The Influence of pH Environments on the Long-Term Durability of Coir Fiber-Reinforced Epoxy Resin Composites

Abstract

This study investigates the effects of different pH environments on the durability of coir fiber-reinforced epoxy resin composites (CFRERCs). The CFRERCs were prepared by combining alkali-treated coir fibers with epoxy resin and exposing them to acidic, alkaline, pure water, and seawater environments for a 12-month corrosion test. The results show that an alkaline environment has the most significant impact on the tensile strength of CFRERCs, with a 55.06% reduction after 12 months. The acidic environment causes a 44.87% decrease in strength. In contrast, tensile strength decreases by 32.98% and 30.03% in pure water and seawater environments, respectively. The greatest reduction in tensile strain occurs in the alkaline environment, with a decrease of 36.45%. In the acidic environment, tensile strain decreases by about 25.56%, while in pure water and seawater, the reductions are 18.78% and 22.65%, respectively. In terms of stiffness, the alkaline environment results in a 49.51% reduction, while the acidic environment causes a 54.56% decrease. Stiffness decreases by 43.39% in pure water and 36.72% in seawater. Field emission scanning electron microscope (FE-SEM) analysis shows that corrosive agents in different pH environments cause varying degrees of damage to the microstructure of CFRERCs. In the acidic environment, corrosive agents erode the fiber–resin interface, leading to delamination and fiber breakage. In the alkaline environment, corrosive agents penetrate the fiber interior, increasing surface roughness and porosity. While pure water and seawater also cause some damage, their effects are relatively mild.

1. Introduction

In recent years, geosynthetics have emerged as an innovative, efficient, and cost-effective solution to address numerous engineering challenges in construction [1]. Geosynthetics are extensively used in road construction, slope stabilization, and the seismic protection of structures [2]. They are pivotal in enhancing the quality, stability, and longevity of highway projects, offering critical reinforcement and support to engineering structures [3].

In the field of geosynthetics, fiber-reinforced composite materials have gained significant attention in both research and practical use due to their superior mechanical properties and durability. These composites offer advantages over conventional materials, including being light weight, high strength, and resistant to corrosion, making them highly suitable for infrastructure projects such as bridges, tunnels, and roads [4]. Researchers worldwide have thoroughly explored fiber-reinforced geosynthetic reinforcements composed of various materials. Among these, glass fiber and basalt fiber are frequently utilized due to their superior mechanical properties and durability [5,6]. However, the high cost and environmental impact associated with these materials have led researchers to investigate natural fiber composites. Natural fibers such as coir, jute, and hemp are abundant, renewable, and environmentally sustainable [7,8,9]. These natural fiber composites are capable of effectively fulfilling the strength and durability requirements of engineering applications [10,11,12]. Although natural fiber composites offer numerous advantages, several challenges persist when incorporating natural fibers into polymer matrix composites. A primary concern is the weak interfacial bonding between the fibers and the matrix, which hampers effective stress transfer and consequently reduces mechanical performance. Furthermore, natural fibers have a tendency to absorb moisture, which can lead to degradation and a decline in material performance under various environmental conditions. Another challenge is the poor dispersion of fibers within the matrix, which can negatively impact the uniformity of the composite.

To overcome these challenges, a variety of chemical and physical methods have been developed. For instance, alkali treatment is frequently employed to remove impurities and increase the surface roughness of fibers, thereby improving fiber–matrix adhesion. Additionally, methods such as coating fibers with hydrophobic nanoparticle copolymers, as well as incorporating additives like carbon black (CB) and graphene oxide (GO), have proven effective in enhancing the interfacial adhesion between fibers and the matrix, which in turn improves the mechanical properties, water resistance, and biodegradability of the composites [13,14]. These approaches enhance the overall performance and durability of natural fiber composites in engineering applications, aligning with the national strategy for low-carbon and sustainable construction.

