Thermal properties and flame-retardant characteristics of layered double hydroxide polymer nanocomposites

Thermal properties and flameretardant characteristics of layered double hydroxide polymer nanocomposites

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Yanshan Gao1, Lei Qiu1, Dermot O’Hare2 and Qiang Wang1 1 College of Environmental Science and Engineering, Beijing Forestry University, Beijing, P.R. China, 2University of Oxford, Oxford, United Kingdom

8.1

Introduction

The PlasticsaEurope statistics suggest that the consumption of polymer-based materials has been increasing rapidly in recent years (Fig. 8.1). Production increased from 1.5 million tons in 1950 to 311 million tons in 2014. This growth is around 9% a year on average. Polymer-based materials are now recognized as key components in many important industries such as construction, automotive, electronics, and aerospace, due to their remarkable combination of properties, low weights, cost-effectiveness, and ease of processing. Most polymers, however, suffer from thermal degradation and are highly flammable, which increases their fire hazards when used in practical applications, significantly reducing service life and severely restricting their uses in many areas (Morgan and Wilkie, 2007; Gao et al., 2014a,b; Hilado, 1998; Feng et al., 2012). Consequently, improving polymer flame retardancy is a major challenge for extending polymer use to most applications. In order to improve the thermal stability properties and flame-retardant performance of polymer resins, effective nano-sized flame-retardant fillers have been added to polymer matrices. Halogenated flame retardants have been in use since the 1930s. They are the most widely produced and used flame retardants due to the advantages of low cost, ease of processing, high flame retardancy, and miscibility (Morgan and Gilman, 2013). However, it has been found that some halogen-containing flame retardants are toxic and will produce smoke and brominated dioxins when burning, which is a great threat to both the environment and people. As the awareness of health care and environmental protection increase, the use of halogen-based additives is diminishing in Europe and the United States. In 2004, two formulations of halogen flame retardants were banned in Europe and North America, while a third was banned in 2008 (Liu and Zhu, 2014). Organic phosphorus compounds can be vapor phase or

Layered Double Hydroxide Polymer Nanocomposites. DOI: https://doi.org/10.1016/B978-0-08-101903-0.00008-2 © 2020 Elsevier Ltd. All rights reserved.

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Figure 8.1 Polymer production in million tons from 2004 to 2014.

condensed phase flame retardants, meaning that they can be useful in low loading levels when combined with polymers that inherently char on their own. But the phosphates in polymers can cause plasticization, which leads to a decrease in the mechanical properties of the polymers. Also, they can generate more smoke and CO during fire conditions because they help inhibit polymer combustion (Morgan and Gilman, 2013). Intumescent flame retardants provide excellent fire protection but tend to be limited to lower-temperature materials and fire protection barriers due to their water absorption issues. Metal hydroxides such as Al(OH)3 and Mg(OH)2 are the largest commercially manufactured flame retardants and are perceived to be very environmentally friendly. They generate greatly lower smoke and reduce overall toxic gas emissions when burning. Further, these fillers are fairly inexpensive and can be easily coated with surfactants to make their use in polymer easier. The main disadvantage of Al(OH)3 and Mg(OH)2 is their high loading (50 70 wt%) and inherent poor compatibility with hydrocarbon-based polymers. In addition, the mineral fillers can delay ignition and slow initial flame growth, but cannot stop it completely if enough constant external heat is applied (Morgan and Gilman, 2013). Layered double hydroxides (LDHs) are a class of lamellar compounds made up of positively charged mixed metal hydroxide layers with an interlayer region containing charge-compensating anions and water molecules. They can be described by the general chemical formula [Mz11 xM31x(OH)2]q1(Xn )q/n
yH2O, where Mz1 represents divalent cations such as Mg21, Zn21, Ca21, etc., while M31 is trivalent cations such as Fe31 or Al31, and Xn is a charge-balancing interlayer anion. LDHs are emerging as a new generation of thermal stabilizer and flame-retardant materials due to their unique chemical composition and layered structure. They are potentially eco-friendly flame retardants for polymer applications. In addition, by properly intercalating certain anions, such as borate into LDHs, LDHs might combine the advantages of both magnesium hydroxide (MH), aluminum hydroxide (AH), and zinc borate (xZnO
yB2O3
zH2O; known in the trade as Firebrake) (Gao et al., 2014a,b). In this chapter, we discuss the techniques for evaluating the thermal stability properties and flame-retardant performances and summarize LDH-based thermal

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stabilizer materials and fire-retardant materials, their applications, and the corresponding mechanisms.

8.2

The techniques for determining thermal stability properties and flame retardancy performance

In order to measure the thermal stability and flame retardancy of polymer composites, different standards and indices need to be considered. To date, thermogravimetric analysis (TGA) is usually used to measure the thermal stability of polymers and four approaches are commonly used to evaluate the fire properties of polymer/LDH nanocomposites, they are: microscale combustion calorimeter (MCC), limiting oxygen index (LOI), cone calorimeter (CONE), and Underwriters Laboratories (UL-94).

8.2.1 The techniques for determining thermal stability properties of polymers TGA is a kind of thermal analysis technology (Doyle, 1961; Wu et al., 2002), it is used to test the thermal stability and composition of polymer materials (Jain et al., 2016). It can offer the relationship between the sample weight and the heating temperature under the control of predefined program. Meanwhile it can be used with other analytical methods in the actual analysis of materials for carrying out comprehensive thermal analysis results (Qiu et al., 2015; Byrn et al., 1995; Zhao et al., 1997; Jeske et al., 2012). Briefly, TGA analysis was carried out with an established heating rate or a corresponding air (or CO2, N2, etc.) flow rate. When the substance which is being measured begins to sublimate, vaporize, decompose gas, or loss the crystal water in the heating process, the weight of measured materials will change. Then the thermogravimetric curve of the sample will not remain straight and instead drop little by little. By analyzing the curves of a material, we can discover the temperatures at which the sample starts to change, including the lost weight and its corresponding percentage. For example, in the TGA curves of ethylene-vinyl acetate copolymer (EVA)/LDH composites, all samples mainly underwent three stages of decomposition (Fig. 8.2) (Wang et al., 2011a,b,c,d). The first stage corresponds to the loss of physically absorbed water and interlayer water in lower temperatures (below 225 C) with a decomposition maximum at about 180 C in the DTG curve. The second step at higher temperatures (in the range of 225 500 C) is associated with the dehydroxylation of the metal hydroxide layers and the degradation of interlayer carbonates (Benito et al., 2010). Finally, the third step occurred at over 500 C, and is attributed to the removal of residual carbonate anions in TGA. Through TGA analysis, changes to crystal properties can be studied, such as the physical phenomena of melting, evaporation, sublimation, adsorption, and other experimental samples. At the same time, it is helpful to study the chemical phenomena of the materials including dehydration, dissociation, oxidation, reduction, and so on.

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Figure 8.2 TGA and DTG curves of pure EVA and its composites with 20 wt% LDHs.

