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Abstract

This study presents a sustainable strategy to enhance the multifunctional properties of high-density polyethylene (HDPE) by reinforcing it with sugarcane bagasse fibers (BFs) at varying weight ratios. The developed bio-composites aim to address critical limitations of conventional HDPE, particularly in flame retardancy, environmental persistence, and moisture sensitivity, there by expanding its applicability in eco-conscious packaging, agriculture, and construction sectors. The composites were thoroughly characterized using Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) to examine interfacial bonding and fiber dispersion, respectively. Mechanical testing revealed a marked enhancement in tensile strength (from 90 kg/cm² in neat HDPE to 234 kg/cm² in the HDPE10/90BFs composite), alongside a rise in Shore D hardness (from 65 to 90), with a concurrent decrease in elongation at break, indicating a trade-off between rigidity and ductility. Thermal analysis via TGA demonstrated a significant increase in char residue and thermal stability at elevated temperatures. Fire performance was substantially improved, with the limiting oxygen index (LOI) rising from 17 to 32.5. Furthermore, toxic gas emissions during combustion were mitigated, as evidenced by reduced CO (from 0.019 to 0.011 ppm) and CO₂ (from 2.082 to 0.645ppm) levels. Crucially, the composites exhibited enhanced environmental responsiveness, with water uptake decreasing and biodegradability increasing proportionally with BFs content reaching up to 46% weight loss after 14 days compared to only 10.18% in neat HDPE. These findings demonstrate that BFs-reinforced HDPE composites not only retain structural integrity but also provide significant environmental and safety advantages.

Graphical Abstract

Keywords

Biodegradation sustainability bio-fiber composites flame resistance

Highlights

⁃ Tensile strength rose from 90 to 234 kg/cm² with added bagasse fibers.

⁃ LOI increased from 17 to 32.5, indicating improved flame resistance.

⁃ Char residue rose to 29.94%, enhancing thermal stability.

⁃ CO and CO₂ emissions dropped significantly with fiber content.

⁃ Biodegradability reached 46% in 14 days with fiber addition.