Coir fiber distinguishes itself from other natural fibers, such as jute, flax, and cotton, due to its exceptionally high lignin content, which can reach up to 45.84%, the highest of any known natural fiber [15,16]. Kumar et al. [17] assessed the mechanical properties of banana/coir fiber-reinforced epoxy composites through experimental modeling and process optimization. Their findings revealed that the mechanical properties, including tensile, compressive, impact, and flexural strengths, were maximized with a fiber/matrix volume ratio of 65:35, a fiber length of 10 mm, and a banana/coir fiber ratio of 75:25. This optimized hybrid composite holds promise for use in the automotive, marine, and aerospace sectors, especially for components subjected to low and medium loads. Yahya et al. [18] employed vacuum-assisted resin transfer molding to produce natural fiber-reinforced epoxy composites using cotton, sisal, coconut shell, and wool fibers, and evaluated their mechanical, wear, and thermal properties. The coir fiber composite exhibited the lowest tensile strength at 15.34 MPa and had a poor wear rate and poor thermal conductivity, consistent with the coarse nature of the fibers. However, it displayed the highest specific heat capacity at 26.313 MJ/m³K, making it an effective insulation material for energy-saving applications. Ru et al. [19] explored the impact of alkali treatment and acetylation on the physical and mechanical properties of coir fibers and their epoxy composites. Using techniques such as thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and SEM, they observed that both treatments significantly improved the tensile strength of coir fibers, with an increase of 69% for alkali-treated fibers. The tensile strengths of alkali-treated long- and short-fiber composites reached 34.51 MPa and 21.14 MPa, respectively, while the flexural strengths of acetylated long- and short-fiber composites were 54.38 MPa and 53.46 MPa, respectively. These modifications notably enhanced the interfacial bonding between the fibers and the matrix, greatly improving the mechanical properties of the composites and paving the way for broader applications of natural fiber composites. Jéssica and José [20] compared the effects of three chemical treatments—sodium hydroxide, oxalic acid, and sodium bicarbonate—on coir fibers in cementitious materials. The study revealed that oxalic acid treatment for 1 h was the most effective, substantially removing extracts, lignin, and hemicellulose from the fibers and increasing tensile strength by 22.9%. FTIR confirmed the removal of some amorphous substances, which increased fiber surface roughness and enhanced the fiber–matrix interaction in the cement, thereby improving the performance of the cementitious materials. This research provides a promising chemical treatment method for the application of coir fibers in construction materials. Yijian et al. [21] examined the changes in the performance of coir fibers after treatment with alkali solution immersion and boiling. The findings indicated that, compared to untreated fibers, the water absorption rate of coir fibers decreased by 40.25% and 48.56% following 2 h of boiling or 5 h of alkali treatment, respectively, while surface static friction increased by 209.41% and 262.35%. Siddika et al. [22] observed that samples treated with 5% NaOH showed improvements in tensile, bending, impact, and hardness properties, whereas samples treated with 10% NaOH exhibited reduced performance. Abdullah and Ahmad [23] highlighted that the use of coir fibers in combination with polyester composites can decrease the water absorption rate and enhance fiber–matrix interactions. Moreover, coir fiber-reinforced polypropylene composites have significantly lower water absorption compared to untreated coir fibers. Sumi et al. [24] discovered that the degradation of coir fibers is primarily driven by microorganisms and other soil factors, such as humidity, temperature, and pH, with degradation beginning at the pores on the fiber surface. However, when a coir geotextile was combined with a cashew nut shell liquid, its resistance to microbial erosion increased by over 70%. Sayida et al. [25] conducted a study in which a coir geotextile was buried in five different soil types for 135 days. They analyzed the biodegradation rate by comparing the tensile strength and load-bearing capacity of the fiber fabric before and after burial. The results showed that the degradation rate in highly compressible clay was the slowest. The initial 30 days saw a slow degradation rate, which then accelerated significantly. The majority of current research predominantly addresses the short-term performance assessment of natural fiber-reinforced composites or the optimization of chemical treatments aimed at improving their mechanical properties [26,27,28]. Nevertheless, research on the long-term durability of these materials in various corrosive environments, including acidic, alkaline, and seawater conditions, remains scarce. Additionally, the use of coconut shell fiber in geotechnical applications is still relatively uncommon.

