One of the most urgent tasks in science is to improve quality of life and life expectancy. As such, new high-tech products are needed from various fields of the economy, including fibrous materials with improved and fundamentally new properties, produced by the textile industry. For example, in a reviews devoted to textile materials, what people want to see modern clothing’s listed: it should be waterproof; fire-resistant; self-cleaning; protect against insects, infections, UV radiation, and chemical and biological agents; be warm in winter and cool in summer; and at the same time be bright and not bulky [1]. Thus, many serious requirements are imposed on the properties of fibrous materials for the manufacture of clothing, and which often contradict each other. The existing fibers and products made from them do not have the necessary characteristics. It is possible to give improved consumer characteristics and special properties to polymer fibrous materials through directed chemical or physical treatments of them; that is, by modification. There is information in the literature reporting that the adhesive ability, chemical resistance, surface energy, hydrophobicity, hydrophilicity, biostability, and many other properties of polymer materials can be determined by a surface layer with a thickness of ~10 nm to several micrometers [2]. Thus, a simple and economical approach to giving fibrous materials improved and previously non-inherent consumer properties is to regulate their surface characteristics; that is, surface modification of fibrous materials.

The global consumption of fibers and threads has shown a steady upward trend. At the same time, there has been an increase in the consumption share of chemical fibers and threads in the market of fibrous materials, due to a reduction in the consumption share of natural fibers. Currently, the share of chemical fibers and threads of world consumption exceeds 70%–75%. According to forecasts, 98% of fabrics will be partially or completely synthetic in the next 5–7 years. Thus, special attention should be paid to the modification of chemical fibers. This is especially difficult, since these fibers have a fine-pored structure and a smooth surface.

It is important to note that surface modification aimed at acquiring special properties should not negatively affect the properties originally inherent in fibrous materials: for example, after applying a modifier, fabrics should retain their softness, drapability, air and vapor permeability ^[3][4][3,4], and strength characteristics [5]. As modern research shows, these effects can be achieved using nanoscale modifiers.

In recent decades, it has been possible to obtain and thoroughly investigate a large number of functional nanoparticles and nanostructured materials that can be used as modifiers. Due to this, as well as due to the development and improvement of technologies for the formation of nano- and micro-dimensional modifying coatings based on such materials, significant progress has been observed in the field of the surface modification of fibrous materials. Many researchers are working on giving the most practically significant properties (individually and in combination) to natural and synthetic textile materials: antimicrobial activity, self-cleaning in light, superhydrophobicity, protection from UV radiation, etc. These goals can be achieved by using a variety of modifying substances and methods of application to fibrous materials.

