Preparation of Nano-Mg(OH)2 and Its Flame Retardant and Antibacterial Modification on Polyethylene Terephthalate Fabrics

The multifunctional polyethylene terephthalate (PET) fabrics were successfully prepared through a dip-coating technology to endow the flame retardant and antibacterial properties of PET fabrics, which are extensively used in many fields. The flame retardant and antibacterial agent was synthesized by a double drop-reverse precipitation method and surface-modified by the mixtures of titanate coupling agents and stearic acid to result in a good compatibility of the hydrophilic nano-Mg(OH)2 and the hydrophobic PET fabrics. The results indicated that the suitable synthesis conditions of nano-Mg(OH)2 are: Mg2+ concentration 1.5 mg/mL, reaction temperature 50 °C and reaction time 50 min, and the optimal modification conditions of nano-Mg(OH)2 are: modifier ratio 5/5, modification temperature 70 °C and modification time 40 min. The flame retardant test and the antibacterial test showed that the multifunctional PET fabrics had excellent flame retardant and antibacterial properties.


Introduction
The polyethylene terephthalate (PET) fabrics, as the most common synthetic fibers, are widely used in a lot of fields due to their excellent stability, friction resistance and mechanical property [1][2][3][4]. However, the PET fabrics are flammable materials due to the limiting oxygen index (LOI) of 20-22%, and the heavy casualties and the huge economic losses caused by the PET burning fires occur frequently [5,6]. Additionally, the PET fabrics have no inherent resistance against bacteria, and the uncontrolled bacteria on the PET fabrics can seriously lead to abominable effects, such as disease, discoloration and malodor [7][8][9]. Thus, the high flammability and the no antibacterial group of the PET fabrics restrict the application scope, especially those where flame retardant and antibacterial properties are required, so that the treatment including flame retardant and antibacterial properties to the PET fabrics is necessary.
Nano magnesium hydroxide (nano-Mg(OH) 2 ) is an inorganic material with nanometer size, which has advantages such as large specific surface area, non-toxic performance, chemical stability and thermal stability [10][11][12]. As halogen-free and phosphorus-free flame retardants, nano-Mg(OH) 2 has high decomposition temperature and soft texture, and especially has smoke suppression, which has good development prospects [13,14]. Nano-Mg(OH) 2 can absorb a lot of heat during decomposition at a high temperature, thereby reducing the temperature of the combustion product and slowing down the combustion reaction [15]. The decomposition product MgO is also a high temperature-resistant substance, which could cover on the surface of PET fabrics to significantly improve the air isolation efficiency and further prevent combustion [16]. After decomposition, a large amount of  (Shanghai, China). was used as model bacteria. The E. coli was grown aerobically in Luria-Bertani (LB) medium (tryptone 1%, NaCl 0.5%, yeast extract 0.5%, pH 7.2) at 37 • C, and the E. coli culture was maintained on LB agar plates (tryptone 1%, NaCl 0.5%, yeast extract 0.5%, agar 2%, pH = 7.2) at 37 • C. The reagents were all analytically pure without further treatment or purification.

Synthesis and Settling Rate Test of Nano-Mg(OH) 2
The NaOH solutions (2 mol/L 50 mL) were added to a 100 mL volumetric flask, and then 1.0% polyethylene glycol (accounting for the mass ratio of MgCl 2 ·6H 2 O) was added to a volumetric flask. After adding the rotor to the volumetric flask, 50 mL MgCl 2 ·6H 2 O solutions were slowly added dropwise to the volumetric flask in a double dropwise manner. The Mg 2+ concentration, reaction temperature and reaction time are shown in Table 1. The reacted suspensions were suction filtered, washed (twice with deionized water; twice with absolute ethanol) and dried at 60 • C for 3 h. Finally, the 15 groups of samples of synthetic nano-Mg(OH) 2 were obtained, and the sample names are shown in Table 1 2 (1 g C-M) were dissolved in 100 mL of deionized water, respectively. Then, the samples were ultrasonically shaken for 1 h. Finally, the dispersed powder suspensions were put into 100 mL measuring cylinder for observation, and the settling time and the settling volume were recorded for analysis.

