Polyimide Copolymers and Nanocomposites: A Review of the Synergistic Effects of the Constituents on the Fire-Retardancy Behavior

Abstract

Carbon-based polymer can catch fire when used as cathode material in batteries and supercapacitors, due to short circuiting. Polyimide is known to exhibit flame retardancy by forming char layer in condensed phase. The high char yield of polyimide is attributed to its aromatic nature and the existence of a donor–acceptor complex in its backbone. Fabrication of hybrid polyimide material can provide better protection against fire based on multiple fire-retardancy mechanisms. Nanocomposites generally show a significant enhancement in mechanical, electrical, and thermal properties. Nanoparticles, such as graphene and carbon nanotubes, can enhance flame retardancy in condensed phase by forming a dense char layer. Silicone-based materials can also provide fire retardancy in condensed phase by a similar mechanism as polyimide. However, some inorganic fire retardants, such as phosphazene, can enhance flame retardancy in gaseous phase by releasing flame inhibiting radicals. The flame inhibiting radicals generated by phosphazene are released into the gaseous phase during combustion. A hybrid system constituted of polyimide, silicone-based additives, and phosphazene would provide significant improvement in flame retardancy in both the condensed phase and gas phase. In this review, several flame-retardant polyimide-based systems are described. This review which focuses on the various combinations of polyimide and other candidate fire-retardant materials would shed light on the nature of an effective multifunctional flame-retardant hybrid materials.

1. Flame Retardancy

Polymeric materials play a critical role in the industry because of their attractive characteristics, such as low density, chemical resistance, and outstanding mechanical properties. Carbon-based polymers backbones are mostly combustible. For a general-purpose application, this is not a fatal flaw. However, the flammability of polymeric materials might severely limit their application in the aerospace, transportation, and electronics industries. Also, in the daily life, one main source of fire hazard is from short circuiting of electric devices. Many polymeric materials are considered to be combustibles under such a hazardous condition. On the other hand, some polymers, including polyimide, have shown great promise for use in energy storage applications. Therefore, it is necessary to understand the flame-retardant behavior of polyimides in order to fully utilize them in a safe and effective manner.

In order to fully describe the flame retardancy of polymeric materials, the mechanism of polymer combustion must be well understood [1]. The traditional model of combustion is known as combustion triangle which suggests that the basic combustion reaction involves three keystones. Polymer materials play the role as combustibles while oxygen in the air is the combustive fuel. The heat generated during combustion will raise temperature of the polymer leading to bond scissions which produces volatile polymer in the form of gas. If the heat released during combustion can reach a critical level, a combustion cycle would be established and maintained. An effective flame-retardant material would inhibit or break this cyclic process.

A recent study described the polymer combustion behavior as a kinetic fire loop model [2]. The fire loop model, initially reported by Emmons [3], is regarded as a more precise description for the combustion behavior of polymer. Combustion of polymer involves two sequences: pyrolysis and oxidation (shown in Figure 1). Pyrolysis, occurring in condensed phase, is the crucial step in the loop and generates the fuel for combustion. While in gaseous phase, the oxidation of fuel flux affects the pyrolysis via heat feeding. Hence, the aim of a flame-retardant system should be the reduction of the rate of one or both reactions. In a self-sustained combustion process, a balance between the generation and consumption of volatile fuel should be established as shown in Equation (1). In the presence of flame retardant, the steady-state pyrolysis product concentration, [G]_SS should be lowered to the flammable limit to cease the fire.

Flame retardants in condensed phase will lower the production rate of flammable polymer volatiles by introducing an extra reaction of the polymer into loops. On the other hand, flame retardant that acts in gaseous phase will react with pyrolysis products and affect the whole process of combustion by consuming more volatile fuel. The controlled fire loop model is described by Equation (2). One key finding from this study is the suggestion that restraining both reactions can result in reduction of the overall combustion process since the synergetic reduction is multiplicative and not additive. Therefore, effective flame-retardant systems should ensure the reduction of the rate of reaction in both the condensed and gaseous phases.

$$\frac{d{\left[G\right]}_{SS}}{dt}=k_{OX}{\left[G\right]}_{SS}-k_G{\left[P\right]}_{SS}=0$$

(1)

where the v_OX and k_OX is the rate and rate constant of fuel (pyrolysis product) oxidation, [OX] is the concentration of the combustion products, [P]_SS and [G]_SS are the steady state polymer concentration and pyrolysis product concentration in volume [2].

$$\frac{d{\left[G\right]}_{SS}}{dt}=k_{OX}{\left[G\right]}_{SS}+k_{FRG}\left[FRG\right]{\left[G\right]}_{SS}-k_G{\left[P\right]}_{SS}\frac{k_G}{k_G+k_{FRC}\left[FRC\right]}=0$$

(2a)

$${\left[G\right]}_{SS}=k_G{\left[P\right]}_{SS}\frac{k_G}{k_G+k_{FRC}\left[FRC\right]}\frac{1}{k_{OX}+k_{FRG}\left[FRG\right]}$$

(2b)

where [FRC] and [FRG] are the concentration of flame retardants in condensed phase and gaseous phase, respectively [2].

Different approaches can be applied to gain flame retardancy based on the model mentioned above. The first one is called physical action. In this case, no chemical reaction is directly involved in the retardancy process. For instance, by cooling, diluting the concentration of fuel gas, or forming of a protective layer, combustion could be inhibited. On the other hand, some fire-retardant agents can react during polymer combustion and interfere with the oxidation process. This type of mechanism is referred as the chemical action.

