Physical and Mechanical Properties of Bamboo Fiber/Glass Fiber Mesh Reinforced Epoxy Resin Hybrid Composites: Effect of Fiber Stacking Sequence

ABSTRACT To further improve the preparation efficiency and properties of bamboo fiber reinforced polymer composites (BFRPs) fabricated by the vacuum-assisted resin transfer molding (VARTM) process. Here, the bamboo fiber/glass fiber mesh reinforced polymer hybrid composites (BGRPs) were fabricated by VARTM to determine the effects of three fiber stacking sequences (namely, GBBGBBG, BGBGBGB, and BBGGGBB; B: bamboo fiber, G: glass fiber mesh) on the physical and mechanical properties, as well as void characteristics of BGRPs (i.e. BGRP-1, BGRP-2, and BGRP-3). The results showed that the incorporation of glass fiber meshes could shorten the injection time of epoxy resin and improve the mechanical properties of BFRPs. BGRP-1 exhibited the lowest water absorption (1.31%) and the highest shear strength (15.55 MPa). Glass fiber meshes on the surface and bottom of BGRP-1, respectively, served as buffer layers to retard mechanical damage, so that BGRP-1 had the best drop hammer impact properties. BGRP-2 represented the highest flexural strength and flexural modulus of 92.22 MPa and 6.69 GPa, respectively. The mechanical properties of BGRP-3 were inferior to those of BGRP-1 and BGRP-2, and more voids were observed in the middle of BGRP-3 in micro-CT slices induced by inadequate epoxy resin impregnation on bamboo fibers.