Samples
Mg

Hydrophobic Modification and Activation Index Test of Nano-Mg(OH) 2
The synthetic nano-Mg(OH) 2 was modified using titanate coupling agent and stearic acid; 200 mL of ethanol and 10 g of nano-Mg(OH) 2 were added into a three-necked flask for ultrasonic dispersion. The titanate coupling agent and stearic acid dissolved in ethanol were added dropwise to the nano-Mg(OH) 2 suspensions. The suspensions were fully stirred (500 rpm), and then centrifuged, washed and dried (60 • C, 3 h) after constant temperature reaction for a certain period of time. The modifier ratio, reaction temperature and reaction time are shown in Table 2.
The 5 g (m) modified nano-Mg(OH) 2 and 100 mL deionized water were added into a 200 mL beaker and stirred for 10 min, which was left to stand for 60 min horizontally. The remaining floating powders (m 1 ) were taken out and dried at 100 • C. The activation index was H = m 1 /m.

Preparation of Functional PET Fabrics
The pure PET fabrics were pretreated by washing and rinsing, and then dried at 90 • C for 0.

Characterizations
The purity and the average size of the crystallite powders were analyzed by X-ray diffraction (XRD, Rigaku D/max-2500/PC) using Cu Kα radiation (λ = 0.15418) at 25 mA and 40 kV, which was acquired from 5 • to 90 • with a step size of 0.05 • /s and calculated by Scherrer equation shown as Equation (1) [32]: where D refers to the particle size (nm); K refers to the Scherrer constant (0.89); λ refers to the diffraction wavelength (0.15418Å); B refers to the half width of the diffraction peak; θ refers to the diffraction angle. The micro morphology of the powders and fabrics was characterized by scanning electron microscopy (SEM, JSM 7500F) after coating with gold on the surface of the samples. The combined way among the powders and the fabrics was characterized by the attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Spectrum 2) at a resolution of 4 cm −1 in a range of wave numbers from 400-4000 cm −1 . Moreover, the thermal behavior was tested by thermogravimetric analysis (TGA, TGA 2), which was performed at a heating rate of 10 • C/min in the range of 40-800 • C under a nitrogen atmosphere with a flow rate of 20 mL/min.

Flame Retardant Performance Test of Fabrics
The vertical burning and limiting oxygen index (LOI) were tested according to GB/T5455-2014 and GB/T 5454-1997. The vertical burning test was tested using a vertical burning tester (YG815B). The PET fabric sample size was 300 × 89 mm, and the ignition time was 60 s (temperature: 10-30 • C; relative humidity: 30-80%). After ignition and reaching 60 s, the igniter was removed and turned off. Then, the timer was turned on to record the duration of continuous combustion. The fabric sample (150 × 50 mm) for testing LOI was placed in a glass covered with a mixture of nitrogen and oxygen flow. The upper end of the sample was ignited with an igniter, and then the minimum oxygen concentration to maintain the flaming combustion of the sample was recorded.

Antibacterial Test of Fabrics
The antibacterial rate (I) of the fabrics was tested by a shake-flask method according to the modified GB/T 20944.3-2008, GB/T 24346-2009 and AATCC 100-2004, which was calculated by Equation (2) [33]: where I refers to the antibacterial rate (%); A refers to the E. coli colonies number of control; B refers to the E. coli colonies in the number of the samples. In addition, there is no impurity peak except for the characteristic peaks of Mg(OH) 2 , suggesting that the purity of the samples synthesized in this research is fairly high. The data of XRD is shown in Table 3, when the Mg 2+ concentration is 1.5 mg/mL, and the average size of the grains is the smallest of 23.7 ± 6.

. ATR-FTIR Spectra Analysis
The surface groups of the synthetic powders are shown in the ATR-FTIR spectra. It can be seen in Figure 1b that the most obvious sharp and high-intensity absorption peak is located at 3697 cm −1 , which is the contraction vibration peak of O-H in the crystal structure of Mg(OH)2. The characteristic peak of Mg-OH bending vibration is located at 1640 cm −1 . The characteristic absorption peak representing the bending vibration of -OH is located at 1451 cm −1 , and another characteristic absorption peak with a broad absorption band is located at 3443 cm −1 . The reason for the absorption peak is the change of the free proton in Mg(OH)2 to the conductive state.

