# Synthesis, Characterization, and Thermal Behavior of Nanoparticles of Mg(OH)2 to Be Used as Flame Retardants

Authors: A. A. Hanna; A. S. Abdelmoaty; M. A. Sherief
Journal: Journal of Chemistry
 (2019)

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## Abstract

To study the effects of the precursor materials on the structure, the morphology, and the thermal stability of Mg(OH)2 particles, five samples were prepared by using the same method. The produced powder was characterized by using FTIR, XRD, SEM, and TEM, and the thermal stability was studied by using the thermogravimetric analysis. This study aims to use the advantage prepared material as a flame-retardant for the polymeric materials. The characterization of the obtained samples shows that Mg(OH)2 is formed in hexagonal phase and arranged in different shapes. The analysis of the data obtained from the TGA shows that Mg(OH)2 in general decomposed in three steps, the first one due to the water content and the other volatile materials, the second step represents the decomposition of Mg(OH)2 to produce MgO, and the third step represents the conversion of MgO to glassy layer. The samples prepared in presence of the surfactants gave a higher value for the formation of the glassy layer so that it is recommended to use this sample as flame retardant for the polymeric materials.


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## Body

## 1. Introduction

The polymer materials were used successfully in different applications because they have excellent chemical, mechanical, and electrical properties [1–8]. The polymer materials have two main drawbacks: the first one is their flammability and the thermal degradation which represent environmental pollution and the second one is smoke that consists of many toxic gases such as nitrogen oxides from polyamides and CO2, CO due to firing the organic materials, and the others due to their low melting point [9]. These gases resist the work of the firemen because of the creation of an atmosphere loaded with smoke. For these reasons, the polymer materials must be modified to overcome these defects. This modification goes in two directions, improving the properties of the polymer materials and adding some flame-retardant materials.The most common flame-retardant materials are the halogenated or phosphorus compounds. These compounds commonly produce some toxic gases. On the other hand, the inorganic flame-retardant materials are the more acceptable compounds because they are hydrated materials which produce water vapour at low temperature and produce some oxides such as MgO, SiO2, and Al2O3 which work as flame-retardant materials and work as smoke suppressants [10] at high firing temperature. Furthermore, their properties could absorb the evolved toxic gases such as CO, CO2, SO2, and nitrogen gases [5]. Among the inorganic flame-retardant materials, Mg(OH)2 draws special attention of different authors [11–15] because it contains hydrated molecules when decomposed thermally at low temperature and its addition improves the mechanical properties of the polymers [16]. The effect of adding Mg(OH)2 on the flammability of polymers focused on the structure and the shape of the produced Mg(OH)2 powder. Henrist et al. and other works [17] found that the structure, the morphology, the size and agglomerations, and the thermal behavior of Mg(OH)2 were influenced greatly by the method of preparation and their conditions as well as the precursor materials.The aim of this work is concerned with the preparation of the nanoparticles of Mg(OH)2 by using different precursor materials under different conditions and study of the thermal behaviors of the produced samples to be used as flame-retardant materials. Also, the work discussed the growth and the nucleation of the produced phases.


## 2. Experimental

### 2.1. Preparation of Mg(OH)2 Samples

Sixteen grams of NaOH, 1 g urea, and 8 ml ethanol are dissolved in a flask containing 50 ml deionized water; then, 50 ml MgCl2 solution was added dropwise with stirring for 10 min at 60°C for 1 h and then kept for 10 h. The precipitate was washed and dried at 60°C. Five samples were prepared as shown in Table 1[18].Table 1
Start materials for preparation of the samples.
Samples
NaOH (g)
MgCl2(ml)
Urea (g)
Ethanol (ml)
Ethylene glycol (ml)
CTAB (g)
A
16
50
1
8
—
—
B
16
50
1
—
—
—
C
16
50
—
8
—
—
D
16
50
1
8
8
—
E
16
50
1
8
—
2


### 2.2. Characterization of the Produced Samples

The five prepared samples were characterized by using different techniques as follows.


