Rice husk Natural Rubber Mechanical New Trends in Natural ... · Rice husk Natural Rubber Mechanical properties The aim of this study is to investigate the effect of rice husk fibers

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Rice husk Natural Rubber Mechanical properties

The aim of this study is to investigate the effect of rice husk fibers in absence and in presence of carbon black on the rheological, physical and mechanical properties of rubber composites and vulcanizates. Alkaline treatment was used to remove most of the lignin and hemicellulose compounds from the rice husk fibers followed by drying, milling and sonication leading to get nano- rice husk particles. Different techniques such as TGA, mechanical testing, mi-crostructural analysis (via SEM and TEM), abrasion, hardness and XRD were carried out to characterize compounds and vulcanizates. It has been found that rice husk fibres has a reinforcing effect and improves the mechanical properties of rubber nano composites. Combination of rice husk and HAF car-bon black established with very valuab-le composites as engineering materials.

Neue Trends in Naturkaut-schuk-Nanokompositen Reisschalen Naturkautschuk Mecha-nische Eigenschaften

Ziel der Studie ist die Untersuchung des Effekts von Reisschalenfasern mit und ohne Gegenwart von Ruß auf die rheo-logischen, physikalischen und mechani-schen Eigenschaften von Kautschuk-kompositen und Vulkanisaten. Eine ba-sische Vorbehandlung wurde eingesetzt, um die meisten Bestandteile des Lignins und der Hemicellulose aus den Reis-schalenpartikeln zu entfernen. Verschie-dene Techniken wie TGA, mechanische Tests, mikrostrukturelle Analysen (über REM und TEM), Abrieb, Härte und XRD wurden eingesetzt, um die Mischungen und Vulkanisate zu charakterisieren. Es wurde festgestellt, dass Reisschalenfa-sern einen verstärkenden Effekt haben und die mechanischen Eigenschaften verbessert werden. Die Kombination von Reisschalenfasern und HAF Rußen führten zu wertvollen Kompositen als Konstruktionsmaterialien.

Figures and tables: By a kind approval of the authors.

IntroductionSeveral million tons of rice husk fibers are generated every year. Rice husk fiber, which is considered as an agricultural waste composes of cellulose, 25-35%, hemicellulose18-21%, of lignin26–31%, of silica15–17%, moisture ca. 7.5% [1]. Most of the previous works deal with burnt rice husk fibers or rice husk fibers ash. Few studies report on the use of rice husk fibers powder in rubber. Egypt is the largest rice producer in the near east re-gion, as rice was probably introduced in-to Egypt in the 7th century. Egypt has an abundance of rice husk fibers as rice is the second most important crop that comes after wheat. It produces about 4.5 million tons according to the world rice production 2015/2016. [2] India and Chi-na are the top producers of rice in the world. Rice husk fibers are largely used as a fuel in small scale, and for electrical power stations and thermal needs in large scale. The ash content of up to 22% and low protein content result in a mate-rial decomposing not as readily as other fibers. It can be used as a fertilizer in ag-riculture [3] or as an additive for cement and concrete fabrication. [4, 5].Because of its high silica content, rice husk fibers have become a source for preparation of elementary silica [6,7] and a number of silica compounds [8] especially silica [9,10], silicon carbide [11,12] and silica nitride. Therefore, the rice husk fibers are one of the excellent sources of high grade amorphous silica.

Waste comes in many different forms and may be categorized in a variety of ways such as industrial wastes, biode-gradable wastes, biomedical wastes, ag-ricultural wastes and many others. Agri-cultural waste is one of the most impor-tant wastes; it can be defined as the res-idues from the growing and processing of raw agricultural products such as fruits, vegetables, meat, poultry, dairy products and crops. [13] Agricultural wastes are found in the form of solid, liquid or slurries depending on the na-ture of agricultural activities. Further-more, agricultural industry residues and wastes constitute a significant propor-tion of worldwide agricultural productiv-ity. The pollution potential of agricultural wastes is high on a long-term basis due

to the burning of these wastes in the air that produces carbon mono and dioxide rather than some poisonous gases.

Rice husk fibers can be very beneficial in the industrial sector if it has been treated correctly and added to some ma-terials that could be used in various ap-plications such as the possibility to use it in rubber compounding aiming to ac-quire rubber product with good mechan-ical and thermal properties.

