Rheological, Thermal and Mechanical Properties of Blown Film Based on Starch and Clay Nanocomposites

Abstract

Growing concern over the environmental impact of conventional plastics has driven the development of biodegradable alternatives. In this context, natural polymers such as starch have emerged as sustainable options. Commercial montmorillonite, implemented as a reference nanomaterial, allows for the enhancement of the properties of biodegradable materials. In this study, commercial cassava starch powder plasticized with water and 35% glycerol, along with commercial nanoclay at concentrations of 0%, 2%, and 4%, was used as film reinforcement. The manufacturing process employed extrusion to evaluate the effectiveness of the nanomaterial in improving the mechanical and functional characteristics of the films. Films with varying concentrations of glycerol and nanoclay were produced to determine the optimal formulation by assessing their rheological, thermal, and mechanical properties. These films were subjected to comprehensive analysis using internationally standardised techniques, including Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), and morphological characterisation via Scanning Electron Microscopy (SEM). Among the properties evaluated, water vapour permeability (WVTR) was of particular interest. Results showed that higher nanoclay content improved moisture retention, thus enhancing the films’ water barrier properties. Mechanical testing indicated that the film with the highest nanoclay concentration, F-g35-NC4, displayed tensile strength values of 0.23 ± 0.02 MPa and elongation of 66.90% ± 4.85, whereas F-g35-NC0 and F-g35-NC2 exhibited lower values. Conversely, the highest tear resistance was also recorded for F-g35-NC4, reaching 0.740 ± 0.009 kg. Contact angle measurements revealed a hydrophilic tendency, with values of 89.93° ± 8.78°. Finally, WVTR analysis confirmed that increased nanoclay content enhanced moisture retention and improved the water barrier performance, with a value of 0.030 ± 0.011 g/m²·day, supporting potential applications in the packaging sector.

1. Introduction

In 2023, global plastic production reached approximately 413.8 million tons. Of this total, 90.4% corresponded to plastics of fossil origin, while 8.7% and 0.1% were attributed to post-consumer plastics recycled through mechanical and chemical methods, respectively. A minor proportion consisted of plastics of biological origin (0.7%) and Carbon-Captured and Used (CCU) plastics (<0.1%) [1]. CCU refers to the process of capturing CO₂ from potential emissions before it reaches the atmosphere, subsequently using the captured CO₂ as a raw material for plastic production. Globally, the packaging sector is the main consumer of plastics, accounting for 48% of total production. Within this sector, 44.9% of generated waste is used for energy recovery, 37.8% is recycled, and 17.3% is disposed of in landfills. Currently, global production capacity for bioplastics indicates that non-biodegradable bioplastics represent 43.7%, while biodegradable bioplastics constitute 56.3%. Among the latter, the most prominent are polylactic acid (37.1%), polymeric compounds containing starch (5.7%), and poly(butylene adipate-co-terephthalate) (4.6%) [1].

Starch-based compounds used as polymeric matrices present a promising opportunity, given that starch is a low-cost biopolymer obtained from various renewable plant sources [2,3]. However, its application in packaging and container manufacturing is limited by several drawbacks. These include low mechanical resistance—particularly tensile strength—and limited thermal stability, resulting in fragile products unsuitable for applications requiring moderate shelf life (6 to 12 months) [4]. Additionally, starch exhibits high hydrophilicity due to the presence of hydroxyl (-OH) functional groups, making it highly susceptible to moisture absorption. This process negatively affects mechanical performance by disrupting the structural conformation and promoting fragmentation of polymer chains [5,6]. Moisture absorption also compromises barrier properties, including water and gas permeability. Under environmental conditions of light and temperature, starch retrogradation has been widely documented; during this process, starch molecules reorganize and crystallize over time, increasing brittleness and reducing malleability [7,8].

To address these limitations, various scientific approaches have focused on modifying the physical, chemical, and biological properties of starch-based materials through adjustments to plasticizer content and the incorporation of fillers or waste materials [9,10]. Among the studied plasticizers, glycerol stands out due to its favorable cost–benefit ratio. Its low volatility and structural compatibility with starch enable its use in various blends and formulations [11]. Glycerol is also applicable in multiple industrial processes for film production, including flat and blown film extrusion, injection molding, and thermoforming [12,13]. Its effectiveness is attributed to the formation of intermolecular hydrogen bonds between polyols and starch [14].

Several authors have reported a synergistic effect between starch, additives, and glycerol, demonstrating increases in tensile strength and improvements in mechanical stability without compromising other essential properties [15]. Similarly, Santana et al. [16] observed changes in opacity, permeability, and mechanical behavior in plastic films produced from jackfruit starch dispersions with varying glycerol concentrations (20–60%). Another study found that high glycerol concentrations (e.g., 40%) increase water absorption and solubility while reducing stiffness from 620.79 MPa to 36.08 MPa as glycerol content rises from 15% to 45% [17].

