Metal–Organic Frameworks as Promising Textile Flame Retardants: Importance and Application Methods

Abstract

We present a review of current research on promising flame retardants using specific methods of applying metal–organic frameworks (MOFs) to the highly flammable fibrous surface of cotton fabric. In this review, we initially address the reasons why the search for new flame retardants has becomes critically important in textile finishing, the area responsible for adhering new functionalities to substrates. This addition of characteristics is closely linked to the nature of the fibers, so the reason for the improvement in cotton fabric in relation to flame retardancy is discussed. Furthermore, the development of highly porous nanomaterials that can generate composites with specific functions is described, as well as their application and methods of integration into textile surfaces. Finally, the main candidates for flame retardant functionality in cellulosic materials are identified. It is also hoped that this work will facilitate researchers to develop and formulate new methods of applying nanomaterials to textile substrates, with a view to becoming a reference for new research into the development of adhesion of emerging materials to traditional materials.

1. Introduction

The textile chain is responsible for transforming fibers and filaments into structures such as woven, knitted, and nonwoven fabrics. Known for its importance in satisfying the human need for clothing, the textile industry is capable of innovating processes and products to meet specific demands, ranging from providing new tactile and visual characteristics to developing intelligent fabrics that improve quality of life. Textile finishing, a sector at the end of the textile chain, is responsible for this function of ennobling the fabric, i.e., improving existing characteristics or providing new functions.

In short, finishing processes tend to satisfy the interests of consumers who are looking for ways to combine practicality, protection, aesthetics, and other factors in a single product. The textile chain tends to develop innovative products with a variety of functions in proportion to market demand. With the advent of nanotechnology, various transformations and changes in human daily life have occurred, and in the same way, textile chemical finishes have followed this evolution. In recent years, increased research on nanoparticles has led to the production of textiles with greater efficiency and added value [1].

Water and oil repellency, stain repellency, self-cleaning, wrinkle resistance, flame retardancy, UV protection, antistatic properties, and better dyeability are some of the functionalities obtained using nanomaterials and nanofibers [2]. The functionalization of these finishes can be achieved by coating or impregnation processes using emulsions, microemulsions, nanoparticles, nanocomposites, and other products.

Like any anthropogenic activity, textile finishing has an impact on the environment due to the high consumption of water, the generation of physical and chemical waste, and the emission of gases of different classes, most of which are harmful. Therefore, the industry is trying to overcome the challenges of sustainability in the textile process when developing products. The constant search for solutions to these challenges leads to the development of new processes and products that combine innovation with sustainable cooperation.

Textile materials with exceptional functionality are defined as functional textiles [2]. One of the functionalizations obtained by finishing is flame retardancy, which is a relevant class of finishes due to its importance in the composition of protective articles, such as military uniforms and the covering of fire-prone areas.

These flame retardant finishes can be applied both in the manufacture of the yarn or fiber and in the textile structure, both of which must inhibit or suppress the combustion process when exposed to fire or high temperatures.

These compounds applied to textiles, called flame retardants (FRs), can slow the spread of flames through physical or chemical action [3]. In turn, there are already products on the market capable of reducing flammability and delaying fire. Although widely used and highly effective, some molecular compounds used in the formulation of halogen-based flame retardants, such as pentabromodiphenyl ether, decabromodiphenyl ether (or oxide), and polychlorinated biphenyls, have been shown to be bioaccumulative and/or environmentally toxic to animals and humans [4].

Due to the prohibition and limitation of the use of halogenated flame retardants (FRs), the market has been encouraged to develop alternatives with greater stability and lower volatility. To meet the environmental safety requirements of FRs, the development of new halogen-free FRs has become inevitable. In recent years, metal–organic frameworks (MOFs) have emerged as a promising class of flame retardants, as they can be synthesized with different metal ions and organic ligands, allowing for the modulation of their properties for specific applications. This chemical flexibility enables the design of highly customized flame retardants, which are optimized for different types of materials and operating conditions.

Unique characteristics such as the high porosity of MOFs allow combustible gases to be adsorbed in the pores during combustion, preventing their release. Additionally, transition metal elements have excellent catalytic oxidation properties, which contribute to the formation of a stable flame retardant char layer [5]. The organic structures of MOFs also facilitate the transport of fire-resistant groups, such as phosphorus, nitrogen, and aromatic derivatives, through appropriate modifications, offering more design and functionalization possibilities than conventional organic–inorganic flame retardants due to their easily adjustable chemical composition and micro–nano morphology [6]. It is worth noting that the hybrid nature of MOFs (organic–inorganic) provides good compatibility with organic polymers and textile surfaces for use as coatings [7].

