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rice husk derived silica
Journal of Industrial and Engineering Chemistry 19 (2013) 1169–1176
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Characteristics of natural rubber hybrid composites based on marble sludge/ carbon black and marble sludge/rice husk derived silica Khalil Ahmed a,*, Shaikh Sirajuddin Nizami b, Nudrat Zahid Raza a a b
Applied Chemistry Research Centre, PCSIR Laboratories Complex, Karachi 75280, Pakistan Department of Chemistry, University of Karachi, Pakistan
A R T I C L E I N F O
Article history: Received 18 August 2012 Accepted 11 December 2012 Available online 20 December 2012 Keywords: Composite Natural rubber Marble sludge Carbon black Rice husk derived silica Mechanical properties
A B S T R A C T
This paper discusses an extensive overview of characterization of marble sludge (MS) with carbon black (CB) and rice husk derived silica (RHS) in hybrid natural rubber (NR) composite. Cure characteristics, mechanical and swelling properties of MS/CB and MS/RHS hybrid NR composite were investigated. As CB and RHS content increases in weight ratio of MS/CB and MS/RHS a decrease in scorch and cure time and increase in the torque, tensile, tear, modulus, hardness, crosslink density were observed, while elongation at break and swelling coefficient decreased. The aging behavior of corresponding hybrid composite materials was also evaluated at two different temperatures. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Marble sludge is a tentative waste, resulting from marble processing industries. The waste or sludge can be defined as the by-product material obtained after the fabrication process and has no further value, especially for prosperous developed economies [1]. Storage and collection of waste are some of the observable signs of successful or unsuccessful solid waste management systems. It is a serious matter as waste material produces tangible effects on soil and environmental system [2,3]. Among such waste materials, marble rejects produced in the cutting process of marble is becoming a perturbing factor for industrialists and environmentalists, due to the growing amount of rejected mud, that is, continuously discarded into land fill, river, pond and lake. Marble is a crystalline metamorphic limestone, containing calcite and dolomite as major components. This practice inflict threats to the eco-system of the environment. Therefore utilization of marble sludge in the fabrication of new material will help to protect the environment. Attempts have been made to use the marble sludge waste for various purposes in cement and the construction industry [4–6] and asphaltic concrete [7], but very few attempts have been made to use it as a filler in rubber composites [8] and polymer concrete from recycled poly(ethyleneterephthalate) [9].
* Corresponding author. Tel.: +92 21 34690350; fax: +92 21 34641847. E-mail address: [email protected] (K. Ahmed).
The rice, another major food crop in Pakistan, generates large amounts of waste which is in the form of rice husk. The uncontrolled rice husk is burned in open air or used as fuel in the rice paddy milling process, which creates environmental problems as pollution. As a result, the use of such ash (silica) has stimulated the development of research into the potentialities of this material. The polymer-based composites, either with rubber or plastic, act as a matrix and conventional fillers like carbon black, silica, clay are reinforced. Fillers improve the physical and mechanical properties. Carbon black is the most important reinforcing part of the rubber compounding recipe [10–12]. Furthermore, among several fillers like carbon black and synthetic silica, mineral fillers can reduce production cost and pollution. It is known that for filled rubber compound, the efficiency of reinforcement depends on a complex interaction of several filler-related parameters including particle size, particle shape, particle dispersion, surface area, surface reactivity, structure of the filler, and bonding quality between the fillers and the rubber matrix [13]. Carbon black improves the overall properties by forming bonds between rubber molecule and particle surface of carbon black [14,15], most important reinforcing filler is precipitated silica. The particles of silica surface have hydrophilic silanol/hydroxyl groups, which result in strong filler–filler interaction by hydrogen bonding [16–18]. Ismail et al. [19–21] observed that the incorporation of rice husk ash in rubber has led to a significantly improved physical
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.12.014
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XRF spectrometer result of marble sludge was obtained on a S4 PIONEER with the Bruker AXS SPECTRA plus software package to analyze the chemical composition or elements present in the sample. Thermogravimetric analysis (TGA) was carried out using METTLER TOLEDO TGA/SDTA 851 under air and N2 atmospheres from ambient temperature to 1000 8C at heating rates (10 8C min1).
properties of the composites which is comparable to that imparted by other commercial fillers such as carbon black and silica. Mehta and Haxo [22] explained the use of rice husk ash as a reinforcing agent for synthetic and natural rubbers. In this work it was observed that RHA did not adversely affect either the vulcanization characteristics or the aging of styrene butadiene rubber, natural rubber, nitrile rubber, butyl rubber, neoprene, and ethylenepropylene-diene rubber. In addition, it was concluded that RHA filler is a satisfactory substitute for carbon black and it can be effectively used as a partial replacement for better and more reinforcing blacks. The aim of this work is to study the reinforcing effect of partial or complete replacement of marble sludge by carbon black and rice husk derived silica, in MS/CB and MS/RHS hybrid NR composite materials. The investigations involved cure characteristics, mechanical and swelling properties of MS/CB and MS/RHS hybrid NR composite materials. Mechanical properties such as tensile strength, 100% modulus, 300% modulus, tear strength, % elongation at break and hardness were analyzed and discussed. Swelling tests were conducted by measuring the swelling coefficient, volume fraction of rubber and the crosslink density of the rubber hybrid composite materials.
Rice husk was washed with water to remove any strange material. Hydrochloric acid solution of 0.4 M was prepared, then 100 g cleaned husk was mixed per liter of prepared acid solution and heated until boiled for 30–45 min. After the reaction, the acid was completely removed from the husk by washing with tap water. It was then dried in an oven at 110 8C for 3–5 h. The treated husk was burned in an electric furnace at 600 8C for 6 h; silica was obtained as white ash. The shape of the silica is similar to the shape of the husk but smaller in size. To reduce its size, a ball mill was used to grind the silica. Then grinded silica passed through sieve to obtain 10 mm size.
2. Experimental
2.4. Sample preparation
2.1. Materials
The rubber was compounded on a laboratory two-roll mill (16 cm 33 cm) employing the formulation given in Table 1. The mixing was done according to ASTM D 3182 (2001). The NR was masticated on the mill and the total amount of filler was incorporated into the rubber (60 part per hundred of the rubber, phr) then the compounding ingredients were added in the following order: activators with balance, accelerators, and then sulfur. After mixing, the rubber compound was passed for 2–4 min through the tight nip gap and finally sheeted out.
Natural rubber: Ribbed smoked sheet (RSS-1), having Moony viscosity (ML1+4 at 100 8C) of 80 and MW of 120,000 with a density of 0.9125 g/cm3, origin from Thailand was obtained from the Rainbow rubber industry Karachi. Marble sludge was collected locally mostly from the local marble cutting/processing industry. The MS was dried in vacuum oven at 80 8C for 24 h and then pulverized in finer form. The ground MS was passed through sieve to obtain 10 mm with a density of 2.67 g/cm3. Carbon black (N330 granulated powder, particle size of 40–50 nm with specific gravity 1.80–1.82. Rice Husk derived silica (RHS) obtained from rice husk. Other compounding ingredients such as zinc oxide, stearic acid, sulphur, tetra methylthiuram disulfide (TMTD), 3-dimethylbutylN-phenyl-p-phenylenediamine (antioxidant) used were commercial grade and all of them were purchased from the local market. Physical properties of natural rubber such as dirt content, ash content, nitrogen content, volatile matter, initial plasticity, and plasticity retention index were determined by ASTM. The results are presented in Table 1. 2.2. Characterization of marble sludge powder by instrumental techniques The characterization of marble sludge powder was carried out with experimental techniques in order to confirm the composition of the sludge. Table 1 Compound recipe of the developed natural rubber hybrid composites. Ingredient
Part per hundred of rubber, phr
NR ZnO Stearic acid TMTDa Antioxidantb Sulfur MSc/CB and MS/RHSc weight ratio
100 05 02 2.4 1.5 1.6 00/00, 60/00, 50/10, 40/20, 30/30, 20/40, 10/50, 00/60
a b c
Tetra methylthiuram disulfide. 3-Dimethylbutyl-N-phenyl-p-phenylenediami. Micro size of MS and RHS particle, 10 mm.
