1. Introduction
Textiles and clothing are essential commodities for human beings. The global demand for textiles is rising as a result of population growth. The apparel manufacturing business has the 6th position in terms of market share on a global scale [
1]. Literature also indicates a growing number of research studies on textile technologies [
2,
3,
4,
5,
6]. The and clothing industry encompass a range of interconnected operations, including yarn manufacturing, fabric manufacturing (weaving and knitting), textile processing industry (dyeing and finishing) and garment manufacturing industry [
7]. Textile and garment sector significantly contribute to environmental pollution [
8,
9]
The availability of clean water is necessary for the sustenance of human existence. The scarcity of adequate clean water for drinking, hygiene, and industrial purposes poses the most significant challenge of the 21st century. The rapid increase in global population and industrial development has exerted significant pressure on our natural resources and placed our ecological system in a precarious state [
10,
11].
The textile industry is a significant consumer of water, posing a major threat to the planet’s water supplies [
12]. Researchers are deeply concerned about the over utilization of potable water by the textile wet processing sector, as well as the release of wastewater and toxic substances into the environment [
13,
14]. The bleaching, printing and finishing processes of textile wet processing industry are the primary culprits for using water as the primary medium for applying dyes and chemicals to textile substrates [
9,
15]. According to literature, it has been observed that dyeing one kilogram of textile substrate requires varying amounts of water. The outcome is dependent upon the specific characteristics of the fibres, the chemical composition of the dyes, and the equipment employed during the dyeing process. Traditional dyeing methods use 100-180 litres of water to colour one kilogram of fibres [
16,
17,
18]
In conventional dyeing and finishing industries, almost 150 cubic metres of water were utilised per metric tonne textile processing. The textile wet processing industry is responsible for the discharge of over 80 percent of industrial wastewater [
18,
19,
20]. Vandevivere et al., [
21] mentioned that after agriculture industry, textile sector is the second worst contaminator of water resources. Currently, there are around 3,600 distinct dyes and 8,000 diverse chemicals used in various processes of bleaching, dyeing, printing and finishing industries. The majority of these compounds provide a potential risk to human health, either directly or indirectly. Furthermore, they pose a significant threat to marine organisms and can potentially contaminate both soil and water. It was determined by Kan et al., that a textile mill which has the capability to process nearly 8000 kg of fabric per day utilizes 1.6 million litters of water on daily basis. On average, the production of finished cloth to cover a sofa requires around 500 gallons of water [
18,
22].
Pang and Abdullah [
23] reported that approximately 280,000 metric tonnes of textile dyes were discharged annually as industrial effluent waste. These waste substances pose a significant threat to marine life. These effluents contain suspended particulates that might obstruct the gills of fish, leading to a decrease in their growth rate. Coloured discharged waste can reduce the penetration of light, hence decreasing the ability of many algae species to produce food and oxygen [
24]. It was noted that synthetic dyes caused toxicity in environment. Furthermore, they are accountable for a multitude of allergic responses in humans. Dyes have a high level of visibility and the ability to stain a significant amount of water, even when present in little quantities. Therefore, in order to reduce and purify these effluents and minimise the amount of water released into the ecological system, it is necessary to establish new laws and regulations for the textile sector [
18,
25,
26].
According to Hasanbeigi and Price, approximately 25 percent of thermal energy utilized in dyeing plants is exhausted by wasted water. The extensive use of water in dyeing and finishing processes is directly correlated with increased energy consumption, as a significant amount of energy is needed to heat the large volume of water to the requisite dying temperature [
18,
27].
The colour of the fabric play a crucial part in the consumer’s decision to purchase it. Nevertheless, this colour is linked to some costs. It is crucial to revamp the strategies for dyeing textile substrates. The policymakers are focusing their efforts on improving water resources to meet future demands. To achieve this objective, several strategies such as the construction of dams, water reservoirs, and infrastructure are implemented. If the water requirements are not met, it will ultimately result in water scarcity, economic downturn, and agricultural decline. As a result of this factor, a significant number of individuals will be rendered unemployed [
18].
Water utility can be categorised into industrial, agricultural and urban applications. In past the main focus was on development of newly discovered reserves and structure as an alternate to preservation of already existing water reservoirs. However, starting in1950s, the focus switched towards on conservation of water sources The water resources policy commission of the American president published a water policy for the American people in the mid-20th century. The report highlighted that the negligent behaviour of individuals towards wastages of water resources is unacceptable. It is imperative for every region in the country to effectively oversee and conserve their water supplies [
28].