Settling Rate
Generally, inorganic nanoparticles could agglomerate to a certain point while the degree of aggregation is different. The dispersion of inorganic nanoparticles in water is commonly expressed by the sedimentation volume. The smaller the sedimentation volume is,

ATR-FTIR Spectra Analysis
The surface groups of the synthetic powders are shown in the ATR-FTIR spectra. It can be seen in Figure 1b that the most obvious sharp and high-intensity absorption peak is located at 3697 cm −1 , which is the contraction vibration peak of O-H in the crystal structure of Mg(OH) 2 . The characteristic peak of Mg-OH bending vibration is located at 1640 cm −1 . The characteristic absorption peak representing the bending vibration of -OH is located at 1451 cm −1 , and another characteristic absorption peak with a broad absorption band is located at 3443 cm −1 . The reason for the absorption peak is the change of the free proton in Mg(OH) 2 to the conductive state.

Settling Rate
Generally, inorganic nanoparticles could agglomerate to a certain point while the degree of aggregation is different. The dispersion of inorganic nanoparticles in water is commonly expressed by the sedimentation volume. The smaller the sedimentation volume is, the slower the settling rate is, which indicates that the dispersion performance of the powder is excellent. On the contrary, the smaller the sedimentation volume, the worse the dispersion performance of the powder. As shown in Figure  The powders at the reaction temperature of 50 • C and the reaction time of 50 min show the more ideal sedimentation volume than that of the others and their reaction temperature and reaction time. In addition, the sedimentation volume of M-M-1.5 is slower than of the C-M. Therefore, the sample of M-M-1.5 exert a minimum sedimentation volume, which shows the best settling rate among all the samples. This is owing to the self-made M-M-1.5 which has an excellent dispersion, while the commercial CM has a poor dispersion so that it is easy to agglomerate.

Microscopic Appearance and Particle Size Distribution
The SEM image and particle size of the M-M-1.5 is shown in Figure 2b. As the M-M-1.5 image shows, the synthetic nano-Mg(OH) 2 of M-M-1.5 are granular in the form of particles with smooth surfaces, and the dispersion performance of M-M-1.5 is better with less agglomeration. It can be seen from the curve that the particle size distribution of the M-M-1.5 belongs to a normal distribution. The particle size of M-M-1.5 is located within the range of 15-55 nm, and the average particle size is 29.61 ± 7.08 nm. Thus, the nano-Mg(OH) 2 synthesized by double drop-reverse precipitation method can obtain the advantage of good uniformity and dispersion. 3.0. The powders at the reaction temperature of 50 °C and the reaction time of 50 min show the more ideal sedimentation volume than that of the others and their reaction temperature and reaction time. In addition, the sedimentation volume of M-M-1.5 is slower than of the C-M. Therefore, the sample of M-M-1.5 exert a minimum sedimentation volume, which shows the best settling rate among all the samples. This is owing to the self-made M-M-1.5 which has an excellent dispersion, while the commercial CM has a poor dispersion so that it is easy to agglomerate.    The XRD pattern of the modified nano-Mg(OH) 2 is shown in Figure 3a, and all the diffraction peaks are well indexed as the structure of Mg(OH) 2 . There is no impure peak other than the characteristic peaks of nano-Mg(OH) 2 which suggests that the high-purity nano-Mg(OH) 2 is obtained. Based on the Scherrer equation, the particle sizes of nano-Mg(OH) 2 corresponding to each diffraction peak are shown in Table 4. The XRD result showed that the grain size of the modified nano-Mg(OH) 2 becomes larger. When the modifier ratio is 5/5, the temperature is 70 • C and the time is 40 min, the grain size is the smallest of 31.1 ± 5.4 nm.

