Development and characterization of sustainable PKS/CaCO 3 /HDPE hybrid composites for enhanced thermal and mechanical performance

Abstract

This study presents a comprehensive evaluation of hybrid composites made from high-density polyethylene (HDPE), calcium carbonate (CaCO₃), and palm kernel shell (PKS) particles, focusing on their thermal, mechanical, and moisture-resistance properties. PKS, a lignocellulosic agro-waste, was pretreated with NaOH and incorporated at varying loadings (10–25 wt%) and particle sizes (125 µm and 500 µm); CaCO₃ content was adjusted so that the total filler loading remained 30 wt%. Fourier-transform infrared spectroscopy (FTIR) confirmed chemical interactions and surface modification of PKS after treatment. Differential Scanning Calorimetry (DSC) revealed minimal variation in melting temperature across formulations but showed distinct changes in enthalpy of fusion and degree of crystallinity, particularly with increasing PKS content. Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) indicated enhanced thermal stability for CaCO₃-rich composites, while PKS-rich formulations showed earlier onset of thermal degradation due to lignocellulosic decomposition. Water absorption results demonstrated that both filler content and particle size significantly influence hydrophilicity, with smaller particles exhibiting reduced water uptake because of better dispersion and packing. Mechanical testing showed that composites with 10 wt% PKS (especially at 125 µm) exhibited superior ultimate tensile strength and impact energy, balancing reinforcement and toughness. Higher PKS contents led to agglomeration, reduced ductility, and diminished strength. Hardness generally increased with filler content and was more pronounced in composites containing larger PKS particles. The results highlight the potential of PKS/CaCO₃/HDPE hybrids as sustainable, high-performance materials for engineering applications, with optimal performance achieved at 10–15 wt% PKS using fine particles.

Introduction

Calcium carbonate (CaCO₃) is a widely distributed inorganic mineral that constitutes about 4% of the Earth’s crust by composition. ¹ It occurs naturally as chalk, limestone, and marble, and is characterized by chemical stability, non-toxicity, and high absorbency. ² These properties have enabled its extensive use in fields such as food production, pharmaceuticals, and cosmetics. Additionally, CaCO₃ is widely used as a cost-effective filler or bulking agent in commodity polymers such as polyethylene (PE) and polyvinyl chloride (PVC) to reduce material costs and enhance processing efficiency. ³

In recent years, the use of CaCO₃ as a filler in composite materials, particularly polymer matrix composites (PMCs), has attracted considerable interest. Incorporating CaCO₃ into polymer matrices has been shown to enhance mechanical properties such as hardness, impact resistance, and tensile strength.^4,5 Ahmed et al. incorporated CaCO₃ into HDPE at varying loadings and reported notable improvements in mechanical performance. The addition of CaCO₃ increased hardness from 65 HD to 77.33 HD and doubled the Young’s modulus from 440 MPa to 860 MPa. The impact strength of the modified blend also reached 69.9 kJ m⁻², demonstrating enhanced stiffness and toughness relative to neat HDPE. ⁶ Similarly, Hongzhen et al. reported that adding 30% wt of CaCO₃ to HDPE increased tensile strength by 12.86% relative to neat HDPE. Impact strength increased marginally from 7.18 kJ.m² to 7.23 kJ.m². ⁷ In another study, Hua Zhou et al. also reported that adding 30 wt% CaCO₃ to HDPE increased impact strength by 15.94% compared with neat HDPE. Tensile strength also increased by 25.7% in that study. ⁸

Although the use of CaCO₃ as a filler in composite materials, particularly PMCs, has attracted considerable interest, its benefits are not unlimited. Incorporation of CaCO₃ can enhance mechanical properties such as hardness, impact resistance, and tensile strength. However, high filler concentrations can reduce impact strength and ductility. This effect arises because rigid filler particles act as stress concentrators, limiting the polymer’s capacity for plastic deformation and impact energy absorption. ⁹ Excess filler can also weaken the interfacial bonding between filler and matrix, reducing the composite’s cohesion and mechanical integrity. ¹⁰ For example, tensile strength at break may decrease beyond critical filler concentrations as the composite becomes more brittle and less flexible. ⁹

Hybrid polymer composites, in which a polymer matrix is reinforced with two or more fillers, have emerged as a promising approach to overcome the limitations of single-filler systems. While CaCO₃ is widely used due to its cost-effectiveness and ability to enhance properties such as hardness and tensile strength, drawbacks such as brittleness and reduced impact strength at high loadings necessitate alternative strategies.^11,12 Researchers have therefore partially replaced CaCO₃ with other fillers to create hybrid systems that better balance and optimize composite mechanical properties. Recent studies have shown that combining bio-based fillers with inorganic particles can produce synergistic enhancements in the mechanical, thermal, and morphological performance of HDPE composites. For instance, Karagöz et al. reported that HDPE reinforced concurrently with walnut shell and nano-calcium carbonate achieved a 19.4% increase in tensile strength, a 23.7% improvement in hardness, and a 16.8% enhancement in impact resistance, along with a 22°C in the degradation onset temperature relative single-filler systems. ¹³ This approach leverages complementary filler properties to enhance tensile strength, hardness, and impact resistance more effectively than single fillers. Setyanto et al. investigated HDPE composites with various filler combinations and reported a 127% increase in tensile strength and 16.6% increase in impact strength for composites containing 25 wt% CaCO₃ and 1% fumed silica. ¹⁴ Awan et al. compared binary systems containing CaCO₃ with ternary systems that incorporated CaCO₃ and nano clay as additional reinforcement. The ternary composite exhibited superior mechanical properties, with a 27.7% increase in tensile strength and a 15% increase in hardness relative to the binary system. ¹⁵

