Perspective Chapter: Functional Sol-Gel Based Coatings for Innovative and Sustainable Applications

Abstract

Alkoxysilanes represent a class of molecules widely employed to achieve the preparation of plenty of functional surfaces by easy, cost-effective and eco-friendly sol-gel methods. In this regard, the advancements of research activities include the proper design of film/patterns/brushes, by starting from opportune alkoxysilane and/or other metal/metalloid precursors, in order to obtain efficient innovative and homogenous functional surfaces showing implemented properties by means of the simple and eco-friendly sol-gel method. Therefore, in light of these aspects, the employment of opportune functional alkoxysilanes, either in combination with other nanofillers or molecules, is a key step for the design, and development of sol-gel based nanohybrid or nanocomposite coatings suitable for different surface properties implementation and applications, spanning from blue-growth sector to smart and technical textiles, from biomedicine to building and cultural heritages, from environmental remediation to catalysis. Some of the most relevant and explicative examples of these innovative and sustainable sol-gel based coatings will be described in this chapter.

1. Introduction

One of the most recent trends in the field of nanotechnology is the development of hybrid functional nanomaterials and nanocomposites, characterized by the presence of functional nanometric organic or inorganic components appropriately dispersed in a (blended) polymeric matrix, thus allowing the fabrication of final hybrid materials with improved performances over that of the sum of the individual components [1, 2, 3]. In particular, these advanced nanostructured materials were often used as surface coatings for a wide range of substrates and therefore for different industrial application fields [4, 5]: among these, those related to the development of high-tech and smart surfaces are of great interest. Creating new functional coatings is crucial in order to give surfaces implemented properties like anti-vegetative [1], antibacterial [2], hydrophobic [3], anti-stain [4], and flame-retardant [5], and release of drugs [6], adsorption and detection of molecules [7]. In this regard, nowadays the sol-gel method could be unquestionably considered among the new, versatile, and sustainable scientific approaches that can lead to addressing the above-mentioned needs. Due to its benefits, such as its low processing temperature, increased homogeneity of the final products, and increased versatility of the corresponding silane precursors, as well as its ability to stably attach to molecules or a variety of substrates, the sol-gel method enables the creation of new functional materials using safe solvents (i.e. water, ethanol, etc.) and reactants with a look on the environment and human health [8].

1.1 Hybrid inorganic–organic materials

The manipulation of matter in the range of 1 to 100 nm is employed to design, engineer, and create nanomaterials in the emerging scientific and application field of nanotechnology.

Nanomaterials can be utilized for a wide range of applications and for developing different kinds of nanocomposites, nanohybrids and nano-devices because of their peculiar features mainly related to their nanoscale dimensions and morphology. As a matter of fact, the presence of their different physical, quantum, and surface features caused by small-scale confinement is the primary distinguishing factor between nanomaterials and the corresponding bulk materials, thus leading to a very interesting and useful wide range of useful physical, surface, and quantum characteristics [9]. Examples of these characteristics exploited at the nanometric scale can be represented by substances that can feature catalytic properties (i.e. gold and platinum), copper that turns transparent from opaque and silicon that turns into a conductor from an insulator [10]. From an application perspective, optical and electrical, chemical–physical, thermal, and mechanical characteristics of nanostructures are the most intriguing.

For example, as the name implies, a “nanohybrid”, also often referred as Hybrid Inorganic–Organic Material (HIOM), points out any material that hosts a compound with nanometric dimensions bonded covalently within a polymeric matrix or structure, or that is produced by a supramolecular assembly of two stably interconnected nanomaterials with distinct former properties from the final formed hybrid itself [11]. More precisely, this wide class of inorganic–organic nanostructured materials can be classified into the following distinguished categories (Figure 1) [12]:

• composites, that is a material combination made up of a micrometric matrix and a dispersion;

• nanocomposites, which are sub-micrometre-sized mixtures of similar-natured materials (1–100 nm);

• hybrids, that is a sub-micrometric amalgamation of substances with a different nature from compound hybrid materials;

• nanohybrids, that is atomic or molecular mixtures of several materials held together by chemical bonds.

Figure 1.

Possibilities of nanocomposites and nanohybrid materials development by the addition of nanometric functional additives into a polymeric matrix.

Moreover, hybrid materials can be classified into two types of categories based on the nature of the interface [13, 14]:

• Class I hybrids, both organic and inorganic components are mixed, and weak connections bond the molecules in the overall structure (ionic bond, hydrogen bond or van der Waals forces).