2.3. Preparation of silica from rice husk
2.5. Cure characteristics The cure characteristics of the mixtures were studied using a Monsanto Moving Die Rheometer (MDR 2000) according to ASTM method D 2084. Samples of about 6 g of the respective compounds were tested at a vulcanization temperature of 140 8C for 20 min. The torque was noted at every 30 s. The cure time, measured as t90, scorch time, tS2, maximum torque, minimum torque, etc., were determined from the rheograph. The compounds were then compression molded at 140 8C using the respective curing time, t90. 2.6. Vulcanization process The compounded rubber stock was then cured in a compression molding machine at 140 8C and at applied pressure of 10.00 MPa for the respective optimum cure time (t = t90) obtained from rheographs. Rheometer tests showed that 90% crosslinking occurs at the corresponding cure time for each MS/CB and MS/RHS hybrid NR composites. After curing, the vulcanized sheet was taken out of the mold and immediately cooled under tap to stop further curing and stored in a cool dark place for 24 h. 2.7. Mechanical properties The properties of MS/CB and MS/RHS hybrid NR composite materials were measured with several techniques based on ASTM. The tensile strength, modulus, and % elongation at break were analyzed by using ASTM D412-03 (2003). Tear strength was determined according to ASTM D624-00 (2000). Samples were punched out from the molded sheets with a dumbbell-shaped die and angular specimens for tear strength. The crosshead speed was
K. Ahmed et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1169–1176
maintained at 500 mm/min at room temperature. The hardness of the sample (Shore A) was determined using Shore Hardness tester, according to ASTM D 2240.
Table 2 Quantitative analysis of marble sludge and rice husk silica using WDX-ray fluorescence Spectrometer Model: S4 Pioneer from Braker – axs Germany. Weight%
Component
2.8. Swelling property The chemical crosslinking density of MS/CB and MS/RHS hybrid NR composite materials, were determined by the equilibrium swelling method. A sample weighing about 0.2–0.25 g was cut from the compression-molded rubber sample. The sample was soaked in pure toluene at room temperature to allow the swelling to reach diffusion equilibrium. After 5 days, the swelling was stopped; at the end of this period, the test piece was taken out, the adhered liquid was rapidly removed by blotting with filter or tissue paper, and the swollen weight was measured immediately. It was then dried under vacuum at 80 8C up to constant weight and the desorbed weight was taken. The Swelling Coefficient (a) of the sample was calculated from following equation [23] (Eq. (1)).
a¼
WS r1 S W1
(1)
where W1 is the weight of the test piece before swelling and WS is the weight of test piece after swelling. The chemical crosslink densities of the composites were determined by the Flory–Rehner equation by using swelling value measurement [24,25] according to the relation (Eq. (2)).
n¼
lnð1 V r Þ þ V r þ xVr2
ro V s Vr1=3 V r =2
¼
1 MC
(2)
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CaO MgO SiO2 Al2O3 Fe2O3 Cr2O3 ZnO TiO Na2O3 K2O
MS
RHS
68.6 22.13 3.89 2.785 0.603 0.24 0.20 0.549 – –
0.62 0.45 94.88 0.86 0.24 – – – 0.18 1.93
2.9. Thermal aging The thermal aging characteristics of the MS/CB and MS/RHS hybrid NR composite materials were studied for 96 h at 70 8C and 100 8C as per ASTM D 573. The properties of accelerated aging were measured after 24 h of aging test. Tensile strength, 100% modulus, 300% modulus, tear strength, % elongation at break, hardness, compression set, rebound resilience and abrasion loss of the MS/CB and MS/RHS hybrid NR composite materials after aging were determined to estimate aging resistance.
3. Results and discussion 3.1. Characterization of marble sludge powder
where Vr is the volume fraction of rubber in the swollen gel, Vs is the molar volume of the toluene (106.2 cm3 mol1), x is the rubber–solvent interaction parameter (0.38 in this study), ro is the density of the polymer, n is the cross-link density of the rubber (mol cm3) and MC is the average molecular weight of the polymer between cross-links (g mol1). The volume fraction of a rubber network in the swollen phase is calculated from equilibrium swelling data as (Eq. (3)). Vr ¼
W r f =r1 W r f =r1 þ W s f =ro
(3)
where Wsf is the weight fraction of solvent, ro is the density of the solvent, 0.867 g/cm3 for toluene, Wrf is the weight fraction of the polymer in the swollen specimen and r1 is the density of the polymer, NR which was taken as 0.9125 g/cm3.
The chemical compositions of MS and RHS were determined using WDX-ray fluorescence spectrometer (model S4 pioneer Bruker AXS, Germany) as shown in Table 2. The chemical composition of MS mainly consists of calcium and magnesium compound in large amount. Silica, aluminum oxide and iron oxide are also present in small amount. The values obtained from relative metal component of marble sludge of atomic absorption spectroscopic study are in close approximation with those obtained from WDX-ray florescence spectrometer study. RHS mainly contains maximum amount of silica in the maximum amount as shown in Table 3. The thermo gravimetric curve discloses one distinctive weight loss stage for MS sample. The result reveals that 42.56% weight lost has been observed due to the evolution of carbon dioxide, which signifies the presence of metal carbonates (Fig. 1). The chemical
Table 3 Data for the scorch time, cure time, minimum torque, maximum torque and cure rate index from cure characteristics of MS/CB and MS/RHS hybrid filler NR composites. Hybrid filler ratio
00/00 60/00 50/10 40/20 30/30 20/40 10/50 00/60
Filler system
Unfilled MS-60 MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS
Cure characteristics at 140 8C for 20 min Scorch time tS2, min
Cure time t90, min
Min. torque, dN m
Max. torque, dN m
CRI, min1
2.47 1.76 1.67 1.63 1.56 1.61 1.48 1.58 1.35 1.55 1.28 1.51 1.20 1.48
4.38 4.86 4.65 4.75 4.57 4.56 4.42 4.47 4.34 4.37 4.27 4.16 4.15 4.06
0.25 0.43 0.47 0.48 0.50 0.50 0.53 0.55 0.57 0.70 0.62 0.87 0.66 1.14
2.63 3.52 3.58 4.05 3.63 4.11 3.69 4.26 3.77 4.51 3.85 4.88 3.93 5.38
0.523 0.322 0.335 0.320 0.332 0.331 0.340 0.349 0.334 0.356 0.334 0.377 0.339 0.387
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chain and results in higher tautness produce in hybrid NR composite materials. The increase in torque showed that gradual incorporation of CB and RHS in MS/CB and MS/RHS hybrid NR composite materials enhanced the crosslink density of the NR matrix. This increase in the maximum torque with increasing CB content is attributed to the increasing amount of CB in the rubber matrix that reduce the macromolecular mobility and consequently increases the torque of the vulcanization. CB is the reinforcing filler and has a particle size smaller than MS and RHS (10 mm). The smaller the particle size, the larger the surface area, and hence greater the interaction between the filler and rubber matrix. The maximum torque of the composite with CB and RHS 60 phr increases about 12% and 53%, respectively, when compared to that of the control composite (MS 60 phr). The curing rate indexes (CRI) of the compounds are calculated according to the equation as given below [28] (Eq. (5)). Fig. 1. Thermo gravimetric (TGA) curve of marble sludge powder.
analysis of XRF and TGA shows that marble sludge powder is mainly composed of calcium and magnesium in major quantities while alumina, silica, iron compounds and other elements are in minor quantities. All spectral data confirmed that carbonates are found in marble sludge powder.
