Chemical Treatment of Coir Fiber Reinforced Polypropylene

Article pubs.acs.org/IECR

Chemical Treatment of Coir Fiber Reinforced Polypropylene Composites Md. Mominul Haque,*,† Md. Ershad Ali,‡ Mahbub Hasan,§ Md. Nazrul Islam,† and Hyungsub Kim∥ †

Department of Chemistry and §Department of Materials & Metallurgical Engineering, Bangladesh University of Engineering & Technology, Dhaka 1000, Bangladesh ‡ Department of Chemistry, Tejgaon College, National University, Gazipur 1704, Bangladesh ∥ Department of Materials Engineering, Hanyang University, Seoul 133-791, Republic of Korea ABSTRACT: Agro-based renewable materials have received significant attention in recent years because of their low cost, light weight, ecofriendliness, and worldwide environmental awareness. In the present work, coir fiber reinforced polypropylene composites were manufactured using injection molding method. Raw coir was chemically treated with benzene diazonium salt in alkali, acidic, and neutral media separately in order to increase the compatibility between the coir fiber and polypropylene composite. During chemical treatment, hydrophilic -OH groups in the raw coir cellulose were converted to hydrophobic -O−Na groups. Both raw and treated coir at 10, 15, 20, 25, 30, and 35 wt % were utilized during composite manufacturing. Microstructural analysis and mechanical tests were conducted. Alkali treated specimens yielded the best set of mechanical properties. On the basis of fiber loading, 30% fiber reinforced composites had the optimum set of mechanical properties among all composites manufactured.

1. INTRODUCTION Synthetic polymers are currently combined with various biodegradable reinforcing fibers in order to improve mechanical properties and obtain the characteristics demanded in actual applications.1−4 Research is going on to replace synthetic fibers with lignocellulosic fibers as reinforcement.5−9 Compared to various synthetic fibers, the lignocellulosic fibers (corn stalk, rice husk, palm, coir, jute, abaca, wheat straw, and grass, etc.) are lightweight, decrease wear in the machine used for their production, and are easily available, renewable, biodegradable, and inexpensive.1,10 The cost of producing lignocellulosic reinforced polymeric composites is quite low. Hence, these composites are becoming increasingly important for the production of a large variety of cheap, lightweight, environmentally friendly composites.11 The physical and mechanical properties of lignocellulosic composites largely depend on the type of matrix, the content and properties of the reinforcing fiber, and fiber−matrix interaction. Better dispersion of the fiber can be achieved by effective mixing of the components and a proper compounding process. The compatibility between the two components can be achieved by physical and chemical modification of the fiber or by use of a suitable coupling agent.3 In the present work, coir fiber was used as the reinforcing material since it is abundant in nature and has a minimal effect on the environment due to their biodegradable properties.11−13 Coir fiber is the cheapest lignocellulosic fiber and most abundantly available in Bangladesh. It is also one of the hardest among the available natural fibers and has many applications, especially in agricultural textiles. One of the major drawbacks of using coir as a fiber material is its hydrophilic nature, responsible for moisture absorption and poor interfacial bonding between the fiber and matrix in the resultant composites. To overcome this problem, coir was chemically © 2012 American Chemical Society

treated with benzene diazonium salt in different media having different pH. A previous study describes the effect of coupling agent on oxidized coir fiber reinforced polypropylene (PP) composite.14 On the other hand, the present work describes the effect of benzene diazonium salt treatment carried out at different pH on coir fiber reinforced PP composites. Benzene diazonium salt can be easily and cheaply prepared in the laboratory. Besides it is nontoxic and nonhazardous. Thus the aim of this study is to prepare composite materials of biodegradable nature and improved mechanical properties using PP reinforced with chemically treated coir. PP is a cheap nontoxic thermoplastic polymer. It has great contribution in the production of relatively inexpensive and tough thermoplastic polymer composites. The chemically treated coir composites might be useful in making lightweight furniture, doors, and windows, etc. When buried and rotten under the soil, the treated coir−PP biodegradable composites will release nitrogen gas into the soil, which in turn maintains the fertility of the soil. The effect of fiber loading and chemical treatment at different pH on physicomechanical properties and the morphology of coir fiber reinforced PP composites are also reported.