• Class II hybrids, in which the two phases are chemically strongly connected (covalent bonds or ionic-covalent bonds).

Low temperatures are necessary for the synthesis of inorganic–organic hybrids since organic components are typically thermally unstable. In this regard, the sol-gel is a quite versatile approach that enables it to meet this need [15].

Hybrid inorganic–organic materials not only offer an intriguing alternative to the production of new compounds and materials useful in several research fields by enhancing their characteristics and properties; they can also be used to create new products in a variety of industrial sectors, including the optical, electronic, mechanical, energy, environmental, biological, and medical sectors [8].

It has been a long time since published research studies in literature examined the properties of organic substances, such as polymers and/or inorganic materials (i.e. metals, ceramics and glasses) in order to develop suitable materials, such as fibers or coatings. The links between the structure and properties of these organic and inorganic materials have been studied and rationalized by means of novel analysis techniques and spectroscopic technologies [16, 17].

In addition to their unique thermal and mechanical qualities, polymers are often used to produce hybrid materials because of their chemical functionalization, chemical stability, biocompatibility, optical and electrical capabilities, and balance between hydrophobicity and hydrophilicity.

The composition, size, crystallinity and structure of the inorganic phase, which can take the form of a variety of compounds, such as silicates, transition metal oxides, metal phosphates, nano clays, and nanostructured metals, determine the mechanical and thermal stability of inorganic molecules, as well as the introduction of new functionalities [17]. In particular, the hybrid magnetic, optical, electrical and redox characteristics can all be enhanced by the inorganic component [18].

Different techniques can be used to produce such hybrid functional materials. For instance, the condensation between oligomers or polymers bearing metal alkoxides could be obtained either by starting from low molecular weight alkyl/aryl(alkoxysilanes) and by introducing an opportune additive (e.g. nanoparticles or other nanomaterials) into an already swollen organic matrix, or by impregnating inorganic gels with a polymeric solution, and by employing precursors of general formula R’-Si(OR)₃, where R’ is a polymerizable functional group (for example an epoxide or amino group) [19, 20].

In addition, the design and development of (multi)functional eco-friendly nanohybrids and nanocomposites that may find a useful application in multidisciplinary transversal fields could considerably contribute to the enhancement of human daily life and well-being, in the interest of long-term sustainability and environmental protection.

1.2 (multi)functional sol-gel based coatings

In recent years, the field of materials engineering has been active in the development of novel functional sol-gel based coatings to alter the characteristics of external surfaces [21]. This type of process, which leads to the formation of organic–inorganic hybrid coatings with a nanocomposite structure, is capable of modifying the surface properties such as abrasion resistance [22], wettability [23], as well as imparting antibacterial characteristics [24], resistance to UV radiation [20] and solvents in general and to release bioactive substances [25]. The sol-gel method is frequently used because of its versatility in the synthesis of inorganic–organic hybrid materials with a wide variety of mechanical, chemical, and morphological characteristics (Figure 2).

Figure 2.

Schematization of the sol–gel process. Reproduced from MDPI ref. [26].

Alkoxides, chlorides of metals or metalloids are typical precursors and they take part in hydrolysis and condensation reactions to create sols, which are colloidal solutions of solid particles with sizes ranging from 1 nm to 1 μm in a liquid phase.

Then the sol phase develops until a continuous inorganic lattice, with an interconnected liquid phase (known as a gel), is formed [27]. Heat treatment is then frequently applied to stabilize the system, by eliminating the liquid phase from the gel and thus enhancing its mechanical characteristics. The sol phase can be processed in a variety of ways to create products with the desired shape; it can be poured into a mold to create massive products (monoliths, membranes, aerogels), used to create powders (nano and microspheres that can be used as finished products or as raw materials for the creation of sintered specimens), and finally, deposited on a surface to create thin or thick films.

Metallic or metalloid elements are often employed as precursors in the sol-gel process to create the colloid within various added ligands that may be bonded; among them, silane alkoxides are examples. Since the early 1800s, these latter are commonly used, due to their well-known chemistry, as well as the easiness with which they may react with water in the so-called hydrolysis reaction step. The silane alkoxides have a general structure of Si(OR)₄, where R is a variable organic functional group depending on the alkoxide in question. Additionally, the inexpensive cost of these precursors makes it possible to use them in larger-industrial scale applications [28].

