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Highlights:
Agave fiber increased the flexural and tensile modulus of biocomposite.
Sodium bicarbonate increased mechanical properties of fibers and impact resistance.
Hybrid biocomposites (agave/glass) had better physical-mechanical performance than single-fiber biocomposites.
The use of agave fibers in biocomposites is an option for agroindustrial waste management.
Introduction
The use of natural fibers has become important worldwide because these materials are a renewable resource (animal or vegetable), with low density, high stiffness and are biodegradable (Luna & Lizarazo-Marriaga, 2022). Fibers such as sisal, flax, palm, fique, coconut and bamboo, mostly agro-industrial waste, have been used in the design of materials that could be used as reinforcement in polymeric matrices, because they offer advantages such as abundance, low cost, and good mechanical properties (Cuellar & Muñoz, 2010), in addition to their low density compared to other composites (Atiqah et al., 2019). Sectors such as aerospace, automotive, furniture and construction have shown great interest in biocomposites reinforced with natural fibers (Thomas et al., 2011; Uddin & Kalyankar, 2011) that, alone or in combination, could replace synthetic fibers, to create highly competitive green composites (Lalit et al., 2018).
Hybrid composites of natural polymers and glass fiber reinforced materials represent a promising area of research because they are renewable, economical, and environmentally friendly materials compared to synthetic composites (Ashik et al., 2018). On this regard, composites with reinforcing agents with more than one type and shape have been developed to compensate for deficiencies, achieving a positive hybrid effect (Kwon et al., 2014), as two types of fibers offer advantages over their individual use in a polymeric matrix. Studies on hybridization combining natural and glass fiber have been carried out to develop composites with superior mechanical properties than only using glass fiber (Jawaid & Khalil, 2011; Md Shah et al., 2021), resulting in materials with lower costs and weights (Pérez et al., 2016).
Other research has evaluated the use of Agave tequilana Weber bagasse fibers as a reinforcement of biocomposites to improve their properties, such is the case of Torres-Tello et al. (2017), who showed that the addition of agave residue fiber (30% weight) in polyhydroxybutyrate (PHB) and hydroxyvalerate P (HB-HV) matrices increased their tensile modulus between 50 and 80 %, flexural modulus increased between 36 and 41 %, while impact strength increased significantly between 44 and 66 %. On the other hand, Mylsamy and Rajendran (2011) showed that alkali treatment of A. tequilana fibers increased mechanical properties of biocomposites (tensile, compression, flexural and impact)
Agave angustifolia Haw. is one of the most widely used plants to produce mezcal, a traditional Mexican beverage (Pérez Hernández et al., 2016; Rodríguez & de la Cerna, 2017). In Mexico, mezcal agave has been used for approximately 400 years in many rural areas from the north in Tamaulipas and Sonora to Oaxaca in the south (Aguirre & Eguiarte, 2013). During the grinding of the maguey segments, a residue called bagasse is generated, a process in which 15 to 20 kg are obtained on a wet basis for each liter of mezcal (Jacques et al., 2007). Data from the Consejo Mexicano Regulador de la Calidad del Mezcal (COMERCAM, 2022) indicate that during 2021, about 160 000 t of bagasse were generated from the production of 8 099 591 L of mezcal.
Bagasse fibers are considered waste because they have no industrial demand; consequently, they have no cost and cause pollution problems because they are deposited directly on the ground without any process. Therefore, in order to achieve an integral use of bagasse, studies have been carried out for its chemical, morphological (Hidalgo et al., 2015) and mechanical characterization (Silva et al., 2009), which show that bagasse fibers have similar characteristics to other lignocellulosic fibers and could have several applications in the field of biocomposites.
The present study aims to evaluate the physical-mechanical performance of laminar biocomposites developed in a low viscosity epoxy polymer matrix, using A. angustifolia bagasse fiber as reinforcement, to improve the impact and flexural strength of the material, considering the length and fiber content, three chemical treatments and a hybridization with glass fiber.
Materials and Methods
This study was divided into three phases: a) material selection; b) biocomposite development; and c) physical-mechanical characterization in three stages (Figure 1).
Figure 1 Methodology to determine the physical-mechanical performance of biocomposites developed in a low viscosity epoxy polymer matrix, using Agave angustifolia bagasse fiber as reinforcement.
Material selection
A low viscosity epoxy composite resin (N-15) with a density of 1.10 g·cm-3, supplied by Epolyglas, was used as matrix. The bagasse of A. angustifolia was collected from a mezcal processing plant. Agave fibers were obtained from the bagasse and conditioned for use as reinforcement. Table 1 shows the main properties of A. angustifolia.
