Development of antibacterial bioplastic from PVA and corn starch for packaging applications

2.1. Materials

2.2. Bioplastic fabrication

Bioplastic films were fabricated by solution casting, as shown in Figure 1. The compositions of the five samples with varying PVA-to-starch ratios are detailed in Table 1. For a representative sample (P6), the fabrication procedure was as follows: First, 6 g of PVA was dissolved in 70 ml of distilled water at 85°C with magnetic stirring at 500 rpm for 1 h to obtain a clear, viscous solution. Separately, corn starch (1 g for P5, see Table 1) was dispersed in 10 ml of cold distilled water under vigorous stirring for 30 min. After the PVA solution cooled down to approximately 50°C, the starch dispersion was added gradually. Subsequently, glycerol (10 ml) and vinegar (10 ml) were added sequentially under continuous stirring. Finally, the antimicrobial agent was added to the film-forming solution to achieve a final concentration of 1% w/v (of the commercial dispersion), followed by stirring for an additional 30 min to ensure homogeneity. The final homogeneous solution was poured onto a clean, leveled 15 cm x 15 cm acrylic sheet. The casting volume was calculated based on the total solids content and casting area to target a consistent dry film thickness of approximately 0.3 mm. The cast solutions were dried in a forced-air convection oven at 45°C for 48 h. After drying, the films were carefully peeled off and stored in a desiccator at room temperature for further characterization. A control film with identical base composition (PVA, corn starch, glycerol, vinegar, and water) but without any silver-based antimicrobial agent was prepared.OPEN IN VIEWER

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Table 1.

Composition of film-forming solutions used for sample preparation.

Sample code	PVA (g)	Corn starch (g)	Glycerol (mL)	Vinegar (mL)	Water (mL)	Antibacterial agent (w/v)
P2	2	4	10	10	70	1%
P3	3	3	10	10	70	1%
P4	4	2	10	10	70	1%
P5	5	1	10	10	70	1%
P6	6	0	10	10	70	1%

2.3. Characterization

3.1. Surface morphology

Optical microscopy analysis, together with surface plot and profile assessments, provides clear insight into how the PVA-to-corn starch ratio governs the surface morphology of solution-cast bioplastic films (Figure 2). Qualitatively, P3, with a 3% PVA and 3 % starch ratio, shows a relatively uniform surface (Figure 2 d₁, d₂, d₃), which may be attributed to improved phase compatibility. However, quantitative roughness analysis from the line profiles reveals a more nuanced trend. The calculated average roughness (Ra) in relative units indicates that surfaces become progressively smoother as PVA content increases. The starch-rich P2 film (2% PVA, 4% starch) exhibits the highest Ra value of 11.95, corresponding to its highly irregular surface with prominent lumps (Figure 2 e₁, e₂, e₃), indicating poor polymer-starch miscibility and inadequate inter-molecular interaction. PVA, being a hydrophilic synthetic polymer with excellent film-forming properties, facilitates the encapsulation of starch granules and contributes to matrix continuity. Consequently, in samples with higher PVA content, such as P4 and P6, the surfaces appear significantly smoother, with Ra values decreasing to 11.61 and 8.94, respectively. This quantitative trend confirms that improved polymer chain entanglement and compatibility at higher PVA concentrations enhance dispersion, reduce granule agglomeration, and minimize surface defects. The surface plots adjacent to each plot profile offer a three-dimensional perspective of these variations, visually corroborating that a PVA-dominated matrix stabilizes the film structure and controls starch particle distribution. The gradual improvement in surface uniformity with increasing PVA content is attributed to sufficient film-forming polymer available to cover and bind the granular starch phase, preventing exposed domains and micro-structural defects. These morphological characteristics, now quantified, directly influence the films’ functional performance, aligning with the observed thermal and mechanical behaviors and underscoring the critical role of composition in determining the physical integrity of the biodegradable matrix (Figure 3).OPEN IN VIEWER

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Figure 3.