#### 2.2.1. X-Ray Diffraction (XRD) Analysis

X-ray diffraction (XRD) patterns were obtained with a power D8 ADVANCE diffractometer (Germany) using CuKa radiation (1.542°A, 40 KV, 40 mA) in the 2Ө range of 4–80. The acquisition parameters were a step size of 0.02 and a step time of 0.4 s.


#### 2.2.2. Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectra of the samples were obtained using a KBr disk technique and FTIR 6500 spectrometer (JASCO, Japan) in the range of 400–4000 cm−1.


#### 2.2.3. Scanning Electron Microscope (SEM)

The surface morphologies of the samples were carried out using a JEOL JSM t20 scanning electron microscope (SEM) (JEOL, Japan) at an accelerating voltage of 5 kV.


#### 2.2.4. Transmission Electron Microscope (TEM)

The morphology and the particle size of the prepared samples were examined by using a transmission electron microscope (TEM) Joel JEM 1230 working at 100 keV.


#### 2.2.5. Thermal Analysis

The thermal analysis (TGA) was performed by using a USA Perkin–Elmer thermogravimeter. Samples of approximately 10 mg were heated from 50°C to 1000°C with heating rate 10/m under a nitrogen atmosphere, and the flow of nitrogen was 50 ml/min.


## 2.1. Preparation of Mg(OH)2 Samples

Sixteen grams of NaOH, 1 g urea, and 8 ml ethanol are dissolved in a flask containing 50 ml deionized water; then, 50 ml MgCl2 solution was added dropwise with stirring for 10 min at 60°C for 1 h and then kept for 10 h. The precipitate was washed and dried at 60°C. Five samples were prepared as shown in Table 1[18].Table 1
Start materials for preparation of the samples.
Samples
NaOH (g)
MgCl2(ml)
Urea (g)
Ethanol (ml)
Ethylene glycol (ml)
CTAB (g)
A
16
50
1
8
—
—
B
16
50
1
—
—
—
C
16
50
—
8
—
—
D
16
50
1
8
8
—
E
16
50
1
8
—
2


## 2.2. Characterization of the Produced Samples

The five prepared samples were characterized by using different techniques as follows.


### 2.2.1. X-Ray Diffraction (XRD) Analysis

X-ray diffraction (XRD) patterns were obtained with a power D8 ADVANCE diffractometer (Germany) using CuKa radiation (1.542°A, 40 KV, 40 mA) in the 2Ө range of 4–80. The acquisition parameters were a step size of 0.02 and a step time of 0.4 s.


### 2.2.2. Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectra of the samples were obtained using a KBr disk technique and FTIR 6500 spectrometer (JASCO, Japan) in the range of 400–4000 cm−1.


### 2.2.3. Scanning Electron Microscope (SEM)

The surface morphologies of the samples were carried out using a JEOL JSM t20 scanning electron microscope (SEM) (JEOL, Japan) at an accelerating voltage of 5 kV.


### 2.2.4. Transmission Electron Microscope (TEM)

The morphology and the particle size of the prepared samples were examined by using a transmission electron microscope (TEM) Joel JEM 1230 working at 100 keV.


### 2.2.5. Thermal Analysis

The thermal analysis (TGA) was performed by using a USA Perkin–Elmer thermogravimeter. Samples of approximately 10 mg were heated from 50°C to 1000°C with heating rate 10/m under a nitrogen atmosphere, and the flow of nitrogen was 50 ml/min.


## 2.2.1. X-Ray Diffraction (XRD) Analysis

X-ray diffraction (XRD) patterns were obtained with a power D8 ADVANCE diffractometer (Germany) using CuKa radiation (1.542°A, 40 KV, 40 mA) in the 2Ө range of 4–80. The acquisition parameters were a step size of 0.02 and a step time of 0.4 s.