Table 1 Mechanical and chemical properties of Agave angustifolia.
| Component | UTS* (MPa) | MOE* (GPa) | Elongation (%) | Cellullose (%) | Hemicellullose (%) | Lignin (%) | Source |
|---|---|---|---|---|---|---|---|
| Leaves | 332-421 | 18-20 | 1.9-2.4 | Silva et al. (2009) | |||
| Bagasse | 42.6-144 | 0.56-1.54 | 0.25-0.62 | 48.04 | 34.08 | 20.69 | Hidalgo et al. (2015) |
*UTS: Ultimate tensile strength. MOE: Modulus of elasticity.
The bagasse was washed with water to remove residues, pulp and soil. The samples were then placed in a greenhouse-type solar dehydrator for 36 h at an average temperature of 45 °C to obtain fibers. To reduce their size, the fibers were cut in a paper guillotine and then chopped in a Wiley mill using grain sizes 10, 14, 20 and 30 with mesh openings of 2.00, 1.41, 0.84 and 0.59 mm, respectively.
Before chopping, the fibers were dehydrated in an oven at 60 °C for 12 h; then, the cuttings were sieved in a Fritsch vibrating machine for 2 min with sieves number 30, 40 and 60 of 0.59, 0.42 and 0.25 mm, respectively. Finally, the fibers were dehydrated for 12 h at 60 °C until moisture content of approximately 6 %.
Chemical treatments
The fibers were modified by implementing the chemical treatments used by Cisneros et al. (2016) and Fiore et al. (2018), who found an increase in the mechanical properties of composites reinforced with natural fibers chemically treated with sodium bicarbonate (NaHCO3), vinyl triethoxy silane (H2C=CHSi(OC2H5)3) and sodium hydroxide (NaOH).
Mercerization was the first chemical treatment performed on the agave fibers, for 24 h in a 5 % NaOH solution at room temperature. It was also used as a pretreatment for the other chemical treatments (vinyl triethoxy silane and sodium bicarbonate), placing the fibers for 15 min in 2 % NaOH at room temperature to activate the hydroxyl groups.
The second treatment was vinyl triethoxy silane (H2C=CHSi(OC2H5)3), which was dissolved at 5 % in an ethanol-water mixture with a 95:5 weight ratio. To ensure hydrolysis, the solution was stirred for 1 h adding acetic acid to maintain the pH between 3.5 and 4. Subsequently, the A. angustifolia fibers were immersed in the solution for 90 min at room temperature. Finally, the hydroxyl silane reaction was placed in an oven at 100 °C for 30 min.
Finally, the third treatment consisted of immersing the fibers in a 10 % NaHCO3 solution at room temperature for 120 h.
The treatments were carried out in a washing solution with a solvent-fiber ratio of 10:1 and mixed in a VWR® magnetic stirrer. At the end, the fibers were rinsed with water to remove residual reagents and recovered by vacuum filtration until the solution was neutralized.
Biocomposite preparation
Twelve 110 mm x 200 mm x 3 mm panels were fabricated using a combination of hand lay-up and cold compression molding techniques (Betelie et al., 2019; Salleh et al., 2018). The matrix was prepared from the epoxy infusion resin and its catalyst with a 10:1 weight ratio. The catalyst was added to the resin by mixing until the substance was homogenized; the required percentage of agave fiber was added and mixed until the fibers were completely wetted. The mixture was poured into a mold coated with release wax and covered with a steel plate also coated with wax to facilitate its removal and to obtain a good surface finish. Simulated pressing of the panel was done with a 20 kg load and allowed to cure for 24 h at room temperature. Figure 2 shows the mechanism used in the molding of the biocomposite panels. For polymerization, the panel was placed in an oven at 45 °C for 4 h.
Figure 2 Mechanism to produce panels from a low viscosity epoxy polymer matrix, using Agave angustifolia bagasse fiber as reinforcement.
The molded boards were cut into rectangular strips according to D790-17 (ASTM, 2017) using an OMGA 500 radial saw, to obtain the specimens for the ultimate tensile strength (UTS), impact and three-point ultimate flexural strength (UFS) mechanical tests. The strips had the following measurements: 175 mm x 19 mm x 3.8 mm for UTS test, 127 mm x 8 mm x 3.8 mm or the impact test and 85 mm x 15 mm x 3.8 mm for UFS; subsequently, the UTS test strips were machined on computerized numerical control equipment to obtain type I specimens. To remove excess, all specimens were roughened with 240 grit sandpaper.