8.2.2 The techniques for determining the flame-retardant performance of polymers MCC, LOI, CONE, and UL-94 are usually used to detect the flame-retardant properties of polymer composites. MCC is also known as pyrolysis combustion flow calorimeter (PCFC), which is a convenient and fast method for laboratory evaluation of flame-retardant properties. It is based on a TGA-like degradation of the polymer in nitrogen, followed by combustion of the gases produced in air (Gao et al., 2014a,b). MCC can quickly and easily measure the key fire parameters of plastics, wood and textiles, and composites. For the MCC test, just a few milligrams (c. 5 mg) of sample is heated to the setting temperature with a heating rate which

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was set up before, a wealth of information on material combustibility and fire hazard can be obtained in minutes, including the heat release rate (HRR), heat release capacity (HRC), total heat release (THR), and temperature of degradation. The LOI of a material is defined as the minimum oxygen concentration (expressed in volume percentage) required for the flame burning of the material to be carried out in a mixture gas system of oxygen and nitrogen. It is measured by passing a mixture of oxygen and nitrogen over a burning sample, and reducing the oxygen level until a critical level is reached. It is widely used to evaluate the flameretardant properties of materials (Weil et al., 1992). LOI values for different plastics are determined by standardized tests, such as the ISO 4589 and ASTM D2863. When doing the LOI test, the samples should be pressed into 120 3 60 3 3 mm sheets according to ASTM D2863. The values obtained by the test should be the averages of several tests for each sample. For LOI results, the higher the value, the more difficult it is for combustion to occur. Generally, we consider that the materials can burn easily when the LOI values are ,22, flammable when the value is between 22 and 27, and it is incombustible when the value is .27. LOI is a method to evaluate the relative combustibility of polymer materials, but it cannot give any useful information about the burning behavior (Alongi et al., 2011). Compared to MCC and LOI tests, CONE can provide useful information about the combustion of polymers and it is the most effective method for the laboratory evaluation of the flame-retardant properties of polymers. Approximately 30 g of composite samples was compression molded into 10 3 10 cm square plaques of uniform thickness (B3 mm) before the tests were performed. A cone-shaped heater with incident flux of 35 kW/m2 was used, and the spark was continuous until the sample ignited. The results obtained from CONE can be used to evaluate material-specific properties, setting it apart from many of the established fire tests which are designed to monitor the fire response of a certain specimen (Schartel and Hull, 2007). The CONE test can gather data regarding the ignition time (tig), average mass loss rate (AMLR), combustion products, HRR, THR, and other parameters associated with burning properties. HRR is generally considered to be the most important parameter for evaluating the flame-retardant performance of polymer composites. For example, pure acrylonitrile-butadiene-styrene (ABS) was observed to burn out within 830 s after ignition, and a very sharp HRR peak appears at the range of 150 700 s, with a peak heat release rate (PHRR) value of 489 kW/m2. While the ABS/MgAl LDH and ABS/ZnMgAl LDH composites showed a PHRR value of 196 and 214 kW/m2 with 60% LDH loading, respectively. The addition of LDHs prolonged the combustion times (Fig. 8.3) (Xu et al., 2012). UL-94 is a plastics flammability standard released by Underwriters Laboratories of the United States. The standard determines the material’s tendency to either extinguish or spread the flame once the sample has been ignited. UL-94 test is carried out with two standards: the vertical burn test (UL-94V) and the horizontal burn test (UL-94 HB) (Gao et al., 2014a,b). UL-94V provides useful information aimed at the dripping behavior of polymer composites. The dripping of the burning melt determines the spread of flame through secondary flaming during real situations

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Figure 8.3 Dynamic curves of HRR versus time for the pristine ABS and two LDH/ABS composites. Surface burn

Vertical burn

Horizontal burn 45º

Doesn’t ignite Under hotter flame UL 94 5VA UL 94 5VB

Self extinguishing UL 94 V-0 (Best) UL 94 V-1 (Good) UL 94 V-2 (Drips)

Slow burn rating Takes more than 3 min to burn 4 inches

Figure 8.4 UL-94 flammability ratings summary.

(Costa et al., 2007). This standard involves five parts including HB, V-2, V-1, V-0, and 5V, as shown in Fig. 8.4. The vertical burning test is measured on sheets 127 3 12.7 3 3.3 mm3 according to the standard UL-94 test ASTM D635. A good flame-retardant material should reach the UL-94 V-0 rating and have no dripping during the test. This is primarily an evaluation that is used to qualify a product. But the UL-94 test is very dependent on operators and which version of the standard is used, so different labs may obtain different results. Jiang et al. (2016) studied PP nanocomposites consisting of Zn2Al-DBS LDHs in combination with zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (ZrP) compounds. Fig. 8.5 shows that the flame for pure PP was very vigorous and spread rapidly. With an increasing load of LDHs in combination with ZrP, the combustion speed of the PP composites slowed significantly, at the same time, in the burning process, the flame of the PP composites was weaker than pure PP. When the loading of LDHs and ZrP was 5 and 15 wt%, respectively, the PP composites can reach UL-94 V-0 rating. UL-94 is the most widely used standard for the flammability of plastic materials which is used to evaluate the ability of materials to be extinguished after being

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Figure 8.5 Images of vertical flame test for: (A) PP, (B) PP LDH20, (C) PP ZrP20, (D) PP LDH10 ZrP10, (E) PP LDH6.7 ZrP13.3, and (F) PP LDH5 ZrP15 composites at different time points.

ignited. But the UL-94 test is very dependent on operators and which version of the standard is used, so different labs may obtain different results. All of the above measures can be used for determining the flame retardancy performance of polymers. Different methods provide different information regarding the burning behavior. A variety of studies have been conducted to show correlations between each of the flammability tests. Weil et al. (1992) reported that the LOI value might be leveled with UL-94 or CONE data to some degree in certain conditions, but it was hard to show close relations between them. Also, it does not mean that higher LOI gives better UL-94V ratings.

8.3

LDH-based thermal stabilizer materials and their applications

8.3.1 Thermal stabilizer introduction The thermal stabilizer is one of the important additives to materials, especially polymers which are sensitive to high temperature. Generally, ideal thermal stabilizers should have properties such as high thermostability, good compatibility with polymer materials, as well as low volatility and proper lubrication. In recent years LDHs have attracted considerable interest from both industry and academia due to their good thermostability (Lin et al., 2005; Xu et al., 2006).

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It was found that this kind of novel heat stabilizer could bring out great environmental protection and economic benefits. For instance, when combining LDHs with organotin, a synergistic stabilization effect was obtained for rigid poly(vinyl chloride) (PVC) and the cost was reduced (Hua et al., 2001). More and more researches on LDHs as thermal stabilizers for polymer materials have been carried out aiming to make sure that the polymers can be used for various aspects.

