The influence of nano-biochar on the mechanical and flame resistance of recycled PLA composites

The growing accumulation of plastic and agricultural waste highlights the urgent need for sustainable material alternatives. This study investigates the incorporation of nano-biochar derived from cashew nut shell biomass to enhance the mechanical and thermal performance of recycled polylactic acid (rPLA). Nano-biochar produced via controlled pyrolysis and high-energy ball milling was incorporated into rPLA at 0–2 wt% loadings through melt compounding and injection moulding. The resulting composites were evaluated for tensile, flexural, impact, and interlaminar shear strength (ILSS), alongside UL-94 flammability testing. A one-way ANOVA followed by Tukey’s HSD post-hoc analysis confirmed statistically significant improvements (p < 0.05) across all mechanical properties. The tensile strength of virgin PLA (32.23 MPa) decreased to 25.92 MPa in recycled PLA due to polymer chain scission; however, the addition of 1.5 wt% nano-biochar increased tensile strength to 49.54 MPa and ILSS from 21.37 MPa to 36.31 MPa. Flexural and impact strengths also rose by 34.19% and 45.85%, respectively, compared to unfilled rPLA. In UL-94 testing, the rPLA1.5 composite achieved a V-0 rating with no dripping, indicating excellent flame retardancy. Overall, nano-biochar reinforcement not only restored but substantially enhanced the mechanical integrity and fire resistance of rPLA, with ANOVA validating the statistical robustness of these improvements. This work demonstrates a viable circular-economy pathway for converting biomass waste into functional nano-reinforcements for sustainable polymer composites. These composites are particularly suitable for automotive interiors, building materials, and consumer goods where improved flame resistance and mechanical durability are required.

The rapid rise in plastic consumption and agricultural waste generation has sparked serious environmental concerns. According to the United Nations Environment Programme, more than 430 million tonnes of plastic are produced each year, with only 9% recycled, 12% incinerated, and the remainder ending up in landfills or the natural environment (Fast Facts, 2025). In parallel, global farming activities generate approximately 140 billion tonnes of agricultural biomass waste each year, with only 40% and 4.5% biomass reuse respectively (Seah et al., 2023). The majority of which is either left unused or openly burned, emitting greenhouse gases into the atmosphere. Combining these two waste streams, recycled plastic and agricultural residues, to create value-added composite materials is a promising and sustainable solution. While most previous research has focused on virgin PLA or other unprocessed polymers, recycled PLA (rPLA) has received little attention, despite being widely available and more environmentally friendly. One of the primary disadvantages of rPLA is its low molecular weight, reduced thermal stability, and increased brittleness, which is frequently caused by heat and moisture damage from previous processing (Hasan et al., 2024; Yousaf et al., 2024; Cisneros-López et al., 2020). The addition of nano-biochar is expected to improve the performance of rPLA through strengthening the polymer physically, provide thermal protection, and form chemical bonds with the weakened PLA chains, these actions may help to slow further degradation, increase decomposition temperatures, and improve load transfer between particles and the polymer, however the optimized biochar content is important. However, excessive addition of nano-biochar may deteriorate properties, as high nanoparticle concentrations can lead to agglomeration (Njuguna et al., 2008; Devaprakasam et al., 2009; Ansari and Berglund, 2016). Hence this study is planned to investigate the effect of nano-biochar on the structural and functional properties of rPLA-based composites and to find the optimized filler content of rPLA-based composites.

Polylactic acid (PLA) has gained significant attention as a sustainable biopolymer for applications in packaging, biomedical implants, textiles, and 3D printing, owing to its biodegradability, mechanical strength, and processability (Bajpai et al., 2014). However, its large-scale utilization is often restricted by high production cost and limited thermal resistance. Recycling PLA offers a practical and eco-friendly strategy to lower material costs and minimize environmental impact, aligning with the principles of a circular economy (Salah et al., 2023). Nevertheless, rPLA typically suffers from chain scission, loss of molecular weight, and reduced mechanical integrity after repeated processing, which limits its reuse in engineering applications (Zenkiewicz et al., 2009). Reinforcing rPLA with carbonaceous fillers such as biochar can restore and even enhance its performance. Among various biomass residues, cashew nutshell waste was selected as the biochar precursor because it is abundantly available from agro-industrial processing, rich in lignin and fixed carbon, and has a naturally porous structure conducive to high-quality carbon yield. Moreover, utilizing this waste prevents open burning and contributes to sustainable resource valorization, transforming an underused agricultural by-product into a high-value nano-reinforcement for polymer composites. This approach not only promotes waste-to-wealth conversion but also supports global efforts toward carbon sequestration and material sustainability.