One frequently reported physical flame-retardant model is called heat sink. A flame-retardant agent, such as hydrated alumina or magnesium hydroxide, could release water vapor under high-temperature exposure [4,5]. Therefore, heat would be consumed and then temperature could drop to a level lower than the polymer combustion temperature. Additionally, the released inert water vapor would dilute the concentration of combustible gas.

Chemical flame retardancy is usually associated to chain scission. Traditional halogen fire-retardant agents would release halogen radicals to couple with highly reactive radicals forming inert molecules. Due to the reduction of fuel, the combustion cycle cannot be sustained. The flame retardants can strongly accelerate the polymer backbones scissions causing polymer to drip away from combustion area. Some fire retardants can catalyze the formation of carbonized or vitreous layer on the surface of the matrix material. This layer will act as a protection barrier.

Flame retardants can also be classified into two categories based on their methods of addition: reactive and additive flame retardants. Reactive flame retardants are introduced into materials in the synthesis stage. They are used as monomer, precursor polymers, or grafting agents. On the contrary, additive fire retardants do not react with polymer directly. They are generally physically mixed with materials. Only under high-temperature condition, do the additive fire retardants become active.

The synergistic effect between flame retardants can be achieved via proper combinations of different mechanism [6,7]. It is a smart approach to ensure the efficiency of flame retardants without sacrificing other properties. Zanetti et al. reported a hybrid polypropylene filled with montmorillonite (MMT) nanoparticles and decabromodiphenyl oxide (DB)-antimony trioxide (AO) [6]. The average heat-release rate of different compositions was measured by cone calorimeter and shown in Figure 2. A decent reduction was achieved from addition of single type of flame retardant. Both decabromodiphenyl oxide and antimony trioxide are flame-retardant additives which are active in gaseous phase. However, a combination of those two even weaken the efficiency of flame retardancy. On the other hand, system containing clay with either AO or DB showed significant reduction in heat-release rate. The highest reduction of the heat-release rate was observed in the system containing all flame-retardant additives.

2. Polyimide

Polyimide is a well-known high-performance polymer material with a wide range of properties that are desired in many commercial applications [8]. For instance, polyimides can effectively resist chemical degradation. Hence, they can be used as coating materials [9,10,11]. Polyimides have relatively low concentration of free charge carries, making them suitable for micro electrical engineering application as an excellent candidate of insulating materials [12,13,14,15,16,17,18]. To meet the specific requirement for use in electronic packaging, proper changes to the preparation procedure must be made. One of the commonly preferred approaches to lowering the dielectric properties of polyimide is by fluorination [19,20,21,22,23,24,25,26]. Fluorinated polyimides have effectively higher hydrophobicity, free volume, and polarizability.

Another frequently used modification strategy is the introduction of nanofillers [27,28], such as graphene. Marashdeh, et al. reported graphene reinforced polyimide composites [12,28]. In these studies, varying weight fractions of graphene were incorporated into the polyimide matrix. It was shown that increasing the weight fraction of graphene resulted in a significant increase in the dielectric constant and that the addition of 30 wt.% graphene resulted in about 55,000 times increase in the dielectric constant over that for neat polyimide.

Polyimide is also believed to be a promising cathode material for energy-storage devices [29,30,31]. Since the 1960s, scientists have developed organic electrode materials, such as dichloroisocyanuric acid [32]. Researchers believe that the presence of a conjugated double bond, similar to that in an aromatic phenyl ring, could promote an electrochemical redox process. Moreover, compared to ceramic materials, polymer materials are preferred because of their excellent strength and relatively low density [33,34,35,36]. With the addition of inorganic fillers, it is feasible to produce high-power density devices. Weidong Sun et al. have evaluated the energy storage properties of polyimide/barium titanium oxide, PI/BaTiO₃ composites. The dielectric permittivity of the composite was significantly changed in the presence of nanoparticles [35].

Fire hazard caused by potential short circuit or overuse is considered to be a major concern in the electronic industry. On the other hand, polyimide can be classified as a high temperature polymer [37,38,39,40,41,42,43]. Generally, polymers with a continuous service temperature greater than 150 °C are known as high-temperature polymers. Based on the presence of heterocyclic imide ring in the backbone, polyimides have characteristically high thermal stability [43,44,45,46]. Moreover, the char yield of polyimide under high-temperature exposure in inert atmosphere is very high ≥55%. Based on these unique characteristics, polyimides have become outstanding candidates for developing flame-retardant materials.

Büger et al. determined the degradation mechanism of polyimide, as shown in Scheme 1 [47]. Heat pyrolysis of polyimide in air atmosphere starts around 550 °C with rupture of imide ring structure. Once the temperature reaches around 700 °C, the decomposition products would become mostly planar polyaromatics, which provide transient protection in the form of char layer. Finally, continuous ramping of temperature will eventually lead to the failure of the polyimide residues.