A second approach involves incorporating nanoclays—specifically inorganic silicate-layered materials—to enhance mechanical strength and moisture barrier properties. Nanoclays have been widely studied as reinforcements in thermoplastic starch systems [18,19,20]. Montmorillonite-type nanoclays are particularly common due to their abundance and accessibility in nature [21,22]. The development of such composite materials has garnered significant interest. Recent studies have examined how clay-layer exfoliation is influenced by the proportion of glycerol in starch-based nanocomposite matrices [23].

In this context, the objective of the present research was to evaluate the rheological, thermal, mechanical, and morphological behavior of a plastic film produced from cassava starch and reinforced with varying proportions of nanoclays and glycerol, using extrusion and extrusion–blow molding processes.

2. Materials and Methods

2.1. Materials

The plastic films were produced from a mixture of modified tapioca starch, glycerol, and nanoclays. The starch was supplied by Ingredium (Cali, Valle del Cauca, Colombia) (reference N-DULGE^® C1) with a moisture content of 11 ± 2%. Glycerol (96% purity, USP grade) was purchased from Suproquim (Cali, Vale del Cauca, Colombia). The nanoclay was obtained from Sigma-Aldrich (St. Louis, MO, USA), with a density ranging from 600 to 1100 kg/m³ and an average particle size of less than 25 µm, analyzed by X-ray diffraction (XRD) (Supplemental Data, Figure S1), XRD experiments were performed using an XPert PANalytical Empyrean diffractometer (Lelyweg 1, 7602 EA Almelo, The Netherlands). The 2θ values were obtained from a step size of 0.0394 using CuKα radiation (l of 1.540598 Å).

2.2. Particle Size Analysis by DLS

The average particle size and size distribution of the starch and nanoclay were determined using dynamic light scattering (DLS). A Zetasizer (Malvern Panalytical Ltd., Malvern, UK) was used, following the methodology proposed by Abdolbaghi, Pourmahdian, and Saadat [24], as well as Huang [25]. Measurements were performed in triplicate using a 0.1% w/v suspension in distilled water. One millilitre of suspension was placed in a polystyrene cell, and measurements were recorded at a 90° detection angle and a constant temperature of 25 °C.

2.3. Determination of the Rheometric Conditions of the Mixture

A Thermo Scientific Haake Rheomix OS (Karlsruhe, Germany) torque rheometer was used under the processing conditions described in previous studies [26]. Three experimental 100 g mixtures were prepared with varying proportions of the plasticizer glycerol at 30%, 35%, and 40% relative to starch. Nanoclay was subsequently incorporated into the selected mixtures (35% and 40%) that demonstrated the best material conformation, at concentrations of 0%, 2%, and 4%. The mixtures were prepared 24 h prior to rheometer processing and dried at 60 °C for 8 h in a forced convection oven. Sample nomenclature reflects composition; for example, S-g40-NC0 denotes a starch-based sample (S) plasticized with 40% glycerol (g) and containing 0% nanoclay (NC).

2.4. Production of Thermoplastic Starch (TPS) with Nanoclays and Pelletization

Extrusion was performed using a counter-rotating intermeshing twin-screw extruder (Rheomex PTW, Thermo Fisher Scientific, Karlsruhe, Germany) equipped with 10 heating zones in the barrel (

Table 1. Proportions of S and NC used to establish process conditions.

Sample	Gly (%)		Nanoclay (%)
	35	40	0	2	4
S-g35-NC0	x		x
S-g35-NC2	x			x
S-g35-NC4	x				x
S-g40-NC0		x	x
S-g40-NC2		x		x
S-g40-NC4		x			x

). The screw speed was maintained at 65 rpm during extrusion, and the temperature profile remained uniform and controlled for all experiments (

Table 2. Temperature profile of the barrel during extrusion.

TS1 (°C)	TS2 (°C)	TS3 (°C)	TS4 (°C)	TS5 (°C)	TS6 (°C)	TS7 (°C)	TS8 (°C)	TS9 (°C)	TS10 (°C)	TS-D1 (°C)
70	70	70	75	80	85	90	95	100	105	110

). A die with a diameter of 10 mm was used. The resulting filaments were pelletized into 1 mm pieces, which were subsequently dried at 60 °C for 12 h and stored for film production and characterization.

2.5. Production of Blown Plastic Films

The starch–clay bionanocomposite films were produced using a Collin E20 blown-film extrusion line (COLLIN Lab & Pilot Solutions GmbH, Maitenbeth, Germany), equipped with a 20 mm diameter extruder and a length-to-diameter (L/D) ratio of 25. The temperature profile was optimized based on stable bubble formation above the annular air-injection nozzle. Processing conditions were maintained at 130 °C and a screw speed of 60 rpm. The resulting tubular blown films were rapidly cooled by air and subsequently stored in a DIES climate chamber at 40 °C and 35% relative moisture until testing.

