Enhanced mechanical and thermal properties of epoxy composites reinforced with date palm seed particles

ABSTRACT

This study aims to evaluate the potential of date palm seed (DPS) particles—an abundant agro-waste byproduct—as a sustainable filler for enhancing the performance of epoxy-based composites. Epoxy composites were fabricated with DPS loadings ranging from 10 to 50 vol%, and their mechanical (compressive strength, impact resistance, hardness), thermal (conductivity), and chemical (FTIR spectroscopy) properties were systematically investigated. The results demonstrated that a DPS content of 30 vol% yielded optimal mechanical performance, with compressive strength reaching approximately 64.5 MPa, alongside notable increases in hardness and impact resistance. Conversely, thermal conductivity decreased progressively with filler content, attaining a minimum of 0.136 W/m·°C at 50 vol% DPS. FTIR analysis confirmed the presence of physical interactions—particularly hydrogen bonding—between the epoxy matrix and DPS particles, while spectra post-impact suggested structural stability with minimal chemical degradation. These findings indicate that DPS is a promising eco-friendly reinforcement for multifunctional epoxy composites, offering improved mechanical resilience and enhanced thermal insulation.

Keywords:
Epoxy composites; Date palm seed (DPS); Agro-waste reinforcement; Mechanical properties; FTIR spectroscopy

1. INTRODUCTION

The growing demand for sustainable material has led to the investigation of using natural fillers and agricultural byproducts as reinforcements in polymer composite [¹, ², ³, ⁴, ⁵, ⁶, ⁷]. Biocomposites reinforced by materials such as almond shells, tamarind seeds, coconut shells, and different parts of date palm tree have been investigated in numerous studies to improve mechanical performance and environmental impact [⁸, ⁹]. With various agro-waste options, the date palm seeds (DPS) particles have emerged as an interesting option considering that date palm seeds are abundant materials, biodegradable, and have favorable mechanical properties [¹⁰, ¹¹, ¹², ¹³]. NWOGU et al. [¹⁴] showed very recently that the date seed granules used as reinforcement in glass fiber reinforced epoxy enhanced tensile, flexural, and hardness properties relative to unreinforced samples particularly at filler loading of 40%. Similarly, SARMIN et al. [²] reported that eggshell and date palm fiber fillers can be used effectively as fillers in bio-epoxy composites, showing improved mechanical behavior and structure application feasibility.

Valorization of these wastes serves as not only an eco-friendly alternative to synthetic reinforcement but also contributes to the circular economy by avoiding issues associated with disposal of these wastes. Furthermore, ABDULRAZAQ et al. [¹⁵] showed that date palm seeds can be utilized in aluminum matrix composites achieving a hardness of 50 HRV and higher compressive strength which promises mechanical usefulness beyond the polymer systems. In addition, the flexural strength of enzymatically treated date palm fibers was comparable to that found in wood and glass fibers composites, highlighting their potential for load and structure bearing applications [⁴].

Research has evaluated multiple date palm components inside epoxy composites which has resulted in growing evidence of their structural potential. RAGHAVENDRA et al. [¹⁶] conducted an investigation which analyzed hybrid laminates composed of date palm fibers together with polyester and carbon fiber mats. Research found that date palm fibers treated with alkalis improved both tensile and flexural properties of composites when used in hybrid arrangements. ALSHAMMARI et al. [¹⁷] expanded the research scope by evaluating date palm leaf sheath and trunk and fruit bunch stalk as epoxy resin fillers. In their study, they established that date palm fruit bunch stalks achieved better mechanical reinforcement through superior tensile and flexural strength measurements.

BENDADA et al. [¹⁸] explored date palm petiole particles with experimental modal and compression testing as well as finite element analysis to verify their structural stiffness contributions and mechanical performance. Through the combination of date palm and bamboo fibers SUPIAN et al. [¹⁹] created hybrid composites which delivered superior impact and tensile strength compared to monofiber systems. The research by PRADHAN et al. [³] involved the addition of blast furnace slag to date palm fiber-reinforced epoxy composites which enhanced their fracture toughness and interlaminar shear strength and confirmed their usage in engineering applications.

Research investigating the impact of DPS particle size along with loading ratios and post-processing conditions on the mechanical and chemical behavior of epoxy composites continues to be insufficiently studied. The majority of existing studies examine fiber-based composites and hybrid forms whereas particulate fillers receive less attention. The research on mechanical properties of such composites lacks complete evaluation since it includes only tensile, compressive and hardness tests alongside spectrum investigations through Fourier-transform infrared (FTIR) analysis for structural modifications under mechanical stress [²⁰].

Although there is an increasing interest in natural fiber-reinforced composites, limited research has been done on the use of date palm seed (DPS) particle as a particulate filler in epoxy matrices, especially on their mechanical properties, thermal conductivity, and chemical stability. A systematic assessment of particulate-filled composites has been lacking, with most research studies being done on fiber reinforcements or hybrid systems. Therefore, the present study aims to: (i) fabricate epoxy composites with varying DPS particle contents (10–50 vol%) using 710 µm fillers; (ii) evaluate their compressive strength, impact resistance, and hardness properties; (iii) assess their thermal conductivity behavior; and (iv) investigate potential chemical interactions and structural modifications using FTIR spectroscopy. The aims are to impart a full understanding of structure-property relationships in DPS-reinforced epoxy composites and determine the applicability of these materials to sustainable engineering practice.

2. MATERIALS AND METHODS

2.1. Materials

The base matrix material selected for this study was epoxy resin (EP) due to its excellent adhesion, thermal stability, and superior mechanical properties, which make it highly suitable for high-performance composite applications. Epoxy resin (EPON 828) and hardener (TETA) were procured from Sigma-Aldrich (analytical grade). All chemicals used in this study were of analytical grade and used as received without further purification.

The reinforcing phase consisted of date palm seed (DPS) particles. Figure 1 depicts the preparation of date palm seed (DPS) particles that are introduced as a reinforcing phase in the epoxy composites. Raw date palm seeds were cleaned and dried naturally to remove external impurities and moisture as illustrated in Figure 1a. Samples were then ground into fine powder using a high-speed electric grinder (Emblems EM 1HP) to obtain a ground sample, as shown in Figure 1b. The resulting powdered product, which can be seen in Figure 1c, was then sieved in stainless steel hand sieve (mesh size = 720 um) to obtain DPS particles of an average size of around 710 um. This sieving step facilitated the uniformity of the particle size and improved the dispersion level of the filler within the polymer matrix. DPS particles were chosen for their sustainability, biodegradability, and ability to enhance the compressive and impact performance of the polymeric matrix. The effect of filler content was studied through systematic additions of 10%, 20%, 30%, 40%, and 50% DPS to the epoxy matrix followed by mechanical, thermal and structural property assessments.

Figure 1
Sample Preparation Process of DPS Particles for Epoxy Composites. (a) Cleaned and dried date palm seeds used as raw material, (b) High-speed grinder (Emblems EM 1HP) used to pulverize the seeds, and (c) Resulting date palm seed powder with average particle size ~710 µm.

2.2. Preparation of composites

Manual fabrication methods produced composite specimens through a process that employed clean aluminum molds with flat surfaces. The entire fabrication process required strict control measures to maintain both the sample homogeneity and structural integrity along with repeatability.

2.2.1. Mold preparation

The aluminum mold (150 mm × 150 mm × 4 mm) received a complete cleansing process with ethyl alcohol to eliminate surface impurities. The walls of the mold received a suitable release agent to help mold removal after curing completion.

2.2.2. Weighing of materials

A high-precision digital balance measured the weights of epoxy resin and DPS particles. The weight fraction (Ψ) and volumetric fraction (Vf) of DPS particles were calculated for each formulation through these equations [²¹]:

(1) $$Vf=\frac{1}{1+\frac{1-\psi }{\psi }\times \frac{{\rho }_f}{{\rho }_m}}$$

(2) $$\psi =\left(\frac{W_f}{W_c}\right)\times 100\%$$

(3) $$W_c=W_f+W_m$$

where W_c, W_m, and W_f denote the total weight of the composite, the weight of the epoxy resin, and the weight of the DPS particles, respectively. The parameters ρf represent DPS density while ρm represents epoxy density with unit’s g/cm³.