Biofillers derived from renewable agricultural by-products not only mitigate the environmental drawbacks of synthetic alternatives but also offer properties that improve the mechanical performance of hybrid polymer composites, including impact resistance, tensile strength, and hardness.^16,17 Incorporating biofillers into polymer matrices provides a sustainable, cost-effective route to valorizing biowastes. However, using untreated biofillers in polymer composites often presents challenges that can compromise mechanical, thermal, and moisture-resistance properties.^18,19 These challenges largely stem from the intrinsic hydrophilicity of lignocellulosic biofillers, which contrast with the hydrophobic nature of most thermoplastic and thermosetting polymer matrices. ²⁰ This polarity mismatch often results in poor interfacial adhesion, producing stress concentrators and early crack initiation under mechanical loading. ²¹

To overcome the limitations caused by poor interfacial adhesion in bio-composites, various surface modification techniques have been employed to improve the compatibility and enhance overall mechanical performance. These techniques are generally categorized as physical, chemical, or physicochemical methods, with chemical treatments being the most adopted due to their effectiveness in reducing hydroxyl group concentration and improving interfacial bonding. ²² Recent studies indicate that chemical surface modifications can significantly enhance composite performance. Alkali-treated fillers typically show 20–30% increases in tensile strength and up to 60% reductions in water absorption, while silane-treated systems often achieve 20–25% higher tensile and flexural strengths, depending on filler type and matrix compatibility.^23,24 Also, alkali treatment effectively removes surface impurities, waxes, and lignin, thereby increasing surface roughness and facilitating stronger mechanical interlocking with the matrix. ²⁵ Other effective methods include maleic-anhydride-grafted compatibilizers (e.g., MAPP and MAPE) and plasma treatment, which improve filler dispersion and interfacial bonding particularly in polyolefin matrice. ²⁶ Additionally, silane coupling agents such as 3 aminopropyltriethoxysilane are often used to form covalent bonds between the treated filler surface and polymer chains, leading to improved structural cohesion. ²⁷ However, quantitative comparisons among these approaches remain limited due to differences in filler size, treatment concentration, and processing conditions. Future work should therefore aim to standardize treatment protocols, evaluate long-term durability under humid or thermal cycling, and explore greener alternatives such as enzymatic or low-energy plasma treatments for sustainable interfacial enhancement.

This study investigates the physico-mechanical performance of a hybrid composite formulated by incorporating surface-treated palm kernel shell into a calcium carbonate-reinforced high-density polyethylene matrix. Palm kernel shell, an abundant agricultural by-product in Ghana, serves as a sustainable filler alternative capable of reducing the environmental impact associated with calcium carbonate mining and processing.^28,29 The study specifically examines the effects of surface treatment and particle size variation on the mechanical, thermal, and physical properties of the developed composites. Characterization was performed using simultaneous thermal analysis techniques, including thermogravimetric analysis and Differential Scanning Calorimetry, while morphological features were examined using optical microscopy and scanning electron microscopy. Mechanical and physical properties were evaluated through hardness, impact, quasi static tensile, and water absorption tests. The findings aim to support the development of cost-effective and environmentally sustainable polymer composites suitable for diverse engineering applications.

Materials and methods

Preparation and surface treatment of palm kernel shell (PKS) particulates

Palm kernel shells (PKS) were sourced from Juaben Oil Mills Ltd, located in Ejisu, Kumasi, Ghana. The raw shells were thoroughly washed with clean water to remove residual impurities and then sun-dried for 48 h. After drying, the shells were milled using a hammer mill (Christy Turner Ltd, Ipswich, UK) and then sieved to obtain two particle sizes: 125 µm and 500 µm. To improve interfacial compatibility with the polymer matrix, the PKS particles were subjected to alkaline surface treatment. Specifically, the sieved particles were immersed in a 5% sodium hydroxide (NaOH, ACS reagent grade, ≥97% purity, Sigma-Aldrich, USA) solution at 60°C and 350 r/min for 2 h to remove lignin, hemicellulose, and other surface impurities. After alkaline treatment, the PKS particles were neutralized by stirring in a 7 vol% acetic acid (ACS Grade ≥99% purity, Merck KGaA, Darmstadt, Germany) for 5 min. The particles were then rinsed thoroughly with distilled water (laboratory grade, pH ˜ 7, Fine Chemicals Ghana Ltd, Accra, Ghana) until a neutral pH was achieved, and subsequently oven-dried (Model DHG-9023A, Yihder Technology Co., Ltd, Taiwan) at 70°C for 24 h to obtain the final surface-treated PKS filler. A schematic overview of the fabrication process of the PKS/CaCO ₃ /HDPE hybrid composite is shown in Figure 1.

Figure 1.

Flowchart outlining the preparation and fabrication process of PKS/CaCO₃/HDPE hybrid composites.

Fabrication of hybrid composites

Hybrid PKS/CaCO₃/HDPE composites were fabricated based on the formulations presented in Table 1. The composite blends were melt-compounded using a Noztek Pro single-screw extruder (Noztek Ltd, Shoreham-by-Sea, West Sussex, UK) operated at 200°C and an extrusion rate of 10 g min⁻¹. The extruder was equipped with a 15 mm diameter screw having a compression ratio of 3:1 and length-to-diameter (L/D) ratio of 18:1. The screw was rotated at 60 r/min, providing a residence time of about 2–3 min, which ensured effective melting and uniform dispersion of the fillers within the HDPE matrix. Each blend was extruded twice to ensure uniform mixing and dispersion of the fillers within the HDPE matrix. The extrudate was pelletized and subsequently hot-pressed using a hydraulic press (Model XLB-DQ, Qingdao Ouli Machine Co., Ltd, Qingdao, China) at 145°C and a pressure of 5 tons. The heating cycle was maintained for 10 min, followed by a 5-min cold-pressing stage under the same pressure to relieve internal stresses and ensure dimensional stability. Standard dog-bone-shaped specimens for tensile testing were produced with dimensions of 56 mm × 10 mm and a gauge length (L₀) of 32 mm, in accordance with relevant testing standards. Impact testing specimens (75 mm × 10 mm × 10 mm) were produced using a custom-built injection molding machine operated at 200°C. The molten composite was injected into the mold cavity under a pressure of approximately 60 bar and held for 25 s before ejection to ensure proper filling and consolidation of the specimens.

