The Flame Retardancy of Polyethylene Composites: From Fundamental Concepts to Nanocomposites

Polyethylene (PE) is one the most used plastics worldwide for a wide range of applications due to its good mechanical and chemical resistance, low density, cost efficiency, ease of processability, non-reactivity, low toxicity, good electric insulation, and good functionality. However, its high flammability and rapid flame spread pose dangers for certain applications. Therefore, different flame-retardant (FR) additives are incorporated into PE to increase its flame retardancy. In this review article, research papers from the past 10 years on the flame retardancy of PE systems are comprehensively reviewed and classified based on the additive sources. The FR additives are classified in well-known FR families, including phosphorous, melamine, nitrogen, inorganic hydroxides, boron, and silicon. The mechanism of fire retardance in each family is pinpointed. In addition to the efficiency of each FR in increasing the flame retardancy, its impact on the mechanical properties of the PE system is also discussed. Most of the FRs can decrease the heat release rate (HRR) of the PE products and simultaneously maintains the mechanical properties in appropriate ratios. Based on the literature, inorganic hydroxide seems to be used more in PE systems compared to other families. Finally, the role of nanotechnology for more efficient FR-PE systems is discussed and recommendations are given on implementing strategies that could help incorporate flame retardancy in the circular economy model.


PE Grades
Structure's Description

FR Approaches and Materials
In this section, the specific approaches and materials used for enhancing the flame retardancy of the PE systems are discussed and their recent works are investigated.

