Flammability properties of polymers and polymer composites combined with ionic liquids

2.1 Ionic liquids combined with polymers

Table 3 shows examples of ionic liquids combined with polymers in studies related to flammability properties. It can be perceived that there are four types of ionic liquids commonly used in combination with polymers, specifically imidazolium-, phosphonium-, pyridinium-, and ammonium-based ionic liquids. Among these, imidazolium-based ionic liquids are usually preferred for combining with polymers compared to the other types (15). The primary reason for this preference is their extensive research and widespread availability in the commercial market. Imidazolium-based ionic liquids offer a diverse range of chemical structures and properties, making them readily accessible for various applications (4). On the contrary, phosphonium- and pyridinium-based ionic liquids are less frequently combined with polymers, particularly in studies related to flammability properties. One possible explanation for this is their limited commercial availability when compared to imidazolium-based ionic liquids, which can make them more challenging and expensive to acquire. As a result, their utilization in research and practical applications may be hindered to some extent. Similarly, ammonium-based ionic liquids are seldom combined with polymers. This can be attributed to their lower compatibility with polymers, which imposes limitations on their use as additives for polymer blending and the preparation of polymer composites. The reduced compatibility can pose challenges in achieving the desired properties and performance when combining ammonium-based ionic liquids with polymers. In short, imidazolium-based ionic liquids have emerged as the preferred choice for combining with polymers due to their extensive availability and versatility, the other types of ionic liquids, such as phosphonium-, pyridinium-, and ammonium-based ionic liquids, face certain limitations and constraints that have hindered their widespread use in polymer-related studies and applications. Figure 1 shows the chemical structures of [Bmim][DBP], [OTP][BScB], [Hpy][PF₆], and [DCEA][DHP].

Figure 1

Chemical structures of (a) [Bmim][DBP], (b) [OTP][BScB], (c) [Hpy][PF₆], and (d) [DCEA][DHP].

2.2 Ionic liquids combined with polymer composites

Table 4 shows examples of ionic liquids combined with polymer composites in studies related to flammability properties. It can be noticed that there are primarily two types of ionic liquids utilized in combination with polymer composites, particularly imidazolium- and phosphonium-based ionic liquids. Among these, imidazolium-based ionic liquids are the preferred choice for adding to polymer composites compared to phosphonium-based ionic liquids (42,43,55). Interestingly, other types of ionic liquids, such as pyridinium- and ammonium-based ionic liquids, are not commonly employed for such purposes. The preference for imidazolium-based ionic liquids in polymer composites can be attributed to their high compatibility with a wide range of polymers. This compatibility enables the effective preparation of polymer composites with enhanced properties. In fact, imidazolium-based ionic liquids with counter anions, such as hexafluorophosphate, tetrafluoroborate, bis(trifluoromethylsulfonyl)imide, and chloride, are frequently studied for their flammability properties. This is due to the good interactions between these ionic liquids and the polymer matrix. Furthermore, these ionic liquids can also act as dispersing agents, facilitating the improved dispersion of fillers within the polymer matrix (52). In contrast, the utilization of phosphonium-based ionic liquids in combination with polymer composites is relatively limited. Only a few specific phosphonium-based ionic liquids have been investigated for their potential to enhance the properties of polymer composites. The scarcity of studies involving phosphonium-based ionic liquids in polymer composites suggests that they are less explored compared to their imidazolium-based counterparts. In short, the extensive use of imidazolium-based ionic liquids in polymer composites can be attributed to their compatibility with various polymers and their role as effective dispersing agents. In contrast, the utilization of phosphonium-based ionic liquids in polymer composites remains limited. Figure 2 shows the chemical structures of [Emim][PF₆], [Ecmmim][BF₄], [Bmim][TFSI], and [Tdcmim][Cl].

Figure 2

Chemical structures of (a) [Emim][PF₆], (b) [Ecmmim][BF₄], (c) [Bmim][TFSI], and (d) [Tdcmim][Cl].

