Investigation of physico-mechanical, thermal, morphological, and antibacterial effects of bio-based epoxidized soybean oil plasticizer on PLA-ZnO nanocomposites as flexible food packaging

2.1 Parallel or Voigt model

Using the mixing law and the isostrain condition, the Parallel or Voigt model determines an upper bound for the tensile modulus of a two-component system (Chow, 1980):

In this equation, $E_{\mathit{II}}$ , $E_m$ , and $E_d$ express the modulus of the blend (or its nanocomposite), matrix, and dispersed phase, respectively. Furthermore, ${\phi }_m$ and ${\phi }_d$ represent the matrix and dispersed phase volume fractions, respectively.

2.2 Series or Reuss model

Unlike the parallel model, the series or Reuss model can predict the minimum value related to the modulus of a blend nanocomposite. This model can be achieved by utilizing the equation provided below (Ahmed and Jones, 1990):

2.3 Guth model

This model can be employed for such blend nanocomposites, considering the ZnO aspect ratio represented by $\alpha$ (Guth, 1945):

2.4 Halpin-Tsai model

Herman and Hill created this model to predict the modulus of unidirectional nanocomposite sheets (Fornes and Paul, 2003; Hill, 1964):

Where $\eta =(E_f/E_m-1)/(E_f/E_m+2\alpha )$ is the shape parameter associated with the geometric features of the filler.

2.5 Hui-Shia model

This model presupposes the existence of excellent interfacial bonding between the matrix and reinforcements (Dayma and Satapathy, 2010):

Where $g={\alpha }^{-1}\pi /2$ .

2.6 Paul model

This model is controlled by the macroscopically uniform stress in the constituents of the nanocomposites (Paul, 1959):

2.7 Maxwell model

The Maxwell model was developed mainly for nanocomposites with uniformly distributed fillers. The expression of this model is as follows (Ohama, 1987):

2.8 Nielsen model

Nielsen modified the Halpin-Tsai theoretical model, which is shown below:

Where $\chi =2.5{(\alpha )}^{0.645}-1$ under the non-slippage condition (Rao, 2007). The parameter ψ relates to the maximum packing volume fraction ( ${\phi }_M$ ) of filler according to the following equation:

2.9 Pukanszky model

This model was formulated to calculate the tensile strength of nanocomposites in the following manner:

The symbols σ_yc and σ_ym represent the nanocomposite and matrix yield strengths, respectively. The parameter β is an empirical value that indicates the degree of interaction between the matrix and filler, specifically in terms of how efficiently the load is transferred between them (Dayma and Satapathy, 2010).

2.10 Nicolais and Narkis model

Nicolais and Narkis introduced the following formula for forecasting the tensile strength of nanocomposites for cubic matrix reinforced with uniformly distributed spherical particles (Narkis and Nicolais, 1971):

3.1 Materials and preparation

PLA (grade Ingeo 4043D), with a 1.21 g/cm³ density, melting point of 145–160 °C, and molecular weight of 1,000,000 g/mol, was obtained from Nature Works Company, USA. Epoxidized soybean oil (ESO, ≥ 99.5%) containing an average of > 6 epoxy groups per molecule and a chemical formula of C₅₇H₉₈O₁₂ was obtained from Aladdin Industrial Corporation (Shanghai, China). ZnO powder, with an average particle size of <100 nm, was purchased from Sigma Aldrich (Germany).

Polylactic acid granules and zinc oxide nanoparticles were placed in an oven at 40 °C for 24h to dehydrate completely. In the first step, PLA, ESO, and ZnO with specific concentrations (Table 1) were blended via mechanical stirring in a well-washed beaker glass. The resulting mixture was kept in a closed container at room temperature for 24h. Afterward, each sample container was put in a laboratory's internal batch mixer with a capacity of 70 g at a temperature of 140 °C and 60 rpm for 15 min to ensure that the nanoparticles were evenly distributed throughout the polymer matrix. Finally, to achieve nanocomposite films with a thickness of 2 cm, all the samples were transformed into uniform film sheets using compression molding at a temperature of 140 °C and a pressure of 10 MPa. Each film follows the same procedures illustrated in Fig. 1.

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Fig. 1

Specimen preparation procedure.

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3.2 Characterization

The chemical structure of each sample was investigated using Fourier Transform Infrared Spectroscopy (Bruker, Equinox 55, USA).