Introduction

Polymer-based composites are an important class of materials used in all fields, such as medicine, automotive, construction, home, textiles, and aviation.¹ The majority of polymers used in composites are petroleum-based, nonrenewable, and nonbiodegradable, polluting the environment and impacting all forms of life on Earth. Growing demand for polymeric composite materials in various sectors is one of the reasons for uncontrolled manufacturing and extensive use of petroleum-based polymers and synthetic fibers, resulting in faster depletion of valuable nonrenewable resources, increased health issues and pollution.^2,3 Rapid growth in the manufacturing industries has prompted material improvements in terms of density, stiffness, strength, and cost-effectiveness while increasing sustainability. Composite materials have been produced as one of the materials with such advancements in properties that serve their promise in a variety of applications.⁴ Composite materials have two or more constituents, one of which is in the matrix phase (biopolymer) and the other in particle or fiber form. Composites have been identified as the most promising material available in the twenty-first century.⁵ Composites reinforced with synthetic or natural fibers are becoming increasingly popular as the market demands lightweight materials with high strength for specialized applications.⁶ The matrix, which largely holds the reinforcement together, is also referred to as resin, especially in the case of polymers.⁷ Biodegradable polymers, whether generated from nature or synthesized, can be degraded by biosphere enzymes in the presence of the right pH and temperature. Biopolymers are plant, animal, and microbial-derived biodegradable polymers.⁸ They are readily available renewable resources that are commonly used to make environmentally acceptable bioplastics.⁹ According to the opposing viewpoint, the most straightforward method for classifying biodegradable polymers is based on their natural and synthetic origins.¹⁰ Natural biodegradable polymers are derived from polysaccharides, proteins, and microorganisms, whereas synthetic biopolymers are those created through microbial fermentation or biotechnological manufacturing.¹¹ Biopolymers can be classified into three types based on their heat response: elastomers, thermosets, and thermoplastics. Biopolymers are used in various applications based on their cost, availability, moisture absorption, thermal stability, mechanical behavior, degradation stability, and biocompatibility.¹² PLA and PHAs are the two most often utilized biopolymers for manufacture and application.¹³ The primary advantage of biodegradable polymers over non-biodegradable polymers is their decomposition by microbes, which returns them to the soil and enriches it.¹⁴ This stabilizes the environment and reduces rubbish volume. The breakdown capability of biodegradable polymers is determined by a variety of parameters, including polymer type, chemical content and environmental circumstances.¹⁵ Despite their widespread use, biopolymers have a few drawbacks, including their hydrophilic nature, limited mechanical strength, and slower breakdown rate in damp settings.¹⁶ Biodegradable polymers can be classified according to their origin, production process, chemical content, and application. They can be broadly categorized according to their origin as natural polymers produced from renewable resources such as plants, animals, and microbes; and biodegradable polymers synthesized from petrochemical products.^17,18 According to Siracusa et al., biodegradable polymers can be divided into three categories: 1. Natural biodegradable polymers derived from natural and renewable resources, such as polysaccharides (starch, cellulose), lipids (oils), and proteins (silk, wool).¹⁹ Synthetic biodegradable polymers, such as polylactic acid (PLA) and polycaprolactone (PCL) 3.²⁰ Polymers produced by microbes and genetically modified bacteria, such as poly (hydroxyalkanoates).²¹ The growing demand for sustainable materials has fueled research into biodegradable polymer composites, particularly those reinforced with natural fibers. These bio-fiber composites are regarded as environmentally beneficial alternatives to traditional materials because of their ability to reduce waste and dependency on fossil fuels.²² Biodegradable polymers, such as PLA and polyhydroxyalkanoates, breakdown spontaneously under a variety of environmental circumstances, making them appropriate for a wide range of applications, including packaging and biomedical equipment.²³ Bio-fiber reinforced composites are not only biodegradable, but also have excellent mechanical qualities. Natural fibers such as jute, flax, and hemp have been integrated into polymer matrices, increasing strength while remaining lightweight. This synergy is critical for applications in industries like automotive, construction, and packaging that value both performance and environmental impact.²⁴ However, there are hurdles for bio-composites, particularly in terms of water resistance and long-term durability. Water absorption by natural fibers can cause swelling and weakening of the composite material. To address this, researchers have looked into several surface treatments and fiber-matrix compatibilizers to increase the water resistance and overall performance of bio-fiber composites under environmental stress.²⁵ Overall, the development of bio-fiber reinforced composites that balance biodegradability and increased water resistance is crucial for their wider use in sustainable materials. The purpose of this study is to evaluate these qualities and provide insights that will help to drive the adoption of bio-composites in advanced material applications.

Materials and Methods

Materials

High-density polyethylene (HDPE 5502 - GA), featuring a density of 0.955 g/cm³ and a melt flow index of 0.2 g/10 min, was sourced from Sidi-Kerir Petrochemicals Company (SIDPEC), Egypt. Bagasse fibers (BFs), a fibrous by-product of sugarcane processing and commonly regarded as agricultural waste, were used as the reinforcing material. Skimmed milk was obtained from local farmers in Egypt.

Methods

Preparation of Bagasse Fibers (BFs)

Bagasse fibers were first cut into 5-10 mm lengths and soaked in distilled water for 24 hours to remove surface impurities. After thorough rinsing, the fibers were dried in an oven at 80°C for 24 hours, then ground using a high-speed mixer and passed through an 80-mesh sieve. The processed fibers were stored in sealed plastic bags. Additionally, a portion of the fibers was chopped into smaller pieces, washed with tap water, and left to dry under sunlight for about 3 days. These were then ground, sieved to obtain particle sizes between 60 and 80 µm, dried again at 80°C for 24 hours, and stored under the same conditions Figure 1. The chemical composition of the bagasse fibers includes approximately 45.47% cellulose, 26.18% hemicellulose, 20.90% lignin, 1.64% ash, and 4.55% extractives.

Figure 1.

Preparation stages of sugarcane bagasse after size reduction: (a) raw bagasse fibers collected from agricultural residues, (b) ground bagasse after mechanical grinding, and (c) fine bagasse powder obtained after sieving.

