A three‐function numerical model for the prediction of vulcanization‐reversion of rubber during sulfur curing

The cross‐linking mechanisms of sulfur vulcanization are not analytically known and, therefore, reticulation kinetics has to be deduced macroscopically from standardized tests. One of the most popular laboratory test to characterize curing and reversion is the oscillating disk rheometer ODR, which gives a quantitative assessment of scorch, cure rate, and state of cure. In this article, a numerical two‐step approach, which is based on the utilization of experimental ODR data and aimed at predicting the degree of vulcanization of thick rubber items cured with accelerated sulfur, is presented. In step one, a composite numerical three‐function curve is used to fit experimental rheometer data, able to describe the increases of the viscosity at successive curing times and at different controlled temperatures, requiring only few points of the experimental cure curve to predict the global behavior. Both the case of indefinite increase of the torque and reversion can be reproduced with the model. In step two, considering the same rubber compound of step one, numerical cure curves at different temperatures are collected in a database and successively implemented in a Finite Element software, which is specifically developed to perform thermal analyzes on complex 2D/3D geometries. As an example, an extruded thick EPDM section is considered and meshed through eight‐noded isoparametric plane elements. Several FEM simulations are repeated by changing exposition time t_c and external curing temperature T_n, to evaluate for each (t_c,T_n) couple the corresponding mechanical properties of the item at the end of the thermal treatment. A recently presented bisectional approach, alternating tangent (AT), is used to drastically reduce the computational efforts required to converge to the optimal solution associated with the maximum value of an output property, tensile strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A new method to predict optimum cure time of rubber compound using dynamic mechanical analysis

The degree of vulcanization of a rubber compound has a big influence on the properties of the final product. Therefore, precisely defining the curing process including optimum cure time is important to ensure the production of final products having high performance. Typically, vulcanization is represented using vulcanization curves. The main types of equipment used for producing vulcanization curves are the oscillating disc rheometer (ODR) and the moving die rheometer (MDR). These can be used to plot graphs of torque versus time at a constant temperature to show how cure is proceeding. Based on the results obtained, optimum cure time (t₉₀) is calculated as the time required for the torque to reach 90% of the maximum achievable torque. In this study, the use of Dynamic Mechanical Analysis (DMA) for assessment of t₉₀ was assessed. DMA was carried out using shear mode isothermal tests to measure the changes in material properties caused by vulcanization. The results revealed that the shear storage modulus (G′), shear loss modulus (G′′) , and tan δ all reflect the vulcanization process, however, tan δ gave the best representation of level of vulcanization. Indeed, the curve of tan δ was able to be used to derive the t₉₀ for rubber compounds and showed good agreement with the results from an MDR. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40008.     

Evidence of ionic components in vulcanization mixtures by means of electric current measurements: Investigations with natural rubber/sulfur/zinc bis(dimethyldithiocarbamate) and comparison mixtures

Continuous low‐level current (CLLC) measurements for detecting ionic species in the course of vulcanization reactions were applied to investigate the vulcanization of a mixture of natural rubber (NR), sulfur (S), and zinc bis(dimethyldithiocarbamate) (ZnDMTC). A dc voltage was applied to the reaction mixture in a special vulcanization mold and the current (e.g., in the range of 10⁻⁹ A) was measured. Temperature‐dependent current maxima were found after reaction times t_max. The simplest explanation is that transitory ionic species occur during vulcanization. An activation energy (E_a ) = 116.4 kJ/mol, similar to that obtained in previous chemical investigations, was determined from the decrease of t_max with increasing temperature. The maxima corresponded to reaction times where a strong increase of polymer crosslinking was observed, as measured using vulcametry. For comparison, dc measurements were carried out with the corresponding mixture without elemental sulfur (NR/ZnDMTC) and mixtures containing zinc stearate (ZnST) instead of zinc bis(dimethyldithiocarbamate) (NR/S/ZnST and NR/ZnST). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2206–2212, 2000     

Polybutadiene rubber/organoclay nanocomposites: Effect of organoclay with various modifier concentrations on the vulcanization behavior and mechanical properties

It has been reported that the cure time t₉₀, scorch time t₂, and their difference (t₉₀?t₂) of Polybutadiene rubber (BR)/organoclay nanocomposites were much reduced over those of BR. This effect can be attributed to the ammonium groups in the organoclay. The possible formation of a Zn complex in which sulfur and ammonium modifier participate may facilitate the formation of crosslinks. If this assumption is true, it is expected that the organoclay with higher ammonium modifier concentration will give larger torque difference and faster vulcanization rate to the BR/organoclay nanocomposites. The effect of organoclay with different modifier concentration on the vulcanization behavior and mechanical properties of BR/organoclay hybrid was investigated in this study. As expected, the order of the torque difference was BR/Cloisite 15A > BR/Cloisite 10A > BR/Cloisite 20A > BR/Cloisite 25A > BR/Cloisite 30B > BR/Cloisite Na⁺, and the order of vulcanization rate also showed similar trends. The organoclay with higher modifier concentration gave larger torque difference and faster vulcanization rate to the BR/organoclay nanocomposites. POLYM. ENG. SCI., 47:308–313, 2007. © 2007 Society of Plastics Engineers.     