Polyimide is usually synthesized by a two-step process [48,49,50,51,52,53,54,55,56,57]. The first step is the synthesis of the precursor, poly(amic acid). In the second step, chemical or thermal method is used to cure poly(amic acid), by a process called imidization, to produced fully imidized polyimide [54]. Imidization involves cyclization and dehydration of poly(amic acid). With the aid of Fourier transfer infrared spectroscopy (FT-IR), the extent of imidization can be determined. For a giving imidization temperature and time, the extent of imidization can be determined by normalizing the area of the imide carbonyl absorption peak with the area of the phenyl ring absorption peak whose intensity remains constant during imidization and dividing this ratio by the peak intensity ratio for the fully cured polyimide [55,56]. It is believed that partially cured polyimide will undergo further imidization when heated to a temperature ≥300 °C, thereby releasing water vapor into gaseous phase. During combustion, the released water vapor will dilute the flammable species [57]. Based on their thermal properties, partially imidized polyimides can serve as excellent alternative to smart flame-retardant materials.

3. Carbon-Based Nanofiller

Carbon-based nanoparticles have been of heightened interest to scientists and engineers recently. Nanofillers such as graphene and carbon nanotubes (CNT), are used to modify the mechanical, electrical, and thermal properties of different materials. It is believed that only a low loading per cent is required to achieve significant flame retardancy for these types of nanofillers. Flame retardancy of nano materials is generally effective in the condensed phase [58,59,60,61,62,63,64,65,66,67,68,69,70,71]. CNT [72,73,74,75,76] and graphene can form of a dense char layer covering of various polymer materials.

Kashiwagi et al. reported the reinforcement of poly(methyl methacrylate) (PMMA) by single-walled carbon nanotube (SWNTs) [77]. The flame retardancy of the composite was analyzed by using the cone calorimeter. The heat-release rate and mass-loss rate of the samples were severely suppressed in the presence of SWNT. Neat PMMA showed a maximum heat-release rate higher than 1200 kW/m². However, with addition of 0.5% SWNT, the heat-release rate was lower than 600 kW/m². Also, the mass-loss rate of neat PMMA was higher than 35 g/(m²s). With addition of SWNT (0.5%), the mass-loss rate of PMMA was decreased to around 10 g/(m²s). In the neat polymer samples, the whole structures were observed to be burnt out without any residues. However, at low SWNT loading per cent, partially protective char was formed. A dense protective layer was formed when SWNT loading was equal to or greater 0.5%. In these cases, no cracking was observed.

Another commonly used nanofiller is graphene. As a monolayer carbon 2D material, graphene or graphene oxide could provide remarkable property enhancement in the composites [78,79,80,81,82,83], such as improved thermal stability and fire retardancy. Graphene usually decomposes around 600 °C in air but can remain stable up to 800 °C in inert atmosphere [84,85,86,87,88,89,90,91,92,93].

Unlike neat amorphous polyimide, the presence of graphene can be detected by X-ray diffraction analysis. Longun et al. reported a series of study on nano graphene sheets/polyimide nanocomposites [94,95,96,97]. Wide angle X-ray diffraction (WAXD) was used to evaluate the dispersion and presence of graphene in the polyimide matrix. There are two important diffraction peaks to be noticed. The peak located at 26.5° (d = 3.36 $\dot{\mathrm{A}}$) indicates the presence of graphene platelets and it was observed in both neat graphene samples and PI-GR composites. Figure 3 shows the graphene peak intensity increases with the graphene fraction in composites. Another peak at 5.65° (d = 15.63 $\dot{\mathrm{A}}$) was observed in composites samples only and it is due to the interaction of graphene with polyimide matrix [94,95,96,97]. The broad peak locating around 18.87° (d = 4.70 $\dot{\mathrm{A}}$) was found in spectrum of polyimide and indicates the amorphous hallow.

Also, functionalized graphene (FG) or graphene oxide (FGO) would have a better dispersion in the polymer matrix. This is crucial to ensure performance enhancement. For instance, Bao et al. reported a graphene oxide functionalized with cyclophosphazene incorporated with polystyrene [86]. Better dispersion of FGO was observed compared to the neat graphene. The fire-retardancy results are shown in Figure 4. It shows the enhancement in thermal stability in the presence of FGO. In fire-safety tests, it was reported that the addition of 3 wt.% FGO resulted in a 53% decrease in the peak heat-release rate when compared to neat polystyrene, PS. The total heat release for this system was also decreased by 33% over that for the neat PS.

Zuo et al. reported polyimide/graphene oxide/montmorillonite (P/G/M) aerogel composites [98]. Montmorillonite (MMT) clay nanoparticle is capable of enhancing the flame retardancy of polymer matrix. The presence of graphene oxide will overcome the possible flaw of heavy aggregation of MMT through a synergistic dispersion effect. The measured LOI values confirm the synergistic effect between MMT and graphene oxide. The combination of PI/GO (5 wt.%)/MMT (10 wt.%) has LOI of up to 55 compared to neat PI of 44.6 and PI/GO of 46 and PI/MMT of 45.5.