Tong et al explored the effects of dif-ferent compositions of rice husk fibers (RH) filler on the mechanical properties of recycled HDPE composite [14]. The composites were prepared with different loading contents of RH fibers. The results exposed improved flexural properties. However, the impact strength of the composites decreased as the RH loading increased. The replacement of commer-cial silica by high purity rice husk fibers- Nano silica as a compounding material has been investigated. [15] The mechan-ical properties of natural rubber incorpo-rated with rice husk fibers Nano silica (RHNS) showed, in general, better out-comes than commercial silica as the spe-cific surface area value of the RHNS is 252 m2/g which is greater than that of commercial silica, [16] Rahman et al. stated that mechanical properties of the composites prepared from alkaline me-dia treated rice husk fibers increase sub-stantially compared to untreated ones. [17]

However, the value of the alkaline media treated rice husk fibers-PE com-posites at all mixing ratios were found to be higher than the untreated rice husk fibers composites.

New Trends in Natural Rubber Nano Composites

AuthorsEl-Sayed M. Abdel-Bary, Mansoura, Haidy Al-Moghazy, Cairo, Egypt Corresponding Author:E. M. Abdel BaryLaboratory of PolymerMansoura University, Mansoura, EgyptE-Mail: [email protected]


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Nadazi et al. discussed the chemical and thermal stability of rice husk fibers against alkali treatment with 2% to 8% w/v NaOH. The results indicated that the proportion of lignin and hemicellulose in rice husk fibers treated by NaOH ranging from 4% to 8% decreased significantly. [18] Attharangsan, S., et al. found that. RH/carbon black hybrid filler can be rec-ommended to be used as efficient rein-forcing filler for the rubber [19].

Chopped carbon fibers are fibers about 5–10 micrometers in diameter and composed mostly of carbon atoms. Several thousand carbon fibers are bun-dled together to form a tow, which may be used by itself or woven into a fabric. The properties of carbon fibers, such as high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion, make them very popular in aerospace, civil engineering, military, and motorsports, along with other com-petition sports [20]. However, they are relatively expensive when compared with other fibers, such as glass fibers or organic fibers.

Usually chopped carbon fibers are used with engineering thermoplastics or thermoset materials; it is rare to be used in rubber. Chopped carbon fiber is very interesting reinforcing filler in polymeric materials, especially when it is sized with epoxy resin. This treatment usually im-proves the interfacial adhesion between fibers and natural rubber

Jacob et al. investigated the use of chopped carbon fiber–reinforced com-posites as crash-energy absorbers pri-marily because the low costs involved in their manufacture make them cost-ef-fective for automotive applications [21]. Of the parameters evaluated, fiber length appeared to be the most critical material parameter determining the spe-cific energy absorption of a composite material, with shorter fibers having a higher specific energy absorption than longer fibers, possibly because of the in-creased concentration of stress raisers in the shorter fiber specimens, resulting in a larger number of fracture-initiation sites.

Fazilitdinove et al. studied the x-ray diffraction analysis of chopped carbon fibers and the material of carbon fibers prepared from poly-acrylonitrile is heter-ogeneous [22]. Its component composi-tion is determined by the fiber prepara-tion conditions and depends on the an-gle at which the coherent scattering do-

mains are oriented relative to the thread axis. The size of coherent scattering do-mains increases in going to components corresponding to lower d002 values and depends on the orientation angle ϕ rela-tive to the thread axis.

Oz soy et al studied the properties of chopped carbon fiber reinforced epoxy composites. Different weight fractions were added as reinforcement to compos-ites [23]. Results of the tests have shown that hardness increases with the increas-ing amount of carbon fiber in compos-ites. Tensile, bending and impact perfor-mances have increased up to 8% of car-bon fiber in the composite and then started to decrease.

Wang et al studied a novel kind of carbon foam reinforced carbon–carbon composite with high density and me-chanical properties by dandifying carbon foam preforms enhanced by chopped carbon fibers [24]. The mechanical prop-erties and densification efficiency of this composite could be improved by adding of fibers. The compressive strength in-creased when the concentration of chopped fibers increased. SEM observa-tion showed that when the additive amount of fibers reached 5%, mi-cro-cracks appeared in carbon foam pre-forms and resulted in the decrease in

compressive strength. The present work is aiming to evaluate the reinforcing ef-fect either in combination with carbon black or chopped carbon fibers, the phys-ical, mechanical and thermal properties of the obtained natural rubber vulcaniza-tes will be evaluated.