Synthesis of Rennet Casein (RC)

Rennet casein was synthesized from skimmed milk following the method reported by Singh et.al.²⁰ The milk was coagulated at 40°C using calf rennet for 60 minutes. The resulting coagulum was then cooked at 50 °C–55 °C for 45 seconds, followed by the removal of whey. The obtained curd was thoroughly washed with deionized water and dried at 40°C to yield purified rennet casein.

Casein Impregnation into Fibers

The prepared RC was incorporated into fibers by immersing them in a 5 wt% aqueous solution of rennet casein. The mixture was heated in an oven for 1 hour, after which the fibers were removed and dried at 70°C for 15 hours.

Compounding and Sheet Molding Processing

Compounding was made using a two roll mill machines (Betol Machinery Ltd- UK) at 150°C. Firstly, the HDPE was added for 3 min and after starting to melt, treated bagasse fibers were add for 3 min, the weight of bagasse fibers and HDPE in the samples sheets was varied from Sheet 1 to Sheet 6 with compositions {100%HDPE},{HDPE50/50BFs},{HDPE40/60 BFs},{HDPE30/70BFs},{HDPE20/80BFs},{HDPE10/90BFs} respectively. This variation in the bio-fiber reinforced polymer composite sheets composition, each with a different ratio of HDPE and BFs treated with rennet Figure 2. A compression molding machine (HEXA PLAST -INDIA) was used to obtain polymer sheets. The temperature was (160 ± 10°C) and the applied pressure was (20 ton) for (10 min). The dimensions of the sheets were (200*200*3 mm).

Figure 2.

Schematic diagram of HDPE/bagasse fiber (BFs) compounding and sheet molding process.

Characterization

Fourier-transform infrared spectroscopy (FTIR) was conducted to analyze the chemical variations in bagasse fibers, HDPE, and their composites. FTIR spectra were recorded using a Perkin Elmer GX model spectrophotometer over a wavenumber range of 400-4400 cm⁻¹. Samples were prepared in disc form for analysis. Thermogravimetric analysis (TGA) was performed using a Shimadzu 50/50H instrument equipped with thermal analysis software. The measurements were conducted under a nitrogen atmosphere at a flow rate of 20 mL/min, with a heating rate of 10 °C/min from 25°C to 600°C, using approximately 5 mg of each specimen. The dispersion of bagasse fibers within the polymer matrix was examined by Scanning Electron Microscopy (SEM) using a Jeol JSM-5300LV instrument. Prior to imaging, the samples were coated with a thin layer of gold using a Fine Coat JFC-110E sputter coater. The SEM micrographs were obtained at an accelerating voltage of 15 kV, under a vacuum pressure of 10⁻⁵ Torr, with magnifications ranging from 500x to 5000× to observe both fiber distribution and interfacial adhesion. The flammability of untreated and treated samples was evaluated using the Limiting Oxygen Index (LOI) method in accordance with ASTM D2863-77 and UL-94 vertical burning tests. Specimens with dimensions of 120 mm × 6.5 mm × 3 mm were tested. During LOI testing, the emission of toxic gases specifically CO and CO₂ was quantitatively measured using a Testo 300 gas analyzer. The UL-94 vertical burning test was performed to classify the flammability level of the composites. Each specimen, measuring approximately 13 mm in width, 4 mm in thickness, and 50–125 mm in length, was vertically positioned and exposed for 10 s to a 25 mm high Bunsen burner flame. After removing the flame, the after-flame time and afterglow time were recorded, and the procedure was repeated for a second ignition. The occurrence of flaming drips and the total combustion time were also monitored. Based on these parameters, the composites were rated as V-0, V-1, V-2, or HB, following the UL-94 classification standards.

