Flame and fire retardancy of polymer-based composites

Materials Research Innovations ISSN: 1432-8917 (Print) 1433-075X (Online) Journal homepage: https://www.tandfonline.com/loi/ymri20 Flame and fire retardancy of polymer-based composites Radhashyam Giri, Lalatendu Nayak & Mostafizur Rahaman To cite this article: Radhashyam Giri, Lalatendu Nayak & Mostafizur Rahaman (2020): Flame and fire retardancy of polymer-based composites, Materials Research Innovations, DOI: 10.1080/14328917.2020.1728073 To link to this article: https://doi.org/10.1080/14328917.2020.1728073 Published online: 15 Feb 2020. Submit your article to this journal Article views: 13 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=ymri20 MATERIALS RESEARCH INNOVATIONS https://doi.org/10.1080/14328917.2020.1728073 Flame and fire retardancy of polymer-based composites Radhashyam Giria, Lalatendu Nayakb and Mostafizur Rahamanc a c Department of Plastics Technology, Central Institute of Plastic Engineering and Technology, India; bPhillips Carbon Black Limited, Kolkata, India; Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia ABSTRACT ARTICLE HISTORY In this article, the recent development concerning the flame and fire-retardant characteristics of polymer-based composites has been discussed. The process of polymer combustion and its inhibition, and the chemistry behind the fire retardancy are mentioned in the background. Focussed are given on mainly inorganic hydroxide, halogen, phosphorus, nitrogen, silicon, boron, clay-layered silicates, metal hydroxides, and carbons based flame retardants as the use of these different nanofiller additives designed the polymer for enhancing flame retardancy. Emphases are given on how these nanofillers are beneficial to retard the flame from spreading during the development of fire. The usefulness to incorporate the conventional flame retardants in nanocomposites is also discussed. Finally, the mechanism of flame retardancy for clay-and carbon-based nanocomposites is reported. Received 29 September 2019 Accepted 3 February 2020 1. Introduction 1.1. Polymers and fire Polymers have been extensively used in many applications because of their light-weight. Many commodity and engineering polymers are replacing conventional metal and ceramic materials, as they comparatively possess many advantages. But the usage of these materials has greatly increased the risk of fire because of their ability to catch fire and possible release of toxic byproducts. However, the majority of organic compounds will burn readily in air or oxygen. The polymers, having high or even lower flammability have a difficult issue and extremely confine their applications [1,2]. Lately, the fire security department has expressed their concern about the necessity of suitable materials in wide area of applications, which result in increasing demand for improved flame or fire-retardant materials at stringent condition [3–5]. The polymer/plastic materials are used in houses, business and transportation sectors. Consequently, the fire-retardant matter can be directly associated with the nature of polymers [6]. The regular fire retardants are halogen-based exacerbates those are prudent and can upgrade the fire retardancy of polymers without debasing their physical property, for example, quality. In any case, lethal species, for example, dioxins and furans, which are evolved during the ignition of halogencontaining composites, may cause serious environmental contamination [7–9]. Therefore, developing halogen-free, low-smoke, and environmental-friendly fire-retardant carbon composites has become increasingly important in recent years. Inorganic hydroxides, such as aluminium hydroxide or magnesium hydroxide are examples of the most extensively used inorganic fire retardants at the present time due to their non-toxic and environmental-friendly characteristics [10–14]. In any case, to meet the purposes, high amount of loading (30 wt%-60 wt %) is expected that prompts extra cost, processing difficulty and a decrease in physical CONTACT Mostafizur Rahaman Riyadh 11451, Saudi Arabia Nanocomposites; flame and fire retardancy; nanofillers; layered silicates properties of polymers. Consequently, the improvement of new exceptionally successful green fire retardants has incited much consideration amid the last ten years [5,6,13,15]. Nano-scale layered metal materials, such as clay and layered double hydroxides (LDHs) have been tested as potential fire retardants that could improve the flame retardancy, while improving also mechanical properties (like tensile strength, elongation at break, etc.) [16–18]. In layered material/polymer frameworks, simultaneous upgrades of numerous properties are ordinarily accomplished, for example, combustibility and biodegradability [19,20]. These enhancements of polymer-nanocomposite properties strongly depend on the dispersion of added substances within the polymer matrix. 1.2. Polymer combustion process Numerous polymers are subjected to some appropriate ignition sources, will experience self-managed ignition in air or oxygen [21]. All in all, non-polymeric materials (e.g., matches, cigarettes, lights, or electric circular segments) are all the basic source of ignition, but polymers themselves are most often in charge of the propagation of fire. A consuming polymer constitutes an exceedingly complex ignition system. Compound responses may occur in three associated regions: inside the condense stage, at an interface between the consolidated stage and gas stage, and in the gas stage. The chemistry of fire retardancy of polymers depends on its chemical structural repeat units and their interaction when a polymer is burnt. It has been mentioned that the highly aliphatic polymers that is having sp3 carbon bonds in their backbone structure are more susceptible to high heat release [22]. On the contrary, the polymer with more aromatic in nature that is having sp2 carbon bonding exhibits low level of heat release. The bonding interaction is high in case of sp2 carbon bonding, and hence in required more heat energy to break it. As a result, polymers with sp2 carbon [email protected]; [email protected] © 2020 Informa UK Limited, trading as Taylor & Francis Group KEYWORDS Department of Chemistry, College of Science, King Saud University, R. GIRI ET AL. 2 Figure 1. Schematic diagram of polymer burning (reproduced from Ref [24].). bonding or aromatic ring will be more fire retardant compare to polymers is having sp3 carbon bonding in their backbone structure. The polymer burning happens like a series of interrelated occasions [23]: (1) (2) (3) (4) Heating of the polymer Decomposition Ignition Combustion Initially, the polymer is warmed to a temperature at where it starts to disintegrate and produce vaporous items that are typically combustible. These items at that point diffuse into flame zone over the consuming polymer. At the time when there is an ignition source, they will experience burning in the gas stage and release heat. Under the intense and increasing state of consuming conditions, a part of the heat is exchanged with the polymer surface, creating more unstable polymer sections to maintain the heating cycle. This process is described in Figure 1 [24]. There are two types of ignition included when polymers are burned: flaming combustion and non-flaming combustion [25]. Flames are self-propagating responses which involve both the fuel and oxidants in the gas stage. Most polymers are based on hydrocarbons; hence, the flame above the surface of consuming polymers is generally a flame containing hydrocarbon. The vital responses in flames are freeradical responses. The most essential radicals present in hydrocarbon flames are basic species, for example H•, O•, and OH•, and a little measure of HO2•, HCO•, and CH3•. Chain-stretching responses in the ignition procedure, for instance H• + O2 → HO• + O• can quicken the consumption of polymers by producing more radicals. The smoke which is formed in flames depends largely on the gaseous fuel’s structure and on the fuel to oxidant ratio. Regularly, polymers containing simply aliphatic auxiliary units deliver moderately little smoke, while polymers containing aliphatic structural units in the fundamental chain deliver intermediate amount of smoke. Non-flaming combustion, includes smouldering and glowing ignition, propagates through the polymer with help of warm front or wave, including the surface oxidation of the pyrolysis items [23]. Shiny burning contrasts form seething ignition; it is joined by pale frames of carbon consuming to form carbon monoxide. Seething ignition, as a rule, happens along with polymeric materials of high surface zone that can shape a carbonaceous scorch. It is joined by the era of smoke because of pyrolysis at the surface or close to it. Glowing burning happens after the underlying roasting of the polymeric material. From a handy perspective, it is additionally vital to take into consideration the related fire risks. The impacts coming about because of polymer burning, which can debilitate human life, incorporate oxygen exhaustion, flame, heat, smoke, hot and lethal burning gasses. The two major causes of fire-related deaths are inhalation of toxic gases and burns [26,27]. 1.3. Fire-retardant chemistry When a polymer degrades and a thermal decomposition occurs, the following types of products may be formed [23]: (1) Combustible gases, e.g. carbon monoxide, methane and ethane; (2) Liquids which are usually the polymers are partially degraded; (3) Non-Combustible gases, e.g., carbon dioxide, hydrogen bromide and hydrogen chloride; (4) Finely divided solid particles which consist of decomposing polymer fragments or soot in the combustion gases; (5) Discrete solids in the form of char or carbonaceous residue. The advancement of extensive volumes of exceedingly combustible gasses will no doubt tend to build the combustibility of polymers. Fluid items are not as precisely flammable as vaporous items; however, fluids can spread heat to adjoining parts of the polymeric structure. Along these lines, diminishing the sum and rate of production of burnable gasses and avoiding the spreading of flame are the most essential procedures to accomplish flame retardancy. For the most part, there are two instruments by which the polymer burning can be repressed [26]. One is strong phase inhibition, including changes in polymer structure. Low thickness, highporosity chars have a tendency to be the most attractive deterioration items from the non-combustibility. The chars can have the valuable capacities including saving basic structural integrity, protecting the basic polymer from the heat of the flame and preventing creation of new fuel and further consuming. However, a char once in a while experiences seething ignition, prompting a large amount of smoke MATERIALS RESEARCH INNOVATIONS formation. This impact typically happens just at elevated temperatures. Another imperative factor in the polymer ignition cycle is the net heat of combustion of the gaseous items, which is the heat discharged by the consolidated burning responses reduce the heat expected to bring the polymer from its underlying state to the ignition. In this way, if a framework can evolve water and some other noncombustible gasses or have vast endothermic requests for thermal disintegration, the flammability can be decreased. Another flame-retardant instrument is vapour phase inhibition, including changes in chemistry of flame. In these frameworks, reactive species, for example, bromine and chlorine are incorporated with the polymer and are changed into freeradical inhibitors that are volatile while burning, for example, hydrogen chloride (HCl). These materials diffuse into the flame and inhibit the radical responses by changing highly reactive species H• and HO• into less reactive Br• and Cl•. Two cases of radical scavenging responses are H• + HCl → 2H + Cl• and HO• + HCl → 2HO + Cl•. Thus, the combustion cycle is interfered. Some materials include both solid and vapour phase inhibition. It is practically difficult to recognise among the distinctive components by which the flaming combustion of a natural polymer is being inhibited. 1.4. Inhibition of polymer combustion The best technique by which we can prevent polymer combustion is to make inherently fire-safe polymers. Such polymers should have high thermal stability, resistance to flame spreading and low consuming rate event if the heat flux is high [28]. Nonetheless, these materials are difficult to process and are exceptionally costly. Another methodology is to utilise flame-retardant additive to hinder the ignition of polymers, particularly for the commodity polymers. Below are the details about these two choices. 1.4.1. Intrinsically fire-resistant polymers Maximum polymers which have high thermal resistance are naturally fire safe. They have elevated decomposition temperature and hence, their initial breakdown will be viably anticipated and process of combustion will not be started. This property of polymers with high-temperature stability can be improved by expanding the associations between chains of polymer or by stiffening of chains [29]. Interactions between the chains can be enhanced by a few means, for example, increasing the crystallinity, the presence of polar group and hydrogen bonding. Stiffening of chains can be achieved modifying the polymer backbone by addition of aromatic or hetrocyclic groups, for example, in poly (p-phenylene), aromatic polyamides, and polyesters. Moreover, polymers containing extensive quantities of Figure 2. Intrinsically fire-resistant polymers. 3 aromatic groups in the basic units have incredible tendency of producing char when heated. They, consequently, deliver less combustible gaseous products into flame. Taking all things together, polymers having high thermal stability and producing less combustible volatiles are the most wanted fire-safe polymers. For intrinsically fire resistant polymers there are three types of structures: linear single-strand polymers comprising of a succession of heterocyclic or cyclic aromatic structures, inorganic polymers, semi-organic polymers and ladder polymers [29]. Up until this point, most fire-resistant polymers based on carbon are prepared by fusing very rigid, stable, aromatic or heterocyclic ring structure specifically in the polymer chain [30–32], for example, polyimide (PI), polybenzoxazole (PBO), polybenzimidazole (PBI), and polybenzthiazoles (PBZT) (Figure 2). The synthetic ways make such polyaromatic heterocyclic polymers include a two-stage process in which a high molecular weight, soluble prepolymers are synthesised and after that stable, rigid rings are formed by chemically or thermally induced build-up of reactive groups on polymer chain. Ladder polymers are an exceptionally extraordinary kind of rigid chain polymers [33,34]. Such polymers are doublestranded structures comprising of the two polymer chains which are intermittently bonded together by chemical bonds, for example, cyclised polyacrylonitrile. On a fundamental level, these materials should show higher thermal stability as of the fact that the polymer chains cannot be disjoined by breaking a single bond. The synthesis of semi-organic and inorganic polymers has aimed for the generation of polymers which have linear chains and stable structures and comprising of repeat units like boron-nitrogen, phosphorus-nitrogen and silicon-nitrogen [35,36]. The non-burning characteristics of inorganic components and the preparation of noncombustible protective coatings are the two principle explanations behind the fire resistance of polymers. 