Introduction
Due to the scarcity and extreme use of fossil fuels and associated environmental problems, lowdensity, low-cost, short-growth-cycle, biodegradable natural plant fibers have been employed to replace synthetic fibers as fiber reinforcements for fabricating fiber-reinforced polymer composites (FRPs), thus reducing energy consumption and environmental pollution caused by FRP production (Puttegowda et al. 2021). Flax fibers, jute fibers, coir fibers, bamboo fibers, etc. are commonly employed fibers, among which bamboo fibers show huge development and application potential with high specific strength and specific modulus (Furtos et al. 2021). Presently, bamboo fiber reinforced polymer composites (BFRPs) are primarily prepared by compression molding, extrusion molding, filament winding molding, and liquid composite molding (Zakikhani et al. 2014). Vacuum-assisted resin transfer molding (VARTM) process is used to fabricate BFRPs by impregnating bamboo fibers through vacuum-driven resin flow to prepare composites at room temperature or under heated conditions (Naik, Sirisha, and Inani 2014). VARTM-prepared BFRPs have lower porosity and better interfacial bonding between bamboo fibers and resin matrix than BFRPs prepared by hand lay-up molding and hot-press molding (Tafur, Mora, and Baracaldo 2018;Xia et al. 2015). In fact, only when the bamboo fibers in the composites reach a certain fiber mass fraction can they fully provide the ideal reinforcement in the resin matrix for BFRPs (Yang et al. 2019). Due to the low density of bamboo fibers, compression is frequently required for bamboo fiber preforms to prepare BFRPs with high fiber mass fraction and low thickness. However, the resin injection time increases markedly with the increasing denseness of bamboo fiber preforms during VARTM, causing decreased preparation efficiency (Ehresmann, Amiri, and Ulven 2016).
Bamboo fiber surface modification, resin matrix modification, and VARTM process parameter optimization have been used to improve the mechanical properties of BFRPs (Muhammad et al. 2018). Bamboo fiber surface modification and resin matrix modification were performed to strengthen the interfacial bonding between bamboo fibers and resin matrix to improve the mechanical properties of the composites . For BFRPs, it is inadequate to use natural bamboo fibers alone in the resin matrix to form a composite with satisfying properties, to which fiber reinforcement hybrid is an effective way (Abd El-Baky 2017). By incorporating other fibers into bamboo fibers to jointly reinforce the same resin matrix, the advantages of other fibers could complement the deficiencies of bamboo fibers, and a balance between cost and performance could be achieved through proper material design (Abd El-Baky 2017). The existence of positive hybrid effects of hybrid composites is affected by hybrid materials, stacking sequences, fiber stacking orientations, hybrid methods, hybrid ratio, etc. (Sujon, Habib, and Abedin 2020). The fibers hybridized with bamboo fibers can be divided into plant fibers and synthetic fibers. The plant fibers for hybrid bamboo fibers include flax fibers, jute fibers, coir fibers, etc. Sathish et al. (2022) compared the effects of jute fibers and coir fibers incorporated into bamboo fiber hybrid composites; they found that jute fibers presented higher modulus and energy absorption than coir fibers. The synthetic fibers used for hybrid bamboo fiber include carbon fibers and glass fibers. Zhou et al. (2020) found that the bending and pendulum impact strengths of hybrid composites with two layers of glass fibers under the surface were 120% and 220.68% greater than those of pure bamboo fiber composites, respectively. Zhang et al. (2021) compared the physical-mechanical properties of bamboo fiber composites with those of glass fiber composites, and the tensile, flexural, and shear strengths of bamboo fiber composites were 45.62 MPa, 105.41 MPa, and 18.31 MPa, respectively, while the tensile, flexural and shear strengths of glass fiber composites were 885.81 MPa, 828.17 MPa, and 37.53 MPa, respectively. The use of glass fibers hybridized with bamboo fibers to improve the mechanical properties of BFRPs is a great approach.
Glass fiber meshes are made of cross-woven glass fibers with a high elongation in all directions. Glass fiber meshes work as the equivalent of a "soft steel bar" in composites owing to their high tensile strength in both warp and weft directions. In the construction field, incorporating glass fiber meshes into the exterior wall insulation system can achieve obvious heat preservation and energy saving, waterproofing, and anti-cracking effects (Xue et al. 2019). In addition, the structure of glass fiber meshes is comparable to that of diversion networks, and their placement in BFRPs can improve resin injection efficiency. In FRPs, the interface is not only a link between fiber reinforcements and resin matrix but also a bridge for stress transfer of FRPs when subjected to external loads. Moreover, the mechanical properties of FRPs depend largely on the interfacial bonding between fiber reinforcements and resin matrix (Qin et al. 2013). BFRP prepared by VARTM highly depends on vacuum-driven resin flow to impregnate bamboo fibers compared to other molding processes (Dhimole, Serrao, and Cho 2021), and the fiber stacking sequence exerts an essential effect on the interfacial bonding between bamboo fibers and resin matrix. Therefore, it is necessary to investigate the effects of different fiber stacking sequences on the preparation and properties of bamboo fiber/glass fiber mesh reinforced epoxy resin hybrid composites (BGRPs) by VARTM.
In this paper, bamboo fibers and glass fiber meshes were prepared into fiber preforms by solution suspension method. BFRPs (BFRP-40 and BFRP-45) with bamboo fiber mass fractions of 40 wt% and 45 wt% and BGRPs (BGRP-1, BGRP-2, and BGRP-3) with three fiber stacking sequences (GBBGBBG, BGBGBGB, and BBGGGBB; B: bamboo fiber, G: glass fiber mesh) were fabricated by VARTM to investigate the effects of fiber stacking sequence on the physical and mechanical properties, as well as void characteristics of the VARTM-prepared BGRPs.

Materials
Bamboo fibers (Dendrocalamopsis oldhami (Munro) Keng f.) with a mean length of 2.54 cm, a mean diameter of 0.25 mm, and an aspect ratio of 102 were obtained from Fujian Haibosi Chemical Technology Co., Ltd., China. Glass fiber meshes, with an aperture of 4 × 4 mm and gram weight of 170 g/m 2 , were purchased from Xilin New Materials Company, China. The epoxy resin (NPEL127E, viscosity: 10255 MPa·s at 25°C, epoxy equivalent: 182.4 g/Eq) was purchased from Nanya Electronic Materials (Kunshan) Co., Ltd., China. The methyl tetrahydrophthalic anhydride (MTHPA) curing agent (WNY1008, viscosity: 36 MPa·s at 25°C) was provided by Dongying Xichen Chemical Co., Ltd., China. N,N-dimethylbenzylamine accelerator was obtained from Hangzhou Lanbo Industrial Co., Ltd., China. The epoxy/anhydride system was prepared using epoxy resin, MTHPA curing agent, and N,N-dimethylbenzylamine accelerator with a ratio of 100:95:1.