2.2.3. Sample pouring and curing

The first step involved pouring a small amount of epoxy resin into the mold so the resulting base layer became uniform. The pre-weighed DPS particles required gradual addition while stirring manually for 10 minutes to produce uniform dispersion and reduce air entrapment. The remaining epoxy resin was added for reaching the targeted filler volume fraction. A methyl ethyl ketone peroxide (MEKP) catalyst was added at a ratio of 100:1.5 (resin to hardener by weight) to initiate polymer crosslinking. The mixture was poured into the mold and left to cure under ambient laboratory conditions (27 ± 1 °C) for 24 hours.

2.2.4. Post-curing heat treatment

After the initial cure, the samples were subjected to post-curing in a thermostatically controlled oven at 50 °C for 1 hour to complete the polymerization process and enhance crosslink density. Following thermal treatment, the samples were cooled gradually inside the oven to avoid thermal shock.

2.2.5. Final preparation

The cured composite sheets were carefully removed from the mold. Edges were trimmed to standardize dimensions, and the samples were stored in sealed plastic bags to prevent contamination before testing. All specimens were fabricated and machined in accordance with ASTM standards relevant to the type of test: ASTM D695 for compression, ASTM D256 for impact, ASTM D2240 for hardness, and appropriate methods for thermal conductivity analysis.

All fabrication steps and experimental evaluations—including compressive strength, impact resistance, hardness testing, thermal conductivity, and FTIR spectroscopy—were carried out in the Advanced Materials Laboratory, Department of Physics, College of Education for Pure Sciences, University of Anbar, Iraq.

Figure 2 illustrates a schematic flowchart summarizing the fabrication process for epoxy–DPS composite specimens, beginning with raw material preparation and proceeding through mixing, molding, curing, and final sample preparation for mechanical and structural testing.

Figure 2
Schematic flowchart of the fabrication process for epoxy- date palm seed particles (DPS) composite specimens.

2.3. Mechanical testing procedures

2.3.1. Compressive test

To evaluate the compressive behavior of the epoxy (EP) matrix reinforced with date palm seed (DPS) particles, cubic specimens with dimensions of 20 mm × 20 mm × 20 mm were prepared in accordance with ASTM D695-15 [²²]. The compressive tests were conducted using a LARYEE Yaur Testing Solution universal testing machine. A compressive load was applied vertically at a crosshead speed of 5 mm/min until specimen failure or significant deformation was observed. The applied load and corresponding deformation were recorded in real-time to determine the compressive strength and strain behavior of each composite formulation. At least three measurements were performed for each DPS loading percentage to ensure reproducibility, and average values with standard deviations were calculated.

2.3.2. Impact test

The Izod Charpy impact test was performed to determine the impact resistance and energy absorption capacity of the epoxy–DPS composites under dynamic loading conditions. Rectangular notched specimens with standard dimensions of 4 mm × 10 mm × 80 mm were prepared in accordance with ASTM D6110-18 [²³]. Impact testing was carried out using a Charpy impact test instrument equipped with a 7 J calibrated pendulum. Each specimen was mounted horizontally in a fixed position and struck centrally by the pendulum. The energy absorbed during fracture (kJ/m²) was measured and recorded automatically by the machine. A minimum of three specimens per composition were tested to account for data variability and statistical reliability.

2.3.3. Hardness test

The surface hardness of the cured epoxy–DPS composites was determined using the Shore D hardness scale, following the protocol specified in ASTM D2240-15 [²⁴]. A Shore D digital hardness tester (Elcometer 120 Shore D, UK) was employed for this purpose. A needle-like indenter was used to apply perpendicular force against sample surfaces and the resulting hardness values were directly measured with the gauge. The representative hardness value incorporates the average of five readings which researchers acquired from different positions across each specimen. Surface wear resistance and indentation resistance measurements were obtained through these tests which examined the composite material properties.

2.4. Thermal conductivity test

The thermal conductivity evaluation of epoxy composites with date palm seed (DPS) particles used Ta Lee’s disk method through a setup produced by Griffen & George. This testing method delivers dependable heat transfer data on polymeric composites by monitoring temperature variations in metallic discs.

The experimental setup required two copper discs of 50 mm diameter and 5 mm thickness which positioned the composite in the middle. A base plate served as a third disc underneath the sample. Direct application of a 0.25 A current at 6 V generated steady thermal conduction across the layered system in the entire disc-sample arrangement. The test incorporated three thermocouples distributed in the copper discs for detecting varying temperature points: TA : Upper disc, TB : Lower disc, and TC : Base disc

Thermal conductivity coefficient (K) measurements followed these calculations [²⁵]:

(4) $$K\left(\frac{T_B-T_A}{T_s}\right)=e\left[T_A+\frac{2}{r}\left(d_A+\frac{1}{4}{d}_s\right)T_A+\frac{1}{2r}{d}_s{d}_B\right]$$

(5) $$\begin{array}{l}H=IV=\pi r^{2}e\left(T_A+T_B\right)\\ +2\pi re\left[d_A{T}_A+d_s\cdot \frac{1}{2}\left(T_A+T_B\right)+d_B{T}_B+d_C{T}_C\right]\end{array}$$

Where:

• K is the thermal conductivity (W/m·°C),

• H is the power input (W),

• e is the rate of heat energy transfer (W/m²·°C),

• T_A, T_B, T_C are the temperatures (°C) of the top, middle, and bottom copper discs, respectively,

• d_A, d_B, d_C are the thicknesses (mm) of the respective copper discs,

• d_S is the sample thickness (mm), and

• r is the radius (mm) of the discs.

The setup’s physical parameters along with measured steady-state temperatures enabled thermal conductivity computations for the composite samples. Results from the investigation analyzed how DPS content affected the thermal insulation capability of the epoxy matrix which revealed potential use cases in thermal management systems.

All mechanical and thermal tests were performed on three replicate samples for each DPS volume fraction to ensure statistical reliability and reproducibility of the results. Data variability and reliability of measurements was visualized by generating error bars representing standard deviation in Microsoft Excel.

2.5. FTIR spectroscopy

The chemical composition and molecular interactions of epoxy (EP) matrix samples containing date palm seed (DPS) particles were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) before and after impact testing. The analytical approach delivered information about molecular-level modifications possibly caused by mechanical stress which could lead to oxidation events and bond scission processes and structural changes. FTIR thus served as a diagnostic tool to correlate chemical integrity with mechanical performance.

The measurements were carried out using a PerkinElmer Spectrum Two FT-IR Spectrometer, equipped with a high-sensitivity detector and a robust signal processing unit. Spectra were acquired in the mid-infrared range of 400 to 4000 cm⁻¹, at a resolution of 4 cm⁻¹, and 16 scans were averaged for each sample to ensure high signal-to-noise ratio and spectral clarity.

Thin films of the cured composite samples (with and without mechanical deformation) were prepared and analyzed in transmission mode. Two distinct types of spectra were collected for comparative interpretation:

• Pre-impact spectra: Obtained from pristine, undamaged epoxy–DPS samples to serve as a reference for the baseline chemical structure.

• Post-impact spectra: Extracted from the fractured regions of impact-tested samples, specifically those containing 30% DPS, to assess changes induced by mechanical loading.

Variations in peak intensity, broadening, or position shifts were interpreted as indicators of chemical modifications resulting from impact-induced stresses, such as chain scission, crosslink density reduction, or oxidative degradation. These spectral changes were critically compared with the mechanical test results to establish a structure–property relationship, highlighting the role of DPS in enhancing or modifying the chemical resilience of the epoxy matrix under dynamic loading conditions.