CRI ¼
1 t 90 t S2
(5)
The CRI results are also given in Table 3. It was observed that, increase in CB and RHS content in MS/CB and MS/RHS hybrid NR composite materials, shows no significant effect on the CRI properties. 3.3. Mechanical properties
3.2. Curing characteristics This study investigated how various fillers affect the cure characteristics, mechanical and swelling properties of partial or full replacement for MS by carbon black and rice husk derived silica filled natural rubber hybrid composite materials. It was also evaluated how those properties change when carbon and rice husk derived silica were gradually added to the MS in NR hybrid composites. Table 3 shows the comparison of partial or complete replacement of MS by CB as well as RHS of the scorch time and cure time of MS/CB and MS/RHS hybrid NR composite materials at 140 8C curing temperature. The result shows that with respect to both properties namely scorch and cure time of the two types of the hybrid filler NR composite materials, the scorch time and cure time decrease with increasing CB and RHS content as compared to the control composite (60 phr MS). The decrease of the curing time was attributed to the high energy input and greater heat builds up during mixing due to the higher viscosity and higher shear heating of the rubber compounds which result is the decrease in scorch time and cure time. A decrease in scorch time with an increasing weight ratio of CB and RHS in MS/CB and MS/RHS hybrid NR composite materials was accredited to the presence of more crosslinked in the MS/CB and MS/RHS hybrid NR composite materials. The scorch time and cure time of MS/CB hybrid NR composite material were not changed much with partial replacement of MS by CB from 50/10 to 10/50 phr loading, however with RHS replaced by MS, both properties change extensively. The effect of partial replacement of MS by the increasing weight ratio of CB and RHS of MS/CB and MS/RHS hybrid NR composite materials on maximum torque and minimum torque are also shown in Table 3. It can be seen that while compared to the control composites (60 phr of MS), the maximum and minimum torque increase with increasing CB and RHS loading in the MS/CB and MS/ RHS hybrid NR composite materials, which is agreement to the work reported by Sezna et al. [26]. The minimum torque and maximum torque mainly depended on both the extent of crosslinking and reinforcement by filler particles in the polymer matrix [27], which reduces the molecular mobility of the rubber
Fig. 2 shows the tensile strength properties of MS/CB and MS/ RHS hybrid NR composite, before and after aging, at room temperature. The total filler loading of the composites is 60 phr. Generally an increase in the tensile property of the hybrid composite was observed with an increase in the content of CB replaced by MS in hybrid NR composite materials. Both hybrid filler NR composites show that the increasing carbon black and rice husk derived silica loading has increased the tensile strength of MS/CB and MS/RHS hybrid NR composite materials. This enhancement again was due to the better interaction of carbon black and rice husk derived silica and natural rubber matrix over marble sludge. The stronger rubber–filler interaction increased the effectiveness of the stress transferred from the rubber matrix to filler particles and consequently enhanced the tensile strength of MS/CB and MS/RHS hybrid NR composite materials. At high levels of CB (60 phr) loading, it may lead to an increase in filler–rubber interactions, and may also cause in filler dispersion resulting in
Fig. 2. Relationship between hybrid filler ratio and tensile strength of filled NR composites.
K. Ahmed et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1169–1176
composites having high mechanical properties. Similar behavior is also observed when the RHS replace by MS content is increased in hybrid NR composite materials. The figures clearly show that the tensile strength of NR hybrid composites is significantly improved by increasing both CB and RHS are replaced by MS in hybrid NR composites. As the weight ratio of CB is increased, the incompatibility in the interfacial region between the CB and rubber matrix also increases. This enhancement in tensile properties is not only attributed to the well-dispersed CB inside the NR matrix, intertubular and interfacial interactions between the CB and rubber matrix but also to the intercalation of CB by NR and other ingredients such as activator and accelerator. The tensile strength of MS/CB and MS/RHS hybrid NR composite materials, exhibit an improvement in tensile strength. This is due to the different degrees of reinforcement imparted by marble sludge, carbon black and rice husk derived silica particles. However the values are different. For maximum reinforcement, the filler particles must be of the same size or smaller than the chain end-toend distance. The degree of filler reinforcement increases with decrease in particle size or increase in surface area. In filled elastomers, the fillers act as stress concentrators. Smaller the particle size of fillers, more efficient will be the stress transfer from the rubber matrix to the fillers [29]. After aging, it can be seen that similar tensile strength trends are observed in samples after aging. The result shows that tensile strength decreased at all of MS/CB and MS/RHS ratios after aging. Thermal aging of MS/CB and MS/RHS hybrid NR composite materials caused the tensile strength to deteriorate, especially at 96 h with 100 8C temperatures of aging [30]. This is explained why both aging temperatures show a lower retention (below 100%), after thermal aging. However, aging at 70 8C for 96 h of MS/CB and MS/RHS hybrid NR hybrid composite materials show higher retention of tensile strength after thermal aging compared to that of 100 8C for 96 h. This could be due to the better thermal stability at lower temperature. Figs. 3 and 4 show the 100% and 300% modulus of MS/CB and MS/RHS hybrid NR composite before and after aging, at room temperature. The results show that the maximum value of 100% modulus and 300% modulus for MS/CB and MS/RHS hybrid NR composite materials were obtained. The mixing of 10/60 weight ratios of carbon black and rice husk derived silica could make a synergistic effect of reinforcement of natural rubber. Gradually replacing of MS by CB and RHS helped to improve dispersion of CB and RHS inside the NR. It can be seen that the 100% and 300%
Fig. 3. Relationship between hybrid filler ratio and 100% modulus of filled NR composites.
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Fig. 4. Relationship between hybrid filler ratio and 300% modulus of filled NR composites.
modulus of MS/CB and MS/RHS hybrid NR composites increased with the increasing of the CB and RHS loading. The increasing of 100% and 300% modulus is because of the increasing stiffness of the composites. The stiffness of composite was enhanced due to the small particle size, of CB and RHS and their ability to have a good dispersion inside the natural rubber matrix in comparison to marble sludge. The results of 100% and 300% modulus of the MS/CB and MS/RHS hybrid NR composites after replacement of MS by CB and RHS indicate that higher the CB and RHS loads the higher the stiffness of the hybrid composites obtained at higher strains. For sample reinforced by 10/50 phr of the MS/CB and MS/RHS the 100% and 300% modulus increase from 1.24 MPa to 1.39 MPa for MS/CB and 1.5 MPa for MS/RHS while for Sample reinforced with 50/10 phr of the MS/CB and MS/RHS increases the 100% and 300% modulus of 1.24 to 1.88 MPa for MS/CB and 1.93 MPa for MS/ RHS. The higher retention in 100% and 300% modulus (more than 100%) MS/CB and MS/RHS hybrid NR composite materials have been shown at 70 8C for 96 h after thermal aging, which might be due to the post cross linking of the rubber composites. However at 100 8C for 96 h lowest retention in 100% and 300% modulus (less than 100%) is shown. Ahagon et al. [31] and Baldwin et al. [32] in their studies of accelerated aging of rubber compound also observed the modulus increase and later reduction, depending on aging mechanism. At 90–110 8C the rate of modulus increase decreases with increasing, aging temperature, however at 70–90 8C the rate of modulus increase increases with decrease in aging temperature. The effect of aging temperature on modulus is due to the complexity of reactions taking place in cured rubber compound. This change in property that can also occur in polymer chain scission caused a reduction in molecular weight and molecular entangling with a high cross link density of NR/MS/CB hybrid composites. The later results in the energy dissipation reduction via molecular mobility restriction. The crosslink density is playing an important role in tensile and tear strength properties. It is also evident from Fig. 9 that crosslink value of MS/CB and MS/RHS hybrid NR composite material is continuously increased with increasing CB and RHS content. This phenomenon is post curing effect which tends to increase aging temperature. Clarke et al. [33] in their study of the aging kinetics of tensile strength of NR compound, also show that both crosslinking reaction and scission reaction increase the rate of reaction with increase in aging temperature. The scission reaction
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Fig. 5. Relationship between hybrid filler ratio and tear strength of filled NR composites.
