Study of the thermal stability and flame retardant properties of graphene oxide-decorated zirconium organophosphate based on polypropylene nanocomposites

Abstract

The aim of this study is to reduce the flammability of polypropylene (PP) by adding an organic nano-sized flame retardant into the PP matrix. In order to compare the effects of adding N-containing zirconium organophosphate and graphene oxide (GO) decorated N-containing zirconium organophosphate to PP, PP/zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (PP/Zr(AE)₃P) and PP/GO-decorated zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (PP/GO-Zr(AE)₃P) nanocomposite films were prepared, and their flame resistance evaluated. The structure and morphology of the novel flame retardants were characterized by Fourier transform infrared (FTIR) spectroscopy, and other properties of the nanocomposites were studied by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), limited oxygen index (LOI), cone calorimeter tests, vertical flame and mechanical tests. The results reveal that both the addition of Zr(AE)₃P and GO-Zr(AE)₃P improve the flame resistance of PP but GO-decorated films exhibited other advantages. Moreover, the evaluation of the thermal properties demonstrated that the addition of Zr(AE)₃P or GO-Zr(AE)₃P to PP improves the stability of PP, and leads to a higher char yield at higher temperatures.

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1. Introduction

Polypropylene (PP) is a traditional thermoplastic that has been widely utilized in various civil and industrial applications¹ due to its excellent mechanical,² crystallinity,^3,4 thermal properties, excellent processability^5,6 and relatively low price.⁷ However, during use, fires are a problem because the neat PP is highly combustible.⁸ Therefore, enhancing the flame retardancy of PP is imperative, especially when used in the electronic industry and as a building material. For improving its flame retardance, one significant strategy is to add inorganic or organic nano-sized flame retardant nanoparticles to neat PP.^9,10

Scheme 1 Preparation routes of PP/Zr(AE)₃P, PP/GO-Zr(AE)₃P nanocomposites.

Halogen-based flame retardants have been proven to be effective on PP in that they have high fire retardant efficiency, are low cost, and require only a low loading.^11,12 However, it can produce large amounts of toxic smoke during combustion,¹³ which poses a great threat to human safety and significantly restricts their applications in many areas. Therefore, aluminum hydroxide (ATH),¹⁴ a very important halogen-free flame retardant, has received much attention because of its environmental friendliness, low cost, low toxicity, low smoke, and corrosive gas is not produced upon combustion.^15,16 However, more than 50 wt% of aluminium hydroxide is generally required for compliance with various flame retardant standards for polymers.^14,17 Moreover the addition of fillers in such high amounts often leads to a processing problem and marked deterioration in mechanical properties of the composites.¹⁸ Thus, halogen-based and ATH flame retardants are gradually replaced by other intumescent flame retardants.

In recent years, intumescent flame retardants have attracted extensive attention as they are environmentally friendly and effective when mixed with polyolefins such as PP and polyethylene through a condensed phase flame retardancy mechanism.⁸ Among them, phosphorus- and nitrogen-containing flame retardants always exhibit higher flame retardant efficiency and enhanced thermal stability.^19–22 P-containing parts act as an acid source, which promotes the char formation in the condensed phase through extracting water from the pyrolysis substrate. While N-containing parts are usually used as a gas source and produce inert gases at high temperature. In order to improve the flame retardancy of P and N-containing organophosphate flame retardants further we try to functionalize graphene oxide (GO) by reacting it with nitrogen containing zirconium phosphonate intumescent flame retardants to form a GO-decorated intumescent flame retardant that integrates the gas and condensed phase flame retardant (Scheme 1). The zirconium phosphonate moiety containing high nitrogen content can release noninflammable and nontoxic gases to dilute oxygen and volatile flammable gas, and facilitate the expansion of the char layers. Furthermore, some gases, such as ammonia and nitrogen, could trap the free radicals during combustion, which can improve the dripping resistance. Thus those factors contribute to high flame resistance. The incorporation of GO moieties containing rich aromatic structures can form intumescent carbonized layers which can reduce heat transfer between the heat source and the polymer surface as a synergist. More importantly, they greatly suppress the generation of smoke during combustion. Furthermore, dehydration of hydroxyl carboxyl groups on the basal planes and edges of GO sheets occurs and water vapor is released. Therefore, it exhibits excellent flame retardancy and is environmentally friendly when used in PP.

