Influence of fiber loading and orientation on properties of pineapple leaf fiber reinforced epoxy composites

Abstract

Research on natural fiber-reinforced epoxy composites has become a potential option in the growing demand for lightweight and environmentally friendly materials. Pineapple leaf fiber (PALF), an agricultural by-product with high strength and toughness, is suitable as a reinforcing material in composite materials. This study evaluated the mechanical properties of pineapple fiber-reinforced epoxy composites by varying the fiber content from 5%, 10%, 15%, and 20% and changing the arrangement of long pineapple fibers in unidirectional and bidirectional directions. The results showed that the composite with 20% unidirectional fibers achieved a tensile strength of 66 MPa, while the sample with 15% bidirectional fibers achieved the highest flexural strength of 135 MPa. The composite with 20% PLAF reinforced in both directions had an impact toughness of 0.18 J/mm². The tribological properties were evaluated by the coefficient of friction and wear rate of the material when arranged in two different orientations, with the wear force direction parallel to and perpendicular to the fiber. In addition, the thermal stability of the materials was evaluated through thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). The morphological structure of the composite material following mechanical damage was investigated using scanning electron microscopy (SEM). The study provides insight into creating high-durability products suitable for technical and household manufacturing applications.

Keywords

PALF epoxy composite mechanical tribological thermal morphological

Introduction

Developing environmentally friendly materials has become a top priority in the manufacturing industry in recent years. Fiber-reinforced composite materials have been supplied in many fields with outstanding advantages such as light weight, strength, and the ability to customize mechanical properties.^1–4 However, using artificial reinforcing fibers, such as glass and carbon fibers, still poses environmental problems due to the production process and the difficulty in recycling.³ Natural fibers such as banana fibers, pineapple fibers, jute fibers, and coconut fibers,^1,2 which are biodegradable, abundant in supply, and low in cost, have been considered a potential alternative to synthetic fibers. Among them, pineapple leaf fiber (PALF) is a natural fiber available in many tropical countries, especially Vietnam, with a total area of pineapple growth estimated at 47,000 hectares. After harvesting, pineapple leaves are agricultural byproducts used for various purposes, such as animal feed and producing glue to make fibers. PALF is utilized to create threads, fabrics, and handicrafts, thereby adding significant economic value. In addition, with the composition including cellulose 81.27 ± 2.45%, hemicellulose 12.31 ± 1.35%, lignin 3.46 ± 0.58%,³ PALF have high mechanical strength tensile Strength 413–1627 MPa, Young’s Modulus 34.5–82.51 GPa,⁴ so PALF are chosen as reinforcing materials to help improve the mechanical properties of composite materials.

PALF has been studied in combination with different polymer matrix materials to fabricate composite materials⁵ such as tapioca biopolymer (TBP),⁶ Polypropylene (PP),⁷ Polylactic Acid (PLA)^8,9, Polyethylene (PE),¹⁰ or High-density Polyethylene (HPPE)¹¹ fabricated by compression molding method. Pineapple fiber is combined with flax in a Polybutylene succinate (PBS) base to form a mixture using the hot-melt mixing method.¹² Conditional, hybridized with other fibers and polymer matrix to create composites of vinyl ester-reinforced kenaf fiber¹³ and PALF or pineapple leaf/glass fiber-based vinyl ester¹⁴ or pineapple leaf/carbon fiber–reinforced unsaturated polyester.¹⁵ In addition to using thermoplastic polymers, thermosetting polymers such as epoxy have also been used as the base. Epoxy has typical advantageous properties, including electrical insulation, heat resistance, good chemical resistance, high mechanical strength, and easy bonding with reinforcing agents.¹⁶ Epoxy is combined with artificial fibers, such as glass fibers^17–19, carbon fiber,^20–22 and various types of organic fibers,²³ including banana fibers,²⁴ aloe vera,²⁵ coconut fiber,²⁶ and pineapple leaf fibers^27,28 for manufacturing composite resin fiber. In addition, hybrid composites of several natural fibers have been developed, such as banana-jute,²⁹ hemp-banana,³⁰ pineapple leaf-banana,³¹ wood dust-abaca-pineapple,³² and PALF-sisal.³³ Furthermore, the simultaneous hybridization of multiple reinforcing epoxy materials has also been evaluated, such as glass-hemp-pineapple-Al₂O₃ nanocomposites,³⁴ high copper-pineapple fiber-Al-T6 mesh,³⁵ and graphene-filled sisal-glass fiber.³⁶

Research on composite materials, specifically epoxy-reinforced PALF, has been published over the past decade. Mittal and Chaudhary³⁷ studied the effect of fiber length and fiber content on the mechanical behavior of pineapple fiber-reinforced epoxy composites. Composite samples with fiber lengths of 10, 15, 20, and 25 mm and contents of 7, 23, 34, and 43 vol% were developed by hand molding and characterized for their mechanical properties according to ASTM standards. The tensile and flexural strength increased with increasing PALF length and content to 15 mm and 34 vol%, respectively. However, the composite with a fiber length of 25 mm and fiber content of 43 vol% exhibited maximum impact strength. Arif Hossain et al.³⁸ published the results of a material investigation using a 15 cm length of raw PALF as reinforcement in composites with content of 10, 15, 20, 25, and 30%. The highest tensile strength, elongation ratio, and impact toughness for the 30% PALF-based epoxy composites were 51.6 MPa, 15.882%, and 14.189 kg/cm, respectively. Meanwhile, the tensile modulus decreased with increasing fiber content. Singh et al.³⁹ tested composites prepared by varying the fiber type, short and long fibers, fiber orientation at 0°, 90°, and 45°, and fiber content in weight 5%, 10%, 15%, and 25%. The mechanical properties analysis indicated that the tensile and flexural strengths reached their maximum values at a 25% short fiber content, with tensile and flexural strengths of 20.85 and 42.70 MPa, respectively. However, the long fiber-reinforced composites with a 5% fiber content achieved maximum tensile and flexural strengths of 35.72 and 56.19 MPa. The maximum flexural and impact strengths were 52.98 MPa and 25.30 J/mm², corresponding to the fibers oriented at 0°. The maximum water absorption value in the composite material is 1.74% for the short-fiber composite and 1.25% for the long-fiber reinforced composite. A recent study by Kumar et al.⁴⁰ introduced pineapple fibers into epoxy matrices at different orientations (45°, 60°, 75°, and 90°) to evaluate their impact on the performance of composites. Neves et al.⁴¹ added graphene oxide (GO) to the pineapple fiber epoxy matrix to improve adhesion. The work evaluated the ballistic performance and energy absorption properties of PALF reinforced composites, upon variation of fiber volume fraction and uniaxial and biaxial arrangement through residual velocity and Izod impact tests. The results showed that composites reinforced with fibers aligned at 90° exhibited effective load transfer and reduced water absorption. The past research has identified that the composite’s mechanical properties depend on the sample’s length, size, and percentage of fiber.