MOF-based flame retardants present significant advantages over conventional alternatives, including high flame retardancy efficiency, smoke suppression, carbonaceous residue formation, chemical versatility, and the reduction in toxic combustion products. Furthermore, the adhesion of these materials to the textile surface has already been reported and is generally achieved through continuous processes such as dip–pad–dry, layer-by-layer, and even direct synthesis (in situ). The study of the development of this functionality acquired through nanotechnology contributes to the creation of new products capable of retarding fire on cellulosic textile surfaces, favoring a reduction in the use of retardant compounds that have an impact on health and the environment.

2. Textile Finishes

Textile processing is one of the oldest and most technologically complex industries. It is an extensive process aimed at transforming fibers into yarns and fabrics. As the last stage of the textile process, finishing is considered essential for the development of textile originality, aesthetic appreciation, and profitable recognition [8].

Generally, finishing can be divided into physical, chemical, or biological [9,10]. The choice of technique depends mainly on the structure, the type of fiber used (natural or chemical), and the objective to be achieved [11].

In particular, chemical finishes can be applied at different stages of the textile chain in the production of a substrate; i.e., they can be incorporated into the spinning process (polymer solution of synthetic and human-made fibers), and they can also be applied during the tertiary processing stage, after the dyeing stage, and before the garment is made [8]. In this way, this class of finishes is able to impart new functionalities to textile materials and make them suitable for special applications [12]. Chemical substances are added to the substrates to achieve the desired function [11].

Functional finishes provide textiles with improved comfort and functional properties through chemical treatment. The study of textile substrates with nanoscale chemical finishes is extensive and is one of the areas to be explored to ensure the functionalization of textiles, both to improve existing methods and to create new techniques [13]. Nanomaterials applied to textiles are an innovative option for producing multifunctional materials [14].

Antimicrobial, anti-UV clothing, catalytic substrates with air purification function, antistatic capacity, flame retardant, moisturizing, insect repellent, antiperspirant capacity, water or oil repellency, and the release of drugs and fragrances are some of the functionalities achieved through nanotechnology.

Metal–organic frameworks, which belong to the class of chemical finishes and nanomaterials, are nanoparticles that can be applied to textiles with the aim of superficially modifying the material, allowing the textile to have various functionalities. In addition, their wide applicability has led to the development of research in catalysis [15], electrodes for water splitting [16], gas or liquid separation processes [17], or even drug carriers [18].

Regardless of the technique used in textile processing, especially in chemical finishing, they are responsible for discharging wastewater containing various excessive chemical products, which leads to the accumulation of innumerable contaminants in the natural environment, causing harmful effects and instability in the ecosystem [19,20]. Thus, they pose serious threats to human health and the ecosystem due to their toxic, mutagenic, carcinogenic, or teratogenic effects [21,22,23,24].

2.1. Flame-Retardant Finishes

Flame-retardant finishes, which belong to the class of chemical textile finishes, generally act by limiting the spread of fire and are referred to as FRs. FRs are chemical products added to materials to prevent combustion and/or slow the spread of fire after ignition [25]. This functionality for many fire-resistant textile applications [26,27]. Flame-retardant additives are generally composed of organic and inorganic compounds of phosphorus (P), nitrogen (N), sulfur (S), halogens (Br, Cl, F), silicon (Si), aluminum (Al), magnesium (Mg), antimony (Sb), tin (Sn), boron (B), zinc (Zn), carbon (C; graphite), zirconium (Zr), titanium (Ti), and calcium (Ca).

There are more than 175 chemical substances classified as flame retardants, and they are divided into two groups: halogenated (brominated and chlorinated) and non-halogenated (phosphorus, nitrogen, and melamine derivatives) [28]. Examples of retardants are Proban^® CC (tetrakis (hydroxymethyl) phosphonium salt) developed by the French company Rhodia Solvay Group; Pyrovatex^® CP (N-hydroxymethyl-3-(dimethoxyphosphinacyl)propionamide) of the Chinese brand Drotex; Goldflam SU, belonging to the Golden Technology group in Brazil.

The flame-retardant treatment of textiles can be carried out using various techniques, such as back-coating [29] (when the back of the fabric is coated so that the outside remains unchanged), spraying, dip coating and filling. In recent years, innovative application techniques have been developed, such as sol-gel [30], layering [31], and plasma grafting [32].

The effect of the flame retardant is achieved by interfering with combustion at some stage of the combustion process and can act during heating, decomposition, ignition, combustion, or flame propagation [33]. There are five mechanisms by which these additives act: by gaseous dilution, thermal extinction, protective coating, physical dilution, chemical interaction, or a combination of these mechanisms [34].

