Keywords

1 Introduction

Self-cleaning textiles are the materials which can clean themselves without laundering. This property can be achieved in textiles by making their surface repellent to water and soil. Water and oil repellency is related to the surface energy and contact angle with the contact fluid. The contact angle is defined as the angle between the solid textile surface and droplet of contact-fluid. The wettability of the solid surface depends on the surface energy of the solid and the liquid. Depending on the difference in surface energy the contact angle is subtended in smooth surface. There are wettability theories proposed in literature for smooth and rough surfaces. For smooth surface, Young model (Eq. 1) was proposed which is based on the three interfacial energies per unit area which are in equilibrium at the droplet resting on solid surface.

$$\gamma_{sv} = \gamma_{sl} + \gamma_{lv} { \cos }\theta$$
(1)

where, θ is the contact angle as shown in Fig. 1, γsv and γsl, γlv are interfacial energies per unit area of the solid-vapor, solid-liquid, and liquid-vapor interfaces, respectively.

Fig. 1
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Contact angle for surface wetting

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If the contact angle subtended by a fluid droplet on the solid surface is less than 90°, the solid surface is termed as a hydrophilic surface corresponding to the same fluid. When the contact angle is >90°, the surface is termed as a hydrophobic surface corresponding to the fluid. If the contact angle is approaching to zero the solid surface is becoming super hydrophilic and inversely, when the contact angle is >150° the surface is super hydrophobic. A super hydrophobic surface creates the water or oil repellency leading to self-cleaning effect. Various means of water and soil repellency finish of textiles have been one of the major research focus since few decades. This increasing interest towards development of self-cleaning textiles is due to their ability to reduce cost of cleaning and henceforth commercial success. There are various materials and techniques are available for preparation of water, oil and soil repellent textiles with self-cleaning effect. They are especially surface treatments to achieve the condition of limited wettability or repellency which leads to the concept of self-cleaning textiles. Super repellent surface or self-cleaning surface is already available in nature itself [1]. A common example of super hydrophobic self-cleaning effect in nature is lotus leaf, where water droplets can roll out of surface without wetting and contaminating. The surface of lotus leaf is thus have ability of self-cleaning by repelling water and dirt. We need to understand such natural phenomenon and apply the knowledge of chemistry and physics to achieve similar self-cleaning effect on textile surface. The lotus leaf consists of two levels of architecture viz. micro-scale bumps and nano-scale hair-like structures coupled with some waxy chemicals. Researchers are developing artificial self-cleaning textiles by the concept of this architecture of lotus leaf. There are irregular epicircular wax crystals present of the surface of the lotus leaves making an uneven micro-texture. Air molecules are thus easily trapped in interstitial spaces of the rough surface resulting less adherence of water and dirt molecules by reducing adhesive forces. Water molecules appear like sphere over such surface and roll over easily and in the course remove the dirt particles away from the surface [2,3,4,5,6,7,8,9]. Self-cleaning effect can be achieved by textile materials either by preparing a hydrophobic surface or by some hydrophilic coatings. Both of these types of surface coatings can help to clean textile surface without laundering.

2 Principles of Preparation of Self-cleaning Textiles

There are two principal methods of preparing self-cleaning textiles. It can be prepared either by coating of textile surface with super hydrophobic materials (such as silicones, fluorocarbons etc.) or by coating with some functional hydrophilic materials by the route of nanotechnology.

A liquid droplet can wet a solid surface when the surface tension of a solid is higher than the surface tension of the liquid. Therefore, the surface tension of the solid need to be reduced than that of the liquid for achieving liquid repellency. Fluorocarbons are the carbon compounds which contain perfluorinated carbon chain possessing a very low surface tension of about 10 dyne/cm. During the application process, the fluorocarbons form a coat of thin layer around the textile surface. As a result, surface tension of the coated textiles becomes lower than that of water and water repellency effect is achieved. A water droplet then does not adhere to the textile surface and rolls out off. Silicones are actually organosilicon compounds which are highly explored for preparation of super hydrophobic textile surfaces. There are various approaches available in literature for preparation of super hydrophobic self-cleaning surfaces using silicones. PDMS (polydimethylsiloxane) is one of the popular silicone, can be used for surface modification of textiles by exciting CO2 pulsed laser to introduce peroxide groups onto the PDMS surface to create a rough surface. These peroxide groups assist graft polymerization of 2-hydroxyethylmethacrylate (HEMA) onto the PDMS. By this method excellent hydrophobic surface achieved with water contact angle of about 175°. But these hydrophobic coating processes have drawback in terms of durability of the coat which is not satisfactory and in case of cotton material this is found to be very poor. Other demerit is hazardous effect of the fluorine compounds which reacts with biological issues and causes skin irritation [1, 3, 10,11,12,13,14]. Nanotechnology is relatively a new approach of achieving self-cleaning effect for textiles. This route is proved to be technically viable as well as economically successful. Various approaches of preparing self-cleaning textiles by this route are proposed in literature by various researchers. Most widely described approach is by the applications of photo catalyst like TiO2. Other methods are using silver nanoparticles, carbon nanotubes, colloidal metal oxide, N halamine, microwaves irradiation, etc.

