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Abstract

Eco-composites reduce environmental impact, promote sustainability, and add value to renewable resources. This study investigated the development of sustainable composites based on poly (lactic acid) (PLA)/microfibrillated cellulose (MC), with and without the addition of epoxidized soybean oil (ESO) as a plasticizer. MC contents of 1, 3, and 5 phr (parts per hundred resin) were evaluated, while ESO was used at 5 phr. The composites were processed using extrusion and injection molding. The addition of MC to PLA resulted in agglomerates and low interfacial adhesion, limiting the mechanical properties. FTIR analysis showed that ESO interacted with PLA, promoting internal plasticization and improving interfacial adhesion with MC, which contributed to the formation of impact strength and flexible composites, as well as with optical transparency. Morphological analysis by optical microscopy and SEM indicated that ESO facilitated better distribution and dispersion of MC in the PLA matrix. The PLA/ESO/MC (5 phr) composite showed increases of 58.1% in impact strength and 467.1% in elongation at break, compared to PLA. Moreover, the composite maintained high stiffness and transparency in the range of 64–80% (400 to 800 nm). These results are promising for sustainable applications such as packaging, contributing to advances in green engineering.

Keywords

PLA microfibrillated cellulose green plasticizer ecomaterials balanced performance

Introduction

The current global transition toward a more sustainable production model goes beyond merely replacing conventional materials with ‘green’ alternatives, it requires a profound rethinking of how we conceive, design, and apply materials.^1,2 In many applications, the main challenge of using eco-friendly materials lies in their property limitations compared to conventional systems.^3,4 In this context, although biodegradable polymers are often promoted as solutions for the plastics processing sector, they face real performance limitations.^5,6 Therefore, eco-friendly strategies for modifying biopolymers must be addressed, aiming to preserve sustainability while tailoring materials for specific applications.^7,8 Poly (lactic acid) (PLA) has been widely studied and applied due to its renewable origin, biodegradability under appropriate composting conditions, and relatively satisfactory tensile mechanical properties.^9,10 However, its excessive stiffness leads to brittleness and low impact strength, limiting its use in more structurally demanding applications under severe collision effects.^11,12 An interesting approach to balancing PLA’s mechanical properties, particularly stiffness versus toughness, is through the production of toughened composites, as demonstrated in the literature.^13–15

PLA composites reinforced with natural fibers represent a promising alternative to conventional synthetic materials, especially in the context of the transition toward eco-materials and environmental compatibility.^16,17 The incorporation of plant-based fibers into PLA not only reduces dependence on fossil resources but also extends the lifecycle of agricultural waste.¹⁸ A systemic perspective can transform PLA/natural fiber composites from a technically viable solution into a central platform for sustainable innovation, as well as foster the development of a more environmentally conscious approach.^19,20 The literature reports the production of PLA-based composites reinforced with natural fibers such as corn straw,²¹ sugarcane,²² sisal,²³ jute,²⁴ flax,²⁵ cotton,²⁶ coconut,²⁷ banana plant,²⁸ rice husk,²⁹ wood flour,³⁰ and bamboo,³¹ suggesting significant technological potential and contributing to the reduction of carbon footprint and environmental impact. Moving toward new directions in eco-composites, natural fibers are being modified through promising engineering approaches, such as the production of microfibrillated cellulose (MC).^32,33

Microfibrillated cellulose (MC) offers a combination of high mechanical strength, low density, and sustainability. Its fibrillated structure can enhance stress transfer and dimensional stability in polymers such as PLA.^34,35 However, this also depends on the type of processing, the degree of distribution and dispersion in the PLA matrix, as well as interfacial compatibility.^36,37 There are reports on the development of polymer composites with microfibrillated cellulose, demonstrating its potential as an eco-friendly additive.^38–42 The inherent polarity of cellulose also poses a challenge for compatibility in polymer composites, as it increases interfacial tension, especially in predominantly hydrophobic matrices.⁴³ As a result, there is a negative impact on mechanical properties, leading to premature failure, particularly in impact strength and elongation at break. To overcome this compatibility barrier and simultaneously enhance the flexibility and processability of PLA/microfibrillated cellulose (MC)-based composites, a solution aligned with green chemistry is the use of eco-friendly plasticizers.^43–47

Several reports in the literature are exploring the potential of eco-friendly plasticizers to enhance the flexibility of PLA-based composites with microfibrillated cellulose and its derivatives, such as polyethylene glycol (PEG),⁴⁸ ionic liquids (ILs),⁴⁹ poly (butylene adipate) (PBA),⁵⁰ cardanol oil (CA),⁵¹ isosorbide dinonanoate (ISN),⁵² and lactic acid oligomer.⁵³ In this context, epoxidized soybean oil (ESO) has gained prominence due to its low cost and large-scale industrial production, making it a viable plasticizing additive for PLA.^54,55 Unlike synthetic plasticizers with environmental impact, ESO is an eco-friendly additive derived from renewable sources.⁵⁶ The oxirane group grafted onto the soybean oil chain can interact with the hydroxyl/carboxyl groups of PLA, promoting internal chain plasticization.^57,58 At the same time, ESO can also interact with microfibrillated cellulose, contributing to a synergistic effect in PLA/microfibrillated cellulose composites without compromising biodegradability at the end of the product’s lifecycle. The development of PLA/MC composites plasticized with ESO is not merely a technical solution, but a material proposal that directly aligns with the principles of bioeconomy and ecodesign.⁵⁹ It represents a synthesis effort that balances technical performance, industrial feasibility, and environmental responsibility, relevance that extends beyond the laboratory to strategic sectors such as the food packaging industry, biomedical devices, consumer goods, and low-impact automotive components.

PLA stands out for its transparency, making it a promising material for rigid packaging applications. However, when producing PLA/MC/ESO composites, a loss in transmittance may occur, as observed in.⁵⁹ This is a result of the difference in the refractive index of the constituent materials. Therefore, further studies are necessary to promote proper distribution and dispersion of microfibrillated cellulose (MC) in the PLA matrix. In this context, an optimized screw profile in an extruder, featuring both distributive and dispersive mixing elements, can reduce agglomeration by breaking down particles and improving their distribution and dispersion in the PLA matrix. This could potentially preserve PLA transparency and enhance its mechanical properties. Nevertheless, studies in the literature on PLA modified with microfibrillated cellulose (MC) and epoxidized soybean oil (ESO) remain limited, particularly regarding processing using high-shear twin-screw extrusion. This highlights the need for further research to better understand how ESO contributes to the dispersion of MC in PLA.

The aim of this study was to produce PLA composites reinforced with microfibrillated cellulose (MC) at loadings of 1, 3, and 5 phr (parts per hundred resin), in order to evaluate them before and after the incorporation of epoxidized soybean oil (ESO), under melt processing using a screw profile configured with distributive and dispersive mixing elements.