In a recent report, Akinyi et al. studied the flammability of polyimide and graphene/polyimide nanocomposites [87,99]. Multilayered graphene sheets with thickness ranging from 50–100 nm was used in these studies. Multilayer graphene is cheaper and more processable than single-layer graphene. The nanocomposites contained up to 50 wt.% graphene. The presence of graphene in the nanocomposite was confirmed by FT-Raman spectroscopy. The absorption peaks for graphene were observed at 1360 cm⁻¹ and 1540 cm⁻¹. Absorption peaks at 1790 cm⁻¹ and 1390 cm⁻¹ were attributed to the imide ring. In Figure 5, it was shown that imide ring intensity increases with the fraction of graphene in composites Elemental composition analysis based on EDAX was reported in Figure 6 [99]. After combustion, the nitrogen and oxygen fractions decreased while a significant increase in the carbon fraction was observed [99]. As shown in the derivative weight-loss DTA curves, there are multiple stages of decomposition of PI and PI-GR composites [99]. The derivative weight-loss curve of neat PI and graphene as well as graphene/polyimide composites are shown in Figure 7a,b. Figure 7a shows that the first stage of decomposition of polyimide starts from 600 °C and extends to 650 °C. This initial degradation profile is associated with the decomposition of the imide ring. At about 700 °C, the polyimide char starts to fail. At a higher temperature of about 800 °C, another decomposition stage is observed both in neat graphene and in polyimide-graphene composites and is attributed to the degradation of graphitic carbon in graphene. Figure 7b shows the effect of graphene on the decomposition of the imide ring. It is clearly shown that the higher the graphene concentration, the better the protection provided to the imide ring (Figure 7b (inset)). This phenomenal behavior is attributed to the ability of graphene to act as a heat sink and shield over the imide ring. It is very noteworthy, therefore, that the presence of graphene strongly influenced the failure of polyimide ring and polyimide char. The calculated char degradation rate is plotted as a function of graphene weight fraction and is shown in Figure 8. The trend of degradation rate shows that the presence of both graphene and polyimide in the composite resulted in overall reduction of the char degradation rate for both the neat PI as well as for graphene. Indeed, the lowest char degradation rate occurred in the composite containing 30 wt.% of graphene (Figure 8). Akinyi et al. and others also determined the heat-release rate based on TGA and DSC analysis and by using Equations (3) and (4) [99,100].

$$\Delta H={{\displaystyle \int }}_{T_1}^{T_2}{\left(\frac{\partial H}{\partial T}\right)}_{p}dT={{\displaystyle \int }}_{T_1}^{T_2}{C}_{p}dT.$$

(3)

$$\dot{q}=\Delta H\times {\dot{m}}_{fuel}$$

(4)

where ΔH is the total enthalpy change of degradation and ${\dot{m}}_{fuel}$. is the weight-loss rate from TGA and DTA. They showed that the total heat of degradation decreased remarkably by an order of magnitude from 1.1 kJ/g for the neat PI to 0.1 kJ/g for PI reinforced with 50 wt.% of graphene [99].

4. Phosphazene

Halogen fire retardants used to be the most widely used reactive flame-retardant additives. Frequently used halogenated additives are shown in Scheme 2. Those additives usually contain up to 10 halogen atoms which can be released into the gaseous phase as free radical (X●). Halogen radical will couple with hydrogen radicals to form non-flammable species HX which would dilute the fuel gases in the form of protective gaseous coating. However, the traditional halogen flame retardants have a serious flaw. Although, this type of flame retardant is effective and only small amount of addition is required to give high flame retardancy. Halogenated fire-retardant agents release toxic gaseous materials during combustion. Therefore, they are prohibited in most countries [102,103,104,105].

Phosphorus based additives have similar flame-retardancy mechanism as halogenated fire retardants. These additives can evaporate into the gas phase as active phosphoxy radicals (PO₂●, PO●, and HPO●) which the act as scavengers of hydrogen H● and hydroxy OH● radicals to effectively inhibit combustion. They are, indeed, effective combustion inhibitors. It was reported that phosphorus containing radicals have 5 times higher flame retardancy than bromine and 10 times higher than chlorine radicals [106,107,108,109,110,111,112]. Red phosphorus is a common option of flame-retardant materials for polyesters, polyamides, and polyurethane [113,114,115]. It is considered to be very effective as a high-concentration source of phosphorus. Another frequently used inorganic compound is ammonium polyphosphate (APP). The best performance of APP is achieved when incorporated with oxygen/nitrogen-containing polymer. The product of pyrolysis of APP could catalyze the self-charring of polymer [116,117,118].

Phosphazenes are one class of organic compounds which has a double bonds linkage between phosphorus (V) and nitrogen, with the formula RN = P(NR₂)₃. With the aid of in-situ FT-IR spectroscopy, several authors have demonstrated both the condensed- and gas-phase fire-retardant mechanisms for cyclophosphazene and its derivatives [119,120,121]. The thermal oxidation of cyclophosphazene involves two important steps. Cleavage of the P-O-C bonds occurs in the temperature range of 250 °C–300 °C depending on the pendant group on the phosphazene ring. CO₂ and H₂O are released during this process. Meanwhile, P-O-P bond will be formed by condensation. The condensation of phosphazene will also release water as shown in Scheme 3. This condensed structure will remain intact until the temperature reaches 500 °C. The P=N bond starts to fail around 300 °C–350 °C, thereby releasing NH₃, N₂, and inhibitor radicals. Once the P-O-P linkage fails, the fire-extinguishing radicals, such as PO₂●, PO●, and HPO●, will then be released into gaseous phase.