2. MATERIALS AND METHODS 2.1. Materials. The thermoplastic polymer PP, used as a matrix material, was supplied by the Polyolefine Co., Private Limited Singapore, in the form of homopolymer pellets. It had a melt flow index of 10 g/(10 min), specific gravity of 0.90− 0.91, melting temperatures of 165− 171 °C, and crystallinity of 82%.15 The coir, used as reinforcing fiber, was collected from a Received: Revised: Accepted: Published: 3958

April 8, 2011 February 17, 2012 February 19, 2012 February 21, 2012 dx.doi.org/10.1021/ie200693v | Ind. Eng. Chem. Res. 2012, 51, 3958−3965

Industrial & Engineering Chemistry Research

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(Model MSC-5/500, Agawn Seiki Co. Ltd., Tokyo, Japan). The tests were performed at a crosshead speed of 10 mm/min. The dimensions of the specimen used were 148 mm × 10 mm × 4.1 mm. 2.5.2. Flexural Test. Static flexural tests were carried out according to ASTM D 790-0019 using the same testing machine mentioned above at the same crosshead speed. The dimensions of the specimen used were 79 mm × 10 mm × 4.1 mm. The flexural strength and modulus were calculated using the following equations.

rural area of Bangladesh. It comprises 43.44% cellulose, 45.84% lignin, 0.25% hemicellulose, 3% pectin, 5.6% ash, and 7.47% other constituents.16 The Young’s modulus and strain to failure of the coir fiber used were 28 GPa and 5%, respectively. The length of the coir fiber used was around 1−2 cm, and the diameter was around 250−300 μm. Benzene diazonium salt was synthesized (Figure 1) by the standard diazotization method using HCl, NaNO2, and C6H5NH2.17

flexural strength: 3PL σf = 2bd2

(1)

flexural modulus:

Figure 1. Synthesis of benzene diazonium chloride.

2.2. Treatment of Coir. Coir was dried at 105 °C for 24 h and then kept in a sealed container. To have diazonium salt in acidic, neutral, and alkali media (pH 3, 7, and 10.5, respectively), 60, 108, and 200 mL of 5% NaOH was mixed with 300 mL of water in a beaker, respectively. A 500 g amount of coir was submerged into the solutions mentioned above separately for 10 min at about 5 °C in an ice bath. A freshly prepared cooled solution of benzene diazonium salt was then poured slowly into the above mixtures with constant stirring for 10 min. Coir was then taken out, washed with soap solution followed by water, and finally dried in open air. 2.3. Fabrication of Composites and Test Specimens. Dried raw and treated coir was initially mixed thoroughly with PP granules at 10, 15, 20, 25, 30, and 35 wt % each. The mixtures were passed through a single screw extruder machine at 60 rpm screw speed and a constant temperature of 135 ± 5 °C. The extruded composites were cut into 15−20 cm long small pieces. All the pieces were then crashed into smaller granules using a grinding machine (Model FFC-23, Machinery Company Ltd. India). The granules were dried in a vacuum oven at 65 °C for 1 h and fed into an injection molding machine for making test specimens. The tensile and flexural test specimens were prepared at a molding temperature of 165 °C. 2.4.1. Microstructural Analysis. Fourier Transform Infrared Spectroscopy. The infrared spectra of the raw and treated coir were recorded on a Nicolet 380 spectrophotometer using attenuated total reflectance (ATR) technique. The transmittance range of the scan used was 370−4000 cm−1. The absorption bands in infrared spectra are discussed in the Results and Discussion. 2.4.2. Scanning Electron Microscopy. The diameter of the coir fiber was measured, and the interfacial bonding between the coir fiber and PP matrix in manufactured composites was examined using a scanning electron microscope (Philips XL 30). Since the coir and composites are not conductive, those were needed to be made conductive. It was done by applying a gold coating sputtering technique. The thin gold coating caused the electron to interact with the inner atomic shells of the sample. The micrographs are presented in the Results and Discussion. 2.5. Mechanical Testing. Tensile, flexural, charpy impact, hardness, and water absorption tests were carried out. In each case, 10 specimens were tested and the average values are reported. 2.5.1. Tensile Test. Tensile tests were conducted according to ASTM D 638-0118 using a Universal Testing Machine

