Effect of coir fiber reinforcement on mechanical properties of vulcanized natural rubber composites

2.1 Materials

The coir fiber used as reinforcement was supplied by Shri Balaji Pressing Unit, Bhosari MIDC, local industry in Pune, Maharashtra, India. The NR used in this investigation was obtained from Balaji Rubber Lining Industry Bhosari MIDC, local industry in Pune, Maharashtra, India. All the materials used were of commercial grade. Figure 1 shows the structures of coir fiber and vulcanized NR. Initial properties of the vulcanized NR are shown in Table 2.

Figure 1:

Optical micrograph of: (A) Raw coir fiber. (B) Raw vulcanized NR.

2.2 Fiber preparation

Coir pith present in the fibers was removed before alkali treatment. The fibers were chopped into approximately 10 mm length and then soaked in 5% NaOH for 48 h. The fibers absorbed most of the solution. The fibers were taken out and washed rapidly under tap water several times and allowed to dry out in air. Figure 2 shows the coir fibers soaked in 5% NaOH solution.

Figure 2:

Coir fibers soaked in 5% NaOH solution.

2.3 Preparation of composites

The composites were prepared on two roll laboratory mill shown in Figure 3. All the machine parameters were kept same for both NR and coir fiber-reinforced NR. The nip gap was maintained at 2 mm. The NR was milled and passed through the roller several times to ensure no air was present in the rubber. The following chemicals were added during the milling of vulcanized NR:

1. Zinc, 10 g

2. Mercaptobenzodithiazole, 10 g

3. Tetramethylthiurum, 5 g

4. Sulfur, 5 g

Figure 3:

Two roll laboratory mill.

The fibers were mixed at the last of the milling process for better fiber alignment. Two different types of sheets were made of the mill. One with reinforcement and the other without reinforcement, then the samples were made of sheets (150×150×2 mm³). The samples were then allowed to cure at room temperature. Initial properties of coir fiber-reinforced vulcanized NR composites are shown in Table 3.

3.1 Initial tensile strength and elongation at break (%)

Mechanical properties such as maximum load, tensile strength, and elongation at break (%) were investigated for both samples without any kind of aging. The specimens were cut into dog bone shape for tensile strength investigation. Both types of samples were analyzed in the same universal testing machine (UTM) according to ATSMD 3039 standard.

The stress-strain variation was recorded by an extensometer. The specimens with coir fiber showed better performance. The tensile strength was increased by 34.70%. Geethamma et al. [3] studied the effect of coir fiber reinforcement on NR composite and calculated various mechanical properties. Elongation at break (%) was analyzed for different mixtures of fiber/rubber. The increase in maximum load was recorded to be 83.32%. Figure 4 shows the initial mechanical properties of vulcanized NR, and Figure 5 shows the initial properties of coir fiber-reinforced vulcanized NR composite. From Tables 2 and 3, it can be concluded that the elongation at break increased because of better fiber/rubber adhesion. The increase in elongation at break (%) was calculated as 8.44%, whereas the percentage increase in the tensile strength was recorded as 34.70 (%).

Figure 4:

Initial mechanical properties of vulcanized NR.

Figure 5:

Initial properties of coir fiber-reinforced vulcanized NR composite.

3.2 Effect of cold resistance at −60°C for 8 h

Multiple samples were made for a better understanding of the behavior of the composite material in different working conditions. Three samples of vulcanized NR and two samples of coir fiber-reinforced vulcanized NR composites were analyzed in cold resistance test. The samples were kept at −60°C for 8 h. Curing was conducted at room temperature. The samples were hardened a little by cold temperature. All the samples were tested in UTM.

Maximum load (N), tensile strength (MPa), and elongation at break (%) were recorded during the test. The stress-strain curve depicts the behavior of material under maximum load (N). Reinforced NR specimens showed higher mechanical properties as compared with raw NR samples. The maximum load was increased by 121.162%, tensile strength by 90.06%, and elongation at break by 84.65%. The coir fiber enhanced the properties of NR for working under cold temperature. The mechanical properties of vulcanized NR after cold resistance at −60°C for 8 h are shown in Table 4. Figure 6 shows the stress-strain curve of vulcanized NR after cold resistance at −60°C for 8 h. Table 5 shows the mechanical properties of coir fiber-reinforced vulcanized NR composite after cold resistance at −60°C for 8 h. Figure 7 displays the stress-strain curve of coir fiber-reinforced vulcanized NR composite after cold resistance at −60°C for 8 h. On comparing these parameters with initial parameters given in Tables 2 and 3, it can be concluded that because of cold resistance, the properties were reduced.