Materials and techniques

MaterialsStearic acid: stearic acid as activator was supplied by Aldrich Company; zinc oxide (ZnO) Dibenzothiazyl Disulfide (MBTS) and N-isopropyl N‘-cyclohexyl p-phe-nylenediamine IPPD (4010) were sup-plied by Aldrich company, Germany. High abrasion furnace carbon black N330 sup-plied by Alexandria Carbon Co. surface area, 79-87 m2/g, particle diameter 28-36 nm, the silicon dioxide (SiO2) Vulcacile C, Bayer AG Leverkusen, Germany., Plasti-cizers (processing oil), vulcanizing agent, sulphur(S).

Alkali treatment of rice husk fibersMechanical treatment of RH fibers to produce a powder for further treatment was accomplished through the high shearing and compressive forces gener-ated by a mechanical pan-mill. Subse-quently, sonication for about 45 min was

1

1 Formulations of natural rubber containing chopped carbon fibersSamples codes Ch0 Ch4 Ch6 Ch8 Ch10 Ch20 Ch30Ingredients, PhrNatural Rubber 100 100 100 100 100 100 100Stearic acid 2 2 2 2 2 2 2ZnO 5 5 5 5 5 5 5Processing oil 10 10 10 10 10 10 10Chopped Carbon fibers 0 4 6 8 10 20 30MBTS* 2 2 2 2 2 2 2IPPD** 1 1 1 1 1 1 1Sulfur 2 2 2 2 2 2 2Dibenzothiazyl disulfide *, N-isopropyl N‘-cyclohexyl- p-phenylenediamine**.

Fig. 1: transmission electron microscopy of rice husk fibers



carried out using a sonicator (Biologics 150VT, VA, USA) in an aqueous medium. Rice husk fibers was immersed first in 4% concentration of NaOH solution with stirring at 100°C for 1 hour, followed by washing with water until it becomes completely free from sodium hydroxide and then left in the oven at 180°C for 48 hours. Rice husk fibers were dried at 120 oC for 24 h immediately before use. [25]

Chopped carbon fibers The carbon fibers were obtained from R.K Carbon fiber Philadelphia USA. The carbon fibers were treated with 1.5% epoxy. Length of fiber: 5 mm, Density: 1.81 g/cm3

Rubber compoundingAll rubber mixtures were prepared ac-cording to ASTM D- 3182 using a two-roll mill of 300 mm length, 150 mm diame-ter, the speed of slow roll 18 rpm, and gear ratio 1.4. The compounded rubber was left for about 24 hours before vul-canization. The compounded rubbers

were shaped into dumbbell-shaped specimens of 1.5 mm thick, 6 mm width, and 100 mm length.

VulcanizationThe vulcanization process was carried out by using an electrically heated hy-draulic press at 143±2°C and under hy-drostatic pressure 15 MPa for 15-20 min depending on formulation and rheo-graph.

Measurements

Rheology measurements:The cure characteristics of rubber com-pounds were carried out using Monsanto Oscillating Disk Rheometer 100, Monsan-to, USA. The measured parameters are:ML: minimum torque.MH: maximum torque.TS2: Time to (2) units of torque increase

above the minimum.MC90: Torque at 90% of full torque devel-

opment, were calculated using the following equations:

( )

−+=

10090*90 LHLC MMMM (1)

tC90: equivalent to optimum cure time is calculated from MC90 data and rheometer curve.

These measurements were accomplished according to ASTM D-2084.

Mechanical properties measurement (stress-strain characteristics)The mechanical test methods cover pro-cedures used to evaluate the tensile properties of vulcanized elastomers are carried out by using a mechanical testing machine of Zwick 1445 according to ASTM D-412. on a Zwick Roell Univerasl testing machine at crosshead speed of 200 mm/min at GUC. Lab.

Dumbbell - shaped specimens were cut from the sheets using a steel die of the standard width of 4mm and length of 115 mm. The thickness of the test spe-cimens was determined by a gauge gra-duated to one hundred of mm.

