Reclaiming of ground rubber tire by a novel reclaiming agent. I. virgin natural rubber/reclaimed GRT vulcanizates.
In the twenty-first century, recycling of waste materials is of growing importance for all the industries in the world. The automotive and transportation industries are the biggest consumers of new rubber products. Rubber waste is usually generated from both the products of the manufacturing process and post consumer products, mainly consisting of scrap tires. At the end of 2003, the U.S. generated ~290 million scrap tires. Developed countries have been paying great attention to the comprehensive utilization of discarded tires to achieve the goals of protecting environment and recycling resources [1, 2]. Almost in all commercial applications recycle or reclaim rubber is used as a component of a blend with fresh rubber. Incorporation and dispersion of reclaim rubber in fresh rubber play important roles towards the product quality, production economy, and market competition. Homogeneous dispersion of reclaim rubber in virgin rubber is essential for optimum vulcanizate properties. Therefore, it is necessary to evaluate the performances of such blends containing reclaim rubber.
Grigoryeva and coworkers  used partially thermo-chemically devulcanized ground tire rubber (GTR) with virgin rubber in 20-80% level. Here, the use of 20% reclaimed GTR in SBR/GTR and BR/GTR revulcanizates results improved tensile mechanical properties and elasticity. The increasing reclaimed GTR content up to 40-80 wt% leads to the significant decrease of elongation at break values of the revulcanizates and some reduction of tensile strength values. De et al.  used mechanochemically devulcanized crosslinked rubber with fresh natural rubber (NR) in 25-60% level. Here reclaiming was carried out by a renewable resource material having the major constituent diallyl disulfide. The increasing reclaim rubber content reduce the tensile strength by about 6% for 25 wt% reclaim containing vulcanizate and 16% for 40 wt% reclaim containing vulcanizate. Klingensmith  used cryoground butyl rubber with fresh virgin rubber in 5-15% level. Such small amount of reclaim rubber appeared to be incorporated as filler with respect to virgin rubber. It is well known that addition of GTR to a number of polymers (e.g. rubbers or thermoplastics) shows poor compound properties [6-15] because of very weak interfacial adhesion between the GTR particles and the matrix forming polymer. This is because of the crosslinked structure of GTR and as a consequence the GTR molecules cannot entangle with the molecules of polymer matrix. In a review paper, Adhikari et al.  have successfully discussed various process of reclaiming in presence of miscellaneous chemicals.
This article involves a development of new elastomer products based on virgin NR and reclaimed GRT. The first step was mechanochemical reclaiming of GRT by tetra methyl thiuram disulfide (TMTD) as reclaiming agent  and a second step was reuse of reclaimed GRT in formulation with virgin NR. Here, the reclaimed GRT was blended and (re)vulcanized with the different proportion of virgin NR to produce new low-cost material products with useful properties, for example, tires. We use the term "(re)vulcanization" because there are two simultaneous processes such as vulcanization of the virgin rubber and revulcanization of partially devulcanized GRT and even the co-vulcanization of them. Curing characteristics and tensile properties before and after aging of such rubber compounds containing reclaim rubber have been studied. Subjects on dynamic mechanical properties and swelling behavior of rubber vulcanizates are still open for discussion, especially with addition of reclaimed rubbers into virgin rubber compounds as these data are still limited in the literature. In this present article, dynamic mechanical properties and swelling behavior of the blends were assessed. An electrical property of virgin NR and reclaimed GRT blend was also evaluated.
GRT, purchased from local market was used. The GRT was an unclassified ground rubber from the tread and sidewalls of passenger and truck tires. The ground particles were of various sizes ranging from a few millimeters to 100 [micro]m. NR (RSS-1, Rubber Board, India), TMTD (ICI Ltd, India), zinc oxide (S. D. Fine Chem, India), stearic acid (Loba Chemie, India), sulfur (S. D. Fine Chem, India), spindle oil (MCI, India), carbon black (N330, Phillips Carbon), and toluene (S. D. Fine Chem, India) were used as received.
Preparation of Reclaim Rubber. One hundred grams of ground rubber was mixed with 2.75 g TMTD and 10 ml spindle oil. The mixture was then mechanically milled in an open two-roll mill and milling was carried out at a friction ratio of 1.2 for 40 min at near ambient temperature. It has been found that with progress of milling the materials become soft, sticky and band formation occurs on the roll. The extent of reclaiming such as gel content, sol content, inherent viscosity of sol rubber, crosslink density, molecular weight between crosslink bonds, and Mooney viscosity of reclaim rubber are shown in Table 1.