3. RESULTS AND DISCUSSION

3.1. Compressive strength

The compressive mechanical behavior of epoxy composites reinforced with date palm seed particles (DPS) demonstrates a pronounced dependence on filler content, as evident from both the stress-strain curves and the summary bar charts with error bars (see Figures 3, 4 and 5). The reference sample of pure epoxy exhibited a characteristic thermosetting polymer response, with an extended elastic-plastic region and a gradual increase in stress with strain, culminating in a high ultimate compressive strength (UCS) of approximately 79.6 MPa and a corresponding strain at failure near 19.4%. This behavior is attributed to the inherent cross-linked molecular network of epoxy resins, which provides substantial resistance to deformation and a delayed onset of microcracking under axial compressive load [²⁶].

Figure 3
Compressive stress-strain curves of epoxy/DPS composites.

Figure 4
Ultimate tensile strength of epoxy/DPS composites.

Figure 5
Strain at UCS of epoxy / DPS composites.

Upon incorporation of 10% DPS by volume, a significant reduction in mechanical performance was observed. The UCS dropped to approximately 45.6 MPa, while the strain at failure decreased sharply to about 5.3%. Physically, this reduction may be attributed to several concurrent mechanisms. Firstly, the introduction of natural fillers interrupts the continuity of the epoxy matrix, leading to localized stress concentrations around the filler-matrix interface. These interfacial regions often act as defect sites when the filler lacks strong adhesion to the matrix, resulting in early debonding and crack initiation. Moreover, at low filler contents, the particle dispersion is often non-uniform, promoting agglomeration that further degrades mechanical integrity [²⁷].

Interestingly, with 20% DPS, a modest improvement was observed compared to the 10% composite, with UCS rising to ~49.2 MPa and strain at failure increasing to ~6.8%. This suggests an initial improvement in filler distribution and stress transfer efficiency as the particle network becomes more integrated within the polymer matrix [²⁸]. However, the most notable enhancement in compressive performance occurred at 30% DPS, where the UCS reached ~64.5 MPa and the strain at failure extended to ~11.8%. At this concentration, the particle-matrix interaction appears to be sufficiently optimized to allow for effective load distribution. Physically, this may be due to the development of a semi-continuous filler network that facilitates crack deflection and energy dissipation mechanisms, including particle bridging, micro-crack pinning, and matrix yielding [²⁹].

At 40% DPS, the mechanical response began to plateau, with a UCS of ~60.1 MPa and a moderate reduction in strain to ~8.9%. While the filler content is higher, the benefits of reinforcement appear to be counterbalanced by the onset of matrix embrittlement and potential filler agglomeration. As filler content increases, the inter-particle spacing decreases, which may constrain the polymer’s ability to deform plastically. Furthermore, the high filler volume fraction increases the likelihood of voids and weak interfacial regions, especially in the absence of surface modification of the DPS [¹⁵].

At 50% DPS, the composite exhibited a noticeable decline in both UCS (~45.8 MPa) and strain at failure (~5.7%), indicating the mechanical limit of reinforcement with unmodified DPS. Physically, the matrix becomes highly constrained by the dense filler phase, reducing its load-bearing capacity and energy dissipation potential [³⁰]. Additionally, the poor stress transfer across the filler-matrix interface at this high loading results in early debonding and brittle fracture. The mechanical deterioration at this level may also be linked to the percolation threshold, beyond which further filler addition no longer contributes to reinforcement but instead promotes the formation of stress-raising inclusions [³¹].

The bar charts summarizing the UCS and strain at failure across all compositions, along with the standard deviation error bars derived from triplicate tests, validate the observed trends. The relatively low standard deviations indicate consistent sample preparation and reliable measurements. Notably, the charts corroborate the inference that the 30% DPS composite achieves the optimal trade-off between stiffness, strength, and ductility. This suggests that at intermediate filler loadings, a synergistic effect between the particle reinforcement and the matrix’s deformation capacity can be achieved. The mechanical improvement at this stage is not solely due to the presence of hard particles but also results from enhanced microstructural interaction, including improved stress distribution and crack-bridging mechanisms at the particle interface.

The current trend in compressive strength is consistent with other natural filler reinforced epoxy systems. As an example, the compressive strength of epoxy composites with Calotropis gigantea fiber filler improved with an increase in the filler up to 35 wt% of filler through enhanced stress transfer but subsequently decreased with the compromise of interfacial bonding and fiber agglomeration [³²]. Similar results were observed by JAYASEELAN et al. [³³] on banana particulate- reinforced epoxy composites, which exhibited a similar trend with compressive strength increasing to 30-35 wt% and then decreasing due to filler clustering and ineffective load transfer at a high content. Furthermore, good fiber-matrix bonding Flax fiber was also found to provide a comparable strength and stiffness optimal point at around 30 wt%, when reinforced in epoxy [³⁴].

These analogies confirm that the mechanical response recorded in the present DPS-filled system- i.e., an initial ramping-up in compressive strength culminating into a drop at high filler contents- is a well-documented pattern in lignocellulosic filler strengthened epoxy composites.

3.2. Impact strength

The results of the impact test for epoxy composites filled with date palm seed (DPS) particles reveal a progressive enhancement in impact strength with increasing filler content (see Figure 6). The base epoxy matrix, in its pure form, exhibited the lowest energy absorption capacity, with an impact strength of approximately 0.33 kJ/m². This low value is characteristic of thermoset resins such as epoxy, which typically fail in a brittle manner under sudden loading conditions due to their highly cross-linked molecular structure and limited capacity for plastic deformation [³²].

Figure 6
Impact strength of epoxy with varying DPS content.

The addition of DPS particles, even at a modest volume fraction of 10%, led to a measurable improvement in impact strength (~0.35 kJ/m²), and this enhancement continued systematically across higher filler contents, peaking at 0.53 kJ/m² for the 50% DPS composite. This rising trend can be attributed to several interrelated physical mechanisms.

Firstly, the presence of DPS particles introduces heterogeneity into the matrix, which acts to interrupt crack propagation paths. When a composite is subjected to impact loading, stress waves travel rapidly through the material. In a homogeneous matrix, these waves can concentrate and initiate brittle fracture. In contrast, in a heterogeneous system with well-dispersed filler particles, the crack front is frequently deflected, bridged, or arrested at the filler-matrix interface. This not only increases the energy required for crack propagation but also facilitates nonlinear deformation mechanisms, such as localized yielding or micro-crack coalescence, that enhance toughness [²⁶, ³⁵].

Secondly, the lignocellulosic nature of DPS particles contributes to energy absorption. Natural fillers tend to exhibit intrinsic flexibility and porosity, which allow them to deform or crush under dynamic loads, absorbing impact energy in the process. At higher volume fractions, such as 40% and 50%, the cumulative effect of this filler-induced toughening becomes more pronounced, enabling the composite to withstand more aggressive impact conditions [³⁶].

The relatively small standard deviations observed across all DPS ratios—ranging from ±0.01 to ±0.025 kJ/m²—indicate good experimental repeatability and support the validity of the observed trend. The error bars reinforce that the increases in impact strength are not artifacts of random variability but rather represent genuine material behavior. It is worth noting that while the increase in impact strength with DPS content is consistent, the rate of improvement appears to diminish at higher loadings. This may be attributed to filler agglomeration or particle–particle interaction, which can create stress concentration zones if not well-bonded or homogeneously distributed. Despite this, the observed trend suggests that the inclusion of DPS particles transforms the epoxy matrix from a brittle to a more damage-tolerant system, particularly at higher filler concentrations [³⁷].

The mechanical behavior observed in epoxy-DPS composites is a manifestation of these different mechanisms of compressive performance and impact performance. Compressive strength first rises to 30% DPS because of better filler dispersion and enhanced stress transfer and crack-bridging networks at the particle-matrix interface. At loads exceeding 30%, however, the compressive strength decreases due to excessive filler loading causing agglomeration, poor interfacial bonding, and voids, introducing stress concentration sites and compromising structural integrity [³³, ³⁸].