Tensile properties were assessed using a universal testing machine (Instron Model 3382), following ASTM 882-570. The test speed was maintained at 50 mm/min, with a gauge length of 50 mm and a load cell capacity of 10 kN. Dog-bone-shaped specimens with dimensions of 100 mm × 10 mm × 3 mm were used. Three specimens, each with a thickness of 3 mm, were tested per formulation, and the average values were reported. Hardness measurements were performed using a Shore D durometer in accordance with ASTM D-2240. Each sample was measured at five different points, and the mean value was taken to minimize local variation. Water absorption tests were conducted based on ASTM D570-98 (85) to evaluate the influence of bagasse fiber content on moisture uptake. Samples were weighed before and after immersion, and the percentage water absorption was calculated as follows:

Water Absorption % = \frac{Wet weight - Dry weight}{Dry weight} \times 100

(1)

All results represent the average of three replicates. Biodegradability of the composite films was examined through a soil burial test, following the procedure described by Medina-Jaramillo et al.²¹ and Patil et al.¹⁹ Film samples (2 × 2 cm) were weighed and buried in a transparent plastic container filled with soil, maintained at an ambient temperature of 27.5°C and relative humidity of 70.5%. Soil moisture was sustained by spraying water twice daily. At weekly intervals (from Week 1 to Week 2), the samples were retrieved, rinsed to remove soil residues, dried, and reweighed to determine the weight loss, indicating the degree of biodegradation. The soil was composed of 45 % sand, 25 % clay, and 30 % silt, with a pH of 6.8 ± 0.2 to simulate natural burial conditions. Each measurement was performed in triplicate (n = 3), and the data are now reported as mean ± standard deviation.

Results and Dissuasion

Structure Investigation

The IR spectrum of BFs exhibits characteristic peaks at 3335 cm⁻¹, 2870 cm⁻¹, 1610 cm⁻¹, and 1021 cm⁻¹, corresponding to OH stretching (primarily from cellulose), CH stretching, C = C stretching, and CO stretching vibrations, respectively, as shown in Figure 3.²⁶ In contrast, the IR spectrum of the HDPE, HDPE/BFs and HDPE/BFs casein(C) composite reveals distinct peaks at 2919 cm⁻¹ and 2859 cm⁻¹, which are attributed to the asymmetric and symmetric stretching vibrations of CH groups, respectively. Additionally, a peak at 1457 cm⁻¹ is associated with CH₂ bending, while the peaks at 717 cm⁻¹ and 870 cm⁻¹ correspond to CH₂ rocking and phosphorus ester formation, respectively.²⁷ The latter indicates a reaction between phosphoric acid and hydroxyl groups present in the fibers as illustrated in Figure 4.

Figure 3.

FTIR Spectrum of Sugarcane bagasse Fiber (BFs).

Figure 4.

Shows the FTIR spectra of HDPE, HDPE/BFs and HDPE/BFs casein(C) composite.

Mechanical Properties

The results demonstrate a significant improvement in tensile strength with increasing BFs content in the HDPE matrix. Tensile strength increased from 90 kg/cm²for pure HDPE (Sheet 1) to 234 kg/cm² for the highest BFs content (Sheet 6), indicating that fiber reinforcement effectively enhances the mechanical strength of the composite Table 1. Similarly, hardness (Shore D) increased steadily with the addition of bio-fibers, from 65 in Sheet 1 to 90 in Sheet 6. This enhancement in hardness reflects improved stiffness, which is desirable in structural and load-bearing applications.²⁸ Conversely, a gradual decrease in elongation at break was observed, dropping from 8.7% in the pure HDPE sample to 3.5% in the sample with the highest fiber content. This decline suggests a reduction in ductility and flexibility due to the rigid nature of the bio-fibers, which restrict the polymer chain mobility and render the composite stiffer but less deformable. Overall, the incorporation of bio-fibers into HDPE matrices improves mechanical strength and rigidity at the expense of flexibility, a trade-off that can be advantageous depending on the intended application.²⁹

Table 1.

Effect of bio-fiber content on the mechanical properties of HDPE/BFs composites.