Gaseous dilution involves the use of additives capable of producing large volumes of non-combustible gases during decomposition. By diluting the oxygen and reducing the amount of combustible material (polymer), flammability is reduced. This mechanism occurs in metal hydroxides, metal salts, and some nitrogen compounds [34].

Thermal quenching occurs through endothermic processes of the flame retardant that cool the substrate to temperatures below those required for pyrolysis to occur. Metal hydroxides, metal salts, and nitrogen compounds act in this manner [34].

Protective coatings limit the amount of combustible substrate available to the flame front by forming an insulating layer that reduces heat transfer from the flame. Phosphorus (P), melamine, and nitrogen compounds are examples of substances that use this mechanism [34].

Physical dilution acts by reducing the amount of combustible material and consequently increasing the flash point of the material [34].

Finally, flame retardants that interact chemically in the gas or condensed phase, such as halogens and some phosphorus compounds, act by dissociating into radicals that compete with the propagation chain stage in the combustion process [34]. In the gaseous phase, the radicals in the additives interact with the free radicals released by the decomposition, neutralizing them and thus drastically reducing the energy release and cooling the system [33,35]. In the condensed phase, the additives act to form a carbonaceous layer. This carbonaceous layer inhibits the diffusion of pyrolysis gases into the gas phase and blocks the feedback of heat flow into the substrate by forming a thermal insulation barrier [33,35,36,37,38].

Although halogen additives are efficient, their use is limited due to their environmental impact. Leila Khan, Leonardo Martin, and Lukasz Pulaski. Leila Khan, Leonardo Martin and Lukasz Pulaski [39] conducted studies to investigate the effects of the major groups of FRs, including halogenated and organophosphate FRs, on animals and humans in vitro and/or in vivo and reported damage to cell membrane integrity, implying DNA damage. As a result, new formulations are being developed and tested, such as phosphorus-based FRs. These substances are very versatile, as they have a condensed and/or vapor-phase flame-retardant effect [40] and produce less smoke and toxic gases [41].

Boron compounds are also an environmentally friendly option for flame retardancy. Depending on the compound and the associated material, they can act in both condensed and gaseous phases, decomposing endothermically to dissipate heat, releasing water, and forming protective layers to reduce the concentration of available fuel [42]. However, these compounds tend to undergo hydrolysis, which leads to low durability, and they are generally used in synergy with other flame retardants to extend their range of effectiveness [43]. Biologically based options are also being studied, such as in the work of Wang et al. [44], which lists chitosan, lignin, phytic acid, polydopamine, tannic acid, and b-cyclodextrin as options for new flame retardants. There are also studies of graphene [45], rare earths [46], organometallic structures [6], dialkylphosphonecarboxylic acid amine (DPCAA) [47], and others as potential retardants.

2.2. Environmental Impact of Flame-Retardant Compounds

Of the two groups, halogenated organic flame retardants (organochlorines and organobromines) are the most effective in inhibiting flame spread because they are able to neutralize free radicals produced during the combustion process [48]. For phosphorus-based flame retardants, it is impossible to describe a single mechanism [49]. However, one of the main mechanisms has been described by Van der Veen, I. and de Boer [4], who stated that the flame retardancy of phosphorus compounds occurs through the formation of a carbon layer generated by phosphoric acid, which protects the material from oxygen, thus preventing the formation of flammable gases.

Halogenated FRs have long been used because they are effective primarily in the gas phase and have been widely developed and marketed. However, they have been characterized as toxic and bioaccumulative [50]. They are therefore classified as persistent organic pollutants, as is the case with hexabromocyclododecane [51].

The substances released during the incineration of halogenated compounds are generally organohalogenates, especially organochlorines and organobromines, which are known to be highly toxic and carcinogenic [33].

When hazardous wastes are released into the air, water, or land, they can quickly disperse throughout the environment, contaminating more areas and causing greater health risks [52]. Thus, there is growing concern about the environmental impact of chemical agents released into the biosphere and the proper disposal of potentially toxic materials.

With increasing environmental responsibility, halogen-based flame retardants are gradually decreasing in the market, and halogen-free additives have become a popular research topic due to their advantages such as less smoke and safety due to non-toxicity. In view of this, conventional flame retardants have become the target of close attention due to their harmful aspects to health, opening space for the study and development of new solutions that offer efficiency and the desired performance.

