Bio-composites: Development and Mechanical ... - Springer Link

Fibers and Polymers 2014, Vol.15, No.4, 847-854

DOI 10.1007/s12221-014-0847-y

Bio-composites: Development and Mechanical Characterization of Banana/Sisal Fibre Reinforced Poly Lactic Acid (PLA) Hybrid Composites B. Asaithambi*, G. Ganesan, and S. Ananda Kumar1 Department of Manufacturing Engineering, Annamalai University, Chidambaram, Tamilnadu, India 1 Department of Chemistry, Anna University, Chennai, Tamilnadu, India (Received June 21, 2013; Revised September 13, 2013; Accepted October 7, 2013) Abstract: The work focuses on the influencing effect of fiber surface treatment by BP towards mechanical properties of BSF reinforced PLA composites. BSF were treated by BP to improve the adhesion between fibres and matrix. BSF (30 wt %) reinforced PLA (70 wt %) hybrid composites were fabricated by means of twin screw extrusion followed by injection molding process. Tensile strength, flexural strength and modulus were tested by means of UTM. The morphological analysis of the untreated and treated BSF reinforced PLA composites in comparison with virgin PLA was carried out by SEM to examine the existence of interfacial adhesion between BSF and PLA. The resultant data reveals that treated BSF restricts the motion of the PLA matrix due to better wettability and bonding. Consequently, mechanical properties like tensile and flexural moduli of BSF reinforced PLA composites were enhanced in comparison to virgin PLA and untreated BSF reinforced PLA composites. The results are discussed in detail. Keywords: Banana/Sisal fibre (BSF), Polylactic acid (PLA), Benzoyl peroxide (BP), Universal testing machine (UTM), Scanning electron microscopy (SEM), Twin screw extrusion process, Injection molding process

considered as a waste product of banana cultivation. Hence without any additional cost input, banana fibres can be obtained as a good reinforcement for industrial purposes [4]. The tensile strength and elongation at break values obtained for the red banana from physico-chemical experimental studies were higher compared with other varieties of banana were reported by Veluraja and Kiruthika [5]. A truck model ‘Manaca’ was developed and tested using banana fiber mixed with epoxy resin and hardener went through many years of performance road tests and demonstrated excellent results [6]. Natural fibres as reinforcement has a drawback associated with its application for reinforcement of polymers especially in higher load bearing applications. Hence, the drawbacks of these fibres have prevented their extended utilization in the composites industries [7]. Therefore; we tried a combination of banana and sisal fibre to overcome the problem of load bearing mentioned above. The hydrophilic nature of NFs leads to incompatibility between the fibre and hydrophobic polymer matrix and this can be reduced by proper chemical surface treatments. Since two fibres combination was used for this study we made use of alkali treatment coupled with BP treatment. Among the biodegradable polymers, PLA is made from plants and is readily biodegradable due to its inherent susceptibility to biological attack [8]. The studies on the mechanical properties of natural fibres reinforced with PLA have been carried out by with different types of NFs as reinforcement to replace synthetic fiber in polymer composites [9]. The advantage of using hybrid composite is that one sort of fibre might complement with another fibre that lacks the desired properties. Hence, we made an attempt to reinforce a banana/sisal fibre combination in PLA matrix. Moreover, a limited literature is available, where a combination of two or more NFs was tried as reinforcement in PLA matrix [10].

Introduction The collapse in prices of engineering and standard plastics, the assumption of the future exhaustion of crude world-wide reserves and increasing environmental concerns have lead to the use of regenerable raw materials for the design and development of new components. Therefore, extensive effort has been devoted on the study and development of natural fibres (NFs) reinforced composites with bio degradable polymers [1]. The renewed interest of NFs over their synthetic fibre counter part is that they are abundant in nature and are also renewable raw materials. Owing to the low specific gravity of NFs as compared to synthetic fibres, they are able to provide a high strength-to-weight ratio in plastic materials. The usage of NFs also provided healthier working condition than the synthetic fibres. Besides, the less abrasive nature of the NFs when compared to that of synthetic fibres offers a friendlier processing environment as the wear of tools could be reduced to a greater extent. Furthermore, NFs offer good thermal and insulating properties, are easily recyclable and also biodegradable. Among the NFs, sisal and banana fibres are noteworthy and find extensive application in the fabrication of biocomposites owing to their exceptional combination of properties [2]. Sisal fiber is coarse and rigid possessing high specific strength, stiffness, durability and resistance to deterioration in saltwater. Therefore; it can be used as a reinforcing material in polymeric resin matrices to make useful structural composite materials that offer unusual combination of properties [3]. In tropical countries fibrous plants are available in abundance and some of them like banana are agricultural crops. Banana fibre at present is *Corresponding author: [email protected] 847

