# Hydrocolloid-based coatings applied on biobased/biodegradable food flexible packaging, A review. Part 1 - Biopolymer to coat and available technologies

Frédéric Debeaufort, Mia Kurek, Maria-Jose Fabra, Yasaman Ghasemi, Mario Ščetar, Kata Galić, Nasreddine Benbettaieb

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Frédéric Debeaufort, Mia Kurek, Maria-Jose Fabra, Yasaman Ghasemi, Mario Ščetar, et al. Hydrocolloid-based coatings applied on biobased/biodegradable food flexible packaging, A review. Part 1 - Biopolymer to coat and available technologies. Food Hydrocolloids, 2025, 169, pp.111624. 10.1016/j.foodhyd.2025.111624. hal-05106103

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# Hydrocolloid-based coatings applied on biobased/biodegradable food flexible packaging, A review. Part 1 - Biopolymer to coat and available technologies

Frédéric Debeaufort<sup>a,b,*</sup>, Mia Kurek<sup>c</sup>, Maria-Jose Fabra<sup>d</sup>, Yasaman Ghasemi<sup>b</sup>, Mario Ščetar<sup>c</sup>, Kata Galić<sup>c</sup>, Nasreddine Benbettaieb<sup>a,b</sup>

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## ARTICLE INFO

**Keywords:**

Biodegradable

Biosourced

Flexible packaging

Coating

Biopolymers

Coating processing

## ABSTRACT

The shift towards sustainable food packaging has intensified research into hydrocolloid-based coatings applied to biobased and biodegradable flexible films. Indeed, most of the biobased and/or biodegradable film available for the food market exhibits too low properties, but that could be compensated by coating applications.

This review explores the potential of hydrocolloids as coating materials to improve the functional properties of sustainable flexible packaging such as polylactides, polyhydroxyalkanoates, thermoplastic starch, poly-caprolactone. Hydrocolloid-based coatings, including proteins, polysaccharides, and their composites, are particularly promising due to their ability to improve barrier, surface and mechanical performance while maintaining biodegradability and food safety.

The choice of technique for coating application depends on the desired coating thickness, adhesion strength, and functional enhancements. Various coating technologies are discussed, ranging from solvent-free (dry) processes, such as extrusion-lamination and hot press techniques, to wet-based (solvent) methods, including solution casting, spray coating, electrospinning, etc. Surface treatments, such as plasma activation, are often required before coating application to enhance adhesion, wettability, and mechanical resistance. This review provides an in-depth analysis of available biodegradable polymer films suitable for coating and food applications and the coating technologies, setting the foundation for the development of next-generation eco-friendly packaging films.

## 1. Introduction

Materials to be used in food packaging must meet several duties. To ensure the quality of the material, control of the mechanical and barrier properties during storage is needed. The industry of packaging is turning towards the usage of environmentally low impact materials, with a variety of natural and synthetic biodegradable polymers (Ghanbarzadeh & Almasi, 2013). These polymers are categorized based on their origin as natural, synthetic, or modified natural polymers. From an environmental standpoint, they are classified according to the origin of the raw materials: renewable resources (biological origin, such as microbial

polymers like cellulose and polyhydroxyalkanoate (PHA), and those from natural monomers like polylactic acid (PLA), polybutylene succinate (PBS), or poly(glycolic acid) (PGA) and non-renewable/fossil resources (oil, natural gas, and coal) like aliphatic and/or aromatic polyesters (polycaprolactons, PCL) or polyvinyl alcohol (PVOH) (Rosenboom et al., 2022). Genetic engineering provides unique opportunities (manipulation of biosynthetic enzymes) to develop non-natural analogues of biopolymers and allows for further modification of polymers. The composition units vary not only based on the carbon source and type of microorganisms, but also on the dilution rate, carbon to nitrogen ratio, time, and temperature. Polymers from renewable

<footnote><sup>a</sup> University Burgundy Europe, IUT-Dijon, BioEngineering dpt., 7 Blvd Docteur Petitjean, BP17867, Dijon, 21078 cedex, France</footnote>

<footnote><sup>b</sup> University Burgundy Europe, Institut AgroDijon, INRAé, Joint Unit UMR PAM Food Processing and Microbiology, PCAV Team, 1 Esplanade Erasme, 21000, Dijon, France</footnote>

<footnote><sup>c</sup> University of Zagreb, Faculty of Food Technology and Biotechnology, Laboratory for Food Packaging, 10000, Zagreb, Croatia</footnote>

<footnote><sup>d</sup> Institute of Agrochemistry and Food Technology (IATA), CSIC, Food Safety and Preservation Department, Carrer del Catedràtic Agustín Escardino Benlloch, 7, 46980, Valencia, Spain</footnote>

<footnote>* Corresponding author. University Burgundy Europe, IUT-Dijon, BioEngineering dpt., 7 Blvd Docteur Petitjean, BP17867, Dijon, 21078 cedex, France.
*E-mail addresses:* frederic.debeaufort@ube.fr (F. Debeaufort), mia.kurek@pbf.unizg.hr (M. Kurek), mjfabra@iata.csic.es (M.-J. Fabra), yasaman.ghasemi@ube.fr (Y. Ghasemi), mario.scetar@pbf.unizg.hr (M. Ščetar), kata.galic@pbf.unizg.hr (K. Galić), nasreddine.benbettaieb@ube.fr (N. Benbettaieb).</footnote>
resources can be prepared from natural polymers using industrial biotechnology (fermentation) and transgenic technologies. While the scientific community is well informed in the properties of natural polymers as self-standing materials, with many published papers available (de Lima Santos et al., 2024; Dey et al., 2024; Nath et al., 2023; Ponnusamy & Mani, 2022; Silva et al., 2021), this review will not focus on their properties. However, it will explore their potential use as a functional layer in multilayer biobased packaging.

Flexible multilayer packaging is rapidly growing in the food pres-
ervation industry, offering sustainability and recyclability advantages. It
leverages substrates like paper and plastic films, providing strong gas
and moisture barriers, transparency, twist retention, and minimal
shrinkage during thermoforming. Using biopolymers or biodegradable
polymers could address current packaging issues, but challenges remain
in competing with petroleum-derived films. Currently, the only coatings
available for biodegradable films, that meet food packaging standards,
are metallized or inorganic nanosheet coatings. This field faces several
challenges, including the large-scale production of biodegradable coat-
ings, the coating's delamination and heat-sealability during service
time, and the migration of the coating's compounds into the packaged
food (Agarwal et al., 2023; Rastogi & Samyn, 2015; Tiwari et al., 2016).
However, trends toward more natural polymers and coating encourage
the industry to look after biobased, biodegradable and safer coatings,
such as hydrocolloids (Cazón et al., 2017; Gigante et al., 2021; Martins
et al., 2022; Tiwari et al., 2016). However significant challenge in the
development of biobased coatings for biobased or biodegradable flexible
packaging, using a multilayer design, is the difficulty of combining
thermodynamically immiscible materials without the incorporation of
synthetic additives. While multilayer packaging structures have been
extensively used in commercial synthetic polymers, their application in
the development of biodegradable food packaging systems has gained
attention in recent decades (Debeaufort, 2021; Fabra et al., 2016).

Biopolymers have many advantages, such as addressing plastic waste and food safety concerns. However, they still have limitations compared to synthetic plastics. Synthetic plastics are easy to handle, cost-effective, highly flexible, and water-insoluble, making them convenient for packaging. The use of biobased polymers in food packaging has faced several functional limitations, including: 1) Brittleness: many biobased polymers are prone to cracking and breaking under stress; 2) Difficult heat sealability: achieving strong seals can be challenging, affecting packaging integrity; 3) Thermal instability: biobased materials may not withstand high temperatures during processing or storage; 4) Low melt strength: this can complicate processing methods like extrusion and molding; 5) Poor barrier characteristics: high permeability to water vapor and oxygen limits their effectiveness in preserving food freshness and reaching desired food shelf-life, and hardly envisaged for recycling (up-cycling) or reuse (Ferreira-Filipe et al., 2021; Ghanbarzadeh & Almasi, 2013; Mendes & Pedersen, 2021; Rosenboom et al., 2022; Russell, 2014).

These shortcomings hinder the widespread adoption of biobased polymers as viable alternatives to petroleum-based plastics in food packaging applications. Addressing these issues through various enhancement strategies is essential for improving their performance.

To enhance the functional properties of biobased polymers, espe-
cially their barrier capacities against gases and water permeation or
chemical/solute migration or scalping, various strategies have been
explored in the literature (Cakmak & Sogut, 2020; Panou & Karabagias,
2023). These include.

- Development of Biobased Films and Coatings: Creating specialized thin films and coatings that can improve barrier performance by increasing the mass transfer resistance.

- Incorporation of Nanoparticles: Reinforcing biopolymers with nanoparticles, such as nanoclay, to enhance mechanical and barrier properties by increasing the tortuosity without disturbing the polymer network.

- Chemical Modification: Techniques like grafting fatty acids can significantly improve water vapor barriers, by reducing the diffusion of permeants and migrants as well as decreasing the surface affinity for these.

- Physical Modification: Methods such as heat treatment and irradiation can alter the physical properties of biopolymers, enhancing their barrier capabilities, very often by improved crosslinking or densification of the network.

- Blending Biobased Materials: Combining two or more biobased ma-
  terials can lead to synergistic effects, improving overall performance,
  indeed, interpenetrated biopolymers network very often increases
  stiffness and reduces diffusivity of small molecules.

These strategies collectively contribute to improving the function-
ality of biobased polymers for various applications. Improving hydro-
phobicity and barrier properties, minimizing costs, and scaling up
production could address the drawbacks by way of coating deposition
onto their surface (Cakmak & Sogut, 2020; Pandit et al., 2018; Shaikh
et al., 2021; Sharma, 2022).

Many hydrocolloids and natural macromolecules could be envisaged as coating materials, such as polysaccharides, proteins and lipids, used alone or in combination. They very often need the incorporation of additives such as plasticizers, tensioactive agents, surface agents, acids or alkaline agents, fillers, crosslinkers, bioactive agents, etc. Most of the properties of hydrocolloids considered for coating applications have been evaluated as self-supported films or coating directly applied onto foods as edible packaging (Debeaufort, 2014; Sason & Nussinovitch, 2021; Yemenicioğlu, 2022). Among the hydrocolloid, the most often considered and potentially interesting for coatings are from plant origin (starches, cellulose derivatives, gluten, soy protein) animal origin (gelatin, caseins, whey protein, chitosan), algae origin (carrageenan, alginates) and in a much less extend due to their high price microbial origin (pullulan or xanthan gums). In a very macroscopic point of view, their main properties can be summed up as follows (Debeaufort, 2014; Farris et al., 2010; Jiménez et al., 2018; Yemenicioğlu et al., 2020).

- proteins provide most efficient barrier properties against oils and greases, and against permanent gases (CO₂, O₂, N₂) when isolated from moisture, and some polysaccharides like chitosan;

- polysaccharides offer better mechanical properties because requires much less amount of plasticizers compared to proteins, except gelatins;

- lipids provides moisture affinity reduction but exhibit very poor mechanical properties; proteins and polysaccharides are most easily sensory compatible, and no oxygen sensitive contrarily to lipid based.

- One of the main draw backs of polysaccharide is related to their high solubility in aqueous media (food), whereas protein solubility could be tuned by pH or crosslinkers, and lipid is almost not soluble.

- Dealing with adhesion properties onto biopolymer packaging mate-
  rials, surface properties could be much easier adapted for protein and
  polysaccharide using surface agents compared to lipids

- Some proteins and polysaccharides such as gelatin hydrolysates or chitosan could also exhibit antimicrobial or antioxidant properties.

By improving barrier properties against water vapor and oxygen, as well as mechanical characteristics like sealability and adhesion, these materials can compete effectively with conventional petroleum-based packaging. They also incorporate antimicrobial properties for added performance. These optimized solutions are suitable for a variety of applications, including greasy foods and fresh or dried fruits and vegetables, providing durable moisture and microbial barriers. Additionally, they help preserve the freshness of fish and meat while reducing oil absorption after frying, representing a valuable step toward sustainable packaging practices. Indeed, up to now, most of the biosourced or biodegradable packaging films available on the market are limited to
dry or semi-dry, non oxygen sensitive and no acidic foods.