Conversely, impact strength increases monotonically with the DPS content. This is attributed to the fact that the presence of DPS particles enhances crack deflection, energy absorbance, and microcrack toughening, which are further magnified by increasing the filler quantity. Porous and flexible properties of DPS allow more energy dissipation under dynamic load, and these mechanisms remain quite active even with the high filler loading, without causing mechanical failure [³², ³⁴].

Collectively, these trends point to a two-fold reinforcement mechanism: DPS functions as a structural reinforcement at moderate filler concentrations (enhancing stiffness and strength), and as a structural toughening agent at high filler concentrations (increasing damage tolerance under impact).

3.3. Hardness (Shore D)

The measured hardness values for epoxy composites filled with date palm seed (DPS) particles show a consistent and progressive enhancement as the filler content increases from 0% to 50% (see Figure 7). Pure epoxy exhibited a Shore D hardness of 71.2, reflecting the inherent rigidity of the cross-linked thermosetting network. Epoxy polymers are known for their high stiffness due to their dense molecular structure, yet their mechanical hardness can still be modified by the addition of suitable fillers [³⁹].

Figure 7
Hardness for epoxy with varying DPS content.

With the incorporation of DPS at just 10% by volume, a marginal increase in hardness to 71.5 Shore D was observed. This initial rise suggests that the dispersed DPS particles begin to restrict the movement of polymer chains locally, thereby contributing to surface rigidity. As the filler content increases further to 20% and 30%, the hardness rises more significantly, reaching 74.2 and 78.0, respectively. This increase is attributed to the growing volume fraction of rigid lignocellulosic particles, which act as mechanical barriers against localized surface deformation during indentation [¹⁴].

From a microstructural perspective, the improvement in hardness can be physically explained by the particle reinforcement effect. DPS particles, due to their intrinsic hardness and irregular surface texture, provide resistance against plastic indentation by increasing the effective contact stiffness. As more particles are introduced, the continuous phase of the epoxy matrix becomes increasingly constrained. The filler particles carry a larger share of the applied load during indentation, reducing the matrix’s contribution to deformation and enhancing the overall hardness of the composite [²⁷].

At 40% and 50% DPS, the hardness further increases to 79.3 and 80.1, respectively. These values reflect the dominance of the filler phase in the composite microstructure. At this stage, the inter-particle spacing is reduced, and the composite approaches a semi-continuous hard phase, where the interaction among particles further inhibits local displacement. Additionally, the polymer chains become more restricted in mobility due to the filler crowding, further reinforcing the composite’s hardness. The relatively small standard deviations observed (ranging from ±0.3 to ±0.6) indicate a high degree of consistency in the measurements, confirming the reliability of the observed trends.

It is also important to note that the hardness increase correlates well with the mechanical reinforcement mechanisms observed in the compressive test. However, unlike compressive strength, which may degrade at higher filler loadings due to poor interfacial bonding or agglomeration, hardness tends to continuously improve with filler content as it reflects surface-level resistance to penetration rather than bulk fracture behavior.

The observed trend of higher DPS filler content translating to higher hardness levels in the present study aligns with the findings of some past studies on epoxy composite materials reinforced with natural fillers. As an example, MŁYNARCZYK et al. [⁴⁰] found that adding lignin, chitosan, and starch in the epoxy system increased the hardness values gradually. The main reason behind this improvement was the higher surface stiffness and better dispersion of particles, which reduced polymer chain mobility and increased resistance to the localized elastic deformation. In a similar manner, walnut shell and oak flour filler experiments showed that hardness increased continuously as filler composition increased. According to the research by SAŁASIŃSKA et al. [⁴¹] and SIENKIEWICZ and CZUB [⁴²], the interaction with the epoxy matrix and the rigidity of such natural particles limited the flexibility of the polymer network. This reduced the flexibility of the chain motions leading to increased resistance to indentation and surface damage, especially at elevated filler loadings. Microstructurally, AYYANAR et al. [⁴³] were able to further confirm that an enhanced fraction of fillers introduced to a natural fiber and reinforced epoxy compound creates a semi-continuous hard phase. The arrangement enhances the filling capacity of the filler together with inter-particle contact, which cumulatively optimize indentation resistance and hardness behaviour.

3.4. Thermal conductivity

The thermal conductivity behavior of epoxy composites reinforced with varying concentrations of date palm seed (DPS) particles reveals a progressive decline in heat transfer capability with increasing filler content (see Figure 8). The neat epoxy matrix exhibits a thermal conductivity of 0.171 W/m·°C, which falls within the typical range for thermosetting polymers known for their low thermal conductance due to the amorphous molecular structure and weak van der Waals interactions between polymer chains [²⁹].

Figure 8
Thermal conductivity for epoxy with varying DPS content.

Upon the addition of DPS at 10%, thermal conductivity slightly decreases to 0.166 W/m·°C, and this downward trend continues steadily, reaching a minimum value of 0.136 W/m·°C at 50% DPS content. This behavior is fundamentally driven by two key physical mechanisms: phonon scattering at heterogeneous interfaces and intrinsic thermal resistance of the DPS particles themselves [⁴⁴]. Natural fillers such as DPS are composed mainly of lignocellulosic constituents—cellulose, hemicellulose, and lignin—which are inherently poor thermal conductors. These materials are rich in organic carbon and possess porous, anisotropic microstructures that impede the orderly propagation of phonons, which are the primary carriers of heat in non-metallic solids [⁴⁵]. As the volume fraction of DPS increases, the number of filler-matrix interfaces also increases, leading to a higher frequency of phonon scattering events. This interfacial disruption hinders heat flow through the composite by acting as thermal barriers.

Moreover, the random and non-aligned distribution of DPS particles further inhibits the formation of continuous thermal conduction pathways. The insulating nature of the DPS phase, combined with the discontinuities at the particle-epoxy boundary, contributes to a composite system with increasing thermal resistance as more filler is introduced [²⁶]. At higher filler loadings (≥30%), the agglomeration and partial percolation of low-conductivity particles create thermally resistive clusters, compounding the reduction in effective thermal conductivity [⁴⁶].

The slight but consistent standard deviation values (±0.0015 to ±0.003) indicate that these measurements are reproducible and that the trend is not due to random variation but an intrinsic material response. This reproducibility reinforces the assertion that the DPS particles systematically degrade thermal transport efficiency in the composite.

In engineering applications, the observed reduction in thermal conductivity may be either beneficial or limiting, depending on the intended use. For thermal insulation applications, the DPS-reinforced composites could offer improved performance due to their enhanced resistance to heat flow. However, for applications demanding efficient thermal dissipation—such as in electronics, LEDs, or automotive components—this decline in conductivity may necessitate additional design considerations or the use of thermally conductive additives to counterbalance the insulating effect of DPS.

