Abstract: The modified nano magnesium hydroxide with a yield of about 98% was synthesized by liquid phase precipitation method with the assistance of surfactant. The best modified sample was determined by the specific surface area and apparent density test results, and the structure and morphology of the best sample were characterized and analyzed by X-ray diffraction, transmission electron microscopy and other means. The results show that the modified sample is a hexagonal crystal system with good dispersibility and high crystallinity. Infrared spectrum analysis shows that during the preparation of magnesium hydroxide, the surfactant is adsorbed on the sample surface. Compared with the unmodified sample, the sedimentation mass fraction of the modified sample increased by 31.2%, 32.8% and 30%, respectively, indicating that the modified sample has good compatibility in non-polar substances.
Surface modification of magnesium hydroxide is to change the surface polarity of magnesium hydroxide by surface modifiers, reduce the surface energy of magnesium hydroxide, and make it more compatible with polymers. One end of the surfactant molecule is a long carbon chain alkyl group, which has a certain compatibility with the polymer molecular chain. The other end is a polar group such as hydroxyl, carboxyl, ether bond, etc., which can react chemically or physically and chemically adsorb on the surface of inorganic materials, thereby effectively changing the surface properties of inorganic materials [1]. The type and properties of surfactants play a decisive role in the effect of surface modification or surface treatment of inorganic materials [2]. Among the commonly used surfactants, the glucose ring of alkyl polyglycoside has a steric stabilization effect, and the hydroxyl group on the ring also interacts with the hydroxyl group on the surface of magnesium hydroxide; the long chain of sodium polyacrylate has a steric stabilization effect and an electrostatic stabilization effect of dissociating to generate a large amount of charge; the phosphate in alcohol ether phosphate reacts with the magnesium ions on the surface of magnesium hydroxide powder to form a magnesium phosphate salt deposit that is coated on the surface of magnesium hydroxide particles, thereby changing the surface properties of magnesium hydroxide powder.
In this paper, a one-step method combining the surfactant control technology that is easy to industrialize and ordinary water phase precipitation is used to synthesize modified nano magnesium hydroxide. Based on the addition amount determined in the previous work [3], the effects of different surfactants on the apparent density and specific surface area of nano magnesium hydroxide are investigated, the best modified sample is determined, and the structure, morphology and compatibility of the best sample are characterized.
1 Experimental part
1.1 Raw materials
Magnesium nitrate hexahydrate, analytical grade, Tianjin Fengchuan Chemical Reagent Technology Co., Ltd.; sodium hydroxide, analytical grade, Beijing Chemical Plant; alkyl polyglycoside APG1214, w=50%; alcohol ether phosphate (MAP), solid mass fraction is 98%; sodium dodecyl sulfate (SDS), w=98%, all are industrial grade, China Daily Chemical Industry Research Institute; sodium polyacrylate (PA-Na), relative molecular mass is 8000, w=45%, industrial grade, BASF, Germany; liquid paraffin, chemically pure, Tianjin Tianxin Fine Chemical Development Center; anhydrous ethanol, analytical grade, Tianjin Beilian Fine Chemical Development Co., Ltd.
1.2 Synthesis and characterization of samples The preparation of nano magnesium hydroxide can be found in reference [3].
Specific surface area: The ASAP2010 specific surface area spectrometer produced by Micromeritics, USA, was used to measure the nitrogen physical adsorption at low temperature, and the amount of nitrogen adsorption was calculated by the area of the desorption peak, and then the BET equation was used to calculate the specific surface area.
Bulk density: The sample was loaded into a 10 mL measuring cylinder in batches, and the sample was weighed after vibrating until the volume no longer changed. The bulk density of the sample was calculated according to the following formula [4]. The average value was taken after multiple measurements.
ρ= (m2-m1)/10，
Where: m1 and m2 are the masses before and after the measuring cylinder is filled with 10 mL of sample.
Infrared spectrum (FTIR): The measurement was performed using the Magna750 Fourier transform infrared spectrometer of Nicolet Corporation of the United States, using the KBr tablet method and scanning at room temperature.
X-ray diffraction spectrum (XRD): The measurement was performed using the D/Max2500 X-ray diffractometer of Rigaku Corporation of Japan, with a tube voltage of 40 kV, a tube current of 100 mA, a scanning speed of 8 (°)/min, and a scanning range of 2θ=5~85°.
Transmission electron microscope image (TEM): The measurement was performed using the JEM1011 transmission electron microscope of Japan Electronics Corporation, with an acceleration voltage of 100 kV. After the sample ultrafine powder is ultrasonically vibrated in ethanol for 20 min, a few drops are removed to the copper grid. After the ethanol is completely volatilized, TEM analysis can be performed.
