semi-EV System Research Articles

Natural rubber/styrene-butadiene rubber blends prepared by solution mixing: Influence of vulcanization temperature using a Semi-EV sulfur curing system on the microstructural properties

Blends of natural rubber (NR) and styrene-butadiene rubber (SBR) were prepared by solution mixing and vulcanized with sulfur and accelerator in a Semi-EV system at 433 K and 443 K in order to study the vulcanization kinetic and the influence of vulcanization temperature on final structure of the blends. The vulcanization kinetic studied through the variation in rheometer curves was analyzed using the Ding and Leonov model, which takes into account the reversion effect during the cure process. The average free nanohole volume and the fractional free volume of samples with different NR/SBR ratio were estimated using positron annihilation lifetime spectroscopy (PALS). Also, the crosslink density was determined by means of swelling tests in a solvent. For all the compounds, a correlation between the free nanohole volume and the delta torque obtained from the respective rheometer curves was established.

Garlic Additive on the Properties of Natural Rubber Vulcanite

The effects of garlic additive and part replacement of sulphur in the conventional curative systems (mercapto benzothiazole-sulphur) MBT/S, (cyclohexyl benzothiazole sulphenamide-sulphur) CBS/S, and semiEV on the mechanical and cure properties of natural rubber compound have been studied. Results show that the addition of garlic powder at content less than or equal to 6pphr to the conventional MBT/S mix improves the tensile strength of natural rubber compound. Part replacement of sulphur with garlic powder in the conventional MBT/S system up to 2ppr also enhances the tensile strength of the compound of natural rubber. MBT/S systems in which sulphur has been replaced with garlic up to a level of 1.5pphr show comparable reversion resistance, cure rate index and scorch resistance to the conventional MBT/S systems containing less or equal to 6ppr garlic Addition of 2pphr powder to semiEV system improves tensile strength, while tensile strength depresses on the addition of 2pphr garlic powder to the CBS/S system. It appears advantageous therefore to include garlic powder in the corresponding formulation of natural rubber compound.

Influence of the cure systems on long time thermal aging behaviors of NR composites

NR composites with different curing systems were aged thermally at 60, 70, 80, and 90 °C for 2–185 days in a convection oven, and the changes in the crosslink density were investigated as a function of the accelerated thermal aging. The overall crosslink densities increased with increasing aging time irrespective of the aging temperatures and curing systems. The changes in crosslink density were enhanced by increasing the aging temperature. The degree of the increased crosslink density was in the following order: “the conventional cure system > the semi-EV system > the EV system”. For short term thermal aging, the change in crosslink density with the aging time was complicated, particularly for low temperature aging. The activation energies of the change in crosslink density with thermal aging using the conventional and semi-EV cure systems increased and then remained relatively constant with increasing aging time, whereas that of the specimen with an EV cure system tended to increase linearly. The experimental results were explained by the dissociation of the existing polysulfidic linkages and the formation of new crosslinks through the crosslinking-related chemicals remaining in the sample.

Effect of curing system on reinforcing efficiency of silane coupling agent

Silane coupling agent has long been used to enhance degree of reinforcement of silica. The mechanical properties of silica-filled vulcanizates are greatly improved in the presence of silane coupling agent, mainly due to an increase in rubber–filler interaction. Recently, various types of silane coupling agent have been developed and commercialized. In the present study, two types of silane coupling agent, i.e. bis-(3-triethoxysilylpropyl) tetrasulfane (Si-69) and 3-thiocyanatopropyl triethoxy silane (Si-264) were selected for comparison of their reinforcing action in various curing systems: conventional vulcanization (CV), semi-efficient vulcanization (semi-EV) and efficient vulcanization (EV). The results reveal that the addition of silane coupling agents not only improves compound processability, but also enhances the mechanical properties of the rubber vulcanizates. The effect of silane type on processability is found to be independent of the curing system. Compared with Si-69, Si-264 can react with the silanol groups on the silica surface more readily, due to its less bulky structure and lower viscosity. In other words, Si-264 can reduce the filler–filler interaction to a greater extent than Si-69, leading to better compound processability. The effect of silane type on the vulcanizate properties, on the other hand, depends greatly on the curing system. For CV and semi-EV systems, Si-264 imparts greater degree of reinforcement than Si-69. This might be attributed to the combined effects of better rubber–filler interaction, better filler dispersion and higher state of cure when Si-69 is replaced with Si-264. However, for the EV system, contrary results are obtained, i.e. Si-69 provides better reinforcement than Si-264. Explanation is given to the effect of sulfur contribution of Si-69 which is dominant only in the EV system.