Fig. 7. Relationship between hybrid filler ratio and hardness of filled NR composites.
has a higher activation energy than crosslink reaction. Hence with decrease in aging temperature, the rate of scission decreases at 70– 80 8C. The rate of crosslink actually increases as temperature decreases. Rapid increase in modulus would become at lower aging temperature. Fig. 5 shows the comparison of tear strength MS/CB and MS/RHS hybrid NR composite before and after thermal aging. The tear strength also follows the same trend as that of tensile strength. Tear strength of the filled MS/CB and MS/RHS hybrid NR composite systems are higher than that of unfilled NR compound and 60 phr of MS filled NR composite. As the CB and RHS weight ratio increases from 10/50 phr to 50/10 phr, the tear stress becomes more uniformly distributed in the NR matrix and the hybrid NR composite exhibits improved tear strength. Fig. 6 shows the influence of increasing CB and RHS content with decreases of MS on the elongation at break of the MS/CB and MS/RHS hybrid NR composites, before and after aging. The values of elongation at break decrease with increasing amount of CB in place of MS in MS/CB hybrid composite materials. This may be because NR matrix allows more rheological flow due to good filler
rubber interaction, i.e., good toughening. Elongation at break for MS/CB hybrid NR composite decreases with increasing CB content. This is also explained in terms of the adherence of the filler to the polymer, which leads to the stiffening of the polymer chain and, hence, resistance against stretching when strain is applied. Fig. 7 shows the comparison of hardness of MS/CB and MS/RHS hybrid NR composite before and after thermal aging, respectively. From the figure it is obvious that the hardness of MS/CB and MS/ RHS hybrid NR composite materials increased by replacement of MS by CB in MS/CB and RHS in MS/RHS hybrid NR composites. This finding is in good agreement with 100% and 300% modulus results mentioned earlier in this article which revealed an increase in the stiffness of the composites by adding CB and RHS in their respective hybrid composites. However, the MS/CB hybrid NR composite has the highest hardness as compared to MS/RHS hybrid NR composite materials. This happened because of the coexistence of two different kinds of fillers with different shapes and dimensions and may produce a new filler structure, which would respond more effectively to the periodic forces. The results show the hardness of composites MS/CB and MS/RHS hybrid NR composite materials are continuously increased with replacement of MS by CB and RHS from 10 to 60 phr loading. It is well known that the stiffness or modulus or hardness of filled rubber compounds is affected by particle size or specific surface area, the structure or the degree of the irregularity and the surface activity of the filler. 3.4. Swelling properties
Fig. 6. Relationship between hybrid filler ratio and % elongation at break of filled NR composites.
Fig. 8 shows the swelling coefficient of MS/CB and MS/RHS hybrid NR composites before and after thermal aging, respectively, versus hybrid filler ratio at room temperature. It can be seen that the swelling coefficient of both hybrid composites decreases with increasing CB and RHS in MS/CB and MS/RHS hybrid NR composite in place of MS. The effect of partial replacement of MS by CB and RHS on swelling coefficient of MS/CB hybrid NR composite materials at room temperature as compared to those of MS/RHS hybrid NR composite. It can be seen that swelling coefficient in the composites decreases with increasing CB content due to better dispersion of CB particles in rubber matrix. For RHS filled NR hybrid composite it can be seen that the swelling coefficient also decreases with the increasing weight ratio of RHS content. This
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calculated by using Flory–Rehner equation. A clear linear increase of the apparent cross-link density with the CB and RHS content in MS/CB and MS/RHS hybrid NR composite materials was observed. This shows as the major contributor a strong interaction between the NR and the CB, which leads to strong physical cross-links as compared to RHS. The increase in rubber–filler interaction observed in MS/CB weight ratio of MS/CB hybrid NR composites might be due to the increasing CB content, which has the highest surface activity and consequently high affinity to rubber. Equilibrium swelling measurements were performed to evaluate the chemical cross-linking density in MS/CB hybrid NR composite materials. 4. Conclusions
Fig. 8. Relationship between hybrid filler ratio and swelling coefficient of filled NR composites.
observation might be due to the particle size of RHS. As a result of weak interaction between RHS particles and MS/NR matrix, the penetration of toluene into the composites is easier as compared to CB filled MS/NR composite. As the interaction between carbon black and MS/NR matrix is better than RHS. When more than 10 weight ratio of carbon black was used, the swelling coefficient reduced which might be due to the penetration hindrance of toluene into the MS/CB hybrid NR composite materials by increasing amount of carbon black. As depicted in Fig. 9, it was found that crosslink density increases with the incorporation of carbon black and RHS weight ratio in MS/CB and MS/RHS hybrid NR composite materials before and after thermal aging, respectively. The cross-links density of polymers is the average molecular weight between the cross-links (directly related to the crosslink density) and was determined from swelling. The cross-link density, n, is defined as the number of elastically active network chains totally included in a perfect network, per unit volume. The cross-links density and the average molecular weight of the polymer between cross-links were
Fig. 9. Relationship between hybrid filler ratio and crosslink density of filled NR composites.
The partial substitution of marble sludge with carbon black and rice husk derived silica into natural rubber was investigated. The minimum and maximum torque of the hybrid composites increase with increasing CB and rice husk derived silica contents in a weight ratio of MS/CB and MS/RHS hybrid NR composites while cure time and the scorch time decrease with increasing CB and rice husk derived silica contents in a weight ratio of MS/CB and MS/RHS hybrid NR composite. The results also show that gradual replacement of MS by CB and RHS increases, the values of tensile strength, modulus, tear strength and hardness. This revealed that MS can be used as a co-filler with carbon black and rice husk derived silica to achieve better dispersion of MS/CB and MS/RHS hybrid filler with the NR to achieve, high mechanical properties such as tensile strength, modulus, tear strength and hardness of MS/CB and MS/RHS hybrid NR composite continuously increases with the increasing CB and RHS from 10 to 60 phr content. The mechanical properties of MS/CB hybrid NR composites were higher than those with MS/RHS hybrid NR composites. Tensile strength, 100% modulus, 300% modulus, tear strength % elongation at break and crosslink density reduced after thermal aging at 100 8C, whereas modulus, hardness and swelling coefficient increased after the aging at 70 8C. On comparing the properties of the two different compositions CB in MS/CB hybrid NR composites has been found to be better than MS/RHS hybrid NR composites. References [1] M. Gandy, Recycling and waste: an exploration of contemporary environmental policy, in: Avebury Studies in Green Research Series, Ashgate publishing Limited, London, 1993. [2] M. Soumare´, F.M.G. Tack, M.G. Verloo, Bioresource Technology 86 (2003) 15. [3] M.F. Abdel-Sabour, M.A. Abo El-Seoud, Agriculture, Ecosystems and Environment 60 (1996) 157. [4] H. Binici, K. Hasan, Y. Salih, Scientific Research and Essays 2 (2007) 372. [5] M. Karasahin, S. Terzi, Construction and Building Materials 21 (2007) 616. [6] W. Acchar, F.W. Vieira, D. Hotza, Materials Science and Engineering A 419 (2006) 306. [7] E. Arslan, S. Aslan, U. Ipek, S. Altun, S. Yaziciog˘lu, Waste Management & Research 23 (2005) 550. [8] S. Agrawal, S. Mandot, S. Bandyopadhyay, R. Mukhopadhyay, Progress in Rubber Plastics Recycling Technology 20 (2004) 229. [9] M.E. Tawfik, S.B. Eskander, Journal of Elastomers and Plastics 29 (2005) 65. [10] S. Salaeh, C. Nakason, Polymer Composites 33 (2012) 489. [11] S. Prasertsri, N. Rattanasom, Polymer Testing 30 (2011) 515. [12] M. Dog˘an, D.D. Oral, B. Yılmaz, S. Melih, S. Karahan, E. Bayraml, Journal of Applied Polymer Science 121 (2011) 1530. [13] Z.A. Ishak, M.A.A. Bakar, European Polymer Journal 31 (1995) 259. [14] P. Bertrand, L.T. Wang, Rubber Chemistry and Technology 75 (2002) 627. [15] D.C. Edwards, Journal of Materials Science 25 (1990) 4175. [16] I.l. J. Kim, W.S. Kim, L. Dong-Hyun, W. Kim, B. Jong-Woo, Journal of Applied Polymer Science 117 (2010) 1535. [17] D. Shin Jeong, C.K. Hong, G.T. Lim, G. Seo, C. Seok, Journal of Elastomers and Plastics 41 (2009) 353. [18] S. Hongsang, N. Sombatsompop, Journal of Macromolecular Science, Part B: Physics 46 (2007) 825. [19] H. Ismail, M.N. Nasaruddin, U.S. Ishiaku, Polymer Testing 18 (1999) 287.
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[20] H. Ismail, M.N. Nasaruddin, H.D. Rozman, European Polymer Journal 35 (1999) 1429. [21] S. Siriwardena, H. Ismail, U.S. Ishiaku, M.C.S. Perera, Journal of Applied Polymer Science 85 (2002) 438. [22] P.K. Mehta, H.E. Haxo, Rubber Chemistry and Technology 48 (1975) 271. [23] G. Unnikrishnan, S. Thomas, Polymer 35 (1994) 5504. [24] P.J. Flory, J. Rehner, Journal of Chemical Physics 11 (1943) 512. [25] H.f. Mark, Chemical crosslinking’, in: Encyclopedia of Polymer Science and Technology, Interscience Publishers, John Wiley & Sons Inc., NY, 1996. [26] J.A. Sezna, H.A. Pawlowski, D. Deconinck, Proceeding of the 136th meeting of the ACS rubber division, Fall, 1989.