8.3.2 Thermal stability properties of LDH-based nanocomposites 8.3.2.1 Effect of inorganic LDHs Due to the unique structure of LDHs, all the members can act as a thermal stabilizer. But different interlayer anions have different results. LDHs composed of Mg, Zn, and Al are preferred as inorganic fillers within the polymer matrix since the usage of these metals preserves the original color of polymers (Yang et al., 2015). Qiu et al. (2015) carried out a series of experiments on polypropylene (PP)/ Mg3Al-CO3 LDH nanocomposites systematically. The morphology-dependent performance of Mg3Al-CO3 LDHs (plate-like, spherical, and flower-like) as nanofillers within PP matrix has been studied. The results showed that the thermal stability of PP/LDH nanocomposites was significantly improved after incorporating Mg3AlCO3 LDHs with different kinds of morphologies. Specifically, the temperature at 50% weight loss (T0.5) of the PP/plate-like LDH nanocomposites (with a LDHs loading of 13.0 wt%) was increased by 61 C compared to that of pure PP. The results also obtained that the influence of LDH morphologies on thermal stability follows the order of plate-like . spherical . flower-like (Fig. 8.6). Gao et al. (2014a,b, 2016a,b) investigated the thermal stability properties of Zn2Al LDHs with different inorganic anions on high-density polyethylene (HDPE) and EVA. The results obviously suggest that different interlayer anions intercalated LDHs could lead to different performances on the same polymer. But the inorganic anions showed the same influence order for HDPE and EVA resin, which is: Zn2Al-Cl LDHs . Zn2Al-CO3 LDHs . Zn2Al-NO3 LDHs . Zn2Al-SO4 LDHs. Lin et al. (2006) investigated the thermal stabilization of PVC with Mg2Al-CO3 and Mg3Al2Zn-CO3 LDHs. The results showed that both LDHs improved the thermal stability of PVC resin, but compared with Mg2Al-CO3 LDHs, Mg3Zn2Al-CO3 LDHs enhanced the thermal stability of PVC in terms of both long-term stability and early coloring. In addition, Wang et al. (2017) added Ni cation into Mg3Al LDHs and studied the performance of Ni0.2Mg2.8Al-CO3 LDHs on EVA. The decomposition temperature of PP with 5 and 10 wt% Ni0.2Mg2.8Al LDHs composites is 34 C higher than pure PP. Moreover, inorganic LDHs had a good effect on enhancing the thermal stability by influencing the reaction process of different kinds of polymers including PVC (Zhao et al., 2008; Zhang et al., 2007), PP(Cui, 2010; Nyambo et al., 2008a,b), etc., either by changing the valence state of the metal cations, interlayer anions, or the species of the metal cations, and even the ratios between the different layers of the metals.

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Figure 8.6 TGA analysis of: (A) PP/spherical LDH, (B) PP/plate-like LDH, (C) PP/flowerlike LDH nanocomposites, and (D) graph of T0.5 versus LDH loading.

8.3.2.2 Effect of organic LDHs The above researches have demonstrated that LDHs with inorganic interlayer anions can improve the thermal stability of polymers effectively. However, pristine LDHs with hydrophilic surface properties are not compatible with hydrophobic polymers such as PP or polystyrene (PS), which will affect the dispersity of LDHs in the polymer matrix (Yang et al., 2015). In view of this problem, organic anionmodified LDHs were reported to improve the dispersion of LDHs in the polymer matrix (Wang et al., 2012a,b,c,d; Manzi-Nshuti et al., 2009,b,c). Yang et al. (2015) modified LDHs with various anionic surfactants, such as lauric acid (LA), palmitic acid (PA), stearate (SA), lauryl phosphate (LP), or dodecyl sulfate (DS), then systematically discussed the thermal stability of PP nanocomposites containing the appropriate hydrophobically modified LDHs. The T0.5 of the PP/organo-LDH nanocomposites was significantly improved by 37 61 C, respectively, depending on the type and loading content of organo-LDHs compared to that of pure PP (Fig. 8.7). The surfactant-dependent (DS and stearic) performance of Mg3Al LDHs as nanofillers for PP matrix was evaluated by Qiu et al. (2018). The results showed that the thermal stability of the PP/LDH nanocomposites was greatly improved in terms of the value of T0.5, especially for the PP/stearic-LDH nanocomposites with a LDH loading of 20 wt%, the T0.5 was increased by 80 C compared to that of pure PP (Fig. 8.8). Wang et al. (2013a,b) investigated the thermal stability of PP/

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Figure 8.7 TG curves of PP and PP/organic LDH nanocomposites depending on: (A) the kind of organic LDHs and (B) the loading content of lauric acid (LA) LDHs.

tartrazine intercalated LDH nanocomposites, and the thermal stability of Mg3Altartrazine LDHloaded PP nanocomposites was significantly enhanced compared to pure PP. With only 0.4 0.8 wt% of LDHs, the T0.1 and T0.5 were increased by 26.2 C and 41.3 C, respectively. In addition, Zhang et al. (2014) synthesized 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid (BP) intercalated Mg2Al LDHs (Mg2Al-BP LDHs) and investigated its thermal stabilization for PVC. Congo Red tests showed that the addition of the Mg2Al-BP LDHs can improve the static thermal stability time of PVC (Fig. 8.9). The dynamic thermal stability behavior of PVC was also enhanced after the addition of Mg2Al-BP LDHs. They also found that the stabilization mechanism may be attributed to the ability of Mg2Al-BP LDHs not only to scatter the incident light but also to absorb the released HCl, which improved the resistance of PVC to both accelerated weathering and thermal degradation. Besides, many studies have proved that other organic LDHs also can improve the thermostability of nanocomposites. For example, Nyambo et al. (2009a,b) prepared poly(methyl methacrylate) (PMMA)/Mg2Al-palmitate (C16) nanocomposites and the T0.1 was increased for all nanocomposites by 15 C. The T0.5 was increased compared to the pure PMMA by 27 35 C.

8.3.2.3 Effect of LDHs with other synergistic thermal stabilizers Some reports have conducted a series of researches on the synergistic effects of LDHs with other additives such as ammonium polyphosphate (APP), MH, and carbon-based materials, etc., as well. LDHs were used as synergistic agents of APP in poly(vinyl alcohol) (PVA) matrix by Zhao et al. (2008). The results showed that

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Figure 8.8 TGA analysis of: (A) PP/DS LDH, (B) PP/stearic LDH nanocomposites, and (C) graph of T0.5 versus LDHs loading.

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Figure 8.9 Thermal stability time of PVC, PVC/Mg2Al-CO3 LDH, PVC/BP, and PVC/ Mg2Al-BP LDH composites measured by Congo Red test.

LDHs exhibit an obvious synergistic effect with APP. The Tinitalis (the initial decomposition temperature, temperature at 5% weight loss) further increased compared to LDHs alone, which may be due to the physical crosslinking effect among layered particles, APP molecules, and polymer chains. Meanwhile, all PVA/APP/LDH composites showed higher char residues than that of PVA/APP at 500 C, 600 C, or 700 C. Besides, Zhang et al. studied the synergistic effects of LDHs with hyperfine magnesium hydroxide (HFMH) in EVA (Zhang et al., 2007). TGA data demonstrated that the addition of LDHs can raise 5 18 C of EVA/HFMH/LDH nanocomposite samples with 5 15 phr (parts per hundred resins) LDHs compared with that of the EVA/HFMH sample when 50% weight loss is selected as a point of comparison. Gao et al. (2016a,b) found that for PP/Mg3Al LDH-oxidized carbon nanotube (OCNT) nanocomposites with 10 wt% LDHs and 0.5, 1, and 2 wt% OCNTs, T0.5 was increased by 41 C, 41 C, and 43 C, respectively. These increases are much higher than observed with PP/LDH nanocomposites without OCNTs. DTG analyses also clearly showed that compared to pure PP (340 C), the temperature of the maxima rose after adding LDHs, OCNTs, or a combination of LDHs and OCNTs. The temperatures of the maxima in DTG lie in the range of 350a390 C, suggesting that LDHs, together with OCNTs, act as a good thermal stabilizer for PP. Many research articles report the addition of LDHs together with other thermal stabilizers does produce a synergistic improvement in thermal stability of the polymeric host material. The physical explanation of this effect is not clear, perhaps the physicochemical properties of host material is changed by the formation of some intermediate products with high thermal stability. To determine the origin of these effects requires further analysis and research.