Preparation of fiber preforms and composites
The preparation of fiber preforms by the solution suspension method is shown in (Figure 1a) The bamboo fibers and glass fiber meshes were laid in order according to different fiber stacking sequences and put in a self-made sieve with the same shape as the mold. Afterward, the sieve was placed into the suspension device and vibrated to achieve even suspension and dispersal of bamboo fibers and glass fiber meshes in water. Finally, fiber preforms were taken out and dried. The moisture content of fiber preforms was controlled to below 2%. As shown in (Figure 1b), BFRPs and BGRPs were prepared by VARTM. Fiber preforms, peel ply, and diversion network were laid in the mold sequentially after mold cleaning, and three layers of release agent were brushed inside the mold. Then, sealing tapes were applied to the female mold surface, and epoxy resin injection was carried out by boundary injection and boundary output with a vacuum level of 0.08-0.085 MPa. When epoxy resin was sucked out of output ports, the injection was continued for 20 min so that the air inside the mold was pumped out as much as possible. Finally, the valves on both sides of the mold were closed after the injection, and BFRPs and BGRPs were cured according to the curing process at 75°C for 1 h, 100°C for 1 h, and 130°C for 2 h. The BFRPs with bamboo fiber mass fractions of 40 wt% and 45 wt% were named BFRP-40 and BFRP-45, respectively. The BGRPs with fiber stacking sequences of GBBGBBG, BGBGBGB, and BBGGGBB (B: bamboo fiber, G: glass fiber mesh) were named BGRP-1, BGRP-2, and BGRP-3, respectively. The bamboo fiber mass fraction of BGRPs was 40 wt%.

Physical property tests
The density of specimens was measured based on the geometric method in GB/T 1463-2005. The water absorption of specimens was assessed using the second method in GB/T 1462-2005: Specimens were immersed in boiling water for 30 min and cooled in distilled water at room temperature for 15 min.

Mechanical property tests
The flexural testing of specimens was conducted according to GB/T 1449-2005 by a mechanical testing machine (5582, Instron Company, USA) with a span length of 160 mm and a crosshead speed of 5 mm/min. The shear testing of specimens was conducted according to ASTM D2344/D2344 M-16 by a mechanical testing machine (5582, Instron Company, USA) with a span length of 40 mm and a crosshead speed of 1 mm/min. The pendulum impact testing and its specimen preparation were performed following the GB/T 1451-2005. An impact experimental machine (XJJ-5, Chengde Kesheng Testing Machine Co., Ltd., China) was employed for the testing with an impact energy of 5 J and an impact speed of 2.9 m/s. The drop hammer impact testing was conducted according to ASTM D7136/D7136 M-15 by a drop hammer impact testing machine of fully digital instrumentation (9250HV, Instron Company, USA) with an impact speed of 5 mm/min. The hammer had a mass of 15 kg and a hemispherical head with a diameter of 16 mm. Five specimens of each type of composite were selected for flexural, shear, and pendulum impact tests.

Microtopography and void analysis
After the flexural and pendulum impact testing, the fracture morphologies of specimens were observed with a scanning electron microscope (SEM, Quanta 200, FEI Company, USA). The specimens were scanned by a three-dimensional (3D) X-ray microscope (micro-CT, SkyScan 2214, Bruker, Belgium) to observe their void characteristics with a tube voltage of 63 kV, a tube current of 88 μA, and a resolution of 9.00 μm.

