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Article

A Study of the Influence of Sodium Alginate Molecular Weight and Its Crosslinking on the Properties of Potato Peel Waste-Based Films

by
Mohsen Sadeghi-Shapourabadi
1,2,*,
Mathieu Robert
1,2 and
Said Elkoun
1,2
1
Center for Innovation in Technological Ecodesign (CITE), University of Sherbrooke, Sherbrooke, QC J1K 2R1, Canada
2
Center for High Performance Polymer and Composite Systems (CREPEC), Montreal, QC H3A OC3, Canada
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(12), 6385; https://doi.org/10.3390/app15126385
Submission received: 9 April 2025 / Revised: 22 May 2025 / Accepted: 28 May 2025 / Published: 6 June 2025
(This article belongs to the Special Issue Design, Characterization, and Applications of Biodegradable Polymers)
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Abstract

:
This study develops a sustainable biopolymer film derived from potato peel waste (PW), enhanced with low- and high-viscosity sodium alginate (SA) through a solution casting method. The effect of calcium chloride crosslinking on the PW/SA composites was also evaluated. Scanning electron microscopy (SEM) analysis revealed that SA incorporation improved the film’s cohesion and uniformity, with both low- and high-viscosity SA showing nearly similar effects. Both the addition of SA and crosslinking led to enhanced tensile strength, as well as improved moisture barrier properties, by lowering the water vapor permeability (WVP) factor. The inclusion of high-viscosity SA (hvSA) resulted in superior mechanical and moisture barrier properties compared to the low-viscosity SA (lvSA), achieving a tensile strength of 5.34 MPa, with a 68% improvement compared to the pure PW film. The WVP analysis showed that hvSA had a superior impact, leading to a 32% reduction in WVP compared to the pure film. Crosslinking further boosted the tensile strength and moisture barrier properties. The crosslinked hvSA/PW composite shows the highest tensile strength among all samples, measuring 6.47 MPa, which accounts for a 104% enhancement compared to the pure film. It also led to a 34% reduction in WVP, reaching a value of 1.58 × 10−12 g/(Pa·cm·s). The findings demonstrate that PW/SA composites, especially the crosslinked hvSA/PW, offer the highest mechanical and barrier properties, making them suitable for biodegradable packaging and biomedical applications.