Sample	Sheet composition (HDPE:BFs)	Tensile strength (MPa)	Hardness (shore D)	Elongation (%)
Sheet 1	100:0	8.83 ± 0.25	65 ± 1.2	8.7 ± 0.3
Sheet 2	50:50	20.1 ± 0.3	84 ± 1.5	4.2 ± 0.2
Sheet 3	40:60	20.6 ± 0.4	86 ± 1.1	3.8 ± 0.2
Sheet 4	30:70	21.3 ± 0.4	86 ± 1.3	3.6 ± 0.3
Sheet 5	20:80	22.1 ± 0.2	88 ± 1.0	3.6 ± 0.1
Sheet 6	10:90	22.9 ± 0.3	90 ± 1.4	3.5 ± 0.1

Thermal Properties

Thermogravimetric analysis (TGA) curves of the HDPE/BFs composite sheets (Figure 5) clearly show three major degradation stages. The initial degradation temperature (T₁) decreases progressively from 254.9°C for pure HDPE (Sheet 1) to 110°C for the composite with the highest BF loading (Sheet 6), reflecting the earlier decomposition of bio-fibers due to their organic composition and lower thermal stability.³⁰ The maximum degradation temperature (Tmax) obtained peaks shifts from 464.3°C for Sheet 1 to 438.6°C for Sheet 6, indicating that the addition of fibers slightly reduces the peak degradation temperature but enhances the char-forming tendency. The residual mass at 590°C (Residue %) significantly increases with BF content, rising from 6.73% in Sheet 1 to 29.94% in Sheet 6, confirming that bio-fibers promote the formation of thermally stable carbonaceous and inorganic residues.³¹ These findings demonstrate that as the fiber content increases, the composites show a dual behavior an earlier onset of degradation at low temperatures but improved char yield and stability at high temperatures. This balance between degradability and stability makes the HDPE/BFs composites promising for eco-friendly applications requiring moderate thermal resistance, such as packaging, automotive, and building materials (Table 2).

Figure 5.

TGA curves showing the thermal stability behavior of the HDPE/BFs composite sheet.

Table 2.

TGA stages and weight loss.

Sample	Initial degradation (°C)	Tmax (°C)	Residue (%) at 590°C
Sheet 1	254.9	464.3	6.73
Sheet 2	210	457.2	12.71
Sheet 3	192	453.8	14.45
Sheet 4	165	447.9	24.27
Sheet 5	130	442.5	26.56
Sheet 6	110	438.6	29.94

(Tg) remains nearly constant across all composite sheets, fluctuating slightly around 127°C, indicating that varying the BFs content does not significantly impact the thermal mobility of the polymer chains in this temperature range. The residual mass after thermal degradation also increases with higher BFs content, rising from 6.73% in Sheet 1 to 29.94% in Sheet 6. This indicates that BFs enhance char formation, contributing to improved thermal stability at elevated temperatures.³² The residual char may serve as a protective layer, limiting further decomposition and supporting the structural integrity of the composite under heat stress.

Flammability Measurements and Toxic Gas Emissions

The limiting oxygen index (LOI), a critical measure of flame retardancy, increases markedly with rising BFs content in the HDPE/BFs composites. LOI values improve from 17% for pure HDPE to 32.5% for Sheet 6, which contains 90% BFs as show in Table 3. This notable enhancement indicates that the incorporation of lignocellulosic bio-fibers, along with the applied resin treatment, significantly boosts the composite’s resistance to ignition in oxygen-rich environments. The underlying mechanism may be attributed to the char-forming ability of the bio-fibers, which form an insulating barrier that slows down heat transfer and volatile gas release during combustion.³³ The Time to Ignition (TTI) exhibits a substantial increase from 25 seconds in Sheet 1 to 217 seconds in Sheet 6. This delay in ignition is likely due to the higher thermal mass and reduced thermal conductivity introduced by the bio-fiber matrix. The natural fibers act as heat insulators and inhibit the diffusion of flammable volatiles, thus extending the induction period before ignition. This improvement in TTI reflects a critical safety enhancement in applications where fire delay is essential.

Table 3.

Limiting Oxygen Index (LOI), UL-94 vertical burning test, Ignition Time, and Gas Emissions of HDPE/BFs composites with varying Bio-fiber content.

Sample	Sheet composition (HDPE:BFs)	LOI (%)	UL-94 rating	Ignition time (s)	CO (ppm)	CO₂ (ppm)
Sheet 1	100:0	17 ± 0.50	NR^b	25	0.019	2.082
Sheet 2	50:50	27.5 ± 0.49	V-1	194	0.017	1.473
Sheet 3	40:60	29.5 ± 0.50	V-1	199	0.016	1.328
Sheet 4	30:70	29.5 ± 0.50	V-0	210	0.014	1.279
Sheet 5	20:80	31 ± 0.50	V-0	215	0.013	1.25
Sheet 6	10:90	32.5 ± 0.49	V-0	217	0.011	0.645

Note. NR^b, not rated.