Phosphazenes are frequently inserted into polymer matrix to enhance the thermal properties or flame retardancy. It has characteristic outstanding thermal stability [122,123,124,125,126,127,128,129,130,131,132]. Also, the P-N group is more environmentally-friendly compared to traditional halogen additives. Wang et al. reported that the presence of phosphazene derivative enhanced flame retardancy of epoxy resin (EP) [133]. Hexachlorocyclotriphosphazene (HCCP) is one common cyclophosphane precursor reported by Wang [133]. Compared to neat EP, cyclophosphane/EP system containing about 10% of the additive can produce residue weight fraction between 6 and 24% when heated in nitrogen atmosphere, as shown in Figure 9. The char retention increases with increasing cyclophosphazene wt.%, in both nitrogen and air atmospheres. Scanning electron microscopic analysis of the char formed showed that the cyclophosphazene/epoxy resin (EP) char has a more compact and dense structure than that for the neat EP [133].

Also, like other phosphorus-based fire retardants, phosphazene is believed to release radicals into the gas phase to inhibit combustion. Rangaraj et al. reported that phosphazene/polypropylene blend produced enhanced thermal properties [134]. In their work, Rangaraj and his co-workers used the HCCP as the precursor material. HCCP was pre-treated with bisphenol-S and aminopropyltrimethoxysilane in order to obtain amine-functionalized phosphazene. Then coupling between modified phosphazene and polypropylene-graft-maleic anhydride (PP-g-MA) was carried out to obtain polypropylene-phosphazene composites. X-ray photoelectron spectroscopy (XPS) was used to perform elemental analysis. Thermal stability and limiting oxygen index (LOI) were found to increase after the introduction of phosphazene as shown in Figure 10. LOI was calculated by using the Van Krevelen equation (Equation (5)) [135]. Both the char yield and LOI increased with increasing weight fraction of amine-functionalized phosphazene, APZS (Figure 10). The Young’s modulus and impact strength for the PP/PZA composites were found to vary with the weight fraction of phosphazene. The Young’s modulus slightly increased with addition of phosphazene until weight fraction of about 5 wt.% after which it slightly decreased until it reaches about 10 wt.% loading. As for impact strength, the impact strength for PP/PZA composite was higher than that for neat PP at low weight fraction of PZA. However, once weight fraction of PZA exceeds 10 wt.%, the impact strength of the composite begins to decrease. The limited oxygen index (LOI) was calculated from Equation (3) by using the char yield obtained during pyrolysis in nitrogen atmosphere. Accordingly, increasing char yield results in increasing LOI.

$$LOI=17.5+0.4\times cy.$$

(5)

where cy indicates the % char yield of the sample.

The chemically modified amine-functionalized phosphazene can be used as comonomer for copolymerization in order to significantly enhance the thermal properties of polymer. Kumar et al. synthesized a polyimide-co-phosphazene copolymer by using diamine terminated cyclophosphazene as the comonomer. 2,2′-dioxybiphenylyl substitution was used to form a diamine terminated phosphazene which was then coupled with the dianhydride monomer to obtain phosphazene modified polyimide shown in Scheme 4 [136]. Unlike the commercial linear polyimide, the char yield in air atmosphere for the phosphazene-co-polyimide copolymer is notably high. The char yield of modified polyimide ranges from 50–58% in air atmosphere. Kumar et al. pointed out that the presence of high percentage of phosphorus in phosphazene ring can provide a reasonable explanation for high char yield. In Figure 11, a comparison of thermal decomposition behavior in air for neat PI, neat phosphazene, PI-PDMS [57] and PI-co-phosphazene [136] is shown. Neat polyimide and hexachlorophosphazene decomposed almost completely in air, like most carbon-based polymers. The presence of linear PDMS blocks in the backbone of PDMS-block-PI increases the char yield up to 10%. The enhancement of fire retardancy for phosphazene-co-polyimide due to the attachment of phosphazene unit in the co-polyimide backbone is remarkable and results in the formation of much higher char yield in air of ≥50%.

It should be noted that the presence of chloride group in phosphazene may lead to undesirable environmental issues. The alternative is to use commercial grade cyclotriphosphazene derivatives. There are serval reports about the flame retardancy of phosphazene derivatives [137,138]. For instance, aziridinyl phosphazene, which is shown in Scheme 5, can be considered as one candidate. Aziridine ring can behave in a way similar to the epoxy ring. It can undergo ring-opening reaction just like the oxirane ring as shown in Scheme 6. This reaction tends to occur when the side substituents of aziridine (R) are electron-withdrawing and strong nucleophiles, such as amines.

5. Silicone-Based Flame Retardants

Silicone-based additives are another promising class of flame-retardant additives [139,140,141,142,143,144]. The dissociation energy of Si-O bond is 108 kcal/mol which is higher than 85.2 kcal/mol for C-C bonds or 82.6 kcal/mol for C-O bonds [51]. This very fundamental property ensures greater thermal resistance for silicone-based materials compared to traditional organic carbon-carbon compounds. Si-O bond can serve as oxygen sink during thermal oxidation and can transform itself into silica, SiO₂. Other silicon based covalent bonds such as Si-Si and Si-N bonds show similar mechanism but with better performance because they can consume more oxygen atoms.

There are plenty of choices among the silicone-based compounds that are flame retardants. The mechanism and addition methods are also varied. The thermal behavior of silicone is dependent on its structure. Polysiloxanes with silanol end group (Si-OH) would decompose by unzipping mechanism [145]. Polysiloxanes without such end-capped functional groups have different mechanism. Inter or intra molecular redistribution occur randomly between siloxane backbones. This thermal degradation mechanism will result in the formation of cyclic oligomers and silica. Such mechanism is called random scission reaction. Moreover, there is one more case of thermal degradation for polysiloxanes. Some polar impurities and additives in polysiloxanes would lead to externally catalyzed degradation. The Si-O backbone will hydrolyze in the presence of polar impurities.