1.4.2. Flame-retardant additive From the manufacturing perspective, the introduction of flameretardant additives without a doubt constitutes the simplest method for making polymer less flammable. There are two sorts of additives: the reactive flame retardants and additive flame retardants [37]. The reactive flame retardants are the compounds which contain heteroatoms giving some level of flame retardance; further, they are incorporated chemically in the polymer chains. On the other hand, the additive flame retardants can be physically blended with given polymer. For this situation, there is no chemical reaction between compound and polymer. Most abundantly utilised flame retardants at the present time are based on six components: bromine, chlorine, aluminium, phosphorous, boron and antimony. Also, silicon 4 R. GIRI ET AL. and nitrogen can likewise give some degree of flame retardance. Other components and their compounds have turned out to be comparatively less effective. Compounds of flame retardants regularly have synergistic or hostile impacts. Once in a while a heteroatom already present in polymer backbone may interface with flame retardant, along these lines, display synergism or hostility. Although additive flame retardants are broadly utilised as a part of polymers, there are a few restrictions, for example, poor compatibility, high volatility, and harmful consequences for the properties of polymers, an increase in generation of smoke and carbon monoxide (CO) [38]. Flame-retardant additives can work by a different mechanism in either gas phase or condensed phase [37]. They can end the free-radical reactions in the condensed phase and act as heat sinks because of their heat capacity, form a non-combustible protective layer or char to protect the combustible polymer from the source of the oxidant and heat and intrude on the combustion of flame in gas phase. It is troublesome, to unequivocally attribute a single mode of action to a particular additive or class of additives. Number of flame retardants gives off an impression of being equipped for working simultaneously by a few distinct components, frequently relying upon the idea of the natural polymers. 1.4.2.1. Flame retardants based on inorganic hydroxide (hydrated additives). Inorganic hydroxides are considered to be an important class of flame retardants as they have low cost, low harmfulness and ease of handling [39,40]. Aluminium oxide trihydrate is utilised in the biggest amounts in polymers as an organic flame retardant [41]. In order to acquire huge flame-retardant impact, these are added into the polymers in large amount (50% by weight). The addition of flame retardants reduces the measure of flammable materials accessible for disintegration. During decay, this filler compound goes about as a heat sink and along these lines, delays the polymer from achieving its degradation temperature [42]. Whenever heat is supplied to it, it decomposes to form anhydrous alumina and discharges water, which is an endothermic response. The heat from the substrate is removed by utilisation of this energy and thus keeps the substrate below its ignition temperature. Additionally, the concentration of combustible gas is diluted by the water discharged into the vapour phase. The oxide deposit produced during decomposition has heat capacity, which can lessen the heat transfer to the substrates. Another advantage of utilising inorganic hydroxides is that they can decrease the measure of smoke created on combustion [43]. Because of its low thermal stability, aluminium oxide trihydrate ought to be utilised beneath 200°C. Other inorganic hydroxides and hydroxycarbonates [44] likewise have some flame-retardant activity. For instance, magnesium hydroxide is more thermally steady and can be utilised over 300°C. 1.4.2.2. Flame retardants based on halogens. Halogencontaining flame retardants contributes to additives in the plastic industry at large scale. Just like reactive flame retardants, other groups containing halogens like alkenes, cycloalkenes and styrene can also be copolymerised with non-halogenated monomers [45]. The organic halogenated compounds are additive flame retardants and are usually used with metal oxides like Antimony Oxide or with phosphorous compounds. The stability of halogen compound decreases from fluorine to iodine as F > Cl > Br > I. Iodine compounds are not steady enough and thus we cannot use them at industrial level, but fluorine compounds are highly stable and hence can be useful. Bromine and chlorine are the most widely used halogen-containing flame retardants. Bromine compounds are more effective than chlorine compounds on a weight basis; however, they are impressively more costly. Halogen-containing flame retardants may work either in the vapour phase or in the condensed phase [45]. The structure of the additive as well as of the polymers influences the mechanism of flame retardant. For the most part, the radicals delivered by thermal degradation of a halogenated flame retardant can interact with the polymer to form hydrogen halide. Radical propagation reaction is inhibited by hydrogen halides. This reaction occurs in flame in response to the dynamic radicals like H• and OH•. It also should be noticed that aromatic brominated compounds can create vast amount of char. In spite of the fact that halogen compounds are generally utilised alone in flame retardants, free-radical initiators and antimony trioxide increase its effectiveness. Halogen systems containing antimony can influence the combustion of polymers as they have the capacity to act in both phases i.e. gas phase and condensed phase. Despite increasing number of laws, brominated and chlorinated flame retardants have occupied the largest share of flame-retardant market. 1.4.2.3. Flame retardants based on phosphorous. Both inorganic and natural phosphorous compounds are valuable for granting flame retardant to numerous polymers. Phosphorous flame retardants incorporate essential red phosphorus, water solvent, insoluble ammonium phosphate, insoluble organophosphates and phosphonates, inorganic phosphates, phosphine oxides and chloroaliphatic and bromoaromatic phosphates [46]. Both additive and reactive flame retardants are commercially available. Additive compounds, particularly phosphates, are generally utilised for much hydroxylated polymers, for example, cellulose. The most widely utilised reactive flame retardants are phosphorus containing polyols which are used in polyurethane foam [47]. Other examples of reactive flame retardants are the one incorporated with vinyl and allyl phosphonates [48]. The flame retardance systems of these phosphorus compounds include free radical restrain for most part proposed for halogen, formation of surface glass which protects the substrate from flame and oxygen and advancing of char [45]. The flame-retardant system for phosphorus relies upon the structure of polymer compound and the type of phosphorous compound. As there is development of phosphorus halides or oxyhalides or P-N bonds the phosphorus flame retardants containing nitrogen or halogen are regularly expressed to display synergistic conduct on decay [49,50]. Recently, the interest for intumescent systems has increased, which can build up a char on the surface of polymer while burning. The compound of melamine, dipentaerythritol and ammonium phosphate is the most ordinarily utilised intumescent flame-retardant framework [51]. For the most part, intumescences require a char former for example, an acid source, a polyol or a catalyst, a phosphate and a spumific (gas generator) like nitrogen source. The mechanism includes decomposition of phosphate to phosphoric acid, esterification of polyol followed by decomposition and recovery of phosphoric acid. The forming char is blown to porous item with help of ammonium polyphosphate with high alkali content. This surface char protects MATERIALS RESEARCH INNOVATIONS the substrate from heat, oxygen and flame. There are also some self-intumescing compounds that contain every one of the three required functions in single molecule (Figure 3). Such intumescent coatings can satisfy ecological and toxicity quality issues as the coatings are halogen free and the decomposition gasses are ammonia and water. 1.4.2.4. Flame retardants based on nitrogen. The presence of nitrogen in regular polymers seems to impart some degree of flame retardance, as appeared by the generally low combustibility of fleece, leather and silk [52]. Synthetic polymers containing nitrogen are not all that impervious to combustion. Various nitrogen-containing natural compounds are utilised in certain polymers as receptive flame retardants, for example, urea, guanidine, cyanuric acid derivatives, etc., [53,54]. Some of these compounds are also utilised as additive flame retardants, frequently with phosphorus compounds, to decrease the combustibility of cellulosic materials. In the later cases, the nitrogen seems to act to a considerable extent by strengthening the attachment of phosphorus to the polymer, yet nothing is certain about the mechanisms of activity. One conceivable clarification is that the release of nitrogen or alkali dilutes the unstable polymer decomposition products making them less combustible. Metal amine complexes and ammonium salts are also broadly utilised as flame retardants for many applications, for example, ammonium phosphates for wood [55,56]. 1.4.2.5. Flame retardants based on silicon. Use of siliconbased flame retardants instead of halogens or phosphorus in Figure 3. Intumescent compounds (reproduced from Ref [38].). 5 gaining interest [57]. Silicones like silicas, organosilanes, silsequioxanes and silicates are all been studied. The flame retardant which is recognised most widely in view of silicon is as polyorganosiloxane, in specific, polydimethylsiloxane (PDMS). A significant decrease has been observed in flammability of block copolymer of different types of polycarbonate (PC) and polyetherimide with PDMS [58]. Silicon can likewise be fused into the polymer chain’s branches [59]. Under specific cases, the expansion of silica can likewise influence the combustibility properties of materials [60]. The arrangement of a protective surface layer which is based on silicon seems, by all accounts to be the flameretardant mechanism for silica and silicone systems. Polycarbosilane, polysilastyrene and polysilsesquioxane preceramic polymers, appeared in (Figure 4), are likewise used to mix with different thermoplastics [61,62]. These studies demonstrate that they are all flame retardants with high efficiencies. They can reduce the peak of heat release rate (HRR) and normal HRR; however, the total amount of discharged stays unaltered. The essential explanation behind the low HRR for the blend is the decrease is mass loss rate. Mass loss rate is the rate at which fuel discharged into the gas phase is reduced because of the presence of ceramic charge. Recently there has been great increase in the combustibility properties of polymer-earth (layered-silicate) nanocomposites [63]. Cone-calorimetry information demonstrate that both the peak and the normal HRR are reduced fundamentally for delaminated and intercalated nanocomposites having low silicate mass (3%-5% by weight), yet there is small change in the yield of char. Polymer-clay nanocomposites are the materials that may satisfy the necessities for a superior, additive- 6 R. GIRI ET AL. Figure 4. Some preceramic polymers. type flame-retardant system. In general, a condense phase mechanism, including a protective surface layer is proposed for silicon-based flame retardants. 1.4.2.6. Flame retardants based on boron. Borate treatments were the only one to be broadly connected to cotton and afterwards to wool initially. Boric acid and borax are as often as possible utilised together [64]. On evaporation of the water of hydration, the polymers swell and an intumescent covering is formed, which protects the main part of the polymer from the heat source. The increase in the development of char, the endothermic dehydration process and the dilution of the gaseous breakdown products by the water discharged could be the explanations behind the flame retardancy of additives containing boron. For cellulose cyclic borate is used as durable additives whereas boric acid and polyols have been fused into rigid polyurethane foams. 1.4.2.7. Flame retardants based on polymers. Polymeric flame retardants have many advantages but are comparatively less studied than their small molecular counterparts. The physical and mechanical properties of polymers are less influenced by addition of polymeric flame retardants. It can also avoid a strategic distance from the outward dissemination in the system and consequent risk of environmental contamination. Examples of such flame retardants are polydibromostyrene and polyphosphazenes [3,65]. In a broad sense, all the fire-resistant polymers can be utilised as polymeric flame retardants to be mixed with some different polymers to upgrade fire retardancy. As a matter of certainty, this is a highly advantageous approach to modify polymer combustibility by organisation. 