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Abstract

Date palm fibers (DPFs) are natural, bio-degradable, green, and eco-friendly materials possessing broad applications. In this study, polypropylene (PP) thermoplastic matrix granules were reinforced with different weight percentages of DPFs (0, 2.5, 5.0, and 7.5 wt%). Two different sizes of DPFs, 75 μm (aspect ratio: ∼4.7) and 150 μm (aspect ratio: ∼2.7), were also taken for investigation. The optimal combination of PP-5DPFs was further reinforced with 2.5 wt% of nanocrystallite FeCrCuMnTi high-entropy alloy (HEA) fillers. These composites were produced in a twin screw extruder at a temperature of 200°C, chopped into granules, and consolidated into bulk samples using a vertical injection moulding machine. The composite samples were characterized using field emission gun scanning electron microscopy (FEG-SEM), differential thermal analysis/thermogravimetric analysis (DTA/TGA), X-ray Diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The effect of DPF size, content, and filler integration with PP matrix on the mechanical, flexural, and impact properties was evaluated and reported. The results revealed that the PP-5DPFs (75 µm)-2.5HEA (FeCrCuMnTi) nanocrystallite composite exhibited significantly enhanced mechanical and impact resistance properties due to improved dispersion, interfacial bonding, and load transfer capability enabled by the HEA fillers. Quantitatively, PP-5DPFs (75 mm)-2.5HEA filler composite exhibited the best performance with improvements in tensile strength (41.9%), flexural modulus (28%), and energy absorption (33.5%) compared to neat PP. Smaller fibers (75 µm) showed superior dispersion, interfacial bonding, and stress transfer, while larger fibers (150 µm) exhibited aggregation and reduced efficiency, especially at higher loadings. HEA fillers enhanced matrix crystallinity, reduced fiber pullout, and distributed stress effectively, mitigating the adverse effects of fiber aggregation. SEM fracture analysis confirmed ductile failure in neat PP and fiber pullout and matrix cracking in composites, with the optimized composition showing strong fiber-matrix adhesion and minimal voids. These findings highlight the potential of hybrid PP-DPF-HEA composites for sustainable, lightweight, and high-performance applications in the automotive, packaging, structural, and advanced manufacturing sectors.

Keywords

Date-palm fibers thermoplastic composites microstructure mechanical properties

Introduction

In the modern world, the development of sustainable, biodegradable, carbon-free, and lightweight materials has the main target to apply to several fields, especially structural and day-to-day life products.¹ For the past two decades, fiber-reinforced thermoplastic materials, namely, carbon fiber, glass fiber, and Kevlar fiber have been commonly used to develop lightweight products that are currently being used in aircraft, automotive, space, and construction industries due to their attractive properties.^2–4 Fiber-reinforced thermoplastic-based composite materials possess an excellent in strength-to-weight ratio, high stiffness-to-weight ratio, outstanding corrosion resistance, and improved surface properties.^5,6 The carbon and glass fibers are manufactured synthetically, which produces a lot of hazardous substances affecting human organs, polluting the environment, and causing climate change. Moreover, the cost of extraction of carbon fiber is too expensive, though it possesses excellent strength with light weight.^7–9 The alternative way to eliminate these serious issues is to adopt the usage of natural fibers from plants into thermoplastic polymers, which make new sustainable composite materials that never pollute the environment.^10,11 In addition, the natural fibers possess biocompatible properties, are easily fabricated at low cost, and are available as a raw material from various sources.^12–14

Material selections for manufacturing products are very important as they affect or influence the performance. Lightweight materials with improved mechanical properties can give us good performances in which thermoplastics as matrix materials embedded with some secondary fiber materials can give expected performances.^15,16 In addition, thermoplastic-based sustainable composite materials possess numerous advantages such as high chemical stability, low wear rate under mild conditions, excellent corrosion resistance, and low value of coefficient of friction. These improved properties can encourage us to replace metal-based products with thermoplastic-based ones for the development of machine tool parts like bearings, gears, slides, cams, and other sectors (automotive and aviation).^17,18 Due to these features, we can eliminate motion resistance in the mechanical components and achieve a reduction in fuel consumption so that the overall efficiency can be improved. Most of the polymers are being produced from crude oil which contributes to climate change due to more CO₂ emissions.¹⁹ Recently, several countries have been affected by climate change in the form of heavy rainfall, severe drought, high heat waves, formation of drastic cyclones, and increasing global warming. These things force us to reduce the consumption of fossil fuels, reuse the available resources, and recycle the used products based on sustainable development. Therefore, the present research is focused on using natural fibers (date palm fibers, DPFs) as reinforcement and polypropylene (PP) as thermoplastic matrix material. Numerous thermoplastic polymers are available in the market, in which polypropylene (PP) thermoplastic material possesses high crystallinity, lightweight, excellent mechanical properties (in terms of strength and stiffness), easily manufactured, chemically inert (resistance against alkalis and acids environment), and more temperature resistant.²⁰ Though the PP thermoplastic material has some improved properties, its toughness and wear resistance is low in PP. Therefore, PP thermoplastic materials are to be embedded with some fillers to improve mechanical and wear performance. The environmental effect produced by conventional polymeric materials can be overcome by reinforcing natural fibers with polymers. Natural fibers are usually produced from renewable sources that need less energy for production. Different natural fibers, namely, banana, flax, henequen, hemp, sisal, coir, kenaf, kapok, and DPFs can be used as fillers in the thermoplastic matrix.^21–27 Among the various natural fibers, the research work related to the usage of DPFs as a filler is limited and needs to be examined further. The biological name of DPF is Phoenix dactylifera, which usually consists of 60 to 75 % holo-cellulose, 20 % lignin, and 1% ash.²⁸ It was reported by Rahul et al.²⁹ that DPFs possess improved properties compared to coir fibers in real-time applications.

Tahir et al.³⁰ developed a composite material consisting of the epoxy matrix (thermosetting plastic) embedded with palm-activated carbon fibers. The authors have taken high-density epoxy and mechanically blended/ground palm kernel activated carbon fibers (PKACFs) with a size of less than 1 mm. The composite samples were manufactured with 70 wt% of epoxy matrix and 30 wt% of PKACFs using a 4:1 ratio of resin and hardener. The prepared composites are then hot pressed at 80°C in a mould for 10 min. The authors investigated the tribological and heat resistance behaviors of the developed composite materials and found the improved performances with the incorporation of activated palm carbon fibers into the epoxy matrix. Om Prakash et al.³¹ fabricated a composite material consisting of an epoxy matrix embedded with 2 wt% of nano-activated carbon as fillers and found improved performance with the incorporation of nano-activated carbon. The authors have taken porous nano-sized (12 nm) carbon powder produced from biomass (Arhar agricultural waste), LY556 grade of epoxy, and HY 951 grade of hardener. A two-step pyrolysis chemical activation methodology was used to extract carbon from the Arhar crop. First, the Arhar crop waste was put inside a ball mill for grinding, and then, chemical activation techniques were used. The hand-lay-up method was used for fabricating the composite samples.

Zuhudi et al.³² manufactured biodegradable hybrid composites consisting of bamboo woven fabric/glass fiber reinforced with a PP thermoplastic matrix. The authors took PP thin sheets (∼55 wt%) with a thickness of around 0.58 mm, put them inside a pre-heated mould (180°C), applied bamboo woven fabric/glass fiber (∼45 wt%), and a compression moulding technique was used for consolidating the composite samples. The authors have successfully manufactured the composite samples and investigated the water absorption behavior. Wojciechowski et al³³ developed a sustainable composite material consisting of a PP matrix embedded with cherry seed powders. Initially, the authors took cherry seeds, and dried them in an oven (80°C for 6 h). The dried seeds were then ground at 6000 rpm to make it into fine powders, and less than 0.8 mm of cherry powders were taken for sample preparation. Cherry seed powder content was varied from 0 to 15 wt% with a step size of 5 wt%. The authors have used 1 wt% maleic anhydride PP as a compatibilizer. The blended composite granules were compounded in a twin extruder, and then, bulk samples were prepared for examining the mechanical and tribological performances using injection moulding. A barrel temperature of 180 °C–205 °C was used in different zones in the twin extruder (L/D = 40/16 mm), which had given pelletized composites. These pelletized composites were subjected to injection moulding for fabricating the bulk samples. The authors have found improved mechanical and tribological performances with the incorporation of cherry seed powders. Liu et al³⁴ manufactured basalt fiber (BF) reinforced with PP matrix using a mould opening foam injection moulding approach. The authors have used homo-polymer-based PP granules and a BF size of 12.7 μm in diameter and 6 mm in length. Both PP and BF were dried at 80°C for 4 h in an oven; BFs with different weight percentages (0, 5, 10, 20 wt%) were blended with PP matrix; blended PP/BFs were fed into twin extruder in which the temperatures at various zones upto die were set as 160, 165, 180, 190, and 185°C. The extruded sustainable composite filaments were pelletized into granules, and then these composite granules were fed into an injection moulding machine for fabricating bulk samples. The authors have found that the 20 wt% BFs reinforced PP composite materials exhibited improved mechanical performances. Jan et al.³⁵ developed a sustainable composite material that consisting of a PP matrix embedded with green wood flour using extrusion and injection moulding processes. Sharma et al.³⁶ developed an epoxy matrix reinforced with food waste fillers (citrus lametta peel fibers) and studied the mechanical and wear behavior. The authors have found that the finest form of citrus limettia peel fiber has given improved properties.