Preparation of NR-RR Vulcanizates. Mixing of fresh NR, various proportions of reclaim rubber, and compounding ingredients was carried out at room temperature on an open two-roll mixing mill. The cure characteristics of the rubber compounds were determined with the help of a Monsanto Oscillating Disc Rheometer, R-100 at 160 C. All cure curves were observed to level off in the region of 60 min, where the torque-time gradient of each sample was constant or did not change significantly.
The compounded rubber stock were then placed in a mold and pressed between the platens of a hydraulic press (Carver, Model 2518). The samples were cured at 160[degrees]C temperature and at applied pressure of 34.5 MPa for the respective optimum cure times (t = [t.sub.90]) obtained from rheographs. After curing, the sheet was taken out of the mold and immediately cooled under tap water to restrict from further curing.
Measurement of Mechanical Properties. A Hounsfield, model H10 KS tensile testing machine was used to measure the modulus, tensile strength, elongation at break of the vulcanizate as per ASTM D 412-51T at room temperature (25 [+ or -] 2)[degrees]C at a uniform speed of separation 500 mm/min. Hardness (shore A) of the vulcanizates were measured by a Hirosima Hardness Tester as per ASTM D 1415-56T. The values reported were based on the average of five measurements for each sample. Accelerated aging test was performed in an air aging oven at (70 [+ or -] 2)[degrees]C for 72 h. The mechanical properties were also measured after accelerated aging condition.
Mooney Viscosities of rubber compounds were determined by a Monsanto Mooney Viscometer 2000 at ML (1 + 4) 100[degrees]C as per ASTM D 1646.
Determination of Cross-Link Density. The crosslink densities of the rubber vulcanizates were determined by the Flory-Rehner equation 
-ln(1 - [V.sub.r]) - [V.sub.r] - [chi][V.sub.r.sup.2] = 2[V.sub.s][[eta].sub.swell] ([V.sub.r.sup.[1/3]] - [2[V.sub.r]/f]), (1)
where [V.sub.r] is the volume fraction of rubber in the swollen gel, [V.sub.s] is the molar volume of the toluene (106.2 [cm.sup.3]/mol in this study), [chi] is the rubber-solvent interaction parameter (0.3795 in this study), [[eta].sub.swell] is cross-link density of the rubber (mol/[cm.sup.3]), and f is functionality of the crosslinks (being four for sulphur curing system).
The volume fraction of a rubber network in the swollen phase is calculated from equilibrium swelling data as:
[V.sub.r] = ([W.sub.2]/[d.sub.2])/[([W.sub.1]/[d.sub.1]) + ([W.sub.2]/[d.sub.2])], (2)
where [W.sub.1] is the weight fraction of solvent, [d.sub.1] is the density of the solvent, [W.sub.2] is the weight fraction of the polymer in the swollen specimen, and [d.sub.2] is the density of the polymer.
Thermo Gravimetric Analysis. The thermo gravimetric analysis (TGA) of the vulcanizate was carried out by using a TGA 50, Shimadzu, Japan, thermo gravimetric analyzer in nitrogen (flow rate 50 ml/min) within the temperature range of 20-800[degrees]C. All these analysis were carried out at heating rate of 10[degrees]C/min.
Measurement of Dynamic Mechanical Properties. The dynamic mechanical properties of NR/RR blends were determined using Dynamic mechanical analyzer (DMA), Model 2980, supplied by TA instrument (USA). The shape of test sample was rectangular, 80 mm long, 10 mm wide and 2 mm thick. The single cantilever mode of deformation was used under the test temperature range from -100 to 70[degrees]C with a heating rate of 1[degrees]C/min, the test amplitude and frequency being 15 [micro]m and 1 Hz, respectively. The cooling was achieved through liquid nitrogen. The results were presented in terms of loss tangent (tan [delta]) and glass transition temperature ([T.sub.g]). In this investigation, loss tangent (tan [delta]) was the ratio of loss modulus (E") to storage modulus (E') and [T.sub.g] was obtained from the loss tangent peak.