The thermal responses are consistent with those of other polymer matrices reinforced with date palm particulates (DPP), and thus, it is appropriate to draw meaningful comparisons. As an example, poly(3-hydroxybutyrate) (PHB) based composites loaded with DPP resulted in even low thermal conductivities of the range of 0.086 to 0.100 W/m K at a filler concentration of 50% pointing to their great insulating capacity [⁴⁷]. Likewise, the unsaturated polyester (UPR) loaded with DPP has a thermal conductivity between 0.126 and 0.138 W/m·K that are in the same range with or better than epoxy based DPH systems at the same filler loading [⁴⁶]. Other polymeric matrices such as polystyrene reached a minimal thermal conductivity of 0.0515-0.0562W/mK within DPP, providing remarkable insulation at the cost of compromised mechanical properties [⁴⁸]. A similar epoxy-based test with 30% DPP reported ~0.138 3 W/m 3 K, not far off this study 0.136 3 W / m 3 K at 50% DPS, which confirms consistency in epoxy performance [²⁹]. The thermal conductivity of date palm seed (DPS) particles reinforced epoxy composites exhibited a predictable decline as filler content was increased, mainly fueled by phonon scattering and the inherent insulating quality of lignocellulosic fillers. The clean epoxy matrix has a thermal conductivity of 0.171 W/mK, which gradually reduces to 0.136 W/mK in 50 dark phosphorus 20 loading. This phenomenon is explained by enhanced phonon scattering at the particle-matrix interfaces, which is also known in polymer composites with organic fillers due to lack of interfacial thermal continuity that prevents heat flow [⁴⁹]. Lignocellulosic fillers, such as DPS, whose main components are cellulose, hemicellulose, and lignin, have low intrinsic thermal conductivity and porous structure, which further impedes phonon conduction. Growing filler-matrix interfaces due to DPS contents strengthens thermal resistance via interface scattering and thermal mismatch influence [⁵⁰]. This effect is compounded at higher loadings where agglomeration of DPS particles forms thermally resistive domains. At the comparative level, other natural fillers e.g., banana fibers and kenaf in polymer matrices also demonstrate the similar trend of thermal conductivity decreasing with the increase of the filler content unless modified with hybrid fillers or alignment mechanism [⁵¹]. Nevertheless, the DPS/epoxy composites offer a highly satisfactory balance between thermal insulation and mechanical behavior. Compared to other biocomposite systems, however, DPS-infiltrated epoxy can be competitive, and recent examples include PHB and UPR, with thermal conductivities of the order of 0.086W/m-K-0.138W/m-K at 50% loading [⁵²].

These comparisons imply that although DPS-filled epoxy is a relatively good compromise of insulation and structural strength, other polymers can be more thermally insulative than epoxy when used directly in insulation applications. Nevertheless, epoxy-based systems are beneficial when both multifunctionality and thermal resistance are desired, with mechanical resilience in particular.

3.5. Integrated discussion and property correlation

The systematic investigation of epoxy composites reinforced with varying concentrations of date palm seed (DPS) particles (0–50%) reveals a complex interplay between mechanical reinforcement and thermal insulation, governed by both the intrinsic properties of the constituents and the interfacial behavior at the microscopic level.

3.5.1. Mechanical–thermal trade-off

DPS had a nonlinear effect on compressive strength and strain-to-failure. Initially, the addition of 10% and 20% DPS reduced the ultimate compressive strength (UCS) due to insufficient matrix-particle adhesion and the introduction of stress-concentration sites. However, an optimal response was observed at 30% DPS, where the UCS reached ~64.5 MPa and the strain extended to ~11.8%. This improvement is attributed to enhanced stress transfer, crack deflection, and increased filler-matrix interaction, forming a quasi-continuous load-bearing network.

In contrast, thermal conductivity demonstrated a monotonic decrease with increasing DPS content, from 0.171 W/m·°C (pure epoxy) to 0.136 W/m·°C (50% DPS). This inverse correlation indicates that the same microstructural features responsible for mechanical reinforcement (e.g., increased filler volume, interface density, and particle dispersion) concurrently disrupt phonon transport, thereby enhancing thermal insulation. Thus, the composites exhibit a mechanical–thermal trade-off, where improved strength and stiffness are accompanied by reduced thermal conduction—a behavior advantageous for load-bearing insulation materials.

3.5.2. Impact toughness vs. hardness

The impact strength showed a consistent and nearly linear increase with filler content, from 0.33 kJ/m² (pure epoxy) to 0.53 kJ/m² at 50% DPS. This enhancement arises from energy-dissipating mechanisms such as crack pinning, filler pull-out, and microcracking at the interface, which are more effective as the filler content increases. Concurrently, hardness (Shore D) also increased from 71.2 to 80.1, reflecting a higher surface rigidity and restricted polymer chain mobility due to the stiff inclusion of DPS particles.

Interestingly, both properties respond positively to filler addition, despite representing different mechanical domains: hardness correlates with localized resistance to indentation, whereas impact toughness relates to energy absorption under dynamic loading. Their simultaneous improvement suggests that the microstructural stiffening effect of the filler phase reinforces both surface and bulk resistance mechanisms without compromising composite integrity.

3.5.3. Optimal filler range and microstructural balance

The composite containing 30–40% DPS emerges as the optimal configuration, achieving a favorable balance between compressive strength, strain capacity, impact toughness, and surface hardness, while maintaining an acceptably low thermal conductivity for structural applications. Beyond 40%, further filler addition yields diminishing returns in mechanical enhancement, likely due to particle agglomeration, matrix discontinuity, and increased interfacial stress.

3.5.4. Structure–property relationships

The observed trends reflect fundamental structure–property relationships:

• Filler-Matrix Interaction: Critical to mechanical load transfer and crack resistance.

• Particle Distribution and Dispersion: Determines uniformity of stress and heat flow.

• Volume Fraction: Influences continuity of both the reinforcing phase (mechanical) and insulating phase (thermal).

• Interfacial Area: Key to energy dissipation (impact), stiffness (hardness), and phonon scattering (thermal).

These correlations suggest that DPS acts not merely as a filler but as a functional phase within the composite, actively governing multi-property behavior through its morphology, dispersion, and interface with the matrix

3.5.5. FTIR spectroscopy

Fourier Transform Infrared (FTIR) spectroscopy was employed to evaluate the chemical structure and interfacial interactions in epoxy composites reinforced with 30% date palm seed (DPS) particles before and after mechanical impact. As shown in Figure 9, the spectra were recorded in the range of ~400–4000 cm⁻¹ for three sample conditions: pure epoxy, epoxy with 30% DPS, and epoxy with 30% DPS post-impact.

Figure 9
FTIR spectra of pure epoxy, epoxy + 30% DPS before impact, and epoxy + 30% DPS after impact.

3.5.5.1. Pure epoxy spectrum (baseline)

The neat epoxy spectrum displays key bands consistent with crosslinked thermoset structures:

• O–H stretching: Broad peak at ~3440–3500 cm⁻¹, attributed to residual hydroxyl groups from incomplete curing or absorbed moisture.

• C–H stretching: Peaks between 2800–3050 cm⁻¹ indicating aliphatic and aromatic hydrogen vibrations.

• C=C aromatic and C–H bending: Bands at ~1600–1500 and ~1450 cm⁻¹ linked to the bisphenol-A backbone.

• C–O–C stretching: Intense peaks from ~1250–1030 cm⁻¹ confirming ether linkages in the epoxide.

• Epoxide ring deformation: Notable band near 915–850 cm⁻¹, representing unreacted oxirane groups [²⁷].

These peaks serve as reference points for detecting any chemical or physical interactions upon DPS addition and mechanical stress.

3.5.5.2. Epoxy + 30% DPS (before impact)

Introducing 30% DPS leads to meaningful spectral changes that suggest specific interfacial interactions:

• O–H band shift and broadening: The peak near ~3440 cm⁻¹ shifts slightly to lower wavenumbers and broadens, indicating hydrogen bonding between DPS hydroxyl groups (from cellulose, hemicellulose, lignin) and epoxy’s polar moieties.

• Increased intensity at 1050–1150 cm⁻¹: Reflects additional C–O and C–C stretching contributions from the polysaccharide-rich filler, confirming its incorporation [⁵³].

• Reduced intensity of epoxy ring band (~915 cm⁻¹): Suggests enhanced crosslinking or physical suppression of epoxide vibrational modes due to the filler’s interaction.

• Subtle shifts in the aromatic and C=O region (1500–1600 cm⁻¹): May indicate π–π stacking or van der Waals interactions between lignin components and aromatic epoxy structures.

Collectively, these changes support the occurrence of non-covalent interfacial bonding and matrix-filler compatibility, which aligns with the improved mechanical performance observed.

