A Talc- and Kaolin-Enriched Acetylated Starch Biocoating: An Alternative to Single-Use Plastic for the Food Industry

Abstract

The increasing production of plastics, driven by modern societal development, has resulted in a significant rise in plastic waste, which poses serious environmental concerns due to its lengthy degradation times. The growing issue of single-use plastics (SUPs), such as packaging for food items and disposable utensils, has led to their reduction and potential future prohibition in the European Union. Cellulose, a natural biopolymer sourced from nature, has been proposed as a viable alternative to SUPs because it degrades without toxicity. However, its limited barrier properties against water and grease have restricted its effectiveness as a substitute. This study focuses on developing an environmentally friendly alternative to SUPs by combining cellulose with acetylated starch and incorporating inorganic fillers like kaolin and talc. These fillers enhance the material’s barrier properties and reduce production costs. The results indicate that the addition of kaolin significantly lowers moisture absorption and water vapor permeability, while a mixture of kaolin and talc provides superior grease resistance. Additionally, incorporating D-sorbitol as a plasticizer improves the mechanical properties of the coated sheets, preventing cracking and enhancing strength. Overall, these coatings offer a promising alternative for packaging applications, such as for sugar, candies, or chocolate.

1. Introduction

Plastics have revolutionized daily life due to their versatility and beneficial properties, such as easy malleability, lightweight nature, and chemical resistance. These qualities make plastics ideal for packaging, automotive components, and electronic manufacturing. They are durable, weather-resistant, and offer a range of mechanical properties from flexibility to rigidity. Additionally, plastics provide effective thermal insulation and are cost-effective to produce, which has driven their widespread industrial and consumer use. Common examples of plastics include high-density polyethylene (HDPE) and low-density polyethylene (LDPE), used in packaging, bags, and pipes; polypropylene (PP), used in packaging and automotive parts; polyvinyl chloride (PVC), used in construction and medical products; polyethylene terephthalate (PET), used in beverage packaging; polystyrene (PS), used in packaging and foam products; and polyurethanes (PURs), used in foams and adhesives. All these plastics are derived from fossil fuels, specifically crude oil. In 2021, it was observed that the global production of the main plastics amounted to 390.7 Mt (million metric tons), in which the percentage of production of PE and PP amounted to 46.2% (Figure 1A) and in which industrial sectors such as packaging account for 44% (Figure 1B) [1,2].

The environmental issues caused by plastic waste have sparked increased interest in developing more sustainable plastics and responsible waste management solutions. Common plastics can take centuries to degrade, leading to significant environmental pollution in our lands and oceans due to their widespread and excessive use. Each year, millions of tons of plastic are produced from fossil sources and, once discarded, these materials persist in the environment for long periods, contributing to growing waste accumulation on our planet [3,4]. In addition to this, according to the European Union (EU) more than 80% of the plastic found in the seas are single-use plastics (SUPs) [5]. Single-use plastics, as their name implies, consist entirely or partially of plastic and are utilized only once. Plastic plates, cutlery, straws, balloon sticks, cotton bands, and food and drink containers made of polystyrene or polyethylene have been classified within this group.

Considering all of the above, there are two major drawbacks in the use of plastics: the scarcity of the raw material used for their manufacture and the generation of environmental pollution. Regarding the used raw material, petroleum, it is expected to be exhausted by 2060 in the Middle East [6], which has generated a search for alternatives of natural providence such as biopolymers or polymers derived from natural sources. Regarding waste, it should be noted that, of the approximately 8000 million metric tons (of accumulated plastic production), 79% has accumulated in landfills or in the natural environment, and annually around 10 million tons of plastic end up in the oceans [4]. This represents 80% of ocean pollution. In addition, taking into account the fact that these materials take between 100 and 1000 years to decompose [7], the amount of plastic accumulated in the ocean will increase exponentially. This fact has meant that there are plans to collect and remove plastic from the oceans. The European Parliament announced that member states must achieve a collection target of 90% of plastic bottles by 2029, and that these will have to contain at least 25% recycled material by 2025 and 30% by 2030 [8,9,10,11].

To address these environmental issues, we need to reduce or replace single-use plastics (SUPs) with more sustainable alternatives. One viable option is cellulose-based paper for packaging. Cellulose, a plentiful and renewable biopolymer, is flexible and biodegradable. However, its low mechanical strength and high porosity limit its effectiveness in packaging, as it allows water and grease to penetrate. Therefore, paper companies are exploring biopolymer coatings to enhance the paper’s barrier properties for various applications.

