This work investigated the properties of materials chosen for biocomposite development. Polylactic acid (PLA) sourced from Green Dot Biopak (City of Gujarat, India) exhibits desirable traits for bio-composites, including low density (1.24 g/cm³), high melting point (170°C), and good mechanical strength (tensile strength: 60 MPa, flexural strength: 108 MPa). FE-SEM analysis confirms a smooth PLA morphology (Fig. 1). Microcrystalline cellulose (MCC), chosen for its potential to enhance bio-composites, is derived from softwood pulp, which was extracted and discussed in detail in the methods part. Dichloromethane, obtained from Sisco Research Laboratory (City of Mumbai, India), acts as a solvent due to its ability to dissolve the PLA and achieve the necessary slurry consistency for composite preparation. In essence, this study lays the groundwork for bio-composite development by characterizing the key materials-PLA, MCC, and dichloromethane. Their properties provide a promising foundation for further research in this area.

PLA goes through depolymerization while processing, which increments the hydroxyl and carboxyl in PLA. This builds the bond development with cellulose. Additionally, hydrogen bonds between cellulose and PLA further enhance the adhesion, leading to better stress transfer from the matrix to the bio-composite fibers. So, it can potential to be interacted with cellulose’s hydroxyl groups and PLA by shown below in Fig. 3.

To investigate the effect of varying cellulose content on the PLA bio-composites, as per Fig. 4, the very first step is the conversion of PLA pellets into a slurry form by adding the required amount (based on pouring condition) of dichloromethane solvent. The second step is the addition of developed cellulose powder into PLA slurry by hand lay-up technique (manual mixing) until a visually homogeneous distribution was achieved using stir stick as per different ratios mentioned in Table 1. The third step is the various ratios of blended composites were poured into the respective moulds designed as per ASTM standards (Ponsuriyaprakash et al. [9,10]) and then dried in the room temperature for 24 h.

Following ASTM D638 standard (Ponsuriyaprakash et al. [9]), PLA/cellulose biocomposite samples were prepared in specific dimensions (Fig. 5), using a glass mold. The samples were placed on an Instron servo-hydraulic machine for in tensile testing. Precision wedge grips securely held the samples when a controlled load was applied at a constant rate of 5 mm/min. The test continued until the sample fractured, providing valuable data on the ultimate tensile strength of the material. The gauge length was set based on the standard at 150 mm for accuracy.

PLA/cellulose biocomposite specimens (Fig. 6) were tested using a standardised three-point flexure test was conducted as per ASTM D790 (Ponsuriyaprakash et al. [10]) on a dedicated UTM (Generic Flexure 3PT). The test, conducted at a controlled strain rate of 5 mm/min, provided valuable insight into the ability of the material to withstand flexural forces. The reported average flexural strength and modulus values demonstrated the consistent performance of the biocomposites, highlighting their potential for applications where both strength and flexibility are required.

To assess the impact strength of the PLA matrix and cellulose-reinforced biocomposites, researchers used an Izod impact test following ASTM D256-07 standard (Ponsuriyaprakash et al. [10]). Six samples (Fig. 7) of standardized dimensions of 63.5 mm × 12.5 mm × 3.0 mm were tested on a dedicated pendulum-type machine (Tinius Olsen Pvt. Ltd., Uttarpradesh, India) at a controlled impact rate of 3.55 m/s, delivering 12.5 J of energy over a distance of 615 mm. The reported average impact strength reflected the consistent performance of the biocomposites under impact conditions, indicating their potential for applications that require resilience to sudden force.

Hardness, a key indicator of resistance of PMCs to deformation, was assessed using Shore D-scale durometer in ASTM D2240 standard (Ponsuriyaprakash et al. [10]). Each biocomposite sample was subjected to five hardness measurements on standardized samples of 30 mm × 30 mm × 6 mm (Fig. 8) at a controlled temperature of 25°C. This approach, employing a reliable durometer (Fasne Test Equipment Pvt. Ltd., Maharastra, India), provided valuable insights for the mechanical behaviors of the biocomposites.

Another set of the same specimens subjected to compressive loading was evaluated by an ultimate compressive strength (UCS) test in accordance with ASTM D3410 (Ponsuriyaprakash et al. [10]). Six standardised specimens (Fig. 9) were tested on a dedicated UTM (Generic compression) at a predetermined load of 50 kN and a controlled deformation rate of 5 mm/min. The average UCS value demonstrated the remarkable ability of the biocomposites to withstand compressive forces, highlighting their potential for applications requiring structural integrity.