Phosphorous and Melamine
Phosphorus-based FRs are the most considerable halogen-free type of FRs used for improving flame retardancy of PE. Common phosphorus-based FRs include red phosphorus (RP), phosphine oxides, phosphines, phosphonates, phosphates, ammonium phosphate, and phosphites [19,20]. Regarding the PE structure, phosphorus compounds are more advantageous in comparison with the halogen-based FRs as they work in two separate phases, gas and condense [21]. The physical and chemical reactions mostly affect flame inhibition and heat reduction because of the controlling melt flow, surface protection by acids, char layer promotion, and char layer protection against oxidation. Phosphorus FRs volatilize into the gas phase and strongly scavenge the hydrogen and hydroxyl radicals [22]. The Equations (1)- (5) demonstrates the radical scavenging ofḢ andȮH by the active phosphorus radicals such as PȮ and HṖO 2 , which are present in the flame (M is a third body species) [21].
The produced HPO can effectively quench the flame and lower the reactivity of the material compared toḢ and OḢ. Some phosphorus-containing compounds can decompose to phosphoric acid and polyphosphoric acid in the condense phase. A molten viscous layer formed by acids protect the surface of the polymers and restrict oxygen penetration [22,23]. Ammonium polyphosphate (APP) has been broadly used in IFR systems as an acidic source and blowing agent. Khanal et al. [24] prepared a novel IFR system containing APP and tris (2-hydroxyethylene) isocyanurate (THEIC) to enhance the fire and flammability performance of HDPE. Flammability of the prepared HDPE/IFR (AAP/THEIC) composite was evaluated by LOI analysis and cone calorimeter tests (CCT) indicating different parameters, but just one of them is investigated, which is peak heat release (PHR) rate here. The LOI analysis indicated that the LOI value of the HDPE/IFR composite with the weight ratio of 3:1 was higher than the LOI value of the pure HDPE and HDPE/APP. It is evident that adding THEIC as a char agent increased the flame retardancy of HDPE/IFR composite while the most striking feature was the optimum weight ratio of APP (3) to THEIC (1). The LOI value of the composite decreased by increasing the amount of THEIC. The CCT analysis illustrated that the combustion time of HDPE/IFR composite is slower than the pure HDPE and the PHR rate decreased and occurred at a longer time.
It is clear that the charring formation attributing to THEIC component caused a significant reduction of PHR rate value.
Melamine (MLM) is another unique material containing 67 wt.% nitrogen and excellent thermal resistance, which could be combined with phosphorus compounds used in FR applications. Among the MLM-containing compounds, MLM phosphate is specific due to the presence of phosphorus. Other commercially available MLM-based FRs are melamine cyanurate (MC), melamine pyrophosphate (MP), and melamine polyphosphate (MPP). Moreover, MLM can form salts with high thermal stability and strong acids. Salasinksa et al. [4] evaluated the effect of incorporating copper phosphate and melamine phosphate (CUMP) into HDPE as a FR compound and compared it with HDPE/APP. The fire property of HDPE containing CUMP was carried out by CCT. Figure 1 indicates the scanning electron microscope (SEM) images of PE/APP and PE/CUMP formed char layer, as well as char chemical composition.
Molecules 2020, 25, x FOR PEER REVIEW 4 of 28 as a char agent increased the flame retardancy of HDPE/IFR composite while the most striking feature was the optimum weight ratio of APP (3) to THEIC (1). The LOI value of the composite decreased by increasing the amount of THEIC. The CCT analysis illustrated that the combustion time of HDPE/IFR composite is slower than the pure HDPE and the PHR rate decreased and occurred at a longer time. It is clear that the charring formation attributing to THEIC component caused a significant reduction of PHR rate value. Melamine (MLM) is another unique material containing 67 wt.% nitrogen and excellent thermal resistance, which could be combined with phosphorus compounds used in FR applications. Among the MLM-containing compounds, MLM phosphate is specific due to the presence of phosphorus. Other commercially available MLM-based FRs are melamine cyanurate (MC), melamine pyrophosphate (MP), and melamine polyphosphate (MPP). Moreover, MLM can form salts with high thermal stability and strong acids. Salasinksa et al. [4] evaluated the effect of incorporating copper phosphate and melamine phosphate (CUMP) into HDPE as a FR compound and compared it with HDPE/APP. The fire property of HDPE containing CUMP was carried out by CCT. Figure 1 indicates the scanning electron microscope (SEM) images of PE/APP and PE/CUMP formed char layer, as well as char chemical composition.  [4].
The SEM images of the residues CCT conducted on HDPE/APP and HDPE/CUMP samples confirmed the char formation of both. Regarding the HDPE/APP, the formed char layer was thin without swollen structure while HDPE/CUMP was found to be porous inside char. The chemical composition analysis demonstrated that the char layer composed of C, O, N, and P components. CUMP were likely to be decomposed to CuO and P2O7, and then take part in the crosslink process, which finally causes a dense char layer formation and mechanical behavior improvement. It was The SEM images of the residues CCT conducted on HDPE/APP and HDPE/CUMP samples confirmed the char formation of both. Regarding the HDPE/APP, the formed char layer was thin without swollen structure while HDPE/CUMP was found to be porous inside char. The chemical composition analysis demonstrated that the char layer composed of C, O, N, and P components. CUMP were likely to be decomposed to CuO and P 2 O 7 , and then take part in the crosslink process, which finally causes a dense char layer formation and mechanical behavior improvement. It was evident that the HDPE/CUMP resulted in more char formation in comparison with pure HDPE. Compared to pure HDPE, HDPE/CUMP showed a significant reduction in PHR rate. Furthermore, the CUMP worked in both condense and gas-phase via char layer formation and emission of more non-flammable gases, respectively. Other works on the flame-retardancy of PE using phosphorous and MLM compounds have been summarized in Table 2.  Molecules 2020, 25, 5157 7 of 28