Preparation and properties of thioglycollic acid modified epoxidised natural rubber and its blends with natural rubber

Solution prepared epoxidised natural rubber (ENR) was modified while still in solution with thioglycollic acid (TGA) (0.3 mol phr) to yield a material with 17% conversion of its initial NR epoxide. This material (ENR–TGA) and its blends with natural rubber were black filled and vulcanised using the semi-EV system. The modified ENR material exhibited low tensile strength (4.4 MPa), but showed better resistance to solvent and mineral oil than a similar natural rubber compound. Blending natural rubber with the modified ENR material at a level of 10 parts ENR–TGA phr increased the modulus values by 46%. At the aforementioned blend composition, the hardness and resistance to swelling in solvents and mineral oil were also increased, but the reverse were the case for tensile strength and elongation at break. Cure behaviour and flex-cracking properties of the blends were also studied.

STYRENE–BUTADIENE RUBBER/EPOXIDIZED NATURAL RUBBER BLENDS: THE EFFECT OF VULCANIZATION SYSTEM ON BLENDS PROPERTIES

The dynamic properties, curing characteristics, and swelling behavior of the blends cured using two types of vulcanization systems [i.e., conventional vulcanization (CV) and semiefficient vulcanization (semi-EV)] were investigated. Results indicate that the maximum elastic torque (S′ @MH) and the torque difference, S′ @MH–S′ @ML (maximum elastic torque minus minimum elastic torque) increased with the increasing epoxidized natural rubber (ENR) composition in the blend. However, a reverse trend was observed for tan δ @MH and viscous torque (S″ @MH). At a similar blend ratio, the CV system gives a higher S′ @MH and torque difference but a lower tan δ @MH and S″ @MH than the semi-EV system. The scorch time, t 2 and cure time, t 90, decreased with increasing ENR composition in the blends. Semi-EV system blends exhibit shorter t 2 and t 90 than CV system blends. The swelling degree decreased with increasing ENR composition in the blends and the CV system blends show better oil-swelling resistance.

Properties of formic acid crosslinked epoxidized natural rubber (FC-ENR) blends with dry natural rubber

Formic acid crosslinked epoxidized natural rubber (FC-ENR) with precrosslinking epoxide level of 33 mol% was prepared and compounded without any further curative and in blends with unmodified natural rubber. A total of four different materials—FC-ENR, NR, FC-ENR/NR blend (25/75) and FC-ENR/NR blend (50/50)—were prepared for analysis and designated mixes K, L, M and N, respectively. All the materials were ISAF black filled at 30 pphr loading. With the exception of the unblended FC-ENR the rest of the materials were cured using the semi-EV system. The unmodified NR vulcanizate served as control. In the present communication we report the results of our studies in the physical properties of the aforementioned vulcanizates. FC-ENR was found to have potential as an effective modifier for natural rubber.

The effects of multifunctional additive and vulcanization systems on silica filled epoxidized natural rubber compounds

The concentration effects of multifunctional additive (MFA) and vulcanization systems on silica filled epoxidized natural rubber (ENR 25) was investigated in the concentration range 0–7.5 phr. Three vulcanization systems were used; conventional vulcanization (CV), semi-efficient vulcanization (semi-EV) and efficient vulcanization (EV). The cure time, ( t 90) and scorch time, ( t 2) of all vulcanization systems decrease with increasing MFA concentration. At a similar MFA concentration, the EV system showed the shortest t 2 and t 90 followed by the semi-EV and CV systems. In all the vulcanization systems, the mechanical properties have been enhanced by the incorporation of MFA. At a similar MFA concentration, semi-EV systems give better tensile and tear strengths followed by EV and CV systems.

Epoxidized Natural Rubber Compounds: Effect of Vulcanization Systems and Fillers

The effects of various vulcanization systems and fillers in epoxidized natural rubber (ENR) at 50% epoxidation (ENR 50 compounds) were examined. ENR compounds cured with the semi-EV (efficient vulcanization) system showed the shortest cure and scorch times but the highest torque values. White rice husk ash (WRHA) filled compounds have the shortest cure and scorch times followed by carbon black and silica. The presence of a silane coupling agent, γ-mercaptopropyl trimethoxy silane enhanced the curing characteristics but the trend is similar. ENR compounds cured with the semi-EV system gave the best overall mechanical properties. A significant enhancement of the mechanical properties was observed in the presence of a silane coupling agent, especially for WRHA-and silica-filled compounds.

Effect of High Crystalline trans-Polyoctenylene (TOR) on Different Cure Systems of NR Compounds

With a view to improve degradation properties of natural rubber on prolonged cure at moderately high temperature, a study is made using a co- vulcanizable processing aid known as trans-polyoctenylene (TOR) on natural rubber filled compounds. Rheometer results show that reversion resistance im proves at all curing temperatures in the presence of TOR-more so with EV (efficient vulcanization) and semi-EV systems compared to CV (conventional vulcanization) systems. Cure rate diminishes and activation energy increases with incorporation of TOR in natural rubber irrespective of the curing system. Modulus and hardness retention also are much better for TOR filled com pounds under anaerobic conditions. Tensile strength, however, drops unlike modulus and hardness. Heat buildup and loss tangent (tanδ) results reveal less heat development in TOR filled compounds. Retention of Vr (volume fraction of rubber) value indicates a similar trend like modulus and hardness, showing preservation of network structure. Such an improvement of reversion prop erties under anaerobic ageing conditions may be attributed to simultaneous crosslink formation on TOR phase in addition to natural rubber phase which has been explained through DSC studies.