Additionally, discussions around green chemistry and sustainability are crucial, as they highlight new ideas and possible directions for the broader application of biodegradable polymeric coatings in food packaging.

This review is the first to specifically summarize biodegradable coatings potentially applied onto biobased and/or biodegradable plastic flexible substrates, emphasizing modifications that enhance the performance of these base materials to meet the necessary properties for food packaging applications. This first part focuses on the potential bio-polymers to be coated and on the technologies available and rational to be applied.

## 2. "Bio-related" polymer: terms and definitions

It is well known that most commercial plastics are made from fossil resources. When made from renewable resources, these materials are usually referred to as bioplastics. As it is discussed and listed in Table 1, the term "bioplastic" should not be used, as recommended by the International Union of Pure and Applied Chemistry, IUPAC (Vert et al., 2012). Definition considering a holistic approach have been also given for some concepts related to sustainable packaging, including bio-"-something" term for the different kinds of materials (Dörnyei et al., 2023).

Indeed, the prefix "bio" can refer to several things: (i) to the monomers when obtained from renewable resources (biomass) and then polymerized through chemical mechanisms; (ii) to the polymer when extracted from biomass; (iii) this polymer is biodegradable; (iv) and to the material when produced through biological processes; or to a combination of these (Geueke, 2015; Rosenboom et al., 2022). Indeed, if the meaning of bioplastic is not specifically detailed, the term of "bioplastic" could be attributed to packaging materials that are 100 % petrol-based, or 100 % not biodegradable, or 0 % sustainable (considering water consumption for production and end of life).

As described in Fig. 1, three parts of the figure are presented with the three families of polymers which can be renewable and/or degradable while the fourth one represents "conventional plastics" (Lackner, 2015; Philp et al., 2013).

As for the terminology point of view, polymer-based substances in contact with living systems are described with terms and definitions recommended to users (polymer producers and users, scientists etc.) by IUPAC publications. However, users of different fields of application (polymer area, packaging, agriculture, biotechnology, medicine, pharmacology, food, etc.), including scientists, have often developed inconsecutive terminologies. Thus, it is important to constantly address this issue and to remind us to use recommended terms. Indeed, for instance, "biofilm" is regularly encountered in the literature, however, it is not related to packaging material or type, but to a community of microorganism able to form a continuous layer on a solid surface (Donlan, 2002); or "biopackaging" that is misleading it could refers to bio-sourced, or biodegradable, or in some few cases to packaging dedicated to bioproducts. The inappropriate or abusive terms could induce misunderstanding or wrong perception of the sustainable aspect of a packaging.

The importance of providing consistent definitions was also seen by the Center for International Environmental Law (CIEL, 2023) working on the document entitled "Compilation of Key Terms Relevant for the Negotiation of a Treaty to End Plastic Pollution". Compilation was made from Multilateral Environmental Agreements (MEAs), European Union (EU), the Organisation for Economic Co-operation and development (OECD), International Union of Pure and Applied Chemistry (IUPAC), the Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP), the International Standardisation Organisation (ISO) as well as from other bodies.

The aim of this section is to provide a summary of IUPAC Recommendations 2012 terminology and of main regulations dealing with bio-

<table><caption>**Table 1**
Terms and their definitions, recommended by the IUPAC (Vert et al., 2012), used in this paper and from standards and regulations.</caption><thead><tr><th>Term</th><th>Definitions comments and related regulation texts</th></tr></thead><tbody><tr><td>Biobased</td><td>Composed or derived in whole or in part of biological products issued from the biomass (plant, animal, and marine or forestry materials).<ul><li>Biobased polymer or polymeric device is not necessarily environmentally friendly nor biocompatible nor biodegradable.</li></ul>According to Article D543-295 of the Environmental Code, a "biosourced material" corresponds to "any material of biological origin excluding materials embedded in geological or fossilised formations". The standards NF EN 16760 and NF EN 16575 propose similar definitions (NF EN 16575, 2014; NF EN 16760, 2015; RF CE D543-295, 2018).<br>According to ISO standard 16620-4:2016, biobased plastics refer to plastics that contain materials wholly or partly of biogenic origin (ISO 16620-1, 2015).<br>According to the European Commission biobased plastics are fully or partially made from biological resources, rather than fossil raw materials (European Commission, 2025). They are not necessarily biodegradable or compostable. It is important to examine the full life cycle of biobased plastics, to ensure that they are beneficial to the environment beyond the reduction in the use of fossil resources. This includes changes in land use.</td></tr><tr><td>Biodegradability</td><td>Capability of being degraded by biological activity.<ul><li>In vitro activity of isolated enzymes cannot be considered as biological activity.</li></ul>According to French regulation, the JORF n° 0297 of December 22, 2016 specifies that the biodegradability of a substance or material is assessed, in environmental terms, according to the degree of decomposition, the time required for this decomposition and the effect of the resulting elements on the environment (JORF 2016-0297, 2016).</td></tr><tr><td>Biodegradable (biorelated polymer)</td><td>Qualifier for macromolecules or polymeric substances susceptible to degradation by biological activity by lowering of the molar masses of macromolecules that form the substances.<ul><li>In the field of biorelated polymers, a biodegradable compound is degradable whereas a degradable polymer is not necessarily biodegradable.</li></ul>• Degradation of a polymer in vivo or in the environment resulting from the sole water without any contribution from living elements is not biodegradation. The use of hydrolysis is recommended (See also degradation).<br>The EC regulation considers Biodegradable plastics could biodegrade in certain conditions at their end of life. Compostable plastics - a subset of biodegradable ones - typically decompose in industrial composting facilities, and first need to be collected. Biodegradable and compostable plastics may be made from biological resources or fossil raw materials (European Commission, 2025).</td></tr><tr><td>Biodegradation (biorelated polymer)</td><td>Degradation caused by enzymatic process resulting from the action of cells.<br>This excludes abiotic enzymatic processes.<br>Degradation of a polymeric item is therefore due to cell-mediated phenomena.<ul><li>Biodegradation is referred to as limited to degradation resulting from cell activity. (See enzymatic degradation).</li></ul>• In vivo, degradation resulting solely from hydrolysis by the water present in tissues and organs is not biodegradation; it must be referred to as hydrolysis or hydrolytic degradation.<ul><li>Ultimate biodegradation is often used to indicate complete transformation of organic compounds to either fully oxidized or reduced simple molecules (such as carbon dioxide/methane,</li></ul>(continued on next page)</td></tr></tbody></table>
<table><caption>**Table 1 (continued)**</caption><thead><tr><th>Term</th><th>Definitions comments and related regulation texts</th></tr></thead><tbody><tr><td>Biomacromolecule</td><td><p>nitrate/ammonium, and water. It should be noted that, in case of partial biodegradation, residual products can be more harmful than the initial substance.</p><ul><li>When biodegradation is combined with another degrading phenomenon, a term combining prefixes can be used, such as oxo-biodegradation, provided that both contributions are demonstrated.</li><li>Biodegradation should only be used when the mechanism is proved, otherwise degradation is pertinent.</li><li>Enzymatic degradation processed abiotically in vitro is not biodegradation.</li><li>Cell-mediated chemical modification without main chain scission is not biodegradation (See bioalteration).</li></ul></td></tr><tr><td>Biomaterial</td><td><p>Macromolecule (including proteins, nucleic acids, and polysaccharides) formed by living organisms.</p><ul><li>Not to be confused with biopolymer, although this term is often used as a synonym.</li></ul></td></tr><tr><td>Bioplastic</td><td><p>Material exploited in contact with living tissues, organisms, or microorganisms.</p><ul><li>This general term should not be confused with the term biopolymer or biomacromolecule.</li><li>The use of "polymeric biomaterial" is recommended when one deals with polymer or polymer device of therapeutic or biological interest.</li></ul></td></tr><tr><td>Biopolymer</td><td><p>Biobased polymer derived from biomass or issued from monomers derived from the biomass and which, at some stage in its processing into finished products, can be shaped by flow.</p><ul><li>Bioplastic is generally used as the opposite of polymer derived from fossil resources.</li><li>Bioplastic is misleading because it suggests that any polymer derived from the biomass is environmentally friendly.</li><li>The use of the term "bioplastic" is discouraged. Use the expression "biobased polymer".</li></ul></td></tr><tr><td>Biorelated</td><td><p>Substance composed of one type of biomacromolecules.</p></td></tr><tr><td>Degradable</td><td><p>Qualifier for actions or substances that are connected to living systems.</p><p>Qualifier to a substance that can undergo physical and/or chemical deleterious changes of some properties especially of integrity under stress conditions.</p><p>A degradable macromolecule is able to undergo chain scissions under specific conditions, resulting in a decrease of molar mass.</p><p>A degradable polymer is a polymer in which macromolecules are able to undergo chain scissions, resulting in a decrease of molar mass. (See degradation (biorelated polymer)).</p></td></tr></tbody></table>

**Degradation**

* The process of degradation may be specified by a prefix or an adjective preceding the term "degradation". For example, degradation caused by the action of water is termed "hydrodegradation" or hydrolysis; by visible or ultraviolet light is termed "photo-degradation"; by the action of oxygen or by the combined action of light and oxygen is termed "oxidative degradation" or "photooxidative degradation", respectively; by the action of heat or by the combined effect of chemical agents and heat is termed "thermal degradation" or "thermochemical degradation", respectively; by the combined action of heat and oxygen is termed "thermos-oxidative degradation".

<table><caption>**Table 1 (continued)**</caption><thead><tr><th>Term</th><th>Definitions comments and related regulation texts</th></tr></thead><tbody><tr><td>Degradation (biorelated polymer)</td><td><p>Degradation that results in desired changes in the values of in-use properties of the material because of macromolecule cleavage and molar mass decrease.</p><ul><li>For biorelated polymers, the definition is purposely and specifically limited to the chemical degradation of macromolecules in order to make a clear distinction with the physical degradation of the material. (See fragmentation and disintegration.)</li><li>In any condition, degradation must be used instead of biodegradation when the mechanism of chain scission is not known or demonstrated as cell-mediated.</li><li>Degradation can result from the action of enzymes (see enzymatic degradation), or from the action of cells, organisms, and/or microorganisms (See biodegradation).</li></ul></td></tr><tr><td>Disintegration or fragmentation</td><td><p>Fragmentation to particles of a defined size that could result from either chemical, physical or biological process. The fragmentation is the breakdown of a material to particles regardless of the mechanism and the size of fragments.</p></td></tr><tr><td>Enzymatic degradation and/or decomposition</td><td><p>Degradation caused by the catalytic action of enzymes. Sometimes called enzymic degradation. Degradation caused by enzymes can be observed under biotic or abiotic conditions but only degradation due to cell bioactivity can be called biodegradation, otherwise enzymatic degradation is to be used, especially in the case of in vitro abiotic conditions.</p></td></tr><tr><td>Compostable</td><td><p>Ability to biodegrade under optimized domestic or industrial conditions of humidity, temperature and oxygenation. Literally, anything that can be composted or be involved in a composting.</p><p><em>Today in France, two standards govern the designation "compostable" for packaging: NF EN 13432 for industrial compostability (2001/524/EC, 2001; NF EN 13432, 2000) and NF T51800 for domestic compostability (NF T51-800, 2015).</em> Plastic packaging that complies with these standards can therefore be recovered organically either by industrial composting (NF EN 13432) or by domestic composting (NF T51800), in the same way as organic waste (food waste, green waste, etc.). Biodegradable and compostable bioplastics (Gontard et al., 2019). According to EU, compostable plastics - a subset of biodegradable ones - typically decompose in industrial composting facilities, and first need to be collected.</p><p>Biodegradable and compostable plastics may be made from biological resources or fossil raw materials (European Union, 2023).</p></td></tr></tbody></table>

related and degradation related terms (Table 1) used by the scientists working and publishing in this area of science, as well as the standard and regulations when existing. It is also worth mentioning that IUPAC Recommendations 2019 is devoted for the nomenclature, terminology and abbreviations related to lactic acid-based polymers (Dörnyei et al., 2023; Vert et al., 2012). Table 1 also gives an explanation for biodegradation (biorelated polymer) as well as terms combined with prefixes such as oxo-biodegradation. This one is very important to address as oxo-degradable (or oxo-biodegradable) plastics are conventional plastics, such as high-density polyethylene (HDPE), commonly used as shopping (carrier) bags. These materials also include additives (transition metal salts or oxides - often with nickel, iron, manganese and cobalt) which promote the oxidation of the material resulting with its fragmentation into micro-fragments or to chemical decomposition. In 2019 the use of oxo-degradable plastics was banned in the European Union (Costa et al., 2023; European Union Parliament, 2019; Mamin et al., 2023).
<figure><img src="image_4.png" /><figcaption>Fig. 1. Types of and examples of materials (adapted from Philp et al., 2013).</figcaption></figure>