Starting in the 1980s, there was a notable increase in awareness about the harmful effects of textile industry. Extensive research was conducted on water resource conservation in the textile industry. Currently, there are strict laws and regulations on environmental protection that pose significant hurdles for the wet processing textile business [
18,
29,
30,
31,
32]. Based on the literature, it was concluded that technology of foam was established in 1906. A scientist from Switzerland utilised pressurised air to transform liquid soap into foam [
33,
34]. This foam was used for the process of degumming silk [
33,
34].
Foam technology is employed to apply various chemicals, such as dyeing and finishing agents, to textile materials by replacing water with foam. It is a low supplement technology in which wet pick up is being lower than that of standard pad dyeing technology. As a result, a smaller quantity of water and energy is needed. The drying time is reduced while production is increased, resulting in less wastage. Foam dyeing method can be used in sizing, printing, finishing, and dyeing processes [
35] since it offers economic and environmental advantages [
36].
Foam was used in synchronised scouring and dyeing of textile substrates in 1930s and this innovative technique was patented in United states of America. In 1979, the textile industry in the United States adopted foam technology to replace water in the processes of textile finishing and dyeing. Dawson highlighted that foam dyeing has the potential for energy and material reductions, as well as increased production and elimination of the washing step [
37]. This research primarily aims to evaluate various procedures and approaches for foam dyeing of different substrates and compositions. It also examines the advantages of foam dyeing and compares it with conventional methods. Foam can be differentiated by gas diffusion in liquid phase of another substance [
38]. The gaseous state often refers to the presence of air, where gaseous bubbles are formed and distributed throughout a liquid. These bubbles are separated from one other by the creation of thin films [
38].
1.1. Foam Dyeing Mechanism
Foam dyeing is a controlled process where a dye solution is combined with a certain amount of foam and applied to a textile substrate using various application methods. During the process of foam dyeing, there is constant formation of foam followed by its crumbling due to breakage of bubbles and the dye liquid is transferred to the textile substrate. The fixing and washing technique of this method is carried out in a manner that is similar to the conventional continuous process. The creation and drainage of foam played a crucial role in facilitating the effective dyeing and fixing of dye process [
39]. Typically, it is necessary to create foam with specific attributes such as consistent bubble size, uniformity, fine quality, and even distribution of foam liquid on and around a textile substrate using appropriate application methods. Furthermore, it is necessary that foam must crumble instantaneously to diffuse dye liquor into textile substrate to assist dye fixation. The dyeing of foam can be carried out either continuously or in batches [
40].
1.2. Types of Foam
Foam can exist in either a gaseous state (formed from a synthesis of gas and liquid) or a solid state (formed from a synthesis of gas and solid). There are two forms of foams, namely condensation foams and dispersion foams[
38]. Condensation foams can be synthesised in within liquid interior elements by a chemical reaction known as chemical vapor deposition method or a physical transformation known as physical vapor deposition methodology. Both these physical and chemical changes are initiated by variation in pressure or temperature [
38]. However, dispersion foam is generated when gas entered and blended from outer source into liquid state. This is the most prevalent type of foam used in the treatment of textile materials. The surfactant acts as a foam-forming agent that aids in conversion of gaseous air into a liquid water solution. Thus, its application to textile fabrics was feasible in liquid state [
38].
1.3. Properties of Foam
Foam can be classified based on its composition, including polyhedron foam, non-spherical foam (honeycomb foam) and spherical foam. When considering applications of foam, it is important to take into account many features such as stability of foam, viscosity of foam, wetting power of foam, size of bubble and dissemination are greatly influenced by foam synthesis methodology and liquor constitution. There are various methods for evaluation of foam properties. A multitude of scientists assessed the characteristics of foam [
40,
41,
42,
43,
44].
1.4. Steps of Foam Processing
Foam processing involves several distinct steps, which are outlined below:
Figure 1.
Schematic diagram of foam processing steps [
40].
Figure 1.
Schematic diagram of foam processing steps [
40].
1.4.1. Liquor Preparation
To synthesise a formula, a careful selection of components is necessary, including a high concentration of dyes, auxiliaries and other substances such as foaming agent, foam stabilizer, and viscosity modifier. These quantities are more than what is often used in standard reagent synthesis methods. The development of foam will take into account the relationship between dye formulation and foaming agent, the stability of both un-foamed and foamed compositions, and the potential impact of the foaming agent on the textile substrate and foam cell structure [
36,
45].