2.6. Characterization

2.6.1. FTIR Analysis

Infrared spectra of the starch and nanoclay samples were acquired following ASTM E1252 [27]. A PerkinElmer Spectrum 3 spectrophotometer (Waltham, MA, USA) was used in attenuated total reflectance (ATR) mode, covering a wavenumber range of 400–4000 cm⁻¹.

2.6.2. Rheological Analysis

Rheological measurements were conducted following the methodology proposed by Caicedo et al. [24] to evaluate the storage and loss module. Analyses were performed on both the TPS reference pellets and the produced films. For film samples, smooth circular discs with a diameter of 25 mm and thickness of 100 μm were prepared. Tests were carried out on a TA Instruments HR2 rotational rheometer (DHR-2, TA Instruments, New Castle, DE, USA) using a 25 mm parallel plate geometry and a 1000 μm gap. Measurements were conducted at 130 °C under angular frequencies ranging from 0.1 to 628 rad/s, with five points per decade.

2.6.3. Thermogravimetric Analysis (TGA)

Thermogravimetric characterization was performed on a TA Instruments Q50 system (TA Instruments, USA, and Netzsch-STA 449 F5) to identify degradation-related weight-loss temperatures for starch and nanoclay samples. Approximately 5 mg of sample was placed in alumina pans and heated from 25 °C to 800 °C at a rate of 10 °C/min under a nitrogen purge flow of 60 mL/min.

2.6.4. Differential Scanning Calorimetry (DSC) Analysis

DSC analysis of the starch–nanoclay bionanocomposites was conducted using a TA Instruments Q2000 calorimeter (TA Instruments, New Castle, DE, USA) to determine gelatinization/plasticizer temperatures. Samples were analyzed under a nitrogen atmosphere at a heating rate of 20 °C/min. The procedure followed ASTM D3418 [28], with an initial temperature of 20 °C and a final temperature of 230 °C, using the theoretical thermal transition temperature of starch as the reference point for the heating program.

2.6.5. Determination of Tensile Strength and Strain

Tensile and elongation tests were conducted under the following conditions: film sections measuring 10 cm × 2.5 cm were prepared from each formulation. Prior to testing, the samples were conditioned in a climate chamber at 40 °C and 35% relative humidity for 4 h. A calibrated gauge length of 50 mm was marked on each specimen. Thickness measurements were taken at three different points to obtain an average value, which was subsequently entered into the testing software. The test parameters were set to a crosshead speed of 12.5 mm/min and a clamping length of 50 mm. Each test recorded material deformation until the point of rupture. Testing was performed using an INSTRON universal testing machine (São José dos Pinhais, PR, Brazil) with a 50 kN load capacity.

2.6.6. Determination of Tear Strength

Tear strength was determined according to ASTM D1922-08 [29]. Ten replicates of each film sample were prepared, each measuring 76 mm in length and 63 mm in width. The thickness of each specimen was measured three times at the center. Two points were marked 9.5 mm from the center at the top of each sample. With the pendulum in the raised position, the specimens were mounted onto the testing apparatus and securely clamped. A 20 mm notch was then introduced into each sample. The pendulum was released using the knife-actuating lever to perform the tearing test. The percentage scale reading was recorded, and tear strength (Rr) was calculated in kilograms using Equation (1):

Rr = p × m/100 × n

(1)

where: p: Scale reading [%], m: Pendulum mass [g] and n: Number of specimens.

2.6.7. SEM

Morphological characterization of the starch and nanoclay samples was conducted using a JSM-6490 JEOL scanning electron microscope (SEM) (Tokyo, Japan). Samples were coated with a 5 nm gold nanolayer using a Cressington 108 Auto Sputter Coater (Ted Pella, Redding, CA, USA). Micrographs were obtained under high vacuum at an accelerating voltage of 10 kV.

2.6.8. Contact Angle

Hydrophobicity was evaluated using the sessile drop method with distilled water as the test liquid. An ST INDUSTRIES (Gerolaan 63A SH Zeist, The Netherlands) Model 20-3500 profiler was employed. Ten film samples measuring 1 cm × 1 cm were randomly selected and cut from the manufactured batches. A 10 μL droplet of water at 25 °C was deposited onto each sample using a micropipette. Photographs were taken at 0 s, 10 s, and 60 s, and contact angles were calculated using Image J software version 1.52v under FIJI distribution (https://imagej.net/software/fiji/, accessed on 17 July 2025).

2.6.9. Determination of Water Vapor Permeability

Water vapor permeability was measured according to ASTM F1249-20 [30] using a PERMATRAN Model 3/33 permeability meter (Ametec Inc., Minneapolis, MN, USA). Circular film sections of approximately 101.6 mm in diameter were prepared. Thickness measurements were taken at three locations within the exposure area before mounting the samples onto the cell, which had an exposure area of 5 cm². All tests were conducted at 37.8 °C and 90% relative humidity. Data were recorded every 30 min.