Measurement of Swelling Behavior. A sorption-de-sorption method was used to determine the swelling behavior of the vulcanizates, this being demonstrated as the mole percent uptake of toluene by 100 g of rubber at 25[degrees]C. Accurately weighed small pieces of rubber samples were immersed in toluene. The swollen samples were taken out periodically and the solvent was blotted from the surface of the sample by light contact pressure with filter paper and weighed immediately. After equilibrium swelling, the sample was taken out of the toluene and weighed until their weights were unchanged. The mole percent uptake of solvent at time t ([Q.sub.t]) is 
[Q.sub.t] = [([[W.sub.1] - [W.sub.0]]/[W.sub.0])/[M.sub.w]] x 100, (3)
where [W.sub.0] and [W.sub.1] are the weights of dry and swollen samples, respectively. [M.sub.w] is the molar mass of toluene (92.14 g/mol).
Measurement of Dielectric Properties. Rubber samples were vulcanized in a mold with a spacer about 0.5 mm thick and 3.8 cm diameter to the corresponding [t.sub.90]. Dielectric measurements were carried out by a Dielectric Spectrometer (model DS100U, Seiko Instruments, Japan) at room temperature in a three terminal parallel plate with working electrode diameter 35.6 mm, allowing adequate overlap of the guard ring by the centrally placed specimen. Dielectric constant and loss factor was measured over the frequency range 10 Hz to 100 KHz at room temperature.
Scanning Electron Microscopy. To study the coherency and homogeneity in the NR/RR vulcanizate, scanning electron microscopic (SEM) studies were done on a JEOL, JSM 5800. The failure behavior was also analyzed using SEM.
RESULTS AND DISCUSSION
Characterization of Reclaim Rubber
The extent of reclaiming of mechanically reclaimed GRT by TMTD is reported in Table 1. The extent of reclaiming was monitored by measurement of sol content, inherent viscosity of sol rubber, gel content, crosslink density of reclaim rubber, molecular weight between crosslinks, swelling ratio, and Mooney viscosity of reclaim rubber.
Compounding of Reclaim Rubber With NR
Compound formulations are presented in Table 2. The amounts of additives such as ZnO, stearic acid, and sulfur were used based on 100 g rubber irrespective of the amount of reclaim rubber used in the compound, because it was observed that the additives in reclaim rubber originated from parent compound were inactive . But the amount of TMTD was maintained at 9 mmol in all the vulcanizates based on the amount of TMTD used during reclaiming of GRT. Formulation 1 contains no reclaim rubber and formulation 2-6 contains various proportion of reclaim rubber from 20 to 60 wt%. Formulations 7-10 were made to study the effect of carbon black on the performances of reclaim rubber in the NR/RR (80/20) blend. Formulation 7 contains only NR with 40 phr carbon black; where as formulation 8-10 contain different proportion of carbon black (20, 30, and 40 phr) in NR/RR (80/20) blend. During compounding with the carbon black it has been observed that with the progressive loading of carbon black, its incorporation and dispersion become gradually difficult. With higher amounts of carbon black loading, the compounds become stiff and the temperature goes high because of higher shearing action required for better dispersion.
Figure 1 shows the cure behavior of NR/RR vulcanizates at a cure temperature of 160[degrees]C. It is seen that the torque attains a maximum at less than 20 min and then reaches a constant level. The maximum rheometric torque increases with increasing reclaim rubber content, because of the presence of crosslinked gel in the reclaim rubber. Curing characteristics of the rubber compounds containing reclaim rubber are given in Table 2 shows that with increase in reclaim rubber content optimum cure time decreases, but scorch time remain unaltered in all the cases. Such low scorch time data shows the reclaim rubber to be scorchy in nature. The increase in the extent of cure is due to higher proportion of reclaim rubber in NR/RR vulcanizates which contain crosslinked gel.
It has been found that with increase in carbon black loading the optimum cure time and scorch time remain unaffected but extent of cure increases with increasing carbon black loading.