## 2.2.2. Fourier Transform Infrared (FTIR) Spectroscopy

FTIR spectra of the samples were obtained using a KBr disk technique and FTIR 6500 spectrometer (JASCO, Japan) in the range of 400–4000 cm−1.


## 2.2.3. Scanning Electron Microscope (SEM)

The surface morphologies of the samples were carried out using a JEOL JSM t20 scanning electron microscope (SEM) (JEOL, Japan) at an accelerating voltage of 5 kV.


## 2.2.4. Transmission Electron Microscope (TEM)

The morphology and the particle size of the prepared samples were examined by using a transmission electron microscope (TEM) Joel JEM 1230 working at 100 keV.


## 2.2.5. Thermal Analysis

The thermal analysis (TGA) was performed by using a USA Perkin–Elmer thermogravimeter. Samples of approximately 10 mg were heated from 50°C to 1000°C with heating rate 10/m under a nitrogen atmosphere, and the flow of nitrogen was 50 ml/min.


## 3. Results and Discussion

### 3.1. Fourier Transform Infrared (FTIR) Spectroscopy

Figure1 (A, B, C, D, and E) illustrates the IR spectrum of the five prepared samples. The IR profiles of these samples behave the same trends and are similar to those reported in the literature [19]. The IR curves indicate that there are some changes in the intensity and the sharpness of the peaks while slight change in their position according to the start materials. For sample A, it is observed that the intensity of the peaks is smaller than that of other samples. These findings are in agreement with the literature [18, 20]. This means that the presence of the urea and the solvent can prevent the growth of the Mg(OH)2 crystals while they help in the formation of the crystals. So, the structure and the morphology of the prepared Mg(OH)2 are controlled by the precursor material, the method of preparation, and its conditions as suggested by Henrist and Yang [17, 21]. The IR spectra of the samples E and B show that the intensity and sharpness of the peaks were increased; the presence of the surfactants improves the growth of the crystals of the Mg(OH)2 as reported previously [18]. On the other hand, the absence of the solvent restricted the nucleation and improves the growth of the crystals. For samples C and D, there are no effects observed. This means that the absence of urea and the presence of the gelating agent help in the formation and nucleation of the crystals. In general, for all samples, the IR charts indicate the formation of Mg(OH)2 as follows:(i)
The appearance of the remarkable sharp and intense peak at 3600 cm−1 is assigned to the OH antisymmetric stretching vibration in the Mg(OH)2 [22](ii)
The appearance of the two peaks at 3450 cm−1 and 1635 cm−1 could be due to the stretching vibration and the bending vibration of the water molecules, respectively(iii)
The appearance of the absorption peak at 500 cm−1 may due to the stretching vibration of Mg-O(iv)
The appearance of previous peaks indicates that Mg(OH)2 crystals were formed in all the cases [23]Figure 1
FTIR of prepared samples.


### 3.2. X-Ray Diffraction (XRD)

The XRD patterns of the prepared samples are illustrated in Figure2 (A, B, C, D, and E); the main diffraction peaks corresponding to the hexagonal phase of Mg(OH)2 according to JCPDS card no. 7-239 appeared. The XRD patterns show some changes in the intensity of peaks depending on the precursor materials.Figure 2
XRD of prepared samples.For sample A, small peaks appeared at 2 theta 17 cm−1, 28 cm−1, 33 cm−1, 37 cm−1, 47 cm−1, 57 cm−1, and 67 cm−1. For sample B, the sharpness of the special peaks increases; this means that the growth of the formed crystals increases. For samples C and D, the sharpness of the specified peaks increased but was less than that of sample B. For sample E, which contains the surfactants (CTAB), the sharpness of the specified peaks increased greatly. From this result, it may be concluded that the growth and the nucleation of the crystals of Mg(OH)2 depend on the precursor materials.