2.6.10. Statistical Analysis

All experimental results were statistically analyzed using analysis of variance (ANOVA) and Tukey’s test to determine significant differences at α = 0.05. Analyses were performed using Minitab 22.0 software.

3. Results and Discussion

3.1. Dynamic Light Scattering (DLS) Analysis

The DLS analysis of the clay particles identified two main peaks: Peak 1, centered at 479.3 nm and representing 58.0% of the volume, and Peak 2, centered at 108.3 nm with a volume contribution of 42.0%. These results demonstrate a bimodal particle size distribution, with a substantial fraction of particles in the nanometer range. The average particle size was 587.6 nm, and the polydispersity index (PDI) of 1.0 indicates a high degree of heterogeneity in the size distribution. The predominance of Peak 1 (58%) may be associated with the formation of lamellar nanoclay agglomerates, likely resulting from insufficient dispersion in the ethanolic solution. This limited dispersion is attributed to interparticle attractive forces arising from Van der Waals interactions. The elevated PDI value further supports the presence of an inhomogeneous particle distribution. Nonetheless, the detection of particles within the micro- and nano-metric scale confirms the partial exfoliation or disaggregation of the clay [31].

3.2. Torque Rheometry

Figure 1 presents the torque evolution as a function of starch mixing time at different glycerol contents. The rheometric behavior exhibited four distinct stages: filling, plasticization, peak torque, and steady state. During the plasticization phase, a rapid increase in torque was observed within the first 15 s, reaching maximum values of 11.2 Nm, 8.5 Nm, and 6.5 Nm for mixtures containing 30%, 35%, and 40% glycerol, respectively. As the mixture absorbed sufficient heat to promote fusion, the torque progressively decreased due to the plasticizing effect of glycerol until reaching stabilization. Final steady-state torque values were 9.8 Nm, 6.2 Nm, and 4.5 Nm for mixtures with 30%, 35%, and 40% glycerol, respectively.

Therefore, bionanocomposite mixtures that enable more efficient plasticization—in terms of both energy consumption and the formation of a viscoelastic mass—were selected. The measured values indicate that increasing the plasticizer content leads to a reduction in torque. The development stage of the bionanocomposite pellets in the extrusion equipment is ongoing. The sample containing 30% plasticizer was excluded due to its high energy requirements and the rigid, brittle texture observed at the end of the mixing process.

Table 3. Maximum torque values obtained in the bionanocomposites obtained in the torque rheometer.

Sample	Maximum Torque [N·m]
S-g35-NC0	8.5
S-g35-NC2	7.6
S-g35-NC4	8.2
S-g40-NC0	6.5
S-g40-NC2	5.4
S-g40-NC4	6.1

presents the maximum torque values of the bionanocomposites. These mixtures demonstrate a reduction in torque (≤0.4 N·m) with the incorporation of nanoclays, which facilitate the plasticization process, as previously reported [32]. The decrease in torque reflects a lower viscosity and reduced resistance to flow during processing. It is also important to note that the physical conformation of the material was considered during selection. In the sample containing 40% plasticizer, an excess amount of plasticizer was observed as segregated phase (see Figure 2).

3.3. FTIR

Figure 3 presents the FTIR spectra of native starch, nanoclays, and glycerol. The starch spectrum exhibits bands around 3275 cm⁻¹ corresponding to the vibrations of hydroxyl functional groups (–OH). The bands between 2928 cm⁻¹ and 2885 cm⁻¹ are associated with the stretching vibrations of C–H groups corresponding to methylene (CH₂). The band at 1636 cm⁻¹ is related to the bending vibration of absorbed water (H–O–H), which is typical of hygroscopic materials such as starch. The bands between 1458 cm⁻¹ and 1417 cm⁻¹ are associated with the bending vibrations of C–H groups and the stretching vibrations of C–O groups in the glucose rings that form starch. The bands at 1336 cm⁻¹ and 1240 cm⁻¹ correspond to stretching vibrations of C–H and C–O groups, respectively. The bands at 1150, 1075, and 998 cm⁻¹ are attributed to stretching vibrations of C–O–C bonds in the starch backbone, as well as to vibrations of C–O bonds in hydroxyl groups. Finally, the bands at 925 cm⁻¹ and 858 cm⁻¹ are associated with vibrations typical of glucose rings in polysaccharides [33,34].