Figure 6:

Stress-strain curve of vulcanized NR after cold resistance at −60°C for 8 h.

Figure 7:

Stress-strain curve of coir fiber-reinforced vulcanized NR composite after cold resistance at −60°C for 8 h.

3.3 Effect of water resistance at ambient temperature for 8 h

To study the effect of water on fibers, the specimens of both types were kept in water for 8 h at ambient temperature. The graphs of both samples were analyzed. Table 6 shows the mechanical properties of vulcanized NR after water resistance at ambient temperature for 8 h. Table 7 highlights the mechanical properties of coir fiber-reinforced vulcanized NR composite after water resistance at ambient temperature for 8 h. Figure 8 shows the stress-strain curve of vulcanized NR after water resistance at ambient temperature for 8 h, and Figure 9 displays the stress-strain curve of coir fiber-reinforced vulcanized NR composite after water resistance at ambient temperature for 8 h.

Figure 8:

Stress-strain curve of vulcanized NR after water resistance at ambient temperature for 8 h.

Figure 9:

Stress-strain curve of coir fiber-reinforced vulcanized NR composite after water resistance at ambient temperature for 8 h.

The results suggested that the reinforced specimens showed higher properties as compared with raw vulcanized NR specimens. For raw vulcanized NR, the maximum load was found out to be 5.635 N (average). For coir fiber-reinforced composite, the maximum load recorded was 8.46 N (average). Thus, it can be concluded that the coir fiber act as a reinforcement for water-based application. The maximum load was increased by 50.13%, tensile strength by 31.428%, and elongation at break by 94.065%. It can be seen that these values of maximum loads are still smaller than the values shown in Tables 2 and 3. Hence, water soaking reduced the fiber/rubber adhesion and decreased the amount of maximum load (N) in both samples. Still, after the effect of water resistance, the reinforced samples showed better results. The coir fiber also has high lignin content.

3.4 Effect of heat resistance at 80°C for 4 h

Three specimens of both NR and coir fiber-reinforced vulcanized NR were examined under this test. NR on its typical application can face temperature variations. It is important to understand its behavior after the heat aging process. After heat aging, the specimens were tested on UTM, and their stress-strain relationship was analyzed. It can be seen that after heat aging, the performance is reduced. Rattanasom and Prasertsri [5] studied the effect of thermal aging on different NR vulcanizates. The strength of samples was decreased after heat aging. Table 8 shows the mechanical properties of vulcanized NR after heat resistance at 80°C for 4 h, and Table 9 displays the mechanical properties of coir fiber-reinforced vulcanized NR composite after heat resistance at 80°C for 4 h. Figure 10 shows the stress-strain curve of vulcanized NR after heat resistance at 80°C for 4 h. Figure 11 shows the stress-strain curve of coir fiber-reinforced vulcanized NR composite after heat resistance at 80°C for 4 h.

Figure 10:

Stress-strain curve of vulcanized NR after heat resistance at 80°C for 4 h.

Figure 11:

Stress-strain curve of coir fiber-reinforced vulcanized NR composite after heat resistance at 80°C for 4 h.

Comparing the results of Tables 8 and 9 with the initial properties given in Tables 2 and 3, it can be concluded that the performance of specimens was reduced because of heat aging. It made the fiber/rubber adhesion weaker. The reinforced samples showed better performance after the heat aging. The maximum load was increased by 80.53%, tensile strength by 26%, and elongation at break by 90.85%. Tensile strength was recorded least in this case. It can be concluded that the behavior of composites was least in this case and optimum in the case of cold aging resistance tests. For the quantitative analysis of reinforcement, the percentage increase has been calculated. Table 10 shows the percentage increase in each property of the composite.