Biopolymers are derived from renewable sources like biomass or microorganisms and must meet specific sustainability criteria to be considered environmentally friendly. These criteria include: (1) Synthesis from renewable materials to reduce reliance on non-renewable resources and lower carbon footprints. (2) Biodegradability or compostability to ensure harmless decomposition. (3) Reduced environmental impact during production, including lower energy and resource usage. (4) A contribution to greenhouse gas reduction by utilizing CO₂-absorbing biomass. (5) The absence of toxic substances to prevent environmental contamination. (6) Efficient production processes that minimize waste and optimize material use. (7) Compatibility with existing waste management systems, such as compostability or recyclability. (8) Economic viability for large-scale production [12,13]. Not all biopolymers meet all of these sustainability criteria, and characteristics can vary based on the biopolymer type and production process. Ongoing research and development are crucial for enhancing biopolymer sustainability and expanding their use. The chemical structure is more critical than the raw material source, so selected materials must meet biodegradability standards [14,15]. Packaging materials need resistance to water and grease, necessitating coatings to achieve these barrier properties. To maintain environmental respect, coatings for cellulose should be from bio-derived or natural sources. Starch, a widely available biopolymer, is a prime candidate for formulating these coatings.

This biopolymer is a polysaccharide formed by the union of amylose and amylopectin chains linked by O-glycoside bonds. Starch is hydrophilic and thus makes it possible to provide water dispersions to obtain good containment. Some studies [16] have revealed that starch coatings offer good grease barrier properties but lack waterproof resistance due to their hydrophilic nature. Modified starches, such as acetylated starch, are crucial here. Acetylation improves the physicochemical and functional properties of starch, enhancing its stability and resistance [17]. Both unprocessed (sourced from corn, potato, tapioca, wheat, and rice [18]) and modified starches (such as acetylated starch) hold significant importance in today’s paper industry. This is because starch effectively enhances paper properties, bolstering its durability and reinforcing inter-fiber bonds [19]. Acetylated starch is vital for producing coatings that impart barrier properties to paper, such as resistance to water and grease. However, native starch struggles with humidity due to its hydrophilic nature, semi-crystallinity, and rapid retrogradation [20].

One of the advantages of using acetylated starch compared to native variants is that it manages to reduce the gelatinization temperature by between 6 and 10 °C [17], thus achieving a reduction in the energy needed to manufacture the coatings. Gelatinization is an extremely important concept and characteristic of starch, since it is, to a large extent, what manages to give it viscosity for its correct deposition on the coating support paper. Gelatinization is the process in which the granules undergo expansion due to the absorption of hot water, which leads to the dissolution of the crystalline structure of amylopectin and the leaching of amylose [21].

The process involves heating starch to form a paste in which hydrated amylose chains surround granules. As the temperature increases past the gelatinization point, gelation occurs, creating a three-dimensional network of amylose and amylopectin linked by hydrogen bonds. However, this process can fragment starch granules, reducing viscosity. Thus, measuring viscosity before coating is crucial. Once gelled, the starch undergoes syneresis, expelling water as it retrogrades to a lower energy state with amylose chains aligning and forming crystalline structures. Retrograded starch should be discarded, as should gelled solutions. Acetylated starch improves hydrophobicity by replacing hydroxyl groups with acetyl groups, reducing interactions with water molecules.

To enhance the water barrier properties of biopolymers, inorganic fillers can be added to the coating dispersion. Fillers improve adhesion and cohesion, providing structural stability, while some also offer antimicrobial properties to inhibit microorganisms. Additionally, fillers can reinforce mechanical strength and facilitate electron transfer in electrochemical processes. The used fillers cover a wide range of materials, such as clay particles, metal nanoparticles, silica, carbon nanotubes, and metal oxides [22,23,24]. The choice of filler depends on the specific objectives of the application and the desired properties in the biofilm. Among the main inorganic fillers, kaolin and talc have been widely used by industry because of their abundance, price (managing to reduce production costs), and capacity to improve the barrier properties of coatings [25]. The use of kaolin and talc as fillers for gel-type coatings seeks to improve the structural stability of the coating, generating an improvement in its barrier and physical properties, achieving bonds and occupying spaces in the amorphous matrix that the starch gel has (Figure 2) [26]. Kaolin is obtained from the kaolinite mineral, with a crystalline structure composed of tetrahedral SiO₄ and octahedral (Al₂Si₂0₅(OH)₄) layers. This clay is highly used in industry since it does not swell and has a great capacity to bind water, which facilitates the homogenization of our coating. Talc is defined as a tri-layered hydrated magnesium silicate (Mg₃ (Si₂O₅)₂(OH)₂) with a laminated structure. When incorporating fillers to improve coating barrier properties, their interaction with the starch matrix is essential. Fillers occupy spaces within the matrix, reducing water and fat permeability. The hydrophilicity of fillers like kaolin and talc can vary due to their processing and specific properties. Kaolin is typically more hydrophilic than talc due to differences in their crystal structures, surface properties, and the presence of hydroxyl groups. Kaolin’s higher negative surface charge and its ability to retain water are influenced by its crystalline structure and impurities, while talc’s structure is less conducive to water retention.