SEM images of PVA/corn starch films (P2-P6) illustrate the effect of starch content on the surface.

3.2. SEM analysis

The micro-structure of the PVA/corn starch films was observed in detail using SEM. A relatively smooth and homogeneous surface was observed in the P6 sample, indicating the formation of a continuous PVA matrix. No visible particles or phase separation were observed, which is consistent with its high tensile strength. In the P5 sample, the addition of a small amount of corn starch resulted in the formation of small circular domains, which were evenly distributed and maintained the matrix’s continuity. This structure is consistent with the high elongation at break observed for this formulation. In the P4 and P3 samples, the particle size and non-uniformity increased with the increase in the corn starch content. In some cases, agglomeration and phase separation were observed, disrupting the matrix’s continuity and reducing mechanical strength. The P2 sample with the highest corn starch content exhibits large agglomerates, voids, and irregular structures, indicating a weak and brittle micro-structure. Overall, the SEM results demonstrate that corn starch acts as a discrete filler phase, disrupting the continuous PVA network. Consequently, the integrity of the PVA matrix is a key determinant of the mechanical performance of this composite system.

3.3. FTIR analysis

The FTIR spectra of the individual components and the composite bioplastic film (P6 formulation) are presented in Figure 4, providing insights into the functional groups present and the molecular interactions within the composite matrix. The spectrum of the bioplastic film is a superposition of its constituents, confirming their successful incorporation.OPEN IN VIEWER

The primary components exhibited characteristic peaks: corn starch showed key bands at 1024 cm^-1 (C-O stretching of glycosidic linkages) and a minor feature at 2350 cm^-1. Glycerol displayed a broad band at 3300 cm^-1 (O-H stretching) and peaks at 1024 cm^-1 (C-O) and 2920 cm^-1 (C-H stretching). PVA presented a strong, broad O-H stretching band at 3300 cm^-1 and a C-O stretching band at 1024 cm^-1. Vinegar (acetic acid) contributed peaks associated with carboxylic acid groups, including O-H stretching (3300 cm^-1) and C=O stretching (1730 cm^-1). The commercial silver-based antimicrobial agent showed a complex spectrum with a distinct peak at 535 cm^-1 (potentially Ag-O or other metal-oxygen vibrations), confirming its organic-inorganic hybrid nature.

The spectrum of the composite bioplastic film contained all major bands of its constituents, indicating successful blending. However, critical spectral changes suggest physicochemical interactions rather than mere physical mixing. The broad O-H stretching band (3300 cm^-1) in the composite was notably broader and showed a subtle shift compared to pure PVA, which is indicative of an enhanced and altered hydrogen bonding network between the hydroxyl groups of PVA, starch, and glycerol. This extensive hydrogen bonding is a key factor in promoting miscibility and improving the mechanical cohesion of the film. Furthermore, the appearance of a new, distinct peak at 1250 cm^-1 in the bioplastic spectrum, absent in the raw materials, suggests the possible formation of new linkages, potentially due to acid-mediated interactions. This peak can be attributed to C-O-C stretching vibrations of ester or ether bonds, likely resulting from an acid-catalyzed cross-linking reaction between the hydroxyl groups of PVA/starch and the acetic acid from vinegar during film processing. The presence of the 535 cm^-1 peak from the antimicrobial agent in the composite spectrum confirms its stable integration into the polymer matrix.

In summary, the FTIR analysis confirms the successful fabrication of a composite material and provides direct spectroscopic evidence for both physical (hydrogen bonding) and potential chemical (cross-linking) interactions between the components. These interactions are crucial for forming a homogeneous, integrated matrix with enhanced structural integrity, thereby directly supporting the observed improvements in mechanical and thermal properties.