Recently, Dhakal et al.³⁷ and Ebrahim et al.³⁸ explored the advancements in DPFs-reinforced composites, who have emphasized the properties improvement using hybrid approaches in processing, and physical and chemical treatments. Khan et al.³⁹ examined the use of date palm microfibers (DPMFs) in polycaprolactone (PCL) biocomposites in rigid packaging. The authors have varied DPMFs from 0 to 10 wt% with and without NaOH treatment. Their results demonstrated that the biocomposites embedded with 5 wt% DPMFs content exhibited improved mechanical properties (20% improvement in tensile strength and 164% in Young’s modulus). Sismanoglu et al.⁴⁰ investigated the effect of alkali and silane surface treatments on DPFs in thermoplastic polyurethane (TPU) matrix eco-composites. The composites were developed using melt blending and injection moulding after the DPFs underwent the surface treatments. Their results explained that the silane-treated TPU-DPFs eco-composites exhibited improved mechanical properties due to proper adhesion between DPFs and the TPU matrix. Raja et al.⁴¹ developed a polylactic acid (PLA) matrix embedded with DPFs and bran filler, who have investigated the fatigue and thermal behaviours. Their results explained that superior fatigue strength of 25 MPa at 5000 cycles, along with enhanced fiber-matrix adhesion was obtained in the composite with 15g bran filler. Dynamic mechanical analysis showed the highest storage modulus of 2400 MPa at 110°C and a damping factor (tan δ) of 0.340 at 130°C. Further, thermogravimetric analysis confirmed its superior thermal stability with an onset degradation temperature of 350°C and the highest residual char of 18% at 500°C. As per Islam et al.,⁴² both thermoset and thermoplastic composites reinforced with DPFs could be a sustainable material for several sectors, including construction and sports products. Singh et al.⁴³ developed a composite consisting of PP matrix reinforced with woven date palm leaves processed via direct compression moulding. Their results demonstrated that the treated composite exhibited improved properties (42.2% improvement in tensile modulus). Abdessemed et al.⁴⁴ developed epoxy resins reinforced with short DPFs of composites and investigated the thermal, water absorption, and viscoelastic behaviour. Maou et al.⁴⁵ have carried out the effect of various chemical modifications on DPFs in polyvinyl chloride (PVC) and high-density polyethylene (HDPE) matrices. Their results demonstrated an improvement in the morphological, thermal, mechanical, dynamic-mechanical, rheological, and water uptake performance of the composites with the addition of modified DPFs.

Although the increasing use of PP in structural and consumer components/parts due to its light weight, high chemical resistance, and ease of processability, the poor characteristics in toughness, wear resistance, and impact strength restrict its applications in more demanding environments such as automotive interior parts, personal protective equipment, and energy absorbing panels.⁴⁶ Whereas, natural fibers like DPFs could improve the stiffness and sustainability; however, excessive fiber content could lead to agglomeration and poor load transfer.⁴⁷ In contrast, metal reinforcements such as FeCrCuMnTi high-entropy alloy (HEA) fillers provide high structural stability, grain refinements, and energy dissipation. Therefore, combining the natural DPFs with HEA nanocrystallites in a PP matrix aims to overcome the mechanical drawbacks of neat PP and fiber-only composites, thereby achieving a sustainable and high-performance hybrid system suitable for real-world impact-prone and structural applications.

Based on the literature survey, most reported studies involving natural fibers, such as, DPFs focus either on thermoset matrices or simple thermoplastic blends without integrating multifunctional reinforcements. However, limited research exists on hybrid systems involving natural fibers and nanocrystallite HEA fillers in thermoplastic matrices. The present study is novel in combining two reinforcing systems (DPFs and FeCrCuMnTi HEA fillers) within a PP matrix using twin-screw extrusion and injection molding. This research targets the enhancement of tensile, flexural, and impact resistance properties through controlled fiber size selection, optimized weight percentages, and the unique properties of HEA fillers. The work specifically addresses the challenges of fiber dispersion, matrix-filler bonding, and impact resistance while aiming to develop a sustainable, high-performance composite system for real-world applications in automotive, construction, and packaging sectors.

Experimental procedure

Materials

The homo-polymer of PP and PP-g-MA (PP-graft-maleic anhydride) acts as a compatibilizer, expected to react with the hydroxyl groups on DPFs and also shows strong interaction with the PP chains, creating covalent and intermolecular bonds. This was used in granule form of 2.5 to 3.5 mm size, which were purchased from SABIC, Saudi Arabia. Commercially available DPFs were purchased in different sizes in the Saudi market. The as-received DPFs were ground in a mixer and sieved in a sieve-shaker (Tencan, China) with different screens (50, 100, 150, 200, 250, and 325 mesh). After sieving, 75 μm with a length of ∼350 μm and 150 μm with a length of ∼400 μm DPFs were selected in this study. DPFs were subjected to 4% NaOH surface treatment for 2h to improve the surface roughness, mechanical interlocking, and hydrogen bonding with the PP matrix.^48,49 Based on reported literature,⁵⁰ the estimated mechanical properties of alkali-treated DPFs used in this study are: tensile strength of 28 MPa, Young’s modulus of 5.7 GPa, and the density of 1.26 g/cm^3. Nanocrystallite FeCrCuMnTi-HEA powder (filler, 2.5 wt%) was also taken to embed with the optimal combination of PP-DPF composite granules. The nanocrystallite HEA metal filler was synthesized via the high-energy mechanical alloying process. The detailed synthesis procedure and characterization of FeCrCuMnTi HEA powder is already published by our research team and explained elsewhere.^49,51,52 The chemical composition of PP, PP-g-MA, PP-x.wt%DPFs (x = 0, 2.5, 5.0, and 7.5 wt%, size: 75 μm & 150 μm), and PP-5 wt%DPFs-2.5 wt%HEA (BCC: 18 nm; FCC: 9 nm and HCP: 31 nm, nanocrystallite powders⁵¹) for manufacturing of monolithic and nanocrystllaite hybrid composites are given in Table 1.

Table 1.

Sample composition, sample ID, and percentage of added raw materials to produce sustainable PP embedded with DPFs and HEA fillers of thermoplastic composites.

Sample composition	Sample ID	PP (wt. %)	DPF-75 μm (wt. %)	DPF-150 μm (wt. %)	HEA, (wt. %)	PP–g–MA, (wt. %)
PP	Neat PP	100	-	-	-	-
PP+5PP–g–MA	PP+5PP-MA	95	-	-	-	5
[PP+5PP–g–MA]-2.5 DPF (75 & 150 μm)	PP+2.5 DPF (75 μm)	92.625	2.5	-	-	4.875
[PP+5PP–g–MA]-2.5 DPF (75 & 150 μm)	PP+2.5 DPF (150 μm)	92.625	-	2.5	-	4.875
[PP+5PP–g–MA]-5.0 DPF (75 & 150 μm)	PP+5.0 DPF (75 μm)	90.250	5.00	-	-	4.750
[PP+5PP–g–MA]-5.0 DPF (75 & 150 μm)	PP+5.0 DPF (150 μm)	90.250	-	5.00	-	4.750
[PP+5PP–g–MA]-7.5 DPF (75 & 150 μm)	PP+7.5 DPF (75 μm)	89.063	7.5	-	-	4.688
[PP+5PP–g–MA]-7.5 DPF (75 & 150 μm)	PP+7.5 DPF (150)	89.063	-	7.5	-	4.688
[PP+5PP–g–MA]-5 DPF (75 & 150 μm)-2.5 HEA	PP+5.0 DPF (75 μm)+2.5 HEA	89.063	5	-	2.5	4.750
[PP+5PP–g–MA]-5 DPF (75 & 150 μm)-2.5 HEA	PP+5.0 DPF (150 μm)+2.5 HEA	89.063	-	5	2.5	4.750

Manufacturing of composites

PP granules (neat PP and PP-g-MA) embedded with different weight percentages of DPFs and different sizes of DPFs (75 nm & 150 nm) of monolithic composite were manufactured using a twin screw extruder (M/s Dongguan Junxin Plastic & Metal Co. Ltd, Qiaotou Town, Dongguan, China). In addition, the optimal composition of PP-5DPFs composites was reinforced with nanocrystallite FeCrCuMnTi HEA fillers to produce hybrid composites using the same twin extruder. Before, as per Table 1, the raw materials were mixed mechanically up to 1 h, and the mixed polymers were pre-heated at 60°C for 1 h in an oven to eliminate the moisture. Figure 1(a) shows the schematic diagram indicating the compounding of designed polymers in a twin screw extruder, followed by chopping to produce composite granules, and photographs showing the developed composite granules (4 to 5 mm). The twin screw extrusion parameters, such as the temperature across the different zones of 185 ± 2°C (Zone 1), 195 ± 2°C (Zone 2), and 205 ± 2°C (Zone 3); mixing time of around 10 minutes per batch; the screw speed of 90 rpm; and feeding rate of 2.5 kg/hr, were applied. The composite granules were fed into a vertical injection moulding machine (M/s Dongguan Junxin Plastic & Metal Co. Ltd, Qiaotou Town, Dongguan, China, 60-ton capacity) for fabricating the bulk samples. The vertical injection moulding parameters, including the set temperature at three different zones of 200 ± 2°C (Zone 1), 210 ± 2°C (Zone 2), and 220 ± 2°C (Zone 3); injection pressure of 100 bar, and cooling time of 30 s were used. The bulk samples were prepared as per ASTM standards for tensile (ASTM D638: dumbbell-shaped, total length = 100 mm, gauge length = 40 mm, width = 15 mm, thickness = 10 mm), three-point bending (ASTM D790: cylindrical, diameter = 15 mm, span = 40 mm), and low-velocity impact (ASTM D7136: square, 60 mm × 60 mm, thickness = 10 mm) tests. Figure 1(b) shows the schematic diagram of a vertical injection moulding process for fabricating the bulk samples and photographs of prepared samples for mechanical testing.