### 3. Suitable biopolymer films for coating

Many polymers and biopolymers have been studied in the scientific literature for packaging materials and dedicated to food sector (Dey et al., 2024; Kabir et al., 2020; Panou & Karabagias, 2023; Popović et al., 2018; Shaikh et al., 2021; Sinha, 2024). However, very few are industrially produced and commercially available due to their lack of technical and/or functional performances, or due economical reasons (Arun et al., 2024; Rosenboom et al., 2022; Tonevitskaya, 2019). This review will then focus only on the commercialized bio-related packaging materials (biosourced and/or biodegradable). The cellulosic materials (paper, cellophanes, etc.) are not considered as biopolymer-based packaging according to BioplasticsEurope project (BioPlasticsEurope, 2025) and PlasticsEurope (PlasticsEurope, 2022) and therefore, in this review, they are scarcely taken into account as support material for coating application i. This review is focused on these main biobased and/or biodegradable polymers. Fig. 2 presents the state of the market of biobased packaging material worldwide in 2024 (adapted from EuropeanBioplastics, 2025), but only about 65 % is dedicated to food packaging. Very few packaging materials being biobased and/or biodegradable for the food packaging sector are industrially produced

and economically competitive to conventional polymer materials. The main biopolymer packaging considered for coating are polylactic acid-based (37 % of biobased production), starch-based (5 %), polyhydroxyalkanoates-based (PHA's, PHB, PHBV, 4.1 %), PBAT (4.6 %), that are both biosourced and biodegradable, and the polycaprolactons (PCL, 2 %) that are biodegradable but obtained from fossil resources.

#### 3.1. *Polylactides (PLA)*

PLA, or poly(lactic acid), is widely accessible as a biobased polymer and is deemed safe for food packaging as approved by the Food and Drug Administration and EU regulation (Conn et al., 1995; O'Loughlin et al., 2023). It is known as biodegradable, compostable, and renewable polymer, and the second most commonly produced biobased polymer following starch (Agwuncha et al., 2017). It is produced from lactic acid obtained through anaerobic fermentation of carbon-containing substrates. These substrates may be pure, such as glucose or lactose, or impure, like starch or molasses, and are fermented with microorganisms such as *Lactobacillus*, *Pediococcus*, *Lactococcus*, and *Streptococcus* bacteria, or specific types of fungi such as *Rhizopus Oryzae*. PLA is a

<figure><img src="image_5.png" /><figcaption>Fig. 2. Biobased plastic production world capacity in 2024 (adapted from EuropeanBioplastics, 2025; Rosenboom et al., 2022).</figcaption></figure>
transparent, colourless thermoplastic that bears some resemblances to polystyrene (Sin & Bee Soo Tueen, 2019; Swetha et al., 2023).

PLA is a thermoplastic polymer commonly used in a growing number of applications, including packaging and 3D printing, to make both rigid and flexible objects. PLA is relatively inexpensive compared with other biopolymers used for packaging, and its mechanical strength is compatible with its intended uses, making it a popular material (Sin, Tueen & (avec Plastics Design Library), 2019). It is mainly used as flexible films for packing dry food (pasta, confectionary, seed and nuts, short-life bakery products like pastries, and confectionary). PLA is a possible alternative of low- and high-density polyethylene (LDPE and HDPE), and poly terephthalate (PET). It is often an alternative of poly-styrene for trays or cups in catering applications (fast-foods, street food, restaurants, etc.) and fresh cut fruits for daily consumption. Its production capacity is estimated to 290 kilo tons (kt) for 2019 (Forecast Report, 2025) and about 459 kt in 2022 (Teixeira et al., 2023), but only half is dedicated to packaging in 2024 (EuropeanBioplastics, 2025).

The noteworthy qualities of PLA include attractive appearance, high mechanical strength, resistant to wear and tear, stability when exposed to ultraviolet radiation and low toxicity because of low migration rates. However, its vulnerability to gas and vapor permeation is a disadvantage, which can lead to an elevated risk of oxidation reactions in packed food. It can undergo processing using similar techniques to conventional plastics, especially in the form of film and fiber. The use of PLA in the packaging industry is growing, in applications necessitating surface hydrophilicity for surface finishing, printing, ink priming, and adhesion based on product design requirements. Nonetheless, the inherent low wettability, inadequate thermal and mechanical characteristics, as well as the insufficient water vapor barrier restricts its potential for use in attractive packaging for end consumers (Bee Soo Tueen, 2019; Mendes & Pedersen, 2021; Sin &; Swetha et al., 2023). One of its main inconvenient is the sensitivity to acids that induces hydrolysis and low mechanical properties that could be compensated by protective coatings (Apicella Apicella, Adiletta, Albanese, Di Matteo, & Incarnato, 2021).

To address these challenges, a range of strategies have been proposed, including the chemical or physical modification of biopolymer molecules, the incorporation of nanoparticles and hydrophobic additives into the polymer matrix, blending it with nonpolar polymeric substances, and creation of bilayer or multilayer films by coatings (Fu & Dudley, 2021). Indeed, Debeaufort et al. (2022) demonstrated that oxygen permeability of PLA coated with fish gelatin can be reduced up to 600 times. Similarly, PLA coated with polyvinyl alcohol based coatings led to a reduction of the oxygen transmission from 2200 to 8 cm³ m⁻²·day⁻¹·bar⁻¹ (Apicella et al., 2022). In another application targeted study, PLA film coated with chitosan embedding essential oils proved its efficacy as an antimicrobial active film for packing oil sensitive to oxygen and other contaminations (Stoleru et al., 2021).

## 3.2. Polyalkanoates (PHAs)

Polyhydroxyalkanoates (PHAs) are biodegradable polymers produced by microorganisms. Under non-favorable conditions such as nutrient scarcity, extreme heat, ultraviolet radiation, and osmotic shock, bacteria produce PHAs as a carbon and energy storage reservoir to survive to the stress (Yadav et al., 2021). PHAs exist in various forms, up to 150 different structures (Mitra et al., 2020). PHAs can then be classified into two categories: 1) short-chain-length PHAs (scl-PHAs), which consist of 3–5 carbon atoms, and 2) medium-chain-length PHAs (mcl-PHAs) that are the more suitable for flexible packaging, containing 6–14 carbon atoms. Sourced from renewable materials such as plant oils and agricultural waste, PHAs are fully biodegradable under aerobic and anaerobic conditions, breaking down into harmless byproducts like carbon dioxide and water (Keshavarz & Roether, 2010; Masood 2017). PHAs are used across a variety of fields, including food packaging, drug delivery and tissue engineering (Samrot et al., 2021).

The PHA market continues to expand as businesses increasingly

adopt sustainable practices, boosting the demand for PHA-based biodegradable materials in packaging, agriculture, and single-use products. Regulations against single-use plastics and shifting consumer preferences are driving the transition toward sustainable materials like PHA. Additionally, advancements in PHA production technologies and increased investment in biobased products are fuelling market growth. However, challenges such as high production costs and limited raw material availability hinder widespread implementation. In 2024, the global market value of PHA was estimated to be 73.12 million U.S. dollars and may double for 2029 (Statista, 2025). In 2021, 28 producers exhibited a production capacity of about out 48 000 tons among these only 30 % is dedicated to packaging (Bioplastics Magazine, 2025). Out of the PHAs family and due to their high elasticity and robustness PHB (poly-3-hydroxy butyrate) and PHBV (Poly-3-hydroxybutyrate-co-3-hydroxyvalerate), PHBH (Poly-3-hydroxybutyrate-co-3-hydroxyhexanoate) continues to be the leading type of PHA in the market (Global Market Insights, 2025).

PHA generally have favorable mechanical properties and a lower glass transition temperature than PLAs, making them suitable for production of flexible packaging and as functional coatings. PHAs are considered strong candidates for replacing petroleum-based plastics for food packaging. This is particularly seen in the context of active food packaging and coatings that require enhanced performance.

Coatings providing barrier properties or active properties for food packaging requirements are being used to improve the overall performance of PHA-based materials. Figueroa-Lopez et al. designed an active multilayer film based on PHA with and without high barrier coatings containing cellulose nanocrystals (CNCs) (Figueroa-Lopez, Enescu, et al., 2020). The multilayers showed transparency, interlayer adhesion, improved barrier to water and limonene vapours by more than 30 %, and intermediate mechanical performance. This improvement in barrier properties is mainly attributed to the CNC thin coating, which is particularly effective at enhancing oxygen resistance (Figueroa-Lopez, Cabedo, et al., 2020). However, CNC coatings have demonstrated significant oxygen permeance reductions PHB and PHBV coated films, achieving decreases of over 90 % when shielded from moisture (Cherpinski et al., 2018). Fabra et al. and Pinheiro Bruni et al. reported a strategy for creating biodegradable multilayer food packaging structures using biomass-derived composites by means of the electrospinning as coating or bioadhesives onto PHAs (Fabra et al., 2016; Pinheiro Bruni et al., 2020).

## 3.3. Thermoplastics starch-based (TPS)

After cellulose-based materials, starch-based were the first biobased and biodegradable materials developed and commercialized for flexible food packaging (Garavito et al., 2024). The starch-based plastics market is segmented between thermoplastic starch (TPS) and starch polymer blends, with a predominance of TPS. TPS is produced by combining starch with water and/or plasticizers such as glycerol or sorbitol, making it a versatile and widely used material (Rodríguez et al., 2006). It is commonly utilized in manufacturing various packaging products, including films, bags, disposable plates and bowls, loose-fill packaging, antistatic materials, shaped protective packaging, and agricultural products. Starch-based polymers are widely used in food packaging and edible films due to their ability to enhance food organoleptic properties, reduce food loss, and extend shelf life. These materials are odorless, non-toxic, and biodegradable, making them highly suitable for food applications. Additionally, starch-based edible films exhibit low oxygen permeability, helping to preserve food quality and freshness (Pelissari et al., 2019). Main advantage of TPS is the ability to be extruded or injected for shaping either films or containers in continuous process and at low costs. The global production capacity is estimated to 185 000 tons in 2025 (Mordor Intelligence, 2025).

The starch-based plastics market was valued at USD 1.63 billion in 2023 and is projected to grow from USD 1.74 billion in 2024 to USD 3.31
billion by 2032, reflecting a compound annual growth rate (CAGR) of 8.07% during the forecast period. Starch-based plastics are widely utilized across various industries, including agriculture, automotive and transportation, consumer goods, and packaging. The rapid expansion of these industries is a key driver of increasing global demand for starch-based plastics (Market Research Future, 2025; Rodríguez et al., 2006). For now, the main uses of TPS for packaging is dedicated to the substitution of paper bags or oil-based plastics (polyolefins) in supermarkets for fruits and vegetable, or for films and containers for catering.

While starch itself is not inherently thermoplastic, it can be transformed into a thermoplastic material when combined with a plasticizer (such as water, glycerol, or sorbitol) and exposed to high temperatures (ranging from 90 to 180 °C) and shearing forces. This process allows starch to melt and become fluid, making it suitable for use in injection molding, extrusion, and blowing equipment, similar to synthetic plastics. During starch extrusion, the combining of shearing, temperature and plasticization allows the obtaining of a melted thermoplastic material (Averous et al., 2001b). Afterwards, this material can be transformed by means of thermoforming or injection molding.