3 Photo-Catalytic Self-cleaning Effect

Unlike hydrophobic surfaces which are based on rolling over of water droplets to clean the adhering dirt from the surface, hydrophilic coatings of a photo-catalyst chemically break down the dirt/foreign molecules in the exposure of sunlight. This is the photocatalytic self-cleaning effect. Titanium dioxide (TiO2) and zinc oxide (ZnO) nano particles are commonly coated over textile surface which are acting as photocatalysts. The thickness of the coating is typically in the range of 20 nm. When these nanoparticles are irradiated with ultra-violet rays of sunlight that has energy higher than their band gap then valance electrons are excited to jump into conduction band. These conductive electrons (e−) form O2− radical ions in presence of atmospheric oxygen. The O2− radical ions are unstable and combined with contaminated dirt particles, pollutants, and micro-organisms which are generally organic compounds. The reaction is resulted in the formation of carbon dioxide (CO2) and water (H2O). TiO2 or ZnO act as catalysts only and never used up in the reaction process and destroy dirt molecules, organic matters and micro-organisms from the textile surface providing self-cleaning effect in presence of sunlight. The mechanism is demonstrated in Fig. 2 [7, 15,16,17,18,19,20,21,22,23].

Fig. 2
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Photocatalytic self-cleaning of titanium dioxide coated on textile surface [19]

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Self-cleaning anti-microbial cotton fabrics are prepared by impregnating the same in a dispersion solution of TiO2 nanoparticles. This TiO2 nanoparticle enriched fabrics can kill bacteria in sunlight [24]. The antibacterial functionality of the TiO2-enriched cotton fabrics is attributed to the destruction of the bacteria cell wall and membrane by O2− radical ions which are generated in course of photocatalytic reactions. In another study, it is reported that TiO2 loaded cotton textiles eliminate the stains of wine, coffee, tea etc. by destroying chromophore(s) of the stains by the irradiation of ultra violet rays of sunlight [25]. Such self-cleaning functionality of the TiO2 loaded textiles is attributed to the formation of highly oxidative intermediates generated at textile surface as shown in Fig. 3. The mechanism of dismissing of the stains and release of CO2 due to light irradiation is shown in Eq. (2).

$${\text{C}}_{x} {\text{H}}_{y} {\text{N}}_{v} {\text{S}}_{w} + hv + {\text{H}}_{ 2} {\text{O}}_{w} + {\text{O}}_{ 2} \to {\text{CO}}_{ 2} + {\text{H}}_{ 2} {\text{O + SO}}_{p} + {\text{NO}}_{q}$$
(2)

In other studies, self-cleaning functionality is bring about on cotton textiles by coating with TiO2 film as well as loading of AgI particles which enables the fabric to be cleaned in visible light only [26]. This visible light irradiated photocatalytic effect of the AgI–Nano-TiO2 coated cotton textiles is reported significantly better than simply TiO2 treated cotton textiles. This reveals that AgI effectively assists the photocatalytic activity of TiO2 and the effect sustains for several numbers of photodegradation cycles [26].

Fig. 3
figure 3

Reprinted from Ref. [25], Copyright (2019) Elsevier

Photocatalytic oxidative intermediates produced by TiO2 on a cotton tissue in the presence of O2 and H2O vapor.

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4 Self-cleaning Effect Using Microwaves

Nanoparticles have poor affinity to textile surface attributed to poor washing fastness. Microwave technology is developed to attach nanoparticles onto textile surface. The functionality of the nanoparticles is enhanced towards better self-cleaning functionality by attaching some chemicals those can repeal water, oil or bacteria. These duel nanoparticles-functional chemicals create protective layer over textile surface and kill bacteria, repel fluid and dirt. This technology is developed by scientists working in U.S. Air Force, and presently applied to prepare anti-microbial t-shirts and underwear which can be worn hygienically for weeks without washing as shown in Fig. 4 [19].