E=

L3m 4bd3

(2)

where P is the maximum applied load, L is the length of support span, m is the slope of the tangent, and b and d are the width and thickness of the specimen, respectively. 2.5.3. Charpy Impact Test. Dynamic charpy impact tests were conducted according to ASTM D 6110-9720 using a universal impact testing machine. Notched composite specimens were used during the experiment. The dimensions of the specimen used were 79 mm × 10 mm × 4.1 mm. 2.5.4. Water Absorption Test. The water absorption tests of the composites were carried out following ASTM D 570-99.21 The specimens having dimensions of 39 mm × 10 mm × 4.1 mm were prepared. The specimens were dried in an oven at 105 °C, cooled in a desiccator using silica gel, and immediately weighed. A Denver Instron balance was used for weight measurement. To allow the composites to absorb water, the weighed specimens were immersed in distilled water and kept for 24 h. The excess water on the surface of the specimens was removed using a soft cloth. The final weight of the specimens was then taken. The percentage of water absorption of the specimens was calculated using the following equation: water absorption/% final weight − original weight = × 100 original weight

(3)

2.5.5. Hardness Test. The hardness tests of the composites were carried out using a Rockwell hardness testing machine. The tests were conducted following ASTM D785-98.22 Results are presented in the next section.

3. RESULTS AND DISCUSSION 3.1. Tensile Properties. The tensile strength of raw and treated coir fiber reinforced PP composites against different fiber loading is presented in Figure 2. It is observed from the figure that the tensile strength of the raw coir reinforced PP composites decreased with fiber loading. Similar results were also reported by other researchers.2,3,5,7,23−25 Coir fiber is hydrophilic in nature, whereas the nature of PP is hydrophobic. Thus, the hydrophilic coir does not interact well with the hydrophobic PP. When the fiber loading was increased, the weak interfacial area between the coir fiber and the PP matrix increased. As a result the tensile strength decreased. It is also observed from the figure that the tensile strength of PP composites reinforced with chemically treated coir changed 3959

dx.doi.org/10.1021/ie200693v | Ind. Eng. Chem. Res. 2012, 51, 3958−3965


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suggests the absorption band at the region near 1732 cm−1. This absorption band may be due to a carboxyl group of acetyl ester in cellulose and carboxyl aldehyde in lignin.26 The coupling reaction between hydroxyl groups and diazonium salt (Figure 5) increased the interfacial bonding between the coir fiber and PP matrix in the composites. There are actually three hydroxyl groups present in the cellulose anhydroglucose unit. One is the primary hydroxyl group at C6 and the other two are secondary hydroxyl groups at C2 and C3. Although the primary hydroxyl group is more reactive than the secondary groups, the diazonium salt breaks the OH group of carbon 6 and carbon 2 during the reaction. This converts the two hydroxyl groups into a diazo group and results in an azo product, 2,6-diazo cellulose, as illustrated in Figure 5. Treatment at different media resulted in a different extent of coupling reaction. Cellulose and lignin are the two main constituents of coir fiber. Cellulose is a hydrophilic glucan polymer that contains hydroxyl groups. Theses hydroxyl groups react with diazonium salt to change the nature of coir fiber from hydrophilic to hydrophobic, which improves the interfacial bonding between the coir fiber and hydrophobic PP. On the other hand lignin does not have any free hydroxyl groups to react with diazonium salt. Alkali treatment dissolves the lignin component and increases the percentage of cellulose of coir. As a result, treatment in alkali media (pH 10.5) improved the interfacial bonding to the highest extent followed by the acidic media (pH 3) and neutral media (pH 7), respectively. Consequently the tensile strength of the composites made of treated coir in alkali media was the highest followed by composites made of coir treated in acidic and neutral media and raw coir, respectively. The range of the tensile strength obtained in the present work is 22.1−30.32 MPa, which is higher than the range obtained in previous research (5−25

Figure 2. Variation of tensile strength at different fiber loading.