2. Flame retardants based on polymer/layered silicate nanocomposites Ordinary flame retardants, as per their nature interfere with the combustion cycle through chemical or physical methods of activity. The principal mechanisms of a flame retardant to reduce combustion in composites are: (i) The quantity of heat transferred to polymer is reduced by arrangement of a protective impervious coating, which limits the diffusion of oxygen to the area of decomposition and impedes, (ii) (ii) The escape of volatiles and inert gasses, at the surface of the consuming polymer which dilute the oxygen supply, and (iii) Advancement of endothermic reactions which cool the substrate to a temperature beneath that required to propagate consuming. Also, the chemical methods of action by which flame retardants work are: (i) carbonaceous layer on the surface of polymer by advancing low energy solid state reactions which leads to the carbonisation of the polymer to the expense of volatiles generation, (ii) Speeding up of degradation of polymer, causing pronounced dripping and hence, the withdrawal of the fuel from the flame source, (iii) Prevention of the oxidation reactions occurring in the gas phase by trapping of free-radical species (particularly, H• and OH•) created during polymer’s decomposition. MATERIALS RESEARCH INNOVATIONS The chain-branching reaction is caused by the hydrogen radical which propagates the combustion of fuel (H• +O2→OH• +O•) while the hydroxyl radical is included with the most exothermic reaction (OH• +CO→H• +CO2) which gives maximum energy preserving the flame. Now, it should be noticed that more often than not these mechanisms do not happen independently however in combination and along these lines, the path according to which a flame retardant works is normally a complicated procedure comprising of single stages with one of them dominating. The determination of an appropriate additive for every step by expecting its mechanism, relies upon factors like its tendency to migrate, stability, toxicity, impact on electrical properties, ability to be coloured, cost-effectiveness and capacity to cause corrosion [66–69]. Progressively restrictive legislation regarding the toxic release of halogen-containing materials has made two categories of flame retardants: halogenated and nonhalogenated. But this characterisation is excessively unsophisticated for an advanced analysis over the addition of conventional flame retardants into polymer nanocomposite. Consequently, inside the system of this study, the category of halogen-free compounds was broken down into littler classes, mostly used in literatures: nitrogenbased compounds, metal hydroxides, intumescent systems and phosphorus flame retardants. In spite of the fact that intumescent systems may contain phosphorous or nitrogen substances, they work uniquely in contrast to their individual segments, hence they are studied independently. All together to justify the more practical approaches of this survey, in every given classifications, the flameretardant nanocomposites inspected are arranged as per the type of matrix of polymer. Components that do not go under the previously mentioned gatherings of flame retardants were incorporated into a specific segment. The study is finished displaying detailed cases of the nanocomposites which contain layered silicates alongside new type of nanoparticles. 2.1. Nanocomposites with halogenated compounds The combustion cycle is interfered by halogen-based flame retardants principally through the mechanism of free-radical scavenging which restrains the propagation of flame in the vapour phase. More in particular, they respond by the degradation radicals of polymers (HX + H• →X• + H2 and HX + OH• →X• + H2O, where X is a halogen) resulting in substantially less active species. Common examples include bromine, chlorine, iodine and fluorine containing compounds, 7 with the initial two containing in the terms of tonnage, the most generally utilised individuals from this group [69,70]. 2.1.1. Polyolefin/layered silicate-based nanocomposites The primary-halogenated flame retardant to be checked, going for the reduced nanocomposite ignitability was decabromodiphenyl oxide (DB) added with antimony trioxide (AO). Metal oxides like AO are incorporated in the compounds that are halogenated as the interaction between them enhances the flame retardancy by generating halides (e.g. antimony trihalide); halides involve strong Lewis acid catalyst capable for promotion of dehydrogenation charring reactions, work like radical scavengers and form a blanket which acts like a barrier between the flue gasses and the condensed phase [71,72]. Zanetti et al. [73] consolidated the DB-AO system in poly (propylene – maleic anhydride) (PP-g-MA) nanocomposites and calculated its productivity by testing with cone calorimetry experiments. The TEM micrographs, XRD patters and tensile properties (100% increase of modulus) of the prepared samples advocate the nanodispersion of clay in the matrix of polymer. The flame retarded polymer (PP-g-MA + 22% by wt DB + 6% by wt AO) displayed HRR peaks having bimodal shape (major peak at 170 s with a weak shoulder at 85 s); while the significant peak vanished when 5 wt % organically treated clay was added, proposing an improved flammability behaviour which is more uniform. Also, when the base polymer was compared with the non-FR nanocomposite, a huge reduction in peak heat release rate (PHRR) (by 60% and 33% separately) and in the normal HRR (by 62% and 66%, separately) was recorded, while the ignition time was expanded. Given the labyrinth impact of layered silicates, the collaboration with the flameretardant system was because of the postponed release of the gas-phase species that were actively developed during combustion (Figure 5), for example, HBr created by DB, SbBr3 created by DB-AO or by the reaction amongst NaCl and AO (show in clay as pollution) and the SbBr3 – RNH3Br created by decomposition of organic modifier [74]. Without the clay, these species were quickly liberated and consumed towards the start of fire, as confirmed by the two-peak curve of the flame retarded polymer (Figure 6). Flame retarded PP/layered silicate nanocomposites, consisting of AO, have been synthesised by Lee et al. [75] in a twin-screw extruder. Because of the poor compatibility between the organoclay and the matrix, PP-g-MA was utilised as a compatibilizer to aid the formation of intercalated/exfoliated morphologies. It was illustrated; indeed, that a more successful reduction Figure 5. Evolution of active gas-phase species during the combustion of polymer nanocomposite containing DB and AO (reproduced from Ref [73].). 8 R. GIRI ET AL. Figure 6. HRR plots of PP-g-MA and PP-g-MA nanocomposite containing DB and AO (heat flux = 35 kW/m2) (reproduced from Ref [73].). in PHRR can be given by the co-expansion of the ingredients given. 2.1.2. Polyamide/layered silicate-based nanocomposites Hu et al have studied the polyamide 6 based selfextinguishing fire-retardant nanocomposites [76] using 5 wt% of organically modified MMT (OMMT), which was treated with a hexadecyl-trimethyl-ammonium salt. They found partially exfoliated/intercalated nanocomposites. The amount of flame-retardant system (15 wt% DB + 5 wt% AO) was somewhat lesser than that utilisation in the investigation of Zanetti et al. [73], yet, sufficient for accomplishing a V-0 order. The PHRR of the PA6 matrix reduced from 1220 to 673kW/m2 when clay filler was present and even lower to 390kW/m2 when the flame retardant was added confirming the synergistic action of the additives. 2.1.3. Acrylonitrile-butadiene-styrene/layered silicate-based nanocomposites The DB-AO system was utilised well by Hu and his associates [77], with a defined aim to prepare flame-retardant system consisting of ABS/OMMT nanocomposites, which could pass the thorough UL94 test. Accordingly, a similar sort of clay (5 wt%) was dispersed in ABS along with 15 wt% DB and 3wt% AO utilising a two roll mill process and the fire properties of the subsequent blend were completely contemplated. In opposition to the neat ABS and the pertinent nanocomposite, the sample containing both the clay filler and the flame-retardant system managed to get a V-0 rating, displaying additionally a LOI value (27.5 vol. %) rose by around 50% and 28%, respectively. Moreover, the performance in cone calorimeter was incredibly enhanced as probed by the 78% lower PHRR of the flame-retardant PLSN (polymer/layered silicate nanocomposite) in respect to that of pure ABS. The challenge to upgrade the thermal stability of ABS nanocomposites along with their resistance to ignite was confronted by Ma et al. [78], replacing DP with a brominated epoxy tar (BER). BER [72] (Figure 7) is considered to be high molecular weight gas-phase flame retardant with 53 wt% bromine content, composed by ICL Industrial Products (beer-Sheva, Israel) and utilised usually in PC/ABS blends with synergist like AO. In this study BER particles had more affinity due to their high polarity than ABS towards clay particles, Figure 7. Chemical structure of BER (reproduced from Ref [78].). encouraging the formation exfoliated and in this way more thermally stable structure. However, the immense preferred standpoint of the connected way was that, with little measure of the halogenated compound (12 wt% BER+4wt%AO), the LOI of ABS containing 2wt% clay raised from 20.5 to 31.4 vol.%, which is a long way past 24 vol.%, the LOI value normally required for a material to acquire a V-0 rating [79]. The combustion procedure and the synergistic impact of clay and BER–AO are introduced schematically in (Figure 8). In addition to the OMMT–DB–AO system, the cooperative energy between OMMT–BER–AO got from silicates forming barriers that stopped BER pyrolysis and the reactions between BER–AO occurring at reduced temperatures. Subsequently, in vapour phase, constant flame retardancy could be achieved all through combustion. In addition, it was recommended that the alkylammonium cations dwelling in the interlayer degraded near 200°C, to parts which could expand clay layers and volatilise promoting silicates dispersion. Exfoliated structures, showing enhanced barrier properties over their intercalated analogues, could postpone all the more effectively heat and mass transfer. Inversely, the reaction between the surfactant’s decayed items and DB–AO taking place at elevated temperatures that brought about the development of radical scavengers. 2.1.4. Polystyrene/layered silicate-based nanocomposites Different cases of organically modified clay nanocomposites mixed with the halogenated compounds were studied in the article of Wang et al. [80]. The creators melt blended PP, PS and PE with 3 wt% of clay (Cloisite 30B, Southern Clay Products, and USA), while acrylic acid pentabromobenzyl ester (ACPB), methacrylate acid pentabromobenzyl ester (MEPB), butyric acid pentabromobenzyl ester (BUPB) or MATERIALS RESEARCH INNOVATIONS 9 Figure 8. Schematic demonstration of the synergistic effect between clay and BER-AO incorporated in ABS (reproduced from Ref [78].). pentabromobenzyl ester polyacrylate (PBPA) and were incorporated as the flame retardant. Also, mass polymerisation was connected to plan flame retarded styrene nanocomposites and nanocomposites of styrene and dibromostyrene copolymer. For this situation, two optional clays were additionally utilised, precisely a fluorine containing ammonium and a dimethyl-n-hexadecyl-4- vinyl benzyl ammonium altered MMT. The two ways used for the sample preparation were ended up being effective for the exfoliation/intercalation of the clay as it was checked through XRD characterisations. Using cone calorimetry, it was brought up that the flame retardancy of PP and PS nanocomposites, with limited concentration of bromine (6 wt %), is increased discernibly. The best execution was displayed by styrene nanocomposites mass polymerised when dibromostyrene was present; the PHRR reduced essentially and a rating of V-0 was achieved with majority of the blends prepared. Whereas, the fire properties of PE-based specimens were not enhanced, inferring that interaction between the clay, polymer and bromine compound, experienced in PS and PP formulations, is absent for this situation. An alternate route increasing the THR and TTI of Polystyrene nanocomposite, rather than the regular polymerclay-flame retardant blending, was studied by Chigwada et al. [81]. The creators at first delivered novel organically/modified clays utilising ammonium salt which containing an oligomeric unit consisting of vinyl benzyl chloride, dibromostyrene and styrene. Therefore, nanocomposites were prepared both by means of melt intercalation and in situ polymerisation at different clay ratios and consequently, bromine. XRD estimations joined by intensive examination of TEM photomicrographs at various magnifications gave a decent idea for the development of intercalated structures. The materials which were polymerised in situ were comparatively more thermally stable, presented inferior THR, lower PHRR and sometimes preserved TTI, compared to the virgin polymer. The drop in PHRR, usually found in nanocomposites, was because of the presence of silicates and not bromine, which, on the other hand, was in charge of the THR drop. It is worth noticing that these improvements were recorded with less than 4 wt% 10 R. GIRI ET AL. Table 1. Fire properties of PMMA nanocomposite containing DB and AO (reproduced from Ref [82]). OMMT Sample (wt%) 1 2 3 5 DB (wt %) 20 20 AO LOI pHRR (wt UL94 (vol. (kW/ %) rating %) m2) - Failed 16.9 1456.8 5 Failed 23.3 490.4 5 V-0 25.6 359.4 Average HRR(kW/ m2) 598.5 306.9 235.9 Average MLR(g/s) 0.138 0.177 0.125 bromine, an amount smaller than that typically utilised for fire-resistant compositions. Then again, the outcomes got with dissolve intercalated nanocomposites showed up not to be so idealistic. 2.1.5. Polyester/layered silicate-based nanocomposites As the combined impact of the DB–AO system and clay was uncovered not to happen only in particular polymer matrix, Si et al. [82] recently investigated the likelihood of planning to make a PMMA nanocomposites which is less flammable, using the same idea; the authors melt blended a commercial organoclay (Cloisite 20A, Southern Clay Products Inc., USA) with DB – AO and the given polymer. By applying secondary ion mass spectrometry (SIMS) and TEM, the addition of clay preset in the matrix in exfoliated/intercalated form was demonstrated valuable for increasing the dispersion of the flame retardants. Also, through Dynamic Mechanical Analysis (DMA), it was interpreted that the viscosity of polymer is increased by silicates, which is important to keep up its shape while burning. The fire tests showed that the PMMA samples of all the three components (5 wt% clay, 20 wt% DB, and 5 wt% AO) showed enhanced properties (Table 1), related to those accomplished with clay or with the flame retardant as it were. A comprehensive examination of specimen that was serially burned, confirmed that in solid phase there were no interactions between the clay filler and flame-retardant system. Also that the silicates play a triplicate role in quenching fire; they promote development of char, encourage the flame retardants to disperse and catalyse. 