An analysis of the influence of the accelerator/sulfur ratio in the cure reaction and the uniaxial stress–strain behavior of SBR

Vulcanized styrene butadiene rubber (SBR) with different cure systems was prepared and analyzed by using the model of rubber elasticity based on the tube concept, applied to the treatment of the stress–strain measurements. Samples with several ratio accelerators to sulfur, Λ, between 0.22 and 3.0 cured at 433 K were studied. The network chain density and the crosslink density of the samples were evaluated. By means of normalized rheometer curves, the kinetics of cure of these samples were evaluated by considering the model of isothermal curing proposed by Kamal and Sourour. In this frame, the parameters of the kinetics model were obtained. A correlation between the order of the kinetic equation, n, and the network chain density of the cure samples was established. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2601–2609, 2004     

Estimation by mechanical analysis of the molecular parameters of SBR vulcanizates at different cure conditions

The changes in the network structure of SBR-1712 during vulcanization were analyzed by means of a study of the stress–strain behavior at uniaxial extension at room temperature. In order to obtain different degrees of crosslinking, samples were cured at 414 K and 433 K at several times and were characterized by a rheometer. The conformational tube model was applied for the treatment of the stress–strain measurements of vulcanized samples. This theory allows the separation of crosslink and constraint contributions to the stress–strain behavior and relevant network parameters can be estimated. In this article the change with the temperature and time of cure of the average molecular mass of the mobile network chains, the crosslink density, the microscopic lateral tube dimension, and the root-mean-square end-to-end distance of the network chain are evaluated. © 1995 John Wiley & Sons, Inc.     

White Rice Husk Ash-Filled Natural Rubber Compounds: The Effect of Bonding Agents

White rice husk ash (WRHA)-filled natural rubber compounds were prepared by using a laboratory-size two-roll mixing mill. Curing using a conventional vulcanization system was used and cure studies were carried out on a Monsanto rheometer. The mechanical testing of the vul-canizates involves the determination of tensile properties, tear strength, hardness, and resilience. Scanning electron microscopy (SEM) and swelling measurement were also done. The effects of bonding agents on the curing and mechanical properties have been investigated using re-sorsinol formaldehyde and hexamethylene tetramine as the bonding agents. Results show that the bonding agents prolonged the cure time t ₉₀ and scorch time t ₂ and, at the same time, improved the mechanical properties of the natural rubber vulcanizates. SEM and swelling studies indicate that the rubber-filler interaction is improved with the addition of bonding agents.     

The role of thiol intermediates in 2‐mercaptobenzothiazole accelerated sulfur vulcanization of rubber model compounds

The model compound, 2,3‐dimethyl‐2‐butene (TME), was vulcanized using 2‐mercaptobenzothiazole (MBT) and sulfur. MBT was not consumed during the vulcanization reaction. The resultant crosslink products were bis(alkenyl) in nature. 2,3‐Dimethyl‐2‐buten‐1‐thiol (TME‐SH) was identified as being present in the vulcanization mixture by a postcolumn derivatization technique. The appearance of thiol was coincident with crosslinking. Polysulfanes (H₂S_n) were formed on crosslinking. Studies of the reaction of TME‐SH and sulfur indicated a rapid reaction to form crosslink products and polysulfanes. No monosulfidic crosslink species were formed in these reactions. Closer investigation revealed the presence of small quantities of what appeared to be highly reactive polysulfidic thiols. This is the first time that such species have been identified in vulcanization systems. Consequently, MBT‐accelerated vulcanization of TME is proposed to occur via the reaction of MBT and S₈ to form polysulfidic MBT, which then reacts with TME to form polysulfidic thiols. These thiols then rapidly react via a metathesis reaction pathway to provide crosslink products and polysulfanes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 47–54, 2003     

Influence of γ-irradiation on proton spin–lattice relaxation of SBR used as antirads and its ESR investigation

Proton spin–lattice relaxation time t₁ was measured on SBR samples with carbon black or kaolin filler using modified linseed oil. The NMR pulse technique at 90MHz was used in the temperature range from 180 to 400 K. The temperature dependence of t₁ indicates that samples filled with carbon black have similar molecular dynamics to the standard unfilled SBR samples. The activation energy for the motion of the main chain for these samples amounts to 16.4kJ/mol. Samples containing linseed oil modified with para-toluidine showed an activation energy of about 14.6kJ/mol and were not affected by γ-irradiation. Values of the minimum relaxation time t^min₁ were increased by γ-irradiation in comparison with a standard SBR sample. ESR measurements carried out at room temperature by means of an X-band spectrometer indicated that unidentified radicals within the rubber were formed during its mastication with vulcanizing additives. The ESR spectra did not change during the vulcanization process. Samples filled with carbon black showed a broadening of the ESR line; this is consistent with the increase in the electrical conductivity.     