differently with the change of treatment media and fiber loading. In alkaline media (pH 10.5) the tensile strength increased (approximately 11%) with an increase of fiber loading up to 10%, and then it decreased to a minimum value at 35% fiber loading. The decrease of tensile strength with fiber loading (from 10 to 35%) is also observed in other media. The overall change of tensile strength due to the change of media of chemical treatment may be due to the change in the structure of the cellulose anhydroglucose unit of coir. The chemical treatment of coir reduced the hydroxyl group of the cellulose anhydroglucose unit by coupling with diazonium salt. The Fourier transform infrared (FTIR) spectroscopic analysis of the raw (Figure 3) and treated coir (Figure 4) confirms this phenomenon. The IR spectrum of treated coir clearly indicates the presence of the characteristic band of the NO group in the region of 1600−1700 cm−1, the absorption band of NN stretching near 1614 cm−1, and C−O stretching at the region of 1000−1300 cm−1. Again the IR spectrum of the raw coir

Figure 3. FTIR spectra of raw coir. 3960



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Figure 4. FTIR spectra of treated coir.

MPa) using the same fiber material.11 This could be due to better interfacial bonding between the coir fiber and PP matrix obtained in the present research, which subsequently improved the tensile strength. Figure 6 shows the comparative Young’s modulus of composites made of treated and raw coir with PP at different fiber loading. The Young’s modulus increase with fiber loading is in accordance with other reported research.5,7,8,23−25,27,28 During tensile loading, partially separated microspaces are

created, which obstructs stress propagation between the fiber and the matrix. As the fiber loading increases, the degree of obstruction increases, which in turn increases the stiffness. The Young’s modulus of composites made of treated coir in alkali media were highest, followed by acidic and neutral media. The raw composites have got the lowest Young’s modulus among all. The range of the Young’s modulus found in the present work is (2.05−3.81 GPa) higher than those obtained in previous research (0.56−1.3 GPa) using the same fiber material.11 3.2. Flexural Properties. The variation of the flexural strength and modulus of both raw and treated coir reinforced PP composites at different fiber loading are shown in Figures 7 and 8 respectively. It is seen from Figure 7 that the flexural strength increased with fiber loading up to 30%; however, at 35% fiber loading, the flexural strength decreased. Flexural strength of composites made of treated coir in alkali media was highest, followed by acidic and neutral media. The composites

Figure 6. Comparative Young’s modulus at different fiber loading.

Figure 7. Variation of flexural strength at different fiber loading.

Figure 5. Coupling reaction of diazonium salt with coir.

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3.4. Hardness Results. Figure 10 shows the hardness of various manufactured composites at different fiber loading. It is

Figure 8. Variation of flexural modulus at different fiber loading.

made of PP have got the lowest flexural strength values among all. It is seen from Figure 8 that the flexural modulus of coir fiber reinforced composites increased with the fiber loading.8,23−25,28−30 The flexural modulus of PP composites reinforced with alkali treated coir was highest, followed by the acidic and neutrally treated coir composites and raw coir fiber reinforced composites, respectively. 3.3. Impact Strength Results. Comparative charpy impact strength with fiber loading for both raw and treated coir fiber reinforced PP composites is shown in Figure 9. The impact

Figure 10. Variation of hardness at different fiber loading.

observed from the figure that the average hardness increased with an increase in fiber loading. This may be due to the increase of stiffness of the respective composites. The treatment with diazonium salt in different media also increased the hardness. The highest value is observed in the case of treatment in alkali media. This could be attributed to both dispersion of the fiber into the matrix with minimization of voids and stronger interfacial bonding between the fiber and matrix. 3.5. Water Absorption Characteristics. Water absorption characteristics of the manufactured composites against the fiber loading are presented in Figure 11. The water absorption (%)

Figure 9. Comparative impact strength at different fiber loading.

strength increased to 30% fiber loading. An increase of impact strength with fiber loading was also observed by other researchers reporting on different natural fiber reinforced composites.23−25,29,31 At 35% fiber loading the impact strength dropped. The impact strength of the fiber reinforced polymeric composites depends on the nature of the fiber, the polymer, and fiber−matrix interfacial bonding.32 It has been reported that high fiber content increases the probability of fiber agglomeration which results in regions of stress concentration requiring less energy for crack propagation.33 As presented in Figure 9, the impact strength of all composites increased with fiber loading. This result suggests that the fiber was capable of absorbing energy because of strong interfacial bonding between the fiber and matrix. Another reason of impact failure at higher fiber loading (%) may be due to fiber pull out from the composites. The impact strengths of PP composites reinforced with coir treated in alkali media were the highest, followed by acidic and neutrally media treated and raw coir, respectively. Previous research also showed the increase of impact strength with chemical treatment.34

