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epoxy hybrid composites: Mechanical and ageing studies
Materials and Design 54 (2014) 644–651
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Technical Report
Development of coir pith/nylon fabric/epoxy hybrid composites: Mechanical and ageing studies R. Narendar a, K. Priya Dasan a,⇑, Muraleedharan Nair b a b
Material Chemistry Division, SAS, VIT University, Tamil Nadu 632014, India Common Facility Service Centre, Malappuram, Kerala 676124, India
a r t i c l e
i n f o
Article history: Received 8 July 2013 Accepted 22 August 2013 Available online 30 August 2013
a b s t r a c t The coir pith epoxy composites were hybridized with nylon fabric/epoxy resin by hand lay up technique followed by compression moulding. A set of composites of same composition having chemically treated coir pith was also prepared. Mechanical properties of composites such as tensile strength, flexural strength, impact strength and hardness were evaluated. Though coir pith acts as a good reinforcement in epoxy resin, the incorporation of nylon fabric and the chemical treatment of coir pith were found to enhance the properties of the composites further. Chemical resistance and flame resistance of composite systems were also found to be improved with hybrid composites. Since water uptake and retentions property of coir pith is a major drawback when it comes to its application in composites, the ageing of composite panels in moist environment was investigated. The results suggested that the presence of nylon fabric and chemically treated pith can contribute to longer durability of the panels in moist conditions. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The use of lignocellulosic materials as reinforcing fillers in polymeric products has increased recently. These natural fillers are ecofriendly, have low density, low-cost, non-abrasive in nature and are biodegradable. Sisal [1], banana fiber [2], cotton [3], flax [4], hemp [5], jute [6] and ramie [7] have been well recognized as reinforcements for natural filler composites. Most of these fillers are categorized under agro wastes and their disposal is a huge responsibility for the government. They pose severe environmental pollution problems and occupy fertile useful land. Therefore developing engineering end use such as building materials and structural parts out of these materials has become a requirement. Application of agro-wastes as particle boards, thermal insulators, building material composites/bricks, cementitious/binder and aggregates [8– 12] have been well studied. Coir pith is one of the major agro wastes found in the southern coastal regions of India. Coir pith is generated in the separation process of the fiber from the coconut husk and is generally dumped as an agro waste. Because of its low degradation in the environment, the hillocks of coir pith collected or dumped pause serious health hazards and loss of fertile lands. Because of the high lignin content left it takes decades to decompose; it only begins to break down when it is 10 years old. The tannins and phenols from coir pith leach out into the soil and water bodies causing pollution. It ⇑ Corresponding author. Tel.: +91 416 2202696; fax: +91 416 2243092. E-mail address: [email protected] (K. Priya Dasan). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.08.080
is estimated that at present there is an accumulated stock of 10 106 metric tons of coir pith in the southern states of India and about 7.5 105 tons of coir pith is produced annually in India [13]. Developing alternate ways to dispose of coir pith is of critical importance. Cost effective technologies that address the development of value added products from coir pith therefore become relevant for countries producing coir pith. The application of coir pith as reinforcement in polymer has not been reported so far. One factor that has prevented a more extended utilization of the agro-wastes in composite industry is the lack of compatibility of these fillers in most polymeric matrices. The hydrophilic nature of natural fillers adversely affects adhesion to hydrophobic matrix and as a result, causes poor mechanical properties. One of the most commonly adopted methods to overcome this issue is the chemical treatment of natural fillers. The effect of chemical treatments on the mechanical and other properties of composites are well documented. Gassan and Bledzki [14] studied the mechanical properties of jute/epoxy composites. Composite strength and stiffness increased as a consequence of the improved mechanical properties of the fibers by alkali treatment. Kenaf/epoxy composites were prepared after subjecting the fiber to mercerization. The failure mechanism and damage features of the materials revealed that reinforcement of epoxy with treated kenaf fibers increased the flexural strength of the composite compare to untreated fibers Yousif et al. [15]. Bachtiar et al. [16] reported the mechanical properties of sugar palm fiber treated with sodium hydroxide. Tensile modulus of composites was much higher than untreated fiber composite specimens, which proved the effectiveness of the
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treatment. Min Zhi Rong et al. [17] studied the sisal fiber reinforced epoxy composites. Results showed that alkali treatment removed cementing materials which were partially replaced by epoxy resin. This greatly improved fiber bundle/matrix bonding and also enhanced mechanical strength of composite. Hybrid composites are made by the incorporation of several different types of reinforcements into a single matrix. They imply a step beyond in the search for novel materials with improved mechanical properties and/or reduced cost. Hybrid polymer composites have been studied many researchers in the past [18–22]. The use of synthetic reinforcements in combination with natural fillers has been shown to exhibit excellent performance and at the same time reduce the environmental impact. The hybrid effect of glass fiber and oil palm empty fruit bunch (OPEFB) fiber on the mechanical properties of phenol–formaldehyde composites was studied by Sreekala et al. [23]. The introduction of small amount of glass fiber improved the impact strength of the composites. Meanwhile density of the hybrid composite decreased as the volume fraction of the OPEFB fiber increased. Mechanical properties of jute/glass fiber reinforced epoxy composites were studied by Koradiya et al. [24]. Experimental results showed that hybrid composites have good mechanical properties than those of jute and glass composites. Jarukumjorn and Suppakarn [25] investigated the effect of glass fiber hybridization on the physico-mechanical properties of sisal–polypropylene composites. Incorporation of glass fiber into sisal/polypropylene composites enhanced tensile, flexural and impact strength. In the present work coir pith/epoxy composites were hybridized with nylon fabric impregnated with epoxy resin. The composites were prepared by hand lay up technique and than compression moulded. A set of composites with same composition having chemically treated coir pith was also fabricated. Mechanical properties, chemical resistance and flame resistance of composites were investigated in detail. Research investigations showed that the exposure of natural filler composites in a wet environment leads to a decrease of the mechanical properties when water spreads in the material [26–30]. Since coir pith has a higher tendency to absorb and retain water, it becomes essential to know how the composites made of coir pith behave in a wet environment. Hence the composite panels were subjected to wet environment and their mechanical properties were evaluated. 2. Materials and methods 2.1. Materials Coir pith was collected from a local coir processing unit (Gudiyathum, Tamilnadu, India). Sodium hydroxide was purchased from Sigma–Aldrich, epoxy resin (LY556) and hardener (HY951) were purchased from Huntsman. Maaxil 402 mould release spray was purchased from Maax lubrication. 2.2. Methods 2.2.1. Chemical treatment Coir pith was soaked in 5% concentration of NaOH solution for 1 h at room temperature followed by washing with distilled water. Afterwards, the samples were oven dried at 70 °C for 2 h. 2.2.2. Composites preparation A square steel plate mould with dimension of 450 450 mm assembled with top plate and base plate was used for the fabrication of composites. To help the complete removal of composites from the mould and to avoid sticking, a polyethylene sheets sprayed with mould release agent is layered on the top and base
plate. Nylon fabric with a uniform coating of epoxy and hardener was spread on the polyethylene sheet. The pith/resin mixture (mixed in an internal mixer) is spread on this followed by another layer of resin coated nylon fabric. The samples were compression molded and cured over-night as shown (Fig. 1a and b). The resin hardener ratio is maintained at 10:1 in all formulations. The composites thus fabricated are denoted as given in Table 1. 3. Characterization 3.1. Fourier transform infrared (FTIR) spectroscopy FT-IR spectroscopy was used to investigate the surface modification in treated and untreated coir pith. FT-IR analysis was carried out in the range of 4000–400 cm1 with a resolution of 2 cm1 using a JASCO 400 Infrared spectrometer. 3.2. Morphology analysis Scanning electron microscope (SEM) of Carl Ziess, EVO make was used to analyze the morphology of coir pith and impact failure surface of epoxy composites. Optical microscopic image of treated and untreated coir pith were obtained using Brucker Carl Zeiss optical microscopy. 3.3. Mechanical properties Tensile test was carried out according to ASTM: D 638-10 using a universal testing machine of AG-IS Shimadzu, TMI make. Flexural tests were performed according to ASTM: D 790-10 using Instron UTM, USA. Impact Izod test was done according to ASTM: D 25610 using Tinius Olsen Model impact analyzer. Hardness was measured by using Shore A hardness tester (Durometer-Mitutoyo Shore A meter). 3.4. Chemical resistance test The chemical resistance properties of the epoxy resin/coir pith/ nylon composites in CCl4, water, NaOH and HNO3 were studied according to ASTM: D543-06. In each case, five pre-weighed samples were dipped in the respective chemical reagents for 24 h. They were then removed and immediately washed in distilled water and dried by pressing them on both sides with a tissue paper at room temperature. The samples were then weighed and the percentage weight loss/gain was determined using the following equation.