Materials and Methods

Materials

Poly (lactic acid) (PLA) provided by NatureWorks, identified by the Ingeo 3D850 code, was used in this study. The material, presented in pellet form, has a density of 1.24 g/cm³ according to ASTM D792, a relative viscosity of 4.0 according to ASTM D5225, and a melt flow index in the range of 7 to 9 g/10 min, determined by ASTM D1238. The glass transition temperature (T_g) of PLA is between 55 and 60°C, as specified by ASTM D3418. The epoxidized soybean oil (ESO) used in this work was purchased from GOTALUBE Aditivos LTDA, identified by the code 010278. According to the technical information provided by the manufacturer, the material has an oxirane content between 6.50 and 7.00%, an iodine value in the range of 0 to 4.00 cg I₂/g, an acid value ranging from 0 to 1.00 mg KOH/g, a density between 0.987 and 0.993 g/cm³ at 25°C, and a maximum moisture content of 0.1%. The microfibrillated cellulose (MC) used as a filler in this study is the Exilva® F 01-V product, provided by Borregaard. According to the supplier’s specifications, the MC has a solid content between 8 and 12%, a viscosity greater than 20,000 mPa·s in an aqueous dispersion at 2%, conductivity lower than 500 µS/cm, and a pH range from 5 to 7 under the same concentration conditions.

Composites Processing

PLA and MC were subjected to a vacuum oven drying process (500 mmHg) for 24 h, maintained at 60°C and 80°C, respectively. This step aimed to remove residual moisture, thereby preventing degradation by hydrolysis during processing. MC contents of 1, 3, and 5 parts per hundred resin (phr) were tested with PLA, considering the high cost of MC (due to the fibrillation process) and previous studies in the literature.^36,60 To minimize a significant decline in the elastic modulus, the ESO content was fixed at 5 phr in the plasticized compositions, aiming for a better balance between toughness and stiffness. MC and ESO were employed as additives in the modification of PLA and were therefore expressed in phr. Since a low concentration of MC was used, polymer concentrates were prepared prior to extrusion to ensure precise control of the MC and ESO proportions in the PLA matrix. As an initial step, PLA/ESO, PLA/MC, and PLA/ESO/MC concentrates were prepared using a laboratory internal mixer (Thermocientific Polylab QC). The operational conditions applied were a temperature of 180°C, rotor speed set to 60 rpm, and a mixing time of 3 minutes. Subsequently, the concentrates were ground in a knife mill to produce pellets suitable for extrusion processing.

The samples of pure PLA, PLA/ESO compound, and polymer composites were processed in a co-rotating twin-screw extruder (model ZSK, Coperion Werner-Pfleiderer), with a screw diameter of 18 mm and an L/D ratio of 40. The extruder screw is configured with high-shear elements to ensure good distribution and dispersion of the dispersed phase. The processing was carried out under a heating profile distributed across the zones as follows: 170°C, 170°C, 175°C, 180°C, 180°C, 185°C, and 190°C. The feed rate was maintained at 3 kg/h, with the screw rotation fixed at 250 rpm. After extrusion, the generated filaments were cooled in a water bath, dried with compressed air flow, and then cut, preparing them for the injection molding step. Table 1 shows the prepared compositions of the polymer composites.

Table 1.

Processed formulations of polymer composites.

Compositions	PLA (wt%)	ESO (phr^a)	MC (phr^a)
PLA	100	-	-
PLA/ESO	100	5	-
PLA/MC	100	-	1
PLA/MC	100	-	3
PLA/MC	100	-	5
PLA/ESO/MC	100	5	1
PLA/ESO/MC	100	5	3
PLA/ESO/MC	100	5	5

^aphr = parts per hundred resin.

Before injection molding, the previously extruded material was dried for 24 hours in a vacuum oven (500 mmHg) at a temperature of 60°C. The sample production was carried out using an Arburg injection molding machine (model Allrounder 207C Golden Edition), operating with the following thermal profile: 170°C, 175°C, 180°C, 190°C, and 190°C. Additionally, an injection pressure of 1200 bar, a holding pressure of 800 bar, a mold temperature set at 20°C, and a cooling time of 20 seconds were established.

Composite Characterizations

An optical microscope (OPTON, model TNP 09 NT) was employed in transmission mode at 40x magnification to examine the dispersion of cellulose microfibrils the samples. The thickness of the PLA and composites samples was approximately 3.2 mm.

Morphological changes in PLA and the polymer composites were investigated by examining the fracture surfaces after impact testing using a TESCAN scanning electron microscope (SEM), model VEGAN 3. Prior to image acquisition, the samples were coated with a thin gold layer (20-30 nm), under a current of 10 mA. Observations were carried out under high vacuum conditions with an accelerating voltage of 10 kV.

Fourier-transform infrared spectroscopy (FTIR) was carried out using a Bruker Alpha II spectrometer equipped with an attenuated total reflectance (ATR) accessory. The analysis was performed on the surface of injection-molded samples with a thickness of 3.2 mm. Spectra were collected over the range of 4000 to 400 cm⁻¹, with a resolution of 4 cm⁻¹ and a total of 32 scans per sample.

Impact strength testing was conducted on notched specimens (Length: 63.5 mm, Width: 12.7 mm, Thickness: 3.2 mm) in accordance with ASTM D256, using a CEAST RESIL 5.5 impact tester. The tests were carried out at room temperature with a pendulum energy of 2.75 J. For each formulation, eight specimens were evaluated, and the results are reported as average values. Tensile properties were evaluated based on the guidelines of ASTM D638, using an Oswaldo Filizola BME universal testing machine, set to a crosshead speed of 5 mm/min and a maximum load capacity of 20 kN. Tests were conducted at room temperature, and the results represent the average obtained from eight tested specimens (Type I: Length: 165 mm, Width: 13 mm, Thickness: 3.2 mm). Shore D hardness was measured using a Metrotokyo durometer, in accordance with ASTM D2240 standards. The procedure involved applying a 50 N load to eight randomly selected locations on each sample, with the indenter held in contact with the surface for 10 seconds at each point. The two-tailed Student's t-test was applied at a 95% confidence level to compare the means of several samples between PLA and PLA/MC, as well as PLA/ESO and PLA/ESO/MC.

Thermomechanical stability was assessed through heat deflection temperature (HDT) testing, following the ASTM D648 standard. The measurements were performed using a CEAST HDT 6 VICAT apparatus, applying a load of 1.82 MPa and a heating rate of 120°C/h. The HDT value was recorded at the point when the specimen (Length: 127 mm, Width: 12.8 mm, Thickness: 3.2 mm) exhibited a deflection of 0.25 mm while immersed in a silicone oil bath.

Thermal characterization was performed using differential scanning calorimetry (DSC) on a Shimadzu DSC-60Plus instrument, operating with a nitrogen purge at a flow rate of 50 mL/min. The thermal program included a heating-cooling-heating cycle (30 °C–200°C, 200 °C–30°C, 30 °C–200°C) at a rate of 10°C/min, with a 2-min isothermal hold at each transition. Approximately 3 mg of material was used for each analysis. The degree of crystallinity (X_c) for PLA and its composites was determined using equation (1).

X_{c} \frac{{Δ H}_{m} - {Δ H}_{cc}}{{Δ H}_{100 %}} * 100 %

(1)

Where, ΔH_100% = 93.7 J/g^61,62; ΔH_m is the enthalpy of melting of the second heating cycle obtained by DSC for PLA and polymer composites; ΔH_cc = enthalpy of cold crystallization.

Film transparency was evaluated by measuring light transmittance (T%) using a Bel UV-Vis spectrophotometer (model UV-M51), scanning across the wavelength range of 200 to 800 nm.

