, the critical average change percent for crosslink density, gel content and torque were 33%, 20% and 15% respectively. BR, NR and SBR formed local primary network during induction period and a continuous fundamental network at the beginning of crosslinking period. This is different from traditional vulcanization theory.     

Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. IV. The reaction of polyisoprene with N-cyclohexylbenzothiazole sulfenamide,sulfur, and zinc oxide

Polyisoprene was vulcanized with N-cyclohexylbenzothiazole sulfenamide (CBS), sulfur, and zinc oxide by heating in a Differential Scanning Calorimeter (DSC) at a programmed rate to given temperatures. The reaction was quenched and the product analyzed. Soluble curatives and reaction intermediates were analyzed by high-performance liquid chomatography (HPLC) and the crosslink density of the network determined by swelling. The delayed action of the CBS accelerator is explained in terms of an exchange reaction between benzothiazole terminated polysulfidic pendent groups on the polymer chain and CBS to yield unreactive amine terminated pendent groups and 2-bisbenzothiazole-2,2′-disulfide (MBTS). MBTS reacts with sulfur to form 2-bisbenzothiazole-2,2′-polysulfides (MBTPs), which also form pendent groups. Crosslinking does not commence until all of the CBS has been consumed and pendent groups are no longer deactivated. 2-Mercaptobenzothiazole (MBT) is released only on crosslinking. When MBT is present in the formulation at the outset of the reaction it traps cyclohexylamine released when CBS adds to the chain as a pendent group. The MBT-amine salt participates in a reaction that regenerates MBTS, which is, thus, not consumed in the vulcanization process. ZnO does not react with CBS, and its role in increasing the crosslink density is attributed to its promoting crosslinking reactions between pendent groups and neighboring chains rather than intramolecular reactions, which lead to cyclization. © 1996 John Wiley & Sons, Inc.     

Vulcanization of carboxylated nitrile rubber (XNBR) by zinc peroxide

The vulcanization of carboxylated nitrile rubber (XNBR) with zinc peroxide, which produces ionic crosslinks, has been studied in relation to vulcanization time. Vulcanized compounds present two transitions, corresponding to the glass transition of the polymer at low temperatures and the ionic transition resulting from the formation of ionic aggregates. Both transitions are displaced to higher temperatures with increasing crosslink density. The ionic associations which give rise to high values of mechanical properties disappear on exposure of the vulcanized compounds to saturated ammonia vapour. This treatment produces a decreased crosslink density resulting in the disappearance of the ionic transition. When the action of ammonia is terminated by immersion in solvent followed by drying, the original crosslink density is recovered and the ionic transition reappears, although at higher temperatures. However, with increasing crosslink density, the difference between the temperatures at which both transitions take place diminishes. All these factors can be interpreted as reflecting the generation of a new and more compatible arrangement of the newly-appearing ionic clusters. © 1999 Society of Chemical Industry     

Effect of water and hydrogen sulfide in zinc dimethyldithiocarbamate accelerated sulfur vulcanization

Rubber and the model compound 2,3‐dimethyl‐2‐butene (TME) are vulcanized with zinc dimethyldithiocarbamate [Zn₂(dmtc)₄] accelerated sulfur formulations. When heated in dry nitrogen, Zn₂(dmtc)₄ is stable at vulcanization temperatures. However, it shows a mass increase when heated in moist nitrogen, indicating strong coordination with water; in a nitrogen/H₂S atmosphere rapid degradation to dimethyldithiocarbamic acid (Hdmtc) and ZnS occurs. Model compound studies show that crosslinked sulfides are essentially bis(alkenyl) and confirm the absence of accelerator terminated pendant groups in the vulcanizates, while the ease with which rubber vulcanizates crystallize on cooling in a density column also suggests that pendant groups are largely absent. However, the rates of crystallization, measured as the time for the crystallization process to go to 50% completion, are slower in lightly crosslinked gels than in peroxide cures of similar crosslink density, particularly in the vulcanizates cured in a vacuum; this is interpreted as an indication that some residual pendant groups are present in Zn₂(dmtc)₄ vulcanizates. Water promotes the rate of crosslink formation in both rubber and TME systems, and it is suggested that the strong coordination of water with zinc in Zn₂(dmtc)₄ promotes its reactivity. The H₂S liberated in the vulcanization process promotes decomposition of Zn₂(dmtc)₄ to Hdmtc, and this reaction makes an important contribution to the amount of Hdmtc that is formed in situ. The importance of Hdmtc as an accelerator and its role in providing alternative routes to crosslink formation in Zn₂(dmtc)₄ accelerated sulfur vulcanization are discussed. It is suggested that water, which is liberated when Hdmtc reacts with ZnO to form Zn₂(dmtc)₄, activates newly formed Zn₂(dmtc)₄ molecules; and this accounts for the beneficial influence of ZnO in Zn₂(dmtc)₄ formulations. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1516–1531, 2002