Tensile Properties and Microstructure of Single-Cellulosic Bamboo Fiber Strips after Alkali Treatment

The study systematically explored the effect of alkali concentration and soaking time on the microstructure and tensile properties of single-cellulosic Buluh Semantan. Scanning electron microscopy and tensile tests were conducted to determine the effects of different alkali treatments on the properties of the single-cellulosic bamboo fibers. In particular, the effects of NaOH concentration and soaking time on the tensile properties of the single-cellulosic bamboo fiber were investigated. The single-cellulosic bamboo fiber was immersed in 2, 4, 6, and 8 wt.% aqueous NaOH solutions for soaking times of 1, 3, 6, 12, 18, and 24 h. The tensile properties of the fiber increased after each alkali treatment. The alkali concentration and soaking time significantly affected the fiber properties. The ultimate tensile strength of the single-cellulosic Buluh Semantan treated with 2 wt.% NaOH for 12 h decreased to 214 MPa relative to the fibers that experienced water retting. The highest tensile strength herein was 356.8 MPa for the single-cellulosic fiber that was soaked for 12 h in 4 wt.% NaOH. Comparatively, the tensile strength of the single-cellulosic bamboo fiber that was soaked for 12 h in 8 wt.% NaOH was 234.8 MPa. The tensile modulus of the single-cellulosic fiber was 12.06 GPa after soaking in 8 wt.% NaOH for 18 h, indicating that a strong alkali treatment negatively affected the stiffness and suitability for use of the fibers in applications. The topography of the fiber surface became much rougher after the alkali treatments due to the removal of hemicellulose and other surface impurities. The alkali treatments substantially changed the morphology of the fiber surface, suggesting an increase in wettability.


Introduction
Currently, due to the increasing awareness of environmentally friendly biomaterials, natural fibers have gained widespread attention as a prospective alternative to synthetic fibers, for their composite applications in the construction and automobile industries [1,2]. The benefits of using natural lignocellulose fibers in place of traditional inorganic fibers made from fossil fuels relate to their specific properties, such as their light weight, low cost, high specific properties, low density, good thermal properties, eco-friendliness, and biodegradability [1]. They can also be used as a replacement for glass fibers during composite manufacturing [3,4]. Indeed, far-ranging studies on natural fibers, including sisal, flax, kenaf, and bamboo, demonstrate that natural fibers possess amazing potential as an effective reinforcing phase in composite materials [1]. The NF exhibits poor resistance to moisture, thus leading to high water absorption and poor mechanical properties and dimensional stability. Alkali treatment is common fiber treatment, to provide fiber/matrix adhesion and good influence on the modulus of elasticity was significant. Wang et al. [1] investigated the influence of alkali concentration on the thermomechanical properties of bamboo fibers. The alkali concentrations were 1, 4, and 7 wt.% NaOH solutions at room temperature, and the treatment time was 1 h. Compared with that for the untreated fibers, there was an improvement in the average tensile strength of the fibers after an alkali treatment at a concentration of 4 wt.%, which can be attributed to the formation of an effective contact area being available for excellent bonding with the matrix after treatment, whereas the high concentration used in the treatment caused a decrease in the tensile strength. This paper investigates the effects of the alkali concentration and soaking time on bamboo strips. The morphologies of the fracture surfaces were examined by using scanning electron microscopy (SEM).

Materials
The bamboo fiber species used in this study was Gigantochloa scortechinii, which is commonly known as semantan bamboo and was obtained from Pahang, Malaysia. This fiber was subjected to a water-retting process before being supplied to the laboratory. The bamboo fibers were then dried under the sun before they were manually extracted. The strips were manually extracted from the bamboo culm after water retting. All fibers were obtained from the outer layer of the bamboo stem, and to ensure consistency, only fibers from the same bunch were used. The sodium hydroxide (NaOH) used in this study was supplied by Mylab company, Malaysia, in a 1 kg container with a small pallet shape.

Fiber Diameter
The bamboo fiber was selected and separated from the bundle fiber and finally separated into single fibers. The bamboo fiber diameter was measured at 4.5 mm intervals, along its length, using a Leica Stereo Microscopic video analyzer 2000 (Model 250 D, Selangor, Malaysia). The mean diameter of single bamboo fiber strips was measured from the average of ten fibers, in which the diameter for every fiber was the average value of the diameter measured ten times at 4.5 mm intervals along the fiber length. The measurement was replicated three times for each treatment condition.