3.1 Preparation of polymers combined with ionic liquids

Table 5 shows the types of polymers, methods, temperatures, and times for combining with ionic liquids. Synthetic polymers offer versatility in combining with ionic liquids, with several methods available, including solution mixing, grinding, and melt mixing. The solution mixing method proves to be a versatile approach, as it can be applied to combine ionic liquids with synthetic polymers of various types, regardless of whether they are thermoplastics or thermosets. This method involves mixing the ionic liquid and the synthetic polymer in a suitable solvent, resulting in a homogeneous mixture (10,13). In the case of thermoplastic polymers, both grinding and melt mixing methods are commonly employed. Grinding involves the mechanical grinding of the thermoplastic polymer and the subsequent addition of the ionic liquid (16). This method ensures effective dispersion and distribution of the ionic liquid within the polymer. On the other hand, melt mixing is specifically used for thermoplastic polymers. It involves melting the polymer, followed by the addition of the ionic liquid into the molten polymer (3). The mixture is then homogenized and processed to ensure proper distribution of the ionic liquid throughout the polymer. It is noteworthy that solution mixing and grinding methods are typically conducted at room temperature, offering convenience and simplicity in the preparation process. In contrast, the melt mixing method requires specific processing temperatures to achieve the molten state of the thermoplastic polymer, ensuring efficient blending of the ionic liquid. Additionally, the duration of the combining process is an important parameter to consider. The solution mixing method generally requires longer combining times compared to the grinding and melt mixing methods. This is mainly due to the need for sufficient time for the polymer and ionic liquid to dissolve and mix thoroughly, ensuring a uniform distribution within the polymer. In brief, synthetic polymers can be combined with ionic liquids through solution mixing, grinding, and melt mixing methods. Solution mixing is applicable for both thermoplastic and thermoset polymers, while grinding and melt mixing methods are primarily used for thermoplastics. Solution mixing and grinding methods are typically conducted at room temperature, while melt mixing requires specific processing temperatures. Moreover, the combining time for the solution mixing method is generally longer compared to the grinding and melt mixing methods, ensuring complete dissolution and thorough mixing of the polymer and ionic liquid.

3.2 Preparation of polymer composites combined with ionic liquids

Table 6 shows the types of polymer composites, methods, temperatures, and times for combining with ionic liquids. Thermoplastic composites are commonly combined with ionic liquids through the melt mixing method, which involves several steps to achieve a well-dispersed composite. Initially, the thermoplastic polymer is melted to form a molten state. Then, the filler and ionic liquid or ionic liquid-treated filler are added to the molten polymer mixture (24,25). The composite constituents are thoroughly mixed and homogenized to ensure an even distribution of the filler particles and ionic liquid or ionic liquid-treated filler particles throughout the molten thermoplastic. This homogenization process is often achieved by extruding the mixture, which helps in the dispersion of the filler and ionic liquid or ionic liquid-treated filler and facilitates their uniform incorporation within the thermoplastic matrix. Finally, the resulting composite is molded into the desired shape through appropriate techniques such as injection molding or compression molding. In the case of thermoset composites, the preferred method for combining them with ionic liquids is the stirring method. This method involves mixing the thermoset resin and filler using a stirrer in a dedicated mixing container. The ionic liquid or ionic liquid-treated filler is gradually added to the mixture during the stirring process to ensure its uniform dispersion throughout the composite (41). The stirring process helps to ensure that the filler and ionic liquid or ionic liquid-treated filler are evenly distributed and incorporated into the thermoset resin matrix. Once the stirring is complete, the resulting composite mixture is poured into a mold and subjected to a curing process, typically involving heating or other curing methods. This curing process promotes the cross-linking and hardening of the thermoset resin, resulting in a solid composite material with enhanced properties. Furthermore, elastomer composites can be combined with ionic liquids using the compounding method. The compounding process starts by masticating the elastomer on a mixing mill, which involves mechanical shearing and blending of the elastomer material. Subsequently, the filler and ionic liquid or ionic liquid-treated filler are added to the masticated elastomer at a slightly elevated temperature (56). The mixture is then further processed and blended to ensure the uniform distribution of the filler particles and ionic liquid or ionic liquid-treated filler particles within the elastomer matrix. Finally, the resulting composite is cured at an elevated temperature and pressure, allowing the elastomer to regain its elastic properties while maintaining the desired dispersion of the filler and ionic liquid or ionic liquid-treated filler. In brief, thermoplastic composites are combined with ionic liquids through the melt mixing method, thermoset composites through the stirring method, and elastomer composites via the compounding method. Each method offers specific steps to ensure the proper dispersion and incorporation of the filler and ionic liquid or ionic liquid-treated filler within the respective polymer matrix, leading to the development of composite materials with improved properties for various applications.