Table 1.

Composition of hybrid composites by weight percentage.

Sample	HDPE (% wt.)	CaCO3 (% wt.)	PKS (% wt.)
70HDPE/30CaCO3/0PKS	70.00	30.00	0.00
70HDPE/20CaCO3/10PKS	70.00	20.00	10.00
70HDPE/15CaCO3/15PKS	70.00	15.00	15.00
70HDPE/5CaCO3/25PKS	70.00	5.00	25.00

Chemical characterization

Fourier-transform infrared spectroscopy (FTIR) was used to analyze the chemical structure and functional groups present in the PKS particulates. Spectra were recorded on a PerkinElmer ATR-FTIR spectrometer (Model Frontier™, PerkinElmer Inc., Waltham, MA, USA) over the wavenumber range of 500–3500 cm^-1. This analysis identified characteristic absorption bands corresponding to specific functional groups, providing insight into the chemical modifications induced by the surface treatment.

Morphological characterization

Particle size analysis was conducted using an optical microscope (Model M158 C, AmScope, United Scope LLC, Irvine, CA, USA) in conjunction with ImageJ software (version 1.8.0). The acquired data were plotted using OriginLab software (version 9.9.0.225) to visualize the distribution of particle sizes. Scanning electron microscopy (SEM) (Hitachi TM3030Plus Tabletop SEM, Hitachi High-Technologies Corporation, Tokyo, Japan) was used to examine the microstructural features of the fractured surfaces of the hybrid PKS/CaCO₃/HDPE composites. Additional imaging was performed using a JEOL JSM-IT300 scanning electron microscope. Prior to imaging, the fracture surfaces of the impact-tested specimens were sputter-coated with a 20 nm layer of gold to enhance conductivity, minimize surface charging, and improve image resolution. SEM imaging was carried out at an accelerating voltage of 10 kV, with images captured at a resolution of 1024 × 784 pixels, a spot size of 2.0, and a working distance of 12–14 mm. This characterization provided insights into filler dispersion, interfacial bonding, and fracture morphology within the composites.

Thermal characterization

The thermal stability and decomposition behavior of the composite samples were investigated using a simultaneous thermal analyzer (Model STA 449 F3 Jupiter®, NETZSCH-Gerätebau GmbH, Selb, Germany), which integrates thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC). Approximately 20 mg of each composite specimen was placed in an alumina crucible and subjected to a programmed heating cycle from 40°C to 800°C at a constant heating rate of 10°C min^-1 under a nitrogen atmosphere flowing at 50 mL min^-1 to inhibit oxidative degradation. TGA was used to monitor the mass loss profiles of the samples, enabling the identification of thermal degradation stages and assessing filler-matrix interactions. DSC simultaneously recorded heat flow changes to determine key thermal transitions, including melting temperature (T_m) and crystallization onset temperature (T₀), thereby providing information on the thermal behavior and crystallinity of the composite materials.

Water absorption behavior

Water absorption testing was conducted in accordance with ASTM D570-22. Specimens were initially weighed using an electronic balance (Model SF-400C, Camry Electronic Co., Ltd, Guangdong, China) with a precision of ±0.01 g to determine their dry mass (W₀). The samples were then immersed in distilled water maintained at ambient temperature (27 ± 2°C). At 24-h intervals, the specimens were removed, gently wiped with tissue paper to remove surface moisture, and reweighed to obtain their wet mass (W₁). The percentage of water absorbed by the composites was calculated using the following equation:

$Water\hspace{0.17em}absorption\hspace{0.17em}(\%)=\frac{w_1-w_0}{w_0}\times 100$

where w ₀ is the initial dry weight (g) and w ₁ is the final wet weight after immersion (g).

Mechanical characterization

The mechanical properties of the hybrid composites were evaluated using hardness, impact, and tensile testing. Hardness measurements were performed using a Shore D electronic durometer (Model HH-338, Landtek Instruments Co., Ltd, Dongguan, China). For each specimen, five readings were taken at different surface locations, and the average value was recorded as the Shore D hardness of the material. Impact strength was evaluated in accordance with the ASTM D256 using a Pendulum Impact Tester (Model HSM55, Tinius Olsen Ltd, Redhill, Surrey, UK) with a maximum energy capacity of 300 J. Unnotched specimens were subjected to controlled pendulum impacts to determine their resistance to sudden shock loading. Tensile properties were determined using a microcomputer-controlled electrohydraulic servo universal testing machine (Model SI-50N, Shenzhen SANS Testing Machine Co., Ltd, Shenzhen, China) in accordance with ASTM D638-10. Tests were conducted on dog-bone-shaped specimens with dimensions of 6 mm in width and 1 mm in thickness. The machine was calibrated prior to testing, and the strain rate along with other test parameters were maintained in accordance with standard requirements.

