用动态剪切模量估算NR与SBR硫化胶的化学交联密度

用美国Ｍｏｎｓａｎｔｏ公司的橡胶加工过程分析仪测定ＮＲ和ＳＢＲ硫化胶或混炼胶料的动态剪切模量（Ｇ＇），进而根据Ｇ＿Ｎ￣０＝ｇ×２Ｘ＿０ＲＴ和Ｇｅ＝ｇ＇×２Ｘ＿（ｐｈ）ＲＴ估算其硫化胶的化学交联密度（Ｘ＿ｃｈ），结果发现：ＮＲ／过氧化二异丙苯体系硫化胶和混炼胶料的Ｇ＇是不同的，但用Ｇ＇估算的Ｘ＿ｃｈ与用化学分析法测定的结果基本相近；对ＳＢＲ／促进剂ＴＭＴＤ体系，用这种方法估算的Ｘ＿（ｃｈ）接近用应力－应变法和ＮＭＲ法的估算值；对ＮＲ／促进剂ＴＭＴＭ体系，用Ｇ＇估算的Ｘ＿（ｃｈ）与用溶胀法获得的数据基本接近。用Ｇ＇估算Ｘ＿（ｃｈ）法，十分简单、方便。     

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.     

Sorption and diffusion of acrylonitrile monomer through crosslinked nitrile rubber

As the first step for the preparation of interpenetrating polymer networks from nitrile rubber and polyacrylonitrile, the sorption and diffusion of acrylonitrile monomer through nitrile rubber crosslinked by three different vulcanization systems—conventional (CV), dicumyl peroxide (DCP), and a mixture consisting of sulfur and peroxide (mixed)—were studied in the temperature interval of 30–70°C. Kinetic curves have been generated for these systems to compute diffusion and sorption coefficients. The equilibrium sorption is found to be maximum for the CV system. The molar mass between crosslinks (M_c) has estimated and compared with affine and phantom models. It was found that the M_c values follow the affine model. The diffusion coefficient values are highest for DCP and lowest for CV. It was observed that the kinetics of liquid sorption followed an anomalous behavior. The temperature dependence of the transport parameters was followed by an Arrhenius relationship, from which the activation energy for diffusion, permeation, and sorption were calculated. It is found that temperature activates diffusion in all cases. The polymer–solvent interaction parameter was determined. The amount of polysulfidic linkages in the rubber network was also estimated. The experimental results were compared with theoretical predictions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 941–952, 2000     

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.     

Differential thermal analysis using high frequency dielectric heating II. Curing kinetics studies

A previous paper described the theory and equipment of Differential Thermal Analysis using High Frequency Dielectric Heating, DTA/HF. Curing studies using DTA/HF on cis-1,4-polyisoprene rubber filled with 50 phr of CaCO₃ and containing from 1.5 to 4.5 phr of dicumyl peroxide (DCP) curative indicated that below about 185°C, the curing reaction was first order with respect to DCP; the mean activation energy was 32.6 kcal/mole; and the heat of reaction ranged from 60 to 80 kcal/mole DCP. These results are in agreement with published results. Above 185°C both thermodynamic and kinetic evidence implied that the reaction mechanism changed; the first order peroxide decomposition no longer controlled the overall rate of reaction. The curing of both stereo styrene-butadiene rubber (SBR) and cis-1,4-polybutadiene rubber (BR) containing 50 phr of CaCO₃ and 0.5 phr of DCP was found to follow first order kinetics as expected. The magnitude of the heat of curing was used to determine the frequency of a vinyl propagation-type cross-linking reaction. Eight to 36 crosslinks per molecule of DCP were obtained. This reaction became more extensive at high temperature, in agreement with published results.     

Effect of elemental sulfur in the crosslinking reaction of poly(vinyl chloride) with tetramethyl thiuram disulfide

The effect of elemental sulfur (S₈) and tetramethyl thiuram disulfide (TMTD) on the chemical crosslinking of PVC is studied. The tensile strength, elongation at break, and gel content of crosslinked and uncrosslinked PVC molded samples, cured at different times in hot air, are evaluated. It was found that crosslinking PVC with TMTD improves its mechanical properties, and adding S₈ produces a slight improvement. Also, the S₈ presence accelerated the PVC/TMTD crosslinking reaction when the crosslinking reaction was carried out under shear forces, but not when the curing was done in hot air, where the S₈ retards the process.     