Fig. 4
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Self-cleaning clothing fibers using microwaves [19]

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5 Self-cleaning Effect Using Carbon Nanotubes

Mimicking of lotus leaf surface is a suitable method of preparing self-cleaning surface where nanosize rods are vertically arranged in regular pattern creating a rough micro-surface. Controlled assembling of carbon nanotubes over textile surface can serve the purpose and which is successfully achieved by researchers. By this method, carbon nanotubes are assembled on cotton surface and water contact angle greater than 150° is achieved. Such cotton fabrics are super hydrophobic in nature and carbon nanotube being electro-conductive the coated fabrics can exhibit various sensory functions [27]. The super hydrophobic functionality of micro-structured surface contained with carbon nanotubes is shown in Fig. 5 [28]. It is reported that the self-cleaning performance of the fabric does not deteriorate even after multiple use. In another study, it has been reported that fluorinated carbon nanotubes have better performance than that of ordinary carbon nanotubes. Fluorinated carbon nanotubes are vertically arranged on textile surface that exhibit better hydrophobicity and excellent self-cleaning performance [29].

Fig. 5
figure 5

Reprinted/adapted from Ref. [28], Copyright (2019) American Chemical Society

Superhydrophobic performance of micro-scale carbon nanotube pillars of 250 µm in width and 100 µm in height. a A10µL water droplet sitting on the surface of vertically aligned carbon nanotubes. b Top view of water droplet sitting on the micro-scale carbon nanotube pillars. c Schematic diagram showing carbon nanotube pillars held at base by polymer adhesive and a water droplet sitting on top of pillars [28].

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6 Self-cleaning Effect Using Silver Nanoparticles

Silver (Ag) nanoparticles are loaded onto textile surface for multipurpose applications. Silver is an excellent antimicrobial agent. Being having higher specific area its nanoparticles is a highly active material. Coating textile materials with silver nanoparticles brings about self-cleaning effect. The silver particles destroy various organic compounds such as dirt, contaminants as well as micro-organisms resulting minimal washing of cloths. The Ag nanoparticles exhibit water repellency effect by creating nano-whiskers over textile surface which are made of hydrocarbons and have about 1/1000th of the size of a typical cotton fiber. The Ag nanoparticles create a fuzz effect on the textile fibre surface without deteriorating the tensile properties of the fibre as shown in Fig. 6. Ag nanoparticles are also used in conjunction with TiO2 particles to coat over textile surface either in colloidal form or in particular form for improved functionality. The fixation of the particles on textile surface can be improved by high temperature curing. The high temperature curing treatment on cotton or polyester fibres produces activated surface induced by oxygen containing diverse polar groups. These polar groups increase the synergy of Ag blending with TiO2 on textile surface. High frequency plasma treatment in presence of oxygen and vacuum UV lead plasma treatments are also tried for increasing adhesion of Ag and TiO2 on textile surface [30].

Fig. 6
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Silver nano-particles used for self-cleaning textile

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7 Self-cleaning Effect by Using Colloidal Metal Oxide

A colloidal solution of suitable metal oxide particles is to be prepared and textile fabric is to be dipped into it followed by a through heat treatment process to create certain roughness on fibre surface in nanometer scale. By this treatment fabric become water repellent with water contact angle above 150°. Synthetic textiles can be coated with TiO2 by this colloid suspension method [31]. These TiO2 coated fabrics are able to remove the stain of tea, coffee, wine etc. under visible light with time. The durability of the TiO2 coating on textile surface achieved by this process is found to be satisfactory for multiple use. In another study, a blended colloidal solution is prepared by mixing TiO2 powder in titanium isopropoxide (TTIP) colloid and wool/polyamide, polyester fabrics are processed through this colloid. The treated textiles are able to discolor wine and coffee stains under solar radiation [31]. The colloidal coating is found to be stable and excellent stain removal potential and it is also can perform under neon light. TiO2–SiO2 sol-gel preparation is proposed to be as a transparent photoactive coating that can apply to textiles at low temperature without damaging textile surface. This preparation is reported to be better photo-catalytic agent than that of TiO2 alone [31].