### 3.3. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM)

To study the effects of the precursor materials on the morphology of the produced Mg(OH)2, the scanning electron microscopy and the transmission electron microscopy were performed as shown in Figures 3 and 4 from A to E. For sample A (Figure 3(a)), the formed particles are hexagonal crystals with definite boundary where the presence of the solvent prevents the agglomeration for the particles. It is also observed that the formed particles were arranged randomly with different sizes. For sample B, the formed particles also have a hexagonal crystal and are arranged in layered structure (Figure 3(b)). For sample C which represents the reaction between the reactant materials without urea, the formed particles tend to form rod-like structures while the photo exhibits some agglomeration. For sample D, the SEM photo exhibits a well rod structure; this may be due to the ethylene glycol being used as a gelating agent. For sample E, which contains the surfactants, the SEM photo exhibits a regular structure with clear boundary. The TEM measurements are in agreement with the SEM photos as shown in Figures 4(a)–4(e). The TEM photos for all prepared samples indicate that the particle size are in the range of nanoparticles of about 5–10 nm.Figure 3
(a) SEM photo of prepared sample A. (b) SEM photo of prepared sample B. (c) SEM photo of prepared sample C. (d) SEM photo of prepared sample D. (e) SEM photo of prepared sample E.
(a)
(b)
(c)
(d)
(e)Figure 4
(a) TEM photo of prepared sample A. (b) TEM photo of prepared sample B. (c) TEM photo of prepared sample C. (d) TEM photo of prepared sample (D). (e) TEM photo of prepared sample E.
(a)
(b)
(c)
(d)
(e)


#### 3.3.1. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) for the five prepared samples was used to evaluate the thermal behavior over the heat range between the room temperature and 1000°C. The thermal gravimetric profiles show that the samples C and E behave in the same trends as shown in Figure5 (C and E) where the weight loss at the temperature below 500°C is about 2% and 5% for the sample C and sample E, respectively. As this stage represents the liberation of the water content, this means that the two samples contain some absorbed surface water. Also, the TG curves indicate that the residual content which represents the formation of the glassy layer from MgO is higher than in the other samples. Generally, it may characterize three stages of the thermal decomposition. The first one starts at the room temperature ending at 200°C; this stage may be attributed to the surface and the hydrated water. The second step starts at 200°C and ends at 480°C corresponding to the decomposition of Mg(OH)2 to form MgO [24, 25]. The third step represents the firing of MgO, converting it to a glassy state and continuing till 1000°C with negligible weight loss. The appearance of the three stages was observed for all the samples with negligible weight loss at the starting and the ending temperature. The difference between the starting and ending temperatures may be due to the amount of the adsorbent water on the surface of the prepared particles and the structure and the shape of the prepared particles. This is confirmed from the percentage of weight loss of the residue amount at the temperature above 500°C. The difference between the residual percentages may be due to the adsorbed and the hydrated water and the other organic solvents which are used in the preparation as well as the shape of the crystals. The TG curves exhibit the formation of the MgO glassy phase which prevents the spread of the flame through the burning materials [25]. The residual for samples C and E is greater than for the other samples, and the residual contents follow the following arrangement E > C > A > B > D. This means that the samples C and E prepared by this method are more preferable for use as flame retardant for the polymeric materials. Also, the thermogravimetric curves indicate that Mg(OH)2 is thermally stable in spite of the presence of the water and the method of preparation.Figure 5
TGA of prepared samples.


