Effect of chemical treatments,hybridizations and water absorption on flexural strength of sisal/polyester composites

Abstract

This study examined the effects of chemical treatments and hybridization on the flexural strength and water absorption-induced degradation of sisal/polyester composites. Sisal fibers were treated with sodium hydroxide and sodium bicarbonate prior to composite fabrication. Hybrid reinforcements included unidirectional glass and carbon fibers, as well as glass and carbon fillers. Various stacking sequences, fiber orientations, and volume fractions were employed. For filler-reinforced composites, four filler contents (2.5 wt%, 5 wt%, 7.5 wt%, and 10 wt%) and two sisal fiber contents (20 wt% and 30 wt%) were tested. Results indicated that chemical treatment notably improved flexural strength, with sodium bicarbonate treatment producing a 25% increase over untreated fibers. Hybridization with unidirectional carbon fibers, especially when positioned in the compressive layer of the laminate, enhanced flexural strength by approximately 140%. Among filler-reinforced composites, those containing 30 wt% sisal fibers combined with 5 wt% carbon fillers showed the highest flexural strength, exceeding filler-free composites by 30.9%. Water absorption reduced flexural strength across all samples; however, degradation was significantly lessened by chemical treatment and hybridization. The composite with 12% carbon fiber volume demonstrated the lowest flexural strength reduction of 2.7%, corresponding to an 84% decrease in degradation compared to untreated samples.

Keywords

sisal hybridization chemical treatment flexural strength degradation filler

Introduction

In recent times, natural fibers are considered as suitable reinforcement materials to make composites for different lightweight applications such as automotive parts, airplane seats, and racing sailboat.¹ The biodegradability, renewability, low densities, and low processing costs of natural fibers are some factors that make them emerge as a better replacement of synthetic fibers.^2,3 Sisal plants are a major source of natural fibers, growing abundantly in the northern region of Ethiopia. However, their use remains traditionally limited to crafts such as bags, baskets, ropes, and rugs. Sisal fibers have not been widely adopted for industrial applications due to several factors: their relatively low strength, incompatibility with polymer matrices, high water absorption properties, and susceptibility to degradation when exposed to water. As a result, recent research is focusing on improving the performance of natural fibers reinforced composites through treatments, hybridization, and optimizing stacking sequences and fiber orientation of the hybrid fibers.^2,4–13

Flexural strength, a critical mechanical property, significantly influences the load-bearing capacity of structures subjected to bending stresses. To enhance the suitability of sisal/polymer composites for structural applications, it is essential to improve their flexural strength. This can be achieved through treatments, hybridization, and adjustments to the sequence of hybrid fibers.^14–17 Rozali et al.¹⁵ reported hybrid composites with kenaf at the inner and glass fiber at the outer layers have displayed the highest flexural strength and modulus. Jarukumjorn and Suppakarn¹⁸ studied the effect of reinforcing glass fiber into sisal polypropylene composites. The flexural strength of the composite was significantly improved without any influence on its flexural modulus. Ganapathy et al.¹ studied the effect of graphene powder on banyan aerial root fibers reinforced epoxy composites. Maximum increment of 5% in flexural strength was reported for a 4 wt% of graphene powder filled composite. Further increase in the grapheme content decreased the flexural strength. Saiteja et al.¹⁹ reported highest flexural strength in a jute reinforced hybrid polymer composite filled with 8% volume of carbon nano-tube fillers. The flexural strength decreased through further addition in the nano-tube fillers due to agglomeration of nano-tube fillers in the composite.

Water absorption significantly reduces the flexural strength of natural fiber reinforced polymer composites.^20–23 Chaudhary et al.²¹ reported a reduction of up to 45.1% in the flexural strength of these composites due to water absorption. However, hybridization and treatments can ameliorate this degradation. For instance, incorporating 10% weight of glass fiber plies into sisal fiber-reinforced composites immersed in water for 15 days reduced the degradation in flexural strength from 26% to 8%.²²

Existing research on sisal/polyester composites often overlooks the combined effects of chemical treatment and hybridization on their mechanical properties and durability. This study addresses this gap by investigating the influence of these factors on flexural strength and water absorption degradation. Sisal fibers were treated with sodium hydroxide and sodium bicarbonate to enhance their interfacial bonding with the polymer matrix, and the effects were compared to untreated fibers. Unidirectional glass and carbon fibers, along with glass and carbon fillers, were incorporated as hybrid reinforcements. The study aims to optimize sisal/polymer composites for structural applications by improving their flexural strength and resistance to water absorption degradation through chemical treatments and hybridizations.

Materials and methods

Materials

Sisal fibers were collected from the outskirts of Mekelle. Commercially available sodium bicarbonate was used to treat the sisal fibers, enhancing surface roughness and improving fiber–matrix adhesion. Glass and carbon fillers (0.1 mm), along with unidirectional carbon and glass fibers, were purchased from Faserverbundwerkstoffe Composite Technology in Germany. The matrix material used in this study was an unsaturated polyester resin (UPR), which was locally sourced. This resin is a thermosetting polymer known for its favorable mechanical properties, low cost, and compatibility with both natural and synthetic fibers. The UPR used had a viscosity of 450–550 MPa·s at 25°C and a density of approximately 1.1–1.2 g/cm³. It exhibited a gel time of 8–15 min at 25°C with 1 wt% catalyst and demonstrated typical mechanical properties such as a tensile strength ranging from 40 to 70 MPa, a flexural strength between 90 and 130 MPa, and a glass transition temperature (Tg) of 60 to 120°C. Butanox M-50, a commercial-grade methyl ethyl ketone peroxide (MEKP), was used as the curing agent and was also locally procured. Butanox is a highly reactive organic peroxide that initiates free radical polymerization in unsaturated polyester systems. It has a density of approximately 1.13 g/cm³ at 20°C, a flash point of around 60°C, and must be stored at temperatures between 10 and 30°C in cool, dry conditions. The recommended dosage for effective curing is typically 1–2 wt% of the resin. In this study, Butanox enabled room-temperature curing of the polyester resin, resulting in a stable and well-crosslinked polymer network.

