This experimental study was conducted to investigate the effect of NaOH concentration and treatment time on the flexural properties of Luffa cylindrica fiber-reinforced epoxy composites. Significant improvement (up to 84.92%) in the flexural properties for the treated fiber composite compared with the untreated fiber composite was observed. Both treated and untreated fiber composites were then subjected to different environmental treatments (saline water, distilled water, and subzero temperature). To find out the changes in flexural strength immediately after treatment, the same test was carried out on the composites. Degradation in the flexural strength of both treated and untreated fiber composites, when subjected to environmental treatments, was observed. They were found within the range of 2%–20% and were found to be least in subzero treatment. The SEM micrograph indicates that alkali treatment is effective in improving the adhesion between the fiber and matrix.
Natural fibers as reinforcement in polymer composites for making low-cost composites have generally been of much interest in recent years. Industries (particularly automobile construction and packaging) today are under tremendous pressure to design their product that will suit to environmental legislation. Accordingly, extensive research is going worldwide to design and develop new materials that can substitute non-conventional non-renewable reinforcing material such as glass fiber . The attractive features of natural fibers over traditional glass fibers are their light weight, high specific modulus, renewability, biodegradability, and potentially low cost , , , . In addition, natural fibers are also claimed to offer environmental advantages such as reduced dependence on non-renewable energy/material sources, lower pollutant emissions, lower greenhouse gas emissions, and enhanced energy recovery. Though advantage over synthetic fiber, natural fibers composites are not totally free from defects. The main bottlenecks in the broad use of natural fibers in various polymer matrixes are poor compatibility with polymer matrixes and inherent high moisture absorption that brings about dimensional instability that leads to a reduction in the mechanical properties of the composites . In order to improve the compatibility between the fiber and matrix that was responsible for the improvement in the flexural properties of the composites, various chemical methods are available . Alkaline treatment is one of the widely used chemical treatments that improve the surface quality of natural fibers. It has been reported that such a treatment improves both the interfacial strength and flexural properties of composite material . Yan et al.  reported that the tensile and flexural strengths of the flax epoxy composite were increased by 21.9% and 16.1%, respectively, compared with the untreated one due to the surface modification of fiber by 5% (conc.) alkali treatment for 30 min. In another paper, Yan  reported that alkali treatment (5% NaOH for 30 min) enhances the compressive strength and compressive modulus, in-plane shear strength and shear modulus, and specific impact strength of both flax-epoxy and linen-epoxy composites. Ray et al.  treated the jute fiber with 5% aqueous NaOH solution from 2 h up to 8 h, and Manshor et al.  treated the durian skin fiber with 4% NaOH for 1 h at room temperature to enhance the mechanical properties of the composite.
Luffa cylindrica (L.) synonym L. aegyptiaca Mill, a forest product commonly called sponge gourd, loofa, vegetable sponge, bath sponge, or dish cloth gourd, is a member of cucurbitaceous family . L. cylindrica is a subtropical plant abundantly available in China, Japan, India, and other countries in Asia as well as in Central and South America . The fruit L. cylindrica can be eaten as a vegetable when it is young. But mature fruits cannot be eaten because of its bitter taste due to the development of purgatives chemicals. Because of its purgative property, it is used as a medicine for remedy of dropsy, nephritis, and chronic bronchitis and lung complaints . The luffa fruit has a fibrous vascular system that forms a natural mat when dried, and it has a unique knitting structure that is generally not found in other natural fibers as shown in Figure 1. The natural luffa mat possesses remarkable strength, stiffness, and energy absorption capacity comparable to metallic cellular material in a similar density range . However, the natural sponge guard/L. cylindrica has no proper utilization except that they are being used as utensils’ cleaning sponge, for making crafts, packing materials, component of shock absorbers, sound proof linings, filters in factories, and part of soles of shoes . As reported by Seki et al. , this fiber c ontains cellulose 63.0%, hemicelluloses 14.4%, lignin 1.6%, ash 0.9%, and others 20.1%. Parida et al.  and Tanobe et al.  studied the effect of fiber surface treatment on the mechanical properties of L. cylindrica polymer composites. They have the opinion that various chemical modifications of luffa fiber enhance the mechanical properties of the composites. Seki et al.  studied the effect of water aging on the mechanical properties of a L. cylindrica polyester composite. They observed that water aging reduces the mechanical properties of the composite.
Survey of the literature on the luffa cylindrica fiber (LCF) composite indicates that the work on Luffa cylindrica as a reinforcing fiber in a polymer composite is scarce. Hence, in this work, an attempt has been made to prepare a L. cylindrica fiber-reinforced epoxy composite. It is also planned to treat the fiber with alkali to increase the adhesion between the fiber and the resin matrix. The flexural strength of the developed composites was studied. The improvement/degradation in the flexural strength of the composites subjected to different environmental conditions is also reported in this study. The morphology of the treated fiber and fracture surface of the composite (environmentally treated) were studied by scanning electron microscopy (SEM).