Physical properties
As shown in (Figure 2a), the densities of epoxy resin, BFRP-40, and BFRP-45 were 1.208 g/cm 3 , 1.234 g/ cm 3 , and 1.251 g/cm 3 , respectively. Numerous bamboo fibers were incorporated into the epoxy resin and thus increasing its density. Among the three BGRPs, BGRP-2 with the fiber stacking sequence of BGBGBGB showed the highest density of 1.258 g/cm 3 and the optimal impregnation effect of epoxy resin on bamboo fibers. Because BGRP-3 contained three glass fiber meshes arranged in adjacent layers, it represented the lowest density and most variability, causing the epoxy resin unable to impregnate bamboo fibers adequately during epoxy resin injection. The total injection time in BFRPs and BGRPs followed the order of BFRP-45 > BFRP-40 > BGRP-2 > BGRP-1 > BGRP-3. The injection time could be shortened by incorporating glass fiber meshes into bamboo fibers. BGRP-2 in three BGRPs presented the longest epoxy resin injection time. BGRP-2 with alternating glass fiber meshes and bamboo fiber layers increased the hybrid interface, prolonging the injection time. The water absorption of epoxy resin was only 0.10%, while that of BFRP-40 rose by 16.2% (Figure 2b) The incorporation of hydrophilic bamboo fibers into epoxy resin significantly enhanced the water absorption of epoxy resin, which was because numerous hydroxyl groups on the bamboo fiber surface could easily combine with water to form hydrogen bonds (Wei et al. 2022). The water absorption of BGRP-1 was the lowest among the three BGRPs at 1.31% due to the placement of two layers of glass fiber meshes on the surface and bottom layers of BGRP-1. Our previous studies indicated that the equilibrium moisture content of bamboo fibers reached a stable value of 21.28% at an RH of 95%, while that of glass fibers was only 0.063%; furthermore, epoxy resin formed better interfacial bonding with glass fiber meshes than with hydrophilic bamboo fibers, and the glass fiber meshes placed on the surface and bottom layers of BGRP-1 could form a water barrier when combined with epoxy resin. The water absorption of BGRP-2 was comparable to that of BFRP-40, while that of BGRP-3 was the highest due to the poor interfacial bonding between the epoxy resin and bamboo fibers.

Void characteristics
BFRP-40, BGRP-1, BGRP-2, and BGRP-3 were scanned with micro-CT. Micro-CT slices are shown in (Figure 3), where the white-gray is bamboo fibers, the white is glass fiber meshes, the gray is epoxy resin, and the black is voids. Due to bamboo fibers being adequately impregnated by epoxy resin with low viscosity and the long epoxy resin injection time, BFRP-40 showed no obvious voids in all three directions. BGRP-1 had a few voids in the XY-upper and XY-middle slices, while more voids appeared in the XY- lower slices. The main reason for the voids in the lower part of BGRP-1 was that the bamboo fibers impregnated with epoxy resin relied on the epoxy resin infiltration in the thickness direction due to gravity. The glass fiber meshes accelerated the epoxy resin flow as they worked as a diversion network. However, the bamboo fibers in the upper part of BGRP-1 were fully impregnated, while the bamboo fibers in the lower part were not. In BGRP-2, only a few voids existed in the XY-upper, XY-middle, and XYlower slices. This was because the better dispersion of the fiber stacking sequence of BGRP-2 enabled the epoxy resin to impregnate all parts of the fiber preforms. More voids were generated in the XY-middle slices of BGRP-3. When the three-layer glass fiber meshes were placed in the middle of BGRP-3, the epoxy resin passed easily and directly through fiber preforms under high vacuum conditions, resulting in poor impregnation of the epoxy resin on the three-layer glass fiber meshes and the surrounding bamboo fibers.