8 Self-cleaning Effect by Using N Halamine

Chlorine is a well known disinfectant used for killing bacteria. The chlorine atom present is N-halamines are successfully explored for biocides applications. N-halamines are heterocyclic organic compounds containing at least one covalent bond between nitrogen atom and a halogen atom (N–X). In case of stable N-halamines the halogen is chlorine (N–Cl) in most of the cases. The stability of N–Cl bond depends on the chlorination reaction by which the bond is formed. The chlorination of amine, amide and imide groups are generally occurs in dilute hypochlorite solution. N-halamines are biocides which can kill a broad range of micro-organisms like bacteria, fungi, viruses etc. This anti-microbial effect is attributed to the capability of electrophilic substitution of chloride ion (–Cl) with hydrogen ion (–H) situated in N–Cl. This substitution reaction occurs in presence of water (H2O) resulting to the transfer of Cl+ ions that can bind to acceptor regions on microorganisms. As a result the enzymatic and metabolic processes of micro-organisms are hindered and they are dismissed. Once the N-halamines perform a reaction to kill bacteria, N–Cl bonds are converted to N–H bond which are inactive and does not have antimicrobial properties. Therefore, regeneration of the same is required by treatment with dilute hypochlorite solution. N-Halamines can be applied to broad range of textile substrates including cellulose, polyamide, and polyester to make them anti-microbial [32]. Though N-halamines contain chlorine it is not toxic since toxic chlorine gas is not generated during the process. The N-halamine-treated textiles can kill microorganisms almost instantly on contact, and therefore, they are found to be best suited for hygiene and medical applications like uniforms, bedding, towels, wipes etc.

9 Applications of Self-cleaning Textiles

Self-cleaning textiles and garments retain their original texture and feel after the chemical treatment. Textiles which can keep themselves clean can save a lot of water, detergent and energy by avoiding frequent washing. Using above mentioned technologies self-cleaning fabrics can prevent dirt, oil, and also act as a disinfectant. Moreover, the self-cleaning effect keeps the textiles long lasting and fresh looking than ordinary fabrics. Anti-dirt, anti-bacterial and self-cleaning clothes can be used in medical, sports, defense, and home textiles widely. Few commercial products are available in global market with this self-cleaning quality. One example is Mincor® TX TT which can be used for tailoring of outdoor textiles like tents, sunshades, flags umbrellas and sails. NanoTex® is another product suitable for apparels like men’s dress materials aprons, gloves, shirts, etc. Nanosphere® is mainly used for preparation of men’s shirts with self-cleaning effect. Nano-whisker surface is created on textile surface and water droplets along with dirt particles rest only on the peaks of the whisker, and as a result there is lower contact area with textile surface as shown in Figs. 7, 8 and 9. Due to lower contact area, surface adhesion is reduced significantly. Water droplet thus rolls out off the textile materials and dirt particles either repelled or can simply be rinsed off automatically. Such effect is called self-cleaning when textiles require no or very less washing. Washing conditions even will be very gentle at low temperature with minimum requirement of soap or detergent. Such products also possess excellent durability in terms of abrasion resistance and washing fastness. The aesthetic appeal, hand and breathability are not affected even after numerous washing cycles. Therefore, NanoSphere® is marketed as an ideal product for use outdoor apparel, sportswear, men’s and women’s wear, work wear, shoes cover and home furnishings [2].

Fig. 7
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The NanoSphere® surface

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Fig. 8
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High level of water resistance

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Fig. 9
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Durable protective function

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10 Limitations of Self-cleaning Fabric

Self-cleaning textiles are becoming practical and economical in the era of advancement of nanoscience and nanotechnology. Advancements in nanoscience and nanotechnology both practical and economical. Industries readily accept the technologies in commercial production of self-cleaning textiles which made the products readily available in market and this could really obsolete the washing machines, laundry detergents. However, there are few shortcomings limiting the efficiency and performance of self-cleaning. Wash fastness of the coated textiles is one of the limitations. All the agents are not suitable for all textile substrates. Another factor is the irradiation time in sunlight. Sunlight as a sole source of energy of photocatalytic self-cleaning required to be sufficient in terms of intensity and duration. A tea-shirt having tea stain required to be exposed in sunlight for a whole day for the removal of the stain. Therefore, it is a time consuming task. Sometimes, it is not problematic for military persons who wear the cloth and stand outside sun for prolong time during his duty and clean their clothes. The intensity of light also plays a big role because the excitation of electrons in the valence band of TiO2 depends on it and unless the electrons hope to conduction band the cleaning process does not commence. The electrons in valance band must react with atmospheric oxygen which causes depletion of oxygen arising environmental concern.

11 Conclusion

There are various approaches reported in literature for preparation of self-cleaning textiles. Broadly they can be divided in two ways such as either by making surface super hydrophobic (coating with silicones, fluorocarbons etc.) making the surface repellent to water, oil soil etc. or by coating with some functional hydrophilic finishes by the route of nanotechnology. Nanotechnology has been found very promising in this regard because by application of fluorocarbons etc. on textile materials made them hazardous and cause skin irritation. Application of nanoparticles, carbon nanotubes, N-halamines etc. in various forms brings about the properties of self-cleaning textiles and researches are going on in this area for further improvement. At present, there are some limitations or demerits with existing self-cleaning textiles in practical and economical point of view. The finish cannot be applied to all kind of material and longer time is required for cleaning treatment in sufficient sunlight.