**4. Liquid Bio-Based Coatings**

One of the main methodologies used in the coating of cellulosic and plastic substrates is represented by the application of a liquid suspension/solution of functional molecules.

Among liquid application techniques, the most used are mentioned in Table 2, summarizing the description and main results of spray drying, electrospray, airbrush spraying, spin coating, dipping, solution casting, flexography and gravure roll coating.


**Table 2.** Liquid coatings techniques and main results regarding liquid bio-based coatings.

Each method can be considered a valid technique for wet coating application and the specific choice depends on the physico-chemical features of the coating and the surface properties of the chosen substrate. For instance, coatings based on polysaccharides or proteins exhibit a considerably polar component in terms of surface energy, while the cutin, composed of ω-hydroxy acids, forms hydrophobic films [135]. Similarly, the surface energy of fossil-based plastics, such as polyolefins, showed a high dispersive component [136,137], bioplastics, such as polyesters, displayed a progressive increase in the polar component [138], whereas polysaccharides showed a predominance of the polar component [139]. It was reported in the literature that good adhesion between coating and substrate strongly depends on the interfacial surface energy and the topography/geometry of the adherent bodies [140]. As the wet coating was applied through the use of a liquid it was necessary for optimal conditions to be established in the substrate and the coating solution/suspension. Commonly the evaluation of surface energy of a liquid on a solid surface is defined by the contact angle expressed by the Young's equation and the relative work of adhesion expressed by Dupré's equation [141,142]. Surface energy of the liquid depends not only on the selected coating but also on the chosen solvent and the presence of surfactants [143–145]. Instead, factors such as concentration [146,147], viscosity [148,149], and wettability also influence the homogeneity of the coating, the drying speed, and the choice of application method. Instead, factors as the concentration and viscosity, in addition to the wettability, also influence the homogeneity of the coating, the drying speed and the

choice of application method. Regarding the morphology, as the liquid coating assumes the shape of the solid, it was important to evaluate the roughness and the absorbency/porosity of the substrate. In literature it was reported that roughness has a strong influence on the wettability of the surface showing lower values of contact angle at higher levels of roughness [150–152]. The presence of porous or high-absorbency substrates highly influences the coating process by increasing the wettability and changing the drying kinetic [153–157]. Although surface roughness and porosity can increase the wettability of a surface, they have a significant influence on the coating morphology and thickness uniformity [158,159]. Other aspects that influence the coating are the process parameters, such as the deposition rate [160], the drying temperature [161] and the use of air or vacuum drying [162,163].

Application of coating with a wet technique had some advantages that were suitable for increasing the development of bio-based coatings. The use of a room temperature application avoids the thermal degradation and hydrolysis of bio-based materials, which are inherently sensitive to these processes [164–166]. Moreover, the use of a liquid medium allows the wettability of this type of coating to be tuned. For instance, a concentrated coating can be more suitable for blade or dipping application than a diluted one, which conversely can be more suitable for spray application. Particular attention must be paid to the choice of solvent/suspending agent, favouring bio-based and non-toxic liquids. The use of non-toxic substances for humans and environment should be deeply investigated because it could interfere with processes such as biodegradation [167–169]. Unfortunately, the preparation of optimal solution and dispersion for coating could not be easy to achieve. Solution guarantees a homogeneous distribution of the coating layer in wet medium, but the coatings are strongly influenced by properties like viscosity and possible formation of gel structures [170]. Dispersion has a weak influence on the physical properties of the coatings but they request a stabilization. In particular, with the increasing availability of nanometric biomolecules, such as the CNs [75,171] or the cellulose nanowhiskers [119], these problems were amplified due to the increase in the surface area. Consequently, high energetic dispersion and homogenization techniques, such as the ultraturrax homogenization [172,173], sonication [174,175] and high pressure homogenization (HPH) [176,177], were increasingly applied. If the operative parameters and the homogenization techniques did not allow an optimal wet coating to be prepared, the use of biosurfactant [178,179] or a bio-based primer [180–182] becomes necessary.