8.3.3 The mechanism of thermostability using LDHs Since LDHs could improve the thermostability of polymer materials, many experts have done a lot of work to explain the mechanism. During the heating process,

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LDHs can release H2O and CO2 effectively, which can delay the scission of polymer chains, making the polymer composites more stable. However, if the loading of LDHs is too high, the T0.5 value will decrease when compared with low LDH loading. The reason is that high LDH loading results in a lot of metal and metal oxide, which in turn accelerates the catalytic degradation of polymer on heating (Jiang et al., 2014). In addition, when LDHs were modified with organic species, the thermal stability and the overall thermal decomposition behavior changed (Costa et al., 2009). The mechanism of chloride-containing polymers, such as PVC, is a little different from polymers without chloride such as PP. The HCl releases from PVC matrix can be absorbed by LDHs, preventing the further self-catalytic reaction of PVC. The addition of LDHs led to an enhanced and excellent thermostability of polymer materials, not only due to their barrier functions, but also concern with the changes of activation energy of thermal degradation, which play an important role in hindering the movement of small molecules during the polymer degradation process.

8.4

LDH-based flame-retardant materials and their applications

8.4.1 Flame retardant introduction As the American Chemistry Council has described, flame retardants are a key component in reducing the devastating impacts of fires on people, property, and the environment. They are added to different materials (e.g., textiles, plastics) to prevent fires from starting, limit the spread of fire, and minimize fire damage. Some flame retardants work effectively on their own, others act as “synergists” to increase the fire protective benefits of other flame retardants. A variety of flame retardants is necessary because materials that need to be made fireresistant are very different in their physical nature and chemical composition, so they behave differently during combustion. The elements in flame retardants also react differently with fire. As a result, flame retardants have to be matched appropriately to each type of material. Flame retardants work to stop or delay fire, but, depending on their chemical makeup, they interact at different stages of the fire cycle. When flame retardants are present in the material, they can act in three key ways to stop the burning process. They may work to: (1) Disrupt the combustion stage of a fire cycle, including avoiding or delaying “flashover,” or the burst of flames. (2) Limit the process of decomposition by physically insulating the available fuel sources from the material source with a fire-resisting “char” layer. (3) Dilute the flammable gases and oxygen concentrations in the flame formation zone by emitting water, nitrogen, or other inert gases. Therefore, the use of flame retardants is essential to stopping or slowing the spread of fire and LDHs have been increasingly used as fire retardants.

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8.4.2 Flame-retardant performance of LDH-based nanocomposites 8.4.2.1 Effect of inorganic LDHs Due to the poor compatibility between inorganic LDHs and the polymer matrix, only a few inorganic anions intercalated in LDHs have been investigated as flame-retardant additives for polymers. Carbonate is the first and the most extensively investigated; carbonate-intercalated LDHs have been shown to be highly efficient in improving the thermal stability and flame-retardant performance of many polymers, such as EVA (Gao et al., 2016a,b; Shi et al., 2005a,b; Jiao et al., 2008; Shi et al., 2005a,b; Nyambo and Wilkie, 2009a,b), PVC (Xu et al., 2006; Zhang et al., 2004; Molefe et al., 2015), ABS (Xu et al., 2012; Zhang et al., 2004), HDPE (Gao et al., 2014a,b; Zhang et al., 2004), and so on. Zhang et al. (2004) studied the fire retardancy of MgAl-CO3 LDHs in various polymers. After PS, ABS, HDPE, and PVC were filled with the nano-LDHs with a loading of 60 wt%, their LOI values could be increased up to 28, 27, 26, and 33, respectively, and the polymers produced less smoke than the materials free of the nanoLDHs during burning. Shi et al. (2005a,b) incorporated 60 wt% MgAl-CO3 and ZnMgAl-CO3 LDHs into EVA-28, the LOI of EVA can be increased from 21 to 34 and 40, respectively. ZnO present in the mixed metal oxide formed by decomposition of ZnMgAl-CO3 LDHs can promote charring of the composite. As a result, incorporation of Zn21 into the layers of the LDHs was found to promote material charring and smoke suppression. In addition, MgAl-CO3 and ZnMgAlCO3 LDHs also showed a good flame-retardant performance to ABS resin. Both ABS/MgAl-LDH and ABS/ZnMgAl-LDH composites exhibit higher LOI, lower smoke density values, and a prolonged combustion time, compared to pristine ABS (Xu et al., 2012). Molefe et al. (2015) observed that MgAl-CO3 LDHs are a promising functional filler for plasticized PVC. Both the thermal stability and flame-retardant performance can be improved with 30 phr LDHs loading. In addition, a series of MgAlFe-CO3 LDHs have been added to EVA by Jiao et al. (2008). The results show that MgAlFe-CO3 LDHs are better than MgAl-CO3 LDHs in improving the flame retardation of EVA at the same additive loading level and reached the UL-94 V-0 rating when the LDH loading was 50 wt%. The addition of ZnAl-CO3 LDHs coated with oleate also can promote charring to retard the generation of flame for PVC (Xu et al., 2006). Gao et al. (2014a,b) synthesized HDPE/Zn2Al 2 X (X 5 CO322, NO32, Cl2, SO422) LDH nanocomposites with different loadings from 10 to 40 wt% using a modified solvent-mixing method. The influence on flame-retardant properties followed the order of SO422 . NO32 . CO322 . Cl2. When adding 40 wt% LDHs, the PHRR was reduced by 54%, 48%, 41%, and 24%, respectively (Fig. 8.10). In 2005, Shi et al. (2005a,b) reported the borate intercalated MgAl LDHs as a flame-retardant filler for EVA for the first time. MgAl-LDHs showed a good flame-retardant performance, the LOI of EVA was increased from 21 to 29 after adding 60 wt% LDHs. EVA/MgAl-borate composites with 60 wt% LDH loading also showed a significantly better smoke suppression, which was 45% less than

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Figure 8.10 MCC analysis of: (A) HDPE/Zn2Al 2 Cl, (B) HDPE/Zn2Al 2 NO3, (C) HDPE/ Zn2Al 2 CO3, and (D) HDPE/Zn2Al 2 SO4 LDH nanocomposites.

that of pure EVA. Later, Nyambo and Wilkie (2009a,b) investigated the fire resistance performance of ZnAl- and MgAl-borate LDHs in EVA. At 40% loading, the reduction in PHHR observed in EVA composites containing LDHs reached to 74% and 77%, respectively. In addition, Wang et al. (2013a,b) synthesized PP/Zn2Al-borate and PP/Mg3Al-borate LDH nanocomposites using a modified solvent-mixing method. The results show that PP/Zn2Al-borate LDH nanocomposites exhibited superior performance to the equivalent PP/Mg3Alborate LDH nanocomposites. By considering both the thermal improvement and the flame-retardant performance, 15 wt% of the highly dispersed Zn2Al-borate LDHs in PP was found to be the optimal loading. The 15 wt% Zn2Al-borate LDHs in pristine (unmodified) PP resulted in a 64% reduction of the PHRR (Fig. 8.11). It is believed that borate promotes the formation of a ceramic-like MgO or Al2O3-based coating that forms over the char, which forms on the surface of a polymer during combustion and subsequently forms a vitreous phase, which acts as a binder to reinforce this ceramic coating, preventing further combustion (Shi et al., 2005a,b). In addition to borate, phosphate-intercalated LDHs were also studied by Ye and Qu (2008). They compared the flame-retardant properties of MgAl-CO3 and MgAlPO4 LDHs in the EVA blends. The LOI values of EVA/MgAl-PO4 samples with different loading levels are 2% higher than those of the corresponding MgAl-CO3 LDHs samples. Meanwhile, both EVA/MgAl-CO3 and EVA/MgAl-PO4 LDH

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composites can pass the V-0 rating when the LDH loading reached 60 wt%. However, the composites with 55 wt% MgAl-PO4 LDHs can pass the V-1 rating while the composites with 55 wt% MgAl-CO3 LDHs cannot pass any rating in the UL-94 test. The flame-retardant mechanism of MgAl-PO4 LDHs can be ascribed to its catalysis degradation of the EVA resin, which promotes the formation of charred layers with the P-O-P and P-O-C complexes in the condensed phase and the compact charred layers formed from the EVA/MgAl-PO4 sample effectively protect the underlying polymer from burning. The SEM observation gives further evidence of this mechanism (Fig. 8.12). To sum up, LDHs intercalated with inorganic anions such as CO322, NO32, Cl2, SO422, borate, phosphate, etc. are potentially promising flame-retardant additives for polymers such as ABS, EVA, PP, and so on. But one problem with inorganic

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Figure 8.12 SEM images of charred residues of (A) EVA/MgAl-CO3 and (B) EVA/MgAlPO4 LDH composites with 60 wt% LDH loading.