3. Flammability of Textile Fibers

To classify polymers in terms of their flammability, their LOI (Limiting Oxygen Index) number must be considered. This indicates the minimum concentration of oxygen required for the material to start burning, i.e., to ignite. To evaluate this property, it is assumed that atmospheric air contains 21% oxygen, so polymers with an LOI > 21% are considered less flammable than those with an LOI < 21%. In addition, materials with an LOI > 25% are considered to be inherently flame retardant [53]. The point at which the material begins to decompose is called pyrolysis, and ignition is the temperature at which the flame starts.

Table 1. Flame behavior of fibers [54,55].

Fiber	Tc (°C)	Tp (°C)	LOI
Wool	600	245	25
Cotton	360–425	335–350	18
Viscose	420	350	18
Triacetate	540	305	18
Polyamide	450	431	21
Polyamide	530	403	21
Polyester	480	420–447	21
Polyacrylic	>250	290	18
Polypropylene	550	469	18
Nomex®	>500	410	28–30
Kevlar®	>550	>590	29

shows examples of fibers and their respective LOI and ignition and pyrolysis temperatures.

It should be noted that cotton has a flammability index (LOI). This is due to its chemical structure, which essentially consists of cellulose, a polysaccharide composed of repeating units of α- and β-glucose, and these molecular compounds are high-energy carbon sources that, when exposed to heat, generate oxidation and are capable of forming solid, liquid, and gaseous flammable products [56].

It can be said that cotton is associated with the earliest origins of clothing and the development of textile production [55]. This fiber is the purest form of cellulose found in nature and is classified as a plant derived from the Gossypiumgenus. According to Lewin [54], cotton fiber has a typical composition of about 95% cellulose, 1.3% protein, 0.9% pectins, 1.2% ash, 0.6% waxes, 0.3% total sugars, 0.8% organic acids, and 1.4% other substances.

In cotton fibers, the degree of polymerization and the length of the molecular chain vary according to the origin of the cellulose, since cellulose makes up a large part of the fiber composition. The ends of the chain are formed by a hydrated aldehyde group, which can be characterized as a reducing agent, and an alkoxy group [57]. The introduction of new chemical species into the chain is possible due to reactions in the hydroxyls, which allow the creation of chemical functions capable of modifying the properties of the fiber, including the incorporation of active materials such as organometallic structures [58], which are extremely important for textile finishing. The most important property of cotton is its permeability, i.e., its ability to absorb water and to be easily dyed and washed in aqueous media.

Despite its chemical simplicity, the diversity of origins and processing to which cellulosic materials are subjected results in a complex range of physical forms of cellulose. For example, cellulose can undergo changes in shape, size, porosity, degree of polymerization, area, molecular shape, crystallinity, and other characteristics. These characteristics are correlated with various parameters related to cotton fiber quality: fiber length, uniformity, fiber strength, whiteness, and elongation.

Like wood, starch, and sugar, cotton fiber is highly flammable. Fibers classified as cellulosic will burn continuously until their entire structure is completely consumed, even if the source of combustion is removed [56].

The two thermal degradation processes that can represent the burning of cotton fiber are combustion and pyrolysis. Under the influence of heat, the combustion of cellulose is the oxidation process that consumes the pyrolysis products and generates excess heat.

Studies of Zhu et al. [56] explain that combustion consists of the moment when cellulose is dehydrated and carbonized to produce water, carbon dioxide and solid residue, as well as the second moment when cellulose produces non-volatile liquid L-glucose by depolymerization; in this way, the cleavage of glucose continues to produce low-molecular-weight products, which are more flammable. If oxygen is present in the medium, the decomposed products of L-glucose will be oxidized, generating much more energy and heat to promote further cellulose cleavage.

At temperatures close to 120 °C, cotton fibers begin their surface degradation process, causing the yellowing of the textile material due to the degradation compounds produced and the formation of oxidized groups in the matrix. Above 150 °C, the degradation process accelerates, and the oxidation of the cellulose is proportional to the time of exposure to heat. Due to its flammability-promoting properties, the development of flame retardant finishes is feasible and has been the subject of several studies [59,60].

4. Metal–Organic Framework

By opening up opportunities for product development, the study of materials chemistry has become invaluable. Combined with nanotechnology, this can lead to innovation and the development of composites that add functionality to materials. Metal–organic materials, or metal–organic frameworks (MOFs), are defined as a coordination networks with organic ligands that have potentially empty cavities [61].

The nomenclatures given to metal–organic frameworks are usually abbreviations that refer to the place or name of the institution where the material was originally synthesized, or even the type of material or the type of structure that is formed after synthesis, and this abbreviation is followed by an integer, which assigns a chronological order of discovery. Examples are MOFs known as MIL-n (MIL—Matériaux de I’Institut Lavoisier), HKUST-n (HKUST—Hong-Kong University of Science and Technology) or UiO-n (UiO—Universitetet i Oslo) [62,63].