The effect of γ-mercaptopropyltrimethoxysilane coupling agent on t90, tensile strength and tear strength of silica-filled ENR vulcanisates

The cure times ( t 90), tensile strengths and tear strengths of silica-filled epoxidised natural rubber (ENR), with or without the presence of γ-mercaptopropyltrimethoxysilane (A-189) and bis(triethoxysilylpropyl) tetrasulphide (Si-69) and cure using a semi-efficient vulcanisation (semi-EV system) were studied. Three grades of ENR having 10, 25 and 50 mole % of epoxidation were used. Mixing was done using a Brabender Plasti-Corder attached with a mixing can. A semi-EV system was found to be better than either a conventional vulcanisation (CV) or an efficient vulcanisation (EV) systems when ENR is mixed with silica, with or without the addition of silane, 2% A-189 was found to be the optimum amount of coupling agent for wetting out the silica. The t 90 of all three ENR grades without the presence of A-189 show cure retardation. When 2% A-189 was added, cure enhancement occurs in all three ENR grades. All three grades of ENR show improvements in the tensile strengths and tear strengths.

Studies on the Cure Characteristics of Elastomer Blends

Abstract Blends of natural rubber (NR) with styrene-butadiene rubber (SBR), polybutadiene rubber (BR), ethylene-propylene terpolymer (EPDM) and acrylonitrile-butadiene rubber (NBR) were vulcanised using an efficient vulcanisation (EV) system and a semi-EV system. Compatible blends show a definite pattern of curing whereas the incompatible blends show no such pattern.

N-(Morpholinothio)Imides as Curing Agents in Semiefficient Vulcanization Systems

Abstract It is well known that both the thermal and oxidative aging characteristics of vulcanizates can be improved by using an efficient vulcanization (EV) or semi-EV system. These desirable aging properties are attributed to the presence of mono- and disulfidic crosslinks in the vulcanizates cured by EV or semi-EV systems. In contrast to a polysulfide crosslink, mono- and disulfide crosslinks are more thermally stable and undergo minimal crosslink modification during aging. One way to achieve such desirable properties is to cure a stock with 2-benzothiazyl 4-morpholino disulfide (BMD) and low levels of sulfur. However, the BMD cure system lacks the scorch protection required for many applications. Moreover, neither N-(cyclohexylthio)phthalimide (CTP) nor N,N′-bis(cyclohexylthio)oxanilide (BCTO) shows an appreciable effect on the scorch time of BMD cure systems. This is true regardless of the level of sulfur used, as shown in Figure 1. The maximum increase in scorch time with 1.00 part CTP was only 35%, and this occurred only when three parts of BMD and no free sulfur was used. When 2.00 parts sulfur was used, scorch time was extended only 18%. Likewise, BCTO did not respond well in BMD cure systems. The poor performance of these retarders in BMD cure systems prompted us to design a cure system in which BMD can be formed during vulcanization.

Effect of Elevated Temperature on the Unaccelerated and Accelerated Sulfur Vulcanization of Natural Rubber

Abstract High cure temperature up to 180°C seems to have no significant effect on the mechanism of unaccelerated sulfur vulcanization of natural rubber. In the case of accelerated systems, the conventional ones undergo more desulfurization and decomposition of the initially formed polysulfidic crosslinks at higher cure temperature. The EV systems show less dependence on cure temperature, and the main maturing reaction seems to be desulfurization of the polysulfidic crosslinks. The semi-EV system shows behavior intermediate between the conventional system and the EV one. Expectedly, the EV systems show better resistance to reversion at high cure temperature than the conventional and semi-EV systems.

N‐(morpholinothio)imides as curing agents in semiefficient vulcanization systems

AbstractSemiefficient vulcanization systems (semi‐EV) are often used when a stock requires good aging properties as well as good fatigue resistance. A partial replacement of sulfur in a conventional cure system with 4,4′‐dithiobismorpholine or 2‐mercaptabenzothiazyl 4‐morpholino disulfide (BMD) is an example of semi‐EV system. However, with BMD the scorch time is not always long enough for multisteps involved in the manufacturing of tires. Such new cure retarders as N‐(cyclohexylthio)phalimide or N, N′‐bis(cyclohexylthio)oxanilide showed very little efficiency in BMD cure systems. This paper describes the usage of new curing agents, (N‐morpholinothio)imides, which can overcome the disadvantage of scorchness in BMD cure system and provide excellent aging properties. This is rationalized by the formation of BMD during vulcanization. The facile formation of BMD from N‐(4‐morpholinothio)phthalimide (MTP) and 2‐mercaptobenzothiazole (MBT) serves to illustrate the feasibility of in situ formation of BMD during cure and to explain long scorch time observed with N‐(4‐morpholinothio)imides.