The properties of plasticized starches are highly influenced by moisture content. The higher the plasticizer content (such as polyols), the higher the moisture content, which in turn alters the material's properties over time (Averous et al., 2001b). Even with controlled moisture and temperature, the material tends to exhibit lower elongation at break and increase rigidity.

Different strategies have been extensively investigated to enhance the mechanical and barrier properties of thermoplastic starch while simultaneously reducing its hydrophilic nature, which significantly limits its applicability in food packaging. Currently, the market trend favors the production of starch-based materials with a high starch content, combined with other biodegradable plastics (30–80 %). This can be achieved either through blending, shaping of multilayer structure or by incorporating dispersed reinforcing agents to create nanocomposites (Averous et al., 2001a). According to Martin et al. (O. Martin et al., 2001), starch films can be combined with thermoplastic polyesters like polycaprolactone (PCL) and processed using traditional methods such as blowing or calendaring. When it comes to biodegradable films, the industrial process can achieve yields of 80%–90% of those obtained for low-density polyethylene (LDPE). Coating technique is an effective and economic method to improve the properties of TPS films. Interfacial adhesion between samples and coating materials is the most important factor to develop the properties of it by the coating technique. However, finding the effect of interfacial adhesion between coating materials with bio-materials upon its properties has not been achieved so far. Jantasakulwong et al. studied the effect of interfacial adhesion between coating materials and TPS on the coated TPS properties (Jantasakulwong et al., 2019).

### 3.4. PolyEthylene adipate terephtalate (PBAT)

Polybutylene adipate terephthalate (PBAT) is an aliphatic-aromatic (co)polyesters and is a biodegradable polymer. The aliphatic-aromatic copolyester also comprises the polybutylene succinate terephthalate (PBST) but much less considered for packaging applications. PBAT is commercially synthesized from 1,4-butanediol (BDO), adipic acid (hexanedioc acid), and purified terephthalic acid (1,4-benzenedicarboxylic acid) in a two-stage reaction process. The first polymerization of PBAT usually involves the formation of an oligomer, while the final polymerization is the polycondensation process. PBAT is a random copolymer with a large amorphous region that provides interesting mechanical properties for flexible materials (Roy et al., 2024). However, there is free volume that allows for easy passage of water and gas molecules, leading to inferior barrier properties.

The global PBAT market was valued at USD 1.5 billion in 2022 and is projected to reach USD 2.8 billion by 2027, (Markets and Markets, 2025). The global production is about 1.14 million tons in China in

2022, main producer country, and estimated to 1.4 million tons for the worldwide production capacity (Luo et al., 2024). There is a growing demand for packaging made from PBAT polymers, particularly for wrapping organic food and premium, branded products that have specific packaging requirements. PBAT is frequently used to produce films due to its excellent stretchability, flexibility, and processability. Depending on the intended application, PBAT films can be transparent, translucent, or opaque. Additionally, PBAT films offer good sealability and printability, making them ideal for flexible packaging applications. But they have relatively weak mechanical strength and barrier properties (Itabana et al., 2024). To improve these properties, several modifications are being made which include blending with other biobased materials and filler incorporation to develop novel PBAT-based blends and biocomposites, as well as coating applications. Recently Melendez-Rodriguez et al. investigated a multilayer structure to improve functional properties of PBAT films (Melendez-Rodriguez et al., 2024). They tested two outer layers made of a PHB/PBAT blown film (30/70) as a mechanical support and water vapor barrier. The coating-layer substantially improved the PBAT multilayer's barrier properties, reducing permeance by 95 %.

### 3.5. Polycaprolactons (PCL)

Among biodegradable polymers, polycaprolactone (PCL) stands out as a hydrophobic, semi-crystalline linear aliphatic polyester with a low melting point of around 60 °C and a glass transition temperature of approximately –60 °C, which facilitates easy processing. It is crystalline and is produced by the ring-opening polymerization of e-caprolactone. It is highly biocompatible and food contact material agreed, exhibits enhanced toughness, and is cost-effective compared to other biobased polymers. Indeed, PCL's suitability for packaging applications, such as films, trays, and containers, stems from its unique properties such as biodegradability, flexibility, durability, and low melting point, offering an environmentally friendly alternative to traditional plastics. PCL could be suitable for packaging refrigerated foods like fresh salads and cheeses for instance (Oney-Montalvo et al., 2024). The global polycaprolactone PCL market size was valued at USD 499.78 billion in 2023 and the worldwide production is not available, however, the main producer (BASF) stated to have about 500 000 metric tons of production in Asia (Data Bridge Market, 2025).

Functional properties of PCL films can be improved after shaping multilayer's structure or after applying of a thin coating on the film surface. For instance, cold plasma treatment was pre-applied to modify the surface of synthetic biodegradable PCL in order to provide a more hydrophilic character and consequently improve the compatibility with starch layer application (Heidemann et al., 2019). The multilayer films increased by 30 % the barrier properties compared to PCL monolayers structure and reinforced the adhesion that withdraw the delamination. Similarly, coating application of PCL on dried chitosan film could be seen as an improvement to film structure while a decrease in the moisture resistance of films was observed (Sogut & Seydim, 2018). However, the bilayer systems demonstrated great capacity as bioactive films with incorporation of grape seed extracts. In a same way, Sharmin et al. displayed that the application of chitosan layer onto PCL film significantly enhanced the tensile strength and young's modulus of the composite films. The oxygen transmission rate (OTR) of the composite film showed a notable decrease compared to pure PCL films (Sharmin et al., 2012).

## 4. Surface treatments prior coating

The naturally low wettability characteristic of most polymeric materials poses a challenge to the development of innovative packaging solutions for consumers. Recent research has focused on enhancing the surface properties of biobased materials, with a notable accent on PLA, achieved through the incorporation of various additives such as
lubricants, plasticizers, and other polymers. However, this method is generally discouraged for food packaging applications due to the potential generation of waste that could adversely affect both the environment and public health. Furthermore, the choice of the coating substrate plays a crucial role in ensuring a successful coating process, which is essential for achieving high-quality properties. The characterization of the substrate in terms of its surface properties is essential for effectively managing a coating process, as it provides critical insights into the appropriate methodologies required to attain an optimal coating finish. The most important property relative to coating process is its surface tension. To overcome this limitation, there are numerous surface modification techniques, both physical and chemical, known to the scientific community but also already available on industrial level. However, chemical treatments frequently generate waste by degrading the polymer surface and therefore posing potential environmental risks. It is crucial that these modifications target only the uppermost surface layer (on the order of tens of nanometers), as this is vital for ensuring functional strength while preserving the material's overall integrity (Abdulkareem et al., 2021). As a result, waste-free and solvent-free treatments are more desirable. Plasma technology-based treatments offer significant benefits in this context, as they are environmentally sustainable and can selectively modify the surface layers without affecting the underlying material. Surface properties of biobased or

biodegradable polymers should thus be characterized to assess the efficiency of the surface treatment and the impact on the further functional properties. Upon treatment, it is important to characterise its efficiency. Mostly used techniques include chemical characterization of the surface, typically achieved through Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which facilitate the quantitative evaluation of chemical alterations or modifications of the surface resulting from plasma treatment; and/or physical characterization of the surface, which includes an analysis of surface topography using scanning electron microscopy (SEM) and atomic force microscopy (AFM) to measure surface roughness as well as gravimetric methods to assess weight variations; and physical-chemical properties by the surface wettability and surface free energy measurements (goniometry).

One of the strategies to modify biobased surface involves the application of a plasma treatment, which aims to enhance the wettability, adhesion, and biocompatibility of the biopolymer in question. Various, currently scalable examples are given in Table 2. In general, plasma treatment is a common technique used to enhance the surface characteristics of polymeric materials, aligning them with their intended functional applications. In recent years, the use of atmospheric plasma has grown remarkably, since it enables covalent surface immobilization of biomolecules and is characterised as a dry, a reagent-free, and a

<table><caption>**Table 2**

Literature examples of surface treatments applied on biobased substrates aimed for use as food packaging materials.</caption><thead><tr><th>Substrate</th><th>Substrate origin</th><th>Treatment</th><th>Surface properties</th><th>Additional coating layer</th><th>Reference</th></tr></thead><tbody><tr><td>Bi-oriented PLA</td><td>Commercial films (Taghleef Industries, Nativia NTSS, Italy)</td><td>Plasma<br>30 W m<sup>-2</sup> min<sup>-1</sup></td><td>improved wettability with water contact angle reduced from 81° to 70° and reduction of surface roughness</td><td>No further coating process</td><td>Rocca-Smith et al. (2016)</td></tr><tr><td>PLA</td><td>Commercial PLA 6201D, pellet form (Nature Works LLC, USA)</td><td>Atmospheric plasma<br>50/60 Hz, 230 V, 16 A, discharge 17 kHz, 20 kV, circular nozzle (1900 rpm), nozzle-substrate distance 10, 14 and 20 mm and advance rate of 100, 300, 700 and 1000 mm s<sup>-1</sup></td><td>Improved wettability of PLA with an increase of the Surface Free Energy (SFE) from 35 to 60 mN m<sup>-1</sup> and roughness decrease from 60 to 15 nm</td><td>No further coating process</td><td>Jordá-Vilaplana et al. (2014)</td></tr><tr><td>PLA</td><td>Commercial PLA<br>2002D, M<sub>w</sub> = 217 000 g mol<sup>-1</sup> (NatureWorks LLC, USA)</td><td>Plasma (high frequency)<br>Low pressure (1.3 MHz, 40 Pa) inside a glass reactor using N<sub>2</sub> atmosphere and 100 W discharge power</td><td>Improved wettability (water contact angle drop from 89 to 53°) and creation of reactive sites onto PLA</td><td>Chitosan, dip coating</td><td>Stoleru et al. (2021)</td></tr><tr><td>PLA</td><td>Commercial PLA films (Goodfellow, UK)</td><td>Oxygen plasma<br>Moderate intensity for 15 min</td><td>A significant reduction in the water contact angle (from 80 to 40°)</td><td>Chitosan, dip coating</td><td>Gomes et al. (2021)</td></tr><tr><td>PLA</td><td>Commercial PLA films (Bi-AX International Inc., Canada; EVLON®)</td><td>Air corona<br>Energy values of 56 dyne/cm</td><td>Reduction by 50 % of the surface water affinity</td><td>Chitosan and/or chitosan/beeswax, film casting</td><td>Mohd Aris et al. (2019)</td></tr><tr><td>PLA</td><td>Commercial PLA films (Bio-FlexR F 2110)</td><td>Air atmospheric plasma<br>Power: 220 V; output frequency: 4–5 MHz<br>transformer of rated voltage 10–48 kV. Noodle distance of 6 mm; five treatments (0, 20, 40, 60 & 80 s)</td><td>Improved adhesion of gelatine and the SFE increased from 21 to 47 mN m<sup>-1</sup></td><td>Gelatin; trilayer, thermo-compression processing</td><td>Valdés et al. (2017)</td></tr><tr><td>PLA</td><td>Commercial PLA granules with a D-isomer content of 4.3 %, M<sub>w</sub> of 2.53 × 10<sup>5</sup> g/mol, films prepared in the study</td><td>Low-temperature plasma at vacuum pressure<br>Plasma discharge generated at radiofrequency powered, at a frequency of 13.56 MHz. A cylindrical Al chamber containing PLA samples vacuumed to 0.2 Torr, plasma treatment for 30–180 s at 80 W</td><td>Improved wettability with an increase of the SFE from 17 to 50 mN m<sup>-1</sup> and contact angle drop from 75 to 50° for water, from 62 to 26° for ethylene glycol</td><td>Fumaric and ascorbic acid grafting at PLA surface</td><td>Abdulkareem et al. (2021)</td></tr><tr><td>PLA</td><td>Commercial PLA (Taghleef Industries, Nativia NTSS, Italy)</td><td>Corona<br>Industrially pretreated</td><td>reduction of surface absorption rate from 26 to 11 × 10<sup>4</sup> µL mm<sup>-2</sup> s<sup>-1</sup> but almost no change in surface contact angle and SFE</td><td>Fish gelatin, Coating from solution casting</td><td>Bozović et al. (2023)</td></tr><tr><td>PLA or PCL</td><td>Commercial PLA (3D LAB Brazil)<br>Commercial PCL (Sigma Aldrich)</td><td>Cold plasma<br>Barrier dielectric discharge (DBD), 50 Hz and 31 kV for 1, 5, 10, 15 and 20 min</td><td>Surface erosion with increased roughness from 10 to 60 nm, improved wettability with a drop of contact angle from 75 to 20°</td><td>Starch coating; casting from solution</td><td>Heidemann et al. (2019)</td></tr><tr><td>PLA/ Polybutylene succinate (PBS)</td><td>Commercial PLA film (Ingeo TM biopolymer, Nature Works LLC. USA)</td><td>Corona<br>4 m/min with a 3-mm air gap between the electrode and the plastic sheet; current of 4, 5, and 6 A</td><td>Contact angle of PLA/PBS sheets decreased for low current (4 A), while increased for 5 and 6 A; increased surface roughness</td><td>Chitosan, dip coating</td><td>Hongsriphan and Sanga (2018)</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Relationship between coating thickness and technologies of applications (adapted from Cazón et al., 2017; Davis et al., 2014; Farkas et al., 2023; Gribova et al., 2021; Griehl et al., 2003; Kausar, 2018; Melentiev et al., 2022; Özenler et al., 2023; Spangler et al., 1990; Van Zele et al., 2018; Walsh & Franses, 1999; Zhang et al., 2022; Zilio et al., 2014).</figcaption></figure>