An initial hydrolysis reaction of the alkoxy groups, followed by a condensation event that creates a cross-linked system, is how a sol-gel system is created. As a result of these overall reaction steps, very large polymeric matrices with -Si-O-Si bridging bonds are produced. The flexibility of the final lattice phase decreases as the number of -Si-O-Si bonds rises, which causes the viscosity of the lattice to grow until it gels. The following variables can have a significant impact on the kinetics of formation of the individual reaction products and, consequently, on the final resulting one [29, 30], in particular:

1. the Rw ratio, also known as the degree of hydrolysis;

2. the solvent;

3. the addition of complexing compounds;

4. the use of acidic or basic catalysts.

Thin sol-gel based films, often employed as paint, coating or finishing layers, have undoubtedly received the most attention in the industrial sector. Moreover, the versatility of the sol-gel method enables a rational design to control the properties of the final silane-based films by controlling its morphological structure at the nanometric scale and its adhesion to the substrate [31]. In addition, the sol-gel method allows the production of thin films using affordable, time-tested and favorable conditions (i.e. alcoholic or aqueous solvents, crosslinking at low temperatures). It is also possible to easily add more than one function to the same coating and therefore, obtain multifunctional coatings thanks to the potential of easily incorporating both organic and inorganic materials into the final sol-gel based formulation. There are several potential uses, formulations, and deposition techniques for the sol-gel materials that are currently used in the protective paints/coatings industry. While the design and development of opportune “sol-gel” necessitate significant expertise, the deposition to generate “xerogels” can be accomplished with straightforward and easy-to-use procedures for the end-users, such as:

• Spray coating [32];

• Dip-coating [33];

• Spin coating [34];

• The doctor-blading procedure [32].

Numerous industrial fields can benefit from the application of thin films made of silica, alumina, zirconium, titanium, or other inorganic oxides. For example, glass or plastic can feature anti-reflective properties and light alloys or polymers can have their hardness and wear resistance increased. The use of sol-gel coatings with an anticorrosion or anti-oxidation function is also fascinating. Hydrophilic glass coatings are appropriate for applications that call for anti-fog and antibacterial qualities. Other functionalities, such as hydrophilicity, hydrophobicity, or dirt-repellency properties, can be added by means of organic–inorganic topcoats [35].

The high degree of properties control and versatility of this method, which has a number of benefits over conventional techniques, as well as some drawbacks, piques curiosity [36, 37, 38].

Low process temperatures, good homogeneity, the ability to produce low-thickness films, the production of mixed oxides due to the stoichiometric control of the starting solution composition, better control of the material porosity by varying the heat treatment and a high degree of purity are all evident advantages.

Reversely, high initial material costs, potential fracture formation during the crosslinking step and lengthy process periods are some of the main disadvantages that need to be paid attention to.

2. Sol-gel based protective coatings for innovative and sustainable applications

2.1 Functional textiles

Due to current trends and client demands for high-tech or high-performance applications, functional nanostructured finishings for textile materials have received the most attention among the many finishing processes [39].

Since the coating process can affect a variety of significant textile properties, including comfort, breathability and the hand of the coated fabric, it is important to optimize the process. Furthermore, the surface of the fabric and the presence of impurities can both affect the coating adhesion to the textile fibers [40]. The main challenge relies on the achievement of both functional hi-tech and smart textiles with outstanding performance, uniformity and good coating distribution on the fabric surface. Textiles are typically prepared in accordance with specified treatments for these reasons.

In the last two decades, innovative applications to enhance the fundamental qualities of textiles have been developed with the help of sol-gel-assisted textile finishing approaches [41, 42]. Sol-gel based textile finishing typically offers greater benefits to make up for the drawbacks of traditional finishing methods (Figure 3).

Figure 3.

Multifunctional properties achieved by the application of sol-gel based finishings for textile materials. Reproduced from MDPI ref. [41].

The primary benefits include environmental friendliness, reduced chemical use, low-temperature processing, safety for human health, protection of the natural former qualities of textile materials and the ability to customize the coating thickness and the durability of completed fabrics [43]. In particular, the sol-gel method is a viable wet method for the deposition of functional hybrid organic–inorganic materials and nanoparticles, leading to a final functionalization of textiles. In comparison to other techniques, the integration of sol-gel technology into extensive industrial applications may be simpler and more convenient (Figure 4).

Figure 4.

Approach to fabric functionalization via sol-gel dip-padding process. Reproduced from MDPI ref. [44].

