The surface modification methods of magnesium hydroxide mainly include surface chemical modification, surface graft modification, and encapsulation modification. Among them, surface chemical modification is the simplest and most common modification method, and it is also the most important surface modification method at present. Modifiers commonly used in surface chemical modification can be roughly divided into coupling agents, surfactants, and composite modifiers. However, choosing different modification methods and using different modifiers in different polymer materials has different application effects.
In this passage, I will introduce performance of surface modified magnesium hydroxide in different formula and kindly noted that all the data was provided based on the laboratory test.
1. Silane coupling agent modified magnesium hydroxide
Ye hong team studied the performance of silane coupling agent modified crystalline magnesium hydroxide in EVA formula. They found the optimal condition is when adding 2% of silane coupling agent, 120 phr magnesium hydroxide to EVA formula, and modified under 110℃ for 0.5 hours, the compound material property shall be oxygen index 33.7%, tensile strength 13.0Mpa, elongation at break 217%. The flame retardant performance is greatly improved, and it also weakened the flame retardant’s effect on the material’s mechanical performance.
Chen Yi team studied the effect of silane coupling agent on the properties of flame retardant composites of HDPE (high-density polyethylene)/nanometer Mg(OH)2. Wet surface modification of magnesium hydroxide can improve the flame-retardant and mechanical properties of the system. When the amount of silane coupling agent was 3.5% of that of magnesium hydroxide and the amount of nano-magnesium hydroxide was 60% of that of HDPE, the compound material achieve its best modification effect—oxygen index is 9.8% higher than that of the unmodified one. The tensile strength is 25.4MPa which increased 20%, and the impact strength is 38.4k J/m2. The elongation at break was 378.76% with no significant change.
2. Stearic acid modified magnesium hydroxide
Huang Honghai team add stearic acid modified magnesium hydroxide powder to the EVA based formula, and it is believed that the modification of stearic acid to Mg (OH) 2 includes weak esterification reaction between – COOH of stearic acid and – OH of Mg (OH) 2, and acid-base reaction between acid and weak base of Mg (OH) 2, so the modification process of Mg (OH) 2 continued along with the adding of stearic acid. By adding modified Mg (OH) 2 which was modified with 5%(mass fraction) stearic acid into EVA based formula, the elongation at break of composite material increased significantly, increasing from 120% to 528%. If the adding amount of modifier exceeds 2% (mass fraction), the vertical combustion grade of composite material will not reach FV-0 grade. However, as long as the amount of modifier is less than 7%, the oxygen index can be maintained at 34%.
3. Sodium Stearate modified magnesium hydroxide
Liu Lihua team studied the wet surface modification process of nanometer magnesium hydroxide with sodium stearate and found that emulsifying the slurry with 5% sodium stearate (mass fraction) in the emulsifier at the speed of 5 000 r/min under 85 ℃ could obtain better-modified magnesium hydroxide powder. Then adding 40phr modified magnesium hydroxide to 100phr soft polyvinyl chloride (PVC) system, the flame retardant properties and mechanical properties of the compound material was improved compared with adding unmodified magnesium hydroxide, such as the oxygen index improved from 25.8% to 27.8%, the tensile strength increased from 15.6 MPa to 19.1 MPa, the elongation at break increased from 135.74% to 220.21%.
4. Zinc stearate modified magnesium hydroxide
Liu Lihua also studied the surface organic modification process of nanometer magnesium hydroxide with zinc stearate as the modifier, and the optimal modification condition is modifying magnesium hydroxide with 5% (mass fraction) zinc stearate under 85 ℃ for 30 minutes. Then add the modified magnesium hydroxide into the flexible PVC based formula, the oxygen index of PVC/Mg (OH) 2 composite material increased from 25.5% to 27.7%, the tensile strength is 18.60Mpa, and the elongation at break is 245.67%. It not only achieved a good flame retardation effect but also had little effect on the mechanical properties.
5. Silane coupling agent/magnesium stearate modified magnesium hydroxide
Li Sanxi team applying magnesium stearate and silane coupling agent to modify the surface of magnesium hydroxide by dry method in high-speed mixer at 80 ℃. The amount of modifier magnesium stearate and silane coupling agent A was 2.5% and 0.5% of the mass of magnesium hydroxide respectively and the chemical adsorption between the modifier and magnesium hydroxide was proved by infrared and other characterization methods. They found that in PE (polyethylene)/Mg (OH) 2 compounding system, modified magnesium hydroxide has better dispersion in polyethylene, but when magnesium hydroxide adding amount reaches 35%, tensile strength of the compounding system decreased from 23.71 MPa to 20.57 MPa, oxygen index increased from 18.6% to 26.0%, thermo-gravimetric residual quantity increased from 0 to 22%.
6. Silane coupling agent/fatty acid modified magnesium hydroxide
Zhang Yuzhong team used silane coupling agent SCA313 and fatty acid STA as composite modifier to prepare a new synergy compound flame retardant of magnesium hydroxide/magnesium silicate aluminum powder. When wet-modified this synergy compound flame retardant with 0.6% (mass fraction) of silane coupling agent and 1% (mass fraction) of aliphatic acid, then adding this flame retardant into EVA based composite material, the oxygen index and tensile strength of the composite were 42.4% and 11.88Mpa respectively. It can be seen from the scanning electroscope that magnesium hydroxide/magnesium silicon aluminum powder is dispersed in the polymer uniformly, which increases the compatibility and force with the polymer, and also improves the mechanical properties of the composite material.
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