1. Introduction

Plastic pollution is currently a major environmental problem, with millions of tons of plastic waste accumulating worldwide [1]. Polymers derived from petroleum-based resources have detrimental effects on ecosystems, prompting scientists and industries to seek methods to mitigate their harmful environmental impact. One solution that has gained attention in recent years is the substitution of conventional polymer products with biobased materials derived from renewable resources [2]. Among these resources, agricultural waste has emerged as a valuable feedstock for producing sustainable materials. Every year, a substantial quantity of agricultural waste is generated globally. Repurposing this waste is a promising approach to reduce environmental concerns while promoting a circular economy by converting waste into valuable resources [3,4].
Potato peel waste (PW) is a major agricultural byproduct, as potatoes are the world’s fourth most important food crop after rice, wheat, and maize [5]. Beyond its common use as a dietary staple, potatoes are widely utilized in various industries, including the production of potato chips, French fries, and starch, leading to the generation of a significant amount of industrial waste [6]. PW is rich in starch, cellulose, pectin, and bioactive compounds such as proteins, polyphenols, and glycoalkaloids. This composition has attracted extensive research for its valorization potential in various domains, including the extraction of thermoplastic starch and the production of biocomposites and packaging materials [7,8], the isolation of bioactive compounds and antioxidants [9,10], and the extraction of nanoparticles, such as starch and cellulose nanoparticles [11,12,13].
Potato waste, a zero-value and abundant biomass, has gained attention as a promising resource for biopolymer production, particularly for film formation intended to replace conventional packaging materials. However, this material has certain limitations. Potato peels are primarily composed of starch, followed by cellulose as the second major component [14]. The high starch content imposes several drawbacks on PW, including insufficient mechanical properties and high moisture sensitivity [15]. To enhance the performance of starch-based matrices, various strategies have been employed, such as blending with polymers that exhibit superior characteristics [16], as well as chemical modification and crosslinking [17]. One of the polymers with more promising features to modify the starch-based compounds is sodium alginate.
Sodium alginate (SA) is a linear polysaccharide primarily extracted from the cell walls of brown seaweeds or produced by certain bacterial species, such as Pseudomonas aeruginosa. It is a copolymer composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G) units arranged in different sequences [18]. The block configuration plays a crucial role in alginate’s physicochemical behavior and its ability to form gel networks upon interaction with multivalent cations. SA is a versatile polymer widely utilized in the food industry, agriculture, and biomedical and cosmetic applications due to its favorable properties, including non-toxicity, excellent film-forming ability, effective thickening, and renewability [18]. The molecular weight of SA is a key factor influencing its behavior in film-forming systems. Higher molecular weight typically results in longer polymer chains, which enhance film cohesion, viscosity, and mechanical strength through greater chain entanglement and enhanced hydrogen bonding. Several studies have incorporated sodium alginate into starch-based matrices to improve mechanical properties or reduce moisture sensitivity [19,20,21].
Plasticizers are small molecules added to polymers to enhance their flexibility and reduce brittleness. They work by reducing the intermolecular forces between polymer chains, which increases polymer chain mobility and lowers the glass transition temperature (Tg). In this study, glycerol was used as a plasticizer due to its compatibility with both starch (the main component of potato waste) and sodium alginate. The increased chain mobility resulting from glycerol’s presence leads to improved flexibility and elongation at break. However, it can negatively impact tensile strength by disturbing inter-molecular interactions, such as hydrogen bonding. Additionally, glycerol can reduce the moisture barrier properties of the films by interacting with water molecules in the polymer matrix, thereby enhancing their hydrophilicity. Understanding the impact of glycerol as a plasticizer is crucial in tailoring the film’s properties for specific applications [22,23].
SA-containing materials are crosslinked through the incorporation of divalent or trivalent cations, such as calcium (Ca2+) or iron (Fe3+) ions [24]. As shown in Figure 1, Ca2+ ions replace the Na+ ions at the carboxylate groups of the guluronic acid (G-block) regions in the alginate chains, resulting in the formation of a stable three-dimensional ionic network known as the “egg-box” structure. This structure enhances film integrity by limiting polymer mobility and reducing water permeability [25]. Several studies have concluded that the crosslinked variant exhibits increased water resistance [26], improved mechanical properties [27], superior barrier properties, and higher thermal stability [28] compared to untreated SA. The crosslinking of SA enhances the suitability of SA-based composites for applications such as biodegradable packaging, wound dressing, and drug delivery [29].
The objective of this study is to develop a waste-derived biopolymer from potato peel waste as a sustainable alternative to petroleum-based polymers. The novelty of this work lies in investigating the role of sodium alginate in reinforcing potato peel waste-based films to develop a fully biodegradable material with enhanced mechanical and barrier properties. In particular, the influence of alginate molecular weight and calcium-induced crosslinking on film performance is evaluated. The final film may have potential applications in packaging purposes or biomedical fields, such as wound healing products. Future studies can explore scalable production techniques, including extrusion or casting, as well as the incorporation of active agents, such as antimicrobial or antioxidant compounds, to enhance the film’s functionality for food preservation.

2. Materials and Methods

2.1. Materials

Fresh potatoes (russet variety) were acquired from the local market for this experiment. The potato peels were separated from the flesh, washed, dried, and subsequently processed via grinding and ball milling. Two grades of sodium alginate, high viscosity (A2033) and low viscosity (W201502), were sourced from (Sigma-Aldrich Saint Louis, MO, USA). According to the supplier, the low-viscosity SA has a viscosity range of 5–40 cps (at 1% w/v, 25 °C), with a molecular weight ranging from 12,000 to 40,000 g/mol. The high-viscosity SA, on the other hand, has a viscosity of 2000 cps (at 2% w/v, 25 °C) and a molecular weight between 80,000 and 120,000 g/mol [30]. Additional reagents used included glycerol (99.0%), anhydrous calcium chloride (CaCl2, 96%), and organic antifoam 204, all supplied by Sigma-Aldrich.