Results and discussion

Chemical characterization of PKS particulates

The Fourier-transform infrared (FTIR) spectrum of surface-treated palm kernel shell (PKS) particles is presented in Figure 2 which exhibits multiple absorption bands characteristic of a complex lignocellulosic structure. These bands correspond to functional groups associated with the major constituents of natural fibers: cellulose, hemicellulose, and lignin. A broad absorption band centered at 3333 cm^-1 is attributed to O-H stretching vibrations. This band arises from hydroxyl groups in the hydrogen-bonded network of cellulose and hemicellulose, and from phenolic structures of lignin. ³⁰ The intensity of this band reflects the hydrophilic nature of PKS, which can affect its water uptake and interfacial adhesion in polymer composites.

Figure 2.

FTIR spectrum of surface-treated palm kernel shell (PKS) particles showing characteristic absorption bands corresponding to major lignocellulosic components.

The absorption peak at 2918 cm⁻¹ corresponds to C-H stretching vibrations of aliphatic -CH₂ groups. These groups occur in both cellulose and lignin backbones, indicating long-chain hydrocarbons that contribute to the structural rigidity of the material. ³¹ This feature contributes to the structural framework that supports mechanical reinforcement in polymer composites. A distinct peak at 1731 cm⁻¹ corresponds to C=O stretching vibrations from ester linkages in hemicellulose or the carbonyl groups in oxidized lignin. The presence of this band suggests partial oxidation or degradation of hemicellulose during processing, which could influence the filler’s thermal stability and compatibility with the polymer matrix. The 1595 cm⁻¹ band is attributed to aromatic C=C stretching vibrations, characteristic of the lignin’s aromatic rings. ³² This band confirms the retention of lignin’s aromatic framework, contributing to the chemical stability of PKS and influencing its thermal decomposition and interfacial behavior in composites. A band at 1454 cm⁻¹ arises from C-H bending vibrations. This confirms the presence of methylene groups within the aliphatic chains of the lignocellulosic structure.

The absorption at 1235 cm⁻¹ corresponds to C-O stretching vibrations associated with ester or ether linkages in cellulose and hemicellulose.^32,33 This groups participate in hydrogen bonding and polar interactions that enhance filler-matrix adhesion, particularly in polar polymer systems. A prominent absorption band at 1030 cm⁻¹ corresponds to C-O-C stretching vibrations from glycosidic linkages in cellulose. ³¹ This confirms the integrity of the polysaccharide backbone and highlights the role of cellulose as a primary load-bearing component. Collectively, these spectral features confirm the chemically diverse lignocellulosic nature of PKS. Understanding these functional groups is essential for predicting the interfacial interactions between PKS and the HDPE/CaCO₃ matrix.

Morphological characterization of PKS particulates

The optical micrographs and corresponding particle size distribution plots of the surface-treated PKS particles are shown in Figure 3. Figures 3(a) and (c) display the morphological features of PKS particles with nominal sizes of 125 µm and 500 µm, respectively, whereas Figures 3(b) and (d) illustrate their corresponding particle size distribution histograms. As shown in Figure 3(a), the 125 µm PKS particles exhibit irregular yet relatively fine geometries, typical of crushed lignocellulosic materials. The particles appear well-dispersed, with minimal agglomeration, indicating effective grinding and sieving during preparation. The size distribution histogram in Figure 3(b) exhibits a normal distribution centered around 125 µm, confirming the uniformity of the sieving process. The relatively narrow distribution indicates a uniform filler size, which is ideal for achieving consistent dispersion and enhanced interfacial contact within the polymer matrix. In contrast, Figure 3(c) depicts the morphology of coarser PKS particles with an average size of 500 µm. These particles exhibit a broader and more angular morphology, indicative of reduced surface-area-to-volume ratio relative to the finer fraction. As shown in Figure 3(d), the particle size distribution is again approximately normal, centered around 500 µm, but with a slightly wider spread than that of the 125 µm fraction. This broader distribution may reduce filler dispersion uniformity and promote to localized stress concentrations in the final composite.

Figure 3.

Optical micrographs and particle size distribution of surface-treated PKS particles. (a) Micrograph of 125 µm particles, (b) particle size distribution for 125 µm fraction, (c) micrograph of 500 µm particles, and (d) particle size distribution for 500 µm fraction.

The irregular, non-spherical morphology observed in both size ranges is characteristic of natural fillers and is known to influence composite behavior through mechanical interlocking and surface interaction effects. The particle size analysis confirms successful processing of PKS into two controlled size fractions, both suitable for incorporation into polymer composites. The choice of particle size can be tailored depending on the desired property enhancement, as finer particles typically promote better interfacial bonding and homogeneity, whereas larger particles may enhance stress transfer and dimensional stability.

Morphological analysis of the PKS-based composites

The scanning electron micrographs of the fractured surfaces of the PKS/CaCO₃/HDPE composites are shown in Figure 4, captured at magnifications of 50× and 100×. These images provide insights into the filler-matrix interaction, dispersion quality, and fracture behavior of the hybrid composites. In Figure 4(a), the composite microstructure reveals embedded PKS particles uniformly distributed within the HDPE matrix. Several filler pull-outs and micropores are visible, indicating regions of weak interfacial bonding between the PKS filler and the surrounding polymer matrix. The observed pull-out regions suggest that, under mechanical loading, stress transfer from the matrix to the filler was inadequate, resulting to debonding and void formation. This phenomenon is common in natural filler-reinforced composites, where polar biofillers interact poorly with non-polar polymer matrices such as HDPE unless they are adequately surface-modified. ³⁴

Figure 4.

Scanning electron micrographs of the fractured surfaces of PKS/CaCO₃/HDPE composites. (a) 50× magnification showing PKS particle distribution, filler pull-outs, and micropores. (b) 100× magnification highlighting interfacial debonding between PKS and the HDPE matrix.

