Water Chestnut (Eleocharis dulcis) Microfiber Reinforced Composite with NaOH Modification

This study aimed to obtain the physical and mechanical properties of a composite made from water chestnut microfiber and e-glass with a polyester matrix. Using water chestnut (Eleocharis dulcis) as reinforcement for composite materials is a good way to utilize local materials and increase the economy of local people. It would need a modified treatment to improve the mechanical properties of water chestnut fiber-reinforced composites. The water chestnut fiber was modified by treating it with 5% volume NaOH solution and continued with 5% volume H2O2 bleaching, 50% volume H2SO4 hydrolysis, and ultrasonication. Then it was washed with water and dried in the oven for 12 hours to obtain the microfiber. The size of the water chestnut microfiber obtained was about 3-9 µm. Composite was produced using the compression molding method at a pressure of 2 MPa and a temperature of 25°C. The variations in the volume fraction of the polyester composite, water chestnut microfiber, and e-glass fiber with Volume Fraction (%) were 60%:40%:0% (sample A), 60%:0%:40% (sample B), 50%:25%;25% (sample C), 60%:40%:0% (sample D) and 70%:15%:15% (sample E). The physical characterization of composites showed that the lowest composite density and water content on samples B and A were 1.22 g/cm3 and 0.59%. In contrast, the mechanical characterization of composites showed the highest MoR and MoE on samples A and C of 87.86 MPa and 10.43 Gpa, respectively. Composite made from water chestnut (Eleocharis dulcis) modified microfiber reinforcing material with 5% volume NaOH solution and e-glass with a polyester matrix at a volume fraction composition of 50%:25%:25% is recommended.


Introduction
Composite materials have been widely used in various fields and the manufacture of composites with natural fiber reinforcement can overcome the limitations of synthetic materials that are difficult to decompose.Water chestnut (Eleocharis dulcis) or Purun Tikus (in the local language) has the potential as a source of natural fiber found in South Kalimantan Province.Based on the SPOT-6 imagery software obtained in 2019, the area of chestnut trees in Hulu Sungai Utara (HSU) is 24,903.22ha, with a testing accuracy of 97.31% and a Kappa index of 0.92 [1].Research using water chestnuts is a good way to increase local materials' economic value.Several studies have been carried out on water chestnut and its utilization, including as a craft material and swamp buffalo feed, biofilter, heavy metal absorbent and cement board composite material [2][3][4][5][6].
Before being used as a natural fiber, water chestnut needs to be treated or modified to reduce unused fiber elements.Alkali treatment is the most common and best method for fiber to be used.It can reduce fiber size and increase the adhesive properties or interfacial adhesion of the elements present in the composite to increase its tensile strength.Alkaline treatment of 5% NaOH reduced water chestnut fibers' size and levels of lignin, cellulose and hemicellulose [7][8][9][10][11].Microfiber can be made from natural fibers by isolating cellulose through delignification and ultrasonication.Ultrasonic treatment of cellulose reduces porosity, increases fiber dispersion in the matrix, and adhesion between the matrix and the fibers, resulting in high tensile strength [12][13][14].
Efforts to improve the mechanical properties of natural fiber-reinforced composites apart from the treatment of modifications to the fibers, are also done by hybridizing them using synthetic fibers.Eglass and polyester resin are often used as composite materials.E-glass is a type of glass fiber with a low concentration of alkaline glass, while polyester resin is cheaper and can bond with natural fibers without causing reactions and gases [15][16][17].This is a consideration in the manufacture of water chesnut hybrid microfiber composites with e-glass and polyster synthetic fibers to obtain composites with good mechanical properties, and on the other hand also save costs.From several composite studies, no one used water chesnut microfiber modified by NaOH hybrid with e-glass fiber.Besides that, composite studies with water chesnut microfiber reinforcing material have not carried out, and this is a novelty of this research.
The composition of the mixture between the matrix and the reinforcement determines the characteristics of hybrid composites.Water chestnut fiber as a reinforcement greatly determines the properties of the composite because it continues the load distributed by the matrix.Water chestnut fiber in microfiber is expected to produce optimal physical and mechanical properties of hybrid composites.This study aimed to obtain the physical and mechanical properties of a hybrid composite made from water chestnut microfiber fiber modified by NaOH and e-glass with a polyester matrix.Water chestnut microfiber is produced through a delignification process by immersing the fiber in 5% NaOH solution, followed by the cellulose isolation process using bleaching, acid hydrolysis, and ultrasonic mechanical treatment.