Fiber treatment

Sodium bicarbonate (NaHCO3), also known as baking soda, is a suitable alkaline treatment for sisal fibers due to its mild nature, safety, environmental friendliness, availability, affordability, versatility, and effectiveness in removing impurities.¹¹ Its alkaline properties enable it to break down impurities, such as lignin, without causing excessive damage to the fibers. Sodium bicarbonate is a safe and non-toxic substance, posing minimal health risks to researchers and workers. Hence, commercially available sodium bicarbonate was selected as one of the treatments for sisal fibers. Thus, the sisal fibers were also treated using baking soda. According to Fiore et al.,¹¹ the optimum time and concentration for achieving maximum mechanical properties are 120 h and 10% weight solution of sodium bicarbonate respectively. Therefore, the fibers were soaked in a 10% weight solution of sodium bicarbonate for 120 h duration. Moreover, sisal fibers were treated with a 6% weight solution of sodium hydroxide (NaOH) for 3 h, following the method recommended by Yahaya et al.⁹ After alkaline treatment, the fibers were washed thoroughly with water to remove any residual NaOH and then allowed to dry at room temperature for 72 h.

Sisal ply preparation

Sisal fibers were cut into 300 mm lengths and manually aligned in a unidirectional arrangement. This alignment method ensured continuous fiber orientation, thereby maximizing the composite’s strength.²⁴ To maintain this alignment, the unidirectional and continuously aligned sisal fibers were sandwiched between sheets of paper on both the top and bottom sides. To prevent any misalignment during handling, the fibers, along with the papers, were stitched together using a sewing machine. After the stitching process, the papers were removed by immersing the sisal fabric in water, allowing for easy separation of the paper from the aligned fibers. The fabric was then left to dry in sunlight for 3 days to ensure it was completely dry and ready for use in composite preparation. The resulting product was a continuously aligned unidirectional sisal fiber fabric, which served as the base material for the fabrication of composite laminates.

Matrix preparation

Firstly, the polyester resin and Butanox hardener were accurately weighed using a digital balance. The matrix was then prepared by thoroughly mixing the polyester resin with the Butanox hardener, following the manufacturer’s recommended mix ratio. This blending process was carried out using mechanical mixing equipment to ensure a homogenous consistency of the mixture. Once properly mixed, the polyester matrix was stored in a suitable container to prevent contamination and premature curing before being used in the laminate preparation.

Furthermore, a modified matrix for filler-hybridized laminates was prepared by adding glass and carbon fillers at weight percentages of 2.5%, 5%, 7.5%, and 10%. Filler loadings were kept below 10% based on previous studies indicating optimal performance at lower filler concentrations.^1,19 The fillers and polyester resin were carefully measured and mixed using a mechanical stirrer to ensure uniform filler distribution within the modified matrix. The desired amount of hardener was then added and mixed for 2 min to ensure proper incorporation.

Fabrication of unidirectional carbon and glass fibers hybridized composite laminates

The composite laminates were fabricated using a combination of hand layup and compression molding techniques. The process involved arranging plies of different fibers in a clean mold and saturating them with a polyester resin mixed with curing agents. The resin was poured onto the plies and then evenly distributed using a brush to ensure complete impregnation. A cylindrical wooden rod was then used to squeeze out air bubbles and excess resin. The mold was closed, and the laminates were cured at room temperature under a constant load of 2 kN for 24 h. After curing, the laminates were carefully removed from the mold, trimmed if necessary, and inspected for quality.

Twenty-three different laminate layups were prepared by varying the stacking sequence, orientation, and relative volume fraction of the sisal and hybrid fibers. While maintaining a constant five-ply configuration and a matrix volume fraction of 55%, literature review results indicate that composites perform best when the weight ratio of reinforcing fiber and matrix ranges from 55% (37% by volume) to 65% (47% by volume) and 35% (53% by volume) to 45% (63% by volume), respectively.^25,26 The layering sequences, volume fractions, laminate designations, chemicals used for treatment, and orientations of the hybrid and sisal fiber plies are presented in Table 1.

Table 1.

Chemicals used for sisal fiber treatment and the stacking sequence, orientation, and volume fraction of fibers and resin.

Laminate name	Stacking sequence	Orientation of sisal fibers	Orientation of hybrid fibers (glass or carbon)	Chemical used for sisal treatment	Volume fraction of samples (%)
Laminate name	Stacking sequence	Orientation of sisal fibers	Orientation of hybrid fibers (glass or carbon)	Chemical used for sisal treatment	Vg	Vs	Vc	V_f
T1	S-S-S-S-S	0°	—	Untreated	0	45	0	45
T2	S-S-S-S-S	0°	—	NaOH	0	45	0	45
T3	S-S-S-S-S	0°	—	NaHCO3	0	45	0	45
T4	S-S-G-S-S	0°	0°	NaHCO3	4	41	0	45
T5	S-S-S-S-G	0°	0°	NaHCO3	4	41	0	45
T6	S-G-S-G-S	0°	0°	NaHCO3	8	37	0	45
T7	S-G-G-S-S	0°	0°	NaHCO3	8	37	0	45
T8	G-G-S-S-S	0°	0°	NaHCO3	8	37	0	45
T9	G-S-S-S-G	0°	0°	NaHCO3	8	37	0	45
T10	G-G-G-S-S	0°	0°	NaHCO3	12	33	0	45
T11	G-S-G-S-G	0°	0°	NaHCO3	12	33	0	45
T12	S-S-C-S-S	0°	0°	NaHCO3	0	41	4	45
T13	S-S-S-S-C	0°	0°	NaHCO3	0	41	4	45
T14	S-C-S-C-S	0°	0°	NaHCO3	0	37	8	45
T15	S-C-C-S-S	0°	0°	NaHCO3	0	37	8	45
T16	C-C-S-S-S	0°	0°	NaHCO3	0	37	8	45
T17	C-S-S-S-C	0°	0°	NaHCO3	0	37	8	45
T18	C-C-C-S-S	0°	0°	NaHCO3	0	33	12	45
T19	C-S-C-S-C	0°	0°	NaHCO3	0	33	12	45
T20	S-G-S-G-S	90°	0°	NaHCO3	8	37	0	45
T21	S-C-S-C-S	90°	0°	NaHCO3	0	37	8	45
T22	S-G-S-G-S	0°	90°	NaHCO3	8	37	0	45
T23	S-C-S-C-S	0°	90°	NaHCO3	0	37	8	45