The heat resistivity and weight loss behavior of PLA/cellulose biocomposites were investigated using TGA. Test samples of weights of 10 to 15 mg were examined in platinum TGA pans in the presence of nitrogen. Once dried, the samples were subjected to a heating ramp of 10°C/min from 30°C to 400°C. Subsequently, the test samples were examined with the same heating ramp.

XRD analysis confirmed the successful incorporation of microcrystalline cellulose into the PLA matrix composite. Distinct peaks corresponding to activated carbon were observed in the XRD patterns (Fig. 10). The peak intensity at 2θ = 49° corroborated the presence and homogeneous dispersion of the reinforcement within the PLA matrix. The stronger PLA peak compared to the microcrystalline cellulose peak reflects the relative abundance of each material in the composite.

FT-IR analysis revealed the presence and distribution of cellulose within PLA-MCC composites (0–50 wt.% MCC). Peaks between 1900–3500 cm⁻¹ corresponded to cellulose stretching vibrations, with the characteristic peak at 1650 cm⁻¹ confirming its presence. The intensity of these peaks (including the one at 2950 cm⁻¹) increased with increasing MCC content (Fig. 11), indicating successful dispersion throughout the PLA matrix. Chemical treatment significantly enhanced the peak at 1050 cm⁻¹ and reduced the intensity of cellulose peaks (740, 1050, 1650, 2350, 2950, and 3490 cm⁻¹). This suggests a decrease in detectable cellulose content, potentially improving the mechanical properties of the composite fibers since high cellulose content can be detrimental. Additionally, chemical treatment seems to eliminate these cellulose peaks entirely.

The FE-SEM images of low and high presence of cellulose fibres in the PLA matrix is presented in Fig. 12. It was observed that the surface became rougher with increasing wt.% of cellulose, as observed at 10 wt.% compared to 50 wt.% in Fig. 9a,b, respectively. With the highest presence of cellulose, the 50:50 wt.% biocomposite sample, the surface showed an uneven distribution of reinforcement. This phenomenon caused an accumulation of reinforcement, leading to failure. In Fig. 3, the yellow mark indicated the presence and distribution of cellulose reinforcement in the matrix. It also confirmed that the behavior of the presence of cellulose influenced the absorption and deflection of the externally applied load. The different magnification scale values were used in the FE-SEM analysis because it was difficult to identify the location of the bond between matrix and reinforcement in samples with different ratios.

Optimizing the cellulose content was crucial to maximizing the tensile strength of the biocomposites, as shown by the data obtained using a universal testing machine. The strength properties of polymer composites depend on various factors such as the aspect ratio of fillers, the crystallinity degree of the matrix, the optimum filler content and the interfacial bond between the filler and matrix. Fig. 13 shows the remarkable strengthening effect with increasing cellulose content, reaching a maximum at 20 wt.%. However, biocomposite samples above this threshold showed a significant decrease in tensile strength. This may be due to some factors such as low strain, non-uniformity in the structure and potential defects. The above findings provided valuable insights for tailoring cellulose reinforcement strategies to achieve optimal tensile performance in similar biocomposites.

Importantly, the present study on PMCs found that the addition of 20 wt.% cellulose to the PLA matrix significantly increased their tensile strength. This can be associated with improved cellulose-PLA interfacial bonding as observed by microstructural analysis in Fig. 14a–c. FE-SEM images provided visual evidence of this improved bonding and uniform distribution of cellulose fibres within the PLA matrix. When the amount of the MCC was above 20 wt.% in the polymer matrix, the tensile strength gradually decreased. This was mainly attributed to the agglomeration problem of the MCC in the PLA matrix. The results showed that the higher content of the MCC fibres adversely affected the interfacial bond between the filler and PLA matrix. Moreover, a little microvoids were also observed in the morphological analysis (Fig. 14). Similar results were observed in previous studies [26,27].

The choice of the right cellulose content was crucial to optimise the flexural strength of the PLA/cellulose biocomposites, as shown in Fig. 15. Pure PLA performed relatively poorly in terms of strength and stiffness compared to other biocomposites, except for the sample with the same ratio or highest MCC. It was observed that the addition of MCC significantly increased the flexural strength of the samples compared to the unreinforced or pure PLA sample. The maximum flexural strength was obtained with the biocomposite sample containing 20 wt.% cellulose. However, exceeding this limit can be detrimental as the material became increasingly brittle and its performance under flexural loading was compromised. This implies that design and engineering of PLA-based biocomposites with optimal flexural strength and functionality can be achieved.