In Figure 4(b), at higher magnification, microscopic interfacial gaps between PKS particles and the HDPE matrix become more apparent. Although the NaOH treatment followed by acetic acid neutralization was designed to improve adhesion by reducing hydroxyl-related incompatibilities, the persistence of these gaps indicates that the interfacial bonding remains suboptimal. This limitation may be attributed to incomplete removal of lignin or hemicellulose, or to insufficient mechanical interlocking arising from the irregular geometry of the filler particles. The observed micropores may also have originated from trapped air during melt compounding or from the release of volatiles during thermal processing. Such defects can act as stress concentrators, promoting premature failure under mechanical loading and a reducing the overall toughness of the composite. These observations are consistent with cavity-driven fracture mechanisms commonly reported in polymer systems. ³⁵

Nonetheless, the PKS fillers appear to be relatively homogeneously distributed, with no evidence of large-scale agglomeration. This suggests that the twin melt compounding process was effective in achieving uniform dispersion throughout the matrix. The rough fracture surface and presence of filler breakage points also suggest a ductile failure mechanism, where both matrix yielding and filler-induced crack initiation contribute to the fracture process. ³⁶ Overall, the SEM analysis confirms that although surface treatment enhances certain aspects of filler-matrix compatibility, further optimization such as incorporating coupling agents or compatibilizers may be required to eliminate voids and improve stress transfer efficiency within the hybrid composite.

Thermal behavior of the PKS-based composites

The thermal behavior of the HDPE-based hybrid composites was analyzed using Differential Scanning Calorimetry (DSC), and the corresponding thermograms are shown in Figure 5. The key thermal parameters, including melting temperature, enthalpy of fusion, and degree of crystallinity, are summarized in Table 2. All composite formulations exhibited distinct endothermic peaks between 130°C and 140°C, corresponding to the melting transition of HDPE. The sharpness and intensity of these peaks reflect crystalline characteristics of the polymer matrix and illustrate the influence of filler type and loading on crystallization kinetics.

Figure 5.

DSC thermograms of HDPE-based hybrid composites with varying PKS and CaCO₃ contents. All samples exhibit melting transitions between 130 and 140°C.

Table 2.

Thermal parameters of PKS/CaCO₃/HDPE hybrid composites.

Composite	Melting temperature (°C)	Enthalpy of fusion (J/g)	Degree of crystallinity (%)
70HDPE/30CaCO3/0PKS	136.13	168.08	81.95
70HDPE/20CaCO3/10PKS	134.71	139.82	68.17
70HDPE/15CaCO3/15PKS	135.96	152.75	74.48
70HDPE/5CaCO3/25PKS	135.99	158.98	77.51

The melting temperature (T _m ) values across the composite formulations remained relatively stable, ranging narrowly from 134.71°C to 136.13°C. The control sample (70HDPE/30CaCO₃/0PKS) exhibited the highest T _m at 136.13°C, whereas the composite containing 10 wt% PKS (70HDPE/20CaCO₃/10PKS) showed the lowest value of 134.71°C. This limited fluctuation in T _m indicates that substituting CaCO₃ with PKS does not significantly affect the overall thermal stability of the HDPE matrix. However, the slight reduction in T _m upon PKS inclusion may indicate minor disturbance in the polymer’s lamellar arrangement, whereas the subsequent recovery in T _m at higher PKS loadings suggests improved filler-matrix compatibility and possible crystallite reinforcement. ³⁷

More insightful trends are observed in the enthalpy of fusion (ΔH _f ) and degree of crystallinity (X _c ), which were calculated using the following relation:

$Dc=\frac{{\mathrm{\Delta }H}_f}{{\mathrm{\Delta }H}_o\times W_p}\times 100\%$

where ΔH _f is the measured enthalpy of fusion of the composite (J/g), ΔH _o denotes the enthalpy of 100% crystalline HDPE (293 J/g), ³⁸ and W _p is the weight fraction of HDPE in the composite. The control sample exhibited the highest ΔHf value of 168.08 J/g, corresponding to a crystallinity (X _c ) of 81.95%, consistent with literature that identifies CaCO₃ as an effective heterogeneous nucleating agent in polyolefin matrices.^39–41 CaCO₃ particles act nucleation sites that promote the growth of polymer crystallites, thereby enhancing the overall degree of crystallinity. Interestingly, incorporating 10 wt% PKS initially disrupted this crystallization process, as reflected by a reduced ΔH _f of 139.82 J/g and a corresponding X _c of 68.17%. This initial decline can be attributed to the amorphous, irregular morphology of PKS, along with its lignocellulosic composition which introduces structural heterogeneity and hinder polymer chain alignment during the early stages of filler substitution.

However, with increasing PKS content to 15 wt% and 25 wt%, a gradual recovery and subsequent enhancement in crystallinity were observed. The highest ΔH _f and X _c values among the hybrid composites, 158.98 J/g and 77.51%, respectively, were recorded for the 70HDPE/5CaCO₃/25PKS sample. This improvement may stem from a synergistic interaction between the residual nucleating action of CaCO₃ and the flexible surface morphology of PKS, which, at higher concentrations could facilitate lamellar crystal growth by acting as semi-structured scaffolds. This observation aligns with previous studies on natural fiber-polymer systems, where the filler’s chemical composition and physical surface characteristics play a crucial role in governing nucleation behavior and thermal performance. Collectively, the DSC results indicates that although replacing CaCO₃ with PKS initially reduces crystalline order, higher PKS loadings can partially restore and even enhance the crystallinity of the composite. These findings underscore the potential of PKS as a sustainable, semi-nucleating biofiller that provides both environmental and thermal performance benefits in hybrid HDPE composites. ⁴²

Thermal stability of the PKS-based composites

The thermal degradation behavior of the HDPE/CaCO₃/PKS hybrid composites was analyzed under an inert nitrogen atmosphere using thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG), as shown in Figure 6(a) and (b), respectively. These thermal profiles provide detailed insights into composites’ thermal stability, degradation mechanisms, and the influence of filler composition. The TGA curves (Figure 6(a)) indicate that all composite formulations maintain initial thermal stability up to approximately 250°C, with minimal weight loss, confirming the absence of volatile components or early-stage decomposition. The primary weight loss stage, observed between 400 and 500°C, corresponds to the thermal decomposition of the HDPE matrix, consistent with literature values reported for polyethylene-based composites. ⁴³

Figure 6.