Using scanning electron microscopy (SEM) (TESCAN®, Czech Republic), the fracture surface of the samples was imaged to assess the morphological features. The localization of ZnO nanoparticles and their dispersion state in nanocomposite samples were investigated using energy-dispersive X-ray spectroscopy (EDS) from Oxford INCA®, UK.

The crystalline structures of nanocomposites were studied via X-ray diffraction analysis (XRD) through a Wide-Angle X-ray Scattering (WAXS) device (FK60-04 D5000, Siemens Co., Germany) operating at 40 kV and 20 mA. Cu Kα radiation (λ = 1.54 nm) was utilized with a detector that moved through 2θ = 5 to 40° and 2θ = 0.02°/sec. Moreover, the crystallinity index (CI) of the samples is calculated according to the following equation (Arshian et al., 2023):

A_c and A_a represent the crystalline and amorphous areas, respectively. These data are generated using the Origin software.

Under a nitrogen environment, DSC (Mettler, Switzerland) was used to test the thermal characteristics of neat PLA and its blends. The specimens, which weighed around 5 mg before drying, were subjected to testing. Next, the specimens were rapidly heated from 0 to 190 °C at 10 °C per minute. After 5 min of heating, they were cooled to −50 °C at 10 °C per minute and kept at that temperature for 5 min to remove any thermal history. The second heating curve was used to measure the PLA's T_g, T_cc, ΔH_cc, T_m, and ΔH_m, which are the glass transition temperature, cold crystallization temperature, cold crystallization enthalpy, melting temperature, and melting enthalpy, respectively. The following is a calculation of the sample's PLA crystallinity (X_c, PLA) (Chen et al., 2015):

Where ΔH_m and ΔH_cc are the observed melting and cold crystallization enthalpy of PLA, respectively, and ${\mathrm{\Delta }H}_m^0$ is the theoretical melting enthalpy of 100% crystalline PLA (=93 J/g) (Xie et al., 2022).

The tensile properties of the rectangular-shaped specimens with a dimension of 5×3 cm were measured using a SANTAM-MTS20, Iran machine, according to ASTM D882-12. The testing speed was 10 mm/min, and a load cell of 6N was used at room temperature. Three separate tests took place, and the average results were recorded.

The contact angle of water and di-iodomethane was examined using a contact angle device (KRUSS G10, Germany) that employed the sessile drop method. Each sample underwent the test three times before reporting the average. Each amorphous film was tested by depositing five 4μL droplets of the testing liquid onto its surface. The picture of the droplet was taken 5 s after deposition to ensure that the liquid had reached thermodynamic equilibrium with the solid surface. Then, the samples' total surface free energy was determined by applying the Owens-Wendt technique (Żenkiewicz et al., 2009).

The nanocomposites' antibacterial properties were assessed using the disk diffusion method. Following impregnation with bacteria, the sample disks were positioned on the surface of Mueller-Hinton agar. After 24h of incubation at 37 °C, the inhibitory zone was examined on the plates.

The TGA instrument (Mettler Toledo) was used to conduct thermal stability studies in a nitrogen atmosphere at a heating rate of 10 °C/min.

4.1 FTIR

Fig. 2 displays the FTIR spectra of PLA nanocomposites. In Fig. 2a, the surface hydroxyl groups of neat PLA are visible at 3450 cm⁻¹. The absorbances at 2946 and 2992 cm⁻¹ indicate the presence of methyl groups in the PLA side chain, corresponding to the stretching vibration of C–H bonds (Jafari et al., 2022). The absorption band at 1748 cm⁻¹ is also mainly due to the stretching of the C=O bond in the ester group. Moreover, the peaks at 1451 and 1370 cm⁻¹ are caused by symmetric and asymmetric C–H stretching vibrations, whereas the absorptions at 1045–1267 cm⁻¹ are related to the C-O stretching vibration (Chen and Zhen, 2021). For neat ZnO nanoparticles (Fig. 2c), the peak observed between 480 and 420 cm⁻¹ corresponds to the stretching mode of Zn-O (PP, 2020). As is clear, the addition of ZnO did not alter any of the PLA peaks, except for a slight decrease in the intensity of the peak at 3450 cm⁻¹ (Fig. 2b). This reduction indicates the formation of secondary forces, such as weak hydrogen bonds, between PLA and ZnO NPs. Fig. 2e shows that ESO has a distinct epoxy functional group structure, indicated by two peaks at 833 and 726 cm⁻¹ (Litke et al., 2023). In addition, the peaks at 1157 and 1738 cm⁻¹ were due to the C-O stretching and C=O (carbonyl) stretching of ester groups, respectively (Kumar et al., 2017). The displacement of these peaks in the blended nanocomposites demonstrates the potential interactions between components. Similarly, the peaks at around 2923 and 2840 cm⁻¹, which belong to the stretching vibration of aliphatic C–H stretching in ESO, are observed in another position in the PLA-ESO-ZnO (Fig. 2d), indicating the likelihood of hydrogen bonding with the ESO's oxirane group, the hydroxyl group of PLA, and the ZnO terminal (Han et al., 2020).