Biochar, a carbon-rich product from biomass pyrolysis, is well known for its high porosity, thermal stability, and environmental friendliness. Converting biochar to the nanoscale (typically <100 nm) significantly enhances its specific surface area (often >300 m 2 /g), aspect ratio, and availability of surface functional groups (–OH, –COOH), which promotes better dispersion and interfacial bonding in polymer matrices. Several studies have shown that the addition of biochar as a filler in polymer composites can lead to significant improvements in mechanical properties (Sundarakannan et al., 2024; Das et al., 2018; Das et al., 2016a; Das et al., 2016b). The extent of enhancement depends on factors such as biochar content, particle size, dispersion, and interfacial bonding with the matrix. Hemlata (Hemlata and Mechanical, 2025) reported that recycled polypropylene (rPP) reinforced with pine needle–derived biochar exhibited optimal mechanical performance at 40 wt% biochar, with increases of 97.30% in tensile modulus, 141.92% in flexural modulus, 54.60% in flexural strength, and 28.85% in compressive strength. These improvements were attributed to the uniform dispersion of biochar and enhanced interfacial adhesion between the filler and the matrix (Botta et al., 2024). The study demonstrated that biochar not only acts as a reinforcing agent but also contributes to the overall toughness and surface durability of the composite. Das et al. (2017) reported that pine wood biochar can improve the fire performance of polypropylene (PP) by lowering the Peak Heat Release Rate (PHRR) and Total Heat Release (THR), while increasing the Limiting Oxygen Index (LOI). This improvement was attributed to the formation of a compact and continuous char layer that acts as a thermal barrier during combustion. However, the studies related to the nano biochar in composites are limited. While Arrigo et al. (2020) explored biochar-PLA composites, their work focused on virgin PLA; our study extends this to recycled PLA, which presents distinct degradation challenges. Peterson and Kim (2020) investigated the effect of biochar particles size on the properties of rubber composites. It was found that when the particles were larger than 10 μm, the mechanical strength of the composite became weaker due to stress building up around the larger particles. To solve this problem, the authors co-milled the biochar with 1 wt% nanosilica, which helped to reduce the particle size. This method allowed them to replace 40% of the carbon black filler in a styrene–butadiene rubber composite without reducing its tensile strength. In fact, the modified composites showed more than 31% improvement in elongation and 24% in toughness. These results show that making biochar particles smaller is important for improving their reinforcing ability and suggest that a similar method could be useful for improving the performance of recycled PLA-based composites.

Previous studies indicate that biochar reinforcement in polymer systems can enhance stiffness and functional performance; however, outcomes vary considerably due to differences in filler dispersion, interfacial compatibility, and biochar characteristics. Hemlata and Mechanical (2025) reported optimal performance at 40 wt% biochar, with increases of 97.30% in tensile modulus and 141.92% in flexural modulus, albeit with a notable reduction in ductility. Similarly, Hoang et al. (2024) found that 20 wt% rice husk biochar increased flexural modulus by 11.27% and improved flame retardancy, with the limiting oxygen index (LOI) reaching 26.64%, underscoring the importance of optimized filler type and loading. These findings highlight that while biochar can enhance polymer strength and stiffness, agglomeration and interfacial debonding at higher filler contents often limit overall performance. To address these limitations, the present study employs nano-biochar, which offers a larger specific surface area, higher surface reactivity, and improved dispersion within the polymer matrix, thereby promoting stronger interfacial adhesion and enhanced mechanical integrity and flame-retardant performance of the resulting rPLA composites.

In this study, recycled polylactic acid (rPLA) was reinforced with nano-biochar produced from cashew shell biomass. The biochar was prepared through pyrolysis and reduced to the nanoscale using high-energy ball milling. Composite samples were developed by incorporating 0–2 wt% of nano-biochar into rPLA using melt compounding followed by injection moulding. The composites were tested for their mechanical properties, including tensile, flexural, and impact strength. Fire performance was evaluated through UL-94 vertical burning tests. This work aimed to assess the effectiveness of nano-biochar as a reinforcing filler in recycled PLA and to explore its potential use in applications such as automotive interiors, building materials, and low-cost consumer goods requiring improved fire resistance.

Materials

Polylactic acid (PLA) pellets (A Grade, ACROTEX, Gujarat, India), sized between 3 and 5 mm with a melting range of 100 °C–160 °C, were used as the base polymer. Recycled PLA (rPLA) was obtained through repeated extrusion using a twin-screw extruder for ten cycles to replicate the thermal and shear degradation effects typical of industrial mechanical recycling. The extruder temperature was maintained at 180 °C across all zones to ensure uniform melting. During each cycle, the material was fed into the hopper, melted, and mixed by screws rotating at 100 rpm. The molten PLA was cooled below its glass transition temperature (below 60 °C) using external cooling and pelletised for reuse. After each cycle, the surface hardness of the recycled PLA pellets was tested to observe the effects of repeated processing. To improve compatibility between the PLA matrix and biochar filler, maleic anhydride grafted PLA (MAPP) was added as a coupling agent.

The biochar used in this study was synthesized from cashew nutshell waste collected from local agro-processing industries in Tamil Nadu, India, ensuring uniform and sustainable feedstock sourcing. Prior to pyrolysis, the nutshells were cleaned, dried at 60 °C for 24 h to remove moisture, and cut into small pieces (approximately 10–15 mm). Pyrolysis was conducted in a horizontal tubular furnace reactor under a nitrogen-controlled atmosphere at a constant flow rate of 100 mL min -1 to ensure oxygen-free conditions. About 200 g of pre-treated cashew shells were processed per batch at 600 °C for 2 h with a heating rate of 10 °C min -1. The average biochar yield was 34 ± 5 wt%. After cooling to room temperature under nitrogen flow, the char was sieved through a 100 µm mesh to remove uncarbonized residues. The purified char was then oven-dried at 60 °C for 6 h, followed by high-energy planetary ball milling for 4 h to achieve an average particle size of 779 nm, confirmed by particle size analysis.