The nanoclay spectrum shows bands in the low-energy region around 3612 cm⁻¹ and within the broad range between 3500 and 3300 cm⁻¹, corresponding to the O–H vibrations of hydroxyl groups associated with Al–OH and Si–OH [35]. This is attributed to the layered structure of the nanoclays, in which tetrahedral silica sheets are bonded to an octahedral alumina sheet [36]. The band at 1631 cm⁻¹ is attributed to bending vibrations of water molecules, indicating moisture content in the nanoclays. A pronounced band at 981 cm⁻¹ is related to the stretching of Si–O bonds in the tetrahedral layer. The band at 1105 cm⁻¹ is associated with Si–O–Si and Al–O–Si vibrations characteristic of clay structures. The regions between 912 cm⁻¹ and 840 cm⁻¹, and the band at 550 cm⁻¹, correspond to Al–Al–OH and Si–O–Al vibrations, respectively, both characteristic of nanoclays.

The FTIR spectrum of glycerol shows a band at 3276 cm⁻¹ attributed to the stretching vibrations of hydroxyl (–OH) groups. Given that glycerol contains three hydroxyl groups, this band is intense and broad, reflecting extensive hydrogen bonding, as glycerol can form multiple bonds both with itself and with water. The bands between 2937 cm⁻¹ and 2879 cm⁻¹ correspond to the stretching vibrations of C–H bonds in methylene (–CH₂) and methyl (–CH₃) groups. The band at 1412 cm⁻¹ is associated with bending vibrations of –OH groups and deformation vibrations of C–H bonds. The bands between 1329 cm⁻¹ and 1219 cm⁻¹ correspond to bending vibrations of C–H bonds and deformation of hydroxyl groups. The bands between 1106 cm⁻¹ and 1030 cm⁻¹ are characteristic of stretching vibrations of C–O bonds in –OH groups, indicating the presence of primary and secondary alcohols [37]. Finally, the bands in the region from 990 cm⁻¹ to 851 cm⁻¹ are associated with deformations of the glycerol carbon skeleton.

The FTIR spectra of the bionanocomposite films show similar bands across all samples. These include bands in the 3200–3600 cm⁻¹ region attributed to O–H stretching vibrations, indicating the presence of hydrogen bonds within the starch chains and plasticizer. In the 2800–3000 cm⁻¹ region, bands corresponding to C–H stretching vibrations are observed; these originate from glucose units in starch and other components of the thermoplastic matrix. The band at 1600 cm⁻¹ is associated with the bending vibration of the O–H group in water. The band between 1020 cm⁻¹ and 1080 cm⁻¹ corresponds to C–O–C stretching vibrations and glycosidic bonds, confirming the polymeric structure of starch. Additionally, this band is associated with Si–O and Si–O–Si bonds in the nanoclays. Notably, the bands at approximately 2800 cm⁻¹ and 2900 cm⁻¹ in the S-g35-NC2 sample are more intense than those in S-g35-NC4 [31,35]. According to C. M. O. Müller, J. B. Laurindo, and F. Yamashita [36], this behaviour may be due to insufficient plasticization of starch and non-uniform dispersion of the nanoclays on the starch surface.

3.4. SEM of Nanoclays and Starch

Figure 4 shows the micrographs of the nanoclays, where two populations of pseudospherical aggregates with irregular edges are observed. These findings are consistent with those reported by K. Moreno-Sader, A. García-Padilla, A. Realpe, M. Acevedo-Morantes, and J. B. P. Soares [20], who described spherical structures with wavy network-like fringes on the surface of the same clay species. Additionally, aggregates of irregular spherical particles with average diameters of approximately 15 µm and 29 µm are evident. Other researchers have also reported that nanoclays exhibit surfaces with pores, interstices, and cavities—morphological features characteristic of montmorillonite-type nanoclays.

SEM micrographs of cassava starch are presented in Figure 5. The starch granules predominantly exhibit a spherical morphology, although some display irregular shapes. With respect to surface characteristics, most granules appear smooth, while others show a rough texture, with diameters ranging from 7 µm to 15 µm. These findings are consistent with those reported by L. Franco, M. Soares, S. Ju, and M. C. Garcia [17], F. Zhu [18], and M. Enriquez, R. Velasco, and A. Fernandez [22], who described native cassava starch granules as spherical with sizes between 5 µm and 17 µm.

Figure 6 shows SEM micrographs of the F-g35-NC0, F-g35-NC2, and F-g35-NC4 films, with the cross and vertical sections of each film obtained.

In general, the SEM micrographs of the films do not show fractures or phase separation. However, when comparing the micrographs in Figure 6d–f, it is evident that the F-g35-NC0 film exhibits cracks and agglomerates in contrast to F-g35-NC2 and F-g35-NC4. The presence of small aggregates or clusters in certain areas may indicate incomplete dispersion of the starch during the mixing process. Although these defects are minor, they may act as points of weakness under mechanical stress or thermal fluctuations, potentially affecting the long-term integrity of the material. The porosities and surface defects observed suggest that the mixture may exhibit limited barrier properties, which would negatively influence its performance in applications requiring resistance to the permeation of gases or liquids [34,36].

