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epoxy composite
Materials and Design 47 (2013) 151–159
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Technical Report
Fiber surface treatment and its effect on mechanical and visco-elastic behaviour of banana/epoxy composite N. Venkateshwaran a,b,⇑, A. Elaya Perumal c, D. Arunsundaranayagam c a
Department of Mechanical Engineering, Anna University, India Faculty of Mechanical Engineering, Rajalakshmi Engineering College, Chennai, India c Department of Mechanical Engineering, Anna University, Chennai, India1 b
a r t i c l e
i n f o
Article history: Received 14 August 2012 Accepted 1 December 2012 Available online 23 December 2012
a b s t r a c t Natural fibers offer many advantages over synthetic fibers but the notable disadvantage of natural fibers is its hydrophilic nature. Due to this nature an incompatibility between the fiber and matrix exist which decreases the properties of the composite. This defect can be overcome by chemical modification of fiber surface so as to make it less hydrophilic. In this work, alkali (NaOH) of various concentrations (0.5%, 1%, 2%, 5%, 10%, 15% and 20%) was used to treat the fiber surface and the effect of these concentrations on the mechanical and visco-elastic behaviour of the composites were carried out. From the experimental investigation, it is found that 1% NaOH treated fiber reinforced composites behaves superiorly than other treated and untreated fiber composite. Further, SEM image analysis also shows the effect of alkali concentration over the fiber surfaces which leads to improving the mechanical properties of the composite. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Natural fibers offer many distinct advantages over its synthetic counterpart and due to these reasons, the composites reinforced with natural fiber has attracted many researchers since it is evolving as an alternative source for synthetic fibers at low cost. Since, natural fibers are derived from lignocelluloses, they are hydrophilic in nature and hence they become weak at the interface between fiber and matrix. Due to poor adhesion, mechanical properties of the natural fiber composite are low and hence it becomes important to improve the adhesion either by physical or chemical methods. The chemical treatment of fiber will activate the hydroxyl group present in the cellulose and lignin or can introduce new moieties that can effectively interlock with the polymer matrices [1]. The effect of alkali on natural fiber is a swelling reaction, due to it; the natural crystalline structure of the cellulose relaxes and converted to form Na-cellulose and cellulose-II. The structure of cellulose-II is more stable than natural cellulose [2]. Since, alkali influence the degree of swelling, the type of alkali and concentration influence the degree of lattice transformation [3]. Further, alkali also influences all the constituents of natural fibers too. Due to the presence of hydroxyl group, natural fibers readily undergo surface modification by chemical treatment. Due to chemical treatment, wetting,
⇑ Corresponding author at: Department of Mechanical Engineering, Anna University, India. Tel.: +91 44 27156750 1031, mobile: +91 9444325794. E-mail addresses: [email protected], [email protected]. in (N. Venkateshwaran). 1 http://www.annauniv.edu. 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.001
adhesion and porosity of fibers were improved which in turn helps to improve the properties of the composite. Das and Chakraborty [4] analysed the effect of alkali concentration on the degree of crystallinity and crystallinity index of the bamboo fibers and observed that by increasing the alkali concentration the crystallinity index also increases up to 15% concentration. Further increasing the alkali concentration causes the reduction in the physical properties of the composite. Suitability of bamboo fibers in polyester matrix was analysed by Wong et al. [5]. Bamboo fibers are treated with 1, 3 and 5 wt.% of NaOH and the effect of chemical treatment on the properties were analysed. The report shows that increasing the alkali percentage reduces the strain at failure and ductility nature of the bamboo fibers. Cao et al. [6] investigated the effect of alkali (NaOH) on the mechanical properties of bagasse fiber-reinforced polyester composites. Of the various concentration of NaOH used, superior properties were obtained for the composites made from 1% NaOH treated bagasse fibers. Vilay et al. [7] compared the effect of NaOH and Acetic Acid on the mechanical and dynamic mechanical properties of the bagasse–polyester composite. The results indicate that the chemical treatment has improved the storage modulus and water resistant capability of the composite. Surface modifications of the coir fibers by alkali treatment, bleaching, and vinyl grafting was carried out by Rout et al. [8]. The effect of alkali immersion time on the mechanical properties of jute fiber composite was analysed by Ray et al. [9]. The results show that increasing the immersion duration up to 8 h increases the crystallinity of the fiber and hence increases the modulus of
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the composite. Further, authors also observed two types of failure mode, between 0 and 4 h of treatment, fiber pull out was predominant whereas between 6 and 8 h, transverse fracture occurs with a minimum fiber pull out. The effect of alkali treatment on the defect concentration on the fatigue endurance of jute fiber composite under normal and liquid nitrogen condition was analysed by Sarkar and Ray [10]. The result indicates that at low temperature, the absorbed moisture was converted into ice and acts as a source of obstacle and thereby increases the damage in the composite. The effect of alkali treatment on jute strand was carried out by Vilaseca et al. [11]. The alkali treatment has improved the mechanical strength of the jute strand/starch composite. Gassan and Bledski [12] analysed the shrinkage behaviour of fibers during alkali treatment and its effect on mechanical properties of the jute–epoxy composite. The authors found that jute exhibit highest properties when shrinkage was zero. Further, they also observed that the formation of rough surface due to alkali treatment will not improve the mechanical properties of the composite. Rong et al. [13] carried out the effect of various chemical treatments (alkalization, acetylation, cyanoethylation, silane coupling agent, and heating) on the fiber surface and internal geometry of the sisal fiber reinforced epoxy composite. The analysis shows that the adhesion between fiber and matrix and that between the cells plays an important role in determining the mechanical properties of the composite. Towo and Ansell [14] established the static properties of sisal–polyester composite under tension–tension mode of fatigue testing. Weyenberg et al. [15] analysed the effect of alkali concentration and immersion time on the mechanical properties of the flax–epoxy composite. The result showed the optimum parameters as 4% alkali content and 45 s as immersion time. At these parameters, it is found that the tensile strength was increased by 30%. Aziz and Ansell [16] studied the effect of alkali treatment on the mechanical and thermal behaviour of the hemp–polyester composite. The effect of alkali treatment was carried out using SEM images. The SEM images show that the alkali treatment removes the impurities over the fiber surface. For, hemp fiber, the optimum alkali parameters were found out as 10% and 40 min of immersion