In addition to mechanical properties, the tribological properties of PALF composites were also examined in references. Singh et al.⁴² investigated the coefficient of friction of epoxy matrix composites as the fiber content increased from 5 to 20 wt.% with fiber lengths ranging from 1 to 6 mm. The results showed that the best composite material was with 5 wt.% PALF exhibited the highest performance in terms of coefficient of friction 0.548, lowest fading rate 36.31%, and lowest specific wear rate 3.49 × 10⁻⁸ cm³/N-m. Ridzuan et al.⁴³ used a magnetic stirrer to produce epoxy composites containing filler form of PALF with different wt%, namely 5, 7.5, and 10 wt%. The COF values recorded for epoxy composites containing PALF fibers corresponded to 0.55, 0.72, and 0.87, respectively. The coefficient of friction is also affected by the fiber arrangement 45°, 60°, 75°, and 90° as investigated by Talib et al.⁴⁴ The minimum COF and wear rate occur at 90°, while the maximum COF occurs at 45°. With the lowest coefficient of friction among different natural fibers, PALF-reinforced composite materials have also been targeted as automotive industry products.⁴²

In this paper, we will present the results of the investigation and evaluation of the properties of epoxy composites reinforced with long PALF arranged in unidirectional and bidirectional directions, with fiber proportions of 5%, 10%, 15%, and 20% by weight. The effects of unidirectional, bidirectional fiber arrangements and fiber proportions on mechanical properties, such as tensile strength, flexural strength, tensile modulus, and impact toughness tests, will be shown. The tribological properties of the material, including the wear resistance and wear rate of the material in the fiber direction, will also be analyzed. In addition, the heat resistance of the material will also be presented. The structure of the material and the morphology of the material after destruction will be analyzed through SEM images. The research results will contribute to selecting optimal samples, providing a scientific basis for applying this material in engineering industries.

Methodology

Materials

Raw PALF is supplied by Ecofa Production and Trading Joint Stock Company (Vietnam), with component: Cellulose 70%–86%, Lignin 5%–10%, extractant 6.85%, and Pectin 1.5% and properties: Tensile strength is 1.45–80 MPa, Specific weight is 1.0–1.7 (g/cm³), and the elongation coefficient is 1.5%–16.3%. Epoxy Resin A50 by Quyet Tri Production and Trading Service Company Limited (Vietnam), with a specific gravity of 1100 kg/m³. Chemical NaOH 96% was supplied by Xilong Scientific Co., Ltd (China).

Manufacturing process

Raw pineapple leaf fibers were combed and separated to remove impurities. Then, the fibers were soaked in a 5% NaOH solution⁴⁵ for 8 h, rinsed several times with clean water, and dried naturally. After treatment, the fibers are combed straight, separated into small bundles, then stretched and flattened. Then, the PALF fiber was arranged in unidirectional and bidirectional directions in the mold, with dimensions 160 mm × 250 mm × 10 mm. Pineapple fiber-reinforced epoxy composite samples were fabricated using the Hand Layup technique.³⁷ The epoxy resin was mixed evenly with the hardener at a 3:1 ratio. The mixture was then stirred at 60 rpm for 5 min. The epoxy was poured into the mold, and the material was compressed and evenly distributed using a roller. The sample was then pressed under a weight of 100 N for 8 h at room temperature. After 1 day, the sample was removed from the mold and machined to standard dimensions to investigate its mechanical properties. The schematic illustration of the sample fabrication protocol is depicted in Figure 1, while the detailed formulations of the unidirectional and bidirectional composite configurations are comprehensively outlined in Table 1 and Table 2, respectively.

Figure 1.

Manufacturing process.

Table 1.

Sample of unidirectional fibers.

No	Epoxy (%)	PALF (%)	Name of samples
No	Epoxy (%)	PALF (%)	Tensile Strength	Flexural strength	Impact toughness
1	100	0	ET0	EF0	EI0
2	95	5	UT5	UF5	UI5
3	90	10	UT10	UF10	UI10
4	85	15	UT15	UF15	UI15
5	80	20	UT20	UF20	UI20

Table 2.

Sample of bidirectional fibers.

No	Epoxy (%)	PALF (%)	Name of samples
No	Epoxy (%)	PALF (%)	Tensile Strength	Flexural strength	Impact toughness
1	100	0	ET0	EF0	EI0
2	95	5	BT5	BF5	BI5
3	90	10	BT10	BF10	BI10
4	85	15	BT15	BF15	BI15
5	80	20	BT20	BF20	BI20

Measurements

Tensile strength

Tensile test specimens, measuring 25 mm × 250 mm × 3 mm (Figure 2(a)), were fabricated according to the ASTM D3039 standard.^33,36

Figure 2.

ASTM 3039 standard sample: (a) and tensile test (b).

The samples were mechanically tested using a ZDM5 universal tensile testing machine. A Bufson BSLZ-1 load cell sensor with a capacity of 700 kg was employed to measure the tensile strength (Figure 2(b)). The specimen was subjected to a tensile loading rate of 0.02 mm/s, and data measurements were recorded using a TMR-Z211 multirecorder.

The tensile strength was determined as follows (2):

σ_{T} = \frac{P_{T \max}}{A}

(2)

Strain was calculated by (3)

ε_{T} = \frac{Δ l}{l_{i}}

(3)

Tensile modulus (E_T) was calculated by formula (4):

E_{T} = \frac{σ_{T}}{ε_{T}}

(4)

Where P_Tmax is the maximum tensile load (N), and A is the cross-section of the bearing part (mm²), l_i is the initial sample length (mm), and ∆l is the length variation during the tensile process (mm).