[FIGURE 1 OMITTED]
Evaluation of Tensile Properties of Rubber Compound
The stress strain curves for NR/RR vulcanizates with various proportion of reclaim rubber and NR/RR (80/20) blend system with different proportion of carbon black loading are shown in Figs. 2 and 3, respectively. In all NR/RR vulcanizates, the moduli at 100 and 200% elongation were increased with increasing reclaim rubber content compared to that of the pure rubber vulcanizates, respectively. The elongation stress was decreased with increasing reclaim rubber content. Especially, the stress strain of NR/RR vulcanizates was decreased after 50% elongation than that of pure rubber vulcanizates. The same trend of stress strain behavior was observed for NR/RR (80/20) vulcanizates with various carbon black loading. Tensile properties and crosslinking value of NR/RR bends are shown in Table 2. From the data in Table 2, it is seen that with increase in the proportion of reclaim rubber (RR) content both 100 and 200% moduli increase but tensile strength and elongation at break decrease. The reason for higher 100 and 200% moduli may be due to higher crosslink density of rubber vulcanizates, arising out of the gel present in reclaim rubber, which is also corroborated by crosslinking value data. As crosslink density increases with increasing reclaim rubber content in the rubber matrix, chain mobility decreases and more load is required for 100 and 200% elongation. When the crosslink density is 5.138 x [10.sup.-5] mol/[cm.sup.3] for control (formulation 1) then the tensile strength is maximum. With further increase in crosslink density value for other formulations containing reclaim rubber tensile strength gradually decreases.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
There is another factor that is also responsible for decrease in the value of tensile strength. Because reclaim rubber contains crosslinked gel, when this is blended with virgin rubber much gel remains as such without dispersing as a continuous matrix with virgin rubber. Such gel remains present as weak sites for stress transmission to its surrounding (continuous matrix) resulting in a lower tensile stress. Elongation at break decreases with the higher percentage of reclaims rubber. Hardness also increases with increasing reclaim rubber content. Mooney Viscosity value increases with increasing reclaim rubber content.
Effect of carbon black loading was studied in NR/RR (80/20) blends. From Table 2, it is seen that with increase in the proportion of carbon black 100 and 200% moduli increase. This also corroborated by corresponding increase of crosslink density data. From crosslink density data, it is evident that with increase in the proportion of carbon black loading crosslink density increases and hence 100 and 200% moduli increases. Tensile strength gradually decreases with increase in carbon black loading. Elongation at break also decreases with increase in carbon black content. Hardness increases because when higher amount of carbon black was incorporated into rubber compounds vulcanizates becomes stiffer. Mooney viscosity data also increased when carbon black loading was increased as the rubber compound became stiffer by increase in carbon black loading and, therefore, processing of rubber compounds also became difficult.
Aging Characteristics of NR/RR Blend System
Because aging characteristics of the rubber compounds containing reclaim rubber need attention, accelerated aging test were also performed with the compounds as formulated in Table 2. The results of aging studies with the formulations 2, 3, 4, 5, and 6 containing reclaim rubber have been compared with those of a control formulation 1 without reclaim rubber. Tensile properties and hardness of NR/RR blend vulcanizates were measured after 72 h aging at (70 [+ or -] 2)[degrees]C in an air aging oven. Percent retention of 100% modulus, 200% modulus, tensile strength, elongation at break, and hardness is shown in Fig. 4. From the figure it is evident that both 100 and 200% modulus increases with increase in the proportion of reclaim rubber during aging because of increase in crosslink density. This is because of the fact that reclaim rubber may contain some active crosslinking sites that form crosslink bonds during aging. It has been found that with increase in the reclaim rubber content, both the percent retention of tensile strength and elongation at break increases. Here on prolonged aging hardness marginally increases with increase in the reclaim rubber content. From the results it has been found that reclaim rubber containing vulcanizates show better aging resistance than that of the virgin rubber. Effect of carbon black loading on % retention of properties after 72 h aged NR/RR (80/20) blend system was shown in Fig. 5. From the figure, it is evident that both 100 and 200% modulus continuously increases with increase in carbon black loading. But the increase in the value is not much pronounced with carbon black loading. A peculiar behavior in retention of tensile and elongation at break properties was observed for different carbon black loaded samples. It has been found, from Fig. 5, that retention of properties is very less when 30 and 40 phr carbon black were added to the rubber compound. But for 20 phr carbon black loading retention of tensile strength and elongation at break values are high when compared with 30 and 40 phr carbon black loaded vulcanizates. These results show the effectiveness of 20 phr carbon black in the 80:20 (NR/RR) blend to get better properties. In this connection, it is interestingly observed that the aging performances of formulations containing reclaim rubber are better than the control formulation that does not contain any reclaim rubber. This phenomenon shows some antiaging characteristics of reclaim rubber.