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Fibers and Polymers 2014, Vol.15, No.4

The aim of this research is to produce a fully biodegradable green composite from renewable resources using PLA as matrix and a combination of BSF as reinforcement. To the best of our knowledge, this combination of BSF as reinforcement with polymer matrix has not been investigated so far. Moreover, the chemical treatment of BSF and its reinforcement to PLA matrix, by twin screw extruder followed by processing by injection molding technique has not been reported so far. Furthermore, the effect of surface treatment and hybridization of BSF with PLA on the mechanical properties were also studied and evaluated.

Manufacturing Process Materials and Fabrication Banana fibres obtained from Genome Group (AAA) of subgroup (red) were supplied by kolvel fibres, Nagerkovil, Tamil Nadu. Sisal fibres were obtained from vibrant nature, Chennai, Tamil Nadu. Both fibres were kept in an oven at 60 oC for 24 hrs to minimize the moisture content. PLA (polymer 3052 D) designed for injection molding has a density of 1.24 g/cm3, crystalline melt temperature (Tm) range 145-160 oC and glass transition temperature (Tg) of about 55-60 oC was kindly provided by Harita NTI Ltd, Chennai, Tamil Nadu. Sodium hydroxide (NaOH) and BP pellets were used in chemical modification of the BSF were purchased from merck, Germany. The photographic images of untreated banana and sisal fibres are shown in Figures 1 and 2 respectively.

B. Asaithambi et al.

pectin and wax materials present in the fibres. Further, the fibres were washed several times with distilled water to remove NaOH solution sticking on the fiber surface. Finally, acetic acid was used to adjust the pH value of the fibres. Banana fibres, after acetic acid treatment were taken out and dried for 2 days at room temperature and then dried at 70 oC for 24 hrs in air circulating oven. These dried fibres were soaked in 6 % BP with acetone solution at temperature of 70 oC for 30 mins and washed several times with distilled water to remove excess BP sticking on the fibre surface. Finally, acetic acid was added to the BP treated banana fibres to adjust pH. The fibres were dried at room temperature for 2 days, followed by an air circulating oven drying at 80 oC for 24 hrs. PLA were dried in an oven at 80 oC overnight and stored in air tight bag. The surface treatment of BSF was carried out as per the reported procedure [7,11]. Chemical Modification of Sisal Fibres The surface modification of sisal fibres using NaOH and BP was carried out in a similar procedure followed for banana fibres. The treated banana and sisal fibres are shown in Figures 3 and 4 respectively. Blending of PLA and BSF with Twin Screw Extruder A two step process, extrusion followed by injection molding to prepare BSF reinforced PLA composite was carried out. Blending was performed by means of three general processes:

Chemical Modification of Banana Fibres Benzoyl Peroxide Treatment For BP treatment, banana fibres need to be pre-treated with NaOH and were immersed in 5 % NaOH solution at room temperature for 2 hrs, to remove hemicellulose, lignin,

Figure 3. Photographic image of alkali combined by BPT-banana fibres. Figure 1. Photographic image of untreated banana fibres.

Figure 2. Photographic image of untreated sisal fibres.

Figure 4. Photographic image of alkali combined by BPT-sisal fibres.

Characterization of BSF/PLA Biocomposites


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Figure 5. Photographic image of compounded and extruded pellets of UT-BSF/PLA composite.

melt blending, extrusion, and pelletizing. The virgin PLA granules and untreated BSF(UT-BSF) with the ratio of 70:30 (250 g) were blended in a twin-screw extruder machine (screw diameter of 28 mm, L/D ratio of 40) comprising 10 heat zones, which were set temperatures at 140 oC, 150 oC, 155 oC, 160 oC, 165 oC, 170 oC, 175 oC, 170 oC, 165 oC and 160 oC from feed zone to die exit at a screw speed of 100 rpm. As a result of the shear stress imposed on fibres during compounding, homogenization of UT-BSF/PLA was carried out by cycling the mixture in the extruder for 15 min and then extruded through a 1 mm gauge strand die at a rate of 10 mm/s. Strands were cooled in a water bath and subsequently pelletized using a pelletizer. Compounded pellets were dried at 90 oC in a vacum oven for 24 hrs and stored in sealed polyethylene bags to avoid moisture infiltration. The blending was carried out in a similar procedure followed for treated BSF (T-BSF) reinforced PLA composites. The images of UT-BSF/ PLA extruded pellets of composite are shown in the Figure 5. Fabrication of Hybrid Composites The UT-BSF/PLA extruded pellets were processed by means of injection molding machine (screw diameter of 30 mm with L/D ratio of 20 of 60 ton capacity). The process was carried out at a temperature profile of 80 oC/160 oC/ 165oC/170 oC and 175 oC in the nozzle, injection pressure of 190 bars, injection time of 0.95 s and mould temperature of 30 oC respectively. The resultant UT-BSF/PLA hybrid composites were subjected to annealing at a temperature of 80 oC in an oven for 24 hrs. Similar procedure was adopted for the fabrication of the T-BSF/PLA hybrid composites [3,12]. Mechanical Tests-tensile and Flexural The tensile and flexural testing was performed for virgin PLA, UT-BSF/PLA and T-BSF/PLA composites by means of UTM (Instron machine-model 5564) as per ASTM D638 and D790 standards. The schematic diagram and photographs of tensile and flexural specimens subjected for the test are shown in Figures 6(a) and 6(b) and Figures 7(a) and 7(b) respectively. Ten samples of tensile and flexural specimen of each were tested and the mean, standard deviation and