A new binary accelerator system for the sulfur vulcanization of natural rubber latex

Vulcanization of latex products are usually carried out at lower temperatures compared to dry rubber products. It has been suggested that, in latex vulcanization systems where thiourea is used as a secondary accelerator, it acts as a nucleophilic reagent facilitating the cleavage of the sulfur bonds in the primary accelerator like TMTD or CBS at lower temperature. But no conclusive proof is given to such a postulate. In the present study 1-phenyl-2,4-thiobiuret (DTB II) and 1,5-diphenyl-2,4-dithiobiuret (DTB III), which are more nucleophilic than thiourea and which vary in their nucleophilic reactivity, were studied as secondary accelerators along with tetramethyl thiuram disulphide (TMTD) and N-cyclohexylbenzothiazyl sulphenamide (CBS) in the vulcanization of natural rubber latex. These binary systems were found to be very effective in reducing the optimum vulcanization time. Also it was noted that 1-phenyl-2,4-dithiobiuret, which is more nucleophilic, is more reactive (as observed from the reduction in optimum cure time) as a secondary accelerator, indicating a nucleophilic reaction mechanism in the vulcanization reactions under review. The optimum dosages of the secondary accelerators required were derived. Physical properties like tensile strength, 300% modulus, and elongation at break of the latex vulcanizates were also studied. There is a definite advantage with respect to many of these properties for dithiobiuret systems compared to the systems containing TMTD alone or TMTD/thiourea. DTB III gives higher values in many of these physical properties than DTB II. Chemical characterization of the vulcanizates was also carried out to correlate the physical properties with the type of chemical crosslinks formed. © 1993 John Wiley & Sons, Inc.     

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.     

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.     

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.     

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.     

Sulfur vulcanization of natural rubber accelerated with 2-mercaptobenzothiazole plus tetramethylthiuram disulfide

The sulfur vulcanization of NR by a MBT-TMTD mixed accelerator system both in the presence and absence of ZnO and stearic acid with or without DCP has been studied. The rate of decomposition of DCP in the presence of both MBT and TMTD is quite similar to that in the MBT system alone. The reduction in crosslinking due to DCP is dependent mainly on MBT. The decomposition product of TMTD may contribute to it only at the later stage of vulcanization. Though TMTD has no influence on the decomposition rate, it reacts with MBT at least in the initial stage of vulcanization and suppresses the retardation caused by MBT on DCP vulcanization in accordance with the free sulfur decrease, the nature of crosslinking formation both in the presence and absence of DCP, and the methyl iodide treatment of the vulcanizates. The vulcanization process of the MBT-TMTD-S-NR system has been interpreted in terms of both free radical and polar mechanisms. The domination of either of these depends on the dominant amount of either MBT or TMTD in the acclerator ratio. According to the initial high rate of crosslink formation, free sulfur decrease and also the initial additiveness of crosslinking in stocks containing DCP, the vulcanization process of MBT-TMTD-ZnO-St. acid-S-NR system has been explained in terms of an ionic mechanism. The pronounced synergistic nature of such systems has been interpreted by the enhanced activation of MBT-S-ZnO-St. acid complex due to the dithiodicarbamate ion formed in the initial stages of vulcanization, and also by the activation of TMTD accelerated vulcanization due to the mercaptobenzothiazylion.     

A study of curative interactions in cis-1,4-polyisoprene. VIII. Network maturing reactions in the cis-1,4-polyisoprene/tetramethylthiuram disulfide/ZnO system

Several aspects concerning the network maturing reactions of the cis-1,4-polyisoprene (IR)/tetramethylthiuram disulfide (TMTD)/ZnO curing system, were evaluated with reference to previously published literature. The crosslink density increased progressively from 1.42 × 10^?5 mol cm^?3 rubber at 140.0°C, to 8.30 × 10^?5 mol cm^?3 at the higher temperature of 190.0°C, which tied in with the fact that natural rubber (NR) or IR/TMTD/ZnO systems show negligible reversion provided sufficient ZnO is present. The increase in the crosslink density value was accompanied by a systematic buildup of monosulfidic crosslinks, and a substantial decrease in the concentration of tetramethylthiuram monosulfide (TMTM). Calculations showed, in addition, that the increase in the concentration of zinc dimethyldithiocarbamate (ZDMC) and the decrease in the TMTM concentration, were interdependent. It is shown that TMTM, rather than ZDMC, was involved in the crosslink shortening reactions.     

Comparative study among TMTD,TBzTD, and ZBEC accelerators in isobutylene-isoprene elastomer vulcanization

Accelerators such as tetramethylthiuram disulfide (TMTD) are responsible for releasing nitrosamines, considered carcinogenic by international organizations. Tetrabenzylthiuram disulfide (TBzTD) and zinc dibenzyldithiocarbamate (ZBEC) are indicated as substituents of TMTD for not releasing noxious nitrosamines. Thus, the objective of this work was to compare the use of TMTD, TBzTD, and ZBEC accelerators in the production of isobutylene-isoprene rubber compounds. It was evaluated the effect of TBzTD/mercaptobenzothiazole and ZBEC/N-cyclohexyl-2-benzothiazolesulfenamide pairs in the rheological properties and the first pair achieved better results. Subsequently, it was evaluated compounds containing TBzTD and ZBEC accelerators separately. The experimental design data of the obtained maximum torque were correlated by an interpolation method (thin-plate spline interpolation method). Physical–mechanical and thermodynamic measurements showed a decrease in the mechanical properties of TBzTD and ZBEC compounds. Thermogravimetric analysis did not indicate thermal differences between the compounds, Fourier transform infrared and swelling degree analyses corroborated the difference found in the crosslink densities of the compounds. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48965.     