[FIGURE 4 OMITTED]
Figure 6 shows the sorption and desorption curves of NR and its blends with various reclaim contents in toluene. Generally, it was found that all cases showed similar patterns: the rate of toluene uptake was relatively fast in the initial stage and reached a plateau at an equilibrium state. Vulcanizates with greater reclaim content required less time to reach equilibrium. At the equilibrium state, the degree of swelling of the NR vulcanizates reduced with increasing reclaim content. The sample with 60% reclaimed rubber was seen to reach the equilibrium first, and exhibited the lowest value of toluene uptake. The decrease in [Q.sub.[alpha]] due to the reclaimed rubber was caused by the increases in crosslink density and carbon black content in the reclaim, which restricted the molecular movement of the polymer. This then made it more difficult for the solvent to penetrate through the rubbers, thus decreasing the swelling. The decrease in [Q.sub.[alpha]] could also indicate higher molecular interactions between carbon black in the reclaim and NR molecules . This can be quantitatively determined by examining the transport mechanism using following equations, where [M.sub.t] and [M.sub.[alpha]] are the weight percent uptakes of the solvent at time t and at equilibrium swelling, respectively .
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[M.sub.t]/[M.sub.[alpha]] = K[t.sup.n], (4)
log([M.sub.t]/[M.sub.[alpha]]) = log K + n log t. (5)
By plotting log([M.sub.t]/[M.sub.[alpha]]) against logt, the values n (slope) and K (intercept) were obtained. The calculated values of n and K are represented in Table 3. From the above equations, K is a constant depending on the structural characteristics of the rubber and interaction with the solvent, whereas n is an indication of diffusion mode. It is seen from the table that the values of n lie in the range of 0.45-0.60, the n values decreasing slightly with increasing reclaim content. The K values were also found to increase with increasing reclaim rubber content. This suggests a higher rubber-solvent interaction due to increased crosslink density of the vulcanizates. Higher rubber-solvent interaction indicated that more contacts between the rubber molecules and the solvent had occurred, these molecular contacts being greater with increasing reclaim rubber content. Under such condition, there would be fewer opportunities for the solvent to pass through the rubber molecules, and thus decreasing swelling.
During desorption, the rate of toluene loss was rapid and reached the original state (unswollen rubber) within 6-7 h, this phenomenon being also found by the workers [20, 23].
Thermo Gravimetric Analysis of NR/RR Blend System
The degradation temperature of NR and various NR/RR blends has been analyzed by thermogravimetric method. Literature shows that the main products of thermal degradation of NR are isoprene, dipentene, and p-menthene . It is found that the NR/RR blend systems exhibit higher thermal stability than the pure NR vulcanizates. The enhancement in thermal stability of blends is explained to the physical compatibility of the virgin NR with RR caused by the molecular level mixing during blending. The temperature interval of degradation stages evaluated from DTG curves, temperature of the stages maximum rate of degradation, sample weight loss at the temperatures, and char residue values are listed in Table 4. The thermal degradation in inert gas atmosphere of reclaim GRT occurs in three temperature regions. The weight loss of the rubber sample at 1st low intensity stage can be related mainly to thermal decomposition of vulcanization crosslinks (sulfur links), decomposition of links formed by zinc oxide, stearic acid, and partial breakage of rubber backbone. Intensive thermal depolymerization process occurs at the 2nd stage with maximum rate of weight loss at 399.4[degrees]C. The weight loss slows down at 3rd stage. Above 700[degrees]C, sample weight becomes constant. From the table, it is also evident that the thermal stability of the vulcanizate increases with increasing the proportion of reclaim rubber. The crosslink density of the vulcanizate increases with increase in reclaim rubber content and crosslinking increased the rigidity of the system, which in turn increased the thermal stability.