TGA and DTG curves of HDPE/CaCO₃/PKS hybrid composites under nitrogen atmosphere. (a) TGA profiles illustrating multi-step thermal degradation. (b) DTG curves showing two principal decomposition stages attributed to PKS and HDPE. Insets show variations in peak intensity and degradation onset with filler composition.

Notably, the degradation process displays a multi-step profile, especially in composites containing higher amounts of PKS. This behavior is typical of lignocellulosic-based systems, which decompose in distinct phases corresponding to the sequential degradation of hemicellulose, cellulose, and lignin. The control sample (70HDPE/30CaCO₃/0PKS) exhibited the highest thermal stability, characterized by a delayed onset of degradation and a broader decomposition window, highlighting the stabilizing role of CaCO₃. In contrast, increasing PKS content led to an earlier onset of degradation and more pronounced mass loss, with the 25 wt% PKS composites showing the lowest thermal resistance. This trend is attributed to the intrinsic thermal sensitivity of PKS, particularly arising from its hemicellulose and cellulose content, which degrade at lower temperatures than the polymer matrix.

The DTG curves (Figure 6(b)) further clarify the degradation behavior, revealing two main degradation peaks; the first degradation peak between 250°C and 350°C, and the second between 400°C and 500°C. This peak corresponds to the decomposition of the lignocellulosic PKS filler, primarily hemicellulose and cellulose. ⁴⁴ The prominence and depth of this peak increase with PKS loading, as indicated by the sharper troughs in the 15 wt% and 25 wt% PKS formulations. This observation confirms the increasing contribution of PKS decomposition to the overall mass-loss behavior. The second peak represents the main degradation stage, corresponding to the thermal breakdown of the HDPE matrix. Interestingly, composites with higher PKS content exhibit the highest peak degradation rates in this region, indicating that biofillers may promote chain scission within the matrix due to localized thermal instability. ⁴⁵

The thermal stabilization effect of CaCO₃ is evident in both the TGA and DTG results. Composites with higher CaCO₃ contents (i.e., 0 wt% PKS and 10 wt% PKS samples) exhibit delayed degradation onset, smoother decomposition curves, and less steep DTG peaks, indicating a more gradual and controlled degradation process. This behavior suggests that CaCO₃ acts as an effective thermal barrier, likely through the formation of a protective char layer and the reduction of thermal conductivity within the composite.^46,47 Such stabilization mechanisms retard the degradation process, enhance thermal resistance, and extend the composite’s service temperature range. These findings align with those of Stambouli et al., ⁴⁸ who reported that CaCO₃ enhances thermal stability by promoting residual char formation and impeding the inward diffusion of heat and volatiles. Consequently, CaCO₃-rich composites are better suitable for applications requiring enhanced thermal durability.

Water absorption behavior of the PKS-based composites

The water absorption behavior of the HDPE/CaCO₃/PKS hybrid composites is presented in Figure 7, illustrating the effects of PKS content and particle size on moisture uptake. As shown, increasing the PKS content from 10 wt% to 25 wt% results in a gradual water absorption across both particle size variants (125 µm and 500 µm). This trend aligns with the inherently hydrophilic nature of PKS, a lignocellulosic biomass primarily composed of cellulose, hemicellulose, and lignin, which contain multiple polar functional groups such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (-C=O).^21,49 These functional groups readily form hydrogen bonds with water molecules, thereby increasing the composite’s affinity for moisture.

Figure 7.

Water absorption of HDPE/CaCO₃/PKS hybrid composites after immersion in distilled water at room temperature. The graph shows the influence of both PKS content (10 wt%–25 wt%) and particle size (125 µm and 500 µm) on moisture uptake.

Furthermore, increasing the filler loading creates additional filler-matrix interfaces that can serve as pathways for moisture diffusion. When interfacial adhesion is suboptimal, as is often the case with natural fillers, these interfaces may develop microvoids or interfacial gaps that significantly increase water uptake. ⁵⁰ Additionally, filler agglomeration at higher loadings can form capillary channels and discontinuities in the polymer network, further facilitating water penetration into the composite matrix. ⁵¹ The presence of such moisture-induced voids and interfacial gaps also weakens stress transfer between the filler and matrix, contributing to the reduction in tensile strength observed at higher PKS contents.^52,53 A key observation from the results is the pronounced influence of particle size on water absorption. Composites containing larger PKS particles (500 µm) consistently exhibit higher water uptake than their finer particle (125 µm) counterparts, at identical filler contents. This behavior is attributed to poorer dispersion and weaker interfacial bonding typically associated with larger particles, which increase void volume and porosity within the polymer matrix.^48,54 These discontinuities act as conduits for capillary water transport, exacerbating moisture diffusion throughout the material. Such interfacial discontinuities not only facilitate water penetration but also act as stress concentrators under mechanical loading, thereby accounting for the lower tensile and impact performance observed in composites reinforced with 500 µm PKS particles. ⁵⁴