Fabrication of carbon and glass fillers hybridized composite laminates

The composites were prepared using the hand layup technique followed by compression molding. In order to facilitate the removal of the composites from the mold after curing, a releasing agent was applied. The amount of sisal fibers used in the composites was determined based on previous studies, with 20 wt% and 30 wt% being reported as the optimum levels.^27,28

Furthermore, carbon and glass fillers were incorporated into the composites to compare and utilize the most favorable outcomes. Four different filler proportions (2.5 wt%, 5 wt%, 7.5 wt%, 10 wt%) were used to fabricate the composites. The matrix employed was polyester resin, both in its pure form and modified with the fillers. Two laminates (S1 and S2) were fabricated using the matrix without fillers and the sisal plies, while the remaining 16 composites (S3 to S18) were prepared using the modified matrix and the sisal plies. The sisal plies were carefully placed in a mold and impregnated with the corresponding matrix by pouring it onto the mat. A roller was then used to ensure an even distribution of the matrix throughout the sisal plies and eliminate any trapped air bubbles. The composites were cured by subjecting them to a constant load of 2 kN for 24 h. The dimensions of the composites measured 270 × 150 mm². The composition of the composites, including the amounts of sisal fibers, glass fillers, carbon fillers, and polyester resin, are provided in Table 2.

Table 2.

Designation and composition of fillers on composites laminates.

Laminate	Designation	Composition
S1	20/80 composite	20 wt% sisal +80 wt% polyester
S2	30/70 composite	30 wt% sisal +70 wt% polyester
S3	2.5 G 20/80 composite	Glass filler (2.5 wt% of the 20/80 composite) + 20/80 composite
S4	5 G 20/80 composite	Glass filler (5 wt% of the 20/80 composite) + 20/80 composite
S5	7.5 G 20/80 composite	Glass filler (7.5 wt% of the 20/80 composite) + 20/80 composite
S6	10 G 20/80 composite	Glass filler (10 wt% of the 20/80 composite) + 20/80 composite
S7	2.5 G 30/70 composite	Glass filler (2.5 wt% of the 30/70 composite) + 30/70 composite
S8	5 G 30/70 composite	Glass filler (5 wt% of the 30/70 composite) + 30/70 composite
S9	7.5 G 30/70 composite	Glass filler (7.5 wt% of the 30/70 composite) + 30/70 composite
S10	10 G 30/70 composite	Glass filler (10 wt% of the 30/70 composite) + 30/70 composite
S11	2.5 C 20/80 composite	Carbon filler (2.5 wt% of the 20/80 composite) + 20/80 composite
S12	5 C 20/80 composite	Carbon filler (5 wt% of the 20/80 composite) + 20/80 composite
S13	7.5 C 20/80 composite	Carbon filler (7.5 wt% of the 20/80 composite) + 20/80 composite
S14	10 C 20/80 composite	Carbon filler (10 wt% of the 20/80 composite) + 20/80 composite
S15	2.5 C 30/70 composite	Carbon filler (2.5 wt% of the 30/70 composite) + 30/70 composite
S16	5 C 30/70 composite	Carbon filler (5 wt% of the 30/70 composite) + 30/70 composite
S17	7.5 C 30/70 composite	Carbon filler (7.5 wt% of the 30/70 composite) + 30/70 composite
S18	10 C 30/70 composite	Carbon filler (10 wt% of the 30/70 composite) + 30/70 composite

Testing methods

Flexural tests were performed according to the ASTM D790-10²⁹ standard using a flexural testing machine. Specifically, a Microcomputer Controlled Electro-hydraulic Servo Universal Testing Machine, Model: SI-1000 KN was used for the tensile tests. The dimensions of the specimens were made 125 × 13 × 3.2 mm³ in accordance with the ASTM D790 standard requirements. He et al.³⁰ investigated that placing the hybrid fibers on the compressive side of the laminates lead to a substantial increase in flexural strength compared to having them on the tensile side. Consequently, in all asymmetric laminates tested in this study, the hybrid fibers are positioned on the compressive side.

To study the effect of water absorption on the flexural properties, wet specimens were created by subjecting the specimens to water immersion. This allowed for the absorption of water by the specimens, potentially leading to changes in their flexural behavior. The samples which are prepared for the flexural testing were stayed immersed in water for 45 days with water absorption with every 24 h measurements taken on the water absorption rate and then immersed immediate back to the water container. All samples dimensions were prepared according to standard requirements for flexural strength testing, as the primary objective of this study is to investigate the effect of water absorption on the flexural strength of the laminates. Water absorption testing was conducted in accordance with ASTM D 570–98 to ensure consistency and reliability of results. Then after 45 days the samples were tested for flexural strength.

The flexural tests were conducted using a three-point bending configuration, in accordance with the requirements of the ASTM D790 standard. The specimens were loaded in this configuration, and a constant rate of loading was applied until failure occurred. During the test, the flexural strength of the specimens was determined by analyzing the recorded data. The flexural strength represents the maximum stress experienced by the specimen at the moment of failure in the bending test. Calculating the flexural strength involves analyzing the load-displacement data obtained during the test. The maximum load recorded just before the specimen fracture is considered the flexural strength. This value is typically reported in units of force per unit area. All experiments were conducted under ambient conditions.

Results and discussion

Water absorption

A systematic evaluation of three strategies—chemical surface treatments, synthetic fiber hybridization, and filler incorporation—was conducted to assess their effectiveness in reducing water absorption in natural fiber-reinforced composites. The results, presented in Figure 1(a), (b), and (c), offer quantitative insights that serve as practical benchmarks for improving moisture resistance in such materials. Chemical surface treatments provided a moderate but meaningful reduction in water uptake as shown in Figure 1(a). Among these, sodium hydroxide (NaOH) and sodium bicarbonate treatments produced comparable reductions in water absorption. NaOH treatment achieved a 45.2% reduction, lowering absorption from 12.40% to 6.80%, while sodium bicarbonate resulted in a similar 43.3% reduction, bringing absorption down to 7.03%. These improvements are attributed to chemical modifications of the fiber surface in both treatments, which reduce the number of hydrophilic hydroxyl groups and enhance fiber-matrix adhesion. In contrast, synthetic fiber hybridization showed significantly greater reductions in water absorption. As detailed in Figure 1(b), increasing synthetic fiber content from 4% to 12% resulted in substantial performance improvements. Glass fiber hybridization achieved a 67.1% reduction in water uptake, bringing absorption down from 7.03% to 2.31%. Carbon fibers demonstrated the highest efficacy, achieving a 74.1% reduction, lowering water absorption to 1.82%. Notably, carbon fibers consistently outperformed glass fibers by a margin of 21.2%, likely due to their lower inherent water affinity and superior compatibility with the polymer matrix.