Composite fabrication

Before processing, all materials—including rPLA, virgin PLA, and biochar—were dried in a hot air oven at 60 °C for 6 h to remove residual moisture. Biochar-reinforced rPLA composites were prepared with biochar loadings of 0.5, 1.0, 1.5, and 2.0 wt%, calculated relative to the rPLA mass. In the first step, rPLA pellets were mixed with the corresponding biochar loadings of 0.5, 1.0, 1.5, and 2.0 wt%, along with 2 wt% MAPP using a twin-screw extruder. The MAPP content was fixed at 2 wt% of the polymer mass and was not included in the filler weight fraction. The barrel temperature was maintained at 180 °C to ensure complete melting of PLA and uniform distribution of the biochar within the matrix. The extrudate was cooled in a water bath, pelletised into uniform granules, and oven-dried again at 60 °C for 5 h to ensure moisture-free moulding. In the second step, composite testing samples were fabricated directly through injection moulding. The moulding barrel temperature was maintained at 180 °C, with an injection pressure of 70 MPa, while the moulds were preheated to 40 °C to minimize internal stresses and improve dimensional accuracy. Two separate moulds were used—one designed for mechanical testing specimens (3 mm thickness) and another for LOI/flame testing specimens (4 mm thickness). Each composite formulation was injection moulded directly into the respective test specimen shape, ensuring consistent geometry and surface finish. The complete composite fabrication methodology is illustrated in Figure 1 and Table 1 shows the composition details of the composite samples fabricated.

Characterization and testing

Mechanical testing

Tensile testing was carried out using a universal testing machine (Instron 5,982, United States) following the ASTM D638 standard. Dog-bone-shaped specimens with dimensions of 250 × 25 × 3 mm were made using injection moulding. The tests were conducted at room temperature (23 °C ± 2 °C) with a crosshead speed of 2 mm/min to maintain a consistent strain rate. Load and displacement were recorded throughout the test, and tensile strength was calculated by dividing the maximum load by the cross-sectional area of the specimen. Flexural strength was measured using the three-point bending method in line with ASTM D790. Specimens measuring 127 × 12.7 × 3 mm were used with a span-to-thickness ratio of 32:1 for proper load distribution. The tests were performed at the same crosshead speed of 2 mm/min using the same testing machine. Flexural strength was calculated based on the maximum force applied before the sample failed. Impact strength was evaluated using a Charpy impact tester according to ASTM D256. Notched specimens with dimensions of 64 × 12.7 × 3 mm were tested under dynamic impact. The energy absorbed during fracture was recorded and divided by the cross-sectional area at the notch to calculate the impact strength in kJ/m 2. ILSS was measured using a three-point short-beam shear test as per ASTM D2344. The tests were conducted on a universal testing machine with a support span-to-thickness ratio of 5:1 and a crosshead speed of 1 mm/min. All tests were conducted at room temperature. For each composite formulation, three separate specimens were tested independently, and the average value of the three measurements was reported for each property.

Limiting oxygen index (LOI) and flammability

In this study, the Limiting Oxygen Index (LOI) test was conducted to determine the minimum oxygen concentration required to sustain combustion over a 50 mm length or for at least 3 min. A higher LOI value indicates stronger flame resistance. The test followed the ASTM D2863 standard, using specimens of 125 × 10 × 4 mm. Flammability behaviour was further evaluated using the UL-94 Vertical Burning (V) test, following the UL-94 standard procedure. Specimens measuring 127 × 12 × 3 mm were vertically mounted and exposed to flame for two 10-s ignition cycles to assess after-flame time, after-glow time, and dripping behaviour. Based on these criteria, samples were assigned V-0, V-1, or V-2 ratings. After each test, the residual char was visually observed to document general differences in char continuity and surface appearance among the samples. These qualitative observations were used only to support the interpretation of flame-retardant behaviour; no additional chemical or microstructural characterization of the char was performed. Together, the LOI and UL-94 V test results provided key insights into the fire resistance and combustion behaviour of virgin PLA, recycled PLA, and their biochar-reinforced composites.

Statistical analysis

All experimental data for mechanical property evaluations—including tensile, flexural, ILSS, impact, and hardness tests—were obtained from three independent replicates (n = 3) for each composite formulation. Mean and standard deviation (SD) values were calculated to evaluate measurement reproducibility and dispersion. A one-way analysis of variance (ANOVA) was conducted to determine whether statistically significant differences existed among the six material groups: Virgin PLA, rPLA, rPLA0.5, rPLA1, rPLA1.5, and rPLA2. The level of significance was set at α = 0.05. Following the ANOVA, Tukey’s Honestly Significant Difference (HSD) post-hoc test was performed to identify which specific pairs of groups exhibited statistically significant differences. Significance thresholds were assigned as p < 0.05, p < 0.01, and p < 0.001, representing increasing confidence levels.