single-step alternative for surface biofunctionalization. The process initiates the activation of species on the surface of the substrate, leading to the formation of novel functional groups, while also facilitating micro etching that contributes to a cleaning effect. The utilization of oxygen as the gaseous medium can result in the generation of low molecular weight oxidized entities in polymeric substrates. Atmospheric plasma operates at ambient temperature in a continuous manner, making it suitable for industrial applications. This method yields consistent and reproducible outcomes and is environmentally sustainable, as it does not generate waste (Walden et al., 2024). Indeed, the impacts of the plasma treatments on the surface properties can be improvements or alterations as described here after:

Upon treatment, it is important to characterise its efficiency. Therefore, impacts of surface treatment are hereafter listed.

a) The plasma treatment might result in alterations in the surface chemistry, like oxidation and increased surface roughness (Abdulkareem et al., 2021; Jordá-Vilaplana et al., 2014; Rocca-Smith et al., 2016). Increase in the intensity of some characteristic peaks like C=O, C-O (secondary alcohols) and COOH can be interpreted as representative of surface oxidation due to the plasma effects. They make new reactive sites available for chemical bonding, for instance with hydrocolloid functional coatings, upon their application on the surface (Stoleru et al., 2021). The XPS analysis of the material indicates the formation of new oxygen-containing groups on the surface, as evidenced by an increase in the O/C ratio. Valdes et al. showed that the oxygen composition (%) tended to increase, whereas carbon composition decreased with increasing treatment time (Valdés et al., 2021). Consequently, an increase in the O/C ratio was found when the PLA surface was treated with corona discharge from 0 to 80 s, with values ranging from 0.18 to 0.37 %, respectively.

b) The untreated material typically exhibits a homogeneous surface characterized by a smooth and consistent appearance, along with a

characteristic texture and uniform topography or morphology. Abdulkareem et al. showed that small spots, grooves, and cavities appear on the PLA substrate treated with plasma discharge (Abdulkareem et al., 2021). Changes in roughness were also observed as result of the physical bombardment of the surface with high-energy particles formed during plasma generation. Accordingly, in corona treated PLA/PBS sheets, the overtreatment of corona energy led to thermal degradation of polymer molecules and resulted in etching on sheet surface, which increased surface roughness (Hongsriphan & Sanga, 2018).

c) From a practical perspective, enhanced wettability leads to improved adhesion between the substrate and the applied material which is particularly crucial for hydrocolloids. The untreated PLA surface has mostly a hydrophobic behaviour with a low total surface free energy (SFE, equivalent to surface tension of liquids) of 37.1 mN m⁻¹ and 33.3 mN m⁻¹ as given by Vilaplana et al. and Gomes et al. respectively (Gomes et al., 2021; Jordá-Vilaplana et al., 2014). Adequate surface treatments might even double the surface free energy values over 60 mN m⁻¹ and 45 mN m⁻¹, respectively. Untreated samples have a relatively low polar component which the value is 6.8 mN m⁻¹ that can be increased up to 32.8 mN m⁻¹ after the surface treatment (Jordá-Vilaplana et al., 2014). The significance of the dispersive component appears to be relatively minor. As the surface is being modified, its hydrophilicity increases which is evidenced by a notable decrease in the contact angle measurement. Gomes et al. showed decrease from 79.2° to value of around 70, Valdes et al. from around 90 to around 50° and Abdulkareem et al. from 70.2 to 55.1° (Abdulkareem et al., 2021; Gomes et al., 2021; Valdés et al., 2021).

Other principles might also be applied, like the one that involves the development of superhydrophobic surfaces, particularly for applications requiring water-repellent characteristics. Superhydrophobic coatings are commonly employed to inhibit fouling and contamination in food
<table><caption>Table 3</caption><thead><tr><th>Technology</th><th>Extrusion-Lamination</th><th>Hot-press-Lamination</th><th>Powder deposition</th><th>Vacuum deposition</th><th>Solution casting</th><th>Die casting</th><th>Blade/air knife coating</th><th>Liquid</th><th>Chemical</th><th>Aqueous & organic</th><th>Chemical</th><th>Electrospinning coating</th><th>Flexography & Lithography</th><th>Adhesive wet coating</th></tr></thead><tbody><tr><td>State of coating for deposition</td><td>Molten solid</td><td>solid</td><td>solid</td><td>Gas & vapor</td><td>Liquid</td><td>Liquid</td><td>Liquid</td><td>Liquid</td><td>Chemical</td><td>Aqueous & organic</td><td>Chemical</td><td>Liquid</td><td>Liquid</td><td>Liquid</td></tr><tr><td>Principle of deposition</td><td>Physical thermal</td><td>Physical thermal</td><td>Physical thermal</td><td>Chemical</td><td>Chemical</td><td>Chemical</td><td>Chemical</td><td>Chemical</td><td>Chemical</td><td>Aqueous & organic</td><td>Chemical</td><td>Chemical</td><td>Chemical</td><td>Chemical</td></tr><tr><td>Solvent</td><td>None</td><td>None</td><td>None</td><td>None</td><td>None or</td><td>None or</td><td>None or</td><td>None or</td><td>None or</td><td>None or</td><td>None or</td><td>Aqueous & organic</td><td>Aqueous & organic</td><td>Organic</td></tr><tr><td>Pre-treatment</td><td>None</td><td>None or</td><td>None or</td><td>None or</td><td>Corona</td><td>Corona</td><td>Corona</td><td>Corona</td><td>Corona</td><td>Corona</td><td>Corona</td><td>Corona</td><td>Corona</td><td>None</td></tr><tr><td>Post-treatment</td><td>None</td><td>None</td><td>None</td><td>Curing</td><td>Curing</td><td>Curing</td><td>Curing</td><td>Curing</td><td>Curing</td><td>Curing</td><td>Curing</td><td>Drying &/or Curing</td><td>Drying &/or Curing</td><td>None</td></tr><tr><td>Thickness range (μm)</td><td>~50–5000</td><td>~15–1000</td><td>~0.005–5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~0.005–0.5</td><td>~10–500</td></tr><tr><td>Investment cost Operating (process) cost</td><td>High Moderate</td><td>Moderate Cheap</td><td>Moderate</td><td>High</td><td>High</td><td>Moderate</td><td>Low</td><td>Moderate</td><td>Low</td><td>Moderate</td><td>Low</td><td>High</td><td>High</td><td>Moderate Cheap</td></tr><tr><td>Advantages</td><td>Fast processing, continuous process, reliable high bond strength</td><td>Fast processing, simple, no cure waiting time, no harmful chemical, reliable high bond strength</td><td>Very thin coating, no harmful chemical or solvents</td><td>Very thin coating, no harmful chemical or solvents</td><td>Adapted for water-based solvent, very versatile, almost no limitation due to viscosity</td><td>Adapted for water-based solvent</td><td>Low Moderate</td><td>Moderate</td><td>Moderate</td><td>Moderate</td><td>Moderate</td><td>Low Expensive</td><td>Low Expensive</td><td>Adapted for water-based solvent</td></tr><tr><td>Limitations</td><td>Quite thick coating, limited to extrudable film-forming solution and hydrocolloid able to melt without thermal degradation</td><td>Heat sensitive, may shrink with heat</td><td>Sensitive to scratch, timely process, discontinuous process, not adapted for heat sensitive film support</td><td>Sensitive to scratch, timely process, discontinuous process, heat curing for melting powder, not adapted for heat sensitive films, use of harmful gas or vapours.</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td><td>Need drying or solvent evaporation, timely process, could require curing</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Solvent-free technologies for coating application (adapted from Apicella Apicella, Barbato, Grimaldi, Scarfato, & Incarnato, 2023; Bacha et al., 2023; Choy, 2003; Kausar, 2018; La Fuente Arias et al., 2023; Mattox, 2003; Miletić et al., 2020; Sundara Rajan & Rothstein, 2024; Vartiainen et al., 2016).</figcaption></figure>

packaging. Furthermore, these coatings contribute to reducing food waste and enhancing consumer satisfaction by facilitating the effortless vision of food from the interior of the packaging (Ruzi et al., 2022). For example, Sun et al. improved the hydrophobicity of poly (l-lactic acid) (PLLA) film by designing film surfaces with multi-scale microstructures via nonsolvent induced phase separation (NIPS) while Liu et al. prepared microporous PLA films via NIPS using dichloromethane as a solvent and N-methyl-2-pyrrolidone as a nonsolvent (W. Liu et al., 2022; X. Sun et al., 2018)

# 5. Coating technologies applications

## 5.1. Classification and choice of the coating method

The strategy of biopolymer-film coating is based on both the nature and properties of the coating (surface tension, viscosity, adhesivity, etc.), the targeted functions (barrier, mechanical, active.) and on the technology (dry versus wet process, thickness range, aqueous or organic solvent, thermal or cold process, pre- or post treatment, etc.) applied (Adibi et al., 2023; Butt, 2022; Kurek & Debeaufort, 2015; Peipei et al., 2021; Tyagi et al., 2021). The final thickness of the coating is often one of the main technology choice criteria. Indeed, for barrier and surface properties improvements, a thin coating is sought. For mechanical resistance, it could be either thin if it is only a surface scratch resistance that is required, or thick if the coating may provide increase overall mechanical resistance to its support. Indeed, if there is a need for an

increased resistance to puncture, elongation or stiffness, a thicker coating must be applied (Kausar, 2018). Fig. 3 illustrates the range of thickness and the related coating technologies.

Most of the scientific papers dealing with the development of hydrocolloid-based coating onto bio-related packaging films in lab-scale investigations use casting (pouring), meter-bar (or) roll coating, dip and layer-by-layer coating technologies. However, these are scarcely applied by the packaging industry that predominantly uses continuous processes like extrusion-lamination, wet adhesive coating, spraying and/or printing technologies. Table 3 gives the principles of the technologies used for polymer and biopolymer coatings, the type of pre- and post-treatment required, the costs of investments and process, the main advantages and inconvenient that are further discussed and illustrated in the following parts.

The main advantages of the dry technology (using solid substrate to be applied onto flexible) are the absence of solvent and of the consequent steps of desolventisation or drying in case of aqueous liquids. Furthermore, optimal ratio between cost and efficiency, a better dispersion, the capacity of applying very thin coatings (lower than 1 µm) are obtained. However, very often, the dry technologies could require high temperature for further curing steps, which could induce degradation of biopolymers, and sometimes possible adhesion drawbacks (Gigante et al., 2021).
## 5.2. Dry (solvent free) processes

The “solvent free” or dry technologies for coating are mostly based on the ability to the coating component to melt before being applied onto packaging film support. However, very few hydrocolloids are able to melt under heating without degradation or denaturation in dry conditions. It is therefore very often necessary to add small amount of solvent, usually water or aqueous based solvent, at very low content (few

percentages of the dry hydrocolloid weight) to help melting and thus flowing of coating forming systems. Except starch-based biopolymer coatings, which are well adapted to extrusion-lamination and hot-press technologies (Su et al., 2009; Versino et al., 2016), and very few proteins like casein, whey-proteins, soy proteins or gelatine, the other hydrocolloids are often not suitable for dry coating processes (Azevedo et al., 2017; Hernandez-Izquierdo & Krochta, 2008; Krishna et al., 2012; P. Kumar et al., 2010; Nur Hanani et al., 2013; Ochoa-Yepes et al., 2019;

<figure><img src="image_8.png" /><figcaption>Fig. 5. Wet-coating technologies for packaging film coating (adapted from (Kurek & Debeaufort, 2015; Technical Coating Intl., 2025)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 5. (continued).</figcaption></figure>

Pranata et al., 2019).