by Pickering et al. [17]. Liu et al. [18] analysed the effect of alkali treatment on the structure, morphology and thermal properties of the grass fibers based composite. The analysis shows that the fibers extracted from the grass can be used as a reinforcement material. Surface modification by 1% NaOH and 1% Acrylic Acid of woven banana fiber polyester composite was carried out Jannah et al. [19]. The results showed that acrylic treatment reduces the water absorption rate that untreated and alkali treated composite. Li et al. [20] made a comprehensive study on various types of chemical treatments applied for the surface modification of the natural fibers. It is found that each treatment has its own advantages and disadvantage. Of the various chemical methods, alkali found to be most economical and easier way of achieving the improved properties by enhancing the better compatibility between fiber and matrix. Shalwan and Yousif [22] in their review analysed that in order to have better properties, interfacial adhesion between fiber and matrix should be high. Alsaeed et al. [23] found that higher concentration of chemical treatment might decrease the strength of natural fiber. Singha and Raj [24] investigated the effect of Malec Anhydride treatment on natural fiber reinforced composite. It shows that composites reinforced with MMA grafted fibers exhibited better mechanical strength as compared to untreated fiber. From the above literature, it is found that alkali (NaOH) is an effective and cheaper method of chemical treatment employed to modify the fiber surfaces. In this work, banana fibers (15 mm length [25]) are treated with alkali of various weight percentages (0.5, 1, 2, 5, 10, 15 and 20) and their effect on tensile, flexural,
impact, water absorption and visco-elastic behaviour of the banana–epoxy composite was carried out. 2. Materials and methods Banana Fiber and Epoxy (LY556) was purchased from the local dealer are used to prepare the composite plate in random orientation and plain weave pattern. The weight ratio of mixing epoxy and hardener is 10:1. The fibers were immersed in the alkali solution for 30 min and then washed with tap water. The washed fibers are wiped with cloth and then placed in oven at 50° C for 45 min for complete removal of moisture. The composites are prepared by reinforcing the treated and untreated fibers using Hand layup technique. The testing was carried out as per ASTMD 3039, ASTMD 790 and ASTM D 256. In each case, five specimens were tested to obtain the average value. Dynamic mechanical analysis was carried out using 0.1db Metravib equipment in the temperature range of 30–140 °C at a heating rate of 10 °C/min and at frequencies of 0.1, 1 and 10 Hz under tensile mode. Morphological analysis of the untreated and treated specimens was carried out by using scanning electron microscope with an accelerating voltage of 20 kV. 3. Results and discussion 3.1. Mechanical properties The most important factor to obtain a good fiber reinforced composite is the adhesion between the matrix and the fiber. Due to the presence of a hydroxyl group in natural fibers, the affinity to moisture absorption is high which leads to poor wetability and weak interfacial bonding between the fibers and the matrices. Therefore, in order to develop composites with better mechanical properties, it is necessary to impart the hydrophobicity to the fibers by suitable chemical treatments. The effect of alkali on the cellulose fiber is a swelling reaction, during which the natural crystalline structure of the cellulose relaxes and hence, there is a lattice transformation. Fig. 1 shows the occurrence of lattice transformation from cellulose-I to cellulose-II structure. Sodium hydroxide can cause a complete lattice transformation from cellulose-I to cellulose-II, in contrast to other alkalis (KOH and LiOH) that produce only partial lattice transformations [15]. The effects of various percentages of alkali treatment on the mechanical properties were carried out as per ASTM standards. The results of tensile properties are shown in Fig. 2. From Fig. 2, it is found that the tensile strength and modulus of 1% NaOH treated composite are more than those of 5%, 10% and 20% NaOH treatment. For 1% NaOH treatment, the tensile strength and modulus are 33.6 MPa and 1.68 GPa respectively which is 52% more than
Fig. 1. Lattice structures of cellulose I and cellulose II.
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Fig. 6. Comparison of the flexural properties of the untreated and treated woven composites.
Fig. 2. Comparison of the tensile properties of the untreated and treated composites.
Fig. 7. Comparison of the impact strength of the untreated and treated woven composites.
Fig. 3. Comparison of the flexural properties of the untreated and treated composites.
Fig. 8. Moisture absorption plot of untreated and treated composite.
Fig. 4. Comparison of the impact strength of the untreated and treated composite.
Fig. 5. Comparison of the tensile properties of the untreated and treated woven composites.
that of the untreated fiber composite. This improved result is due to the conversion of cellulose-I to cellulose-II structure. Further, increasing the NaOH percentage causes the tensile properties to
decline, due to the damage of the surface of the fiber caused by the alkali. Hence, 1% NaOH is the optimum alkali concentration that provides better interface adhesion between the fiber and the matrix. The bending properties of the untreated and alkali treated composites are predicted under the three point bending mode in the universal testing machine. The results of the flexural tests are shown in Fig. 3. Fig. 3 also shows that the flexural properties of the treated composite are more than those of the untreated fiber composite. It also shows that of the various percentages of alkali treatment, 1% NaOH has better flexural properties than the other treatment percentages. For 1% NaOH treatment, the flexural strength and modulus are 69.03 MPa and 13.22 GPa respectively, which is 16.65% more than that of the untreated composite. Further increase in the alkali concentration results in decreasing the properties of the composite. This decreasing trend is due to the damage caused by the alkali over the fiber surface. Impact strength is the capability of the material to withstand the suddenly applied loads in terms of energy. It measures the impact energy required to fracture a sample. In order for a material or object to have higher impact strength the stresses must be distributed evenly throughout the object. Fig. 4 shows the impact
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N. Venkateshwaran et al. / Materials and Design 47 (2013) 151–159 Table 1 Comparison of the dry and wet tensile properties of the randomly oriented composite. % Of NaOH
Dry specimen
Untreated 0.5 1 2 5 10 20
Wet specimen
Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strength (MPa)
Tensile modulus (GPa)
14.51 23.61 33.60 31.85 30.33 28.21 24.32
0.725 0.851 1.68 1.61 1.616 1.41 1.216
12.11 19.95 29.33 27.36 26.32 21.09 16.68
0.605 0.72 1.466 1.39 1.316 1.054 0.834
Percentage reduction in tensile properties
16.54 15.5 12.7 14.1 18.6 25 31
Table 2 Comparison of the dry and wet flexural properties of the randomly oriented composites. % Of NaOH
Dry specimen
Untreated 0.5 1 2 5 10 20
Wet specimen
% Reduction
Flexural strength (MPa)
Flexural modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
57.53 62.12 69.03 60.57 55.31 39.66 35.78
11.81 12.31 13.22 12.14 11.54 9.05 7.84
47.17 51.37 58.67 49.6 43.7 30.14 26.04