Results and Discussion

Fourier Transform Infrared Spectroscopy

The FTIR analysis was used to identify possible chemical interactions between PLA and ESO. In this regard, an effort was made to establish a correlation between these interactions and the mechanical performance of the materials, especially regarding impact strength and elongation at break. Figure 1 shows the FTIR spectra for pure PLA, the PLA/ESO mixture, and the polymeric composites, both with and without the addition of ESO. In relation to pure PLA, bands at 2996 cm⁻¹ and 2948 cm⁻¹ are observed, suggesting the CH₃ group (asymmetric and symmetric stretching of the C-H bond).⁶³ An intense absorption band was observed at 1749 cm⁻¹, indicating a response from the ester group of PLA, specifically the stretching of the carbonyl group (C = O).⁶⁴ The response of the C-H bond under bending and deformation was observed in the absorption bands at 1451 cm⁻¹, 1381 cm⁻¹, and 1358 cm⁻¹⁶⁵ Other typical PLA bands are seen at 1181 cm⁻¹ and 1080 cm⁻¹, resulting from the stretching of the C-O bond of the C-O-C group.⁶⁶ The absorption bands at 869 cm⁻¹ and 756 cm⁻¹ correspond to the amorphous and crystalline fractions of PLA, respectively.⁶⁷

Figure 1.

FTIR spectra for pure PLA, the PLA/ESO system, and the polymer composites.

The main absorption bands of PLA were preserved in the PLA/ESO system, as shown in Figure 1. However, it was observed that the PLA/ESO system exhibited a shift in the bands to 2952 cm⁻¹ and 2853 cm⁻¹, compared to pure PLA and the composites. This behavior suggests that the oxirane functional group of ESO likely interacted with the carboxyl/hydroxyl groups of PLA. Additionally, the PLA/ESO system significantly increased the intensity of the carbonyl group compared to PLA, reinforcing the hypothesis of good interaction between the phases. Consequently, it favored the plasticizing effect on the PLA chain, confirming the trend of improvement in impact strength and elongation at break, as seen later. Zhou et al.⁶⁸ also observed the shift in the FTIR bands of PLA plasticized with ESO, attributing it to the reactive functional groups, which contribute to greater miscibility and intermolecular interactions.

The PLA/MC composites, regardless of the MC content, maintained the typical PLA bands. The most noticeable difference was observed in the carbonyl absorption band at 1749 cm⁻¹, with an increase in intensity compared to pure PLA, except for the PLA/MC (1 phr) composite. Apparently, for 3 phr and 5 phr of MC, there were secondary interactions between the hydroxyl groups of the MC and the carbonyl groups of the PLA, leading to an increase in the intensity of the 1749 cm⁻¹ band. The same trend was observed for the PLA/ESO/MC composites, with a more prominent modification in the 1749 cm⁻¹ band. The PLA/ESO/MC composites increased the intensity of the carbonyl band, surpassing both pure PLA and the PLA/ESO system. This indicates that there was a synergistic effect between the oxirane functional group of the ESO with both PLA and MC, generating a stronger response in the carbonyl group. The suggested interaction mechanism between PLA/ESO and the polymeric composites is shown in Figure 2. The main natural interaction between PLA and MC occurs through hydrogen bonds, especially between the MC groups and the carbonyl and hydroxyl groups of PLA. In contrast, the PLA/ESO/MC composites exhibit a more complex interaction mechanism. The oxirane ring of ESO can react with the carboxyl terminal groups of PLA, as well as form hydrogen bonds between the oxirane ring and the terminal hydroxyl groups of PLA. At the same time, there is a possibility that the MC can also form hydrogen bonds with PLA and ESO, thus contributing to a synergistic effect on the mechanical properties.

Figure 2.

Proposed interaction mechanism for the PLA/ESO/MC composite.

ESO is plant-derived and biodegradable, which aligns with the new trends of producing greener materials. ESO is not only a physical plasticizer but also chemically reactive.^69,70 The oxirane ring groups can interact with the OH or COOH functional groups of PLA, as shown in Figure 2.

Impact Strength

Polylactic acid (PLA) has garnered significant interest in the manufacturing of eco-composites, aiming to develop sustainable alternatives for various applications.^71,72 However, its brittle nature represents a significant limitation when subjected to more demanding mechanical requirements.^73,74 In this context, several modifications have been applied to PLA to enhance its ductility.⁷⁵ The information obtained regarding the impact strength of pure PLA, the PLA/ESO blend, and the polymeric composites is presented in Figure 3.

Figure 3.

Impact strength behavior of PLA and polymer composites.

The behavior of pure PLA was characterized by its brittleness, showing a low impact resistance of 28.7 J/m, which aligns with the SEM morphology. This confirms the importance of modifications to make it more suitable for applications that require higher impact strength. This result is consistent with data reported in the literature.⁷⁶ The addition of ESO to PLA resulted in a notable increase in impact strength by approximately 50%, suggesting an improvement in toughness. This finding can be attributed to the plasticizing effect of ESO, favoring greater mobility of the polymer chains and, consequently, a higher energy dissipation capacity under impact in the PLA matrix.⁷⁷

In Figure 3, the production of PLA/MC composites with different concentrations of 1, 3, and 5 phr resulted in a slight reduction in impact strength compared to pure PLA, indicating a stress concentration effect in the presence of MC. However, considering the experimental error margin of the PLA/MC composites, the results are similar to pure PLA. The two-tailed Student's t-test was applied at a 95% confidence level to assess whether the performance of the PLA/MC (5 phr) composite was similar to that of pure PLA (under impact). The selection of 5 phr of MC was based on its lower impact strength. For 14 degrees of freedom and a significance level of α = 0.05 (two-tailed), the critical t-value was 2.145 and the calculated t-value was 1.8. Since the calculated t < critical t, it is concluded that there is no significant difference between the impact strength at the 95% confidence level. The decline in impact strength for the PLA/MC composites, although slight (ranging from 26.3 to 27.5 J/m), suggests that, at these concentrations, there was no efficient energy dissipation mechanism to toughen PLA, only maintaining its brittleness under impact. As observed in OM and SEM (see later), the size of the MC particles in the PLA matrix was large, and the degree of interfacial adhesion was low, which possibly did not favor an effective mechanism to retard crack propagation, leading to premature failure. However, the combination of ESO and MC in the PLA matrix showed a differentiated behavior compared to the PLA/MC composites.

In the PLA/ESO/MC composites, the impact strength values exceed 44 J/m at all MC contents, reaching a maximum of 45.4 J/m with 5 phr. This corresponds to a 58.1% increase compared to pure PLA, suggesting a higher level of ductility as a result of a synergistic interaction between the two additives, ESO/MC, in the PLA matrix. Statistical analysis using the Student’s t-test (α = 0.05) revealed no significant difference between the mean values of the PLA/ESO and PLA/ESO/MC (5 phr) samples. The calculated t-value was 1.83, lower than the critical value (t₀.₉₇₅,14 = 2.145), and the 95% confidence interval was considered. These results indicate that the addition of 5 phr of MC to PLA/ESO did not cause a statistically significant change in impact strength, suggesting that the material’s behavior remains similar. In the morphology obtained by SEM (see later), it was observed that the PLA/ESO/MC composites had smaller MC particles, better distribution, and greater interfacial wetting between PLA/MC. Apparently, ESO played a dual role in the PLA/ESO/MC composites, both plasticizing the PLA matrix and stabilizing the interface between PLA and MC, reducing interfacial stress and dissipating energy more efficiently in the PLA, leading to improved impact performance. Figure 4 presents a hypothetical representation of the behavior of ESO, MC, and the combined effect of ESO/MC in the PLA matrix.