1.3 Sedimentation volume test
Weigh about 1.00 g of the synthetic sample into a 25 mL stoppered measuring cylinder, add liquid paraffin to the scale line, ultrasonically vibrate for 20 min, shake well and let stand, and read the volume of the sample suspension at different times. The surface of unmodified magnesium hydroxide is hydrophilic, and its compatibility with liquid paraffin is poor. It cannot be well dispersed in it and has a fast sedimentation rate; the surface of modified magnesium hydroxide is hydrophobic, and its compatibility with liquid paraffin is good, and it can be well dispersed in it. The mass fraction of the particle suspension w (%) is calculated as follows:
w= (V/25)×100%，
Where V is the volume of the particle suspension at 660 min.
2 Results and discussion
2.1 Analysis of particle size, apparent density and specific surface area
| Sample | Particle size/nm | Bulk density/(g·cm-3) | Specific surface area/（m2 ·g-1） |
| Unmodified (M0) | 12 | 0.39 | 57.34 |
| APG（M1） | 14.5 | 0.42 | 54.75 |
| PA-Na（M2） | 10.8 | 0.3 | 64.6 |
| MAP（M3） | 11.7 | 0.33 | 61.02 |
| SDS/APG （M4） | 9.4 | 0.24 | 67.96 |
| SDS/PA-Na （M5） | 9.2 | 0.22 | 70.25 |
| SDS/MAP（M6） | 10.6 | 0.27 | 64.16 |
Based on the addition amount determined in the previous work [3], the effect of different surfactants on the modification effect of magnesium hydroxide was investigated. The particle size, apparent density and specific surface area of the samples synthesized with different surfactants are listed in Table 1. From the data in the table, it can be seen that the specific surface area of magnesium hydroxide increases from 57.34 m2/g of the unmodified sample (M0) to 70.25 m2/g of the M5 sample with the addition of different surfactants; the increase in specific surface area is due to the decrease in the bulk density and particle size of the sample. The addition of surfactants effectively inhibits the agglomeration of magnesium hydroxide particles and increases the porosity of the aggregate. The influence of different surfactants on the bulk density and specific surface area of magnesium hydroxide varies to a certain extent, because the physicochemical properties of the surfactants determine the growth process and properties of the nanoparticles [5]. The specific surface area of the flame retardant filler is closely related to the performance of the product. The larger the specific surface area, the smaller the particle size, and the better the flame retardant effect [6].
2.2 Structure and morphology analysis
In order to compare the effects of different surfactant systems on the structure and morphology of magnesium hydroxide, the untreated sample M0, the sample M4 with SDS/APG, the sample M5 with SDS/PA-Na, and the sample M6 with SDS/MAP were selected.
2.2.1 XRD analysis
The curves in Figure 1 (a)-(d) are the XRD spectra of samples M0, M4, M5, and M6, respectively. It can be seen that the positions of the diffraction peaks before and after modification and the interplanar spacing are consistent with the standard diffraction spectrum of magnesium hydroxide (PDF44-1482), indicating that the synthesized sample is magnesium hydroxide with a hexagonal crystal structure. In the XRD spectrum, no diffraction peaks of other phases appear, indicating that the reaction is complete and the product is pure magnesium hydroxide crystals. Compared with the unmodified sample M0, the diffraction peaks of M4, M5, and M6 have obvious broadening phenomena, and the characteristic diffraction peaks are high in intensity and sharp in peak shape, indicating that the crystal size of the modified magnesium hydroxide is reduced and the structure is more regular. This can be explained as follows: when the sample M0 without the addition of surfactant is subjected to precipitation reaction, the aggregation speed of the nucleation ions is greater than the orientation speed, and the crystal form is incomplete and the grains are easy to agglomerate because there is not enough time for lattice arrangement. After adding the surfactant, the surfactant will be adsorbed on the surface of the newly generated magnesium hydroxide particles, inhibiting the growth rate of magnesium ions on the surface of the already generated magnesium hydroxide particles, providing enough time for lattice arrangement, so that the crystal form of the modified sample is more regular, which is consistent with the following TEM analysis results. According to the Scherer equation D = kλ/ (βcosθ) [7], the average crystal size of Mg(OH)2 can be estimated, as shown in Figure 1. Adding surfactant can reduce the particle size of magnesium hydroxide, and M4 and M5 are more obvious than M6, which is consistent with the XRD results. It can also be seen from the figure that although the broadening phenomenon of the diffraction peak of M6 is not as obvious as that of M4 and M5, its characteristic diffraction peak intensity is higher, which shows that SDS/MAP can make the morphology of magnesium hydroxide more regular. This can be analyzed in combination with the following TEM images and Table 1.
