Mechanical and Morphological Properties of Chemically Treated Coir

Sep 18, 2009 - Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh. Ind. Eng. Chem. Res. , 2009, 48 (23), ...
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Ind. Eng. Chem. Res. 2009, 48, 10491–10497

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Mechanical and Morphological Properties of Chemically Treated Coir-Filled Polypropylene Composites Md. Nazrul Islam,* Md. Mominul Haque, and Md. Monimul Huque Department of Chemistry, Bangladesh UniVersity of Engineering and Technology, Dhaka, Bangladesh

In the present work, coir was chemically treated first with sodium perchlorate and then with 2,4-dinitrophenyl hydrazine (DNPH) to improve the mechanical properties of the coir-PP composites. Untreated, oxidized, and DNPH-treated coir samples at different mixing ratios were utilized to prepare the composites. Mechanical properties of the composites prepared from both perchlorate and DNPH-treated coir were found to be better than those of untreated ones. The tensile strengths of both untreated and treated coir-PP composites decreased with an increase in fiber content. However, the values were found to be higher than those of corresponding values of untreated ones. Treated coir-PP composites were found to absorb a lower amount of water than the untreated ones. To understand why the mechanical properties of composites prepared under different conditions of coir were different, surface morphologies of the tensile fractured surfaces of the specimens were recorded using scanning electron microscopy (SEM). The SEM images clearly revealed that there were fewer fiber agglomerations, microvoids, and fiber pull out traces in both perchorate and DNPH-treated coir-PP composite than in the untreated one, indicating that better distribution of the fiber into the matrix as well as stronger fiber matrix interfacial adhesion occurred upon treatment of coir. Introduction In recent years, significant efforts have been devoted to the use of agro-based residues as reinforcing fillers in thermoplastics. These efforts stem in part from their eco-friendliness, biodegradable nature,1 and worldwide environmental awareness with a view to conserve forest resources through reducing uncaring and massive use. From an economic viewpoint, natural fiber reinforced polymer composites are gaining increasing popularity as they are low cost, lightweight, require low processing temperature, and reduce wear in processing equipment.2–5 With the new economic dimensions, increasing volume of agro-based natural fiber has received a wide variety of industrial uses for manufacturing of housing, interior automotive, and packaging products.6–8 Other emerging industrial applications of natural fiber reinforced polymer composites include flower pots, fixtures, furniture, and tiles. Furthermore, since composites prepared by reinforcing natural fibers require low processing temperature, their proper use could unhook the extensive use of fossil fuel, which will consequently reduce environmental pollution by lowering the amount of carbon dioxide. From the viewpoint of worldwide environmental awareness, proper use of agro-based natural fiber could reduce the volume of refuse and emission of greenhouse gases, particularly carbon dioxide to the atmosphere that would otherwise cause environmental pollution if thrown away or burnt down.8 The main purpose of incorporation of fillers into thermoplastic polymer matrixes is to improve the specific physical and mechanical properties of the composites. The factors that determine the physical properties and improve mechanical strength of composites are the extent of filler loading, size and shape of the filler, and the filler-matrix interfacial adhesion.9 Therefore, the proper selection of fillers for a particular polymer matrix is an important factor for the improvement of the filler-matrix interfacial adhesion. Complete fusion of matrix * To whom correspondence should be addressed. Tel.: +8801715784778 (extension 7340). Fax: +880-2-863046. E-mail: [email protected].