The widely used sol-gel coating application procedures are based on dip-coating, padding or spray, producing smart or functional textiles with precisely designed properties. From an economic standpoint, the dip-pad-cure method was shown to be the most popular due to its simplicity and viability [45]. Using a padder, the fabric is immersed or dipped into a coating material solution while moving at a constant speed. After drying and curing, the process is repeated. A layer with a thickness of about 100 nm is generated on the surface of the cloth throughout the drying and curing process (Figure 4). The functional groups of the precursors are thus attached to the surface of the textile to form covalent or stable hydrogen/ionic bonds, which greatly enhance the adhesion of the hybrid film to the textiles [44]. Through the attaching of coating chemical moieties to the surface of the materials themselves, the sol-gel method, together with other process technologies, successfully modifies the surface of textile materials.

The recent last years have seen the application of hydrophobic treatment to textile surfaces for antifouling, self-cleaning, anti-ice and oil–water separation [46]. Additionally, due to their low surface energy and oil/water-repellent qualities, stain-resistant surfaces and anti-stain coatings have important applications in a variety of industries, including textiles, construction, cars and electronics [47, 48, 49]. For instance, the ability to repel water could aid in clearing bio-settlement off the surface and preventing textile damage. Specifically, because of the property of biomaterials, it immediately aids in the prevention of biofouling and bacterial colonization. Mechanical, chemical and coating functionalization techniques can all introduce these desired characteristics [50].

Water-repellence chemicals are added to the textile fiber to obtain the desired hydrophobic effect with less impact as possible on other functional intrinsic attributes of the fabric such as flexibility, breathability and strength. The use of water-resistant finishes is intended for everyday items like tablecloths, pricey silk garments, uniforms, protective apparel, filter fleece and carpet. Utilizing a straightforward water-based sol-gel procedure, MohdZa’im et al. successfully created a hydrophobic coating for polyester fabric. As a precursor, hexyltrimethoxysilane (HTMS) was diluted in a solution of water, ethanol, and sulfuric acid. The coated polyester fabric’s water contact angle was measured at 136.2 degrees, and the HTMS sol-gel coating achieved the development of excellent surface morphology with undetectable fractures that can induce surface roughness on the coated polyester fabric. As a result, these qualities contribute to gaining a textile with hydrophobic properties without altering the fabric’s inherent qualities, particularly its softness, breathability and smoothness [51].

Fluoroalkyl silanes were used to further increase the surface water repellency. A different method was developed by Simončič et al. using commercially available aqueous organic–inorganic hybrid precursors as finishing agents, such as fluoroalkyl-functional siloxane (FAS) and 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride (SiQAC). Two application processes, a one-step treatment and a two-step treatment, were used to apply the hybrid sols to the cotton fabric using the dip-coating and pad-dry-cure methods. One-step treatments (coating FAS-SiQAC) employed a sol mixture of both precursors, whereas two-step treatments (coating SiQAC + FAS) used SiQAC sol and FAS sol. However, in cotton materials, both treatments demonstrated superhydrophobic, oleophobic and antibacterial characteristics [52].

The ECHA committees most recently proposed to restrict the use of particular perfluoroalkyl compounds (PFAS) in certain application sectors. As a result, examples of environmentally friendly, fluorine-free textile finishings with stain- and water-repellent properties are also documented.

In order to create oleophobic textiles with effective stain resistance, Lei et al. discussed the alteration of fabric substrates with an organic silicone tris-trimethylsilypropyl (M₃T) containing methacrylate copolymer [4]. Additionally, Yu et al. used the dry-pad-cure method to cross-link an amino long-chain alkyl polysiloxane functional agent to create a water-repellent fluorine-free cotton fabric [53].

Modified silica hydrosols were employed by Xu et al. to create superhydrophobic cotton fabric. In order to develop a homogenous emulsion, sodium dodecylbenzenesulfonate (SDBS) and methyltrimethoxy silane (MTMS) were dissolved in 100 mL of water and mixed. The solution was then supplemented with hexadecyltrimethoxysilane (HDTMS) while being constantly agitated. Using the dip-pad-cure method, the modified SiO₂ hydrosols were employed to coat some cotton fabric specimens using a single step. The SiO₂ nanoparticles’ surface roughness and the HDTMS modification’s low surface energy worked together to give the coated cotton fabrics outstanding superhydrophobicity [54].