## 3.1. Fourier Transform Infrared (FTIR) Spectroscopy

Figure1 (A, B, C, D, and E) illustrates the IR spectrum of the five prepared samples. The IR profiles of these samples behave the same trends and are similar to those reported in the literature [19]. The IR curves indicate that there are some changes in the intensity and the sharpness of the peaks while slight change in their position according to the start materials. For sample A, it is observed that the intensity of the peaks is smaller than that of other samples. These findings are in agreement with the literature [18, 20]. This means that the presence of the urea and the solvent can prevent the growth of the Mg(OH)2 crystals while they help in the formation of the crystals. So, the structure and the morphology of the prepared Mg(OH)2 are controlled by the precursor material, the method of preparation, and its conditions as suggested by Henrist and Yang [17, 21]. The IR spectra of the samples E and B show that the intensity and sharpness of the peaks were increased; the presence of the surfactants improves the growth of the crystals of the Mg(OH)2 as reported previously [18]. On the other hand, the absence of the solvent restricted the nucleation and improves the growth of the crystals. For samples C and D, there are no effects observed. This means that the absence of urea and the presence of the gelating agent help in the formation and nucleation of the crystals. In general, for all samples, the IR charts indicate the formation of Mg(OH)2 as follows:(i)
The appearance of the remarkable sharp and intense peak at 3600 cm−1 is assigned to the OH antisymmetric stretching vibration in the Mg(OH)2 [22](ii)
The appearance of the two peaks at 3450 cm−1 and 1635 cm−1 could be due to the stretching vibration and the bending vibration of the water molecules, respectively(iii)
The appearance of the absorption peak at 500 cm−1 may due to the stretching vibration of Mg-O(iv)
The appearance of previous peaks indicates that Mg(OH)2 crystals were formed in all the cases [23]Figure 1
FTIR of prepared samples.


## 3.2. X-Ray Diffraction (XRD)

The XRD patterns of the prepared samples are illustrated in Figure2 (A, B, C, D, and E); the main diffraction peaks corresponding to the hexagonal phase of Mg(OH)2 according to JCPDS card no. 7-239 appeared. The XRD patterns show some changes in the intensity of peaks depending on the precursor materials.Figure 2
XRD of prepared samples.For sample A, small peaks appeared at 2 theta 17 cm−1, 28 cm−1, 33 cm−1, 37 cm−1, 47 cm−1, 57 cm−1, and 67 cm−1. For sample B, the sharpness of the special peaks increases; this means that the growth of the formed crystals increases. For samples C and D, the sharpness of the specified peaks increased but was less than that of sample B. For sample E, which contains the surfactants (CTAB), the sharpness of the specified peaks increased greatly. From this result, it may be concluded that the growth and the nucleation of the crystals of Mg(OH)2 depend on the precursor materials.


## 3.3. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM)

To study the effects of the precursor materials on the morphology of the produced Mg(OH)2, the scanning electron microscopy and the transmission electron microscopy were performed as shown in Figures 3 and 4 from A to E. For sample A (Figure 3(a)), the formed particles are hexagonal crystals with definite boundary where the presence of the solvent prevents the agglomeration for the particles. It is also observed that the formed particles were arranged randomly with different sizes. For sample B, the formed particles also have a hexagonal crystal and are arranged in layered structure (Figure 3(b)). For sample C which represents the reaction between the reactant materials without urea, the formed particles tend to form rod-like structures while the photo exhibits some agglomeration. For sample D, the SEM photo exhibits a well rod structure; this may be due to the ethylene glycol being used as a gelating agent. For sample E, which contains the surfactants, the SEM photo exhibits a regular structure with clear boundary. The TEM measurements are in agreement with the SEM photos as shown in Figures 4(a)–4(e). The TEM photos for all prepared samples indicate that the particle size are in the range of nanoparticles of about 5–10 nm.Figure 3
(a) SEM photo of prepared sample A. (b) SEM photo of prepared sample B. (c) SEM photo of prepared sample C. (d) SEM photo of prepared sample D. (e) SEM photo of prepared sample E.
(a)
(b)
(c)
(d)
(e)Figure 4
(a) TEM photo of prepared sample A. (b) TEM photo of prepared sample B. (c) TEM photo of prepared sample C. (d) TEM photo of prepared sample (D). (e) TEM photo of prepared sample E.
(a)
(b)
(c)
(d)
(e)