3.4. Antibacterial assessment

The antibacterial activity of the PVA/corn starch bioplastic film was evaluated against both Gram-positive (S. aureus, ATCC 6538) and Gram-negative (E. coli) bacteria. After 24 h of incubation, clear inhibition zones were observed around the films containing the silver-based antimicrobial agent (Figure 5). The measured total diameters of the inhibition zones, including the disc, diameter were obtained as 12 mm for S. aureus and 10 mm for E. coli. This corresponds to a relatively narrow inhibition halo beyond the film edge, indicating moderate antibacterial activity. No inhibition zone was observed for the control film with identical base composition but without any silver-based antimicrobial agent, confirming that the antibacterial effect originates solely from the incorporated antimicrobial component rather than the PVA/corn starch matrix. This also indicates that the solution casting process did not deactivate the antimicrobial functionality and that bioactive silver ions remain available at the film surface. The antibacterial mechanism is likely associated with the release of silver ions (Ag⁺), which interact with thiol groups in bacterial proteins, disrupt enzymatic systems, compromise membrane integrity, and ultimately lead to cell death. The slightly larger inhibition zone observed for S. aureus compared to E. coli may be attributed to differences in cell wall structure, as Gram-negative bacteria possess an additional outer membrane that can limit the penetration of antimicrobial agents. Direct comparison between the two bacterial strains should be interpreted cautiously due to differences in disc diameter. This suggests potential applicability in active packaging, where even limited surface inhibition can contribute to reducing microbial contamination. However, the long-term stability and sustained release behavior of the antimicrobial agent were not investigated in this study. Further work is required to evaluate durability, release kinetics, and performance under real storage conditions. Overall, the findings indicate that incorporating a silver-based antimicrobial agent into a simple PVA/corn starch system can impart measurable antibacterial activity while maintaining material simplicity.OPEN IN VIEWER

3.5. Thermal stability analysis

The thermal stability and decomposition pathways of the bioplastic films were evaluated using TGA. While derivative thermogravimetry was not employed to precisely resolve individual decomposition peaks, the obtained weight-loss curves (Figure 6) clearly delineate a multi-stage degradation profile and allow for a comparative assessment of thermal resilience as a function of composition. All formulations exhibited a three-stage degradation profile characteristic of plasticized starch/PVA blends. The initial minor weight loss (<10%) below 150°C corresponds to the evaporation of physically adsorbed and bound water, as well as volatile components from vinegar. The onset of the first major decomposition occurred between 200°C and 250°C. This region is primarily attributed to the thermal degradation of the corn starch component and the glycerol plasticizer, which possesses lower thermal stability than the synthetic PVA matrix. The extent of weight loss in this region increased with higher starch content, with samples P2 and P3 being affected more visibly. The principal and most substantial degradation event occurred between 290°C and 340°C, corresponding to depolymerization and backbone chain scission in PVA. The film’s composition exerted a profound influence here. Sample P6, with the highest PVA content (6% PVA, 0% starch), demonstrated the greatest resistance, retaining 36.62% of its mass at 316°C. In contrast, sample P5 (5% PVA, 1% starch) showed a markedly lower residual mass of 26.13% at the same temperature. This distinct difference quantifies the role of PVA as the primary thermal stabilizer; as a continuous, cohesive PVA network provides a higher energy barrier to decomposition. The introduction of starch granules disrupts this network integrity, creating sites for earlier thermal breakdown and effectively reducing the composite’s overall thermal stability. The final residues (char) at 550°C further reflect the composite’s composition, increasing from 1.21% for the starch-rich P2 film to 2.21% for the PVA-rich P6 film. This trend suggests that PVA contributes to the formation of a more stable carbonaceous char, potentially via a different decomposition pathway than starch. In summary, the TGA analysis provides quantitative evidence that thermal stability is a tunable property in this system, directly governed by the PVA content. The P6 formulation, with its superior thermal resilience (major decomposition >290°C, highest char residue), indicates sufficient stability for potential use in environments requiring brief thermal exposure. This structure-property relationship is fundamental for material selection in applications with defined thermal constraints.OPEN IN VIEWER