Figure 6. (a) Schematic diagram for virgin PLA, UT-BSF/PLA and BPT-BSF/PLA tensile specimens and (b) photographic image of virgin PLA specimens for tensile test.

Figure 7. (a) Schematic diagram for virgin PLA, UT-BSF/PLA and BPT-BSF/PLA flexural specimens and photographic image of virgin PLA specimens for flexural test.

Figure 8. Tensile strength of virgin PLA, UT-BSF/PLA and BPTBSF/PLA composites with standard error bars.

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Figure 9. Flexural strength of virgin PLA, UT-BSF/PLA and BPTBSF/PLA composites with standard error bars.


Figure 12. Elongation at break of virgin PLA, UT-BSF/PLA and BPT-BSF/PLA composites with standard error bars.

Figure 10. Tensile modulus of virgin PLA, UT-BSF/PLA and BPT-BSF/PLA composites with standard error bars.

Figure 13. (a) Schematic diagram for virgin PLA, UT-BSF/PLA and BPT-BSF/PLA Izod-impact specimens and (b) photographic image of virgin PLA specimens for Izod-impact test.

Figure 11. Flexural modulus of virgin PLA, UT-BSF/PLA and BPT-BSF/PLA composites with standard error bars.

standard error values were calculated and the results are given as Figures 8-12 with error bar graphs for virgin PLA, UT-BSF/PLA and T-BSF/PLA composites. Impact Test The notched Izod Impact testing was performed using an

Figure 14. Impact strength of virgin PLA, UT-BSF/PLA and BPTBSF/PLA composites with standard error bars.



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Figure 15. Tested specimens of tensile, flexural & Izod-impact BPT-BSF/PLA composites.

EMIC pendulum machine according to ASTM D 256. The schematic diagram of specimens subjected for impact test and photographs of specimen taken before the test are shown in the Figures 13(a) and (b). Ten samples of impact specimen were tested and the mean, standard deviation and standard error values were calculated for virgin PLA, UT-BSF/PLA and T-BSF/PLA composites with error bar graph depicted in Figure 14. Photographs of tested specimens of BP treated BSF/PLA (BPT-BSF/PLA) composites subjected to tensile, flexural and impacts are shown in the Figure 15. SEM Investigation Scanning electron microscopy (SEM-JEOL JSM 6060) characterized the fractured surfaces of tensile, flexural and

Figure 16. SEM fractograph of tensile specimen of (a) virgin PLA, (b) UT-BSF/PLA composite, and (c) BPT-BSF/PLA composite.

Figure 17. SEM fractograph of flexural specimen of (a) virgin PLA, (b) UT-BSF/PLA composite, and (c) BPT-BSF/PLA composite.

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impact specimens of virgin PLA, UT-BSF/PLA and T-BSF/ PLA composite. The samples were coated with a thin layer of gold before scanning observation in order to increase the


sample conductivity and also to avoid electrostatic charging during sample examination. The images were analyzed to investigate the distribution of natural fibres in the polymer matrix and their nature of interaction with each other. The results of SEM analysis are depicted in Figures 16(a)-(c), Figures 17(a)-(c), and Figures 18(a)-(c) respectively.

Results and Discussion

Figure 18. SEM fractograph of impact specimen of (a) virgin PLA, (b) UT-BSF/PLA composite, and (c) BPT-BSF/PLA composite.