In this work, we synthesize zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (Zr(AE)₃P, Zr[O₃P(CH₂)₂NH(CH₂)₂NH (CH₂)₂NH₂]₂·H₂O) and GO-decorated zirconium 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonate (GO-Zr(AE)₃P, GO-Zr[O₃P(CH₂)₂NH(CH₂)₂NH (CH₂)₂NH₂]₂·H₂O) and prepared a series of PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposite films, and studied the influence of Zr(AE)₃P and GO-Zr(AE)₃P loading on the morphology of the residual char, thermal stability, mechanical properties and flame retardancy of films. The structure and morphology of the flame retardants were analyzed by Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM), and the properties of the nanocomposites were studied by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), limited oxygen index (LOI), cone calorimeter tests, vertical flame and tensile tests. The fundamental structure–property relationship of PP/Zr(AE)₃P-based and GO-Zr(AE)₃P-based PP nanocomposites were also discussed.

2. Experimental section

2.1. Materials

A commercially available PP was manufactured by Lanzhou Petroleum Chemical Incorporation (Gansu, China), with a melt flow index of 2.5 g/10 min (190 °C, 2.16 kg) and a density of 0.90–0.91 g cm⁻³. Graphite powder was supplied by Shanghai Huayi Group Hua Yuan Chemical Company Limited (Shanghai, China). Hydrogen peroxide 30% (H₂O₂, 30%) was supplied by Chengdu Kelong Chemical Reagent Company (Chengdu, China). Ethanol, sulfuric acid (H₂SO₄, 98% v/v) and hydrochloric acid were purchased from Chongqing Chuandong Chemical Reagent Factory (Chongqing, China). Others were purchased from Taixing Chemical Company (Chongqing, China). The water used was distilled and deionized.

2.2. Synthesis of (AE)₃P

2-(2-(2-Aminoethylamino)ethylamino)ethylphosphonic acid ((AE)₃P) was synthesized as follows: 2.89 g 2-chloroethyl phosphoric acid, 220 mmol diethylenetriamine, 0.3 g KI and 160 mL cold NaOH solution were added into 500 mL ice water. After the reaction at 18 °C for 120 h, the mixture was left at room temperature for 24 h and then evaporated to form crystals. Finally, the product was filtered and recrystallized twice with ethanol. The obtained powder was dried under vacuum for 48 h at 60 °C.

2.3. Preparation of Zr(AE)₃P

A solution of 4.2 g 2-(2-(2-aminoethylamino)ethylamino)ethylphosphonic acid ((AE)₃P) in 50 mL water was added into 3.5 g ZrOCl₂·8H₂O in 50 mL water with vigorous stirring and refluxing at 100 °C for 24 h. The precipitate was filtered and washed using deionized water at pH = 5–6 and dried in vacuo at 60 °C.

2.4. Preparation of GO-Zr(AE)₃P

GO was prepared by oxidation of natural graphite powder according to the well-known Hummers method.²³ Briefly, 2 g graphite powder and 120 mL H₂SO₄ were mixed in the beaker under stirring in an ice-bath. Next, 15 g KMnO₄ was added slowly into the beaker under stirring and the temperature was controlled below 20 °C. Then the ice-bath was removed and the system was kept for 2 h at 35 °C. Subsequently, 250 mL water and 10 mL 30% H₂O₂ were added slowly, respectively, and the mixture was stirred for another 2 h, resulting in a yellow-brown mixture. Finally, the resulting product was collected by centrifugation and washed with hydrochloric acid solution and water until the pH value of the upper layer suspension neared 7. GO powder was obtained under vacuum for 48 h at 50 °C.