### 5.2.1. Extrusion-lamination and co-extrusion coating

In the extrusion-lamination coating process, substrate is heated and melted and then extruded through a slit die as a thin web of molten biopolymer cast on the support film prior to being cold laminated between two rolls to fix the coating layer (Figs. 4–1). Coextrusion is

frequently combined with extrusion coating or lamination (Figs. 4–2). In coextrusion, two or more layers of different polymers are extruded simultaneously through a single die. The primary advantage of coextrusion is that it consolidates multiple laminating steps into a single process, simplifying the production of complex multilayer laminates (Kausar, 2018; La Fuente Arias et al., 2023).

Co-extrusion is often used to develop biodegradable bilayer films for
food packaging (Apicella Apicella, Barbato, Grimaldi, Scarfato, & Incarnato, 2023). The inner layer, made from a blend of Ecovio F2332 (PBAT) and PLA, provides heat sealability and flexibility, while the outer layer, a mix of PBS and a semi-crystalline grade of PLA, acts as an oxygen barrier. By adjusting the thicknesses of each layer, properties like water resistance, oxygen barrier, heat-sealability, and strength were modulated and improved. The films showed good potential for food packaging by effectively blocking oxygen, which can help reduce food spoilage. The best performance was seen in a sample that achieved a 53 % reduction in oxygen permeability and a 78 % increase in flexibility after co-extrusion coating. In another study, gelatin was used as barrier and active coating applied by extrusion onto PLA film or onto paper (Cheng et al., 2015). Miletic et al. used co-extrusion with a lignin-chitin based for improving the surface and thermomechanical resistance of PLA films and also providing antimicrobial activity (Miletić et al., 2020). In a same way, polyglycolic acid coatings have been co-extruded onto some polymer films for optimized oxygen barrier performances (Vartiainen et al., 2016).

### 5.2.2. Hot press technologies

Hot pressing is a process used to shape plates or thick sheets. Both systems, a batch-discontinuous (Figs. 4–3) or an industrial continuous based on extrusion melting prior hot-pressing (Fig. 4–), could be employed. This offers several advantages, including low cost, short cycle times, high-volume production capability, dimensional accuracy, improved impact strength, and uniform shrinkage when using the continuous process with high pressured heated rolls. The process involves heating a thermoset resin under pressure in a mold cavity in batch process, or between two heated-rolls in continuous industrial process. In the batch system, the resin powder or granules are placed on the lower mold plate, and then the upper and lower plates are pressed together. After the material cools, the pressed product is removed from the mold. It is also used for the assembly of two layers under a controlled temperature and pression (La Fuente Arias et al., 2023). In the continuous process, two preformed films are hot-pressed between heated rolls and melting one of them allowed the coating (adhesion) of the second.

Nilsuwan et al. developed easy-to-handle, transparent bilayer films made from PLA and fish gelatin with added natural antioxidant, using thermo-compression molding process suitable for large-scale production (Nilsuwan et al., 2020). Adding 12 % active compound improved the film's properties, including water and UV light resistance, controlled antioxidant release, and enhanced durability. When used to package fish slices, these films helped to preserve the fish quality for 7 days at 4 °C, showing lower bacterial growth, reduced spoilage, and higher freshness compared to other non coated packaging.

### 5.2.3. Powder deposition

The powder deposition method is not so common for polymer coatings used as packaging, except for metallization. Very few case considered the powder deposition of polymers, like LDPE, as thin particles onto another polymer (Sundara Rajan & Rothstein, 2024). The powder could be sprayed using a dry gas (air) as propulsor or deposited as an aerosol onto the surface prior being melt by heating or agglomerated and/or graft by a chemical reaction that could be induced by, for example, irradiation techniques. The powder could also be heated prior to being applied, but most often, cold spraying is preferred for heat sensitive polymers (Bacha et al., 2023). Powder could be sprayed using either an electrostatic spray gun or a bi-fluid spray gun with air or gas for powder carriage flow.

The powder deposition coating process is an alternative to wet technologies like spraying or solution casting for soluble or partly soluble hydrocolloid-based coatings. Indeed, one of the most common drawbacks of wet processes is the longer process time of water evaporation compared to most organic solvents. Most importantly, aqueous coating, even if well adapted for soluble biopolymers, is not suitable for either moisture sensitive or for non-water-soluble substances including

essential oils, unsaturated fats etc. (Adhikari et al., 2018). Nevertheless, the powder deposition technique for biopolymer coating could require a pre-wetting of the surface or uses the electrostatic forces to enhance the adhesion of the powder, and very often need a curing step after the deposition (Figs. 4 and 5). For medical purpose, a semi-dry process based on powder deposition onto flat surface and possibly for packaging materials using biopolymer or hydrocolloid powders are used. Indeed, a pre-wetting of the surface with a very low amount of solvent mixture considers spraying of a plasticized aqueous solution onto the surface and then the powder is deposited immediately before being cured. The pre-wetting allows a partial solubilization that favors agglomeration and finally coalescence of wet powder particles to merge into a continuous coating layer (Figs. 4 and 5) as described by Yang et al. (Yang et al., 2020).

### 5.2.4. Vacuum deposition or chemical vapor deposition (CVD)

Chemical vapor deposition (CVD) is a widely used method for producing functional nanomaterials or metal deposition as very thin layers. It is frequently employed to create high-quality, high-performance coated films on large-area support or substrates with complex patterns. This technique is valued for its ability to deliver precise and uniform coatings, making it a popular choice in various industries for applications requiring advanced material properties (Choy, 2003). However, in case of hydrocolloids or biopolymer based, the coating should be converted into vapor or through nebulization to be and can be introduced in the vacuum chamber for its chemical deposition on the film surface as shown in Figs. 4–6. This is a batch process that requires time and energy. A typical CVD system consists of several key components: a vapor pressure feed system, a CVD reactor for heating the substrate up to its vaporization or a system generating a nebulization or aerosol, and an effluent gas handling system. The process has some drawbacks, including the potential release of toxic gases during reactions and potential high temperature for the vaporization that is rarely compatible with biopolymers or hydrocolloids (Mattox, 2003).

CVD was used to apply thin layer based on polydimethylaminomethylstyrene for providing antimicrobial properties of nylon (T. P. Martin et al., 2007). Coated nylon exhibits low amount deposition of 40 µg cm⁻² but enough to reduce by 6 Log the E. coli contamination after 1 h. Coatings of different nature, ranging from 5 nm to 0.1 µm, and applied on polymers allowed to be grafted, meaning a chemical deposition onto the polymer surface. Such coatings provide surface mechanical resistance, and barrier performances (Alf et al., 2010). These techniques could also be coupled, during coating application process to plasma deposition technology when the gas is able to generate plasma under vacuum and exposed to microwave or electric field (Choy, 2003).

### 5.3. Wet coating processes

Wet processing technologies for coating polymer or biopolymer films are based on the dissolution or dispersions of the hydrocolloids in an adequate solvent prior to be applied. These technologies make a further step mandatory for stabilizing the coating, shifting from liquid or gel state to solid state, by drying or heat-curing or UV curing or electron-bean curing. The coating application post-treatment induces consequently timely process and operating cost due to required energy for the solvent evaporation which is one of the major limitations of choosing wet methods (Farris et al., 2010; Swarupa & Thareja, 2024). However, for sustainable approach, water-bone coatings are mostly preferred than those using organic solvents, making the process more sustainable, but also because most of the hydrocolloids able to be used are soluble only in aqueous or hydro-alcoholic liquid mixture and precipitates in organic solvents (Farris et al., 2011; Khuntia et al., 2022). Almost all these technologies could be applied at a lab-scale for testing coating formula and processes and thus are the most often cited in scientific literature (Benbettaïeb et al., 2019; Wang, Chen, Zhu, McClements, Liu, & Liu,
2022; Rastogi & Samyn, 2015; Reichert et al., 2020; Shaikh et al., 2021; Stoleru et al., 2021; Tyagi et al., 2021). Fig. 5 displays the principle of the main wet coating technologies. The main advantages of the wet techniques using liquid for coating application are the non-thermal degradation, the tailored wettability of the surfaces, tuning the viscosity to control the thickness and the amount deposited. However, this technique could require toxic or inflammable solvent, could need additives for improving adhesion onto the solid surface, could display inhomogeneous dispersion and requires a lot of energy for solvent removal (drying) if water-based solutions or suspensions are used (Gigante et al., 2021).

### 5.3.1. Solution casting

Solution casting is the widest used method for fabricating films and coatings from biopolymers, composites, and blends at laboratory scale (Othman et al., 2023; Rhim et al., 2006; Vogelsang et al., 2014). This technique involves dissolving or dispersing the desired material in a solvent, spreading the solution onto a flat surface, and allowing the solvent to evaporate, leaving behind a thin, solid film or coating. Different solution casting methods are summarized in the following paragraphs.

One method for obtaining multi-layer films is known as the casting method, which consists in pouring each film-forming solution sequentially. To apply the second and subsequent layers, the first layer must transition into a gel state (Figs. 5–1). However, not all types of biopolymers are suitable for the first layer, as some do not turn to gel during the drying process. Biopolymers such as starch, chitosan, and carboxymethylcellulose (CMC) fall into this category (C. Sun et al., 2022). Despite this limitation, most of the biopolymers can still be used as the final (top) layer in multi-layer films (Grzebieniarz et al., 2023). Miletić et al. showed that the chemically modified chitin–lignin complexes by grafting onto PLA chains is suitable for lactide oligomers. Grafting also occurred as a coating process by the solution casting method (Miletić et al., 2020). The aim of the first step was to improve the interaction between the hydrophobic PLA matrix and the hydrophilic chitin–lignin nanoparticles.

### 5.3.2. Spray coating

Spray coating is a process used to create thin to thick films by pulverizing a colloidal dispersion or solution onto a heated or cold substrate. When the aerosol contacts the solid substrate, it undergoes solvent evaporation, if the surface is hot, or requires a further solvent removal step. This sequence of events results in the formation of the desired coating, as illustrated in Figs. 5–2. The thickness of the film produced through spray coating can be controlled by adjusting the number of spraying cycles performed and or the time of spraying, but it also mainly depends on the dry content of the coating-forming suspension or solution (Kunam et al., 2024). By increasing the number of cycles, a thicker coating can be achieved, allowing for precise tailoring of the film thickness according to specific application requirements, particularly important when coating is aimed to provide bioactive properties to the coated films, and thus incorporation of significant amount of active agents is needed. Two type of nozzles could be used, the bi-fluid uses a gas (air/N₂, etc.) as a carrier to make nebulization of the liquid at the outlet of the nozzle, and the mono-fluid nozzle, that uses high pressurization of the liquid to generate the nebulization (Jahangiri et al., 2024; Kurek & Debeaufort, 2015; Wu et al., 2023).

Spraying application of pullulan/graphene bio nanocomposites as coating for packaging film showed excellent antibacterial properties against both gram-negative *E. coli* and gram-positive *S. aureus* (Shanmugam et al., 2022). Spraying allowed to control the amount of coating to reach the inhibitory concentration required for a better efficacy of the antimicrobial film. Spraying systems are also well adapted for the deposition of coatings containing nanofibrils, (such as cellulose, or other nanoparticle), as it allows a more homogeneous distribution of the particles in the coating (Nadeem et al., 2022). Films made from spray

deposited nano fibrils cellulose showed excellent mechanical properties and oxygen barrier performance, while possessing limited moisture barrier performance. This technique also complies with the nature of liquids to spray, that could be aqueous based, or organic solvent based. It could be also used for molten fats or waxes as demonstrated by Ahuja and Rastogi with insect wax extracts applied on paper-based sheets (Ahuja & Kumar Rastogi, 2023).