Biochar characterization

The nano-biochar produced from cashew nut shell was studied using FTIR, XRD, particle size analysis, and SEM to understand its surface chemistry, structure, and morphology. These techniques helped to evaluate the functional groups, carbon structure, particle size, and surface appearance, which are important for its use in composite and environmental applications.

FTIR analysis

During pyrolysis, cashew nutshell biomass undergoes staged thermal decomposition of its hemicellulose (200 °C–350 °C), cellulose (300 °C–400 °C), and lignin (400 °C–600 °C) components, producing volatile compounds, tars, and a carbon-rich solid residue (biochar). These transformations generate characteristic functional groups that appear in the FTIR spectrum. Figure 2a shows the FTIR spectrum of the nano-biochar prepared at 600 °C. The major wavenumbers have been labelled directly in the graph to highlight the corresponding vibrational modes. A broad and intense band at ∼3,450 cm -1 corresponds to O–H stretching vibrations, arising from hydroxyl groups or trace moisture retained on the biochar surface (Dai et al., 2025). A smaller peak at ∼2,920 cm -1 is attributed to C–H stretching of aliphatic–CH 2 groups, indicating incomplete depolymerisation of the original biomass (Zhang et al., 2019). The prominent dual peaks at ∼1,600 and ∼1,510 cm -1 represent aromatic C=C stretching vibrations, with the 1,600 cm -1 peak indicating asymmetric stretching and the 1,510 cm -1 peak corresponding to symmetric stretching of the conjugated aromatic system (Abdelhadi et al., 2017). This characteristic doublet confirms the development of condensed aromatic structures and polyaromatic compounds derived from lignin decomposition, indicating well-developed aromaticity typical of biochar carbonized at 600 °C.

The presence of a carbonyl band at ∼1735 cm -1 (C=O stretching) is indicative of oxygenated functional groups such as carboxylic acids, ketones, and quinones, demonstrating that the biochar retains significant oxidized surface chemistry despite the high pyrolysis temperature (Janu et al., 2021; Sahoo et al., 2021). The strong band around ∼1,090 cm -1 is due to C–O stretching from alcohols, esters, or ether linkages, reflecting the diversity of oxygen-containing functional groups present on the biochar surface. The characteristic peak at ∼800 cm -1 corresponds to aromatic C–H out-of-plane bending vibrations, further confirming the aromatic ring substitution patterns and condensed polycyclic aromatic structures that form during pyrolysis (Janu et al., 2021; Sahoo et al., 2021). These oxygenated functional groups—hydroxyl (–OH), carbonyl (C=O), carboxyl (–COOH), and ether linkages—which persist even after pyrolysis at 600 °C, enhance the chemical reactivity of biochar and contribute to its partially functionalised, amorphous carbon structure. This observation aligns with previous studies (Qin et al., 2020; Bartoli et al., 2022), which demonstrated that such surface functionalities improve interfacial coupling in carbon-based fillers and promote composite reinforcement.

The improvement in interfacial bonding between biochar and the polymer matrix occurs through both chemical interactions and physical anchoring mechanisms. The oxygenated groups identified in the FTIR spectrum—hydroxyl (–OH), carbonyl (C=O), and carboxyl (–COOH)—on the biochar surface can form hydrogen bonds or ester linkages with the terminal hydroxyl and carbonyl sites of PLA during melt processing (Esmaeelnejad et al., 2017). These interactions promote interfacial adhesion by enhancing chemical compatibility between the polar biochar surface and the slightly polar PLA chains. In parallel, the rough and porous morphology of nano-biochar, characterized by its diverse aromatic structure and surface functionalities, provides mechanical interlocking sites that aid efficient stress transfer across the filler–matrix interface (Bin Mobarak et al., 2024). The combined effects of chemical bonding (hydrogen bonding and covalent interactions mediated by C=O and O–H groups), aromatic π-π interactions between the biochar structure and PLA chains, and mechanical interlocking lead to more uniform load distribution, higher tensile and flexural strength, and improved composite integrity. Therefore, the functional groups identified in the FTIR spectrum not only signify incomplete carbonization at 600 °C but also play a crucial role in interfacial compatibility and energy dissipation under load. The presence of both aromatic C=C character (1,600/1,510 cm -1 doublet) and reactive oxygen-containing groups (C=O, O–H, C–O) creates an optimal functional balance for biochar reinforcement, thereby contributing to the enhanced stability and mechanical performance of the rPLA–biochar composite.

