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Hybrid composites prepared from Industrial waste: Mechanical and swelling behavior

In this assessment, hybrid composites were prepared from the combination of industrial waste, as marble waste powder (MWP) with conventional fillers, carbon black (CB) as well as silica as reinforcing material, incorporated with natural rubber (NR). The properties studied were curing, mechanical and swelling behavior. Assimilation of CB as well as silica into MWP containing NR compound responded in decreasing the scorch time and cure time besides increasing in the torque. Additionally, increasing the CB and silica in their respective NR hybrid composite increases the tensile, tear, modulus, hardness, and cross-link density, but decreases the elongation and swelling coefficient. The degradation property e.g., thermal aging of the hybrid composite was also estimated. The overall behavior at 70 °C aging temperature signified that the replacement of MS by CB and silica improved the aging performance.


Introduction
Pollution that is created as the outcome of human activities has banged onto the environment that life is about to face unexpected calamity. At present, there are hundreds of tones of gases, liquid and solid industrial wastes that spoil the soil, water and atmospheric environment and have unhealthy effect on human.
A gigantic quantity of marble waste is generated in marble carving industry as a by-product during the cutting/polishing process of marble blocks and is carried away by the drainage system or thrown away on open grounds. Consequently, employing of marble waste in the fabrication of new substances will assist to defend the environment. Polymer composites can be the best application to utilize marble sludge waste in large quantities to substitute the conventional fillers, clay and other materials. A quite few attempts have been made to use marble waste in road making, soil filling, and building construction materials [1] and asphaltic concrete [2] but very little effort together with our own have been made to employ as filler in rubber composites [3][4][5][6].
The polymer based composites, act as matrix while carbon black, silica and clay are act as reinforcing material.
Fillers enhance the mechanical properties for example, tensile, tear, hardness and abrasion resistance of the final product. Carbon Black and silica usually plays the vital role as reinforcing material of the rubber compounding and can also reduce the production cost [7][8][9][10][11][12]. Nowadays, mineral filler also added to polymers to achieve the improved product with low cost [13][14][15][16][17][18][19][20]. It is well-known that for cured filled rubber, the worth of reinforcement depends on the interaction of various filler related considerations counting the particle size, shape, dispersion, surface area, surface reactivity, structure of the filler, and bonding quality between the fillers and the matrix.
Currently different hybrid filler synchronization systems have been examined by numerous researchers. CB-silica hybrid filler system glances to be the most popular and successful. The dual phase filler is at present commercially manufactured by Cabot Corporation for the applications of truck tire [21]. The CB-silica hybrid filler system recommends generally overall improved mechanical properties compared to individual one. It also illustrates the most favorable balance of various properties for example wet traction, wear resistance and rolling resistance [22].
Rattanasom et al. [23] used the CB/silica hybrid filler with natural rubber and found better overall mechanical properties at 30/20 and 20/30 hybrid filler ratio. Besides that, RNP/carbon black and RNP/silica hybrid [24] and carbon black/nanoclay [25], carbon black/clay [26] have also been evaluated as reinforced hybrid systems.
Another low cost composite is carbon black and silica amended with newspaper (recycled). The use of combined fillers in polymers has been recognized for many years [27,28] persuaded by the demand for high performance materials.
Herein the study, the effects of partial or full replacement of marble waste by carbon black as well as silica, in hybrid composite on the overall properties is reported. Tensile, tear strength, modulus, elongation at break and hardness was investigated moreover swelling properties was also carried out to calculate the swelling ratio, crosslink density and shear modulus of hybrid filled NR composites.

Materials
The raw elastomer used was a natural rubber, NR (grade RSS-1) poly cis-isoprene, supplied by Rainbow rubber industry. The conventional reinforcing filler was carbon black (N330, particle 40-50 nm, specific gravity 1.80-1.82). Precipitated silica (Zeosil-175) by Rhodia. Marble waste as a sludge form, was obtained from home marble industry.
The MWP was dehydrated in the oven and then pulverized in finer form and passed through sieve to obtain 37 lm.
In addition to rubber, filler and other components such as Tetramethyl thiuram disulfide (TMTD), zinc oxide, sulfur as curing agent, 3-Dimethylbutyl-N-phenyl-p-phenylenediamine as antioxidant, were used as commercial grade and procured from the market.

Characterization of MWP by XRF spectrometer
The characterization of MWP was carried out with a XRF spectrometer (PIONEER with the Bruker AXS SPECTRA).

Preparation of composites
The basic formulation is given in Table 1. The composites were prepared as described previously [5,6,29].

Cure characteristics
The cure characteristics of mixes were studied as in our earlier works [30,31].

Testing of physical properties
Testing of mechanical, swelling and aging properties of MWP/ CB and MWP/Silica hybrid NR composites were carried out as described previously using standard procedures [4,32].
Obviously the composition of MWP shows calcium and magnesium compound in large amount. Silica, aluminum oxide and iron oxide also present in small amount.

