Effect of Substituted Dithiobiurets—Tetramethyl Thiuram Disulfide (TMTD) Binary Accelerator Systems in the Vulcanization of NR-SBR Blends

1-Phenyl 2,4-dithiobiuret and 1,5-diphenyl 2,4-dithiobiuret were studied as secondary accelerators along with tetramethyl thiuram disulfide (TMTD) in the vulcanization of a blend of natural rubber (NR) and styrene-butadiene rubber (SBR). These binary systems were found to be very effective and reduced the optimum vulcanization time considerably. 1-Phenyl 2,4-dithiobiuret, which is more nucleophilic than 1,5-diphenyl 2,4-dithiobiuret, reduced the vulcanization time more, indicating a nucleophilic reaction mechanism in the vulcanization reactions under review. In both cases the optimum dosage of the secondary accelerator was derived. Physical properties such as modulus, tensile strength, elongation at break, hardness, compression set, heat buildup, resilience, etc., of the vulcanizates were studied before and after heat aging and compared with a reference mix. There is substantial increase in many of these properties compared with TMTD alone or with the reference mix. Chemical characterization of the vulcanizates was also carried out to correlate the physical properties with the type of chemical crosslinks formed in these systems.     

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     

New binary systems containing TMTD‐amidino phenyl thiourea and CBS–amidinophenyl thiourea for the vulcanization and rheological behavior of natural rubber latex

The use of accelerators in rubber latex is basically different from their use in dry rubber. In the present study, N amidino N'phenyl thiourea (APT) which is more nucleophilic than thiourea was studied as a secondary accelerator along with tetra methyl 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 vulcanization time. The optimum dosages for this non‐toxic secondary accelerator required were derived. Rheological studies of the compounded latex show that the introduction of APT in these systems does not have adverse effect in processing in comparison with the thiourea systems. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Studies on novel binary accelerator system in sulfur vulcanization of natural rubber

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Thiourea and its derivatives are important secondary accelerators in this context. It is suggested that thiourea containing binary systems of rubber vulcanization proceed by a nucleophilic reaction mechanism. Amidinothioureas (ATUs), which are derivatives of thiourea, have been investigated extensively as secondary accelerators. One of the aims of this study was to get further proof with regard to the theory of the nucleophilic reaction mechanism in such binary systems. In the present study phenyl substituted ATU was used as a secondary accelerator along with mercaptobenzothizyl disulfide, tetramethyl thiuram disulfide, or cyclohexyl benzthiazyl sulfenamide in the sulfur vulcanization of natural rubber. The results showed an appreciable reduction in the cure time for the mixes containing the ATU compared to the reference mixes. These results are indicative of a nucleophilic reaction mechanism in the vulcanization reaction under review. These vulcanizates also showed comparatively better tensile properties and good retention of these properties after aging. The optimum dosages of the secondary accelerator required for these vulcanization reactions were also derived. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1–8, 2001     

Studies on novel binary accelerator system in sulfur vulcanization of natural rubber

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Binary accelerator systems are being widely used in industry and are becoming increasingly popular because of the fact that such mixed systems can effectively prevent prevulcanization, permit the vulcanization to be carried out at a lower temperature in a shorter time, and produce a vulcanizate with superior mechanical properties compared to those of a stock cured with a single accelerator. Thiourea and its derivatives are important secondary accelerators in this context. It is suggested that thiourea containing binary accelerator systems cause rubber vulcanization to proceed by a nucleophilic reaction mechanism. In the present study 1‐phenyl‐5‐ortho, ‐meta, and ‐para‐tolyl derivatives of 2,4‐dithiobiurets, which are more nucleophilic than thiourea and vary in their nucleophilic reactivity, are used as secondary accelerators along with 2‐morpholinothiobenzothiazole in the vulcanization of natural rubber. The results show an appreciable reduction in the cure time for the mixes containing the dithiobiurets compared to the reference mix. These results are indicative of a nucleophilic reaction mechanism in the vulcanization reaction under consideration. These vulcanizates also demonstrate comparatively better tensile properties and good retention of these properties after aging. An attempt is also made to correlate the variation in physical properties to chemical crosslink formation in the various vulcanizates. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3173–3182, 2003     

Effect of fillers in the binary systems containing TMTD–amidinothiourea and MBTS–amidinothiourea in NR vulcanization