LDHs is the high loading as nanofillers. In order to obtain a high-efficiency flame retardant with a low inorganic intercalated LDHs loading, more work needs to be done in the future.

8.4.2.2 Effect of organic LDHs Although inorganic anion-intercalated LDHs are potentially promising flameretardant nanofillers for some polymers, the hydrophilic surface of the inorganic LDHs is incompatible with hydrophobic polymers, which severely inhibits homogeneous dispersion of LDH layers within the polymer matrix (Manzi-Nshuti et al., 2009,b,c; Bao et al., 2008). Furthermore, the high charge density in the metal hydroxide layer leads to a strong electrostatic interaction between the hydroxide sheets, making separation of these sheets (exfoliation) very difficult. Therefore, it is important to modify LDHs with suitable organic anions to increase the gallery distance as well as reduce the hydrophilic character of the surface (Gao et al., 2014a, b). Till now, many organic anions have been intercalated into LDH interlayers including oleate, DS, SA, undecenoate, dodecyl benzene sulfonate (DBS), C16,N(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexyl)formamide-2-propenyl acid (DPHPA), 2-carboxylethyl-phenyl-phosphinic acid (CEPPA), etc. Among the organic anions, oleate is most extensively studied. The long chain of oleate acts to compatibilize the LDHs with many polymers, such as PP, PE, EVA, PMMA, and poly(ethylene-co-butylacrylate) (PEBuA). Oleate exhibits an excellent combination of high thermal stability, good water solubility, and relatively low cost, and as a result it is usually preferred to other possible surfactants (ManziNshuti et al., 2009,b,c). It was found that ZnAl-oleate LDHs revealed a good flame-retardant performance for PE, which PHRR reduction was 58% with 10 wt% LDHs loading, followed by PMMA (28%) and PEBuA (2%) (Manzi-Nshuti et al., 2009,b,c). Wang et al. (2011a,b,c,d) investigated the EVA/ZnAl-oleate nanocomposites, the result showed that the PHRR reduction was 33% with 10 wt% LDH loading. In addition, MgAl-oleate LDHs showed a similar flame-retardant performance, in which PHRR reduction was 36%, with the same LDH loading in EVA resin. In addition, Manzi-Nshuti et al. (2009,b,c) synthesized PP nanocomposites with a

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series of oleate-intercalated ZnMg LDHs. It was found that Zn0.5Mg1.5Al-oleate LDHs showed the largest PHRR reduction, 38% with 4 wt% LDHs loading. For Zn2Al-oleate LDHs, the PHRR reductions are 25% and 5% with 2 and 4 wt% LDH loading. DS is an important anion applied to modify LDHs for flame-retardant applications. Ye and Wu (2012) investigated the flame-retardant properties of low-density polyethylene (LDPE)/LDH nanocomposites with DS-modified MgAl-LDHs. When the LDH loading was 5 phr, the PHRR values were reduced by 14.5% and 5%, respectively. In addition, Wang et al. (2012a,b,c,d) proved that DS-intercalated NiAl LDHs and EVA matrix had good compatibility and when the LDHs loading was 20 wt%, the PHRR was reduced by 74.9%. DBS is another important anion applied to modify LDHs as polymer additives. Costa et al. (2007) investigated the flammability properties of the nanocomposites based on LDPE and MgAl-DBS LDHs. The PHRR values were found to be reduced significantly with increasing LDH concentration. When the LDHs loading was 16.2% (PE-LDH6), the PHRR value of the nanocomposites was reduced by 68% (Fig. 8.13A). Tig, a parameter defined as the time at which the test samples catch fire, is also significantly increased with increasing LDH content. The pure LDPE has a Tig below 100 s and that increased to above 120 s with the addition of 16.2 wt % LDHs (Fig. 8.13B). Except oleate, DS, DBS anions, 1,4-butane sultone (BS) (Wang et al., 2015a,b), phenyl phosphate (PP) (Edenharter and Breu, 2015), C16 (Majoni, 2015; Nyambo et al., 2009a,b), 2-carboxy lethyl-phenyl-phosphinic acid (CEPPA) (Ding et al., 2015), [((1,1,3,3-tetramethyldisiloxane-1,3-diyl)bis(propane-3,1-diyl))bis(2-methoxy-4,1-phenylene)bis(phenylphosphonochloridate)(SIEPDP) (Li et al., 2015), N(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-hexylacetamide-2-propyl acid (PAHPA) (Huang et al., 2012), N-(2-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-ylamino)-N-hexylformamide-2-propenyl acid (DPHPA) (Huang et al., 2011), and so on, were also intercalated into the LDH interlayer as flame-retardant nanofillers. For example, with only 6% cardanol BS modified MgAl LDHs (m-LDHs), the epoxy resin (EP) composite reached an LOI of 29.2% and UL-94 V-0 rating. The PHRR, THR, and total smoke production (TSP) values of EP/m-LDH-6% were decreased by 62%, 19%, and 45%, respectively, compared to those of pure EP (Wang et al., 2015a,b). Edenharter and Breu (2015) found that with a filler content of 5 wt%, MgAl-PP LDHs could be shown to significantly improve the flame-retardant properties of PS as compared to MgAl-CO3 LDHs, the PHRR values were 47% and 22%, respectively (Fig. 8.14A). Moreover, the heat release of the PP-LDH nanocomposites was spread over a wider range of time resulting in a higher burnout time (from 550 to 695 s), which indicates a slower transfer of mass and heat during the combustion of the polymer. The longer burning time at lower HRR may be related to the formation of a thin layer of char and residues of metal oxides that insulate the polymer from heat radiation (Fig. 8.14B). The addition of 5% and 10% of MgAl-C16 LDHs to PS also resulted in a substantial reduction in PHRR (47% and 61%, respectively) of the polymer (Majoni, 2015). In addition, Kang et al. (2013) investigated the effect of dye structure (acid yellow 36 and acid red 88)

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intercalated MgAl LDHs (d-LDHs) on the flammability of PP-g-MA composite. Compared with MgAl-NO3 LDHs, d-LDHs can significantly decrease the PHRR and THR of the composite, when the LDH loading was 5 wt%, the PHRR reduced by 22% and 33% for acid red 88 and acid yellow 36 intercalated LDHs, respectively, while it reduced by 11% for NO3-LDHs. In conclusion, organic intercalated LDHs show significantly enhanced flame retardancy compared with pure polymer matrix. Compared with the inorganic LDHs, the organic modified LDHs have much better compatibility with polymers, and much lower LDH loading is required to obtain a similar flame-retardant performance.