Figure 11. Comparative water absorption at different fiber loading.

increased with fiber loading. As mentioned earlier, the hydroxyl group (-OH) of coir is responsible for the water absorption characteristics. With the increase in fiber loading, the number of hydroxyl groups in the composites increased, which in turn increased the amount of water absorption. Composites reinforced with raw coir had the highest water absorption, followed by neutral media treated coir composites, acidic media treated coir composites, and alkali media treated coir fiber reinforced composites, respectively. Most of the hydrophilic -OH groups in the raw coir was converted to hydrophobic -O− Na groups during treatment in basic media. The hydrophobic -O−Na groups have less affinity to water compared to -OH groups. This is why the basic media treated composites had the lowest water absorption. During treatment in acidic and neutral media, only a few hydrophilic -OH groups in the raw coir were converted to hydrophobic -O−Na groups. Thus, the acidic and 3962



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neutral media treated composites had higher water absorption compared to the basic media treated ones. 3.6. SEM Morphology. Scanning electron micrographs of the raw, neutral, acidic, and alkali media treated 30% coir reinforced PP composites are shown in Figures 12−15,

Figure 14. SEM micrograph of the 30% acidic media treated coir fiber composite.

Figure 12. SEM micrograph of the 30% raw coir fiber composite.

respectively. The raw coir fiber can be clearly seen in the composite micrograph due to the weak interfacial bonding

Figure 15. SEM micrograph of the 30% alkali media treated coir fiber composite.

between the fiber and matrix (Figure 12), whereas, in Figure 15, the fiber and matrix are not clearly differentiable due to improved interfacial bonding between them. The acidic and neutral media treated coir reinforced PP composite structures (Figures 13 and 14, respectively) are between the two mentioned above due to medium improvement in the interfacial bonding between the fiber and matrix.

Figure 13. SEM micrograph of the 30% neutral media treated coir fiber composite. 3963



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Overall the chemically treated coir composites had better physicomechanical properties. During chemical treatment, hydrophilic -OH groups in the raw coir cellulose were converted to hydrophobic -O−Na groups. Because PP is hydrophobic, the interaction and interfacial bonding between the coir fiber and the PP matrix was increased after chemical treatment, which in turn increased the physicomechanical properties of the resultant composites. The conversion rate of -OH groups into -O−Na groups was higher during alkali media chemical treatment compared to acidic and neutral media treatment. As a result, alkali media treated composites showed better performance compared to the other two media treated composites.

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4. CONCLUSION In the present work, coir fiber reinforced PP composites were manufactured using an injection molding method. Raw coir was chemically treated with benzene diazonium salt in alkali, acidic, and neutral media separately in order to improve the compatibility between the hydrophilic coir and hydrophobic PP. Both the raw and treated coir were utilized for composite manufacturing. Chemical treatment of the raw coir decreased the water absorption capacity of the resultant composites and improved the interfacial bonding between the fiber and matrix. The difference of interfacial bonding between the fiber and matrix in raw and treated coir reinforced PP composites is clearly seen in the scanning electron microscopy. The level of the fiber loading was varied at 10, 15, 20, 25, 30, and 35 wt %. Manufactured composites were subsequently characterized using the microstructural analysis (Fourier transform infrared spectroscopy and scanning electron microscopy) and mechanical testing (tensile, flexural, impact, hardness, and water absorption). The tensile strength of the composites decreased with the coir fiber loading. However, there was an increase in the tensile strength of the alkali and acidic media treated 10% coir reinforced composites compared to PP alone. The Young’s and flexural moduli of the composites increased with the fiber loading. Flexural strength and charpy impact strength increased with the fiber loading; however, the 35% fiber loaded composites had lower flexural strength and impact strength values compared to the 30% ones. Rockwell hardness results show an increase in average hardness values with fiber loading. The tensile strength, Young’s modulus, flexural strength, flexural modulus, impact strength, and hardness values of the alkali media treated coir reinforced PP composites were the highest followed by the acidic media treated coir, neutral media treated coir, and raw coir reinforced PP composites, respectively. The water absorption (%) increased with the fiber loading, whereas the alkali media treated coir composites yielded lower water absorption capacity compared to the other ones. The authors propose that the alkali media treated coir composites yielded the best mechanical properties, while the 30% coir fiber reinforced PP composites had the optimum set of mechanical properties in comparison with other manufactured composites.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +88029665614. Fax: +88029665622. Notes