Flexural strength

The flexural strength test specimens, measuring 12.7 mm × 127 mm × 3.2 mm (Figure 3(a)), were prepared according to the ASTM D790 standard.^36,37

Figure 3.

ASTM D790 standard sample (a) and flexural strength test (b).

The bending test specimens were tested using the three-point bending method (Figure 3(b)). The samples were placed on two supports 80 mm apart, and the loading speed was 0.04 mm/s. The flexural strength was measured using a Kyowa DTHA-50 measurement head.

The flexural strength was calculated using the following formula (5):

σ_{F} = \frac{3 P L}{2 b t^{2}}

(5)

Flexural modulus (E_F) was determined in (6):

E_{F} = \frac{m L^{3}}{4 b t^{3}}

(6)

Where P is the breaking load (N), L is the length of the support span (mm), b is the width (mm), d is the thickness of the sample (mm), and m is the slope of the load-deflection curve (N/mm).

Impact toughness

The Izod impact test specimens were prepared following the ASTM D256 standard.³⁷ They measure 12.7 mm × 64 mm × 3.2 mm and have a notch depth of 2 mm in Figure 4(a).

Figure 4.

ASTM D256 standard sample (a) and Izod Impact test (b).

The sample’s impact toughness is measured using the Izod method, with a hammer mass of 10 kg, a pendulum length of l = 0.6 m, a hammer elevation angle of β = 30°, a fracture angle of α, and a destructive force of P(N) in Figure 4(b).

Impact toughness is determined by the work required to break the specimen.

Work consumed to break the specimen presentation in formula (7):

A_{k} = P l (\cos β - \cos α)

(7)

Impact toughness a_k is determined in equation (8):

a_{k} = \frac{A_{k}}{F}

(8)

Where F is the cross-section of the bearing part (mm²).

Abrasion wear behavior

The Tribometer UMT-3MT (CETR, USA) was used to measure the sample’s wear rate and coefficient of friction (COF). The test follows the ASTM G133 standard,⁴³ with the sample prepared to 25 mm × 30 mm × 3.2 mm in Figure 5(a).

Figure 5.

ASTM G133 standard sample (a) and abrasion wear test (b).

Abrasion wear test with parameter measurement: humidity 60%, room temperature 25°C, loading force 10 N, time 600 s, 5 Hz, sliding stroke 10 mm, and total sliding length 60 m. The stainless steel ball above the sample has a hardness of 62 HRC. The coefficient of friction and wear rate of the material are investigated in two directions, as illustrated in Figure 5(b): one direction along the sample’s longitudinal axis and one across the sample’s transverse axis. The results of these parameters are recorded and calculated directly by the UTM software.

The coefficient of friction (COF) can be calculated over time by (9):

C O F (t) = \frac{F_{f} (t)}{F_{n}}

(9)

The wear rate (mm/s) is usually calculated using (10):

Wear rate = \frac{Δ Z}{Δ t}

(10)

Where COF(t) is the coefficient of friction at time t, F_f(t) is the friction force at time t (N), and F_n is the constant normal force acting on the test piece (N). ΔZ is the change in height of the test specimen (mm), and Δt is the test time (s).

TGA/DTG

Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) were conducted using a Netzsch 209F1 Phonic device (Germany) under a nitrogen atmosphere. The temperature range was set from 30°C to 700°C, with a heating rate of 10°C/min and a gas flow rate of 20 cm³/min.

SEM analysis

The PALF structure and structural materials composite were examined using a Hitachi Regulus 8100 scanning electron microscope (SEM; Japan) and a Metallographic microscope, Model GX41-Olympus (Philippines).

Results and discussion

Morphological analysis of PALF

Significant morphological changes in the surface of the pineapple leaf fibers before and after alkali treatment were shown by SEM images, as shown in Figure 6. Figure 6(a) and (b) depict the rough, untreated PALF surface with fiber diameters ranging from 90 to 100 μm. The fiber surface is covered with impurities such as wax, pectin, hemicellulose, and lignin,^2,7,8 which appear quite smooth, hindering the adhesion between the fiber and the substrate, leading to difficulty in the penetration of epoxy resin into the fiber, resulting in poor efficiency and bonding between the fiber and the substrate. After being treated with 5% NaOH solution,⁴⁵ the fibers are thinner and smaller, with diameters of about 50–60 μm (Figure 6(c)). The rougher surface morphology increases adhesion, facilitating mechanical bonding between the fibers. The small cellulose fibers with diameters ranging from 4 to 5 μm were exposed (Figure 6(d)), resulting in an increased surface charge in contact with the matrix, leading to increased wettability and promoting chemical bonding through the available hydroxyl groups of the fibers and epoxy.⁴⁵ These morphological changes improve load transfer efficiency across the fiber-matrix interface in composite systems.

Figure 6.

PALF before (a and b) and after treated NaOH 5% (c and d).

Mechanical properties

Tensile strength

The effect of PALF content and unidirectional (U), bidirectional (B) fiber orientation on the tensile strength (T) properties of the composites is shown in Figure 7. Increasing the fiber content from 5% to 20% significantly increased tensile strength and tensile modulus, with the highest values observed for the UT20 specimen, 66 MPa, 2.5 GPa, respectively. This increase was attributed to improved stress transfer through the reinforcing fibers adjacent to the matrix.^46,47 At varying fiber contents, unidirectional alignment consistently outperformed bidirectional alignment, as fiber orientation parallel to the loading direction promoted efficient load transfer.⁴⁸ In contrast, bidirectional composites containing fibers oriented parallel and perpendicular to the loading showed lower strength and less effective stiffness enhancement, consistent with Afkari et al.⁴⁹ This explanation is demonstrated in BT20, which has the duplicate fiber content as UT20 but shows a sudden drop in tensile strength of 40 MPa and 2.3 GPa in tensile modulus, respectively. This difference is also reflected in the fracture surfaces of the composite specimens after testing. The fracture lines appear relatively straight and aligned in the fiber direction for the unidirectional specimens. It suggests that the failure occurs primarily through matrix cracking or fiber-matrix debonding since the fibers aligned with the tensile load bear the stress until fracture or pullout occurs (Figure 7(b)). The cracks are more localized, indicating that the stress propagation is concentrated in the fiber direction. As the fiber content increases from UT5 to UT20, the fracture edges become sharper and more pronounced, which is typically associated with more brittle failure and higher tensile strength. In contrast, the bidirectional specimens show more complex and irregular fracture patterns. Cracks propagate in multiple directions, reflecting a combination of tensile and shear failure mechanisms due to the multidirectional fiber arrangement, where not all fibers contribute effectively to the tensile load-carrying capacity.⁴³ In general, increasing fiber content increases the stiffness and strength of composites, but fiber arrangement plays a more dominant role in determining the fracture behavior. Unidirectional specimens exhibit more controlled and tensile-dominated failure, while bidirectional specimens fail more diffusely and mixedly.

