Concentration effect of stearic acid on scorch behavior of epoxidized natural rubber

The effect of stearic acid on Mooney scorch time of epoxidized natural rubber (ENR 25 and ENR 50) and one grade of unmodified natural rubber (SMR L) was investigated in the concentration range of 0.5 to 14.5 phr. Other parameters, namely accelerator systems, temperature, and fillers (carbon black and silica), on the scorch property of ENR 25 in the presence of excess loading of stearic acid were also studied. Results indicate that scorch time increases with stearic acid loading for all the rubbers investigated, the rate of increase being fastest in ENR 50, followed by ENR 25 and SMR L. Mooney scorch time of ENR shows strong dependence on stearic acid loading for delay-action accelerators and at lower temperature of vulcanization. For a fixed filler loading, the dependence of scorch time on stearic acid concentration is similar to that of gum stock. The retardation effect exhibited by excess stearic acid on the vulcanization of ENR may be attributed to complex formation of chelates and the reduction in activation of adjacent double bonds in ENR resulting from interaction between stearic acid and the epoxide group of ENR. © 1995 John Wiley & Sons, Inc.     

Cure characteristics of unaccelerated sulfur vulcanization of epoxidized natural rubber

The curing characteristics of unaccelerated sulfur vulcanization of ENR 25 and ENR 50 were studied in the temperature range from 100–180°C. The range of sulfur loading was from 1.5 to 6.5 phr. The scorch time was determined by Mooney Shearing Disk Viscometer whereas the initial cure rate, maximum torque, and reversion properties were obtained from the Moving Die Rheometer (MDR 2000). Results shows that ENR 25 gives a longer scorch time than ENR 50, an observation similar to that in an accelerated system reported earlier. For temperature < 120°C, scorch time depends exponentially on sulfur loading for both rubbers. However, this dependence diminishes as temperature is increased. This observation is attributed to the availability of activated sulfur molecules for vulcanization. The initial cure rate and maximum torque increases with increasing sulfur loading. ENR 50, however, exhibits higher value than ENR 25, suggesting faster cure in the former. For a fixed sulfur loading, reversion is a time and temperature-dependent phenomenon. It decreases with increasing sulfur loading because of the increase of cross-linking density for both rubbers stuided. © 1996 John Wiley & Sons, Inc.     

Effects of palm oil fatty acid on curing characteristics,reversion and fatigue life of various natural rubber compounds

The effects of palm oil fatty acid concentration (0, 1, 3, 5, 7 phr) and epoxidation on curing characteristics, reversion and fatigue life of carbon black filled natural rubber compounds have been studied. Three different types of natural rubber, SMR L, ENR 25 and ENR 50 having 0, 25 and 50 mol% of epoxidation and conventional sulphur vulcanization were used. The cure time t₉₀, scorch time t₂, M_HR − M_L (maximum torque − minimum torque) and fatigue life of all rubbers were found to increase with increasing palm oil fatty acid concentration. However, the reversion of all rubbers decreases with increasing palm oil fatty acid concentration. At similar concentrations of palm oil fatty acid, ENR 50 compounds exhibit the shortest scorch and cure times followed by ENR 25 and SMR L compounds. For reversion, SMR L compounds show the lowest value followed by ENR 50 and ENR 25 compounds, whereas for fatigue life, the highest value is obtained with ENR 50 compounds followed by ENR 25 and SMR L compounds. © 1999 Society of Chemical Industry     

Properties of Halloysite Nanotube (HNT) Filled SMR L and ENR 50 Nanocomposites

Various HNTs loading filled SMR L and ENR 50 were prepared. Addition of HNTs caused increments in scorch time, cure time, tensile modulus, and thermal stability of nanocomposites. Optimum tensile strength of nanocomposites was achieved at 20 phr loading. Elongation at break, swelling percentage, and fatigue life decreased with increasing HNTs loading. ENR 50 nanocomposites show shorter scorch time, longer cure time, and lower curing rate index than SMR L nanocomposites. ENR 50 nanocomposites also show higher tensile modulus and thermal stability than SMR L nanocomposites. SEM images show that HNTs can be dispersed more uniformly at lower filler loading.     