Hardness measurementsDetermination of hardness Shore A was measured according to ASTM D-2240, using a shore ‘A’ hardness tester of the type PTC, pacific transducer corp. Los Angelo’s. CA 90064 USA. Five specimens of each sample, each sample in the form of a disc of 3 cm diameter and 1.2 cm thick, accor-ding to the standard methods listed above.

Abrasion test measurements

Test Methods: Determination of the abrasion resistance for rubbers and elastomers are per-formed according to standard method DIN 53516 listed. The abrasion loss per-cent (Ab) is defined as:

%100%0

0 ×−

=m

mmAb (2)

Where (m0) and (m) are the weight of the sample before and after abrasion, re-spectively.

transmission electron microscope (TEM) TEM images and particle sizes were ob-tained using a (JEOL JEM-1230 operated at 120 kV). For TEM image the CNCs pow-der was dispersed in water by using ultra-sonic dispersant and a drop of the suspen-sion placed onto the carbon-coated grids.

X-ray diffractionThe X-ray diffraction (XRD) measure-ments were performed on a PANalytical

2 Physico-mechanical properties of natural rubber containing different concentrations of chopped carbon fibers Samples codes Ch0 Ch4 Ch6 Ch8 Ch10 Ch20 Ch30Ingredients, PhrNR 100 100 100 100 100 100 100Chopped carbon fiber 0 4 6 8 10 20 30

Phsico-mechanical PropertiesTensile strength (N/mm2) 17.3 19 20.5 22.2 24.5 24.2 24.9Elongation (%) 1196 950 890 840 798 774 668Swelling in toluene (%) 388 372 365 354 347 340 335Hardness (shore. A) 40 42 45 47 50 57 65 *All other ingredients of rubber formulations are kept constant as shown in Table 1.

2

Fig. 2: effect of chopped carbon fiber concentration. on tensile strength.



Empyrean diffractometer with Cu radia-tion (kα1 =1.54060 Å) operated at a volt-age of 45 kV and a current intensity of 40 mA. The pattern was recorded by a PIX-cel3D solid state detector in the angular range of 5-80° (2θ) with steps of 0.01° and a scan speed of 0.1S-1. Measure-ments of X-ray diffraction were carried out at National Institute of Standards in Cairo.

Results and Discussions

Microstructure of treated rice huskThe TEM micrographs Fig. 1 shows that the nanoparticles exhibited nearly uni-form spherical shape and the particle size distribution has the average size of 14.75- 64.60 nm.

Effect of chopped carbon fiber on the properties of rubber mixturesUsually chopped carbon fibers are used with engineering thermoplastics or ther-moset materials to get excellent engi-neering materials. It is rare to be used in rubber, chopped carbon fibers is very in-teresting reinforcing filler in polymeric materials, especially when these chopped carbons are sized with epoxy resin. This treatment usually improves the interfacial adhesion between fibers and natural rubber. Rubber formulations containing surface treated chopped car-bon fiber with epoxy resin are given in Table 1.

Mechanical properties The rubber and other ingredients given in Table 1 were mixed as usual using open two- roll mill, followed by vulcanization in hydraulic press at optimum time of vulcanization as mentioned before in the material and techniques and the me-chanical properties were evaluated.

From the mechanical properties given in Table 2 it can be seen the mechanical and physical properties values of rubber mixes containing different concentra-tions of chopped carbon fibers. The con-centration of chopped carbon has been changed from 4 to 30phr. The results ob-tained show that the tensile strength of the vulcanized rubber increases by incre-asing the concentration of chopped car-bon fiber. At the same time elongation at break decreases from 950% at 4phr to 668% at 30phr. In contrast, modulus 100% and hardness values increase.

Some published papers about chop-ped carbon fiber reported that the opti-mum concentration occurs at 8% while in

our work we have found that increasing concentrations give good mechanical properties.

Figure 2 shows the effect of chopped carbon fibers on tensile strength values. It can be seen that it is a typical behavior of effect of crosslinking density on the mechanical properties. The reinforcing effect of chopped carbon fiber plays the same role in affecting crosslinking densi-

ty in the reinforcement of rubber as we got a maximum value for tensile strength, further increase in the concen-tration leads to interrupt the orientation of rubber chains similar to that in case of changing crosslinking density.

As can be expected, the elongation at break decreases with increasing the chopped carbon fiber concentration as shown in Fig. 3, because increasing the

3

Fig. 3: effect of chopped carbon fiber conc. on elongation at break.