Coated papers for packaging must match the packaging material’s requirements. If the coating fails, its barrier properties are compromised. Starch-based coatings often have low mechanical properties and can be brittle under stress. One solution to this problem is the use of plasticizers, since these chemicals increase the flexibility of the polymers, reducing the glass transition temperature (T_g) and, therefore, reducing the possible breakage of the materials [27]. There are numerous types of compounds that can be used as plasticizers: polyols, glycerol, sorbitol, and polyethylene glycol (for example); however, the most widely used are glycerol and sorbitol [27]. Sorbitol was the plasticizing agent that gave us the best results, this being the one chosen to continue the investigation. It should be noted that this plasticizer helps improve mechanical properties and some barrier properties, such as water vapor permeability [27]. In turn, an important factor when using plasticizers is to know the quantity of these that are used, since with a small quantity the coatings would break because their effect is null and if an excessive quantity is added, the properties of the coating would be lost [28]. Therefore, the optimal amount of plasticizer will be of vital importance in order to improve the properties of the coating.

Finally, it is important to address the environmental impact of biocoatings, exploring their behavior in anaerobic digestion and composting processes. The degradation of these coatings, which include talc and kaolin, is a crucial aspect in terms of assessing their environmental sustainability. According to the literature, biopolymeric enriched coatings can behave differently in composting and anaerobic digestion, depending on the additives used. For example, studies indicate that talc and other fillers can affect the degradation rate and the quality of the resulting compost [29,30,31]. These references provide a basis for understanding how biofortified coatings affect the composting process and their potential impacts on the environment.

This work aims to replace single-use plastics (SUPs) in food packaging with biopolymer-coated paper from renewable and biodegradable sources. Water-based solutions were prepared using starch biopolymer as the matrix. We optimized coating dispersion parameters (temperature, starch amount, acetic acid volume, and plasticizer) and studied the effects of inorganic fillers (kaolin, talc, and their mixtures) at varying concentrations (3, 6, 9, and 12 wt.%) on barrier properties.

2. Materials and Methods

2.1. Materials

As a biofilm holder, cellulosic sheets of 40 g per square meter (GSM) and 120 mL·min⁻¹ porosity, supplied by Aralar S.A. (Amezketa, Spain), were used. The GSM value refers to the weight of the paper sheet, also sometimes referred to as its grammage. The rest of the reagents were used for the biocoating dispersion preparation: (1) as a matrix, acetylated corn starch with an acetylation degree of substitution (DS) lower than 2% provided by Lyckeby S.A. (Kristianstad, Sweden) was used; (2) glacial acetic acid, used to improve the acetylation, and glycerol and D-sorbitol as plasticizers, were supplied by Sigma Aldrich; (3) as inorganic fillers, commercial laminated kaolin and talc were used (supplied by Imerys); and, (4) for the KIT test, castor-oil, toluene from Fluka, and n-heptane from Lab-Scan, were used.

2.2. Biocoating Dispersion Preparation

Biocoating dispersions were prepared mixing 5 g of corn starch in 120 mL distillate water by magnetic agitation at room temperature. After 5 min, 0.75 mL of acetic acid was added. Once the starch was totally dispersed, D-sorbitol (3 g) was added. On the other hand, in order to improve the barrier properties of the final biocoating, fillers were introduced to the dispersion. Kaolin, talc, or mixtures of both were added at 3, 6, 9, and 12 wt.% with respect to the starch weight. All the compounds were added one by one except for the talc, which was stirred manually before introducing it into the dispersion to facilitate its mixing. Once all the compounds were in the beaker, the stirring speed was increased to 500 rpm for 15 min. Subsequently, the temperature of the dispersion was increased until it reached to 90 °C, and the stirring was continued for 30 min so that the starch gelatinized and the dispersion viscosity increased. After this time, the viscosity of the dispersion was measured during heating and after 10 min of cooling (Figure 3).