Weight loss=gainð%Þ ¼
Final weight Original weight Orginal weight
ð1Þ
3.5. Ageing studies The ageing of composites on exposure to water was evaluated by keeping the samples immersed in water. Five specimens of each sample were kept immersed in distilled water at 30 °C for 31 days. The samples were taken out, dried at room temperature and the impact strength was measured as mentioned above. 3.6. Flammability test Flammability of polymer composites were evaluated as mentioned in a previous report [31]. The tests closely simulate the Federal Aviation Regulation, FAR 25.853 60 s vertical burn test specification [46]. 290 mm 70 mm sized samples were suspended vertically using a clamp on a lab stand. An LPG Bunsen flame was applied to the leading edge of the bottom surface of
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Fig. 1. (a) Schematic representation of composite fabrication (b) composites panel.
Table 1 Material code and its abbreviation. Materials code
Materials abbreviations
BLANK EN2L EN3L ERCP ENA ERCPN2L ENAN2L
Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy layers Epoxy Epoxy layers
ERCPN3L ENAN3L
resin resin/nylon two layer resin/nylon three layer resin/raw coir pith resin/sodium hydroxide treated coir pith resin/raw coir pith/nylon two layer resin/sodium hydroxide treated coir pith/nylon two resin/raw coir pith/nylon three layers resin/sodium hydroxide treated coir pith/nylon three
composite. The time required to catch fire is taken as ignition time. The flame time from the first mark (25 mm from the ignition end) until the second mark (100 mm from the ignition end) was measured to determine the linear burning rate (V) of the sample. The linear burning rate (V), was calculated in millimeters per second using the equation
V ¼ 60L=t
ð2Þ
L is the burned length in millimeters and t is the time in seconds. 4. Results and discussion 4.1. Tensile strength The tensile strength of pure epoxy and different composite samples are given in Table 2. The composite samples exhibit better tensile strength than the pure epoxy resin. The incorporation of raw coir pith increased the tensile strength of epoxy resin by 19.34%. Meanwhile two and three layered nylon/epoxy composites showed 25.74% and 30.49% higher tensile strength than the raw coir pith/ epoxy composites. This indicates nylon fabric as better reinforcement for epoxy resin. Nylon fabric forms chemical bonding with epoxy resin and provides superior mechanical properties
(Scheme 1). The hybrid composites showed higher tensile values than the individual composites samples with the two layered hybrid composites showing almost similar tensile strength as that of three layered nylon/epoxy composites. The presence of nylon fabric enhances the tensile properties in hybrid composites also. Three layered composites samples showed 33.00% higher tensile strength than two layered composites samples. Henceforth it can be concluded that the hybrid composites system with coir pith/nylon fabric/epoxy composites exhibits maximum tensile strength and the property improves with number of nylon fabrics. In the present case nylon fabric act as skin and coir pith as the core material. The tensile strength will be higher, when the high strength material is used as the skin, which is the main load bearing component in the tensile measurements [32]. 4.1.1. Effect of chemical treatment The tensile strength of composite samples with treated and raw coir pith is also given in Table 2. Pure wettability of natural filler due to its hydrophilic nature is one of the major reasons for the mechanical failure of natural filler reinforced composites. This results in increased void content and structural flaws resulting in low stress transfer between the polymer and filler. To prevent this, the filler surface has to be modified in order to encourage adhesion. Chemical treatment results in surface modification and gives rise to more groups on the filler surface and thus facilitates efficient coupling with the matrix [16]. Many authors have indicated an increase in mechanical properties with chemical treatment [33,34]. In a recent work on coir fiber reinforced polypropylene, the authors observed an increase in mechanical properties and fiber wettability with chemical treatment [35]. For the present experiment, composite samples with chemically treated coir pith showed better tensile strength values compared to the samples with untreated coir pith. Compared to ERCP composite, tensile strength for ENA composites increase by 19.44%. When coir pith is subjected to alkaline environment, the waxy material gets removed from the pith surface and increases its surface roughness. And also it promotes the activation of hydroxyl groups of cellulose unit by breaking hydrogen bonds, which in turn makes it less hydrophilic [36]. The chemically treated coir pith shows a rougher surface
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R. Narendar et al. / Materials and Design 54 (2014) 644–651 Table 2 Mechanical strength of composites. Sample code
Tensile strength (MPa)
Flexural strength (MPa)
Impact strength (J/m)
Impact retention (%)
Hardness (shore A)
BLANK ERCP ENA EN2L EN3L ERCPN2L ENAN2L ERCPN3L ENAN3L
4.21 ± 0.2 5.22 ± 0.3 6.48 ± 0.2 7.03 ± 0.3 7.51 ± 0.2 7.57 ± 0.3 8.51 ± 0.2 11.3 ± 0.2 12.5 ± 0.2
27.32 ± 0.4 32.87 ± 0.3 38.95 ± 0.3 42.31 ± 0.4 48.90 ± 0.3 53.19 ± 0.4 68.56 ± 0.2 75.22 ± 0.3 106.52 ± 0.4
83.14 ± 0.5 101.35 ± 0.4 132.64 ± 0.5 167.38 ± 0.5 189.97 ± 0.4 214.29 ± 0.6 248.13 ± 0.5 311.52 ± 0.4 359.88 ± 0.5
95.1 ± 0.2 85.7 ± 0.2 96.7 ± 0.3 96.8 ± 0.4 97.0 ± 0.2 87.6 ± 0.3 91.3 ± 0.2 93.1 ± 0.4 97.8 ± 0.3
94.00 ± 0.3 95.60 ± 0.4 96.09 ± 0.4 94.50 ± 0.4 95.56 ± 0.3 96.62 ± 0.3 97.58 ± 0.2 98.61 ± 0.3 99.73 ± 0.4
Treated Coir pith
Coir pith CH2 Epoxy resin
Nylon fabric
C6H5-C(CH3) 2-C6H4-O-CH2 Scheme 1. Hydrogen bonding interaction between treated coir pith/nylon 6 fabric/epoxy resin.