Three-stage physico-mechanical characterization
Effect of size and fiber content (E1)
Mechanical tests were performed on 12 combinations derived from four lengths (1, 3, 6 mm, and a mixture in equal proportion) and three fiber contents (18, 24 and 30 %), and compared with a control of resin panels without fiber, for the selection of the biocomposite with the best mechanical performance.
Mechanical properties were determined with an INSTRON-3282 universal testing machine equipped with a 100 kN and 500 W load cell with digital loading software, using specimens for the UTS and MOE tests.
Effect of chemical treatment (E2)
To increase the strengths (mechanical and impact) of the fiber-matrix interface, the samples were immersed for 12 h in NaHCO3, H2C=CHSi(OC2H5)3 and NaOH, before and then washed with water (Benkhelladi et al., 2020). Impact resistance was measured with an INSTRON-CEAST 9050 machine equipped with a 1 J Charpy hammer.
The specimens were subjected to hardness tests to know the effect of the treatments on the mechanical properties of the material and to determine the differences in penetration resistance. A 6.35 mm ball indenter Mitutoyo-ARK equipment was used to determine the Rockwell hardness on the surface of the material; in addition, a morphological analysis was included, using scanning electron microscopy (QUANTA 504), to observe the fracture surface of the specimens. Finally, the influence of the treatments on the density of the specimens was determined.
Effect of hybridization on flexural strength (E3)
For the MOE test, the agave fiber content of the panel was set at 30 % weight added with five percentages of glass fiber (10, 15, 20, 25 and 30 %), based on the best combination of the first experiment.
Glass fiber, mainly composed of silica (SiO2), is one of the main reinforcements used in biocomposite technology that improves properties compared to conventional materials, as it stands out for its low cost, as well as its resistance to fatigue and deformation (Tanzi et al., 2019), whose values range from 70 to 80 GPa and 1 700 to 2 200 MPa, respectively; in addition, they resist temperatures above 1 700 °C (Park & Seo, 2011).
Table 2 shows the tests carried out at each stage according to the standards, dimensions, number of samples and parameters of each test.
Table 2 Physical-mechanical tests performed on the biocomposites developed in a low viscosity epoxy polymer matrix with Agave angustifolia bagasse fiber as reinforcement.
| Stage | Test | Standard | Treatment | Replication | Total | Dimension (mm) | Parameter |
|---|---|---|---|---|---|---|---|
| E1 | Tensile | D638-22 (ASTM, 2022) | 13 | 6 | 78 | Tipo I | 3.85 mm·min-1 |
| 23 °C | |||||||
| E1 | Flexure | D790-17 (ASTM, 2017) | 13 | 8 | 104 | 85 x 15 x 3.8 | 1.85 mm·min-1 |
| 23 °C | |||||||
| E2 | Tensile | D638-22 (ASTM, 2022) | 4 | 8 | 32 | Tipo I | 3.85 mm·min-1 |
| 23 °C | |||||||
| E2 | Flexure | D790-17 (ASTM, 2017) | 4 | 10 | 40 | 85 x 15 x 3.8 | 1.85 mm·min-1 |
| 23 °C | |||||||
| E2 | Impact | D6110-18 (ASTM, 2018) | 5 x 2 | 10 x 2 | 100 | 127 x 8 x 3.8 | 3.4 m·s-1 |
| 23 °C | |||||||
| E2 | Hardness | D785-23 (ASTM, 2023) | 5 | 8 | 32 | 2 x 2 x 2 | 960 h |
| 23 °C | |||||||
| E2 | Moisture absorption | D570-22 (ASTM, 2022) | 4 | 10 | 40 | 85 x 15 x 3.8 | 960 h, |
| 22 °C | |||||||
| E2 | Density | - | 5 | 10 | 50 | 85 x 15 x 3.8 | Geometrical method |
| E2 | Fracture | - | 4 | 5 | 20 | 3 x 3.8 x 0.5 | 15 kV, |
| 23 °C | |||||||
| E3 | Flexure | D790-17 (ASTM, 2017) | 5 | 10 | 50 | 85 x 15 x 3.8 | 1.85 mm·min-1 |
| 23 °C |
Statistical Analysis
The physico-mechanical properties of the biocomposite were analyzed with a completely randomized experimental design, comparing, for each treatment, the data of UTS, UFS, impact, hardness, tensile (TMO) and flexural modulus (FMO), density and moisture resistance.
The first experiment was conducted with a 4 x 3 factorial arrangement (four percentages and three fiber sizes). The second experiment was under a one-factor design with three chemical treatments. The third experiment was carried out with one factor (hybridization) with five percentages of glass fiber.