2.2. Nanocomposites based on non-halogenated compounds 2.2.1. Nanocomposites with phosphorous-based compounds The phosphorus-containing substances involve, value wise the second biggest class and presumably the fastest developing section of flame retardants because of the expanding environmental awareness of the issues emerging from the utilisation of additives based on halogen [83]. Phosphorous compounds increase the flame retardancy essentially by advancing the development of carbon layer (char) on polymer’s surface, giving rise to phosphoric anhydrides and anhydrides of related acids acting as dehydrating agents; dehydrating reactions causes the double bond formation, which results in carbonised or Figure 9. Chemical structure of PEBI (reproduced from Ref [85].). crosslinked structures at elevated temperatures. Volatile phosphorous compounds (PO•, PO2•, and HPO•) can perform well in the gas phase, displaying the properties of free-radical trapping. The generated acids can also form a superficial liquid or thin glassy coating on the condensed phase, bringing down the oxygen diffusion and also mass and heat transfer [67,72,84]. 2.2.1.1. Polyolefin/layered silicate-based nanocomposites. The flame retardancy of PP nanocomposites was studied by Song et al with an aim to supports the concept that the silicates and phosphorus combinedly add some flame retardancy in polymers [85]. Because of the non-polarity of PP lattice, the creation of nanocomposites of PP/clay consistently needs to use a compatibilizer, for example, PP-g-MA which offers polar groups can be intercalated into silicates producing affinity between the filler and the matrix. In polymer matrix, the dispersion of clay can be additionally enhanced by high amount of shearing, for example by using a co-intercalating monomer having large steric volume. Considering this, the fundamental thought of the authors was to utilise a phosphorouscontaining monomer, with the goal to achieve high flame retardancy accomplished by the improvement in clay dispersion. To confirm the validity of this theory, N-imidazol-O-(bicyclo pentaerythritol phosphate)O-(ethyl methacrylate) phosphate (PEBI) (Figure 9) was utilised and its impact on combustion performance and thermal stability of melt compounded PP nanocomposites was assessed. As expected, the interlayer spacing of silicates was increased due to addition of bulky PEBI and therefore encouraged the intercalation of polymer chains into galleries, upgrading the thermal properties of PP matrix. Enhanced dispersion alongside PEBI using as an internal plasticisation agent, the glass transition temperature (Tg) was shifted to lower values because of DMA. The cone calorimetry (Table 2) displays that the flame retardancy of prepared nanocomposites was increased by phosphorous component of PEBI and it further enhances all the fire properties of PP; even the average SEA (specific extinction area) reduced, reducing a significant smoke suppression. Table 2. Fire properties of PP nanocomposite containing PEBI (reproduced from Ref [85].). Sample 1 2 3 4 PP (wt %) 100 97 97 82 OMMT (wt %) 3 - PEBI-OMMT (wt %) 3 3 PP-g-MA (wt %) 15 TTI (s) 30 30 32 32 pHRR (kW/ m2) 390 185 170 161 Average pHRR (kW/ m2) 205 104 92 85 Time to pHRR(s) 85 90 94 96 MLR (g/ s) 0.035 0.029 0.028 0.025 Average SEA (m2/ kg) 555 440 400 380 MATERIALS RESEARCH INNOVATIONS 2.2.1.2. Polyamide/layered silicate-based nanocomposites. Red phosphorus (known part RP) is a well-of this group of flame retardants. Since the revelation in year 1965 about its to capacity protect polymers against fire, RP has achieved lot of popularity and its interest increased because of its advantages like thermal stability, low concentration efficiency and low impact on polymer’s mechanical performance [86]. In the work by Hao et al. [87] RP and an organically modified MMT were extruded with polyamide 66 (PA 66) to make flame-retardant nanocomposites. Two formulations were created containing 5 wt% clay and 10 or 15 wt% RP. When compared with virgin PA66 it was found that the LOI of first formulation increased from 25.5 to 30.4 vol. %, while no impact was observed in mechanical properties. The increase in the LOI of 5 wt% clay – 15 wt% RP – PA66 was less intense (8%); however, UL94/V-0 was accomplished, recommending the likelihood of synergism amongst RP and clay particles at certain loading levels. Other than the known activity of MMT (promotes the formation of superficial coating), RP served for flame retardancy playing its settled double role: (1) RP decomposes to form volatile white phosphorus during combustion, migrates to the surface, and then it is oxidised to derivatives of phosphoric acid which act as char-forming agents, forming a coat that protect the substrate along with silicates. (2) RP occurring on polymer’s surface ingests ambient oxygen generating PO• species that trap-free radicals like H• and HO• responsible for sustaining combustion. Here, silicates were additionally recognised to synergise well with phosphate flame retardant (PFR) for diminishing the ignitability of Polyamide 6. Characteristically, the LOI of Polyamide 6 with 3 wt % clay was 20.2 vol. %, that of the PA6 with 5 wt% PFR was 22.5 vol. % while the LOI of the correlated flame-retardant nanocomposite was 24.3 vol. %. 11 2.2.1.3. Polystyrene/layered silicate-based nanocomposites. The types of phosphorus compounds utilised as flame retardants are extremely wide since this component exists in many oxidation states. Especially, the phosphorus compounds based on organic substances have for quite some time been utilised to supply styrenic polymers along with fire resistance [86]. The case of polystyrene (PS) nanocomposites was handled by Chigwada and Wilkie [88] who created various phosphorous-containing PS/clay nanocomposites by bulk polymerisation process. More than 30 diverse phosphorous-based substances were utilised and assessed as potential effective flame retardants by means of a high throughput procedure created by the authors. In short, this strategy included screening for flammability resistance small amount (1.5 g) of the samples, after exposure (1 min) to a methane flame. Samples that did not burn by any means were viewed as promising and in this manner, its flame retardancy was explored more altogether. Every formulation included 15% wt of flame retardant and 3% wt of commercial clay filler (Cloisite 10A, Southern Clay Products, USA). Out of all specimens, lack of ignition was observed just for those containing tricresylphosphate (TCP), trixylylphosphate (TXP) or resorcinoldiphosphate (RDP); the effectiveness of these compounds was then inspected through vast scale tests, including UL94 and cone calorimetry. In cone calorimeter, the flame-retarded nanocomposites indicated critical decrease in the PHRR also, THR, relative to the concentration of the added phosphate (Table 3). Regardless of the flame-retardant type, the amount of reduction in PHRR and THR was beyond that achieved by incorporating each component separately. Then again, the TTI corresponding well with the TGA information varied and as reflected by SEA, the smoke creation in all cases increased even by 100%. To accomplish a V-0 classification in the UL 94 test, 10 wt% of clay alongside 30 wt% of phosphate were required, negatively yet influenced the elongation of the polymer; there was Table 3. Fire properties of PP nanocomposite containing PEBI (reproduced from Ref [85].). Sample PS PS+3%Clay 15%TCP+PS 15%TCP+3%Clay 30%TCP+3%Clay 30%TCP+5%Clay 30%TCP+10%Clay 5%TCP+3%Clay 10%TCP+3%Clay 10%TCP+5%Clay RDP5%+3%Clay RDP5%+5%Clay RDP15%+3%Clay RDP30%+3%Clay RDP15%+PS RDP15%+3%Clay RDP15%+5%Clay RDP15%+10%Clay RDP30%+PS RDP30%+3%Clay RDP30%+5%Clay RDP30%+10%Clay 15%TXP+PS 15%TXP+3%Clay 15%TXP+5%Clay 15%TXP+10%Clay 30%TXP+PS 30%TXP+5%Clay 30%TXP+10%Clay TTI (s) 62 57 59 59 43 53 55 60 49 48 67 59 68 75 63 68 74 73 77 75 55 63 64 69 58 61 57 38 59 pHRR (kW/m2) (% reduction) 1419 610 (56) 1122 (20) 495 (65) 378 (74) 342 (76) 324 (79) 704 (50) 485 (65) 508 (64) 502 (64) 458 (67) 474 (66) 358 (74) 710 (49) 474 (66) 433 (69) 424 (70) 499 (64) 358 (74) 110 (92) 307 (78) 890 (36) 390 (72) 449 (68) 475 (66) 864 (38) 313 (78) 372 (73) THR (MJ/m2) 109.7 85.5 63.4 59.1 49.5 45.8 47.3 75.3 62.4 70.7 69.8 79.1 58.3 42.3 56.8 58.3 57.5 60.1 41.0 42.3 43.1 44.7 58.5 62.4 59.4 63.2 53.9 45.5 49.4 MLR (g/sm2) 17 14 14 14 14 14 14 15 15 14 14 14 14 14 15 14 14 14 14 14 14 14 14 12 13 13 15 13 13 Average SEA (m2/kg) 1097 1695 1560 1803 2401 2310 2285 1560 2159 1660 2057 2641 1995 2157 1551 1995 2391 1905 1852 2157 2322 1892 1443 1763 1882 1700 2122 2287 2028 12 R. GIRI ET AL. Figure 10. Chemical structure of polystyrene-co-vinyl phenyl phosphatecovinyl phenyl ammonium salt (R: phosphate moiety) (reproduced from Ref [89].). no change in elongation just when the substance of phosphate was 15 wt% at maximum. Proceeding with the study on PS nanocomposites, Zheng and Wilkie [89] endeavoured to check the interaction between phosphates and clay fillers by adding the phosphate in the organic treatment of the clay filler instead of incorporating it into the polymer matrix directly. In that regard, pristine sodium MMT was ion exchanged with an ammonium salt which contained an oligomeric material constituting of styrene, vinyl benzyl chloride and vinyl phosphate responding with dimethy l hexadecylamine (Figure 10). A while later, the polymer was blended with the modified clay by either melt mixing or solution blending to plan PS nanocomposites, which were found to introduce intercalated morphology and higher heat stability. Before continuing to cone-calorimetry experiments, thermogravimetric analysis combined with Fourier transform infrared spectroscopy (TGA/FTIR) was applied on the materials, as a device to pick up bits of knowledge in the flame-retardant activity. It was demonstrated that when the thermal degradation occurred, the phosphate species were freed from the clay surfactant and responded with the degrading polymer to scavenge radical species in the gas phase. Besides, the cone calorimetry information clearly showed that the amount and impact of the imparted fire resistance depended exceptionally on the constituents of the phosphate, and on preparation technique, announcing best approach to be the melt-blending technique. The synergy between the layered silicates and the phosphate was again significantly apparent, looking at that as a reduction of 70% or even 80% in PHRR was accomplished differentiated to 60% which is generally obtained when a typical clay (without adding phosphate) is embedded in a PS matrix. Nevertheless, the increased levels of smoke evolved and the loss of mechanical integrity caused by the huge quantity of phosphate (acts as a plasticiser decreasing melting/softening point) were the other symptoms to deal with. 2.2.1.4. Acrylonitrile–butadiene–styrene/layered silicatebased nanocomposites. The process which was applied for imparting flame retardancy in PS/clay nanocomposite, Kim et al. [90] intercalated triphenylphosphate (TPP) in the displays of a business naturally adjusted MMT (Cloisite 30B, Southern Clay Products, USA) and melt mixed the clay and ABS to subsequent plan nanocomposites. This process has an advantage that by protecting the typically volatile TPP present in the silicates, can suppress its evaporation during melt compounding, taking into consideration more proficient flame retardancy and a larger scope of processing conditions. However, if this approach enabled nanocomposite preparation was not certified in the article as no characterisation of the materials morphology was reported. The incorporation of clay increased the thermal stability of ABS because of the late release of TPP but the LOI remained practically unaltered. A huge enhancement in LOI was accomplished when an epoxy-novolac system was considered. Also, an improvement in thermal properties was considered in samples consisting of silane agents, which also favoured coupling between epoxy and silicates. It is worth seeing that the LOI of 85/9/6 wt% ABS/epoxy/(clay – TPP + silane) formulation raised to 41.2 vol.% while the corresponding value for the sample of 15 wt% TPP was 20.2 vol.%, only 9% over that of the virgin polymer (18.2 vol.%). Optical micrographs of the combustion residue uncovered that the change in flammability was related to the development of a more coherent char, lacking of gaps and crevices (Figure 11). The synergistic impact of this system was likewise affirmed when rather than TPP, a tetra-2, 6-dimethylphenyl resorcinol diphosphate (DMP – RDP) was utilised. 2.2.2. Nanocomposites with nitrogen-based compounds Because of the increasing requirement for flame retardants that are halogen free, the contribution of the overall industry of compounds based on nitrogen, chiefly melamine and its subsidiaries, has experienced a critical development over the earlier time. This is mainly due to its ability because of which it can operate in all stages of burning process via different methods like endothermic reactions, free-radical scavenging, dilution of inert gas and the fact that materials containing flame retardants based on nitrogen can be recycled easily [67 69,]. Specifically, melamine cyanurate (MC), a salt of cyanuric acid and melamine, includes the most vital agent, broadly utilised as a part of the nylon industry since the mid-1980s. Over 320°C, MC separates to its parts by interfering with the combustion cycle as a heat sink and an inert gas source. The produced melamine sublimes by absorbing the heat generated to inert gasses which dilute the oxygen and combustible volatiles. Then, again evolve melamine and cyanuric acid or the depolymerisation catalysed by ammonia, upgrading melt flow, in this manner making the polymer drip away from the flame source [90–92]. 