2.2. Preparation Method

In order to obtain potato peels, the locally sourced potatoes were initially washed to remove soil residues, and then the peels were carefully separated from the tubers. The peels were subsequently processed to prepare a fine powder. For this purpose, the peels were first dried in the drying apparatus at 50 °C for 24 h. Following that, the dried material was subjected to grinding and then processed using a Retsch MM400 ball mill machine (Retsch GmbH, Haan, Germany) at a frequency of 30 Hz for a duration of four minutes to achieve finely powdered potato peel waste (PW). The resulting powder was then sieved using a 200-mesh sieve to ensure uniform particle size and stored for use in film fabrication.
To evaluate the impact of sodium alginate, its molecular weight, and the crosslinking on the properties of pure PW film, three film types were analyzed: pure PW, binary films (PW/SA), and crosslinked films with calcium chloride. The binary and crosslinked films were formulated using two grades of SA: low-viscosity grade SA (lvSA) and high-viscosity SA (hvSA).
Pure PW films were prepared by dissolving 10 g of PW powder in 200 mL of distilled water, followed by stirring for 5 min. The 5% solid content was selected based on previous studies, indicating that this concentration provides an optimal balance between tensile strength, flexibility, moisture barrier, and film integrity [22,23]. The mixture was then processed in a high-speed homogenizer at 4500 rpm for 15 min. To prevent foam formation, a few drops of organic antifoam were added during the homogenization process. The solution was subsequently subjected to ultrasonic irradiation using a probe ultrasonic device (Q700, Qsonica, LLC, Newtown, CT, USA) for 15 min at 70% of the total amplitude to enhance solution uniformity and improve film integrity. After obtaining a well-dispersed solution, glycerol was introduced as a plasticizer at a concentration of 35% relative to the dry weight. The mixture was then stirred at 400 rpm at 80 °C for 1 h. For each sample, the specified amount of the solution was cast onto Petri dishes and allowed to dry in the oven at 40 °C for 24 h, after which the films were removed for analysis.
In the binary mixtures, an 80:20 ratio of PW powder to SA was evaluated. To prepare the solution, 2 g of SA was dissolved in 100 g of distilled water in a separate beaker, stirring at 400 rpm and 80 °C for 50 min. This solution was then added to the homogenized and ultrasonicated PW solution (containing 8 g of PW powder in 100 g of distilled water), with 35% glycerol added as a plasticizer. The mixture was stirred for 1 h at 400 rpm and 80 °C. Following the cooking process, the solution was cast onto Petri dishes for drying. For the crosslinked mixtures, a CaCl2 solution was prepared, and an amount equivalent to 7% of the total SA was added dropwise to the PW/SA mixture using a syringe and needle during agitation. To prevent gelation and ensure a uniform solution, the mixture underwent high-speed homogenization at 1000 rpm for 10 min. Finally, a predetermined amount of the solution was poured onto Petri dishes. The amount of SA was determined by the total weight of the dried samples, while the percentage of CaCl2 was calculated based on the content of SA within the samples. Figure 2 presents a photograph of the PW, PAlv, and PAlvx films after casting.

2.3. Characterization

2.3.1. Thermogravimetric Analysis (TGA)

The thermal stability of samples was determined through the analysis of TGA and derivative thermogravimetric (dTG) data obtained using a Perkin Elmer thermogravimetric analyzer (TGA4000—Perkin Elmer Inc., Waltham, MA, USA). The analysis was conducted under a nitrogen atmosphere, with a temperature range of 30 to 700 °C at a heating rate of 10 °C per minute. Each sample was tested in triplicate.

2.3.2. Dynamical Mechanical Properties (DMA)

The viscoelastic properties of the produced polymer film samples were evaluated using dynamic mechanical analysis (DMA) utilizing a Q800 instrument (TA Instruments, New Castle, DE, USA). The analysis was performed in film clamp mode with a heating rate of 2 °C/min across the temperature range of −90 °C to 150 °C. During testing, a frequency of 1 Hz and an oscillatory strain amplitude of 0.04% were utilized. The samples were prepared and analyzed with approximate dimensions of 7 × 20 mm and a thickness of 15 µm. Each sample was examined in triplicate to ensure reproducibility.

2.3.3. Tensile Characterization

The mechanical properties of the films were characterized using a universal testing machine (Z050, Zwick/Roell, Ulm, Germany) equipped with a 100 N load cell. Film samples were cut into 50 mm × 10 mm strips and subjected to conditioning at 60% relative humidity for 24 h at room temperature. The tensile strength (TS) and strain at break (SB) were measured following ASTM D882 [31], using a crosshead speed of 20 mm/min and an initial gauge length of 30 mm. To ensure statistical reliability, each test was replicated five times.

