Abstract

Nanosols are innovative coating agents useable for the functionalization and finishing of textile materials. Nanosols are based on inorganic particles in nanometer scale produced by sol–gel technology, which are stabilized in solvents – mainly ethanol, iso-propanol or water. One main advantage of this type of coating agent is that the agents can be modified with simple methods in a broad range, and by this, a variety of new and smart functions can be realized on textile surfaces. However, for application on textiles, there is also a main challenge; this is the realization of sol–gel systems using water as the main solvent. Due to stability reasons, nanosols are often prepared with organic solvents, which are from the point of flammability, safety reasons and cost often not wished.

This chapter will introduce the preparation and modification of nanosols especially dedicated to the application on textiles to realize functional and smart materials. Examples of the realization of water-based sol–gel coating agents are given. Special focus is set on photocatalytic/light responsive materials, antimicrobial systems and controlled release systems.

Keywords: Sol–gel technology, inorganic nanoparticle coating, photocatalysis, antimicrobial, controlled release

3.1 Introduction

The sol–gel technology is probably one of the most successful and promising methods to produce new, advantageous and functional materials in all areas of life, science and industry. The number of new materials realized and developed by sol–gel method is nearly uncountable. As well, the fields of applications and realized functions are manifold [1–5].

To give here an impression on the broad field of sol–gel technology, just take a look on the recent sol–gel conference held last September 2017 in Liège (Belgium). This conference takes place every second year, and in 2017, more than 300 contributions from different fields of sol–gel technology were presented by scientists from all over the world. The idea of functional sol–gel coatings is mainly to create an inorganic coating with a certain functional property and to apply it onto a certain substrate (e.g. metal, glass, textile, wood, leather, etc.) [6–11].

By this, the functional property is transferred to the coated substrate. This created inorganic sol–gel coating can be self-functional from its own nature such as the metal oxide coating, e.g. to improve the abrasion resistance of glass fiber fabrics [3]. However, in most cases, the function is introduced by special additives carrying the functional properties. A simple example can be given with colored sol–gel coatings. Mixed metal oxides of spinel type can be realized by sol–gel method and contain a self-inherent colorant [12]. In comparison, an uncolored silica coating realized by sol–gel method can be modified by incorporation of a dye stuff that carries the color as a functional property [13–21].

Because of the many different functions, which can be applied by sol–gel coatings, it is suitable to place the functions in several specific main groups, as shown in Figure 3.1.

Optical functions summarize all properties modifying the interaction with light or in a broader view with any type of electromagnetic radiation. To this field count the ordinary coloration but also functions as UV-protection, IR-absorption or X-ray protection properties [15–17, 22–24]. The chemical functionalization is related to all types of coatings modifying the chemical properties of the coated substrates. This can be a coating introducing flame-retardant properties [3, 25–30]. Also, the application of catalytic properties is possible [30, 31]. In this field, many developments have been achieved with photocatalytic coatings often based on titania or zinc oxide materials [33–38].

The biological functionalization is related to any property interacting with living organisms. Main field in this area is the antimicrobial function acting against microorganisms [39–44]. Sol–gel coatings can be also modified to realize coatings with biocompatibility [3, 45–48].

The term “surface functional” summarizes all coatings modifying the interaction of the fiber surface to other media. This can be an enhanced abrasion stability, stabilizing the textile surface against mechanical influences in contact with other materials [3].

Besides, repellent coatings are surface functional. Water-repellent hydrophobic coatings can be realized on textiles by sol–gel method as well as oleophobic coatings [46, 49–51]. A repellent modification can be also described as antiadhesive. Such antiadhesive coatings can be used to realize antiadhesive wound bandages, which are less sticky to the healing wound [8, 52].

A hydrophilic coating increases the ability of the coated textile substrate for water uptake. The content of water on a textile substrate is directly related to its antistatic properties, because a static charging is conducted away by the uptaken water [53–55]. This approach can be broadened in a double functional sol–gel coating combining the water-repellent and antistatic properties. This combination is especially challenging, because two contradictive properties (hydrophobic and hydrophilic) are combined and realized in one single application [56, 57].