4.1 Flammability properties of polymers combined with ionic liquids

Both the LOI and UL-94 tests are essential measures of the flammability properties of various materials, including polymers and polymer composites. They are widely recognized and typically used in industries where fire safety is a concern. LOI and UL94 have distinct advantages and complement each other in assessing different aspects of fire performance. The LOI value is a measure of the flammability of a material. It is determined by measuring the minimum concentration of oxygen in a mixture of oxygen and nitrogen that will support the combustion of materials under specified test conditions (48). The LOI value is expressed as a percentage of oxygen in the mixture, with higher values indicating a material that is less flammable.

Besides, the UL-94 rating system assigns materials to one of several classifications: V-0, V-1, V-2, or HB. The V-0, V-1, and V-2 classifications apply to materials that are used in vertical applications, while the HB classification applies to materials used in horizontal applications. The V-0 classification indicates that the material has passed the most stringent requirements of the UL-94 test and will self-extinguish within 10 s of the removal of the test flame. The V-1 classification indicates that the material will self-extinguish within 30 s, while the V-2 classification indicates that the material will self-extinguish within 30 s but may drip flaming particles (9,37). The HB classification indicates that the material will burn slowly and may produce burning droplets but will not propagate the flame horizontally.

Table 7 shows the flammability properties of polymers combined with ionic liquids. Xiao et al. have combined EP with [Vdpim][Br] through the solution mixing method (1). The LOI value of the EP/[Vdpim][Br] was 34.9%, which is higher than that of neat EP (25.9%). Furthermore, the UL-94 rating for the EP/[Vdpim][Br] was V-0, whereas the neat EP had no rating. The results showed that the addition of [Vdpim][Br] provided a homogenous and compact phosphorus-rich char residue that inhibited the heat transfer and further decomposition of EP, which are vital to reduce the flammability of EP.

Ou et al. have combined EP with [TPP][PF₆] via the solution mixing method (7). The LOI value of the EP/[TPP][PF₆] was 30.3%, which is higher compared to the pure EP (23.2%). Moreover, the UL-94 rating for the EP/[TPP][PF₆] was V-0, while the pure EP had no rating. The results displayed that the addition of [TPP][PF₆] enhanced the creation of the compact char layer to forbid the pyrolysis gases into the combustion zone. [TPP][PF₆] also thermally decomposed into phosphorus-containing fragments, which can be free radical destroyers.

Jiang et al. have combined EP with [Bmim][DBP] through the solution mixing method (8). The LOI value of the EP/[Bmim][DBP] was 24.0%, which is higher than that of neat EP (19.4%). Additionally, the UL-94 rating for the EP/[Bmim][DBP] was V-1, whereas the neat EP had no rating. The results exhibited that the addition of [Bmim][DBP] released non-flammable gas and diluted the combustion space, as well as increased the holes of residual chars of EP. The results also suggested the contribution of phosphorus in residual chars to stimulate network structure formation.

Guo et al. have combined EP with [OTP][BScB] by the solution mixing method (9). The LOI value of the EP/[OTP][BScB] was 35.6%, which is higher compared to the pure EP (26.3%). In addition, the UL-94 rating for the EP/[OTP][BScB] was V-0, while the pure EP had no rating. The results demonstrated that the addition of [OTP][BScB] induced the formation of char, which behaved as a protective shield in the condensed phase, released gaseous phosphorus-containing compounds as free radical scavengers in the gas phase, and exposed flame-retarding capability in both phases.