Dynamic Mechanical Analysis of NR/RR Blend System
The storage modulus of pure NR vulcanizates and various NR/RR vulcanizates was shown in Fig. 7. The room temperature storage moduli of the blends increases with increase in % reclaim rubber content (Table 5). From Table 5, it is evident that room temperature storage modulus continuously increases with increasing reclaim rubber content. From Fig. 8, it is also evident that beyond room temperature storage modulus increases with increasing reclaim rubber content. This is due to the fact that with increasing reclaim rubber content the crosslink density of the blend vulcanizates increases, which makes the vulcanizates more rigid. Here all the storage modulus graphs show three distinct region, that is, glassy, transition, and rubbery regions. The sharp fall of storage modulus in the transition region was due to the mobility of polymer chains that increases with temperature. Generally, it is well known that if the storage modulus value becomes higher, the traction property will be more excellent. It indicates that the elastic and storage modulus of the rubber vulcanizates becomes improve with increasing reclaim rubber content.
[FIGURE 7 OMITTED]
Temperature dependence of loss modulus for pure NR and different NR/RR vulcanizates was represented in Fig. 8. From the figure, it is clear that the loss modulus of NR/RR (50/50) and NR/RR (40/60) vulcanizates became higher than that of the pure NR vulcanizates. Since the loss modulus is related to the viscoelasticity of rubber, therefore it indicates that the viscoelasticity of the vulcanizates is enhanced with higher reclaim rubber content such as 50 and 60 phr. Where as the loss modulus values of other NR/RR blends became lower than that of the pure NR vulcanizates. This indicates that the heat build up for these particular vulcanizates will be to a lower extent. This is an added advantage for the vulcanizates containing various proportion of reclaim rubber such as 20, 30, and 40 phr.
[FIGURE 8 OMITTED]
The glass transition temperature of the pure NR and different NR/RR blends were measured from tan [delta] peak. The values are presented in Table 5. It is found from the Table that glass transition temperature decreases with increase in reclaim rubber content up to a certain level and then again increases. But the [T.sub.g] values of all the blends are higher compared to that of the pure NR vulcanizates. From this observation, we can conclude that at higher proportion of reclaim rubber an incompatible blend is obtained which increases the glass transition temperature of the vulcanizate. Where as for lower proportion of reclaim rubber containing vulcanizate a compatible blend is obtained. For compatible blend system, glass transition should go down due to the favorable interaction between the systems.
The flex temperature ([T.sub.flex]) of the pure NR and different NR/RR blends were measured from storage modulus curve. The values are presented in Table 5. Flex temperature is the temperature at which rubbery plateau starts. It is generally measured by the intercept of two tangents, from transition region and rubbery plateau region of storage modulus curve. It is found from the table that flex temperature goes down because of incorporation of reclaim rubber in fresh NR. Thus, incorporation of reclaim rubber in fresh NR shows low temperature flexibility of the vulcanizates in its storage modulus region.
Temperature dependence of tan [delta] for pure NR and different NR/RR vulcanizates was presented in Fig. 9. The dynamic property, loss tangent (tan [delta]) is a function of viscoelasticity. The rubber for tire should be excellent for driving stability and handling stability during drive. The properties are related to the viscoelasticity of rubber. The viscoelasticity of rubber should be optimized with temperature variation. It is well known that the value of tan [delta] becomes higher; the traction properties become improve in a low temperature range. From Fig. 9, it is found that with increase in the reclaim rubber content up to 50 phr, tan [delta] increases and for NR/RR (40/60) vulcanizates a relatively lower tan [delta] is obtained. Therefore, it can be summarized that NR/RR (80/20), NR/RR (70/30), NR/RR (60/40), and NR/RR (50/50) blend vulcanizates have better traction property in a low temperature range compared to that of the other vulcanizates, where as NR/RR (40/60) blend vulcanizate have better rolling resistance property compared to that of the other vulcanizates.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
The dielectric loss, [epsilon]", for NR, RR, and their blends were measured over a frequency range 10 Hz to 100 kHz. The measurements were carried out at room temperature (27 [+ or -] 1)[degrees]C. The measured values of [epsilon]" versus log of applied frequency are illustrated graphically in Fig. 10. It is evident from Fig. 10 that loss factor [epsilon]" is lowest for NR in the whole frequency region and the loss factor is much more pronounced in the RR and NR/RR blend system. This may be explained by preferential distribution of reclaim rubber in NR phase and localization of reclaim rubber at the boundary of the polymer phase. The absorption in the lower frequency region might be attributed due to Maxwell-Wagner effect [25-27] which is very much dependent on the presence of foreign substance. The absorption in the high frequency region may be explained by the mobility of the main chain and its related motions . This a.c. current result from the significant difference in permittivity and resistivity of the blend and the ingredients added to the rubber. It is apparent that loss factor observed increases with increase in reclaim rubber content and the highest loss factor was observed for only RR vulcanizates in the entire frequency region. The absorption peak (interfacial polarization) observed at low frequency is caused by the alternating accumulation of charges at the interfaces of the constituent rubbers and is influenced by phase separation. In the blend system, the loss factor ([epsilon]") is always higher than that of the pure NR vulcanizate in the entire frequency region. This suggests that in the blend systems there are a large number of interfaces because of phase separation, which evidently becomes responsible for higher dielectric loss. This dielectric loss for NR/RR vulcanizates increases with increase in reclaim rubber content.