morphology as is evident from the SEM photographs (Fig. 2a and b). The optical photographs support this further as shown in Fig. 2c and d. The rougher morphology of treated coir pith results in enhanced mechanical bonding by an interlocking method [16]. This results in better stress transfer among the components in composites systems. Factually, epoxy is able to fill up the apparent flaws in treated coir pith and results in better load sharing.
double layer of nylon fabric epoxy composites [20,21]. The hybrid composites showed maximum flexural properties. The three layered hybrid composites showed a flexural strength value of 75.22 MPa which is 34.99% higher than nylon/epoxy composites. Composite samples with chemically treated coir pith showed better flexural properties than the samples with raw coir pith. The enhanced interfacial interaction among the composites constituents is expected to be responsible for this.
4.2. Flexural strength 4.3. Impact strength Flexural strength of epoxy composites are shown in Table 2. The reinforcement increases the flexural strength of epoxy resin. The incorporation of coir pith increased the flexural strength of epoxy by 16.88%. Meanwhile the incorporation of nylon fabric enhanced the flexural strength of resin by 35.42%. The three layered nylon fabric had 7.94% higher flexural strength that two layered nylon epoxy composites. The presence of double and triple layer fabric plays an important role in sandwich composites. In three layers of nylon fabric composites, the upper layer is put into compression, the lower layer into tension and the core or middle layer of nylon act as shear. Upper and lower layer of nylon fabric are subjected to compression/tension and are largely responsible for the strength of the sandwich laminate. Though the usage of middle layer of nylon is to support the upper and lower layer of nylon, so that they drastically reduce the maximum stress and deformation of outer layer by increasing the moment of inertia of the sandwich beam. Based on the sandwich theory, it can be expected that triple layer of nylon fabric epoxy composites withstand high stress during bending and good resistance to propagation of cracks compare to
Impact properties of materials are directly related to the overall toughness of the material. Toughness means the ability of the material to absorb applied energy. Impact strength of composites is given in Table 2. The composites showed higher impact strength than pure epoxy resin. The impact strength of composite increased with increasing number of nylon fabric. It is also clear from the Table 2 that sodium hydroxide treated coir pith increases the reinforcement efficiency of the pith in epoxy matrix. SEM observation of impact fracture surface of ERCPN2L and ENAN2L composites is shown in Fig. 3a and b. The involvement of fillers in the failure is due to the separation of fillers and matrix and loss of stress transferring capability. Fig. 3a. indicates poor adhesion between the filler and matrix. This may be due to hydrophilic nature of coir pith resulting in fragile bond formation with hydrophobic epoxy resin. SEM photograph of fractured surface shows pith ejection at a number of places giving rise to holes. The presence of voids, debris and air entrapment visible in the SEM photographs indicates poor bonding between the pith and
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Fig. 2. (a) Scanning electron microscope photographs of raw coir pith (b) scanning electron microscope photographs of sodium hydroxide treated coir pith (c) optical microscopic images of raw coir pith and (d) optical microscopic images of sodium hydroxide treated coir pith.
Fig. 3. SEM images of (a) impact failure fracture surface of ERCPN2L composites and (b) impact failure fracture surface of ENAN2L composites.
matrix, ultimately effecting the composite properties. The fracture surface of ENAN2L composites appears smooth with lesser number of holes, when compared to ERCPN2L composite fracture surface. This is an evidence of good bond existing between coir pith and matrix due to chemical treatment. 4.4. Hardness properties Hardness of composite samples is given in Table 2. Compared to blank sample, the composite samples show higher hardness values.
The presence of nylon fabric slightly enhances the hardness of composites further. The hybrid composite shows the highest hardness values. An increase in hardness is observed with chemically treated coir pith. Raw coir pith is highly amorphous and fluffy material because of its high lignin content. Chemical treatment results in the formation of cellulose rich product which is more crystalline nature [37,38]. During chemical treatment, the lignin part of the coir pith gets removed resulting in cellulose rich product. The FTIR spectra of treated and untreated coir pith are shown in Fig. 4. The peak at 1732.16 cm1 in untreated coir pith
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is attributed to acetyl or uronic ester linkage of hemicelluloses and or ester linkage of carboxylic group of ferulic or p-coumeric acids of lignin or hemicelluloses [39]. The aromatic peak C@C stretch from aromatic ring of lignin gives two peaks at 1612 cm1 and 1456 cm1. The peak at 1240.11 cm1 in untreated coir pith is due to hemicelluloses; the intensity of peak decreased in treated coir pith indicating the removal of hemicelluloses content in treated coir pith. The peak at 1388.36 cm1 in raw coir pith corresponds to ACH symmetric stretching of aromatic lignin and this peak is shifted in treated coir pith, indicating the deformation of lignin in treated pith. NaOH treated coir pith shows a peak at 3444 cm1 corresponding to cellulose-OH. The increased intensity of this peak in treated samples indicates increased cellulose content with treatment. 4.5. Ageing studies – in water The composites samples were subjected to ageing to ascertain their utility during applications. The percentage retention of impact strength of composites after exposing to water is given in Table 2. The composites samples with coir pith showed a slight decrease in impact strength on exposure to water. This result indicates that the presence of coir pith do not favor application of composites samples in moist environment. The hygroscopic nature of coir pith results in high water uptake by the composites samples. Water filled voids at the interface results in interfacial de-bonding. This causes cracks and micro-voids in the surface of composites [40]. The water filled voids at the interface results in interfacial de-bonding. Once water penetrates inside composites materials, pith start swelling and matrix tend to chain reorientation resulting in poor mechanical properties. However, the composites with treated pith showed improved water resistance and retention of mechanical properties even on being exposed to water environment. The chemical treatment of coir pith effectively improved pith–matrix adhesion. The alkaline sensitive hydroxyl groups present among the molecules are broken down. They react with water molecules and move out from the coir pith structure. The remaining reactive molecules from pith–cell–ONa groups between the cellulose molecular chains (Scheme 1). Due to this, hydrophilic hydroxyl groups are reduced and increase the coir pith moisture resistance properties [36]. The nylon/epoxy composites were almost not affected on exposure to water. This may be due to the hydrophobic nature of nylon fabric. The nylon/coir pith/epoxy hybrid composites with treated coir pith showed maximum retention of impact strength which is almost same as 3 layered nylon/epoxy
105
Transmittance %
100 95
composites. These results indicate the importance of incorporation of nylon fabric and chemical treatment of coir pith in the present investigation. 4.6. Flammability of composites Composite materials are increasingly being used in applications in which their fire response is a critical consideration. Combustibility of a natural fiber composite depends on a number of factors such as the type of natural fibers and polymers used for preparation of composite, its density, structure, thermal conductivity and humidity. Table 3 shows the quantitative results from the vertical burn experiments. The addition of coir pith increased the ignition time of composite. The decreased flammability with natural fibers has been reported earlier also [31,41]. The ignition time for coir pith/epoxy composites is found to be higher than the nylon/epoxy composite. The hybrid composite with treated coir pith showed maximum ignition time. This may be due the low percentage of lignin present in the composites systems due to the chemical treatment of pith. Manfredi et al. [42] in his work on polyester reinforced with different natural fibers have indicated better thermal stability for fibers with low lignin content. The same trend is observed for rate of burning. The composites samples with coir pith showed lower flammability than pure epoxy and nylon/epoxy composites. The hybrid samples with coir pith and nylon showed least rate of burning. 4.7. Chemical resistance The ability of the composite samples to resist chemical environment such as acid, alkali, solvent and water were tested and is given in Table 4. This clearly indicates that all composites have not lost the weight. The incorporation of raw coir pith decreased the chemical resistance of epoxy resin. The presence of nylon fabric was found to overcome this to some extent though the values were found to be higher than blank epoxy. The hybrid composites showed a lesser solvent uptake or increased chemical resistance compared to coir pith/epoxy composite. Though the composite samples with chemically treated samples showed better solvent resistance, it was found to be still higher than pure epoxy resin. This may be due to increased interfacial bonding between pith and epoxy on chemical treatment resulting in reduced void content. This increased interfacial interaction makes the polymeric segments around the filler immobile. These factors offer higher resistance to the movement of solvent molecules into the composites system [43]. Restricted equilibrium technique has been used as a tool by many researchers to analyze the filler–matrix bonding in the composites system [44,45]. It is reported that increased interfacial interaction results in lower solvent uptake by composites systems. Therefore the solvent uptake in solvent resistance for composites systems with treated coir pith may due to the increased interfacial interaction between filler and matrix.