3.2 Preparation of Nanosols as Coating Agents

Nanosols are solutions containing particles with small diameters in the range of a few nanometers up to 80 nm. The nanosol particles are often built up by semimetal oxide or metal oxide compounds. These solutions are meta-stable, meaning that the particles would agglomerate and precipitate or gelate because of their small size [45]. The driving force of agglomeration is the large surface area of the small particles compared to their small particle volume. In case of nanosols, particle agglomeration is prevented by different influences, e.g. surface charges or sterical reasons [58]. Another and more traditional definition of nanosols could be the description as colloidal solution.

Nanosols can be used and applied as liquid coating agents on textile substrates by different methods such as padding, dipping or spraying. After application on the substrate, the solvent evaporates and the remaining nanoparticles aggregate forming a three-dimensional network. This network forms the final sol–gel coating. A further thermal treatment leads to condensation of the coating and finally to the formation of a ceramic coating (Figure 3.2) [3, 59].

The preparation and application of a silica-based SiO₂-nanosol is schematically depicted in Figure 3.2. This silica sol is shown as an example, because most reported nanosols are related to silicon-based systems. Other prominent examples for materials building up nanosols are titania (TiO₂), zirconia (ZrO₂), zinc oxide (ZnO) or barium titanate (BaTiO₃) [3, 23].

In case of SiO₂ sols, the preparation starts by hydrolysis of metal alkoxide compounds. In Figure 3.2, the example tetraethoxysilane (TEOS) is given. TEOS has the advantage of medium reactivity, compared to tetramethoxysilane (TMOS) with higher and tetrapropoxysilane (TPOS) with lower reactivity [58]. The hydrolysis can be performed under acidic or alkaline conditions and leads to the formation of silanol groups Si–OH in the first step. A following condensation of silanol groups leads to the formation of nanosized SiO₂ particles and the nanosol is formed.

To functionalize nanosols, agents containing the desired functions are added to the nanosol or to the precursors before hydrolysis is started (Figure 3.2). These agents are embedded into the sol–gel coating during the coating and drying processes. This embedding is also named as physical modification of the sol–gel coating. A chemical modification is performed, if a covalent bond between the SiO₂ matrix of the coating and the functional agent is also formed, besides the simple embedding [3, 45].

3.3 Application on Textiles

In a first and easy view, a nanosol agent can be applied on any kind of substrate to gain the wished function. However, each kind of substrate has its own demands to realize an optimal sol–gel coating.

Textile substrates contain higher flexibility compared to other substrates, e.g. wood, stone or glass. This flexibility is a typical property essential for many textile applications, so the flexibility should not be negatively influenced by the functional coating. Other properties such as air permeability also have to be kept even after the coating application. To achieve flexible coatings, nanosols should be applied in low concentrations [3, 43, 60].

Another important point is the stability of the coatings in case of usage, which is related to abrasion stability and washing stability. For this purpose, anchoring of the coating to the textile surface is required. For industrial applications, nanosols, which are free from flammable or toxic organic solvents, are often required.

Table 3.1 summarizes some basic nanosols and their preparation parameters. These sols can be used after further modification for textile treatments. All of these sols contain water as the solvent. The used silane precursors contain hydrophobic properties and are, for this reason, not mixable with water, so a strong stirring has to be performed during preparation to force the components together to an emulsion. After the hydrolysis of the silane precursor is finished and the SiO₂ particles are formed, a clear, transparent and homogeneous solution is obtained. The reaction time can be as long as two days. After the reaction, it has to be confirmed that no precipitation occurs in the sol. Such a precipitation is a hint for an insufficient sol formation and an unstable recipe. The stability of these nanosols varies in a broad range. The mentioned silica sol is stable at room temperature for several weeks. The duration until the gelation process is increased, if the compound 3-glycidyloxypropyltriethoxysilane (GLYEO) is added as the second precursor [42]. In comparison, the mentioned alkaline silica sol is stable against gelation for a period of one year or even longer (Table 3.1).

3.4 Nanosols and Smart Textiles

Nanosols that are used to realize functional materials are described in the following subsections. The application of these sols onto textile substrates is in their first steps or even when the sol formation has not started yet. However, the perspective functional properties are highly attractive, worth to mention here and to highlight the chances for realization of functional and advantageous textiles in the future.