Nitrogen
Nitrogen-based FRs have also been used in PE systems to improve its flame resistance. Essential environmentally friendly groups of FRs are nitrogen-comprising ones because of their low toxicity, efficiency, recyclability, and their low evolution of smoke during combustion [47,48]. Commonly known nitrogen-based FRs are ammonia, MLM, and their derivatives, whilst other types based on urea and guanidine are also identified. Generally, MLM and derivatives are the basic components of an intumescent system [49,50]. Nitrogen-containing compounds showed a well-effective synergism with the fire retardants containing phosphorus [51]. As a case in point, MP and APP materials, taking advantage of N-P synergism, are the most used P and N-containing fire retardants. Besides, MLM/ammonium salts with organic or inorganic acids such as boric acid (BA), cyanuric acid, phosphoric acid, or pyro/poly-phosphoric acid are typically used to provide higher thermal stability along with lower volatility [52]. Polymeric nitrogen compounds based on cyanuric acid have been recently developed and MC, as a two-dimensional, highly thermally stable structure, showed strong synergism with phosphorus compounds [53,54].
The flame retardancy action mode of MLM and its salts is somewhat different [55]. Upon heating of the pure MLM, MLM starts volatilization by heat absorption caused by cooling down the surface of the polymeric matrix. Under high temperature, MLM undergoes further endothermic decomposition to produce cyanamid [56]. In the meanwhile, thermally stable condensates, i.e., melam, melem, and melon, as well as ammonia gas, may form. The chemical structure of the produced condensates is illustrated in Figure 2. These residue condensates contribute to the creation of an insulative layer in the condensed-phase. In addition, the ammonia evolution dilutes the burning atmosphere with inert/non-combustible gases, causing a gas phase flame retardancy contribution. In MLM salts, if MLM reforms upon dissociation of the salts exposed to the heat, a same decomposition mechanism is expected. Because of more progressive condensation in MLM salts rather than the pure MLM, their condensed-phase contribution is dominant [57]. Likewise, N-containing synergistic systems either form char through condensed-phase or promote gas phase reaction to scavenge free radicals [52,58]. Moreover, the ammonium salts decrease the spread rate of the flame but evolve potentially toxic gases [58].
Molecules 2020, 25, x FOR PEER REVIEW 8 of 28 radicals [52,58] . Moreover, the ammonium salts decrease the spread rate of the flame but evolve potentially toxic gases [58]. In an interesting study reported by Luyt et al. [18], six FR compositions obtained by mixing commercial N-and P-containing FRs were embedded into different grades of LDPE and LLDPE to evaluate their effects on thermal stability and fire-resistance. LDPE grade produced in an autoclave reactor showed the best flame retardancy performance (UL 94 V0, char residue: 10 ± 1% at 800 °C) when 35 wt.% of the triazine (TRZ) derivative and APP formulated FR was incorporated. This was due to the phosphorus-nitrogen synergism forming the highly thermally stable phosphorous oxynitride residue. In contrast, LDPE from the tubular reactor and LLDPE, contained TRZ derivative and APP formulated FR, represented poorer fire resistance since their strongly entangled molecules hindered well dispersion of FRs and lessened their activity due to the premature thermal decomposition. Results of the nitrogen-based FR revealed that nitrogen-based compounds alone could not achieve a V0 rating in any grades of PE. Table 2 indicates the works conducted by using nitrogen-based FRs in PE systems during the past 10 years.
The nitrogen and phosphorus-based flame retardants used as treatments in PE, LDPE, LLDPE, and HDPE are listed in Table 2 with emphasis on the effect each additive have on the polymer matrix. In an interesting study reported by Luyt et al. [18], six FR compositions obtained by mixing commercial N-and P-containing FRs were embedded into different grades of LDPE and LLDPE to evaluate their effects on thermal stability and fire-resistance. LDPE grade produced in an autoclave reactor showed the best flame retardancy performance (UL 94 V0, char residue: 10 ± 1% at 800 • C) when 35 wt.% of the triazine (TRZ) derivative and APP formulated FR was incorporated. This was due to the phosphorus-nitrogen synergism forming the highly thermally stable phosphorous oxynitride residue. In contrast, LDPE from the tubular reactor and LLDPE, contained TRZ derivative and APP formulated FR, represented poorer fire resistance since their strongly entangled molecules hindered well dispersion of FRs and lessened their activity due to the premature thermal decomposition. Results of the nitrogen-based FR revealed that nitrogen-based compounds alone could not achieve a V0 rating in any grades of PE. Table 2 indicates the works conducted by using nitrogen-based FRs in PE systems during the past 10 years.
The nitrogen and phosphorus-based flame retardants used as treatments in PE, LDPE, LLDPE, and HDPE are listed in Table 2 with emphasis on the effect each additive have on the polymer matrix. Meanwhile, some additives greatly improve the flame retardancy of the polymeric composites others, such as the Phenyl phosphinic arid di-4-[1-(4-pheny phodphonic acid monophenyl ester-yl)-methyl-ethyl] phenyester dimelaminium (PDEPDM), caused a reduction in the mechanical properties of PE. Hence, it is more appropriate to apply FRs that enhance fire properties without compromising their mechanical properties.