In earlier studies,^1,2 we have shown that in MBTS–amidinothiourea and TMTD–amidinothiourea binary system of rubber vulcanization, amidinothiourea functions effectively as a secondary accelerator and improves the accelerator activity of these systems. These secondary accelerators enhance the physical properties of the vulcanizates and show better cure characteristics of the mixes compared to the reference systems studied. In this paper, we present the results of our study on the effect of various fillers in the sulfur vulcanization of natural rubber using amidinothiourea as the secondary accelerator. We have taken MBTS–amidinothiourea and TMTD–amidinothiourea binary accelerator systems and vulcanizates were prepared with carbon black, precipitated silica, and china clay as fillers. Different compositions of amidinothiourea were used in various mixes using standard recipes. Reference mixes were also studied. Curing characteristics of the mixes and various physical properties of the vulcanizates were evaluated. Optimum dosage of amidinothiourea required has also been derived. Among the different secondary accelerators tried including amidinothiourea, it may be stated generally that the more nucleophilic the secondary accelerator, the less the optimum cure times. This fact points to a nucleophilic reaction mechanism in the systems under review. In the evaluation of tensile and other physical properties of the vulcanizates, some of the systems containing amidinothiourea show better properties while others give comparable values with the reference mixes. Estimation of crosslink density also supports the above conclusion. © 1996 John Wiley & Sons, Inc.     

Studies on a new binary accelerator system containing TMTD and amidinothiourea in sulfur vulcanization of natural rubber

It is well known that the use of binary accelerator systems in rubber vulcanization provide better physical and chemical properties to the vulcanizates. The present work reports the results obtained on the study of amidinothiourea as a secondary accelerator along with tetramethyl thiuram disulfide (TMTD) in the sulfur vulcanization of natural rubber. Vulcanizates containing amidinothiourea have an appreciable increase in mechanical properties and better retention of these properties after aging compared to the reference mixes used. These mixes containing amidinothiourea showed appreciable reduction in optimum cure time as well. This suggests a nucleophilic reaction mechanism in the vulcanization reactions under review. Mixes with varying concentrations of amidinothiourea were tried; and the optimum level of amidinothiourea required has been derived based on the cure characteristics of the mixes and physical properties of the vulcanizates. © 1994 John Wiley & Sons, Inc.     

Effect of vulcanizing system on the crosslink density of nitrile rubber compounds

The synergistic activity of binary accelerator systems in rubber vulcanization is well known. Binary accelerator systems are being widely used in industry and are becoming increasingly popular because of the fact that such mixed systems can produce a vulcanizate with superior mechanical properties compared to those of stock cured with a single accelerator. The authors have studied the performance of a binary accelerator system based on cyclohexyl benzothiazole sulfenamide (CBS), tetramethyl thiuram disulphide (TMTD) in the sulfur vulcanization of nitrile rubber. The amount of sulfur and accelerator was varied to change the network crosslink density of vulcanizates. The observed mutual activity has been discussed based on the mechanical properties and crosslink density. The physical crosslink density of the various nitrile rubber mixes was estimated using the Kinetic Theory of Elasticity. The mechanical properties of the various rubber compounds were related to the corresponding crosslink density estimated for each compound. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2440–2445, 2005     

Structural characterization of vulcanizates. XII. Efficient vulcanization using a sulfenamide–thiuram disulfide accelerator system

Vulcanizates obtained from the sulfur vulcanization of natural rubber using a combination of the accelerators N-cyclohexylbenzothiazole-2-sulfenamide (CBS) and tetramethylthiuram disulfide (TMTD) have been analyzed in terms of the numbers of poly-, di-, and monosulfide crosslinks, network-bound accelerator residues, and cyclic sulfide chain modifications as a function of cure time. The vulcanization system produced mainly monosulfide crosslinks at optimum cure, although there were differences in detail between this mixed system and a previously reported efficient sulfur vulcanization (EV) system using a high ratio of CBS to sulfur.     

New binary accelerator systems for sulphur vulcanisation of styrene butadiene rubber

Abstract

It is suggested that binary systems of rubber vulcanisation containing thiourea proceed by a nucleophilic reaction mechanism. The exact mechanism of the chemical reactions involved when binary accelerator systems are used in rubber vulcanisation is not fully understood even now. It differs with the systems and the nature of the elastomer used. In the present work, a derivative of thiourea, N-amidino N'-phenyl thiourea (APT), was studied as a secondary accelerator along with tetramethyl thiuram disulphide (TMTD)/mercaptobenzothiazyl disulphide (MBTS) in the sulphur vulcanisation of styrene butadiene rubber. Thiourea binary systems were used as controls. The binary systems containing APT are very effective in reducing the optimum vulcanisation time. APT being more nucleophilic than thiourea, is more reactive here (as observed by the reduction in cure time). A nucleophilic reaction mechanism is indicated in the vulcanisation reaction under review. Different concentrations of APT were tried in various mixes using standard recipes, and reference mixes were also evaluated. The optimum dosage of APT required has been derived based on the cure characteristics of the mixes and the physical properties of the vulcanisates.     