2.2.2.1. Polyamide/layered silicate-based nanocomposites. Hu et al. made the first attempt to investigate the influence of MC on nanocomposites [76,93]. He prepared flame retarded PA6 nanocomposites (of intercalated/exfoliated structure) and characterised the IR combustion properties by cone calorimetry experiments and UL94 tests. At first, with a specific end goal to reveal the dominating mechanism of MC for flame hindering polyamides without silicates, 15 wt% MC were added to the polymer. The PHRR of PA6/MC reduced just by 95kW/m2 in its MLR curve and was practically similar to that of virgin polymer, showing the condense phase fire retarded activity of MC. Then again, the incorporation of wt% MMT in a 15 wt% MC-containing sample prompted a reduction in PHRR from 925 to 515kW/m2, because of the carbonaceous char formed on the nanocomposite’s surface, which limits the degradation of polymer (Figure 12). Concerning UL94 test, a clear antagonism was detected between the flame retardant and the clay filler, MATERIALS RESEARCH INNOVATIONS 13 Figure 11. Optical micrographs of the chars formed after LOI test: (a) ABS/Clay 99/1; (b) ABS/NanoTPP 85/15; (c) ABS/(NanoTPP/Silane) 85/15; (d) ABS/Epoxy/(Nano TPP/Silane) 85/9/6 (reproduced from Ref [90].). Figure 12. HRR vs. time of the PA6 and PA6 nanocomposite containing MC (heat flux = 50 kW/m2) (reproduced from Ref [76].). bringing about materials that could not qualify the V-0 order proved unable to be accomplished when nanocomposite was incorporated with 25 wt% MC. This behaviour originated from the melt viscosity of nanocomposites which was higher than that compared with unfilled polymers. The dripping effect of MC was also nullified because of it. The barrier properties play a small role in UL94 tests, while alterations in the dripping characteristic becoming fundamental factor, this property was endeavoured by Kiliaris et al. [93] and he tried to improve the dripping capacity of PA6 nanocomposites, studying the efficiency of MC in increasing the melt flow of polyamides due to polymerisation. Blends of varying composition 1–5 wt% (Nanofil 9, Süd-Chemie, Germany) and 2.5–5 wt% MC were extruded and studied like flammability and other vital engineering properties, for example, thermal properties, mechanical properties and molecular weight. Nanocomposites of blended intercalated/ exfoliated morphology were synthesised, with reduced fraction of exfoliation/intercalation in the presence of MC. It was observed that while extruding the polymer, MC promoted 14 R. GIRI ET AL. the development of PA6 macromolecules of increased size and complexities, excessively bulky groups, making it impossible to diffuse into the galleries of clay. Besides, it was observed that though clay particles made a confined environment for the crystallisation of PA6, MC expanded crystalline assuming a heterogeneous nucleation part. Mechanical tests show that both the additives emphatically affected tensile modulus but the ductility reduced. Regarding flammability, as found out by UL94 test, it was proved that MC was not capable at the concentrations examined of increasing the tendency of nanocomposites to flow; the specimens were completely burned. Another study on the flame retardancy of PA6 nanocomposites containing MC was done by Zhang et al. [94]. Formulations with OMMT (Cloisite 25A, Southern Clay Products, USA) or pristine MMT were inspected, while he also considered the effect of incorporation of Polyvinyl Pyrrolidone (PVP) (Table 4). The highest UL94 (V-0) rating and LOI were gotten with 13 wt% MC and kept up to 0.2 wt% clay content, over which the viscosity increased bringing about bigger and heavier drops, contrarily influenced flammability. Because of the incorporation of PVP (5 wt %), a partial restoration of UL94 rating held, as there was decrease in viscosity of PVP nanocomposite melt by limiting the miscibility of polymer–clay composite. Essentially, the not well-dispersed pristine clay, connecting with polymer macromolecules to lower degree did not bring fundamental changes in the dripping characteristics of the flame retarded polymer, safeguarding the UL94 classification. Regarding cone calorimetry, a gradual reduction in PHRR was observed when OMMT content was increased and the similar method was used for TTI also. The MLR curves which were practically indistinguishable to HRR blends showed that the activity of MC does not take place in the gas phase, but rather is expected to the weakening of the network and of the flame by ammonia and sublimed melamine. The advancement of melamine was ceased on the account of nanocomposite’s viscous melts, prompting a progressive increment of the effective heat o combustion (EHC), when the clay was fused with MC formulations. The previously mentioned properties appeared not to be fundamentally influenced when PVP was present, while on account of pristine clay, the qualities acquired were like those of PA 6–13 wt% MC. Tensile tests performed on the flame retarded nanocomposites showed decrease in elongation and tensile strength along with a slight increase in modulus when compared with the neat polymer. NR, not rated; BC, burns to clamp aPristine clay The above findings tell that the ignorable conflict between the effect of MC (decrease in melt viscosity) and that of silicates (increase of melt viscosity) is witnessed only when the organically modified clay is added at small concentrations. Additional proof of this statement was brought by Hao et al. [87], who concentrated on PA66. They blended Polyamide 66 with 10 wt% MC and 7 wt% clay in a twinscrew extruder and compared the resultant material and the virgin polymer on the premise of ignitability. The LOI of flame-retardant nanocomposite as indicated by LOI test was 22.8 vol. %, recognisably lower than the virgin PA66 (25.5 vol. %). 2.2.2.2. Polyester/layered silicate-based nanocomposites. The blend of an organophillic clay and MC was likewise researched by Gianelli et al. [95] on account of poly (butylene terephthalate) (PBT) and a commercial copolyester elastomer with the trade name PIBIFLEX E5601 (P-Group, Ferrara, Italy). The clay was first ion exchanged with dimethyl-hydrogenated tallow-benzyl-ammonium chloride followed by melt compounding with the matrix of polymer using a twin-screw extruder under the conditions which were demonstrated very efficient for creating basically intercalated nanocomposites, as dictated by TEM and XRD characterisations. The fire performance was analysed by method of cone calorimetry for samples containing 5 wt% clay and 10 wt% of the flame retardant, showing a critical decrease in PHRR of both PBT (69%) and co-polyester elastomer (71%). Reduction was additionally studied for the THR, around 15% and 11%, individually. Moving on to the ignition resistance, the TTI of PBT increased from 38 to 64 s, when the additives were present, though it was unaltered on account of the co-polyester elastomer. A rise in TTI was followed (from 44 s to 55 s) when 7.5 wt% clay and 20 wt% melamine cyanurate were included, while the various properties of the co-polyester elastomer were not really influenced (Table 4). When compared with the samples including just one of the two additives, the properties of quaternary formulations were enhanced to the best, thus indicating a synergistic action of silicates and MC in the course of the cone calorimeter. Nevertheless, flammability tests were not performed in this examination to evaluate whether the hostility between these two parts confirms on a case of polyamide matrices (as depicted in Section 2.2.2.1), and is likewise met in polyesters. 2.2.3. Nanocomposites with metal hydroxides Metal hydroxides contain the most important segment in the market of flame retardants. They are inorganic compounds which serve to release heat by discharging extensive amounts Table 4. Fire properties of PA6 nanocomposite containing MC (reproduced from Ref [94].). Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 MC (wt%) 13 13 13 13 13 13 13 13 13 13 13 - OMMT (wt%) 0.2 0.5 1 3 5 3 5 0.2a 0.5a 3a 5 PVP (wt%) 5 5 - UL94 rating Failed V-0 V-0 V-2 V-2 Failed Failed V-2 V-2 V-0 V-2 V-2 Failed LOI (vol.%) 25.3 32.8 29.2 21.0 21.2 21.9 22.6 21.7 21.5 32.0 31.6 30.2 22.3 TTI (s) 55 57 45 42 39 27 55 36 pHRR (kW/m2) 1629 1543 1171 905 706 676 1450 834 Average MLR (g/s) 0.11 0.094 0.095 0.089 0.074 0.083 0.097 0.077 EHC (MJ/kg) 30.2 25.3 27.2 28.0 28.9 26.7 26.2 31.2 MATERIALS RESEARCH INNOVATIONS of water at a similar temperature go or beneath polymer’s disintegration temperature. The produced water vapours can also dilute combustible gasses and may incite the build-up of an oxygen displacing protective layer. Another contribution of metal hydroxides to flame retardancy is smoke suppression. The correct mechanism has not been yet illustrated, however, the most possible clarification is by all accounts that carbon, resulting from polymers’ degradation, is deposited on the oxide (produced by the decomposition of metal hydroxide) and after that, this is volatilised as carbon dioxide, discharging no smoke [68,69,96]. 2.2.4. Nanocomposites with miscellaneous compounds Other than the previously mentioned cases, distinctive sorts of flame retardants have been moreover joined in PLSN. Among them, silicon-containing flame retardants have been accounted to synergise well with clay fillers. For quite a while, silicon compounds were known to be proficient coadditives in flame-retardant systems. Recent advances, however, have shifted a great deal of attention to their use (especially polysiloxanes) for preparation of highperformance protective coatings. Coatings which are based on inorganic siloxane or organic–inorganic siloxane hybrids are inherently resistant to temperature and oxidation. They also provide protection to polymers exposed to fire [97]. Few years ago Quede et al. [98] demonstrated that the utilisation of organosilicon coating can also offer a gainful path for additionally enhancing the flame retardancy of PA6 nanocomposites. The dual advantage of this approach is that the mechanical and physical properties of materials remain unchanged without employing an additive-type flame retardant and secondly that the protection is focused on the part of the material most affected by fire, i.e. the surface. The coating of organosilicon was a homogeneous film created by the polymerisation of 1, 1, 3, 3-tetramethyldisiloxane monomer premixed with oxygen, applying a cool remote nitrogen plasma process. Expanding the film thickness (from 0.6 to 18.1 m) reduced nanocomposites flammability, as reflected by the LOI values; the most maximum value (48 vol. %, increase of 130%) was gotten for the coating of 1.5 m thickness. Despite what might be expected, the coating, regardless of the thickness, the LOI of the neat polymer was barely influenced, which remained practically unaltered at 22 vol. %. The fire properties of coated nanocomposites, measured in a cone calorimeter, did not show spectacular enhancement but the synergic activity between the film and silicates was still clear. The deposit of the coating on the surface of nanocomposite brought an extra reduction of 11% in PHRR, 5% in THR and 10% in smoke creation compared to that offered by the incorporation of 2 wt% clay in the PA6 network (34%, 63%, and 32%, separately). By analysing the residue, it was concluded that amid the burning of coated nanocomposites, a carbonaceous-silicate and silica like layer was created, the volatilisation of active fragments was reduced by it and absorbed and dissipated heat more effectively than the char of the uncoated nanocomposite or the polysiloxane coating alone (covered PA6). Huo et al. [99] explored the likelihood of a silicon-based coating to present synergism with layered silicates in the flame retardancy of poly (ethylene terephthalate) (PET). Rather than depositing on the surface of polymer, a highly crosslinked PBSiO containing phenyl was synthesised to coat a commercial clay 15 (Nanomer I.34TCN, Nanocor, USA), which was subsequently melt blended with PET. Over conventional polysiloxanes, PBSiO presents the merit of superior performance at high temperatures and oxidising conditions. The decay of PBSiO coatings leads to the development of borosilicate glassy structures, which keep the substrate away from degradation. In combination with the organoclay, PBSiO was found to build up a defensive borosilicate-carbonaceous char on the surface of PET, improving significantly its performance towards fire. The addition of 5 wt% PBSiO and 2.5 wt% OMMT was adequate to get a reduction of 59% in pHRR and of 51% in smoke yield. The cone calorimetric information of samples containing uncoated clay unfortunately is not detailed in the paper, to completely welcome the contribution of the two components. However, the higher decomposition temperatures recorded for PET/PBSiO/ OMMT formulations with respect to PET/OMMT give a sign of synergistic activity between PBSiO and clay particles. Rather than coating the silicates, Zhu et al. [100] prepared silicon methoxide-altered clay by means of treating pristine MMT with 3[-(trimethoxysilyl) propyl] octadecyldimethyl-ammonium chloride. The clay was utilised to blend (intercalated) PS nanocomposites of enhanced fire retardancy and thermal stability with respect to the virgin polymer. But the improvement was near to that of PS by regular alkyl ammonium modified MMTs. Introducing layered silicates flame-retardant elements is well known to add brittleness in the polymer matrix, therefore restricting the use of the resulting material in applications where the elongation ability is essential. With an aim to overcome the above drawback, Dong et al. [101] prepared a novel compound, namely the silicone elastomeric nanoparticles (S-ENP) and used it to incorporate in PA6 for heat resistance, good processability, high toughness and stiffness. Other novel elastomeric flame retardant (S-ENPC) was synthesised by spray drying S-ENP latex mixed with the slurry of an unmodified MMT. Meltblending S-ENPC with PA6 proved that S-ENP and silicates co-operate to further improve the flame retardancy and mechanical properties; compared with PA6/S-ENP, the elongation at break increased by 30%, the PHRR of PA6/S-ENPC decreased by 8%, while and the flexural modulus by 19%. The values of mean effective heat combustion, mean CO yield and mean CO2 yield were almost same for the neat polymer and the composites, suggesting that the improvement in flammability was due to changes in the condensed phase decomposition process and not the gas-phase. A barrier of two layers made of silica and silica/clay, respectively, was created on the surface of PA6/S-ENPC, which provides superior performance towards fire than the single layer formed on PA6/S-ENP. By applying the same procedure, Wang et al. [102] fabricated poly (vinyl chloride) (PVC) nanocomposites containing sodium MMT and nitrile rubber nanoscale particles. Contrary to the previous case, nanoparticles significantly deteriorated the flame retardancy of the polymer matrix, acting as defective sites which loosened the compact char formed superficially on PVC (typical charforming polymer) during combustion. Ammonium sulfamate (NH2SO3NH4, symbolised as AS), which belongs to the family of sulphur-containing compounds, joint with Di comprise another flame-retardant system whose ability to exert synergistic effect with clay particles was examined by Lewin et al. [103]. The system acts through the development 16 R. GIRI ET AL. of a three-dimensional network in the condensed phase that carbonises upon ignition and generates a carbonaceous char by migrating to the polymer surface. Referring to their publication on PA6 with MC [94], the experiments were performed by the authors in which the PA6-AS + Di samples were treated with pristine MMT or the organically modified MMT (Cloisite 25A, Southern Clay Products, USA). The introduction of 2.0 wt% AS and 0.7 wt% Di in PA6 matrix yielded materials of 35.7 vol. % LOI and of UL-94 V-0 classification that preserved with the co-addition of 1wt% OMMT. Increasing clay content reduced the LOI values and UL-94 ratings, because of the catalytic effect of clay on matrix degradation and the partial absorption of AS + Di in the clay galleries that prevented their contribution to flame retardancy. Adding 5.0 wt% PVP moderately retained the original values of LOI as it seemed to enter the interlayer replacing AS + Di and thus neutralising the catalysis. In comparison with the perfect matrix, the TTI of PA6/AS + Di were higher, moving downwards within the sight of OMMT. Then again, the PHRR was increased with AS + Di addition, but reduced to a value below that of the original PA6 when 3 wt% OMMT was added. The hostility did not appear to exist when pristine MMT was utilised (ideal at 3.0 wt %). The poor dispersion of pristine clay, barely influencing mechanical properties, prevented extended associations with the flame retardant and the polymer for the advantage of fire resistance; a uniform char without floccules was formed upon combustion leading to decrease of 30% in PHRR values, a slight increment of TTI, a LOI of 35.4 vol. % and a V-0 classification. 