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K.A. Malini, P. Kurian, M.R. Ananthraman, Materials Letters 57 (2003) 3381. E.M. Dannenberg, Rubber Chemistry and Technology 48 (1975) 410. M.J. Wang, Rubber Chemistry and Technology 71 (1998) 520. A.K. Bhowmick, J.R. White, Journal of Materials Science 37 (2002) 5141. A. Ahagon, M. Kida, H. Kaidou, Rubber Chemistry and Technology 63 (1990) 683. [32] M. Baldwin, D.R. Bauer, K.R. Ellwood, Rubber Chemistry and Technology 78 (2005) 336. [33] J. Clarke, E.F. Ngolemasango, M. Bennett, Journal of Applied Polymer Science 102 (2006) 3732.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Characteristics of natural rubber hybrid composites based on marble sludge/ carbon black and marble sludge/rice husk derived silica Khalil Ahmed a,*, Shaikh Sirajuddin Nizami b, Nudrat Zahid Raza a a b
Applied Chemistry Research Centre, PCSIR Laboratories Complex, Karachi 75280, Pakistan Department of Chemistry, University of Karachi, Pakistan
A R T I C L E I N F O
Article history: Received 18 August 2012 Accepted 11 December 2012 Available online 20 December 2012 Keywords: Composite Natural rubber Marble sludge Carbon black Rice husk derived silica Mechanical properties
A B S T R A C T
This paper discusses an extensive overview of characterization of marble sludge (MS) with carbon black (CB) and rice husk derived silica (RHS) in hybrid natural rubber (NR) composite. Cure characteristics, mechanical and swelling properties of MS/CB and MS/RHS hybrid NR composite were investigated. As CB and RHS content increases in weight ratio of MS/CB and MS/RHS a decrease in scorch and cure time and increase in the torque, tensile, tear, modulus, hardness, crosslink density were observed, while elongation at break and swelling coefficient decreased. The aging behavior of corresponding hybrid composite materials was also evaluated at two different temperatures. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Marble sludge is a tentative waste, resulting from marble processing industries. The waste or sludge can be defined as the by-product material obtained after the fabrication process and has no further value, especially for prosperous developed economies [1]. Storage and collection of waste are some of the observable signs of successful or unsuccessful solid waste management systems. It is a serious matter as waste material produces tangible effects on soil and environmental system [2,3]. Among such waste materials, marble rejects produced in the cutting process of marble is becoming a perturbing factor for industrialists and environmentalists, due to the growing amount of rejected mud, that is, continuously discarded into land fill, river, pond and lake. Marble is a crystalline metamorphic limestone, containing calcite and dolomite as major components. This practice inflict threats to the eco-system of the environment. Therefore utilization of marble sludge in the fabrication of new material will help to protect the environment. Attempts have been made to use the marble sludge waste for various purposes in cement and the construction industry [4–6] and asphaltic concrete [7], but very few attempts have been made to use it as a filler in rubber composites [8] and polymer concrete from recycled poly(ethyleneterephthalate) [9].
* Corresponding author. Tel.: +92 21 34690350; fax: +92 21 34641847. E-mail address: [email protected] (K. Ahmed).
The rice, another major food crop in Pakistan, generates large amounts of waste which is in the form of rice husk. The uncontrolled rice husk is burned in open air or used as fuel in the rice paddy milling process, which creates environmental problems as pollution. As a result, the use of such ash (silica) has stimulated the development of research into the potentialities of this material. The polymer-based composites, either with rubber or plastic, act as a matrix and conventional fillers like carbon black, silica, clay are reinforced. Fillers improve the physical and mechanical properties. Carbon black is the most important reinforcing part of the rubber compounding recipe [10–12]. Furthermore, among several fillers like carbon black and synthetic silica, mineral fillers can reduce production cost and pollution. It is known that for filled rubber compound, the efficiency of reinforcement depends on a complex interaction of several filler-related parameters including particle size, particle shape, particle dispersion, surface area, surface reactivity, structure of the filler, and bonding quality between the fillers and the rubber matrix [13]. Carbon black improves the overall properties by forming bonds between rubber molecule and particle surface of carbon black [14,15], most important reinforcing filler is precipitated silica. The particles of silica surface have hydrophilic silanol/hydroxyl groups, which result in strong filler–filler interaction by hydrogen bonding [16–18]. Ismail et al. [19–21] observed that the incorporation of rice husk ash in rubber has led to a significantly improved physical
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.12.014
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XRF spectrometer result of marble sludge was obtained on a S4 PIONEER with the Bruker AXS SPECTRA plus software package to analyze the chemical composition or elements present in the sample. Thermogravimetric analysis (TGA) was carried out using METTLER TOLEDO TGA/SDTA 851 under air and N2 atmospheres from ambient temperature to 1000 8C at heating rates (10 8C min1).
properties of the composites which is comparable to that imparted by other commercial fillers such as carbon black and silica. Mehta and Haxo [22] explained the use of rice husk ash as a reinforcing agent for synthetic and natural rubbers. In this work it was observed that RHA did not adversely affect either the vulcanization characteristics or the aging of styrene butadiene rubber, natural rubber, nitrile rubber, butyl rubber, neoprene, and ethylenepropylene-diene rubber. In addition, it was concluded that RHA filler is a satisfactory substitute for carbon black and it can be effectively used as a partial replacement for better and more reinforcing blacks. The aim of this work is to study the reinforcing effect of partial or complete replacement of marble sludge by carbon black and rice husk derived silica, in MS/CB and MS/RHS hybrid NR composite materials. The investigations involved cure characteristics, mechanical and swelling properties of MS/CB and MS/RHS hybrid NR composite materials. Mechanical properties such as tensile strength, 100% modulus, 300% modulus, tear strength, % elongation at break and hardness were analyzed and discussed. Swelling tests were conducted by measuring the swelling coefficient, volume fraction of rubber and the crosslink density of the rubber hybrid composite materials.
Rice husk was washed with water to remove any strange material. Hydrochloric acid solution of 0.4 M was prepared, then 100 g cleaned husk was mixed per liter of prepared acid solution and heated until boiled for 30–45 min. After the reaction, the acid was completely removed from the husk by washing with tap water. It was then dried in an oven at 110 8C for 3–5 h. The treated husk was burned in an electric furnace at 600 8C for 6 h; silica was obtained as white ash. The shape of the silica is similar to the shape of the husk but smaller in size. To reduce its size, a ball mill was used to grind the silica. Then grinded silica passed through sieve to obtain 10 mm size.
2. Experimental
2.4. Sample preparation
2.1. Materials
The rubber was compounded on a laboratory two-roll mill (16 cm 33 cm) employing the formulation given in Table 1. The mixing was done according to ASTM D 3182 (2001). The NR was masticated on the mill and the total amount of filler was incorporated into the rubber (60 part per hundred of the rubber, phr) then the compounding ingredients were added in the following order: activators with balance, accelerators, and then sulfur. After mixing, the rubber compound was passed for 2–4 min through the tight nip gap and finally sheeted out.
Natural rubber: Ribbed smoked sheet (RSS-1), having Moony viscosity (ML1+4 at 100 8C) of 80 and MW of 120,000 with a density of 0.9125 g/cm3, origin from Thailand was obtained from the Rainbow rubber industry Karachi. Marble sludge was collected locally mostly from the local marble cutting/processing industry. The MS was dried in vacuum oven at 80 8C for 24 h and then pulverized in finer form. The ground MS was passed through sieve to obtain 10 mm with a density of 2.67 g/cm3. Carbon black (N330 granulated powder, particle size of 40–50 nm with specific gravity 1.80–1.82. Rice Husk derived silica (RHS) obtained from rice husk. Other compounding ingredients such as zinc oxide, stearic acid, sulphur, tetra methylthiuram disulfide (TMTD), 3-dimethylbutylN-phenyl-p-phenylenediamine (antioxidant) used were commercial grade and all of them were purchased from the local market. Physical properties of natural rubber such as dirt content, ash content, nitrogen content, volatile matter, initial plasticity, and plasticity retention index were determined by ASTM. The results are presented in Table 1. 2.2. Characterization of marble sludge powder by instrumental techniques The characterization of marble sludge powder was carried out with experimental techniques in order to confirm the composition of the sludge. Table 1 Compound recipe of the developed natural rubber hybrid composites. Ingredient
Part per hundred of rubber, phr
NR ZnO Stearic acid TMTDa Antioxidantb Sulfur MSc/CB and MS/RHSc weight ratio
100 05 02 2.4 1.5 1.6 00/00, 60/00, 50/10, 40/20, 30/30, 20/40, 10/50, 00/60
a b c
Tetra methylthiuram disulfide. 3-Dimethylbutyl-N-phenyl-p-phenylenediami. Micro size of MS and RHS particle, 10 mm.