Functional alkyl(trialkoxy)silane-modified hybrid nanostructured coatings for cotton fabrics were successfully produced by Sfameni et al., specifically via the sol-gel process and pad/dry/cure applications, featuring hydrophobic and water-based stain resistance (Figure 5).

Figure 5.

Cotton fabric functionalization with alkyl(trialkoxy)silane polymer shell by “grafting to” or “grafting from” covalent grafting techniques (a) for the obtaining of hydrophobic and water-based stain-resistant coatings (b). Reproduced from MDPI ref. [55].

The specific objective of the study was to investigate different functional alkyl(trialkoxy)silanes as precursors to synthesize efficient and stable hybrid sol-gel (3-glycidyloxypropyl)trimethoxysilane (GPTMS) based coatings and further reduce cotton surface energy to enhance textile hydrophobicity and water-based stain resistance. In fact, this entire synthetic process has been demonstrated to be a simple, affordable and environmentally friendly method, making it suitable for potentially beneficial application for finishing and functionalizing common textiles in the future [55].

Flame-retardancy is another feature that can be exploited by the design of proper sol-gel based coatings [56]. Textile end uses are restricted and constrained in a wide range of applications because of their possible inherent flammability tendency. For cotton fabric finishing using the sol-gel process, numerous flame-retardant agents have been utilized including halogen-containing (chloride, bromide), nitrogen-containing (melamine, urea), boron and phosphorus compounds [57]. The development of more eco-friendly formaldehyde-free flame-retardant coatings is therefore necessary to reduce the environmental impact of these treatments.

Castellano et al. developed a novel coating for cotton fabrics based on phosphorus, nitrogen and silicon compounds that exhibit self-extinguishing features. In particular, the formaldehyde- and halogen-free coating was synthetized by the use of (3-glycidyloxypropyl)triethoxysilane modified with N-(phosphonomethyl) iminodiacetic acid (PGPTES) and co-hydrolysed and co-condensated with tetraethylorthosilicate (TEOS) as silane cross-linker [58].

Sol-gel process can be exploited also using different inorganic precursors like titanium. Titanium (IV) butoxide, in ethanol, hydrochloric acid and water solution, was employed by Bentis et al. for the formulation of a functional sol with boric acid to obtain sol-based boron-doped titania. Key qualities of cotton samples, in particular their appearance and crystallinity, are unaffected after the application of the TiO₂-boron-based coating by the pad-dry-cure process. The treated specimens demonstrated the ability to absorb more heat than the raw cloth because of the creation of a protective layer that serves as a physical barrier. However, the vertical fire test showed that the coated textiles could demonstrate their fire resistance in actual environments due to their self-extinguishing properties [59].

Drug release and wearable sensors are a recent trend in smart textile research [60, 61, 62, 63]. A thermoresponsive polymer, Pluronic F-127, a non-ionic triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) and pH-responsive polymers N,N,N-trimethyl chitosan and polyethylene glycosylated hyaluronic acid, were combined by Chatterjee et al. to create a dual-responsive hydrogel (pH/temperature). Gallic acid was added to the obtained functional hydrogel to aid at it use in textile-based transdermal therapy as a potential medication for the treatment of atopic dermatitis [64].

In regard to other smart applications, a healthcare monitoring system using wearable temperature sensors based on lanthanum-doped aluminum-oxide dielectrics operating at low voltage and high frequency was developed by Park et al. by the sol-gel process on polyimide substrate [65]. Textile substrates can be also coated with such functional sol-gel based formulations to obtain active and stimuli-responsive surfaces for wearable sensors as well i.e. with the use of organic dye chromogenic materials [66]. Cotton fabric coatings with electrical conductivity were moreover created by Trovato et al. using silica sol-gel precursors that had been doped with vertically aligned carbon nanotubes nanofillers dispersed in the functional sol with the help of surfactants in an organic solvent-free aqueous solution [67].

2.2 Blue-growth

Metal products in seawater must fight with two main processes: corrosion and fouling. Corrosion is a serious issue that might seriously compromise the integrity of the product. Meanwhile, biofouling can affect the ship weight and increase fuel costs by the colonization of organisms (bacteria, algae, plants, and other organisms) on wet surfaces. The two phenomena are also complementary to one another [68].

For the period of the building’s necessary life, any structure must be protected to withstand corrosive forces to avoid damage caused by the corrosion process. Every method that can slow down or inhibit anodic or cathodic processes, or that can close the conductive channel that connects anodic and cathodic sites, can be utilized to reduce, or stop corrosion. The surface protection process typically entails two (pre)treatments that are applied in succession: the preparation pretreatment, which cleans the metal to be protected from any impurities or remnants of earlier corrosive operations and the actual protection treatment [69].