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Fig. 2

FTIR spectra of the neat PLA, ZnO, PLA-ZnO, and PLA-ESO-ZnO.

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4.2 SEM-EDS

Figs. 3-7 depict the cryo-fractured surfaces of neat PLA, PLA-ZnO, and PLA-ESO-ZnO blend nanocomposites as detected by SEM-EDS. Based on the observations, the fractured surface of pure PLA exhibited a smooth and consistent appearance (Fig. 3). However, adding 3 wt% of ZnO NPs resulted in roughness on the PLA surface, suggesting the presence of a more complex fracture propagation route while maintaining a uniform distribution, except for specific clusters of the ZnO NPs, as shown by white dots in Fig. 4b) (Shankar et al., 2018). It is well known that good inter-component interactions and the formation of secondary forces, such as hydrogen bonds between the PLA polymer and ZnO, cause good dispersion of the nanoparticles (Kim et al., 2019; Tan et al., 2023; Yu et al., 2021).

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Fig. 3

SEM image of the neat PLA.

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Fig. 4

a: SEM and b: ESD images of the PLA containing 3 wt% of ZnO.

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Fig. 5

SEM image of the 95–5 blend containing 3 wt% of ZnO.

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Fig. 6

SEM-EDS images of the a-d: 90–10-1, b-e: 90–10-3, and c-f: 90–10-5.

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Fig. 7

SEM and EDS images of the 80–20 blend containing 3 wt% of ZnO.

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Upon blending with 5 wt% of ESO, the blended nanocomposites exhibited spherical holes on their cracked surfaces (Fig. 5), showing the matrix-droplet structure (Silverajah et al., 2012). In the case of the 90–10-5 blend nanocomposites, there was a significant reduction in the diameter and number of holes. These results validate that adding more ZnO NPs enhanced the compatibility between PLA and ESO, indicating the presence of robust contact forces between PLA, ESO, and ZnO. In other words, ZnO has prevented the coalescence phenomenon and enhanced the characteristics at the interface (Díez-Pascual and Diez-Vicente, 2014; Rokbani and Ajji, 2018; Tayouri et al., 2022). Moreover, ZnO increases the viscosity of the PLA-ESO system, and at the same shear rate, the increased shear stress can cause smaller ESO droplets within the matrix (Jalalifar et al., 2020). Fig. 7 shows that the morphological structure of the blend nanocomposite is greatly affected by the addition of 20 wt% ESO. PLA underwent morphological modifications because of the increased ESO (as a low-molecular-weight phase) concentration, transitioning from brittle to ductile fracture. A similar discovery can be found in the literature (Raghunath et al., 2018).

4.3 XRD results

The microstructure and crystallinity properties of sample films were analyzed using XRD patterns in Fig. 8. Fig. 8a displays a broad diffraction peak, from 2θ=15.0° to 20.0°, which arises from the inherent amorphous structure of the PLA (Arshian et al., 2023; Paydayesh et al., 2022). Upon mixing with ESO, no bulge shifting observation indicates no changes in the corresponding interlayer spacing in PLA structure (Fig. 8b and 8c) (Giita Silverajah et al., 2012). Besides, no discernible crystalline peaks occur, accounting for the substantial interactions between the PLA O–H group and the epoxy group of ESO through hydrogen bonding (Raghunath et al., 2018; Jian et al., 2017). Fig. 9 depicts the most feasible interactions that dominate the system.

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Fig. 8

XRD pattern of the PLA samples.

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Fig. 9

The possible interactions between the components of the PLA-ESO-ZnO blend nanocomposites.