2.2.5. Nanocomposites with new types of nanoparticles Without a doubt, the usage of layered silicates represents right now the most utilised course of nanotechnology to improve polymers with flame retardancy. This study mainly focuses on natural aluminosilicates; thus only a portion of the known structures of these nanofillers with different sorts of nanoparticles is examined in this segment. In the previous 10 years, rich writing work has been composed and committed on the utilisation of carbon nanotubes (CNTs) which can be utilised as superior nanofillers. CNTs comprises of a novel group of carbon materials which are developed of carbon molecules orchestrated in pentagons and hexagons (graphite structure) framing chambers. Three primary strategies are normally utilised for their creation, including circular segment discharge, chemical vapour testimony and laser removal, albeit most extreme research movement is spent on more economic techniques. Amid amalgamation, contaminations as formless carbon, nontubular structures and impetus particles are likewise acquired; in this manner the sanitisation of CNTs is required and performed more often than not through oxidation or acid refluxing strategies. Normally, CNTs comprise both of one (single-wall carbon nanotubes, SWCNTs) or progressively (multi-wall carbon nanotubes, MWCNTs) concentric round and hollow shells of graphitic sheets. Due to the nearness of their symmetric structure, these confine like carbon chambers gangs remarkable mechanical, thermal and electric properties. In the handling of polymer nanocomposites, homogeneous dispersion and distribution of nanotubes inside the polymer matrix and upgraded CNTmatrix wetting are basic issues (not effectively tended to) with the goal that proficient fortification can be accomplished [104–106]. The subject of blending clays with CNTs has been considered in many papers, the greatest piece of them created by Beyer. His first production [107] regarding this matter managed the synthesis of flame-retardant nanocomposites by means of melt-blending EVA with MWCNTs or/and a commercially accessible OMMT (Nanofil 15, Süd-Chemie, Germany). MWCNTs were integrated by catalytic decomposition of acetylene on Co–Fe/Al(OH)3 impetus and then by purging by means of disintegration of the impetus bolster in bubbling concentrated sodium hydroxide and disintegration of the impetuses in concentrated hydrochloric acid. The organised nanocomposites were thought about on premise of fire execution utilising cone calorimetric tests under 35kW/m2 heat flux. The acquired results are referred to in (Table 5), certainly illustrates diminish in PHRR by including separately both the nanofillers (2.5 or 5 phr), more articulated for MWCNTs. Extra reduction in the PHRR bend (Figure 13) was considered for the specimen which contained 2.5 phr of MWCNTs and 2.5 phr of clay, giving confirmation for the synergism between the two portions. The two fillers responded in the consolidate phase in the midst of combustion and framing char that was more resistant to cracking by virtue of MWCNTs as a result of their long aspect proportion. Concentrating again on (Figure 13), plainly regardless of the nearness of MWCNT, the TTI diminished along with the nearness of silicates in view of the thermal degradation of the clay surfactant occurring over the traverse of a fire. The thermal protection, flame retardancy and pliable properties of EVA copolymer soften blended with clay particles and MWCNTs was contemplated in a writing of Peeterbroeck et al. [108], in which Beyer was one of the co-creators. The impact of crude and cleansed MWCNTs on the properties of nanocomposites was researched. Thermogravimetric scans with a heating slope of 20 K/ min, under air atmosphere, demonstrated an unmistakable synergistic effect between MWCNTs (FUNDP, Belgium) and Cloisite 30B (Southern Clay Products, USA). Most extreme delay in degradation temperature of polymer matrix was conveyed by the blend of 3 wt% of Cloisite 30B and 1 wt% of refined nanotubes, which moved by 56 ◦C towards higher temperature. Exactly when included freely, neither of the two nanofillers caused such a change, regardless of the substance. Filtered nanotubes were demonstrated to be more useful due to the weakening of MWCNTs with synergist bolster by virtue of the crude nanotubes; MWCNTs were polluted by 30 wt% alumina. Mechanical tests demonstrate that both the nanofillers opened up stiffness with similar effectiveness independent of the substance proportion of clay/MWCNT, while saving the great ductility of the slick polymer. The cone calorimetric data, which were represented in this writing alluded to tests containing Nanofil 15 (Süd-Chemie, Germany) as clay filler; these results were likewise presented and deciphered in the past papers of Beyer [107]. Better understanding about the consolidated impact of MWCNTs Table 5. Fire properties of EVA nanocomposite containing OMMT and/or MWCNTs (reproduced from Ref [107].). Sample 1 2 3 4 5 6 OMMT (phr) 2.5 5 2.5 MWCNTs (phr) 2.5 5 2.5 TTI (s) 84 85 83 70 67 71 pHRR (kW/m2) 580 520 405 530 470 370 MATERIALS RESEARCH INNOVATIONS 17 Figure 13. HRR vs. time: EVA + 5.0 phr OMMT (A); EVA + 5.0 phr MWCNT (B); EVA + 2.5 phr OMMT+ 2.5 phr MWCNTs (C) (heat flux = 35 kW/m2) (reproduced from Ref [107].). Figure 14. Morphology of chars produced from clay/EVA, clay/MCWNT/EVA and MCWNT/EVA nanocomposites after cone calorimeter test (top row) and the natural burning (bottom row) (reproduced from Ref [109].). and layered silicates on the fire execution of EVA nanocomposites was attempted by Gao et al. in his investigation as shown in Figure 14 [109]. The legitimacy of this speculation was demonstrated by subjecting small plate examples of the three composites to broad oxidation; all the more particularly, the specimens were burnt in a mute furnace at 600°C for 20 min. It was seen in the cone calorimeter that the higher reactivity char of MWCNT-overhauled composite was burnt out while the char shaped on the example doped with clay held its honesty. The set up synergism amongst nanotubes and silicates in upgrading polymer flame retardancy was furthermore avowed by Beyer [110,111] when certifiable things were created. A link creating extruder was utilised to plan two insulating wires having flame retardancy based on ATHcontaining EVA/PE-clay nanocomposites, either with or without MWCNTs. The total filler loading was kept predictable in the two cases; for the generation of the second wire, 50 wt% of the organoclay was substituted by a comparable measure of MWCNTs. Conducting small-scale fire tests, where the insulations were presented to a Bunsen flame (IEC 60,332–1), showed that no dripping and equivalent charred length was watched for the two formulations. Likewise, the char was fortified by the guide of nanotubes. The updated char structure achieved prevalent fire properties, as assessed by cone calorimetric examinations. Concentrating on PHRR, which is believed to be the primary reason for flame, it was found that the 1:1 blend of clay and nanotubes procured diminish in PHRR (more vital than including clay alone) to levels that allow the age of flame retardant connect compounds. As opposed to simultaneously including the nanotubes and layered silicates in the matrix. Tang et al. [112] explored the possibility of enhancing the fire protection of PLSN by means of the in situ advancement of CNTs amid combustion. The viability of this idea was approved by blending PP/clay nanocomposites with a nickel impetus (nickel bolstered on silica – alumina); the combusted residue of the formulation comprised of fibre-like charring particles intermixed with MMT layers. Likewise, 18 R. GIRI ET AL. the measure of the residue was hoisted in correlation with that of the specimens containing either the clay (Cloisite 15A, Southern Clay Products, USA) or the impetus. Clearly, the catalysed carbonisation of the polymer was expanded within the sight of clay, prompting a subjective too quantitative improvement in the development of char. In the limited environment made by layered silicates the decomposition results of the polymer could not escape effortlessly, along these lines they stayed for additional time in contact with the impetus, experiencing dehydrogenation and aromatisation to yield char with CNTs, since nickel is an impetus for the arrangement of CNT. Resulting in the protective layer developed on the surface of an example doped with 5 wt% clay + 5 wt% impetuses which was superior to that of PP with 10 wt% clay as far as PHRR decrease. In any case, no progressions in THR and TTI were watched, inferring that the applicable gas-phase mechanism is missing. A few questions on how the carbon nanotubes and layered silicates synergise to build the flame retardancy of polymers are replied in the truly far-reaching writing of Ma et al. [113] by learning about ABS copolymer. The three parts were melt blended in a mixer at 190°C for 10 min at a screw speed of 60 rpm to make nanocomposites of blended intercalated/ exfoliated morphology. Without a doubt, some MWCNTs were thought to have been embedded in the clay galleries for the advantage of clay dispersion. The fire properties were assessed by cone calorimetry and demonstrated that clay went with MWCNTs diminished the PHRR and the entire combustion procedure of ABS was deferred, more viably than MWCNTs or clay alone. A network structure which firmly affected flame retardancy by constituting a barrier to oxygen, heat and flammable gasses was shaped by presentation of nanoparticles in polymer matrix. Rheological estimations demonstrated that the conjunction of nanotubes and silicates, blocking moreover the development of macromolecules, prompted a more permeated network. The propelled fire protection of ABS/MWCNTs/clay was additionally related with the char shaped on its surface amid combustion, which was thicker and denser, introducing less cracks than that of the surface of ABS/clay or ABS/MWCNTs. TEM pictures uncovered that some MWCNTs kept running crosswise over between silicates, inferring a solid cooperation between them. The microstructure of char was examined utilising XRD examination and Raman spectroscopy; it was discovered that when the clay was embedded in the nanotubes-containing formulation, expanded the level of graphitisation of char henceforth ensured it against thermal oxidation. The level of graphitisation mirrors the change degree of a carbon material from turbostratic (cluttered) to graphitic (idealise) structure. Accordingly, the confined environment induced by silicates provoked the rearrangement of carbon crystallites, eliminating any dislocations and defects. Likewise, Al2O3, which comprises of one of the parts of MMT, is a famous impetus of graphitisation. In any case, the mix of MWCNTs and clay was discovered lacking breeze through the UL94 test for ABS. Metal oxides like titanium oxide (TiO2) and iron oxide (Fe2O3) are another developing group of nanoparticles with a recognised beneficial outcome on the flame retardancy and thermal security of PS [114] and PMMA [115]. Laachachi et al. [116] examined whether metal oxides could likewise contain promising synergists to organoclays, with the goal that materials of higher fire protection could be made. So the Table 6. Fire properties of PMMA nanocomposite containing metal oxides (reproduced from Ref [116]). OMMT Sample (wt%) 1 2 10 3 5 4 5 Fe2 TiO2 O3 (wt (wt TTI %) %) (s) - 69 - 74 5 - 86 5 53 Total burning (s) 250 480 630 590 pHRR THR SEA Char (kW/ (MJ/ (m2/ residue 2 2 m ) m ) kg) (%) 620 110 430 0 320 110 810 6 360 100 530 8 350 100 600 10 creators arranged PMMA – clay nanocomposites melt exacerbated with TiO2 or Fe2O3. TGA bends of the readied tests demonstrated that the incorporation of metal oxides changed the beginning degradation temperature of PMMA – clay nanocomposite to higher esteems. A synergistic impact between layered silicates and oxide nanoparticles on upgrading the fire protection of PMMA was seen through cone calorimetric tests; hoisted TTI, lessened THR and PHRR, diminish in released smoke and impressive ascent in total burning time were apparent at higher degree in the nanocomposites doped with oxides than in the specimens containing just a single of the two parts (Table 6). The addition of oxide nanoparticles improved the covering of the material’s surface when the polymer ablation took place and the decomposition of the organic modifier disorganised the deposition of clay particles on the sample’s exterior. Better results were observed in the case of clay–TiO2 formulations. With the high thermal stability, TiO2 allowed its function as a heat sink, limiting the thermal conduction in polymer bulk. Moreover, the increase in melt viscosity of the polymer hampered the emission of volatiles. Finally, convection forces generated during combustion probably promoted particles migration to the surface of the nanocomposites reinforcing the char layer. 