Figure 7.

Tensile strength (a) and image of the damaged sample (b).

Flexural strength measurement

As shown in Figure 8, the flexural strength (F) and modulus increased with the PALF content up to 15%. Like the tensile test, fiber content and orientation affected the fracture morphology of unidirectional and bidirectional PALF reinforced composites after flexural testing. The epoxy matrix dominated the mechanical response at 5% fiber loading (UF5, BF5), resulting in low flexural strength, and fracture specimens (Figure 9(b)) showed wide, diffuse cracks, indicating low flexural strength and matrix-dominated failure, where insufficient fiber reinforcement resulted in poor stress-bearing capacity. Increased reinforcement was observed at 10%–15% in both fiber orientations, with the highest being in the BF15 specimen with a flexural strength of 135 MPa and flexural modulus of 7.5 GPa (Figure 9(a)), with the bidirectional fiber orientation promoting better stress distribution and retarding the failure process. However, at 20% PALF, the flexural strength and flexural modulus decreased due to increased brittleness and fiber agglomeration, limited fiber–matrix interface bonding, resulting in swelling consistent with references.⁵⁰ The fracture images of UF20 and BF20 showed interlaminar separation and uneven crack propagation (Figure 8(b)). Such behaviors indicate matrix saturation and reduced stress dispersion, similar to the failure modes reported in previous studies of natural fiber composites with excessive fiber content.⁵⁰

Figure 8.

Flexural strength (a) and image of the damaged sample (b).

Figure 9.

Impact strength (a) and image of the damaged sample (b).

Impact toughness

The impact toughness (I) of PALF-reinforced epoxy composites was evaluated using Izod tests, with results presented in Figure 9. Figure 9(b) illustrates the post-impact fracture morphology of unidirectional and bidirectional specimens with different fiber contents. The impact toughness remained relatively low at 5%–10% fiber contents, ~0.14–0.152 J/mm² (Figure 9(a)), indicating that the fibers only contributed limitedly to the energy absorption capacity, while the epoxy matrix dominated the fracture behavior. The specimens UI5 and BI5 showed wide, irregular fracture paths with large cracks and fiber pull-outs. The fiber content increased to 15% and 20%, and the impact toughness improved significantly, reaching 0.17–0.18 J/mm² (Figure 9(a)) for all orientation configurations. This improvement is attributed to the role of the fibers in dissipating impact energy and hindering crack propagation.⁵¹ Despite similar content, the unidirectional composites exhibited more effective crack arrest behavior due to the arrangement of the fibers parallel to the loading direction, promoting uniform stress distribution and enhancing energy dissipation.⁵² In contrast, the bidirectional samples exhibited slightly lower impact toughness, possibly due to the misaligned fibers contributing to less efficiency under dynamic loading. The fracture path of the sample supports this explanation as the fracture became more subtle, with signs of partial fiber bridging and less matrix fragmentation. This suggests improved fiber-matrix interface bonding and greater energy absorption during fracture.⁵¹ The UI specimens exhibited straighter, more cohesive fracture surfaces than the corresponding BI specimens, indicating more efficient crack deflection along the fiber direction in unidirectional composites. At 20% fiber loading (UI20, BI20), the fracture became more brittle and localized, with sharper cracks and occasional fiber aggregates visible, indicating reduced matrix saturation and energy dissipation, consistent with the findings of Rana et al.⁵² The BI20 specimen exhibited delamination characteristics, possibly due to poor fiber wettability and stress concentration at the fiber intersections. The fracture behavior confirmed that the optimal fiber arrangement and content increased the impact toughness, while excessive fiber loading could cause brittleness and interface failure.

Tribological behaviors

The tribological properties investigation of the materials was carried out for the epoxy samples and the samples reinforced with 20% unidirectional and bidirectional fibers, as shown in Figure 10, and the worn surface of the materials, as shown in Figures 11 –13.

Figure 10.

(a) Coefficient of friction and (b) Curve of wear rate of epoxy and composite reinforced 20% PALF-treated arranged in unidirectional and bidirectional.

Figure 11.

Image of the friction measurement of epoxy.

Figure 12.

Image of friction measurement of composite reinforced 20% PALF-treated arranged in unidirectional: (a) U1 direction, (b) U2 direction.

Figure 13.

Image of friction measurement of composite reinforced 20% PALF-treated arranged in bidirectional: (a) B1 direction, (b) B2 direction.