2.2.1. Viscosity Measurement

The viscosity of the prepared dispersion was measured using a Brookfield RVT viscometer with an RV-04 spindle at 200 rpm during the heating process and after 10 min at ambient temperature (60 °C, before applying onto the paper).

2.2.2. Film Deposition

The paper holder was coated using the prepared dispersion mixture at 60 °C with Doctor Blade equipment. Once the Doctor Blade was calibrated by setting the desired 250 μm thickness measurement, the biocoating was deposited onto the paper. A line of the biocoating must be extended across the top of the paper, and then, with the help of the Doctor Blade, spread across the paper. Finally, coated papers were inserted into an oven at 60 °C for 24 h in order to dry them (Figure 3).

2.3. Film Characterization

To characterize the coated sheets, first the GSM was measured. After confirming the GSM value, TGA was performed and the barrier and mechanical properties were characterized. For the barrier properties, the water absorption was checked by the COBB-60 and contact angle tests; the water vapor absorption by water vapor permeability (WVP); and the grease barrier properties were assessed using KIT test. The effect of the D-sorbitol plasticizer on the cracking property was checked using scanning electron microscopy (SEM) and the mechanical properties were evaluated by a tensile test (Figure 4). All the tests were performed three times in order to confirm the redundancy.

2.3.1. Film Biocoating Weight

The film biocoating’s weight was measured in order to know the grams per square meter (GSM) of the material used. This value limits the amount of biocoating to be used, which, for possible industrial use, is set at a limit of 4 GSM without using a load and 8 GSM using a load. An increase in the amount of material used to make the biocoating would generate an increase in the cost of production, generating low profits or a loss for the company. To calculate the GSM value, 3 squares with a size of 5 × 5 cm² were cut out of the coated and uncoated paper. Each of the samples was weighed and the GSM value was calculated using Equation (1)

$$\mathrm{G}\mathrm{S}\mathrm{M}\left({\displaystyle \frac{\mathrm{g}}{{\mathrm{m}}^2}}\right)={\displaystyle \frac{\overline{{\mathrm{m}}_{\mathrm{i}}}-\overline{{\mathrm{m}}_{\mathrm{o}}}}{\mathrm{A}}}$$

(1)

where $\overline{{\mathrm{m}}_{\mathrm{i}}}$ is the mean of the three coated squares’ masses, $\overline{{\mathrm{m}}_{\mathrm{o}}}$ is the mean of the three uncoated (blank) squares’ masses, and A is the area of the squares (25 cm²).

2.3.2. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) makes it possible to determine the temperature up to which coated papers can be exposed before degradation. Through this technique, it was possible to determine the mass changes in the samples as the temperature increased. This enabled the thermal stability of the biocoating and paper to be determined [32].

The thermogravimetric analysis was carried out using a SHIMADZU DTG-60 thermobalance, with a method programmed for a sample (between 17 and 20 mg) from 25 to 800 °C with a heating ramp of 10 °C∙min⁻¹, under a controlled nitrogen atmosphere. After completing the analysis, a thermal decomposition curve (TG) was obtained, in which the initial degradation temperature (T_o) and the final one (T_f) were graphically obtained. Moreover, the degradation temperature for each stage (T_d1, T_d2…), which were obtained by the first-order derivative of the thermogravimetric curve (DTG), as well as the final residue at 800 °C, could be determined. The initial degradation temperature (T_o) was established for all samples when the weight loss reached 2 wt.%.

2.3.3. Water Resistance

To check the water resistance of the biocoatings, the COBB-60 test (ISO 535:2014) was carried out. This method makes it possible to calculate the amount of liquid water that the paper is capable of absorbing by gravimetry. The maximum limit for water absorption, so that the developed product can be used industrially, is 17 g·m⁻².

To carry out this test, 3 squares of paper must be cut with dimensions of 12 × 12 cm² so that the COBB ring (102 cm²) fits well and there is no leakage. Each paper was weighed. In the center of each square (a zone delimited by the ring), 100 mL of distilled water was poured. After 45 s, the water was emptied, and the superficial water adsorbed by the paper was removed by placing it between two absorbent papers passing a 10 kg roller. Finally, the paper was folded twice and weighed.

To obtain the absorbed amount of water, Equation (2) is used.

$$bsorbedwater\left({\displaystyle \frac{g}{m^2}}\right)={\displaystyle \frac{m_i-m_o}{A_{circle}}}$$

(2)

where $m_i$ is the mass of the wet paper, $m_o$ is the mass of the dry paper, and the area of the circle (A_circle) is determined by the ring’s radius of 5.7 cm.

