# Metal-Organic Frameworks as Promising Textile Flame Retardants: Importance and Application Methods
## 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 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 \(\alpha\)- and \(\beta\)-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 [PERSON] [54], \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fiber** & **Tc**(\({}^{\circ}\)**C**) & **Tp**(\({}^{\circ}\)**C**) & **LOI** \\ \hline 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 & \textgreater{}250 & 290 & 18 \\ Polypropylene & 550 & 469 & 18 \\ Nomex® & \textgreater{}500 & 410 & 28–30 \\ Kevlar® & \textgreater{}550 & \textgreater{}590 & 29 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Flame behavior of fibers [54; 55]. 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 cellulose 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 [PERSON] et al.