This study aimed to investigate the durability evolution of coconut shell fiber-reinforced composites under prolonged exposure to acidic and alkaline geotechnical environments. A comprehensive 12-month evaluation was conducted on alkali-treated coconut shell fiber-reinforced epoxy composites in various corrosive conditions, including acidic, alkaline, pure water, and seawater. Changes in the material’s mechanical properties, such as tensile strength, strain, and stiffness, were assessed under different environmental conditions, and scanning electron microscopy (SEM) was used to analyze the microstructural changes. The degradation mechanisms and durability evolution of the composites during long-term environmental exposure were analyzed. The findings not only deepen the theoretical understanding of the long-term durability changes of coconut shell fiber-reinforced composites but also provide essential theoretical support and experimental evidence for their application as sustainable materials in the geotechnical field. This study holds significant engineering application value and environmental implications, especially in promoting low-carbon and green construction practices.

2. Materials and Methods

2.1. Experimental Materials

In this study, coir fibers, with diameters ranging from 250 to 300 μm and a bulk density of 1.25 g/cm³, were sourced from Haikou, Hainan. Hydrochloric acid (HCl, 0.1 mol/L concentration) and sodium hydroxide (NaOH, solid granules, 96% purity) were obtained from Shanghai Boer Chemical Reagent Co., Ltd. (Shanghai, China) Furthermore, oxalic acid dihydrate (C₂H₂O₄∙2H₂O, solid granules, 99.5% purity) was purchased from Xilong Chemical Co., Ltd. (Xiamen, China). The E51 epoxy resin and 650 curing agent, both of industrial grade, were provided by Sinopec Baling Petrochemical (Yueyang, China) and Dongguan Elike New Materials Co., Ltd. (Dongguan, China), respectively.

2.2. Principal Equipment and Instruments

The instruments utilized in this experiment are detailed in

Table 1. Key equipment and instruments.

Instrument Name	Model	Manufacturer	Application
Electric Blast Drying Oven	FX101-3	Shanghai Shuli Instrument Co., Ltd. (Shanghai, China)	Drying samples
Microcomputer-Controlled Electronic Universal Testing Machine	WDW-100C	Shanghai Hualong Testing Instrument Co., Ltd. (Shanghai, China)	Tensile strength testing
Field Emission Scanning Electron Microscope (FE-SEM)	S-3000N	Hitachi, Ltd., (Tokyo, Japan)	Observing microstructural changes

.

2.3. Sample Preparation and Experimental Procedures

2.3.1. Pretreatment of the Coir Fibers

To enhance the interfacial bonding between the coir fiber and the epoxy resin, the fibers underwent the following treatment. They were first thoroughly rinsed with water to remove dust and impurities. After cleaning, the fibers were dried at 50 °C for 24 h and then stored for later use. At room temperature, the dried fibers were immersed in a 5% sodium hydroxide solution for 6 h, with a fiber-to-solution mass ratio of 1:10. After soaking, the fibers were washed several times with a 2% oxalic acid solution to neutralize any remaining alkali, followed by rinsing with deionized water until the rinse water was neutral. Finally, the fibers were dried in a forced-air oven at 50 °C for 24 h to ensure complete dryness.

2.3.2. Preparation of CFRERCs

To enhance the surface activity of the coir fibers, they underwent alkali treatment. After the treatment, the fibers were cut into uniform segments, each measuring 1 cm in length. The materials were weighed with a mass ratio of 1:99 for coir fibers to epoxy resin. A composite reinforcement mold with 1 cm rib spacing was selected. A mixture of epoxy resin and curing agent, with a mass ratio of 1:3, was injected into the mold using a syringe until the mold was half-filled. The mold was then vibrated to eliminate any air bubbles trapped in the resin. The cut coir fibers were evenly distributed over the resin layer to ensure consistent fiber alignment. The remaining resin mixture was injected into the mold to completely fill it, and the surface was leveled with a spatula to ensure a smooth finish. The mold was then left to cure at room temperature for 48 h. After curing, the composite material was demolded. The final composite was cut to the desired specifications, with attention to ensuring smooth and clean edges for subsequent testing and analysis. The procedure is depicted in Figure 1.

2.3.3. Experimental Design

In this study, the pretreated CFRERCs were exposed to alkaline, acidic, pure water, and seawater conditions for 12 months (see Figure 2). The pH of each solution was maintained at a specific value: 5 for the acidic solution, 10 for the alkaline solution, 7 for pure water, and 8 for seawater. The pH of the acidic and alkaline solutions was regularly adjusted by adding HCl or NaOH to compensate for any changes due to evaporation or chemical reactions. Additionally, the pure water and seawater were replaced weekly to maintain consistency. Samples from each environment were retrieved monthly and analyzed for tensile properties using a microcomputer-controlled universal testing machine to track changes in tensile strength, strain, and stiffness. To gain deeper insights into the effects of these environments on the coir fiber in CFRERCs, FE-SEM was employed to perform a detailed examination of the fibers exposed during tensile failure. This methodology enables a thorough evaluation of the long-term impact of environmental exposure on the mechanical properties and microstructural characteristics of CFRERCs.