Figure 1.

(a) Schematic diagram representing the development of sustainable thermoplastic composite granules of used raw materials (as-received PP, PP-g-MA, DPF-75 μm, DPF-150 μm and HEA filler), twin extrusion process, and photographs of PP-g-MA, PP-2.5DPF (150 μm), PP-5.0 DPF (150 μm), and PP-5.0 DPF (150 μm)-2.5 HEA (FeCrCuMnTi filler); (b) Schematic diagram representing the vertical injection moulding process for manufacturing of sustainable thermoplastic composite of PP embedded with different weight percentages of DPF (75 mm), DPF (150 mm),2.5% FeCrCuMnTi HEA and photographs of fabricated samples for tensile, flexural and low-velocity impact tests.

Characterization of methods

The developed composites were characterized using FEG-SEM (Apreo FEG-HRSEM, 30 keV applied voltage with 1.3 nm resolution) for examining the surface topography/morphology of modified DPFs, FeCrCuMnTi HEA fillers, and internal structures on bulk samples. Energy-Dispersive X-ray Spectroscopy (EDAX) with elemental mapping was also carried out to check the homogenisation of dispersed elements and report the chemical composition. The phase formation and crystallite structural evaluation were carried out using an X-ray diffractometer (Empyrean, Malvern Panalytical, source: CuKa = 1.54 Å). Samples were scanned at 2° per minute for the diffraction angle (2θ) from 5° to 90°. X-pert high score plus software was used to analyse the results. Fourier transform infrared spectroscopy (FTIR, model: Shimadzu IR affinity model 1S; wavelength range: 4000 cm⁻¹ to 600 cm⁻¹; resolution: 0.5 cm⁻¹; number of scans: 45) was used to characterize the developed composites for examining the functional groups formed and the corresponding chemical bonds present in the samples after compounding. A differential thermal analyser (DTA) was used to examine the thermal stability and phase transitions. In addition, a thermo-gravimetric analyser (TGA) was also used to determine the decomposition temperature and mass loss that occurs with the function of temperature. A Netzsch STA 449, Germany, instrument was used for both TGA/DTA test (temperature range: 25 – 800°C; heating rate: 10°C/min; atmosphere: nitrogen with 60 mL/min).

The tensile properties of developed composites were determined using an M/s MTS Universal testing machine (250 kN capacity). Dumbbell shape composite samples (Figure 1(b)) were loaded with a loading rate of 5 mm per minute. At least five trials in each composition were tested, and the average was used for examination. Tensile yield strength, ultimate strength, modulus of elasticity, and elongation at break were determined using the following equations:

σ_{T} = \frac{F_{T}}{w h}

(1)

ϵ_{T} = \frac{Δ l}{l_{0}}

(2)

E_{T} = \frac{Δ σ}{Δ ϵ}

(3)

Where,

σ_{T}

indicates the tensile stress (MPa);

F_{T}

represents the applied tensile force (N);

w

is the width/breadth of the sample (mm);

h

is the thickness of the sample (mm);

ϵ_{T}

is the tensile strain;

Δ l

represents the longitudinal elongation (mm);

l_{0}

indicate the initial gauge length (mm);

E_{T}

represents the tensile modulus of elasticity (MPa) within elastic limit.

A flexural test using a three-point bending method (Figure 1(b)) was carried out with a loading rate of 2 mm per minute. Five replicas in each composite were used in the test, and the average was taken for investigation. Flexural properties in terms of bending stress ( $σ_{b}$ ), bending strain ( $ϵ_{b}$ ), flexural modulus ( $E_{b}$ ), and flexural toughness (area under flexural stress-strain curve) were determined based on the results obtained from the machine using the following equations.

σ_{b} = \frac{{8 F}_{b} L}{π d^{3}}

(4)

ϵ_{b} = \frac{6 Δ d}{L^{2}}

(5)

E_{b} = \frac{4 L^{3} m}{3 π d^{4}}

(6)

Where,

F_{b}

is applied bending force (N),

Δ d

represents a change in deflection at mid-span of the sample, L is the span between two supports (mm),

m

indicates the first slope value obtained from bending force-deflection, which is usually calculated between 0.25

{F_{b}}_{\max}

to 0.75

{F_{b}}_{\max}

The majority of applications in the automotive industries (components like bumpers, panels and interior parts), construction sectors (light-weight panels or protective barriers), aerospace field (components of aircraft and drones), and sports equipment (helmets, protective gears, and sporting goods), and packaging industries, the components are usually subjected to sudden or repeated impact load emphasizing the importance of low-velocity impact test which describes the composite materials ability to absorb and dissipate energy. Five trials in each material were tested and the average was used for investigation (Figure 1(b)). A low-velocity impact (LVI) test was executed to explore the peak impact force, absorbed impact energy, velocity, and displacement profiles, which are calculated using equations (7)–(9)^53,54:

v (t) = v_{i} + \int_{0}^{t} \frac{F (t)}{m} d t

(7)

D (t) = D_{i} + v_{i} t + \frac{g t^{2}}{2} - \int_{0}^{t} (\int_{0}^{t} \frac{F (t)}{m}) d t

(8)

E (t) = \frac{m [v_{i}^{2} - v {(t)}^{2}]}{2} + m g D (t)

(9)

Where v represents the input velocity (m/s), v_i indicates the impactor incident velocity (m/s), t is the time (ms), g represents the gravity (m/s²), F(t) represents the impact force (N), m is the mass (kg), D(t) represents the displacement (mm), and D_i denotes the initial position (mm).

Results and discussion

Characterization of raw materials

Figure 2(a1)–(c2) shows the SEM images of developed DPFs (both low and high magnification, 75 μm and 150 μm), revealing distinct morphological features. The DPFs exhibit a rough and irregular surface texture, which was expected to benefit for enhancing mechanical interlocking and improving the bonding strength with the PP matrix. The DPFs-75 μm (Figure 2(a2)) show finer fibrils with more pronounced surface undulations compared to DPFs-150 μm (Figure 2(c2)), which display a relatively smoother surface with fewer microfibrils. It was observed that both DPFs produce intrinsic pores and voids distributed along their surfaces, which is more in DPFs-150 μm due to its larger diameter. The formation of minor surface cracks and pits was attributed to the mechanical processing or chemical treatment employed during the fiber preparation. These defects are less prevalent in 75 μm, indicating better structural integrity. The SEM EDAX results of Figure 2(d1) and (d2) confirm the presence of carbon (C), oxygen (O), and other trace elements such as fluorine (F), sodium (Na), silicon (Si), and calcium (Ca). The presence of high carbon content (∼53.95 wt%) indicates the predominance of lignocellulosic components (cellulose, hemicellulose, and lignin), which is expected to contribute to the mechanical and thermal properties of DPFs. The presence of oxygen content (∼41.47 wt%) aligns with the hydroxyl groups present in cellulose and hemicellulose. The presence of trace elements (F, Na, Si, and Ca) is attributed to mineral residue inherent to natural fibers or remnants from chemical treatments.

Figure 2.

SEM images of synthesized raw materials of: (a₁) DPFs – 75 μm, low magnified SEM image & (a₂) corresponding high magnified image of (a₁); (b₁) DPFs – 150 μm, low magnified SEM image & (b₂) corresponding high magnified image of (b₁); (c₁) low magnified FeCrCuMnTi HEA fillers & (c₂) corresponding high magnified image of (c₁); SEM with EDAX analyses of: (d₁) high magnified DPF-75 μm & (d₂) corresponding EDAX spectrum; (e₁) SEM image of FeCrCuMnTi HEA filler & (e₂) corresponding EDAX spectrum.

Figure 2(c1) and (c2) show the SEM images of the FeCrCuMnTi HEA fillers, revealing the homogeneous microstructure and alloy formation, which confirms the potential suitability as reinforcement particles in polymeric composites. The HEA powder particles produce an irregular and angular morphology with well-defined edges. These surface characteristics are expected to enhance the mechanical performance. The synthesized HEA fillers show a broad particle size distribution with smaller particles (∼1 μm to 5 μm) contributing to increased surface area and larger particles (∼5 μm to 15 μm) enhancing load transfer applications. The high-magnification SEM image (Figure 2(c2)) exhibits surface asperities, suggesting the potential for strong mechanical interlocking at the interface when incorporated into the polymeric matrix. The SEM-EDAX results of Figure 2(e1) and (e2) demonstrate the presence of primary alloying elements (Fe, Cr, Cu, Mn, and Ti) with near equi-molar ratios.