Despite their high efficacy, phosphating and chromating are costly processes with a significant environmental impact. This is because they use toxic materials like phosphates and chromium, which react with metal supports and form toxic sludge, which must be disposed of in large quantities [70]. It also involves using a lot of thermal energy to heat the reaction baths to the proper temperature. There are numerous additional realistic approaches to lower the rate of corrosion for metallic constructions. Utilizing nobler materials, such as stainless-steel rather than mild carbon steel, can help to naturally form a protective layer (passive layer) on the surface. However, due to cost and processing issues, using noble materials is not always the best option. In reality, applying coatings is an additional method of corrosion prevention. In particular, corrosion caused by exposure to a more or less harsh environment over time can be prevented or delayed by protective coatings which operate as an artificial protective layer on the metal surface. Protective coatings can be made by employing a variety of materials and polymers [71].

To replace potentially harmful chromate-based pretreatment layers, silane-based coatings and films coated with organic and inorganic corrosion inhibitors are now frequently used in corrosion mitigation surface treatment. In this regard, different examples of innovative and sustainable coatings for metal surface protection are reported in the literature. Balestriere et al. developed a sol-gel based formulation based on TEOS and MTMS incorporating borosilicate bioactive glass in order to enhance the anticorrosive and surface performance of stainless-steel implants by a double-layer system obtained through a dip-coating approach [72].

Different additives and nanofillers can be also incorporated in sol-gel based formulations to improve their protective and anticorrosive features like carbon-based nanomaterials and natural derivatives. Different examples are reported in literature about the use for example of graphene oxide (GO) as functional nanofiller. Graphene oxide (GO) is a carbon-based nanomaterial characterized by different oxygen functional groups (such -OH and -COOH) on its basal planes and edges and has received a lot of attention in coating applications as its functional groups ensure its strong compatibility with matrices and additives. Additionally, GO has a great barrier property because of its high aspect-ratio flake-like structure [73]. The effects of GO-filled sol-gel sealing on the corrosion resistance and paint adhesion of anodized aluminum were investigated by Ye et al. In particular, the GO-filled sol was obtained by a mixture of organosiloxane sol and zirconium alkoxides sol. Anhydrous ethanol, GPTMS and GO aqueous solution were mixed for the preparation of the organosiloxane sol, meanwhile the Zr sol was prepared by mixing anhydrous ethanol, ethyl acetoacetate and tetrapropyl zirconate. The coating was then applied on aluminum substrates by a dip-dry-cure approach, revealing good anticorrosion performances and paint adhesion [74].

Flavonoids and coumarin derivatives are examples of polyphenolic molecules that exhibit a wide range of features as well as biological and chemical activity. They are typically found as glycosides in nature, particularly in plants, to ensure their water solubility and facilitate their absorption by the organisms. They may perform a variety of functional functions, such as antibacterial and antimicrobial properties, sun protection, or antioxidant activity. The use of such natural compounds with good ligand characteristics is a hot research issue in the area of metal corrosion inhibition [75].

Tamarind shell tannin-doped hybrid sol-gel coatings based on GPTMS and TEOS were developed by Abdulmajid et al. for the improvement of the corrosion protection of mild steel in the acidic medium [76].

In the area of corrosion resistance, phytic acid is another natural derivative that has a wide range of uses because of its non-toxicity. The alloy’s corrosion resistance is increased by the chemical adsorption of phytic acid which forms a dense network structure on the alloy’s surface and prevents the anion from coming into contact with the substrate [77]. A functional sol based on GPTMS and (3-aminopropyl)triethoxysilane (APTES) was employed in combination with GO and phytic acid for the production of anticorrosive coatings for steel and aluminum substrates [78].

Regarding marine biofouling, which is the other significant issue affecting materials in the marine environment, new coatings and strategies for fouling mitigation have been developed during the past several years [79].

The two most developed types of antifouling coatings include biocide-release coatings and non-biocide-release coatings (Figure 6), also indicated as antifouling (AF) and fouling-release (FR) coatings.

Figure 6.

Types of antifouling coatings for the marine environment. Reproduced from MDPI ref. [61].

In particular, the biocide-release coatings are based on the dispersion of biocides from various polymeric hosting matrices and they release biocides into saltwater gradually over time.

