Effect of oxygen plasma treatment on tensile strength of date palm fibers and their interfacial adhesion with epoxy matrix

2.1 Preparation of materials

The raw date palm meshes surrounding the stems of date palm trees were collected from a farm in Boushehr, Iran. The epoxy resin E-8128 and the curing agent H-3895 were both supplied by A.C.R. Tech Co., Taiwan. The fibers were washed with tap water several times to remove dirt and sand. In order to facilitate the extraction of fibers, they were retted in warm water at 50°C for 1 h. After separation of fiber bundles into individual fibers, they were dried at room temperature for 24 h and kept in polyethylene bags until they were used. The diameter of the fibers was in the range of 310–800 μm (with the average diameter of 510 μm and coefficient of variation of 19.61%). Figure 1 shows DPFs after cleaning and extraction.

Figure 1:

Extracted date palm fibers after cleaning.

2.2 Oxygen plasma treatment

In this work, the DPFs were treated by oxygen plasma with two discharge powers (100 and 200 W) and in three different exposure times (3, 5 and 8 min). Plasma treatment was carried out at 13.56 MHz using the low-pressure plasma equipment, Junior plasma, from Europlasma Co., Belgium. The sample chamber was evacuated to 100 mTorr and maintained at this pressure during process. Then, oxygen was introduced with a flow rate of 100 sccm (standard cubic cm per minute). Seven samples were prepared by changing the fiber treatment conditions as listed in Table 3. Sample code “R” represents untreated fibers.

2.3 Characterization techniques

2.3.4 Microbond test

The microbond test was carried out as per the procedure reported by Miller et al. [25]. The treated and untreated DPFs were mounted across a cardboard window with paper tabs as shown in Figure 2. The epoxy resin was mixed with hardener in a ratio of 2:1 as recommended by the manufacturer. Microdroplets of liquid epoxy resin were delivered to each fiber using a needle and left to be cured at room temperature for 24 h. Then, the diameter of each fiber and the embedded length were measured using a Projectina microscope to compute the embedded area, which is the surface area of the fiber in contact with the resin. Figure 3 shows an enlarged image of an epoxy droplet on a date palm fiber. The microbond test was performed using the tensile testing machine EMT-3050. A pair of blades was designed to grip the droplet (Figure 4). The small gap between the blades was controlled so that only the main body of the fiber can pass through it. As illustrated in Figure 4, the blades were mounted on the lower jaws of the tester. The fiber was moved by the upper jaws between the blades at a rate of 1 mm/min until complete debonding of the droplet from the fiber occurs. Thirty specimens from each sample were tested. The average peak load was then determined and used to calculate the interfacial shear stress using Equation (4) [25]:

Figure 2:

Schematic representation of template used in the preparation of samples for microbond test.

Figure 3:

Enlarged image of epoxy microdroplet on DPF surface; (A) embedded length and (B) fiber diameter.

Figure 4:

Schematic diagram of the microbond test.

(4)τ=F/dfπle

where F is the peak load recorded during the experiment, l_e is the embedded length, and d_f is the diameter of the fiber.

3.1 Effect of plasma treatment on the surface characteristics of the fibers

The micrographs of the fibers before and after oxygen plasma treatment can be seen in Figures 5 and 6, respectively. The raw DPF is cylindrical in shape and there are some impurities as shown in Figure 5. Moreover, a waxy layer can be observed on the surface of the fiber. This layer has covered the trichomes in some regions. As can be seen in Figure 6, plasma treatment removes the waxy layer and thus improves the surface wettability of the fiber. It is seen that by removing the waxy layer, the trichomes appear in more regions. Furthermore, by increasing the exposure time to 8 min, the trichomes are destroyed and the grooves underneath them appear in some regions. In some studies on the effect of alkaline concentration on the DPF, similar observations have been reported [2], [10]. As can be seen in Figure 6C,D, increasing the plasma power to 200 W, leads to higher surface roughness on the fibers. This is attributed to the etching effect caused by the bombardment of the oxygen plasma species on the fiber surface [15], [26].

Figure 5:

Micrographs of untreated DPFs.

Figure 6:

Micrographs of plasma-treated DPFs under different conditions: (A) A3, (B) A8, (C) B3, and (D) B8.