The XRD diffractometer result of Figure 3(a) shows the observed peaks of modified DPFs-75 μm, DPFs-150 μm, and FeCrCuMnTi HEA fillers. The peaks corresponding to DPFs reveal the characteristics associated with the lignocellulosic type and indicate its semi-crystalline nature. The major peaks observed at approximately 2θ = 16° and 2θ = 22° correspond to (1 1 0) and (2 0 0) planes of cellulose. Here, the peaks are more pronounced in DPFs-75 μm, indicating a higher degree of crystallinity compared to DPFs-150 μm fibers. Further, a broad diffraction humb around 2θ = 18° was observed due to the amorphous regions of hemicellulose and lignin. These features are more prominent in DPFs-150 μm, suggesting a higher proportion of amorphous components in larger size fibers. In addition, the crystallinity based on intensity height was determined, which was 88%, and 86% for DPFs-75 μm and DPFs-150 μm, respectively. This higher crystallinity in DPFs-75 μm fibers could enhance the mechanical strength and thermal stability, making them more suitable as reinforcements in polymeric composite applications.

Figure 3.

(a) XRD patterns of synthesized raw materials (DPFs-75 mm, DPFs-150 mm, and FeCrCuMnTi HEA fillers); (b) DTGA curves of modified DPFs-75 μm and DPFs-150 μm; and (c) FTIR curves of modified DPFs-75 μm and DPFs-150 μm.

Figure 3(b) shows the DTGA curves of modified DPFs-75 μm and DPFs-150 μm, which exhibit four distinct stages of weight loss. In Stage I (28°C to 125°C, small broad peak), a minor weight loss of around 8 to 12% was observed, indicating the evaporation of physically bound water and any other volatile compounds. DPFs-75 μm sample produced slightly higher moisture loss compared to DPFs-150 μm due to its higher surface-to-volume ratio. In stage II (125°C to 232°C, no peak), both DPFs absorbed the heat and withstood the temperature by retaining almost negligible weight loss (∼2% compared to Stage I). In stage III (232°C to 350°C, a sharp peak), a significant weight loss (∼43% to 46%) was recorded, indicating the thermal degradation of hemicellulose (155°C to 250°C) and cellulose (250° to 350°C). The DPFs-75 μm fibers exhibited a slightly earlier onset of degradation, indicating higher thermal reactivity due to smaller fiber size. In stage IV (350°C to 550°C, a small peak), a gradual weight loss (∼18% to 19.5%) was noticed due to the decomposition of lignin, which occurred a broader temperature range due to its complex and cross-linked structure. The residual weight loss at 550°C was higher in DPFs-150 μm fibers (∼25%) compared to DPFs-75 μm fibers (∼21.5%), indicating a higher fraction of non-degradable components or ash content in the coarser fibers. The onset temperature of degradation for DPFs-150 μm fibers was slightly higher than that of DPFs-150 μm fibers, suggesting better thermal stability. Further, DPFs-75 μm fibers demonstrated a sharp degradation peak, indicating a more rapid thermal decomposition.

FTIR results of modified DPFs are shown in Figure 3(c), which is used to identify the functional groups and chemical bonds in the DPFs (chemical composition of cellulose, hemicellulose, lignin, and other organic constituents). The observed peak range from 3330 cm⁻¹ to 3350 cm⁻¹ represents the O-H stretching. This is a hydrogen-bonded hydroxyl group in cellulose and hemicellulose. The intensity of this peak (∼3335 cm⁻¹) indicates high hydroxyl content, reflecting the hydrophilic nature of fibers. This peak is stronger in DPF-75 μm fibers than in DPF-150 μm fibers, indicating reduced hydroxyl content. The sharp peaks at 2908 cm⁻¹ and 2852 cm⁻¹ are related to C-H stretching. This is associated with aliphatic C-H bonds in cellulose and hemicellulose, which confirms the presence of organic compounds in the developed DPFs. These peaks are stronger in DPFs-75 μm fibers than in DPFs-150 μm fibers. The observed peak at 1743 cm⁻¹ indicates C = O stretching, which corresponds to the carbonyl groups in hemicellulose and esterified lignin. The peak at 1627 cm⁻¹ indicates the aromatic C = C stretching. This represents the aromatic skeletal vibrations in lignin, and the observed peak confirms the presence of lignin in the fiber structure. The major peak-trough at 1032 cm⁻¹ represents the C-O bonds in cellulose and hemicellulose presence in the DPFs. A sharp peak reflects the polysaccharide backbone of the developed fibers. The peak at 2355 cm⁻¹ was associated with the stretching vibrations of CO₂ in the environment during the FTIR measurement process. The peaks at 3750 cm⁻¹ and 3850 cm⁻¹ are generally associated with the stretching vibrations of free hydroxyl (-OH) groups, which are typically from water molecules on the fibers surface.

Characterization of PP matrix embedded With date palm fibers and FeCrCuMnTi HEA fillers

XRD analyses

Figure 4 shows the XRD diffractometer patterns of the PP matrix embedded with different weight percentages of DPFs and 2.5 wt% FeCrCuMnTi HEA fillers. Table 2 lists the centre of peak, peak intensity, area of peak, full-width half maximum (FWHM), and percentage crystallinity (X_c) obtained from XRD peaks. The percentage crystallinity (X_c) is calculated using:

X_{c} = \frac{A_{c}}{A_{c} - A_{a}} \times 100

(10)

where,

A_{c}

is the area under crystalline, and

A_{a}

is the area of amorphous diffraction patterns. These results explain the crystalline structure, phase distribution, and changes in crystallinity of PP reinforced with DPFs and HEA filler. The XRD profile of neat PP demonstrates the formation of α-crystalline PP phase with reflections at 2θ = 13.87° (1 1 0), 16.69° (0 4 0), 18.36° (1 3 0), 21.66° (1 1 1/0 4 1). The formation of these peaks indicates the dominance of α-PP, which is a typical isostatic PP. There were no other significant peaks corresponding to β-PP or γ-PP phases, indicating the pure crystalline behaviour after extrusion. The XRD patterns of the PP-MA sample are similar to neat PP with a slight increase in peak intensity of the α-PP phase. These results indicate that PP-g-MA acts as a good compatibilizer without disrupting the intrinsic crystallinity of PP. Therefore, the incorporation of 5 wt% of PP-g-MA as a compatibilizer was expected to improve the interfacial interaction between the PP and the DPFs & HEA filler as reinforcements, leading to improved crystalline alignment. The XRD patterns of PP-2.5 DPFs (75 μm) and PP-2.5DPFs (150 μm) exhibit α-PP phase with slightly decreased intensity compared to neat PP. The decrease in peak intensity suggests a marginal reduction in crystallinity due to the addition of DPFs. This effect is more in the PP-2.5DPFs (150 mm) sample, as larger fiber sizes tend to disrupt the crystallinity due to weaker dispersion. Further, a weak broad around 24.5° to 25.5° was observed, indicating the amorphous nature of lignocellulosic components of DPFs. The XRD patterns of PP-5DPFs (75 mm) and PP-5DPFs (150 μm) exhibit a significant reduction in crystallinity compared to 2.5DPFs samples. This was attributed to the increase of lignocellulosic phase with an increase of DPFs, which disrupts the crystalline region of PP. This effect was more in the DPFs-150 μm sample, indicating a less efficient fiber-matrix interaction. The XRD patterns of PP-5DPFs (75 μm)-2.5 HEA and PP-5DPFs (150 μm)-2.5 HEA samples produced α-phase peaks of PP alongside the peaks of the HEA filler at 2θ = ∼35.5°, 43°, and 48.5°, which correspond to HCP, FCC, and BCC phases, respectively. These reflections confirm the better alignment of the HEA filler with the PP matrix. In addition, the smaller DPFs, along with the HEA filler sample, produced better crystallinity compared to larger DPFs due to improved dispersion and effective interfacial bonding.

Figure 4.

XRD diffractometer peaks of PP matrix embedded with a different weight percentage of DPFs and HEA fillers: (a) DPFS-75 mm; (b) DPFs-150 mm (drawn in the same scale for comparison).

Table 2.

XRD peak parameters analyses of developed PP matrix embedded with DPFs and HEA filler.