1.4.2. Development of Foam
Foam is created by introducing air into liquid water, such as dye liquor, combined with a foaming agent and surfactant, which help to form bubbles. When the surfactant adheres to bubbles, it forms a coating that stabilises the bubbles [
40]. There are two techniques, namely the air blown approach and the stirring method. In the air blown process, pressurised air is put into the beverage. However, in the process of stirring, the liquid is agitated for a certain duration in order to achieve a uniform distribution. Primarily, a combination of both approaches can be employed to achieve rapid foam development. The foam that was produced needed to be impregnated onto textile materials with a specific level of confidence and should be easily crumpled after being impregnated onto the textile substrate [
40]. In order to dye continuous strands of yarn, a heated solution was transformed into foam for the purpose of dyeing these threads. The machinery utilised for the production of this particular foam will result in a uniform dyeing effect on yarn [
46].
1.4.3. Foam Application Methods in Textiles
There are two methods for applying foam on surface of textile substrate: direct system and indirect system. In direct system, foam application can be either pressurised or non-pressurized. In pressurised system, foam is subjected to pressure inside a container and then applied directly to a textile substrate by passing it through a rotating screen or slot applier. During this procedure, textile material is subjected to compression against a roller located at the back. In the pressurised system methodology, the foam tank does not require any pressure. Nevertheless, the application of foam to a textile substrate necessitates a certain amount of pressure. This methodology involves the utilisation of a carrying agent, such as drum or covering layer to transport foam onto a textile substrate. Applying foam to a textile substrate necessitates the interaction between the transporting agent and textile material [
46].
1.4.4. Crumpling of Foam
The foam should be rinsed off promptly after application to a textile substrate. By applying and crumpling the foam, the dye liquor sprayed onto textile material can be used as a means of transporting and dispersing dye before the dye is fixed onto the textile substrate.
Foam can be eliminated by either vacuum evacuation systems or squeezing systems, depending on the circumstances. The primary technologies available for foam removal are as follows [
40]:
- i.
Utilization of temperature: Enhancement in drying temperature might cause decline in viscosity, increment in molecular movements in bubble that may cause loss in foaming attributes due to which formation of foam can be prevented.
- ii.
Addition of defoaming agent: these agents are useful in disappearance of foam to avoid foam in bubbles.
- iii.
Application of pressure: The bursting of bubbles under extreme pressure
In addition, hydrophobic oil can also function as a defoaming agent, aiding in the removal of foam when applied to a textile substrate [
47].
1.4.5. Drying and Fixing
To dry and fix foam quickly, a high speed and low temperature are needed since the foam requires less water after being applied to the textile substrate. However, precise management of lower temperature is necessary to provide the desired amount of remaining moisture, in order to avoid issues related to the transfer of dyes. Pre-existing equipment such as stenters or curing compartments can be utilized for the purposes of drying and fixing [
40].
3. Benefits of Foam Dyeing
Previous literature confirms that foam dyeing method offers significant cost savings in terms of drying energy usage, thanks to reduced wet pickup. An 80% reduction in the cost of drying energy was reported [
81]. An reported decrease in drying energy resulted in a lower drying temperature, while productivity increased. Furthermore, the consumption of water was reduced by a range of 30 percent to 90 percent. In addition, a study indicated a 60 percent reduction in pick up as compared to pad dyeing [
57]. Consequently, the quantity of discharged wastewater was decreased, leading to a reduction of 50 to 60 percent in the treatment of the liquid waste.
When comparing foam dyeing to traditional dyeing of cotton, foam dyeing exhibited superior physical qualities and enhanced colour strength. Moreover, there as significant decline in consumption of water and chemicals [
76]. Also, foam dyeing exhibited superior sustainability, durability, and productivity in comparison to conventional pad dyeing methods. Thus, the foam dyeing is the most practical and effective answer for the issues faced by the textile wet processing industry. Foam dyeing technology allows dyes to penetrate rapidly, in contrast to conventional dyeing procedures. The impact of mechanical activity is reduced due to the protection of the fibre structure, allowing for uniform dispersion of dyes across the substrate surface. As a consequence, there will be an enhancement in the management of textile substrate [
39].
Kumar et al. [
82] highlighted the advantages of foam dyeing, specifically the effective and long-lasting bonding of dyes to the substrate as a result of rapid wetting. As a result, the dispersion of dyes is enhanced, leading to an increase in their stability. Thus, a significant quantity of energy is conserved, and instances of waste are reduced. In addition, cost of equipment of foam dyeing is lower than that of conventional pad dyeing. Moreover, chemicals utilized in traditional treatment of textile substrate can be employed in foam media if appropriate foam is generated [
36].