3.2 Attenuated total reflectance-Fourier transform infrared spectroscopy analysis

The ATR-FTIR spectra of raw and plasma-treated DPFs are shown in Figure 7. In these spectra very strong and broad bands in the range of 3000–3600 cm⁻¹ are due to stretching vibrations of O–H groups. The bands in the range of 2800–3000 cm⁻¹ are related to stretching vibrations of C–H in saturated hydrocarbons. The peak at around 1726 cm⁻¹ is due to carbonyl (C=O) stretching of acetyl groups of hemicellulose or ester linkages of carboxyl groups of lignin and hemicellulose [13], [15], [27]. After plasma treatment, decreasing intensity of this peak in the spectra may be due to the decrease in hemicellulose content. The bands at 1510 and 1604 cm⁻¹ are attributed to aromatic skeleton vibrations assigned to lignin [13], [28], [29], [30]. In the spectra of the treated fiber, these bands shifted towards 1505 and 1596 cm⁻¹, respectively. This is possibly due to the decrease in lignin components. After plasma treatment, intensity of the band at 1420 cm⁻¹ has decreased. In general, the bands in the range 1400–1430 cm⁻¹ are related to the C–H deformation for methoxy group in the structure of lignin [30]. The band near 1369 cm⁻¹ is possibly due to a CH₃ bending that in the treated fiber spectra is shifted towards 1374 cm⁻¹ [2], [31]. The peak at 1239 cm⁻¹ may be attributed to C–O–C stretching vibration and O–CH deformation from phenolic group [28], [30]. After plasma treatment, this band was observed at 1245 cm⁻¹. The strong band about 1031 cm⁻¹ is ascribed to the C–O and C–OH stretching vibration which belongs to cellulose [26]. In the treated fiber spectra, this peak is near 1026 cm⁻¹.

Figure 7:

FTIR spectra of (A) raw and treated DPF with discharge power of 100 W and (B) exposure time of 5 min.

3.3 Single fiber tensile properties

Average tensile strength and modulus values of untreated (raw) and treated single fibers are compared in Figure 8. It is seen that plasma treatment improves both tensile strength and Young’s modulus. According to Morent et al. [32] changing the physical properties of yarns and fabrics, and more in particular their increase in tensile strength, bursting strength and wear resistance are some positive effects caused by plasma treatment on textile structures. The significant increase in tensile strength and Young’s modulus of banana fibers after plasma treatment is also reported by Oliveira et al. [19]. However, it should be noted that in some previous works the decrease in mechanical properties of natural fibers after plasma treatment was reported. It was attributed to the heat generated during the plasma treatment which causes dehydration of the fibres, the etching effect of plasma which degrades their macrofibril structure and generation of micropits and voids on the surface of fibers [33], [34].

Figure 8:

Treatment conditions: two discharge powers (100 W and 200 W) and three exposure times (3, 5 and 8 min). (A) Tensile strength and (B) Young’s modulus of treated and untreated DPFs.

The maximum tensile strength and Young’s modulus values are obtained for A8 and B3 samples, respectively. The values of tensile strength of the treated fibers at 200 W power are slightly lower than samples at 100 W power. Similar observations are seen in Young’s modulus values for 5 and 8 min treatment time. It may be due to producing more cracks and damages on the surface of the fiber with increasing intensity of plasma, as shown in Figure 6D. One-way ANOVA analysis showed significant differences between untreated and treated samples. However, the effects of plasma power and exposure time on tensile properties of DPF are not found significant.

3.4 Weibull distribution of the tensile strength of DPF

Figure 9 shows the Weibull distribution of the tensile strength of raw and treated DPF. The Weibull modulus values for natural date palm fiber are much lower compared to that of synthetic fibers such as glass fiber [12]. This could be due to the nonhomogeneous nature of natural fibers and irregular fiber diameter. On the other hand, the Weibull modulus of DPFs after plasma treatment has not increased significantly. This indicates that plasma treatment could not decrease the variability in fiber strength due to the nature of fibers.

Figure 9:

Weibull distribution of the tensile strength of DPFs: (A) R, (B) A1, (C) A2, (D) A3, (E) B1, (F) B2, and (G) B3.

3.5 Microbond test analysis

It is well known that mechanical properties of composites are largely controlled by the interface, which is usually required to be strong in polymer matrix composites, thus transferring load from the matrix to the fibers efficiently [14]. The IFSS values which were calculated using Equation (1) are shown in Figure 10. It can be seen that plasma treatment has increased the IFSS values of DPFs. The maximum IFSS value is obtained at 100 W and 5 min. This is attributed to the cleaning process and etching effect of plasma species that result in fiber-matrix adhesion improvement. The surface of the fiber has roughened and the waxy layer and impurities have been removed. Plasma treatment improves the fiber-matrix adhesion largely by increasing friction and mechanical interlocking between the fiber and the matrix [15]. According to the ANOVA analysis, there is a significant difference between the IFSS values of treated and untreated samples. However, the effects of plasma power and exposure time on IFSS values of DPF are not found significant. Figure 11 shows a treated specimen after microbond test.

Figure 10:

IFSS values of the samples.

Figure 11:

Micrograph of debonded epoxy microdroplet on a treated fiber surface.

Plasma treatment positively affects the strength of DPF and its interfacial adhesion with the matrix. Reducing lignin and hemicellulose content, etching and cleaning surface, and increasing mechanical interlocking between fibers and matrix can improve fiber-matrix adhesion.