XRD analysis

The XRD pattern of nano-biochar derived from cashew nut shell is shown in Figure 2b. A broad diffraction band centered around 21.6° (2θ) corresponds to the (002) reflection of amorphous carbon, indicating a disordered turbostratic structure with no long-range graphite ordering, which is characteristic of biochar produced from lignocellulosic biomass at moderate pyrolysis temperatures. This broad feature is typical of randomly oriented aromatic layers and supports the presence of predominantly amorphous carbon domains. The amorphous nature arises from the partial carbonization and aromatization of lignin, cellulose, and hemicellulose structures during pyrolysis at approximately 600 °C, conditions under which complete reorganization into crystalline graphite does not occur (Animali et al., 2025; Bin Mobarak et al., 2024). In addition to the main amorphous hump, several weak and narrow reflections are observed at approximately 26.6°, 28.3°, and 40.7° (2θ). The peak near 26.6° (2θ) can be attributed to trace silica (SiO 2) and other ash constituents naturally present in biochar. The minor peak at 40.7° (2θ) likely arises from poorly crystalline carbon and mineral residues such as potassium and calcium oxides. Some of the crystalline features may have developed during the ball-milling process, as the repeated mechanical impact and friction can induce local lattice rearrangements and expose embedded mineral particles. The coexistence of an amorphous carbon matrix and dispersed crystalline mineral phases indicates that the nano-biochar primarily consists of disordered carbon domains with minor inorganic inclusions. These mineral residues, originating from the raw biomass and introduced during ball milling, can serve as rigid microfillers and heat-dissipation centers, enhancing interfacial bonding, stiffness, and thermal resistance within the matrix. Such inclusions restrict polymer chain mobility and promote efficient stress transfer across the interface, thereby improving the mechanical and thermal performance of the composite (Xu et al., 2017; Clemente et al., 2018). Thus, the XRD analysis confirms that the nano-biochar exhibits a predominantly amorphous turbostratic carbon structure interspersed with trace crystalline residues, contributing positively to the mechanical integrity and thermal performance of the polymer composite.

Particle size distribution and SEM

Figure 2c presents the particle size distribution of the nano-biochar derived from cashew-nut shell after high-energy ball milling. The average particle diameter was approximately 779 nm, with most particles distributed between 500 nm and 1,200 nm. Although the measured mean size lies within the submicron range rather than the strict nanometer regime (<100 nm), the term “nano-biochar” is used here following prevailing conventions in the literature (Chaubey et al., 2024; Wu et al., 2022). The SEM micrograph shown in Figure 2d further confirms the fine morphology of the milled biochar. The particles appear irregular in shape with fractured surfaces predominantly below 1 μm, consistent with the particle-size data. The high-energy impacts during milling caused fragmentation and partial collapse of larger macropores, resulting in smoother and denser external surfaces. Nevertheless, the material retains nanoscale internal porosity, micro-/mesopores, and surface defects that are not easily visible under SEM but significantly enhance its specific surface area and surface reactivity. Previous studies have shown that ball-milled biochars retain a considerably high specific surface area, which is attributed to the persistence of nanoscale pores and structural defects within the carbon matrix (Chaubey et al., 2024).

Tensile strength

Figure 3 shows the tensile strength and stress–strain behaviour of virgin PLA, recycled PLA (rPLA), and rPLA reinforced with different amounts of biochar. Each reported value represents the mean ± standard deviation of three independently tested specimens for each formulation. Virgin PLA had a tensile strength of 32.23 MPa, which dropped to 25.92 MPa for rPLA. This 19.57% decrease was due to the breakdown of polymer chains during the recycling process. The addition of biochar to rPLA improved its tensile strength. For example, rPLA with 0.5 wt% biochar (rPLA0.5) reached 33.51 MPa, showing a 29.25% increase compared to rPLA. This was supported by the stress–strain curve, which had a steeper slope and a longer stress plateau, indicating better strength and load-bearing ability. At 1 wt% biochar (rPLA1), the tensile strength further increased to 43.56 MPa, representing a 67.98% rise compared to rPLA and 35.17% higher than virgin PLA. The corresponding stress–strain curve exhibited a distinct stress peak and greater strain at failure, signifying improved ductility and stronger interfacial bonding between the biochar and polymer matrix. The maximum tensile strength was recorded for rPLA1.5 (1.5 wt% biochar) at 49.54 MPa, which was 91.13% higher than rPLA and 53.69% higher than virgin PLA. Its stress–strain curve showed the most pronounced peak and longer strain before fracture, confirming strong reinforcement and efficient energy absorption. Although the highest tensile strength was observed at 1.5 wt% biochar, the error bars based on standard deviation (SD) indicate that the performance at 1 wt% and 1.5 wt% are comparable within experimental uncertainty. The minor variation arises from sample-to-sample differences but remains small, confirming that moderate filler loadings (1–1.5 wt%) deliver consistent and optimal mechanical performance for rPLA–biochar composites. However, at 2 wt% biochar (rPLA2), the tensile strength fell to 38.98 MPa. The decline at higher filler loading was attributed to biochar agglomeration, forming weak interfacial regions that reduced stress transfer efficiency—consistent with previous reports on biochar and nanoparticle clustering effects in polymer composites (Zouari et al., 2022; Kumar et al., 2014). Similar observations were reported in PLA composites reinforced with beechwood biochar, where excessive filler loading resulted in reduced tensile strength due to the agglomeration and weak interfacial adhesion between the PLA and biochar particles (Zouari et al., 2022). The stress–strain curve for rPLA2 also showed a lower stress peak and shorter strain, confirming the decline in performance. These results suggest that while biochar improves rPLA strength at lower amounts, too much filler can reduce performance. The best results were achieved at 1.5 wt% biochar, showing its potential for creating stronger, more sustainable materials from rPLA.