3.4.1 Photocatalytic and Light Responsive Materials

Light responsive materials are systems exhibiting any kind of action in case of exhibition to light. In case of textile materials, the light responsive action that is discussed mostly in literature is the photocatalytic activity. In this field of photocatalytic materials, the photooxidation promoted by catalytic TiO₂ species is the most prominent one.

For application onto textiles, titania sols in aqueous solutions are often wanted as the finishing agents for textiles. However, the reactivity of titanium precursors with water is higher compared to silane precursors, so it is quite challenging to prepare water-based TiO₂ sol coating agents.

One change to prepare photoactive TiO₂ particles in aqueous solutions is to use a special pH-regime and to stabilize titanium alkoxides in the presence of amino compounds. For this purpose, compounds like triethanolamine and polymers containing amino groups can be used [33]. A prominent example here is polyvinylamine, which is also presented in Figure 3.3. Polyvinylamine is also well known under the trade name Lupamin (BASF product).

After preparation of the water-based titania sols, a thermal treatment is performed on the liquid recipes to transfer the previously formed amorphous TiO₂ into the photoactive anatase modification. This thermal treatment can be done under ambient pressure in reflux or under solvothermal conditions with high pressure in an autoclave device. The presence of crystalline anatase is determined by X-ray diffraction (XRD) measurements. By this analytical method, the formation of brookite as second crystalline modification of TiO₂ is also determined. Obviously the brookite is formed as well in small amounts during the mentioned preparation technique. The prepared TiO₂ sols can be used as coating agents for textile treatments. The photoactivity of the coated textiles is measured by color changes of dye solutions under illumination with UV A light. The related measurement procedure is described in the literature [31, 32]. The photoactivity A [%] is given in relation to the color change during a reference measurement with an analogous but uncoated textile. The measurement results are given in Table 3.2.

The method used to determine the photoactivity is simple and fast. However, it has to be kept in mind that the determined photoactivity is related to the used dye stuff [32]. Also the type and number of used UV light sources and their distance have significant influence on the measurement result. For this, a comparison of percentage values of photoactivity is only possible if the measurements are performed with the same dye stuff, same dye concentration and similar measurement arrangement. Nevertheless, measurement results for the TiO₂ coatings onto polyester fabrics are given in Table 3.2; some main results can be summarized. The addition of the amino compounds increases the photoactivity. The addition of polymers containing amino groups especially leads to effective results.

The preparation of TiO₂ sols can be modified by addition of water-soluble silver salts, e.g. silver nitrate, silver acetate or silver lactate. By this, crystalline elementary silver particles are also formed similar to the formation of crystalline anatase (Figure 3.4). Formation of the metallic silver during the thermal treatment is probably the result of reduction of the silver ions by the added amino compounds [40]. The silver-modified TiO₂ sol was investigated using transmission electron microscopy (TEM) (Figure 3.4). The formed crystalline areas of anatase exhibited diameters around 7 nm. In comparison, the formed silver particles were significantly larger possessing diameters around 30 nm.

Besides pure TiO₂ coatings, combinations of TiO₂ and SiO₂ are also used in sols to prepare photocatalytic coatings on textile substrates. Gregori et al. [61] report on suitable aqueous and alcohol-based suspensions and solutions. The same group of researchers recently presented an interesting modification of TiO₂/SiO₂ systems by using gold nanoparticles and gold nanospheres [62, 63]. Even by addition of only 1 wt% gold particles, the photocatalytic degradation rate is doubled in the case of illumination with UV light. The often aimed activity with visible light is not introduced by the addition of gold particles [62, 63].

In case of light responsive materials, coatings leading to coloration by interference effects are also interesting and fascinating. Such interference effects can be achieved by application of hybrid organic/inorganic materials [64]. Interference colored materials like opals can be realized by coatings of highly ordered SiO₂ spheres with diameters of several hundred nanometers [65]. This approach is quite close to sol–gel techniques. A recently developed approach in this field contains structurally colored multilayer films made of a silk-based material. This approach is inspired from nature, where specific species of beetles receive their color via interference effects [66].