Flexural properties
As shown in (Figure 4a), the flexural properties of BGRP-1 and BGRP-2 were enhanced by incorporating glass fiber meshes into BFRPs, compared to those of BFRP-40. For example, BGRP-2 represented   (Figure 4b), displayed a linear decrease of BFRPs and BGRPs after reaching the maximum flexural load, indicating that the inferior interfacial properties of the bamboo fibers and the epoxy resin were primarily induced by the flexural damage of BFRPs and BGRPs (Zhang et al. 2018). As shown in (Figure 4c), when the specimens were subjected to flexural loads, the damage was primarily incurred by cracks in the middle of the specimens, and the bamboo fibers in composites were easily separated from the epoxy resin (Figure 4d), The poor interfacial bonding of the epoxy resin with the bamboo fibers and glass fiber meshes in the middle part of BGRP-3 was observed in the previous micro-CT slices, serving as the main cause for the delamination damage of BGRP-3 (Madhu et al. 2020).

Shear properties
The mechanical properties of FRPs largely depend on the interfacial bonding between fiber reinforcements and resin matrix, and shear properties are the most effective mechanical characterization parameter to quantify the interfacial bonding (Qin et al. 2013). As shown in (Figure 5a), the incorporation of bamboo fibers could improve the shear strength of epoxy resin, and the shear strength of BFRP-45 was improved by 73.19% compared to that of epoxy resin. The shear strength of BGRP-1 and BGRP-2 was higher than that of BFRP-40 and BFRP-45 when glass fiber meshes were incorporated into BFRPs, with BGRP-1 showing the highest shear strength of 15.55 MPa. In contrast, BGRP-3 presented a lower shear strength than BFRP-40. In the three BGRPs, when the mass fractions of bamboo fibers and glass fiber meshes were consistent, the fiber stacking sequence of BGRPs and the interfacial bonding between the fibers and the epoxy resin were the keys to affecting their shear properties. BFRP-1 had two layers of glass fiber meshes on the surface and bottom layers, which made the glass fiber meshes break first, followed by the bamboo fibers inside when BFRP-1 was subjected to external loads. Moreover, the strength of the glass fiber meshes was higher than that of the bamboo fibers, enabling BGRP-1 to have a slightly higher shear strength than BGRP-2 with a better impregnation effect of epoxy resin on bamboo fibers. The middle part of BGRP-3 had an inadequate impregnation effect of epoxy resin on bamboo fibers due to their fiber stacking sequence. Voids in FRPs reduced the strength of FRPs, particularly short beam shear strength (Tretiak, Kawashita, and Hallett 2022). Visible cracks could be observed after the damage of BGRP-3 (Figure 5c), The shear displacement-load curves (Figure 5b), indicated that BGRP-3 showed a slow decline after reaching the maximum load, which was attributed to the strong shear properties of the three-neighboring-layer glass fiber meshes (Furtos et al. 2022).

Pendulum impact properties
The incorporation of glass fiber meshes substantially improved the impact toughness of BFRPs ( Figure 6a). Compared to that of BFRP-40, the impact toughness of BGRP-1, BGRP-2, and BGRP-3 increased by 549%, 621%, and 779%, respectively. Among them, BGRP-3 had the highest impact toughness of 41.41 kJ/m 2 . According to analyzing the macroscopic fracture morphologies of BFRPs and BGRPs after the pendulum impact tests (Figure 6b), BFRP-40 and BFRP-45 specimens were completely broken after being impacted. Glass fiber meshes in the middle and bottom layers of BGRP-1 were broken, but those in the surface layer was still connected. In contrast, in BGRP-2, all three layers of glass fiber meshes were broken, and the specimens transferred stress more uniformly upon impact. In BGRP-3, the glass fiber meshes were torn, and the three layers of glass fiber meshes were stronger when placed adjacently and less likely to break completely upon impact (Ahmed, Vijayarangan, and Naidu 2007). In (Figure 6c), more bamboo fibers were pulled out on the fracture morphologies of BFRP-40 and BFRP-45 due to the interfacial bonding between the bamboo fibers and the epoxy resin. Glass fiber meshes incorporated into BFRPs worked as buffer layers so that fewer bamboo fibers were pulled out from the epoxy resin after being impacted in BGRP-1 and BGRP-2 (Figure 6d), Although BGRP-3 had the highest impact toughness, large voids were left by the bamboo fibers being pulled out from the epoxy resin on the fracture morphologies of BGRP-3 due to the fiber stacking sequence. In addition, the fracture morphologies of BGRP-3 were observed to be more disorganized.