Furthermore, the integration of BFs has a pronounced effect on toxic gas emissions. Carbon monoxide (CO) emissions drop significantly as the BF content increases from 0.0189 ppm in Sheet 1 to 0.0115 ppm in Sheet 6. The presence of bio-fibers may facilitate more complete combustion or promote char layer formation, both of which reduce the evolution of CO, a highly toxic gas. Similarly, carbon dioxide (CO₂) emissions decrease from 2.082 ppm in Sheet 1 to 0.645 ppm in Sheet 6, suggesting lower total combustion and volatile release due to the presence of more thermally stable fibrous content. These reductions in CO and CO₂emissions contribute not only to enhanced fire safety but also to environmental benefits, particularly during material disposal via incineration. Collectively, these results demonstrate that increasing bio-fiber content in HDPE composites significantly improves flame retardancy, delays ignition, and reduces harmful gas emissions through synergistic effects of thermal insulation, char formation, and reduced flammable gas generation during combustion.³⁴

Water Absorption

The results demonstrate a progressive decline in water absorption as the BFs content increases within the HDPE/BFs composites. The control sample (Sheet 1), made entirely of HDPE, and recorded the highest water uptake at 0.0194%, while the lowest absorption (0.0014%) was observed in Sheet 6, containing 90% BFs as show Figure 6. This trend indicates that integrating BFs enhances the composite’s resistance to moisture. Although natural fibers are inherently hydrophilic, their interaction with the HDPE matrix likely limits the accessibility of hydroxyl groups, thereby reducing water affinity.³⁵ Moreover, higher fiber loading contributes to a denser and more homogeneous structure, which minimizes voids and impedes moisture penetration.³⁶ Consequently, these composites, particularly those with elevated BF content, exhibit improved water resistance, making them promising candidates for use in humid environments where dimensional stability is critical.

Figure 6.

Water absorption behavior of HDPE/BFs composites at different fiber ratios.

Biodegradability of HDPE/BFs Composites

The biodegradation behavior of HDPE/BFs composite sheets was evaluated by monitoring weight loss over 7 and 14 days of exposure to a microbial environment. The results demonstrate a clear and consistent increase in weight loss with higher BF content, indicating that the addition of natural fibers enhances the degradability of the composites. Sheet 1, composed entirely of HDPE, exhibited minimal degradation, with a weight loss of only 7.30% after 7 days and 10.18% after 14 days as show in Figure 7 and Table 4. This limited biodegradation is consistent with the hydrophobic, non-polar nature of HDPE, which resists microbial colonization and enzymatic attack due to its chemical inertness and crystalline structure. In contrast, Sheet 6, containing 90% BF, showed the highest degradation, losing 34.00% of its weight after 7 days and 46.00% after 14 days. This substantial increase in biodegradability is attributed to the presence of lignocellulosic bagasse fibers, which are rich in cellulose and hemicellulose components readily degraded by soil microorganisms.³⁷ The organic and porous structure of these fibers provides a favorable environment for microbial activity, facilitating the breakdown of the composite matrix. These findings confirm that increasing the BFs ratio in HDPE composites significantly enhances their susceptibility to biological degradation. This property is particularly advantageous in applications requiring temporary material performance or environmental compatibility, such as biodegradable packaging, agricultural films, or disposable construction materials.^38–40

Figure 7.

Weight loss percentage of the sheets over time during the biodegradation test.

Table 4.

Biodegradability assessment of HDPE/BFs composite sheets over time.