Inorganic Hydroxides
Inorganic hydroxides are widely applied to develop the fire retardancy of PE products because of their benefits, such as low toxicity, cost efficiency, minimal corrosion, and contribution to declining smoke emission during the combustion process. In addition, releasing water at the temperature above 200 • C is a distinct characteristic of inorganic compounds. The two main types of inorganic hydroxide are ATH and magnesium hydroxide (MH). There are typically two mechanisms of action in flame retardancy of ATH and MH compounds including flame dilution and the catalytic effect, which leads to charring enhancement [59]. The anhydrous alumina and magnesia are white powerful refractor powders, which can reflect the heat and assist to improve heat insulation with aggregating on the surface. Regarding the ATH, water releasing occurs at 220 • C, while MH releases water at 330 • C. The largest commercially use of ATH and MH FRs is in wire and cable insulation applications. There are several advantages associated with ATH, such as low cost, non-toxicity, and excellent flammability behavior. Generally, flame retardancy analysis requires at least 35% of metal hydroxide. Increasing the amount of metal hydroxide could lead to degrading physical properties as well as low-temperature flexibility. As a result, one of the most important techniques is to combine metal hydroxides with other FRs, such as phosphorus compounds, boron compounds, and nanoclays [60,61]. In some cases, surface modification is effective to increase flame retardancy of hydroxide compounds. Moreover, ATH is able to work in two different phases, gas and condense. The main mechanism of ATH in the condensed phase is heat absorption during the decomposition process. The ATH decomposition occurs at the range of 220-400 • C based on the reaction as shown in Equation (6) [62].
The decomposition reaction of ATH is extremely endothermic and absorbs about 1kJ/gr heat. The heat absorption reaches its maximum at 300 • C. The most striking feature is the formation of water vapor from the hydroxyl groups bonded to aluminum. Furthermore, the combustion is hindered by releasing the water vapor into the fire, diluting the flammable gases concentration, and limiting the accessibility of oxygen to the surface of the composite [62]. On the contrary, to the halogenated-FRs, the produced gases from the decomposition reaction of ATH are non-toxic and non-corrosive. Generally, the required characteristics of inorganic hydroxides FRs in commercial products are: (a) low cost, (b) highly accessible with low surface area and small particle size, (c) low toxicity, (d) exhibiting endothermic decomposition reaction between 100-300 • C, (e) capable of being used at high loading, and (f) colorless [22].
Layered double hydroxides (LDH) are synthetic materials containing negatively charged layers of inorganic/organic anions, which alternately located in the interlayer of positively charged layered of metal hydroxide [63,64]. The general formula of LDH is presented in Equation (7) [64].
LDH is considerably important to be used in commercial polymer industries, as they are environmentally friendly FRs. These types of FR compounds are attractive due to their endothermic decomposition against high temperature, nontoxicity, and high level of smoke suppression [64,65]. Until now, various studies have been conducted to evaluate the effect of incorporating metal hydroxides with polymers and nanocomposites.
Recent studies on the flame-retardancy of PE using inorganic hydroxides have been summarized in Table 3.  Recent studies on the flame-retardancy of PE using inorganic hydroxides have been summarized in Table 3.   From Table 3, Huntite and hydromagnesite caused a reduction in the tensile strength and elongation of the LLDPE material. Some of the notable additives that must be taken into consideration with regards to the flame retardancy of PE and its grades are the Organopalygorskite (OPGS), Molybdenum sulfide (MoS 2 ), and MH, which all resulted in a drastic reduction of peak heat release rate. Furthermore, the addition of MH, TiO 2 and LDPE composites had an excellent fire resistivity and mechanical performance. Aside from Modified MH, all the additives of HDPE produced better results and can therefore be utilized when developing flame retardant strategies for PE.