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The 90–10-1 nanocomposite film shows a clear crystallization peak near 2θ of approximately 16.5°. This peak corresponds to the diffraction of the (2 0 0)/(1 1 0) planes of the typical orthorhombic PLA crystal, similar to previous findings in the literature (Pantani et al., 2013; Wu and Wu, 2006). In addition, the planes (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1) of the hexagonal crystal of ZnO NPs are corresponded to the distinctive peaks at 2θ = 31.6, 34.4, 36.2, 47.4, 56.7, 62.7, 66.2, 67.9, and 69.0°, respectively (Agrawal et al., 2010). As confirmed, an increase in the amount of ZnO leads to a decrease in the intensities of the crystalline peak at a 2θ of about 16.5°. However, the presence of the ZnO NPs enhanced the total crystallinity of the blended nanocomposites from 9.5 for 90–10-1 to 11.3% for 90–10-5. The comparison of Fig. 8d and 8e indicates that a lower concentration of ZnO (as a nucleation agent) can lead to improved arrangement of PLA chains. This ordered structure of PLA occurs because the interactions between PLA and ESO chains are interrupted by weak hydrogen bonds formed between the ZnO and the PLA polymer matrix (Tan et al., 2023). Nevertheless, the PLA chain will be entrapped at the higher level of ZnO, leading to lower intensities of the PLA characteristic peaks (Shankar et al., 2018; Murariu et al., 2011; Jayaramudu et al., 2014).

4.4 DSC results

DSC was used to investigate the thermal properties of neat PLA and PLA-ESO-ZnO blend nanocomposites (Fig. 10 and Table 2). It is observed that the addition of ESO reduced the T_g values of blend nanocomposites. The plasticizer increased chain mobility by replacing the interaction between the original PLA chains, leading to a lower T_g (Wang et al., 2021; Ebadi-Dehaghani et al., 2015; Lee et al., 2003). Moreover, the T_g of PLA-ESO blends decreased when 5 wt% of ZnO was added to the mixtures. This behavior can be associated with the catalytic effect of the ZnO NPs on the PLA chain's degrading reaction, consistent with the findings in the literature (Anžlovar et al., 2018; Lizundia et al., 2019).

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Fig. 10

DSC curves of the samples.

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As evident, the ZnO did not positively affect the crystallization of the 90–10 blend. Although NPs can increase crystallization rates as nucleating agents, they can also slow down spherulite development by restricting the order of polymer chain segments and raising the T_cc value (Kim et al., 2019; Quiles-Carrillo et al., 2018). The 90–1-5 has lower values of ΔH_cc than 90–10-1, which might indicate less conversion of amorphous regions into crystalline domains (Agbakoba et al., 2023). On the other hand, 80–20-5 blend nanocomposites had higher crystallinity and a lower ΔH_cc than other samples. The crystallite's quality is responsible for such contrary results (Kaczor et al., 2022). PLA-ESO has a comparatively low T_g, accompanied by a reduction in T_cc as ESO increases in the blended nanocomposites (Chu et al., 2017). As a result, the spherulitic growth rate of PLA-ESO-ZnO improves with decreasing PLA content and increasing the ZnO concentration (Han et al., 2017; Farrag et al., 2022).

As indicated, the T_m of PLA marginally decreases with the addition of ESO and ZnO (Table 2). In simple terms, the reduced melting point results from the increased mobility of the polymer chains facilitated by plasticizers and nanofillers (Chapple et al., 2013; Diep et al., 2019). This section showed that increasing NPs to higher levels (about 5 wt%) had a better nucleation effect on the 80–20 blend nanocomposites than the 90–10 matrix. These specific results are highly dependent on the quality of the crystals and morphological features. Fig. 11 illustrates the intermolecular interactions of the components with each other, in which ZnO NPs play the role of compatibilizers and nucleation agents.

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Fig. 11

Possible interaction mechanisms of PLA-ESO-ZnO blend nanocomposites.

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4.5 Tensile properties

Tensile tests are used to study the influence of ESO and ZnO on PLA’s mechanical properties, and the resulting data are shown in Table 3 and Figs. 12–15. The tensile strength and modulus of PLA decreased with the addition of ESO, demonstrating its plasticizer effect (Fig. 12). Generally, plasticizers disrupt the intermolecular interactions between polymer chains, resulting in lower chain cohesion and strength (Darie-Niţă et al., 2016). When ESO and PLA are blended, they can become partially miscible, dissolving the connection sites and weakening the forces between the ESO fatty chains. Consequently, phase separation for ESO would be improved (Khoo and Chow, 2017). In this regard, we can see that the blend with 10 wt% ESO was more rigid in structure and had a different effect on lowering the tensile modulus of the PLA compared to those with 5 and 20 wt% plasticizer. As can be seen, the blend 90–10 significantly increased (141.4%) in elongation-at-break compared to neat PLA, indicating that ESO improved the flexibility and mobility of PLA chains. At the initial stage of ESO loading, PLA's elongation-at-break increased dramatically, then decreased continuously. This can be attributed to poor phase separation of ESO and their weak interactions with the PLA matrix when ESO content exceeded 10 wt% (Zhao et al., 2012).Fig. 16..