These compounds are formed by coordinating secondary building units (SBUs) with organic ligands. SBUs refer to metal nodes containing metal cations so that the nodes can be directly coordinated with organic ligands to form three-dimensional structures [64]. Over the last two decades, studies have been conducted on MOFs with different metals and ligands. With this feature of using different metal ions, the structures can form numerous geometric shapes. Figure 1 shows the common formation of metal–organic frameworks from SBUs and ligands.

These materials have properties that have been described as favorable, such as high porosity, large internal specific area, the presence of coordinately unsaturated sites, the ability to incorporate functionalities and active species, and the flexibility for different processes due to the combination of the metal and the organic networks [65]. Thermal stability, well-ordered structures, very low density, facile synthesis routes, and the ability to control pore size, shape, and functionality due to their structural and chemical diversity [66,67,68] are all relevant properties for various applications such as gas separation [69], purification [70], adsorbents [71], catalysts [15], sensors [72], and drug delivery [73].

Pioneering work on MOF synthesis began in the late 1990s, and conventional methods for MOF synthesis mostly result in crystalline powders with crystallite sizes ranging from nanometers to micrometers [74]. In addition, an increasing number of studies have led to the development of new synthesis routes that aim to reduce the use of reagents or even produce them from waste depolymerization, such as the work of Ko et al. [75], Dyosiba et al. [76], and Dutra et al. [77]. MOFs are mostly crystalline, have high porosity and are resistant to structural collapse after evacuation in the presence of adsorbate [78].

Classified as finishes, MOFs aim to modify the textile surface to achieve a specific assigned functionality. Thus, the incorporation of MOFs into the fabric can provide specific applications in textile articles due to their mechanical stability, chemical stability, selectivity, and other properties mentioned. MOFs such as UiO-66, ZIF-8, MOF-808, and HKUST-1 can impart specific properties that increase their attractiveness in technical textiles in the areas of gas and liquid waste treatment, gas storage, and even flame retardancy.

Table 2. Textile functionalizations from MOF.

Functionality	MOF	References
Insect repellency	Cu-BTC	[79]
UV protection	MIL-68(In)-NH2	[80]
	MIL-125(Ti)-NH2
Pesticide Removal	UiO-66	[81]
Photocatalytic effect	AI-PMOF	[82]
Antimicrobial	HKUST-1	[83]
Antibacterial	HKUST-1/CF	[84]
	CuBTC	[85,86]
Fuel purification	CuBTC	[87]
Antimicrobial	Cu(BDC)	[77]
Integrated radiative cooling and photocatalytic antibacterial properties.	ZIF-8	[88]
UV protection	UIO-66	[45]
Chemical protection	UIO-66	[89,90]
Adsorption of water contaminants	ZIF-8	[91]

shows some of the functionalities obtained by incorporating organometallic structures into the textile substrate.

4.1. Application of Metalorganic Materials in Textiles

With the advent of nanotechnology, various methods for functionalizing textiles have been explored, and metal–organic frameworks (MOFs) have great potential for development as they allow substrates to acquire new properties for use in various fields. Classified as chemical finishes, MOFs can be adhered to the polymeric mass of fibers or fixed to the final textile substrate. MA et al. [92] summarize recent MOF/fiber fabrication methods and applications. In addition to directly dipping the substrate into or dispersing it in MOF particles, the techniques for attaching the structures to the final textile substrate focus on continuous processes such as dip–pad–dry, direct synthesis (in situ growth), and layer-by-layer (LBL) assembly. In addition to these continuous processes, plasma, sol-gel, and spray technologies are gaining importance in the application MOFs to substrates. Chemical modifications can also be made to the textiles prior to contact with the organometallic structures, which can promote the formation of reactive groups such as carboxylates and hydroxyl groups, allowing the nucleation of MOFs in the fibers [92]. Figure 2 shows the main continuous processes used for MOF–textile adhesion.

The dip–pad–dry process is a common and suitable method for treating cellulosic fabrics, in which a solution is used to fill the finishing liquid [93]. The dip consists of immersing the fabric in the finishing liquid, the pad refers to the act of passing the fabric through cylinders with stable pressure and speed to help the textile absorb the immersed finishing liquid evenly. Drying is the process of drying the moisture contained in the fabric and retaining the amount of finishing liquid required by the process.

