1. 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 water vapor is generated, which consumes part of the heat and dilutes combustible gases such as CO to a certain extent. Furthermore, nano-Mg(OH)
2 can also absorb smoke and plays a role in eliminating smoke [
17]. In addition, the nano-Mg(OH)
2 has been found to have a broad-spectrum antibacterial property, which is representative of inorganic antibacterial material due to its excellence in stability and persistent antibacterial property, and the nano-Mg(OH)
2 has great antibacterial property for loading on PET fabrics [
18,
19,
20]. It has been reported that nano-Mg(OH)
2 has antibacterial properties even in the dark, which implies that nano-Mg(OH)
2 can be effective without light irradiation [
21]. The antibacterial mechanism of nano-Mg(OH)
2 is commonly attributed to the production of reactive oxygen species (ROS) on nano-Mg(OH)
2 surface, which can cause bacterial lipid peroxidation and death [
22,
23,
24]. Thus, the nano-Mg(OH)
2 as flame retardant and antibacterial agent is suitable for loading on PET fabrics.
Among the methods which are commonly used for nano-Mg(OH)
2 synthesis are sol-gel technique, microwave/ultrasound-assisted technique, precipitation of a magnesium salt with an alkaline solution and solvothermal treatment [
25,
26,
27]. The microstructure of the nano-Mg(OH)
2, i.e., the particle size, shape and agglomeration are crucial in flame retardant and antibacterial applications. The double-dropping technique could improve the instantaneous supersaturation of the reactants in the reaction system, which is beneficial to the formation of the particles with a uniform particle size [
28,
29]. The conventional chemical precipitation method is to add an alkaline substance as a precipitant to the salt solutions, while the reverse precipitation method is to drop the magnesium salt solutions into the sodium hydroxide alkaline solutions for the reaction, and the pH value is always higher than the isoelectric point of the Mg(OH)
2 in water [
30,
31]. During the precipitation process, the net charge on the surface of the Mg(OH)
2 crystal nucleus is always negative, and the electrostatic repulsion between the negative charges will prevent the particles from agglomerating.
In this work, the nano-Mg(OH)2 as flame retardant and antibacterial agent is synthesized by 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 modified nano-Mg(OH)2 is loaded on the PET fabrics through dip-coating technology. A variety of characterizations including X-ray diffraction (XRD), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and thermogravimetric analysis (TGA) were used to characterize the powders and the fabrics. Additionally, the flame retardant and antibacterial properties of the fabrics were tested in this research.
2. Materials and Methods
2.1. Materials
The magnesium chloride hexahydrate (MgCl2·6H2O) and the sodium hydroxide (NaOH) purchased from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China), were used to synthesize the nano-Mg(OH)2. The commercial nano-Mg(OH)2 (C-M) for comparison was purchased from Xuancheng Jingrui New Material Co., Ltd. (Xuancheng, China). The polyethylene glycol and the absolute ethanol were purchased from Shanghai McLean Biochemical Technology Co., Ltd. The titanate coupling agent and stearic acid were purchased from Nanjing Chuangshi Chemical Auxiliary Co., Ltd. (Nanjing, China), and Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China). The PET fabrics were purchased from Wujiang Haixu Textile Co., Ltd. (Wujiang, China). The Escherichia coli (E. coli, ATCC 25922) purchased from Shanghai Luwei Technology Co., Ltd., (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.
2.2. 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
2O) was added to a volumetric flask. After adding the rotor to the volumetric flask, 50 mL MgCl
2·6H
2O 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. As shown from the label names are the M-M means (nano-Mg(OH)
2 sample)-(Mg
2+ concentration), the M-T means (nano-Mg(OH)
2 sample)-(reaction temperature) and the M-t means (nano-Mg(OH)
2 sample)-(reaction time).
The settling rate of the synthetic nano-Mg(OH)2 are expressed by the sedimentation volume. The synthetic samples (1 g M-M-0.5, M-M-1.0, M-M-1.5, M-M-2.0, M-M-3.0, M-T-20, M-T-30, M-T-40, M-T-50, M-T-60, M-t-20, M-t-30, M-t-40, M-t-50 and M-t-60) and the commercial nano-Mg(OH)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.
2.3. 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 (m1) were taken out and dried at 100 °C. The activation index was H = m1/m.
2.4. Preparation of Functional PET Fabrics
The pure PET fabrics were pretreated by washing and rinsing, and then dried at 90 °C for 0.5 h. The 50, 100, 150, 200, 250 g/L flame retardants including commercial nano-Mg(OH)2 (CM), synthetic nano-Mg(OH)2 (M) and modified nano-Mg(OH)2 (GM) were loaded onto the PET fabrics by dip-coating method (30 ℃ for 30 min), and then the fabrics were dried at 90 °C for 3 h. The prepared fabrics were named as F-0, CM-50, CM-100, CM-150, CM-200 and CM-250; M-50, M-100, M-150, M-200 and M-250; GM-50, GM-100, GM-150, GM-200 and GM-250.
2.5. 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.
2.6. 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.
2.7. 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.