Figure 1.

Water absorption: (a) effect of alkali treatment (b) effect of glass and carbon fiber hybridization (c) effect of glass and carbon filler hybridization.

Filler incorporation also significantly influenced moisture resistance, as shown in Figure 1(c). Composites with a 30/70 matrix-to-fiber ratio exhibited initial water absorption of 7.17%, which was 5.4% higher than the 6.80% observed in the 20/80 system. Despite this difference, both formulations showed similar trends in response to filler addition. In the 30/70 system, the inclusion of 10% glass filler reduced water absorption from 7.17% to 4.11%, representing a 42.7% reduction, while the addition of 10% carbon filler further lowered absorption to 3.36%, corresponding to a 53.1% reduction. In comparison, the 20/80 composite system with 10% carbon filler achieved an even greater reduction of 55.0%, lowering absorption from 6.80% to 3.06%. These results underscore the effectiveness of filler incorporation, particularly carbon-based fillers, in limiting moisture uptake. The superior performance of carbon fillers relative to glass can be attributed to their lower intrinsic hydrophilicity, better dispersion within the matrix, and greater ability to disrupt water transport pathways.

For applications demanding maximal moisture resistance, the synergistic integration of alkali surface treatment with carbon fiber hybridization presents an optimal engineering strategy. This bimodal approach demonstrates the potential to yield a total water absorption reduction of approximately 84.9% (from 12.04% to 1.82%), relative to untreated natural fiber composites. The pronounced synergistic effect derived from these combined chemical and structural modifications underscores the intrinsic value of multi-modal strategies in the development of high-performance, moisture-resistant composite materials. Consequently, these findings furnish researchers with a lucid, data-driven foundation for tailoring natural fiber composites for deployment in humid or aqueous environments, while concurrently optimizing for mechanical properties, economic viability, and environmental sustainability.

Flexural strength

Influence of alkali treatment

Figure 2 displays the flexural strength results obtained from the experiments. It is evident that the flexural strength of the sisal/polyester composites is significantly enhanced when treated with sodium hydroxide and baking soda. The untreated composites (T1) exhibited an average tensile strength of 142.2 MPa. In contrast, the sodium hydroxide treated composites (T2) showed an average tensile strength of 176.4 MPa, representing an increase of approximately 24%. The baking soda treated composites (T3) demonstrated the highest average tensile strength of 177.6 MPa, corresponding to a 25% increase compared to the untreated composites. This enhancement aligns with similar findings by Negi et al.,³¹ who reported a comparable 25.55% increase in the flexural strength of kenaf (Hibiscus cannabinus L.) fiber-reinforced epoxy composites through sodium bicarbonate treatment.

Figure 2.

Effect of sodium hydroxide and sodium bi carbonate treatments on flexural strength of sisal/polyester composites.

The observed improvement in flexural strength can be attributed to the alkali treatments’ ability to improve the adhesive characteristics of the sisal fiber surface. Sodium hydroxide and baking soda treatments effectively remove impurities such as hemi cellulose, resulting in a rougher surface topography. This rough surface enhances the interface between the sisal fibers and the polyester matrix, leading to improved bonding and load transfer.^32,33 Furthermore, the rough surface created by the alkali treatments exposes more reactive sites on the fiber surface, facilitating a stronger bond with the matrix material. This enhanced bonding between the sisal fibers and the matrix reduces the possibility of fiber-matrix debonding and improves load transfer, resulting in improved flexural strength.³⁴

In conclusion, the alkali treatments of sodium hydroxide and baking soda significantly enhance the flexural strength of sisal/polyester composites. The improved adhesive characteristics of the sisal fiber surface, achieved through the removal of impurities and the creation of a rough surface topography, contribute to the enhanced bonding between the fibers and the matrix. This, in turn, reduces fiber-matrix debonding and improves load transfer, resulting in improved flexural properties. These findings highlight the potential of alkali treatments as a viable method for enhancing the mechanical performance of sisal/polyester composites.

ANOVA

Table 3 presents the ANOVA results for the effects of sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO₃) treatments on the flexural strength of sisal/polyester composites. For the NaOH treatment, the analysis shows a between-group sum of squares (SS) of 2924.1 with 1 degree of freedom, and a within-group SS of 36 with 8 degrees of freedom, yielding a mean square (MS) of 2924.1 for the treatment and 4.5 for the error term. The corresponding F-value is 649.8, which is well above the F critical value of 5.318, indicating a statistically significant effect. The p-value is 6.01 × 10^-9, confirming that the likelihood of these differences occurring by random chance is exceedingly small. Similarly, for the NaHCO₃ treatment, the between-group SS is 3132.9 and the within-group SS is 40, with the same degrees of freedom structure as the NaOH treatment (1 and 8, respectively). The resulting F-value is 626.58, with a p-value of 6.94 × 10^-9, again demonstrating a highly significant difference in flexural strength between treated and untreated samples. The F critical value remains 5.318, and the calculated F-value far exceeds this threshold. These findings indicate that both NaOH and NaHCO₃ treatments significantly influence the flexural strength of the composite materials. The high F-values and extremely low p-values suggest that the alkali treatments markedly improve flexural strength, likely through chemical modification of the fiber surface that enhances fiber-matrix adhesion. While both treatments are statistically effective, the slightly higher sum of squares and F-value for the NaHCO₃ treatment may suggest a marginally stronger effect.

Table 3.

ANOVA for effect of alkali treatment on flexural strength.