4

Fig. 4: effect of chopped carbon fiber conc, on M100% and hardness.

5

Fig. 5: effect of hybrid chopped carbon fibers and carbon black on tensile strength of vulca-nized samples.



concentration of chopped carbon fibers restricts the movement of the rubber chains and consequently, decreases the flexibility of the rubber product and , consequently decreases the elongation at break.

Figure 4 shows that modulus values of vulcanizates M (modulus at 100%), which reflects the stiffness of vulcanized rubber increases with increasing the con-centration of chopped carbon fiber due to its reinforcing effect. Also, the hard-ness values increase as the chopped car-bon fiber concentration increases, me-aning that changing the chopped carbon fiber concentration can control the hard-ness value of natural rubber.

The empirical equations given below show the polynomial relation between modulus y, hardness y’ and chopped car-bon fiber concentration x

Y’ = 0.9628x + 39.445 for Hardness (3)y = 0.3361x + 2.6964 for Modulus 100% (4)

Equilibrium swelling Measurements of swelling of vulcanized rubber samples are very important as they show the effect of fillers and vul-canizing systems on this behavior. Both the density of chemical and physical crosslinks affects the degree of swelling. It has been found that the degree of swelling of vulcanized rubber in toluene

decreases with increasing of concentra-tion of chopped carbon fiber (Fig. 5), as carbon fibers does not swell in toluene as mentioned before.

Effect of binary mixture of chopped car-bon fiber and carbon black in rubber mixturesIn this section, we investigate the effect of chopped carbon fiber concentration on rubber vulcanizes containing fixed amount of carbon black (40phr); we started with increasing the concentra-tion of chopped carbon up to 30phr, while rice husk and silica concentrations were kept constant. Table 3

Rheological properties The rheological properties were deter-mined and given in Table 4. From the rheological data, it can be seen that the maximum torque of rubber mixes and chopped carbon fiber is practically equal to the maximum torque of rice husk at the same concentration. The optimum time of vulcanization is slightly lower than that of rice husk.

Mechanical properties The found mechanical properties are summarized in Table 5.

From the results obtained before in our previous work, it was found that in-creasing the concentrations of rice husk lead to a reduction in the mechanical properties, while in these results it can be seen that the mechanical properties increase because in this section both chopped carbon fibers and carbon black exist in the rubber mixtures and both are good reinforcing fillers. Their presence in the mixtures compensate the weak rein-forcement effect of rice husk alone.

From the results given in Table 5 it can be seen that carbon black alone in the rubber vulcanizate gives tensile strength value 21.7 (N/mm2). On adding chopped carbon fiber starting from 4phr to 30phr, the tensile strength values of the rubber vulcanizates containing this constant amount of carbon black increases from 23.2 to 27.8 N/mm2, which reflects the effect of chopped carbon fibers on incre-asing the tensile strength as shown in Fig. 5.

As can be expected, Fig. 6 shows that elongation at break decreases from 680% to 455% at 30phr. The decrease in the elongation at break is a typical behavior of vulcanized rubber reinforced either with carbon black or with chopped car-bon fiber.

3 formulations of natural rubber containing different concentrations of chopped carbon fiber in presence of carbon blackIngredients,Phr

C40Ch0 C40Ch4 C40Ch6 C40Ch8 C40Ch10 C40Ch20 C40Ch30

Natural rubber 100 100 100 100 100 100 100ZnO 5 5 5 5 5 5 5Stearic acid 2 2 2 2 2 2 2Processing oil 10 10 10 10 10 10 10Silica 15 15 15 15 15 15 15Rice husk 20 20 20 20 20 20 20Carbon black 40 40 40 40 40 40 40Chopped carbon fibers

0 4 6 8 10 20 30

MBTS 2 2 2 2 2 2 2IPPD 1 1 1 1 1 1 1Sulphur 2 2 2 2 2 2 2

4 rheology parameter of rubber mix containing chopped carbon fiber C40Ch0 C40Ch10 C40Ch30

ML (dN.m) 0.8 0.88 0.96MH (dN.m) 6 8.44 9.32ts1 (mins.) 6.12 3.25 3.02t90 (mins.) 12.4 7.62 7.31