3.5.5.3. Epoxy + 30% DPS (after impact)

Post-impact spectra provide insight into structural integrity and molecular rearrangements:

• Reduced transmittance across 1000–1300 and 2800–3500 cm⁻¹: May signal microstructural densification or fracture-induced exposure of functional groups.

• Further broadening of O–H stretch: Implies increased hydrogen bonding, possibly due to newly exposed hydroxyl sites at fracture surface [²⁶].

• Red-shifting of peaks: Minor wavenumber decreases suggest molecular disorder or polymer chain relaxation following mechanical loading.

• Persistent suppression of the epoxy ring band: Indicates the cured network remains chemically stable, with no reformation of unreacted epoxide after impact.

These observations reveal that the composite undergoes physical rearrangement under stress but retains its chemical structure, emphasizing its impact resilience and interfacial durability.

The FTIR analysis indicates that the addition of 30 percent DPS to the epoxy matrix can promote interfacial interactions via bonding through hydrogen bonds (broadening and shifting of OH bands) and supplementary polysaccharide-related vibrational modes, and it is agreeable with the existing data on natural filler epoxy systems [⁵⁴, ⁵⁵]. Following impact, hydrogen bonding and red-shifting have increased, evidence of further exposure of polar groups and breach of chemical integrity as a result of fracture processes but, importantly, the essential bands remain. These spectral shifts remain consistent with providing good interfacial matching and interfacial creep behavior and structural integrity after mechanical loading, which in turn, contributes to the enhanced impact toughness and post-impact resiliency of the composite.

4. CONCLUSIONS

The integration of date palm seed (DPS) particles into epoxy resin successfully produced biocomposites with enhanced mechanical and thermal characteristics. The 30% DPS formulation was identified as the optimal composition, yielding superior compressive strength, strain capacity, impact toughness, and surface hardness. The progressive increase in filler content contributed to improved energy absorption and surface resistance, while simultaneously reducing thermal conductivity due to phonon scattering and the intrinsic insulating nature of DPS. FTIR spectroscopy confirmed the chemical compatibility and interfacial bonding between the matrix and filler, and revealed no significant degradation after impact testing. Collectively, the results demonstrate that DPS-reinforced epoxy composites present a promising route toward sustainable engineering materials suitable for load-bearing and thermally insulating applications, while promoting agricultural waste valorization and environmental sustainability.

Despite the promising findings, the study is limited by the absence of long-term environmental aging tests, microstructural characterization (e.g., SEM analysis), and evaluation under cyclic loading conditions. Additionally, the interfacial adhesion between epoxy and DPS could be further improved through surface functionalization or chemical treatments. Future research should explore hybridization with other natural or synthetic fibers, optimization of particle size distribution, and life cycle assessment (LCA) of the developed composites. Furthermore, numerical modeling and finite element simulation could be employed to predict composite behavior under diverse mechanical and thermal conditions.

The importance of this research is that it demonstrates the potential application of agro-waste products, which are date palm seed (DPS) particle materials, as functional reinforcement in epoxy composites. Besides being economical and readily accessible, DPS usage can provide significant improvements in compressive strength, impact toughness, surface hardness, and thermal insulation. The multifunctionality of DPS, in terms of its simultaneous enhancement of mechanical strength and thermal conductivity retardation, makes it a high potential component in sustainable engineering. Moreover, DPS adoption cultivates the valorization of agricultural waste, leading to the implementation of circular economy approaches and environmental sustainability. Hence, the results serve as a robust basis of further green composite development in structural designs, insulation, and environmentally responsible product design.