Then, 200 mg of GO was dispersed in 250 mL of water with the assistance of ultrasonication for 1 h, and then 4.8 g of Zr(AE)₃P and hydrochloric acid was added into the GO dispersion. The mixture was stirred at 70 °C for 24 h. Afterwards, the resulting GO-Zr(AE)₃P was filtered, washed with water and dried under vacuum.

2.5. Fabrication of PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites

PP/Zr(AE)₃P nanocomposites containing 0, 0.4, 0.8, 1.2, 1.6, 2.0 wt% of Zr(AE)₃P (designated as PP, PP/Zr(AE)₃P-1, PP/Zr(AE)₃P-2, PP/Zr(AE)₃P-3, PP/Zr(AE)₃P-4, PP/Zr(AE)₃P-5) were fabricated via melt blending using a Thermo Haake Rheomix at 180 °C with a speed of 60 rpm for 10 min. The mixed materials were transferred to a mould and preheated at 180 °C for 4 min, then pressed at 10 MPa for 6 min followed by cool pressing for 10 min. PP/GO-Zr(AE)₃P nanocomposites containing 0.4, 0.8, 1.2, 1.6, 2.0 wt% of GO-Zr(AE)₃P (designated as PP/GO-Zr(AE)₃P-1, PP/GO-Zr(AE)₃P-2, PP/GO-Zr(AE)₃P-3, PP/GO-Zr(AE)₃P-4, PP/GO-Zr(AE)₃P-5) were synthesized using the same process.

2.6. Characterization

Fourier transform infrared (FTIR) spectra of the samples were recorded on a Nicolet 170SX Fourier transform infrared spectrometer (Madison, WI, USA) in the wavelength range of 4000–500 cm⁻¹ in the attenuated total reflection mode. For transmission electron microscopy (TEM) experiments, the suspension of Zr(AE)₃P and GO-Zr(AE)₃P were dropped onto a copper grid individually which was coated with a carbon film and dried in a vacuum drying oven for 3 h, after that, the samples were examined using a JEM-1200EX TEM (JEOL, Tokyo, Japan) at an accelerating voltage of 120 kV. Mechanical tests were performed using a Microelectronics Universal Testing Instrument Model Sans 6500 (Shenzhen, China) with cross-head speed of 10 mm min⁻¹ at room temperature and the initial grip separation was set at 50 mm. To study the thermal stability of the PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites, thermogravimetric analysis (TGA) was carried out at a heating rate of 10 °C min⁻¹ from room temperature to 550 °C under nitrogen flow (20 mL min⁻¹) using TA-STDQ600 (New Castle, USA). An empty pan was used as a reference. DSC was measured with a NETZSCH DSC 200 F3 (Netzsch Co, Selb/Bavaria, Germany) from −30 °C to 250 °C at a heating rate of 20 °C min⁻¹ under a nitrogen atmosphere. Nitrogen at a rate of 20 mL min⁻¹ was used as the purge gas. Heat flow versus temperature scans from the second heating run was plotted and the glass transition temperature (T_g) was read at the mid-point of the inflexion curve resulting from the typical second heating.

The morphology of the residual chars obtained was examined by means of a Quanta FEG450 SEM instrument (FEI, USA) at an accelerating voltage of 20 kV. The specimens were previously coated with a conductive layer of platinum. Vertical flame tests were performed with a CZF-3 vertical burning tester (Nanjing, China) according to ASTMD6413. The samples (100 × 10 mm × 0.8 mm), held 19 mm over the Bunsen burner, were exposed to the flame for a period of 5 s. Heat of combustion of nanocomposites were performed using a HR-15 oxygen bomb calorimeter (Hunan, China). LOI values were measured on a Stanton Redcrat Flame Meter (England) according to ASTM D2863-2009; at least five samples for each formulation were tested. The dimensions of each sample were 100 × 50 × 3 mm³. The cone calorimeter tests were carried out on a cone calorimeter (FTT, UK). The squared specimens (100 × 100 × 3 mm³) were wrapped with aluminium foil and placed in a frame without a grid. The specimens were irradiated at a heat flux of 35 kW m⁻², corresponding to a medium fire scenario. Each sample was tested at least twice.