3. Fire retardants based on nanocomposites Recently nanoparticles have attracted many considerations in materials science since they regularly show properties that are quite different from those of their counterpart polymer microcomposites containing the matrices of same inorganic components. The nanofiller’s surface areas are definitely expanded with the goal that polymer nanocomposites appear large scale/small scale/nano-interfaces. Incorporation of CNTs will not only enhance the mechanical properties but along with that the functionalities also, for example, thermal, flammable and electrical properties of composites. CNTs are amongst the most representative nano-materials which are used to give remarkable properties to polymers. Innovation for the production of CNTs on large scale has been produced, reducing the cost of CNTs to ~$100/kg in 2013. Thus, some CNT that are based on nanocomposites have started showing up. For instance, Evonik Industries are creating moulding PA12 CNT-containing compounds for fuel lines [117]. The fundamental advantage of this material includes avoiding the ignition caused by electrostatic charges. Fire hazard can be considerably decreased by producing percolation systems of CNTs in polymers. Adding CNTs to polymers likewise changes their flammability. Nanofiller-based flame retardants indicate high flame-retardant efficiencies. Including just a little sum (i.e., 5%) of nanofiller can diminish the pinnacle warm discharge rates (PHRRs) of polymers and therefore lessen the speed at which flames spread all through MATERIALS RESEARCH INNOVATIONS them. Further, the little measure of nanofiller does not decrease polymer processability and can enhance the mechanical properties of polymers. Be that as it may, including no one but nanofiller cannot deliver self-extinguishing (V-0, −1, and −2) polymers, which are required for most fireretardant items. The nanofillers ought to be joined with other customary flame retardants to give a superior adjust of flammability/mechanical properties. 3.1. Fire-retardant mechanism for clay-based nanocomposites Montmorillonite is the most commonly used clay since it is regularly present, can be obtained at high virtue and negligible cost, and shows particularly rich intercalation chemistry, which implies that it can be easily organically adjusted. The typical clay surface is hydrophilic, so the clay effortlessly disperses in aqueous solutions, however not in polymers. Common clays are routinely adjusted using regular cations, for instance, alkylammonium and alkylphosphonium cations, framing hydrophobic organomodified clays that can be promptly dispersed in polymers. Clay-based nanocomposites are regularly gathered into three classifications since clay properties are exceptional: (1) immiscible (likewise called as microcomposites), (2) intercalated, and (3) exfoliated (additionally called delaminated). Exfoliated nanocomposites are normally wanted in light of the fact that they demonstrate enhanced mechanical properties [118]. Clay-based nanocomposite loaded with 5% clay is starting at now used as a commercial flame retardant by virtue of its upgraded mechanical properties and flame retardancy. The fire-retardancy systems for clay and carbon-based nanocomposites are for all intents and purposes indistinguishable. One fire-retardancy component is the abatement in PHRR on account of the advancement of a defensive surface barrier/insulation layer comprising of clay platelets collected with a little measure of carbonaceous char [119,120]. The clay platelets gather on the surface that the clay staying at first glance from polymer decay and clay migration was pushed by different rising bubbles of degradation things. 19 The surface quality appears to choose the flame-retardant proficiency. Another instrument proposed by Wilkie et al. is that the paramagnetic iron in the matrix traps radicals and redesigns thermal solidness. Actually, including only 0.1 wt% iron containing clay diminished the polystyrene (PS) PHRR by 60% [121]. This impact was not watched for carbon-based nanocomposites and on the grounds that their contact with the polymer is negligible [122]. (Figure 15) indicates HRR blends for polymer and clay-based nanocomposites [119]. Including clay can diminish the PHRR and generally lessens the ignition time (IT); nonetheless, it cannot change the total heat release rate (THRR). Carbon-based nanocomposites exhibit comparative results in light of the fact that the fire-retardancy components for clay-and carbon-based nanocomposites; i.e., the barrier/insulation impacts, are comparable. Subsequently, the key factor in choosing clayand carbon-based-nanocomposite flame retardancy is the advancement of a surface system layer. The barrier/insulation impact relies upon the external heat flux. Schartel et al. investigated the relations among HRR and external heat flux, as showed up in (Figure 16) [123]. THRR does not depend upon external heat flux for the PP and nanocomposites. The PP PHRR, on the other hand, increments with expanding external heat flux, regardless of the way that the nanocomposite PHRR does not change. The fire-retardant proficiency emphatically depends upon irradiance with the ultimate objective that the nanocomposite fire retardancy decreases with diminishing irradiance. The results gained through extrapolation to small irradiances identify with flammability circumstances, for instance, LOI and the UL 94 test. These results clear up why including nanofillers cannot improve LOI and UL characterisation. In addition, they suggest that including nanofiller is successful for polymers showing high HRRs. The flame-retardancy adequacy of clay-based nanocomposites depends upon the sort of matrix [20,62,67,120– 133]. The IT of polymers, for instance, PP, polyethylene (PE), PS, ethylene-vinyl acidic acid determination (EVA), and PMMA ordinarily lessens, when nanofiller is included, in view of the way that the clay itself is possibly catalytic. Strikingly, the IT of the PA6 nanocomposite expanded Figure 15. HRR plots for pure PP, PP/C18, PP/Na-MMT, PP/H-MMT and PP/OMMT (reproduced from Ref [119]. 20 R. GIRI ET AL. Figure 16. PHRR and THR plotted against the external eat flux for PP (PP-g-MA-I) and nanocomposite (PPC20A-I) (reproduced from Ref [123].). when the nanoclay was included. Besides, rate at which PHRR diminishes depends whereupon polymer matrix is used. PA6 and PS both lessening PHRR ~40–75% [6, 127,128]. PMMA, of course, just decreases PHRR 10–30% [20,131,132]. Wilkie et al. exhibited that polymer nanocomposite, for instance, PA6 and PS, which generally diminish PHRR, indicate essential intermolecular reactions and that the degradation pathway changes by consolidating nanoclay; however, the PMMA did not show any adjustment in the polymer degradation pathway or any significant HRR diminish [133]. Despite the way that adding the nanoclay to the PMMA does not basically impact PMMA flammability, adding nanosilica to high-consistency PMMA lessens PHRR by 50% in light of the fact that silica covers the entire surface [134]. Including nanoclay into PMMA is possibly convincing when the nanoclay covers the entire PMMA surface. The nanoclay was less viable in improving PMMA fire retardancy may be because of the fact that the low thickness of the PMMA kept the nanoclay from covering the entire PMMA surface. Adjusting the clay surfaces is the most key parameter for upgrading the fire retardancy of clay-based nanocomposite. Microcomposites are procured instead of nanocomposite when unmodified clays are fused to polymers. The flammability of the microcomposites is for the most part essentially like or now and again more loathsome than those of the neat polymers. Organomodifying clays convey intercalated or exfoliated nanocomposites. Additionally, the char substance and cone-calorimetry experiments of organomodified nanocomposites depend upon the organomodifier content: expanded organomodifier content prompts a more articulated catalytic impact and more escalated char development [124]. Figure 15 shows the effect of surface adjustment on the HRR conduct of PP. Consolidating organomodified montmorillonite (OMMT) diminished the PHRR the most inferable from the nano-dispersed clay and the catalytic properties of the organomodifier. The second most imperative factor in upgrading nanocomposite fire retardancy is clay loading. Not at all like CNT loading, expanding clay loading enhances nanocomposite fire retardancy and there is no perfect clay loading in the range 15 wt% [67,128]. It is difficult to frame a crack-free clay-network layer. In this manner, the major flame-retardancy mechanism is through the arrangement of barrier against the heat source as opposed to gasses. Photos of deposites procured from degraded PS/OMMT tests containing distinctive OMMT substance are shown in Figure 17. The residues from the degraded PS/OMMT tests containing 6-and 15-wt% OMMT demonstrate cracks. Thicker floccules can be gained by including more clay. The advancement of thick floccules can basically lessen HRR. Clay-based nanocomposite flame Figure 17. Digital photos showing the residue morphology of different PS/OMMT composites after degraded at 400 ºC for 3 h (reproduced from Ref [128].). MATERIALS RESEARCH INNOVATIONS retardancy could be additionally improved if polymer-clay nanocomposites could be tuned to shape steadier crack-free networks in the midst of consuming. The effect of nano morphology on flame retardancy has just been analysed in the writing [127,135,136]. Most scientists have finished up that polymer/clay nanocomposites ought to in any event show PHRR decrease if nanomorphology is accomplished through exfoliation and intercalation. The distinction in nanomorphologies does not altogether influence polymer/clay nanocomposite flame retardancy. Nanocomposites can be obtained by organomodifying clays, and is easily achieved through melt-compounding. However, organomodified clay surfaces degrade at high temperatures, rendering organomodification problematic in melt-compounding and decreasing nanocomposite flame retardancy [62]. 3.2. Fire-retardancy mechanism of carbon-based nanocomposites Nanocomposites can be classified into three classes according to the quantity of dimensions of the nanofillers (100 nm) scattered in polymers: (1) lamellar, (2) nanotubular and (3) spherical polymer nanocomposites. Carbon-based nanomaterials demonstrating such morphologies are in this way named graphene, carbon nanotubes (CNTs), and carbon black (CB), separately. Graphene is the completely exfoliated structure of graphite (single layer). The technique for delivering graphene was built up as of late, so graphene has pulled in critical research intrigue [137–141]. CNTs are mostly used as fillers to improve the mechanical, electrical, and flame retardancy properties of nanocomposites. The flame retardancy mechanism of CNTs was shown by Kashiwagi et al. [122,142,143,]. Since CNTs are the well-established material, we present mainly their fireretardancy mechanism and some of their disadvantages here. Figure 18 demonstrates the cone-calorimetry results for single-wall nanotube and poly (methyl methacrylate) (PMMA) composites. Ignition time (IT), PHRR and THRR 21 are the essential parameters in cone calorimetry to characterise material flammability. PHRR is the most indispensable parameter used to delineate flammability and is normal as the fundamental source of the fire. Including CNTs can diminish the PHRR; that is, the combustion heat power. In any case, it cannot change the THRR on the grounds that CNTs do not act in the vapour phase, inferring that the measure of fuel gas required for combustion is not changed by including CNTs. CNTs accelerate flame ignition (i.e., they diminish the IT). Most polymer/CNT composites demonstrate these propensities. Kashiwagi et al. watched the residues (Figure 19) after the cone calorimetry tests. The rate of PHRR decrease was little for the nanocomposite containing 0.2 wt% filler, as showed up in (Figure 18). Many black discrete spots had formed amid the test. Then again, the surfaces of the nanocomposites containing 0.5 wt% CNTs were totally secured with uniform, crack free, opening free CNT network layers, which basically decreased the PHRRs. The key motivation behind nanocomposite fire retardancy is the course of action of a uniform CNT layer. CNTcontaining nanocomposites absorb more radiation than polymers amid fires; hence, nanocomposite temperature increments are faster than polymer ones. The materials reduce as the CNTs absorb a ton of radiation. Polymers begin to burn when they are heated to temperatures at which thermal degradation begins. The degradation things are superheated and nucleated to form bubbles. The bubbles burst at heated surfaces, discharging their substance as fuel vapour into the gas phase. There are a couple of possible mechanisms through which CNTs accumulate at material surfaces: the energy of different rising bubbles in the midst of combustion pushes the CNTs to the material surface or the power of the polymer dying down from the material surface in the midst of pyrolysis, abandoning the CNTs. The fire-retardancy mechanism of the CNT or char layer is depicted in Figure 20. Kashiwagi et al. demonstrated that almost 50% of the occurrence flux was lost through emission from the hot nanotube surface layer and that the rest of the flux was traded to the nanotube-network layer and the virgin Figure 18. Effects of SWNT concentration on mass loss rate of PMMA/AWNT in a nitrogen atmosphere (reproduced from Ref [142].). 