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Figure 8.14 (A) Heat release rate for pristine polystyrene (blue) and nanocomposites filled with CO3-LDH (black) or PP-LDH (red) and (B) residue of the cone test for Polystyrene/PPLDH nanocomposite.

8.4.2.3 Effect of LDHs with other synergistic fire retardants A problem of inorganic LDH nanofillers for polymers is that a very high loading is required. Many other flame-retardant additives were combined with inorganic LDHs, such as red phosphorus (RP), APP, carbon nanotubes (CNTs), graphene oxide (GO), melamine (MA), mesoporous silica (m-SiO2), and TiO2, etc. APP is an important flame-retardant additive, and previous studies showed that using LDHs together with phosphorus-containing flame retardants can help to improve the dispersion of these additives within the polymer matrix (Nyambo et al., 2008a,b). Zhao et al. (2008) combined different cation (ZnAl-CO3, ZnFe-CO3, NiAl-CO3, and NiFe-CO3) LDHs as synergistic agents with APP to improve the flame retardancy of PVA matrix. When the content of the LDHs in PVA is 0.3 wt% and the content of APP is 14.7 wt%, the LOI of PVA increased from 20 to 31, 33, 34, and 34, respectively. Furthermore, the amount of residue increased in the order: PVA/APP , PVA/APP/ZnAl , PVA/APP/ZnFe , PVA/APP/NiFe , PVA/APP/ NiAl. Among the PVA/APP/LDH samples, PVA/APP/NiAl showed the best flameretardant performance, which may be attributed to the slightly different decomposition behavior around 450 C from other ternary composites. Furthermore, a study on the effect of MgAlZnFe-CO3 LDHs on the flame-retardant properties of APP and melamine (mass ratio 5:1) poly(butylene succinate) (PBS) composites was investigated by Liu et al. (2014). It was revealed that IFR-PBS composites exhibited both excellent flame retardancy and antidripping properties when the content of MgAlZnFe-CO3 LDHs and IFR was 1% and 19%, respectively, for a goal of UL-94 V-0 rate and a limiting oxygen index value of 35. The results showed that a suitable amount of MgAlZnFe-CO3 LDHs had a noticeable synergistic effect on IFR-PBS composites. A possible interaction between APP and LDH was also proposed byZhao et al. (2008). During burning, APP is first thermally decomposed to form poly(phosphoric acid), which may undergo a further dehydration in two traditional ways. The phosphate ester may react with the PVA chain or itself, which subsequently crosslinks with the formation of a three-dimensional network structure.

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After adding the LDHs as synergistic agents, the approach of char formation contributed more to flame-retardant PVA. In addition to the traditional dehydration methods, poly(phosphoric acid) may react with the LDH layers, releasing water molecules and producing bridges between APP chains. The formation of a small number of such bridges will induce a stabilization of APP and a decrease in the volatility of phosphorus, and thus more APP will be available for phosphorylation and char formation (Song et al., 2006; Chen et al., 2005). The crosslinks can increase the degree of polymerization of poly(phosphoric acid), which will increase the viscosity of the melt during pyrolysis and combustion, and therefore enhance the formation of compact and dense charred layer. CNTs are another kind of flame-retardant additive for polymers. It has been found that adding a small amount of CNTs to polymers can improve the flameretardant performance of the polymer composites significantly (Xie et al., 2016). Considering the fact that LDHs and CNTs possess different flame-retardant mechanisms, the potential synergistic effect between them in polymers was investigated by Gao et al. (2016a,b). They found that a system with 10 wt% of aqueous miscible organic (AMO)-LDH and 1 wt% OCNT showed a PHRR reduction of 40%, even greater than the PHRR reduction with PP/20 wt% AMO-LDH (31%) (Fig. 8.15A). The increased PHRR reduction after adding OCNTs is because the dense network of nonflammable OCNTs acts as a physical barrier to the diffusion of oxygen and also slows the escape of combustion products formed during decomposition, which can shield the polymer resin from external radiation and heat feedback from the flame (Kashiwagi et al., 2002; Song et al., 2008; Wang and Jiang, 2011a). They also found the degree of mixing between AMO-LDH and OCNT has a significant effect on the flame-retardant properties. Better mixing can lead to better flame-retardant performance (Fig. 8.15B). In addition, the incorporation of AMO-LDH-OCNT hybrids led to better thermal stability and mechanical properties. CNT and ZnAl-CO3 LDHs with good solubility in liquid media were also synthesized by Xie et al. (2016). It was established that CNT/ZnAl-CO3 LDHs could improve the thermal stability while reducing the PHRR and the total smoke release of polyurethane (PU) foams efficiently. Jiang et al. (2014) combined m-SiO2 and CoAl LDHs to improve their flame retardancy effectiveness in EP. The m-SiO2@CoAl LDHs were synthesized through a layer-by-layer assembly process. The strong Si and O signal across the sphere confirms the m-SiO2 core, while the Co and Al signals both detected in the surface region clearly suggest the adsorption of CoAl LDH particles (Fig. 8.16A). Incorporation of m-SiO2@CoAl LDHs into EP led to an increase of the char yield and a decrease of PHRR as well as THR values. Compared to pure EP, the addition of 2 wt% m-SiO2@CoAl LDHs brings about a 39.3% maximum decrease in PHRR (Fig. 8.16B), and a 36.2% maximum decrease in THR. In addition, the incorporation of m-SiO2@CoAl LDHs results in a significant improvement of the char yield (Fig. 8.16C). The results exhibit that the EP/m-SiO2@CoAl LDH nanocomposites present a good flame retardancy. They also proposed the mechanism for the improved fire-resistant property of EP/m-SiO2@CoAl LDH nanocomposites. During the combustion process, m-SiO2 with catalytic activity leads to the

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Figure 8.15 PHRR reduction of PP and the various nanocomposites prepared.

formation of pyrolysis products with lower carbon numbers, which can be easily catalyzed by carbonization in the presence of metal oxides. Meanwhile, CoAl LDHs can catalyze carbonization of degradation products. Moreover, m-SiO2 plays a role as a barrier that can absorb degraded products to extend the contact time with metal compound catalyst. Furthermore, the degraded products are dehydrogenated and catalytically converted into char by the combination of the mSiO2 labyrinth effect and the CoAl LDH catalysis effect. In addition, Jiao and Chen (2011) proved that TiO2 has a good flame-retardant synergistic effect with LDHs in the EVA/LDH/TiO2 blends. Only 2 phr TiO2 can make the EVA/LDH/TiO2 pass the UL-94 test. The PHRR values of the composite samples decreased with increasing loading of TiO2, and their burning was also prolonged to 600 650 s from 80 250 s. The mechanism of the reduction in PHRR is

Figure 8.16 (A) Dark-field STEM image and elemental mapping of m-SiO2@Co 2 Al LDH, (B) HRR curves of EP and its nanocomposites, and (C) digital photos of the residues from EP and its nanocomposites.