Drop hammer impact properties
As shown in (Figure 7a-b), incorporating bamboo fibers into epoxy resin could significantly increase the forces that epoxy resin could carry after being impacted and delay its damage time. After impact, the epoxy resin cracks grew quickly and fractured in 0.02 s, with the breaking force, maximum force, and breaking energy of 3.34 kN, 16.76 kN, and 25.12 J, respectively (Table 1), For BFRP-40, the break time was extended to about 3 s, and the breaking force, maximum force, and breaking energy were 6.18 kN, 31.28 kN, and 131.44 J, respectively. Moreover, the total absorbed energy of BFRP-40 increased by 110.77 J compared to epoxy resin due to the prolonged energy loss time in the late fracture stage with the growing crack extension time of BFRP-40. The incorporation of glass fiber meshes into BFRPs allowed for better impact properties of BGRP-1 and BGRP-2 than those of BFRP-40 but poorer impact properties of BGRP-3 than those of BFRP-40. The high-strength glass fiber meshes in BGRP-1 and BGRP-2 could exert their performance after being impacted, and the hybrid structure with many hybrid interfaces could restrain the crack expansion. The total absorbed energy of all BGRPs was lower than that of BFRPs due to the smaller thickness of the glass fiber meshes, which were inferior to thicker bamboo fiber mats in retarding crack expansion. BGRPs followed an order of BGRP-1 > BGRP-2 > BGRP-3 in terms of breaking force, maximum force, fracture energy, and total absorbed energy. The impact properties of BGRP-1 were superior to those of BGRP-2 and BGRP-3. BGRP-1 had two layers of glass fiber meshes on the surface and bottom layers, such that the fiber stacking sequence of BGRP-1 made the glass fiber meshes damaged first upon impact and acted as buffers for the bamboo fibers inside (Zhou et al. 2020). Yield force is the force at which the material stress reaches its yield value. The yield force of BGRP-2 was higher than that of BGRP-1, ascribed to the most hybrid interfaces of BGRP-2 and the best dispersion of fiber layers, allowing for more effective stress transfer between different fiber layers.

Conclusions
In this study, three bamboo fiber/glass fiber mesh reinforced polymer hybrid composites (BGRP-1, BGRP-2, and BGRP-3) were fabricated by vacuum-assisted resin transfer molding (VARTM) to determine the effects of the corresponding three fiber stacking sequences (GBBGBBG, BGBGBGB, and BBGGGBB; B: bamboo fiber, G: glass fiber mesh) on the properties and preparation efficiency of BGRPs. Glass fiber meshes could shorten epoxy resin injection time, increasing the effectiveness of the bamboo fiber reinforced polymer composite (BFRP) preparation by VARTM. The epoxy resin had the optimal impregnation effect on bamboo fibers when the fiber stacking sequence was BGBGBGB. In contrast, when the fiber stacking sequence was BBGGGBB, the epoxy resin was not adequately impregnated on the bamboo fibers, and more voids were observed in the middle of BGRP-3 in micro-CT slices. BGRP-1 presented the lowest water absorption of 1.31% and the highest shear and drop hammer impact properties. This was mainly because two layers of glass fiber meshes on the surface and bottom of BGRP-1, respectively, served as buffer layers to retard water infiltration and mechanical damage. BGRPs followed an order of BGRP-1 > BGRP-2 > BGRP-3 in terms of breaking force, maximum force, fracture energy, and total absorbed energy. BGRP-2 exhibited the highest flexural strength (92.22 MPa) and flexural modulus (6.69 GPa), which were improved by 42. 82% and 8.08%,  respectively, compared to the BFRPs without the incorporation of glass fiber meshes. This was due to more hybrid interfaces of BGRP-2, better dispersion of fiber layers, and more effective stress transfer between fiber layers. Since BGRP-3 contained more voids than BGRP-1 and BGRP-2, its mechanical properties were less favorable.