3. Results and discussion

3.1. Structural and morphological characterization

FTIR spectra analysis. Fig. 1 displays the FTIR of Zr(AE)₃P and GO-Zr(AE)₃P. The most significant bands of Zr(AE)₃P were 3445, 3106, 1619, 1400, 1224 and 1029 cm⁻¹, which were due to the N–H stretching, O–H stretching of water, the N–H bending vibration, C–N stretching vibration, P=O stretching vibration and P–O stretching vibration, respectively. The above analysis confirms the successful synthesis of the Zr(AE)₃P. For the GO-Zr(AE)₃P, characteristic peaks appeared at 3445, 3043, 1648, 1630, 1605, 1341 and 1020 cm⁻¹, and were assigned to N–H stretching vibration, C–H of benzene, C=O stretching vibration, C=C stretching vibration corresponding to the remaining sp² character, N–H bending vibration, P=O stretching vibration and P–O stretching vibration, respectively. The above analysis confirms the successful synthesis of the GO-Zr(AE)₃P.

Fig. 1 FTIR spectra of Zr(AE)₃P and GO-Zr(AE)₃P.

Morphological analysis. Fig. 2 presents the TEM images of Zr(AE)₃P and GO-Zr(AE)₃P sheets dispersed in water. The TEM image of Zr(AE)₃P displays a slightly wrinkled structure with ultrathin nanosheets. The TEM image of GO-Zr(AE)₃P shows a wrinkled laminar structure that was much plumper than Zr(AE)₃P and has abundant gaps between layers.

Fig. 2 TEM images of Zr(AE)₃P (A), and GO-Zr(AE)₃P (B).

3.2. Mechanical properties

To understand the effect of the GO-decorated Zr(AE)₃P intumescent flame retardant on the mechanical properties, we studied the Young’s modulus (E), tensile strength (σ_b) and elongation at break (ε_b) of the films with adjustable compositions. From Fig. 3, it was found that Zr(AE)₃P and GO-decorated Zr(AE)₃P had an obvious reinforcing effect on the PP matrix. The tensile strength of PP (Fig. 3A) was only 32.15 MPa. For the PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites, the maximum tensile strength reached was 34.74 MPa and 34.47 MPa (improved by 8.1%, 7.2%) respectively, indicating a considerable reinforcing effect from Zr(AE)₃P and GO-Zr(AE)₃P nanoparticles. However, it is clearly illustrated that further addition of Zr(AE)₃P or GO-Zr(AE)₃P could not considerably improve the tensile strength. Compared with neat PP, all of the Young’s modulus values of nanocomposite films (Fig. 3B) sharply increased while the elongation at break values (Fig. 3C) showed a sharp reduction trend. These results suggested that Zr(AE)₃P and GO-Zr(AE)₃P could improve the strength and stiffness of PP at the expense of flexibility, however, the GO-decorated Zr(AE)₃P (GO-Zr(AE)₃P) exhibited more promise. The improvement in the mechanical properties was due to the good dispersion of Zr(AE)₃P or GO-Zr(AE)₃P within the PP matrix and the strong interfacial interactions between Zr(AE)₃P or GO-Zr(AE)₃P and the PP matrix. Good dispersion also helped prevent slippage among PP molecules, thus, adding a small amount of Zr(AE)₃P or GO-Zr(AE)₃P decreased elongation at the break-point of PP films. At higher contents of Zr(AE)₃P or GO-Zr(AE)₃P, the tensile strength of nanocomposites decreased abruptly, this was due to the agglomeration of nanoparticles which can weaken the interfacial interaction.

Fig. 3 Tensile strength (A), elongation at break-point (B), and Young’s modulus (C) of PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites.