The Effect of Filler Loading and Epoxidation on Paper-Sludge-Filled Natural Rubber Composites

The effect of filler loading and epoxidation on curing characteristics, dynamic properties, tensile properties, morphology, and rubber-filler interactions of paper-sludge-filled natural rubber compounds have been studied. Two different types of natural rubber, SMR L and ENR 50, having 0% and 50% of epoxidation and conventional vulcanization were used. Paper sludge was used as a filler and the loading range was from 0 to 40 phr. Compounding was carried out using a laboratory-sized two-roll mill. The scorch time for both rubber compounds decreased with filler loading. The cure time was found to decrease with increasing filler content for SMR L vulcanizates, whereas for ENR 50, the cure time seemed to be independent of the filler loading. Dynamic properties, i.e., maximum elastic torque, viscous torque, and tan delta, increase with filler loading in both grades of natural rubber. Results also indicate that both rubbers show increment in tensile modulus but inverse trend for elongation at break and tensile strength. However, for a fixed filler loading, ENR 50 compounds consistently exhibit higher maximum torque, modulus at 100% elongation, and modulus at 300% elongation, but lower elongation at break than SMR L compounds. In the case of tensile strength, ENR 50 possesses higher tensile strength than SMR L at 10 to 20 phr, but the difference is quite small at 30 and 40 phr. These findings might be associated with better rubber-filler interaction between the polar hydroxyl group of cellulose fiber and the epoxy group of ENR 50.     

Abrasion Property of Epoxidized Natural Rubber

The abrasion resistance of two grades of epoxidized natural rubber (ENR 25 and ENR 50) and one grade of styrene-butadiene rubber (SBR) were studied using an Akron abrasion tester. An accelerated sulfur vulcanization system with 2-mercaptobenzothiazole (MBT) as the accelerator is used throughout the study. Carbon black (N 330), precipitated silica, and calcium carbonate were chosen as the fillers. The range of sulfur and filler loadings was from 1 to 5 phr and 10 to 50 phr, respectively. Mixing was done on a two-roll mill. Results obtained show that for all the rubbers studied, the volume loss due to abrasion decreases with increasing sulfur loading and passes through a minimum at about 3 phr of sulfur. This observation is attributed to the changes of cross-link types from monosulfidic to polysulfidic crosslink as sulfur concentration is increased. However, further sulfur loading would cause a “tight” cure, thus increasing the abrasion loss. For sulfur loading less than 3 phr, ENR 25 indicates the highest abrasion loss, followed by SBR and ENR 50. For the filled stock, minimum loss is observed at about 35–40 phr of filler. Reinforcing filler such as carbon black exhibits better abrasion resistance than calcium carbonate, a nonreinforcing filler. The abrasion loss increases at higher filler loading due to the dilution effect of fillers. Ozone plays an important role in the abrasion property of unsaturated rubbers, as reflected by the higher abrasion loss in the presence of ozone.     

Dependence of Mooney scorch time of SMR L,ENR 25, and ENR 50 on concentration and types of antioxidants

The effect of concentration of antioxidants on the Mooney scorch time of two grades of epoxidized natural rubbers (ENR 25 and ENR 50) and one grade of natural rubber (SMR L) was studied using a Monsanto automatic Mooney viscometer (MV 2000). Three types of antioxidants, viz., 2,2′‐methylene‐bis(4‐methyl, 6‐tertbutylphenol) (AO 2246), N‐isopropyl‐N′‐phenyl‐p‐phenylenediamine (IPPD) and poly‐2,2,4‐trimethyl‐1,2‐dihydroquinoline (TMQ) were used, and the concentration range was varied from 0 to 5 phr. The conventional vulcanization system with 2‐mercaptobenzothiazole (MBT) as the accelerator was used throughout the study. Results show that increasing the phenol‐based antioxidant (AO 2246) concentration will increase the scorch time of ENR at a lower temperature of vulcanization while its effect on SMR L is not significant. This retardation effect is attributed to the “solvation” of epoxide group by the phenolic group in AO 2246, thus reducing the activation of adjacent double bond in ENR. The scorch time, however, is shortened by the amine‐based antioxidants (IPPD and TMQ) for the three rubbers studied, a phenomenon associated with the ability of the amine group to enhance the formation of more active sulfurating agent and subsequently increases the cure rate as the concentration of the amine‐based antioxidants is increased. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2940–2946, 1999     