It is also possible to synthesize the MOF on the textile substrate so that during the process of forming the structures, the fabric is immersed in the reagents, thus achieving direct synthesis on the fabric through exhaustion. It is also possible to apply MOFs layer-by-layer, which is a technique of alternating solutions to make multilayer coatings, with nanoscale thickness control of the films generated on the surface of the material [94].

Table 3. Methodologies used to functionalize textiles.

Method	MOF	Composition	Functionality	References
Direct synthesis	Cu-BTC	Cotton, Linen and Silk	Insect repellent	[79]
Direct synthesis	MIL-68(In)-NH2 MIL-125(Ti)-NH2	Cotton and Silk	UV protection	[80]
Direct synthesis	HKUST-1	Wool	Antibacterial	[83]
Dip–pad–dry	ZIF-8	Polyester	Odor adsorption and antimicrobial finish	[74]
Dip–pad–dry	Uio-66	Cotton	Fire safety and UV protection	[45]
Direct synthesis	UiO-66, UiO-66-NH2, UiO-66-NDC, e UiO-67	Cotton	Arsenate adsorption	[91]
Dip–pad–dry	ZIF-8 e ZIF-67	Cotton	Adsorption of pollutants	[97]
Direct synthesis	Cu(BDC)	Polyester	Antibacterial	[77]
Dip–pad–dry	ZnO derivado de MOF	Cotton	Photocatalytic and antibacterial activities	[98]
Direct synthesis	Ln-MOF	Viscose	Self-cleaning	[99]
Direct synthesis	MOF-derivado Ni3S2@C	Cotton	Electrode potential	[100]
Direct synthesis	ZIF-8	Cotton	Absorption of acid dyes	[101]
Direct synthesis	MOF-199, MOF-74, MOF-5 e MIL-53	Cotton	Antimicrobial	[102]
Direct synthesis	MOF Zr4+	Cotton	Removal of toxic oxyanions	[7]

shows examples of methodologies used to functionalize textiles using MOFs. The LBL technique is a process that was developed in the 1990s and is produced by alternating deposition of the tissue in charged solutions, as well as electrostatic interactions, hydrogen bonds, covalent bonds, and donor–acceptor interactions that support this assembly [95,96].

It is also possible to attach the MOF to the substrate without using water/solvents as a transport medium. This is the case with embossing, which can contribute to the adhesion of organometallic materials with the help of resins incorporated into the embossing paste that crosslink with the textile. This ensures complete fixation by creating a kind of film under the surface. The table shows the methodology used in the work, including the MOF applied, the composition of the material, and the functionality achieved.

4.2. Metal–Organic Frameworks for Flame Retardants and Their Mechanism

Concern about the environmental impact of halogenated flame retardants has led to the development of new products to replace them, with a focus on environmental responsibility. In this context, MOF-based flame retardants have emerged as a new class of additives and have been the subject of several studies. Works by Pan, Y.T., Zhang, Z. and Yang, R [103]; Lyu, P. et al. [104]; and Nabipour, H., Wang, X., Song, L. and Hu, Y. [105] review the application of organometallic materials for flame-retardant polymers.

As mentioned above, flame retardancy is achieved by interfering with combustion at some stage of the process, and metal–organic frameworks comply with this principle. MOFs are capable of creating metal oxides on their surface during the thermal decomposition phase; this layer serves as a protective coating, preventing the associated material from continuing to combust. It is worth noting that the porous configuration of the MOFs is capable of efficiently capturing smoke nuclei and particles, thereby reducing the density of the smoke in the polymer matrix [106,107]. In addition to providing a physical barrier and capturing particles, these materials can perform the gas-dilution mechanism. When they combust, they produce non-flammable gases, which reduce the concentration of flammable gases by diluting them. Both mechanisms interfere with the amount of fuel available, whether in physical or gaseous form, thus reducing the release of heat [103,105,108,109].

The transition metals zirconium and zinc are examples of compounds capable of forming layer of carbon from oxides. Classified within the typical MOFs [110], ZIFs (ZIF for Zeolitic Imidazolate Framework) and UiOs (UiO for University of Oslo) are MOFs that feature zinc and zirconium nodes, respectively, and are known for their thermal, hydrothermal and chemical stability, being the target of several studies in the incorporation of flame retardant polymeric matrices. Figure 3 shows the schematic scanning electron microscopy (SEM) images of the structure and junctions of the metal nodes (SBUs) of the ZIF-8 and Uio-66 MOFs.

Cellulose is a biopolymer found in cotton fibers. Despite its properties such as absorbency, light weight, stability, and chemical simplicity, the use of cotton fiber is limited due to its high flammability. Given the high performance and effectiveness of MOFs in polymeric matrices, the potential benefits of incorporating these compounds into cotton fiber textile substrate are increasing.