9.68 10.2 11.23 9.94 9.11 6.87 5.7
Table 3 Comparison of the dry and wet impact strength of the randomly oriented composites. % Of NaOH
Dry specimen Impact strength (J/m)
Wet specimen Impact strength (J/m)
% Reduction
Untreated 0.5 1 2 5 10 20
2.23 10.5 12.25 11.85 10.74 8.27 7.32
1.85 8.8 10.51 9.79 9.56 6.34 5.16
17.04 16.2 14.2 17.3 19.32 23.33 29.50
Table 4 Comparison of the dry and wet tensile properties of the plain woven composite. % Of NaOH
Dry specimen Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strength (MPa)
Wet specimen Tensile modulus (GPa)
Untreated 1
20 42.15
0.95 2.04
16.3 37.13
0.77 1.8
% Reduction
18.5 11.91
Table 5 Comparison of the flexural properties of the plain woven composite before and after water immersion. % Of NaOH
Dry specimen
Wet specimen
Flexural strength (MPa)
Flexural modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
Untreated 1
59.5 78.96
11.93 14.75
48.67 69.17
9.75 12.92
% Reduction
18 17.3 15.07 18.1 20.9 24 27.22
Table 6 Comparison of the Impact Strength of the Plain Woven Composite before and after water immersion. % Of NaOH
Dry specimen Impact strength (J/m)
Wet specimen Impact strength (J/m)
% Reduction
Untreated 1
8.7 13.25
7 11.63
19.54 12.22
1% NaOH provides better impact strength than the other percentages. The impact strength of the 1% NaOH treated composite is 12.25 J/m, which is 81.8% more than that of the untreated fiber composite. From Figs. 2–4, it is found that 1% NaOH concentration is the optimum alkali treatment percentage for banana fiber, to obtain the maximum mechanical properties. Hence, the plain woven composite was treated with 1% NaOH concentration for 30 min and then dried in sunlight for 3 days. The tensile, flexural and impact test results are shown in Figs. 5–7. Fig. 5 shows that the tensile strength and modulus of the treated composite are 42.15 MPa and 2.04 GPa respectively, which is 52.55% more than that of the untreated composite. Fig. 6 shows the comparison of the flexural properties of untreated and treated plain woven composites. From Fig. 6 it is found, that the flexural strength and modulus of 1% NaOH treated composite are 78.96 MPa and 14.75 GPa respectively, which is 24.73% more than that of the untreated composite. Similarly, the impact strength of the treated composite obtained from Fig. 7 is 13.25 J/m, which is 34.33% more than that of the untreated woven composite. 3.2. Moisture absorption studies
18.2 12.39
strength of the untreated and treated composites. It shows that the impact strength of the treated composite is higher than that of untreated composite. Of the various concentrations of alkali used,
Moisture absorption is one of the most undesirable factors in natural fibers because it reduces the interface adhesion between the fiber and matrix. The effects of alkali treatment on the water absorption properties are investigated. Fig. 8 shows the plot of moisture absorption percentage vs. duration (Days) of the untreated and treated composites. From Fig. 8, it is found that the moisture absorption percentage of the untreated fiber composite
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Fig. 9. Storage modulus vs. temperature curves of the alkali treated and untreated composites at 0.1 Hz frequency.
Fig. 10. Storage modulus vs. temperature curves of the alkali treated and untreated composites at 1 Hz frequency.
Fig. 11. Storage modulus vs. temperature curves of the alkali treated and untreated composites at 10 Hz frequency.
is 7.9% whereas for 1% NaOH treated composite it is 6.72% which is 15% lesser than that of untreated composite. The figure also shows that after treatment (except 1%), the moisture absorption rate of the composite increases because the increase in alkali concentration damages the fiber and thereby exposing more surface area for absorbing moisture. Further, Fig. 8 also shows that irrespective of the alkali concentration, the saturation point is reached after 12 days of immersion. After obtaining the equilibrium time from the Fig. 8, specimens of tensile, flexural and impact tests are immersed in water until saturation time and the same immersed specimens are tested within an hour’s duration to assess the effect of moisture on the mechanical properties. After reaching the equilibrium time, the samples are taken out and wiped before testing. The comparative results of the tensile properties before and after water immersion
are tabulated in Table 1. Table 1 shows that the presence of moisture has a tremendous effect on the tensile properties of the randomly oriented banana/epoxy composite. Of the various treatment percentages, 1% NaOH has the least decrement in the tensile properties of the composite. As mentioned earlier, the moisture absorption rates for various alkalis treated composites are more; the same affects the tensile properties of the composite here too. The percentage decrease in the tensile properties for 1% NaOH treated composite is 12.7%, which is 23.2% lesser than that of the untreated composite. The flexural properties obtained after water immersions are shown in Table 2 along with dry flexural properties. Here also, a similar kind of trend prevails as in the tensile properties study. Of the various alkali treated composites, 1% NaOH has the least decrease in the flexural strength and modulus, which are 58.75 MPa
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The measurements were carried out in the tensile mode of the equipment, and the corresponding visco-elastic properties were determined as a function of temperature and frequency. Experiments are carried out in the temperature range of 30–140 °C at 0.1, 1 and 10 Hz frequencies. The results are plotted, as Storage modulus (E0 ) vs. temperature and Tan d vs. temperature.
Fig. 12. Tan d vs. temperature curves of the alkali treated and untreated composites at 0.1 Hz frequency.
and 11.23 GPa respectively. The flexural properties of 1% NaOH treated composites are 19.6% lesser than those of the untreated composite. Table 3 compares the effect of water immersion on the impact strength of the randomly oriented composite. This Table 3 also shows that 1% NaOH treated composite has the least decrement in the impact strength of the composite, of all the treated and untreated composites compared. For 1% NaOH treated composite the percentage decrease in the impact strength is 14.2, which is 16.67% lesser than that of the untreated composite. The effect of moisture absorption on the mechanical properties of the plain-woven composite are studied and the results are tabulated in Tables 4–6. From the tables, it is found, that the tensile, flexural and impact properties of woven composites are affected by the presence of moisture. Further, it also shows that 1% NaOH treatment has decreased the mechanical properties of the composite. Alkali treatment has improved the tensile properties by 55%, flexural properties by 47% and impact properties by 59% when compared with the untreated plain woven composite. Hence 1% NaOH treatment has decreased the moisture absorption rate of the composite, which in turn, improves the mechanical properties of the woven composite after water immersion. The decrease in the mechanical properties is attributed to poor fiber–matrix adhesion because the presence of moisture causes swelling of the fibers, which could fill the gaps between the fiber and the polymer–matrix, and eventually lead to a decrease in the mechanical properties of the composites. 3.3. Visco-elastic behaviour The effect of alkali treatments on the visco-elastic behaviour of composites are studied using the dynamic mechanical analyser.