Effect of 2‐mercaptobenzothiazole level on kinetics of natural rubber vulcanization

The kinetics of natural rubber vulcanization were investigated by use of a vulcameter. The vulcanization process before t_dis (the time when the accelerators and/or intermediates react to depletion) was expressed in an equation as ln(M_H ? M_t) = ln A ? k₁(t ? t₀)^α, which is different from the famous equation of Vu_t = ?[α(k₃/k₄)]ln[(k₂e ? k₁e)/(k₂ ? k₁)] deduced by Coran. It was found that the rate constants of two vulcanization processes with different reaction mechanisms before and after t_dis increase and their activation energies decreased with an increase in 2‐mercaptobenzthiazole (MBT) level. The considerable effect of MBT level on the activation energies of the vulcanization process before t_dis and the obvious temperature dependency of the reaction rate of vulcanization process after t_dis were observed. The time t_dis was shortened with an increase in MBT level, whereas the degree of vulcanization at t_dis remained unchanged. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3260–3265, 2004     

About the activation energies of the main and secondary relaxations in cured styrene butadiene rubber

This article studies the influence of the network structure on the activation energies of the α and β relaxations in vulcanized styrene butadiene rubber, SBR. A cure system based on sulphur and TBBS (N‐t‐butyl‐2‐benzothiazole sulfenamide) was used in the formulation of several compounds cured at 433 K. The activation energies were evaluated from internal friction (loss tangent) data of the compounds using an automated subresonant forced pendulum in a wide frequency range and between 80 K and 273 K. The internal friction data of the samples reveal two transitions, α and β, characterized by the temperatures T_α and T_β, due to the glass transition and the phenyl group rotation of the copolymer, respectively. Although T_α increases at higher crosslink density, it shows also a dependence with the amount of polysulphide and monosulphide linkages present in the samples. The highest activation energy for this process is obtained for the samples with high crosslink density and 30% of monosulphides in this structure. In the case of the β‐relaxation, there is a pronounced change in the activation energy between the uncured and the cured samples. The type of structure formed during vulcanization has an important effect in the activation energy of the segmental mode‐process. In the case of the β‐process, the cis‐trans isomerization that takes place during vulcanization in the butadiene part of the SBR, might be the cause of conformational changes in the surrounding of the phenyl rings that affect the energy barrier associated to the phenyl rotation. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Benzothiazole‐accelerated sulfur vulcanization. I. 2‐Mercaptobenzothiazole as accelerator for 2,3‐dimethyl‐2‐butene

2,3‐Dimethyl‐2‐butene (TME) was used as a model compound for polyisoprene in a study of 2‐mercaptobenzothiazole (MBT)‐accelerated sulfur vulcanization. Mixes that contained curatives only were heated in a DSC to various temperatures, while those that also contained TME were heated isothermally at 150°C in evacuated, sealed glass ampules. Heated mixtures were analyzed for residual curatives, intermediates, and reaction products by HPLC. It is proposed that MBT forms polysulfidic species (BtS_xH) in the presence of sulfur and that these react with TME via a concerted, substitutive reaction pathway to form polysulfidic hydrogen‐terminated pendent groups of varying sulfur rank (TME–S_xH). MBT is released as a by‐product of this reaction. Crosslinking occurs slowly as a result of the interaction of polythiol pendent groups, the rate being dependent on the pendent group concentration. H₂S is released on crosslinking. 2,3‐Dimethyl‐2‐butene–1‐thiol was synthesized and reacted in the presence of sulfur to confirm the formation of crosslinked products (TME–S_x–TME). Benzothiazole‐terminated pendent groups (TME–S_xBt) were not observed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1377–1385, 2000     

