Preparation of modified magnesium hydroxide and its application in PA66

There are the advantages of flame retardant, filling, smoke suppression, non-corrosion processing equipment for environmental flame retardant magnesium hydroxide, so it is widely used in engineering plastics, rubber and other materials. The dispersibility and compatibility of magnesium hydroxide in polymer materials are poor. Therefore, it is necessary to modify the surface of magnesium hydroxide. Taking MgCl2 and NaOH as raw material and fatty alcohol polyoxyethylene ether-9(AEO-9)/ Sodium Dodecyl Sulfate (SDS) as composite modifier, modified magnesium hydroxide was prepared by direct precipitation method. The modification results were characterized by oil absorption value, activation index, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and other methods. When modification time is three hours and modifier dosage is 4.0g/mL, the modification effect of Mg(OH)2 prepared by the AEO-9/SDS treatment process is the best. It was shown that the composite modifier only acted on the surface of Magnesium Hydroxide (MH) powder and did not change its crystal structure, and its surface was changed from hydrophilic to hydrophobic, and the dispersibility and thermal stability were significantly improved compared with unmodified magnesium hydroxide. The Mg(OH)2 before and after the modification were applied to Polyamide 66 (PA66) to test its flame retardant and mechanical properties. The results show the modified Mg(OH)2 has good compatibility with PA66.


Introduction
Flame retardants have been used in more and more fields, such as construction, textile, transportation, aviation, electronics, plastic materials and so on (2018). Therefore, it is very important to study efficient flame retardants to reduce the combustion risk of polymers(2008).
As a new kind of environmental retardant, magnesium hydroxide plays an excellent role in fire prevention. Magnesium hydroxide is synthesized easily , the cost is low compared with other flame retardant. Magnesium hydroxide decomposition is at 380℃, decomposition produces less smoke and produces a lot of water vapor absorbing a lot of heat(2017). When magnesium hydroxide breaks down, it not only can reduce the temperature of the material, but also can control combustible gas and smoke emission.
Magnesium hydroxide as a new type of inorganic flame retardants, there is good thermal stability, non-toxic, smoke suppression and efficient effect on promoting substrate into charcoal. In addition to producing no corrosive gases, there is the function of neutralizing acidic and corrosive gases generated in the combustion process for magnesium hydroxide(2019). As a kind of environment-friendly green flame retardant, there is a good market prospect for magnesium hydroxide(2014, 2019 and 2011). There is the triple functions of flame retardant, filling, and smoke suppression for magnesium hydroxide. However, its compatibility with high molecular polymers is very poor. It is not easy to disperse and cause agglomeration phenomenon, and lacks affinity with the polymer matrix. Therefore, ultrafine treatment or surface modification of the magnesium hydroxide particles is required. Magnesium hydroxide particles modified can reduce the surface energy of magnesium hydroxide particles, reduce the hydrophilicity of magnesium hydroxide particles, increase the lipophilicity, and is easy to be dispersed. The compatibility and wettability of magnesium hydroxide and polymer materials are improved, and the dispersibility of magnesium hydroxide in matrix materials is also improved.
Althouh there is high strength, good wear resistance and fine feel, application of PA66 is limited due to its poor flame retardancy. Modified magnesium hydroxide was prepared to improve its compatibility in PA66, and added to PA66 to improve the flame retardant performance of PA66.The application range of PA 66 composite material is expanded.
SHB-B95 circulating water multi-purpose vacuum pump is used, Bruker D8 ADVANCE X-ray diffractometer is used and conditions of use are ,Cu target Kα-ray (λ = 0.15406nm, tube pressure 40kV, tube flow 30mA, scanning range (10~90)°. The structure of the modified magnesium hydroxide is analyzed by USING JSM-6700F scanning electron microscope, and tested under high vacuum condition after gold spraying.AK36 twin screw extruder , CJ50E Ⅱ precision injection molding machine,HC-2 oxygen index tester and CZF-5 horizontal vertical combustion tester are also used.