22 R. GIRI ET AL. Figure 19. Residues of PMMA/SWNT after the gasification tests in a nitrogen atmosphere a PMMA, b PMMA/SWNT (0.2%), c PMMA/SWNT (0.5%), d PMMA/SWNT (1%) (reproduced from Ref [142]. Figure 20. Fire-retardant mechanism for nanoparticles. example [142]. The nanotube-network layer emits radiation from the material surface and goes about as a barrier against the decomposed gas provided from the bulk polymer and against oxygen diffusing from the air into the material, which accelerates polymer decomposition. The nanotube-network layer must be smooth, crack free, and opening free with the objective that it might go about as a viable gas barrier [144]. Surface-layer cracks deteriorate nanocomposite flame retardancy in the midst of combustion. Rheological properties appear to rule the making of smooth CNT networks or char layers for all carbon-based nanocomposites [141,144,145]. Figure 21 demonstrates rheological properties ordinary of PP/CB nanocomposites [145]. Perfect PP demonstrates regular low-frequency G′−ω scaling, where ω speaks to the oscillatory frequency. Conversely, the low-frequency G′ scaling vanishes, and G′ turns out to be almost steady at low frequency for the nanocomposites containing 5 wt% CB, implying that the nanocomposite changes from a liquid to a solid, which goes with the arrangement of a mechanically stable network structure. It is realised that nanocomposites carrying on like fluids cannot convey smooth CNT network layer on the material surface. The bubbles bursting at the surface bothers the course of action of accumulation layer. Nanocomposites acting like solids contain bubbles that stay little in the high-thickness layer and transport to the material surface, which tends not to bother the plan of the accumulation layer [142]. Reliably dispersed nanocomposites demonstrate rheological properties like those of authentic solids. Thus, the dispersion of carbon-based nanofillers chooses the idea of the surface layer framed in the midst of combustion and in this way impacts the nanocomposite flame retardancy [146]. Picking appropriate CNTs is fundamental. Barus et al. contemplated the properties of three sorts of CNTs and found that the dispersivity of the CNTs themselves impacts the thermal degradation of nanocomposites [147]. The CNT stack is likewise fundamental in choosing fire retardancy, and really, the perfect CNT loading diminishes PHRR, as appeared in (Figure 18). Including more CNTs, once the uniform surface layer has shaped disintegrates flame retardancy since it energises the agglomeration of CNTs and upgrades thermal conductivity. The CNT aspect proportion impacts fire retardancy, and higher aspect ratios prompt more imperative abatements in PHRR [148], demonstrating that a system for compounding thermoplastics to disperse CNTs and to abandon them longer is satisfactory. From a handy perspective, a twin-screw extruder can be utilised to first compound thermoplastics, which will then be injection moulded all together to develop a method for large-scale manufacturing thermoplastic-based items. MATERIALS RESEARCH INNOVATIONS 23 Figure 21. The rheological properties of neat PP and PP/CB nanocomposites (reproduced from Ref [145].). Figure 22. TGA (a) Effects of concentration of MWNT in PP on heat release rate of PP/MWNT nanocomposite at 50 kW/m2. (b) SEM picture of MWNT dispersion in the PP/MWNT (4%) nanocomposite after solvent removal of PP. (c) Collected residues after the gasification experiment at 50 kW/m2 in nitrogen about PP/MWNT (1%). (reproduced from Ref [122].). The relations amongst handling and flame-retardancy viability should be talked about to apply CNTs in commercial items, for example, flame retardants. Actually, dispersing CNTs through a twin-screw extruder is more terrible than the nanocomposite prepared by utilising a closed kneader. Besides, injection moulding can break down material strands. How these influences of nanocomposite flammability should be talked about. Pötschke et al. explored the relations between CNT dispersion and handling conditions [149–151]. They concentrated on the electrical conductivities of nanocomposites and directed extensive investigations. Successful compounding techniques are vital for growing low-CNT-loading fire-retardant nanocomposites. Other carbon-based materials, for example, CB and graphene have additionally as of late been explored as flame retardants. Dittrich et al. demonstrated that graphene was the best carbon-based fire-retardant material [141]. Moreover, Wen et al. discovered new fire-retardancy mechanism for CB [145]. They demonstrated that peroxy radicals, the principle factor affecting the thermal degradation of polypropylene (PP), could be successfully gotten in CB at raised temperatures to outline a gelled-ball crosslinked composition. The PHRR was decreased 75% and the LOI enhanced from 18% to 27.6% by joining CB (to trap the peroxy radicals) and CNT (to make the networked layer) [152]. Surprisingly, adding CB and CNTs reduced the THRR; other nanocomposite systems do not demonstrate this inclination. The new fire-retardancy 24 R. GIRI ET AL. Figure 23. (a) Cone calorimeter experiments, heat release rate versus time curves: comparison of the effect of the 3wt% different nanofillers (Na-MMT, MWNTs and graphene). (b) Morphologies of the final chars after cone calorimeter tests of PVA-Graphene 3wt% composite. (Reproduced from Ref [153].). mechanism for CB can possibly additionally enhance the flame retardancy of carbon-based nanocomposites. Kashiwagi et al. [122] examined the flammability of a CNT polymer nanocomposite material and a carbon dark (CB) polymer composite. CNTs have alternate morphology as compared with nanosilicate or graphene; silicate or silicate clay has two-dimensional plate-type morphology, while CNTs have one-dimensional wire like morphology. However, CNT composites have comparative mechanisms, CNT additionally creates residual form, have CNT structural cone. As shown at (Figure 22) CNT polymer nanocomposite indicates low HRR graph compared with novel polypropylene. One per cent CNT/PP composite demonstrates the least heat release rate; the explanation behind the most minimal PHRR is expected to the balance between the impact of thermal conductivity and the protecting execution of external radiant flux (and heat feedback from the flame) contingent upon the concentration of MWNT in the sample. A nanotube network layer comprising of carbon nanotubes is formed, and it covers the whole sample surface with no noteworthy crack formation amid burning. Graphene has a chemical structure similar to CNTs, composed of sp2 carbon atoms arranged in a honeycomb structure, and the same morphology as nanosilicate clay’s twodimensional sheet. When MMT, CNT, and graphene are compared with the same 3 wt% of nanometric polymer composite Figure 24. (a) Heat release rate versus time chart for unfilled PP and ternary PP nanocomposite (10 wt. % Sep + 2 wt. % MWNT). SEM images of (b) PP + CNT (c) PP + Sep and (d) PP ternary system post-cone calorimeter test. (Reproduced from Ref [154].). MATERIALS RESEARCH INNOVATIONS 25 for flammability using a cone calorimeter, they have different behaviours, as shown in (Figure 23). Compared to pure PVA, the PHRR of PVA-G3 is reduced by 49% and is lower than those of PVA-MMT and PVA-MWNTs. To explain this unexpected fire behaviour, morphologies of the final chars after cone calorimeter tests of the PVA composites were based on graphene nanosheets by using SEM. The SEM images (Figure 23 (b)) for the residues of PVA-G3 after cone calorimeter tests showed that many quadrate carbonaceous particles (1–3μm) joined each other and formed the compact, dense and uniform char. The graphene can promote the formation of compact char layers in condensed phase during combustion of the polymer matrix. Furthermore, the char structure effectively prevents the inside thermal decomposition products into the flame zone and that of the O2 into the underlying polymer matrix [153]. These nanometric composites indicated great flammability compared with novel polymer resin. Also, they can have a synergetic impact on flammability, when they are utilised together in a polymer matrix as additives. Sepiolite nanoclay (Sep) has an extraordinary morphology in the class of nanoclay, and CNT makes a network, forming a tight char. Clay and CNT both make char layers. When they are utilised together, the clay and CNT composite makes a much higher density network frame, when compared with other CNT/PP or clay/PP composite cone calorimeter deposit, appeared in (Figure 24). In this manner, the PHRR can be powerfully reduced from a novel PP of 1,933 kW/m2 to PP/CNT/Sep composite of 355 kW/m2 for the 10wt% Sepiolite + 2wt% MWNT ternary nanocomposites system, these outcomes began from a higher density char residue. The utilisation of sepiolite nanoclay in combination with multi-walled CNTs demonstrated that the PHRR was fundamentally decreased by 82% compared with the neat polymer in the cone calorimeter as shown in (Figure 24) [154]. Marosfoil et al. likewise contemplated the flame retardancy performance of CNT filled PP and figured out how to reduce the PHRR from 2,755 kW/m2 to 760kW/m2 for PP and PP/CNT, respectively [154,155]. In this way, utilising diverse morphological nanometric additives can prompt synergetic flammability execution. mechanism of activity is achieved so new innovative leaps forward could be spearheaded. In that regard, the principle target of this investigation was to audit the collaborations between polymer nanocomposites and conventional flame retardants, as dissected in open writing, planning to give a useful device to those directing exploration in this field. Attempting an inside and out investigation of these frameworks uncovered that, in spite of the current advance, some essential inquiries on a few aspects still stay unanswered. One inquiry, for instance, is whether the responsive refers to living on silicates are idle amid fire or respond with the flame retardant impacting its viability. Then again, there exists no hypothetical foundation to portray quantitatively the correlation\between the diminishment in pHRR and dispersion of silicates. Polymer-nanocomposite retardancy towards supplanting halogenated flame retardants with safe, ecoaccommodating polymer nanocomposite. Nanocomposites should be joined with legitimate flame retardants to accomplish a similar flame retardancy that halogenated mixes appear. The principle mechanism of nanofiller-based flameretardant materials is that the nanofillers act as barriers against gas flow and oxygen dissemination at the condensed phase. In this manner, delivering solid, thick, crack-free nanocoating surface layers amid combustion is the key factor in creating compelling polymer nanocomposite flame retardants. Nanofiller dispersion and distribution are essential elements adding to flame retardancy synergistic impacts, which are hopeless when nanofillers are incorporated into other flame retardants since poor nanofiller distribution dissipates surface layers rendering them unfit to go about as barrier layers. Nanofiller/flame-retardant cooperations likewise add to nanofiller flame retardant – proficiency. The representative nanometric materials, including nanoclay, carbon nanotube and graphene, referred to in this chapter can be used as flame retardancy additives. In addition, an organic intumescent system was summarised regarding its chemical composition and mechanism. Occasionally nanometric additives, for achieving high flammability, have priority for disparity in polymer composites. 4. Conclusions Author Contributions The flame retardancy of polymer/layered silicate nanocomposites, assessed in this section, is the result of a few mechanisms; the level to which every last one of them impacts material’s execution is exceedingly connected with the fire situation under core interest. The development of a shallow carbonaceous-silicate charred layer is for the most part considered as the primary method of ‘nanocomposites’ flame-retardant activity in creating fires. The likelihood of formulating selfextinguishing nanocomposites is given by conventional flame retardants which can enhance fire execution without truly disintegrating mechanical properties, when utilised at unassuming loadings. All conceivable methods for embedding flame retardants into polymer/layered silicate nanocomposites (PLSN) are discovered: direct blending, chemical connection to the clay cation, and incorporation in the polymer backbone to form flame-retardant copolymer nanocomposites. In many cases, the results bring up that flame retardants serve to advance the fire properties of nanocomposites. The extensive scale commercialisation, in any case, of flame-retardant PLSN will be expert just when more profound learning on their R.G., L.N., and M.R. all have partly contributed to writing the manuscript. Disclosure statement The authors declare no conflict of interest. Funding This research received no external funding. Notes on contributors Dr. Radhashyam Giri is presently working as an Assistant Professor & H.O.D of the Plastics Engineering Department CIPET: IPT Bhubaneswar, Odisha India. He received Master of Technology (M. Tech) degree in Plastics Engineering and Technology in 2007 from Biju Pattanaik University of Technology (BPUT) Rourkela, Orissa, India. Dr. Giri received Doctor of Philosophy (Ph.D) from Rubber Technology Centre, Indian Institute of Technology, Kharagpur, West Bengal, India in 2012. Dr. Giri has nineteen international peer reviewed 26 R. GIRI ET AL. journal papers, three book chapters, one book published, 43 international oral conference papers on his credit and five more journal papers are in the pipeline. Apart from this Dr. Giri has so many achievements towards academic & research activity: He received R & D Institution runner up 5th National Awards for Technology Innovation in Petrochemical and downstream Plastic Processing Industry from Late Shri Ananth Kumar, Hon'ble former Minister for Chemicals & Fertilizers, Government of India on 21st February, 2015 at Hotel Lalit Ashok, Bengaluru. He is Editorial Board member for the esteemed journal, International Journal of Engineering Development and Research (IJEDR). Dr. Lalatendu Nayak is a Senior Manager in Specialty division at Phillips Carbon Black’s Global Research and Development Centre, Palej, Gujarat, India. He has completed his PhD (Polymer) from Indian Institute of Technology, Kharagpur, West Bengal, India, MTech (Polymer) from Central Institute of Plastics Engineering and Technology, Bhubaneswar, Odisha, India, and M.Sc. (Organic Chemistry) from Fakir Mohon University, Odisha, India. He has published more than 20 research articles in different international journal and published two book chapters. He is an active reviewer in different international journals. He has more than 10 years’ research experience in the field of conductive polymer composites and nanocomposites, synthesis and surface treatment of nanomaterials. His research interest includes surface modification of fillers for ink, paint and coating applications, synthesis of nanomaterials for energy storage applications, conductive polymer nanocomposites for electrostatic discharge (ESD), electromagnetic interference shielding (EMIS), and stealth technology applications Dr. Mostafizur Rahaman is an Assistant Professor at the Department of Chemistry at the College of Science, King Saud University, Riyadh 11451, Saudi Arabia. He obtained his M. Sc. (Chemistry) from T. M. Bhagalpur University, India and Ph. D. (Chemical/Polymer Chemistry) from the Indian Institute of Technology Kharagpur, India. He completed his M. Tech. in Plastics Engineering at the Central Institute of Plastics Engineering and Technology (CIPET), Bhubaneswar, Orissa, India. He has published 60+ articles in international journals, 9 book chapters, and 15 research articles in international conference proceedings. He has also published 1 patent and 3 books. Dr. Rahaman has 11 years of teaching and 12 years of research experience. He has completed eight research projects and attended/presented at various international conferences/seminars. He has been an active reviewer for various international journals and member of journal advisory boards. An expert in handling sophisticated instruments. His research interests include polymer nanotechnology/nanocomposites; polymer membrane, polymer thin film; polymer-based sensors; catalytic synthesis of polymers and conducting polymers; polymer-based coating for corrosion protection; polymer fuel cell and solar energy and bio-polymers for biomedical applications. 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