Effect of blend ratio on Mooney scorch time of rubber blends

The Mooney scorch times of three rubber blends [epoxidized natural rubber (ENR) 50/SMR L, ENR 50/styrene butadiene rubber (SBR), and Standard Malaysian Rubber SMR L/SBR] were studied in the temperature range of 120–160°C using an automatic Mooney viscometer. N-Cyclohexyl-2-benzothiazyl sulfenamide was used as the accelerator, and the rubber formulation was based on the conventional vulcanization system. Results for the blends investigated indicate that a negative deviation of scorch time from the interpolated value was observed, especially for temperatures lower than 130°C. This observation was attributed to the induction effect of the ENR 50 in the ENR 50/SMR L and ENR 50/SBR blends to produce more activated precursors to crosslinks, thus enhancing interphase crosslinking. To a lesser extent, SMR L also exhibited such an induction effect in the SMR L/SBR blend. At 120°C, maximum induction effect occurred at around a 40% blend ratio of ENR 50 and SMR L in the respective blends. For the filled stock at 140°C, carbon black exhibited less effect on the scorch property of the blends compared to silica. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1301–1305, 1998     

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     

Mooney scorch time of epoxidized natural rubber

The scorch property of accelerated sulfur vulcanization of three grades of expoxidized natural rubber (viz. ENR 10, ENR 25, and ENR 50) was studied by using Mooney Shearing Disk Viscometer in the temperature range of 100–180°C. Effects of accelerator types, concentration of accelerator, and carbon black on ENR 10 were also determined. Results obtained indicate a similar scorch behavior as that reported earlier for SMR L. However, some differences in the magnitude of scorch times in the temperature and concentration studies are observed between ENR and SMR L. These differences are attributed to the activation of a double bond by the adjacent epoxide group in ENR, the effect being more significant for a higher degree of epoxidation of natural rubber. In the case of ENR 50, differential scanning calorimetry measurement suggests that additional crosslink occurs via a ring-opening reaction at about 155°C. Based on first-order reaction kinetics, the apparent activation energy of vulcanization for the rubbers studied is estimated and discussed.     

Role of geopolymer as a cure activator in sulfur vulcanization of epoxidized natural rubber

Geopolymer (GP) was synthesized and used as activators in sulfur vulcanization of epoxidized natural rubber (ENR). Influences of GP on cure characteristics, crosslink density, mechanical, thermal, and morphological properties were investigated and compared to the conventional rubber formulation with ZnO activator. The ZnO is a hazardous chemical for the environment and has proclaimed that its application in rubber technology should be reduced and controlled. It was found that the GP-activated ENR compounds showed significantly higher vulcanization rate than cases with the conventional ZnO compound. This was indicated by the GP activated compounds having shorter scorch time, cure times, and lower activation energy but higher cure rate index (CRI). Also, the GP activated ENR compounded with stearic acid exhibited the highest conversion. This matches well the highest torque difference and crosslink density, observed by temperature scanning stress relaxation (TSSR) and swelling measurements. Furthermore, the GP-activated vulcanizate had better thermal stability than the ZnO-activated ENR material. In addition, the GP-activated ENR vulcanizate with stearic acid exhibited high 100% moduli, tensile strength, and hardness. This proves that GP has a high potential for use as activators in sulfur vulcanization of rubber compounds, as an alternative to the conventional ZnO. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48624.     