2.3. Preparation of silica from rice husk
2.5. Cure characteristics The cure characteristics of the mixtures were studied using a Monsanto Moving Die Rheometer (MDR 2000) according to ASTM method D 2084. Samples of about 6 g of the respective compounds were tested at a vulcanization temperature of 140 8C for 20 min. The torque was noted at every 30 s. The cure time, measured as t90, scorch time, tS2, maximum torque, minimum torque, etc., were determined from the rheograph. The compounds were then compression molded at 140 8C using the respective curing time, t90. 2.6. Vulcanization process The compounded rubber stock was then cured in a compression molding machine at 140 8C and at applied pressure of 10.00 MPa for the respective optimum cure time (t = t90) obtained from rheographs. Rheometer tests showed that 90% crosslinking occurs at the corresponding cure time for each MS/CB and MS/RHS hybrid NR composites. After curing, the vulcanized sheet was taken out of the mold and immediately cooled under tap to stop further curing and stored in a cool dark place for 24 h. 2.7. Mechanical properties The properties of MS/CB and MS/RHS hybrid NR composite materials were measured with several techniques based on ASTM. The tensile strength, modulus, and % elongation at break were analyzed by using ASTM D412-03 (2003). Tear strength was determined according to ASTM D624-00 (2000). Samples were punched out from the molded sheets with a dumbbell-shaped die and angular specimens for tear strength. The crosshead speed was
K. Ahmed et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1169–1176
maintained at 500 mm/min at room temperature. The hardness of the sample (Shore A) was determined using Shore Hardness tester, according to ASTM D 2240.
Table 2 Quantitative analysis of marble sludge and rice husk silica using WDX-ray fluorescence Spectrometer Model: S4 Pioneer from Braker – axs Germany. Weight%
Component
2.8. Swelling property The chemical crosslinking density of MS/CB and MS/RHS hybrid NR composite materials, were determined by the equilibrium swelling method. A sample weighing about 0.2–0.25 g was cut from the compression-molded rubber sample. The sample was soaked in pure toluene at room temperature to allow the swelling to reach diffusion equilibrium. After 5 days, the swelling was stopped; at the end of this period, the test piece was taken out, the adhered liquid was rapidly removed by blotting with filter or tissue paper, and the swollen weight was measured immediately. It was then dried under vacuum at 80 8C up to constant weight and the desorbed weight was taken. The Swelling Coefficient (a) of the sample was calculated from following equation [23] (Eq. (1)).
a¼
WS r1 S W1
(1)
where W1 is the weight of the test piece before swelling and WS is the weight of test piece after swelling. The chemical crosslink densities of the composites were determined by the Flory–Rehner equation by using swelling value measurement [24,25] according to the relation (Eq. (2)).
n¼
lnð1 V r Þ þ V r þ xVr2
ro V s Vr1=3 V r =2
¼
1 MC
(2)
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CaO MgO SiO2 Al2O3 Fe2O3 Cr2O3 ZnO TiO Na2O3 K2O
MS
RHS
68.6 22.13 3.89 2.785 0.603 0.24 0.20 0.549 – –
0.62 0.45 94.88 0.86 0.24 – – – 0.18 1.93
2.9. Thermal aging The thermal aging characteristics of the MS/CB and MS/RHS hybrid NR composite materials were studied for 96 h at 70 8C and 100 8C as per ASTM D 573. The properties of accelerated aging were measured after 24 h of aging test. Tensile strength, 100% modulus, 300% modulus, tear strength, % elongation at break, hardness, compression set, rebound resilience and abrasion loss of the MS/CB and MS/RHS hybrid NR composite materials after aging were determined to estimate aging resistance.
3. Results and discussion 3.1. Characterization of marble sludge powder
where Vr is the volume fraction of rubber in the swollen gel, Vs is the molar volume of the toluene (106.2 cm3 mol1), x is the rubber–solvent interaction parameter (0.38 in this study), ro is the density of the polymer, n is the cross-link density of the rubber (mol cm3) and MC is the average molecular weight of the polymer between cross-links (g mol1). The volume fraction of a rubber network in the swollen phase is calculated from equilibrium swelling data as (Eq. (3)). Vr ¼
W r f =r1 W r f =r1 þ W s f =ro
(3)
where Wsf is the weight fraction of solvent, ro is the density of the solvent, 0.867 g/cm3 for toluene, Wrf is the weight fraction of the polymer in the swollen specimen and r1 is the density of the polymer, NR which was taken as 0.9125 g/cm3.
The chemical compositions of MS and RHS were determined using WDX-ray fluorescence spectrometer (model S4 pioneer Bruker AXS, Germany) as shown in Table 2. The chemical composition of MS mainly consists of calcium and magnesium compound in large amount. Silica, aluminum oxide and iron oxide are also present in small amount. The values obtained from relative metal component of marble sludge of atomic absorption spectroscopic study are in close approximation with those obtained from WDX-ray florescence spectrometer study. RHS mainly contains maximum amount of silica in the maximum amount as shown in Table 3. The thermo gravimetric curve discloses one distinctive weight loss stage for MS sample. The result reveals that 42.56% weight lost has been observed due to the evolution of carbon dioxide, which signifies the presence of metal carbonates (Fig. 1). The chemical
Table 3 Data for the scorch time, cure time, minimum torque, maximum torque and cure rate index from cure characteristics of MS/CB and MS/RHS hybrid filler NR composites. Hybrid filler ratio
00/00 60/00 50/10 40/20 30/30 20/40 10/50 00/60
Filler system
Unfilled MS-60 MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS MS/CB MS/RHS
Cure characteristics at 140 8C for 20 min Scorch time tS2, min
Cure time t90, min
Min. torque, dN m
Max. torque, dN m
CRI, min1
2.47 1.76 1.67 1.63 1.56 1.61 1.48 1.58 1.35 1.55 1.28 1.51 1.20 1.48
4.38 4.86 4.65 4.75 4.57 4.56 4.42 4.47 4.34 4.37 4.27 4.16 4.15 4.06
0.25 0.43 0.47 0.48 0.50 0.50 0.53 0.55 0.57 0.70 0.62 0.87 0.66 1.14
2.63 3.52 3.58 4.05 3.63 4.11 3.69 4.26 3.77 4.51 3.85 4.88 3.93 5.38
0.523 0.322 0.335 0.320 0.332 0.331 0.340 0.349 0.334 0.356 0.334 0.377 0.339 0.387
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chain and results in higher tautness produce in hybrid NR composite materials. The increase in torque showed that gradual incorporation of CB and RHS in MS/CB and MS/RHS hybrid NR composite materials enhanced the crosslink density of the NR matrix. This increase in the maximum torque with increasing CB content is attributed to the increasing amount of CB in the rubber matrix that reduce the macromolecular mobility and consequently increases the torque of the vulcanization. CB is the reinforcing filler and has a particle size smaller than MS and RHS (10 mm). The smaller the particle size, the larger the surface area, and hence greater the interaction between the filler and rubber matrix. The maximum torque of the composite with CB and RHS 60 phr increases about 12% and 53%, respectively, when compared to that of the control composite (MS 60 phr). The curing rate indexes (CRI) of the compounds are calculated according to the equation as given below [28] (Eq. (5)). Fig. 1. Thermo gravimetric (TGA) curve of marble sludge powder.
analysis of XRF and TGA shows that marble sludge powder is mainly composed of calcium and magnesium in major quantities while alumina, silica, iron compounds and other elements are in minor quantities. All spectral data confirmed that carbonates are found in marble sludge powder.