XRD Result
The XRD pattern of the modified nano-Mg(OH)2 is shown in Figure 3a, and all the diffraction peaks are well indexed as the structure of Mg(OH)2. There is no impure peak other than the characteristic peaks of nano-Mg(OH)2 which suggests that the high-purity nano-Mg(OH)2 is obtained. Based on the Scherrer equation, the particle sizes of nano-Mg(OH)2 corresponding to each diffraction peak are shown in Table 4. The XRD result showed that the grain size of the modified nano-Mg(OH)2 becomes larger. When the modifier ratio is 5/5, the temperature is 70 °C and the time is 40 min, the grain size is the smallest of 31.1 ± 5.4 nm.

Activation Index
The activation index of the modified nano-Mg(OH) 2 is shown in Figure 4, and the higher the activation index, the better the modification effect of nano-Mg(OH) 2 . As shown in the activation index, when the modifier ratio is 5/5, the modification temperature is 70 • C and the modification time is 40 min, the modification effect of the GM-B-5/5 is excellent.

Microscopic Appearance and Particle Size Distribution
The SEM image and particle size of the GM-B-5/5 is shown in Figure 4b. The GM-B-5/5 image indicates that the modified nano-Mg(OH)2 is granular in the form of particles with smooth surfaces, and the dispersion performance of GM-B-5/5 is better with less agglomeration. The curve indicates that the particle size distribution of the GM-B-5/5 belongs to a normal distribution. The particle size of GM-B-5/5 is located within the range of 20-60 nm, and the average particle size is 36.45 ± 8.12 nm. Thus, the particle size of the modified powders is larger than that of the powders before modification.
The XRD result indicates that the purity of the synthetic nano-Mg(OH)2 and the modified nano-Mg(OH)2 in this research are all fairly high, while the grain size of the modified nano-Mg(OH)2 (23.7 ± 6.7 nm) becomes a little larger than that of the synthetic nano-Mg(OH)2 (31.1 ± 5.4 nm). The ATR-FTIR spectra show that the modified nano-Mg(OH)2 stretching vibration peaks of C-H at 2952 cm -1 and 2855 cm -1 are stronger than that of the synthetic nano-Mg(OH)2, which is due to the new chemical bonds which are formed between the nano-Mg(OH)2 and the modifier. From the microscopic characterization analysis, it can be seen that the particle size of the modified nano-Mg(OH)2 (20-60 nm, 36.45 ± 8.12 nm) is larger than that of the synthetic nano-Mg(OH)2 (15-55 nm, 29.61 ± 7.08 nm). Thus, the particle size of the powders is improved during the modification process. Meanwhile, the SEM images showed that the modification process improves the uniformity and the dispersion of the particles.

Microscopic Appearance and Particle Size Distribution
The SEM image and particle size of the GM-B-5/5 is shown in Figure 4b. The GM-B-5/5 image indicates that the modified nano-Mg(OH) 2 is granular in the form of particles with smooth surfaces, and the dispersion performance of GM-B-5/5 is better with less agglomeration. The curve indicates that the particle size distribution of the GM-B-5/5 belongs to a normal distribution. The particle size of GM-B-5/5 is located within the range of 20-60 nm, and the average particle size is 36.45 ± 8.12 nm. Thus, the particle size of the modified powders is larger than that of the powders before modification.
The XRD result indicates that the purity of the synthetic nano-Mg(OH) 2 and the modified nano-Mg(OH) 2 in this research are all fairly high, while the grain size of the modified nano-Mg(OH) 2 (23.7 ± 6.7 nm) becomes a little larger than that of the synthetic nano-Mg(OH) 2 (31.1 ± 5.4 nm). The ATR-FTIR spectra show that the modified nano-Mg(OH) 2 stretching vibration peaks of C-H at 2952 cm -1 and 2855 cm -1 are stronger than that of the synthetic nano-Mg(OH) 2 , which is due to the new chemical bonds which are formed between the nano-Mg(OH) 2 and the modifier. From the microscopic characterization analysis, it can be seen that the particle size of the modified nano-Mg(OH) 2 (20-60 nm, 36.45 ± 8.12 nm) is larger than that of the synthetic nano-Mg(OH) 2 (15-55 nm, 29.61 ± 7.08 nm). Thus, the particle size of the powders is improved during the modification process. Meanwhile, the SEM images showed that the modification process improves the uniformity and the dispersion of the particles.