Effect of Fibre Surface Treatment by BP on Tensile Properties The effect of fiber treatment of BSF on the tensile properties of virgin PLA, UT-BSF/PLA and T-BSF/PLA composites was evaluated based on our findings from Table 1. The addition of untreated BSF into virgin PLA resulted in a significant change in its tensile properties. The increase in the tensile strength of virgin PLA from 45 MPa to 57 MPa of UT-BSF/PLA composites suggest that inclusion of the UTBSF into virgin PLA, rendered reinforcement by offering a stress bearer to the PLA matrix as reported by Huda et al. [13]. The alkali treatment followed by BP further improves the adhesive characteristics of the UT-BSF surface by eliminating natural and artificial impurities as well as lignin and hemicelluloses, which severely affect the tensile properties of BSF. After the removal of hemicelluloses, fibres were stretched leading to rearrangements, which result in a rough surface of BSF, thereby promoting improved PLA/BSF interpenetration [10]. Thus T-BSF/PLA composite showed drastic improvement of tensile strength (79 MPa) than UTBSF/PLA composites (57 MPa). The presence of peroxides further introduces polarity in the PLA matrix, thereby increasing its degree of dispersion between fibre/matrix interface and fibre surface modification removed pectin, lignin from BSF promoted adequate compatibility and wettability between fiber/matrix and also generates cellulose radicals with subsequent promotion of the grafting of cellulose on to PLA matrix. There by an increase in tensile strength and modulus of T-BSF/PLA composites than UT-BSF/PLA composites was observed [1,3,7]. It is clear that the UT-BSF/PLA composites distinctively have lower value of elongation at break (1.8 %) compared to the virgin PLA matrix whose elongations at break (2.5 %) can be associated with uneven aligning of BSF fibres with PLA matrix.tis indicates the existence of physical bonding showing inadequate wetting of the UT-BSF with PLA matrix and thereby introduces numerous

Table 1. Mechanical properties of virgin PLA, UT-BSF/ PLA and BP treated BSF reinforced PLA composites Materials Virgin PLA UT-BSF/PLA composite BPT-BSF/PLA composite

Tensile strength (MPa) 45 57 79

Tensile properties Flexural properties Impact properties Elongation Tensile modulus Flexural strength Flexural modulus Impact strength at break (%) (GPa) (MPa) (GPa) (kJ/m2) 2.5 1.01 79 3.7 45 1.8 1.7 91 4.2 31.5 1.09 4.1 125 5.6 47.8


voids leading to a lower elongation value of UT-BSF/PLA composites, which behave as plastic material resisting the deformation to a greater extent than virgin PLA [8,14]. On the other hand, T-BSF/PLA composites (1.09 %) posses lower value of elongation at break than UT-BSF/PLA composites and virgin PLA matrix respectively. This may be attributed to an increased strength and stiffness achieved from the TBSF/PLA composites. Higher elongation imparted by UTBSF/PLA composites than T-BSF/PLA composites was due to a weak bonding between PLA matrix and BSF [10]. The maximum tensile modulus of elasticity exhibited by T-BSF/ PLA composites (4.1 GPa) may be due to the even load transfer between the fibre and matrix through strong fibre/ matrix interaction. Furthermore, alkali along with BP treatment leads to breaking down fibre bundles in to individual fibres (fibrillation) thereby enhancing the surface area and facilitates improved tensile modulus ultimately [8,15,16]. The lower values of tensile modulus observed in the case of UT-BSF/ PLA composites (1.7 GPa) may be attributed to the restrictions of macromolecular mobility and deformability imposed by the presence of UT-BSF in PLA matrix. Such macromolecular restriction was absent in the case of virgin PLA and hence offered the least value of tensile modulus (1.01 GPa) of elasticity among all [17]. The SEM image of tensile fractured surface of the virgin PLA was relatively smooth and flat, though it had some matrix deformation/tearing and ridges as seen from Figure 16(a), which depicts its large brittle nature with some limited ductility. Figure 16(b) illustrates the fractured surface of UTBSF/PLA composites that contains cavities, fibre pull-out and fracture with aggregation of fibres. UT-BSF/PLA composites shows weaker fibre/matrix interface causing the fibres to be pulled out from the matrix easily when stress was applied, leading to high porosity through which entrapment of air on the surface or inside the composite [10,18]. The SEM image of T-BSF/PLA composites exhibited in Figure 16(c) showing evidence for the matrix deformation and fibre fracture instead of fibre drawn out [10,12] indicating minimum porous nature. It also reveals that injection molding process was believed to have contributed to the enhancement in tensile properties of the T-BSF/PLA composites whose T-BSF was evenly distributed within the PLA matrix imparting improved interfacial adhesion between them [14]. Effect of Fibre Surface Treatment by BP on Flexural Properties The flexural strength and modulus values seen from Table 1were found to be higher for T-BSF/PLA composites than UT-BSF/PLA composites and virgin PLA respectively. The improved flexural properties observed for T-BSF/PLA composites may be due to the process of mercerization that converts crude cellulose structure I to a refined cellulose structure II with reactive groups, producing short length crystallites capable of making intimate bonding with PLA