2 Materials and methods
Epoxy LY 556 (bisphenol-A-diglycidyl-ether) is used as matrix material. Hardener HY-951 (2-amineethylethane-1, 2-diamin) is used as a curing agent. Both the epoxy resin and the hardener are supplied by Ciba Geigy Ltd. (Mumbai, India). Luffa cylindrica fibers are collected locally (Rourkela, Odisha, India). The details of the L. cylindrica fiber from collection to preparation are shown in Figure 2. The density of epoxy resin is 1.1 g/cm3. The densities of luffa cylindrical fibers are 0.56 g/cm3.
2.1.1 Alkali treatment of luffa cylindrical fibers
To enhance interfacial bonding with matrix material and to reduce moisture absorption, surface modification (alkali treatment) of lignocellulose luffa fibers was performed. Prior to alkali treatment, luffa fibers were washed thoroughly with fresh water. The NaOH solution of 1%, 3%, 5%, and 7% concentration was prepared separately in a separate water bath . Then the washed luffa fibers were immersed in different concentrations of NaOH solution maintaining a liquor ratio of 15:1 (w/w) for 2, 4, and 6 h with subsequent stirring , . Next, the alkali solution was drained out and the fibers were washed properly with fresh water to remove any NaOH sticking to the fiber surface. Finally, the fibers were washed with distilled water containing little dilute acetic acid and pH of 7 was maintained . The fibers were then air-dried for 48 h followed by oven-drying at 70°C for 6 h.
2.2 Fabrication of composites
Composites were fabricated by a general hand lay-up technique by a wooden mold of dimensions 140 mm× 100 mm×6 mm. The composites were cast with a single, double, and triple layer of a natural mat Luffa cylindrica fiber in three different weight proportions (8, 13, and 19 wt.%). The weight percentage of the fiber is calculated by using the following formula:
For different wt.% of fibers, a calculated amount of epoxy resin and hardener in the ratio of 10:1 was thoroughly mixed with gentle stirring to minimize air entrapment. For easy removal of the composite from the mold, a releasing agent (silicon spray) was used. Each ply of the Luffa cylindrica fiber was of dimensions 140 mm×100 mm. The average diameter of the L. cylindrica fiber was 226 μm (see Figure 1). The cast of each composite was cured under a load of 25 kg for 72 h. Specimens of required dimensions were cut using a diamond cutter for physical characterization and mechanical testing.
2.3 Experimental design
The composites were manufactured by varying the fiber weight fraction, and chemical treatment of fibers is summarized in Table 1. The first group of the composites were prepared with single-layer (6.5 wt.%), double-layer (13 wt.%), and triple-layer (19 wt.%) Luffa cylindrica fibers. From the first group of samples, the maximum flexural strength was obtained for the composite prepared with double layer. Based on this result, the second group of samples were prepared with a double-layer luffa fiber treated with 1%, 3%, 5%, and 7% concentrations of NaOH solution. From the second group of samples, the maximum improvement of flexural strength was obtained for the composite prepared with 5% (conc.) NaOH-treated fibers. Then the third group of the samples were prepared with a double-layer luffa fiber treated with 5% (conc.) NaOH solutions for 2, 4, and 6 h. The optimum result was found for 4-h treatment. Finally, the fourth group of composites were prepared with a 4-h alkali-treated (5% conc.) double-layer luffa fiber to study the mechanical properties (namely flexural strength) of the composites under different environmental conditions such as saline water, distilled water, and subzero temperature.
|Group||Sample||Weight fraction analysis||Type||Remarks|
|Weight fraction analysis||1||SL (08 wt.%)||Untreated luffa fiber||DL (13 wt.%) optimum|
|2||DL (13 wt.%)||Untreated luffa fiber|
|3||TL (19 wt.%)||Untreated luffa fiber|
|Group 2||Concentration of alkali (%)|
|Treatment of fiber||4||DL (13 wt.%)||0||5% Optimum|
|5||DL (13 wt.%)||1|
|6||DL (13 wt.%)||3|
|7||DL (13 wt.%)||5|
|8||DL (13 wt.%)||7|
|Group 3||Hours of treatment|
|Treatment of fiber||9||DL (13 wt.%)||2||4 h of treatment optimum|
|10||DL (13 wt.%)||4|
|11||DL (13 wt.%)||6|
|Group 4||Environmental treatments|
|Composite subjected to different environmental conditions||12||DL (13 wt.%)||Saline water|
|13||DL (13 wt.%)||Distilled water|
|14||DL (13 wt.%)||Subzero temperature|
2.4 Experimental procedure
The prepared composites (fourth group) were subjected to different environmental conditions such as distilled water, saline water (5% NaCl concentration), and subzero temperature (−25°C), until saturation, that is, for a period of 156 h. The effect of different environmental treatments on the flexural properties of the composites has been studied and reported in this paper.