Figure 4.

Hypothetical scheme of the distribution of ESO, MC and MC/ESO in the PLA matrix. ^* The droplet-like representation of MC in the schematic is only intended to illustrate the possible combined effect between MC/ESO, considering that the actual morphology tends to be fibrillar.

Unlike the PLA/MC composites, the increase in MC concentration in the PLA/ESO/MC formulations did not result in a loss of impact strength compared to the base PLA/ESO system. Although the results are comparable within the experimental margin of error, ESO acted by stabilizing the PLA/MC system, possibly promoting MC as a barrier to crack propagation, thus maintaining good energy dissipation under impact. FTIR demonstrated a interaction between PLA and ESO, which favored internal plasticization of the PLA chain. At the same time, the oxirane group of ESO has the ability to interact with MC through hydrogen bonds, generating a synergistic effect, as shown in Figure 4.

Tensile Properties

Figure 5 (a)–(c) shows the results of the tensile mechanical behavior for pure PLA, the PLA/ESO system, and its polymeric composites, with and without the presence of ESO.

Figure 5.

Mechanical properties under tensile for pure PLA, the PLA/ESO system, and the polymer composites.

In Figure 5(a), PLA demonstrated a rigid and brittle polymer behavior, with a relatively high elastic modulus of 2728 MPa. When modifying PLA with 5 phr of ESO, the elastic modulus decreased by approximately 12.1% compared to pure PLA, suggesting a plasticizing effect and increasing the mobility of the polymer chains. As observed in the impact test, ESO was able to increase the flexibility of the PLA chain, resulting in a reduction in rigidity. The PLA/ESO system became more ductile but less resistant to elastic deformation. This result is consistent with previous studies^78,79 that report the flexibilization of the PLA matrix using plant-based epoxidized oils as sustainable plasticizers. The incorporation of MC into PLA, on the other hand, had an opposite effect. The PLA/MC composites showed a slight tendency to increase the elastic modulus compared to pure PLA, indicating that MC restricted molecular mobility, which corroborates with the impact strength. However, the results for the elastic modulus of the PLA/MC composites were not significant, considering that the maximum gain was 4% for 5 phr of MC. In other words, PLA and PLA/MC composites maintained comparable results, considering the experimental error margin. The combined ESO/MC effect on PLA resulted in a partial recovery of the elastic modulus, which had been reduced with the exclusive use of ESO. Although ESO decreases the rigidity of PLA, the addition of MC compensates for this effect, resulting in PLA/ESO/MC composites with rigidity approaching that of pure PLA. For example, the PLA/ESO/MC (5 phr) composite showed an 11.7% increase in the elastic modulus compared to the PLA/ESO system. The Student’s t-test (α = 0.05) revealed a statistically significant difference between the mean elastic modulus values of the PLA/ESO and PLA/ESO/MC (5 phr) samples. The calculated t-value (13.44) exceeded the critical value (t₀.₉₇₅,14 = 2.145), indicating that the incorporation of 5 phr of MC led to a substantial improvement in the elastic modulus property. The use of ESO helped promote the interaction between the PLA/MC components, improving the distribution of MC and providing more effective accommodation in the PLA matrix, leading to the recovery of the elastic modulus. This finding supports the hypothesis that there was good interaction between the oxirane functional group of ESO with the PLA and MC phases, as observed in the FTIR. Therefore, the combined effect in the PLA/ESO/MC composites allows for a balance between flexibility and rigidity, maintaining properties close to those of pure PLA.

As shown in Figure 5(b), pure PLA exhibited the highest tensile strength among all formulations, with a value of approximately 62.9 MPa, reflecting its inherently rigid characteristic and requiring a high load to deform. The addition of 5 phr of ESO to PLA resulted in a reduction in tensile strength, reaching around 46.1 MPa. This decline can be attributed to the plasticizing effect of ESO, which promotes greater mobility of the PLA chains and, consequently, helps reduce the mechanical load required to deform. Although it favors ductility, ESO compromises the mechanical strength under tensile. When evaluating the addition of MC to PLA, a reduction in tensile strength was observed compared to pure PLA. In the PLA/MC composites, the best performance was observed with 1 phr of MC, showing 59.2 MPa, a value slightly lower than pure PLA. Increasing the amount of MC to 3 phr and 5 phr did not restore the tensile strength of the PLA/MC composites, resulting in a decrease in strength (57.9 and 58.9 MPa, respectively). MC was not able to act as a reinforcing filler for PLA, possibly due to agglomerations and low interfacial adhesion, as demonstrated in the SEM (see later). Consequently, there were points of stress concentration in the PLA/MC composites, leading to premature fracture. In the PLA/ESO/MC composites, the tensile strength remained at levels similar to the PLA/ESO system, ranging from 45.2 to 47.1 MPa, regardless of the MC concentration. The results are quite similar between PLA/ESO and the PLA/ESO/MC composites, staying within the experimental error margin. Although the ESO improved the distribution and interfacial adhesion in PLA/MC, the plasticizer inhibited the potential reinforcing effects of the MC, indicating that flexibility predominated.

In Figure 5(c), PLA showed a rupture strain of 4.32%, a value consistent with the literature.^80,81 The low level of deformation in PLA confirms its brittle mechanical behavior, as well as a limited capacity for plastic deformation. On the other hand, the addition of microfibrillated cellulose (MC) in different proportions (1, 3, and 5 phr) to PLA did not improve the elongation at break. Instead, a slight reduction in the rupture strain values was observed, indicating inhibition of the deformation mechanism. The presence of agglomerates and poor interfacial adhesion in the PLA/MC composites, as observed in the SEM (see later), contributed to the fracture at low deformation levels. Additionally, PLA has a brittle behavior, and MC is a rigid filler, which did not favor the increase in ductility. The effect of modifying PLA with 5 phr of ESO was quite significant, resulting in an improvement in the rupture strain to 28.8%, which represents an increase of over 560% compared to pure PLA. This result highlights the plasticizing role of ESO, which, due to its functional nature and flexible structure, was able to reduce the intermolecular cohesion forces in PLA. Thus, it promoted greater mobility of the PLA chains, consequently improving its deformation capacity under mechanical stress. Notably, the simultaneous incorporation of ESO and MC led to the formation of flexible composites with high rupture elongation values, close to those obtained for PLA/ESO. Although there was a slight decrease with the increase in MC concentration: 28.4% (1 phr), 27.5% (3 phr), and 24.5% (5 phr), the results suggest a synergistic effect in the ductility process, confirming the trend observed in impact strength. It seems that the presence of ESO had a positive effect in PLA/MC, promoting better distribution of the microfibrils and strengthening the interfacial adhesion, thereby mitigating the negative effects of adding MC on ductility. However, at higher concentrations of MC (5 phr), the adverse influence of the rigidity of the microfibrils tends to outweigh the flexibility, resulting in a gradual reduction in rupture elongation.