Samples	Initial weight	Loss % after 7 days	Loss% after 14 days
Sheet 1	1.2	7.30 ± 0.25	10.18 ± 0.35
Sheet 2	1.3	33.08 ± 0.45	43.08 ± 0.60
Sheet 3	1.2	38.33 ± 0.50	48.33 ± 0.55
Sheet 4	1.2	34.17 ± 0.40	43.33 ± 0.50
Sheet 5	1.1	35.45 ± 0.35	42.73 ± 0.45
Sheet 6	1	34.00 ± 0.30	46.00 ± 0.50

Surface Morphology

The fiber matrix interfacial region plays a critical role in defining the mechanical strength, thermal stability, and water resistance of polymer composites. Effective interfacial bonding facilitates stress transfer from the matrix to the reinforcement, thereby enhancing overall composite durability. In the SEM micrograph of pure HDPE (Sheet 1), the surface is smooth, compact, and homogeneous characteristics typical of a hydrophobic thermoplastic with low porosity and poor biodegradability. Conversely, Sheet 6, which incorporates the highest content of treated BFs, displays a heterogeneous morphology with clearly embedded fibrous textures. This indicates successful fiber dispersion and good interfacial interaction between the modified fibers and the HDPE matrix, without distinct phase separation. A closer look at the treated BFs shows a roughened surface with adsorbed particles identified as casein-based flame retardants. These particles are either coated on the fiber surface or intercalated among fiber bundles, enhancing adhesion through mechanical interlocking. The surface treatment introduces functional groups and micro-roughness that improve compatibility with the hydrophobic matrix and reduce water ingress pathways.^41,42 The improved water resistance observed in the treated composites can be attributed to two key factors. First, the flame retardant enhances fiber dispersion and interfacial integrity by promoting a more uniform distribution of the bagasse fibers within the HDPE matrix. This uniformity minimizes the formation of interfacial voids, which typically serve as pathways for moisture penetration. Second, chemical modification of the fibers occurs through esterification reactions between the hydroxyl groups on the bagasse fibers and phosphoric acid derivatives released from the flame retardant. This reaction reduces the number of hydrophilic –OH groups on the fiber surfaces, thereby significantly lowering the moisture absorption capacity of the composite can be represented as

R ‐ OH + HO ‐ PO ‐ {(OH)}_{2} \to R ‐ O ‐ PO ‐ {(OH)}_{2} + H_{2} O

(2)

These structural and chemical changes diminish the hydrophilic nature of the fibers, improving the composite’s dimensional stability in humid environments. The central SEM image confirms that treated BFs are coated with granular casein, evidencing surface modification and increased reactivity. In Sheet 6, the dense, uniform fiber distribution and indistinct interfacial boundaries imply enhanced compatibility and strong fiber matrix adhesion. This tight integration limits micro-gaps and voids that typically allow water diffusion, likely improving mechanical performance as well. Figure 7 displays SEM images of Sheet 1 (pure HDPE), treated BFs, and Sheet 6 (composite with maximum BF content). In the cross-sectional SEM micrographs (Figure 8(d)–(e)), Sheet 1 exhibits a dense and continuous structure with no visible voids or fiber inclusions, confirming the homogeneous nature of pure HDPE. In contrast, the cross section of Sheet 6 reveals a well-distributed fibrous network embedded within the polymer matrix. The fibers appear tightly bonded to the HDPE, indicating strong interfacial adhesion and effective load transfer. The absence of interfacial gaps or pull-outs further demonstrates the successful integration of treated BFs. The rough, interlocked morphology of the fiber matrix interface suggests mechanical anchoring facilitated by surface modification and the deposition of casein-based flame retardant particles. This compact and coherent cross-sectional architecture supports the observed enhancement in both mechanical strength and water resistance of the composite. As illustrated, treated BFs are well integrated within the HDPE matrix, resulting in a uniform structure with minimal visible phase boundaries. The observed flame retardant particles on the fiber surfaces support the conclusion that surface modification has occurred, as noted in similar findings by Branca and Di Blasi.³³ In terms of material composition and water resistance, Sheet1 composed solely of HDPE exhibits superior hydrophobicity and water resistance but is limited in biodegradability. Sheets 2-6 progressively increase the proportion of BFs (from 50% to 90%), which introduces hydrophilic characteristics and can compromise water resistance. However, in composites like Sheet 4 and Sheet 6, the addition of casein-based flame retardants significantly improves moisture resistance. This enhancement is due both to better fiber dispersion and to the esterification process, which reduces the number of hydroxyl groups on the BFs.⁴³

Figure 8.