Sample name	Plane of major peak	Center of peak, 2θ°	Peak intensity, cps	Area of peak, a.u	FWHM	% Crystallinity
Neat PP	(1 1 0)	13.85215	13,508.24518	10,179.19	0.6012	24.6395
PP-MA	(1 1 0)	13.91855	13,226.24671	9781.766	0.5900	25.0668
PP-2.5DPFs (75 μm)	(1 1 0)	13.92848	11,057.52152	8881.893	0.6409	22.7608
PP-2.5DPFs (150 μm)	(1 1 0)	13.89159	7165.72993	5510.711	0.6136	14.1218
PP-5DPFs (75 μm)	(1 1 0)	13.95034	11,190.39888	8886.107	0.6335	22.0716
PP-5DPFs (150 μm)	(1 1 0)	13.87187	6193.17097	4743.762	0.6111	12.1564
PP-2.5DPFs (75 μm)-2.5HEA	(1 1 0)	13.84408	5922.58044	5033.479	0.6781	12.8988
PP-2.5DPFs (150 μm)-2.5HEA	(1 1 0)	13.79856	5039.60683	3786.41	0.5994	9.7030

DTGA analyses

The thermal stability and degradation temperature of PP embedded with different percentages of DPFs and HEA fillers were analysed using TGA and derivative TG (DTG) analysis. Figure 5 shows the TGA (showing weight loss) and DTG (rate of decomposition) curves, and Table 3 lists the degradation temperature at different weight losses of developed composites. The results demonstrate that neat PP exhibits a single-step degradation process with an onset temperature of 404.8°C (5% wt.% loss), a peak degradation temperature of 450.23°C (at 50% wt. loss), and a temperature of 5% residue of 468.69°C. This thermal degradation of neat PP was attributed to chain scission reactions leading to volatile hydrocarbons. With the addition of 5 wt% of PP-g-MA, there were no changes in the degradation properties of PP, indicating good compatibility. The incorporation of DPFs with PP accelerates the onset temperature due to the thermal decomposition of hemicellulose and cellulose in DPFs. The onset temperature of DPFs-150 μm fiber samples produced a higher value compared to DPFs-75 μm fiber samples, representing more thermal stability in DPFs-150 μm samples. However, the addition of FeCrCuMnTi HEA fillers enhanced the onset temperature indicating, good thermal stability. The incorporation of HEA fillers was expected to act as a thermal barrier, consequently enhancing polymer-filler interfacial bonding. The lowest thermal stability was observed in PP-7.5DPFs (75 μm, T_onset = 345.97°C). Usually, at higher DPF loading, the thermal degradation is dominated by the DPFs decomposition of lignocellulosic components.

Figure 5.

TGA and derivative weight loss curves of: (a) Neat PP and PP-MA; (b) PP embedded with different wt% of DPFs (75 μm) and HEA filler; (c) PP embedded with different wt% of DPFs (150 μm) and HEA filler.

Table 3.

TGA of developed PP embedded with different weight percentages of DPFs and HEA fillers.

Name of sample	Degradation temperature, (°C)				5% residue
Name of sample	Wt. loss at 5%, T₅ (°C)	Wt. loss at 10%, T₁₀ (°C)	Wt. loss at 20%, T₂₀ (°C)	Wt. loss at 50%, T₅₀ (°C)	5% residue
Neat PP	404.08 ± 0.35	421.12 ± 0.28	433.90 ± 0.44	450.23 ± 0.27	468.69 ± 0.49
PP-MA	404.81 ± 0.42	421.88 ± 0.54	434.66 ± 0.27	450.99 ± 0.36	470.16 ± 0.34
PP-2.5DPFs (75 μm)	387.01 ± 0.51	418.24 ± 0.34	435.28 ± 0.31	451.61 ± 0.43	471.49 ± 0.48
PP-2.5DPFs (150 μm)	400.35 ± 0.63	421.26 ± 0.42	435.86 ± 0.62	450.76 ± 0.52	469.26 ± 0.53
PP-5DPFs (75 μm)	365.24 ± 0.18	417.34 ± 0.46	436.25 ± 0.47	451.87 ± 0.40	471.04 ± 0.37
PP-5DPFs (150 μm)	386.49 ± 0.22	419.19 ± 0.25	436.19 ± 0.33	451.79 ± 0.23	470.95 ± 0.55
PP-7.5DPFs (75 μm)	345.97 ± 0.43	415.53 ± 0.19	436.83 ± 0.40	452.45 ± 0.65	472.33 ± 0.30
PP-7.5DPFs (150 μm)	352.61 ± 0.20	415.81 ± 0.30	435.84 ± 0.52	452.01 ± 0.22	471.89 ± 0.28
PP-2.5DPFs (75 μm)-2.5HEA	364.94 ± 0.33	417.94 ± 0.47	435.94 ± 0.45	452.28 ± 0.44	475.04 ± 0.17
PP-2.5DPFs (150 μm)-2.5HEA	391.04 ± 0.26	420.14 ± 0.36	435.76 ± 0.37	451.38 ± 0.50	471.94 ± 0.26

FTIR analyses

Figure 6 shows the FTIR curves of neat PP, PP-MA, and PP embedded with different percentages of DPFs (both 75 μm and 150 μm). The neat PP sample exhibited asymmetric and symmetric stretching vibrations of -CH₃ and -CH₂ groups around 2874 cm⁻¹, bending vibrations of -CH₂ and -CH₃ groups around 1458 cm⁻¹, skeletal vibrations of isotactic PP, and stretching vibrations of the C = O anhydride group. The observed spectrum demonstrates the characteristic bands of neat PP, which confirms the chemical nature. PP-MA sample produced all peaks of neat PP with a slight increase in peak-trough at 2874 cm⁻¹, indicating the interaction of incorporated PP-g-MA compatibilizer with neat PP, which was expected to improve the interfacial bonding between the PP, DPFs, and HEA filler. PP-2.5DPFs (75 μm) and PP-2.5DPFs (150 μm) samples retained the peaks of PP and PP-MA. In addition, C-O stretching vibrations occur around 973 cm⁻¹, which was attributed to the presence of cellulose and hemicellulose in DPFs. This peak-trough was decreased with the increase of DPFs loading. In addition, a C-O-C stretching of lignin occurred around 1160 cm⁻¹. It is to be noticed here that smaller fibers (75 μm) exhibit sharper peaks, suggesting better dispersion and interfacial interactions. With the addition of FeCrCuMnTi HEA filler in the PP matrix, the sample produced all peaks like PP embedded with DPFs. Further, the suppression of O-H and C-O peak intensity occurred compared to the PP-DPFs of composite samples. In addition, metal-oxygen stretching vibrations around 522 cm⁻¹ occurred, representing the effective interaction of HEA fillers with the PP matrix. Here, smaller fibers (75 μm) exhibit better interaction with HEA fillers as the PP-5DPFs (75 μm)-2.5 HEA sample produced sharp peaks, indicating effective dispersion.

Figure 6.

FTIR curves of: (a) Neat PP and PP-MA; (b) PP embedded with different wt% of DPFs (75 μm) and HEA filler; (c) PP embedded with different wt% of DPFs (150 μm) and HEA filler.

SEM analyses

The microstructure in terms of topography, DPFs & HEA fillers dispersion, and its interfacial interactions could be examined via SEM micrographs which is shown in Figure 7. SEM micrograph of neat PP (Figure 7(a1) and (a2)) sample exhibited a relatively smooth and homogeneous surface with no significant phase separation or defects. PP-5PP-g-MA sample (Figure 7(b1) and (b2)) demonstrates a similar smooth surface to neat PP, but with some increased roughness and localized variations, indicating improved interfacial bonding of the incorporated compatibilizer (PP-g-MA). SEM micrographs of PP-2.5DPFs (75 μm) (Figure 7(c1) and (c2)) and PP-2.5DPFs (150 μm) (Figure 7(d1) and (d2)) samples show scattered DPFs embedded in the PP matrix. The DPFs appear well distributed and embedded with the PP matrix. The higher magnification of Figure 7(c2) and (d2) indicates the effective interfacial bonding of incorporated DPFs with the PP matrix. PP-5DPFs (75 μm) sample (Figure 7(e1) and (e2)) exhibited still better dispersion of DPFs with PP matrix. In contrast, the PP-5DPFs (150 μm) sample (Figure 7(f1) and (f2)) shows pronounced clustering of DPFs with poor dispersion of DPFs within PP matrix. Increasing the DPF content worsens the dispersion challenges, especially in larger DPFs (150 μm). The formation of agglomerations and improper dispersions of larger DPFs (150 μm) are expected to reduce the overall composites performance. SEM micrographs of PP-7.5DPFs (75 μm) (Figure 7(g1) and (g2)) and PP-7.5DPFs (150 μm) (Figure 7(h1) and (h2)) samples reveal more significant DPFs clustering and agglomerations, especially in the case of DPFs-150 μm. The results explain that higher DPF content significantly affects the interfacial bonding with the PP matrix, which further limits the DPF potential as reinforcement beyond 5%. PP-5DPFs (75 μm)-2.5 HEA (Figure 7(i1) and (i2)) and PP-5DPFs (150 μm)-2.5 HEA (Figure 7(j1) and (j2)) samples exhibited significantly improved fiber dispersion compared to the same composition without HEA fillers. In particular, the smaller DPFs-75 μm shows excellent distribution with the PP matrix. The larger DPF-150 μm shows some clustering, but better dispersion compared to the same composition without HEA fillers. The results explain that the HEA metal filler particles are expected to act as bridges, improving the interaction between DPFs and the PP matrix. As a result, the DPFs-PP matrix adhesion was expected to be improved, decreasing agglomerations and enhancing the overall composite structure. Figure 8 shows the SEM elemental mapping and EDAX analyses of PP-5DPFs (75 μm)-2.5HEA composites, demonstrating the distribution of each element and the chemical composition. The carbon atoms dominate in the PP matrix, and it is distributed/dispersed uniformly. The high carbon content was attributed to the PP matrix and the organic components of DPFs confirming the polymeric nature of the developed composites. The next higher intensity of oxygen corresponds to the DPF cellulose, hemicellulose, and lignin content. The distribution of Fe, Cr, Cu, Mn, and Ti elements indicates their uniform dispersion within the composites. The even distribution suggests the effective incorporation of HEA particles into the PP matrix, which is expected to enhance both mechanical and thermal properties.

Figure 7.