The data obtained for the PLA/ESO/MC composites indicate that the addition of ESO was effective in increasing the ductility of PLA, while the incorporation of MC must be optimized to avoid compromising this property. The PLA/ESO/MC formulation with 1 to 3 phr of MC represents a promising solution, combining the advantages of the plasticizer and the natural reinforcement while maintaining deformation capacity. This is desirable for applications requiring materials with balanced mechanical properties and environmental sustainability.

Figure 6 illustrates the behavior of pure PLA, the PLA/ESO system, and the polymer composites, both with and without ESO. Additionally, the post-tensile test deformation of selected samples is shown.

Figure 6.

Stress vs. strain responses for PLA, PLA/ESO system, and polymer composites. Appearance of deformation after tensile testing for: (a) pure PLA; (b) PLA/ESO; (c) PLA/MC (5 phr); (d) PLA/ESO/MC (5 phr).

The stress–strain curves obtained for the different systems reveal significant changes in the mechanical properties of PLA due to the addition of the ESO additive and MC. Pure PLA exhibited a typical brittle behavior, with premature failure after reaching its tensile strength limit. The curve shows a profile with low strain at break, characteristic of a material with limited molecular mobility. The production of PLA/MC composites was accompanied by a slight reduction in strain at break, maintaining the brittle nature. Additionally, a decline in the maximum stress of the PLA/MC composites was observed compared to pure PLA, as shown in the stress–strain curve.

The plasticization of PLA with ESO led to a deterioration in tensile strength; however, it was accompanied by a significant increase in elongation at break. This change in the deformation mechanism suggests that ESO acted as an efficient plasticizer for PLA, reinforcing the interaction hypothesis presented in the FTIR analysis. The stress–strain curve of the PLA/ESO system began to exhibit a more pronounced yield region, indicating a transition toward a more ductile behavior. This confirms the trend observed in impact strength, as a result of greater PLA chain mobility and increased capacity for deformation before fracture. The PLA/ESO/MC composites display a plastic deformation profile with a ductile mechanism. The presence of ESO tends to partially compensate for the brittleness induced by MC, resulting in curves with higher elongation at break compared to PLA/MC. This indicates a synergistic effect between the plasticizer and MC, with ESO promoting better MC distribution and good interfacial adhesion, minimizing stress concentrations and enhancing ductility.

In Figure 6(a) and (c), both pure PLA and the PLA/MC composite exhibit abrupt fracture, with no clear signs of plastic deformation along the specimen body. The absence of visible elongation indicates low ductility and a brittle fracture, confirming the limited strain at break. In contrast, the PLA/ESO system and the PLA/ESO/MC composites show more pronounced deformation prior to fracture, as evidenced by visible elongation in the samples. The occurrence of stress whitening is a clear indicator of increased ductility, particularly in the deformed region. These visual cues confirm the plasticizing effect of ESO, which enhances the ductility of PLA, contributing to improved toughness and delaying premature failure.

Shore D Hardness

Shore D hardness is a relevant mechanical property for rigid polymeric materials, particularly in structural applications where resistance to surface deformation is desired. Figure 7 shows the Shore D hardness behavior for pure PLA, the PLA/ESO system, and the PLA/ESO/MC composites. Pure PLA exhibited a Shore D hardness of 78.5, consistent with its inherently rigid nature, as demonstrated in the elastic modulus. A similar Shore D hardness behavior was reported by Candela et al..⁸² The addition of 5 phr of ESO to the PLA matrix resulted in a 6% reduction in hardness compared to pure PLA, indicating decreased rigidity. This is in line with the plasticizing effect of ESO, which enhances the mobility of PLA chains, as seen in the increased elongation at break, likely due to greater molecular spacing. However, the use of a low ESO concentration in PLA did not significantly affect resistance to surface penetration. Regarding the PLA/MC composites, the incorporation of MC at varying concentrations (1, 3, and 5 phr) led to slight increases in Shore D hardness. However, these gains were not considered significant, as the results were close to those of pure PLA and within the range of experimental error. In the PLA/ESO/MC composites, a gradual recovery in Shore D hardness was observed with increasing MC content, compared to the base PLA/ESO system. Although the difference was not substantial, the PLA/ESO/MC (5 phr) composite reached a Shore D hardness of 77.8, representing a modest 4% increase relative to PLA/ESO. It appears that the combination of ESO and MC in the PLA matrix helped offset the loss in rigidity typically caused by the plasticizer, which is consistent with the trend observed in the elastic modulus. This behavior suggests that MC was well accommodated the PLA chains, contributing to a balance with the ESO plasticizer and enabling the formation of composites that are both stiffer and flexible than the base PLA/ESO system.

Figure 7.

Shore D hardness of PLA and ESO plasticized systems.

Heat Deflection Temperature (HDT)

The heat deflection temperature (HDT) is a key property for structural applications of polymers and their composites, as it measures the material’s stability under combined thermal and mechanical stress. As such, it has become an essential quality parameter for determining the maximum service temperature at which the material can be used without undergoing deformation due to heat.⁸³ Figure 8 presents the HDT behavior of PLA and its composites as a function of MC concentration and in the presence of ESO.

Figure 8.

Evolution of HDT behavior for pure PLA and composites, with different MC contents and in the presence of ESO.

The results show that pure PLA exhibited an HDT of 56°C, which is consistent with values reported in the literature.⁸⁴ The incorporation of ESO into the PLA matrix led to a slight decrease in HDT to 51.9°C, representing a reduction of 7.3% compared to pure PLA. However, this decrease in HDT was not significant, especially when considering the gains in flexibility and impact strength observed in the PLA/ESO system. This behavior can be attributed to the plasticizing nature of ESO, which tends to reduce intermolecular interactions between PLA chains, increasing molecular mobility and, consequently, lowering thermomechanical resistance. Furthermore, as later confirmed by DSC, the PLA/ESO system caused a more pronounced reduction in the degree of crystallinity, which also contributed to the decrease in HDT values. When crystallinity is reduced, there are fewer ordered regions, and the material exhibits greater molecular mobility, thus deforming more easily under heat.

When producing the PLA/MC composites, regardless of MC concentration, no significant increases in the HDT of PLA were observed, only slight variations within the range of 56.1°C to 56.5°C. This suggests that low MC content (1 to 5 phr) did not enhance the thermomechanical stability of PLA, likely because the glass transition temperature (T_g) is close to 60°C, as later confirmed by DSC analysis. In other words, the PLA’s Tg (∼60.8°C) limits its HDT, since approaching this temperature causes the material to lose stiffness and deform under load.

The HDT behavior of the PLA/ESO/MC composites showed a slight improved thermomechanical resistance with increasing MC concentration when compared to PLA/ESO, reaching values between 53.5°C and 53.9°C. However, these composites did not achieve the HDT levels of either pure PLA or the PLA/MC system. The synergistic effect between ESO and MC in the PLA matrix suggests a balance between the rigid filler (MC) and the plasticizer (ESO), likely due to the interactions illustrated in Figure 2. It appears that the MC helped offset the HDT reduction caused by ESO, contributing to PLA/ESO/MC composites with a better compromise between flexibility (from ESO) and thermomechanical stability (provided by MC).

Differential Scanning Calorimetry (DSC)

Figure 9 shows the second heating cycle curves for pure PLA, PLA/ESO, and the polymer composites containing the MC and ESO additives. The corresponding results are presented in Table 2.