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matrix. Moreover, mercerization reduces the diameter of BSF and increases the aspect ratio offering an anchoring effect to T-BSF/PLA composites. The BP treatment, in addition to alkali treatment, increased the intensity of chemical bonding between BSF and PLA matrix in composites [1,9,10]. In case of UT-BSF/PLA composite, the presence of wax, oil, and surface impurities provide an insulative layer on the surface of the fibres led to a weak interfacial bond leading to failure [18]. The fractured flexural surfaces of virgin PLA depicted in Figure 17(a) appear rougher; bumpy and mottled supporting its brittle behavior with a limited ductile nature [18]. The fractured image of UT-BSF/PLA composite depicted in Figure 17(b), showing gaps between PLA and the UTBSF leading to detachment of fibre from the PLA matrix that prevented the interaction between the hydrophilic fibre and hydrophobic polymer matrix [9,15]. The morphology of T-BSF/PLA composites illustrated in Figure 17(c) shows individual separation and dispersion of the BSF fibres within the PLA matrix which might have occurred during the extrusion process and as a result of this, extruded T-BSF/ PLA composites showed the best flexural properties among all [4,19]. Effect of Fibre Surface Treatment by BP on Impact Properties It is evident that impact strength of UT-BSF/PLA composites (31.5 kJ/m2) was found to be lower than that of virgin PLA (45 kJ/m2) and T-BSF/PLA composites (47.8 kJ/m2). Fibre pull out was the deciding factor and the failure mechanism may be due to the cracks being easily propagated through void regions of UT-BSF/PLA composites in Figure 18(b), Furthermore, the presence of UT-BSF led to agglomeration, with subsequent creation of stress concentrations that required less energy to elongate the crack propagation in UT-BSF/ PLA composites than T-BSF/PLA composites [8]. Moreover, shear of BSF due to applied load may exceed the fibre/ matrix surface bond strength leading to detachment. Consequently, the strain level exceeds the BSF strength and ultimately BSF fracture happens [4]. The slight improvement in impact strength observed for T-BSF/PLA composites compared to virgin PLA may be explained by means of void coalescence observed from Figure 18(c) and this void coalescence prevented the crack growth, its propagation and microscopic plastic deformation of PLA matrix T-BSF/PLA composites [16]. The absence of such voids (Figure 18(a)) was the main reason, for virgin PLA matrix to have offered higher impact strength than UT-BSF/PLA composites [20].

Conclusions and Final Considerations This study concludes that hybridization of BP treated BSFs and its subsequent reinforcement with PLA matrix reveals sensible mechanical properties. The alkali treated fibre followed by BP treatment gave higher values tensile,

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flexural and impact strength for T-BSF/PLA composites than UT-BSF/PLA composites and virgin PLA matrix. The above chemical treatment enhanced fibre/matrix interaction by eliminating hemicelluloses and allied impurities, which further improved the impregnation of BSF within the PLA matrix. The results of the study discovered that BP treated BSF improves the compatibility between the PLA matrix and BSF through cross linking. Furthermore, extrusion and injection molding of BSF reinforced PLA composite has been found to be an ideal route to fabricate BSF/PLA composites exhibiting better mechanical properties. The SEM micrographs of UT-BSF/PLA and T-BSF/PLA composites clearly indicate that the nature of bonding established between fiber and matrix was physico - chemical. Thus, we tend to conclude that the systematic and protracted analysis of this present study may pave an avenue to an increased scope and future for BSF reinforced PLA composite for intended end use of applications.

Acknowledgement The authors thank Dr. Ravichandran, Professor and Head, Department of Rubber and Plastics Technology, MIT, Anna University, at Chennai in Tamilnadu, India for providing processing facilities to fabricate the bio composites of the present study.

Appendices BSF: Banana /Sisal fibre PLA: Polylactic acid BP: Benzoyl peroxide UTM: Universal testing machine SEM: Scanning electron microscopy GPa: Giga Pascal MPa: Mega Pascal UT-BSF/PLA: Untreated banana and sisal fibre reinforced poly-lactide composites T-BSF/PLA: Treated banana and sisal fibre reinforced poly-lactide composites kJ/m2: kilo joules per square meter BPT: Benzoyl peroxide treated


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