Figure 11 shows that the epoxy sample has prominent and noticeable wear marks, with the sample surface severely damaged and displaying many deep scratches. When the image is magnified at 5× and 20×, multiple layers of material are observed to be peeling off. The wear marks form deep grooves and are dark in the middle, indicating that debris accumulation may have occurred during the wear process. This is evidenced by the significant coefficient of friction of 0.569 and the rapid wear rate of 0.594 × 10⁻⁴ mm/s (Figure 10), resulting in poor wear resistance of the epoxy substrate.⁵³ Meanwhile, Figures 12 and 13 observe the wear process of the composite sample reinforced with 20% unidirectional fibers and arranged in two directions at two locations, as shown in Figure 5(b). The results indicated that the unidirectional fiber pattern differs between the two measurement directions, U1 and U2 (Figure 12). In the U1 direction (Figure 12(a)), the force applied along the fiber direction only produced faint scratches on the sample surface, which did not affect the quality of the composite material. The composite material had a minimum coefficient of friction of 0.093 and a low wear rate of 0.175 × 10⁻⁴ mm/s. In the U2 direction (Figure 12(b)), long scratches appeared on the surface and, when magnified, the epoxy matrix cracked adjacent to the pineapple fibers, with a coefficient of friction of 0.236 (Figure 10(a)). These results are consistent with previous studies showing that the fiber arrangement parallel to the sliding process reduces the interface stress and wear.⁵⁴ Meanwhile, the two-dimensional sample exhibited wear marks in the B1 and B2 directions (Figure 13), with the wear process visible in the significant wear marks on the sample surface. The abrasion marks are extensive in the B1 direction (Figure 13(a)), and some areas are severely damaged. In the magnified image, the scratches appear irregular; some areas are more strongly peeled off, penetrating deep into the fiber layer. The abrasion phenomenon can increase suddenly at specific points. This is evidenced by the abrasion rate, which is divided into two distinct stages (Figure 10(b)). The first stage is linear at 0.338 × 10⁻⁴ mm/s, then increases sharply to 1.14 × 10⁻⁴ mm/s. It is observed that when the surface layer is broken in the fiber direction in the first layer, the hard alloy ball tip is rapidly eroded deep into the inner layer of the second layer, perpendicular to the first layer (Figure 1), and gradually breaks the structure. The eroded material causes burrs along the abrasion path. The significant average COF of this B1 orientation, at 0.558 (Figure 10(a)), also supports this observation. For the B2 orientation (Figure 13(b)), the wear rate was relatively high at 0.789 × 10⁻⁴ mm/s, and the wear track was flatter than that of the B1 orientation, with a coefficient of friction of 0.536 (Figure 10(a)). These findings suggest that the multidirectional fiber orientation increases the interface shear force and debris formation during sliding, reducing wear resistance.^54,55 The wear measurement results showed that the unidirectional sample exhibited better wear resistance than the bidirectional sample. Because the composites have fibers arranged in the sliding direction, friction and wear tend to be lower.⁴³ Fibers arranged in the non-sliding direction have higher friction and wear rates because they tend to be more susceptible to degradation under shear stress. Overall, unidirectional PALF composites aligned in the sliding direction provide superior wear resistance due to better stress distribution and fiber bridging effect.³⁸

Thermogravimetric analysis

The thermal stability of neat epoxy, PALF, and 20% PALF reinforced composites with uniaxial and bidirectional fiber alignments was evaluated by thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG), as illustrated in Figure 14.

Figure 14.

TGA and DTG results of epoxy (a), PALF-treated (b), and composite reinforced 20% PALF-treated arranged in unidirectional (c) and bidirectional (d) orientations.

Figure 14(a) shows that the pure epoxy exhibits the first decomposition step with a mass loss of 8.34% at 270°C due to the evaporation of OH groups.³⁸ The primary decomposition step has a start time of approximately 330°C and a maximum decomposition temperature of 371.9°C. The 82.99% mass loss is typical of thermosetting decomposition.⁵⁶ For the treated pineapple fiber, in the first stage, the mass loss is 4.96%, peaking at 63.2°C, and is due to water loss in the fiber.³⁸ In the next stage, cellulose, the main component in PLAF, is destroyed at a maximum temperature of 361°C,³⁸ corresponding to the stage with the highest mass loss in the thermosetting decomposition process, at 75.77%. The final stage is the degradation of wax and lignin,⁵⁶ which occurs at temperatures above 500°C. The incorporation of pandan leaf fiber (PALF) into epoxy composite materials affects the thermogravimetric degradation behavior due to the inherent thermal properties of the lignocellulose components.⁵⁷ Natural fibers, such as PALF, which contain hemicellulose and cellulose, degrade at lower temperatures than the polymer matrix, thereby delaying the onset of degradation as fiber content increases.⁵⁸ Furthermore, increasing fiber content alters thermal stability relative to the matrix, confirming that higher fiber content, particularly of less thermally stable fibers, accelerates degradation in the early stages.⁵⁹ Composite materials reinforced with 20% PALF in unidirectional and bidirectional arrangements exhibit distinct degradation behaviors. The UD composite material (Figure 14(c)) signified a DTG peak at 373.5°C and a total mass loss of 83.15%. The peak temperature was slightly lower than that of the pure epoxy, suggesting that the decomposition of the original composite sample occurred earlier than that of the matrix. However, PALF increased the carbon formation, demonstrating its barrier effect during thermal decomposition.^38,59 Notably, the bidirectional composite material (Figure 14(d)) indicated slightly improved thermal performance, with a DTG peak at 374.6°C and the highest remaining carbon mass at 15.54%. The result suggests that the cross-linked multilayer fiber structure may hinder heat transfer and volatile byproducts, leading to slower decomposition and improved structural preservation under thermal loading. This result is consistent with findings in woven or multiaxial natural fiber composite materials, where the interaction between the fiber and matrix increases, reducing the thermal diffusion capacity.^51,60 Compared with other natural fibers, such as jute or banana, in epoxy resins, the major weight loss usually occurs at about 375°C–380°C; the presence of fiber mixtures can support stable interactions between fiber-resin and improve the relative thermal stability.⁶¹ Or for hemp fiber, the DTG curve of the 20% material reaches the maximum mass loss T_max at 327.7°C, lower than PALF, due to its higher hemicellulose content, which leads to earlier decomposition.^62,63 Overall, fiber-reinforced materials improve thermal stability, especially in a biaxial configuration, promoting more uniform heat distribution and slowing down degradation, a trend consistent with many previous studies.^38,60,61 These results support the use of PALF as a thermally stable fiber reinforcement for polymer composite materials.

Morphological fracture structure

The unidirectional and bidirectional fiber orientations of the 20% fiber-reinforced epoxy matrix specimens upon mechanical failure are shown in Figure 15(a) and (b), and the fracture surface morphology of the specimens, as affected by mechanical damage due to tensile, bending, and impact on the surface, is further examined by SEM analysis in Figure 15(a1)–(b3). Figure 15(a1) and (b1) show transversely fractured specimens, destroying the fiber and epoxy structure. The unidirectional pineapple fiber specimen fractured along the longitudinal direction of the fiber arrangement upon impact. Meanwhile, the bidirectional specimen fractured where the fibers were aligned perpendicular to the parallel weave surface, exposing a large fiber bundle at the fracture surface.