SEM micrographs of PP matrix embedded with DPFs and HEA fillers produced from injection moulding samples of: (a₁) neat PP, (a₂) magnified view of (a₁); (b₁) PP-5PP-g-MA, (b₂) magnified view of (b₂); (c₁) PP-2.5DPF (75 μm), (c₂) magnified view of (c₁); (d₁) PP-2.5DPF (150 μm), (d₂) magnified view of (d₁); (e₁) PP-DPF (75 μm), (e₂) magnified view of (e₁); (f₁) PP-5DPF (150 μm), (f₂) magnified view of (f₁); (g₁) PP-7.5DPF (75 μm), (g₂) magnified view of (g₁); (h₁) PP-7.5DPF (150 μm), (h₂) magnified view of (h₁); (i₁) PP-5DPF (75 μm)-2.5HEA, (i₂) magnified view of (j₁); (h₁) PP-5DPF (150 μm)-2.5HEA, (j₂) magnified view of (j₁).

Figure 8.

SEM elemental mapping and EDAX analyses of PP-5DPFs-2.5HEA composites.

Tensile properties of PP embedded with DPFs and HEA filler

Figure 9(a) and (b) and Table 4 illustrate the tensile properties of the PP matrix embedded with different percentage of DPFs (both 75 and 150 μm) and FeCrCuMnTi-HEA fillers. The results demonstrated that neat PP exhibited the lowest mechanical properties (modulus of elasticity, E: 2113 MPa, and ultimate tensile strength, UTS: 36.66 MPa) among all samples due to its isotropic and unreinforced nature. PP with 5%PP-g-MA samples possessed E of 2128 and UTS of 37.38 MPa. The slight increase in tensile properties indicates molecular interactions with the PP matrix. PP-2.5DPFs (75 μm) sample exhibited E:2307 MPa (9.2% increase from neat PP) and UTS:42.02 MPa (14.6% improvement from neat PP). Whereas the PP-2.5DPFs (150 μm) sample produced E: 2647 MPa (25.2% higher than neat PP) and UTS: 40.63 MPa (10.8% more compared to neat PP). Both composite samples show significant improvement in tensile properties due to DPF reinforcement, with larger fibers (150 µm) offering better load transfer capabilities but slightly reduced strain performance. PP-5DPFs (75 μm) sample exhibited E:3338 MPa (58% more than PP) and UTS:46.35 MPa (26.3% higher than PP). PP-5DPFs (150 μm) sample exhibited E:3854 MPa (82.4% higher than neat PP) and UTS:42.26 MPa (15.3% higher than PP). These results explained that increasing DPF content enhances stiffness and strength, but the strain and toughness decreased due to fiber aggregation at higher weight percentages (Figure 7). PP-7.5DPFs (75 μm) sample showed E:4709 MPa (122.8% higher than neat PP) and UTS: 40.05 MPa (only 9.3% improvement over neat PP). The result of the PP-7.5DPFs (150 μm) sample showed E:7539 MPa (256.8% more compared to neat PP) and UTS:34.29 MPa (6.4% reduction compared to neat PP). The result clearly explains that higher DPF content increases stiffness significantly, but the UTS was decreased after 5%DPFs, especially for 150 µm fibers. PP-5DPFs (75 μm)-2.5HEA sample exhibited E:3132 MPa (48.2% higher than neat PP) and UTS:48.55 MPa (32.4% more than neat PP). Whereas, the PP-5DPFs (150 μm)-2.5HEA sample produced E:3673 MPa (73.8% higher than neat PP) and UTS:44.16 MPa (20.5% more than neat PP). The results of the incorporation of FeCrCuMnNi HEA fillers with the optimum DPF samples exhibited enhanced strength and toughness due to improved interfacial bonding and filler dispersion. The 75 µm DPFs outperform the 150 µm DPFs due to better compatibility and reduced aggregation.

Figure 9.

Engineering tensile stress-strain curves of developed composites: (a) neat PP, PP-MA, PP-2.5DPFs (75 μm), PP-5DPFs (75 μm), PP-7.5DPFs (75 μm), and PP-5DPFs (75 μm)-2.5 HEA; (b) (a) PP-2.5DPFs (150 μm), PP-5DPFs (150 μm), PP-7.5DPFs (150 μm), and PP-5DPFs (150 μm)-2.5 HEA; Flexural stress-strain curves of developed composites: (c) neat PP, PP-MA, PP-2.5DPFs (75 μm), PP-5DPFs (75 μm), PP-7.5DPFs (75 μm), and PP-5DPFs (75 μm)-2.5 HEA; (d) PP-2.5DPFs (150 μm), PP-5DPFs (150 μm), PP-7.5DPFs (150 μm), and PP-5DPFs (150 μm)-2.5 HEA.

Table 4.

Tensile and flexural properties of PP embedded with different weight percentages of DPFs (both 75 μm & 150 μm) and HEA fillers.

Materials	Tensile properties					Flexural properties
Materials	Modulus of elasticity (E), MPa	Yield strength, MPa	Strain at yield point, %	Ultimate tensile strength (UTS), MPa	Strain at ultimate point, %	Maximum flexural strength (FS_max), MPa	Flexural modulus (E_b), MPa
Neat PP	2113 ± 3.52	24.123 ± 1.85	0.0195 ± 0.0014	36.66 ± 1.61	0.078 ± 0.008	47.128 ± 1.11	2262 ± 2.35
PP-MA	2128 ± 4.23	24.856 ± 0.95	0.0195 ± 0.0021	37.38 ± 1.23	0.076 ± 0.006	48.965 ± 0.88	2458 ± 1.98
PP-2.5DPFs(75 μm)	2307 ± 2.65	25.325 ± 0.87	0.0176 ± 0.0034	42.02 ± 1.31	0.072 ± 0.007	54.772 ± 0.94	2576 ± 2.07
PP-2.5DPFs(150 μm)	2647 ± 3.21	29.871 ± 1.22	0.0231 ± 0.0017	40.63 ± 0.98	0.064 ± 0.004	53.182 ± 0.81	2350 ± 1.24
PP-5DPFs(75 μm)	3338 ± 2.78	32.058 ± 2.44	0.0175 ± 0.0029	46.35 ± 1.04	0.057 ± 0.005	57.304 ± 1.02	2806 ± 1.52
PP-5DPFs(150 μm)	3854 ± 2.55	29.314 ± 1.88	0.0133 ± 0.0072	42.26 ± 0.88	0.041 ± 0.007	55.010 ± 0.78	2538 ± 2.33
PP-7.5DPFs(75 μm)	4709 ± 4.11	28.651 ± 2.07	0.0114 ± 0.0064	40.05 ± 1.05	0.031 ± 0.004	58.490 ± 0.65	2910 ± 2.11
PP-7.5DPFs(150 μm)	7539 ± 2.45	26.544 ± 3.20	0.0074 ± 0.0047	34.29 ± 1.48	0.019 ± 0.006	54.630 ± 0.59	2736 ± 1.84
PP-5DPFs(75 μm)-2.5HEA	3132 ± 3.84	34.231 ± 1.57	0.0207 ± 0.0036	48.55 ± 2.54	0.058 ± 0.005	59.823 ± 0.64	2895 ± 0.85
PP-5DPFs(150 μm)-2.5HEA	3673 ± 4.06	33.256 ± 2.08	0.0166 ± 0.0041	44.16 ± 2.66	0.047 ± 0.003	56.873 ± 0.83	2998 ± 1.44

Based on the tensile properties of developed composites, it was observed that neat PP exhibited pronounced necking behaviour, which is a typical characteristic of semi-crystalline thermoplastics due to localized yielding and plastic flow of aligned macromolecular chains. This necking results from interlamellar slippage and chain disentanglement, where crystalline regions resist deformation, but the amorphous regions stretch, lead to strain localization. The incorporation of DPFs disrupted the uniform plastic deformation by impeding chain mobility, which reduced the extent of necking. For instance, in PP–5DPFs (75 µm) composite, the enhanced interfacial bonding and mechanical interlocking between fibers and the PP matrix restricted chain alignment, thus delaying or suppressing neck propagation. The addition of FeCrCuMnTi HEA nanocrystallites further restricted macromolecular segment motion by acting as nucleation and stress transfer sites which leads to an increase in strain hardening and a more distributed deformation zone rather than localized necking.

Flexural properties of developed composites

The flexural properties of PP reinforced with different percentages of DPFs (75 μm and 150 μm) and 2.5 FeCrCuMnTi-HEA (Figure 9(c) and d and Table 4) were evaluated based on maximum flexural strength (FS_max) and flexural modulus (E_b). These parameters indicate the composites resistance to bending and stiffness under applied loads. Neat PP shows the lowest flexural properties due to its isotropic and unreinforced nature. The addition of PP-g-MA improves interfacial adhesion within the polymer matrix, leading to enhanced load transfer and slightly better flexural properties as the PP-MA sample exhibited FS_max:48.965 MPa (3.9% more than PP) and E_b:2458 MPa (8.7% improvement over PP). PP-2.5DPFs (75 μm) sample exhibited FS_max:54.772 MPa (16.2% higher) and E_b:2576 MPa (13.9% improvement), whereas the PP-2.5DPFs (150 μm) sample produced FS_max:53.182 MPa (12.8% higher) and E_b:2350 MPa (3.9% improvement). The addition of 2.5 wt% DPFs increases stiffness and flexural strength due to fiber reinforcement. Smaller DPFs (75 µm) exhibit better dispersion and interfacial bonding (Figure 7), resulting in superior flexural properties. At 5 wt% DPFs, the improvement in flexural properties becomes more pronounced, particularly in the PP-5DPFs (75 μm) sample (FS_max:57.304 MPa, which is 21.6% higher, and E_b:2806 MPa, which is 24.1% more), due to better stress transfer and fiber-matrix interaction. Larger fibers of the PP-5DPFs (150 μm) sample show reduced performance due to potential agglomeration and weaker adhesion. Increasing DPF content to 7.5 wt% further enhances flexural stiffness, but flexural toughness decreases due to fiber brittleness. The addition of HEA fillers enhances flexural strength and modulus due to their stiffening effect and improved stress distribution. PP-5DPFs (75 μm)-2.5 HEA sample exhibit the best overall performance (FS_max:59.823 MPa and E_b:2895 MPa) due to their superior dispersion and better fiber-matrix compatibility.