90 Table 3 Flammability test results.
Raw coir pith
85 80
Treated coir pith
75 70 4000
3500
3000
2500
2000
1500
1000
-1
Wave number (Cm ) Fig. 4. FTIR spectra of raw and treated coir pith.
500
Sample code
Ignition time (s)
Linear burning rate (mm/s)
BLANK EN2L EN3L ERCP ENA ERCPN2L ENAN2L ERCPN3L ENAN3L
9.0 ± 0.2 11.0 ± 0.2 12.3 ± 0.1 13.4 ± 0.2 14.1 ± 0.3 13.3 ± 0.2 15.7 ± 0.1 14.8 ± 0.2 16.4 ± 0.3
64.28 ± 0.2 61.43 ± 0.3 59.36 ± 0.2 58.35 ± 0.3 57.69 ± 0.1 58.87 ± 0.2 56.16 ± 0.3 57.58 ± 0.2 53.35 ± 0.2
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Table 4 Chemical resistance of epoxy composites. Samples code
HNO2 (%)
H2O (%)
NaOH (%)
CCl4 (%)
BLANK EN2L EN3L ERCP ENA ERCPN2L ENAN2L ERCPN3L ENAN3L
0.603 0.616 0.618 0.711 0.651 0.687 0.651 0.648 0.635
0.506 0.515 0.514 1.012 0.616 0.865 0.575 0.732 0.566
0.598 0.612 0.616 0.781 0.583 0.728 0.678 0.709 0.644
0.583 0.598 0.599 0.652 0.628 0.641 0.620 0.630 0.621
5. Conclusion The results of present study showed that composites with good mechanical strength could be developed successfully by hybridizing coir pith and nylon fabric in epoxy resin. Tensile strength, flexural strength, impact strength and hardness of hybrid composites were much higher than the composites. The mechanical strength, chemical resistance and flammability of composites were found to be improved with chemical treatment. The properties of composites were considerably increased due to alkali treatment of pith. Thus in coir pith reinforced nylon/epoxy composite, improvement in bonding between the pith/nylon/epoxy resin could be achieved by surface treatment of the pith by alkali environment, which could promote its application in light weight materials industry. Acknowledgements The authors thank the management of VIT University, Vellore for their whole heart support to research activities. References [1] Fung KL, Xing XS, Li RKY, Tjong SC, Mai YW. An investigation on the processing of sisal fiber reinforced polypropylene composites. Compos Sci Technol 2003;63:1255–8. [2] Pothan LA, Oommen Z, Thomas S. Dynamic mechanical analysis of banana fiber reinforced polyester composites. Compos Sci Technol 2003;63:283–93. [3] Prachayawarakorn J, Sangnitidej P, Boonpasith P. Properties of thermoplastic rice starch composites reinforced by cotton fiber or low-density polyethylene. Carbohydr Polym 2010;81:425–33. [4] Arbelaiz A, Fernandez B, Ramos JA, Retegi A, Ponte RL, Mondrago I. Mechanical properties of short flax fiber bundle/polypropylene composites: influence of matrix/fiber modification, fiber content, water uptake and recycling. Compos Sci Technol 2005;65:1582–92. [5] Pracella M, Chionna D, Anguillesi I, Kulinski Z, Piorkowska E. Functionalization, compatibilization and properties of polypropylene composites with hemp fibers. Compos Sci Technol 2006;66:2218–30. [6] Singh B, Gupta M, Verma A. The durability of jute fiber–reinforced phenolic composites. Compos Sci Technol 2000;60:581–9. [7] Lodha P, Netravali AN. Characterization of stearic acid modified soy protein isolate resin and ramie fiber reinforced ‘green’ composites. Compos Sci Technol 2005;65:1211–25. [8] Noor-ul Amin. Use of bagasse ash in concrete and its impact on the strength and chloride resistivity. J Mater Civ Eng 2011;23:717–20. [9] Mannan MA, Ganapathy C. Concrete from an agricultural waste-oil palm shell (OPS). Build Environ 2004;39:441–8. [10] Pinto Jorge, Paiva Anabela, Varum Humberto, Costa Ana, Cruz Daniel, Pereira Sandra, et al. Corn’s cob as a potential ecological thermal insulation material. Energy Build 2011;43:1985–90. [11] German Quintana, Jorge Velasquez, Santiago Betancourt, Piedad Ganan. Binderless fiber board from steam exploded banana bunch. Ind Crops Prod 2009;29:60–6. [12] Rajput D, Bhagade SS, Raut SP, Ralegaonakar RV, Mandavgane Sachin A. Reuse of cotton and recycle paper mill waste as building material. Constr Build Mater 2012;32:470–5. [13] Ghosh PK, Sarma US, Ravindranath AD, Radhakrishnan S, Ghosh P. A novel method for accelerated composting of coir pith. Energy Fuels 2007;21: 822–7. [14] Gassan J, Bledzki AK. Possibilities for improving the mechanical properties of jute/epoxy composites by alkali treatment of fibers. Compos Sci Technol 1999;59:1303–9.