SEM photographs of the tensile fractured surface of NR, NR/RR (80/20), and NR/RR (40/60) (Fig. 11a-c) were taken to study the phase morphological structure and homogeneity of the vulcanizate. The micrograph of pure NR vulcanizate showed unidirectional ripples (lines of reinforcement) along the direction of flow, showing weak rubbery failure with higher tensile strength. The reclaim rubber containing vulcanizates contains several number of crack paths in different directions with number of holes, making the vulcanizate vulnerable under mechanical stress. From the micrograph it is also evident those with increase in the proportion of reclaim rubber, the number of crack paths in different directions and number of holes increases. The micrograph also indicated that the fractured surface of the NR/RR (40/60) vulcanizate had less homogeneity than that of the NR/RR (80/20) vulcanizate, because of the presence of higher proportion of reclaim rubber in the blend vulcanizate. Here in case of reclaim rubber containing vulcanizate, the fracture mode showed the failure mode that less rubbery in nature, because of higher crosslink density of the vulcanizate.
[FIGURE 11 OMITTED]
GRT is reclaimed mechanically in presence of a multifunctional reclaiming agent, TMTD. The reclaim rubber, prepared in this investigation, when blended with fresh NR has been found to reduce the tensile strength by about 7% for 20% reclaim containing vulcanizate and 46% for 60% reclaim containing vulcanizate. It is observed that the aging performances of the reclaim rubber containing vulcanizates are better than the control formulation, which does not contain any reclaim rubber. TGA shows that the thermal stability of the vulcanizate increases with increasing reclaim rubber content. Rubber vulcanizates with higher contents of reclaimed rubber required less time to reach equilibrium. The degree of swelling at the equilibrium state of the NR vulcanizates reduced with reclaim content.
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Debapriya De, (1) Debasish De, (2) G.M. Singharoy (3)
(1) Chemistry Department, MCKV Institute of Engineering, Liluah, Howrah 711204, India
(2) Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India
(3) Birla Tyres, Balasore, Orissa, India
Correspondence to: Debapriya De; e-mail: email@example.com
Contract grant sponsor: Department of Science and Technology (DST), New Delhi, India.
TABLE 1. Properties of reclaim rubber used for the preparation of NR/RR blend. Properties of reclaim rubber Sol content (%) 30.3 Inherent viscosity of sol rubber 0.3944 Gel content (%) 69.7 Crosslink density x [10.sup.-4] (mol/[cm.sup.3]) 6.587 Molecular weight between crosslinks x [10.sup.-3] 33.093 Swelling ratio 4.099 Mooney viscosity [ML (1 + 4) 100[degrees]C] 70.6 TABLE 2. Mix formulation and curing characteristics of NR/RR Compounds. Ingredients (phr) 1 2 3 4 5 Natural rubber (NR) 100 80 70 60 50 Reclaim rubber (RR) -- 20 30 40 50 Zinc oxide 5 5 5 5 5 Stearic acid 2 2 2 2 2 TMTD 2.16 1.61 1.335 1.06 0.785 Sulfur 0.5 0.5 0.5 0.5 0.5 Carbon black -- -- -- -- -- (N330) Spindle oil -- -- -- -- -- Curing characteristics Optimum cure time 3.5 3.0 3.0 2.75 2.5 ([t.sub.90], min) Scorch time 1.5 1.0 1.0 1.0 1.0 ([t.sub.s2], min) Extent of cure (dNm) 40 43 45.3 50.4 53.5 Cure rate index 50 50 50 66.7 57.14 ([min.sup.