As a summary on the load-bearing mechanisms and particle size/geometric effects of the reinforcing fillers, the PP matrix embedded with DPFs (75 μm) provides efficient stress transfer due to its large surface area-to-volume ratio and improved aspect ratio, which might have enhanced the tensile and flexural properties through strong fiber-matrix interlocking and mechanical anchoring. Further, the incorporation of 2.5 wt% FeCrCuMnTi HEA nanocrystallite fillers (9 – 31 nm) act as rigid load-bearing agents, which might have hindered dispersion, and increased stress concentration zones that promote effective load redistribution. In addition, at moderate DPFs contents (2.5 to 5.0 wt%), uniform dispersion and appropriate geometry of DPFs along with HEA powders enable improved interface bonding and stress bridging, which was expected to improve the stiffness and load-bearing capacity. However, at higher DPFs loadings (7.5 wt%), agglomeration and stress localization occurred due to DPFs clustering and poor wettability, lead to a decrease in ductility and lower mechanical performance. The hybrid system of PP-5DPFs (75 mm)-2.5 HEA composite exhibited the highest mechanical performance due to the synergetic combination of DPFs reinforcements and nanocrystallite HEA fillers, which promotes a multi-scale reinforcement mechanism across fiber-matrix and particle-matrix interfaces.

Low-velocity impact behaviour of developed composites

The low-velocity impact performance of neat PP embedded with different weight percentages of DPFs (both sizes, 75 & 150 μm) and HEA fillers was investigated based on force-displacement and absorbed energy-displacement curves (Figures 10 and 11), and the corresponding properties are listed in Table 5. These properties were evaluated to assess the developed DPFs-based composites resistance to impact, energy dissipation, and deformation behaviour. The results demonstrate that neat PP exhibits the lowest peak force (5331 N) and a broader displacement range, indicating low stiffness and significant deformation before failure, which limits its application for withstanding high-impact loads. PP-MA sample produced the peak force of 5443 N (2.1% improvement over neat PP), reflecting that the incorporation of PP-g-MA acts as a good interfacial adhesion between the matrix and DPFs reinforcements/HEA fillers. PP-2.5DPFs (75 μm) sample exhibited a peak force of 5739 N (7.7% more), whereas the PP-2.5DPFs (150 μm) composite produced a peak force of 5917 N (11% improvement). The results revealed that the addition of DPFs improves the stiffness and resistance to deformation. Larger DPFs (150 μm) with 2.5% level loading provide slightly better peak force due to increased load-bearing capability compared to DPFs (75 μm). PP-5DPFs (75 μm) sample exhibited a peak force of 6146 N (15.3 % higher than neat PP), whereas the PP-2.5DPFs (150 μm) sample produced a peak force of 6051 N (13.5% higher). Significant improvement occurred in 5% DPF loading with PP, in which smaller DPF (75 μm) composites showed superior performances due to good dispersion and interfacial bonding, leading to better load distribution and higher force resistance. Further increasing DPFs beyond 5%, the peak force drops to 4785 N in the DPFs-75 μm loaded sample and 4352 N in the DPFs-150 μm loaded sample. This was attributed to improper dispersion/aggregation leading to stress concentrations, which reduced the performance. However, with the introduction of 2.5%HEA fillers with 5%DPFs, the mechanical performance (both load transfer and energy dissipation) is improved, in which the PP-5DPFs (75 μm)-2.5HEA sample exhibited the peak force of 6279 N (17.8% improvement) and the PP-5DPFs (150 μm)-2.5HEA sample produced 6127 N (15% higher). Smaller fibers of DPFs-75 μm combined with HEA fillers showed the best performance due to improved dispersion and effective interfacial bonding.

Figure 10.

Force-displacement curves obtained from low-velocity impact test of developed composites.

Figure 11.

Absorbed energy-displacement curves obtained from low-velocity impact test of developed composites.

Table 5.

Input parameters used during low-velocity impact test and the corresponding properties of the developed composites.

Impact test input parameter	Materials	Low-velocity impact properties
Impact test input parameter	Materials	Maximum absorbed force, N	Maximum impactor absorbed energy, J	Area under absorbed energy-displacement, J-mm
Applied impact energy: 50 J Input velocity: 2.94 m/s Insert type: hemi-spherical (20 mm in diameter) Test temperature: 20°C Falling height: 440 mm Impactor head mass: 15.5 kg	Neat PP	5331 ± 8.567	17.963 ± 1.254	269.238 ± 3.875
	PP-MA	5443 ± 6.351	18.838 ± 1.023	281.764 ± 5.871
	PP-2.5DPFs(75 μm)	5739 ± 4.635	19.241 ± 0.985	292.288 ± 2.587
	PP-2.5DPFs(150 μm)	5917 ± 3.785	17.917 ± 1.057	278.357 ± 3.565
	PP-5DPFs(75 μm)	6146 ± 7.365	20.562 ± 0.857	318.956 ± 5.645
	PP-5DPFs(150 μm)	6051 ± 4.325	18.976 ± 0.958	310.888 ± 4.572
	PP-7.5DPFs(75 μm)	4785 ± 4.856	14.152 ± 0.869	209.452 ± 3.55
	PP-7.5DPFs(150 μm)	4352 ± 5.365	12.499 ± 0.924	211.824 ± 4.897
	PP-5DPFs(75 μm)-2.5HEA	6279 ± 3.478	23.976 ± 0.798	357.559 ± 2.587
	PP-5DPFs(150 μm)-2.5HEA	6127 ± 6.354	21.568	344.794 ± 5.678

Figure 11 shows the variation of absorbed impact energy with the function of displacement for the developed DPFs embedded composites. Neat PP sample exhibited the absorbed energy of 17.96 J, the lowest compared to PP-MA, 2.5DPFs (75 μm), 5DPFs (75 & 150 μm), and 5DPFs (75 & 150 μm)-2.5 HEA loaded samples. PP-MA sample absorbed the energy of 18.84 J (4.9% more) indicating the improved interfacial adhesion. PP-2.5DPFs (75 µm) sample produced an absorbed energy of 19.24 J (7.1% more), whereas PP-2.5DPFs (150 µm) exhibited an absorbed energy of 17.92 J (0.2% lower than neat PP). These results demonstrate that smaller DPFs-75 μm loading enhance energy absorption due to better load transfer, while larger DPFs-150 μm loading show limited improvement in energy absorption due to poor dispersion. The PP-5DPFs (75 µm) sample exhibited an absorbed energy of 20.56 J (14.5% more), whereas the PP-5DPFs (150 µm) sample possessed an absorbed energy of 18.98 J (5.7% more). Increasing the DPFs content up to 5% with the PP matrix, improved energy absorption behaviour was noticed, especially with smaller DPFs-75 μm, due to better load distribution and stress dissipation. With an increase in DPFs beyond 5%, the absorbed energy drops significantly to 14.15 J (21.2% lower) in PP-7.5DPFs (75 µm) and 12.5 J (30.4% lower) in PP-7.5DPFs (150 µm) due to aggregation and stress concentrations. In contrast, the incorporation of 2.5% HEA fillers with 5% DPFs loading enhanced the absorbed energy to 23.98 J (33.5% more) in PP-5DPFs (75 µm)-2.5 HEA and 21.57 J (20% improvement) in PP-5DPFs (150 µm)-2.5HEA. This was attributed to the mixing entropy effect of the incorporated HEA as fillers.

