**1. Introduction**

The paper industry is increasingly becoming interested in developing efficient and innovative solutions to guarantee high-quality products [1]. For this purpose, research activities have always been focused on the development of additives capable of adding functional properties to cellulosic substrates. Typical properties required by the paper industry are related to the water and grease barrier, antimicrobial properties, or antioxidant activities [2–5].

**Citation:** Panariello, L.; Coltelli, M.-B.; Giangrandi, S.; Garrigós, M.C.; Hadrich, A.; Lazzeri, A.; Cinelli, P. Influence of Functional Bio-Based Coatings Including Chitin Nanofibrils or Polyphenols on Mechanical Properties of Paper Tissues. *Polymers* **2022**, *14*, 2274. https://doi.org/10.3390/ polym14112274

Academic Editor: Fernão D. Magalhães

Received: 28 April 2022 Accepted: 29 May 2022 Published: 2 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In parallel with innovative pharmaceutical products [6,7], considering it a sustainable and circular economy perspective, the use of functional molecules from natural sources or industrial wastes is increasingly being used [8–10].

Extracts derived from agro-food wastes and forest residues represent a valid source of a wide range of functional molecules [11,12]. Polyphenols are considered interesting and widely available active molecules obtainable from wastes, and they are mainly used as natural antioxidants. For instance, different studies dealing with the extraction of polyphenols from tomato, lemon, orange, carrot peels or seeds [13,14], fennel stems, foils, and outer sheaths [15] but also from by-products such as cashew nuts, coconut shells, or groundnut hulls [16] have been reported.

Other products of great interest, extracted from natural wastes, are chitin nanofibrils and chitosan, which are used for their natural antimicrobial properties [17]. Chitin can be naturally obtained from marine sources, such as exoskeletons of crustaceans (crabs and shrimps) and molluscs (squid pens and mussel shells) [17–19], but also from terrestrial sources, such as insects [20] or mushrooms [21]. The extraction process generally involves demineralization (only for animal shells) and deproteinization steps [22–24]. The obtained chitin can be converted into chitosan by a deacetylation process or to chitin nanofibrils thanks to milder processes [25].

Moreover, the availability and yield of these active molecules from biomass waste have been recently improved by using innovative extraction techniques such as ultrasoundassisted extraction (UAE) [26], microwave-assisted extraction (MAE) [26–28], ultrafiltration (UF) and nanofiltration (NF) [29], and hydrodynamic cavitation [30]. The application of these molecules on cellulosic substrates has received great attention due to their intrinsic properties, which guarantees the biodegradability of the substrates and their compatibility with industrial-scale production [31,32]. Their use is very important, especially in skin care products [33,34], as they exhibit properties such as UV radiation protection [34,35] and anti-age action, acting as anticancer or moisturizer agents [36,37] and skin inflammatory reaction modulators [38,39].

One of the most used methods for the application of functional molecules involves using water as a solvent or suspension medium [40–44], thanks to its good environmental and economic advantages. Many techniques have been developed for the application of water dispersions or solutions of active molecules, such as flexography [45], roll-to-roll [46,47], wire-bar [48], blade [49], and spray [50,51]. In particular, in the paper industry, the application of a coating on the surface of paper substrates enables the increase in properties such as water vapor or gas barriers [52,53], and antimicrobial [54] or antioxidant activities [55]. Even if it is important to verify the effectiveness of functional additives to transfer the desired properties, it is also necessary to investigate their effect on the morphology and mechanical properties of the substrates. The thickness of the paper substrate and coating plays a key role in the analysis of the mechanical behaviour. Substrates constituted by paperboard for packaging are often affected by cracking of the coated layer during creasing and folding [56,57], but mechanical properties are minimally affected by the presence of a coating due to its negligible thickness compared to the substrate. Conversely, the mechanical properties of materials with limited thickness, such as paper tissues or towels, are affected by the presence of a coating and its application technique [58–60]. The main issues are represented by the change in coated tissues in terms of softness and dry/wet mechanical strength but also properties such as hydrophobicity, surface anisotropy, absorbency, and colour [44,61,62]. In solvent-mediated applications, it is pivotal consider that a greater interaction between the coating and substrate often means a greater modification of properties. It was reported that surface roughness and porosity distribution of the paper are the main factors that affect the interaction between the liquid and substrate and the absorption properties of the fluids [63,64]. Furthermore, it is also important to consider the molecular size of the active molecules and their affinity with the substrate. Small hydrosoluble molecules or micrometric particles, such as calcium carbonate, kaolin, talc, alumina, and titanium oxide [65–67], often used as pigments, can penetrate deeper than

macromolecules such as polyphenols (tannic acid, catechin) [68,69], polysaccharides (starch, chitin) [70,71], or other polymers (natural rubbers, polyesters, polysiloxane) [58] used for surface modification [67].

In this paper, two coatings based on chitin nanofibrils and polyphenols were applied onto paper tissues by using spray techniques. FTIR spectra of treated tissues were recorded on different surface points and compared with the spectra of raw coatings to evaluate the homogeneity and penetration of the performed treatment on the tissues. Antioxidant and antibacterial properties of, respectively, polyphenols and chitin were measured to verify their activity as functional molecules. Their effect on the mechanical properties of tissues was investigated by different mechanical tests that included puncture resistance, and tensile and tearing tests. Mechanical properties were discussed correlating their trend with the microstructure, observed through field emission scanning electron microscopy (FESEM).