3.3. Thermal analysis

DSC. Fig. 4A and B show the melting and crystallization curves of the PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P composites, respectively. The melting temperature (Table 1) decreased in nanocomposites with Zr(AE)₃P from 167 to 162 °C, and that decreased to 164 °C in the case of nanocomposites with GO-Zr(AE)₃P. We can deduce that the existence of Zr(AE)₃P or GO-Zr(AE)₃P in PP blends gave rise to lower melting temperatures, and the melting temperature of PP/Zr(AE)₃P-n was lower than that of PP/GO-Zr(AE)₃P-n. The nanocomposites with Zr(AE)₃P had a higher crystallisation temperature than PP, which seemed to slightly increase as the concentration of Zr(AE)₃P increased. The temperature peak of crystallization for PP/GO-Zr(AE)₃P-n composites was higher than that of PP/Zr(AE)₃P-n. Both Zr(AE)₃P and GO-Zr(AE)₃P slightly enhanced the heterogeneous nucleating ability on the crystallization of PP, and GO-Zr(AE)₃P exhibited more promise.

Fig. 4 The melting (A), and crystallization (B) curves of PP, PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites.

Table 1 Selected results from TG, DSC and heat of combustion analysis

Sample	IDT^a (°C)	FDT^b (°C)	T_50%^c (°C)	Tmax^d (°C)	W^e (wt%)	Tc^g (°C)	Tm^h (°C)	Q^f (kJ g^−1)
a The initial decomposition temperature.b The final decomposition temperature.c The temperature at 50% weight loss.d The temperature at the maximum rate of mass loss.e The char yield after combustion.f The heat of combustion.g The melting temperature.h The crystallisation temperature.
PP	371	496	439	450	0.20	110	165	46.76
PP/Zr(AE)3P-1	377	498	443	451	0.80	111	167	46.09
PP/Zr(AE)3P-2	374	506	446	459	5.76	111	165	45.53
PP/Zr(AE)3P-3	379	507	447	453	4.80	111	164	43.86
PP/Zr(AE)3P-4	382	508	452	459	10.59	111	164	43.50
PP/Zr(AE)3P-5	383	514	453	460	11.68	112	162	42.92
PP/GO-Zr(AE)3-1	379	503	447	456	5.11	111	165	45.79
PP/GO-Zr(AE)3-2	384	506	452	458	9.28	111	165	44.41
PP/GO-Zr(AE)3-3	386	509	453	462	11.14	112	165	43.19
PP/GO-Zr(AE)3-4	387	511	456	463	15.73	112	164	42.97
PP/GO-Zr(AE)3-5	389	518	459	465	20.99	112	164	42.35

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TG. The thermal stability of neat PP and the nanocomposites were studied by TG and DTG analysis. As shown in Fig. 5, the TG (Fig. 5A) curves show one thermal decomposition platform, indicating a one step process. All of the samples displayed similar degradation profiles, suggesting that the existence of the Zr(AE)₃P and GO-Zr(AE)₃P did not significantly alter the degradation mechanism of the PP matrix. The summary of TGA results are listed in Table 1. It was found that all PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites showed a higher decomposition temperature than neat PP, the reason was attributed to the formation of a char which acted as a mass transport barrier and an isolator between the bulk polymer matrix and the surface, where combustion occurred. However, the decomposition temperature of almost all PP/GO-Zr(AE)₃P-n nanocomposites was higher than that of PP/Zr(AE)₃P-n nanocomposites with the same loading. This is due to the fact that the amount of the char of PP/GO-Zr(AE)₃P-n nanocomposites was higher that of PP/Zr(AE)₃P-n nanocomposites. Compared with the neat PP, the DTG curves indicate that the maximum degradation temperature (T_max) also increased upon addition of the Zr(AE)₃P or GO-Zr(AE)₃P, as shown in Fig. 5B. However, the T_max of almost all PP/GO-Zr(AE)₃P-n nanocomposites was higher than that of PP/Zr(AE)₃P-n nanocomposites with the same loading. Those results clearly indicate that the thermal stability of Zr(AE)₃P or GO-Zr(AE)₃P loaded PP nanocomposites was enhanced compared to pure PP, and that of PP/GO-Zr(AE)₃P nanocomposites exhibited more promise. This may explain why fire resistance of PP can be improved by both Zr(AE)₃P and GO-Zr(AE)₃P, and GO-Zr(AE)₃P is better than Zr(AE)₃P.