3.3.1. Storage modulus (E0 ) The variation of the storage modulus as a function of temperature at 0.1,1 and 10 Hz frequencies, are shown in Figs. 9–11 respectively. On comparing the storage modulus of the untreated, 1% NaOH and 5% NaOH treated composite, it is found that the alkali treatment increases the storage modulus of the composite. The 1% NaOH treatment has increases the storage modulus of the composite by about 27.4%, and 5% NaOH treatment has increased the storage modulus by about 14.3% when compared with the untreated composite at 30 °C. This increase in the storage modulus is due to the improved interfacial adhesion between the fibers and the matrix. As the temperature increases, the storage modulus of the composite, irrespective of the treatment, starts to decreases and attain almost the same value around 140 °C. Irrespective of whether the composites are made of treated fiber or not, the storage modulus decreases with an increase in the temperature. The reduction in the modulus with an increase in temperature is associated with softening of the matrix at higher temperature. Further, on changing the frequency, the storage modulus of the composite also changes the behavioural pattern. By changing the frequency from 0.1 Hz to 1 and 10 Hz, the storage modulus value decreases on increasing the frequency. This shows the effect of frequency on the storage modulus value of the composites. The modulus values were found to decrease drastically from 60 to 110 °C. It is observed that there was an increase in the storage modulus with an increase in the frequency, which was more prominent when the frequency was increased from 0.1 to 1 and 10 Hz; after that the modulus value remained unchanged. The storage modulus measured over a shorter time (high frequency) results in higher values, whereas measurements performed over a long time (low frequency) result in lower values. This is because the material undergoes molecular rearrangement in an attempt to minimize the localized stresses [21]. 3.3.2. Damping parameter (Tan d) Tan d is a damping term that can be related to the impact resistance of a material. Since the damping peak occurs in the region of the glass transition, where the material changes from a rigid to a more elastic state, it is associated with the movement of small groups and chains of molecules within the polymer structure, all of which are initially frozen. Figs. 12–14 show the plot between
Fig. 13. Tan d vs. temperature curves of the alkali treated and untreated composites at 1 Hz frequency.
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Fig. 14. Tan d vs. temperature curves of the alkali treated and untreated composites at 10 Hz frequency.
Fig. 17. SEM image of 5% NaOH treated banana fiber.
Fig. 15. SEM image of the untreated banana fiber.
Fig. 18. SEM image of 20% NaOH treated banana fiber.
Fig. 16. SEM image of 1% NaOH treated banana fiber.
the damping parameter and temperature. From these figures it is clear that the Tan d values were also affected by frequency. On increasing the frequency, the Tan d peak shifted to a higher temperature, i.e., from 90 °C at 0.1 Hz frequency to 100 °C at 10 Hz frequency. The damping peak is associated with the partial loosening of the polymer structure, which is indicative of the glass transition temperature.
Fig. 19. SEM image of the untreated fiber.
3.3.3. Fractography study In order to evaluate the fracture behaviour, interface adhesion, etc., of the composite, a fractography study becomes inevitable. Figs. 15–22 show the images of the fiber before and after treatment
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Fig. 20. SEM image of the 1% NaOH treated composite.
Fig. 22. Fractured tensile specimen of the 1% NaOH treated composite.
The 1% NaOH treatment (Fig. 20) revealed a compressed cellular structure. In other words, the treatment destroyed the cellular structure of the banana fiber, hence reduced the void content of the fiber. Figs. 21 and 22 show the SEM images of the tensile fractured specimens of the untreated and 1% NaOH treated composite. Fig. 21 shows the agglomeration of fibers occurs in the untreated composite due to poor wettability and interface adhesion. Due to these reasons the tensile properties of the untreated composite are lower than those of the treated composite. Fig. 22 shows the existence of a fiber bundle even after the composite fractures. The existence of the fiber shows the good interface adhesion between the fiber and matrix. This causes an increment in the tensile properties of the treated fiber composite. 4. Conclusions
Fig. 21. Fractured tensile specimen of the untreated composite.
with alkali. Fig. 15 shows the front view of the untreated fibers with the presence of impurities, responsible for poor fiber–matrix interface adhesion. Due to this, the compatibility between the fiber and matrix is reduced and hence lower the mechanical properties when compared with the treated fiber composite. Fig. 16 is the image of 1% NaOH treated fiber, shows the smooth surfaces of the fiber and the fibrillation phenomenon. According to Cao et al. [6], the phenomenon of the breaking down of the untreated fiber bundle down into smaller ones by dissolution of the hemi-cellulose is called fibrillation. The fibrillation is reported to increase the effective surface area available for contact with the matrix, and hence the interfacial was improved. Figs. 17 and 18 show the 5% and 20% NaOH treated fibers, with the fiber damage and peeling of the fiber surface, due to the increase in the alkali concentration. Due to these damages, interface adhesion and wettability become poor, which decreases the composite properties. Figs. 19 and 20 show the top view of the SEM micrographs of the untreated and 1% NaOH treated fibers. It reveals the cross section of untreated and 1% NaOH treated fibers. Fig. 19 exhibits the porous cellular structure of the fiber.
From the above study, it is found that the alkali treatment plays a significant role in improving the mechanical properties and decreasing the moisture absorption rate of both the randomly oriented and the plain woven composite. Of the various percentages of alkali treatments, it is found that 1% provides better mechanical properties. Increasing the alkali concentration results in fiber surface damage, which in turn decreases the mechanical properties of the composite. This is evident from the scanning electron microscope images. The 1% NaOH treatment results in nearly 50% increase in the properties of the composite when compared with the untreated fiber reinforced composite. Jannah et al. [19] also observed similar kind of results and Maya and Rajesh [2] too concluded the same. This work indicates that the alkali treatment is simple and efficient way of improving the mechanical properties and decreasing the moisture absorption rate of the natural fiber composite when compared with other chemical treatments. From DMA, it is found that the frequency has an immense effect on the storage modulus and damping parameter. On increasing the frequency, there is a shift and widening of values glass transition temperature of the composite, irrespective of whether it is treated or untreated. This is because the material undergoes molecular rearrangement in an attempt to minimize the localized stresses. References [1] Susheel Kalia, Kaith BS, Inderjeet Kaur. Pretreatments of natural fibers and their application as reinforcing material in polymer composites – a review. Polym Eng S 2009;49:1253–72. [2] Jacob Maya, Anadjiwala Rajesh. Recent developments in chemical modification and characterization of natural fiber-reinforced composites. Polym Compos 2008;29:187–207.