The surface of the F-g35-NC2 film is rough and contains prominent aggregates, indicating incomplete or insufficient dispersion of the components, possibly due to inadequate integration into the matrix. In contrast, F-g35-NC4 displays a smoother surface with few visible imperfections, demonstrating improved interaction between the clay layers and the polymer matrix. This enhanced integration results in superior mechanical properties and greater thermal stability [34]. The increased uniformity of F-g35-NC4 enhances its barrier properties by reducing porosity and increasing mechanical strength. Moreover, the lower occurrence of surface defects suggests improved performance in applications requiring high resistance to gas or liquid transmission.

3.5. DSC and TGA

Figure 7 presents the TGA thermograms and their respective derivatives (DTG) for starch (Figure 7a) and nanoclays (Figure 7b). In the DTG curve of starch, two main stages of weight loss are observed. The first stage, occurring around 58 °C, corresponds to a small initial loss attributed to the removal of surface or adsorbed water, which is typical of hygroscopic materials. The second stage, between 260 °C and 350 °C, exhibits a maximum peak at 312 °C, indicating the thermal decomposition of starch. At this stage, the polysaccharide chains break down, resulting in rapid and significant weight loss characteristic of the primary thermal degradation of starch. These findings are consistent with those reported by X. Chen [23], who observed a degradation temperature of 319 °C for native cassava starch.

In the DTG curve of the nanoclay, a significant weight loss is observed around 105 °C, corresponding to the elimination of adsorbed water on the nanoclay surface. This behavior is similar to that reported by Qin et al., who found a notable loss around 120 °C in three samples analyzed between 500 °C and 720 °C. A second stage of weight loss is also evident, associated with the decomposition of the nanoclay structure and the elimination of surface functional groups—mainly hydroxyl groups—through the dehydroxylation of structural water present in the clay [24,25].

Figure 7c shows the DSC curves obtained for starch and nanoclay samples. The starch thermogram displays an endothermic peak with a maximum at 94 °C, corresponding to its plasticizer point. At this temperature, the starch absorbs heat and undergoes gelatinization, leading to the loss of crystalline structures within the granules [17,18]. The DSC thermogram of the nanoclay exhibits an endothermic peak associated with the desorption of surface or physically adsorbed water, a common phenomenon in materials with high specific surface area. This peak occurs at 115 °C and is attributed to the elimination of interlaminar water located between the nanoclay layers. A second endothermic peak, observed at 173 °C, corresponds to the elimination of organic molecules or volatile contaminants trapped within the nanoclay structure and is accompanied by slight structural changes induced by thermal energy. Additionally, partial dehydroxylation of the nanoclay may occur, involving the removal of hydroxyl groups as temperature increases [26].

3.6. Characterization of Thermoplastic Starch (TPS) by DSC and TGA

Figure 8 presents the TGA thermograms (Figure 8a) and their corresponding derivatives (Figure 8b) for thermoplastic starch containing 35% plasticizer: TPS without nanoclays (S-g35-NC0), with 2% nanoclays (S-g35-NC2), and with 4% nanoclays (S-g35-NC4), obtained after the extrusion process. In Figure 8b, an initial mass loss is observed between 25 °C and 150 °C in all samples, associated with the evaporation of surface and adsorbed water, as well as with interactions between the plasticizer and the nanoclays. The second mass loss corresponds to the thermal decomposition of the starch and nanoclay structures.

The maximum degradation temperature of TPS was 310 °C, very similar to that of native starch (312 °C). In S-g35-NC2, this temperature decreased slightly to 306 °C. However, in S-g35-NC4, the degradation temperature increased to 316 °C. This increase indicates that the addition of 4% nanoclays enhances the thermal resistance of the material due to several factors: (i) the interaction between nanoclays and the polymer matrix, which acts as a thermal barrier; (ii) the high degree of nanoclay dispersion within the matrix; (iii) the formation of interactions between nanoclay functional groups and starch, increasing material cohesion; and (iv) the higher decomposition temperature of inorganic nanoclays [34].

Moreover, the uniform distribution of nanoclays restricts molecular mobility within the matrix, contributing to the delay of thermal decomposition processes [35].

In Figure 9, the TPS sample exhibits an endothermic peak at 113 °C, corresponding to the plasticizer temperature of the matrix. This relatively low temperature indicates that the unreinforced matrix has reduced thermal stability. In the S-g35-NC2 sample, an endothermic peak is observed at 138 °C, reflecting a substantial increase in the thermal transition temperature compared to TPS. The incorporation of 2% nanoclays enhances the thermal stability of the starch matrix, likely due to interactions between the nanoclays and the starch that promote the formation of a network more resistant to temperature variations. Finally, the S-g35-NC4 sample also shows an endothermic peak at 138 °C, similar to S-g35-NC2. This suggests that increasing the nanoclay content to 4% does not significantly raise the thermal transition temperature beyond that achieved with 2%, indicating that the thermal enhancement effect of nanoclays may reach a plateau at higher concentrations [35].