Synthesis of ultrafine modified magnesium hydroxide
Magnesium chloride hexahydrate solution and sodium hydroxide solution were used in the experiment.The modified magnesium hydroxide was prepared by direct precipitation method and synthesized by selecting suitable reaction conditions such as modification time dosage of modifier and type of modifier. The reaction was carried out as follows: 1mol/L magnesium chloride solution and 2mol/L sodium hydroxide solution was prepared beforehand, and the concentration ratio of the magnesium chloride and sodium hydroxide was 1:2. 100ml of NaOH solution was added to 500ml beaker and place on a magnetic stirrer. 1g of AEO-9 and 1 g of SDS were dissolved in 100mL distilled water. Then 100ml MgCl 2 solution were added to the constant pressure funnel and temperature was set to 50℃. MgCl 2 solution was dropped into NaOH solution under the condition of frequent agitation after reaching the temperature. 3-20mL AEO-9/SDS solution was added to the solution and stirred for 1-5 hr after the dripping. After stopping stirring, the solution was kept at the same temperature for 1.5 hours. After cooling, the filter cake was drained and washed with water first and then with anhydrous ethanol twice.

Determination of oil absorption value and activation index
Oil absorption value is measured as follows:The oil absorption value is expressed as the milliliter number of DOP absorbed by 1g magnesium hydroxide volume X (mL), calculated as X=V/m. Where V is the volume consumed by DOP, mL; M mass of sample, g.
Activation index is measured as follows: 1 g of magnesium hydroxide sample was accurately weighed and added to a beaker of 200 mL distilled water. Magnesium hydroxide solution was vibrated 30 minutes and set for 10 h, The powder floating on the water surface was collected, drained and dryied.The dried product was cooled to room temperature and weighed. The calculation formula of activation index is as follows:A=m/M×100% .Where, A-Activation index; m-The mass of powder floating on water ; M-Total sample mass.

Preparation of PA66 flame retardant material
Related components were added according to the proportion of modified magnesium hydroxide, AEO-9/SDS and PA66. The mixing condition was 80℃, vacuum and drying for 8 hours. After drying, the mixture is extruded and granulated into a twin-screw extruder.

Performance measurement
Oxygen index is measured on the basis of GB/T2406-93.Vertical combustion is measured on the basis of GB4609-84. 250℃-270℃ is the extrusion temperature of the twin screw extruder, and the screw speed is 60 r/min. The temperature of injection is 270℃ and the sample is tested after 24 hours.

Influence of different modifier on modification effect of Mg (OH) 2
The influence of different modifiers on the modification effect of Mg(OH) 2 is shown in Table 1. It can be seen that the activation index of unmodified Mg(OH) 2 is 0, which is easily soaked by water and has strong hydrophilicity, and its oil absorption value is 1.022 mL/g. The surface polarity of unmodified MG (OH) 2 is strong and is easy to agglomerate, which lead to the gap between the particles is large and the ineffective absorption of DOP is too much and the oil absorption value is also large. When only modifiers were added, the activation index increased rapidly to 91.03%, showing obvious hydrophobicity. The oil absorption value decreased at this time, because the addition of modifiers improved the hydrophilicity of Mg(OH) 2 , but could not effectively improve its dispersity. At the same time, the activation index and oil absorption value of Mg(OH) 2 treated by composite modifier were improved greatly, the activation index was 96.98%.The oil absorption value was 0.542mL/g and the dispersity was improved.

Effect of modification time on modification effect of Mg (OH) 2
With the treatment of composite modifier, AEO-9/SDS dosage was kept at 4 g/mL , and the time interval (1 hr, 2 hr, 3 hr, 4 hr,5 hr) on the modification effect of Mg (OH) 2 . As can be seen from Table 2, activation index and oil absorption value of modified Mg(OH) 2 change within a small range with change of time. The oil absorption value decreased from 1.022 mL/g of unmodified Mg (OH) 2 to 0.684mL/g after 2 hr. The activation index reached more than 95% at this time and the modification effect was obvious. With the increase of time, the oil absorption value decreases gradually, and the corresponding activation index increases gradually. At 3 hr, the lowest oil absorption value is 0.471mL/g, and the activation index reaches the maximum value of 97.68%. When the time was extended to 5 hr, the oil absorption value increased but the change was small and the activation index decreased slightly. 3 hr is selected to ensure the best modification effect.