4.2.1. UIO-66

The metal–organic framework UiO-66 was first developed at the University of Oslo, which is reflected in its name (UiO). This porous metal–organic network, based on zirconium (Zr), has the formula Zr₆O₄(OH)₄(BDC)₆. The inorganic component consists of a core unit Zr6O4(OH)4 linked by twelve carboxylates (-CO₂) from dicarboxylic acids, forming a face-centered cubic network [111,112,113,114]. The Zr₆O₄(OH)₄ nodes feature six zirconium ions (Zr⁴⁺) arranged with octahedral geometry, with four oxygen or hydroxyl atoms at the centers of each of the octahedral faces [115]. This structure provides UiO-66 with high chemical, thermal, and mechanical stability while supporting structural defects around the metal core r and presenting a high surface area [111].

Variation in the binder can result in different metallic structure, as demonstrated by MOF-808. Also based on Zr₆O₄(OH)₄(CO₂)₆(HCOO)₆ as SBU, MOF-808 has six BTC (trimesic acid) units acting as an organic binder to form a three-dimensional porous structure [116,117]. Although MOF-808 has zirconium clusters similar to those of UiO-66, this structure is connected by the BTC binder and has two different hole cages of approximately 18 and 14 [118].

UiO-66 has a 7.5 Å tetrahedral cage and a 12 Å octahedral cage, with a pore opening of 6 Å and a theoretical pore volume of 0.77 cm³ g⁻¹, and its surface area depends on the preparation method and the presence of defects, which can vary from 800 to 1200 m² g⁻¹ [112,115,119]. MOFs synthesized from transition metal elements represented by zirconium can catalyze the char formation reaction at high temperatures [117]. Upon decomposition, Zr-MOFs generally produce zirconia, which acts as a physical barrier on the substrate, as well as non-flammable gases that dilute the concentration of flammable gases, thus facilitating smoke suppression [105,120,121].

This MOF exhibits high hydrothermal and chemical stability as well as mechanical stability, and in addition to being stable at 500 °C and able to maintain its stable structure in water, DMF, benzene, or acetone, it has strong acid stability and some alkaline stability [114]. Therefore, as a robust material accommodates various types of synthesis, whether modulated or not, and different synthetic post-modifications, it is an attractive material for numerous applications, including as a flame retardant.

4.2.2. ZIF-8

Zeolitic Imidazolate Frameworks (ZIFs) are characterized as a class of MOFs using imidazoles as organic ligands [122]. In general, they consist of divalent metal cations (Zn²⁺, Co²⁺, Cu²⁺, Cd²⁺) linked by organic imidazole ligands. Zinc imidazolate framework-8 (Zn-ZIF-8) is a member of the ZIF subfamily based on zinc and 2-methylimidazole [123]. The three-dimensional structure of Zn-ZIF-8 forms a cubic unit cell: the zinc ion (Zn²⁺) is bound to the nitrogen atoms of the 2-methylimidazole by a coordination bond.

The angle of the M-IM-M structure is 145° to allow a topology similar to zeolite, traditional molecular sieves, so the structures of ZIFs constitute several advantages of MOFs, thus allowing a wide range of shapes [124,125]. ZIF-8, a widely studied class of ZIFs, shows good properties due to the closer interaction between the metal cations and the nitrogen atoms of the imidazolate ligand compared to carboxybenzene, resulting in high chemical, thermal, and hydrothermal stability [103,126,127].

ZIFs have properties similar to MOFs and zeolites, such as exceptional thermal and chemical stability, ultra-high surface area, and high crystallinity. Thus, the unimodal micropores and other properties provide a wealth of functional possibilities [128]. Like UiO-66, ZIF-8 can be used as a flame retardant due to its special composition—central metal atom and organic ligand, where the transition metal zinc is considered to have an excellent catalytic effect in the flame retardant field [129,130].

The ammonia, NH₃, produced during the combustion of ZIF-8 is able to dilute flammable gases, and the oxidation of zinc hinders the development of the fire by acting as a carbonizing agent, forming carbonized layers that protect the substrate [105]. In view of this, ZIF-8 has been studied for its adhesion to polymeric coatings in order to obtain a flame retardant material.

5. Challenges and Future Perspectives

In the textile industry, the durability of finishes is crucial, especially for advanced functionalities such as flame retardancy, antimicrobial properties, and controlled release. Recent research has focused on using metal–organic frameworks (MOFs) to achieve these functionalities. In addition to satisfying the functional requirements, the application of MOFs to textile substrates must ensure durability—a key aspect of textile quality. Researchers are exploring alternatives to enhance this durability, including structural modifications to the substrate or the MOF itself, and the use of cross-linking techniques.