### 5.3.3. Dip and layer-by-layer (L-b-L) coating methods

The dip-coating process, displayed in Figs. 5–3, is likely the most widely used sol-gel technique for applying coatings onto both faces of flexible packaging on laboratory scale (Atay, 2020). This method involves immersing a substrate in a colloidal solution and then withdrawing it, resulting in the formation of a liquid film that adheres to the surface. The sol-gel transition occurs as the solvent drains and evaporates, leading to the consolidation of the coating, which can happen within a few seconds depending on the solvent’s volatility. Additionally, the thickness of the resulting thin film can be controlled by adjusting the number of dips and the withdrawal rate (Butt, 2022; Z. Zhang et al., 2022). Dip coating offers several notable advantages over other deposition methods, such as low cost and simple operating requirements, making it accessible for various applications. The process can be conducted at room temperature with simplified setup and operation. Additionally, dip coating is highly versatile, allowing for the coating of large surface substrates with diverse shapes. Its scalability also makes it suitable for producing coatings on an industrial scale, enhancing its appeal for commercial use (Atay, 2020). Fang et al. examined how different dip coating parameters, including withdrawal velocity, solution concentration, and viscosity, affect the thickness of the PLA coatings applied onto metal-based packaging-coated films (Fang et al., 2008). Experimental results indicate that withdrawal velocity affected the PLA film.

Dip coating was used for applying coating of PLA solution onto thermoplastic starch (TPS) for improving functional properties of the TPS biodegradable films (Jantanasakulwong et al., 2019). Coatings allowed tensile strength at break to increase twice, prevent the plasticizer bleeding, and significantly reduce the swelling of TPS film when immersed in water, as well as their wettability.

Coated-films can also be produced using the Layer-by-Layer (L-b-L) technique which is mainly based on the dipping method (Figs. 5–4). This method was first developed through the use of electrostatic interactions between polyelectrolytes, offering an extremely simple and cost-effective way to create multi-layer composites of various sizes and shapes and allows to coat both faces of the polymer film. The L-b-L technique involves the sequential deposition of two or more materials that adhere to one another (Abbadessa et al., 2023; Grzebieniarz et al., 2023). In this process, a substrate with a negative surface charge is immersed in a solution containing positively charged materials. The L-b-L method leverages the electrostatic attraction between successive layers. After rinsing off any excess solution, the positively charged substrate is then immersed in another solution with a negative charge, facilitating the formation of the subsequent layer (Wang et al., 2022).

However, spraying and casting based methods could also be employed for the layer-by-layer coatings of flexible films dedicated to packaging when only one face of the packaging film has to be coated, as displayed on Fig. 5– and 5-6 (Thuy et al., 2021). Moreover, spraying systems are often preferred for industrial continuous process of L-b-L coating. Indeed, L-b-L technique is one of the easiest, most flexible, efficacious, and economical methods for fabricating nanoscale multi-layer composite films with accurate control of structure, diameter, and qualities (Ganjeh et al., 2024). Layer-by-Layer films enable the controlled release of drugs while also exhibiting antimicrobial and antifungal properties. This capability makes L-b-L films particularly valuable in food packaging applications, where precise active release and prevention of microbial growth are critical. The sequential layering allows for fine-tuning of the release rates and bioactivity, enhancing the
efficacy of the films in food applications. However, this method has only mainly comprised packaging for fresh vegetables and fruits, although there are no contraindications for their use in the storage of other food products (Grzebieniarz et al., 2023). Xu et al. compared the L-b-L method for associating sodium caseinate and carboxymethylcellulose, hydrocolloids having opposite charges to the same biopolymers associated as a blend, both containing epigallocatechin gallate as a natural preservative (Xu et al., 2023). Depending on what was considered, the results were opposite, exhibiting better barrier properties of the films prepared from the blend, whereas the quality of fresh fruits (strawberries and bananas) were much better preserved with the L-b-L method.

### 5.3.4. Slot-die coating

Slot-die coating, also called tape-casting, is a technique used to apply thin films of solutions, hot-melts, or extruded materials onto flat substrates like glass, metal, paper, fabric, plastic, or metal foils (Kumar et al., 2017). Originally developed in the 1950s for producing photographic papers, it has since become important in various commercial industries and nanomaterials research. In slot-die coating, thin films are formed through solution processing (Figs. 5–7). The coating material, often dissolved or suspended in a precursor solution or slurry (also called “ink”), is applied to the substrate via a precise slot-die coating head. This head, featuring a high aspect ratio outlet, ensures controlled delivery of the coating liquid onto the substrate and could be fitted for hydrocolloid and biopolymer support films (Pemble et al., 2021).

Indeed, the slot-die coating technique, for applying cellulose nanocrystals and/or chitosan on paper-based films, is used for improving barrier properties as well as reinforcing mechanical strength of produced material (Ruberto et al., 2024). In the same way, chitin-cellulose-based coating applied by slot-die technique onto cellulose acetate biodegradable films exhibit oxygen barrier performances similar to PET films (Jung et al., 2022).

### 5.3.5. Doctor blade/air knife coating

Doctor blade coating/casting (also called knife-on-roll coating) and air-knife coating methods are classically used for applying coating in a continuous process onto packaging films. It consists of depositing in excess a semi-viscous liquid onto a packaging material and using a blade/knife or air-knife to control the thickness of the coating (Arun et al., 2024; Kunam et al., 2024). In air knife coating, an excess coating material is blown off from the substrate surface by an air jet that comes from an air knife instead of doctor blade (Figs. 5–8). The rheological property of the liquid coating is essential as the thickness of the coating is very low in the knife/blade coating method. Polymer, biopolymer and hydrocolloids-based solutions have a good hold over the rheological properties as viscosity could be easily monitored by temperature, concentration or additives (Kunam et al., 2024; McHugh, 2015). This technique is well adapted to coating of polymers or flexible material sensitive to high temperature (McKeen, 2012). This technique is commonly employed for coating based on hydrocolloid-based (proteins, polysaccharides), latex-based, fluorine-based, polyesters, and composites onto paper or onto “plastics” (Basak et al., 2024). It is very often coupled to spreading techniques when edible coating using either hydrocolloids or lipids are employed (Valdés et al., 2017).

Blade coating was used by Kim et al. to coat PLA films with polyvinylalcohol-based thin layers while maintaining the transparency of films and their biodegradability, for improving gas barrier performances without using organic or toxic solvents. They also stated this method was easily scalable to industrial production (J.-H. Kim et al., 2020; M. Kim et al., 2024). To make sealable paper, coatings based on cottonseed proteins and distillery by-products have been applied by the blade method onto paper (Ge et al., 2023).

### 5.3.6. Meter-bar (Mayer-rod) coating

The meter bar/rod, also called Mayer rod, is one of the most popular methods of adhesive coating application onto flat surfaces like films or

sheets. The packaging industry extensively uses bar coating for applying barrier coatings or inks/varnish to flexible packaging materials. These coatings improve the shelf life of packaged goods by providing moisture, oxygen, and chemical barriers (OSSILA, 2025). Bar coating is a basic technique used to deposit a thin layer of coating solution on a substrate, using a wet process. It involves placing a bar (Mayer rod) over the flat support and sliding it over a deposit of coating liquid (Figs. 5–9). This allows spreading the liquid homogeneously into a thin film or layer. Bar coating can be carried out either manually or automatically, for greater precision and efficiency. In the case of automated coating, the coating bar (such as a wire-wound bar) is moved at a uniform speed by an automatic film applicator, ensuring even, smooth film deposition. The bar is designed with a wire spiraling around it, the gap between the bar (or encasing wire) and the flat support controls how much solution is allowed through and determines the final thickness of the coating. If the coating solution can flow easily after coating, this method is suitable for liquids with a viscosity of between approx. 1 and 1000 cP. Coating thickness with normal tightly-wound wire rods is around 4 to 160 µm, and for greater thicknesses, up to 1000 µm, the rod winding must be looser and adapted.

Mayer rod coating was used for deposition of a primer onto PBAT and PHA's films prior their coating by PHBV suspensions containing cellulose nanocrystals for improved oxygen barrier properties. Meter-bar coating allowed to coat the support films with final thickness lower than 1 µm. This highly increased the adhesion of the barrier coating that was applied by electrospinning (Figueroa-Lopez, Torres-Giner et al., 2020). Number of publications describe the hydrocolloid-based coating and impregnation/calendaring on paper-based or polymer flexible packaging using meter bar coating technique (Abdenour et al., 2023; Fierascu et al., 2019; Izdebska-Podsiadly et al., 2021; Kara et al., 2016; Melendez-Rodriguez et al., 2024; Stuppa et al., 2024; Vishnuvarthanan & Rajeswari, 2019). These papers focus very often on two targets, providing barrier properties to support films, particularly on paper and biobased/biodegradable films, or on encapsulating bioactive agents (antimicrobial or antioxidant) in a coating layer. For instance, Barbosa-Pereira used the bar coating for developing antimicrobial coated films containing brewery by-products incorporated into a thin layer of polyvinilic resin applied onto low density polyethylene films for beef oxidative stability (Barbosa-Pereira et al., 2014). Wired-rod or Mayer-rod coating of whey-protein coating applied onto Pet film allowed to reduce by about 10 times the oxygen permeability of the films (Schmid et al., 2017).

### 5.3.7. Spin coating

In the spin coating process, a small volume of solution or suspension of low viscosity is applied to a flat substrate that is spinning at a controlled speed (Das & Chanda, 2016; Mendhe, 2023). This rotation generates centrifugal force, which spreads the solution into a wet thin film across the surface. While the substrate rapidly rotates, the solvent evaporates, resulting in the formation of a thin uniform coating, as illustrated in Figs. 5–10. The formation of multilayer coatings can be carried out through successive spin cycles. However, this technique is costly for industrial transfer as only small size sample could be treated and require time if water-based solvent is used (Butt, 2022). Moreover, the spin coating method has relatively low efficiency, as approximately 95 % of the sol precursor is expelled from the substrate during the spinning process.

To compete effectively with traditional materials, PLA needs improved barrier properties and antimicrobial activity for food packaging that have been solved by spin-coating applications. Shojaeiarani et al. investigated how spin-coating techniques with varying concentrations of cellulose nanocrystals (CNC) could improve the properties of PLA films with thin layer from few microns to 200 µm (Shojaeiarani et al., 2020). Spin-coated films exhibited higher crystallinity and glass transition temperatures, higher surface hydrophobicity and lower water vapor permeability, depending on the amount of cellulose nanocrystals
introduced. Additionally, these films demonstrated lower water vapor permeability and higher contact angles, indicating they were less hydrophilic compared to solvent-cast films. Antimicrobial tests were conducted against foodborne pathogens, revealing that both the manufacturing technique and the concentration of CNCs significantly affected microbial reduction. In conclusion, the need for safer, more environmentally friendly packaging has led to the development of antibacterial materials from biobased polymers like PLA. This study found that PLA films reinforced with CNCs performed better when produced using the spin-coating method, which improved CNCs dispersion, crystallinity, and hydrophobicity. While the method of fabrication had a greater impact on these properties than the CNCs concentration, both factors played a role in enhancing the antimicrobial effectiveness of the films against pathogens.

### 5.3.8. Electro-spray and electrospinning

Electrospraying is also an electrohydrodynamic processing that employs an electrical field to generate fine droplets of a liquid solution, which are then deposited onto a substrate (Lim, 2021). Indeed, it is a technique that utilizes electrostatic forces to create thin fibers (or filaments or fibrils) from a polymer solution, which are then deposited onto films surface under an electrostatic field (Wang & Su, 2024; Y. Zhang et al., 2024). This method is widely used for creating thin and uniform coatings, particularly in applications requiring nanoscale precision, such as food packaging, biomedical devices, and functional surfaces. The main advantage of electrospraying compared to conventional spraying deposition method is that the aerosol generated is attracted by the surface to coat, and then the yield of coating rises. Firstly, a liquid solution containing the desired coating material (e.g., polymers, nanoparticles, or bioactive compounds) is prepared (Figs. 5–11). Then the solution is fed through a nozzle, where a high-voltage electric field is applied. This induces a charge on the liquid, causing it to form a conical shape known as a Taylor cone. The charged liquid at the apex of the Taylor cone is ejected as fine droplets. These droplets evaporate quickly, depositing the coating material onto the target substrate. Finally, the substrate is placed at a specific distance from the nozzle to collect the fine particles or droplets, forming a thin, uniform coating (Alehosseini et al., 2018).