Partial replacement of NR by GTR in thermoplastic elastomer based on LLDPE/NR through using reactive blending: Its effects on morphology,rheological, and mechanical properties

Attempts have been made to use different amount of ground tire rubber (GTR) powder as a partial substitute for natural rubber (NR) in thermoplastic elastomer based on linear low‐density polyethylene (LLDPE, 60 wt%) and NR (40wt%). Maleic anhydride (MA) and dicumyl peroxide (DCP) were used, during melt mixing of the compound, to modify GTR and vulcanize the rubber phases of the blends. Morphology of the blends was studied by scanning electron microscopy and rheological behavior investigated through rheomechanical spectroscopy. Mechanical properties of the blends were also measured, and the effect of GTR concentration on properties was evaluated. Obtained results showed that modification of GTR with MA and using DCP in the blends containing GTR improves the bonding between GTR and matrix. This leads to a distinctive rheological behavior and enhances tensile strength and elongation at break compared to its corresponding simple blend. It can be said that using of MA and DCP during melt mixing of thermoplastic elastomers based on LLDPE/NR containing GTR, concludes to a better dispersion of GTR and formation of morphology similar to that of a dynamic vulcanized thermoplastic elastomer, which improves interfacial bonding between phases and causes a dramatically increase in mechanical properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of plasticizer and curing system on freezing resistance of rubbers

At glass transition temperature, T_g the rubber compound becomes stiff and brittle and it loses all its rubbery characteristics. This article deals with the changes in T_g of rubber blends based on natural rubber and polybutadiene rubber of varying vinyl content having different types and content of plasticizers, different curing systems and its effect on physico‐mechanical properties to improve its freezing resistance. The plasticizers used were dioctylphthalate (DOP), tricrecylphosphate (TCP), dioctyladipate (DOA), and oil type plasticizers like parafinic oil (P#2) and aromatic oil (A#2). Among the plasticizers, when DOP and DOA content was high, an appreciable decrease of T_g was found compared to TCP. Moreover, there was a remarkable decrease of T_g using DOA plasticizer, which shows more effective on freezing resistance. However, there was not much change in T_g with oil‐type plasticizers with high oil content compared to TCP plasticizer. The effect of cross‐linking systems such as conventional sulfur vulcanization (CV), efficient sulfur vulcanization (EV), and dicumyl peroxide (DCP) and rubber blends with varying vinyl content in polybutadiene rubber were also carried out. It was found that T_g in different cross‐linking system decreased in this order: CV < EV < DCP. It reveals that DCP cross‐linking system affect more for improving freezing resistance. Physico‐mechanical properties such as tensile strength, tear strength, hardness were also measured. The ratio of initial slope (M₀) to steady‐state slope (M₁), M₀/M₁ in tensile curves of different blends were verified, which in turn related to the physico‐mechanical properties and freezing resistance of rubber compounds. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39795.     

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.     

Studies on sulfur vulcanization of natural rubber accelerated with diphenylguanidine both in presence and absence of dicumyl peroxide

Diphenylguanidine acceleration of sulfuration of natural rubber both in the presence and absence of dicumyl peroxide (DCP) has been studied. DPG enhances the rate of decomposition of DCP, and the crosslinking maxima due to DCP are lowered by DPG to some extent, which increases with the concentration up to a limiting value. The rate of sulfur decrease is very high and is dependent on DPG concentration but practically independent of the presence of ZnO and stearic acid. It is observed that with constant level of DCP and sulfur, crosslinking increases with the increase of DPG concentration and finally becomes additive when the molar proportion of DPG: S₈ approximately equals 1:1. In presence of ZnO and stearic acid, with each increment of DCP concentration crosslinking is retarded for both systems. Mechanisms have been suggested in terms of polar and radical process.     

交联硫化体系对PP／POE／EPDM热塑性弹性体性能的影响

采用动态硫化法制备聚丙烯／聚烯烃弹性体／三元乙丙橡胶（PP/POE／EPDM）共混型热塑性弹性体,研究了交联前后不同POE／EPDM并用对比体系力学性能的影响。采用交联剂过氧化二异丙苯（DCP）及DCP／S硫化体系对PP／POE／EPDM体系进行硫化,研究了力学性能的变化。结果表明,EPDM可有效降低材料的硬度和断裂永久变形。助交联剂硫黄（S）对PP／POE／EPDM体系有较好的硫化作用,固定DCP用量为3份,S用量为0．4份时,体系力学性能最佳,交联对体系硬度影响很小。