On the potential of organoclay with respect to conventional fillers (carbon black,silica) for epoxidized natural rubber compatibilized natural rubber vulcanizates

Onium modified montmorillonite (organoclay) was compounded with natural rubber (NR) in an internal mixer and cured by using a conventional sulfuric system. Epoxidized natural rubber with 50 mol % epoxidation (ENR 50) in 10 parts per hundred rubber (phr) was used as a compatibilizer in this study. For comparison purposes, two commercial fillers: carbon black (grade N330) and silica (grade vulcasil‐S) were used. Cure characteristics were carried out on a Monsanto MDR2000 Rheometer. Organoclay filled vulcanizate showed the lowest values of torque maximum, torque minimum, scorch, and cure times. The kinetics of cure reaction showed organoclay could behave as a cocuring agent. The mechanical testing of the vulcanizates involved the determination of tensile and tear properties. The improvement of tensile strength, elongation at break, and tear properties in organoclay filled vulcanizate were significantly higher compared to silica and carbon black filled vulcanizates. In terms of reinforcing efficiency (RE), organoclay exhibited the highest stiffness followed by silica and carbon black filled vulcanizates. Scanning electron microscopy revealed that incorporation of various types of fillers has transformed the failure mechanism of the resulting NR vulcanizates compared to the gum vulcanizates. Dynamic mechanical thermal analysis (DMTA) revealed that the stiffness and molecular relaxation of NR vulcanizates are strongly affected by the filler–rubber interactions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2438–2445, 2004     

Cure index and activation energy of vulcanization of natural rubber and epoxidized natural rubber vulcanized in the presence of antioxidants

The cure index and apparent activation energy of vulcanization of one grade of natural rubber (SMR L) and two grades of epoxidized natural rubbers (ENR 25 and ENR 50) were studied in the presence of three types of antioxidants [viz., 2,2′methylene‐bis(4‐methyl‐6‐tertbutylphenol) (AO 2246), poly‐2,2,4‐trimethyl‐1,2‐dihydroquinoline (TMQ), and N‐isopropyl‐N′‐phenyl‐p‐phenylenediamine (IPPD)] in the temperature range of 120–180°C by using a Monsanto automatic Mooney viscometer. Accelerated sulfur vulcanization system and up to 5 phr of antioxidant concentration was used throughout the investigation. Results indicate that both cure index and apparent activation energy of vulcanization are dependent on the type and concentration of the antioxidant used. AO 2246 (a phenol‐based antioxidant) would retard vulcanization as reflected by the higher cure index and activation energy, an observation which is attributed to the solvation and steric hindrance effects of the antioxidant. On the contrary, both TMQ and IPPD (amine‐based antioxidants) exhibit reverse behavior due to the catalytic effect of the antioxidants in generating more active sulfurating agents for vulcanization. In all cases, SMR L gives higher cure index and apparent activation energy than the corresponding ENR, a phenomenon which is associated with the activation of the adjacent double bond by epoxide group in the latter. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3234–3238, 2000     

Effects of silane coupling agents on the vulcanization characteristics of natural rubber

The vulcanization characteristics of silica‐filled natural rubber (NR) were studied in the presence of silane coupling agents, 3‐octanoylthio‐1‐propyltriethoxysilane (NXT) and bis [(3‐triethoxysilylpropyl) tetra sulfide] (TESPT, or Si‐69). The scorch time, cure rate index, and the rate constant of NR were measured using an Oscillating Disk Rheometer in the temperature range of 140–170°C. The scorch time of NR decreases with increasing TESPT and NXT concentrations, and the scorch time of NR compound with NXT is longer than that with TESPT. The apparent activation energies of vulcanization for the compounds with NXT and TESPT both decrease with increasing silane concentration. The apparent activation energy of vulcanization of NR compound with NXT is lower than that with TESPT at various concentrations. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1511–1518, 2004     

Acrylic rubber‐fluorocarbon rubber miscible blends: Effect of curatives and fillers on cure,mechanical, aging,and swelling properties