2.2.2 TEM analysis
Figure 2 (a)-(d) are TEM images of samples M0, M4, M5, and M6 respectively. It can be seen that the primary particles of the samples are basically flaky. According to literature reports [8-9], when the shape of the filler is flaky, the polymer-inorganic nanocomposite material can produce some special mechanical and physical properties. In addition to enhancing the mechanical properties of composite materials, nanosheets also have the so-called “fence effect”, which makes them have a significant flame retardant effect even when the amount of filler added is very low. In addition, flaky nano magnesium hydroxide can be dehydrated at high temperatures to form a dense and masking magnesium oxide ceramic film, with an oxygen index higher than that of ordinary micron-grade magnesium hydroxide flame retardants, and the difference is more obvious when the amount added is higher. Sample M0 without surfactant added has interlaced growth of flakes, resulting in a larger secondary particle size. Compared with M0, the side lengths of M4, M5 and M6 are smaller and the dispersion has changed significantly. This may be due to the uneven distribution of atoms on different crystal faces of magnesium hydroxide, which may lead to different adsorption amounts of surfactant molecules on different crystal faces. These adsorptions promote or inhibit the growth of crystal faces by changing the specific surface energy and growth rate constant of the crystal face [10-11], and adjust the interface characteristics, thereby guiding the regular arrangement of molecules between organic matrix and inorganic ions at the interface, achieving the nucleation and growth order of inorganic crystals under the modulation of organic matrix, improving the growth uniformity of magnesium hydroxide grains, and making their arrangement regular. Therefore, a structural sample with high crystallinity and regular morphology is prepared. Compared with M4 and M5, the crystal outline of M6 is more obvious and the regularity is better, which is consistent with the previous XRD analysis results.
2.3 FTIR spectrum anaalysis
Figure 3 (a)-(c) are the infrared spectra of samples M4, M5 and M6. It can be seen that the sharp and high-intensity peak at 3698 cm-1 belongs to the stretching vibration peak of -OH in the magnesium hydroxide crystal structure, and the peaks at 1634 and 1417 cm-1 belong to the bending vibration peaks of Mg-OH and -OH, respectively, which is consistent with the report in the literature [12]. The absorption peaks of asymmetric and symmetric stretching vibrations of aliphatic CH2 appear at 2926 and 2854 cm-1, respectively, and the absorption peak at 1123 cm-1 is the stretching vibration peak of S-O, which indicates that the surfactant is adsorbed on the sample surface during the preparation of magnesium hydroxide, which plays a role in controlling the growth of nanoparticles and preventing the agglomeration of nanoparticles.
2.4 Compatibility test
The compatibility of magnesium hydroxide samples in non-polar substances before and after modification with surfactants was investigated by measuring the sedimentation stability of magnesium hydroxide samples in liquid paraffin. The test results are shown in Figure 4. In the modification of nanomaterials, sedimentation volume test is a relatively important research method because it can be used to evaluate the modification effect [13] and to obtain the compatibility of modified magnesium hydroxide with organic matrices. Generally speaking, inorganic particles are prone to sedimentation due to their strong hydrophilicity and poor compatibility with oily solvents. Unmodified magnesium hydroxide nanoparticles show the same tendency. However, after magnesium hydroxide is modified with surfactants, the surface of magnesium hydroxide changes from hydrophilic to lipophilic, which improves the dispersion stability of magnesium hydroxide in organic media. Specifically, it maintains a high stability in liquid paraffin and does not settle for a long time. Compared with the unmodified sample, the sedimentation mass fraction of the modified sample increased by 31.2%, 32.8% and 30%, respectively. This indicates that the surfactant adsorbed on the sample surface changes its surface properties and enhances its surface hydrophobicity. This is consistent with the previous FTIR analysis results.
A stable platform of about 1% rises slowly. At 1 kHz, P1200 has a dielectric loss peak near 290 °C, slightly lower than the peak temperature of the dielectric peak of 310 °C. Above 310 °C, the dielectric loss increases sharply. The reason may be due to the conductivity of ions or space charges in the sample at high temperature [7].
3 Conclusions
1) Among several modifiers, PA-Na has the best modification effect. The modified sample has the largest specific surface area, the smallest particle size, the smallest apparent density, and the suspension mass fraction reaches 94.8%, while the suspension mass fraction of the unmodified sample is 62%, which has the best compatibility with the organic matrix.
2) The results of infrared spectroscopy analysis show that during the preparation of magnesium hydroxide, the surfactant is adsorbed on the sample surface, which plays a role in controlling the growth process of nanoparticles and preventing nanoparticles from agglomerating.
3) This method has a short process and simple process, with obvious modification effect and broad industrial prospects.