Cure and mechanical properties of filled SMR L/ENR 25 and SMR L/SBR blends

The effect of blend ratio of natural rubber/epoxidized natural rubber (SMR L/ENR 25) and natural rubber/styrene‐butadiene rubber (SMR L/SBR) blends on scorch time (t₂), cure time (t₉₀), resilience, hardness, and fatigue properties were studied in the presence of carbon black and silica. An accelerated sulfur vulcanization system was used throughout the investigation. The scorch and cure times of the rubber compound were assessed by using a Moving‐Die Rheometer (MDR 2000). Resilience, hardness, and fatigue life were determined by using a Wallace Dunlop Tripsometer, a Wallace Dead Load Hardness Tester, and a Fatigue to Failure Tester, respectively. The results indicate that t₂ and t₉₀ decrease with increasing ENR 25 composition in the SMR L/ENR 25 blend whereas both values increase with increasing SBR content in the SMR L/SBR blend. This observation is attributed to faster cure in ENR 25 and higher saturation in SBR. Resilience decreases with increase in % ENR and % SBR but hardness shows the reverse behavior in their respective blends. The fatigue life increases with % ENR, but it passes through a maximum with % SBR in the respective blends. In all cases, aging lowers the fatigue life, a phenomenon that is caused by the breakdown of crosslinks in the vulcanizate. Differences in all the observed values between carbon black‐filled and silica‐filled blends are associated with the varying degrees of interaction and dispersion of the two fillers in the rubber blend matrix. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 47–52, 2001     

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.     

Nano ZnO as cure activator and reinforcing filler in natural rubber

Zinc oxide (ZnO) nanoparticles of size 20–90 nm and surface area 9.56 m²/g were synthesized from ZnCl₂ and Chitosan and characterized by X‐ray diffraction, high resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM). Natural rubber (NR) vulcanizates containing nano ZnO was prepared by mill mixing and characterized by SEM, energy dispersive X‐ray analysis (EDAX), and HRTEM. Cure characteristics, free volume studies, bound rubber, crosslink density, and dynamic mechanical properties were evaluated and compared with that of NR vulcanizate containing conventional micro ZnO. Considering the cure characteristics, it was found that NR vulcanizate with 0.5 phr (parts per 100 g rubber) of nano ZnO showed low values of optimum cure time (t₉₀) and very high cure rate index compared with 5 phr of conventional micro ZnO. The study shows that micro ZnO can be successfully replaced with nano ZnO for accelerated sulfur vulcanization process in NR, and preparation of vulcanizate containing nano ZnO with better properties as that of micro ZnO. The optimum dosage of nano ZnO as a cure activator in NR vulcanization was found to be 0.5 phr compared with conventional grade micro ZnO. This will lead to substantial cost reduction in the manufacture of rubber products and alleviate environmental pollution due to excess ZnO in rubber compounds. POLYM. ENG. SCI., 2013 © 2013 Society of Plastics Engineers     

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.     

Evaluation of physical,rheological, and structural properties of vulcanized EVA/SBS modified bitumen

In this study, the performance and modification mechanism of EVA (ethylene‐vinyl‐acetate) modified (EM), EVA/SBS (Styrene‐Butadiene‐Styrene) modified (ESM), and EVA/SBS/sulfur modified (ESSM) bitumens were evaluated. The physical, rheological, morphological, and structural properties were determined before and after aging, and compared with those of base bitumen. These properties were evaluation using conventional physical methods, Fourier transform infrared spectrometer, optical microscopy, dynamic shear rheometer, and bending beam rheometer, respectively. The results showed that sulfur was useful in bitumen, EVA, and SBS modification by forming a vulcanized crosslinking polymer network. The vulcanization improved most of the physical properties of ESM bitumen, especially high‐ and low‐temperature performance, and toughness and tenacity (previously not evaluated in the literature). Meanwhile, vulcanization improved the compatibility between polymers and bitumen and increased the aging resistance of ESM bitumen. Vulcanization reactions took place without new functional groups being presented in the infrared spectrum. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44850.     

Thermal aging properties and chemical resistance of blends of natural rubber and epoxidized low molecular weight natural rubber

Studies into solvent resistance and aging properties of blends of natural rubber and epoxidized low molecular weight natural rubber were carried out. Vulcanization of the blends using the semi‐efficient vulcanization (semi‐EV) system was found to have curing advantages over conventional vulcanization (CV) and efficient vulcanization (EV) systems. The rheological properties (cure time, t₉₀, and scorch time, t₂), solvent resistances, and aging properties of the vulcanizates were found to improve as the level of epoxidized low molecular weight natural rubber in the blends increases. The mechanical properties of the blends were also found to be within the accepted level for NR vulcanizates. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1733–1739, 2005     

Vulcanization characteristics of asphalt/SBS blends in the presence of sulfur

The vulcanization of asphalt/styrene–butadiene–styrene (SBS) triblock copolymer blends in the presence of elemental sulfur was followed with a strain-controlled rheometer. The vulcanization of the blends took place at temperatures greater than 140°C. From 150 to 180°C, the curing rate of the blends increased significantly with increasing temperature, and the apparent activation energy of vulcanization was 45.2 kJ mol⁻¹. A suitable processing temperature for good mechanical and thermally stable properties was between 170 and 180°C. Both the structure of SBS and the sulfur level had important effects on the vulcanization of the blends. A plot of the electric current versus time showed the process of the dynamic vulcanization of the blends. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 989–996, 2001     

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.