Figure 15.

Fracture of composite reinforced 20% PALF-treated arranged in unidirectional (a), bidirectional (b), and the fracture surface of samples in tensile (a1, b1), flexural (a2, b2), and impact (a3, b3) tests.

Figure 15(a1) shows the fiber fracture phenomenon after tensile fracture, in which the PALF fractured along the length of the fiber. The fibers may be completely fractured or only stretched, retaining partial bonding to the epoxy matrix.⁶⁰ The result indicates that the interfacial adhesion between the fibers and the epoxy matrix is insufficient under tensile loading. These fracture characteristics are typical of composites with natural fibers when the loading direction is parallel to the fiber direction. In contrast, the bidirectional sample Figure 15(b1) exhibits more fiber separation and fracture, indicating more efficient stress transfer due to multidirectional load distribution, where bidirectional bonding increased tensile ductility and redistributed stress. Figure 15(a2) and b(2) depict the fiber deformation behavior under bending. The fibers stretch, separate, and bend at the epoxy contact area. It is observed that the fibers are entangled, leading to increased energy absorption.³⁹

In addition, the bond between the fibers and the epoxy matrix is broken, reducing the adhesion between the composite components and the material’s load-bearing capacity. Figure 15(a2) shows localized bending, fiber pull-out, and matrix cracking, highlighting stress concentration and limited resistance to out-of-plane loading. The bidirectional composite (B2) viewed fibers bending with minimal fiber pull-out, indicating better fiber-matrix load sharing and improved flexural strength. According to the bending test results in Figure 8, the bidirectional samples exhibit higher flexural strength than the unidirectional composite materials. Figures 15(a3) and (b3) show large cracks and indentations in the epoxy matrix of the material after an intense, rapid impact. The unidirectional composite in Figure 15(a3) fails by brittle fracture, fiber separation, and surface cracking, indicating poor energy absorption and stress localization. The result of the stress being transferred along the fibers to a much larger volume of material, causing more cracks in that volume and thus absorbing a significant amount of energy.²⁷ For Figure 15(b3), the pineapple fibers can separate from the epoxy matrix, resulting in areas of fiber extrusion. At this point, the epoxy surface becomes pitted or concave, indicating that the pineapple fibers have been pulled out of the matrix, resulting in severe failure or damage to the specimen.

In summary, the coexistence of fiber fracture and the mechanism of fiber pulling out of the sample indicates a shift from primarily matrix-mediated fracture to fiber-supported fracture. Composite materials exhibiting both matrix fiber pulling out and matrix fracture generally show improved impact resistance and fracture toughness, while the predominant fiber fracture indicates more efficient stress transport and higher stiffness. Therefore, these SEM observations provide strong corroborating evidence for the microstructure of the mechanical test results presented above.

Conclusion

The results demonstrate the influence of varying the percentage and arrangement of fibers in the unidirectional and bidirectional directions on the mechanical properties of the composites. Pineapple fiber-reinforced epoxy composites exhibit the best mechanical properties when the fiber proportion is between 15% and 20%. Specifically, a 20% fiber proportion in the unidirectional arrangement provides the highest tensile strength, up to 66 MPa. A 15% bidirectional arrangement at 135 MPa achieves the optimum flexural strength. Elongation ranges from 0.15 to 0.18 J/mm² in the unidirectional and bidirectional samples. Friction wear results evaluated that the unidirectional arrangement exhibits better wear resistance than the bidirectional arrangement, with the wear force acting along the two axes of the sample. In addition, TGA/DTG analysis also confirmed that the degradation process of composite samples occurred more slowly than pure epoxy, demonstrating high thermal stability when fiber reinforcement was added. The structure of the material was examined after the sample was destroyed by mechanical force. The image showed that the fracture path differed when the fibers were arranged in one direction and two directions. However, the final results showed that the fiber structure and the epoxy were destroyed due to the weak mechanical bond between the fiber and the matrix. Consequently, the pineapple fiber-reinforced epoxy composite material opens up potential applications in technical and industrial fields with high mechanical strength requirements.

Footnotes

Acknowledgements

We gratefully acknowledge the use of facilities of the at the Photonics Laboratory IOP, VAST.

ORCID iD

Nga Thi Hang Nguyen

Ethical considerations

Ethical approval was not required in this study.

Consent to participate

Informed consent was obtained from all individual participants included in the study.

Author contributions

Nga Thi Hang Nguyen: writing original draft preparation, review, and editing; Doanh Duc Mai: calculation and analysis; Minh Dao Huu Hoang: methodology and measurement; Quang Khai Le: mechanical measurement and analysis; Toan Duc Nguyen and Lien Thi Ha Lien: OM and SEM measurement and analysis, Hang Thi Pham: review and editing.

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.

Data availability statement

Data is available on request from the authors.

References

Faheed

. Advantages of natural fiber composites for biomedical applications: a review of recent advances. Emerg Mater 2024; 7: 63–75.

Ravindran

Mahesh

Bheel

, et al. Usage of natural fibre composites for sustainable material development. Global Research Productivity Analysis. Buildings 2023; 13: 1260.

Azman

Asyraf

MRM

Khalina

, et al. Natural fiber reinforced composite material for product design: a short review. Polymers 2021; 13: 1917.

Nugraha

Nuryanta

Sean

, et al. Recent progress on natural fibers mixed with CFRP and GFRP: properties, characteristics, and failure behaviour. Polymers 2022; 14: 5138.

Yadav

Verma

Kardam

, et al. Pineapple leaf fiber in polymer composites: structure, characterization, and applications. Materials Chemistry and Physics: Sustainability and Energy 2025; 2: 100011.

Jaafar

Siregar

Oumer

, et al. Experimental investigation on performance of short pineapple leaf fiber reinforced tapioca biopolymer composites. BioResources 2018; 13(3): 6341–6355.

Motaleb

KZMA

Shariful Islam

Hoque

. Improvement of physicomechanical properties of pineapple leaf fiber reinforced composite. Int J Biomater 2018; 2018: 7384360.