SEM fracture surface analyses after tensile test

The fracture mechanisms and interfacial bonding of developed neat PP and DPFs/HEA reinforced composites samples can be examined using SEM over the fracture sample surface after the tensile test, and the same is shown in Figure 12. These analyses were carried out to explore the morphology of the fracture surface, DPFs-PP matrix interaction, and evidence of dispersion/aggregation, which directly correlate with the tensile properties. SEM fracture surface morphology of neat PP (Figure 12(a1) and (a2)) showed a smooth appearance with ductile tear marks and limited roughness. The appearance of a smooth surface indicates the plastic deformation of the polymer during tensile load. PP-MA sample fracture surface (Figure 12(b1) and (b2)) appears to have increased surface roughness compared to neat PP, with some localized regions showing minor fibrillation. The addition of PP-g-MA enhances the interfacial bonding and energy dissipation during fracture. In addition, the rough surface is expected to improve the resistance to crack propagation, leading to slightly better tensile properties. SEM fracture surface micrograph of PP-2.5DPFs (75 μm) (Figure 12(c1) and (c2)) exhibits well-dispersed smaller DPFs-75 μm in the PP matrix with limited fiber pull-out and de-bonding. Some DPFs are fractured, indicating good load transfer. Smaller fibers (75 μm) produced strong interfacial adhesion with the PP matrix, promoting efficient load transfer and improving tensile properties. The result of the PP-2.5DPFs (150 μm) sample (Figure 12(d1) and (d2)) has shown a noticeable DPFs pull-out and gaps at the fiber-matrix interface. This result explains that larger DPFs-150 μm tend to have a weaker bonding with the PP matrix, leading to premature failure and reduced tensile strength compared to smaller DPFs-75 μm (Table 4). PP-5DPFs (75 μm) sample (Figure 12(e1) and (e2)) exhibited densely distributed DPFs with minimum pull-out and tightly bonded DPFs to the PP matrix. The increased DPF content up to 5% with a good dispersion enhances the stress transfer, leading to a significant improvement in tensile properties (Table 4). Whereas, the PP-5DPFs (150 μm) sample (Figure 12(f1) and (f2)) exhibited slight aggregation of DPFs, which leads to localized stress concentrations affecting tensile properties. Both PP-7.5DPFs (75 μm, Figure 12(g1) and (g2)) and PP-7.5DPFs (150 μm, Figure 12(h1) and (h2)) samples produced a very rough surface with evidence of DPFs clustering, some matrix cracking around aggregated fibers, extensive pull-out, and PP matrix cracking around voids. The poor dispersion of DPFs at high content leads to severe stress concentrations and brittle failure, resulting in poor tensile properties. SEM fracture surface micrographs of PP-5DPFs (75 μm)-2.5HEA and PP-5DPFs (150 μm)-2.5HEA samples demonstrated the uniform dispersion of DPFs with excellent DPFs-PP bonding. The incorporated HEA particles are visible, well integrated, and show no significant voids in smaller DPFs (75 μm). The results further explain that the addition of HEA as fillers improves interfacial bonding, strengthens the PP matrix, and enhances the load transfer. The SEM fracture surface analyses revealed that the combination of smaller DPFs-75 μm and HEA fillers provides the most efficient load transfer and energy dissipation, leading to superior properties. This finding implies the importance of optimal DPFs content, and HEA filler integration for achieving high-performance composites.

Figure 12.

SEM fracture surface micrographs of PP matrix embedded with DPFs and HEA fillers after tensile test samples of: (a₁) neat PP, (a₂) magnified view of (a₁); (b₁) PP-5PP-g-MA, (b₂) magnified view of (b₂); (c₁) PP-2.5DPF (75 μm), (c₂) magnified view of (c₁); (d₁) PP-2.5DPF (150 μm), (d₂) magnified view of (d₁); (e₁) PP-DPF (75 μm), (e₂) magnified view of (e₁); (f₁) PP-5DPF (150 μm), (f₂) magnified view of (f₁); (g₁) PP-7.5DPF (75 μm), (g₂) magnified view of (g₁); (h₁) PP-7.5DPF (150 μm), (h₂) magnified view of (h₁); (i₁) PP-5DPF (75 μm)-2.5HEA, (i₂) magnified view of (j₁); (h₁) PP-5DPF (150 μm)-2.5HEA, (j₂) magnified view of (j₁).

Overall, SEM images (Figures 7 and 8) indicate that the addition of PP-g-MA as a compatibilizer improved fiber–matrix adhesion by forming strong polar interactions with hydroxyl groups on the alkali-treated DPFs as evidenced by reduced fiber pull-out and cleaner fracture surfaces. The PP–5DPF (75 µm)–2.5HEA hybrid composite showed excellent filler dispersion with minimal agglomeration of HEA particles. This homogeneity was attributed to the nanometric size and high surface energy of HEA particles, which facilitate uniform distribution within the PP matrix during melt compounding. In contrast, samples with higher fiber loadings (7.5 wt%) exhibited visible fiber clustering, particularly with the 150 µm DPFs, which results in localized stress concentrations and reduced interfacial bonding. Improved adhesion between the PP matrix and DPFs was evident from fewer voids and better embedding of fibers in composites with optimal DPF loading and compatibilizer presence. HEA fillers contributed to the formation of a rigid interphase, which restricts the polymer chain movement and promotes stress transfer under mechanical loading. This effect was most pronounced in hybrid composites, which validates the synergistic reinforcement mechanism. The fracture surfaces of tensile-tested samples (Figure 12) supported the above findings, where ductile tearing, matrix deformation, and filler–matrix integration were more prominent in optimized hybrid composites than in neat or under/overfilled composites.

Conclusions

In this study, thermoplastics of PP matrix embedded with different sizes of DPFs (75 & 150 μm), different percentages of DPFs (0, 2.5, 5.0, and 7.5 wt%), and 2.5 % of FeCrCuMnTi-HEA fillers of composites were successfully developed via twin extrusion and injection moulding processes. The following conclusions were made through this research work:

❖ The XRD results of bulk samples explain that the addition of DPFs decreased the crystallinity of PP due to their amorphous nature, but this effect could be mitigated using 5% of PP-g-MA as a compatibilizer. The TGA and DTG results revealed that the addition of DPFs enhances/retains the thermal stability of PP-based composites, and the identified best sample of PP-5DPFs-2.5 HEA exhibited a balance between thermal stability and mechanical reinforcement.

❖ FTIR result concludes that the incorporation of DPFs and HEA fillers into PP introduces additional functional groups while maintaining the PP characteristic peaks. Smaller DPFs (75 μm) exhibited better interaction with HEA fillers as PP-5DPFs (75 μm)-2.5 HEA sample produced sharp peaks indicating, effective dispersion.

❖ SEM surface topography analysis of injection moulded samples revealed that the combination of 5DPFs-75 μm and 2.5 HEA fillers showed the best results in terms of DPFs dispersion and interfacial interaction, leading to a more homogeneous microstructure. The SEM-EDAX and elemental mapping confirmed the successful incorporation and uniform distribution of DPFs and HEA fillers in the PP matrix.

❖ The incorporation of DPFs into the PP matrix significantly enhances the mechanical, flexural, and impact properties, with the optimal performance observed at 5 wt% DPFs (75 µm). Smaller fibers (75 µm) exhibit superior dispersion and interfacial bonding compared to larger fibers (150 µm), resulting in better stress transfer and energy dissipation. The addition of FeCrCuMnTi-HEA fillers to PP-DPF composites further enhances the tensile strength, flexural properties, and impact resistance. The best performance was observed in the PP-5DPFs (75 µm)-2.5HEA composite, which demonstrated the highest tensile strength, flexural modulus, and impact energy absorption with improvements of 41.9%, 28%, and 33.5%, respectively, over neat PP.

❖ Based on the significant improvements in tensile, flexural, and low-velocity impact properties obtained in PP–5DPF (75 µm)–2.5HEA hybrid composites, these materials show excellent structural integrity and energy absorption which are highly desirable for automotive components (e.g., bumper systems, interior trims) and protective sporting goods (e.g., helmets, guards).

❖ The use of natural DPFs in PP enhances biodegradability and reduces dependency on petroleum-based raw materials. This aligns with sustainable practices and makes the composites suitable for eco-friendly packaging applications with enhanced mechanical stability.

❖ The introduction of FeCrCuMnTi HEA nanofillers can enhance conductive or antistatic behavior. While this aspect was not directly tested in this study, the material system developed holds future potential for electrical enclosures, antistatic packaging, or EMI shielding applications, which has been briefly highlighted for further exploration.

Footnotes

Acknowledgements

The authors gratefully acknowledge Qassim University, represented by the Deanship of Scientific Research, on the financial support for this research under the number (2023-SDG-1-BSRC35663) during the academic year 1445 AH/2023 AD.

Author contributions

Abdulaziz S. Alaboodi: Formal analysis, data curation, Investigation, Writing-review and editing. S. Sivasankaran: Methodology, Formal analysis, Investigation, Validation, Data curation, Roles/Writing-original draft. Hany R. Ammar: Resources, Project administration, Data curation, Investigation, Writing - review and editing.

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Deanship of Scientific Research; 2023-SDG-1-BSRC35663.

Declaration of generative AI in scientific writing

During the preparation of this work, the author(s) have not used any AI tool.

ORCID iD

S Sivasankaran

Data Availability Statement

The experimental datasets obtained from this research work and then the analyzed results during the current study are available from the corresponding author upon reasonable request.*

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Influence of date-palm-fibers (DPFs) content and nanocrystallite FeCrCuMnTi high-entropy alloy fillers in polypropylene matrix on mechanical and structural performance of thermoplastic composite materials

Abstract

Keywords

Introduction

Experimental procedure

Materials

Manufacturing of composites

Characterization of methods

Results and discussion

Characterization of raw materials

Characterization of PP matrix embedded With date palm fibers and FeCrCuMnTi HEA fillers

XRD analyses

DTGA analyses

FTIR analyses

SEM analyses

Tensile properties of PP embedded with DPFs and HEA filler

Flexural properties of developed composites

Low-velocity impact behaviour of developed composites

SEM fracture surface analyses after tensile test

Conclusions

Footnotes

Acknowledgements

Author contributions

Declaration of conflicting interests

Funding

Declaration of generative AI in scientific writing

ORCID iD

Data Availability Statement

References