He et al. have combined PA6 with [Pmim][CH₃SO₃] using the melt mixing method (17). The LOI value of the PA6/[Pmim][CH₃SO₃] was 26.2%, which is higher than that of neat PA6 (20.5%). Furthermore, the UL-94 rating for the PA6/[Pmim][CH₃SO₃] was V-2, whereas the neat PA6 had no rating. The results indicated that the addition of [Pmim][CH₃SO₃] boosted the char formation of PA6 during combustion. Nonetheless, the char layer of the PA6/[Pmim][CH₃SO₃] was overly thin and weak to extinguish the fire of PA6 sufficiently, which manifested a little flame-retardant effect for PA6.

Liu et al. have combined PVA with [DCEA][DHP] through the solution mixing method (18). The LOI value of the PVA/[DCEA][DHP] was 30.1%, which is higher compared to the pure PVA (19.0%). Moreover, the UL-94 rating for the PVA/[DCEA][DHP] was V-0, while the pure PVA had no rating. The results showed that the addition of [DCEA][DHP] prompted the creation of a continuous and dense phosphide-containing carbon layer in the combustion process, which efficiently split the internal PVA from the flame.

Table 7 also shows that the EP/[Vdpim][Br] possessed low flammability properties, as indicated by the LOI value of 34.9% and the UL-94 V-0 rating. The addition of 4 wt% of [Vdpim][Br] seemed to improve the flame-retardant properties of EP significantly. The EP/[TPP][PF₆] also exhibited poor flammability properties, with an LOI value of 30.3% and a UL-94 V-0 rating. Although the LOI value is slightly lower than the EP/[Vdpim][Br], the addition of 2 wt% of [TPP][PF₆] still provided effective flame-retardant properties. However, the flammability properties of EP with 10 wt% of [Bmim][DBP] are comparatively higher than the EP/[Vdpim][Br] and EP/[TPP][PF₆]. The LOI value of 24.0% indicated a reduced ability to resist ignition and sustain combustion. The UL-94 V-1 rating suggested that it might not self-extinguish as quickly as the V-0-rated materials. The EP/[OTP][BScB] exhibited inferior flammability properties, as indicated by the LOI value of 35.6% and the UL-94 V-0 rating. The addition of 10 wt% of [OTP][BScB] seemed to significantly enhance its flame-retardant properties, which made it comparable to the EP/[Vdpim][Br]. The PA6 with 30 wt% of [Pmim][CH₃SO₃] had a lower LOI value of 26.2% compared to the EP/ionic liquid, which indicated increased flammability properties. The UL-94 V-2 rating suggested that it might not self-extinguish and had a higher tendency for dripping during the burning test. The PVA/[DCEA][DHP] demonstrated poor flammability properties, with an LOI value of 30.1% and a UL-94 V-0 rating. The addition of 20 wt% of [DCEA][DHP] appeared to improve its flame-retardant properties, similar to the EP/[Vdpim][Br] and EP/[OTP][BScB]. Indeed, the flammability properties of polymers combined with ionic liquids are dependent not only on the types of ionic liquids but also on the contents of ionic liquids.

4.2 Flammability properties of polymer composites combined with ionic liquids

Table 8 shows the flammability properties of polymer composites combined with ionic liquids. Li et al. have combined HDPE/WF/EG with [Bmim][H₂PO₂] via the melt mixing method (24). The LOI value of the HDPE/WF/EG/[Bmim][H₂PO₂] composite was 31.5%, which is slightly higher than that of the HDPE/WF/EG composite (31.2%). Additionally, the UL-94 rating for the HDPE/WF/EG/[Bmim][H₂PO₂] composite was V-0, whereas the HDPE/WF/EG composite had no rating. The results displayed that the addition of [Bmim][H₂PO₂] produced a two-component system, which had better flame retardancy compared to the single-component system. Besides, [Bmim][H₂PO₂] had a synergistic effect on the flame retardancy of HDPE/WF/EG composite.

Ding et al. have combined LLDPE/Mg(OH)₂ with [Tdcmim][Cl] through the melt mixing method (25). The LOI value of the LLDPE/Mg(OH)₂/[Tdcmim][Cl] composite was 36.8%, which is marginally higher compared to the LLDPE/Mg(OH)₂ composite (35.6%). In addition, the UL-94 rating for the LLDPE/Mg(OH)₂/[Tdcmim][Cl] and LLDPE/Mg(OH)₂ composites was H-0. The results exhibited that the addition of [Tdcmim][Cl] caused the LLDPE composite to possess better flammability resistance than the LLDPE composite without the ionic liquid, which also met the requirement of self-extinguishing in the horizontal burning test.