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Technical Report
Fiber surface treatment and its effect on mechanical and visco-elastic behaviour of banana/epoxy composite N. Venkateshwaran a,b,⇑, A. Elaya Perumal c, D. Arunsundaranayagam c a
Department of Mechanical Engineering, Anna University, India Faculty of Mechanical Engineering, Rajalakshmi Engineering College, Chennai, India c Department of Mechanical Engineering, Anna University, Chennai, India1 b
a r t i c l e
i n f o
Article history: Received 14 August 2012 Accepted 1 December 2012 Available online 23 December 2012
a b s t r a c t Natural fibers offer many advantages over synthetic fibers but the notable disadvantage of natural fibers is its hydrophilic nature. Due to this nature an incompatibility between the fiber and matrix exist which decreases the properties of the composite. This defect can be overcome by chemical modification of fiber surface so as to make it less hydrophilic. In this work, alkali (NaOH) of various concentrations (0.5%, 1%, 2%, 5%, 10%, 15% and 20%) was used to treat the fiber surface and the effect of these concentrations on the mechanical and visco-elastic behaviour of the composites were carried out. From the experimental investigation, it is found that 1% NaOH treated fiber reinforced composites behaves superiorly than other treated and untreated fiber composite. Further, SEM image analysis also shows the effect of alkali concentration over the fiber surfaces which leads to improving the mechanical properties of the composite. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Natural fibers offer many distinct advantages over its synthetic counterpart and due to these reasons, the composites reinforced with natural fiber has attracted many researchers since it is evolving as an alternative source for synthetic fibers at low cost. Since, natural fibers are derived from lignocelluloses, they are hydrophilic in nature and hence they become weak at the interface between fiber and matrix. Due to poor adhesion, mechanical properties of the natural fiber composite are low and hence it becomes important to improve the adhesion either by physical or chemical methods. The chemical treatment of fiber will activate the hydroxyl group present in the cellulose and lignin or can introduce new moieties that can effectively interlock with the polymer matrices [1]. The effect of alkali on natural fiber is a swelling reaction, due to it; the natural crystalline structure of the cellulose relaxes and converted to form Na-cellulose and cellulose-II. The structure of cellulose-II is more stable than natural cellulose [2]. Since, alkali influence the degree of swelling, the type of alkali and concentration influence the degree of lattice transformation [3]. Further, alkali also influences all the constituents of natural fibers too. Due to the presence of hydroxyl group, natural fibers readily undergo surface modification by chemical treatment. Due to chemical treatment, wetting,
⇑ Corresponding author at: Department of Mechanical Engineering, Anna University, India. Tel.: +91 44 27156750 1031, mobile: +91 9444325794. E-mail addresses: [email protected], [email protected]. in (N. Venkateshwaran). 1 http://www.annauniv.edu. 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.001
adhesion and porosity of fibers were improved which in turn helps to improve the properties of the composite. Das and Chakraborty [4] analysed the effect of alkali concentration on the degree of crystallinity and crystallinity index of the bamboo fibers and observed that by increasing the alkali concentration the crystallinity index also increases up to 15% concentration. Further increasing the alkali concentration causes the reduction in the physical properties of the composite. Suitability of bamboo fibers in polyester matrix was analysed by Wong et al. [5]. Bamboo fibers are treated with 1, 3 and 5 wt.% of NaOH and the effect of chemical treatment on the properties were analysed. The report shows that increasing the alkali percentage reduces the strain at failure and ductility nature of the bamboo fibers. Cao et al. [6] investigated the effect of alkali (NaOH) on the mechanical properties of bagasse fiber-reinforced polyester composites. Of the various concentration of NaOH used, superior properties were obtained for the composites made from 1% NaOH treated bagasse fibers. Vilay et al. [7] compared the effect of NaOH and Acetic Acid on the mechanical and dynamic mechanical properties of the bagasse–polyester composite. The results indicate that the chemical treatment has improved the storage modulus and water resistant capability of the composite. Surface modifications of the coir fibers by alkali treatment, bleaching, and vinyl grafting was carried out by Rout et al. [8]. The effect of alkali immersion time on the mechanical properties of jute fiber composite was analysed by Ray et al. [9]. The results show that increasing the immersion duration up to 8 h increases the crystallinity of the fiber and hence increases the modulus of
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the composite. Further, authors also observed two types of failure mode, between 0 and 4 h of treatment, fiber pull out was predominant whereas between 6 and 8 h, transverse fracture occurs with a minimum fiber pull out. The effect of alkali treatment on the defect concentration on the fatigue endurance of jute fiber composite under normal and liquid nitrogen condition was analysed by Sarkar and Ray [10]. The result indicates that at low temperature, the absorbed moisture was converted into ice and acts as a source of obstacle and thereby increases the damage in the composite. The effect of alkali treatment on jute strand was carried out by Vilaseca et al. [11]. The alkali treatment has improved the mechanical strength of the jute strand/starch composite. Gassan and Bledski [12] analysed the shrinkage behaviour of fibers during alkali treatment and its effect on mechanical properties of the jute–epoxy composite. The authors found that jute exhibit highest properties when shrinkage was zero. Further, they also observed that the formation of rough surface due to alkali treatment will not improve the mechanical properties of the composite. Rong et al. [13] carried out the effect of various chemical treatments (alkalization, acetylation, cyanoethylation, silane coupling agent, and heating) on the fiber surface and internal geometry of the sisal fiber reinforced epoxy composite. The analysis shows that the adhesion between fiber and matrix and that between the cells plays an important role in determining the mechanical properties of the composite. Towo and Ansell [14] established the static properties of sisal–polyester composite under tension–tension mode of fatigue testing. Weyenberg et al. [15] analysed the effect of alkali concentration and immersion time on the mechanical properties of the flax–epoxy composite. The result showed the optimum parameters as 4% alkali content and 45 s as immersion time. At these parameters, it is found that the tensile strength was increased by 30%. Aziz and Ansell [16] studied the effect of alkali treatment on the mechanical and thermal behaviour of the hemp–polyester composite. The effect of alkali treatment was carried out using SEM images. The SEM images show that the alkali treatment removes the impurities over the fiber surface. For, hemp fiber, the optimum alkali parameters were found out as 10% and 40 min of immersion by Pickering et