Figure 9.

Curves obtained by DSC referring to the second heating cycle for PLA, the PLA/ESO system and the polymer composites.

Table 2.

Thermal properties obtained by DSC.

Samples^a	T_g (°C)	T_m (°C)	ΔH_cc (J/g)	ΔH_m (J/g)	X_c (%)
PLA	60.8	175.1	7.61	45.41	40.3
PLA/ESO	56.1	173.6	20.2	37.49	18.4
PLA/MC (1 phr)	61.2	174.7	2.40	48.25	48.9
PLA/MC (3 phr)	62.2	174.8	---	44.07	47.0
PLA/MC (5 phr)	62.3	175.0	---	47.45	50.6
PLA/ESO/MC (1 phr)	56.8	173.4	13.75	48.84	37.4
PLA/ESO/MC (3 phr)	57.1	173.9	16.62	55.68	41.6
PLA/ESO/MC (5 phr)	56.9	173.6	20.01	58.78	41.3

^aT_g = glass transition temperature; T_m = crystal melting temperature; ΔH_cc = enthalpy of cold crystallization; ΔH_m = enthalpy of crystalline melting; X_c (%) = degree of crystallinity.

The PLA sample exhibited a glass transition temperature (T_g) of 60.8°C, a typical value for this polymer.⁸⁵ For the PLA/ESO system, a pronounced reduction in T_g was observed, dropping to 56.1°C, which supports the plasticizing effect of ESO. This finding suggests that ESO played a role in weakening the intermolecular cohesive forces in PLA, resulting in internal plasticization of the polymer chain and increased mobility, thereby explaining the decline in T_g. This greater conformational freedom in PLA/ESO reduces the energy required for the glass transition and explains the more ductile behavior observed mechanically. Given this, the reduction in the glass transition temperature (Tg) observed in the PLA/ESO system indicates the effectiveness of the plasticizer in increasing the mobility of the polymer chains, likely promoting an increase in free volume and a decrease in intermolecular interactions. Consequently, the energy required for the segmental motion of the chains is reduced, shifting the T_g of PLA to lower temperatures. On the other hand, the incorporation of MC at different concentrations shifted the T_g to the range of 61 to 62.5°C, resulting in an increase compared to pure PLA. This indicates that the cellulose microfibrils restricted the molecular mobility of the amorphous phase of PLA, possibly through hydrogen bonding interactions between the hydroxyl groups of cellulose and the functional groups of PLA (see Figure 2), promoting the formation of a stiffer physical network. The PLA/ESO/MC composites, regardless of the MC concentration, exhibited T_g values slightly higher than that of the PLA/ESO system. However, this change was not drastic, only a slight increase, confirming the trend observed in the elongation at break. The T_g behavior of the PLA/ESO/MC composites suggests that some level of interaction occurred between the phases, leading to a subtle shift in T_g to the range of 56.5 to 57.5°C. Apparently, the slight increase in T_g in the PLA/ESO/MC systems indicates that the hydrogen bonds between MC and the PLA/ESO chains partially restrict the mobility induced by the plasticizer, which is consistent with the balance between flexibility and stiffness.

The crystalline melting temperature (T_m) of PLA was around 175.1°C, a value close to that reported by Colaers et al..⁸⁶ In the PLA/MC composites, Tm values remained relatively constant, ranging between 174°C and 175°C. This suggests that the MC likely did not significantly affect the perfection or thermal stability of the crystals formed in the PLA. The PLA/ESO system showed a slight reduction in T_m to 173°C compared to pure PLA. Apparently, there was a minor alteration in PLA’s crystalline perfection in the presence of ESO, possibly resulting in thinner lamellae. Consequently, less heat was required to melt the crystals, leading to a decrease in T_m. This effect of ESO persisted in the PLA/ESO/MC composites, with values similar to those of the PLA/ESO system.

PLA exhibited a cold crystallization enthalpy (ΔH_cc) of approximately 7.61 J/g. The addition of MC to PLA inhibited the cold crystallization process, suggesting a barrier to the organization of the amorphous phase during heating, in other words, it did not promote crystalline formation in PLA upon heating. On the other hand, when PLA was modified with ESO, an increase in ΔH_cc was observed. The PLA/ESO/MC composites also exhibited cold crystallization. Therefore, ESO was able to overcome the inhibitory effect of MC, enabling crystalline formation during heating. Thus, the presence of the plasticizer ESO facilitated the reorganization of amorphous regions upon heating. Interestingly, the simultaneous addition of MC and ESO resulted in ΔH_cc values lower than those of the PLA/ESO formulation, suggesting a possible antagonism between the mobility induced by ESO and the rigidity imposed by the MC.

Pure PLA exhibited a crystallinity degree of 40.3%, consistent with a semicrystalline polymer.⁸⁷ A significant reduction in crystallinity was observed in the PLA/ESO system, with a value of 18.4%. The decrease in crystallinity observed in PLA/ESO is associated with the interference of the plasticizer in the molecular organization of PLA. This is likely attributable to the plasticizing effect of ESO, which increases chain mobility but in a disorganized manner. As a result, it hinders the orderly rearrangement during the molecular packing of PLA chains, inhibiting the formation of well-defined crystals. Additionally, the free molecular volume and chain flexibility of ESO introduce steric disorder and reduce the packing efficiency of PLA. Therefore, it suppresses nucleation and crystal growth, leading to a lower degree of crystallinity, as shown in Table 2. The behavior of PLA/ESO is typical of compounds in which the plasticizer shows good compatibility with PLA, promoting flexibility and reducing the stiffness of the material. This finding also explains the lower performance of the PLA/ESO system in terms of elastic modulus (see Figure 5(a)). Regarding the PLA/MC composites, an increase in the degree of crystallinity was observed compared to pure PLA, with results ranging from 47–51%. In this case, the MC acted as a nucleating agent in the PLA matrix, that is, the addition of MC promoted nucleation and stabilization of nuclei, leading to the formation of more crystals during the solidification process. This effect suggests the heterogeneous nucleating agent behavior of cellulose microfibrils, which serve as nucleation sites for PLA. In other words, they act by reducing the critical free energy required for nucleus formation, generating more crystals and increasing the degree of crystallinity. A shift in crystallinity behavior was noted in the PLA/ESO/MC composites, with a recovery of crystallinity to levels comparable to pure PLA. As shown in Table 2, ESO tends to reduce crystallinity by increasing amorphous mobility, as observed in PLA/ESO, whereas MC promotes nucleation, as demonstrated in PLA/MC composites. The result is an intermediate degree of crystallinity, close to that of pure PLA. Therefore, this suggests a good interaction between the oxirane groups of ESO and the surface of the MC (see Figure 2), contributing to improved surface wettability, as seen in SEM analysis (see further below), and favoring an organization that promotes crystalline nucleation in PLA. This supports the positive mechanical performance of PLA/ESO/MC composites, showing a better balance between stiffness and flexibility than the base PLA/ESO system.

Optical Property by Transmittance (T%)

Optical transmittance in polymers refers to the material’s ability to allow light to pass through its structure. A material with high transmittance can be transparent or translucent, while one with low transmittance is considered opaque. In practical packaging applications, such as for food, it may be desirable to maintain visibility in certain situations, whereas in others, blocking light is necessary to protect light-sensitive products. The optical transmittance property for pure PLA, the PLA/ESO system, and the polymer composites is shown in Figure 10.