Thermal Performance
The TGA curves of the fabrics during the decomposition procedure are shown in Figure 5b. The weight loss attributed to water evaporation and thermal decomposition of the fabrics is calculated as a function of temperature. In terms of thermal stability, the fabrics show three stages of weight loss: the F-0 shows the weight loss between 28-

Thermal Performance
The TGA curves of the fabrics during the decomposition procedure are shown in Figure 5b. The weight loss attributed to water evaporation and thermal decomposition of the fabrics is calculated as a function of temperature. In terms of thermal stability, the fabrics show three stages of weight loss: the F-0 shows the weight loss between 28-340,

SEM
The SEM images of the fabrics are shown in Figure 5c. The surface of the PET fabric (F-0) is smooth and the fiber diameter is about 9 µm. After loading the CM, there are particulate matters of CM on the fabrics. However, the CM agglomerates seriously on the surface of the fabrics. As shown in the M-250 image, the M agglomerates on the surface of the fabrics, while in the GM-250 image, the GM has a better dispersion on the surface of the fibers. Thus, the modified nano-Mg(OH) 2 enhance the dispersion performance, which obtain an excellent binding force with the fabrics.

Flame Retardant Property
The flame retardant property of the fabrics is shown in Table 5. The pure PET fabrics have a poor flame retardant property due to the LOI which is 20%, the damaged length is On the one hand, the MgO will be deposited on the surface and inside of the fibers, forming an inorganic protective film that blocks combustible gases and heat, thereby preventing combustion. On the other hand, the water vapor will reduce the concentration of combustible gases and prevent combustion.  Table 6 show the antibacterial property against E. coli of the fabrics. The E. coli colony can be visually observed on the LB agar plates of control, which is covered with E. coli colony (307 ± 5.9), while the E. coli colony number decreases as the content of the powders on the PET fabrics. When the content of the CM reaches 250 g/L, there is no colony on the LB agar plates (CM-250). As shown in Figure 6b, the reduction percentage of the E. coli colony number is calculated, and the antibacterial rate of CM-50, CM-100, CM-150, CM-200 and CM-250 reached 76.9 ± 0.3, 85.7±1.0, 95.4 ± 0.5, 99 ± 0.3 and 100 ± 0%, respectively. When the content of the M reaches 100 g/L, the CM-100 has no colony on the LB agar plates, and the antibacterial rate is 100%. When the content of the GM reaches 150 g/L, the GM-150 has no colony on the LB agar plates, and the antibacterial rate is 100%. The above results indicate that the fabrics have a certain antibacterial property against E. coli, which entirely depended on the powder content on the fabrics. Moreover, the antibacterial property are as follows: M > GM > CM. Thus, the modification process slightly reduced the antibacterial property of the synthetic nano-Mg(OH) 2 .
Polymers 2023, 15, x FOR PEER REVIEW 15 of 18 respectively. When the content of the M reaches 100 g/L, the CM-100 has no colony on the LB agar plates, and the antibacterial rate is 100%. When the content of the GM reaches 150 g/L, the GM-150 has no colony on the LB agar plates, and the antibacterial rate is 100%.
The above results indicate that the fabrics have a certain antibacterial property against E. coli, which entirely depended on the powder content on the fabrics. Moreover, the antibacterial property are as follows: M > GM > CM. Thus, the modification process slightly reduced the antibacterial property of the synthetic nano-Mg(OH)2.    The breaking strength of the fabrics are listed in Table 7 with the F-0 as a control. The breaking strength is slightly increased with the increase of powder content on the PET fabrics, which indicates that the addition of the powders can enhance the breaking strength of the PET fabrics. The results demonstrate that modification of the PET fabrics using nano-Mg(OH) 2 has almost no obvious effect on the breaking strength of the PET fabrics, and the functional PET fabrics have an excellent physico-mechanical property as the pure PET fabrics.