3.6. Tensile property

The tensile properties, fundamental for packaging materials, were characterized to evaluate the structural integrity and flexibility of the PVA/corn starch films. The films had an average thickness in the range of 0.30-0.36 mm, as detailed in Section 2.3. The mechanical performance was intrinsically linked to the material composition, specifically the PVA-to-starch ratio, which governs the film’s micro-structure. Representative stress-strain curves reveal a transition from ductile to brittle behavior as starch content increases. Quantitatively, the tensile strength decreased from 3.61 ± 0.18 MPa for the P6 film (6% PVA, 0% starch) to 0.79 ± 0.04 MPa for P2 (2% PVA, 4% starch), directly correlating with the reduction of the continuous, cohesive PVA matrix that provides primary load-bearing capacity. The incorporation of starch granules introduces discontinuities and stress concentration points, compromising strength. Conversely, ductility, measured by elongation at break, showed an optimal peak of approximately 294% for the P5 formulation (5% PVA, 1% starch), indicating that a minor starch content combined with glycerol plasticizer maximizes polymer chain mobility before excessive starch reintroduces brittleness, as seen in P3 (44% elongation). The material’s stiffness, as measured by Young’s modulus, showed a complex relationship with composition. While the highest modulus was observed for P6 (178 ± 9 MPa), the P4 sample (4% PVA, 2% starch) exhibited a higher modulus (162 ± 8 MPa) than P5 (142 ± 7 MPa), suggesting that at specific intermediate filler loadings, the dispersed starch phase can restrict polymer chain movement and enhance rigidity, a phenomenon observed in particulate-filled composites. This structure-property analysis enables strategic tuning of film mechanics: P6 offers the best balance of strength and stiffness, whereas P5 provides superior flexibility. To contextualize these properties within materials engineering for packaging, the achieved tensile strength and exceptional elongation are competitive with other bioplastic blends designed for flexibility, such as chitosan/PVA films (0.21-0.24 MPa) and tamarind starch/glycerol films (0.21 MPa). A direct comparison with starch-based systems highlights the role of the PVA matrix; the tensile strength of the present P6 film (3.61 MPa) is markedly higher than that of a simple tamarind starch-glycerol film (0.21 MPa) and compares favorably to more complex, cross-linked corn starch composites (5.62-6.93 MPa). This result indicates that, in simplified formulations, the presence of a synthetic polymer such as PVA and its continuous network formation are the main determinants of mechanical performance, rather than the botanical source of starch. The tensile strength obtained in this study (up to 3.61 MPa) falls within the range reported for typical starch-based bioplastics (approximately 2-5 MPa), which varies depending on formulation and processing conditions. Similar mechanical behavior patterns have been seen in PVA/starch-based films, where a higher starch content results in a lower tensile strength because the polymer matrix is disrupted.

Compared with typical starch-glycerol systems, where tensile strength is typically very low (approximately 0.2-1 MPa), the results of the present study demonstrate significant improvements resulting from the formation of a continuous PVA matrix. On the other hand, tensile strengths above 5 MPa are usually achieved through chemical cross-linking, the addition of nanofillers (e.g., nanocellulose), or complex multi-component systems, which increase the material’s complexity and cost.

In this context, the PVA/corn starch/vinegar system, especially the P6 and P5 samples, can be considered as a potential material for producing flexible packaging (e.g., bags or wrapping materials), where high ductility is more important than high strength. In particular, the elongation at break of about 294% in the P5 sample further demonstrates its suitability for such applications.

The interpretation of the mechanical data is based on the well-established material properties. PVA forms strong, continuous films, whereas starch typically yields brittle, less cohesive structures. The progressive reduction in tensile strength with increasing starch content (Figure 7(c)) can therefore be attributed to the disruption of the continuous PVA matrix, with starch acting as a dispersed phase that introduces structural discontinuities and stress concentration sites.OPEN IN VIEWER

These findings suggest that the PVA network’s continuity is the dominant factor governing mechanical performance in this simplified composite system. Further enhancement may be achieved through green cross-linking strategies or the incorporation of bio-based nano reinforcement.