The last example related to light responsive materials given here are carbon particles used for fluorescence effects. Fluorescence effects on textiles are demanded for working clothes or clothes with high visibility. Usually fluorescence textiles are prepared by application of organic fluorescence dyes. Manifold types of fluorescence dyes are developed, and the application on several types of textiles such as polyester, polyamide or cotton is easily performed. However, one main disadvantage of organic dye stuffs could be their limited stability under light exposure. The exposure to light can also cause photochemical reactions leading to the decomposition of dyes [3, 37].

In contrast, carbon nanoparticles (as well named as carbon nanodots) exhibit high light stability. An excellent review on carbon nanodots embedded in mesoporous materials is given by Innocenzi et al. [67]. Such mesoporous materials can be realized by the sol–gel approach. The shown preparation of bulk materials can be transferred, in a next step, to the development of sol–gel-based coating agents used for the application of thin inorganic mesoporous coatings with embedded carbon nanodots for textile treatments.

Carbon nanodots can be functionalized on their surface by introduction of amino groups. Epoxy modified silane compounds like GLYEO can be covalently bonded to these amino groups. By this, the chemical modification of silica sol coatings with carbon nanodots is possible, and photoluminescence coatings for textile finishing can be realized easily. One advantage for the development of these coating recipes is the good water solubility of the modified carbon nanodots [68].

Alternative to SiO₂ coatings, carbon nanodots can also be embedded in ZnO coatings [69]. In contrast to SiO₂, ZnO is a semi-conductive material and can be activated by UV light. The electron band structure of zinc oxide can be modified via embedding of carbon nanodots leading to narrowing of the ZnO band gap. By this, an activation with visible light can be introduced [69].

3.4.2 Antimicrobial and Bioactive Systems

Most applications on antimicrobial sol–gel coatings are probably related to recipes using the embedding of silver ions, silver compounds or metallic silver particles [39–41, 70–72].

An application developed especially for the antimicrobial treatment of cotton is reported by Xing et al. [73]. This group uses sol–gel coating agents prepared from silica water glass modified with silver nitrate. An advantage of this recipe is its higher water content compared to earlier reported recipes containing significant amounts of organic solvents. Besides silver-containing systems, copper-containing coatings are also investigated for antimicrobial sol–gel coatings as an alternative [42, 74]. For further information on the broad field of antimicrobial sol–gel coatings, several excellent review papers can be referred to [8, 75, 76].

One interesting approach is sol–gel coatings releasing antibacterial acting gases [3]. These antibacterial sol–gel coatings act against bacteria, even if there is no contact to the coated surface or no intermediate liquid medium. A possible process in this field is the use of nitric oxide (NO) releasing sol–gel systems [77, 78]. However, two points have to be kept in mind. First, the released gases can also influence the human health. Second, the release into the gas phase can exhaust the antimicrobial depot after short duration. By view on both limitations, it is clear that these releasing systems would find main applications as textile packaging materials.

Just have a view on other more innovative and forecasting bioactive sol–gel materials [79]. Sol–gel materials can be set to the interface of inorganic chemistry and biology by bioencapsulation of biomolecules, e.g. proteins, enzymes, polysaccharides, lipids and nucleic acids. The resulting materials are often also named as biocers (from bioceramics). Potential applications for biocers are the development of new biosensors, bioreactors, filter systems for waste water cleaning or drug release systems [79–82].

Especially attractive is the embedding of enzymes in sol–gel coatings. Enzymes are highly active biomolecules working as biocatalysts. By fixation of these enzymes on textile surfaces, their specific catalytic properties are transferred to the coated textile substrates. A simple example is the embedding of the enzyme lysozyme in sol–gel coatings applied onto polyester foils [83]. Lysozyme presents antimicrobial activity, and by its introduction into the coating recipe, the antimicrobial properties are transferred to the coated polyester substrates. The advantageous part here is the realization of a biobased antibacterial coating, which could be a big issue also for marketing reasons.

The realization of biocatalytic coatings prepared by sol–gel encapsulation of enzymes is described in references [84, 85]. For this, silica sol coatings are mainly used. The use of water-based recipes without organic solvents or solvents at lowest concentration is absolutely necessary to avoid the denaturation of the embedded enzymes. The thermal fixation performed after the coating process is also limited to moderate temperatures to avoid any denaturation. If these demands are fulfilled, the enzyme containing coating agents can be used to functionalize textiles with biocatalytic functions. Applications can be found as additives in cleaning agents or in bioreactors.