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Effect of glass fiber hybridization on properties of sisal fiber–polypropylene composites. Composites Part B 2009;40: 623–7. [26] Akil HMD, Cheng LW, Ishak ZAM, Bakar AA, Rahman MAA. Water absorption study on pultruded jute fiber reinforced unsaturated polyester composites. Compos Sci Technol 2009;69:1942–8. [27] Alix S, Lebrun L, Morvan C, Marais S. Study of water behaviour of chemically treated flax–fibers-based composites: a way to approach the hydric interface. Compos Sci Technol 2011;71:893–9. [28] Assarar M, El Scida D, Mahi A, Poilane C, Ayad R. Infiuence of water ageing on mechanical properties and damage events of two reinforced composite materials: flax–fibers and glass–fibers. Mater Des 2011;32:788–95. [29] Chen H, Miao M, Ding X. Influence of moisture absorption on the interfacial strength of bamboo/vinyl ester composites. Composites Part A 2009;40:2013–9. [30] Chow CPL, Xing XS, Li RKY. Moisture absorption studies of sisal fiber reinforced polypropylene composites. 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Technical Report
Development of coir pith/nylon fabric/epoxy hybrid composites: Mechanical and ageing studies R. Narendar a, K. Priya Dasan a,⇑, Muraleedharan Nair b a b
Material Chemistry Division, SAS, VIT University, Tamil Nadu 632014, India Common Facility Service Centre, Malappuram, Kerala 676124, India
a r t i c l e
i n f o
Article history: Received 8 July 2013 Accepted 22 August 2013 Available online 30 August 2013
a b s t r a c t The coir pith epoxy composites were hybridized with nylon fabric/epoxy resin by hand lay up technique followed by compression moulding. A set of composites of same composition having chemically treated coir pith was also prepared. Mechanical properties of composites such as tensile strength, flexural strength, impact strength and hardness were evaluated. Though coir pith acts as a good reinforcement in epoxy resin, the incorporation of nylon fabric and the chemical treatment of coir pith were found to enhance the properties of the composites further. Chemical resistance and flame resistance of composite systems were also found to be improved with hybrid composites. Since water uptake and retentions property of coir pith is a major drawback when it comes to its application in composites, the ageing of composite panels in moist environment was investigated. The results suggested that the presence of nylon fabric and chemically treated pith can contribute to longer durability of the panels in moist conditions. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The use of lignocellulosic materials as reinforcing fillers in polymeric products has increased recently. These natural fillers are ecofriendly, have low density, low-cost, non-abrasive in nature and are biodegradable. Sisal [1], banana fiber [2], cotton [3], flax [4], hemp [5], jute [6] and ramie [7] have been well recognized as reinforcements for natural filler composites. Most of these fillers are categorized under agro wastes and their disposal is a huge responsibility for the government. They pose severe environmental pollution problems and occupy fertile useful land. Therefore developing engineering end use such as building materials and structural parts out of these materials has become a requirement. Application of agro-wastes as particle boards, thermal insulators, building material composites/bricks, cementitious/binder and aggregates [8– 12] have been well studied. Coir pith is one of the major agro wastes found in the southern coastal regions of India. Coir pith is generated in the separation process of the fiber from the coconut husk and is generally dumped as an agro waste. Because of its low degradation in the environment, the hillocks of coir pith collected or dumped pause serious health hazards and loss of fertile lands. Because of the high lignin content left it takes decades to decompose; it only begins to break down when it is 10 years old. The tannins and phenols from coir pith leach out into the soil and water bodies causing pollution. It ⇑ Corresponding author. Tel.: +91 416 2202696; fax: +91 416 2243092. E-mail address: [email protected] (K. Priya Dasan). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.08.080
is estimated that at present there is an accumulated stock of 10 106 metric tons of coir pith in the southern states of India and about 7.5 105 tons of coir pith is produced annually in India [13]. Developing alternate ways to dispose of coir pith is of critical importance. Cost effective technologies that address the development of value added products from coir pith therefore become relevant for countries producing coir pith. The application of coir pith as reinforcement in polymer has not been reported so far. One factor that has prevented a more extended utilization of the agro-wastes in composite industry is the lack of compatibility of these fillers in most polymeric matrices. The hydrophilic nature of natural fillers adversely affects adhesion to hydrophobic matrix and as a result, causes poor mechanical properties. One of the most commonly adopted methods to overcome this issue is the chemical treatment of natural fillers. The effect of chemical treatments on the mechanical and other properties of composites are well documented. Gassan and Bledzki [14] studied the mechanical properties of jute/epoxy composites. Composite strength and stiffness increased as a consequence of the improved mechanical properties of the fibers by alkali treatment. Kenaf/epoxy composites were prepared after subjecting the fiber to mercerization. The failure mechanism and damage features of the materials revealed that reinforcement of epoxy with treated kenaf fibers increased the flexural strength of the composite compare to untreated fibers Yousif et al. [15]. Bachtiar et al. [16] reported the mechanical properties of sugar palm fiber treated with sodium hydroxide. Tensile modulus of composites was much higher than untreated fiber composite specimens, which proved the effectiveness of the
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treatment. Min Zhi Rong et al. [17] studied the sisal fiber reinforced epoxy composites. Results showed that alkali treatment removed cementing materials which were partially replaced by epoxy resin. This greatly improved fiber bundle/matrix bonding and also enhanced mechanical strength of composite. Hybrid composites are made by the incorporation of several different types of reinforcements into a single matrix. They imply a step beyond in the search for novel materials with improved mechanical properties and/or reduced cost. Hybrid polymer composites have been studied many researchers in the past [18–22]. The use of synthetic reinforcements in combination with natural fillers has been shown to exhibit excellent performance and at the same time reduce the environmental impact. The hybrid effect of glass fiber and oil palm empty fruit bunch (OPEFB) fiber on the mechanical properties of phenol–formaldehyde composites was studied by Sreekala et al. [23]. The introduction of small amount of glass fiber improved the impact strength of the composites. Meanwhile density of the hybrid composite decreased as the volume fraction of the OPEFB fiber increased. Mechanical properties of jute/glass fiber reinforced epoxy composites were studied by Koradiya et al. [24]. Experimental results showed that hybrid composites have good mechanical properties than those of jute and glass composites. Jarukumjorn and Suppakarn [25] investigated the effect of glass fiber hybridization on the physico-mechanical properties of sisal–polypropylene composites. Incorporation of glass fiber into sisal/polypropylene composites enhanced tensile, flexural and impact strength. In the present work coir pith/epoxy composites were hybridized with nylon fabric impregnated with epoxy resin. The composites were prepared by hand lay up technique and than compression moulded. A set of composites with same composition having chemically treated coir pith was also fabricated. Mechanical properties, chemical resistance and flame resistance of composites were investigated in detail. Research investigations showed that the exposure of natural filler composites in a wet environment leads to a decrease of the mechanical properties when water spreads in the material [26–30]. Since coir pith has a higher tendency to absorb and retain water, it becomes essential to know how the composites made of coir pith behave in a wet environment. Hence the composite panels were subjected to wet environment and their mechanical properties were evaluated. 2. Materials and methods 2.1. Materials Coir pith was collected from a local coir processing unit (Gudiyathum, Tamilnadu, India). Sodium hydroxide was purchased from Sigma–Aldrich, epoxy resin (LY556) and hardener (HY951) were purchased from Huntsman. Maaxil 402 mould release spray was purchased from Maax lubrication. 2.2. Methods 2.2.1. Chemical treatment Coir pith was soaked in 5% concentration of NaOH solution for 1 h at room temperature followed by washing with distilled water. Afterwards, the samples were oven dried at 70 °C for 2 h. 2.2.2. Composites preparation A square steel plate mould with dimension of 450 450 mm assembled with top plate and base plate was used for the fabrication of composites. To help the complete removal of composites from the mould and to avoid sticking, a polyethylene sheets sprayed with mould release agent is layered on the top and base
plate. Nylon fabric with a uniform coating of epoxy and hardener was spread on the polyethylene sheet. The pith/resin mixture (mixed in an internal mixer) is spread on this followed by another layer of resin coated nylon fabric. The samples were compression molded and cured over-night as shown (Fig. 1a and b). The resin hardener ratio is maintained at 10:1 in all formulations. The composites thus fabricated are denoted as given in Table 1. 3. Characterization 3.1. Fourier transform infrared (FTIR) spectroscopy FT-IR spectroscopy was used to investigate the surface modification in treated and untreated coir pith. FT-IR analysis was carried out in the range of 4000–400 cm1 with a resolution of 2 cm1 using a JASCO 400 Infrared spectrometer. 3.2. Morphology analysis Scanning electron microscope (SEM) of Carl Ziess, EVO make was used to analyze the morphology of coir pith and impact failure surface of epoxy composites. Optical microscopic image of treated and untreated coir pith were obtained using Brucker Carl Zeiss optical microscopy. 3.3. Mechanical properties Tensile test was carried out according to ASTM: D 638-10 using a universal testing machine of AG-IS Shimadzu, TMI make. Flexural tests were performed according to ASTM: D 790-10 using Instron UTM, USA. Impact Izod test was done according to ASTM: D 25610 using Tinius Olsen Model impact analyzer. Hardness was measured by using Shore A hardness tester (Durometer-Mitutoyo Shore A meter). 3.4. Chemical resistance test The chemical resistance properties of the epoxy resin/coir pith/ nylon composites in CCl4, water, NaOH and HNO3 were studied according to ASTM: D543-06. In each case, five pre-weighed samples were dipped in the respective chemical reagents for 24 h. They were then removed and immediately washed in distilled water and dried by pressing them on both sides with a tissue paper at room temperature. The samples were then weighed and the percentage weight loss/gain was determined using the following equation.