2.4. Testing Methodology

2.4.1. Tensile Testing

Tensile strength testing: In accordance with the SL 235-2012 standard [29], the tensile properties of the CFRERCs were assessed using a WDW-100C microcomputer-controlled universal testing machine, as depicted in Figure 3. The specimens, measuring 20 mm × 150 mm × 5 mm, were prepared for testing by first adjusting the distance between the machine’s grips to match the 120 mm gauge length of the samples. The tensile speed was set to 24 mm/min. Each specimen was clamped securely before the machine applied a continuous tensile force until fracture occurred. Testing was conducted on a group of six specimens, with the maximum force recorded and converted to tensile strength per unit width. The average value was used as the final result. The calculation for tensile strength is provided in Equation (1).

$$T={\displaystyle \frac{F\times N}{n}}$$

(1)

where T denotes the tensile strength of CFRERCs in kN/m, F is the maximum tensile force exerted on the specimen in kN, N refers to the number of ribs per meter of specimen width in ribs/m, and n represents the number of ribs in the specimen.

2.4.2. FE-SEM Analysis

The microstructure of the CFRERCs was examined using a field emission FE-SEM. A small segment was first excised from the CFRERCs and secured onto a metal stub with conductive adhesive, as depicted in Figure 4. The secured sample was then coated with gold for 5 min using a sputtering process at an accelerating voltage of 10 kV to enhance surface conductivity, as shown in Figure 5. Finally, the sample was placed in the SEM for detailed examination, as illustrated in Figure 6.

3. Results and Discussion

3.1. Durability Evaluation of CFRERCs

Figure 7 illustrates the stress–strain behavior of coir fiber-reinforced epoxy resin composites (CFRERCs) across 12 months in four distinct pH environments: pure water, seawater, acidic, and alkaline. Data for each month is based on six sample groups, with averages calculated after excluding outliers.

Figure 7a–d demonstrates that the mechanical properties of CFRERCs are significantly affected by different pH environments. In all the tested environments, a gradual decline in both stress and strain was observed month by month. A closer analysis of the fluctuations in the stress–strain curves reveals that the curve in the acidic environment is the smoothest, indicating greater stability, followed by the pure water environment, which shows slight but manageable fluctuations [30]. In contrast, the curves in the seawater and alkaline environments display significant instability, with more pronounced fluctuations. These variations are primarily due to the differential degradation effects caused by corrosive agents in the respective pH environments [31,32].

The stress–strain curves for all environments display a yield plateau. This plateau is observed when the stress of CFRERCs remains nearly constant over a strain range of about 0% to 1%, after which the stress increases with further strain. The presence of the yield plateau can be attributed to the composite reinforcement’s tensile zone, which includes two longitudinal reinforcements and four transverse ribs, as shown in Figure 3. Upon application of tensile force, the longitudinal reinforcements shift toward the specimen’s central axis, while the transverse ribs bear the load. The interaction between the coir fibers and the resin matrix causes a redistribution of internal stress within the tensile zone, leading to a situation where tensile strain increases without an immediate rise in stress. As the external load continues to increase, the coir fibers begin to slide or debond, pushing the specimen into an accelerated failure phase, characterized by increasing stress with increasing strain. This phenomenon underscores the energy absorption and deformation capabilities of CFRERCs, highlighting their crucial performance attributes in practical use.

Figure 8 depicts the changes in tensile strength of CFRERCs over a 12-month period in four distinct pH environments: acidic, alkaline, pure water, and seawater.