Alkali treatment	Source of variation	SS	df	MS	F	p-value	F crit
NaOH treatment	Between groups	2924.1	1	2924.1	649.8	6.01E-09	5.318
	Within groups	36	8	4.5
	Total	2960.1	9
NaHCO₃ treatment	Between groups	3132.9	1	3132.9	626.58	6.94E-09	5.318
	Within groups	40	8	5
	Total	3172.9	9

Influence of hybridization

Effect of glass fibers

The data presented in Figure 3 clearly illustrates the incorporation of glass fibers significantly enhanced the flexural strength of the composites. The hybrid composites displayed noticeably higher flexural strength values compared to the sisal/polyester composites T3. This improvement can be attributed to the good interfacial bonding between the fibers, which facilitate a uniform load transfer.²² Additionally, the inherent strength of glass fibers, which is higher than that of sisal fibers, further contributed to the improved flexural strength of the hybrid composites. Among the various laminates tested, laminate T10 exhibited the highest flexural strength, measuring 390.4 MPa. This remarkable result represents a substantial increase of approximately 120% compared to the flexural strength of the baking soda treated and non-hybridized laminate T3, which measured 177.6 MPa. Similarly, the second highest performing laminate, T11, demonstrated a noteworthy improvement of approximately 114% compared to T3. Furthermore, the laminates containing 12% volume of glass fiber hybridization (T10) displayed more than 20% higher value than the maximum flexural strength observed in laminates containing 8% volume of glass fiber hybridization (T8). Moreover, the maximum flexural strength among laminates containing approximately 8% volume of glass fiber hybridization (T8) was approximately 33% higher than that observed in laminates containing approximately 4% volume of glass fiber hybridization (T5).

Figure 3.

Effect of glass fiber hybridization on flexural strength of sisal/polyester composites.

Thus, a carefully selection of the volume of glass fiber hybridization in composite materials is important to achieve the desired flexural strength. By varying the volume fraction of glass fibers, engineers and researchers can optimize the flexural strength of the composites for specific applications, ensuring that the materials meet the required performance criteria.

Effect of carbon fibers

As depicted in Figure 4, similar to glass fibers, the inclusion of carbon fibers in sisal/polyester composites significantly enhances the flexural strength of the laminates. The results clearly demonstrate that laminates with higher volumes of carbon fibers consistently exhibit higher flexural strength, regardless of their stacking sequences. Among the laminates, T18 and T19, with the highest volume fraction of carbon fibers (12%), demonstrate the highest flexural strength values. T18 laminates have an average flexural strength of 426.6 MPa, while T19 laminates show 412 MPa. The difference in flexural strength between T18 and T19 can be attributed to their variation in stacking sequence.

Figure 4.

Effect of carbon fiber hybridization on flexural strength of sisal/polyester composites.

Laminates T18 exhibit the highest flexural strength among all the laminates. They surpass the maximum flexural strength of laminates with 8% volume fraction of carbon fibers (T16) by approximately 17% and exceed the maximum flexural strength of laminates with 4% volume fraction of carbon fibers (T13) by about 55%. Furthermore, T18 laminates display a significantly higher flexural strength of 140% compared to baking soda-treated sisal/polyester laminates (T3).

Figure 4 also provides additional data points representing laminates with varying volume fractions and remarkable flexural strength values. In general, results showed a substantial influence of volume fraction on the flexural strength properties of sisal/polyester hybrid composite laminates, with laminates having higher volumes of carbon fibers exhibiting superior flexural strength.

Effect of stacking sequence

Figures 3 and 4 demonstrate that the flexural strength of sisal/polyester hybrid composites is highly dependent on the stacking sequence of the hybrid fibers. Specifically, placing the hybrid fibers in the compressive side of the laminate leads to higher flexural strength compared to placing them in the core or symmetrically in the outer layer. Moreover, the laminates with hybrid fibers in the compressive side exhibit slightly higher flexural strength compared to symmetrical and alternately arranged laminates. For instance, among the glass fiber hybridized sisal/polyester composite laminates, T10 and T11 were made with the same volume of glass fibers (12%). However, T10 demonstrated about 3% higher flexural strength than T11. Furthermore, T8 exhibited about 8% higher flexural strength than T6 despite both the laminates having the same volume of glass fibers (8%). Additionally, in the case of carbon hybrid composites, the flexural strength of asymmetric laminates T18 was approximately 4% higher than laminates T19, despite both laminates having the same volume of carbon fibers. Similarly, asymmetric laminates T16 demonstrated about 10% higher flexural strength than laminates T15 even though both laminates are hybridized with 8% volume of carbon fibers. Additionally, among the 4% volume of glass fiber hybridized laminates, laminates T13 showed about 7% flexural strength than laminates T12. This indicates that the arrangement of fibers within the laminate plays a significant role in flexural strength, in addition to the volume fraction. Similar research by He et al.³⁰ also support these findings, reporting that situating rigid and stronger carbon fibers on the outer compressive side of composites leads to enhanced flexural strength.

Overall, the results indicated that the stacking sequence significantly influences the overall flexural strength performance of the laminates. Specifically, the arrangement of hybrid fibers within the laminate, particularly their placement on the compressive side, significantly improved the flexural strength values.

Effect of fiber orientation

Figures 3 and 4 demonstrate that the flexural strength of laminates is also influenced by the orientation of the reinforcing fibers. Hybridized cross-plied laminates generally exhibited lower flexural strength compared to unidirectional laminates with the same volume fraction of fibers. For example, the glass hybridized cross-plied laminate T20 exhibited approximately 34% lower flexural strength than the unidirectional laminate T8, both with the same volume fraction of glass fibers. Similarly, the glass hybridized cross-plied laminate T22 exhibited flexural strength about 41% lower than laminate T8. Carbon fiber hybridized laminates T23 and T21 demonstrated approximately 47% and 34% lower flexural strength, respectively, compared to the unidirectional laminate T16. The lower flexural strength of cross-plied laminates can be attributed to the presence of fibers oriented at 90°, perpendicular to the applied load. These fibers have limited ability to withstand bending forces, resulting in decreased flexural strength. In contrast, unidirectional laminates with fibers aligned at 0° have higher flexural strength due to their longitudinal alignment, which allows them to better resist bending forces. Among the cross-plied laminates, those with the hybrids (glass or carbon fibers) oriented at 90° and the sisal fibers oriented at 0° exhibited lower flexural strength compared to laminates with the sisal fibers oriented at 90° and the hybrids at 0°. This is because the stronger glass and carbon fibers, which are oriented at 0°, can better withstand bending forces, resulting in higher flexural strength. Thus, the orientation of reinforcing fibers also plays a significant role in determining the flexural strength of laminates. Laminates with longitudinally aligned fibers tend to have higher flexural strength than those that are cross-plied or have fibers oriented perpendicular to the load.