2.3.4. Contact Angle Measurements

The contact angle measurement was used to determine the hydrophobicity of the biocoating. The equipment used for this test is an OCA 15EC. A 5 μL drop of water was applied vertically to 3 different areas of the coated paper surface and a photograph was taken which, with the help of the SCA20_U software, enables the calculation of the contact angle. If the contact angle of the drop on the film is less than 90°, the biocoating will be hydrophilic and, conversely, if it is higher than 90°, it will be hydrophobic.

2.3.5. Water Vapor Permeability

By measuring the water vapor permeability (WVP) the barrier of the biocoating against water vapor molecules was determined. The setup requires test tubes with an inlet 7.065 cm² in area and 10 cm long, in which 25 g of silica gel is added, and inside the test tubes, paper stoppers 3 cm in diameter are collocated. Test tubes containing dry paper biocoatings and silica gel were weighted. Once the tubes were prepared, they were kept for 24 h in the climate-controlled oven (Clima Temperature System, C-70/200), simulating extreme humidity conditions (90% humidity and 30 °C).

The tubes were weighed before and after the test, just like the coated papers. In this way, the amount of water that has been absorbed by the paper and the amount of water that has passed through paper and has been absorbed by the silica was determined. Three tests were carried out by type of biocoating.

To determine the water absorbed by the paper during this test, Equation (2) is used, while Equation (3) is used for the amount of water absorbed by the silica gel, that is, to calculate the water vapor permeability (WVP) of the paper.

$$WVP\left({\displaystyle \frac{g}{m^{2}h}}\right)={\displaystyle \frac{m_i-m_o}{A_{circle}·time}}$$

(3)

where $m_i$ is the mass of the tube after taking it out of the oven, $m_o$ is the mass of the tube before putting it in the oven, A is the area of the circle (7.065 cm²), and time is the time spent exposed to the humidity conditions, in this case 24 h.

2.3.6. Grease Resistance

The grease resistance of the obtained films is quantified by the KIT test (ISO 16532-1: 2008). This analysis has the aim of mimicking grease using twelve different solutions (

Table 1. KIT test solution proportions.

KIT Number	Castor Oil, mL	Toluene, mL	n-Heptane, mL
1	100	0	0
2	90	5	5
3	80	10	10
4	70	15	15
5	60	20	20
6	50	25	25
7	40	30	30
8	30	35	35
9	20	40	40
10	10	45	45
11	0	50	50
12	0	45	55

), where number one is the greasiest and number twelve the wateriest. Each solution contains different amounts of castor oil, toluene, and n-heptane to mimic the effect explained. In this assay, a drop of the mentioned solutions was poured onto the coated papers for 15 s. After a given time, it was observed which was the first solution to leave a stain on the paper, and this stain determines the KIT number up to which the coated paper has a resistance to grease.

2.3.7. Scanning Electron Microscopy

The objective of this analysis is to verify the resistance to cracking of the biocoating; that is, to observe if any type of micro-break, fissure, or crack is generated on the surface of the biocoating after applying an effort, as well as after bending or torsion. This is of vital importance, since if a small crack is generated in the biocoating after folding, it will lose all its barrier properties. To perform this test, a piece of coated paper was folded twice in half, a 10 kg roller was passed over it and the surface of the central area was analyzed by scanning electron microscopy (SEM). The SEM microscope used was a Hitachi S-4800 model, at an accelerating voltage of 5.0 kV with a magnification of 500×.

2.3.8. Tensile Strength

The tensile strength of the coated and uncoated papers was measured using an MTE-1 testing machine from METROTEC S.A., with a load cell of 500 N. The test conditions were: a preload of 1 N and a test speed of 10 mm min⁻¹. The distance between the two pieces holding the paper was 4 cm (data to necessary determine the elasticity of the paper). The analyzed coated and uncoated paper holder sample dimensions were 15 cm in length, 5 mm in width (W), and 0.05 mm in thickness (e). Eight tests were carried out for each sample. The section (S) is calculated using Equation (4).

$$\mathrm{S}={\displaystyle \frac{\mathrm{W}}{\mathrm{e}}}$$

(4)

The calculated value of the section makes it possible to calculate the stress supported by the paper (σ) until it breaks, using the force value (F) in Newtons and applying Equation (5). By employing this equation, one can derive the elastic limit and maximum allowable stress (RUT) of the paper subjected to the test.

$$\mathsf{\sigma }={\displaystyle \frac{\mathrm{F}}{\mathrm{S}}}$$

(5)

3. Results and Discussion

This study aims to investigate the individual and combined effects of inorganic fillers, namely kaolin and talc, on the matrix of acetylated starch gel used as a barrier biocoating on paper substrates. Prior to this, it was essential to establish the optimal conditions for the biopolymer matrix, which serves as the primary support. Consequently, the study has been divided into two main sections. The first focuses on optimizing the preparation of starch biopolymer dispersions and their application as biofilms on paper, while the second evaluates the barrier and mechanical properties of the coated paper.