Boron
In addition to the water-soluble boron compounds including sodium borate (borax), BA, and boron oxide, water-insoluble ones and more commonly ZBs, are widely used as boron-derived FRs, former for application in cellulosic materials and latter in thermoplastics [52]. Boron based FRs possess low cost, high thermal stability, non-toxicity, and ease of handling, which have resulted in their wide use in PE systems [90]. Employment of ZB, with the most commercial importance, not only improves flame retardancy but also helps with the smoke suppression and anti-arcing in condensed phase [52]. Although ZB FRs are often used in halogen-containing systems, it has also been utilized in halogen-free and FR polymers [91]. Based on the studies conducted by Li et al. [92] and Wu et al. [93], the introduction of ZB into PE systems can result in the increase in residual char and thermal stability improvement. An extensive review of various types of ZB and their application was conducted recently by David M. Schubert [94]. In the most reported literatures [95,96], boron compounds were used together with other synergistic retardants such as nitrogen, phosphorus, silicon, and other synergies that has been comprehensively investigated in the following section. Therefore, other boron-based FRs, including calcium borate, melamine borate, boron phosphate, ammonium pentaborate, borosiloxane, etc., have become potential candidates only in recent years [52].
Upon heating and polymer combustion, depending on the grades of ZBs, endothermic dehydration occurs, in which the ZB loses its chemically bonded water molecules. This water vaporization not only provides a heat sink, delaying the combustion, but also dilutes the concentration of the oxygen and gaseous flammable components in the flame zone, causing an enhancement in the residual char formation [97]. Furthermore, at elevated temperatures, a glassy protective layer may form on the polymer/char surface, acting as a barrier to the transfer of heat, oxygen, and decomposition products, resulting in char strength and further combustion retardant [98]. Hence, another function of the ZB would be inhibiting the oxidation of the char (afterglow suppression), as well as suppressing smoke formation. Accordingly, a change in the oxidative decomposition direction of the halogen-free polymers (e.g., PE) is demonstrated when ZB is used. However, it is hesitated due to the suppression effect of boron oxides on hydrocarbons' decomposition [99], or graphite oxidation in the char [59], or just because of the insulative layer formation. Besides contributing to the condensed phase retardancy, in the presence of halogens, a gas phase flame retardancy is also attributed to the ZB due to the production of halogenated compounds, scavenging hot radicals, during the reaction between ZB and halogens. It is believed that BA and ZB are following the same flame retardancy mechanism just a few studies on flame retardancy effects of BA are released [100,101]. On the other hand, boron oxide can function only in the condensed phase by forming an insulative layer [52].
Recently, Sultigova et al. synthesized a certain chemical composition of ZB (2ZnO•3B 2 O 3 •3,5H 2 O), using borax and zinc sulfate as the precursors in aqueous solution, aiming at producing composites based on HDPE [102]. The composites were obtained via extrusion of the mixture at several prescribed temperatures. It is found out that the polymeric composites burnt much more slowly, without the formation of polymer melt droplets. Furthermore, the percentage of LOI and coke residue (CR) of the composites was 20.6 and 8.6, respectively, which were much higher than those values for the initial polymer. The results revealed that the incorporation of ZB into HDPE, increased the fire resistance of the starting polymer without diminishing its mechanical properties. Another recent study is developed by Abdulrahman et al. to investigate the effect of BA and borax on the thermal and viscoelastic properties of natural rubber (NR)/LDPE/high abrasion furnace carbon black composites [103]. For both fillers, the residual char yield was increased in the related composites. The loading of the fillers showed a considerable impact on the flammability behavior of the composites altering it from slow-burning to self-extinguishing (LOI: 28.5%) for BA and to the upper range of slow-burning (LOI: 27.8%) for borax ( Figure 4). Boron-containing FRs can show an advantageous synergistic interaction with MH, phosphorus, carbon, Si-and N-comprising compounds. For instance, Boron phosphate or metal borophosphate are produced in the intumescent systems containing phosphorus compounds (i.e., APP) and BA or ZB and boost the char formation and integrity. In the case of boron-nitrogen synergies, generation of boron nitride during fire may change the dominant fire retardancy mode of action. Furthermore, for the boron-nitrogen synergetic systems, it is believed that borosilicate glass or ceramic is formed because of borate/silica fusion at high temperatures. This phenomenon increases the fire resistance in the condensed phase [97]. Other works in the past 10 years on the flame-retardancy of PE using boron compounds have been summarized in Table 4.