CRI ¼
1 t 90 t S2
(5)
The CRI results are also given in Table 3. It was observed that, increase in CB and RHS content in MS/CB and MS/RHS hybrid NR composite materials, shows no significant effect on the CRI properties. 3.3. Mechanical properties
3.2. Curing characteristics This study investigated how various fillers affect the cure characteristics, mechanical and swelling properties of partial or full replacement for MS by carbon black and rice husk derived silica filled natural rubber hybrid composite materials. It was also evaluated how those properties change when carbon and rice husk derived silica were gradually added to the MS in NR hybrid composites. Table 3 shows the comparison of partial or complete replacement of MS by CB as well as RHS of the scorch time and cure time of MS/CB and MS/RHS hybrid NR composite materials at 140 8C curing temperature. The result shows that with respect to both properties namely scorch and cure time of the two types of the hybrid filler NR composite materials, the scorch time and cure time decrease with increasing CB and RHS content as compared to the control composite (60 phr MS). The decrease of the curing time was attributed to the high energy input and greater heat builds up during mixing due to the higher viscosity and higher shear heating of the rubber compounds which result is the decrease in scorch time and cure time. A decrease in scorch time with an increasing weight ratio of CB and RHS in MS/CB and MS/RHS hybrid NR composite materials was accredited to the presence of more crosslinked in the MS/CB and MS/RHS hybrid NR composite materials. The scorch time and cure time of MS/CB hybrid NR composite material were not changed much with partial replacement of MS by CB from 50/10 to 10/50 phr loading, however with RHS replaced by MS, both properties change extensively. The effect of partial replacement of MS by the increasing weight ratio of CB and RHS of MS/CB and MS/RHS hybrid NR composite materials on maximum torque and minimum torque are also shown in Table 3. It can be seen that while compared to the control composites (60 phr of MS), the maximum and minimum torque increase with increasing CB and RHS loading in the MS/CB and MS/ RHS hybrid NR composite materials, which is agreement to the work reported by Sezna et al. [26]. The minimum torque and maximum torque mainly depended on both the extent of crosslinking and reinforcement by filler particles in the polymer matrix [27], which reduces the molecular mobility of the rubber
Fig. 2 shows the tensile strength properties of MS/CB and MS/ RHS hybrid NR composite, before and after aging, at room temperature. The total filler loading of the composites is 60 phr. Generally an increase in the tensile property of the hybrid composite was observed with an increase in the content of CB replaced by MS in hybrid NR composite materials. Both hybrid filler NR composites show that the increasing carbon black and rice husk derived silica loading has increased the tensile strength of MS/CB and MS/RHS hybrid NR composite materials. This enhancement again was due to the better interaction of carbon black and rice husk derived silica and natural rubber matrix over marble sludge. The stronger rubber–filler interaction increased the effectiveness of the stress transferred from the rubber matrix to filler particles and consequently enhanced the tensile strength of MS/CB and MS/RHS hybrid NR composite materials. At high levels of CB (60 phr) loading, it may lead to an increase in filler–rubber interactions, and may also cause in filler dispersion resulting in
Fig. 2. Relationship between hybrid filler ratio and tensile strength of filled NR composites.
K. Ahmed et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1169–1176
composites having high mechanical properties. Similar behavior is also observed when the RHS replace by MS content is increased in hybrid NR composite materials. The figures clearly show that the tensile strength of NR hybrid composites is significantly improved by increasing both CB and RHS are replaced by MS in hybrid NR composites. As the weight ratio of CB is increased, the incompatibility in the interfacial region between the CB and rubber matrix also increases. This enhancement in tensile properties is not only attributed to the well-dispersed CB inside the NR matrix, intertubular and interfacial interactions between the CB and rubber matrix but also to the intercalation of CB by NR and other ingredients such as activator and accelerator. The tensile strength of MS/CB and MS/RHS hybrid NR composite materials, exhibit an improvement in tensile strength. This is due to the different degrees of reinforcement imparted by marble sludge, carbon black and rice husk derived silica particles. However the values are different. For maximum reinforcement, the filler particles must be of the same size or smaller than the chain end-toend distance. The degree of filler reinforcement increases with decrease in particle size or increase in surface area. In filled elastomers, the fillers act as stress concentrators. Smaller the particle size of fillers, more efficient will be the stress transfer from the rubber matrix to the fillers [29]. After aging, it can be seen that similar tensile strength trends are observed in samples after aging. The result shows that tensile strength decreased at all of MS/CB and MS/RHS ratios after aging. Thermal aging of MS/CB and MS/RHS hybrid NR composite materials caused the tensile strength to deteriorate, especially at 96 h with 100 8C temperatures of aging [30]. This is explained why both aging temperatures show a lower retention (below 100%), after thermal aging. However, aging at 70 8C for 96 h of MS/CB and MS/RHS hybrid NR hybrid composite materials show higher retention of tensile strength after thermal aging compared to that of 100 8C for 96 h. This could be due to the better thermal stability at lower temperature. Figs. 3 and 4 show the 100% and 300% modulus of MS/CB and MS/RHS hybrid NR composite before and after aging, at room temperature. The results show that the maximum value of 100% modulus and 300% modulus for MS/CB and MS/RHS hybrid NR composite materials were obtained. The mixing of 10/60 weight ratios of carbon black and rice husk derived silica could make a synergistic effect of reinforcement of natural rubber. Gradually replacing of MS by CB and RHS helped to improve dispersion of CB and RHS inside the NR. It can be seen that the 100% and 300%
Fig. 3. Relationship between hybrid filler ratio and 100% modulus of filled NR composites.
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Fig. 4. Relationship between hybrid filler ratio and 300% modulus of filled NR composites.
modulus of MS/CB and MS/RHS hybrid NR composites increased with the increasing of the CB and RHS loading. The increasing of 100% and 300% modulus is because of the increasing stiffness of the composites. The stiffness of composite was enhanced due to the small particle size, of CB and RHS and their ability to have a good dispersion inside the natural rubber matrix in comparison to marble sludge. The results of 100% and 300% modulus of the MS/CB and MS/RHS hybrid NR composites after replacement of MS by CB and RHS indicate that higher the CB and RHS loads the higher the stiffness of the hybrid composites obtained at higher strains. For sample reinforced by 10/50 phr of the MS/CB and MS/RHS the 100% and 300% modulus increase from 1.24 MPa to 1.39 MPa for MS/CB and 1.5 MPa for MS/RHS while for Sample reinforced with 50/10 phr of the MS/CB and MS/RHS increases the 100% and 300% modulus of 1.24 to 1.88 MPa for MS/CB and 1.93 MPa for MS/ RHS. The higher retention in 100% and 300% modulus (more than 100%) MS/CB and MS/RHS hybrid NR composite materials have been shown at 70 8C for 96 h after thermal aging, which might be due to the post cross linking of the rubber composites. However at 100 8C for 96 h lowest retention in 100% and 300% modulus (less than 100%) is shown. Ahagon et al. [31] and Baldwin et al. [32] in their studies of accelerated aging of rubber compound also observed the modulus increase and later reduction, depending on aging mechanism. At 90–110 8C the rate of modulus increase decreases with increasing, aging temperature, however at 70–90 8C the rate of modulus increase increases with decrease in aging temperature. The effect of aging temperature on modulus is due to the complexity of reactions taking place in cured rubber compound. This change in property that can also occur in polymer chain scission caused a reduction in molecular weight and molecular entangling with a high cross link density of NR/MS/CB hybrid composites. The later results in the energy dissipation reduction via molecular mobility restriction. The crosslink density is playing an important role in tensile and tear strength properties. It is also evident from Fig. 9 that crosslink value of MS/CB and MS/RHS hybrid NR composite material is continuously increased with increasing CB and RHS content. This phenomenon is post curing effect which tends to increase aging temperature. Clarke et al. [33] in their study of the aging kinetics of tensile strength of NR compound, also show that both crosslinking reaction and scission reaction increase the rate of reaction with increase in aging temperature. The scission reaction
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Fig. 5. Relationship between hybrid filler ratio and tear strength of filled NR composites.