2. Preliminary Activation of the Surface of Fibrous Materials

Pre-activation is often used to increase the adhesion of coatings to the surface of fibrous materials. This consists in various kinds of treatment soft the fibers, as a result of which their near-surface layer is transformed, new active oxygen-containing groups are formed, and the fiber becomes less smooth. In addition to increasing the surface content of reactive functional groups and increasing the roughness of the fiber surface, an important criterion for the effectiveness of pre-activation is the preservation of a high level of strength of the fibrous material, which can decrease with intense exposure to the fiber [6]. Surface activation is especially used for the pretreatment of synthetic fibers, many of which consist of chemically inert polymer materials. The authors [7] divide the solvent (or “wet”) activation methods for fibrous materials into those based on hydrolysis, oxidation, halogenation, complexation, and the formation of layers that promote adhesion. However, we do not consider the formation of coatings on fibers that promote the adhesion of functionalizing drugs is not considered as an activation method, since these usually have a significant effect on the properties of fibers. The method of modification of aliphatic and aromatic polyamides based on complexation causes significant changes in the bulk properties and structure of fibers ^[8][9][10][8,9,10]. Therefore, it cannot be attributed to the methods of surface activation. Fiber activation methods based on weak alkaline hydrolysis of the surface can be considered traditional. However, they are still being developed and improved. These methods are particularly suitable for polymer materials containing ester bonds ^[11][12][11,12]. Thus, it was found in ^[13][14][13,14] that it is possible to select a reagent, its concentration, and the duration of hydrolysis in such a way that a significant number of active groups are formed on the surface of the polyester fiber, which cause the fixation of functional preparations. In particular, the treatment of polyester fiber with a urea solution of a concentration of 0.05–0.1 mol/L at a boiling point for 15–20 min led to the formation of hydroxyl groups, the number of which was six-times higher than the initial one [14]. At the same time, the strength of the fiber remained at the initial level. Another method of hydrolysis is based on the use of enzymes. Enzyme proteins act as biocatalysts. Of particular interest for fiber modification are the hydrolases that provide controlled degradation of the fiber surface, with the formation of functional end groups and an increase in roughness, similar to alkaline hydrolysis ^{[15][16][17][18]}[15,16,17,18]. Optimization of the results of enzymatic treatment can be carried out by regulating parameters such as the treatment temperature, pH value, concentration, and duration of exposure ^[19][20][19,20]. Oxidative methods for the surface activation of fibrous materials are based on the action of nitric, chromic acids, or potassium permanganate ^[21][22][21,22]. Such treatments usually lead to the formation of new functional groups on the fiber surface, such as hydroxyl, carbonyl, and carboxyl groups. Another method of oxidation of polymer fibers is treatment with phosphoric acid [23]. When using aqueous solutions of sulfuric acid, sulfonation can also occur simultaneously with oxidation [24]. This activation method was used in [25], the authors of which treated the fiber with concentrated H₂SO₄ to increase the adhesion of polypropylene fiber, before forming a coating based on modified graphene oxide on its surface. Halogenation is based on the substitution of a hydrogen atom by halogens, through radical reactions or addition and substitution reactions. Chlorination is used to activate aromatic polyamide fibers in a wet state ^[26][27][26,27]. In previous works ^[26][27][26,27], this process was implemented using active chlorine-containing reaction agents, such as solutions of sodium dichloroisocyanurate or sodium hypochlorite. As a result, the wettability of the fiber was increased and the formation of active chlorine-containing groups was observed. In addition to “wet” activation methods, so-called “dry” methods are often used, which do not require water consumption. These are considered more environmentally friendly. Thus, oxidative activation of fibrous materials can also be carried out by the “dry” method using ozone treatment [28]. Ozone is a powerful oxidizer, whose interaction with fiber leads to the formation of carboxyl, hydroxyl, and amino groups, in the case of polyamide fibers ^[29][30][29,30]. The morphology of the fiber surface changes and the roughness increases ^[31][32][31,32]. Ozone treatment can lead to a loss of fiber strength ^[31][32][33][31,32,33]. A popular method of “dry” pre-activation of fibrous materials is low-temperature plasma treatment ^[34][35][36][34,35,36]. When plasma interacts with polymer substrates, several reactions and processes can occur, most of which lead to the formation of free radicals and unsaturated organic products, cross linking, destruction of macromolecules, and the formation of gaseous products. Under the action of plasma, chemical and physical changes of the fibrous material occur in a thin surface layer, without affecting the entire volume of the polymer material. The authors in [37] estimated the thickness of a plasma-discharge modified layer at about 10 nm. The changes caused were regulated by several factors, such as the type of gas used, pressure, frequency, power, and processing time, as well as the nature of the fibrous material ^[34][35][36][34,35,36]. First of all, plasma pretreatment is a tool for improving the adhesive properties of fibers ^[37][38][37,38]. For these purposes, plasma at both low ^[39][40][39,40] and atmospheric pressure can be used, although atmospheric pressure plasma is more often used ^[37][41][42][37,41,42]. For example, the authors of ^[43][44][43,44] used plasma pretreatment of cellulose fabrics to increase the absorption of chitosan and silver nanoparticles, and in ^{[37][45][46][47]}[37,45,46,47] plasma activation was used to increase the degree of fixation of TiO₂ on fibrous materials. It should be noted that the effect of plasma on the surface of a fibrous material associated with the formation of new functional groups may decrease over time ^[48][49][48,49]. In recent years, the attention of researchers has also been attracted by the effect on fibrous materials of a plasma discharge ignited in electrolyte solutions. It was shown in ^[50][51][50,51] that the treatment of polyester thread with diaphragm discharge plasma in an electrolyte solution ensured the formation of hydroxyl groups on the surface of the polymer material, the number of which was 3.2 times higher than the initial number. In this case, carbonyl and carboxyl groups were generated (in larger quantities than under the conditions of the chemical activation method) at an acceptable level of thread strength loss. Each of the considered methods of preliminary activation of fibrous materials has certain advantages and disadvantages. The choice of method is determined by the type of fibrous material, as well as the requirements for the properties, structure, and morphology of the formed coatings.