Silicon
Silicon-containing FRs always have been in the frontline of co-additives in FR systems for PE products due to their high versatility, compatibility, low toxicity, and environmentally friendly characteristics [47,97]. This class of compounds can be mainly categorized into silicones, silica, organosilanes, silsesquioxanes, and silicates, functioning in FR systems as additives or other forms [52,90]. Silicon-comprising materials are inherently thermal stable and during a fire, they can produce an insulative layer upon decomposition. Through the formation of a highly thermally stable char, further substrate decomposition would be suppressed, and the rate of heat release would be lowered. Thus, the combustion of silicones is only associated with the emission of a negligible amount of toxic gases and smoke [124]. The reduction of HRR, PHR rate in CCT, and the rate of combustibles are most evident in silicone and siloxane. Besides fire retardancy through the condensed phase, the functionalization of silicone-based FRs with phosphorous or nitrogen groups makes them more efficient through contribution to the gas phase by trapping of dynamic radicals in the vapor phase [90,125].
Silicon dioxide, known as silica, in forms of silica gel, fumed and fused silica, is the most common silicon-based FR tested in various polymeric matrixes. Employment of functionalized silica and nano-silicates, in which oligomers or polymers attached through silanol groups, is a promising approach to produce the most efficient silicon-based fire retardants. This group of organic silicon compounds attracted intensive attention in recent years and the amount of research introducing these novel additives is growing [81,110]. Scarfato et al. reported the incorporation of a novel SC1 into the LDPE matrix to investigate its thermal and burning behavior together with its synergism with MH [81]. The SC1 was synthesized through the functionalization of MMT with (3-glycidyloxypropyl) trimethoxysilane (GOPTMS) by a silylation procedure (Figure 5a). LDPE/SC1, LDPE/MH, and LDPE/SC1/MH composites with various loading of the fillers were prepared. For binary LDPE/8SC1 system, the LOI value increased only from 17.5 up to 18.6 vol%, demonstrating a minor change that may be still within the margin of uncertainty. This is probably because of inadequate protection against direct contact to flames, provided by the inorganic residue layer formed by nanoclays. On the other hand, the addition of SC1 to LDPE/MH composites lowered their LOI indicating an adverse effect on the flame retardancy of the system, which is likely due to the worsening of the protective fire residue quality. Furthermore, with increasing SC1 content in LDPE/SC1 composites, the time to ignition (TTI) was decreased (Figure 5b).  Table 4 shows that POSS can reduce the dripping effect of PE. Nano clays are effective in extending the ignition times and reducing fire growth capacity, PHR rate as well as increasing the mechanical properties of PEs. The presence of ZB enhances the crystallinity of PE whiles an increase in the ratio of BA/BX has an adverse effect on ignition time, HRR, smoke production rate. It is therefore of great importance to maintain the blends at desirable ratios.

The Role of Nanotechnology in Flame Retardancy of Polymer Nanocomposites
Nowadays, the most important role of nanomaterials in polymer nanocomposites is the improvement of mechanical properties, such as impact strength and stiffness [131,132]. First, the employment of nanocomposites as FRs is receiving great attention because of nanomaterials with high aspect ratio, and many studies are conducted in this field [133]. Although some of the research in this field indicated improvement of flame retardancy via the incorporation of nanomaterials, some of them showed negative effects of using them [134][135][136]. Recently, researchers turned to the simultaneous use of nanomaterials and FRs to improve the flammability of polymer nanocomposites. Enormous studies have been conducted about the combination of nanomaterials and FR compounds, which showed synergistic effects in different properties of nanocomposites [137][138][139]. Exploiting the combination of these nanoscale materials not only reduces the loading of nanomaterials and FR In addition to the aforementioned organic silicones, noticeable research works have been conducted recently on polydimethylsiloxane (PDMS), as one of the most important polyorganosiloxanes, to modify the fire-resistance properties of organic polymers, through direct mixing with the polymers, deposition of PDMS on the fillers or synthesis of copolymers [126]. Owning to limited thermodynamic miscibility of PE with PDMS, ethylene-methyl acrylate copolymer (EMA) is often used as the chemical compatibilizer in a LDPE-PDMS mixture [127]. A recent review on flame resistance of PDMS systems by Zielecka et al. is a comprehensive reference for more information [124]. Moreover, polyhedral oligomeric silsesquioxane (POSS) with their specific hybrid organic-inorganic structures, also attracted significant attention in recent years [128]. An overview of the fire retardancy properties of polymer/POSS nanocomposites is represented by Zhang et al. [129]. Monofunctional POSS can contribute to the polymerization processes to produce, for instance, PE-POSS, poly(methyl methacrylate)-POSS, or other nanocomposites [130].
Whilst the addition of 2.5 wt.% octamethyl POSS into the PE-calcium carbonate-silicone (CaSiEMAA) composite system showed poor performance in the CCT, it eliminated dripping completely which is likely because of promoting ceramization of the silicon at the surface [121]. In essence, the rich chemistry of silicon compounds, especially, their inherent thermal stability, makes them strong candidates for FR applications. Table 4 shows a summary of recent studies in the past 10 years on PE systems using boron-based FRs. Table 4 shows that POSS can reduce the dripping effect of PE. Nano clays are effective in extending the ignition times and reducing fire growth capacity, PHR rate as well as increasing the mechanical properties of PEs. The presence of ZB enhances the crystallinity of PE whiles an increase in the ratio of BA/BX has an adverse effect on ignition time, HRR, smoke production rate. It is therefore of great importance to maintain the blends at desirable ratios.