Mohamed

Hapizi

Norizan

, et al. Pineapple leaf fibers as a reinforcement of biocomposites - an overview. Polimery 2021; 66: 559–573.

Darsan

Retnam

Sivapragash

Tensile, impact, flexural and morphological characteristics of pineapple leaf fiber reinforced polylactic acid composites. Int J Mech Eng Technol 2019; 10(01): 1383–1391.

10.

Praveena

Shetty

Vinayaka

., et al. Mechanical properties and water absorption behaviour of pineapple leaf fibre reinforced polymer composites. Adv Mater Process Technol 2022; 8(2): 1336–1413.

11.

Rajagukguk

Ansyori

Isnugroho

, et al. The optimization of mechanical properties of HDPE-pineapple fiber composites using the Taguchi method. Science & Technology Asia 2024; 29(4): 235–246.

12.

Amornsakchai

Duangsuwan

Mougin

, et al. Comparative study of flax and pineapple leaf fiber reinforced poly(butylene succinate): effect of fiber content on mechanical properties. Polymers 2023; 15: 3691.

13.

Jothiprakash

Gurijala

Sathish

, et al. Mechanical and tribological behavior of pineapple leaf and kenaf fiber reinforced vinyl ester hybrid composites. Proc Inst Mech Eng Part E: J Process Mech Eng 2026; 240(1): 360–368.

14.

Ramakrishnan

Ramasubramanian

Subbarayalu

, et al. Analysis of mechanical properties of pineapple leaf/glass fiber-vinyl ester hybrid composite. Rev Mater 2022; 27: 2.

15.

G V. Experimental investigations of physical and mechanical properties of pineapple leaf/carbon fiber–reinforced unsaturated polyester composites at liquid nitrogen environment. Biomass Convers Biorefinery 2024; 14: 22019–22034.

16.

Jin

Luo

Wang

, et al. Composition optimization of a high-performance epoxy resin based on molecular dynamics and machine learning. Mater Des 2020; 194: 108932.

17.

Gao

Yang

Cheng

, et al. Epoxy resin composite containing nanocarbon-coated glass fiber and cloth for electromagnetic interference shielding. J Mater Res Technol 2021; 13: 1759–1766.

18.

Sundeep

Limbadri

Manikandan

, et al. Study of mechanical properties of pineapple leaf fiber and E-glass fiber reinforced hybrid epoxy matrix composite materials. Mater Today Proc 2023; July: 2214–7853.

19.

Justin Abraham Baby

Arul

Thandavamoorthy

, et al. Elevating mechanical and thermal performance on alkali-treated pineapple/glass fibre sandwich composite. Polym Int 2025; 74: 170–177.

20.

Yang

Wang

Zhang

, et al. Carbon fiber reinforced epoxy resin composites with excellent recyclable performance via dynamic acylsemicarbazide moieties. ACS Appl Polym Mater 2023; 5(7): 5189–5199.

21.

Sayam

Rahman

ANMM

Rahman

, et al. A review on carbon fiber-reinforced hierarchical composites: mechanical performance, manufacturing process, structural applications and allied challenges. Carbon Lett 2022; 32: 1173–1205.

22.

Hashim

MKR

Majid

MSA

Jamir

MRM

, et al. The effect of stacking sequence on fatigue behaviour of hybrid pineapple leaf fibre/carbon-fibre-reinforced epoxy composites. Polymers 2021; 13: 3936.

23.

Jeyapragash

Srinivasan

Sathiyamurthy

. Mechanical properties of natural fiber/particulate reinforced epoxy composites – a review of the literature. Mater Today Proc 2020; 22(3): 1223–1227.

24.

Gupta

Dhamarikar

Dharkar

, et al. Study on the effects of fibre volume percentage on banana-reinforced epoxy composite by finite element method. Adv Compos Hybrid Mater 2020; 3: 530–540.

25.

Kumar

Singh

. Effect of aloe vera natural fibre orientation on the mechanical properties of epoxy composites. Adv Mater Process Technol 2023; 9(4): 1859–1873.

26.

Al Mahmud

Chowdhury

Rayhan

, et al. Epoxy resin composites reinforced with upcycled fabrics: mechanical, thermal, and morphological analysis. SPE Polym 2024; 5(4): 624–636.

27.

Klinthoopthamrong

Thanawan

Schrodj

, et al. Synergistic toughening of epoxy composite with cellulose nanofiber and continuous pineapple leaf fiber as sustainable reinforcements. Nanomaterials 2023; 13(11): 1703.

28.

Hoque

Mollah

MZI

Faruque

MRI

, et al. Review on the mechanical properties of pineapple leaf fiber (PALF) reinforced epoxy resin based composites. ET J 2021; 6(04): 855–860.

29.

Rajesh

Jayakrishna

Sultan

MTH

, et al. The hydroscopic effect on dynamic and thermal properties of woven jute, banana, and intraply hybrid natural fiber composites. J Mater Res Technol 2020; 9(5): 10305–10315.

30.

Venkataravanappa

Lakshmikanthan

Kapilan

, et al. Physico-mechanical property evaluation and morphology study of moisture-treated hemp–banana natural-fiber-reinforced green composites. J Compos Sci 2023; 7: 266.

31.

Madhukiran

Raju

Madhusudan

, et al. Mechanical and morphological properties of epoxy based hybrid composites reinforced with banana-pineapple fiber. Adv Mater Res 2018; 1148: 3–11.

32.

Maruthi Prashanth

Gowda

Ramesh

, et al. Effect of teak wood dust as filler material on mechanical properties of abaca-pineapple-epoxy hybrid composite. J Nat Fibers 2024; 21(1): 2434603.

33.

Deeban

Maniraj

Ramesh

. Experimental investigation of properties and aging behavior of pineapple and sisal leaf hybrid fiber-reinforced polymer composites. e-Polymers 2023; 23: 20228104.

34.

Sangeethkumar

Ramanathan

Jaikumar

, et al. Experimental study on mechanical effects of Al₂O₃ filled E-glass / hemp /pineapple/epoxy hybrid nano composite. AIP Conf Proc. 2024; 3122(1): 100042.