al. [17]. Liu et al. [18] analysed the effect of alkali treatment on the structure, morphology and thermal properties of the grass fibers based composite. The analysis shows that the fibers extracted from the grass can be used as a reinforcement material. Surface modification by 1% NaOH and 1% Acrylic Acid of woven banana fiber polyester composite was carried out Jannah et al. [19]. The results showed that acrylic treatment reduces the water absorption rate that untreated and alkali treated composite. Li et al. [20] made a comprehensive study on various types of chemical treatments applied for the surface modification of the natural fibers. It is found that each treatment has its own advantages and disadvantage. Of the various chemical methods, alkali found to be most economical and easier way of achieving the improved properties by enhancing the better compatibility between fiber and matrix. Shalwan and Yousif [22] in their review analysed that in order to have better properties, interfacial adhesion between fiber and matrix should be high. Alsaeed et al. [23] found that higher concentration of chemical treatment might decrease the strength of natural fiber. Singha and Raj [24] investigated the effect of Malec Anhydride treatment on natural fiber reinforced composite. It shows that composites reinforced with MMA grafted fibers exhibited better mechanical strength as compared to untreated fiber. From the above literature, it is found that alkali (NaOH) is an effective and cheaper method of chemical treatment employed to modify the fiber surfaces. In this work, banana fibers (15 mm length [25]) are treated with alkali of various weight percentages (0.5, 1, 2, 5, 10, 15 and 20) and their effect on tensile, flexural,
impact, water absorption and visco-elastic behaviour of the banana–epoxy composite was carried out. 2. Materials and methods Banana Fiber and Epoxy (LY556) was purchased from the local dealer are used to prepare the composite plate in random orientation and plain weave pattern. The weight ratio of mixing epoxy and hardener is 10:1. The fibers were immersed in the alkali solution for 30 min and then washed with tap water. The washed fibers are wiped with cloth and then placed in oven at 50° C for 45 min for complete removal of moisture. The composites are prepared by reinforcing the treated and untreated fibers using Hand layup technique. The testing was carried out as per ASTMD 3039, ASTMD 790 and ASTM D 256. In each case, five specimens were tested to obtain the average value. Dynamic mechanical analysis was carried out using 0.1db Metravib equipment in the temperature range of 30–140 °C at a heating rate of 10 °C/min and at frequencies of 0.1, 1 and 10 Hz under tensile mode. Morphological analysis of the untreated and treated specimens was carried out by using scanning electron microscope with an accelerating voltage of 20 kV. 3. Results and discussion 3.1. Mechanical properties The most important factor to obtain a good fiber reinforced composite is the adhesion between the matrix and the fiber. Due to the presence of a hydroxyl group in natural fibers, the affinity to moisture absorption is high which leads to poor wetability and weak interfacial bonding between the fibers and the matrices. Therefore, in order to develop composites with better mechanical properties, it is necessary to impart the hydrophobicity to the fibers by suitable chemical treatments. The effect of alkali on the cellulose fiber is a swelling reaction, during which the natural crystalline structure of the cellulose relaxes and hence, there is a lattice transformation. Fig. 1 shows the occurrence of lattice transformation from cellulose-I to cellulose-II structure. Sodium hydroxide can cause a complete lattice transformation from cellulose-I to cellulose-II, in contrast to other alkalis (KOH and LiOH) that produce only partial lattice transformations [15]. The effects of various percentages of alkali treatment on the mechanical properties were carried out as per ASTM standards. The results of tensile properties are shown in Fig. 2. From Fig. 2, it is found that the tensile strength and modulus of 1% NaOH treated composite are more than those of 5%, 10% and 20% NaOH treatment. For 1% NaOH treatment, the tensile strength and modulus are 33.6 MPa and 1.68 GPa respectively which is 52% more than
Fig. 1. Lattice structures of cellulose I and cellulose II.
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Fig. 6. Comparison of the flexural properties of the untreated and treated woven composites.
Fig. 2. Comparison of the tensile properties of the untreated and treated composites.
Fig. 7. Comparison of the impact strength of the untreated and treated woven composites.
Fig. 3. Comparison of the flexural properties of the untreated and treated composites.
Fig. 8. Moisture absorption plot of untreated and treated composite.
Fig. 4. Comparison of the impact strength of the untreated and treated composite.
Fig. 5. Comparison of the tensile properties of the untreated and treated woven composites.
that of the untreated fiber composite. This improved result is due to the conversion of cellulose-I to cellulose-II structure. Further, increasing the NaOH percentage causes the tensile properties to
decline, due to the damage of the surface of the fiber caused by the alkali. Hence, 1% NaOH is the optimum alkali concentration that provides better interface adhesion between the fiber and the matrix. The bending properties of the untreated and alkali treated composites are predicted under the three point bending mode in the universal testing machine. The results of the flexural tests are shown in Fig. 3. Fig. 3 also shows that the flexural properties of the treated composite are more than those of the untreated fiber composite. It also shows that of the various percentages of alkali treatment, 1% NaOH has better flexural properties than the other treatment percentages. For 1% NaOH treatment, the flexural strength and modulus are 69.03 MPa and 13.22 GPa respectively, which is 16.65% more than that of the untreated composite. Further increase in the alkali concentration results in decreasing the properties of the composite. This decreasing trend is due to the damage caused by the alkali over the fiber surface. Impact strength is the capability of the material to withstand the suddenly applied loads in terms of energy. It measures the impact energy required to fracture a sample. In order for a material or object to have higher impact strength the stresses must be distributed evenly throughout the object. Fig. 4 shows the impact
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Dry specimen
Untreated 0.5 1 2 5 10 20
Wet specimen
Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strength (MPa)
Tensile modulus (GPa)
14.51 23.61 33.60 31.85 30.33 28.21 24.32
0.725 0.851 1.68 1.61 1.616 1.41 1.216
12.11 19.95 29.33 27.36 26.32 21.09 16.68
0.605 0.72 1.466 1.39 1.316 1.054 0.834
Percentage reduction in tensile properties
16.54 15.5 12.7 14.1 18.6 25 31
Table 2 Comparison of the dry and wet flexural properties of the randomly oriented composites. % Of NaOH
Dry specimen
Untreated 0.5 1 2 5 10 20
Wet specimen
% Reduction
Flexural strength (MPa)
Flexural modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
57.53 62.12 69.03 60.57 55.31 39.66 35.78
11.81 12.31 13.22 12.14 11.54 9.05 7.84
47.17 51.37 58.67 49.6 43.7 30.14 26.04
9.68 10.2 11.23 9.94 9.11 6.87 5.7
Table 3 Comparison of the dry and wet impact strength of the randomly oriented composites. % Of NaOH