### 3.3.1. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) for the five prepared samples was used to evaluate the thermal behavior over the heat range between the room temperature and 1000°C. The thermal gravimetric profiles show that the samples C and E behave in the same trends as shown in Figure5 (C and E) where the weight loss at the temperature below 500°C is about 2% and 5% for the sample C and sample E, respectively. As this stage represents the liberation of the water content, this means that the two samples contain some absorbed surface water. Also, the TG curves indicate that the residual content which represents the formation of the glassy layer from MgO is higher than in the other samples. Generally, it may characterize three stages of the thermal decomposition. The first one starts at the room temperature ending at 200°C; this stage may be attributed to the surface and the hydrated water. The second step starts at 200°C and ends at 480°C corresponding to the decomposition of Mg(OH)2 to form MgO [24, 25]. The third step represents the firing of MgO, converting it to a glassy state and continuing till 1000°C with negligible weight loss. The appearance of the three stages was observed for all the samples with negligible weight loss at the starting and the ending temperature. The difference between the starting and ending temperatures may be due to the amount of the adsorbent water on the surface of the prepared particles and the structure and the shape of the prepared particles. This is confirmed from the percentage of weight loss of the residue amount at the temperature above 500°C. The difference between the residual percentages may be due to the adsorbed and the hydrated water and the other organic solvents which are used in the preparation as well as the shape of the crystals. The TG curves exhibit the formation of the MgO glassy phase which prevents the spread of the flame through the burning materials [25]. The residual for samples C and E is greater than for the other samples, and the residual contents follow the following arrangement E > C > A > B > D. This means that the samples C and E prepared by this method are more preferable for use as flame retardant for the polymeric materials. Also, the thermogravimetric curves indicate that Mg(OH)2 is thermally stable in spite of the presence of the water and the method of preparation.Figure 5
TGA of prepared samples.


## 3.3.1. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) for the five prepared samples was used to evaluate the thermal behavior over the heat range between the room temperature and 1000°C. The thermal gravimetric profiles show that the samples C and E behave in the same trends as shown in Figure5 (C and E) where the weight loss at the temperature below 500°C is about 2% and 5% for the sample C and sample E, respectively. As this stage represents the liberation of the water content, this means that the two samples contain some absorbed surface water. Also, the TG curves indicate that the residual content which represents the formation of the glassy layer from MgO is higher than in the other samples. Generally, it may characterize three stages of the thermal decomposition. The first one starts at the room temperature ending at 200°C; this stage may be attributed to the surface and the hydrated water. The second step starts at 200°C and ends at 480°C corresponding to the decomposition of Mg(OH)2 to form MgO [24, 25]. The third step represents the firing of MgO, converting it to a glassy state and continuing till 1000°C with negligible weight loss. The appearance of the three stages was observed for all the samples with negligible weight loss at the starting and the ending temperature. The difference between the starting and ending temperatures may be due to the amount of the adsorbent water on the surface of the prepared particles and the structure and the shape of the prepared particles. This is confirmed from the percentage of weight loss of the residue amount at the temperature above 500°C. The difference between the residual percentages may be due to the adsorbed and the hydrated water and the other organic solvents which are used in the preparation as well as the shape of the crystals. The TG curves exhibit the formation of the MgO glassy phase which prevents the spread of the flame through the burning materials [25]. The residual for samples C and E is greater than for the other samples, and the residual contents follow the following arrangement E > C > A > B > D. This means that the samples C and E prepared by this method are more preferable for use as flame retardant for the polymeric materials. Also, the thermogravimetric curves indicate that Mg(OH)2 is thermally stable in spite of the presence of the water and the method of preparation.Figure 5
TGA of prepared samples.


## 4. Conclusion

From the analysis of the obtained results, it may be concluded that the formed nanoparticles of Mg(OH)2 have a hexagonal phase and their morphology depend on the nature of the precursor materials when they are prepared by the same method. Also, it is observed that the prepared Mg(OH)2 is thermally stable and by heating, it is converted to glassy layers and liberates water. The value of the glassy layer may be arranged as follows E > C > A > B > D. The obtained samples may be used as flame-retardant materials especially those prepared in presence of the surfactants.



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*Source: 1805280-2019-06-17.xml*