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mainly due to the physical processes in the condensed phase (Kong et al., 2006; Tang and Lewin, 2008). The accumulated TiO2 consequently formed a charred layer by collaborating with LDHs, which acts as a heat insulation barrier. This charred layer prevented heat transfer and transportation of degraded products between the melting polymer and surface, thus reducing the HRR and related parameters. Except APP, CNT, SiO2, and TiO2, microcapsulated red phosphorus (MRP) and intumescent flame retardant (IFR) also show good synergy with inorganic LDHs in polymers. The synergistic effect in the flame retardation between MgAlCO3 LDHs and MRP in EVA has been found by Jiao et al. (2006). The LOI value, the mechanical properties, and thermal stability are improved for the composites. When the loading of LDH and MRP is 70 and 10 phr, respectively, the LOI value increased from 32 to 39. Liu et al. (2014) studied the synergistic effect of IFR consisting of APP and melamine, and MgAlZnFe-CO3 LDHs. It was revealed that IFR-PBS composites exhibited both excellent flame retardancy and antidripping properties when the content of MgAlZnFe-CO3 LDHs was 1% (the total loading of flame retardant was 20%), for a goal of UL-94 V-0 rate and a limiting oxygen index value of 35. Although organically modified LDHs have much better flame-retardant performance than inorganic LDHs, organic anions are inherently combustible, thus the loading cannot be too high or both the thermal stability and flame-retardant properties will decrease. In addition, LDHs or organic LDHs alone, even at high concentrations, are not sufficient to obtain a high LOI value or V-0 rating in UL-94 testing. Thus, many synergistic flame-retardant additives are also combined with organic LDHs in order to achieve a desired result. MH is an example of a toxic-free, smoke-suppressing, halogen-free flameretardant additive. In order to improve the flame-retardant performance of polymer, a series of DS-intercalated LDHs, such as MgAl, ZnAl, and MgFe have been added to EVA/MH composites (Ding et al., 2011). The results show that the distribution of inorganics in EVA/MH/LDH composites is more homogeneous than the distribution in sample EVA/MH, which means that the addition of LDH can improve the distribution of MH in EVA. Composites containing LDH show good flame retardancy, when adding 45 wt% MH and 5 wt% LDHs, the PHRR reduction reached 88%, 68%, and 85% for MgAl LDHs, ZnAl LDHs, and MgFe LDHs, respectively. Especially for EVA/MH/MgAl LDH composite, it displays a remarkable reduction in PHRR of almost 60% relative to that of EVA/MH composite without LDHs. APP is a high-molecular-weight phosphate-based chain, it serves as both an acid source and a blowing agent in intumescent formulations to promote char formation during polymer decomposition. Phosphoric acids produced during pyrolysis promote charring, while the evolved NH3 improves swelling, hence slowing or preventing heat and mass transfer to and from the pyrolysis zone (Nyambo et al., 2008a,b). MgAl undecenoic acid LDH and APP were added to neat PS individually or in combinations at weight fractions no greater than 10% by Nyambo et al. (2008a,b). PS composites containing 5% and 10% LDH show reductions in PHRR of 17% and

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27%, respectively. When APP is added to PS at the same weight fractions, lower PHRR reductions were observed, 11% and 22%, respectively. Even though LDHs alone is more effective in reducing the PHRR than APP, their combination produced a better result than simply an additive effect. The observed reduction in PHRR for PS/LDH/APP was significantly higher, and was 42% with 5 wt% LDH and 5 wt% APP. Furthermore, APP/MgAl-DS LDH were added to poly(butyl acrylate vinyl acetate) (P(BA-VAc)) (Zhao et al., 2011). Pure P(BA-VAc) is a readily flammable polymeric material with an LOI value of 20.0%, and it cannot pass the UL-94 test. The LOI value of flame-retardant P(BA-VAc) can reach up to 30.7 and UL-94 V-0 rating, particularly when the contents of organic LDH and APP in P (BA-VAc) are 0.5 and 14.5 wt%, respectively. IFR is an efficient flame-retardant system for polymer matrix. It is halogenfree and has low toxicity. A typical IFR system is APP and pentaerythritol (APP/ PER). Ding et al. (2010) proved that the addition of LDH nanofillers into the PP composites can obtain good flame-retardant synergistic effects with APP/PER additives at appropriate LDH loadings. Only small amounts of LDH fillers (lower than 5 wt%) can evidently increase the LOI values. For example, 5 wt% loading of LDH can increase the LOI value up to 35 of PP/IFR/ZnAl-DS LDH composites from 19 of pure PP. This indicates that a small amount of LDH fillers can give a good synergistic effect on flame-retardant properties with APP/PER additives. Wang et al. (2012a,b,c,d) synthesized maleic anhydride grafted ethylenepropylene-diene terpolymer (mEPDM)/IFR/MgAl-DBS composite; the results showed that the introduction of a small amount of LDH in the flame-retardant mEPDM led to a significant decrease in HRR. The PHRR value reduced by 55.2% when the IFR and LDH loading is 38 and 2 phr, respectively. In addition, when adding 30 phr IFR and 2 phr DBS-modified MgAl LDH into EPDM, the LOI value increased to 27% from 17.5% (pure EPDM), and the composite reached a V-0 rating in the UL-94 test (Shen et al., 2013). Furthermore, PMMA/IFR/ MgAl-DS LDH composites were investigated by Huang et al. (2014) when incorporating 5 wt% LDH and 10 wt% IFR, the PHRR reduction was 37.9%, the LOI value increased to 26.1 from 17.4 of pure PMMA. Wang et al. (2015a,b) investigated the effect of DBS-modified binary MgAl- and ternary MgZnAl-LDHs on flammability of flame-retardant PP composites in combination with IFR additives. The synergism between either binary or ternary LDH and IFR occurred during the combustion. The reduction of PHRR value was 79.2% and 77.7% for PP/18 wt% IFR/2 wt% MgAl-DBS and PP/18 wt% IFR/2 wt% MgZnAl-DBS LDH composites, respectively (Fig. 8.17A). In contrast to the MgAl LDHs, the MgZnAl LDHs showed superior char-formation ability and smoke suppression due to the presence of the element zinc. When IFR is partly substituted by LDH, most LOI values are slightly higher than that with IFR alone, except PP/16 wt% IFR/4 wt% MgAl-DBS LDH composites. An optimum is observed at 2.0 wt% of MgZnAl LDHs and 18.0 wt% of IFR, exhibiting the highest LOI of 32.5% and a UL-94 V0 rating in vertical burning test (Fig. 8.17B). The combustion process for PP/IFR/LDH composites can be divided into four stages based on the HRR curves: (1) predegradation of PP; (2) main burning

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Figure 8.17 (A) Heat release rate versus time curves of PP and its flame-retardant composites and (B) the LOI values and UL-94 results of PP and its flame-retardant composites.

process; (3) char formation; and (4) oxidation of char residues (Fig. 8.17A). The incorporation of both binary and ternary LDH has a significant influence on the fourth stage, which enhances the thermal oxidative resistance of the char layer, answering for the disappearance of the second peak of HRR curves for PP/IFR/ LDH composites. Except MH, APP and IFR, zinc borate (Wang et al., 2011a,b,c,d), triphenol phosphate (TPP) (Manzi-Nshuti et al., 2009,b,c), melamine (Manzi-Nshuti et al., 2008), and ZrP (Jiang et al., 2016) were also combined with organic LDHs as a synergistic flame-retardant additive. For example, as an effective synergistic flameretardant, the addition of the LDH/ZrP composites resulted in a significant decrease in the HRR compared with pure PP. When the loading of LDHs and ZrP was 5% and 15% (a ratio of 1:3), respectively, the PHRR was reduced by 28.2%. The improved flame-retardant performance may be because LDH and ZrP decomposed

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to produce noncombustible gases, which diluted the combustible gases, and a compact char layer that acted as a fire barrier (Jiang et al., 2016). Also, Huang et al. (2014) studied the flame-retardant effect of IFR/RGO/LDH on PMMA, when filled with 10 wt% IFRs, 1 wt% RGO, and 5 wt% LDHs; it can achieve an LOI value of 28.2% and UL-94 V-1 grade. Compared with neat PMMA, the PHRR of PMMA/ IFRs/RGO/LDHs is reduced by about 45%. Previous studies show that both the physical and chemical interactions between the LDHs and the halogen-free flame-retardant (HFFR) materials are responsible for the observed synergy in fire performance (Nyambo et al., 2008a,b). The LDHs are thought to impact char formation in the polymer/HFFR system. This char is very effective, making the polymeric substance less prone to combustion (Wang et al., 2010).