Alkaline Treatment
For the first step, the bamboo fiber strips were subjected to a water-retting process before they were supplied to the laboratory. The selected Buluh semantan strips were cut into an average length of 240 mm, with a diameter of 95.12 µm, and soaked in an alkaline aqueous solution, at different soaking times, at room temperature. Mishra et al. [28] reported that the NaOH concentration can vary from 1 to 10 wt.%. The researchers who published [23,29] reported that high concentrations of NaOH can worsen the mechanical properties of bamboo fiber strips. In this work, four different alkaline concentrations were prepared (2, 4, 6, and 8 wt.%). After the treatment, the bamboo fiber strips were washed, using fresh water, to remove the alkali content from them. A litmus paper was used to confirm that the fibers were neutralized. This was followed by drying under the sun for approximately two days before they were dried in an oven at a temperature of 60 • C for 24 h. The dried bamboo fiber strips were then kept in a sealed plastic bag, to avoid water absorption from the atmosphere before tensile testing.

Fabrication of the Samples
Before folding the adhesive tapes, the bamboo fiber strips were further glued with a cyanoacrylate adhesive liquid, to prevent the fibers from slipping during the tests and to enhance the adhesion of the fibers to the tape surface. All samples were fixed to a gauge length of 210 mm. Figure 1 shows a schematic diagram of the sample preparation method for the testing. The samples were then clamped tightly on a 2 kN Zwick/Roell machine (Zwick company, Berlin, Germany) ( Figure 1e). The tensile test was carried out according to the ASTM-D 3822-07 standard. The crosshead speed was set to 1.5 mm/min. The tests were performed on fibers with 2, 4, 6, and 8 wt.% NaOH treatments. Seven samples from each condition were tested, and the average values were reported by calculating the maximum of five values, because we excluded from analysis the samples that broke near the edge of the clamps.
Fibers 2020, 8, x Seven samples from each condition were tested, and the average values were reported by calculating the maximum of five values, because we excluded from analysis the samples that broke near the edge of the clamps. .

Fiber Morphology
The surface morphology of the bamboo strips after various alkali treatment conditions was examined with SEM (ZEISS/Carl ZEISS Sdn. Bhd.). The fractured surface of the tensile test samples was used with the SEM observations on an instrument operated at an accelerating voltage of 20 kV.  Figure 3a-c shows the individual cellular elements on the clean fiber surface after alkali treatment and relatively rough surfaces were observed on the single-cellulosic bamboo fibers. The researchers [30,31] reported that the surface roughness increases with increasing aqueous NaOH concentration. Rough surface morphologies are typical for treated fibers because of the removal of hemicellulose and other surface impurities. Thus, alkali treatment can result in substantial changes in the fiber surface morphology, which can lead to an increase in the fiber wettability.