The results were examined by an ANOVA with SAS v. 9.0 (Statistical Analysis System, 2002). The F test was used to determine the level of significance in each treatment, while significant differences between means were identified with the Tukey test (P = 0.05).
Results and Discussion
Samples cut with the mesh (number 14) and sieved on the sieve (number 30) measured 6 mm in length (Figure 3). The fiber cut and sieved with the mesh (number 20) and the sieve (number 40) measured 3 mm on average, while the size of the fiber cut with the mesh (number 30) and sieved on the sieve (number 60) was 1 mm (Figure 3).
Effect of fiber size and fiber content on tensile and flexural strength
The significant effect of fiber size and fiber content, in addition to their interaction with the biocomposite, was detected, deducing the influence of these factors on the mechanical properties (Figure 4). Fiber content reduced UTS and UFS values, while TMO and FMO increased.
Figure 4 Biocomposite properties in relation to the length and fiber content of Agave angustifolia. Different letters between levels of each factor indicate statistically significant difference: ***P < 0.001, **P < 0.01, *P < 0.05, ns = non-significant. UTS: ultimate tensile strength, UFS: ultimate flexural strength TMO: tensile modulus and FMO: flexural modulus.
Mechanical strength decreased with the incorporation of agave fibers to biocomposite. The highest value was recorded in the 1mm18 configuration (1 mm length and 18 % fiber), where the UTS was 30.5 MPa compared to 36 MPa in the pure epoxy resin (PR) matrix, while the other variants showed no significant difference. UFS decreased in all treatments; the highest values were found in 1mm18 and 1mm24 with 42 MPa and 41.5 MPa, respectively, compared to 61.26 MPa in RP. For 3mm30 TMO (4.5 GPa) was higher compared to RP (3.2 GPa), as well as FMO = 3.96 GPa compared to FMO = 2.93 GPa in RP.
The improvement in TMO is due to the incorporation of a rigid phase (fiber) in the matrix, as indicated by Cisneros et al. (2016) and Yan et al. (2012), who had similar results. Mylsamy and Rajendran (2011) found that equilibrium in mechanical properties with Agave americana L. fibers was observed in 3-mm samples and a fiber-resin ratio of 3:7.
For UTS, in the Tukey test analysis, the means of the treatments were compared with a critical value of the estudentized range of 4.8632 and 4.8118, and a minimum significant difference (MSD) of 7.0231 and 10.781, while for UFS the same critical value was used, but with an MSD of 0.8316 and 0.4330.
Effect of chemical treatment on physical and mechanical properties
Hardness and Density
The hardness values of the chemical treatments versus RP, incorporating the treated fibers, increased significantly in contrast to the untreated fiber composites (UFC). The NaHCO3-treated fiber (FBIC) treatment increased 46.4 %, followed by mercerized fiber surface (MERF) treatment with 34.7 % and fiber treated with vinyl triethoxy silane (FSIL) with 16.7 % compared to UFC, given the good affinity between the fiber-matrix interface, which allowed a better distribution of the fibers and decreased the formation of holes and defects in the material (Cisneros et al., 2016). Koffi et al. (2021) reported 24 % increases in the strength of a high-density polyethylene matrix reinforced with birch fiber treated with maleic anhydride.
According to Mahfoudh et al. (2013), good contact between the fiber-matrix interface should increase the density of biocomposites; however, in this study, the incorporation of treated fibers reduced this parameter. This could be attributed to the fiber surface treatment that modifies its properties (Cisneros et al., 2016; Fiore et al., 2018); for example, NaHCO3 treatment removes a portion of hemicellulose, lignin and waxy substances covering the outer surface of the fiber (Fiore et al., 2018). FBIC treatment reduced 11 % density regarding CFU and 17 % regarding RP.
Moisture content
Moisture absorption of the developed biocomposites was evaluated as the difference between dry and wet weight of the samples. When determining the weight gain according to time, the materials reinforced with treated fiber had higher moisture compared to CFU, exhibiting the highest percentage in FSIL (6.5 %) and FBIC (11 %), due to the elimination of extractive substances and waxes during the treatment (Pérez et al., 2014). Cisneros et al. (2016) reported similar results in A. tequilana fibers treated with silane, with an increase in moisture content of 8 %.
For the MERF treatment, from 2 h of immersion in water, a lower weight gain was observed compared to UFC, since 6 % less water absorption was recorded. This trend was maintained until 960 h; that is, this treatment presented better compatibility between the fiber-matrix interface, which made the biocomposite more hydrophobic, increased moisture resistance and gave greater applications to the material (Zhang et al., 2013).