Zhang et al. have combined PA6/MPP/MMT with [Emim][CF₃SO₃] by the melt mixing method (46). The LOI value of the PA6/MPP/MMT/[Emim][CF₃SO₃] composite was 31.5%, which is lower than that of the PA6/MPP/MMT composite (38.0%). However, the UL-94 rating for the PA6/MPP/MMT/[Emim][CF₃SO₃] and PA6/MPP/MMT composites was V-0. The results demonstrated that the addition of [Emim][CF₃SO₃] probably interfered with the effectiveness of MPP as an intumescent flame retardant, which might have hindered or weakened the flame-retardant mechanism, which resulted in a lower LOI value. But the synergistic effect between MMT/[Emim][CF₃SO₃] and MPP produced a carbonaceous-silicious char layer and shielded the heat and mass transport, as well as prohibited the escape of volatile products created and eradicated the melt dripping.

Jia et al. have combined PLA/APP with [TBP][BF₄] using the melt mixing method (26). The LOI value of the PLA/APP/[TBP][BF₄] composite was 28.3%, which is marginally higher compared to the PLA/APP composite (26.3%). Furthermore, the UL-94 rating for the PLA/APP/[TBP][BF₄] composite was V-0, while the PLA/APP composite rating was V-2. The results indicated that the addition of [TBP][BF₄] improved the flame retardancy of PLA/APP composite from V-2 to V-0 rating in the UL-94 test and increased the LOI value by up to 7.6%. The results also revealed that the addition of [TBP][BF₄] enhanced the flame retardance performances of APP.

Hu et al. have combined PLA/MWCNTs with [Hemim][TMP] through the melt mixing method (27). The LOI value of the PLA/MWCNTs/[Hemim][TMP] composite was 26.0%, which is higher than that of the PLA/MWCNTs composite (23.0%). Moreover, the UL-94 rating for the PLA/MWCNTs/[Hemim][TMP] composite was V-1, whereas the PLA/MWCNTs composite rating was V-2. The results showed that the addition of [Hemim][TMP] formed MWCNTs/[Hemim][TMP], behaved as an effective carbonization agent for PLA, which promoted the production of a protective char layer and increased the anti-dripping effect, consequently improving the flame-retardant properties of the PLA composite.

Chen et al. have combined TPU/AHP with [Emim][PF₆] via the melt mixing method (48). The LOI value of the TPU/AHP/[Emim][PF₆] composite was 35.8%, which is higher compared to the TPU/AHP composite (32.5%). Additionally, the UL-94 rating for the TPU/AHP/[Emim][PF₆] composite was V-0, while the TPU/AHP composite rating was V-1. The results displayed that the addition of [Emim][PF₆] into the TPU/AHP system created a synergistic effect with AHP that provided a high LOI value, and the UL-94 reached a V-0 rating, which effectively increased the flame-retardant level of the TPU composite.

Jiao et al. have combined TPU/HGM with [Ecmmim][BF₄] through the melt mixing method (30). The LOI value of the TPU/HGM/[Ecmmim][BF₄] composite was 25.0%, which is slightly higher than that of the TPU/HGM composite (23.5%). In addition, the UL-94 rating for the TPU/HGM/[Ecmmim][BF₄] composite was V-0, whereas the TPU/HGM composite rating was V-1. The results exhibited that the addition of [Ecmmim][BF₄] enhanced the structure of char residue from the TPU composite. The char residue acted as a protective layer on the TPU surface during thermal decomposition.

Wan et al. have combined EP/MPP/GO with [Bmim][PF₆] by the stirring method (41). The LOI value of the EP/MPP/GO/[Bmim][PF₆] composite was 29.2%, which is marginally higher compared to the EP/MPP/GO composite (28.0%). Furthermore, the UL-94 rating for the EP/MPP/GO/[Bmim][PF₆] composite was V-0, while the EP/MPP/GO composite rating was V-1. The results demonstrated that the addition of [Bmim][PF₆] formed an intumescent char layer during combustion, which shielded the inside of the composite. [Bmim][PF₆] also provided the best synergistic effect to the composite.