Preparation of sample
Water chestnut (Eleocaris dulcis) samples were taken from the Kayu Tangi area of Banjarmasin with stem lengths of around 100-160 cm.The samples were cleaned and dried in the sun for 2 x 8 hours.Furthermore, the stems are cut into pieces with a maximum length of 2 cm and then blended into thin shavings.The resulting fibers were washed with water with stirring and heated at 80°C for 1 hour.

Experimental methods
Firstly, the water chestnut fibers were modified to micro size by the delignification method to reduce lignin content, and the cellulose isolation method was microfiber.The delignification method was carried out by soaking the fibers in 5% NaOH solution for 3 hours and the cellulose isolation process was carried out using chemical treatments: bleaching, acid hydrolysis, and ultrasonic mechanical treatment.Hydrolysis did by using 50% H2SO4 with immersion time of 60 minutes at a temperature of 45 o C and ultrasonication for 60 minutes at a temperature of 50 o C with a frequency of 37 kHz.These modifications were expected to produce water chestnut fibers in a micro diameter, so that the tensile strength of the composite is greater [18].
In the manufacture of composites, water chestnut microfiber fibers are substituted with synthetic fibers, which is CSM 3445 MPa e-glass fiber, compressive strength of 1080 MPa, 6.73 GPa which meets the standards [19] and the matrix used is polyester resin brand Yukalac 157 BTQN-EX with MEKPO (Methyl Ethyl Ketone Peroxide) catalyst (hardener).The mixed comparison of polyester resin with MEKPO catalyst was 2:1.The function of the MEKPO catalyst is to accelerate the process of hardening the resin on the composite.While the synthetic fiber used was e-Glass fiber in the form of random chopped strand mat (CSM).
The design of hybrid composites made with various volume fraction compositions, as shown in Table 1.The composites were made using the compression moulding method at a pressure of 2 MPa and a temperature of 250oC.The composite was left to dry for 1-2 days and then released from the mould.

Characterisation of water chestnut microfiber
Table 2 shows the moisture content and density of water chestnut fiber without any modification treatment and with modification to water chestnut microfiber.Water chestnut microfiber showed decreased fiber water content and met the standard as a natural fiber reinforcement material in the manufacture of composites.However, this value was still higher than the study by Hairiyah et al. [20], which is 2.47%.The density value of water chestnut microfiber also decreased, where the lower the density value of the reinforcing material, the lighter the composite produced.In addition, modifications have also changed the chemical composition and surface morphology of water chestnut fibers.These results are demonstrated by FTIR (Fourier Transform Infrared Spectroscopy) spectra and SEM (Scanning Electrone Microscope) images.Figure 1 shows the difference in surface morphology of water chestnut and water chestnut fibers using SEM-EDX images.The morphology of the fiber without modification (Figure 1(a)), indicates that the fiber is still protected by lignin and hemicellulose components.The fiber's diameter appears large, about 6-13 µm.After the modification (Figure 1(b)), the fiber looks cleaner and the fiber diameter was smaller, around 3-9 µm.This proves that the modification steps have reduced the components of lignin, hemicellulose and other impurities and have also reduced the fiber's size and cleaned the fiber.The smaller fiber diameter, the strength of the composite increases, because it increases the contact between their surfaces.This also improves its mechanical properties.
Figure 1(c) and 1(d) were EDX images of water chestnut fibers without and with modification.Unmodified fiber is composed of elements C: 39.05%, O: 41.24%, Na: 1.27%, Si: 5.15%, Mg: 0.64%, S: 2.75%, Al: 1.83%, K: 1.95%, Cl: 6.13%.While water chestnut microfiber obtained elements C: 26.02%, O: 49.71%, Na: 0.25%, Si: 21.78%, Mo: 2.23%.The modification appears to have reduced the content of C, Na and other chemical substances such as the elements Mg, S, Al, K, and Cl not found in water chestnut microfibers.The functional groups of water chestnut fibers have also changed, as shown in Figure 2. Water chestnut fibers are composed of a functional group of 3274 cm -1 which is an O-H stretching vibration.These functional groups are present in the cell wall as lignin, hemicellulose and cellulose.The peak of 2917 cm -1 corresponds to the aldehyde group's C-H strain vibration of the aldehyde group, 1730 cm -1 relates to the stretching of C=O, a hemicellulose component belonging to the scetil group and carboxylic acid.The groups 1629 cm -1 , 1500 -1156 cm -1 are characteristic of lignin and 1031 cm -1 are related to the O-H vibrations found in cellulose and lignin.After modification, the microfiber fiber only left a few wave number peaks, namely 3303 cm -1 , 1160 cm -1 and 1031 cm -1 peaks where the peak intensity also decreased.This shows that the modified treatment through several stages has succeeded in cleaning the microfiber fiber from unnecessary chemical components, such as lignin, hemiocellulose and other chemicals.