Fiber Morphology
The surface morphology of the bamboo strips after various alkali treatment conditions was examined with SEM (ZEISS/Carl ZEISS Sdn. Bhd.). The fractured surface of the tensile test samples was used with the SEM observations on an instrument operated at an accelerating voltage of 20 kV.  Figure 2a-c shows the SEM micrographs of the single-cellulosic bamboo fiber strips. The fractured cellulose fibers were observed for the low-alkali-concentration sample, suggesting an unsuitable concentration. Figure 3a-c shows the individual cellular elements on the clean fiber surface after alkali treatment and relatively rough surfaces were observed on the single-cellulosic bamboo fibers. The researchers [30,31] reported that the surface roughness increases with increasing aqueous NaOH concentration. Rough surface morphologies are typical for treated fibers because of the removal of hemicellulose and other surface impurities. Thus, alkali treatment can result in substantial changes in the fiber surface morphology, which can lead to an increase in the fiber wettability.  The surface materials started to dissolve after treatment with the strong alkali concentrations (Figure 4a-c). Zhang et al. [26] reported that a decrease in fiber strength after treatment with high alkali concentrations can be attributed to the partial removal of cellulose. Holes and grooves were also observed, indicating the capability of the alkali treatment to remove very large amounts of soluble substances from the layers. Hence, an excessive alkali treatment and soaking time can damage the strength of singlecellulosic bamboo fiber strips, suggesting that these variables are also unsuitable for treating bamboo fibers ( Figure 5).   The surface materials started to dissolve after treatment with the strong alkali concentrations (Figure 4a-c). Zhang et al. [26] reported that a decrease in fiber strength after treatment with high alkali concentrations can be attributed to the partial removal of cellulose. Holes and grooves were also observed, indicating the capability of the alkali treatment to remove very large amounts of soluble substances from the layers. Hence, an excessive alkali treatment and soaking time can damage the strength of singlecellulosic bamboo fiber strips, suggesting that these variables are also unsuitable for treating bamboo fibers ( Figure 5). The surface materials started to dissolve after treatment with the strong alkali concentrations (Figure 4a-c). Zhang et al. [26] reported that a decrease in fiber strength after treatment with high alkali concentrations can be attributed to the partial removal of cellulose. Holes and grooves were also observed, indicating the capability of the alkali treatment to remove very large amounts of soluble substances from the layers.  The surface materials started to dissolve after treatment with the strong alkali concentrations (Figure 4a-c). Zhang et al. [26] reported that a decrease in fiber strength after treatment with high alkali concentrations can be attributed to the partial removal of cellulose. Holes and grooves were also observed, indicating the capability of the alkali treatment to remove very large amounts of soluble substances from the layers. Hence, an excessive alkali treatment and soaking time can damage the strength of singlecellulosic bamboo fiber strips, suggesting that these variables are also unsuitable for treating bamboo fibers ( Figure 5).  Hence, an excessive alkali treatment and soaking time can damage the strength of single-cellulosic bamboo fiber strips, suggesting that these variables are also unsuitable for treating bamboo fibers ( Figure 5).    Figure 7 shows the tensile-strength results for the fibers treated with different alkali concentrations and different soaking times, relative to those that underwent water retting. The 4 wt.% NaOH treatment for 12 h had the best effect on the tensile strength of the bamboo fiber strips, whereas the high alkali concentration (i.e., 8 wt.% treatment) for different soaking times yielded the lowest result herein. Similar observations have been reported by [1], who investigated the influence of alkali concentration on tensile properties of bamboo fiber. The fibers were alkalized with 1, 4, and 7 wt.% NaOH solution for 1 h. They found that the maximum tensile strength is achieved at 4 wt.% alkali treatment as a result of the low-alkali-concentration removal of hemicellulose, lignin, and other impurities that makes those fibrils more capable of rearranging themselves along the direction of tensile deformation, resulting in better packing of cellulose chains. Moreover, leaching of these noncellulosic materials makes a rough topography that appears on the fiber surface, offering better interfacial adhesion with the matrix, causing an improvement in the fiber strength. They also found that the tensile strength decreased by 4% when they increased the alkali concentration to 7%, as a result of weaker fiber with a high alkali concentration. The authors of [32] studied the soaking time effect on tensile strength of single pineapple leaf fiber. Their results showed that a 3-hour soaking time is insignificant for increasing tensile strength for both 3% and 6% NaOH treatment. However, a 6-hour soaking time at 6 wt.% NaOH is able to remove hemicellulose and lignin and enhance surface area adhesion for fiber/matrix. Likewise, a 12-hour soaking time and 6 wt.% NaOH treatment did not      Figure 7 shows the tensile-strength results for the fibers treated with different alkali concentrations and different soaking times, relative to those that underwent water retting. The 4 wt.% NaOH treatment for 12 h had the best effect on the tensile strength of the bamboo fiber strips, whereas the high alkali concentration (i.e., 8 wt.% treatment) for different soaking times yielded the lowest result herein. Similar observations have been reported by [1], who investigated the influence of alkali concentration on tensile properties of bamboo fiber. The fibers were alkalized with 1, 4, and 7 wt.% NaOH solution for 1 h. They found that the maximum tensile strength is achieved at 4 wt.% alkali treatment as a result of the low-alkali-concentration removal of hemicellulose, lignin, and other impurities that makes those fibrils more capable of rearranging themselves along the direction of tensile deformation, resulting in better packing of cellulose chains. Moreover, leaching of these noncellulosic materials makes a rough topography that appears on the fiber surface, offering better interfacial adhesion with the matrix, causing an improvement in the fiber strength. They also found that the tensile strength decreased by 4% when they increased the alkali concentration to 7%, as a result of weaker fiber with a high alkali concentration. The authors of [32] studied the soaking time effect on tensile strength of single pineapple leaf fiber. Their results showed that a 3-hour soaking time is insignificant for increasing tensile strength for both 3% and 6% NaOH treatment. However, a 6-hour soaking time at 6 wt.% NaOH is able to remove hemicellulose and lignin and enhance surface area adhesion for fiber/matrix. Likewise, a 12-hour soaking time and 6 wt.