impregnated with filler through strong interfacial adhesion between the two and matrix-to-filler stress transfer efficiency are the prime requirements for the production of durable and reliable composites having specific mechanical properties that can withstand mechanical shocks and dimensional changes due to moisture absorption.10–12 However, the inherent hydrophilic nature of natural fibers does not allow them to couple strongly with the hydrophobic polymer matrix, resulting in composites with inferior mechanical properties. To address this drawback, a number of fiber treatment methods have appeared in the literature.13–23 Upon treatment, the hydrophilic nature of the cellulose is significantly reduced, giving better filler-matrix interfacial adhesion. In this regard, a number of coupling agents possessing both hydrophilic and hydrophobic parts have also been introduced while preparing composites that can chemically bridge the hydrophilic filler on one side and facilitate wetting of the hydrophobic matrix with the filler on the other side2 Sanadi et al. reported that the hydrophilic nature of natural fiber reinforced polymer composites can substantially be reduced by acetylation of hydrophilic OH group present in the fiber.2,15 Karmarkar et al.16 studied the wood fiber reinforced PP composites and showed that improved mechanical properties of the composites can be achieved if a compatibilizer with an isocyanate functional group is introduced between the two components. In our previous reports, we showed that mechanical properties of natural fiber reinforced polymer composites can be achieved if the fiber is pretreated with benzene diazonium salt21 and post-treated with urotropine.22 In the present study, we endeavored to present the physical and mechanical properties of chemically treated coir-filled polypropylene (PP) composites at different treatment conditions and fiber compositions. Coir is a natural fiber, which is obtained from the husk of matured coconuts. Cellulose is the main constituent (43%) of this fiber, which is a hydrophilic glucan polymer consisting of a linear chain of 1,4-β bonded anhydroglucose unit that contains alcoholic hydroxyl groups.18,19 Here, we have shown a two-step fiber treatment technique to modify the hydrophilic nature of coir, which is a substantial extension

10.1021/ie900824c CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009

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Scheme 1. Treatment of Coir with Sodium Perchlorate and 2,4-Dinitrophenyl Hydrazine

to our previous studies.20–23 The main objective of chemical treatment of coir used in the present study is to improve its interaction with the PP matrix. This will consequently improve the mechanical properties of the composite and reduce the microvoids in the composites as well as the hydrophilic nature of coir responsible for moisture absorption. Water absorption behavior of the composites was characterized to understand the effect of chemical treatment on the hydrophilicity of the composites. To understand why the surface properties of the composites prepared under different treatment conditions are different, SEM images of the tensile fractured surface of the samples were also recorded. Materials and Methods The thermoplastic polymer, polypropylene (PP), used as matrix material, was supplied by the Polyolefin Company Private Ltd., in the form of homopolymer pellets. Its specific gravity was 0.90-0.91, melt flow index was 10 g/10 min, and melting temperature was 165-171 °C. The coir, used as reinforcing filler, was obtained from a local coconut oil factory in Bangladesh. The chemicals used to treat coir were NaClO4 (Merck) and 2,4-dinitrophenyl hydrazine (DNPH) (Merck). Treatment of Coir. Coir was first treated with an aqueous solution of NaClO4 to produce cellulose dialdehyde, which was further treated with DNPH to obtain the adduct as shown in Scheme 1. Before chemical treatment, coir was cleaned and then dried in an oven at 105 °C for 24 h to obtain 1-2% moisture content. The dried coir was then kept in a sealed container. After oxidation, coir fiber was washed and then dried in air. For coupling reaction, DNPH was dissolved in an ethanol/water mixture in a 1000-mL beaker. The pH of the solution was adjusted to 3 by adding H2SO4. Coir (500 g) was then immersed

into the solution for about 4 h at 70 °C for coupling reaction with DNPH. After the reaction, coir was taken out of the beaker, washed with distilled water, and finally dried in open air. Fabrication of Composites and Test Specimens. Coir processed as mentioned above was initially mixed thoroughly with PP granules at 10/90, 15/85, 20/80, and 25/75 wt % mixing ratios. The mixture was then passed through an extruder at a constant temperature of 165 ( 5 °C. The extruded composites were cut into 2-4-cm-long pieces. All the pieces were then crushed into smaller granules using a grinding machine. The granules were dried in a vacuum oven at 65 °C for 1 h and then fed into an injection molding machine for making specimens. The specimens for tensile and flexural tests were prepared from dried granules using the injection-molding machine at a molding temperature of 165 °C. Details of experimental procedure and tests of the specimens can be found elsewhere.21 Water Absorption. To measure the water uptake capacity of the composites, rectangular specimens of dimensions 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. The water absorption tests were carried out by immersing the specimens in a water bath for 24 h at room temperature. After immersion, the excess water was removed using a piece of soft cloth and final weight of the specimens was taken. From the difference of the final and initial weights percentage of water uptake was calculated. Infrared Spectra. The infrared spectra of raw and treated coir were taken on a Shimadzu FT-IR 81001 spectrophotometer with coaddition of 64 scans at a resolution 4 cm-1 to characterize the chemical change of coir upon treatment with sodium perchlorate and DNPH. Scanning Electron Microscopy (SEM). The morphology of the coir-PP composites and interfacial adhesion between the

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Figure 1. Infrared spectra of raw, oxidized, and DNPH-treated coir.