SEM images of (a) bagasse fibers (BFs), (b) pure HDPE matrix (Sheet 1), (c) BF/HDPE composite (Sheet 6), (d) cross section of Sheet 1, and (e) cross section of Sheet 6.

Comparative Analysis

Table 5 compares the mechanical, thermal, and biodegradation properties of the HDPE/BFs composites developed in this study with previous reports on natural fiber-reinforced HDPE systems. As shown, the current composites demonstrate superior tensile strength, flame retardancy, and biodegradability. The discussion highlights the reasons for these improvements, including effective fiber dispersion, strong interfacial adhesion, and enhanced char formation during combustion.

Table 5.

Comparison of HDPE/BFs composites with reported natural fiber reinforced HDPE systems.

Polymer&Fiber type	Fiber content (wt%)	Tensile strength (MPa)	LOI value	Weight loss % biodegradability (14 days)	References
HDPE + Rice husk fiber	15	18.6	27.0	30	Shah AU, et al.⁴⁴
PP + Jute fiber + PP	10	20.2	28.5	32	Chatterjee A et al.⁴⁵
NR + Sisal fiber	10	21.1	25.8	28	Abdel‐Hakim A, et al.⁴⁶
Kenaf fiber + HDPE	12	22.5	29.2	35	Sonnier R, et al.⁴⁷
HDPE + spent coffee grounds	10	16	-	-	Sanyang ML, et al.⁴⁸
Recycled HDPE + rice Husk/Wood flour	30.4	22.1	21.06	-	Sun J. et al.⁴⁹
PP + Hemp fiber	8	19.3	26.7	25	Rahman MA et al.⁵⁰
HDPE + Bagasse fiber	10	23.4	32.5	46	Current study

Conclusion

The incorporation of sugarcane bagasse fibers (BFs) into HDPE significantly enhanced the biodegradability, mechanical, and thermal properties of the composites compared to pure HDPE. After 14 days, weight loss increased from 10.18% for pure HDPE to 42.73%–48.33% for BF-reinforced samples, confirming the positive effect of natural fibers on microbial degradation. The composites also showed up to 27.5% higher tensile strength, 20.2% higher hardness, improved thermal stability, and reduced CO/CO₂ emissions during combustion. These results demonstrate the potential of HDPE/BF composites as sustainable, eco-friendly materials for industrial applications. Future work should explore optimized fiber treatments and hybrid reinforcement systems.

Future Work

Although the incorporation of sugarcane bagasse fibers (BFs) into HDPE showed significant improvements in biodegradability and mechanical performance, the study was limited to short-term biodegradation (14 days) under controlled laboratory conditions, which may not fully represent real environmental scenarios. Additionally, only one type of natural fiber and fixed fiber contents were investigated, without extensive optimization of surface treatments or fiber dispersion. Future studies should include cone calorimetry testing to comprehensively evaluate the flame-retardant behavior of the composites, as this analysis could not be conducted in the current work due to equipment maintenance. Long-term and field investigations are also recommended to further assess the degradation behavior, durability, and large-scale applicability of these sustainable composites.

Footnotes

Acknowledgements

The authors would like to express their gratitude to Department of Materials Science, institute of graduate studies and research, Alexandria University, Egypt.

Declaration of conflicting interests

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

Funding

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

ORCID iD

Ashraf Morsy

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Bio-fiber reinforced high-density polyethylene composites - A sustainable approach to improve flame retardancy,biodegradation,and water resistance

Abstract

Keywords

Highlights

Introduction

Materials and Methods

Materials

Methods

Preparation of Bagasse Fibers (BFs)

Synthesis of Rennet Casein (RC)

Casein Impregnation into Fibers

Compounding and Sheet Molding Processing

Characterization

Results and Dissuasion

Structure Investigation

Mechanical Properties

Thermal Properties

Flammability Measurements and Toxic Gas Emissions

Water Absorption

Biodegradability of HDPE/BFs Composites

Surface Morphology

Comparative Analysis

Conclusion

Future Work

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References