Tensile strength
The reinforced biocomposites had a slight increase in UTS compared to UFC, indicating an improvement in the fiber-matrix interface; in particular, the FBIC treatment had a minimum increase of 6 % compared to RP. Despite the positive effect that the treatments had on UTS, they affected the TMO of the material, showing 34.3 % less than UFC and 14.7 % less than RP, because the adhesion mechanism gave a ductile profile to the interface, as reported by Zhou et al. (2022), who reported similar results in wood and plastic composites.
Flexural strength
The reinforced biocomposites showed a slight increase in flexural strength compared to UFC. In particular, the FBIC treatment generated a maximum increase of 24 % versus RP, while FMO was 34.7 % lower than UFC and 9.3 % lower than RP.
Impact resistance
Impact resistance for UFC increased 2 % compared to RP and significant increases were found in biocomposites with treated fiber; FBIC increased 36 % compared to UFC and up to 38 % compared to RP. Recent studies show that NaOH treatment plays an important role in the bonding of concrete composites because it improves the mechanical properties of natural fiber reinforced materials (Shah et al., 2021).
Fracture analysis
The impact tested specimens were observed under a scanning electron microscope to analyze the behavior of the treated fiber-matrix interface. Fiber pullout due to low interfacial bonding, as well as the presence of holes due to the manufacturing process of the materials, were identified as major factors of impact fracture (Figure 5).
The UFC and FSIL biocomposites showed a failure mode comprising pulled-out fibers, as well as less breakage and presence of holes (Figures 5a-b). This indicates lower interfacial adhesion; in addition, FSIL exhibited signs of crack propagation at the end of the sample. Figure 5d shows the fracture mode of FBIC biocomposites, which presented a higher number of breaks than MERF (Figure 5c), as well as less presence of holes, indicating better interaction at the interface and better fiber distribution.
Chaitanya and Singh (2018) had similar results on sisal fibers treated with NaHCO3 and NaOH. On the other hand, Cisneros et al. (2016) and Robledo et al. (2020) worked with fiber reinforcements treated with maleic anhydride grafted polyethylene as a compatibilizer to improve adhesion and dispersion of the materials.
Figure 5 Fractographies of impact on panels developed from low viscosity epoxy polymer with Agave angustifolia bagasse fiber subjected to chemical treatments: a) untreated fiber (UFC), b) fiber treated with vinyl triethoxy silane (FSIL), c) mercerized fiber (MERF), and d) fiber treated with sodium bicarbonate (FBIC).
Effect of moisture on mechanical properties
After immersing the biocomposites in water, the FBIC treatment showed the widest range of reduction in tensile strength, flexural strength, and impact strength with 47, 50 and 27 %, respectively (Figure 6). The deterioration of mechanical properties is due to insufficient tensile transfer in the biocomposites affected by sample immersion (Alomayri et al., 2014), which decreased the fiber-matrix interface interaction (Haameem et al., 2016).
Figure 6 Mechanical properties of biocomposites reinforced with Agave angustifolia bagasse fiber subjected to chemical treatments, before and after immersion in water (P = 0.05). UFC = untreated fiber, FSIL = fiber treated with vinyl triethoxy silane, MERF = mercerized fiber con NaOH, and FBIC = fiber treated with sodium bicarbonate.
Effect of hybridization on flexural strength
UFS was higher in the RP panel compared to the panel with only agave fiber (30FA-0GF); however, it increased with the gradual addition of 5 % glass fiber, reaching its maximum value at 30FA-25GF (30 % agave fiber and 25 % glass fiber), higher by 6 % and 77.3 % than RP and 30FA-0GF, respectively. Zhou et al. (2022a) reported similar behaviors, where flexural strength, impact strength and peak impact strength of the hybrid composites were higher than that only with the bamboo fiber composite.
The agave fiber content increased MOE, reaching its maximum value in 30FA-25GF with 65 and 62 % higher compared to RP and 30FA-0GF, respectively. Zhou et al. (2022a) also found this trend when glass fiber was added.
Conclusions
Laminar biocomposites, developed in a low viscosity epoxy polymer matrix, improve their physical-mechanical performance when reinforced with Agave angustifolia fibers. These biocomposites could represent an alternative for industrial applications and fibers would be an option to be used. The physical-mechanical improvements of the biocomposites were observed by combining length, size, and chemical treatments to the agave fibers. The tensile modulus, impact strength and flexural strength of the biocomposite increased when varying the length and fiber content. Also, chemical treatment with sodium bicarbonate increased the mechanical properties and impact resistance of the fibers but decreased the material density and moisture resistance. On the other hand, agave/glass hybridization increased up to 2/3 the impact strength and flexural modulus compared to the agave or pure resin mixture.
