**3. Combustion and thermal behaviors of nylon textiles**

To come up with a suitable flame retardant approach along with an applicable flame-retarding agent for nylon textiles, it is thus needed to understand their thermal and flammability behaviors. In general, the polymeric materials release gases like CO, NO2 and HCN, etc. when they are burned [15] and it is also observed that the evolution of CO differs from fiber to fiber. Meanwhile, polyamide fibers show self-extinguishing behavior due to its extensive shrinkage and dripping in combustion [16]. During burning, polyamide ignites with molten droplets and drip away from the flame; most of the heat is carried away with the droplet, making the material selfextinguishing. However, if the molten droplets burn continuously, this will encourage a greater fire hazard and pose a secondary fire risk. In inert atmosphere and at a higher temperature range (i.e., above 300°C), the main decomposition products released by polyamides are about 95% non-volatiles and the remaining volatile compounds mainly consist of CO2, CO, water, ethanol, benzene, cyclopentanone, ammonia, others aliphatic and aromatic hydrocarbons, etc. [17, 18]. However, in air atmosphere and at temperatures below 200°C, the degradation pattern of polyamide is different [17] where the volatile products to be likely water 52%, CO2 33%, CO 12%, and methanol, formaldehyde and acetaldehyde are around 1% each. Moreover, the pyrolysis process also causes de-polymerization of its structure [19]. The suggested oxidative decomposition mechanism of polyamide structure is given in **Figure 2** [20].

From **Figure 2**, it is assumed that like the oxidative degradation of hydrocarbons, the oxygen molecule initiates the chain process of oxidation of polyamides (**Eq. 1**). At first, hydrogen atom will be abstracted and subsequently, either

*Flame Retardant and Thermally Insulating Polymers*

**retardant mechanism**

with significant researches with a focus on alternate flame retardant chemistries and methodologies, including the use of more environmentally benign raw materials and eco-friendly approaches in the synthesis and application of new flame retardants. Nylon is the oldest man-made fiber (MMF) among the synthetic textiles, which remains as an important fiber in the synthetic fiber community till the date. Initially Nylon, also in the name of polyamide, is developed for a limited number of end uses; however, these days, the fibers belonging to the nylon/polyamide group share a big market, from regular apparels to technical textiles. For example, carpet is a significant application for nylon and accounts for 17.5 percent of total usage globally. Other applications of nylon include airbag, heavy-duty tires, intimate apparel, military apparel, sheer hosiery and swimwear, etc. [7]. Among, different types of polyamides, polyamide 66 (Nylon 66) and polyamide 6 (Nylon 6) represents one of the most used technical fibers. Both of them possess almost similar physical properties, namely high mechanical properties (tensile strength is higher than that of wool, silk, rayon or cotton), high chemical stability, high melting point, resistance to shrinkage and abrasion [8]. However, like other common textiles these fibers are also flammable due to their organic structure; alongside they also show serious dripping. Thus, the nylon textiles cannot meet industrial and civil requirements in many cases, which ultimately limit their uses in the mentioned sectors [9–11].

**2. General combustion behaviors of textiles and strategies of flame** 

*Combustion cycle of a typical textile material. [7], Copyright 2020. Reproduced with permission from* 

In general, combustion of a typical polymer substrate happens in contact of a fire source and in the presence of air or oxygen. Prior to the combustion process, the textile materials degrade thermally, while some of the degraded species turn into combustible volatiles and serially, in the presence of oxygen, they kindle the flame. In a logical way, while the heat generation exceeds the threshold to sustain the combustion process, the excessive heat transmitted to the textile material, usually accelerates the degradation process and form a self-sustaining combustion cycle as presented in **Figure 1** [7]. In line, we also need to study the mechanism of action of various flame retardants on textiles to evaluate a particular flame-retardant system

**90**

**Figure 1.**

*Elsevier ltd.*

$$\begin{array}{ccccccccc}\text{\text{\textdegree C}\_{2}\text{\text{\textdegreeCONCHCH}\_{2}\text{CH}\_{2}\text{\text{\textdegree C}}\_{2}} & + \text{O}\_{2} & \rightarrow & \text{\text{\textdegree C}\_{2}\text{\text{\textdegreeCONCHCH}\_{2}\text{\text{\textdegree C}}} & + \text{H}\_{2} & (0) & \text{(1)} & \text{(2)} & \text{(3)} & \text{(4)} & \text{(5)} & \text{(6)} & \text{(7)} & \text{(8)} & \text{(9)} & \text{(1)} & \text{(1)} & \text{(1)} & \text{(2)} & \text{(1)} & \text{(2)} & \text{(1)} & \text{(2)} & \text{(1)} & \text{(2)} & \text{(1)} & \text{(2)} & \text{(1)} & \text{(2)} & \text{(2)} & \text{(1)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{(2)} & \text{($$

**Figure 2.**

*Possible oxidative decomposition mechanism for polyamides [20].*

peroxide radical or a hydroperoxide will be formed (**Eq. 2 and 3**). Later, with the decomposition of hydroperoxide, water will be formed (**Eq. 4**). The formation of water can lead to the hydrolysis of the polymer and thus, on decarboxylation, CO2 will be produced. Apart from the decomposition of peroxide, the peroxide radicals may also break down in the degradation process. These radicals may also go through isomerization via making reaction with the free valence of the adjacent C-C bond. Ultimately, this kind of isomerization as well as the breakdown of peroxide radicals causes the collapse of molecular chains to form a molecule with a terminal aldehyde group (a) and a radical (b) (**Eq. 5**). Afterwards, these aldehyde groups go for further decomposition to the form CO (**Eq. 6**), while the radicals (b) cause the rupture of the C-C bond to form a secondary C-O bond and thus, in turn direct the creation of formaldehyde (**Eq. 7**).