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Fig. 12

Effect of ESO on the mechanical properties of the neat PLA.

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Fig. 13

Effect of ZnO on the mechanical properties of the neat PLA.

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Fig. 14

Effect of ZnO content on the 90–10 blend nanocomposites.

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Fig. 15

Effect of ZnO content on the 80–20 blend nanocomposites.

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Fig. 16

Comparison between the experimental and theoretical tensile modulus of (a) 90–10-ZnO and (b) 80–20-ZnO nanocomposites.

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According to Fig. 13, introducing ZnO at 1 wt% decreased the tensile strength and modulus of the PLA. However, As the amount of ZnO NPs increased, the tensile modulus improved, while the tensile strength peaked at 3 wt% of the NPs due to enhanced filler-matrix interactions (Ahmed et al., 2016; Tayouri et al., 2023). As expected, when ZnO was added to PLA film, the elongation-at-break value dropped from 16% for neat PLA to 3.4% for PLA containing 5 wt% ZnO. The chain entanglement effect causes this phenomenon.

We witness that the tensile strength and modulus of the blended nanocomposites improved as the rigid ZnO content increased, creating a better loading and stiffening physical network. Besides, adding 5 wt% of ZnO nanoparticles in PLA-ESO-ZnO films enhanced ductility since ZnO NPs, with their high aspect ratio, were uniformly dispersed in the matrix and entrap the polymeric chains (Jayaramudu et al., 2014; Hocker et al., 2018). As a result, the packing mode of the ordered PLA structure was reduced, and elongation at break will increase. In addition, there is speculation that this behavior is likely linked to the polylactic acid matrix degradation during melt mixing, which could be caused by the catalytic action of ZnO (Quiles-Carrillo et al., 2018; Keshavarzi et al., 2019). Another contributing factor might be the enhancement of ZnO content, which improved the morphology of ESO within the PLA matrix and increased chain mobility. Ultimately, due to fine structural features, the 90–10-5 sample shows the highest strength, modulus, and elongation-at-break (about 35%) properties among all samples (Mauck et al., 2016).

The tensile modulus of the PLA-ESO-ZnO blend nanocomposites was determined using experimental data and Equations (1)–(11). The obtained findings were compared against each other, revealing that the Maxwell, Paul, and Hui-Shia models offer more accurate predictions for the 90–10-ZnO. Even though the Maxwell equation accurately predicts the experimental findings, the Paul and Hui-Shia models yield better when the nanoparticle content is higher. As for the 80–20-ZnO blend, the best prediction can be derived from the upper model for lower nanoparticle content, whereas the Paul and Halpin-Tsai models are suitable for higher loadings.

As illustrated in Fig. 17, the tensile strengths of the nanocomposites were determined using the models proposed by Pukanszky (Equation (12) and Nikolais and Narkis (Equations (13). The 90–10-ZnO and 80–20-ZnO nanocomposites' empirical β parameters were found by comparing the experimental data with the Pukanszky model, assuming that σ_ym is the tensile strength of the blend nanocomposites. The β parameter value is affected by several variables, including the nanofiller's average size, the interface strength between the polymer and nanofiller, and the average contact area between the components. These variables impact the nanocomposite's load-bearing ability, and the higher value of β will cause the curve associated with the Pukanszky equation to move to a larger σ_yc. Compared to the Nikolais and Narkis model, the Pukanszky model yields more precise predictions of the tensile strengths.

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Fig. 17

Comparison between the experimental data and theoretical tensile strengths of (a) 90–10-ZnO and (b) 80–20-ZnO nanocomposites.