Figure 10.

UV–visible spectra for PLA, the PLA/ESO system, and the polymer composites. Films overlaying a symbol with pure PLA, PLA/ESO, PLM (5 phr), and PLA/ESO/MC (5 phr).

In Figure 10, within the 400–800 nm range, PLA exhibited a high level of transmittance between 72–83%, indicating a typically transparent behavior. This result aligns with findings reported by Scaffero et al.⁸⁸ and Morais et al..⁸⁹ The degree of crystallinity of PLA was 40.3%, as shown in the DSC analysis; however, it did not result in a significant loss of transparency. This suggests that the formation of small crystals, which scatter minimal light (typically smaller than visible light wavelengths, i.e., <100 nm), allows for high transmittance and, consequently, greater transparency.⁹⁰ When PLA was modified with ESO, a reduction in light transmittance was observed, decreasing to a range of 58–79%. Although the degree of crystallinity significantly dropped to 18.4%, this did not lead to an increase in transmittance compared to pure PLA. It appears that ESO induced the formation of a disordered structure in the PLA matrix, along with imperfect crystals, which contributed to increased optical scattering. Another possible explanation is the good interaction between PLA and ESO, as evidenced by FTIR analysis, which may have resulted in a phases’ mixture system with differing refractive indices, thus creating light-scattering points.

The PLA/MC and PLA/ESO/MC composites displayed an intermediate behavior between pure PLA and the PLA/ESO system, with transmittance ranging from 64% to 82%. Overall, the incorporation of MC led to a decrease in PLA’s transmittance, which can be attributed to light scattering caused by interfaces between phases with differing refractive indices. In this case, the MC dispersed in the PLA likely acted as light-scattering centers. However, the reduction was less pronounced compared to the PLA/ESO system. Additionally, the combined effect of ESO and MC in PLA contributed to improved transmittance compared to the PLA/ESO formulation. This suggests that ESO alone is more detrimental to PLA transparency, while the hybrid system containing both ESO and MC helps mitigate the negative impact on transmittance caused by ESO. The addition of MC to the PLA/ESO system promoted the formation of a better optical path, with less interference in light propagation, compared to the base PLA/ESO system. MC likely favored the breakup of ESO droplets dispersed in the PLA, reducing heterogeneity and increasing uniformity for light propagation, leading to greater transparency.

Optical Microscopy

Figure 11 shows the morphology obtained by optical microscopy (OM) for pure PLA, the PLA/ESO system, and the polymer composites with ESO and MC. The analysis was conducted using the transmission method to observe the distribution of MC in the PLA matrix, at 40x magnification. In Figure 11(a) and (b), both PLA and PLA/ESO exhibited a uniform morphology, with no visible dispersed phases. In contrast, the PLA/MC and PLA/ESO/MC composites displayed a heterogeneous morphology, with MC particles distributed throughout the PLA matrix. The OM analysis revealed significant differences between the composites containing or not containing epoxidized soybean oil (ESO), highlighting the influence of this plasticizer on the distribution of MC in the PLA matrix.

Figure 11.

Morphology obtained by MO in the transmission method, with 40x magnification, for: (a) PLA; (b) PLA/ESO; (c) PLA/MC (1 phr); (d) PLA/ESO/MC (1 phr); (e) PLA/MC (3 phr); (f) PLA/ESO/MC (3 phr); (g) PLA/MC (5 phr); (h) PLA/ESO/MC (5 phr).

In Figure 11(c), it was observed that the MC showed a relatively uniform distribution in the PLA matrix, although the presence of large agglomerates was noticeable. Increasing the MC concentration to 3 phr led to more agglomeration, indicating difficulties in dispersing the MC in pure PLA, as seen in Figure 11(e). The morphology of the PLA/MC (5 phr) composite became even more irregular, with a greater presence of large agglomerates. This can result in stress concentration points, which compromised mechanical properties such as elongation at break and impact strength, as demonstrated in mechanical tests. Overall, the PLA/MC composites tended to form agglomerates, especially with higher MC content, suggesting difficulty in dispersion and deagglomeration. In contrast, the addition of ESO to the PLA/MC composites promoted visible improvements in morphology, particularly by assisting in the deagglomeration of MC. In Figure 11(d), the PLA/ESO/MC (1 phr) composite exhibited a more homogeneous distribution of MC, with a significant reduction in agglomerates compared to PLA/MC (1 phr). For the PLA/ESO/MC (3 phr) composite, as shown in Figure 11(f), despite the increase in MC content, the MC distribution remains homogeneous, with fewer agglomerations compared to PLA/MC (3 phr). Even at high MC concentrations (5 phr), as seen in Figure 11(h), the presence of ESO maintains a more uniform morphology compared to the PLA/MC (5 phr) system. Interestingly, in Figure 11(h), a slight modification in the PLA texture can be observed, with the presence of ESO flow lines, reinforcing its role as a plasticizer. In general, the PLA/MC composites exhibited low distribution and the formation of large MC agglomerates. On the other hand, the PLA/ESO/MC composites showed better distribution and refinement in the MC particle size. Apparently, the presence of ESO in these composites acted as an effective plasticizer, contributing to the reduction of the MC agglomeration level.

Scanning Electron Microscope

Figure 12(a) and (b) shows the fracture behavior obtained by SEM of pure PLA and the PLA/ESO system, with a 1000x magnification. In Figure 12(a), it can be seen that pure PLA exhibited a predominantly smooth fracture surface, with flat unevenness and no ductility, indicating a brittle behavior. This morphology is typical of materials with low toughness, such as pure PLA, and is characterized by rapid crack propagation with little energy dissipation.⁹¹ The surface of the PLA/ESO system, on the other hand, showed greater roughness, cavities, and plastic deformation, suggesting a more ductile behavior. This morphological evidence indicates a greater capacity for energy dissipation before fracture, in line with the impact strength, and elongation at break results presented in mechanical tests.

Figure 12.

Evolution of the morphology obtained by SEM, with a magnification of 1000x, for: (a) PLA; (b) PLA/ESO.

The morphological analysis of the fracture surfaces of PLA/MC and PLA/ESO/MC composites was performed by SEM, with a magnification of 20,000x, as shown in Figure 13. The micrographs reveal significant differences in the interfacial interaction and distribution of microfibrillated cellulose (MC) in the PLA matrix, especially in the presence of ESO. Regarding the PLA/MC composites, large MC particles were observed in the PLA, which aligns with the behavior observed in optical microscopy. Additionally, MC showed poor dispersion and distribution in the PLA matrix, leading to the formation of aggregates. At the same time, large MC particles without interfacial adhesion to PLA were noted, suggesting high stress in the interfacial region, which did not favor impact strength and elongation at break, as observed previously. However, for small MC particles, the degree of interfacial adhesion was reasonable in the PLA matrix, possibly due to secondary interactions between MC hydroxyls and PLA carbonyls. Apparently, the MC aggregates cause strong interfacial stress with PLA, generating a poorly adhered interface, which may compromise the efficiency of mechanical stress transfer.

Figure 13.