2.3.4. Water Vapor Permeability (WVP)

The moisture barrier properties of film composites are assessed using water vapor permeability (WVP), a key metric calculated based on the water vapor transmission rate (WVTR). These parameters were measured using a Labthink WVTR machine (C390H, Labthink Inc., Jinan, China), which operates according to the ASTM F1249 standard [32]. The experimental setup employed an infrared sensor to detect moisture transmission across two cells, with the film composite mounted between them. One cell contained humidified nitrogen gas, while the other contained dry nitrogen. Moisture transmitted from the humid side to the dry side was detected by the infrared sensor [33].
The samples were prepared for testing as follows. Each specimen was cut into 5 cm × 5 cm squares and mounted into aluminum masks with a 4 cm × 4 cm window. To prevent moisture leakage and ensure data integrity, the samples were sealed with high vacuum silicone grease (Dow Corning, Midland, MI, USA) and secured with sheathing tape. The tests were conducted at 38 °C and 68% relative humidity in the humidified cell [34,35]. Prior to the test, a 2 h preheating time was implemented to stabilize the setup and avoid inconsistencies. Each specimen underwent nine measurement cycles, each lasting 30 min, to ensure the reproducibility of WVP measurements.

2.3.5. Scanning Electron Microscopy (SEM)

The cross-sectional morphology of the produced films was examined using a JEOL JCM-7000 scanning electron microscope (JEOL Ltd., Tokyo, Japan). The samples were mounted on SEM stubs and coated with a thin layer of gold in order to enhance their surface conductivity. The microscope operated at an accelerating voltage of 10 kV, and the images were captured at 500× magnification.

2.4. Statistical Analysis

For each category of samples, 3 to 5 test replications were performed. The average and standard deviation (SD) for key properties were calculated. To compare the performance of different samples, the average data were directly compared to assess which sample exhibited superior properties.

3. Results and Discussion

3.1. TGA Analysis

The thermal stability of the film composites was assessed using thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (dTG) graphs, as shown in Figure 3. The parameters of thermal decomposition are summarized in Table 1.
The thermal degradation pattern of pure PW, the sodium alginate-modified compounds, and the crosslinked samples exhibited similar overall trends. All samples displayed two distinct weight loss stages occurring at 50–150 °C and 200–350 °C. The first weight loss stage (Stage I) is primarily attributed to the evaporation of moisture, resulting from the hydrophilic properties of both PW and SA. A clear difference was observed between PAlv and PAhv composites. The weight loss for PAlv in the 50–150 °C range was 15%, while for PAhv, it was 8.5%. This difference is due to the varying water retention capacities of SA with different viscosities. Low-viscosity SA has a lower moisture retention capacity, leading to quicker moisture loss in dry conditions or faster moisture uptake in humid environments [36]. In contrast, high-viscosity SA forms stronger polymer networks in hydrogel structures, allowing it to retain moisture for longer periods [37]. The next major degradation stage (Stage II) occurs within 200–350 °C and corresponds to the decomposition of polysaccharides, which are the primary components of both potato waste and sodium alginate [38]. The total mass degradation in this temperature range was between 55 and 60% across all formulations, including pure PW, binary composites, and their crosslinked variants.
The incorporation of SA into the PW resulted in lower thermal stability, regardless of its viscosity. The pure PW film exhibited a single degradation peak at around 290 °C. However, after the incorporation of SA, a new peak emerged at around 225 °C, corresponding to the thermal breakdown of SA. This observation aligns with the findings reported by Jiang et al. [39]. The primary degradation peak for the PW component remained at 290 °C, which is consistent with the decomposition temperatures of starch (280 °C) [40] and cellulose (300 °C) [41], the major subcomponents of PW.
Crosslinking had different effects on PAlv and PAhv composites. In PAlv composites, crosslinking did not cause a noticeable impact on thermal degradation behavior. However, for PAhv composites, crosslinking led to a notable shift toward higher temperatures. In the PAhv composite, thermal degradation occurred at 220 °C and 270 °C, whereas after crosslinking, the maximum degradation temperatures in PAhvx increased to 240 °C and 290 °C. This shift suggests that crosslinking enhances the thermal stability of composites containing hvSA by reinforcing polymer interactions, thereby delaying thermal decomposition.