The fire retardancy of silicones-based materials show both mechanisms. Many reports show that thermally stable residues and silica are formed under high-temperature treatment. The heating rate mainly decides the fraction of residues [146]. For instance, at slow heating rate, cyclic oligomers would be the main product. On the other hand, increasing heating rate creates more tetramer. Radicals are also formed during bonds scission. The production of radicals can only occur at high temperature, leading to the release of methane and macro radicals. Coupling between radicals would strongly decrease the flexibility of polysiloxanes and slow down the degradation of oligomers. One important advantage of most silicone-based fire retardants is that the combustion of silicone-based materials produces little or no toxic gasses. The possible pathways of thermal oxidation of silicone-based materials are shown in Figure 12 [146].

Flame retardancy can be affected by the structures of silicone-based materials, like other aspects of performance, including the end-group, side-group, and polymer main chain. As was discussed above, the end-group affects the thermal decomposition mechanism of PDMS. Methyl end-groups provide better thermal stability for PDMS than hydroxyl terminated structures [147]. When vinyl groups are added to the system, the thermal stability would be lower than that for methyl terminated PDMS. Also, the nature of the side group has influence on polysiloxanes decomposition. It was reported that methyl side group structure is more stable than Si-H side group [148]. The thermal stability of polysiloxanes can be enhanced if phenyl groups were incorporated into the chain. PDMS is naturally stable in neutral environment around 340 °C. In the presence of small amount of methylphenylsiloxane or diphenylsiloxane, the onset degradation temperature would be increased to about 400 °C [149]. The effect of molecular weight is also to be noted. Based on published reports, longer siloxane chain would give shorter ignition time in cone calorimetry [150]. The reason of this effect is that the length of chain decides the amount of combustible hydrocarbon in system. On the other hand, the mass of residues is proportional to the molecular weight of polysiloxanes.

Silicone elastomeric nanoparticles (S-ENP) prepared by spray drying the silicone latex was physically mixed with Nylon-6 in an extruder followed by extrusion as was reported by Dong et al. [151]. In cone calorimeter tests, modified nylon composites show better flame retardancy than neat nylon 6 shown in

Table 1. Dependence of Peak HRR and Time to Peak HRR on composition for nylon-6 and PA/clay/silicone nanocomposites (heat flux of 35 kW/m²) [153].

Samples	Peak HRR (kW/m2)	Time to Peak HRR (s)	Mean HRR (kW/m2)
Nylon-6	790	375	370
PA-1 a	317	685	129
PA-2 b	249	380	110

^a: 10 wt.% of S-ENP, ^b: 10 wt.% S-ENPC(S-ENP/clay (4/1)).

. With incorporation of exfoliated clay, the flame retardancy of the system can be further enhanced. An improved flame retardancy has been confirmed based on observation of the combustion residues [96].

Silica/polyimides nanocomposite aerogel was reported 2020 by Zhang et al. [152]. Aerogels are well-known as outstanding thermal isolation due to high porosity and low density [153]. A novel co-gel preparation strategy was highlighted here to obtain the hierarchically porous structure of aerogels. The LOI of PI/silica composites could be raised up to 48 compared to neat PI with 34 for LOI.

Polyhedral Oligomeric Silsesquioxanes (POSS)

Nanoparticles are now attracting lots of attention in materials engineering. Organic-inorganic composite materials are believed to lead to revolution of high-performance materials. Combination of inorganic components and polymers could provide enhancements of both high rigidity due to the inorganic materials and flexibility of polymer [154,155,156,157,158].

POSS was first reported by Scott in 1946 [159]. It is regarded as one of the smallest silica nanoparticles with diameters of 1–3 nm [160,161]. The empirical formula of Silsesquioxanes is RSiO_1.5, where R can be a hydrogen atom or different group such as alkyl, alkylene, or acrylate. A wide range of options for the end-groups can be achieved by chemical modification. Structures of silsesquioxanes can be found in different shapes as shown in Figure 13, where R represents a functional group, such as hydroxyl and amine.

POSS has inherent high thermal resistance and excellent mechanical properties [162,163,164,165,166,167,168,169,170,171]. For instance, Handke et al. reported that ladder-like polysilsesquioxanes decompose at temperatures >400 °C [172]. Highly crosslinked POSS produces about 70% residues under nitrogen pyrolysis. Some researchers reported that incorporation of polysiloxane blocks into carbon-based polymer might severely decrease the flexibility of polymers [173]. POSS-based copolymers can overcome this flaw and effectively enhance mechanical and thermal properties. Feng et al. reported that the char yield of PI increased in the presence of increasing amount of ladder-like POSS [174].

Fan et al. has reported a crosslinked polyimide with functionalized cage POSS as crosslinker [175,176]. The improvement of thermal properties of PI was confirmed by DSC and TGA tests, including increase in the T_g and thermal stability. The most significant outcome is the LOI increase from 46.5% for neat PI to 57% with 5.43 wt.% of POSS.

6. Hybrid Systems

To achieve optimal flame retardancy, a combination of multiple flame-retardant additives must be considered. One important advantage of a hybrid system is that flame retardancy can be achieved in several ways or different mechanisms. Hence, high efficiency of fire retardancy can be achieved [177,178,179,180,181,182,183,184,185,186,187,188,189,190,191].