Bis(N‐benzyl piperazino) thiuram disulfide and dibenzothiazyl disulfide as synergistic safe accelerators in the vulcanization of natural rubber

Some investigation regarding the effect of binary system of accelerators comprising one safe thiuram disulfide, namely bis(N‐benzyl piperazino) thiuram disulfide (BPTD) and dibenzothiazyl disulfide (MBTS), on the vulcanization of NR is carried out. The results are compared with those obtained with conventional tetra methyl thiuram disulfide (TMTD) presently considered as unsafe. The vulcanizates obtained from safe synergistic pair of accelerators (BPTD‐MBTS) possess comparable mechanical properties [modulus, tensile strength, and elongation at break (%)] and exhibits some improvement in heat resistant behavior when compared with those obtained with TMTD‐MBTS system. In the light of mechanical properties, safe BPTD‐MBTS system introduces the safe noncarcinogenic rubber accelerator in the vulcanization of rubber. Same type of synergistic activity may be due to comparable activation energy for both the TDs with the combination of MBTS. Although rate constant values are low for BPTD‐MBTS (6 : 3) compared to TMTD‐MBTS (6 : 3) in the high temperature vulcanization, the crosslinking efficiency of former is very large compared to later. This may be responsible for improvement in heat resistance behavior of the novel accelerator in the combination with MBTS. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synergic effect of N‐benzylimine aminothioformamide secondary accelerator during sulfur vulcanization of a styrene‐butadiene‐natural rubber blend

In this study we reported synergic activity of a novel secondary accelerator N‐Benzylimine aminothioformamide (BIAT) along with tetramethylthiuram disulfide (TMTD) in improving cure and mechanical properties of gum and filled mixes of Styrene‐Butadiene Rubber (SBR). The feasibility of application of BIAT in sulfur vulcanization of an ideal blend of SBR and natural rubber (NR) has also been investigated. The mechanical properties like t ensile strength, tear resistance, hardness, compression set, and abrasion loss were measured. Swelling values were also determined as a measure of crosslink densities of the vulcanizates. The binary accelerator system BIAT‐TMTD was found very effective in improving cure properties of the mixes of pure SBR and a 50/50 blend of SBR and NR.There was also found simultaneous improvement in mechanical properties of vulcanizates of both pure and blend. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The behavior of dithiocarbimate derivative as safety accelerator of natural rubber compounds

A conventional vulcanization system containing tetrabutylammonium bis(4‐methylphenyldithiocarbimato)zincate(II) (ZNIBU) was used for curing of natural rubber (NR) compounds. Rheometric (t_s1, t₉₀, and CRI) and mechanical properties, such as tensile and tear strengths and modulus at 300%, were measured to evaluate the acceleration potential of ZNIBU. Commercial accelerators (TMTD, MBTS, and CBS) and a binary system CBS/ZNIBU were also tested for comparison purposes. It was observed that ZNIBU alone does not give either safe scorch time or cure rates appropriate for industrial applications. Nevertheless, mechanical properties are comparable to those given by the other accelerators used. As for the binary system, positive synergistic effects can be found in tear strength and modulus of NR vulcanizates. Besides, ZNIBU does not contribute for the formation of nitrosamines in the vulcanization process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

N‐benzoyl‐N′N′‐disubstituted thioureas—A new binary accelerator system and its effect of nucleophilicity in sulfur vulcanization of natural rubber

The effect of several new binary accelerator systems were studied with special emphasis to the relationship of nucleophilicity to their rheological and mechanical properties. In this study, dialkyl/azacycloalkyl substituted benzoylthioureas (BTU 1‐5) were used as secondary accelerators (SA) along with three different primary accelerators (PA) viz., Cyclohexylbenzthiazylsulfonamide (CBS), Mercaptobenzothiazole (MBT) or Zincdithiocarbamate(ZDC) in the sulfur vulcanization of natural rubber. The effect of these secondary accelerators on the rheological and mechanical properties was found to be improved when compared to the reference mixes. It is noted that the N‐benzoyl‐N′N′‐piperidinylthiourea (BTU 4) is more effective as a secondary accelerator due to the higher nucleophilicity of the same when compared with the test compounds. The ZDC‐BTU gives the best result as binary accelerator system in natural rubber. Chemical characterization was carried out by determining the total crosslink density. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Studies on the effect of thiuram disulfide on NR vulcanization accelerated by thiazole-based accelerator systems