The cure characteristics and mechanical properties of gum and filled acrylic rubber (ACM), fluorocarbon rubber (FKM), and their blends of varying compositions were studied both under unaged and aged conditions. The rheometric study showed that optimum cure properties were obtained using a mixed curing system of blocked diamine, hexamethylenediamine carbamate (Diak #1), and ammonium benzoate. From varying the curing agents, the optimum levels of Diak #1 and ammonium benzoate were found to be 1.5 and 2.5 phr, respectively. The addition of different fillers and their loading influenced the cure properties, with increased torque and reduced scorch safety. The gum and filled 50:50 (w/w) ACM‐FKM showed overall performance in strength properties. Postcuring improved the strength of all the systems, especially the systems with a higher proportion of FKM. None of the properties changed significantly during aging of the blends. FKM and the blends containing a higher proportion of FKM were affected least by aging. Swelling of the blends was reduced by the addition of fillers. Dynamic mechanical thermal analysis showed a single tan δ peak corresponding to a single phase transition for both cured and filled blends. The storage modulus of the blend increased from the gum blend to the filled blend, indicating the presence of polymer‐filler interaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1442–1452, 2003     

Bio‐processing aids based on jatropha seed oil and its epoxidized derivatives in carbon black‐reinforced natural rubber

Jatropha seed oil (JSO) is renewable oil with unique characteristics including a high flash point, low viscosity and saponification value, and a solubility parameter close to that of natural rubber. The incorporation of JSO and its epoxidized derivatives containing 50% (EJSO‐50) and 100% mole epoxide groups (EJSO‐100) in carbon black‐filled natural rubber was compared with the incorporation of paraffinic, naphthenic, and treated distillate aromatic extract oils in a range of 2–10 phr. The results showed that JSO promoted good rubber processability similar to that achieved with paraffinic and naphthenic oils, with the advantages of the shortest cure time and scorch time. However, it was found that the epoxide groups especially in EJSO‐100 can interfere in the vulcanization reaction resulting in poorer cure behaviors and mechanical properties. The abrasion resistance of the rubber containing JSO was the highest and progressively improved in a range of 4–8 phr. Moreover, using the various oils at this range of values can improve filler dispersion, sustain the tensile properties, and achieve a balance between wet grip and rolling resistance. J. VINYL ADDIT. TECHNOL., 26:62–76, 2020. © 2019 Society of Plastics Engineers     

Swelling and mechanical properties of (acrylonitrile‐butadiene rubber)/(hydrogenated acrylonitrile‐butadiene rubber) blends with precipitated silica filled in gasohol fuels

This work studied the effects of hydrogenated acrylonitrile‐butadiene rubber (HNBR) and precipitated silica (PSi) loadings in acrylonitrile‐butadiene rubber (NBR) filled with 60 parts per hundred of rubber (phr) of carbon black (CB) for oil‐resistant seal applications in contact with gasohol fuel. The cure characteristics, mechanical properties, and swelling behavior of HNBR/NBR blends reinforced with PSi before and after immersion in ethanol‐based oils (E10, E20, and E85) were then monitored. This work studied the effects of PSi loading in rubber compounds on the mechanical properties of the rubber blends. The results suggested that the scorch time of CB‐filled NBR/HNBR was not affected by HNBR loading, but the cure time, Mooney viscosity, and torque difference increased with HNBR content. The swelling of the blends in E85 oil were relatively low compared with those in E10 and E20 oils. The recommended NBR/HNBR blend ratio for oil‐resistant applications was 50/50. Tensile strength and elongation at break before and after immersion in gasohol oils increased with HNBR loading, and the opposite effect was found for tensile modulus and hardness. PSi filler had no effect on scorch time, but decreased the cure time of the blends. The swelling level of the blends slightly decreased with increasing PSi content. The recommended silica content for optimum reinforcement for black‐filled NBR/HNBR blend at 50/50 was 30 phr. The results in this work suggested that NBR/HNBR blends reinforced with 60 phr of CB and 30 phr of silica could be potentially used for rubber seals in contact with gasohol fuels. J. VINYL ADDIT. TECHNOL., 22:239–246, 2016. © 2014 Society of Plastics Engineers     

Curing Characteristics,Fatigue and Hysteresis Behaviour of Feldspar Filled Natural Rubber Vulcanizates