3.2. DMA Analysis

The impact of SA incorporation, with different viscosities, and its crosslinking on the viscoelastic properties of the PW-based matrix was evaluated using dynamic mechanical analysis (DMA). One practical datum obtained from DMA analysis is the glass transition temperature (Tg) of the films, as listed in Table 1. This was determined from the main peak temperature in the Tan δ versus temperature curve, as shown in Figure 4. Tg is an important parameter for understanding the flexibility of polymeric materials, defining the temperature range in which the material transitions from a rigid to a flexible state. In the case of packaging applications, Tg provides insights into the material’s brittleness and the temperature range in which the material can be used effectively.
Across all samples, a distinct peak in Tan δ was observed at approximately −50 °C. This peak corresponds to the relaxation temperature of the glycerol-rich phase, indicating that glycerol had a similar effect on all formulations. The primary Tg values of each composite were identified at temperatures above 0 °C [42].
The pure PW film exhibited a Tg of 60 °C. The incorporation of SA, regardless of its viscosity, caused a decrease in Tg, indicating increased flexibility at ambient temperatures. This reduction can be attributed to the higher hydrophilic nature of SA, which leads to increased moisture absorption in the composite. Water molecules act as plasticizers, enhancing polymer chain mobility and consequently lowering Tg [43]. Additionally, SA incorporation increases the free volume within the PW/SA matrix due to the irregular shape and size of SA molecules, further contributing to the decrease in Tg [44].
Typically, a higher molecular weight in polymers leads to an increased Tg due to greater chain entanglement. However, despite its higher molecular weight, PAhv exhibited a lower Tg (38 °C) compared to PAlv (48 °C). This discrepancy is attributed to the greater hydrophilicity of hvSA relative to lvSA. The longer chain length of hvSA contributes to increased moisture absorption, as the higher number of carboxyl groups in these extended chains enhances the material’s hydrophilic properties.
Crosslinking further influenced the Tg of the composites. Compared to their non-crosslinked counterparts, Tg decreased by 3 °C in PAlvx and by 8 °C in PAhvx composites. Generally, crosslinking is expected to increase Tg due to restricted polymer chain mobility. However, in this case, a significant increase was not observed. This behavior can be explained by the dual effects of crosslinking in polymer systems: increasing free volume and restricting chain mobility. On one hand, crosslinking increases free volume, allowing greater chain mobility and potentially lowering Tg [45]. On the other hand, crosslinking by Ca2+ ions restricts polymer chain movement due to the formation of an “egg-box” structure between SA chains and calcium ions, which would typically increase Tg [46]. The overall effect of crosslinking on Tg depends on the balance between these two opposing factors.

3.3. Mechanical Properties

The impact of incorporating sodium alginate in low- and high-viscosity grades, as well as crosslinking, on the mechanical properties of PW-based films is presented in Figure 5. Table 2 summarizes the average values and standard deviations (SDs) for each material, representing the overall variability across five replicates. The analysis focuses on two key parameters: tensile strength (TS) and elongation at break (Eb).
This poor mechanical performance can be attributed to its microstructure and composition. Scanning electron microscopy (SEM) revealed the presence of voids and microcracks, particularly at the interface between starch and cellulose, the primary components of potato peel waste [47]. These structural defects facilitate crack propagation during tensile testing, thereby reducing the film’s mechanical properties. In addition to microstructural defects, the inherently low mechanical strength of starch further contributes to the poor mechanical performance of the pure PW film. Starch, which constitutes approximately 46% of potato peel waste mass [22], has lower tensile strength compared to SA. Since starch is the predominant component in the pure PW film, its inherent weakness explains the observed mechanical behavior.
The incorporation of SA significantly improved the mechanical characteristics of the resultant films due to several factors. Firstly, SA possesses higher intrinsic tensile strength compared to the PW matrix [19]. Secondly, SA demonstrates excellent film-forming properties, which upon integration into the PW matrix, bridge the gaps within the PW structure [48]. This results in a more uniform film that prevents crack initiation and propagation while also enhancing stress distribution throughout the film when subjected to mechanical stress. Moreover, the formation of new hydrogen bonds between the carboxyl groups of SA and the hydroxyl groups of starch and cellulose further contributes to the enhanced mechanical performance [49].
The degree of improvement varied based on SA’s viscosity. Compared to the pure PW film, lvSA incorporation increased TS by 44% and Eb by 116%, while hvSA incorporation led to a 70% increase in TS and a 76% increase in Eb. The higher TS in PAhv films can be attributed to the longer polymer chains of SA, which form stronger hydrogen bonds with starch and cellulose, facilitating more efficient stress transfer. However, this increased molecular interaction also restricts polymer mobility, leading to a lower Eb compared to the PAlv composite [50].
The crosslinking process further enhanced the TS of the composites while reducing their Eb. The TS of PAlvx and PAhvx increased by 19% and 13%, respectively, compared to their non-crosslinked counterparts. However, Eb decreased by 18% for PAlvx and 10% for PAhvx. This phenomenon can be attributed to the formation of an egg-box structure, wherein calcium ions (Ca2+) form a three-dimensional network with SA chains [51]. This structure restricts polymer chain mobility, leading to reduced elongation after crosslinking. The egg-box formation enhanced TS, which can be explained by the establishment of new bonds between the material structures [52].
Among the untreated composites, the PAhv film exhibited the highest TS at 5.34 MPa. In the crosslinked variants, the PAhvx film demonstrated the highest TS at 6.47 MPa, marking a significant enhancement compared to the pure film (3.17 MPa). In terms of elongation at break, the PAlv composite exhibited the greatest improvement, reaching 54%, representing a 116% increase over the pure PW film.
Table 3 shows a comparison between the tensile properties of films developed from pure PW or modified with different reinforcements such as bacterial cellulose, sweet lime pomace, and curcumin. As no previous studies have specifically investigated PW/SA composites, comparable research on starch/SA-based films was considered instead. These films involved additional reinforcements, such as ground coffee waste or montmorillonite (MMT). The tensile characteristics observed in this study are generally lower than those reported in other PW-based films. This difference may be due to variations in testing conditions, inherent inconsistencies in waste material properties, and processing techniques. Compared to the films containing only PW, the current PW/SA composites—particularly the PAhvx sample—showed moderate tensile strength along with high elongation at break, indicating a balanced mechanical performance. Compared to the starch/SA compositions, they exhibited higher tensile strength, which could be attributed to their higher SA-to-starch ratios of 1:1 and 7:3 in the Nguyen and Zhang studies, respectively, compared to the 8:2 ratio used in this study. Overall, all compositions in this study exhibited considerable elongation at break along with acceptable tensile properties, supporting their potential suitability for packaging applications.