Effect of dosage of modifier on modification effect of Mg (OH) 2
The influence of the dosage of modifier on the modification effect was investigated, and the dosage of AEO-9/SDS was controlled as 1.0g/mL, 4.0g/mL, 7.0g/mL and 10.0g/mL, respectively during the modification time of 3 hr. As shown in Table 3, the oil absorption value of modified Mg(OH) 2 decreases firstly and then increases slightly and tends to be gentle with the increase of dosage. When the dosage of AEO-9/SDS is 4g/mL, the oil absorption value reaches the minimum of 0.465 mL/g, indicating a significant modification effect. The activation index changes obviously with the dosage of AEO-9/SDS . When the dosage is 1g/mL, the activation index is only 51.82% and the modification effect is not good.
With the gradual increase of dosage, the activation index increases significantly and can reach 98.01% when the dosage is 4g/mL. It shows that excessive modifier will not improve the modification effect continuously. When the dosage of AEO-9/SDS was small, Mg(OH) 2 could not be completely coated by modifier, and part of Mg(OH) 2 was not modified. When the dosage of modifier was increased to reach the required dosage of Mg(OH) 2 , the modification effect was the best and the oil absorption value was small and the activation index was high. When the dosage of Mg(OH) 2 is exceeded, the modification effect will not be enhanced, the oil absorption value and activation index are basically unchanged. The optimal modifier dosage was 4.0g/mL of Mg(OH) 2 .  Figure 1 shows the XRD patterns of magnesium hydroxide powder surface modification. It can be seen that strong characteristic diffraction peaks of magnesium hydroxide appear at 2θ is 18.6, 28.0, 50.9, 58.6, etc.They correspond to planes (001), (101), (102) and (110) of the crystal, respectively, which are consistent with the position of the standard atlas (JCPDS75-1527), and are hexagonal crystal structure. There is no new diffraction peak appeared in the spectrogram of the modified sample, indicating that the surface modification of magnesium hydroxide by composite modifier did not damage the crystal structure of magnesium hydroxide, but only adsorbed on the crystal surface. The upper part of Fig. 1 is modified Mg(OH) 2 , and the lower part is unmodified Mg(OH) 2 . As can be seen from Fig. 1, the modified and unmodified spectra are roughly similar and there is no other miscellaneous peaks. It can be seen that the ultrafine modified Mg(OH) 2 is hexagonal crystal, and the structure of the modified Mg(OH) 2 is not changed. The spectral peak of ultrafine modified Mg(OH) 2 is sharper than that before modification, indicating that modified Mg(OH) 2 has smaller crystal shape, more stable structure, stronger crystal strength and better flame retardant performance. After modification, there is no diffraction peak of other impurities except the characteristic peak of magnesium hydroxide, indicating that the sample is pure phase magnesium hydroxide. Loose particles can be clearly seen in the Fig. 2. The size and shape of particles reflect the geometry of Mg(OH) 2 , and the dispersion of magnesium hydroxide is better.

PA66 Burning Tester
Modified magnesium hydroxide (MH) were added into PA66 in different proportions to investigate the effect of MH on the flame retardancy of PA66. The experimental results are shown in Table 4. Magnesium hydroxide is added into polymer materials as flame retardant, oxygen index is an important index.As can be seen from Table 4, pure PA66 thick spline UL94 is V-2 (3.2mm), and liquid droplets appear in the burning process.The oxygen index (LOI) of PA66 material is 28.5% and the vertical combustion is V-2 When 30 (Phr) modified magnesium hydroxide is used. The organic alkyl part of the modified magnesium hydroxidecan form covalent bond with PA66, and there is good compatibility with PA66. Therefore,modified magnesium hydroxide can be evenly dispersed in PA66 matrix and there is good flame retardant effect. After modification, the surface of Mg(OH) 2 is coated with an organic layer, which has good lipophilicity and is more evenly distributed in PA66. In addition, the coating of organic compounds enhances the thermal stability of the composite, so the LOI has a small increase.The results show that the flame retardant of PA66 composite can be improved by adding 36Phr modified magnesium hydroxide.

Mechanical performances of PA66
The mechanical performances of PA66 with MH were measured. The results are shown in Table 5.As can be seen from table 5, the tensile strength of PA66 did not decrease obviously.

Conclusion
The influence of different conditions on the modification of Mg(OH) 2 ,such as the amount of modifier, type of modified , the modification time was discussed.The modification conditions of Mg(OH) 2 by AEO-9/SDS composite modifier were obtained. The crystal structure of modified Mg(OH) 2 was not damaged. Scanning electron microscopy (SEM) analysis showed that modified magnesium hydroxide was obtained. After modification, the properties of Mg (OH) 2 was changed from hydrophilic to oleophilic, and the dispersity was improved. The modification agent is only applied to Mg (OH) 2 surface and the modified Mg(OH) 2 is more suitable to be added into polymer materials. MH not only plays an excellent flame retardant performance, but also improves the compatibility and dispersion of the modified Mg(OH) 2 with the polymer substrate, and has little influence on the mechanical properties of the material itself.