Characterization of water chestnut microfiber composites
Table 3 shows the physical-mechanical properties of water chestnut microfiber composites based on variations in composition.The addition of the percentage of water chestnut microfiber reinforcement causes an increase in moisture content.Composition A has the lowest moisture content and the highest density.Low moisture content was caused by polyester fibers and e-glass fibers do not like water [21].While high density was caused by it is influenced by the density of these two constituent materials, namely, 0% fiber, the highest e-glass fiber (40%) and 60% polyester.The moisture content of composites have increased 100% with the addition of microfiber.While the variations in the composition of e-glass and polyester fibers did not have a significant effect.This shows that microfiber affects the moisture content of the composite [22], because it is a biomass fiber, which quickly absorb water.However, the density value of the composite remains high, due to the influence of the density of polyester 1.21 gr/cm 3 and e-glass 2.74 g/cm 3 , even though the density of microfiber is 0.58 gr/cm 3 .Based on SNI 01-4449-2006 natural fiber reinforced composites should have a maximum moisture content of 13%.
Water chestnut is one of the natural fibers that can be used as a composite reinforcement, because it has one advantage: lower density value than synthetic fibers.The addition of the percentage of microfiber as a composite reinforcement can cause a decrease in density.This can be seen in the composite compositions C, D and E. The research conducted [23] utilized waste and natural materials, the maximum density obtained was 1.2 g/cm 3 .Based on SNI 01-4449-2006 hybrid fiber composites are classified as a type of high-density fiberboard.The mechanical properties test conducted in several part, including testing the maximum force, MoR, MoE and Strain were shown in Table 3.This test shows the fracture and elasticity of the composite when a force is applied according to ASTM D790-03.Synthetic fibers have superior mechanical properties when compared to natural fibers.The MoR is greater in composite A with a value of 87.86 MPa and MoE 6.73 GPa and meets the standards [19].When compared to composite B, it has lower mechanical properties.However, 25%:25% microfiber and e-glass fibers a higher MoE value than the other fiber compositions.Even if you only use natural fibers, the MoE value is the smallest compared to the others.The modulus of elasticity is the resistance value of a material to experience elastic deformation when a force is applied to the material.When subjected to loads (within their elastic limits), materials with high stiffness will experience elastic deformation but only slightly.The material's stiffness is usually indicated by the modulus of elasticity, where the greater the elastic modulus of the composite, the stiffer the composite material is.Mixing the percentage of material composition can affect the composite's MoR and MoE, and the addition of matrix volume can increase the bond between fibers.Besides that, the bond between the water chestnut microfiber fiber and the e-glass fiber to the polyester matrix, which is not strong, also has an effect.The lack of strong bonds between the matrix as a binder and the fibers as reinforcement will cause de-bounding (the bond between the fibers and the matrix is released) [24,25].
Several studies found that the composite's mechanical properties decreased if the NaOH immersion time was more than 2 hours [22] and 5% NaOH modification with 3 hours time [10].In addition, different methods of making composites also produce different mechanical properties [10,19].
Figure 3 shows the composite stress and strain analysis.Figure 3(A) shows that the composite with e-glass reinforcement has stiff, strong, and tough properties.Composite A1 has an ultimate strength value of 128 MPa and a yield strength of 122 MPa with an average strain of 1.33%.Research made fiberglass composites using long and plaited fibers, producing a greater composite strain of 2.4% [26].Figure 3(B) shows the strain of the microfiber fiber composite with an average of 3.03% greater than the e-glass fiber composite.In this sample, the composite has flexible, brittle properties, but its strength is lower than the e-glass composite.The ultimate strength value of the composite B is 35 MPa, and the yield strength is 31 MPa.The factor that affects the flexibility of the microfiber composite is fiber size [22].Composites with microfiber will easily bond between fiber surfaces with the polyester matrix as an adhesive [20].However, the composite strain range is lower.Figure 3 shows a fracture with a brief plastic deformation [27].This is due to the nature of natural fibers, which have lower mechanical properties than synthetic fibers [22].
Figure 3(C) obtained an average strain of 2.10%.The strain of the hybrid fiber composite in this composition increases when compared to the e-glass fiber composite.Composite C3 shows a larger strain analysis curve than the other samples.However, the necking area occurs very briefly so that the plastic deformation in the sample occurs more quickly.The ultimate strength value shown in the curve is 203 MPa, and the yield strength is 170 MPa. Figure 3(D) shows the strain results on one composition with three samples.The strain on the composition with an average of 1.91% decreased compared to the 50%:25%:25% hybrid fiber composition.The obtained curve looks different for the same composition.Composite D2 has more elasticity than the other samples.The ultimate strength value of the composite is 37 MPa, and the yield strength is 30 MPa. Figure 3(E) shows the analysis results of the relationship between stress and strain, averaging 1.56%.The figure shows a yield strength of 49 MPa and an ultimate strength of 65 MPa.The necking area occurs quite briefly in samples E2 and E3.For sample E1, plastic deformation occurs longer before fracture occurs.Variations in the composition of hybrid fiber composites show a decrease in strain when the reinforcing composition is lower [19].The composite stress and strain curves show differences in the strain hardening and necking areas.The yield strength is found at the top of the curve in the strainhardening area.Plastic deformation is a permanent change in the shape of the composite until a fracture occurs in the necking area [27].Several factors influence the results of the analysis.The stress and strain analysis shows the effect of mixing microfiber reinforcement with e-glass fiber.The method of making composites can also affect the strain results obtained.This study uses the compress molding method with short fiber types.The use of random fibers can affect the composite's mechanical properties due to the irregular arrangement [28].