% NaOH treatment did not  Figure 7 shows the tensile-strength results for the fibers treated with different alkali concentrations and different soaking times, relative to those that underwent water retting. The 4 wt.% NaOH treatment for 12 h had the best effect on the tensile strength of the bamboo fiber strips, whereas the high alkali concentration (i.e., 8 wt.% treatment) for different soaking times yielded the lowest result herein. Similar observations have been reported by [1], who investigated the influence of alkali concentration on tensile properties of bamboo fiber. The fibers were alkalized with 1, 4, and 7 wt.% NaOH solution for 1 h. They found that the maximum tensile strength is achieved at 4 wt.% alkali treatment as a result of the low-alkali-concentration removal of hemicellulose, lignin, and other impurities that makes those fibrils more capable of rearranging themselves along the direction of tensile deformation, resulting in better packing of cellulose chains. Moreover, leaching of these non-cellulosic materials makes a rough topography that appears on the fiber surface, offering better interfacial adhesion with the matrix, causing an improvement in the fiber strength. They also found that the tensile strength decreased by 4% when they increased the alkali concentration to 7%, as a result of weaker fiber with a high alkali concentration. The authors of [32] studied the soaking time effect on tensile strength of single pineapple leaf fiber. Their results showed that a 3-hour soaking time is insignificant for increasing tensile strength for both 3% and 6% NaOH treatment. However, a 6-hour soaking time at 6 wt.% NaOH is able to remove hemicellulose and lignin and enhance surface area adhesion for fiber/matrix. Likewise, a 12-hour soaking time and 6 wt.% NaOH treatment did not show much fluctuation and fall drastically to the lowest point, as the high alkali concentration may decrease tensile strength because it removes impurities, causes lignocellulosic degradation, and ruptures the fiber surface.  As shown in Figure 8, the trend of the tensile modulus is similar to that of the tensile strength, and the tensile modulus increased smoothly up to the 6 wt.% concentration; moreover, the effect of the treatment time seemed to be significant. The optimum increase in the tensile modulus was achieved at a 6 wt.% concentration and 12 h of soaking. Similar observations were reported by [3]. They found the tensile modulus of banana fiber has been increase by 38% and 102% for 10 and 15 wt.% alkali concentration, respectively, and the same studies reported a decreased in the tensile modulus when increase alkali concentration to 20 wt.%; due to degradation of cellulose, there is a reduction in tensile strength tenacity by 10% and tensile modulus by 66%. Reddy and Gowda [33] reported that alkali treatment of sisal fibers improved the cellulose crystallinity and removed impurities, such as hemicellulose and lignin. The sisal fibers became much more ductile after the removal of some of the impurities, resulting in a substantially increased fiber stiffness due to the increased crystallinity of the hard cellulose. The enhanced tensile modulus of bamboo strips fiber is also due to the same factor.  As shown in Figure 8, the trend of the tensile modulus is similar to that of the tensile strength, and the tensile modulus increased smoothly up to the 6 wt.% concentration; moreover, the effect of the treatment time seemed to be significant. The optimum increase in the tensile modulus was achieved at a 6 wt.% concentration and 12 h of soaking. Similar observations were reported by [3]. They found the tensile modulus of banana fiber has been increase by 38% and 102% for 10 and 15 wt.% alkali concentration, respectively, and the same studies reported a decreased in the tensile modulus when increase alkali concentration to 20 wt.%; due to degradation of cellulose, there is a reduction in tensile strength tenacity by 10% and tensile modulus by 66%. Reddy and Gowda [33] reported that alkali treatment of sisal fibers improved the cellulose crystallinity and removed impurities, such as hemicellulose and lignin. The sisal fibers became much more ductile after the removal of some of the impurities, resulting in a substantially increased fiber stiffness due to the increased crystallinity of the hard cellulose. The enhanced tensile modulus of bamboo strips fiber is also due to the same factor.  As shown in Figure 8, the trend of the tensile modulus is similar to that of the tensile strength, and the tensile modulus increased smoothly up to the 6 wt.% concentration; moreover, the effect of the treatment time seemed to be significant. The optimum increase in the tensile modulus was achieved at a 6 wt.% concentration and 12 h of soaking. Similar observations were reported by [3]. They found the tensile modulus of banana fiber has been increase by 38% and 102% for 10 and 15 wt.% alkali concentration, respectively, and the same studies reported a decreased in the tensile modulus when increase alkali concentration to 20 wt.%; due to degradation of cellulose, there is a reduction in tensile strength tenacity by 10% and tensile modulus by 66%. Reddy and Gowda [33] reported that alkali treatment of sisal fibers improved the cellulose crystallinity and removed impurities, such as hemicellulose and lignin. The sisal fibers became much more ductile after the removal of some of the impurities, resulting in a substantially increased fiber stiffness due to the increased crystallinity of the hard cellulose. The enhanced tensile modulus of bamboo strips fiber is also due to the same factor.  Figure 9 shows the strain-at-break results for the different alkali concentrations for various soaking times, relative to those that experienced water retting. No significant differences in the strain rate were observed for the fibers with the 2, 4, 6, and 8 wt.% treatments. The difference occurred between 0.7% and 1.6%. The strain-at-break decreased slightly following the 4 wt.% NaOH treatment. The data were consistent with an increase in the stiffness and brittleness of the fibers after the mid-phase of the alkali treatment in this study. The maximum strain-at-break of 1.602% was found for the fiber with the 4 wt.% alkali concentration and 12 h of soaking, whereas the minimum of 0.76 was found for the fiber with the 8 wt.% alkali concentration and 24 h of soaking. No obvious variations in the trend for the fracture strain as a function of the soaking time were established.  Figure 9 shows the strain-at-break results for the different alkali concentrations for various soaking times, relative to those that experienced water retting. No significant differences in the strain rate were observed for the fibers with the 2, 4, 6, and 8 wt.% treatments. The difference occurred between 0.7% and 1.6%. The strain-at-break decreased slightly following the 4 wt.% NaOH treatment. The data were consistent with an increase in the stiffness and brittleness of the fibers after the midphase of the alkali treatment in this study. The maximum strain-at-break of 1.602% was found for the fiber with the 4 wt.% alkali concentration and 12 h of soaking, whereas the minimum of 0.76 was found for the fiber with the 8 wt.% alkali concentration and 24 h of soaking. No obvious variations in the trend for the fracture strain as a function of the soaking time were established.