Effect of glass fillers

Figure 5 showed that the addition of glass fillers increased the flexural strength of the composites. Among the 20/80 composites, the highest flexural strength of 131.4 MPa was achieved with 7.5 wt% glass fillers (S5). Similarly, 30/70 composites reached a maximum flexural strength of 152.4 MPa with 5 wt% glass fillers (S8). However, further addition of fillers decreased the flexural strength for both 20/80 and 30/70 composites. This reduction could be attributed to inadequate bonding between the matrix and reinforcement due to glass filler agglomeration in the polyester material.^1,28,35 The 30/70 composites showed a decline at lower filler concentrations compared to the 20/80 composites, primarily due to the higher volume of polyester in the 20/80 composites, which allowed for the addition of more glass fillers (up to 7.5 wt%) before agglomeration occurred.

Figure 5.

Effect of glass fillers on flexural strength of sisal/polyester composites: (a) 20/80 composites and (b) 30/70 composites.

The maximum increase in flexural strength for the 20/80 and 30/70 composites was 25.4% and 28.5%, respectively, for laminates S5 and S8. Laminate S8 exhibited a flexural strength more than 15% higher than laminate S5. Therefore, adding glass fillers to 30/70 composites is preferable to 20/80 composites for achieving superior flexural strength.

Effect of carbon fillers

As shown in Figure 6, the addition of carbon fillers in composites resulted in a significant increase in flexural strength, similar to glass fillers. The highest flexural strength achieved for the 20/80 composites with carbon fillers was 134.6 MPa, while the 30/70 composites reached a maximum flexural strength of 155.2 MPa. This represents a 28.4% increase in flexural strength for the 20/80 composites and a 30.9% increase for the 30/70 composites compared to the corresponding composites without fillers.

Figure 6.

Effect of carbon fillers on flexural strength: (a) 20/80 composites and (b) 30/70 composites.

The higher increase in flexural strength observed in the carbon-filled composites can be attributed to the superior reinforcing properties of carbon fillers, which effectively distribute and absorb stress within the composite material, enhancing its ability to withstand bending forces. While glass fillers also contributed to an increase in flexural strength, the magnitude of improvement was slightly lower. Therefore, based on the results obtained, incorporating carbon fillers in the composites is more advantageous for achieving improved flexural strength compared to glass fillers.

ANOVA

ANOVA analysis was conducted to evaluate the influence of hybridization parameters—including fiber type, filler type, volume fraction, stacking sequence, and fiber orientation—on the flexural strength of composite materials. This statistical approach helps determine whether differences in mechanical performance across experimental groups are due to actual treatment effects or random error. The results strongly indicate that all the studied parameters have a significant impact on flexural behavior. Glass and carbon hybridization, along with their volume fraction, stacking sequence, and orientation, showed a significant influence, as demonstrated in Tables 4 and 5. In the case of carbon fiber hybridization, the analysis (Table 5) showed that volume fraction, stacking sequence, and fiber orientation each have a statistically significant effect. For volume fraction, the ANOVA revealed a between-group sum of squares (SS) of 93,975.6 and a within-group SS of 141.6. The corresponding F-value was 3539.57, far exceeding the critical F-value of 3.239, with a p-value of 8.76 × 10^-23. This indicates a very strong influence of fiber content on flexural strength, likely due to increased stiffness and load-bearing capacity from higher proportions of high-modulus carbon fibers. For stacking sequence, an F-value of 91.32 (p = 2.8 × 10^-10) was calculated, confirming that different ply arrangements affect internal stress distribution and the composite’s resistance to flexural loading. Fiber orientation produced an even higher F-value of 2668.21 (p = 1.28 × 10^-16), showing that the alignment of fibers relative to the loading direction is critical in optimizing flexural performance. Overall, volume fraction and orientation exhibited particularly dominant effects, emphasizing the importance of these parameters in composite design. When examining the role of glass fillers (Table 6), the ANOVA results demonstrated a highly significant effect on flexural strength. The between-group SS was 12,020.32, with a within-group SS of 235.2, producing an F-value of 227.14 and a p-value of 1.92 × 10^-31. These results indicate that variations in glass filler type, content, or dispersion can dramatically alter the mechanical response of the composite under bending loads. Properly integrated glass fillers improve stiffness, distribute stresses more evenly, and enhance resistance to cracking. Conversely, poor dispersion or incompatibility with the matrix can reduce flexural performance. The statistical evidence confirms that glass filler hybridization plays a key role in defining the structural capabilities of composite systems. Similarly, carbon filler hybridization (Table 7) was shown to have a pronounced effect on flexural strength. The analysis yielded a between-group SS of 10,478.08 and a within-group SS of 275.6, resulting in an F-value of 168.97 and a p-value of 6.1 × 10^-29. These findings underscore the benefits of using carbon fillers, which are known for their excellent mechanical properties. When properly incorporated into the polymer matrix, they significantly enhance stiffness and resistance to deformation under load. The high F-value indicates not only statistical significance but also a meaningful impact on the material’s real-world performance. .In conclusion, the ANOVA results clearly demonstrate that both fiber and filler hybridization strategies—along with key design factors like stacking sequence, volume fraction, and orientation—significantly affect the flexural strength of composite materials. The extremely low p-values and high F-values across all factors confirm that these effects are real and not due to random variation. Among all the variables, volume fraction and orientation (for fibers) and filler type (glass or carbon) showed the most dominant effects. These insights highlight the critical need for careful material selection and structural design to achieve high-performance, durable composites suitable for demanding applications.

Table 4.

ANOVA for effect of glass fiber hybridization on flexural strength.