Dry specimen Impact strength (J/m)
Wet specimen Impact strength (J/m)
% Reduction
Untreated 0.5 1 2 5 10 20
2.23 10.5 12.25 11.85 10.74 8.27 7.32
1.85 8.8 10.51 9.79 9.56 6.34 5.16
17.04 16.2 14.2 17.3 19.32 23.33 29.50
Table 4 Comparison of the dry and wet tensile properties of the plain woven composite. % Of NaOH
Dry specimen Tensile strength (MPa)
Tensile modulus (GPa)
Tensile strength (MPa)
Wet specimen Tensile modulus (GPa)
Untreated 1
20 42.15
0.95 2.04
16.3 37.13
0.77 1.8
% Reduction
18.5 11.91
Table 5 Comparison of the flexural properties of the plain woven composite before and after water immersion. % Of NaOH
Dry specimen
Wet specimen
Flexural strength (MPa)
Flexural modulus (GPa)
Flexural strength (MPa)
Flexural modulus (GPa)
Untreated 1
59.5 78.96
11.93 14.75
48.67 69.17
9.75 12.92
% Reduction
18 17.3 15.07 18.1 20.9 24 27.22
Table 6 Comparison of the Impact Strength of the Plain Woven Composite before and after water immersion. % Of NaOH
Dry specimen Impact strength (J/m)
Wet specimen Impact strength (J/m)
% Reduction
Untreated 1
8.7 13.25
7 11.63
19.54 12.22
1% NaOH provides better impact strength than the other percentages. The impact strength of the 1% NaOH treated composite is 12.25 J/m, which is 81.8% more than that of the untreated fiber composite. From Figs. 2–4, it is found that 1% NaOH concentration is the optimum alkali treatment percentage for banana fiber, to obtain the maximum mechanical properties. Hence, the plain woven composite was treated with 1% NaOH concentration for 30 min and then dried in sunlight for 3 days. The tensile, flexural and impact test results are shown in Figs. 5–7. Fig. 5 shows that the tensile strength and modulus of the treated composite are 42.15 MPa and 2.04 GPa respectively, which is 52.55% more than that of the untreated composite. Fig. 6 shows the comparison of the flexural properties of untreated and treated plain woven composites. From Fig. 6 it is found, that the flexural strength and modulus of 1% NaOH treated composite are 78.96 MPa and 14.75 GPa respectively, which is 24.73% more than that of the untreated composite. Similarly, the impact strength of the treated composite obtained from Fig. 7 is 13.25 J/m, which is 34.33% more than that of the untreated woven composite. 3.2. Moisture absorption studies
18.2 12.39
strength of the untreated and treated composites. It shows that the impact strength of the treated composite is higher than that of untreated composite. Of the various concentrations of alkali used,
Moisture absorption is one of the most undesirable factors in natural fibers because it reduces the interface adhesion between the fiber and matrix. The effects of alkali treatment on the water absorption properties are investigated. Fig. 8 shows the plot of moisture absorption percentage vs. duration (Days) of the untreated and treated composites. From Fig. 8, it is found that the moisture absorption percentage of the untreated fiber composite
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Fig. 9. Storage modulus vs. temperature curves of the alkali treated and untreated composites at 0.1 Hz frequency.
Fig. 10. Storage modulus vs. temperature curves of the alkali treated and untreated composites at 1 Hz frequency.
Fig. 11. Storage modulus vs. temperature curves of the alkali treated and untreated composites at 10 Hz frequency.
is 7.9% whereas for 1% NaOH treated composite it is 6.72% which is 15% lesser than that of untreated composite. The figure also shows that after treatment (except 1%), the moisture absorption rate of the composite increases because the increase in alkali concentration damages the fiber and thereby exposing more surface area for absorbing moisture. Further, Fig. 8 also shows that irrespective of the alkali concentration, the saturation point is reached after 12 days of immersion. After obtaining the equilibrium time from the Fig. 8, specimens of tensile, flexural and impact tests are immersed in water until saturation time and the same immersed specimens are tested within an hour’s duration to assess the effect of moisture on the mechanical properties. After reaching the equilibrium time, the samples are taken out and wiped before testing. The comparative results of the tensile properties before and after water immersion
are tabulated in Table 1. Table 1 shows that the presence of moisture has a tremendous effect on the tensile properties of the randomly oriented banana/epoxy composite. Of the various treatment percentages, 1% NaOH has the least decrement in the tensile properties of the composite. As mentioned earlier, the moisture absorption rates for various alkalis treated composites are more; the same affects the tensile properties of the composite here too. The percentage decrease in the tensile properties for 1% NaOH treated composite is 12.7%, which is 23.2% lesser than that of the untreated composite. The flexural properties obtained after water immersions are shown in Table 2 along with dry flexural properties. Here also, a similar kind of trend prevails as in the tensile properties study. Of the various alkali treated composites, 1% NaOH has the least decrease in the flexural strength and modulus, which are 58.75 MPa
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The measurements were carried out in the tensile mode of the equipment, and the corresponding visco-elastic properties were determined as a function of temperature and frequency. Experiments are carried out in the temperature range of 30–140 °C at 0.1, 1 and 10 Hz frequencies. The results are plotted, as Storage modulus (E0 ) vs. temperature and Tan d vs. temperature.
Fig. 12. Tan d vs. temperature curves of the alkali treated and untreated composites at 0.1 Hz frequency.
and 11.23 GPa respectively. The flexural properties of 1% NaOH treated composites are 19.6% lesser than those of the untreated composite. Table 3 compares the effect of water immersion on the impact strength of the randomly oriented composite. This Table 3 also shows that 1% NaOH treated composite has the least decrement in the impact strength of the composite, of all the treated and untreated composites compared. For 1% NaOH treated composite the percentage decrease in the impact strength is 14.2, which is 16.67% lesser than that of the untreated composite. The effect of moisture absorption on the mechanical properties of the plain-woven composite are studied and the results are tabulated in Tables 4–6. From the tables, it is found, that the tensile, flexural and impact properties of woven composites are affected by the presence of moisture. Further, it also shows that 1% NaOH treatment has decreased the mechanical properties of the composite. Alkali treatment has improved the tensile properties by 55%, flexural properties by 47% and impact properties by 59% when compared with the untreated plain woven composite. Hence 1% NaOH treatment has decreased the moisture absorption rate of the composite, which in turn, improves the mechanical properties of the woven composite after water immersion. The decrease in the mechanical properties is attributed to poor fiber–matrix adhesion because the presence of moisture causes swelling of the fibers, which could fill the gaps between the fiber and the polymer–matrix, and eventually lead to a decrease in the mechanical properties of the composites. 3.3. Visco-elastic behaviour The effect of alkali treatments on the visco-elastic behaviour of composites are studied using the dynamic mechanical analyser.