Curing characteristics
The values of curing characteristics were determined from the corresponding curing isotherms measured at 155°C. Figs. 1 and 2 shows the assessment of replacement of MWP by CB as well as silica on scorch and cure time of the hybrid composite. The result illustrates that the scorch time as well as cure time reduced by increasing the amount of CB as well as silica in their particular hybrid system as compare to the unfilled and 60 phr MWP in NR composites. The high energy and heat develops during compounding, owing to greater viscosity with better shear heating of the rubber compounds results in diminishing scorch time as well as cure time. Reduction in scorch time by increasing CB as well as silica was suitable to develop additional cross-linked in the MWP/CB and MWP/silica dual filler system. The minimum and maximum torque increases by increasing CB as well as silica amount in their respective hybrid NR composite that is shown in Figs. 3 and 4. The torque generally lied on the degree of cross link and strength of the filler particles within the matrix [33] which trim downs the molecular manipulability of the matrix chain, moreover the result in rigid construction of the composite. Enhancement in the torque proved that continuing insertion of CB and silica in their corresponding NR hybrid composite developed the crosslink density of the NR compounds.
CB as well as silica is reinforcing fillers with having smaller particle and so much better interaction between the filler and NR as compare to MWP.
From obtained data it is clear that the addition of CB as well as silica in MWP with its content in NR compound influences the properties of prepared hybrid composites in various ways.
The CRI result given in Fig. 5 illustrates no considerable outcome on the ratio of CB and silica in NR composite.

Mechanical properties
The stress-strain curves of MWP/CB and MWP/silica hybrid composite were evaluated and results are shown in Figs. 6 and 7. From Figs. it is clear that the stiffness as governed by the slope of the initial linear part increases with increasing the concentration of CB and silica that acts as reinforcing filler in MWP/CB and MWP/silica hybrid composites. Consequently, elongation at break decreases. The stress-strain parameters; modulus of the samples as realized from the stress-strain curves were found to be significantly affected by increasing CB or silica concentration. Both results show that CB as well as silica content dependence of these parameters. Generally, it is clear that modulus increases with increasing     CB and silica content and reaches a maximum value at 60 phr of CB and silica amount in their particular NR composites. On the other hand, strain shows a minimum value around the same content. The consequent formation of compact structure [34] with increasing the contents of CB or silica may be conscientious for the minimum in strain and the maximum of stress observed at 60 phr of CB or silica amount.
The tensile strength of hybrid composites, before and after aging is revealed in Table 2. As expected, incorporation of CB as well as silica replaced by MWP in their relevant hybrid NR composite has improved tensile strength. This improvement was owing to the adequate interaction of carbon black as well as silica with NR over MWP. The well-built filler-rubber interaction increased the ability of stress to shuffle from matrix to filler therefore, boosted the tensile strength.
This is also attributed to good filler dispersion in the NR, inter-tubular and interfacial interactions of the filler and rubber besides to the filler intercalation with rubber constituents.
The filler, reinforcing level improves with reduce particle size or besides extend in surface area. Additional proficient will be stress shift from matrix to filler, when diminish the particle size [35].
The same tensile trend is viewed in samples after aging. However, a decrease is found in all MWP/CB and MWP/silica hybrid composite [36]. Thermal aging of MWP/CB and MWP/silica hybrid composite caused the tensile to be worse, particularly 100°C for 96 h [37]. Both aging properties show less than 100% retention though, aging at 70°C of hybrid composite observe higher retention value of tensile as compare to 100°C. This is due to better thermal stability at lower aging temperature.
According to the results, the tensile strength is much higher for the marble sludge/carbon black-filled NR hybrid composite than for the marble sludge/silica filled hybrid ones. This may be explaining as the reinforcing level and the crosslink density of both different types of filler. The reinforcing altitude is improved for the better dispersion of filler in matrix. Silica is poorer dispersion as compare to carbon black because silica has strong filler-filler interaction by silanol groups. The tensile strength of unfilled NR compound was not broken. Non-failure specimen has less peak load and higher elongation at break. However, when hybrid composites prepared the tensile strength gradually increases and the failure behavior raised with higher peak load and lower the strain. Table 3 shows the 200% modulus (before and after aging) of MWP/CB and MWP/silica hybrid composite. The result shows that the highest value of modulus for hybrid composite was obtained. Progressive substitution of MWP assisted better dispersion of CB as well as silica within matrix. It is observed that the modulus of MWP/CB and MWP/silica hybrid composites improved with the increasing CB as well as silica in their individual hybrid composite. This is due to the small particle size, CB as well as silica and their aptitude to have excellent dispersion within the matrix as compare to MWP that increased the rigidity of composites. Higher retention value have been revealed at 70°C, of 200% modulus of hybrid composite that may be due to the post curing and cross-linking, but at 100°C the% retention is less than 100 in 200% modulus is shown.
The modulus at 90-110°C decreases, conversely at 70-90°C the pace of modulus increase due to the intricacy of reactions taking place in vulcanized rubber [38,39].
Clarke et al. [40] investigated the thermal aging kinetics of NR compound and point up that cross-linking reaction and scission reaction ascend the rate of reaction through increase in aging temperature. The scission reactions have greater activation energy as compare to crosslink reaction. Therefore, with decrease in aging temperature, the rate of scission decreases at 70-80°C. The rate of crosslink in fact increases when aging temperature decreases. Table 4 represents the tear strength trend of MWP/CB and MWP/silica hybrid composite. The tear strength also shows similar as to tensile strength. Tear strength of filled hybrid composite is higher than that of the unfilled as well as 60 phr of MWP filled NR composite. With addition of CB as well as silica in the particular hybrid system, the tear stress equivalently dispersed in NR and hybrid composite shows enhanced in tear strength. Table 5 summarizes the% elongation at break of MWP hybrid system through CB as well as silica NR composite with accelerated thermal aging. When CB as well as silica is added in-place of MWP, the elongation at break decreased. This reduction indicates that ductility became worse when more conventional fillers were added into NR composites. The CB and silica are non-deformable particle so the addition of CB as well as silica in their particular composites restricts the molecular chains movement.  The decrease in the elongation at break also due to the striking forces between the filler and the polymer molecules leading to the development of a cross-linked structure so as to limit the free mobility of the polymer chains, hence increases the resistance to accelerate upon the execution of tension [41]. Table 6 depicts the variation in hardness of correspondingly MWP/CB and MWP/silica hybrid composite throughout accelerated thermal aging. It is evident that hardness of corresponding composite increased via replacement of MWP by CB as well as silica. This discovery is in well conformity with 200% modulus outcomes cited prior in this research works that disclosed an increase in rigidity of the composites with addition of CB as well as silica within hybrid composites.
However, the MWP/silica hybrid composite has relatively higher hardness as compared to MWP/CB hybrid composite due to the two dissimilar filler with different form and dimensions which design a new filler establishment. After aging data of hardness of all the samples changed to some extent.    up-take propensity as contrasted with the unfilled. All MWP/ CB and MWP/silica hybrid composites also displayed a similar pattern of sorption, where the filled NR composites absorbed solvent was very fast within the initial 36 h, followed by gradual increase until achieving a saturated point. Obviously, the toluene uptake of the NR composites decreased as CB or silica content increased in both hybrid systems. The efficient reinforcement in the hybrid NR composite systems is visible from the swelling ratio graphs. Vulcanization causes to limit the movements of the NR molecules in presence of fillers, although departs their fragmental mobility high [35,42]. Filler changes this state. The attachment of molecule chains to the filler has been achieved through sopping wet. CB and silica can wet rubber segments because the increased prospect of various absorptions. The improvement of interfacial adhesion along with filler and matrix also reduced, toluene addition in interfacial spaces, therefore, limiting the penetration of toluene into the composites. The highest bound rubber is found with CB and silica particles in their particular hybrid NR composite systems. This reinforcement restricts the free movement of rubber chains, thus improves the toluene resistance.