-1]) Mechanical properties 100% Modulus (MPa) 1.103 1.214 1.366 1.796 2.045 200% Modulus (MPa) 1.474 1.737 1.980 2.639 3.026 Tensile strength (MPa) 14.166 13.196 11.704 10.417 9.123 % Elongation at break 1240.12 1140.58 908.29 805.43 671.88 Hardness. (Shore A) 40 45 49 53 55 Crosslinking value, 0.212 0.261 0.288 0.330 0.370 (1/Q) Crosslink density x 5.138 6.829 7.514 9.060 10.444 [10.sup.-5] (mol/[cm.sup.3]] Mooney viscosity 6.0 6.7 9.2 14.1 15.4 [ML(1 + 4) 100[degrees]C] Ingredients (phr) 6 7 8 9 10 Natural rubber (NR) 40 100 80 80 80 Reclaim rubber (RR) 60 -- 20 20 20 Zinc oxide 5 5 5 5 5 Stearic acid 2 2 2 2 2 TMTD 0.51 2.16 1.61 1.61 1.61 Sulfur 0.5 0.5 0.5 0.5 0.5 Carbon black -- 40 20 30 40 (N330) Spindle oil -- 4 2 3 4 Curing characteristics Optimum cure time 2.25 2.5 2.25 2.25 2.25 ([t.sub.90], min) Scorch time 1.0 1.0 1.0 1.0 1.0 ([t.sub.s2], min) Extent of cure (dNm) 61 58 53 60.5 64.2 Cure rate index 80 66.7 80 80 80 ([min.sup.-1]) Mechanical properties 100% Modulus (MPa) 2.568 3.361 2.592 3.482 4.310 200% Modulus (MPa) 3.966 5.296 3.928 5.325 6.739 Tensile strength (MPa) 7.709 19.394 18.416 17.152 16.39 % Elongation at break 501.56 767.84 872.99 709.13 578.17 Hardness. (Shore A) 60 62 60 65 70 Crosslinking value, 0.435 0.302 0.314 0.342 0.357 (1/Q) Crosslink density x 13.135 -- -- -- -- [10.sup.-5] (mol/[cm.sup.3]] Mooney viscosity 16.2 28.1 17.3 29.3 31.5 [ML(1 + 4) 100[degrees]C] TABLE 3. Values of n and K of pure NR and different NR/RR vulcanizates. Sample code n K x [10.sup.2] NR (100) 0.46 9.98 NR/RR (80/20) 0.60 5.39 NR/RR (70/30) 0.51 8.73 NR/RR (60/40) 0.51 8.58 NR/RR (50/50) 0.47 10.48 NR/RR (40/60) 0.45 11.79 TABLE 4. Degradation temperature and % weight loss of NR/RR vulcanizates. Degradation Sample code Start Peak End % Weight loss % Residue NR 321.24 388.53 478.94 89.93 4.47 NR/RR = 80/20 307.65 391.76 490.59 82.55 11.35 NR/RR = 70/30 321.10 388.53 494.22 78.42 14.37 NR/RR = 60/40 297.33 386.90 504.07 78.03 16.34 NR/RR = 50/50 298.11 394.41 501.37 73.17 20.41 NR/RR = 40/60 304.84 391.61 499.13 69.72 22.92 1st degradation 2nd degradation Start Peak End % Wt loss Start Peak End % Wt loss RR 160 274 336 13.28 336 400 507 49.99 3rd degradation Start Peak End % Wt loss % Residue RR 507 566 671 25.79 9.81 TABLE 5. Values of G'. [T.sub.g], and [T.sub.flex] of pure NR and different NR/RR vulcanizates. G' (MPa) [T.sub.g] Sample code at 25[degrees]C ([degrees]C) [T.sub.flex] ([degrees]C) NR (100) 0.42 -50.1 -28.1 NR/RR (80/20) 0.49 -35.3 -41.2 NR/RR (70/30) 0.63 -40.4 -38.4 NR/RR (60/40) 0.81 -47.4 -42.7 NR/RR (50/50) 0.96 -45.1 -40.0 NR/RR (40/60) 1.29 -39.6 -37.0
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|Title Annotation:||ground rubber tire|
|Author:||De, Debapriya; De, Debasish; Singharoy, G.M.|
|Publication:||Polymer Engineering and Science|
|Date:||Jul 1, 2007|
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