5. BIBLIOGRAPHY

• ^[1] SIENKIEWICZ, N., DOMINIC, M., PARAMESWARANPILLAI, J., “Natural fillers as potential modifying agents for epoxy composition: a review”, Polymers, v. 14, n. 2, pp. 265, 2022. doi: http://doi.org/10.3390/polym14020265. PubMed PMID: 35054672.
  » https://doi.org/10.3390/polym14020265
• ^[2] SARMIN, S., JAWAID, M., ZAKI, S., et al, “The effect of eggshell fillers on the physical, mechanical, and morphological properties of date palm fibre reinforced bio-epoxy composites”, Journal of Polymers and the Environment, v. 31, n. 11, pp. 5015–5027, 2023. doi: http://doi.org/10.1007/s10924-023-02924-9.
  » https://doi.org/10.1007/s10924-023-02924-9
• ^[3] PRADHAN, M., PATNAIK, A., BANERJEE, M.K., et al, “Effect of micro-sized blast furnace slag on physical, mechanical, and thermo-mechanical properties of bi-directional date palm fiber reinforced polymer composite”, Materialwissenschaft und Werkstofftechnik, v. 56, n. 2, pp. 266–280, 2025. doi: http://doi.org/10.1002/mawe.202400153.
  » https://doi.org/10.1002/mawe.202400153
• ^[4] BELGACEM, C., SERRA-PARAREDA, F., TARRÉS, Q., et al, “Valorization of date palm waste for plastic reinforcement: macro and micromechanics of flexural strength”, Polymers, v. 13, n. 11, pp. 1751, 2021. doi: http://doi.org/10.3390/polym13111751. PubMed PMID: 34071915.
  » https://doi.org/10.3390/polym13111751
• ^[5] GORAR, A.A.K., QAISARANI, M.E., LIU, W.-B., “Epoxy composites reinforced with almond shell and date seed particles: mechanical properties, machinability, conductivity, thermal stability, and stress analysis”, IEEE Transactions on Dielectrics and Electrical Insulation, v. 32, n. 1, pp. 145–152, 2025. doi: http://doi.org/10.1109/TDEI.2024.3396439.
  » https://doi.org/10.1109/TDEI.2024.3396439
• ^[6] IBRAHEEM, E.K., BDAIWI, W., “Enhancing mechanical and thermal properties of unsaturated polyester composites through sidr leaves’ particle reinforcement”, Revue des Composites et des Materiaux Avances., v. 34, n. 3, pp. 269–275, 2024. doi: http://doi.org/10.18280/rcma.340301.
  » https://doi.org/10.18280/rcma.340301
• ^[7] IBRAHIM, M., SAPUAN, S., FAIEZA, A., “Mechanical and thermal properties of composites from unsaturated polyester filled with oil palm ash”, Journal of Mechanical Engineering Science, v. 2, pp. 133–147, 2012. doi: http://doi.org/10.15282/jmes.2.2012.1.0012.
  » https://doi.org/10.15282/jmes.2.2012.1.0012
• ^[8] NAIK, S., HALEMANI, B., RAJU, G., “Investigation of the mechanical properties of tamarind seed particles reinforced epoxy composites”, AIP Conference Proceedings, v. 2057, pp. 020023, 2019. doi: http://doi.org/10.1063/1.5085594.
  » https://doi.org/10.1063/1.5085594
• ^[9] RAFEEQ, S., ABDULMAJEED, I., SAEED, A., “Mechanical and thermal properties of date palm fiber and coconut shell particulate filler reinforced epoxy composite”, Indian Journal of Applied Research, v. 3, n. 4, pp. 89–92, 2011. doi: http://doi.org/10.15373/2249555X/APR2013/153.
  » https://doi.org/10.15373/2249555X/APR2013/153
• ^[10] ALARIFI, I.M., “Investigation into the morphological and mechanical properties of date palm fiber-reinforced epoxy structural composites”, Journal of Vinyl and Additive Technology., v. 27, n. 1, pp. 77–88, 2021. doi: http://doi.org/10.1002/vnl.21785.
  » https://doi.org/10.1002/vnl.21785
• ^[11] BOURMAUD, A., DHAKAL, H., HABRANT, A., et al, “Exploring the potential of waste leaf sheath date palm fibres for composite reinforcement through a structural and mechanical analysis”, Composites. Part A, Applied Science and Manufacturing, v. 103, pp. 292–303, 2017. doi: http://doi.org/10.1016/j.compositesa.2017.10.017.
  » https://doi.org/10.1016/j.compositesa.2017.10.017
• ^[12] MISHRA, A., SINGH, P., “Mechanical analysis of waste leaf sheath date palm fibres for composite reinforcement”, Journal of Futuristic Sciences and Applications, v. 5, n. 2, pp. 45–53, 2022. doi: http://doi.org/10.51976/jfsa.522207.
  » https://doi.org/10.51976/jfsa.522207
• ^[13] MATHU KUMAR, S., REJIKUMAR, R., ANAND, M.J., et al, “Investigation of mechanical and physical properties of date palm stem fibre reinforced epoxy composites”, Materiali in Tehnologije, v. 58, n. 2, pp. 203–208, 2024. doi: http://doi.org/10.17222/mit.2023.1033.
  » https://doi.org/10.17222/mit.2023.1033
• ^[14] NWOGU, C.N., NWAIWU, U., UDO, V.U., et al, “Effect of date seed granules on the mechanical properties of glass fibre reinforced epoxy composite”, Cleaner Materials, v. 6, pp. 100160, 2022. doi: http://doi.org/10.1016/j.clema.2022.100160.
  » https://doi.org/10.1016/j.clema.2022.100160
• ^[15] ABDULRAZAQ, A., AHMED, S., MAHDI, F., “Agricultural waste and natural dolomite for green production of aluminum composites”, Cleaner Engineering and Technology, v. 11, pp. 100565, 2022. doi: http://doi.org/10.1016/j.clet.2022.100565.
  » https://doi.org/10.1016/j.clet.2022.100565
• ^[16] RAGHAVENDRA, S., SIVARAM, N., SADIK, T., et al, “Mechanical properties of hybrid composites with date palm fibre reinforcement”, Applied Medical Informatics, v. 7, n. 3, pp. 78, 2018. doi: http://doi.org/10.11648/j.am.20180703.14.
  » https://doi.org/10.11648/j.am.20180703.14
• ^[17] ALSHAMMARI, B., SABA, N., ALOTAIBI, M., et al, “Evaluation of mechanical, physical, and morphological properties of epoxy composites reinforced with different date palm fillers”, Materials, v. 12, n. 13, pp. 2145, 2019. doi: http://doi.org/10.3390/ma12132145. PubMed PMID: 31277304.
  » https://doi.org/10.3390/ma12132145
• ^[18] BENDADA, A., BOUTCHICHA, D., KHATIR, S., et al, “Mechanical characterization of an epoxy panel reinforced by date palm petiole particle”, Steel and Composite Structures, v. 35, pp. 627–634, 2020. doi: http://doi.org/10.12989/SCS.2020.35.5.627.
  » https://doi.org/10.12989/SCS.2020.35.5.627
• ^[19] SUPIAN, A., JAWAID, M., RASHID, B., et al, “Mechanical and physical performance of date palm/bamboo fibre reinforced epoxy hybrid composites”, Journal of Materials Research and Technology, v. 15, pp. 1330–1341, 2021. doi: http://doi.org/10.1016/j.jmrt.2021.08.115.
  » https://doi.org/10.1016/j.jmrt.2021.08.115
• ^[20] HASSANA, D.J., ALI, N.A., “Evaluation of mechanical properties for epoxy reinforced with palm oil/zinc oxide composites”, Iraqi Journal of Physics., v. 20, n. 2, pp. 26–37, 2022. doi: http://doi.org/10.30723/ijp.v20i2.978.
  » https://doi.org/10.30723/ijp.v20i2.978
• ^[21] PETERS, S.T., Handbook of composites, 2 ed., New York, Springer, pp. XVIII, 2013.
• ^[22] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM D695-15 Standard test method for compressive properties of rigid plastics, West Conshohocken, ASTM, 2015.
• ^[23] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM D6110-18 Standard test method for determining the charpy impact resistance of notched specimens of plastics, West Conshohocken, ASTM, 2018.
• ^[24] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM D2240-15 Standard test method for rubber property - durometer hardness, West Conshohocken, ASTM, 2021.
• ^[25] BARRAGÁN, V.M., MAROTO, J.C., PASTUSCHUK, E., et al, “Testing a simple Lee’s disc method for estimating throuh-plane thermal conductivity of polymeric ion-exchange membranes”, International Journal of Heat and Mass Transfer, v. 184, pp. 122295, 2022. doi: http://doi.org/10.1016/j.ijheatmasstransfer.2021.122295.
  » https://doi.org/10.1016/j.ijheatmasstransfer.2021.122295
• ^[26] DHANABALAN, D., RATHANASAMY, R., VEDIAPPAN, V., et al, “Experimental investigation of date seed and neem powder reinforced natural fiber composites”, Medziagotyra, v. 30, n. 3, pp. 334–339, 2024. doi: http://doi.org/10.5755/j02.ms.34033.
  » https://doi.org/10.5755/j02.ms.34033
• ^[27] ELKHOULY, H., ABDEL-MAGIED, R., ALY, M., “An investigation of date palm seed as effective filler material of glass–epoxy composites using optimization techniques”, Polymers & Polymer Composites, v. 28, n. 8-9, pp. 541–553, 2020. doi: http://doi.org/10.1177/0967391119888313.
  » https://doi.org/10.1177/0967391119888313
• ^[28] NAGARAJ, N., BALASUBRAMANIAM, S., VENKATARAMAN, V., et al, “Effect of cellulosic filler loading on mechanical and thermal properties of date palm seed/vinyl ester composites”, International Journal of Biological Macromolecules, v. 147, pp. 53–66, 2020. doi: http://doi.org/10.1016/j.ijbiomac.2019.11.247. PubMed PMID: 31887386.