3.4. WVP Analysis

To assess the moisture barrier properties of the films, the water vapor transmission rate (WVTR) and water vapor permeability (WVP) were measured. Since the WVTR value is influenced by the film thickness, WVP was used to ensure a more accurate comparison across different specimens [53]. The analysis is reported in Figure 6.
The results indicate that the pure PW exhibited the highest WVP at 2.4 × 10−12 g/(Pa·cm·s), suggesting poor moisture barrier properties. This can be attributed to the structural characteristics of the PW matrix, which contains cavities and inherent porosities in the structure of the film matrix. Unlike high-performance homogenizers, such as microfluidizers or high-shear mixers, the ball mill grinding process used in this study did not achieve ultra-fine particles, leading to insufficient integration of PW-derived starch and cellulose components and thereby facilitating moisture diffusion.
The inclusion of SA significantly enhanced moisture barrier properties by reducing the WVP factor. The addition of lvSA and hvSA resulted in a WVP reduction of 1.83 × 10−12 and 1.63 × 10−12 g/(Pa·cm·s), indicating decreases of 24% and 32%, respectively. This improvement is attributed to SA’s excellent film-forming ability, which fills the gaps in the PW matrix, creating a denser and more cohesive structure that restricts moisture diffusion [54]. Additionally, the PAhv composite exhibited greater moisture resistance compared to PAlv, probably due to its longer polymer chains and higher carboxyl group content, which enhance hydrogen bonding interactions with water molecules [55]. The presence of carboxylic functional groups not only increases hydrophilicity but also strengthens the polymer network, further improving the film’s barrier efficiency [56].
Crosslinking further improved the moisture barrier properties of the composites. Upon crosslinking, the WVP values decreased to 1.66 × 10−12 and 1.58 × 10−12 g/(Pa·cm·s) for PAlvx and PAhvx, respectively, corresponding to an additional 9% and 3% improvement. This enhancement can be attributed to two primary factors. Firstly, crosslinking reduces the hydrophilicity of the compound, thereby enhancing its water barrier performance [57]. Secondly, the crosslinking produces a more interconnected and compact structure, which effectively limits water vapor diffusion [58].
Table 3 presents a comparison of WVP values for the PAhvx composite in this study and those reported in similar studies, with all values converted to g/(Pa·cm·s) for consistency. The results indicate that PAhvx exhibits a notably lower WVP than most composites containing PW powder, suggesting superior moisture barrier properties. While there are no previous studies on the combination of PW with SA, the composites of SA and starch were selected for comparison. The WVP of the SA/starch/montmorillonite composite reported by Zhang is similar to that of the current study. In contrast, the addition of ground coffee powder to starch/SA films in other studies resulted in higher WVP values compared to the composites in this work. This difference is likely due to the heterogeneity introduced by the coffee powder, which adversely affects barrier performance. Overall, the composites developed in this study demonstrate lower and more promising WVP values than most previously reported materials. This suggests that the addition of SA, regardless of its viscosity, does not compromise the barrier properties and makes these composites strong candidates for packaging applications requiring high moisture resistance.
Table 3. Comparison of tensile properties and WVP of the present study and the previous literature on PW, PW starch, or SA/starch composites.
Table 3. Comparison of tensile properties and WVP of the present study and the previous literature on PW, PW starch, or SA/starch composites.
CompositesTensile Strength (MPa)Elongation at Break (%)WVP
(×10−12 g/Pa·cm·s)		Ref.
Current study
(PAhvx)		6.47431.41
PW + 5% bacterial cellulose (5–10)13.870.11306Xie [22]
PW + sweet lime pomace (1:0.5)2.47.60.02Borah [59]
PW (3–0.3–0)9.485.338.31Kang [23]
PW starch/curcumin
(CS-DES-Cur5)		14.468.7913.2Liu [60]
Starch/SA/ground coffee
(TPS-Alg/SCG-10)		67.65.29.6Nguyen [21]
Starch/SA/MMT
(S-6%M)		30411.28Zhang [20]