2.4.1 Flexural strength
The flexural (three-point bend) test of normal and environmentally treated fiber-reinforced composite was carried out on a Computerized Universal Testing Machine (INSTRON H10KS) in accordance with ASTM D790-03. Specimens 140 mm long, 15 mm wide, and 4 mm thick were tested at a constant crosshead speed of 2 mm/min with a span length of 70 mm.
2.4.2 SEM studies
A scanning electron microscope Nova NANO SEM 450 was used to characterize the morphology of both untreated and alkali-treated fiber surface and fractured surface of environmental degraded composites after the flexural test. The samples were mounted on the stub by carbon tape. A thin film of gold was vacuum evaporated on the surface of the sample in order to avoid charging before the photomicrographs were taken.
3 Results and discussion
Figure 3 shows the variation of the flexural strength of single-, double-, and triple-layer untreated luffa fiber-reinforced epoxy composites. From this figure, it is clearly observed that the flexural strength of the composite is increasing with an increase in fiber loading up to the double layer of the luffa fiber. Similar observations were also reported by El-Shekeil et al.  when they worked on cocoa pod husk fiber-reinforced thermoplastic polyurethane composites. However, a decrease in the flexural strength for the triple-layer composite was observed. This may be due to poor fiber-matrix adhesion that might have promoted microcrack formation at the interface as well as nonuniform stress transfer due to fiber agglomeration within the matrix , . Based on these obtained results, for further experimentation, a double-layer (13 wt.%) LCF epoxy composite was taken, as indicated in Table 1.
Figure 4 shows the effect of alkali treatment on various concentrations (1%, 3%, 5%, and 7%) of alkali on the flexural strength of a luffa fiber (DL) composite. It is observed from the figure that the flexural strength of composites increases as the NaOH concentration increases up to 5%. With the further increase of NaOH concentration to 7%, the flexural strength is found to decrease. This might have happened due to higher fibrillation caused at this concentration level, which gives rise to larger fiber ends responsible for crack initiation. This finally lowers down the effective stress transfer at the interface, which leads to a decrease in the flexural strength . The greatest improvement in the flexural strength of the composite is observed for 5% NaOH-treated fibers, which is about 68% higher than the untreated LCF epoxy composite. This improvement in the flexural strength may be due to better interfacial adhesion between the fiber and matrix due to fibrillation of the fiber with alkali treatment. Kushwaha and Kumar  and Mishra et al.  also reported similar types of results when they worked on a bamboo-reinforced epoxy resin composite and sisal fiber-polyester composite, respectively.
The flexural strengths of the composite for double-layer luffa fibers treated with 5% (conc.) NaOH for 2, 4, and 6 h are shown in Figure 5. From this figure, it is clear that the flexural strength was found to be highest for the composites prepared with 4-h-alkali-treated fibers and was about 84.92% more than the untreated composites. Similar types of results were obtained by Ray et al.  when they worked on jute fiber-reinforced vinyl ester resin composites. Hence, in the present work, we have restricted our investigation to only 4-h alkali-treated fibers for the preparation of composites. The composites were then subjected to different environmental conditions to find out the changes in the flexural properties.
Figure 6 shows the flexural strength of untreated and 4-h alkali-treated (5% conc.) fiber-reinforced composite samples under different environmental conditions. For untreated composites when subjected to different environmental conditions such as distilled water, saline water, and subzero temperature, the flexural strength decreases by 20%, 16%, and 4%, respectively. For the composite prepared with 4-h alkali-treated fibers, the flexural strength decreases by an amount of 12.89%, 6.9%, and 2.6% when subjected to distilled water, saline water, and subzero temperature, respectively. From this, it is clear that the degradation of the flexural strength of alkali-treated composites is less than the untreated composites when subjected to different environmental conditions. The degradation in the mechanical properties of the untreated fiber composite subjected to different moist environments is found to be higher than that in the treated fiber composite. This behavior of the composite is due to the hydrophilic nature of the untreated lignocellulosic luffa fiber that contains many –OH groups which form a large number of hydrogen bonds with water molecule when composites are immersed in water and absorb more water causing degradation of the luffa fiber and interfacial bonding between the fiber and matrix. This in turn leads to poor stress transfer between the fiber and matrix and finally a decrease in the flexural properties of the composite . The mechanisms of the degradation of natural fiber-based composites that contain cellulose, hemicellulose, wax, and lignin of varying quantities are well explained by Yan et al. . Abral et al.  also reported similar types of observation, that is, a decrease in the flexural strength due to water absorption, when they worked on water hyacinth fibers-polyester composites.