Flexural strength

The flexural strength results are shown in Figure 4. The flexural strength results followed a trend similar to the tensile strength findings, especially at the optimal biochar content of 1 wt% (rPLA1), where both properties showed noticeable improvement. Virgin PLA had a flexural strength of 50.99 MPa, which dropped to 42.19 MPa in recycled PLA (rPLA), reflecting a 17.26% decrease caused by degradation of the polymer chains during recycling. The addition of biochar improved the flexural performance of rPLA. For example, rPLA with 0.5 wt% biochar (rPLA0.5) reached 53.66 MPa, a 27.22% increase over rPLA, indicating that biochar improved stiffness and load distribution within the material. At 1 wt% biochar (rPLA1), the flexural strength further increased to 56.59 MPa, showing a 34.19% rise compared to rPLA and a 10.96% improvement over virgin PLA. This improvement was likely due to the even distribution of biochar particles and stronger bonding at the interface between the filler and the matrix. However, with 1.5 wt% biochar (rPLA1.5), flexural strength decreased to 49.57 MPa, although it remained similar to that of virgin PLA. This drop was possibly due to the beginning of particle agglomeration at higher filler content, which reduced the reinforcement effect. The largest drop in flexural strength occurred at 2 wt% biochar (rPLA2), where the value fell to 36.55 MPa—a 13.37% decrease compared to rPLA. This was attributed to severe clustering of biochar particles, creating weak points in the structure and reducing the composite’s ability to resist bending.

At 1.5 wt% biochar, tensile strength reached its peak, while flexural strength began to decline after 1 wt%. This difference may be due to the way stresses are distributed under tensile and bending conditions. Under tensile load, the added biochar improved load transfer and stiffness, filling gaps and strengthening the connection between the matrix and filler. In contrast, flexural loading involves both tensile and compressive forces across the material. At 1.5 wt% loading, excess biochar may have formed clusters, especially in the compressed areas of the specimen, acting as stress concentration points that weakened the structure. Also, higher filler content may have disrupted the flow and continuity of the PLA matrix in these zones, reducing flexibility and making the material more brittle during bending. These findings suggest that although tensile properties continued to improve with biochar content, flexural performance was more sensitive to particle agglomeration at higher loadings.

Impact strength

The impact strength of virgin PLA, recycled PLA (rPLA), and rPLA reinforced with different amounts of biochar is shown in Figure 5. Virgin PLA showed an impact strength of 29.38 kJ/m 2, which reduced to 25.06 kJ/m 2 in rPLA. This 14.73% drop was due to the weakening of polymer chains during the recycling process. The addition of biochar noticeably improved the impact strength of rPLA. At 0.5 wt% biochar (rPLA0.5), the impact strength reached 33.82 kJ/m 2, a 34.98% increase over rPLA. This improvement suggests that biochar helped to enhance the material’s toughness and its ability to absorb energy during impact. The highest impact strength was recorded at 1 wt% biochar (rPLA1), reaching 36.55 kJ/m 2. This was 45.85% higher than rPLA and 24.42% higher than virgin PLA, showing that the biochar was well distributed within the matrix, allowing better energy dissipation when the material was struck. However, at 1.5 wt% biochar (rPLA1.5), the impact strength dropped to 32.29 kJ/m 2. Although still better than unfilled rPLA, it showed an 11.65% decrease compared to rPLA1. This reduction was likely caused by the clustering of biochar particles at higher loading, which created weak points in the structure and reduced its toughness. The lowest impact strength among the composites was found at 2 wt% biochar (rPLA2), where the value fell to 26.19 kJ/m 2 —close to that of plain rPLA. This sharp decline highlighted that too much biochar led to particle agglomeration, which interrupted the material’s structure and weakened its ability to absorb impact energy. Overall, the best impact performance was observed at 1 wt% biochar, where the filler was most effective. These results match the trends found in tensile and flexural tests and show the importance of keeping a uniform distribution of biochar particles in the composite. Avoiding agglomeration is essential for maintaining the material’s strength and toughness.