Thiuram disulfides form synergistic combinations with thiazole and thiazole-based accelerators, namely, N-cyclohexy-2-benzothiazole sulfenamide (CBS), 2-mercaptobenzothiazole(MBT), and 2-mercaptobenzothiazyl disulfide (MBTS). Unfortunately, widely used thiuram disulfides (TD) generate carcinogenic N-nitrosoamine. It is reported that the nitrosamines from N-methylpiperazine and dibenzylamine are free from this menace. So, some investigations were carried out with the binary combinations of each of bis(N-methylpiperazino)thiuram disulfide (MPTD), tetrabenzylthiuram disulnde (TBzTD), and tetramethyl-thiuram disulfide (TMTD) separately with CBS, MBT, and MBTS. It was observed that all the TD are activated by the CBS, MBT, or MBTS in the combinations studied. The intensity of activation is manifested in the enhancement of torque, modulus, tensile strength, cure rate, hardness, and decrease of elongation at break values and is very much dependent upon the ratio of the accelerators used. Considering the torque, modulus, tensile strength, and the elongation at break values, it apears that MPTD and TBzTD are capable of competing with the hitherto unbeaten TMTD as suitable accelerators for the vulcanization of rubber. Some investigations in respect to heat- and age-resistance behavior have also been carried out and the observed differences in the activities of various binary combinations have been explained through a mechanism. The results obtained with filled vulcanizates indicate that the binary systems comprising TD and MBTS provide fruitful results of which the TBzTD–MBTS combination seems to give the best cure and physical data for practical vulcanizates. © 1996 John Wiley & Sons, Inc.     

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.     

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.     

促进剂对氯丁胶/反式-1,4-聚异戊二烯并用胶性能的影响

将促进剂TMTD、NA-22和DM用于氯丁橡胶（CR）和反式-1,4-聚异戍二烯（TPI）并用胶中,研究其对并用胶性能的影响。结果表明,与促进剂NA-22和DM相比,促进剂TMTD可以降低混炼胶的门尼粘度,提高胶料的流动性;缩短正硫化时间,有利于改善加工工艺,提高硫化效率。加入促进剂DM的胶料焦烧时间比较长,加工安全性提高,但正硫化时间过长,硫化效率低。加入促进剂TMTD的胶料拉伸强度、拉断伸长率、撕裂强度等物理机械性能优异,屈挠性能提高显著;耐老化性能略有下降。当促进剂TMTD的用量在1．0～1．3份时胶料的综合性能最好。     

Influence of sulfenamide accelerators on cure kinetics and properties of natural rubber foam

The effect of types of sulfenamide accelerator, i.e., 2‐morpholinothiobenzotiazole (MBS), N‐t‐butylbenzothiazole‐2‐sulfenamide (TBBS), and N‐cyclohexyl benzothiazole‐2‐sulfenamide (CBS) on the cure kinetics and properties of natural rubber foam was studied. It has been found that the natural rubber compound with CBS accelerator shows the fastest sulfur vulcanization rate and the lowest activation energy (E_a) because CBS accelerator produces higher level of basicity of amine species than other sulfenamide accelerators, further forming a complex structure with zinc ion as ligand in sulfur vulcanization. Because of the fastest cure rate of CBS accelerator, natural rubber foam with CBS accelerator shows the smallest bubble size and narrowest bubble size distribution. Moreover, it exhibits the lowest cell density, thermal conductivity and thermal expansion coefficient, as well as the highest compression set as a result of fast crosslink reaction. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44822.     

Cure characteristics and mechanical properties of hydrogenated natural rubber/natural rubber blends

The cure characteristics and mechanical properties of blends consisting of hydrogenated natural rubber (HNR) and natural rubber (NR) blends were investigated. The HNR/NR blends at 50/50 wt ratio were vulcanized using various cure systems: peroxide vulcanization, conventional vulcanization with peroxide, and efficient vulcanization with peroxide. The HNR/NR vulcanizates cured by the combination of peroxide and sulfur donor (tetramethylthiuram disulfide, TMTD) in the efficient vulcanization with peroxide exhibited the best mechanical properties. It was also found that the hydrogenation level of HNR did not affect the tensile strength of the vulcanizates. The tensile strength of the blends decreased with increasing HNR content because of the higher incompatibility to cause the noncoherency behavior between NR and HNR. However, the HNR/NR vulcanizate at 50/50 wt ratio showed the maximum ultimate elongation corresponding to a co‐continuous morphology as attested to by scanning electron micrographs. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009