Factor	Source of variation	SS	df	MS	F	p-value	F crit
Volume fraction	Between groups	59227.8	3	19742.6	2460.137	1.6E-21	3.239
	Within groups	128.4	16	8.025
	Total	56020.93	14
Stacking sequence	Between groups	1072.188	3	357.40	53.11	3.36E-07	3.490
	Within groups	80.75	12	6.729
	Total	1152.938	15
Orientation	Between groups	33880.93	2	16940.47	1254.849	1.16E-14	3.885
	Within groups	162	12	13.5
	Total	34042.93	14

Table 5.

ANOVA for effect of carbon fiber hybridization on flexural strength.

Factor	Source of variation	SS	df	MS	F	p-value	F crit
Volume fraction	Between groups	93975.6	3	31325.2	3539.571	8.76E-23	3.239
	Within groups	141.6	16	8.85
	Total	94117.2	19
Stacking sequence	Between groups	3184.95	3	1061.65	91.325	2.8E-10	3.239
	Within groups	186	16	11.625
	Total	3370.95	19
Orientation	Between groups	53897.7	2	26948.87	2668.205	1.28E-16	3.885
	Within groups	121.2	12	10.1
	Total	54018.9	14

Table 6.

ANOVA for effect of glass fillers hybridization on flexural strength.

Source of variation	SS	df	MS	F	p-value	F crit
Between groups	12020.32	9	1335.591	227.14	1.92E-31	2.124
Within groups	235.2	40	5.88
Total	12255.52	49

Table 7.

ANOVA for effect of carbon filler hybridization on flexural strength.

Source of variation	SS	df	MS	F	p-value	F crit
Between groups	10478.08	9	1164.231	168.974	6.1E-29	2.124
Within groups	275.6	40	6.89
Total	10753.68	49

Influence of water absorption

Figures 7, 8, and 9 illustrate the effect of water absorption on the flexural strength of composite laminates. The results clearly demonstrate that water absorption significantly affects the flexural strength of the composites, resulting in a decrease in their bending resistance performance. The reduction in flexural strength in water-aged samples is primarily due to water absorption’s detrimental effects. Water plasticizes the polyester matrix, reducing its stiffness. Critically, water weakens the hydrophilic fiber-matrix interface by disrupting chemical bonds and causing interfacial debonding. Additionally, water absorption by sisal fibers leads to swelling, inducing internal stresses and delamination. These combined mechanisms compromise stress transfer efficiency, resulting in a significant decrease in the composite’s flexural strength.²²

Figure 7.

Effect of alkali treatment on flexural strength degradation due to water absorption of sisal/polymer composites: (a) Average flexural strength (b) % of reduction in flexural strength.

Figure 8.

Effect of glass and carbon fiber hybridization on flexural strength degradation due to water absorption of sisal/polymer composites: (a) Average flexural strength (b) % of reduction in flexural strength. (a) Glass hybridized composites (b) % reduction in flexural strength of glass hybridized composites (c) carbon hybridized composites (d) % reduction in flexural strength of carbon hybridized composites.

Figure 9.

Effect of glass and carbon fillers on flexural strength degradation due to water absorption of sisal/polymer composites: (a) Average flexural strength (b) % of reduction in flexural strength. (c) 30/70 composites.

Effect of alkali treatment

When sisal fibers are subjected to alkali treatment before being incorporated into composites, the degradation of the laminates is significantly diminished. This is because alkali treatments, such as with sodium hydroxide and baking soda, effectively modify the surface properties of the fibers. They work by removing amorphous constituents like hemicellulose and lignin from the sisal fiber surface, which not only increases the surface roughness and exposes more reactive cellulose crystallites, but also crucially reduces the inherent hydrophilicity of the fibers. This makes the treated fibers less prone to water absorption and improves their wettability by the hydrophobic polyester resin, facilitating a stronger and more stable fiber-matrix interface. Consequently, while untreated laminates (T1) experience a substantial decrease in flexural strength of 20% due to their higher susceptibility to water absorption, leading to a rapid rate of debonding and material degradation, the treated counterparts show superior resistance. Sodium hydroxide and baking soda treatments have been found to reduce degradation to 17% and 16.8%, respectively. This minimized debonding and improved interfacial integrity, resulting from the alkali-induced surface modifications, leads to enhanced resistance against water-induced degradation. This improved resistance to water absorption ultimately enhances the flexural strength of wet composites, ensuring better bending resistance performance even after moisture exposure. Figure 7 shows the effect of alkali treatment on the degradation of flexural strength due to water absorption.

Effect of glass and carbon fiber hybridization

Figure 8 illustrates that incorporating glass and carbon fibers into the laminates significantly enhances their resistance to the degradation of flexural strength caused by water absorption. This enhanced performance is attributed to several key factors. Firstly, synthetic fibers like glass and carbon are inherently more hydrophobic than natural sisal fibers. Their presence, especially on the outer surfaces or at higher volumes, creates a more effective barrier against water ingress into the composite, thus reducing the amount of water reaching the hydrophilic sisal fibers and the fiber-matrix interface. This minimizes matrix plasticization and interfacial debonding.

Secondly, these hybrid fibers contribute to improved load transfer efficiency. Carbon fibers, with their high stiffness and strength, and glass fibers, provide robust reinforcement that maintains structural integrity even under moisture exposure. The strategic placement of these hybrid fibers is also critical. For instance, consider Laminates T18 (C-C-C-S-S) and T19 (C-S-C-S-C), both containing 12% volume of carbon fiber. While T18 exhibited a higher initial flexural strength of 426.6 MPa compared to T19’s 412 MPa, T19 demonstrated superior durability under moisture aging, showing the lowest reduction in flexural strength at only 2.7%. This highlights that T19’s alternating carbon fiber placement, especially on its outer surfaces, creates a more effective hydrophobic barrier and provides more uniform protection against water ingress and interfacial degradation throughout the laminate, thereby minimizing strength reduction in wet conditions. Conversely, T18’s concentrated carbon layers, while contributing to higher dry strength, appear less effective at preventing degradation when exposed to moisture. Similarly, in glass fiber composites, Laminates T10 (S-G-S-G-S) and T11 (G-S-G-S-G), both with 12% glass fiber volume, showcased the impact of stacking. Laminate T10 achieved approximately 3% higher flexural strength than T11. This indicates that even with glass fibers, their precise arrangement within the laminate significantly influences the composite’s ability to resist bending forces. Overall, these comparisons underscore that strategic fiber placement, particularly positioning stronger or more hydrophobic fibers to bear critical stresses or act as barriers, is paramount for optimizing both the initial flexural strength and long-term durability, especially in environments where moisture absorption is a concern. These water-aged laminates demonstrated significant reductions in degradation compared to laminate T3 (treated solely with baking soda), achieving reductions in degradation of 84% and 82.7% respectively.