Interlaminar shear strength

The ILSS results of virgin PLA, rPLA, and rPLA reinforced with various amounts of biochar are presented in Figure 6. Virgin PLA exhibited an ILSS of 26.30 MPa, whereas rPLA showed a noticeable reduction to 21.37 MPa, corresponding to a 18.74% decrease. This decline is attributed to the degradation and chain scission of PLA molecules during the recycling process, which reduces interfacial bonding and load transfer efficiency within the matrix. The incorporation of biochar significantly enhanced the ILSS of rPLA composites. At 0.5 wt% biochar (rPLA0.5), the ILSS reached 26.14 MPa, representing a 22.34% improvement over rPLA. This indicates that a small amount of biochar can reinforce the polymer matrix by increasing interfacial adhesion and resistance to shear deformation. The ILSS further increased to 31.76 MPa for 1 wt% biochar (rPLA1) and peaked at 36.31 MPa for 1.5 wt% biochar (rPLA1.5). These increases—48.64% and 69.93%, respectively, compared to rPLA—demonstrate that biochar effectively enhanced the interfacial shear resistance between polymer chains, likely through mechanical interlocking and better stress distribution under load. However, at higher filler loading (2 wt% biochar, rPLA2), the ILSS dropped to 27.17 MPa, though it remained higher than that of plain rPLA. This reduction can be ascribed to biochar agglomeration at elevated concentrations, which created stress-concentration sites and weakened the bonding efficiency between the filler and the matrix. Overall, the highest ILSS was achieved at 1.5 wt% biochar, indicating that an optimal filler concentration is crucial to maximize interfacial bonding and shear strength. Beyond this point, particle clustering counteracts the reinforcing effect. These findings align well with the trends observed in tensile and impact properties, confirming that uniform dispersion of biochar plays a vital role in enhancing the mechanical integrity of rPLA composites.

Hardness

The Shore D hardness results for virgin PLA, rPLA, and rPLA reinforced with different amounts of biochar showed trends that support the role of biochar as an effective reinforcement (Figure 7). Virgin PLA had a hardness of 73.32, which dropped to 67.32 in rPLA, showing an 8.18% decrease due to the degradation of polymer chains during recycling. The addition of biochar led to a noticeable improvement in hardness. At 0.5 wt% biochar (rPLA0.5), the hardness increased to 75.91, which was 12.77% higher than that of rPLA. This suggests that even a small amount of biochar can improve surface stiffness and resistance to indentation. At 1 wt% biochar (rPLA1), the hardness rose further to 83.36—an increase of 23.78% over rPLA and 13.70% compared to virgin PLA. This improvement indicates that the biochar was well dispersed in the matrix, limiting polymer chain movement and making the surface more rigid. A slight increase in hardness was also seen at 1.5 wt% biochar (rPLA1.5), reaching 83.85, which was 24.56% higher than rPLA and 14.38% greater than virgin PLA. This shows that the reinforcement effect of biochar was still effective at this level. However, at 2 wt% biochar (rPLA2), a small drop in hardness was observed, with the value decreasing to 81.70. Although still higher than both virgin PLA and rPLA, this reduction could be due to biochar agglomeration at higher loading, which may have created uneven distribution and local stress points. These results show that biochar not only strengthens rPLA mechanically but also improves its surface hardness, with the best performance achieved between one and 1.5 wt%. The slight decline at 2 wt% confirms the need to keep filler content within an optimal range to avoid agglomeration and ensure uniform reinforcement. The hardness trends closely follow those observed in tensile and flexural strength, further confirming biochar’s effectiveness in reinforcing recycled PLA composites.

Overall, the enhancement in rPLA mechanical performance with nano-biochar can be attributed to three primary mechanisms that differ slightly from those of conventional porous biochars. First, although the high-energy ball milling process reduces visible porosity, the fractured and irregular surfaces of nano-biochar particles still enable partial mechanical interlocking with the PLA matrix, facilitating effective load transfer. Second, the presence of oxygen-containing functional groups (–OH, –COOH) on the biochar surface allows for the formation of hydrogen bonding and ester linkages with the PLA chains, thereby improving interfacial adhesion. Third, the nanostructured morphology and fine particle size of the biochar act as nucleation sites during crystallization, restricting polymer chain mobility and enhancing the overall stiffness of the composite. Similar strengthening mechanisms for carbonaceous fillers in polymer matrices have been reported by Xia et al. (Xia et al., 2024), and George et al. (George et al., 2019). These combined effects explain the observed increase in mechanical strength up to the 1.5 wt% biochar loading level. It is also understood from previous studies that the properties of biochar-based polymer composites are highly dependent on the type of biomass precursor, pyrolysis temperature, and resulting biochar characteristics (Mozrall et al., 2023; Minugu et al., 2021; Das et al., 2015). Consequently, the mechanical behaviours of such composites often vary widely across different systems. However, when biochar is converted into nanoscale form, these variations can be significantly reduced. The reduction in particle size enhances surface uniformity, interfacial compatibility, and dispersion within the polymer matrix, leading to more consistent reinforcement behaviour. This observation aligns with the current findings, where the use of nano-biochar improved the overall homogeneity and performance of the rPLA composites despite the inherent variability in biomass-derived precursors. The summary of mechanical property results is shown in Table 2.

Results of limiting oxygen index and flammability tests

The Limiting Oxygen Index and UL-94 vertical burning results for virgin PLA, rPLA, and the rPLA–biochar composites are summarised in Table 3. Both virgin PLA and rPLA exhibited an LOI of 23% and received a UL-94 V-2 rating, demonstrating that the recycling process does not substantially alter PLA’s inherent flammability. However, rPLA displayed slightly reduced dripping, which may be attributed to changes in its thermal degradation pathway. Chain scission during reprocessing can promote marginally higher char residue, helping stabilise molten polymer flow and reducing the extent of dripping. A marked improvement in fire performance was observed when 0.5–1.5 wt% biochar was incorporated into rPLA. These composites achieved a UL-94 V-0 rating with no dripping, signifying strong resistance to melt flow and self-sustained burning. This improvement is attributed to the formation of a more continuous and coherent protective char layer during combustion, which restricts heat transfer, limits oxygen diffusion, and stabilises the underlying polymer matrix. A slight reduction in LOI values (to 21%–22%) was observed in these samples; however, despite the lower LOI, the UL-94 performance improved significantly. This behaviour indicates that the flame-retardant action is dominated not by oxygen-index requirements but rather by the barrier-forming and heat-shielding properties of biochar.