Weight loss=gainð%Þ ¼
Final weight Original weight Orginal weight
ð1Þ
3.5. Ageing studies The ageing of composites on exposure to water was evaluated by keeping the samples immersed in water. Five specimens of each sample were kept immersed in distilled water at 30 °C for 31 days. The samples were taken out, dried at room temperature and the impact strength was measured as mentioned above. 3.6. Flammability test Flammability of polymer composites were evaluated as mentioned in a previous report [31]. The tests closely simulate the Federal Aviation Regulation, FAR 25.853 60 s vertical burn test specification [46]. 290 mm 70 mm sized samples were suspended vertically using a clamp on a lab stand. An LPG Bunsen flame was applied to the leading edge of the bottom surface of
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Fig. 1. (a) Schematic representation of composite fabrication (b) composites panel.
Table 1 Material code and its abbreviation. Materials code
Materials abbreviations
BLANK EN2L EN3L ERCP ENA ERCPN2L ENAN2L
Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy layers Epoxy Epoxy layers
ERCPN3L ENAN3L
resin resin/nylon two layer resin/nylon three layer resin/raw coir pith resin/sodium hydroxide treated coir pith resin/raw coir pith/nylon two layer resin/sodium hydroxide treated coir pith/nylon two resin/raw coir pith/nylon three layers resin/sodium hydroxide treated coir pith/nylon three
composite. The time required to catch fire is taken as ignition time. The flame time from the first mark (25 mm from the ignition end) until the second mark (100 mm from the ignition end) was measured to determine the linear burning rate (V) of the sample. The linear burning rate (V), was calculated in millimeters per second using the equation
V ¼ 60L=t
ð2Þ
L is the burned length in millimeters and t is the time in seconds. 4. Results and discussion 4.1. Tensile strength The tensile strength of pure epoxy and different composite samples are given in Table 2. The composite samples exhibit better tensile strength than the pure epoxy resin. The incorporation of raw coir pith increased the tensile strength of epoxy resin by 19.34%. Meanwhile two and three layered nylon/epoxy composites showed 25.74% and 30.49% higher tensile strength than the raw coir pith/ epoxy composites. This indicates nylon fabric as better reinforcement for epoxy resin. Nylon fabric forms chemical bonding with epoxy resin and provides superior mechanical properties
(Scheme 1). The hybrid composites showed higher tensile values than the individual composites samples with the two layered hybrid composites showing almost similar tensile strength as that of three layered nylon/epoxy composites. The presence of nylon fabric enhances the tensile properties in hybrid composites also. Three layered composites samples showed 33.00% higher tensile strength than two layered composites samples. Henceforth it can be concluded that the hybrid composites system with coir pith/nylon fabric/epoxy composites exhibits maximum tensile strength and the property improves with number of nylon fabrics. In the present case nylon fabric act as skin and coir pith as the core material. The tensile strength will be higher, when the high strength material is used as the skin, which is the main load bearing component in the tensile measurements [32]. 4.1.1. Effect of chemical treatment The tensile strength of composite samples with treated and raw coir pith is also given in Table 2. Pure wettability of natural filler due to its hydrophilic nature is one of the major reasons for the mechanical failure of natural filler reinforced composites. This results in increased void content and structural flaws resulting in low stress transfer between the polymer and filler. To prevent this, the filler surface has to be modified in order to encourage adhesion. Chemical treatment results in surface modification and gives rise to more groups on the filler surface and thus facilitates efficient coupling with the matrix [16]. Many authors have indicated an increase in mechanical properties with chemical treatment [33,34]. In a recent work on coir fiber reinforced polypropylene, the authors observed an increase in mechanical properties and fiber wettability with chemical treatment [35]. For the present experiment, composite samples with chemically treated coir pith showed better tensile strength values compared to the samples with untreated coir pith. Compared to ERCP composite, tensile strength for ENA composites increase by 19.44%. When coir pith is subjected to alkaline environment, the waxy material gets removed from the pith surface and increases its surface roughness. And also it promotes the activation of hydroxyl groups of cellulose unit by breaking hydrogen bonds, which in turn makes it less hydrophilic [36]. The chemically treated coir pith shows a rougher surface
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R. Narendar et al. / Materials and Design 54 (2014) 644–651 Table 2 Mechanical strength of composites. Sample code
Tensile strength (MPa)
Flexural strength (MPa)
Impact strength (J/m)
Impact retention (%)
Hardness (shore A)
BLANK ERCP ENA EN2L EN3L ERCPN2L ENAN2L ERCPN3L ENAN3L
4.21 ± 0.2 5.22 ± 0.3 6.48 ± 0.2 7.03 ± 0.3 7.51 ± 0.2 7.57 ± 0.3 8.51 ± 0.2 11.3 ± 0.2 12.5 ± 0.2
27.32 ± 0.4 32.87 ± 0.3 38.95 ± 0.3 42.31 ± 0.4 48.90 ± 0.3 53.19 ± 0.4 68.56 ± 0.2 75.22 ± 0.3 106.52 ± 0.4
83.14 ± 0.5 101.35 ± 0.4 132.64 ± 0.5 167.38 ± 0.5 189.97 ± 0.4 214.29 ± 0.6 248.13 ± 0.5 311.52 ± 0.4 359.88 ± 0.5
95.1 ± 0.2 85.7 ± 0.2 96.7 ± 0.3 96.8 ± 0.4 97.0 ± 0.2 87.6 ± 0.3 91.3 ± 0.2 93.1 ± 0.4 97.8 ± 0.3
94.00 ± 0.3 95.60 ± 0.4 96.09 ± 0.4 94.50 ± 0.4 95.56 ± 0.3 96.62 ± 0.3 97.58 ± 0.2 98.61 ± 0.3 99.73 ± 0.4
Treated Coir pith
Coir pith CH2 Epoxy resin
Nylon fabric
C6H5-C(CH3) 2-C6H4-O-CH2 Scheme 1. Hydrogen bonding interaction between treated coir pith/nylon 6 fabric/epoxy resin.