35.

Ramesh

Gokilakrishnan

Jayaraja

, et al. High-content Al-T6/pineapple fiber/brass mesh reinforcements on nanosilica-toughened epoxy hybrid natural fiber metal laminate composite for aircraft applications. Biomass Convers Biorefinery 2024; 14: 14017–14025.

36.

Kamesh

Gude

Maddamsetty

, et al. Investigation of the mechanical, absorption, flammability and swelling properties of graphene filled sisal/glass fiber reinforced polymer hybrid nanocomposites. Cogent Eng 2024; 11(1): 2342433.

37.

Mittal

Chaudhary

. Effect of fiber length and content on the mechanical properties of pineapple leaf fiber reinforced-epoxy composites. Adv Sci Technol 2021; 106: 68–77.

38.

Arif Hossain

Sahadat Hossain

Khan

, et al. Preparation and characterization of pineapple leaf fiber reinforced epoxy composite: effect of gamma radiation. Adv Appl Sci 2022; 7(3): 65–72.

39.

Singh

Sharma

Singh

, et al. Effects of fiber type, content, orientation, and surface treatments on the mechanical properties of PAFRP composite. Eng. Res. Express 2024; 6: 015011.

40.

Kumar

Singh

, et al. Mechanical, water absorption, and tribological behavior of polymer composites: role of pineapple leaf fiber orientation. Lubricants 2025; 13: 161.

41.

Neves

Costa

Bezerra

WBA

, et al. Dynamic and ballistic performance of uni- and bidirectional pineapple leaf fibers (PALF)-reinforced epoxy composites functionalized with graphene oxide. Polymers 2022; 14: 3249.

42.

Singh

Pruncu

Gangil

, et al. Comparative performance assessment of pineapple and Kevlar fibers based friction composites. J Mater Res Technol 2020; 9(2): 1491–1499.

43.

Ridzuan

MJM

Abdul Majid

Khasri

, et al. Effect of pineapple leaf (PALF), napier, and hemp fibres as filler on the scratch resistance of epoxy composites. J Mater Res Technol 2019; 8(6): 5384–5395.

44.

Talib

AAA

Jumahat

Jawaid

, et al. Effect of wear conditions, parameters and sliding motions on tribological characteristics of basalt and glass fibre reinforced epoxy composites. Materials 2021; 14: 701.

45.

John

Thomas

. Biofibres and biocomposites. Carbohydr Polym 2008; 71(3): 343–364.

46.

Feng

Zhang

, et al. Mechanical properties and failure behavior of pineapple leaf fiber reinforced composites. Polym Compos 2020; 41(12): 5198–5208.

47.

Rajagukguk

Sitorus

Manurung

, et al. Mechanical characteristics of pineapple leaf fiber-reinforced epoxy composites: effect of fiber content and orientation. SciTechAsia 2024; 9(1): 12–20.

48.

Sreenivasan

Somasundaram

Ravindran

, et al. Microstructural, physico-chemical and mechanical characterisation of Sansevieria cylindrica fibres – an exploratory investigation. Mater Des 2011; 32(1): 453–461.

49.

Afkari

Pratama

Juwono

, et al. Mechanical properties of pineapple leaf fiber/epoxy composites with 0°/0°/0°/0° and 0°/90°/0°/90° fiber orientations. Jurnal Sains Materi Indonesia 2022; 23: 83–89.

50.

Ramesh

Palanikumar

Reddy

. Mechanical property evaluation of sisal–jute–glass fiber reinforced polyester composites. Compos B Eng 2013; 48: 1–9.

51.

George

Sreekala

Thomas

. A review on interface modification and characterization of natural fiber reinforced plastic composites. Polym Eng Sci 2001; 41(9): 1471–1485.

52.

Rana

Mandal

Bandyopadhyay

. Short jute fiber reinforced polypropylene composites: effect of compatibiliser, impact modifier and fiber loading. Compos Sci Technol 2003; 63(6): 801–806.

53.

Ghosh

Bhowmik

Khatun

. Dry sliding wear behavior of natural fiber reinforced polymer composites – a review. Mater Today Proc 2019; 19: 2261–2266.

54.

Suresha

Kumar

. Investigation of wear behavior of glass–epoxy composite with graphite filler. Wear 2009; 267(5–8): 775–782.

55.

Laoutid

Bonnaud

Alexandre

, et al. New prospects in flame retardant polymer materials: from fundamentals to nanocomposites. Mater Sci Eng R Rep 2009; 63(3): 100–125.

56.

Fiore

Scalici

Nicoletti

, et al. A new eco-friendly chemical treatment of natural fibres: effect of sodium bicarbonate on properties of sisal fibre and its epoxy composites. Compos B Eng 2016; 85: 150–160.

57.

Nurazzi

Asyraf

MRM

Rayung

, et al. Thermogravimetric analysis properties of cellulosic natural fiber polymer composites: a review on influence of chemical treatments. Polymers 2021; 13(16): 2710.

58.

Asim

Jawaid

Nasir

, et al. Effect of fiber loadings and treatment on dynamic mechanical, thermal and flammability properties of pineapple leaf fiber and kenaf phenolic composites. J Renew Mater 2018; 6(4): 383–393.

59.

Jawaid

Abdul Khalil

HPS

. Cellulosic/synthetic fibre reinforced polymer hybrid composites: a review. Carbohydr Polym 2011; 86(1): 1–18.

60.

Nagarajan

Babu

Palanivelu

, et al. Mechanical and thermal properties of PALF reinforced epoxy composites. Macromol Symp 2016; 361: 57–63.

61.

Neto

JSS

de Queiroz

HFM

Aguiar

RAA

, et al. A review on the thermal characterisation of natural and hybrid fiber composites. Polymers 2021; 13(24): 4425.

62.

Gargol

Klepka

Klapiszewski

, et al. Synthesis and thermo-mechanical study of epoxy resin-based composites with waste fibers of hemp as an eco-friendly filler. Polymers 2021; 13: 503.

63.

Hiremath

Reddy

Mutra

, et al. Thermal degradation and fire retardant behaviour of natural fibre reinforced polymeric composites- a comprehensive review. J Mater Res Technol 2024; 30: 4053–4063.