The effect of sodium hydroxide treatment and fiber length on the tensile property of coir fiber-reinforced epoxy composites

2.1 Materials

The coconut husks were soaked in tap water in a container for 5 months. This process is called retting, which can partially decompose the pulp on the shell, allowing the fiber to be easily removed from the husk. After retting, the husks were beaten with a hammer. The coir fibers were removed from the shell and separated with a comb. After drying at room temperature, the coir fibers were combed in a carding frame to further separate the fibers into an individual state. Epoxy LY556 was used as a matrix with HY951.

2.2 Coir fiber treatment

The coir fibers were treated in the NaOH solution at room temperature (27–29°C) with various concentrations of 2%, 4%, 6%, 8%, and 10%. Each group of the coir fiber was treated for 10 days. The treated fiber was washed with water to remove the excess NaOH sticking to the fibers. Final washing was carried out with distilled water and the fibers were dried in hot air. Finally, the fibers were cut into 10-, 20-, and 30-mm lengths for molding the composites.

2.3 Preparation of coir fiber-reinforced epoxy composite

The coir fiber-reinforced epoxy composites were manufactured using a mold box of size 300 mm×300 mm×3 mm. The fabrication of the composite material was carried out through the hand lay-up technique. The top and bottom surfaces and the walls of the mold were coated with remover and allowed to dry. The chopped fibers were mixed manually with epoxy resin. The epoxy resin and coir fiber mixture was transferred to the mold and the mold was closed; then it was pressed in a compression testing machine and left for 24 h for curing. The setup is illustrated in Figure 1. After the curing process, the samples were cut into the required size prescribed in the ASTM standards.

Figure 1

Mold preparation setup.

2.4 Tensile test

The fiber strength testing of the coir fiber was carried out by using the model INSTRON 5500R single fiber tensile strength tester (SITRA, Coimbatore, TN, India). Thirty samples were tested and the average value is reported. The distance between the two clamps before the test was measured as 60 mm and displacement rate was set as 10 mm/min as most of the researchers have used this strain rate. It was selected in order to compare our results with that available in the literature. Images of the coir fiber before and after the alkali treatment were taken by using a scanning electron microscope (Model KYKY 2800, Amirtha University, Coimbatore, TN, India). The diameter of the coir fiber before and after the alkali treatment was measured using an image analyzer from SITRA (Coimbatore, TN, India).

From the fabricated coir fiber-reinforced epoxy composite, test specimens were cut as per ASTM D3039 and tested for tensile strength using a computerized universal testing machine (INSTRON model 3369). The setup is illustrated in Figure 2. The distance between the two clamps before the test was maintained at 150 mm, and the testing speed was 2 mm/min, the value normally used. Five samples were tested and the average value is reported.

Figure 2

Universal tensile tester model Instron 3369.

3.1 Image of the coir fiber

Figures 3–5 represent the surface appearance of the coir fiber before and after the alkali treatment, respectively. Figure 3 shows that the surface of the coir fiber is covered with a layer of substance, which may include pectin, lignin, and other impurities. The surface is not smooth, but spread with nodes and irregular strips. Figures 4 and 5 represent the surface of the coir fiber after NaOH treatment. The figures show that most of the lignin and pectin are removed, resulting in a rougher surface. There are rows of pits on the surface. These would increase the mechanical bonding between the coir fiber and the matrix in the composite fabrication.

Figure 3

Surface appearance of the coir fiber before alkali treatment.

Figure 4

Surface appearance of the coir fiber after 2% NaOH treatment.

Figure 5

Surface appearance of the coir fiber after 4% NaOH treatment.

3.2 Fiber tensile strength

The tensile strength of NaOH-treated fibers is listed in Table 1 for comparison. The tensile strength of the untreated fiber is also included in the table because of the incredible variation of the fiber fitness; the tensile strength of the fiber varies greatly. To observe the influence of the coir fiber variation, 30 samples were used for each group. Table 1 represents the tensile strength of the coir fiber before and after the alkali treatment. The results revealed that a decreasing trend is seen in the fiber tensile strength with increasing alkali density. The difference in the tensile strength among each group was proved to be significant. An increase in alkali concentration causes a greater amount of lignin and pectin to leach out; this would be detrimental to fiber strength. Figure 6 represents the comparison of the coir fiber diameter before and after the alkali treatment. One may note that a decreasing trend is seen in the fiber diameter with increase in alkali density. Figure 6 proved that the NaOH treatment reduces the fiber strength drastically because the pH value of NaOH is high. After the alkali treatment, a rough fiber surface resulted; this might improve the bonding ability of the fiber with the matrix.

Figure 6

Coir fiber diameter before and after NaOH treatment.

3.3 Tensile strength of the coir fiber- reinforced epoxy composite

Figure 7 shows the appearance of the specimen of the coir fiber-reinforced epoxy composites. The weight fraction of the coir fiber in the composite was 30%. Each piece of the fabricated board was cut into sets of specimens. Results of the tensile strength tests of coir fiber-reinforced epoxy composites with and without alkali treatment are summarized in Tables 2 and 3.

Figure 7

(A) Specimen for impact test (4% NaOH and 20-mm length). (B) Specimen for flexural test (4% NaOH and 10-mm length). (C) Specimen for tensile test (4% NaOH and 30-mm length).

3.4 Effect of fiber length on the tensile property of coir fiber-reinforced epoxy composites

Joseph et al. [19] studied the tensile properties of short sisal fiber/polyethylene composites in relation to processing methods and the effects of fiber content, length, and orientation. For this experiment, it was shown that the chopped fiber distribution in epoxy is random, so the fiber could not hold the load when the matrix was transferred. Arib et al. [20] compared the experimental and theoretical tensile strengths for pineapple leaf fiber-reinforced polypropylene composites and found that the equation for rule of mixture fails to provide a good fit, and the discrepancy increases with the increase in fiber volume fraction. The fiber is not perfectly aligned, and the presence of voids in the composites may also be the factor contributing to the lower experimental values. According to Baiardo et al. [21], the mechanical properties of short fiber-reinforced composites are expected to depend on (i) the intrinsic properties of matrix and fibers, (ii) aspect ratio, content length distribution, and orientation of the fibers in the composites, and (iii) fiber-matrix adhesion that is responsible for the efficiency of load transfer in the composites. Table 3 shows, as expected, that the tensile strength gradually increases with increase in fiber length, reaching a maximum at about 30 mm (13.782 MPa). The reason for this increase in the tensile properties of these composites is that the chemical bonding between the fiber and the matrix may be too strong to transfer the tensile load.

3.5 Effect of alkali treatment on the tensile property of coir fiber-reinforced epoxy composites

We may note the significant difference in the tensile strength between the alkali-treated and the untreated coir fiber-reinforced epoxy composites. The tensile strength of the composites when using the alkali-treated coir fibers is usually greater because after alkali treatment, most of the impurities like lignin and pectin that covered the fiber surface were removed, which improved the fiber adhesive character in combination with the matrix. Table 3 clearly shows the important difference in the tensile strength of NaOH-treated coir fiber-reinforced epoxy composites with varying fiber length and chemical concentration. There is no significant difference in the tensile strength in the case of 4% and 6% NAOH concentration. To achieve a composite with better strength, NaOH concentration of 4% may be recommended for economical considerations. Significant difference is noted for the elongation at break values between the composites made by using the alkali-treated and the untreated coir fiber. The higher elongation at break for the alkali-treated coir fiber composites indicates that the removal of the lignin and pectin improved the elasticity of the coir fiber.