Conversely, the finer 125 µm PKS particles exhibit better compatibility with the HDPE matrix, resulting in improved dispersion and filler packing density. This improves filler-matrix interfacial contact, reduces the number of microvoids, and minimizes the available free volume, thereby decreasing water absorption.^50,55 The larger surface area of the smaller particles facilitates more intimate bonding with the matrix, creating a more tortuous path for water molecules and reducing their diffusion rate. This enhanced interfacial integrity promotes more efficient mechanical load transfer, which corresponds to the higher tensile strength and hardness observed in the 125 µm composites. ⁵⁶

These findings align with the moisture diffusion theory for natural fiber- and particle-reinforced composites, which states that finer fillers, due to their high surface-to-volume ratio, promote more uniform distribution and denser microstructures that more effectively resist water penetration. ⁵⁰ Thus, both particle size and filler content are critical parameters in optimizing the moisture resistance of biofiller-reinforced polymer composites. The observed correlation between reduced water uptake and improved mechanical performance suggests that enhanced filler dispersion and interfacial adhesion act synergistically to limit moisture diffusion pathways while reinforcing the composite’s structural integrity. ⁵⁷ In future work, we intend to incorporate diffusion/saturation curve analysis to complement equilibrium water absorption results, enabling the modeling of moisture transport kinetics and extraction of diffusion coefficients for a more comprehensive understanding of water uptake mechanisms in the composites.

Hardness and impact strength of the PKS-based composites

Figure 8(a) and (b) present the hardness and impact energy characteristics of HDPE/CaCO₃/PKS hybrid composites, with comparisons drawn between two PKS particle sizes (125 µm and 500 µm) across different filler loadings. The impact energy profiles reveal a non-linear dependence on filler composition for both particle sizes. For composites incorporating 500 µm PKS particles, the maximum impact energy of 20.16 J was achieved at 15 wt% PKS (15CaCO₃/15PKS). In contrast, the composite reinforced with 125 µm particles exhibited a lower peak impact energy of 14.35 J at 10 wt% PKS (20CaCO₃/10PKS). This enhanced performance observed for larger particle sizes at moderate loadings is attributed to their enhanced ability to dissipate impact energy via crack deflection and microcrack formation, mechanisms that mitigate crack propagation during dynamic loading. ⁵⁸ However, a decline in impact energy was observed at 25 wt% PKS for both particle sizes variants. This reduction can be attributed to the increased rigidity caused by higher PKS loading, which restricts polymer chain mobility and diminishes the composite’s capacity for plastic deformation under impact, thereby increasing brittleness. ⁵⁹ This effect is more pronounced in the 125 µm particles, where the smaller size and higher surface area foster stronger interfacial bonding but limit energy dissipation due to fewer crack deflection pathways.

Figure 8.

Hardness and impact energy of HDPE/CaCO₃/PKS hybrid composites for (a) 125 µm and (b) 500 µm particle sizes, across different filler compositions.

The hardness results reveal a steady increase in Shore D values with increasing PKS content, an effect that is more pronounced in composites reinforced with 500 µm particles. The maximum hardness value of 66.85 HD was observed for the 25 wt% PKS (5CaCO₃/25PKS) composite with 500 µm particles, whereas the lowest value of 60.17 HD was recorded at 10 wt% PKS. This trend is attributed to the inherently abrasive and rigid nature of PKS particles, which strengthen the polymer matrix and improve resistance to surface indentation and plastic deformation.^60,61 For composites incorporating 125 µm PKS particles, the increase in hardness is less pronounced yet remains clearly noticeable. The smaller particles disperse more uniformly within the matrix, promoting the formation of consistent and well distributed filler-matrix interfaces. This uniform dispersion reinforces the polymer at the microstructural level, impeding localized deformation during indentation and thus contributing to the overall increase in hardness. ⁴⁴ Furthermore, the 3.6% higher hardness observed in the 500 µm composites at equivalent PKS content suggest that larger particles, despite exhibiting poorer dispersion, function as localized stiff inclusions that effectively resist indentation forces. ⁶²

The mechanical performance of the composites demonstrates a characteristic trade-off between hardness and impact toughness. Although higher PKS loadings enhance surface hardness through reinforcement by rigid particulates, they concurrently create stress concentration zones that reduce impact resistance by constraining matrix ductility. ⁶³ This trade-off underscores the need for an optimized filler loading strategy that balances mechanical reinforcement with preservation of toughness, particularly for applications requiring both durability and impact resistance. These findings are consistent with the observations of Arrakhiz et al., ⁶² who reported comparable hardness-toughness trade-offs in agro-waste reinforced composites. The results further affirm that the mechanical performance of composite is governed by the synergistic interplay among filler morphology, particle size, and loading content, each critically influencing the polymer matrix and overall behavior.

Tensile properties of the PKS-based composites

Figure 9(a)–(d) illustrate the tensile performance of the HDPE/CaCO₃ composites reinforced with NaOH-treated palm kernel shell (PKS) particles of 125 µm and 500 µm, evaluated across various CaCO₃:PKS ratios. The analysis focus is on the ultimate tensile strength (UTS), as a critical parameter that reflects material’s mechanical resistance to uniaxial deformation before fracture. The stress-strain curves (Figure 9(a) and (c)) and corresponding UTS bar plots (Figure 9(b) and (d)) reveal a consistent trend: the inclusion of PKS initial enhancement in tensile strength upon PKS incorporation reaching a maximum at 10 wt% PKS for both particle sizes. Specifically, the composite containing 125 µm particles exhibited the highest tensile strength of approximately 13.5 MPa, whereas the 500 µm variant reached about 9.8 MPa at the same filler content. The observed increase in tensile strength at lower PKS loadings can be attributed to the synergistic reinforcement effect of the rigid PKS particles and CaCO₃, which collectively improve stress transfer efficiency within the composite. Moreover, effective interfacial adhesion at this optimal filler concentration facilitates efficient mechanical load transfer from the polymer matrix to the reinforcing phases, thereby maximizing tensile performance.^44,64

Figure 9.