5 Physico-mechanical properties of natural rubber containing different concentrations of chopped carbon fiber in the presence of carbon blackSamples codes C40Ch0 C40Ch4 C40Ch6 C40Ch8 C40Ch10 C40Ch20 C40Ch30Ingredients, phrNatural rubber 100 100 100 100 100 100 100Rice husk 20 20 20 20 20 20 20Carbon black 40 40 40 40 40 40 40Chopped carbon fiber

0 4 6 8 10 20 30

Tensile strength (N/mm2)

21.7 23.2 24.8 25.9 27.5 27.6 27.8

Elongation (%) 680 620 576 533 502 469 455Modulus at 100%

3.9 4.3 5.6 6.8 7.4 8.2 9.8

Swelling (%) 152 148 143 139 133 125 120Hardness(shore A)

43 45 48 51 55 62 68



The crystallinity of rice husk fibers has not been disturbed by compounding as will be seen later by conducting XRD

Modulus reflecting the stiffness of the samples increases with increasing the concentration of chopped carbon fi-ber with respect to carbon black as shown in Fig. 8. The empirical equation shown below represents the polynomial relation between the chopped carbon concentration and the modulus. Where X is the concentration of chopped carbon fiber and Y is the modulus, by substitu-ting X one can get the new modulus.

Hardness value is represented in figure 8, from this figure it can be seen that hard-ness increases with the increase of chop-ped carbon fiber concentration in presence of carbon black. This means that one can control the hardness of natural rubber eit-her by changing the concentration of chop-ped carbon or by the addition of rice husk at fixed concentration of carbon black.

The microstructure of combination of chopped carbon fibers, carbon black and rice husk

Scanning electron-microscopeThe microstructure of combination of chopped carbon fibers, carbon black and rice husk is given in (figure 9). This figure shows that chopped carbon fibers are well adhered to rubber in a way better than rice husk. One can differentiate be-tween them as the holes present in the matrix are due to the pull out of the rice husk fibers, while the fibers surrounded by the matrix are the chopped carbon fibers. This is clear because the chopped carbon fibers we used are sized with epoxy resin (5%), which improved the in-terfacial adhesion with the rubber. X-Ray diffraction (XRD) of rubber mixes containing chopped carbon fiber and carbon blackFrom figure 10 the x-ray diffraction shows that chopped carbon appears at 2θ20 with sharp peak and high intensity, while its peak is overlapped in the x-ray diffraction of carbon black and rice husk appeared between 2θ10 and 2θ29. An-other peak appears in the spectrum with different intensities, which may be at-tributed to residual contents of the filler and the rubber itself. The x-ray shows also high crystalline phase of chopped carbon fibers, which has not been affect-ed during mixing on open mill,

It is probable that carbon black is a heterogeneous mixture of particles which

range from single graphite layers up to graphite crystals several layers thick. The intense small angle scattering is due to the difference between grain density and average density, caused by the loose pa-cking of the extremely small grains. ConclusionFrom the results obtained the following conclusion can be derived

■ The results obtained from the diffe-rent concentrations of chopped carbon fiber samples, proved that the optimum concentration was attained at 10phr of chopped carbon fiber.

■ The maximum torque of rubber mix-tures with chopped carbon is practically equal to the maximum torque of that of rice husk at the same concentration, while the optimum time for vulcanizati-

6 Fig. 6: Effect of rice husk fibers con-centration on the hard-ness values of natural rubber vul-canizates.

7

Fig. 7: Effect of rice husk fibers concentration on the equilibrium swelling values of natural rubber vulcanizates.

8

Fig. 8: effect of hybrid of chopped carbon fibers and carbon black on Hardness.



on is slightly lower than that of rice husk. ■ SEM micrographs illustrated that the

chopped carbon fibers sized with epoxy resin improve the interfacial adhesion with the rubber.

■ X-ray diffraction indicated that chop-ped carbon fiber appears at 2θ=20 with sharp peak at high intensity accompa-

nied by high crystalline phase and has not been affected during rubber proces-sing.

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9

Fig. 9: SEM of rubber mixes containing chopped carbon fibers and carbon black.

10

Fig. 10: X-ray diffraction of rubber vulcanizates containing rice husk, chopped carbon fiber and carbon black.

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Rice husk Natural Rubber Mechanical New Trends in Natural ... · Rice husk Natural Rubber Mechanical properties The aim of this study is to investigate the effect of rice husk fibers