The authors declare no competing financial interest. 3964



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(22) ASTM Standard D 785-98. Standard Test Method for Rockwell Hardness of Plastices and Electrical Insulating Materials; Annual Book of ASTM Standard; ASTM International: West Coshohocken, PA, U.S.A., 2002. (23) Haque, M. M.; Hasan, M.; Islam, M. S.; Islam, M. S.; Islam, M. N.; Huque, M. M. Physico-Mechanical Properties of Chemically Treated Palm Fiber Reinforced Polypropylene Composites. J. Reinf. Plast. Compos. 2010, 29, 1734−1742. (24) Islam, M. N.; Rahman, M. R.; Haque, M. M.; Huque, M. M. Physico-Mechanical Properties of Chemically Treated Coir Reinforced Polypropylene Composites. Composites, Part A 2010, 41, 192−198. (25) Haque, M. M.; Hasan, M.; Islam, M. S.; Ali, M. E. Physicomechanical Properties of Chemically Treated Palm and Coir Fiber Reinforced Polypropylene Composites. Bioresour. Technol. 2009, 100, 4903−4906. (26) Ismail, H.; Edyhan, M.; Wirjosentono, B. Bamboo Fiber Filled Natural Rubber Composites: the Effects of Filler Loading and Bonding Agent. Polym. Test. 2002, 21, 139−144. (27) Segal, L.; Wakelyn, P. J. Handbook of Fiber Science and Technology: Fiber Chemistry; Marcel Dekker: New York, NY, U.S.A., 1985. (28) Josep, S.; Sreekala, M. S.; Oommen, Z.; Koshy, P.; Thomas, S. A Comparison of Mechanical Properties of Phenol Formaldehyde Composites Reinforced with Banana Fibers and Glass Fibers. Compos. Sci. Technol. 2002, 62, 1857−1868. (29) Jamil, M. S.; Ahmed, I.; Abdullah, I. Effects of Rice Husk Filler on the Mechanical and Thermal Properties of Liquid Natural Rubber Compatibilized High-Density Polyethylene/Natural Rubber Blends. J. Polym. Res. 2006, 13, 315−321. (30) Lin, J. C.; Chang, L. C.; Nien, M. N.; Ho, H. L. Mechanical Behavior of Various Nanoparticle Filled Composites at Low-Velocity Impact. Compos. Struct. 2006, 27, 30−36. (31) Lou, C. W.; Lin, C. W.; Lei, C. H.; Su, K. H.; Hsu, C. H.; Liu, Z. H.; Lin, J. H. PET/PP Blend with Bamboo Charcoal to Produce Functional Composites. J. Mater. Process. Technol. 2007, 192−193, 428−433. (32) Joseph, P. V.; Mathew, G.; Joseph, K.; Groeninckx, G.; Thomas, S. A. Dynamic Mechanical Properties of Short Sisal Fiber Reinforced Polypropylene Composites. Composites, Part A 2003, 34, 275−290. (33) Karmakar, A.; Chauhan, S. S.; Modak, J. M.; Chanda, M. Mechanical Properties of Wood-Fiber Reinforced Polypropylene Composites. Composites, Part A 2007, 38, 227−233. (34) Bachtiar, D.; Sapuan, S. M.; Hamdan, M. M. The Influence of Alkaline Surface Treatment on the Impact Properties of Sugar Palm Fibre-Reinforced Epoxy Composites. Polym.-Plast. Technol. Eng. 2009, 48, 379−383.

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Chemical Treatment of Coir Fiber Reinforced Polypropylene

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