In 2019, Revathi et al. reported a nanocomposite of phosphazene core-base POSS-reinforced polyimide materials [192]. The hybrid composite was synthesized by coupling of phosphazene to naphthalene tetracarboxylic dianhydride (NTDA) to form phosphazene imide (PZI) copolymer. Additionally, a cage shaped POSS with amino ended group was then reacted with PZI copolymer to complete the last puzzle and form the hybrid system [192] as shown in Scheme 7).

This POSS-PZI nanocomposite was evaluated for different properties. TGA analysis shows that POSS/PZI composite containing 10 wt.% POSS has 14% higher char yield in nitrogen atmosphere at 800 °C than neat PZI as shown in Figure 14. LOI calculated by using Krevelen equation [135], shows that that the LOI of PZI/POSS composites increased with increasing amount of POSS. The glass transition temperature of POSS-PZI obtained by differential scanning calorimetry, DSC, shows increased T_g of PZI/POSS with increasing amount of POSS, from 238 °C for neat PZI to 297 °C for PZI/POSS containing 10 wt.% of POSS. The LOI of POSS-PZI composites were also higher than that for neat PZI. It increased from 39.9% for neat PZI to 45.5% for PZI/POSS containing 10% POSS. This paper also reported that anti-bacterial effect and UV shielding behavior were also improved with addition of POSS. However, this system can be improved further. First, the comparison was only done for PZI and PZI-POSS composites. The role of phosphazene was not fully understood. Secondly, the polyimide studied in this work was synthesized by using amino terminated phosphazene and naphthalene tetracarboxylic dianhydride, NTDA. This would create a highly crosslinked structure and shorter linear polymer chain. Such a system might strongly decrease the mechanical properties and processability for ultra-high stiffness of chains. Hence, significant improvements and modifications can be made.

7. Comparison of Techniques Used for Flame Retardancy

In this section, serval important techniques that are used to measure decomposition and flammability behavior of polyimide and polyimide nanocomposites are reviewed. A brief compassion of these techniques is shown in

Table 2. Comparison of commonly used flame-retardancy techniques.

	TGA	DSC	LOI/UL-94
Sample Scale	mg	mg	g
Strength	Monitor weight change under controllable heat conditions. Char yield, Td, LOI	Calorimeter method, measuring phase transition (Tg, Tm) temperature, heat capacity and enthalpy change	Commercially preferred standard flame-retardant test, easy, and low-cost
Weakness	Lack of information on heat release	Usually test under Td, cannot provide weight-loss conditions	Insufficient sensitivity
	Cone Calorimeter		MCC
Sample Scale	g		mg
Strength	Provide detailed information on combustion, including mass loss, heat-release rate (HRR), total heat released (THR), time to ignition, and time of combustion/extinction		Non-flaming combustion methods separately control and study the pyrolysis and thermal oxidation
Weakness	Require large scale and specific dimension samples		Accuracy influenced by potential side products of decomposition, such as water vapor and carbon oxide

(T_d: Degradation temperature, T_g: Glass transition temperature, and T_m: Melting temperature).

.

Thermal gravimetric analysis (TGA) is a fundamental tool for thermal properties study. TGA can measure the mass change of samples under programmed and controlled heating procedure. The weight change caused by breakage of covalent bonds and loss of small molecule, such as water, can be monitored. By comparing the weight of residues which is regarded as char yield (Cy) and degradation temperature (T_d), the thermal stability of a material can be determined. Also, the derivative weight-loss analysis based on the integral of the area under the derivative weight-loss curve is used to calculate the decomposition rate of materials. TGA analysis can be done under different gaseous atmosphere, including inert nitrogen atmosphere and oxidizing oxygen or air atmosphere. By means of the Kissinger–Akahira–Sunose (KAS) model (shown in Equation (6)), the activation energy of the respective thermal process can be determined from the TGA data obtained at a handful of heating rates. The activation energy for decomposition is usually obtained from the slope of lnβ/T² vs. 1/T where β is the heating rate [193]. Besides, there are two other methods which can be used to evaluate the decomposition processes, including the Friedman method (Equation (7)) and the Ozawa–Flynn–Wall method (Equation (8)) [194,195,196,197]. TGA char retention data can also be used to estimate the limiting oxygen index (LOI) from the Van Krevelen equation (Equation (5)).

$$\ln \frac{\beta }{T^2}=\ln \left(\frac{AR}{E_{a}g\left(\alpha \right)}\right)-\frac{E_a}{RT}$$

(6)

where α is the degree of conversion, β is the heating rate, g(α) is the integral conversion function, A is the pre-exponential factor, R is the gas constant, and Ea is the activation energy.

$$\ln \frac{d\alpha }{dt}=\ln \beta \frac{d\alpha }{dT}=\ln \left[Af\left(\alpha \right)\right]-\frac{E}{RT}.$$

(7)

where the E is the activation energy, A is the pre-exponential factor (s⁻¹), and f(α) is the conversion function. In the Friedman method, ln dα/dt vs. 1/T is plotted for different heating rates. A straight line is drawn through the same α value for the thermogram with different heating rates. The activation energy is then obtained from the slope of this isoconversional line.

$$G\left(\alpha \right)=\frac{A}{\beta }{{\displaystyle \int }}_{T_0}^{T_p}{e}^{-\frac{E}{RT}}dT={{\displaystyle \int }}_0^{{\alpha }_p}\frac{d\alpha }{f\left(\alpha \right)}.$$

(8)

where T₀ is the initial temperature (when α = 0), and T_p is the peak temperature in DTG curves. The Ozawa–Flynn–Wall method extracts the activation from the slope of the linear line of lnβ vs. 1/T curve.