At 2 wt% biochar, the flame rating deteriorated to V-2, and moderate dripping reappeared. This decline is likely associated with filler agglomeration at higher biochar loadings, which disrupts the formation of a uniform char barrier and reduces the efficiency of the flame-retardant mechanisms. Excessive particulate clustering may also introduce weak points in the matrix, leading to premature dripping and insufficient char cohesion during burning. The improved flame resistance at 0.5–1.5 wt% biochar is supported by several experimental indicators: (i) the transition from V-2 to V-0 rating, (ii) the complete elimination of dripping, and (iii) visually denser, more coherent char residues after UL-94 testing.

These observations collectively support the proposed mechanisms—heat absorption, partial quenching of free-radical combustion pathways, and enhanced char barrier formation. The denser char layer is particularly important, as it serves as a physical shield, reducing heat flux to the substrate and acting as a diffusion barrier against oxygen. Thus, even though LOI slightly decreases, the overall flame-retardant efficiency improves substantially due to these synergistic effects. Taken together, these results show that biochar content must be carefully optimised: low loadings (0.5–1.5 wt%) effectively enhance flame resistance and suppress dripping, whereas excessive loading (2 wt%) may counteract these benefits due to agglomeration and impaired char formation.

One-way ANOVA and tukey HSD post-hoc analysis

A one-way ANOVA was carried out for each mechanical property to assess the influence of biochar loading on the recycled PLA matrix. As summarised in Table 4, all F-statistics exceeded the critical value (F > 3.11) with p < 0.05, confirming that biochar reinforcement significantly affected the strength, stiffness, and hardness of the composites. Tukey HSD comparisons (Table 5) identified the specific group pairs responsible for these significant differences. In particular, rPLA1 and rPLA1.5 consistently outperformed both Virgin PLA and rPLA, while the difference between rPLA1.5 and rPLA2 was statistically insignificant, indicating saturation of reinforcement efficiency beyond 1.5 wt% biochar.

The statistical comparison revealed that all mechanical properties varied significantly with composite formulation. Tensile and ILSS values increased progressively up to 1.5 wt% biochar, achieving 49.5 MPa and 36.3 MPa, respectively representing 53% and 38% improvements over Virgin PLA (p < 0.001). Flexural and impact strengths peaked at 1 wt% loading, indicating optimal stress transfer and energy absorption before filler agglomeration at higher concentrations. Hardness increased steadily, reaching 83.9 Shore D for rPLA1.5, consistent with enhanced surface densification and matrix rigidity. Tukey HSD results (Table 5) confirmed that differences between Virgin PLA and rPLA1/rPLA1.5 were highly significant (p < 0.01), while rPLA1.5 vs. rPLA2 showed no significant difference, suggesting that further biochar addition beyond 1.5 wt% does not yield additional mechanical benefits.

This study demonstrates the potential of nano-biochar–reinforced recycled PLA (rPLA) composites as sustainable, high-performance materials for structural and flame-retardant applications. Incorporating nano-biochar derived from cashew nut shell biomass significantly enhanced the mechanical and flammability characteristics of rPLA. The tensile strength reached a maximum of 49.54 MPa at 1.5 wt% biochar, corresponding to a 91.13% increase over rPLA and 53.69% over virgin PLA. Flexural and impact strengths peaked at 56.59 MPa and 36.55 kJ/m 2, marking respective improvements of 34.19% and 45.85% compared to unfilled rPLA. Interlaminar shear strength increased from 21.37 MPa for rPLA to 36.31 MPa at 1.5 wt% biochar, indicating superior interfacial adhesion between matrix and reinforcement. Statistical validation using one-way ANOVA and Tukey’s HSD post-hoc tests confirmed that all observed improvements were significant at p < 0.05, ensuring the reliability of mechanical enhancements and identifying 1–1.5 wt% as the optimal loading range. Flammability performance also improved markedly, with rPLA0.5–rPLA1.5 composites achieving UL-94 V-0 ratings and exhibiting no dripping behaviour, attributed to the formation of a protective char layer that increased thermal stability. At higher loadings (2 wt%), a marginal decline in performance was observed, likely due to nano-biochar agglomeration and uneven dispersion, underscoring the importance of maintaining optimal filler–matrix interaction. Overall, the findings establish nano-biochar as an eco-efficient reinforcement that not only restores the degraded mechanical properties of recycled PLA but also imparts superior flame resistance and structural integrity, contributing to the advancement of circular-economy polymer composites for sustainable engineering applications.