3.5. SEM Analysis

In order to investigate the morphology and microstructure of the prepared film composites, SEM images of the cross-sectional area were obtained, as shown in Figure 7.
The SEM image of pure PW revealed a rough and porous structure with multiple microcracks, indicating poor structural integrity. This can be attributed to the heterogeneous composition of potato peels, particularly the presence of non-starch substances, such as cellulose, which prevents uniform film formation [47]. The incorporation of SA improved structural integrity by reducing small gaps and forming a denser and more cohesive structure [20]. However, minor cracks were still present, which may be due to the lower proportion of SA compared to PW in the compound. When comparing the impact of low- and high-viscosity SA, PAhv films exhibited a slightly denser structure than PAlv. However, the difference was not significant, and some voids remained in both composites. This could be due to the dual effect of hvSA. While it enhances binding and promotes uniformity, its longer polymer chains may hinder homogeneous dispersion with PW, increasing the risk of phase separation or agglomeration [18]. As a result, potential cohesion gains may be offset by microstructural defects.
In general, crosslinking with calcium chloride improves structural cohesion [61]. It resulted in a more uniform structure with fewer microcracks in both PAlvx and PAhvx composites compared to their non-crosslinked counterparts. The reduction in microfractures correlated with the improved tensile strength observed in mechanical tests, where crosslinked samples exhibited higher tensile strength than their non-crosslinked counterparts.

4. Conclusions

This study developed sustainable biopolymer films from potato peel waste reinforced with sodium alginate (SA), achieving significant improvements in mechanical strength and moisture barrier properties through the use of high-viscosity SA (hvSA) and calcium chloride crosslinking. These composites demonstrated over a 100% increase in tensile strength and reduced water vapor permeability compared to pure potato peel films, indicating their promise as biodegradable alternatives for packaging and biomedical applications.
Mechanical characterization revealed that pure PW films had poor performance, with a tensile strength of 3.17 MPa and 25% elongation at break. Incorporating SA improved these properties, with hvSA and its crosslinked form outperforming low-viscosity SA (lvSA). The hvSA composite increased tensile strength by 68% (5.34 MPa), while its crosslinked version increased it by 104% (6.47 MPa). LvSA films showed greater elongation improvements, reaching 54%, which is more than double that of the pure PW film. WVP tests revealed poor moisture barrier performance in pure PW (2.4 × 10−12 g/(Pa·cm·s)). Adding SA improved barrier properties, with hvSA performing better than lvSA. Crosslinking further reduced permeability, with the crosslinked hvSA film achieving the lowest WVP (1.58 × 10−12 g/(Pa·cm·s)). The DMA results showed that pure PW had a Tg of 60 °C, which decreased with SA addition. Tg values dropped to 48 °C and 38 °C for lvSA and hvSA composites, respectively. TGA analysis indicated that SA-modified films degraded at lower temperatures than pure PW (peak at 290 °C), with a new peak near 225 °C. Crosslinking slightly enhanced thermal stability, shifting this peak to about 240 °C. SEM analysis confirmed that SA incorporation improved film structural integrity by filling voids caused by potato peel heterogeneity.
By valorizing agricultural waste, this work advances the development of eco-friendly materials with practical potential to replace petroleum-based plastics for packaging or biomedical applications. Future efforts could focus on scaling up production and incorporating functional additives to tailor film properties for specific uses.