8.4.3 Posttreatment of LDHs as flame retardants Because of the intrinsic hydrophilic nature of LDHs, they generally do not exhibit native compatibility with nonpolar polyolefin hosts (e.g., PP), which may affect the flame-retardant characteristics of the materials. Therefore, to enhance the compatibility of the LDHs with the polymer matrices is very important. Till now, there are the following two methods to improve the dispersibility of LDH particles in polymers.

8.4.3.1 Organic modification of LDHs In order to apply LDHs as an inorganic filler in polymer nanocomposites, hydrophobic modification of LDH is necessary, and can be achieved by intercalating anionic surfactants with hydrophobic aliphatic carbon chains, such as fatty acids and dodecyl sulfate. Hydrophobic polymer chains can easily access the interlayer of LDH when the hydrophobic interlayer is swelled, resulting in polymer nanocomposites with highly dispersed LDH. Or, the LDH materials are treated by organic modification with anionic surfactants before they are incorporated into the polymers. The anionic surfactants include fatty acid, fatty acid metal salt, silane coupling agent, or titanate coupling agent, etc. (Feng et al., 2012; Tao et al., 2009). For example, pristine Mg2Al LDH was modified with three different organic acids (laurate, palmitate, and stearate) to increase its hydrophobicity. TEM analysis showed most of the layers of LDH modified with laurate and palmitate were sufficiently separated from each other, and randomly dispersed in the PP matrix, indicating that most of the LDH layers were exfoliated in the PP matrix (Fig. 8.18A,B). But in PP/ stearate-LDH nanocomposites, a swelled LDH structure with several layers (3 6 layers) and sufficiently separated layers was observed, indicating that both the PP intercalated LDH layers and the exfoliated LDH layers existed concurrently (Fig. 8.18C) (Yang et al., 2015). Treatment of the LDHs with 1 10 wt% of anionic surfactant, for example, sodium stearate or sodium oleate results in improvement of dispersibility and fluidity.

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Figure 8.18 Transmission electron microscopy images of (A) 3 phr laurate-LDH/PP, (B) 3 phr palmitate-LDH/PP, and (C) 3 phr stearate-LDH/PP, respectively.

8.4.3.2 Aqueous miscible organic solvent treatment In order to improve the compatibility between LDH and polymer, one solution is to intercalate surfactant anions into the LDH interlayers. The other method to improve the dispersibility of LDH particles in polymer matrix is using the solvent-mixing method. Currently, most polymer/LDH nanocomposites are prepared using the melt-mixing method, by which the polymer and dried LDH powders are mixed in an extruder at an elevated temperature. However, one problem is that the LDH nanoparticles aggregate severely when being dried, which can result in poor dispersion of the LDH nanoparticles in the polymer matrix. Therefore, in order to make highly dispersed polymer/LDH nanocomposites, solvent mixing is preferred. Recently, Wang et al. (2012a,b,c,d) reported a new method that makes it possible to disperse inorganic anion-intercalated LDHs in nonpolar solvents (e.g., xylene). Polymer/LDH nanocomposites can then be prepared by the solvent-mixing method. Due to the intrinsic hydrophilic nature of LDHs, they generally cannot be dispersed in nonpolar solvents and so they do not exhibit native compatibility with nonpolar polyolefin hosts. After aqueous miscible organic solvent treatment, these solvent-treated LDH nanoparticles can now be dispersed in xylene to give a stable suspension (see Fig. 8.19A). UV-visible data also indicate that the LDH suspension in xylene is optically transparent and stable (Fig. 8.19B) (Wang et al., 2012a,b,c,d). By using this method, polypropylene/Mg3Al-LDH nanocomposites were synthesized successfully by dissolving PP into a clear dispersion of Mg3AlLDH in xylene. Further, ZnAl and MgAl LDHs with different inorganic anions (such as Cl2, NO32, CO322, and SO422) were dispersed in the HDPE, EVA, and PP polymer matrix using the solvent-mixing method. The thermal stability and flame-retardant performance of the nanocomposites were significantly enhanced due to the good dispersion of LDHs (Gao et al., 2014a,b).

8.4.4 The mechanism of flame retardancy using LDH The origin of the excellent flame retardancy and smoke suppression properties of LDHs is derived from their unique chemical composition and layered structure.

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Figure 8.19 The effect of solvent treatment on the Mg3Al-LDH nanoparticles: (A) unmodified LDH cannot be dispersed in xylene, after the washing treatment the LDHs nanoparticles can be dispersed in xylene, (B) transmission UV-visible spectra of solventmodified Mg3Al-LDH dispersions in xylene.

Although no mechanism has been proposed for flame-retardant LDH nanocomposites (Nyambo et al., 2009a,b; Zhu et al., 2001; Chen et al., 2007), it is generally believed that the flame-retardant mechanism of LDHs is different from that of silicate clay-based nanocomposites (Nyambo et al., 2008a,b). During thermal decomposition, the LDH gradually loses its interlayer water when heated in the range from 50 C to 220 C. At higher temperature, it further loses its hydroxyl groups in the host sheet and carbonate anions in the interlayer spacings to produce water and carbon dioxide, and then it is converted into mixed metal oxides. The water vapor and carbon dioxide released from the LDHs can dilute flammable gases and prevent contact of the materials with oxygen and eventually stop the combustion when there is not enough fuel to propagate the reaction, and promote the formation of an expanded carbonaceous coating or char. Char formation protects the bulk polymer from exposure to air, thus reducing the heat release during the combustion and suppressing smoke production (Zammarano et al., 2005; Chen et al., 2002). The mechanism of action of the LDHs as the flame retardant can be described as the endothermic decomposition reducing the fire intensity, the shielding due to char formation, and stabilization of char and the dilution effect (Feng et al., 2012). Consequently, the mass loss rate will be significantly reduced due to the combination of the above-mentioned three functions (Gao et al., 2014a,b).

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Conclusions and future development

In this chapter, we have summarized the current research into flame-retardant polymer/LDH nanocomposites. As flame retardant nanofillers, the role of LDHs is summarized as having four functions: (1) heat absorption (endotherm), (2) gaseous dilution, (3) char formation, and (4) dispersion. A proper loading of LDH could also improve the thermostability of polymers because the released H2O and CO2 can delay the scission of polymer chains, so making the polymer composites more stable. In addition, the synergistic effect between LDHs and other HFFR additives was discussed. As a synergistic additive, LDH can not only enhance the flameretardant properties of polymer/LDH nanocomposites, but also reduce the loading of HFFR agents in polymer matrix so as to improve the thermal stability and mechanical properties of polymer nanocomposites. The synergistic function of LDHs is considered to impact the char formation of polymer/HFFR systems. But the detailed mechanism between LDHs and HFFR needs to be explored in future work.

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