Fig. 7. Relationship between hybrid filler ratio and hardness of filled NR composites.
has a higher activation energy than crosslink reaction. Hence with decrease in aging temperature, the rate of scission decreases at 70– 80 8C. The rate of crosslink actually increases as temperature decreases. Rapid increase in modulus would become at lower aging temperature. Fig. 5 shows the comparison of tear strength MS/CB and MS/RHS hybrid NR composite before and after thermal aging. The tear strength also follows the same trend as that of tensile strength. Tear strength of the filled MS/CB and MS/RHS hybrid NR composite systems are higher than that of unfilled NR compound and 60 phr of MS filled NR composite. As the CB and RHS weight ratio increases from 10/50 phr to 50/10 phr, the tear stress becomes more uniformly distributed in the NR matrix and the hybrid NR composite exhibits improved tear strength. Fig. 6 shows the influence of increasing CB and RHS content with decreases of MS on the elongation at break of the MS/CB and MS/RHS hybrid NR composites, before and after aging. The values of elongation at break decrease with increasing amount of CB in place of MS in MS/CB hybrid composite materials. This may be because NR matrix allows more rheological flow due to good filler
rubber interaction, i.e., good toughening. Elongation at break for MS/CB hybrid NR composite decreases with increasing CB content. This is also explained in terms of the adherence of the filler to the polymer, which leads to the stiffening of the polymer chain and, hence, resistance against stretching when strain is applied. Fig. 7 shows the comparison of hardness of MS/CB and MS/RHS hybrid NR composite before and after thermal aging, respectively. From the figure it is obvious that the hardness of MS/CB and MS/ RHS hybrid NR composite materials increased by replacement of MS by CB in MS/CB and RHS in MS/RHS hybrid NR composites. This finding is in good agreement with 100% and 300% modulus results mentioned earlier in this article which revealed an increase in the stiffness of the composites by adding CB and RHS in their respective hybrid composites. However, the MS/CB hybrid NR composite has the highest hardness as compared to MS/RHS hybrid NR composite materials. This happened because of the coexistence of two different kinds of fillers with different shapes and dimensions and may produce a new filler structure, which would respond more effectively to the periodic forces. The results show the hardness of composites MS/CB and MS/RHS hybrid NR composite materials are continuously increased with replacement of MS by CB and RHS from 10 to 60 phr loading. It is well known that the stiffness or modulus or hardness of filled rubber compounds is affected by particle size or specific surface area, the structure or the degree of the irregularity and the surface activity of the filler. 3.4. Swelling properties
Fig. 6. Relationship between hybrid filler ratio and % elongation at break of filled NR composites.
Fig. 8 shows the swelling coefficient of MS/CB and MS/RHS hybrid NR composites before and after thermal aging, respectively, versus hybrid filler ratio at room temperature. It can be seen that the swelling coefficient of both hybrid composites decreases with increasing CB and RHS in MS/CB and MS/RHS hybrid NR composite in place of MS. The effect of partial replacement of MS by CB and RHS on swelling coefficient of MS/CB hybrid NR composite materials at room temperature as compared to those of MS/RHS hybrid NR composite. It can be seen that swelling coefficient in the composites decreases with increasing CB content due to better dispersion of CB particles in rubber matrix. For RHS filled NR hybrid composite it can be seen that the swelling coefficient also decreases with the increasing weight ratio of RHS content. This
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calculated by using Flory–Rehner equation. A clear linear increase of the apparent cross-link density with the CB and RHS content in MS/CB and MS/RHS hybrid NR composite materials was observed. This shows as the major contributor a strong interaction between the NR and the CB, which leads to strong physical cross-links as compared to RHS. The increase in rubber–filler interaction observed in MS/CB weight ratio of MS/CB hybrid NR composites might be due to the increasing CB content, which has the highest surface activity and consequently high affinity to rubber. Equilibrium swelling measurements were performed to evaluate the chemical cross-linking density in MS/CB hybrid NR composite materials. 4. Conclusions
Fig. 8. Relationship between hybrid filler ratio and swelling coefficient of filled NR composites.
observation might be due to the particle size of RHS. As a result of weak interaction between RHS particles and MS/NR matrix, the penetration of toluene into the composites is easier as compared to CB filled MS/NR composite. As the interaction between carbon black and MS/NR matrix is better than RHS. When more than 10 weight ratio of carbon black was used, the swelling coefficient reduced which might be due to the penetration hindrance of toluene into the MS/CB hybrid NR composite materials by increasing amount of carbon black. As depicted in Fig. 9, it was found that crosslink density increases with the incorporation of carbon black and RHS weight ratio in MS/CB and MS/RHS hybrid NR composite materials before and after thermal aging, respectively. The cross-links density of polymers is the average molecular weight between the cross-links (directly related to the crosslink density) and was determined from swelling. The cross-link density, n, is defined as the number of elastically active network chains totally included in a perfect network, per unit volume. The cross-links density and the average molecular weight of the polymer between cross-links were
Fig. 9. Relationship between hybrid filler ratio and crosslink density of filled NR composites.
The partial substitution of marble sludge with carbon black and rice husk derived silica into natural rubber was investigated. The minimum and maximum torque of the hybrid composites increase with increasing CB and rice husk derived silica contents in a weight ratio of MS/CB and MS/RHS hybrid NR composites while cure time and the scorch time decrease with increasing CB and rice husk derived silica contents in a weight ratio of MS/CB and MS/RHS hybrid NR composite. The results also show that gradual replacement of MS by CB and RHS increases, the values of tensile strength, modulus, tear strength and hardness. This revealed that MS can be used as a co-filler with carbon black and rice husk derived silica to achieve better dispersion of MS/CB and MS/RHS hybrid filler with the NR to achieve, high mechanical properties such as tensile strength, modulus, tear strength and hardness of MS/CB and MS/RHS hybrid NR composite continuously increases with the increasing CB and RHS from 10 to 60 phr content. The mechanical properties of MS/CB hybrid NR composites were higher than those with MS/RHS hybrid NR composites. Tensile strength, 100% modulus, 300% modulus, tear strength % elongation at break and crosslink density reduced after thermal aging at 100 8C, whereas modulus, hardness and swelling coefficient increased after the aging at 70 8C. On comparing the properties of the two different compositions CB in MS/CB hybrid NR composites has been found to be better than MS/RHS hybrid NR composites. References [1] M. Gandy, Recycling and waste: an exploration of contemporary environmental policy, in: Avebury Studies in Green Research Series, Ashgate publishing Limited, London, 1993. [2] M. Soumare´, F.M.G. Tack, M.G. Verloo, Bioresource Technology 86 (2003) 15. [3] M.F. Abdel-Sabour, M.A. Abo El-Seoud, Agriculture, Ecosystems and Environment 60 (1996) 157. [4] H. Binici, K. Hasan, Y. Salih, Scientific Research and Essays 2 (2007) 372. [5] M. Karasahin, S. Terzi, Construction and Building Materials 21 (2007) 616. [6] W. Acchar, F.W. Vieira, D. Hotza, Materials Science and Engineering A 419 (2006) 306. [7] E. Arslan, S. Aslan, U. Ipek, S. Altun, S. Yaziciog˘lu, Waste Management & Research 23 (2005) 550. [8] S. Agrawal, S. Mandot, S. Bandyopadhyay, R. Mukhopadhyay, Progress in Rubber Plastics Recycling Technology 20 (2004) 229. [9] M.E. Tawfik, S.B. Eskander, Journal of Elastomers and Plastics 29 (2005) 65. [10] S. Salaeh, C. Nakason, Polymer Composites 33 (2012) 489. [11] S. Prasertsri, N. Rattanasom, Polymer Testing 30 (2011) 515. [12] M. Dog˘an, D.D. Oral, B. Yılmaz, S. Melih, S. Karahan, E. Bayraml, Journal of Applied Polymer Science 121 (2011) 1530. [13] Z.A. Ishak, M.A.A. Bakar, European Polymer Journal 31 (1995) 259. [14] P. Bertrand, L.T. Wang, Rubber Chemistry and Technology 75 (2002) 627. [15] D.C. Edwards, Journal of Materials Science 25 (1990) 4175. [16] I.l. J. Kim, W.S. Kim, L. Dong-Hyun, W. Kim, B. Jong-Woo, Journal of Applied Polymer Science 117 (2010) 1535. [17] D. Shin Jeong, C.K. Hong, G.T. Lim, G. Seo, C. Seok, Journal of Elastomers and Plastics 41 (2009) 353. [18] S. Hongsang, N. Sombatsompop, Journal of Macromolecular Science, Part B: Physics 46 (2007) 825. [19] H. Ismail, M.N. Nasaruddin, U.S. Ishiaku, Polymer Testing 18 (1999) 287.
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