The non-biocidal strategy, related to the fouling release, however, can act in one of two ways:

1. the separation of stabilized biofoulants, which aims to lessen the force with which microorganisms stick to surfaces in order to facilitate their removal through the weight of the deposits or the flow of water produced during navigation;

2. the prevention of the attachment of biofoulants, which attempts to prevent the development of a stable fouling coating and, in turn, the adhesion of organic molecules that would start the bio-settlement process.

Moreover, sol-gel coating technology is probably one of the most pertinent tools for the development of environmentally friendly antifouling and fouling release formulations given the current social expectation that new clean, flexible and effective solutions will be adopted rather than the previously mentioned harmful ones.

Numerous techniques have been developed to give innovatively designed coatings to directly interfere with microbe adherence because of topography or surface chemistry, demonstrating fouling release activity, due to the limitations of utilizing biocides in antifouling marine systems. By mixing silicones with fluoropolymers, the critical surface tension can be reduced. Additionally, extensive research has been done on developing hydrophobic surfaces that can prevent the onset of microfouling settlements using proper sol-gel AF-FR formulations and employing different alkyl alkoxysilanes, that is hexadecyltrimethoxysilane, triethoxy(octyl)silane and triethoxy(ethyl)silane (Figure 7) [80] and either fluorinated alkoxysilanes [61].

Figure 7.

Hydrophobic and foul-release sol-gel based coating was developed using different alkyl alkoxysilanes. Reproduced from MDPI ref. [80].

Two novel organoalkoxysilanes were also synthetized by Tan et al. including 2-(2-hydroxy-3-(3-(trimethoxysilyl)propoxy)propyl)benzo[d]isothiazol-3(2H)-one and (N-methoxyacylethyl)-3-aminopropyltriethoxysilane. A series of AF coatings featuring zwitterionic and antibacterial (1,2-benzisothiazolin-3-one) functionalities were obtained through the combination of the two mentioned precursors and TEOS [81]. Given the possible environmental implications of nanomaterial-based nanofillers, surfaces with microstructures incorporating nanomaterials have been widely employed for marine antifouling coatings. Natural cellulose-derived nanofillers have been also explored for antifouling coating preparation. In this regard, Duan et al. developed a bioinspired superhydrophobic coating based on cellulose nanocrystals, CNCs, which are naturally occurring nanomaterials widely employed for the improvement of the mechanical features of materials by their incorporation in polymeric blends [82, 83]. In detail, a sol-gel method with the precursors tetrapropyl zirconate, GPTMS and MTMS, was employed for facile synthesis of an eco-friendly marine antifouling coating incorporating CNCs applied with the spin coating approach in glass substrates [84].

Sol-gel functional coatings can feature also both anticorrosion and antifouling properties [85]. Zhang et al. produced a sol-gel based silver nanoparticle/polytetrafluorethylene coating for stainless-steel with enhanced antibacterial and anticorrosive properties employing titanium (IV) butoxide as precursor and silver nanoparticles [86]. In another study, Zhang et al. (developed a low-surface-free-energy coating based on GO and a fluorine-silicon copolymer featuring self-healing properties for anticorrosion and antifouling applications [87].

2.3 Cultural heritages and buildings

Finally, the protection of architectural and cultural heritage artifacts (based on stone, wood, glass, metals and textiles) should be further mentioned, in order to provide a comprehensive view of the application of novel and environmentally friendly sol-gel based coatings for various surfaces, beyond those made of textiles and metals. Over the last several decades, a variety of protective coatings have been developed to maintain the integrity of cultural heritages and stop the degrading process by reducing the pace of deterioration brought on by weathering and environmental processes (Figure 8).

Figure 8.

Weathering phenomena that may cause the stone deterioration. Reproduced from MDPI ref. [88].

The ideal coating for such an application sector must feature different qualities like transparency, reversibility, compatibility with the surface, long lifetime, ease of synthesis, low-cost maintenance and non-toxicity [89].