From Fig. 16, the microstructural analysis of the optimal biocomposite showed that it had the most improved interfacial bonding among the different samples, which allowed for improved load transfer capability within the biocomposite system. Therefore, this study concluded that the addition of the MCC to PLA played a significant role in enhancing the flexural behaviour of the PMCs. Furthermore, the FE-SEM images showed a clear interfacial bonding reaction and uniform distribution of cellulose within the PLA matrix (Fig. 16).

The impact strengths of the different biocomposite samples varied significantly with their different reinforcement weight % as shown in Fig. 17. There was a 41.66% increase in impact strength of 17 J/m² for the 10% w/w sample compared to pure PLA. A further increase in cellulose content to 20% gave the highest impact strength of 23 J/m² with a huge significant improvement of 91.66% over pure PLA. This maximum performance can be attributed to the enhanced interfacial bonding together with the uniform distribution of reinforcement, which effectively absorbs energy and prevents crack propagation. However, the addition of more than the threshold of 20 wt.% cellulose resulted in a decrease in impact strength due to the increased brittleness of the biocomposite samples with higher cellulose content.

Impact tests revealed a remarkable improvement in load transfer within the PLA/cellulose biocomposites at the optimum 80:20 wt.%, as shown in Fig. 18. This improvement can be attributed to the enhanced bonding between PLA and cellulose as demonstrated by microstructural analysis.

The hand lay-up process is very important for the production of high-quality biocomposites, as described by the hardness data of cellulose-reinforced PLA biocomposites. Fig. 19 shows that increasing the cellulose content from 10 to 50 wt.% consistently increased the hardness of the materials, unlike other mechanical properties investigated, which later decreased, especially at the same 50:50 wt.% content (Table 1). This remarkable trend not only highlighted the reinforcing effect of cellulose but also established the successful development of biocomposites with minimal pores, voids and uniform fibre distribution, considering their fabrication by hand lay-up process. The 50:50 wt.% biocomposite sample, with its impressive 91 Shore D hardness, showed an increase of 35.82% compared to the pure PLA sample, demonstrating the potential of the hand lay-up method for the production of healthy and high-performance biocomposites.

The compressive strength or bond between cellulose content and PLA of the biocomposites was commendable as shown in Fig. 20. The addition of cellulose up to 20 wt.% indicated its good reinforcing property. This was evident from the 21.54% increase in compressive strength from 65 to 79 MPa compared to pure PLA. This increase supported better interfacial bonding of PLA with cellulose, which promoted efficient load transfer between PLA and cellulose particles. However, the compressive strength of the samples decreased when their cellulose content exceeded the threshold of 20 wt.%, as the stiffness before deformation was compromised.

Overall, the combination of microstructural analysis and FE-SEM imaging from Fig. 21 provided compelling evidence that the incorporation of cellulose in the optimal ratio significantly improved the mechanical properties of the biocomposites. These results can support the development of more efficient and high-performance biomaterials for various applications.

The percentage weight loss for all samples at specific temperatures using TGA is given in Table 2. A key objective of adding cellulose fibre to PLA was to extend the temperature range before significant degradation occurs. The optimum 80:20 wt.% sample showed the lowest weight loss at 400°C (around 20%), indicating an optimum reinforcement content and the strongest PLA-cellulose interactions. The lowest weight loss compared to pure PLA indicated a stabilising effect of cellulose.

Significantly, the higher cellulose content resulted in a faster rate of weight loss. This suggests that the cellulose started to degrade at higher temperatures than the PLA, as shown in Fig. 22. It may have been shielded by the more thermally stable PLA surrounding it. The increased thermal resistance of the 80:20 wt.% sample can be attributed to two factors: (i) the cellulose may have acted as a shield, trapping harmful free radicals generated during PLA degradation and (ii) the fibre network hindered the escape of volatile degradable products.

Parameswaranpillai et al. [12] concluded that cellulose nanocomposites reinforced with PLA fabricated using the melt compounding method result in better enhancement in tensile strength, but noted that the processing cost will be higher.

Bhasney et al. [26] determined that commercially available cellulose powder reinforced with PLA and PP using a twin screw extruder blending technique showed improved characterization results in a composite containing 80% PLA, 19.7% PP, and 0.3% MCC. This composite also withstood higher weight loss at a low temperature of 120°C due to its 99.7% polymer content. The present research facilitated comprehension of the overall performance of the extracted and developed MCC fiber powder with PLA composites. Consequently, it was concluded that the overall mechanical and thermal properties of PLA were moderately optimized through the addition of extracted MCC fibre powder.