Conclusions
An experimental investigation was conducted in this study, to characterize the mechanical behavior and microstructure of single-cellulosic bamboo fiber strips. We found that the mechanical characteristics of the strips were affected by the various soaking times. The soaking time of 12 h resulted in optimized tensile properties, while increasing the soaking time to 18 and 24 h yielded the weakest performance herein. The tensile strength of the bamboo fiber strips was significantly improved after treatment with 4 wt.% NaOH for 12 h. In contrast, the fibers that experienced water retting only had a 28% increase in the tensile strength. Moreover, the tensile modulus of the bamboo fiber strips was significantly improved after treatment with 6 wt.% NaOH for 12 h, but the tensile modulus value at this condition was somewhat similar to that after water retting. We also found that an insufficient alkali solution decreased the tensile properties, whereas an excessively high NaOH solution can easily erode the fibers. The performance of the fibers was enhanced with the optimum drying and soaking times. The morphological changes determined from SEM observations indicated that alkali treatments at moderate concentrations enhanced the roughness of the Buluh semantan surface area, leading to an increase in the wettability. An excessive alkali concentration and soaking time damaged the fibers and subsequently affected their tensile properties.

Conclusions
An experimental investigation was conducted in this study, to characterize the mechanical behavior and microstructure of single-cellulosic bamboo fiber strips. We found that the mechanical characteristics of the strips were affected by the various soaking times. The soaking time of 12 h resulted in optimized tensile properties, while increasing the soaking time to 18 and 24 h yielded the weakest performance herein. The tensile strength of the bamboo fiber strips was significantly improved after treatment with 4 wt.% NaOH for 12 h. In contrast, the fibers that experienced water retting only had a 28% increase in the tensile strength. Moreover, the tensile modulus of the bamboo fiber strips was significantly improved after treatment with 6 wt.% NaOH for 12 h, but the tensile modulus value at this condition was somewhat similar to that after water retting. We also found that an insufficient alkali solution decreased the tensile properties, whereas an excessively high NaOH solution can easily erode the fibers. The performance of the fibers was enhanced with the optimum drying and soaking times. The morphological changes determined from SEM observations indicated that alkali treatments at moderate concentrations enhanced the roughness of the Buluh semantan surface area, leading to an increase in the wettability. An excessive alkali concentration and soaking time damaged the fibers and subsequently affected their tensile properties.