The data indicate a consistent decline in tensile strength across all environments, with significant differences in the rate and extent of this decline among the different pH conditions. In the alkaline environment (NaOH), the tensile strength of CFRERCs experiences a sharp decline from 54.83 kN/m to 24.64 kN/m, a reduction of 55.06%. In the acidic environment (HCl), the tensile strength decreases from 54.08 kN/m at the start to 29.81 kN/m after 12 months, representing a decrease of 44.87%. In pure water, the tensile strength drops from 53.80 kN/m to 36.05 kN/m, reflecting a 32.98% reduction. In seawater, the strength falls from 54.93 kN/m to 38.43 kN/m, showing a reduction of 30.03%. These results indicate that the tensile strength of CFRERCs is most significantly impacted by the alkaline environment, followed by the acidic, pure water, and seawater conditions. The alkaline environment (NaOH) exerts the strongest corrosive effect on the material, resulting in a rapid decrease in strength. Although the acidic environment (HCl) also causes significant corrosion, it is less severe compared to the alkaline environment (NaOH) [33]. The effect of pure water is primarily due to a gradual decline in physical properties from prolonged exposure. Seawater, being weakly alkaline, has a milder corrosive impact, with the presence of salts and microorganisms forming a protective layer on the coir fibers, thereby maintaining relatively better material properties [34]. The above research results are consistent with the findings of Sumi et al. [35], Ahad et al. [36], and Obada et al. [37].

The CFRERCs exhibited a marked decline in tensile strength during the first 9 months in the alkaline, acidic, and pure water environments, followed by a stabilization phase in the subsequent 3 months. In the alkaline environment (NaOH), tensile strength decreased from 54.83 kN/m to 25.74 kN/m over the first 9 months, a reduction of 29.09 kN/m, then dropped slightly further to 24.64 kN/m in the following 3 months, a decrease of 1.10 kN/m. In the acidic environment (HCl), the tensile strength decreased from 54.08 kN/m to 31.78 kN/m, a decrease of 22.30 kN/m, and dropped further to 29.81 kN/m in the next 3 months, a reduction of 1.97 kN/m. In pure water, the tensile strength fell from 53.80 kN/m to 37.50 kN/m in the first 9 months, a decrease of 16.30 kN/m, and slightly reduced to 36.05 kN/m in the subsequent 3 months, a decrease of 1.45 kN/m. The reduction rates in the final 3 months were only 11.40%, 26.51%, and 25.71% of the corresponding reductions in the first 9 months for the alkaline, acidic, and pure water environments, respectively. These results indicate that the significant erosion effects of acidic and alkaline conditions on CFRERCs occur predominantly in the first 9 months. This is likely because the initial presence of more susceptible substances, such as lignin and pectin, in the coir fibers leads to greater corrosion and a more rapid reduction in tensile strength [38]. After 9 months, the fibers reveal more corrosion-resistant cellulose, slowing down the rate of strength reduction. In contrast, in pure water, CFRERCs undergo less aggressive physical immersion corrosion, resulting in a more gradual change in tensile strength after 9 months.

In the seawater environment, the tensile strength of CFRERCs decreased from 54.93 kN/m to 41.39 kN/m over the first 9 months, a drop of 13.54 kN/m. In the following 3 months, it slightly declined further to 38.43 kN/m, an additional decrease of 2.96 kN/m. The rate of reduction in the last 3 months was 61.31% of that in the initial 9 months. Unlike the alkaline environment, the erosion effect of seawater on CFRERCs has not yet diminished, as the tensile strength continues to decline. This is likely because, initially, other substances in seawater may have provided some level of protection to the coir fibers, slowing the erosion process. By the 12th month, the surface still had a substantial amount of easily corroded substances, leading to ongoing erosion and a continued decrease in tensile strength.

Figure 9 presents the tensile strain trends of coir fiber-reinforced epoxy resin composites (CFRERCs) subjected to four distinct pH environments (acidic, alkaline, pure water, and seawater) over a 12-month period.

Figure 9 presents the tensile strain data for CFRERCs across different pH environments over a 12-month period. In an alkaline environment (NaOH), the tensile strain initially decreased to 4.31% in the first month, continuing to decline to 2.76% by the 12th month. In an acidic environment (HCl), tensile strain initially increased to 4.48% in the first month, then gradually decreased to 3.23% by the end of the 12 months. In pure water, tensile strain rose to 4.71% in the first month and subsequently decreased to 3.53% by the 12th month. In seawater, tensile strain peaked at 5.03% in the first month and then gradually declined to 3.36% by the 12th month. The data suggest that the initial increase in tensile strain could be attributed to the moisture-induced swelling of the CFRERCs during the initial phase [39]. This swelling led to a temporary increase in strain during the first month. Additionally, the largest reduction in tensile strain occurred in the alkaline environment (36.45%), followed by the acidic environment (25.56%) and the seawater environment (22.65%), with the smallest reduction observed in the pure water environment (18.78%). This indicates that the alkaline environment has the most pronounced effect on the tensile strain of CFRERCs, while the pure water environment has the least impact.