Swelling properties
Swelling coefficient (a) of MWP/CB as well as MWP/silica hybrid composites is presented in Fig. 10. From the diagram it is noticeable that, swelling coefficient of hybrid composites is decreased via replacement of MWP by CB as well as silica owing to better diffusion of CB as well as silica in rubber. However, some fragile interaction between silica and MWP/NR, the penetration of toluene into the composites is quite easier as compare to CB filled hybrid composite which has better interaction with NR than silica. Hence, the 'a' of CB containing hybrid composite reduced owing to the diffusion barrier of solvent (toluene) into the MWP/CB hybrid composite.
The crosslink density data are illustrated in Fig. 11 that is increasing with gradual increasing amount of CB as well silica in MWP/CB and MWP/silica system.
The increase in filler-rubber interaction examined in MWP/ CB and MWP/silica hybrid composites, due to increasing CB   as well as silica content that has best surface activity and therefore high affinity of powerful interaction to rubber.

Conclusions
The partial replacement of industrial waste like MWP with conventional fillers as CB as well as silica in NR was investigated. The following conclusions have been depicted from the present assessments.
The cure time as well as scorch time decrease, but the torques increases with addition of CB as well as silica in the hybrid composite.
Remarkable mechanical performance, particularly tensile strength, tear strength, modulus and hardness (superior values), and% elongation at break (inferior values) were obtained with replacement of MWP by CB and silica. MWP/CB hybrid composite was higher than those with MWP/silica hybrid NR composites.
Addition of CB as well as silica hybrid composites had an effect on swelling ratio and cross link density of composites.
Low mechanical properties of the some MWP/CB and MWP/silica hybrid composite showed after thermal aging while modulus, hardness and swelling coefficient increased at 70°C.
This assessment revealed that MWP can be utilized as a cofiller with CB as well as silica to obtain better dispersion of MWP/CB and MWP/silica hybrid filler with NR to gain, high mechanical properties.
Indentation results furthermore disclose that MWP can be utilized as a co-filler with CB as well as silica to make the better mechanical properties.