The Role of Nanotechnology in Flame Retardancy of Polymer Nanocomposites
Nowadays, the most important role of nanomaterials in polymer nanocomposites is the improvement of mechanical properties, such as impact strength and stiffness [131,132]. First, the employment of nanocomposites as FRs is receiving great attention because of nanomaterials with high aspect ratio, and many studies are conducted in this field [133]. Although some of the research in this field indicated improvement of flame retardancy via the incorporation of nanomaterials, some of them showed negative effects of using them [134][135][136]. Recently, researchers turned to the simultaneous use of nanomaterials and FRs to improve the flammability of polymer nanocomposites. Enormous studies have been conducted about the combination of nanomaterials and FR compounds, which showed synergistic effects in different properties of nanocomposites [137][138][139]. Exploiting the combination of these nanoscale materials not only reduces the loading of nanomaterials and FR additives but also improves the mechanical and flame retardancy properties simultaneously.

The Role of Nanomaterials in Improving Flame Retardancy of PE Systems
Szustakiewicz et al. investigated on flame retardancy of HDPE/clay nanocomposites with MPP and APP FRs. They used two different types of organoclay: hydrophobic and hydrophilic. Based on the results of two flammability test methods (LOI and CCT), the simultaneous addition of organoclay and FRs to HDPE simultaneously results in a synergistic effect of flame retardancy. In this case, the LOI increases because of two factors, firstly, hydrophobic clay forms a reinforced structure that hinders the heat transfer of heat and secondly, APP intumescent char formation. As a result, the combination of these two effects makes the material burning more slowly [140].
Chuang et al. also found that the incorporation of nano-dispersed layered silicate and low smoke non-halogen (LSNH) FRs to the EVA/HDPE polymer blend caused a synergistic effect on the flame retardancy and smoke suppressing. According to the results, during combustion, the HRR of the FR-EVA/LDPE-n nanocomposite is 40% lower than the FR-EVA/LDPE polymer blend. Furthermore, they investigated the effect of organoclay contents on the flammability of FR-EVA/LDPE-n. It was found that there is an optimum loading of organoclay (3 phr), where the nanocomposite has the highest performance in flame retardancy [141]. In another study by Yu et al., the effects of adding MWCNTs and Ni 2 O 3 on the flame retardancy performance of LLDPE were investigated. The results of CCT show a synergistic effect of a combination of MWCNTs and Ni 2 O 3 in improving the flame retardancy of LLDPE, such that nanocomposite containing 3 wt.% MWCNTs and 5 wt.% Ni 2 O 3 shows 73% reduction in PHR rate compared to LLDPE and the yield of residual char is 13.7%. The improvement of flame retardancy of LLDPE by incorporating MWCNTs and Ni 2 O 3 was attributed to the physical effect of MWCNTs (formation of a network like structure because of the good dispersion of MWCNTs), chemical effect of Ni 2 O 3 (catalytic carbonization), and the combination of physical and chemical effect. Figure 6 schematically illustrates the aforementioned mechanisms [142].
of LLDPE, such that nanocomposite containing 3 wt.% MWCNTs and 5 wt.% Ni2O3 shows 73% reduction in PHR rate compared to LLDPE and the yield of residual char is 13.7%. The improvement of flame retardancy of LLDPE by incorporating MWCNTs and Ni2O3 was attributed to the physical effect of MWCNTs (formation of a network like structure because of the good dispersion of MWCNTs), chemical effect of Ni2O3 (catalytic carbonization), and the combination of physical and chemical effect. Figure 6 schematically illustrates the aforementioned mechanisms [142].   [143]. Table 5 represents some results about the influence of nanomaterials on the flammability of PE nanocomposites.  Han et al. applied different contents of well-exfoliated graphene nano-platelets to enhance flame retardancy of polyethylene/alumina trihydrate (PE/ATH) composites and showed the addition of 0.2 wt.% of GNP decreased the PHR by 18% of that of the PE/ATH composite. A possible explanation of this behavior is a char layer of GNPs acting as a heat shield and a barrier against mass transport. Images of the PE/ATH/GNPx (x, contents of GNP) after cone testing are represented in Figure 7.
of LLDPE, such that nanocomposite containing 3 wt.% MWCNTs and 5 wt.% Ni2O3 shows 73% reduction in PHR rate compared to LLDPE and the yield of residual char is 13.7%. The improvement of flame retardancy of LLDPE by incorporating MWCNTs and Ni2O3 was attributed to the physical effect of MWCNTs (formation of a network like structure because of the good dispersion of MWCNTs), chemical effect of Ni2O3 (catalytic carbonization), and the combination of physical and chemical effect. Figure 6 schematically illustrates the aforementioned mechanisms [142].   [143]. Table 5 represents some results about the influence of nanomaterials on the flammability of PE nanocomposites.   Table 5 represents some results about the influence of nanomaterials on the flammability of PE nanocomposites. Organic-modified montmorillonite, 10 wt.% MHSH, 30 wt.% 84% reduction in PHR rate and increase in t ign observed from CCT. [146] Organic-modified montmorillonite, 5 wt.% IFRs, 15 wt.% 51% reduction in PHR rate observed from CCT [122] Halloysite nanotubes, 2 wt.% IFRs, 28 wt.% 92% and 75% decrease in PHR rate and THR, respectively. [147] Graphene, 1 wt.% Brominated polystyrene/antimony trioxide, 6.2 wt.% Increase LOI value from 23.4% to 24.1%, change UL-94 grades from NG to V-2. [148]