Conclusion
The physical properties of the hybrid composite obtained the lowest density of 1.21 g/cm3 was a hybrid composite with a volume fraction of 25% water chestnut microfiber.The addition of water chestnut microfiber as a reinforcement can cause a decrease in the density of the hybrid composite.Its modulus of elasticity (MoE) decreases from the volume fraction of polyester by 50%, 60%, and 70%.The highest MoE value was found in the composite with water chestnut, e-glass, and polyester 20%: 20%: 60% composition of 4.99 GPa.The MoR value also tends to increase with the increase in the volume fraction of polyester, with the highest composite value in the composition of microfiber, e-glass, and polyester, namely 15%: 15%: 70% of 68.34 MPa.
Composites were moulded in sizes of 165 mm × 20 mm × 5 mm (radius of fillet: 76 mm) and 120 mm × 25 mm × 5 mm for MoR (Modulus of Rupture) and MoE (Modulus of Elasticity) tests according to ASTM D790-92 and 20 mm × 20 mm × 5 mm for testing density and moisture content.

Figure 1 .
Characteristics of SEM EDX water chesnut with and without modification Fiber morphology without modification (a), After modification (b), (c) and (d) are EDX images of mouse Purun fibers without and with modification.

Figure 2 .
Figure 2. FTIR spectra of water chestnut fiber with and without modification.

Table 1 .
Identification of composites with variations in volume fraction composition.

Table 2 .
Moisture content and density of water chestnut microfiber.

Table 3 .
Analysis of moisture content and composite density.