MOF–textile adhesion can occur either ex situ or in situ within cellulose-based textiles. The metal ions of MOFs coordinate with the hydroxyl groups of cellulose, which acts as a mixed ligand with an organic acid in the formation of MOFs [80,87,131,132]. Modifying cellulose can create chemical groups that serve as anchors for MOF adhesion, thereby increasing durability. Xue Bi and collaborators [5] developed a durable coating made of UiO-66, cationic starch (St), and polyphosphonitrile (PZS) on cotton fabrics modified with citric acid, without affecting the fabrics’ color, whiteness, or stiffness. In in situ synthesis, Hardeep Singh Jhinjer and team [133] obtained a multifunctional fabric with visual aesthetics and washing durability through the carboxymethylation of cotton fabrics—a strategy also employed by other researchers and substrates [81,86,134]. Nour F. Attia and collaborators [45] used a different MOF–textile adhesion method, developing coatings from UiO-66 decorated with spherical polypyrrole nanoparticles and wrapped with chitosan chains, achieving high flame retardancy, UV protection, and tensile strength.

Another approach considered by research groups involves using cross-linking agents to promote more cohesive bonds between the metal–organic structures and textile substrates. Reda M. Abdelhameed and collaborators [135] reported using 3-glycidyloxypropyltrimethoxysilane as a bonding agent in natural cotton, viscose, and linen fibers to create functional fabrics. Yingying Yang and colleagues [136] performed the in situ growth of ZIF-8 on cotton fibers at room temperature, followed by coating with polydimethylsiloxane (PDMS), ensuring the durability of superhydrophobic and antibacterial fabrics. Hossam E. Emam [137] and his team used silicate as a cross-linker to produce durable functional fabrics with attention to coloration. Metal–organic structures can also act as adsorbents for dyes, improving durability. Dongdong Liu [101] demonstrated this by using cotton fabric to achieve dyeing with acid dyes.

Beyond fixation properties, the colorimetric aspects of MOFs applied to textiles are also crucial due to their impact on the visual appearance of fabrics. MOFs can have intrinsic colors that vary depending on the metals and organic ligands used in their synthesis. Incorporating colored MOFs can alter the visual appearance of textiles, adding new shades [138,139] or intensifying existing colors. For instance, Shuai Zhang [140] used ZIF-8 and colored nanospheres composed of polymethylhydrosiloxane (PMHS) through a spraying process to create multifunctional colored cotton fabrics.

Typically, ZIF-8 and UiO-66 in powder form are white to slightly yellowish due to the synthesis reagents. Reda M. Abdelhameed’s [132] team reported the yellowing of cotton fabric after modification with UiO-66. ZIF-8, known for its white appearance, was chosen by Dongdong Liu [141] to modify cotton fabrics through acid dye adsorption. The colorimetric aspects of MOFs applied to textiles are complex and multifaceted. Researchers are exploring the intrinsic color of MOFs, the uniformity of application, and interaction with dyes to optimize the visual appearance of treated fabrics. Thus, while the application of MOFs to textiles can confer advanced functional properties, it must be carefully controlled to produce only the desired effects. Continuous research into new MOFs and application techniques is essential to balance durability, appearance, and texture, ensuring that functional benefits do not compromise other qualities of the substrates.

6. Conclusions

The textile chain has a great capacity for product innovation, contributing to the development of multifunctional substrates with specific performance. In this paper, the reasons why the search for new flame retardants has become a focus of research are outlined, highlighting the severe reduction in brominated composites. Flame retardant finishes on cotton fabrics become considerable due to the composition of the fiber, mainly cellulose, which lowers its oxygen limit index and thus favors rapid ignition and high smoke generation.

Current research has focused on metal–organic materials as promising flame retardants. This class of material is widely investigated due to its porosity characteristics, large specific area, chemical stability, and ease of synthesizing and incorporating new functionalities. MOFs have attracted the attention of researchers because of their flame retardant mechanism since they absorb the gases that fuel combustion and also generate a layer of charcoal that inhibits the evolution of the flame. The application of these emerging materials has been reported, presenting three main methods: dip–pad–dry, direct synthesis (in situ growth), and layer-by-layer (LBL) assembly. From the perspective of developing new flame retardant materials, this study serves as inspiration for the search for substitutes for brominated compounds to encompass the trend towards the use of versatile structures such as MOFS. Finally, to strengthen textile innovation, MOFs should be increasingly explored as a component of substrate functionalization, with the aim of linking their characteristics to application methods in order to improve the performance of textile articles.