3.7. DSC Thermograms of the Films

Figure 10 presents the DSC thermograms for the bionanocomposite films. Sample F-g35-NC0 exhibits an endothermic peak at 88 °C, corresponding to the gelatinization temperature of the film-forming mixture. Sample F-g35-NC2 displays an endothermic peak at approximately 120 °C, indicating a higher thermal transition temperature compared to F-g35-NC0. The incorporation of nanoclays increases the plasticizer temperature due to interactions between the starch matrix and the nanoclays, which restrict molecular mobility and enhance the material’s thermal resistance. Sample F-g35-NC4 shows an endothermic peak at 136 °C, the highest among the three samples, suggesting that increasing the nanoclay content further improves the material’s thermal stability. This behavior indicates that a higher nanoclay concentration reinforces the film structure by more effectively limiting polymer chain mobility.

Sample F-g35-NC0 exhibits an endothermic peak at 88 °C, corresponding to the gelatinization temperature of the film-forming mixture. Sample F-g35-NC2 presents an endothermic peak at approximately 120 °C, indicating a higher thermal transition temperature compared to F-g35-NC0. The presence of nanoclays increases the plasticizer temperature due to interactions between the starch matrix and the nanoclays, which limit molecular mobility and enhance the material’s thermal stability. Sample F-g35-NC4 exhibits an endothermic peak at 136 °C, the highest among the three samples, suggesting that increasing the nanoclay content significantly improves the material’s thermal stability. This behavior indicates that a higher nanoclay concentration reinforces the film structure by further restricting polymer chain movement.

Similar results were reported by E. Gil and M. Mesa [37], who found that the addition of nanoclays improved thermal stability and reduced starch recrystallization. Notably, sample F-g35-NC4 exhibits a narrower plasticizer range compared to samples F-g35-NC0 and F-g35-NC2, reflected in a sharper peak. This behavior may be attributed to a greater degree of molecular ordering within the sample and strong interactions between the nanoclays and starch, generating a thermal insulation effect that contributes to increased material stability [34].

3.8. Rheological Analysis of the TPS and the Obtained Films

Dynamic rheological properties serve as valuable indicators of the thermoplastic behavior of the extruded bionanocomposite samples (S-g35-NC) and their corresponding films (F-g35-NC). Figure 11 presents the complex viscosity and storage modulus of these samples at varying shear frequencies. In the low-frequency region, interactions associated with molecular chain entanglement markedly influence both complex viscosity and storage modulus, providing insight into the effects of processing on the samples [38].

At an angular frequency of 100 rad/s, a substantial increase in the storage modulus was observed for all bionanocomposites. This behavior has also been reported for cassava starch samples and is attributed to the formation of an elastic network structure generated when the material is subjected to external stress. Additionally, the incorporation of nanoclay reduces the viscosity of the composite, as shown in Figure 11 for samples S-g35-NC4 and F-g35-NC4, facilitating greater deformation of the TPS secondary phase under the heating conditions applied during processing [39].

Moreover, Figure 11 shows that the storage modules and complex viscosity of TPS exhibit only minor changes with increasing shear rate. This indicates that the molecular weight of the TPS produced at different processing speeds remains relatively constant. In other words, elongational rheology contributes positively to maintaining the molecular weight of TPS while ensuring an adequate plasticization state of the materials [40].

3.9. Contact Angle

Table 4. Contact angle values obtained when a water droplet came into contact with the surface of P, 2% PNA, and 4% PNA films.

Films	Average Contact Angle 0 s	Average Contact Angle 10 s	Average Contact Angle 60 s
P
	52.00° ± 1.37 a	56.48° ± 3.33 a	50.66° ± 6.64 a
PNA 2%
	73.90° ± 3.22 b	55.21° ± 4.62 a	46.12° ± 4.87 a
PNA 4%
	89.93° ± 8.78 c	58.17° ± 1.59 a	51.13° ± 2.61 a

Values with different letters in the same column (a–c) are significantly different (p < 0.05).

presents the contact angle results of the films and the corresponding ANOVA for each measurement time (0, 10, and 60 s). At 0 s, the F-g35-NC0 film shows the lowest contact angle (52.00° ± 1.37), which is significantly different from those of F-g35-NC2 (73.90° ± 3.22) and F-g35-NC4 (89.93° ± 8.78). The incorporation of nanoclays results in a more hydrophobic surface at both concentrations. The data show that the contact angle remains highest at all times for sample F-g35-NC4, with significant differences compared to sample F-g35-NC2 at 10 and 60 s.

According to G. Giridhar, R. K. N. R. Manepalli, and G. Apparao [41], contact angle values below 90° are indicative of hydrophilic surfaces. However, as noted by T. Gutiérrez, R. Ollier, and V. Alvarez [42], the classification of a material as hydrophobic or hydrophilic may vary depending on its application (e.g., packaging, coatings, or biomedical uses). In composite materials, contact angles between 50° and 90° are typically considered slightly hydrophobic. Therefore, the contact angle values exceeding 65° for the films containing 2% and 4% nanoclays in this study can be classified as indicative of hydrophobic surfaces.