Considering these features, sol-gel based coatings find a place as the best recent innovative research results, featuring protective and multifunctional performances, as well as cultural heritages. Due to its advantageous characteristics, including its high chemical stability, non-toxicity, strong photoreactivity and affordability, ZnO is widely used in a variety of technical fields. Due to its antibacterial activity, it is recently useful in preventing the biodeterioration of cultural heritage structures. Doped ZnO nanostructures provide superior antibacterial capabilities than their undoped analogues as reported in some examples. Weththimuni et al. developed a polydimethylsiloxane (PDMS)/ZrO₂-doped ZnO nanocomposites as a protective and self-cleaning coating for stone materials applied by the brushing method. ZrO₂-ZnO core-shell nanoparticles were synthetized and incorporated with octamethylcyclotetrasiloxane and CsOH, employed as a catalyst for the ring-opening polymerization and subsequently, hexamethyldisiloxane to produce in-situ the ZrO₂-ZnO-PDMS nanocomposite [90]. In another recent work, an isobutyltriethoxysilane modified silica (IBTES@SiO₂) nano-coating was produced by Wang et al. to protect from water, ultraviolet radiation and the destructive effect of condensation some outdoor sandstone cultural relics. In this regard, the functional coating featuring self-cleaning, porous and superhydrophobic properties was obtained with a one-step spraying method starting from a formulation based on isobutyltriethoxysilane and nano-SiO₂ prepared at room temperature [91]. A multifunctional protective formulation was developed by Azadi et al. containing methyl methacrylate, 3-(trimethoxysilyl)propyl methacrylate, perfluorooctyl-trichlorosilane and TiO₂ to improve the weathering resistance, self-cleaning properties and hydrophobic behavior of the coated stones with the final nanohybrid film [92].

Glass cultural heritages represent another class of interest in the protective coating development field. Some methods have already been employed to protect glass surfaces from mechanical and climatic hazards such as passive glass object protection with an exterior glazing system placed in front of the stained-glass window. Other techniques involve directly applying chemical inhibitors and consolidants to the surface of the glass after appropriate treatment. Generally, a good protective coating should satisfy some conservation needs (maintenance of the original aspect of the objects and their optical properties) and enhance certain properties (resistance to aggressive environments, slowing down the rate of degradation processes) [93].

Historical glazed wall tiles are an uncommon kind of outdoor artistic expression that is used in structures from many different countries; hence they are frequently prone to biodeterioration. The study carried out by Coutinho et al. was determined whether protective coatings for glazed tiles might be used to stop biological colonization. The anti-biofouling qualities of thin coatings of titanium dioxide produced by sol-gel using titanium (IV) isopropoxide as the precursor in ethanol and acidic water (HNO₃), were tested after their application by spin coating on glazed tiles in order to determine the sustainability and effectiveness of this approach for use in cultural heritage applications [94].

Also, wood cultural heritage elements must be preserved. It has been recently investigated by Andriulo et al. on how to deacidify and consolidate alum-treated wood using multifunctional organic/inorganic hybrid systems. In detail, the formulation was prepared by mixing poly(dimethylsiloxane) hydroxy-terminated (PDMS-OH), TEOS or MTES. Subsequently, the Nanorestore paper® Propanol 5 and 5 g/L Ca(OH)₂ nanoparticles dispersion in 2-propanol was added to the functional sol. The designed formulations, which combine nanostructured Ca(OH)₂ with a network of polysiloxanes, have shown promise in terms of penetration, deacidification, consolidation and preservation of wood artefact characteristics [95].

3. Conclusions

Nowadays, the advancements of nanotechnology in the field of innovative material and implemented surfaces must face the more sustainable expectations and environmental needs of modern society. Despite the new rules and laws about the limitation of the use of toxic and harmful additives for surface finishings, today there is still the challenge to find solutions that are safe for the environment and human health because also of the large use of organic solvents.

In this regard, sol-gel technology aims to achieve environmentally friendly, easy and innovative solutions to address these tasks, by employing different approaches mainly related to water-based or alcohol-based formulations. This chapter described the most recent developments in the field of inorganic–organic hybrid materials and the importance of the choice of different inorganic precursors and functional additives to fine-tune the final desired product properties, also by means of sol-gel synthesis. In particular, the use of titanium, zirconium, zinc and silane-based formulations is reported and moreover, their functionalization with different functional alkoxysilanes, nanofillers (i.e. carbon-based nanomaterials) or functional natural molecules. Thus, their applications for the implementation of the protective, hydrophobic, anticorrosive, flame-retardant, drug release, antifouling and antibacterial features of different surfaces (including metals, textiles, wood, glass and stones) are shown. Therefore, the described solutions may address important advancements in functional and smart textiles, cultural heritage protection and blue-growth applications.

Acknowledgments

CNR and MUR are gratefully acknowledged for the financial support.

Conflict of interest

The authors declare no conflict of interest.