3.1 Factographic analysis
Generally natural fibers are lignocellulosic in nature and mainly consist of crystalline cellulose surrounded and cemented together with hemicellulose and lignin. The external surface of the fiber cell wall is covered by wax, oil, and other surface impurities. The alkali reaction  between the luffa fiber and NaOH is as follows:
NaOH reacts with the free hydroxyl (OH) group of the luffa fiber and brings on the destruction of the cellular structure and thereby fiber splits into filaments. Figures 7A, B and 8A, B show the morphology of the fiber surface before and after treatment. From Figure 7, it is noticed that filaments of the untreated fiber surface are closely bundled together. Figure 8 shows the morphology of the fiber surface after alkali treatment. It is observed that the surface of the fiber was modified with the removal of the outer cellular layer (wax, oil, and other impurities). The packed structure split and fibrillation of the fiber structure took place that increased the surface roughness of the fiber. As the rough fiber surface produced by the removal of natural and artificial impurities due to alkali treatment that facilitates the mechanical anchoring between fiber and matrix and hence the flexural strength of the composite increases. Similar types of observation were also reported by Cao et al.  when they worked on bagasse fiber.
Figure 9A shows the micrograph of the fracture surface of composites subjected to distilled water treatment. It is evident from the figure that during fracture fibers are removed without breaking from the matrix. During alkali treatment, the dissolution of cellulose took place due to which voids were created in the fiber structure. This probably gives rise to swelling of the composite and makes the fiber weaker. This type of failure of the composites might have occurred due to the destruction of the network structure that leads to splitting of fibers into filaments.
Figure 9B shows the fracture of the composite subjected to saline water treatment. Breaking of fibers is seen, but no pulling of fiber from the matrix is visible. However, matrix cracking at some place is visible. Pulling out of fiber in saline treatment is restricted probably due to the improvement of fiber-matrix interface bonding. This might be due to less propagation of moisture through the fibril interface or may be due to ion exchange (Na+ Cl−) in between the fiber and matrix . This might be the reason for higher strength with regard to distilled water treatment.
Figure 9C shows the fracture surface of the composites subjected to subzero temperature. It clearly indicates the breaking down of net structure into bundles of small fibers. Cleavages are formed on the matrix surface when subjected to subzero temperature. Matrix cracking from fiber to fiber is seen but pulling out of fibers is restricted. This might have happened due to higher strength attainment of the matrix at this temperature (−25°C), which does not allow the fibers to come out from the matrix. The absorption of water is also less due to less intermolecular hydrogen bonding and hence higher strength of the composite when subjected to subzero temperature .
In the present work, the effect of fiber treatment with varying concentration of NaOH and treatment time on the flexural properties and the effect of moisture absorption on mechanical properties of both untreated and treated luffa cylindrical reinforced epoxy composites were studied. Based on this study, the following conclusions are drawn.
The flexural strength of the composite is found to be maximum for the composite prepared with the double-layer fiber treated with alkali (5% conc.) for 4 h, which is about 84.92% higher than the untreated fiber-reinforced composite.
During alkali treatment, the dissolution of hemicellulose gives rise to fibrillation in the fiber structure. The increase in the flexural strength of the composite is observed because alkali treatment improves the roughness of the fiber surface that in turn increases the mechanical interlocking between the fiber and polymer.
The degradation of the flexural strength of alkali-treated composites is less than that of the untreated composites when subjected to different environmental conditions such as distilled water, saline water, and subzero temperature.
It is generally observed that the mode of failure in different natural fiber-reinforced composites is due to fiber pullout from the matrix. This is because of lower adhesion between the fiber and matrix. However, in our case, the morphology of the fractured surface of alkali-treated luffa fiber-reinforced composites subjected to different environmental conditions indicated that fiber breakage is the predominant mode of failure instead of fiber pullout from the matrix. The alkali-treated fiber improved the fiber-matrix adhesion, which is mainly responsible for the increase in the flexural strength of composites.
The Luffa cylindrica epoxy composite can be used in structural applications such as door panel, window panel, partition boards, and false ceiling. In addition to improving the durability performance of the LC epoxy composite, the fiber/matrix interfacial adhesion has to be improved. The bonding between fiber and matrix could be improved by chemical modifications of fiber/matrix and the use of adhesion promoters such as (1) acetylation, (2) benzyl chloride treatment, (3) alkali treatment, (4) permanganate treatment, (5) stearic acid treatment, (6) peroxide treatment, (7) isocyanate treatment, (8) anhydride treatment, (9) silane treatment, and (10) plasma treatment , . Also the hybridization of luffa with the synthetic fiber such as carbon or glass could be effective to improve the durability performance , .
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