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Figure 2. Tensile strength of coir-PP composites: (1) untreated coir, (2) oxidized coir, and (3) DNPH-treated coir.

filler and the PP matrix was examined using a scanning electron microscope (JSM-6701F) supplied by JEOL Company Limited. The samples were viewed perpendicular to the fractured surfaces. The micrographs were taken at a magnification of 300. Results and Discussion Tensile Properties. In the present study, surface modification of the coir fiber was carried out to achieve better mechanical properties of composites and the results were compared with those of the untreated ones. The presence of hydroxyl groups of the cellulose in coir is responsible for its inherent hydrophilic nature. As a result, it becomes difficult to compound it with the hydrophobic polymer matrix, resulting in poor performance in the mechanical properties as well as dimensional change of furnished products due to moisture absorption of the composite. To overcome these problems, coir was chemically treated first with sodium perchlorate and then with DNPH. Scheme 1 shows the chemical changes of the cellulose in coir upon a two-step treatment with sodium perchlorate and DNPH. Upon treatment with perchlorate, the hydroxyl groups at C2 and C3 and C6 are transformed into aldehyde, which further undergoes coupling reaction with DNPH. The corresponding IR spectra of raw and chemically treated coir are shown in Figure 1. The IR spectrum of raw coir shows a band in the region near 1646 cm-1, which is probably due to the CO group of acylester in hemecellulose or of aldehyde group in lignin24 The IR spectrum of perchloratetreated coir shows a band near 1617 cm-1, which is maybe due to the carbonyl groups produced from the oxidation of the hydroxyl groups of cellulose. On the other hand, DNPH-treated coir shows a clear band in the region near 1504 cm-1, which is maybe due to the stretching frequency of nitro group present in the aromatic ring of DNPH. These results suggest that chemical modification of the cellulose has occurred upon treatment with perchlorate and DNPH. Figure 2 shows the tensile properties of raw and treated coir-PP composite as a function of filler loading. It is clear from the figure that values of tensile strengths of raw coir-PP composites gradually decrease with an increase in filler content. With increasing the composition of filler in the composite, weak filler-matrix interfacial area increases, which consequently results in a decrease in tensile strength.10 Chemical modification of coir has reduced the hydrophilic nature of coir by reducing the number

Figure 3. Effect of fiber loading on the Young’s modulus of (1) raw coir-, (2) oxidized coir-, and (3) DNPH-treated coir-PP composites.

of hydroxyl groups in the cellulose. As a result, interfacial adhesion between the filler and matrix has improved. This in turn improved the tensile strengths of both perchlorate and DNPH-treated coir-PP composites. Significant improvement in tensile strengths is found for DNPH-treated coir-PP composites. The increase in tensile strength might be due to the cross-linking network formation between the filler and the matrix. This indicates that fiber treatment can improve the fibermatrix interfacial adhesion, leading to better stress transfer efficiency from the matrix to the filler with consequent improved mechanical properties of the composites. Figure 3 shows the Young’s modulus of the composites at different filler loading. As expected, the addition of fiber increases the modulus of the composites, resulting from the inclusion of rigid fiber into the soft PP. This observation suggests that the incorporation of rigid filler into the soft thermoplastic PP increases the stiffness of the composite. The chemically treated coir-PP composites are found to show higher modulus compared to those of the untreated ones. This indicates that homogeneous dispersion of coir particles and better filler-matrix interaction has occurred upon treatment of coir. It has been reported that crystallites possess higher modulus compared to

