**5. Solid Functional Bio-Based Coatings**

In recent years, solid coatings have been developed in an exponential way and the necessities of functional coatings have also gradually been fortified [183]. As described in Table 3, among the widely used solid coating application techniques, the most common are: co-extrusion, compression molding, fluidized bed dipping, electrostatic spray and roll coating.

A differentiation can be made between hot melt coatings (HMCs) and powder coatings. HMCs have been in use since the fifties, they relies on thermoplastic solid materials achieved without the use of solvents, which are inherently solid below 80 ◦C and they become low-viscosity fluids at higher [184,185].

HMC is made of thermoplastic materials that can be easily spread upon heating. When the hot melt is in a fluid state, it flows onto the substrate. When the hot melt is then cooled, the coating solidifies and forms a bond to the substrate [186].

Today, HMCs are involved in the production of items in many manufacturing fields, from packaging to paper industry, and their development is increasing considering the step ahead made in the hot melt coating application methods [187].


**Table 3.** Description of solid coatings application techniques and main results on solid coatings.

As they form a strong bond quickly, simply by cooling, they are compatible with many materials Achanta et al. [198] stated that HMC methods of applications are very attractive in all sectors in which there is a fundamental necessity to develop novel, simple, efficient, precise, and cost-effective coating processes.

The driving force for the employment of HMCs (and their strength compared to water-based film-coating technology) is to avoid the use of hazardous and toxic solvents as described by several literature works [199,200].

On this premise, since there is no necessity for solvent evaporation, the time for the process to be completed is shorter; consequently are eliminated all solvent disposal/treatment associated with organic solvents., making HMC environment-friendly materials [183].

Although water-based coating systems are useful, they are not completely flawless. A difficult problem encountered with waterborne coating systems is the variation in the dispersion of the coating. In fact, it is virtually impossible to control the presence or growth of microbes in coating dispersions without damage [201]. In addition, HMCs offer significant technical advantages, i.e., faster and cheaper coating processes and less risk of dissolution of biomolecules during treatment [202].

However, although this technique has been described well by many review papers, like by Lopes et al. [203], its application is scarce in producing coatings out of the pharmaceutical sector. The motivation is the necessity to mix in the correct way "active molecules", able to guarantee the achievement of the desired HMC properties, with oligomers, which act as primers during a low-temperature extrusion process (to ensure that the hot melt has the right melt strength to be processed) [204].

Improving the solubility of water-insoluble molecules remains a real challenge in the development of HMC formulations, as the bioavailability of active ingredients is controlled by their solubility in water [205]. Improving the solubility of the couple "active molecule–oligomer" is one of the challenges nowadays.

Finally, it is possible to conclude that HMCs represent the best strategy to develop coatings for bioplastics and cellulose with highly diffused industrial technologies, such as extrusion coating, in which the adhesion of the coating to the bioplastic substrate is very critical, as pointed out by Correlo et al. [206].

Another solid coating can be achieved in the form of a powder. In fact, with environmental regulations becoming more stringent, an urgent problem is to reduce the use of volatile organic compounds (VOCs). An approach based on powder coatings, which is

inherently solvent free is perfect from this point of view. Such coatings represent the final destination along the road to VOC reduction [207].

Because of their superior application properties and environmental friendliness, the use of powder coatings has grown very rapidly in recent years and the demand for functional powder coatings has gradually intensified. The components of powder coatings are extruded, crushed and screened to obtain powder for coating [208]. Powder coatings are usually operated first by electrostatic spraying and fluidised bed impregnation methods. Then, the powder is heated until it melts and hardens.

The most commonly used method for thermoplastic systems is the fluidised bed process. Here, a hot metal test piece is immersed in the fluidised powder. The powder dissolves, melts and cures, resulting in a smooth polymer surface on the test piece [209]. Due to the partial crystallisation of polyester resin, the effect on the properties of the powder coating film, especially the mechanical properties, cannot be ignored in industrial applications [210].

The production of a polymeric powder coating by extrusion is, actually, a multi-step process. Indeed, it can be labelled a "batch process" because it involves weighing, premixing, extrusion and milling, weighting the "ingredients" in prescribed ratios, pre-mixing them in the solid state, feeding them into an extruder so to obtain a molten homogenous mixture. The molten material, after cooling, is subsequently crushed into flakes of about 10–20 mm and then finally ground by disc or hammer mill to obtain particles with size in the range of 2–100 μm with a distribution peak of about 50 μm [211].

Powder coating formulations exist on the market either as thermosetting or thermoplastic but they are fossil-based. Concerning biopolyester thermoplastic-based powder coatings there are still many steps ahead to reach an industrial application. Interestingly, Van Haverman et al. [212] developed alkyd resins for high-solid powder coatings completely based on commercially available renewable resources.

As interest continues to focus on improving more sustainable technologies, and as the prices of fossil raw materials are set to rise, the coming decades will inevitably see an increase in renewable-based coatings, combining them with unique properties.