  » https://doi.org/10.1016/j.ijbiomac.2019.11.247
• ^[29] GHAZI, I., JADDAN, R., “Thermal conductivity characterization of epoxy based composites reinforced with date palm waste particles”, Journal of Physics: Conference Series, v. 1973, n. 1, pp. 012144, 2021. doi: http://doi.org/10.1088/1742-6596/1973/1/012144.
  » https://doi.org/10.1088/1742-6596/1973/1/012144
• ^[30] INBAKUMAR, P., RAMESH, S., BOPPANA, S., “Influence of mechanical properties in the surface modification of palm fiber/epoxy matrix composite”, Journal of Natural Fibers, v. 19, n. 14, pp. 9791–9802, 2022. doi: http://doi.org/10.1080/15440478.2021.1993409.
  » https://doi.org/10.1080/15440478.2021.1993409
• ^[31] MOHAMED YUSOFF, M.Z., SAPUAN, S., ISMAIL, N., et al, “Mechanical properties of short random oil palm fibre reinforced epoxy composites”, Sains Malaysiana, v. 39, pp. 87–92, 2010.
• ^[32] PANDI, G., RAJA, K., VIJAYAN, V., et al, “Investigation on the mechanical, water absorption, and tribological performance of calotropis gigantea and abaca fiber reinforced epoxy composites”, Journal of Polymer Research, v. 31, n. 10, pp. 308, 2024. doi: http://doi.org/10.1007/s10965-024-04157-3.
  » https://doi.org/10.1007/s10965-024-04157-3
• ^[33] JAYASEELAN, C., PADMANABHAN, P., ATHIJAYAMANI, A., et al, “Comparative investigation of mechanical properties of epoxy composites reinforced with short fibers, macro particles, and micro particles”, BioResources, v. 12, n. 2, pp. 2864–2871, 2017. doi: http://doi.org/10.15376/biores.12.2.2864-2871.
  » https://doi.org/10.15376/biores.12.2.2864-2871
• ^[34] AKARSLAN KODALOĞLU, F., KODALOĞLU, M., “Mechanical properties of natural fibre reinforced sustainable epoxy composites”, Uluslararası Sürdürülebilir Mühendislik ve Teknoloji Dergisi, v. 8, n. 2, pp. 108–118, 2024. doi: http://doi.org/10.62301/usmtd.1506650.
  » https://doi.org/10.62301/usmtd.1506650
• ^[35] BOULEBNANE, A., DJEGHADER, D., TIOUA, T., “Weibull analysis of charpy impact test in short date palm fiber reinforced epoxy composite”, Periodica Polytechnica. Civil Engineering, v. 68, n. 1, pp. 122–130, 2023. doi: http://doi.org/10.3311/PPci.22500.
  » https://doi.org/10.3311/PPci.22500
• ^[36] DEHURY, J., MOHANTY, J., NAYAK, S., et al, “Comprehensive characterization of date palm petiole fiber reinforced epoxy composites: effect of fiber treatment and loading on various properties”, Journal of Natural Fibers, v. 19, n. 14, pp. 9457–9470, 2022. doi: http://doi.org/10.1080/15440478.2021.1982834.
  » https://doi.org/10.1080/15440478.2021.1982834
• ^[37] GHORI, S.W., RAO, G., “Fiber loading of date palm and kenaf reinforced epoxy composites: tensile, impact and morphological properties”, Journal of Renewable Materials, v. 9, n. 7, pp. 1283–1292, 2021. doi: http://doi.org/10.32604/jrm.2021.014987.
  » https://doi.org/10.32604/jrm.2021.014987
• ^[38] GC, J.R., GIRI, H., SUJAKHU, S., et al, “Experimental study of mechanical properties of natural fiber polymer composite”, IOP Conference Series. Materials Science and Engineering, v. 1314, n. 1, pp. 012005, 2024. doi: http://doi.org/10.1088/1757-899X/1314/1/012005.
  » https://doi.org/10.1088/1757-899X/1314/1/012005
• ^[39] ALI, M., “Epoxy-date palm fiber composites: study on manufacturing and properties”, International Journal of Polymer Science, v. 2023, pp. 1–12, 2023. doi: http://doi.org/10.1155/2023/5670293.
  » https://doi.org/10.1155/2023/5670293
• ^[40] MŁYNARCZYK, K., LONGWIC, F., PODKOŚCIELNA, B., et al, “Influence of natural fillers on the thermal and mechanical properties of epoxy resin composites”, Polimery, v. 67, n. 3, pp. 102–109, 2022. doi: http://doi.org/10.14314/polimery.2022.3.2.
  » https://doi.org/10.14314/polimery.2022.3.2
• ^[41] SAŁASIŃSKA, K., BARCZEWSKI, M., GÓRNY, R., et al, “Evaluation of highly filled epoxy composites modified with walnut shell waste filler”, Polymer Bulletin, v. 75, n. 6, pp. 2511–2528, 2018. doi: http://doi.org/10.1007/s00289-017-2163-3.
  » https://doi.org/10.1007/s00289-017-2163-3
• ^[42] SIENKIEWICZ, A., CZUB, P., “Modification of epoxy compositions by the application of various fillers of natural origin”, Materials, v. 16, n. 8, pp. 3149, 2023. doi: http://doi.org/10.3390/ma16083149. PubMed PMID: 37109985.
  » https://doi.org/10.3390/ma16083149
• ^[43] AYYANAR, B., KUMAR, R., HELAILI, S., “Experimental and numerical analysis of natural fillers loaded and e-glass reinforced epoxy sandwich composites”, Journal of Materials Research and Technology, v. 32, pp. 1235–1244, 2024. doi: http://doi.org/10.1016/j.jmrt.2024.07.142.
  » https://doi.org/10.1016/j.jmrt.2024.07.142
• ^[44] DAHAD, H.A., HASAN, S.F., ALWAN, A., “Study the effect of different percentages of natural (orange peels and date seeds) and industrial materials (carbon and silica) on the mechanical and thermal properties of polymeric reinforced composites”, Al-Khwarizmi Engineering Journal, v. 14, n. 4, pp. 16–23, 2018. doi: http://doi.org/10.22153/kej.2018.04.001.
  » https://doi.org/10.22153/kej.2018.04.001
• ^[45] PERERA, H., BANU, H., “Recent developments in composite reinforcement using date palm fibers for improved performance through physical and chemical modifications”, International Journal of Polymer Analysis and Characterization, v. 27, n. 7, pp. 446–463, 2022. doi: http://doi.org/10.1080/1023666X.2022.2110088.
  » https://doi.org/10.1080/1023666X.2022.2110088
• ^[46] ABU‐JDAYIL, B., MOURAD, A., HUSSAIN, A., et al, “Thermal insulation and mechanical characteristics of polyester filled with date seed wastes”, Construction & Building Materials, v. 315, pp. 125805, 2022. doi: http://doi.org/10.1016/j.conbuildmat.2021.125805.
  » https://doi.org/10.1016/j.conbuildmat.2021.125805
• ^[47] MLHEM, A., TEKLEBRHAN, T., BOKURETSION, E., et al, “Development of sustainable thermal insulation based on bio-polyester filled with date pits”, Journal of Bioresources and Bioproducts, v. 9, n. 1, pp. 74–89, 2024. doi: http://doi.org/10.1016/j.jobab.2023.12.004.
  » https://doi.org/10.1016/j.jobab.2023.12.004
• ^[48] ABU‐JDAYIL, B., HITTINI, W., MOURAD, A., “Development of date pit–polystyrene thermoplastic heat insulator material: physical and thermal properties”, International Journal of Polymer Science, v. 2019, pp. 1–10, 2019. doi: http://doi.org/10.1155/2019/1697627.
  » https://doi.org/10.1155/2019/1697627
• ^[49] TSEKMES, I., KOCHETOV, R., MORSHUIS, P., et al, “How different fillers affect the thermal conductivity of epoxy composites”, In: Proceedings of the 2014 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), pp. 647–650, 2014. doi: http://doi.org/10.1109/CEIDP.2014.6995843.
  » https://doi.org/10.1109/CEIDP.2014.6995843
• ^[50] SAHU, Y.K., Study on the effective thermal conductivity of fiber reinforced epoxy composites”, M.Sc. Thesis, National Insititute of Technology, Rourkela, 2014.
• ^[51] XIA, C., GARCIA, A., SHI, S., et al, “Hybrid boron nitride-natural fiber composites for enhanced thermal conductivity”, Scientific Reports, v. 6, n. 1, pp. 34726, 2016. doi: http://doi.org/10.1038/srep34726. PubMed PMID: 27703226.
  » https://doi.org/10.1038/srep34726
• ^[52] KUMAR, R., MISHRA, A., SAHOO, S., et al, “Epoxy-based composite adhesives: effect of hybrid fillers on thermal conductivity, rheology, and lap shear strength”, Polymers for Advanced Technologies, v. 30, n. 6, pp. 1365–1374, 2019. doi: http://doi.org/10.1002/pat.4569.
  » https://doi.org/10.1002/pat.4569
• ^[53] ORHADAHWE, T., AJIDE, O., ADELEKE, A., “Physico-chemical characterization of date palm seed as a reinforcement in aluminium matrix composites”, In: Proceedings of the 2nd International Conference on Multidisciplinary Engineering and Applied Science (ICMEAS), pp. 1–5, 2023. doi: http://doi.org/10.1109/ICMEAS58693.2023.10429872.
  » https://doi.org/10.1109/ICMEAS58693.2023.10429872
• ^[54] SUPRIYA, A., “Studies on pre-treated coir dust reinforced polymer composites”, D.Sc. Thesis, National Insititute of Technology, Rourkela, 2017.
• ^[55] KOCAMAN, S., AHMETLI, G., “Eco-friendly natural filler based epoxy composites”, International Journal of Chemical and Molecular Engineering, v. 10, n. 4, pp. 471–474, 2016.

Publication Dates

• Publication in this collection
  30 Jan 2026
• Date of issue
  2026

History

• Received
  10 May 2025
• Accepted
  01 Aug 2025