1. Introduction
Textiles are the most popular materials for the protection and comfort of humans of all ages [
1], particularly, due to health and safety issues in recent years [
2]. Therefore, diversification and performance optimization of textile materials is continuously in demand, for example, for functional textiles (heat and flame resistant clothing, fire-protective clothing, radiant heat protective clothing, protective clothing for coal miners, protective clothing for racing drivers, protective clothing for astronauts, protective clothing for armed forces, and antimicrobial protective clothing) [
3,
4,
5]. As a result, scientists have focused on fabricating novel applications including sensors and effective development of textile polymeric materials that may also have environmental impact [
6,
7,
8,
9].
Since textile materials are readily combustible, their thermal stability and flame retardant behavior have many complexities and are therefore a challenging topic [
10]. However, with advancements in technologies, researchers have developed new and innovative merchandises in textiles because these could be transformed into highly protective flame retardant [
11]. Flame retardant materials can react physically and chemically in the gas phase (halogens), liquid phase (phosphorus), and solid phase (borate) depending upon their nature and characteristics [
12]. Halogen derivatives and phosphorus compounds have a severe effect on the environment due to the generation of toxic corrosive gases, smoke, and formaldehyde [
13,
14]. However, boron and silicon are non-toxic and formaldehyde-free flame retardants and have thus drawn much attention in the field of textiles given their eco-friendly characteristics [
15].
Many techniques, including nanotechnology, have been applied to textile fabrics for flame retardant functionalization [
16,
17]. Nanoscale particles have been used to increase the effects in properties of textile fibers and fabrics through either assimilation or surface treatment on the surface during the finishing process [
18]. Similarly, graft polymerization, chemical modification, the layer by layer assembly method (LLAM), physical vapor deposition (PVD), chemical vapor deposition (CVD), laser vaporization, electroplating or electroless plating, plasma deposition, pad-dry-cure and sol-gel processes have also been used for textile modifications [
19,
20,
21,
22]. The sol-gel process, introduced by Textor, has been considered the most effective and simplest surface modification for textile fabrics and is coated with a high degree of homogenous nanoparticles [
23]. This approach has demonstrated exceptional potential for eco-friendly association by reducing harmful effects [
15].
The sol-gel process has been extensively studied in recent years for flame-retardants [
24,
25,
26,
27]. Synthesis is based on two-step reactions, i.e., hydrolysis (converted to unstable hydroxides) and condensation (formation of covalent bonds) of a metal-organic precursor such as an alkoxide, tetraethoxysilane (TEOS), tetramethoxysilane (TMOS), and titanium tetraisopropoxide (TTIPO) [
28,
29]. Alongi et al. examined the synergistic influence of silica sol doping with aluminum phosphinate [
30]. NeclaYaman et al. used silica and phosphoric acid with PAN fibers via a sol-gel process to produce flame retardant fabric [
31]. Qianghua Zhang et al. applied the boron-doped silica to wool fabric through the sol-gel process for fire retardant treatment [
32]. Zhiang-hua Zhang et al. fabricated silk fabric via boron hybrid silica through the sol-gel method using TEOS and boric acid (H
3BO
3) [
33]. They achieved fire retardancy; however, the tensile strength and handle-ability of the treated samples were reduced. In the review of literature data, a lot of studies have also described layer-by-layer techniques [
34,
35,
36], green synthesis methods, and natural approaches [
9] for thermal stability and FR performances; however, due to the involvement of high cost of technology and loss of deposited minerals during repeated washings, the FRs predicament has yet to be resolved [
37]. Recently, organosilicon derivatives (silanes and polysiloxanes) have been applied to cotton fabrics for high FR efficiencies and protection coatings [
38]. They exhibit high reactivity and cross-linking for siloxane functionalization to enhance high washing durability and thermal stability [
39]. Thus, in this respect, these properties are attributed to the most suitable, easy, and promising fixation of FR elements into the molecular structure of treated fabrics [
40].
The goals of this investigation are to examine the synergistic effects of silica (SiO
2) and zinc oxide (ZnO) on the flame retardant properties of cotton and polyester-cotton (PC) fabrics through the sol-gel process, since inorganic metallic ions (silica, zinc oxide, alumina, and zirconia) have more stability than organic metals that also incorporate readily with cellulosic fibers and their blends (like PC) [
41]. SiO
2 has the highest water content and therefore reduces burning kinetics and smoke production [
42], while ZnO (a photocatalyst) promotes the formation of a char layer to enhance the flame retardant action [
43]. Many attributes, such as UV resistance, antibacterial activity, and outstanding hydrophobicity, have been investigated; however, fire-retardant properties through silica and zinc oxide nanoparticles are still very limited. To our knowledge, there have been no investigations into the synergistic effect of silica and zinc oxide coating via sol-gel technique on fire-retardant properties. Therefore, in this research, the flame retardancy on COT and PC fabrics treated with different concentrations of silica and zinc nanoparticles through the sol-gel finishing technique (acts as barrier to heat and oxygen transfer through substrate) [
34] have been investigated and evaluated for thermal stability.