Figure 9 illustrates that the tensile strain of the CFRERCs decreased by 35.49% over the first 8 months in the alkaline environment (NaOH) and by an additional 1.48% in the following 4 months. In the acidic environment (HCl), there was a significant decrease of 20.96% in tensile strain during the first 8 months, with a minimal reduction of 5.84% in the subsequent 4 months. For pure water, the tensile strain dropped by 15.84% in the initial 8 months and by 3.47% in the final 4 months. In seawater, the strain decreased by 16.31% over the first 8 months and by 7.56% in the last 4 months. The most rapid decline in tensile strain occurred in the alkaline environment (NaOH), likely due to its stronger damaging effect on CFRERCs compared to other environments [40,41,42]. The chemical reaction between NaOH and the coir fibers and epoxy resin accelerates material degradation, leading to severe damage at the fiber–matrix interface and a significant drop in tensile strain. The rate of strain reduction slowed after the 8th month in the alkaline environment, possibly because the CFRERCs had already undergone significant degradation and reached a stable phase, slowing further deterioration. This phenomenon may be attributed to the formation of a protective layer of corrosion products on the material’s surface, which reduces the ingress of corrosive agents [43,44]. Additionally, after the initial water absorption and swelling phase, the internal structure of the material stabilizes, resulting in a natural reduction in the rate of strain change.

From the first to the seventh month, the tensile strain of the CFRERCs in the seawater environment was always higher than that in the pure water environment. However, between the seventh and ninth months, the tensile strain of the CFRERCs in the seawater environment began to be lower than that in the pure water environment. Specifically, in the seawater environment, the tensile strain decreased from 3.80% in the seventh month to 3.54% in the ninth month, while in the pure water environment, the tensile strain only decreased from 3.69% in the seventh month to 3.67% in the ninth month. This phenomenon may be caused by several factors. During the early corrosion phase, the salts and other chemical components in seawater caused the CFRERCs to absorb more water and expand, resulting in higher ductility in the seawater environment than in the pure water environment [34]. As a result, the tensile strain in the seawater environment was greater than that in the pure water environment. However, the overall weak alkalinity of seawater leads to more intense corrosion of CFRERCs compared to pure water, causing the corrosion rate of the CFRERCs in the seawater environment to be faster. In contrast, the corrosion process in pure water occurs at a relatively slow and steady rate, resulting in a more gradual degradation. Throughout the corrosion period, the tensile strain decay rate of the CFRERCs in the seawater environment was higher than that in pure water for the majority of the time, only becoming lower than that in pure water in the later stages.

Figure 10 depicts the variation in stiffness of coir fiber-reinforced epoxy resin composites (CFRERCs) over a 12-month duration in acidic, alkaline, pure water, and seawater environments.

Figure 10 illustrates that the stiffness of the CFRERCs decreased most significantly in the acidic environment (HCl), dropping to 9.71 kN/m by the 12th month, representing a 54.56% reduction. The stiffness in the alkaline environment (NsOH) also decreased markedly, reaching 10.78 kN/m after 12 months, with a 49.51% reduction. In contrast, the reduction in stiffness in both the pure water and seawater environments was relatively modest, with final values of 12.09 kN/m and 13.52 kN/m, corresponding to decreases of 43.39% and 36.72%, respectively. Long-term observations suggest that the acidic environment has the most significant impact on the stiffness of CFRERCs, followed by the alkaline, pure water, and seawater environments.

3.2. SEM Analysiss

Figure 11 presents FE-SEM images that depict the changes in the microstructure of coir fiber-reinforced epoxy resin composites (CFRERCs) in their initial form and after 12 months of exposure to acidic, alkaline, pure water, and seawater conditions. The SEM scans were performed on the ruptured parts of the samples, focusing on the lateral textures of the coir fibers, to investigate the fiber–matrix interface bonding and changes in structural integrity.