Fig. 5 TG (A), and DTG (B), curves of PP, PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites.

3.4. Flame retardant performance analysis

Heat of combustion. The heat of combustion of PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites are presented in Table 1. Compared with neat PP, all of the PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites had a lower heat of combustion, and both PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n showed a reduction in heat of combustion with increasing content of filler. Moreover, the value of total heat released by PP/GO-Zr(AE)₃P-n nanocomposites was lower than that by PP/Zr(AE)₃P-n with the same loading. These results demonstrate that the addition of Zr(AE)₃P or GO-Zr(AE)₃P can effectively improve the flame retardancy of PP, and GO-Zr(AE)₃P exhibits the best performance. Moreover, the flame retardancy of nanocomposites increased with increasing loading of Zr(AE)₃P or GO-Zr(AE)₃P.

Vertical flame test. Further assessment of the flammability properties for PP, PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites was provided by vertical flame test. The burning behaviors of the nanocomposites was recorded by a digital camera and is presented in Fig. 6. For pristine PP, the flame was very vigorous and rapidly spread upward releasing flaming drips, and the glow could be obviously seen on the top side after removing from the burner after 2 s, indicating that PP is highly flammable. However, when PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P nanocomposites were exposed to the same flame, the composites were consumed by the flame relatively slowly and just combusted slightly with a small blaze. Especially for PP/GO-Zr(AE)₃P-n nanocomposites, the glow nearly vanished after removing from the burner after 2 s. At the end of the burning experiment, the residue of PP/GO-Zr(AE)₃P-n nanocomposites was more uneven than PP/Zr(AE)₃P-n nanocomposites, which is consistent with SEM results. On the basis of these results, it was clear that the PP/GO-Zr(AE)₃P-n nanocomposites formed an effective char which was able to prevent the heat transfer and flame spread during combustion. Thus, GO-Zr(AE)₃P is an excellent flame retardant for PP.

Fig. 6 Images of vertical flame test for PP (A), PP/Zr(AE)₃P-1 (B), PP/Zr(AE)₃P-3 (C), PP/Zr(AE)₃P-5 (D), PP/GO-Zr(AE)₃P-1 (E), PP/GO-Zr(AE)₃P-3 (F), and PP/GO-Zr(AE)₃P-5 (G) nanocomposites.

Analysis of the char residues. The Zr(AE)₃P and GO-Zr(AE)₃P results of char yield and heat of combustion are listed in Table 1. Compared with neat PP (0.19%), the char yield of PP/Zr(AE)₃P-n (the maximum amount was 11.68%) and PP/GO-Zr(AE)₃P-n (the maximum amount was 20.99%) significantly improved, and the amount of char of PP/GO-Zr(AE)₃P-n was almost twice as high as that of PP/Zr(AE)₃P-n with the same loading. This indicates that both Zr(AE)₃P and GO-Zr(AE)₃P can promote char formation and GO had synergic effects in enhancing the char formation further. The formation of a stable char layer can prevent flames from spreading both in the gaseous phase and condensed phase, which can result in enhancing the thermal stability of PP at a high temperature.