Chen et al. have combined EP/HPCP/Gra with [Bmim][PF₆] using the stirring method (52). The LOI value of the EP/HPCP/Gra/[Bmim][PF₆] composite was 33.8%, which is higher than that of the EP/Gra composite (26.1%). Moreover, the UL-94 rating for the EP/HPCP/Gra/[Bmim][PF₆] composite was V-0, whereas the EP/Gra composite had no rating. The results indicated that the addition of [Bmim][PF₆] increased the dispersion of Gra filler in the EP matrix, which improved the flame retardancy of the EP composite. The phosphorus-containing material (HPCP) also assisted in the improvement of flame retardancy.

Li et al. have combined EP/Mo-MOF/GO with [Bmim][BF₄] through the stirring method (42). The LOI value of the EP/Mo-MOF/GO/[Bmim][BF₄] composite was 27.6%, which is slightly higher compared to the EP/Mo-MOF/GO composite (26.7%). Additionally, the UL-94 rating for the EP/Mo-MOF/GO/[Bmim][BF₄] composite was V-1, while the EP/Mo-MOF/GO composite had no rating. The results showed that the addition of [Bmim][BF₄] contributed to the charring cross-linking effect, which caused catalytic carbonization. The composite burned out to produce a charcoal shell, and the inside EP matrix was maintained.

Bi et al. have combined EP/ANP with [Bmim][PF₆] via the stirring method (53). The LOI value of the EP/ANP/[Bmim][PF₆] composite was 30.3%, which is higher than that of pure EP (24.4%). In addition, the UL-94 rating for the EP/ANP/[Bmim][PF₆] composite was V-1, whereas the pure EP had no rating. The results displayed that the addition of [Bmim][PF₆] generated ANP/[Bmim][PF₆], which acted as an effective flame retardant for EP composite. The high content of ANP/[Bmim][PF₆] increased the char residue of EP composite and attained excellent flame retardancy.

Gao et al. have combined PU/EG/APP/GO with [Bmim][PF₆] through the ultrasonication method (38). The LOI value of the PU/EG/APP/GO/[Bmim][PF₆] composite was 29.0%, which is marginally higher compared to the PU/EG/APP/GO composite (28.2%). Furthermore, the UL-94 rating for the PU/EG/APP/GO/[Bmim][PF₆] composite was V-0, while the PU/EG/APP/GO composite rating was V-1. The results exhibited that the addition of [Bmim][PF₆] gave an excellent synergistic effect with EG and APP, which inhibited the interior layer structure from further burning and enhanced the flame retardancy of the PU composite.

Gao et al. have also combined UPR/APP/EG/GO with [Bmim][PF₆] by the ultrasonication method (36). The LOI value of the UPR/APP/EG/GO/[Bmim][PF₆] composite was 28.2%, which is higher than that of the UPR/APP/EG/GO composite (27.5%). Moreover, the UL-94 rating for the UPR/APP/EG/GO/[Bmim][PF₆] composite was V-0, whereas the UPR/APP/EG/GO composite rating was V-1. The results demonstrated that the addition of [Bmim][PF₆] produced GO/[Bmim][PF₆] with excellent properties, which had a good synergistic effect with APP/EG and catered great flame-retardant performance.

Hu et al. have combined UPR/pEG with [Mim][ATP] using the stirring method (37). The LOI value of the UPR/pEG/[Mim][ATP] composite was 36.5%, which is higher compared to the UPR/EG composite (31.0%). Additionally, the UL-94 rating for the UPR/pEG/[Mim][ATP] composite was V-0, while the UPR/EG composite rating was V-2. The results indicated that the addition of [Mim][ATP] created an excellent synergistic effect with EG, which increased the carbon density of UPR composite during combustion and self-extinguished quickly after the ignition.