Figure 11 reveals that in the initial state, the coir fibers, after alkali treatment, had some pectin removed from their surfaces and developed a pronounced wrinkled structure. This wrinkled structure enhances the interface bonding area between the fibers and the epoxy resin, thereby improving the interface strength and overall mechanical performance of the composite material. After 12 months of corrosion in different environments, the coir fibers exhibited varying degrees of structural damage and degradation. In the alkaline environment, the damage was most pronounced, with complete erosion of the pectin on the fiber surfaces, exposing the fibers and leading to torn fiber strands [45,46]. The corrosion not only roughened the fiber surfaces but also created noticeable gaps between the epoxy resin and the coir fibers, significantly reducing the mechanical properties of the CFRERCs due to interface delamination. In the acidic environment, the fibers also experienced considerable corrosion. Although the pectin was fully eroded, the slower erosion of lignin and cellulose in the acidic environment meant that 12 months was insufficient to cause severe internal damage to the fibers [47]. Thus, lignin and cellulose did not exhibit significant degradation. In the pure water environment, while the pectin was not entirely eroded, it underwent noticeable deformation and partial hydrolysis after 12 months of soaking [48]. This hydrolysis did not severely damage the fiber’s main structure but led to swelling and softening of the pectin layer, potentially affecting the interface bonding strength. In the seawater environment, corrosion was relatively minor. After 12 months of immersion, salts from the seawater formed solid deposits in the gaps between the epoxy resin and the coir fibers [36,49]. These deposits acted as a protective barrier, slowing the corrosion rate, and filled gaps between the resin and fibers, enhancing the interface bonding and leading to a slower decline in the mechanical properties of the CFRERCs.

To summarize, the effects of various corrosion environments on coir fibers exhibit notable differences. Alkaline environments inflict the most severe damage, followed by acidic environments, with pure water and seawater environments having a comparatively lower impact. The structural alterations in the coir fibers are closely linked to the type of corrosion environment, and these alterations significantly affect the mechanical properties and long-term stability of CFRERCs.

4. Conclusions

(1)

The study reveals a consistent degradation of tensile strength in CFRERCs across all pH environments over a 12-month period. The greatest decline occurs in the alkaline environment (NaOH), with a 55.06% reduction, predominantly within the first 9 months. This is followed by a 44.87% decrease in the acidic environment (HCl), also mainly during the first 9 months. The pure water and seawater environments show more gradual reductions of 32.98% and 30.03%, respectively, with the seawater degradation continuing beyond 9 months. The initial rapid degradation in alkaline and acidic conditions is attributed to the breakdown of lignin and pectin, leaving corrosion-resistant cellulose, which moderates further strength loss. The seawater environment causes continuous degradation due to ongoing exposure to salts and microorganisms.

(2)

Distinct trends in tensile strain were observed across pH environments. The largest reduction of 36.45% occurs in the alkaline environment, followed by 25.56% in acidic conditions and 22.65% in seawater. The smallest reduction, 18.78%, is in pure water. Initially, tensile strain increases due to moisture-induced swelling. In seawater, the initial higher tensile strain is influenced by the protective effects of salts and microorganisms, but by the ninth month, tensile strain decreases more rapidly than in pure water. This accelerated reduction is due to seawater’s mildly alkaline nature, which increases the degradation rate compared to pure water.

(3)

Long-term exposure to the acidic environment has the most significant impact on stiffness, with a 54.56% decrease. The alkaline environment follows with a 49.51% reduction, while pure water and seawater show smaller decreases of 43.39% and 36.72%, respectively.

(4)

FE-SEM analysis reveals the distinct effects of corrosive environments on the fibers’ structural integrity. Alkaline environments cause severe damage, including complete pectin erosion and extensive fiber surface roughening, leading to substantial reductions in mechanical strength. Acidic environments also erode pectin, but the slower degradation of lignin and cellulose results in less severe damage. In pure water, pectin undergoes deformation and partial hydrolysis, leading to swelling and softening that may weaken interface bonding without significantly damaging the fiber structure. Seawater exposure results in minimal corrosion, with salt deposits forming a protective layer that slows degradation, helping to maintain mechanical properties better than other environments.