For the preparation and optimization of the biopolymer dispersion, several parameters were considered, including the starch-to-water ratio, the temperature, the volume of acetic acid, the effects of plasticizers (D-sorbitol and glycerol), and the incorporation of fillers. To ensure proper application, the dispersion’s viscosity was optimized through temperature adjustments, and different film thicknesses were applied using the Doctor Blade technique. Subsequently, the coated papers were dried in an oven, with both temperature and drying time optimized. The quantity of deposited biopolymer was determined in grams per square meter (GSM), and thermogravimetric analysis (TGA) was conducted to ensure the biocoating’s thermal stability under industrial conditions.

To assess the coated papers’ barrier and mechanical properties, a series of tests were conducted. For barrier performance, the COBB-60 and contact angle tests were used to measure water absorption, water vapor permeability (WVP) was tested for moisture resistance, and the KIT test was employed to evaluate grease resistance. In terms of mechanical properties, scanning electron microscopy (SEM) was used to examine the film surface for cracking, and tensile strength tests were performed to assess the films’ structural integrity.

3.1. Starch-Based Dispersion Preparation Optimization and Deposition onto the Holding Paper

Acetylated starch serves as the primary matrix for the preparation of dispersions, and the amount used directly influences the resulting barrier properties. Starch acetylation involves substituting the hydroxyl groups of starch with ester groups, thereby reducing the polarity of the chain and increasing hydrophobicity by decreasing the potential for hydrogen bonding. To enhance the acetylation of the starch used in this study, varying volumes of acetic acid (HAc) were added to the dispersion, and their effects were systematically analyzed. Additionally, the influence of two plasticizers, glycerol and D-sorbitol, was assessed. The optimization of the dispersions was carried out by evaluating different concentrations of acetylated starch (X), different volumes of HAc (Y), and a fixed quantity of plasticizer (3 g). Specifically, dispersions were formulated with X g of acetylated starch in 120 mL of water, Y mL of HAc, and 3 g of plasticizer.

Initially, the effect of the acetylated starch mass in 120 mL of water was studied without the addition of HAc but with the inclusion of 3 g of plasticizer. Starch quantities of 4.5, 5.0, and 5.5 g were evaluated using the COBB-60 test to measure water absorption. The results indicated that the best performance was obtained with 5 g of starch; however, the KIT value for grease resistance improved from 7 to 8 with 5.5 g of starch (Figure S1A, Supplementary Materials). As observed, increasing the starch content enhances grease resistance but not moisture resistance. Given the hydrophilic nature of starch and the requirement for hydrophobic biocoatings, the decision was made to proceed with 5 g of starch, which yielded lower water absorption results.

Next, the effect of HAc addition (Y) was studied. Once the optimal starch concentration was determined, various volumes of HAc were added to the dispersions. As previously mentioned, reducing moisture absorption is challenging due to the inherent hydrophilicity of the starch matrix. To address this, the degree of starch acetylation was increased by adding different volumes of acetic acid. Dispersions containing 5 g of acetylated starch in 120 mL of water and various amounts of HAc (0.1, 0.5, 0.75, and 1.0 mL) were prepared with 3 g of D-sorbitol. The COBB-60 test revealed that the lowest water absorption value (40 GSM) was achieved with the addition of 0.75 mL of HAc (Figure S1B, Supplementary Materials). However, in the KIT test, the grease resistance only increased slightly, from 3 to 4, and higher volumes of HAc did not further improve this property. Therefore, based on these findings, 0.75 mL of HAc was selected to minimize water absorption in the subsequent formulations.

Following this, the effects of the plasticizers, D-sorbitol and glycerol, were examined. The primary function of plasticizers is to reduce material stiffness and enable the coated paper to bend, fold, or crease without causing cracks or fractures in the biofilm. Both plasticizers are hydrophilic due to their hydroxyl groups, and their impact was evaluated using the COBB-60 test. Maintaining the previously optimized formulation—5 g of starch, 0.75 mL of HAc, and 120 mL of water—3 g of plasticizer was added. The results showed that D-sorbitol yielded a water absorption value of 37.12 GSM, while glycerol produced a higher value of 52.66 GSM. Consequently, D-sorbitol was chosen for further use.