For measurement times of 10 s and 60 s, no significant differences were observed among the treatments.

3.10. Determination of Mechanical and Barrier Properties

Table 5. Results for the mechanical and barrier properties of the films.

Films	Tensile Strength	Percentage of Deformation	Water Vapor Transmission Rate	Tear Resistance
	(MPa)	(%)	(g/m2·day)	(kg)
F-g35-NC0	0.48 ± 0.10 a	6.80 ± 1.01 a	0.170 ± 0.095 a	0.090 ± 0.007 a
F-g35-NC2	0.20 ± 0.03 b	44.57 ± 3.67 b	0.054 ± 0.007 b	0.680 ± 0.019 b
F-g35-NC4	0.23 ± 0.02 b	66.90 ± 4.85 c	0.030 ± 0.011 c	0.740 ± 0.009 b

Values with different letters in the same column (a–c) are significantly different (p < 0.05).

presents the results for tensile strength, strain, water vapor transmission rate, and tear resistance of the films. The tensile strength data show that the F-g35-NC0 film exhibits a value of 0.48 ± 0.10 MPa, which is significantly higher than the values obtained for F-g35-NC2 (0.20 ± 0.03 MPa) and F-g35-NC4 (0.23 ± 0.02 MPa). The incorporation of nanoclays at the studied concentrations reduces tensile strength [43]. This effect may be attributed to a saturation phenomenon that hinders the adequate dispersion of nanoclay particles within the polymer matrix and promotes the formation of aggregates. These aggregates disrupt interparticle interactions and increase mixture viscosity, ultimately negatively affecting the material’s mechanical performance [44].

With respect to strain, the reinforced films exhibit mechanical characteristics comparable to those of synthetic plastics. These films show elongation values of 44.57% and 66.90%, classifying them as semi-rigid materials. This classification reflects their limited elastic deformation capacity and their ability to recover shape after force application. This behavior is likely related to the plasticizing effect produced by glycerol and nanoclay within the polymer matrix, which enhances material flexibility [34]. Such flexibility is advantageous for applications requiring deformation without fracture. The deformability of this bioplastic broadens its potential applications, positioning it as a viable alternative to synthetic materials such as polypropylene and polystyrene [45].

The water vapor transmission rate is a key variable for evaluating film barrier properties. In this study, the base film F-g35-NC0 exhibits a transmission rate of 0.17 ± 0.095 g/m²·day. The incorporation of the hydrophilic plasticizer (glycerol) into the polymer matrix increases this rate by reducing molecular interactions and enlarging the free volume between polymer chains [46]. In contrast, formulations containing nanoclays show substantially lower transmission rates, with values of 0.054 ± 0.071 g/m²·day and 0.030 ± 0.011 g/m²·day for the 2% and 4% nanoclay samples, respectively. These results demonstrate a marked decrease in water vapor permeability with increasing nanoclay concentration. This trend is consistent with the barrier effect provided by nanoscale silicate structures, which introduce a tortuous path that increases the transit time of water vapor molecules through the material [47]. The resulting more complex internal structure improves the films’ barrier performance [48]. Reducing water-vapor permeability is essential in packaging and coating applications where moisture protection is required to preserve product quality [49].

The tear resistance results show significant differences among the film formulations. The base film F-g35-NC0 presents a resistance of 0.090 ± 0.272 kg, which is considerably lower than that of films containing 2% (0.680 ± 0.187 kg) and 4% (0.740 ± 0.009 kg) nanoclays. These data indicate that nanoclay incorporation substantially enhances tear resistance. This improvement may be attributed to increased cohesion and a more robust polymer matrix structure. Nanoclays act not only as fillers but also contribute to mechanical integrity through interactions between their functional groups and the starch–plasticizer system [50]. This property is crucial for applications requiring materials with high durability and resistance during handling.

4. Conclusions

The incorporation of nanoclays at a 4% concentration resulted in a significant improvement in the thermal properties, deformation percentage, and tear resistance of the plastic films. These findings highlight the potential of nanoclays as effective reinforcing agents, making them a viable alternative for packaging applications that require enhanced mechanical strength and durability. Moreover, the addition of nanoclays at concentrations of 2% and 4% led to a reduction in the water-vapor transmission rate, indicating that nanoclays not only improve mechanical performance but also enhance moisture barrier properties. This behavior is particularly important for packaging applications where effective control of vapor permeability is essential. The contact angle results for films containing nanoclays show an increase in this parameter, reflecting a reduced affinity for water. This increase suggests that nanoclays contribute to improved hydrophobicity, which would be advantageous in applications requiring greater resistance to moisture.