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Figure 4. Variation of flexural modulus of PP composites reinforced with (1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

Figure 5. Effect of fiber loading on the flexural modulus of (1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir-reinforced-PP composites.

amorphous substances16 Upon chemical treatment of coir with peroidate and DNPH surface crystallization of coir probably dominates over its bulk nature. Furthermore, incorporation of fiber into the polymer matrix reduces the matrix mobility. As a result, the modulus of the composites upon treatment with perchlorate and DNPH has increased with an increase in filler content. Flexural Properties. The flexural strength and modulus of both untreated and treated coir-PP composites are shown in Figures 4 and 5, respectively. As shown in Figure 4, the values of flexural strength of both raw and treated coir-PP composites initially increased and showed a steady behavior with further increases in the filler content. The steady behavior of flexural strength of the composites could be a balance in the favorable entanglement of the polymer chain with the filler and opposing weak filler-matrix interfacial adhesion with increasing filler content. It is evident from Figure 5 that the addition of coir fiber to PP has significantly increased the modulus of the composites, which is found to be in agreement with the results of previous reports.16,25 This

Figure 6. Variation of impact strength of PP composites reinforced with (1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

observation suggests that addition of coir fiber into the thermoplastic matrix has improved the stiffness of the composite. Since coir is a high modulus material, higher fiber concentration in the composites demands stronger stress for the same amount of deformation. Consequently, flexural modulus of the composites increases with an increase in the fiber content. Chemically treated composites show much higher strength and modulus. This could be due to better filler-matrix interfacial adhesion and effective stress transfer from the matrix to the fiber. Impact Strength. Figure 6 shows the variation of impact strengths of both raw and chemically treated coir-reinforced-PP composites at different filler loading. Impact strength is a measure of the tolerability, when the composite is subjected to a sudden impact that results in crack propagation through the material. For fiber-reinforced polymeric composites, it depends on a number of factors, such as the nature of the fiber, polymer matrix, and the polymer-matrix interfacial bonding.26 Sanadi et al. reported that high fiber content increases the possibility of fiber agglomeration, which results in regions of stress concentration that require less energy for crack propagation and that an increase in the resistance of crack propagation occurs if fiber bridges the crack in the composites.2 As shown in Figure 6, impact strengths of both treated and untreated coir-PP composites show a slight increasing trend with an increase in the filler loading, indicating that the filler is capable of absorbing energy because of strong filler-matrix interfacial adhesion. It has been reported that improved interfacial bonding provides an effective resistance to crack propagation during impact tests.11,26 Thus, higher impact strengths of the treated coir-PP composites suggest a better interfacial bonding compared to those of untreated ones. This could be due to better kneading of the matrix-filler system during the preparation of composites, their grinding and then specimen fabrication in injection molding method. Slightly higher impact strength for perchlorate and DNPH-treated coir-PP composites is probably due to the favorable interaction between the treated coir and the hydrophobic PP chain of the matrix. So-called fiber pullout and fiber agglomeration could be responsible for lower impact strengths of untreated coir composites.

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2,3,27

Figure 7. Hardness of (1) raw coir-, (2) oxidized coir-, and (3) DNPHtreated coir-PP composites at different fiber loading.

Figure 8. Water absorption behavior of PP composites reinforced with (1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

Hardness. Hardness of a composite material refers to its resistance to shape changes when force is applied on it. For composites, it depends on the distribution of the filler into the matrix.26 Usually the presence of a more flexible matrix causes the resultant composites to exhibit lower hardness.15 As shown in Figure 7, incorporation of both treated and untreated coir into the PP matrix has reduced the flexibility of the matrix, resulting in more rigid composites. The hardness of both treated and untreated composites is found to increase with an increase in the filler loading. The incorporation of filler particles into the PP matrix has reduced the mobility of the polymer chain in the rigid composites. The treated coir composites seem to have better hardness compared to untreated ones. This could be attributed to both better dispersion of the fiber into the matrix with minimization of voids and stronger interfacial adhesion between the matrix and the filler. Water Absorption Behavior. Water absorption characteristics of the composites against filler loading are shown in Figure 8. Water absorption (%) increased with an increase in filler loading. Usually natural fiber-polymer composites without compatibilizer show remarkably water absorption due to the