An interesting approach is to stabilize the embedded biomolecules against thermal denaturation [86]. Some proteins have the property to take up and bond bivalent metal ions as Ca²⁺. In case of bonding to Ca²⁺, these proteins can receive an increased stability against higher temperatures but also under exposition to the organic solvent ethanol [86]. This is of course a very attractive approach for preparation of biocer coatings with sol–gel technology.

It is a further step to immobilize and embed complete cells of microorganisms in sol–gel coatings; this is done for algae, bacteria and cyanobacteria [3, 87]. The final idea of this approach is to realize coatings containing living bioorganisms working as a kind of bioreactors. By embedding cyanobacteria, the development of novel photobioreactors is aimed [87].

It is of course a smart idea to apply these coatings onto textile substrates and to realize these textile-based bioreactors, which can be modified in various constructions and geometry by textile technology processes. However, even if this is a very attractive idea, the main difference in embedding a biomolecule like protein compared to a living cell should be kept in mind. In contrast to proteins, living cells need ambient temperature conditions, humidity and nutrients to survive. For this, some efforts have to be made to keep the functional properties of coatings containing living cells even after the coating process is finished.

3.4.3 Controlled Release Systems

Active agents, perfumes, fragrances or even medical substances can be impregnated onto textile substrates. In contact with water or other solvents, these substances are released into the solvent. In this situation, the rate of release depends on the solubility of the substances in the present solvent and on the adhesive properties of the textile surface. Usually the release is faster than wished for the aimed application, meaning that the concentration in the solvent decreases too fast and the depot function of the textile diminishes soon. In many cases, it is useful to decelerate the rate of releasing by a coating. Sol–gel coatings can be used also for embedding of active substances or drugs, and the velocity of release is no longer determined by the solubility of the drug [88–90].

Now the release is determined or, in other words, controlled by the type of coating used for the embedding. The addition of penetration agents such as sorbitol can be used to introduce to the silica coatings capillary channels that enable the embedded drug molecules to escape from the coating. By adjusting the concentration of sorbitol added to the sol–gel recipe, the rate of release can be controlled [88–90]. Such mesoporous silica materials can be used for release of DNA fragments as well, e.g. for the modification of living cells [91, 92]. The controlled release of antibiotics is useful for the preparation of antimicrobial coatings [93].

3.5 Summary

Nanosols are versatile and multifunctional tools to functionalize textile substrates. The realized new functions can lead to completely new materials and applications, which is never expected for textiles in the traditional world. These new functional textiles can be best described by the term “smart textiles.”

The functional properties that can be realized on textiles with nanosol technology could be best described in a schematic overview distinguishing between optical, chemical, biological or surface functionalities. Many of the actually mentioned and summarized properties could be also understood as effects gained with traditional textile finishing, e.g. coloration, dyeing or water-repellent effect. All these functions can be realized as well by nanosol treatment of the textile. However, one aim of this chapter is to emphasize possible and extraordinary functional properties, which can be especially realized by nanosol application. In the section of optical functionalization, nanosols can be used to realize photocatalytic and light responsive textile materials. Of high potential are especially nanosol recipes containing carbon quantum dots (C-dots) with fluorescence properties. These C-dots are supposed to be innovative fluorescence materials with high lightfastness and low toxicity. Also innovative biofunctional textiles can be realized. These biofunctions are especially related to the embedding of biopolymers, enzymes and complete cells onto textile surface by using nanosol coatings. Prospective applications are found for textiles as carrier of enzymatic properties, e.g. as part of bioreactors or for waste water cleaning processes. Other potential applications could be developed in the fields of biosensors, bioreactors or drug release systems.

Acknowledgements

The author would like to thank Prof. Dr. T. Textor (Fachhochschule Reutlingen, Germany) for many helpful discussions and long-term cooperation. For help with electron microscopic measurements, many thanks are owed to M. Reibold (TU Dresden, Germany). All product and company names mentioned in this chapter may be trademarks of their respective owners, also without labeling.

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