Compared to the pure polyvinyl alcohol (PVOH) film, the chitosan coated PVOH films using the electrospray method showed greater elongation at break, lower oxygen permeability, higher water barrier properties, and greater antibacterial activity (Y. Liu et al., 2018). Electrospray-coated paper with bioactivated shellac gum demonstrated the technique ability to obtain thin defect-free coatings, with barrier properties significantly enhanced to permit the preservation of fish products (Poddar et al., 2024). Pavon et al. prepared this technique bilayer systems, consisting of electrosprayed gum rosin microspheres onto polycaprolactone films (Pavon et al., 2021). Coatings provided an ultrahydrophobic surface that repeals water, prevent wetting, as well as improved UV barrier properties. Similar properties were obtained when polyethylene terephthalate films have been electrospray-coated with nanostructured silica, while maintaining film transparency (Pardo-Figuerez et al., 2018).

A typical electrospinning setup consists of three main components: (i) a high-voltage power supply to generate an electrically charged jet, (ii) a capillary tube (such as a pipette or needle) with a small diameter to produce fibers ranging in size from tens of nanometers to a few micrometers, and (iii) a film to collect the fibers as described in Figs. 5–12. In this voltage-driven process, one electrode is connected to the reservoir holding the biopolymer solution, while the other is attached to the film (Wang & Su, 2024). It is important to recognize that while nearly all soluble polymers can be processed by electrospinning, several factors influence the spinnability of biopolymers. Key parameters include the viscosity, surface tension, and conductivity of the biopolymer solution. Conductivity is particularly crucial when selecting a biopolymer. Polycationic and polyanionic biopolymers inherently provide good conductivity, whereas uncharged biopolymers require the addition of ions to

the solution. In addition to solution properties, technical parameters such as the flow rate, applied electric potential, and the distance between the capillary tip and the film also affect the electrospinning outcome (Chalco-Sandoval et al., 2017; Fabra et al., 2014). A key advantage of electrospinning is its versatility. It enables the fabrication of fibers with a wide variety of morphological structures, while also allowing for the creation of different fiber assemblies on the substrate. These assemblies can be arranged in various patterns, including aligned or random configurations, offering flexibility in design and application. One main limitation is that coating applied by electrospinning very often forms a mat and not a continuous layer, that could be interesting for releasing systems as active coatings (Wen et al., 2017) but not for barrier properties to gas and vapours. In some cases, a thermal treatment like curing or drying allows to induce coalescence of electrospun fibers aiming to obtain continuous layer for barrier efficacy improvement.

Recent studies have demonstrated that multilayer systems based on the electrospun coatings based on zein and applied onto polyhydroxy alkanoate (PHA) biopolymer films significantly enhance the oxygen barrier (Fabra et al., 2014). Additionally, a coating based on medium-chain-length polyhydroxyalkanoates (mcl-PHA), combined with apple extract, has been applied to cellulose nanopapers through a casting method, resulting in improved hydrophobicity and antioxidant activity (Urbina et al., 2019). Marcoaldi et al. developed biobased electrospun multilayer films designed to have strong barrier properties and nearly superhydrophobic behavior (Marcoaldi et al., 2023). They used two biopolymers: a low-melting, fully biobased synthetic aliphatic copolyamide and a microbial-based poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), combined with food-safe silica (SiO₂) micro-particles. The multilayer structure was created by thermal treatment, which laminated the components together through interfibre coalescence. While the single layer copolyamide didn't meet barrier expectations, the multilayer format significantly improved the barrier and mechanical properties, achieving a high water contact angle of about 146° and ameliorated barrier functions. A huge amount of literature exists on the electrospun coating but very few industrial applications have been commercialized for food packaging (Y. Zhang et al., 2024).

### 5.3.9. Print technologies for coating application: flexography and lithography

Flexography and lithography are roll-gravure technologies currently employed for printing flexible packaging, from paper, conventional plastics, bioplastics like polyesters (PLA) or laminated like brick materials (Gigante et al., 2021). Both methods require plates or rolls (cyclinders) to transfer the ink onto the flat substrate (film, sheet, cardboard, etc.). However, for continuous process, both techniques are roll-to-roll coating (Bautista et al., 2017; Gigante et al., 2021). Indeed, a first roll collects the coating solution from a tank to cover the printing roll with the liquid to be applied onto the flexible film (Figs. 5–13). Flexo printing uses flexible roll/plate, often made with elastomers, while litho-printing employs metal or aluminum inkers. These techniques are very fast method for applying thin coatings of high quality (homogeneous covering) but could require surface pretreatments when hydrophilic coatings (aqueous-based solvents) should be applied onto hydrophobic films. Litho-coating typically has higher setup costs than flexo-coating because it requires more time and effort to set up the press and uses more coating solutions. Consequently, flexography is the major printing method able to apply active or barrier thin coating on film surface and is suitable for hydrocolloid-based coating (Sonmez et al., 2022).

Flexography was used by Widsten et al. (Widsten et al., 2017) for printing antimicrobial coatings onto commercial multilayer barrier films at a rate of 30 m treated per minute immediately after a corona surface treatment. The corona is aimed at improving the adhesion of the different formulations based on mango or feijoas extracts containing phenolic antimicrobial compounds. Films coated by this technique have been tested as antimicrobial against more than 15 strains of gram+ and gram-bacteria and demonstrated high efficiency to reduce the bacteria
population. It is concluded that the flexographic printing method is ideally suited to the application of coatings with non-volatile compounds on the surface of commercial films. Chitosan-based coating have been applied onto PE and PET film for providing antimicrobial activity by the incorporation of volatile essential oil compounds using a technique that mimic the industrial flexography and validate on industrial production at a rate of 30–40 m/min (Kurek, Brachais, et al., 2012; Kurek, Descours, et al., 2012). Modified chitosan-Schiff base coatings including fluorescent probes were used for anti counterfeiting systems (Muthamma et al., 2023). These authors applied very thin layers of this coating by flexography printing onto polymer and biopolymer films. The Schiff base was then used as a multifunctional material: a binder and a fluorescent dye to formulate a coating for flexible coatings. Invisible in daylight, but revealed by UV light, the coatings showed good scratch and rub resistance, and good lightfastness, demonstrating the potential of flexography for biopolymer-based materials. Oxygen and/or water barrier layer based on nanoparticles incorporated into hydrocolloid-based solution or suspension were also applied on various flexible materials by flexography in some studies (Bautista et al., 2017; F. Ding, Hu, et al., 2020; L. Ding, Hu, et al., 2020; Fu & Dudley, 2021; Hong et al., 2004; Tarnowiecka-Kuca et al., 2023). Moreover, Hong et al. displayed flexographic printing of whey protein-based coating onto PVC (polyvinyl chloride) films allowed to improve the glossy, mechanical and water wettability (Hong et al., 2004)

## 6. Conclusions

The development of hydrocolloid-based coatings for biobased and biodegradable food packaging represents a promising step toward more sustainable materials. While biopolymers such as PLA, PHAs, TPS, PBAT, and PCL offer significant environmental benefits, their limitations—such as brittleness, poor barrier properties, and limited thermal stability—continue to hinder widespread industrial adoption. Surface treatments, including plasma activation and corona discharge, and mainly coating depositions based on renewable and safe components (hydrocolloids) have shown potential in improving adhesion and wettability, thus enhancing the performance of biopolymer coated films.

Despite advances in coating technologies, challenges remain in achieving large-scale production and maintaining the stability of coatings throughout storage and usage. While dry coating processes, such as extrusion-lamination and hot press techniques, offer solvent-free solutions with reduced environmental impact, wet-based methods like solution casting and spray coating provide greater flexibility in functionalizing the coatings. However, these wet techniques require additional drying steps, increasing energy consumption and production costs.

Future research should focus on optimizing coating formulations to balance biodegradability with performance requirements. This includes developing hybrid or multilayer coatings that integrate biopolymers with reinforcing nanoparticles or biobased plasticizers to improve mechanical and barrier properties. Additionally, exploring novel deposition methods, such as electrospinning and layer-by-layer assembly, could further enhance the functionality of coatings.

From an industrial perspective, the integration of these coatings into existing packaging production lines remains a key challenge. Collaborations between academia and industries are essential to scale up innovative solutions and align them with regulatory and market demands. Moreover, lifecycle assessments and end-of-life considerations must be further studied to ensure that these coatings provide a genuinely sustainable alternative to conventional plastic packaging. In conclusion, hydrocolloid-based coatings hold great promise for the future of eco-friendly food packaging, but continued research and technological advancements are needed to achieve widespread commercialization. Since biopolymer coatings are of the top scientific interests in material engineering as well as eco-designed sustainable economy, the detailed relevant theoretical basics on the preparation (formulation),

modification, and also mechanisms involved or targeted by the coating applications will be provided in another review article (Hydrocolloid-based coatings applied on biobased/biodegradable food flexible packaging – A review: Part 2: Hydrocolloid properties for applications and scale-up considerations), as the continuation of this one.

## CRediT authorship contribution statement

**Frédéric Debeaufort:** Writing – review & editing, Writing – original draft, Conceptualization. **Mia Kurek:** Writing – review & editing, Writing – original draft, Conceptualization. **Maria-Jose Fabra:** Writing – review & editing, Writing – original draft, Conceptualization. **Yasaman Ghasemi:** Writing – original draft. **Mario Šcetar:** Writing – original draft. **Kata Galić:** Writing – review & editing, Writing – original draft. **Nasreddine Benbettaieb:** Writing – review & editing, Writing – original draft, Conceptualization.

## Fundings

For all partners, this work was supported by the EVOLVEPACK project, a PRIMA-Horizon 2020.

For the University of Zagreb (authors Kurek, Ščetar, Galić), the EVOLVEPACK Grant is PCI2024-153409, work is also supported by the Croatian Ministry of Science, Education and Youth.

For the IATA (author Fabra), the EVOLVEPACK Grant is Grant PCI2024-153409, funded by MICIU/AEI/10.13039/501100011033 and, by ERDF A way of making Europe. The Accreditation asCenter of Excellence Severo Ochoa CEX2021-001189-S funded by MCIU/AEI/10.13039/501100011033 is also fully acknowledged.

For the university of Burgundy Europe (authors Debeaufort, Benbettaieb and Ghasemi), the EVOLVEPACK grant is PCI2024-153409 and is also funded by the National Agency for research (ANR), referenced ANR-24-P013-0008-04, and contributes also to NanoBap project grant n° ANR-23-CE09-0003-02.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The authors are in Editorial Board Member/Editor-in-Chief/ Associate Editor/Guest Editor for *Coatings, Foods, Antioxidants (Debeaufort & Kurek)* and for *Food Hydrocolloids (Fabra)* and was not involved in the editorial review or the decision to publish this article.

## Acknowledgement

The authors sincerely thank prof. Audrey Bentz for the revision of English spelling.

The EVOLVEPACK project (Innovative Sustainable Packaging to Safeguard Our Food and Our Planet) is a contribution to PRIMA programme (Partnership for Research and Innovation in the Mediterranean Area, https://prima-med.org/) funded by the European Union Horizon 2020 programme and national research funding agencies of partners. The authors wish to thanks all other partners involved in this project: Prof F. Poças from the Catholic Portuguese University (Portugal), Dr Göksen from Tarsus University (Turkey), Dr. Lavric from Pulp and Paper Institute (Slovenia), Dr. Rodov from Volcani Institute of Agricultural Research Organisation of Israel (Israel), Prof Nechad from ENCG-university Essaadi (Morrocco), and company Tecselor from Spain.

## Data availability

No data was used for the research described in the article.
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