presence of voids. With an increase in filler loading, the number of hydroxyl groups as well microvoids in the composites increased, which results in an increase in water absorption. Chemically treated coir-reinforced composites are found to show lower water absorption capacity compared to the untreated ones, indicating that the hydrophilic nature of coir has substantially decreased upon chemical treatment with both NaClO4 and DNPH. This can directly be attributed to the decrease in the number of hydroxyl groups responsible for the hydrophilic nature of the cellulose that converted into aldehyde group and subsequently coupled with DNPH. No dimensional change is observed upon immersion of the composites in water. This indicates that fiber is thoroughly encapsulated in the matrix. At the same time, it can also be ascribed that, due to favorable interaction between the matrix and the treated filler, microvoids in the composites have substantially minimized, showing lower water uptake capacity. Morphological Study. The morphology of the tensile fractured surface gives information as to why mechanical properties of the composites prepared under different treatment conditions are different. The tensile fractured surface morphologies of untreated and treated coir-PP composites prepared with 25 wt % coir are shown in Figure 9. The SEM images of the untreated coir-PP composite show a number of fiber agglomerations and fiber pullout traces in the composites (image A). These features suggest fiber-fiber interaction as well as weak interfacial bonding between the hydrophilic filler and the hydrophobic matrix. On the other hand, chemically treated coir-PP composites show almost uniform dispersion of the filler into the matrix, which results in better interfacial adhesion between the filler and the matrix with improved mechanical properties. This also implies that hydroxyl groups are being oxidized to aldehyde group, which upon coupled with DNPH has reduced the hydrophilicity of coir, providing favorable interaction with the PP chain with improved mechanical properties (images B and C). It is evident from images B and C that both fiber pullout traces and fiber agglomeration as well as the microvoids in the composites have significantly reduced in the composite upon treatment of coir with perchlorate and DNPH. This result suggests that interfacial bonding between the filler and the matrix has become much more favorable for treated coir and the matrix compared to that of the untreated one. Conclusions The present work reveals that low cost renewable materials can be used to prepare useful composites with good mechanical properties. The tensile strength values of the composites of untreated coir showed a decreasing trend with increasing filler loading. On the other hand, the tensile strength values of the DNPH-treated composites showed an increasing trend up to 15% filler loaded composite and then decreased with further increases in filler content. It is important to note here that, at all mixing ratios, the tensile strengths of the treated coir-PP composites showed higher values compared to those of the untreated ones. In both cases, the Young’s modulus, flexural strength, flexural modulus, impact strength, and hardness are also found to increase with an increase in filler loading and the values are found to be higher for treated coir-PP composites than those of the untreated ones. It is concluded that interaction between the filler and the matrix has become more favorable upon chemical treatment of coir. Water absorption (%) increased with filler loading; however, treated coir composites showed lower water uptake capacity compared to those prepared from

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with almost no fiber pullout traces and agglomeration of the treated coir-PP composites. Acknowledgment We thank the members of the Board of Postgraduate Studies (BPGS) of the Department of Chemistry, BUET for helpful discussion. The financial assistance (CASR-216/23) approved by the Committee for Advanced Studies and Research (CASR) BUET for carrying out the present work is highly appreciated. Literature Cited

Figure 9. SEM images of PP composites reinforced with 20% (1) raw coir, (2) oxidized coir, and (3) DNPH-treated coir.

untreated coir, indicating that upon chemical treatment the number of hydroxyl group in the cellulose of coir has substantially decreased, giving reduced the hydrophilic nature of coir. At the same time, it can be said that, due to favorable interaction between the treated coir and PP, microvoids in the composites have largely minimized, showing lower water uptake capacity of the composites. The improved mechanical properties are supported by SEM images of the fractured surfaces that show better dispersion of the filler in the matrix

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ReceiVed for reView May 20, 2009 ReVised manuscript receiVed August 18, 2009 Accepted August 30, 2009 IE900824C