Incorporation Methods of Nanomaterials in Polymer Matrices
A variety of methods are applied for the incorporation of nanomaterials into PE, depending on the nature of the nanomaterials [149]. These methods include in situ polymerization, solvent casting, and melt mixing, which will be elaborated in the following sections.

In Situ Polymerization
In this method, at first nanomaterials and monomers are mixed in a solvent with a proper shear rate, which obtains a stable suspension [150]. In this stage, some interfacial agents are added to enhance the stability of the mixture. Then, the obtained mixture is fed to the reactor where the processing conditions mostly are the same applied to the synthesis of the base polymer. When the polymerization is complete, the solvent is removed [151]. This method is used for a wide range of polymer nanocomposites.

Solvent Casting
This method is mostly used in cases that there is not enough dispersion of nanomaterials in the polymer [152]. In other words, the thermodynamic affinity between polymer and filler is not favorable for homogeneous dispersion. Therefore, the most important step of this method is the breaking of the agglomerates of nanomaterials, and ultra-sonication is the best method to achieve this goal. This process is especially suitable for the exfoliation of clay layers and thermosetting polymers [153,154]. To conserve dispersed/intercalated/exfoliated structure, some interfacial compatibilizers, such as maleic anhydride grafted polymers, are used.

Melt Mixing
Melt compounding is the most common method for the production of nanocomposites because of the simplicity and availability of equipment [155,156]. Depending on the amount of the product, internal mixer and twin-screw extruder are used for processing. There are two major methods of feeding for melt compounding: direct and master-batch which the later method is more common. In the master-batch feeding method, first, a concentrated master-batch of filler is prepared and then is diluted in the base polymer [157]. To obtain a good dispersion of filler in polymer and improve compatibility among components, some grafted polymers are utilized [158]. The amount of shear rate, mixing time, temperature, and design of the screw profile can determine the final microstructure and properties of the nanocomposites [159,160].

Summary and Perspective
Flame retardancy of polymeric materials has become an area of keen interest recently following the rampant fire outbreaks. This research focused on the FRs and different application mechanisms available for the treatment of PE and its grades. In this study, a list of phosphorus, melamine, nitrogen, inorganic hydroxide, boron and silicon-based retardants with their loading amounts and effects on the grades of PE have been presented. The desirable FRs are the additives that presents a balance between fire resistivity and maintaining or improving the mechanical properties of the composite. It was realized from the research that the addition of FRs such as POSS, nanoclays, Organopalygorskite (OPGS), Molybdenum sulfide (MoS 2 ), MH, TiO 2 , etc., greatly improved the fire and mechanical properties of the PE samples. Possible concerns for future research should be to investigate the effect of FR polymers on the environment. Most of the studies analyzed failed to assess the effects of the additives on the environment. Depending on the lifetime of FR polymers, for the short lifetimes, the biodegradability of polymer is an important issue, and the recyclability of FRs is an important factor for the polymers with a long lifetime. The innovative natural sources for the polymer FR additives are highly recommended possibilities for decreasing environmental issues. Moreover, during burning, some additives of the FRs can produce toxic compounds that must be controlled or even substituted. Addressing these concerns will be a step towards the betterment of the circular economy model.