Among the laminates with 8% volume of glass and carbon fibers, laminates T9 and T17, which have hybrid fibers on their outer surfaces, displayed a minimal flexural strength reduction of 3.9%. This highlights that the extent of flexural strength degradation is intricately linked to the volume, type, and critically, the stacking sequence of the hybrid fibers. Notably, increasing the volume of hybrid fibers consistently contributed to enhanced resistance against degradation, as evidenced by the superior performance of laminates T11 and T19. Therefore, by precisely optimizing the volume, stacking sequence, and type of hybrid fibers, it is possible to significantly enhance the flexural strength and effectively reduce water absorption-induced degradation in water-aged laminates.

Effect of glass and carbon filler hybridization

Figure 9 illustrates the influence of glass and carbon fillers on the degradation of flexural strength in water-aged composites. Among the composites tested, laminate S2 exhibited the highest degradation at 15.5%, followed by laminate S1 at 12.2%. The increased degradation in laminate S2 can be attributed to the higher concentration of hydrophilic sisal fibers, which facilitated a more rapid rate of water absorption, thereby contributing to the degradation of flexural strength. However, the incorporation of glass and carbon fillers was found to mitigate this degradation. The study revealed that the degradation of flexural strength decreased with the addition of glass and carbon fillers up to a concentration of 10 wt% for the 20/80 composites. Conversely, for the 30/70 composites, the rate of degradation reduction diminished after adding more than 7.5 wt% of fillers. The lowest degradation in flexural strength, at 2.6%, was observed in the 10 wt. % carbon-filled 20/80 composites (S14), representing a significant 78.7% reduction compared to S1 laminates. The second lowest rate of degradation, at 2.8%, was observed in the 7.5 wt% carbon-filled 30/80 composites (S17), representing an 81.9% reduction compared to S2 laminates. Additionally, similar degradation of 2.8% was observed in the 10 wt% glass-filled 20/80 composites (S6) and the 7.5 wt% carbon-filled 20/80 composites (S13), both of which demonstrated a 77% reduction compared to S1 laminates. Therefore, incorporating fillers, such as glass and carbon fillers, can significantly mitigate degradation and enhance the flexural strength of wet composites.

ANOVA

Table 8 presents the ANOVA results evaluating the effect of water absorption on the flexural strength of composite materials. The analysis reveals a highly significant influence of water uptake on flexural performance. The between-group sum of squares (SS) is 2,964,660 with 62 degrees of freedom, while the within-group SS is 1428 with 247 degrees of freedom, resulting in mean squares of 47,817.1 and 5.78, respectively. The calculated F-value of 8270.89 is substantially higher than the critical F-value of 1.368, and the p-value is effectively zero, indicating an extremely significant difference among the groups. The results clearly show that the reduction in flexural strength is not due to experimental error or random variation, but is directly attributable to the effect of water absorption. As composites absorb moisture, mechanisms such as matrix plasticization, degradation at the fiber–matrix interface, and the formation of micro-cracks lead to a measurable and consistent decline in flexural strength. The extremely high F-value further supports that this reduction is systematic and significant across the different levels of water absorption tested. In conclusion, the ANOVA confirms that water absorption causes a statistically and practically significant reduction in flexural strength, highlighting the need for improved moisture resistance in composite materials intended for use in humid or wet environments.

Table 8.

ANOVA for effect of water absorption on flexural strength.

Source of variation	SS	df	MS	F	p-value	F crit
Between groups	2964660	62	47817.1	8270.89	0	1.368
Within groups	1428	247	5.78
Total	2966088	309

Conclusion

This study comprehensively investigated the impact of chemical treatments, hybridization, and water absorption on the flexural strength of sisal/polyester composites.

• The findings demonstrate that both sodium hydroxide and sodium bicarbonate treatments significantly enhance the flexural strength of these composites, with sodium bicarbonate treatment producing a 25% increase over untreated fibers and sodium hydroxide treatment an approximately 24% increase. This improvement is attributed to the alkali treatments’ ability to enhance the adhesive properties of the sisal fiber surface by removing impurities and creating a rougher topography, which in turn leads to improved fiber-matrix bonding and efficient load transfer.

• Hybridization with synthetic fibers, particularly carbon fibers, proved highly effective in further increasing flexural strength. Composites with unidirectional carbon fibers, especially when positioned in the compressive layer of the laminate, showed a notable enhancement in flexural strength by approximately 140%. Among the filler-reinforced composites, those containing 30 wt% sisal fibers combined with 5 wt% carbon fillers exhibited the highest flexural strength, exceeding filler-free composites by 30.9%. Furthermore, optimized stacking sequences and fiber orientations consistently resulted in improved flexural strength, with specific configurations showing increases of up to 8%.

• Water absorption consistently reduced the flexural strength across all samples, highlighting the importance of chemical treatments and hybridization in mitigating this degradation. The composite with 12% carbon fiber volume demonstrated the lowest flexural strength reduction due to water absorption, indicating a substantial decrease in degradation compared to untreated samples.

• The ANOVA results confirmed the statistical significance of all investigated factors—including chemical treatments, hybridization (with glass and carbon fibers), stacking sequence, fiber orientation, fillers (glass and carbon), and water absorption—on the flexural strength. For all these factors, the p-values were less than 0.05, demonstrating a highly significant effect and confirming that the observed changes are direct results of the material modifications and structural designs.

These findings collectively underscore the potential of combining chemical treatments and hybridization strategies for developing high-performance sisal/polyester composites with enhanced mechanical properties and improved resistance to environmental degradation for various structural applications.

Footnotes

ORCID iD

Abrha Gebregergs Tesfay

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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