Table 8 also shows that the HDPE/WF composite with 30 wt% of EG-[Bmim][H₂PO₂] had a good flammability resistance with an LOI value of 31.5% and a UL-94 rating of V-0. The addition of EG-[Bmim][H₂PO₂] likely enhanced the flame retardancy of the composite, which made it less susceptible to burning and reduced the spread of fire. The LLDPE/Mg(OH)₂ composite with 2.3 wt% of [Tdcmim][Cl] exhibited even higher flammability resistance with an LOI value of 36.8% and a UL-94 rating of H-0. The presence of Mg(OH)₂ and [Tdcmim][Cl] likely acted as effective flame retardant additives, which improved the ability of the composite to withstand fire and self-extinguish. The PA6/MPP composite with 3 wt% of MMT-[Emim][CF₃SO₃] shared a similar LOI value of 31.5% as the HDPE/WF composite, which indicated a moderate flammability resistance. The addition of MMT-[Emim][CF₃SO₃] might provide some flame retardancy, which resulted in a V-0 UL-94 rating. The PLA/APP composite with 2.5 wt% of [TBP][BF₄] had a relatively lower LOI value of 28.3% compared to the previous composites. However, it still possessed a V-0 UL-94 rating, which suggested a good flammability resistance. The addition of [TBP][BF₄] likely contributed to its flame-retardant properties. The PLA composite with 5 wt% of MWCNTs-[Hemim][TMP] exhibited a lower LOI value of 26.0% and a UL-94 rating of V-1. The addition of MWCNTs-[Hemim][TMP] improved flammability resistance compared to the neat PLA, but it did not meet the V-0 rating, which indicated some susceptibility to fire. The TPU/AHP composite with 0.06 wt% of [Emim][PF₆] demonstrated good flammability resistance with an LOI value of 35.8% and a UL-94 rating of V-0. The addition of [Emim][PF₆] positively enhanced the flame retardancy of the composite and self-extinguishing properties. The TPU composite with 0.25 wt% of HGM-[Ecmmim][BF₄] had a lower LOI value of 25.0%, which indicated comparatively lower flammability resistance. However, it still possessed a V-0 UL-94 rating, which suggested that the addition of HGM-[Ecmmim][BF₄] helped to improve its flame-retardant properties.

The EP/MPP composite with 0.1 wt% of GO-[Bmim][PF₆] showed a moderate LOI value of 29.2% and a UL-94 rating of V-0. The addition of GO-[Bmim][PF₆] likely contributed to the flame retardancy of the composite. The EP/HPCP composite with 1.8 wt% of Gra-[Bmim][PF₆] demonstrated relatively higher flammability resistance with an LOI value of 33.8% and a UL-94 rating of V-0. The addition of Gra-[Bmim][PF₆] contributed to its improved flame-retardant properties. The EP composite with 3 wt% of Mo-MOF/GO-[Bmim][BF₄] exhibited a lower LOI value of 27.6% and a UL-94 rating of V-1. While the addition of Mo-MOF/GO-[Bmim][BF₄] improved the flammability resistance compared to the neat EP, it did not achieve the V-0 rating. The EP composite with 6 phr of ANP-[Bmim][PF₆] demonstrated a moderate LOI value of 30.3% and a UL-94 rating of V-1. The addition of ANP-[Bmim][PF₆] provided some flame-retardant properties but fell short of achieving the V-0 rating. The PU/EG/APP composite with 0.05 phr of GO-[Bmim][PF₆] displayed an LOI value of 29.0% and a UL-94 rating of V-0, which indicated good flammability resistance. The addition of GO-[Bmim][PF₆] likely enhanced the flame-retardant properties of the composite. The UPR/APP/EG composite with 0.1 wt% of GO-[Bmim][PF₆] exhibited an LOI value of 28.2% and a UL-94 rating of V-0, which indicated good flammability resistance. The addition of GO-[Bmim][PF₆] contributed to its flame retardancy. The UPR composite with 9 wt% of pEG-[Mim][ATP] demonstrated high flammability resistance with an LOI value of 36.5% and a UL-94 rating of V-0. The addition of pEG-[Mim][ATP] significantly enhanced its flame-retardant properties. The flammability properties of polymer composites combined with ionic liquids are also dependent on the types and contents of ionic liquids.