Tensile properties of HDPE/CaCO₃/PKS hybrid composites. (a) Stress-strain curves for composites with 125 PKS particles. (b) Ultimate tensile strength for 125 µm PKS composites. (c) Stress-strain curves for composites with 500 µm PKS particles. (d) Corresponding ultimate tensile strength values. (e) Tensile modulus and elongation at break for 125 µm PKS composites. (f) Tensile modulus and elongation at break for 500 µm PKS composites.

However, increasing PKS content to 15 wt% and 25 wt% results in a reduction in tensile strength. This decline is more pronounced for the finer 125 µm composites. The performance drop is largely attributed to particle agglomeration and dispersion inefficiencies at higher filler loadings. Such agglomerates act as stress concentrators, disrupting uniform stress distribution and promoting microcrack initiation and propagation under tensile loading. ²¹ This observation supports prior studies on lignocellulosic-filler-reinforced thermoplastics, which report deterioration in mechanical performance beyond optimal filler thresholds.^65,66 Comparing the two particle sizes, the 125 µm composites consistently outperformed their 500 µm counterparts in tensile strength across all filler compositions. This enhanced performance is primarily due to the higher surface-area-to-volume ratio of the finer particles, which promotes improved dispersion within the HDPE matrix and stronger interfacial adhesion. ⁶⁷ Consequently load-transfer efficiency between the matrix and filler particles is improved during tensile deformation. In contrast, larger 500 µm particles are more susceptible to incomplete matrix wetting and the formation of micro voids, which compromise structural integrity and reduce tensile strength.

It is also worth noting that, despite the reduction in tensile strength beyond 10 wt% PKS, the hybridization approach; combining organic PKS and inorganic CaCO₃ fillers, still outperforms CaCO₃-only composites at equivalent total filler loading. This suggests that the dual-filler system, when optimized, can provide a balanced mechanical profile. The results indicate that the optimal filler composition for tensile strength is 20 wt% CaCO₃/10 wt% PKS, particularly when using 125 µm PKS particles. Beyond this threshold, tensile strength is compromised by agglomeration, poor filler dispersion, and weakened interfacial bonding.

In addition, Figure 9(e) and 9(f) present the variation in tensile modulus and elongation at break for the HDPE/CaCO₃/PKS composites. A clear trade-off between stiffness and ductility is observed. For both particle sizes, the tensile modulus initially increases with hybridization, reaching a maximum at the 15:15 CaCO₃: PKS composition, after which it declines with further PKS addition. This enhancement at intermediate filler loading reflects the synergistic reinforcement effect between the rigid CaCO₃ and lignocellulosic PKS, which collectively restrict polymer chain mobility and improve stiffness. ⁶⁸ Conversely, elongation at break decreases progressively with increasing PKS content, indicating reduced matrix flexibility due to restricted chain mobility and potential microstructural discontinuities at higher filler loadings. ⁶⁹ Notably, the 125 µm PKS composites exhibit higher tensile modulus values compared to those containing 500 µm PKS particles, confirming that finer particles promote stronger interfacial bonding and more efficient stress transfer. ⁵⁴ However, the larger PKS particles retain slightly higher elongation at break at comparable filler ratios, likely due to lower filler-matrix interfacial area and reduced constraint on chain deformation.

Conclusion

This study successfully developed and evaluated HDPE-based hybrid composites reinforced with both inorganic (CaCO₃) and organic (NaOH-treated PKS) fillers. This study revealed that the thermal, mechanical, and moisture-resistance properties of the composites were significantly affected by both PKS particle size and filler loading ratio. FTIR analysis confirmed that alkaline treatment effectively modified the PKS surface, enhancing its interfacial compatibility with the HDPE matrix. DSC analysis showed that the melting temperature remained relatively stable across formulations, whereas the degree of crystallinity was highest in CaCO₃-rich composites and increased again with higher PKS content due to synergistic nucleation effects. TGA and DTG further revealed that thermal stability decreased with increasing PKS content but improved with CaCO₃ addition, attributed to its char-forming capability and barrier effect.

Based on these findings, mechanical testing showed that 10 wt% PKS with 125 µm particle size yielded optimal tensile strength, whereas the 500 µm particles enhanced impact energy and hardness through size-induced toughening and surface resistance effects. However, excessive PKS loading (25 wt%) caused particle agglomeration, resulting in reduced tensile performance. Water absorption increased with both PKS content and particle size, highlighting the hydrophilic nature of PKS and the need for particle refinement in moisture-sensitive applications. Overall, the optimal formulation balanced filler size and composition, with the 70HDPE/20CaCO₃/10PKS (125 µm) composite exhibiting the most favorable combination of strength, crystallinity, thermal stability, and water resistance. Together, these results demonstrate that the hybrid PKS/CaCO₃/HDPE formulations developed, with optimal performance observed at approximately 10 to 15 wt% PKS, offer a practical route to convert abundant agro-waste into value-added thermoplastic composites suitable for rigid packaging, lightweight construction boards, non-structural building elements, and interior or low-load automotive parts. These findings highlight PKS as a low-cost, sustainable filler for polymer composites and underscore its potential in lightweight construction, packaging, and environmentally friendly products.