Author Contributions

Conceptualization, M.S.-S., S.E. and M.R.; validation, M.S.-S., S.E. and M.R.; Methodology, M.S.-S.; Formal analysis, M.S.-S.; Investigation, M.S.-S.; Resources, M.R. and S.E.; Data curation, M.S.-S.; Writing—original draft, M.S.-S.; Writing—review & editing, M.S.-S., S.E. and M.R.; Supervision, M.R. and S.E.; Project administration, M.R. and S.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

During the preparation of this work, the authors used ChatGPT-4o to check grammar and spelling. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
PWPotato peel waste
SASodium alginate
lvSALow-viscosity sodium alginate
hvSAHigh-viscosity sodium alginate
PAlvCompound of PW/lvSA
PAhvCompound of PW/hvSA
PAlvxCrosslinked compound of PW/lvSA
PAhvxCrosslinked compound of PW/hvSA

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Figure 1. The “egg-box“ model of crosslinking of sodium alginate by Ca2+ ions.
Figure 1. The “egg-box“ model of crosslinking of sodium alginate by Ca2+ ions.
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Figure 2. Photograph of dried films after casting of PW, PW/SA (PAlv), and crosslinked PW/SA film (PAlvx).
Figure 2. Photograph of dried films after casting of PW, PW/SA (PAlv), and crosslinked PW/SA film (PAlvx).
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Figure 3. (a) TGA and (b) DTG curves of PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
Figure 3. (a) TGA and (b) DTG curves of PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
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Figure 4. DMA graph of PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
Figure 4. DMA graph of PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
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Figure 5. Stress–strain graph for PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
Figure 5. Stress–strain graph for PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
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Figure 6. Water vapor permeability (WVP) for PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
Figure 6. Water vapor permeability (WVP) for PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
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Figure 7. Cross-sectional SEM images of films: PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA (PAlvx, PAhvx).
Figure 7. Cross-sectional SEM images of films: PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA (PAlvx, PAhvx).
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Table 1. Summary of thermal properties: Tg, TGA weight loss (Δm) for two major stages, and decomposition temperatures.
Table 1. Summary of thermal properties: Tg, TGA weight loss (Δm) for two major stages, and decomposition temperatures.
Sample Stage I (50–150 °C)Stage II (200–350 °C)
Tg (°C)Δm (%)Decomposition Temperatures (°C)Δm (%)
Pure PW60 ± 211 ± 0.8290 ± 162 ± 1.4
PAlv48 ± 515 ± 1.1225 ± 1; 275 ± 157 ± 0.5
PAlvx45 ± 516 ± 1230 ± 2; 280 ± 155 ± 0.3
PAhv38 ± 48.5 ± 1.4220 ± 2; 270 ± 160 ± 2.8
PAhvx30 ± 38.5 ± 1.2240 ± 1; 290 ± 160 ± 1.5
All values are reported as mean ± standard deviation (SD) based on three replicate measurements.
Table 2. Mechanical characteristics of PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
Table 2. Mechanical characteristics of PW, PW/SA (PAlv, PAhv), and crosslinked PW/SA films (PAlvx, PAhvx).
CompositionTensile Strength (MPa)Elongation at Break (%)
Pure PW3.17 ± 0.4225 ± 2.9
PAlv4.55 ± 0.2854 ± 2.3
PAlvx5.4 ± 0.444 ± 5.6
PAhv5.34 ± 0.7448 ± 6.7
PAhvx6.47 ± 0.1643 ± 4.8
All values are reported as mean ± standard deviation (SD) based on five replicate measurements.
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MDPI and ACS Style

Sadeghi-Shapourabadi, M.; Robert, M.; Elkoun, S. A Study of the Influence of Sodium Alginate Molecular Weight and Its Crosslinking on the Properties of Potato Peel Waste-Based Films. Appl. Sci. 2025, 15, 6385. https://doi.org/10.3390/app15126385

AMA Style

Sadeghi-Shapourabadi M, Robert M, Elkoun S. A Study of the Influence of Sodium Alginate Molecular Weight and Its Crosslinking on the Properties of Potato Peel Waste-Based Films. Applied Sciences. 2025; 15(12):6385. https://doi.org/10.3390/app15126385

Chicago/Turabian Style

Sadeghi-Shapourabadi, Mohsen, Mathieu Robert, and Said Elkoun. 2025. "A Study of the Influence of Sodium Alginate Molecular Weight and Its Crosslinking on the Properties of Potato Peel Waste-Based Films" Applied Sciences 15, no. 12: 6385. https://doi.org/10.3390/app15126385

APA Style

Sadeghi-Shapourabadi, M., Robert, M., & Elkoun, S. (2025). A Study of the Influence of Sodium Alginate Molecular Weight and Its Crosslinking on the Properties of Potato Peel Waste-Based Films. Applied Sciences, 15(12), 6385. https://doi.org/10.3390/app15126385

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