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4.6 Contact angle and antibacterial properties

The Owens-Wendt method was used to calculate the total surface-free energies of the nanocomposites. Accordingly, the contact angles of each sample were measured against polar solvent (water) and nonpolar solvent (diiodomethane) (Thanakkasaranee et al., 2018). Table 4 summarizes the resulting data. The neat PLA sheet has a water contact angle of 63.969°, while adding 3 wt% of ZnO increased the water contact angle by 1.9°, suggesting an enhancement in the PLA's hydrophobicity and surface roughness (Li et al., 2021; Yang et al., 2012; Mora-Fonz et al., 2017). This observation can be attributed to the interactions between ZnO nanoparticles and the hydrophilic groups on PLA chains (Ismail et al., 2022). The results also show that adding plasticizer to the blend nanocomposites enhanced their hydrophilicity and increased total surface energy due to polar groups such as oxirane in ESO (Aydın et al., 2017; Meadows et al., 2018). Fig. 18 indicates that the 90–10-3 has comparatively lower contact angles than 80–20-3 blend nanocomposites.

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Fig. 18

Contact angle analysis of (a) and (b) for water and (c) and (d) for diiodomethane of 90–10-3 and 80–20-3 blend nanocomposites, respectively.

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The effect of increasing ZnO content on the contact angle measurements of the 90–10 blend is illustrated in Fig. 19. The figure indicates that adding ZnO NPs to the blend can increase the contact angles of water and diiodomethane and reduce both polar and dispersive surface energies due to the low molecular interactions of non-London (dipole–dipole) and London dispersion forces, respectively (Zhuang and Hansen, 2009). It can be stated that the crystalline domains caused by the presence of the ZnO NPs can block the access of water molecules to the hydrophilic groups of the polymer components (Arshian et al., 2023; Hosseinvand et al., 2022).

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Fig. 19

Contact angle analysis of the 90–10 blend containing different ZnO content.

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The PLA materials' antibacterial abilities against Gram-positive (S. aureus) and Gram-negative (E. coli) were studied using the disk diffusion method. The obtained results are shown in Fig. 20 and Table 5. As can be seen, PLA materials had no antibacterial activity against E. coli, indicating the structural distinction of the bacteria's cell walls, which was emphasized in the literature (Espitia et al., 2012; Mizielińska et al., 2018). Another difference between E. coli and S. aureus is how much negative charge is on their membranes. S. aureus with less negative charge interacts more strongly with the negatively charged ROS (reactive oxygen species) in the ZnO NPs' antibacterial mechanism (Zhang et al., 2017). As depicted, the ESO had no significant effect on the antibacterial properties of the PLA. The 90–10-5 blend nanocomposite showed a prominent antibacterial circle, revealing the highest inhibitory ability of about 6 mm against S. aureus. Indeed, zinc ions impede bacterial development and damage the microbial surface (Shankar and Rhim, 2019; Restrepo et al., 2019). These findings suggest that, in terms of food preservation, the produced materials are appropriate for use in food packaging applications.

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Fig. 20

Disk diffusion test against (a) Staphylococcus aureus and (b) Escherichia coli.

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4.8 TGA results

TGA was performed to determine the thermal stability of the 90–10 blend nanocomposites (as a candidate sample) in Fig. 21, and Table 6 presents the results comparing the thermal parameters, such as temperature at the initial stage (T_i) and maximum degradation temperature (T_max). As the temperature increased, the neat PLA exhibited a T_i of 265.9 °C and a T_max of 353.1 °C, which increased to 275.8 and 358.3 °C, respectively, once ESO was blended with the PLA matrix. In addition to the stabilizer effect of the ESO (Burkov et al., 2021), the interfacial interaction between PLA and ESO can lead to lower thermal decomposition of PLA’s molecular chain (Zhao et al., 2021; Mahmud et al., 2019).

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Fig. 21

TGA curves of PLA blend nanocomposites.

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It is shown that the thermal degradation of the 90–10-1 blend nanocomposites started at higher temperatures than in neat polymer blends. This indicates that ZnO NP layers make the polymers more thermally stable by creating a more tortuous path within the polymeric matrix and blocking the movement of heat and gases (Pantani et al., 2013). Moreover, the presence of the ZnO improved the compatibility of the PLA and ESO, making the blends more resilient against thermal decomposition (Lim et al., 2019). However, adding 5 wt% of ZnO led to lower thermal stability of the blend nanocomposites. The reduction in thermal stability can be related to the catalytic activity of ZnO on the plyometric matrix at high temperatures. Other studies have also reported a similar observation, showing that ZnO NPs in PLA lead to less thermally stable materials. Among the investigated samples, the PLA-ESO-ZnO (90–10-1) blend exhibited the maximum values of T_i (365.3 °C) and T_max (425.1 °C), respectively.