3.3.1. Storage modulus (E0 ) The variation of the storage modulus as a function of temperature at 0.1,1 and 10 Hz frequencies, are shown in Figs. 9–11 respectively. On comparing the storage modulus of the untreated, 1% NaOH and 5% NaOH treated composite, it is found that the alkali treatment increases the storage modulus of the composite. The 1% NaOH treatment has increases the storage modulus of the composite by about 27.4%, and 5% NaOH treatment has increased the storage modulus by about 14.3% when compared with the untreated composite at 30 °C. This increase in the storage modulus is due to the improved interfacial adhesion between the fibers and the matrix. As the temperature increases, the storage modulus of the composite, irrespective of the treatment, starts to decreases and attain almost the same value around 140 °C. Irrespective of whether the composites are made of treated fiber or not, the storage modulus decreases with an increase in the temperature. The reduction in the modulus with an increase in temperature is associated with softening of the matrix at higher temperature. Further, on changing the frequency, the storage modulus of the composite also changes the behavioural pattern. By changing the frequency from 0.1 Hz to 1 and 10 Hz, the storage modulus value decreases on increasing the frequency. This shows the effect of frequency on the storage modulus value of the composites. The modulus values were found to decrease drastically from 60 to 110 °C. It is observed that there was an increase in the storage modulus with an increase in the frequency, which was more prominent when the frequency was increased from 0.1 to 1 and 10 Hz; after that the modulus value remained unchanged. The storage modulus measured over a shorter time (high frequency) results in higher values, whereas measurements performed over a long time (low frequency) result in lower values. This is because the material undergoes molecular rearrangement in an attempt to minimize the localized stresses [21]. 3.3.2. Damping parameter (Tan d) Tan d is a damping term that can be related to the impact resistance of a material. Since the damping peak occurs in the region of the glass transition, where the material changes from a rigid to a more elastic state, it is associated with the movement of small groups and chains of molecules within the polymer structure, all of which are initially frozen. Figs. 12–14 show the plot between
Fig. 13. Tan d vs. temperature curves of the alkali treated and untreated composites at 1 Hz frequency.
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Fig. 14. Tan d vs. temperature curves of the alkali treated and untreated composites at 10 Hz frequency.
Fig. 17. SEM image of 5% NaOH treated banana fiber.
Fig. 15. SEM image of the untreated banana fiber.
Fig. 18. SEM image of 20% NaOH treated banana fiber.
Fig. 16. SEM image of 1% NaOH treated banana fiber.
the damping parameter and temperature. From these figures it is clear that the Tan d values were also affected by frequency. On increasing the frequency, the Tan d peak shifted to a higher temperature, i.e., from 90 °C at 0.1 Hz frequency to 100 °C at 10 Hz frequency. The damping peak is associated with the partial loosening of the polymer structure, which is indicative of the glass transition temperature.
Fig. 19. SEM image of the untreated fiber.
3.3.3. Fractography study In order to evaluate the fracture behaviour, interface adhesion, etc., of the composite, a fractography study becomes inevitable. Figs. 15–22 show the images of the fiber before and after treatment
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Fig. 20. SEM image of the 1% NaOH treated composite.
Fig. 22. Fractured tensile specimen of the 1% NaOH treated composite.
The 1% NaOH treatment (Fig. 20) revealed a compressed cellular structure. In other words, the treatment destroyed the cellular structure of the banana fiber, hence reduced the void content of the fiber. Figs. 21 and 22 show the SEM images of the tensile fractured specimens of the untreated and 1% NaOH treated composite. Fig. 21 shows the agglomeration of fibers occurs in the untreated composite due to poor wettability and interface adhesion. Due to these reasons the tensile properties of the untreated composite are lower than those of the treated composite. Fig. 22 shows the existence of a fiber bundle even after the composite fractures. The existence of the fiber shows the good interface adhesion between the fiber and matrix. This causes an increment in the tensile properties of the treated fiber composite. 4. Conclusions
Fig. 21. Fractured tensile specimen of the untreated composite.
with alkali. Fig. 15 shows the front view of the untreated fibers with the presence of impurities, responsible for poor fiber–matrix interface adhesion. Due to this, the compatibility between the fiber and matrix is reduced and hence lower the mechanical properties when compared with the treated fiber composite. Fig. 16 is the image of 1% NaOH treated fiber, shows the smooth surfaces of the fiber and the fibrillation phenomenon. According to Cao et al. [6], the phenomenon of the breaking down of the untreated fiber bundle down into smaller ones by dissolution of the hemi-cellulose is called fibrillation. The fibrillation is reported to increase the effective surface area available for contact with the matrix, and hence the interfacial was improved. Figs. 17 and 18 show the 5% and 20% NaOH treated fibers, with the fiber damage and peeling of the fiber surface, due to the increase in the alkali concentration. Due to these damages, interface adhesion and wettability become poor, which decreases the composite properties. Figs. 19 and 20 show the top view of the SEM micrographs of the untreated and 1% NaOH treated fibers. It reveals the cross section of untreated and 1% NaOH treated fibers. Fig. 19 exhibits the porous cellular structure of the fiber.
From the above study, it is found that the alkali treatment plays a significant role in improving the mechanical properties and decreasing the moisture absorption rate of both the randomly oriented and the plain woven composite. Of the various percentages of alkali treatments, it is found that 1% provides better mechanical properties. Increasing the alkali concentration results in fiber surface damage, which in turn decreases the mechanical properties of the composite. This is evident from the scanning electron microscope images. The 1% NaOH treatment results in nearly 50% increase in the properties of the composite when compared with the untreated fiber reinforced composite. Jannah et al. [19] also observed similar kind of results and Maya and Rajesh [2] too concluded the same. This work indicates that the alkali treatment is simple and efficient way of improving the mechanical properties and decreasing the moisture absorption rate of the natural fiber composite when compared with other chemical treatments. From DMA, it is found that the frequency has an immense effect on the storage modulus and damping parameter. On increasing the frequency, there is a shift and widening of values glass transition temperature of the composite, irrespective of whether it is treated or untreated. This is because the material undergoes molecular rearrangement in an attempt to minimize the localized stresses. References [1] Susheel Kalia, Kaith BS, Inderjeet Kaur. Pretreatments of natural fibers and their application as reinforcing material in polymer composites – a review. Polym Eng S 2009;49:1253–72. [2] Jacob Maya, Anadjiwala Rajesh. Recent developments in chemical modification and characterization of natural fiber-reinforced composites. Polym Compos 2008;29:187–207.
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