The final parameter studied was the effect of temperature, which directly influences the viscosity of the dispersion. As is well known, starch granule swelling behavior is temperature-dependent and can be classified into three phases: gelatinization (low viscosity) between −20 and 80 °C, gelation (maximum viscosity) at 100 °C, and retrogradation, where continued heating beyond 100 °C leads to granule breakdown and viscosity reduction [17,33,34]. For the preparation of a homogeneous biocoating, after the addition of the main reagents, the temperature was raised to 70–90 °C to achieve maximum viscosity, with stirring at 500 rpm for 30 min.

Once the dispersion formulation was optimized, the effect of different filler proportions was evaluated. From this point onward, the final formulation used was: 5.0 g acetylated starch, 0.75 mL acetic acid, and 3.0 g D-sorbitol in 120 mL of water. The fillers were characterized by scanning electron microscopy (SEM) (Figure S2, Supplementary Materials) to verify their particle sizes. Although the dispersions were prepared at 70–90 °C, they were allowed to cool to 55 °C for 10 min before deposition onto the paper substrate. During this cooling period, the viscosity increased, reaching approximately 500 cP, which facilitated the proper application of the dispersion onto the substrate. The dispersion was evenly applied using the Doctor Blade technique, setting a thickness of 250 µm for all samples. After deposition, the samples were dried in an oven at 60 °C for 24 h to remove residual moisture. Once dried, the GSM of each sample was measured.

Figure 5 illustrates the behavior of each system (individual fillers and a mixture of both fillers in equal proportions) with increasing filler content from 3 to 12% by weight. Without filler addition, the GSM was 4, which is below the industrial threshold. However, with the addition of fillers, distinct behaviors were observed. Kaolin increased the GSM value rapidly above 8, whereas talc kept the GSM value below 8 until a concentration of 9% by weight was reached. For the mixed fillers, the GSM remained below 8 up to 6% by weight. Therefore, these latter formulations appear to be viable for potential industrial applications.

Figure 6 presents the thermogravimetric analysis (TGA) results for both the uncoated paper (Figure 6A) and the paper coated with a 6 wt.% mixed-filler biocoating (Figure 6B). The graphs display the degradation profiles (black curve) along with their corresponding derivative curves (red curve), which provide a more precise identification of the peak degradation temperatures for the various components of the paper and the biocoating.

In Figure 6A,B, two large weight losses can be seen. The first one, which is between 80 and 120 °C, is due to the absorbed water contained in the paper [32]. The second one, the degradation of 80 wt.% of the paper, is due to the pyrolysis of the cellulose [35], and occurs between 230 and 370 °C. In turn, in the DTG curve of Figure 6B around 250 °C, another subtle peak is observed as a small shoulder in the main peak, that shows the degradation of the starch [36]. So, this analysis demonstrates that the use of this type of biocoating on paper does not affect its degradation.

3.2. Barrier Properties of the Biofilm-Coated Paper Holder

After optimizing the dispersion preparation and deposition process, the barrier properties of the various systems were evaluated. Moisture barrier performance was first assessed using COBB-60 and contact angle measurements. As shown in Figure 7, a clear and logical correlation is observed between COBB-60 values and contact angles: as COBB-60 values decrease, the contact angle increases. With the exception of the talc-containing biocoatings, all systems exhibited contact angles exceeding 90°, indicating that hydrophobic biocoatings were successfully achieved, even without the inclusion of fillers.

When comparing the two inorganic fillers individually (Figure 7A for COBB-60 and Figure 7B for contact angle), kaolin outperformed talc in moisture resistance. Increasing the kaolin content resulted in lower COBB-60 values and higher contact angles, whereas talc exhibited the opposite trend, with higher water absorption and lower contact angles as the filler content increased. This behavior is attributed to the hydrophilic nature of talc. Additionally, the combination of both fillers improved water resistance, with the lowest COBB-60 value of 30 g·m⁻² was achieved at 6 wt.% of the filler mixture. This represents a significant improvement compared to the uncoated sample, which had a value of 63 g·m⁻², and approaches the industrial water absorption limit of 17 g·m⁻². Simultaneously, the contact angle reached a maximum of 114° at a 6 wt.% filler content (Figure 7B).