Fig. 7 shows SEM photographs of the residue of PP, PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P. In Fig. 7A, a smooth and flat surface of pure PP can be observed because the residue is composed of melted and partially decomposed PP. Compared with PP, both chars of PP/Zr(AE)₃P (Fig. 7B) and PP/GO-Zr(AE)₃P (Fig. 7C) have cellular structures. Fig. 7B indicates that the residual char of 0.8% Zr(AE)₃P has little microconvexities with different sized holes on the surface, which might be caused by the Zr(AE)₃P promoting gas release and char formation. The charring of the PP matrix in the condensed phase restricts the rapid volatilization of the degradation products and results in the formation of char at high temperatures, thus improving the flame retardant efficiency of PP. The residual char of PP/GO-Zr(AE)₃P in Fig. 7C has an even more uneven structure like foam. This can be explained as the introduction of GO sheets of GO-Zr(AE)₃P can further promote char formation and this char serves as a superior protective barrier against heat, oxygen, or volatile products, and therefore prevents the polymers from burning further. These results are consistent with the results of char yield and heat of combustion previously mentioned.

Fig. 7 SEM micrographs of the residues of inner surface of PP (A), PP/Zr(AE)₃P (B), and PP/GO-Zr(AE)₃P (C), after vertical flame test, and EDX pattern of PP (D), PP/Zr(AE)₃P (E), and PP/GO-Zr(AE)₃P (F).

As shown in Table 2, the EDX analysis shows that only carbon and oxygen elements were found in the char residues for pristine PP. Phosphorus and zirconium elements were detected in the char residues for PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites. However, the char residues of PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites exhibited higher relative amounts of carbon than that of pristine PP, moreover, the amount of carbon from PP/GO-Zr(AE)₃P nanocomposites was much higher than that from PP/Zr(AE)₃P. The results reveal that the introduction of Zr(AE)₃P and GO-Zr(AE)₃P to PP significantly enhance the carbonization degree during combustion.

Table 2 Elemental compositions of char residues for PP, PP/Zr(AE)₃P, and PP/GO-Zr(AE)₃P nanocomposites

Char residue	Carbon (C, wt%)	Oxygen (O, wt%)	Phosphorus (P, wt%)	Zirconium (Zr, wt%)
PP	82.20	16.84	0	0
PP/Zr(AE)3P	95.15	2.61	0.43	0.83
PP/GO-Zr(AE)3P	96.7	1.25	0.1	1.02

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LOI analysis. The LOI values of cured PP and its nanocomposites are shown in Fig. 8. The LOI value of pure PP was 17.2, which indicates that PP is a flammable polymeric material. It can be seen that the LOI value of PP increased with the addition of Zr(AE)₃P and GO-Zr(AE)₃P. Compared with PP, the PP/GO-Zr(AE)₃P-5 composite had the highest LOI value, which improved by 20.9%, and also was much higher than that of the PP/Zr(AE)₃P-5 nanocomposite, indicating the increased effectiveness of GO-Zr(AE)₃P as a flame retardant. During the burning process, PP/Zr(AE)₃P nanocomposites rapidly carbonized and released large amounts of nonflammable gases such as ammonia,^24,25 leading to highly expanded and stable foamed char residues, which are also proven by char yield and SEM images of the char residue.

Fig. 8 The LOI values of PP and its nanocomposites.

Analysis of cone calorimetry test. Cone calorimetry is one of the most effective small-sized polymer fire behavior tests and provides a wealth of information on combustion behavior.²⁶ The curves of the heat release rate (HRR), total heat release (THR) against time of PP and its composites are presented in Fig. 9, and the detailed data are shown in Table 3. It can be seen that the neat PP burned very fast after ignition, and a sharp HRR peak appeared with a peak heat release rate (PkHRR) value of 665.0 kW m⁻² and average heat release rate (AvHRR) value of 125.0 kW m⁻². The PkHRR and AvHRR of PP/Zr(AE)₃P nanocomposite were reduced to 597.1 kW m⁻² (reduced by 10.2%) and 95.3 kW m⁻² (reduced by 23.8%), while that of PP/GO-Zr(AE)₃P nanocomposites were reduced to 508.2 kW m⁻² (reduced by 23.6%) and 51.9 kW m⁻² (reduced by 58.5%). Compared with pure PP, PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites burnt relatively slowly and the peak THR of PP/Zr(AE)₃P nanocomposite decreased from 96.9 MJ m⁻² to 81.0 MJ m⁻². However, in the case of PP/GO-Zr(AE)₃P nanocomposite, the peak HRR exhibited a further decrease to 59.0 MJ m⁻². The PkHRR, AvHRR and peak THR values of both PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites were significantly reduced. These results indicate that both Zr(AE)₃P and GO-Zr(AE)₃P provide PP with excellent flame retardency, further demonstrating that more organic structures in the PP participated in the carbonization process and kept in the condensed phase, rather than converting to “fuel” in the gas phase. This was also evidenced by the increased char residues (as listed in Table 1).