morphology as is evident from the SEM photographs (Fig. 2a and b). The optical photographs support this further as shown in Fig. 2c and d. The rougher morphology of treated coir pith results in enhanced mechanical bonding by an interlocking method [16]. This results in better stress transfer among the components in composites systems. Factually, epoxy is able to fill up the apparent flaws in treated coir pith and results in better load sharing.
double layer of nylon fabric epoxy composites [20,21]. The hybrid composites showed maximum flexural properties. The three layered hybrid composites showed a flexural strength value of 75.22 MPa which is 34.99% higher than nylon/epoxy composites. Composite samples with chemically treated coir pith showed better flexural properties than the samples with raw coir pith. The enhanced interfacial interaction among the composites constituents is expected to be responsible for this.
4.2. Flexural strength 4.3. Impact strength Flexural strength of epoxy composites are shown in Table 2. The reinforcement increases the flexural strength of epoxy resin. The incorporation of coir pith increased the flexural strength of epoxy by 16.88%. Meanwhile the incorporation of nylon fabric enhanced the flexural strength of resin by 35.42%. The three layered nylon fabric had 7.94% higher flexural strength that two layered nylon epoxy composites. The presence of double and triple layer fabric plays an important role in sandwich composites. In three layers of nylon fabric composites, the upper layer is put into compression, the lower layer into tension and the core or middle layer of nylon act as shear. Upper and lower layer of nylon fabric are subjected to compression/tension and are largely responsible for the strength of the sandwich laminate. Though the usage of middle layer of nylon is to support the upper and lower layer of nylon, so that they drastically reduce the maximum stress and deformation of outer layer by increasing the moment of inertia of the sandwich beam. Based on the sandwich theory, it can be expected that triple layer of nylon fabric epoxy composites withstand high stress during bending and good resistance to propagation of cracks compare to
Impact properties of materials are directly related to the overall toughness of the material. Toughness means the ability of the material to absorb applied energy. Impact strength of composites is given in Table 2. The composites showed higher impact strength than pure epoxy resin. The impact strength of composite increased with increasing number of nylon fabric. It is also clear from the Table 2 that sodium hydroxide treated coir pith increases the reinforcement efficiency of the pith in epoxy matrix. SEM observation of impact fracture surface of ERCPN2L and ENAN2L composites is shown in Fig. 3a and b. The involvement of fillers in the failure is due to the separation of fillers and matrix and loss of stress transferring capability. Fig. 3a. indicates poor adhesion between the filler and matrix. This may be due to hydrophilic nature of coir pith resulting in fragile bond formation with hydrophobic epoxy resin. SEM photograph of fractured surface shows pith ejection at a number of places giving rise to holes. The presence of voids, debris and air entrapment visible in the SEM photographs indicates poor bonding between the pith and
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Fig. 2. (a) Scanning electron microscope photographs of raw coir pith (b) scanning electron microscope photographs of sodium hydroxide treated coir pith (c) optical microscopic images of raw coir pith and (d) optical microscopic images of sodium hydroxide treated coir pith.
Fig. 3. SEM images of (a) impact failure fracture surface of ERCPN2L composites and (b) impact failure fracture surface of ENAN2L composites.
matrix, ultimately effecting the composite properties. The fracture surface of ENAN2L composites appears smooth with lesser number of holes, when compared to ERCPN2L composite fracture surface. This is an evidence of good bond existing between coir pith and matrix due to chemical treatment. 4.4. Hardness properties Hardness of composite samples is given in Table 2. Compared to blank sample, the composite samples show higher hardness values.
The presence of nylon fabric slightly enhances the hardness of composites further. The hybrid composite shows the highest hardness values. An increase in hardness is observed with chemically treated coir pith. Raw coir pith is highly amorphous and fluffy material because of its high lignin content. Chemical treatment results in the formation of cellulose rich product which is more crystalline nature [37,38]. During chemical treatment, the lignin part of the coir pith gets removed resulting in cellulose rich product. The FTIR spectra of treated and untreated coir pith are shown in Fig. 4. The peak at 1732.16 cm1 in untreated coir pith
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is attributed to acetyl or uronic ester linkage of hemicelluloses and or ester linkage of carboxylic group of ferulic or p-coumeric acids of lignin or hemicelluloses [39]. The aromatic peak C@C stretch from aromatic ring of lignin gives two peaks at 1612 cm1 and 1456 cm1. The peak at 1240.11 cm1 in untreated coir pith is due to hemicelluloses; the intensity of peak decreased in treated coir pith indicating the removal of hemicelluloses content in treated coir pith. The peak at 1388.36 cm1 in raw coir pith corresponds to ACH symmetric stretching of aromatic lignin and this peak is shifted in treated coir pith, indicating the deformation of lignin in treated pith. NaOH treated coir pith shows a peak at 3444 cm1 corresponding to cellulose-OH. The increased intensity of this peak in treated samples indicates increased cellulose content with treatment. 4.5. Ageing studies – in water The composites samples were subjected to ageing to ascertain their utility during applications. The percentage retention of impact strength of composites after exposing to water is given in Table 2. The composites samples with coir pith showed a slight decrease in impact strength on exposure to water. This result indicates that the presence of coir pith do not favor application of composites samples in moist environment. The hygroscopic nature of coir pith results in high water uptake by the composites samples. Water filled voids at the interface results in interfacial de-bonding. This causes cracks and micro-voids in the surface of composites [40]. The water filled voids at the interface results in interfacial de-bonding. Once water penetrates inside composites materials, pith start swelling and matrix tend to chain reorientation resulting in poor mechanical properties. However, the composites with treated pith showed improved water resistance and retention of mechanical properties even on being exposed to water environment. The chemical treatment of coir pith effectively improved pith–matrix adhesion. The alkaline sensitive hydroxyl groups present among the molecules are broken down. They react with water molecules and move out from the coir pith structure. The remaining reactive molecules from pith–cell–ONa groups between the cellulose molecular chains (Scheme 1). Due to this, hydrophilic hydroxyl groups are reduced and increase the coir pith moisture resistance properties [36]. The nylon/epoxy composites were almost not affected on exposure to water. This may be due to the hydrophobic nature of nylon fabric. The nylon/coir pith/epoxy hybrid composites with treated coir pith showed maximum retention of impact strength which is almost same as 3 layered nylon/epoxy