Appearance of the fracture surface at 20,000x magnification, for: (a) PLA/MC (1 phr); (b) PLA/ESO/MC (1 phr); (c) PLA/MC (3 phr); (d) PLA/ESO/MC (3 phr); (e) PLA/MC (5 phr); (f) PLA/ESO/MC (5 phr).

The PLA/ESO/MC composites exhibit a fracture with better dispersion of MC in the PLA matrix, although the distribution remains irregular. For example, in Figure 13(b) ESO was able to improve the dispersion of MC in PLA; however, there was a region rich in MC, while other areas had a low concentration of MC. In Figure 13(d), the MC particles were refined, distributed, and dispersed in the PLA, and at the same time, elongated fibrils were present, suggesting a ductile fracture. For the PLA/ESO/MC (5 phr) composite, the refinement of MC was noticeable, with good distribution in the PLA matrix. Additionally, an important effect was the better degree of interfacial adhesion in the PLA/ESO/MC composites. The presence of ESO promotes better interfacial adhesion in PLA/MC, possibly due to its functional nature with the oxirane group, contributing to its ability to interact with both PLA and the hydroxyl groups of MC.

The comparison between the micrographs of PLA/MC and PLA/ESO/MC composites reveals that, even at higher MC concentrations (5 phr), the addition of ESO minimized the formation of MC aggregates in PLA. This finding favored a more homogeneous distribution of MC in the plasticized composites. This behavior is consistent with the mechanical results discussed previously, where the PLA/ESO/MC composites showed better properties, particularly in impact strength and elongation at break. The morphology observed by SEM reinforces the effectiveness of ESO in the interfacial modification of PLA/MC composites, promoting a more stable morphology, such as reduced aggregation and improvement in the interfacial region.

Final remarks on PLA/ESO/MC Composites

The proposed formulations exhibited excellent industrial feasibility since all composites were processed by melt blending using twin-screw extrusion followed by injection molding, techniques widely employed in commercial PLA packaging production. These processing routes ensure scalability and compatibility with existing manufacturing lines, without requiring solvents or additional purification steps. From a performance standpoint, the PLA/ESO/MC composites exhibited a balanced combination of stiffness and toughness, along with good transparency (64–80% in the 400–800 nm range), showing properties comparable to those of commercially available rigid PLA packaging. Thus, the studied formulations are not only compatible with conventional industrial PLA processing but also retain suitable mechanical and optical characteristics for sustainable packaging applications.

From an environmental standpoint, the addition of ESO and MC can also influence the degradation behavior of PLA. Microfibrillated cellulose (MC), due to its hydrophilic nature, can promote water diffusion into the PLA matrix, favoring hydrolysis initiation at the interfaces and contributing to faster degradation under composting proper conditions.^92,93 Epoxidized soybean oil (ESO), derived from renewable vegetable sources, can accelerate the biodegradation of biopolymers, as reported by Intranuwong et al.⁹⁴ and Goswami et al..⁹⁵ This is because the oxirane groups in ESO can interact with PLA terminal groups, increasing molecular mobility and improving moisture diffusion. However, when used simultaneously, ESO and MC can establish a more balanced structure between mechanical performance and controlled biodegradation. This is advantageous for applications that require environmentally friendly materials with defined lifetime and end-of-life behavior. The addition of ESO (natural plasticizer) and MC (natural reinforcement) to PLA contributes significantly to the environmental performance, particularly by reducing the need for synthetic additives and enabling the production of composites with a greater ecological footprint. Given this, the PLA/ESO/MC composites developed in this work combine mechanical performance with an enhanced potential for biodegradation, reinforcing their alignment with sustainability principles.⁹⁶

Conclusion

PLA-based biocomposites reinforced with microfibrillated cellulose (MC) were processed and molded in the molten state, using epoxidized soybean oil (ESO) as a plasticizer. The incorporation of MC and ESO into the PLA matrix enables the development of sustainable and flexible composites, with mechanical properties balanced between toughness and stiffness. When used alone, MC provided slight improvements in PLA stiffness; however, it also led to agglomeration, poor dispersion, and weak interfacial adhesion, which negatively affected impact strength, elongation at break, and tensile strength. The simultaneous presence of MC and ESO in the PLA/ESO/MC composites resulted in materials with more balanced properties, showing improved distribution and dispersion of the cellulose microfibers, enhanced interfacial adhesion, and improved mechanical performance. Morphological analysis, FTIR, impact strength, tensile testing, and DSC confirmed the role of ESO as a green plasticizer, promoting interactions between PLA and MC while reducing interfacial stress. These findings highlight the potential of hybrid composites as an alternative for technical applications requiring balanced properties. From an environmental perspective, the PLA/ESO/MC composites align with the growing demand for green materials, offering reduced environmental impact and supporting the bioeconomy. The use of renewable and biodegradable raw materials under appropriate conditions, such as PLA, MC, and ESO, helps decrease reliance on petrochemical inputs and minimizes waste generation. Notably, the composites demonstrated a favorable balance between impact strength and elastic modulus while maintaining high transparency, indicating their suitability possible for packaging applications.

Overall, PLA/ESO/MC composites show potential for reducing CO₂ emissions, mitigating waste accumulation, and lessening the impact on natural ecosystems when compared to conventional polymer composites. The use of bio-based inputs, as in the PLA/ESO/MC system, decreases dependence on fossil resources and may facilitate the transition to eco-materials, especially under industrial conditions. Accordingly, these composites represent a more environmentally responsible alternative for applications in packaging, lightweight engineering materials, and disposable devices.

Footnotes

Acknowledgements

The authors would like to thank the Federal University of Campina Grande (UFCG) for its institutional support and the excellent infrastructure of the polymer processing laboratory. They also thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for granting the research scholarship, which enabled them to dedicate time to the development of this study. Special thanks go to the Research Support Fundação de Apoio à Pesquisa do Estado da Paraíba (FAPESQ) for the funding that made possible the acquisition of essential equipment, such as the tensile testing machine and the Fourier Transform Infrared spectrometer (FTIR), critical resources for the characterization of the analyzed materials. The authors also thank GOTALUBE additives for donating the epoxidized soybean oil.

Declaration of conflicting interests

The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: There are no competing or potentially conflicting interests among the authors related to the research, authorship, or publication of this manuscript. Furthermore, responsibility for the content of the article is equally shared by all authors.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study is supported by CoordenaÃ§Ã£o de AperfeiÃ§oamento de Pessoal de NÃ‐vel Superior; 001; Conselho Nacional de Desenvolvimento CientÃ‐fico e TecnolÃ3gico; 350025/2023-1; 303426/2021-7; 312014/2020

ORCID iD

Carlos Bruno Barreto Luna

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Flexible and transparent bio-based composites of Poly(lactic acid) and microfibrillated cellulose (MC): Mechanical,thermal,optical,thermomechanical properties and morphological evolution

Abstract

Keywords

Introduction

Materials and Methods

Materials

Composites Processing

Composite Characterizations

Results and Discussion

Fourier Transform Infrared Spectroscopy

Impact Strength

Tensile Properties

Shore D Hardness

Heat Deflection Temperature (HDT)

Differential Scanning Calorimetry (DSC)

Optical Property by Transmittance (T%)

Optical Microscopy

Scanning Electron Microscope

Final remarks on PLA/ESO/MC Composites

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References