Fig. 9 Heat release rate (A), and total heat release (B), curves for samples from cone calorimeter testing.

Table 3 Combustion parameters obtained from the cone calorimeter test

Sample	TTI (s)	PkHRR (kW m^−2)	AvHRR (kW m^−2)	THR (MJ m^−2)	PkCOP (g s^−1)	PkSPR (m^2 s^−1)	TSP (m^2)
PP	38	665.0	125.0	96.9	0.0065	0.37	94.1
PP/Zr(AE)3P	39	597.1	95.3	81.0	0.0055	0.25	69.5
PP/GO-Zr(AE)3P	45	508.2	51.9	59.0	0.0046	0.21	38.7

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Smoke in a real fire means more risk of suffocation, even more fatal than heat release. The CO production (COP) and smoke production rate (SPR) cures for PP and its composites are displayed in Fig. 10, and the detailed data are shown in Table 3. It was clear that both Zr(AE)₃P and GO-Zr(AE)₃P decreased both the SPR and COP. The peak COP decreased from 0.0065 g s⁻¹ for neat PP to 0.0055 g s⁻¹ for PP/Zr(AE)₃P nanocomposite, and 0.0046 g s⁻¹ for PP/GO-Zr(AE)₃P nanocomposite. Compared with neat PP, the SPR peak of PP/Zr(AE)₃P decreased from 0.37 m² s⁻¹ to 0.25 m² s⁻¹ and the reduction in peak of SPR was 32.4%. While the value of PP/GO-Zr(AE)₃P nanocomposite was further reduced to 0.21 m² s⁻¹ which corresponds to a 43.2% reduction. From Table 3, it can be seen that the total smoke production (TSP) of PP/Zr(AE)₃P nanocomposites was reduced by 26.1%, while that of PP/GO-Zr(AE)₃P nanocomposites was reduced by 58.9%. These results indicate that both Zr(AE)₃P and GO-Zr(AE)₃P suppress the formation of smoke very well, but the smoke suppression effect of GO-Zr(AE)₃P is better than that of Zr(AE)₃P. Thus, Zr(AE)₃P and GO-Zr(AE)₃P are advantageous in reducing fire risk.

Fig. 10 CO production (A), and smoke production rate (B), of PP, PP/Zr(AE)₃P and PP/GO-Zr(AE)₃P nanocomposites.

4. Conclusion

In this work, a series of PP/Zr(AE)₃P-n and PP/GO-Zr(AE)₃P-n nanocomposites were obtained by melt incorporation of the Zr(AE)₃P or GO-Zr(AE)₃P. The results from tensile testing and TGA indicate that polymer mechanical properties and the thermal stability were improved by adding fillers, and the mechanical properties and the thermal stability of PP/GO-Zr(AE)₃P-n nanocomposites were better than PP/Zr(AE)₃P-n nanocomposites with the same loading. Results from SEM, heat of combustion, LOI, cone calorimeter tests, and vertical flame tests show that the fire resistant properties of PP were enhanced by Zr(AE)₃P and GO-Zr(AE)₃P, and the fire resistant properties of PP/GO-Zr(AE)₃P-n nanocomposites were better than that of PPZr(AE)₃P-n nanocomposites, which can probably be attributed to the higher char yield of GO during combustion, where the char layer provides an effective shield to protect the underlying polymers against flames.

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