Subsequently, water vapor barrier properties (WVPs) were assessed. As shown in Figure 8A, the WVP values for talc and the mixed-filler systems remained relatively constant and were not significantly affected by filler percentage. In contrast, the lowest water vapor permeability (approximately 10 g·m⁻²·h⁻²) was achieved with kaolin, while talc and the filler mixture exhibited similar values of 20 and 19 g·m⁻²·h⁻², respectively. This aligns with previous results, where talc was found to be more hydrophilic than kaolin.

In this test, the water vapor passing through both the biopolymer coating and the paper substrate was measured. For the talc-containing biopolymer, a portion of the water vapor (approximately 4 g·m⁻²) was absorbed into the film itself, indicating that talc retains more moisture within the coating. This is an undesirable effect and was also observed to a lesser extent in the other biocoatings, as the starch matrix allows water to penetrate and be retained in gel form. In the other biocoatings, the absorbed water was lower, around 3 g·m⁻².

Figure 8B presents the results of the KIT test, used to evaluate the grease barrier properties of each system. A significant improvement in KIT values is observed when comparing the blank sample (without filler) to those with varying filler percentages. The uncoated paper exhibited a KIT value of 5, which increased upon the addition of any filler.

For kaolin, the KIT value increased progressively, starting from 9 at 3 wt.% and reaching a maximum of 11 at 6 wt.%. In contrast, with talc and the filler mixture, the improvement in KIT values was more pronounced, reaching a maximum value of 12 with just 3 wt.% of filler. These high grease resistance values, as indicated by the KIT test, suggest strong potential for various industrial applications, such as biocoatings for chocolates [37].

3.3. Mechanical Properties

The barrier properties of the coatings are highly dependent on their structural integrity. If the coating cracks, moisture and grease can easily penetrate, compromising the paper and rendering it ineffective. To mitigate this issue, D-sorbitol was used as a plasticizer to reduce the stiffness of the material, enhancing its elasticity and allowing it to wrap food more effectively or improve its overall performance.

Figure 9 presents SEM images of the sample coated with 6 wt.% of filler mixture. Although only this sample is shown, similar images were obtained for the other systems. The absence of cracks in the biocoatings confirms that their barrier properties remain intact even after bending, indicating that acetylated starch-based coatings are flexible and resistant to breakage. This makes them well-suited for packaging applications.

The mechanical properties of all formulations were further evaluated through tensile tests. Figure 10 illustrates the maximum force that the paper samples (both with 6 wt.% of filler biocoating and without a coating) can withstand under elongation. To calculate the tensile strength (RUT), Equation (5) was applied, considering that the cross-sectional area of the paper is 0.25 mm². The maximum force values recorded were 10.53 N for uncoated paper, 14.79 N for paper with kaolin, 15.44 N for paper with talc, and 13.5 N for paper coated with the mixed filler.

From these values, the calculated RUT for each paper sample is as follows: 42.12 MPa for uncoated paper, 59.16 MPa for paper with kaolin, 61.74 MPa for paper with talc, and 54 MPa for paper with mixed fillers. In all cases, the addition of a biocoating increased the paper’s tensile strength, enhancing its resistance and ductility, allowing for greater elongation before breakage.

4. Conclusions

In this study, we have investigated the effects of inorganic fillers—namely kaolin, talc, and their combinations—on acetylated starch biopolymer systems for the development of biofilms applied to cellulosic paper. The results demonstrate that these biofilms are applicable for industrial use, as the achieved GSM values align with market standards. The incorporation of fillers into the matrix system enhances the barrier properties of the paper, with the 1:1 filler mixture yielding the most significant improvements in the biocoatings, providing a balanced enhancement of water, moisture, and grease resistance. Among the evaluated systems, the 6 wt.% of filler mixture offers the optimal balance of barrier properties, meeting market requirements effectively. The study reveals that acetylated starch-based biocoatings, utilizing kaolin and talc individually or in combination, substantially improve grease resistance and significantly enhance water barrier properties. Furthermore, the inclusion of D-sorbitol in the formulation mitigates cracking and substantially improves the mechanical properties of all coated samples. Despite these advancements, a notable challenge remains with the water vapor absorption capacity of acetylated starch. Consequently, these biocoatings are best suited for applications such as wrapping fatty foods stored in dry environments, rather than exposure to moist conditions, exemplified by their potential use in packaging for chocolates or cookies.

In conclusion, this environmentally friendly biocoating material supports circular economy principles and offers a viable alternative to current single-use plastics (SUPs). The use of water as a solvent, reliance on natural resources for reagents, and the compostable and biodegradable nature of the final product, combined with minimal biocoating application, make this material a promising candidate for large-scale production and paper repulpability.