The differential scanning calorimetry, DSC is another important analytical technique used to determine heat of thermal transitions and thermal transition temperatures. It is also used to determine the heat of reaction as well as the heat of decomposition. In DSC, the heat change due to state/phase transition is measured. The glass transition temperature (T_g), crystallization temperature, T_c and the melting temperature, T_m can be determined by DSC. To avoid the influence of thermal oxidation and other chemical reaction such as thermal degradation, the temperature range for DSC test is usually set to be lower than decomposition starting point and samples are tested under inert atmosphere, such as nitrogen or argon atmosphere. When DSC is carried out at high temperatures ≥ 400 °C and under an oxidizing atmosphere, the heat of decomposition and the heat capacity of decomposition can be determined.

There are three commonly used techniques for flammability tests of materials: UL-94 test, limiting oxygen index (LOI) test, and calorimeter test. All these three techniques can provide quantitative information about how flammable a material is [198,199,200,201,202,203,204,205,206]. For LOI test, the minimum concentration of oxygen in an oxygen–nitrogen mixture atmosphere needed for materials combustion is measured. However, recent study has indicated that LOI test has some limitations. Extinguishing of flame might occur because of the dripping of materials that might take the flame away from combustion zone, thereby giving a higher LOI values than expected. Also, some researchers pointed out that LOI might unexpectedly vary with the concentration of flame retardant. In some cases, LOI might decrease after reaching the peak of effectiveness along with the addition of flame retardant [207]. Therefore, an empirical calibration curve is required for this system. As for UL-94 test, it is widely applied in both industry and academia. The materials are ignited by a controlled burner in UL-94. The after-flame time, which is the time required for the flame to extinguish, is recorded. To ensure the accuracy, cohesive samples are generally preferred to avoid potential error created by samples flowing.

Calorimetry in the form of Cone calorimetry, or Microscale combustion calorimetry, MCC is capable of providing more quantitative parameters including the heat-release rate (HRR), total heat of combustion, and onset temperature for combustion, making it reliable in measuring flame retardancy. In cone calorimeter test, a forced combustion of organic sample is accomplished by projected radiant heat [208,209]. The heat-release rate, time to ignition (TTI), total heat release (THR), and mass-loss rate are monitored by using a series of sensors. The heat of combustion (h_c) can be calculated by using Equation (9).

Recently, microscale combustion calorimeter (MCC) has gained lots of attention. One commonly known advantage of MCC, is that it consumes less energy and small amount of samples, usually in the milligram scale, which makes it more preferable and adaptable. MCC can simulate the combustion condition based on a different mechanism as cone calorimeter [210,211,212,213,214,215]. In contrast to real flaming tests, MCC creates a well-controlled premixed burning which can sustain the pyrolysis and combustion (oxidation of volatile fuel) separately. On the contrary, flaming combustion faces the huddle of controlling of the entire process which might lead to insufficient combustion. The most important part in MCC set-up is a pyrolysis-combustion flow calorimeter which consists of individually controlled reactors of fuel generation and heat generation. This arrangement makes it possible to alter the heating conditions and composition of atmosphere in condensed phase and gaseous phase separately.

$$h_c=\frac{THR}{1-Cy}$$

(9)

8. Conclusions

This review is focused primarily on flame-retardant behavior of polyimides, an engineering high-temperature polymer, and its composites and copolymers. The synergetic fire-retardancy effect of the polyimide and its copolymers and commonly used flame-retardant additives is described. Because polyimide is considered as a potential candidate for dielectric and energy storage application, the potential for fire hazard must be a concern as is the case with all carbon-based materials. Because of the presence of aromatic phenyl ring in its backbone, polyimide can serve as a fire retardant in the condensed phase by forming protective char layer during pyrolysis. Neat polyimide can produce about 55% char yield when heated under inert atmosphere, however, combustion of PI in air leads to catastrophic failure. Hence, the flame retardancy of polyimide could be improved by introducing materials such as POSS and phosphazene to improve its flame retardancy in oxidizing and inert atmospheres. Carbon-based nano composites like carbon nanotubes and graphene can improve char yield. When reinforced with graphene, the composites char failure rate is 40% lower than that for neat polyimide. Silicone base nanoparticles such as POSS can also enhance the flame retardancy in both the condensed and gaseous phases and the product of its pyrolysis is ultra-thermally stable. The heat-release rate of silicone nanoparticles incorporated into polyimide is about 60% lower than that for the neat polyimide. Phosphorus-based fire retardants can release radical into the gaseous phase, which couples with flammable radical to form inactive species in the gaseous phase. For a polyimide copolymer containing phosphazene in the backbone, the char yield in air atmosphere is as high as 50%. A combination of polyimide matrix and other flame retardants can provide a new paradigm for design and construction of smart fire retardants capable of dual mechanism of fire retardancy while maintaining high thermomechanical properties. A phosphazene core-based POSS-PI copolymer was reported to show increased char yield with increasing amount of POSS. However, the effect of POSS on the processability and mechanical performance of the copolymer is not fully understood and needs to be determined.