105
Transmittance %
100 95
composites. These results indicate the importance of incorporation of nylon fabric and chemical treatment of coir pith in the present investigation. 4.6. Flammability of composites Composite materials are increasingly being used in applications in which their fire response is a critical consideration. Combustibility of a natural fiber composite depends on a number of factors such as the type of natural fibers and polymers used for preparation of composite, its density, structure, thermal conductivity and humidity. Table 3 shows the quantitative results from the vertical burn experiments. The addition of coir pith increased the ignition time of composite. The decreased flammability with natural fibers has been reported earlier also [31,41]. The ignition time for coir pith/epoxy composites is found to be higher than the nylon/epoxy composite. The hybrid composite with treated coir pith showed maximum ignition time. This may be due the low percentage of lignin present in the composites systems due to the chemical treatment of pith. Manfredi et al. [42] in his work on polyester reinforced with different natural fibers have indicated better thermal stability for fibers with low lignin content. The same trend is observed for rate of burning. The composites samples with coir pith showed lower flammability than pure epoxy and nylon/epoxy composites. The hybrid samples with coir pith and nylon showed least rate of burning. 4.7. Chemical resistance The ability of the composite samples to resist chemical environment such as acid, alkali, solvent and water were tested and is given in Table 4. This clearly indicates that all composites have not lost the weight. The incorporation of raw coir pith decreased the chemical resistance of epoxy resin. The presence of nylon fabric was found to overcome this to some extent though the values were found to be higher than blank epoxy. The hybrid composites showed a lesser solvent uptake or increased chemical resistance compared to coir pith/epoxy composite. Though the composite samples with chemically treated samples showed better solvent resistance, it was found to be still higher than pure epoxy resin. This may be due to increased interfacial bonding between pith and epoxy on chemical treatment resulting in reduced void content. This increased interfacial interaction makes the polymeric segments around the filler immobile. These factors offer higher resistance to the movement of solvent molecules into the composites system [43]. Restricted equilibrium technique has been used as a tool by many researchers to analyze the filler–matrix bonding in the composites system [44,45]. It is reported that increased interfacial interaction results in lower solvent uptake by composites systems. Therefore the solvent uptake in solvent resistance for composites systems with treated coir pith may due to the increased interfacial interaction between filler and matrix.
90 Table 3 Flammability test results.
Raw coir pith
85 80
Treated coir pith
75 70 4000
3500
3000
2500
2000
1500
1000
-1
Wave number (Cm ) Fig. 4. FTIR spectra of raw and treated coir pith.
500
Sample code
Ignition time (s)
Linear burning rate (mm/s)
BLANK EN2L EN3L ERCP ENA ERCPN2L ENAN2L ERCPN3L ENAN3L
9.0 ± 0.2 11.0 ± 0.2 12.3 ± 0.1 13.4 ± 0.2 14.1 ± 0.3 13.3 ± 0.2 15.7 ± 0.1 14.8 ± 0.2 16.4 ± 0.3
64.28 ± 0.2 61.43 ± 0.3 59.36 ± 0.2 58.35 ± 0.3 57.69 ± 0.1 58.87 ± 0.2 56.16 ± 0.3 57.58 ± 0.2 53.35 ± 0.2
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Table 4 Chemical resistance of epoxy composites. Samples code
HNO2 (%)
H2O (%)
NaOH (%)
CCl4 (%)
BLANK EN2L EN3L ERCP ENA ERCPN2L ENAN2L ERCPN3L ENAN3L
0.603 0.616 0.618 0.711 0.651 0.687 0.651 0.648 0.635
0.506 0.515 0.514 1.012 0.616 0.865 0.575 0.732 0.566
0.598 0.612 0.616 0.781 0.583 0.728 0.678 0.709 0.644
0.583 0.598 0.599 0.652 0.628 0.641 0.620 0.630 0.621
5. Conclusion The results of present study showed that composites with good mechanical strength could be developed successfully by hybridizing coir pith and nylon fabric in epoxy resin. Tensile strength, flexural strength, impact strength and hardness of hybrid composites were much higher than the composites. The mechanical strength, chemical resistance and flammability of composites were found to be improved with chemical treatment. The properties of composites were considerably increased due to alkali treatment of pith. Thus in coir pith reinforced nylon/epoxy composite, improvement in bonding between the pith/nylon/epoxy resin could be achieved by surface treatment of the pith by alkali environment, which could promote its application in light weight materials industry. Acknowledgements The authors thank the management of VIT University, Vellore for their whole heart support to research activities. References [1] Fung KL, Xing XS, Li RKY, Tjong SC, Mai YW. An investigation on the processing of sisal fiber reinforced polypropylene composites. Compos Sci Technol 2003;63:1255–8. [2] Pothan LA, Oommen Z, Thomas S. Dynamic mechanical analysis of banana fiber reinforced polyester composites. Compos Sci Technol 2003;63:283–93. [3] Prachayawarakorn J, Sangnitidej P, Boonpasith P. Properties of thermoplastic rice starch composites reinforced by cotton fiber or low-density polyethylene. Carbohydr Polym 2010;81:425–33. [4] Arbelaiz A, Fernandez B, Ramos JA, Retegi A, Ponte RL, Mondrago I. Mechanical properties of short flax fiber bundle/polypropylene composites: influence of matrix/fiber modification, fiber content, water uptake and recycling. Compos Sci Technol 2005;65:1582–92. [5] Pracella M, Chionna D, Anguillesi I, Kulinski Z, Piorkowska E. Functionalization, compatibilization and properties of polypropylene composites with hemp fibers. Compos Sci Technol 2006;66:2218–30. [6] Singh B, Gupta M, Verma A. The durability of jute fiber–reinforced phenolic composites. Compos Sci Technol 2000;60:581–9. [7] Lodha P, Netravali AN. Characterization of stearic acid modified soy protein isolate resin and ramie fiber reinforced ‘green’ composites. Compos Sci Technol 2005;65:1211–25. [8] Noor-ul Amin. Use of bagasse ash in concrete and its impact on the strength and chloride resistivity. J Mater Civ Eng 2011;23:717–20. [9] Mannan MA, Ganapathy C. Concrete from an agricultural waste-oil palm shell (OPS). Build Environ 2004;39:441–8. [10] Pinto Jorge, Paiva Anabela, Varum Humberto, Costa Ana, Cruz Daniel, Pereira Sandra, et al. Corn’s cob as a potential ecological thermal insulation material. Energy Build 2011;43:1985–90. [11] German Quintana, Jorge Velasquez, Santiago Betancourt, Piedad Ganan. Binderless fiber board from steam exploded banana bunch. Ind Crops Prod 2009;29:60–6. [12] Rajput D, Bhagade SS, Raut SP, Ralegaonakar RV, Mandavgane Sachin A. Reuse of cotton and recycle paper mill waste as building material. Constr Build Mater 2012;32:470–5. [13] Ghosh PK, Sarma US, Ravindranath AD, Radhakrishnan S, Ghosh P. A novel method for accelerated composting of coir pith. Energy Fuels 2007;21: 822–7. [14] Gassan J, Bledzki AK. Possibilities for improving the mechanical properties of jute/epoxy composites by alkali treatment of fibers. Compos Sci Technol 1999;59:1303–9.
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