Curing characteristics, fatigue, and hysteresis behaviour of feldspar filled SMR L vulcanizates and feldspar filled ENR 50 vulcanizates were studied. Two different types of natural rubber, SMR L and ENR 50 having 0 and 50 mol% of epoxide groups were used. The feldspar filled natural rubber vulcanizates were compared at similar filler loading which were used at 0, 10, 20, and 30 phr of filler loading. The curing characteristics such as scorch time (t ₂) and cure time (t ₉₀) slightly increased with increasing feldspar loading for both rubber vulcanizates. Besides t ₂ and t ₉₀, maximum torque (M _HR) significantly increased for both rubbers with increasing feldspar loading. The fatigue test showed that fatigue life decreased with increasing extension ratio, strain energy and filler loading. As the filler loading increased, the poor wetting of the feldspar by the rubber matrix gave rise to poor interfacial adhesion between filler and rubber matrix. Results also indicate that the vulcanizates with the highest feldspar loading exhibited the highest hysteresis. The feldspar filled SMR L vulcanizates showed higher fatigue life and lower hysteresis compare to feldspar filled ENR 50 vulcanizates.     

THE COMPARISON PROPERTIES OF RECYCLE RUBBER POWDER,CARBON BLACK,AND CALCIUM CARBONATE FILLED NATURAL RUBBER COMPOUNDS

The effects of filler loading on the curing characteristics, swelling behavior, and mechanical properties of natural rubber compounds were studied using a conventional vulcanization system. Recycle rubber powder (RRP), carbon black (CB) (N550), and calcium carbonate (CaCO₃) were used as fillers and the loading range was from 0 to 50 phr. Results show that the scorch time, t ₂, and cure time, t ₉₀, decrease with increase in filler loading. At a similar filler loading, RRP shows shortest t ₂ and t ₉₀ followed by CB and calcium carbonate. The tensile strength, tensile modulus, and hardness increase with increase in CB loading, whereas elongation at break, resilience, and swelling properties show opposite trend. For RRP and calcium carbonate filled natural rubber compounds, the tensile strength increases up to 10 phr and starts to deteriorate at higher filler loading. The other properties such as tensile modulus, hardness, elongation at break, resilience, and swelling percentage show a small change (increase or decrease) with increase in RRP and calcium carbonate loading in natural rubber compounds. Overall results indicate that RRP can be used as a cheapener to replace calcium carbonate in natural rubber compounds where improved mechanical properties are not critical.     

Effect of stearic acid concentration on the reversion behavior of epoxidized natural rubber

The effect of stearic acid concentration on the reversion behavior of epoxidized natural rubber (ENR 25 and ENR 50) was carried out in the temperature range of 150–180°C and 0.5–14.5 phr of stearic acid loading. Three common accelerators; namely, zinc dimethyldithiocarbamate (ZDMC), tetramethylthiuram disulfide (TMTD), and N-morpholinylbenzothiazole-2-sulfenamide (MBS) and conventional sulfur vulcanization system was used throughout the study. A Monsanto oscillating-disk rheometer was used to determine the reversion behavior of the rubber. Results indicate that for all the accelerators used, reversion decreases with increasing stearic acid concentration. The rate of decrease is more gradual up to about 6.5 phr of stearic acid, after which a rapid drop of reversion is observed for both ENR 25 and ENR 50. This observation is attributed to the increasing amount of mono- and disulfidic crosslinks as a result of desulfuration of polysulfidic crosslinks. The higher the stearic acid loading, the greater is the retardation effect on vulcanization; that is, more time is available for desulfuration. Increasing temperature would increase the reversion of the rubber vulcanizate because of the increase in thermal energy to decompose more crosslinks, including mono- and ether crosslinks for temperature higher than 160°C. ZDMC (an ultrafast accelerator) gives a higher reversion than MBS (a delay-action accelerator), because desulfuration occurs more slowly in the former system. The observed decrease in reversion as stearic acid concentration increases is technologically importance, because the aging property of the rubber vulcanizate is significantly improved. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1165–1169, 1999