Machenochemistry/Crystallization Synthesis And Application Of Nano-hydrotalcite

Hydrotalcit is one kind of layered double hydroxides, and belongs to anionic clay materials. Hydrotalcite is widely used in the fields of chemical engineering, material engineering, environmental protection, medicine, and so forth, owing to its unique capability of catalytic activity, adsorbability, iron exchange and thermal stability. With the increasing wide use, the demands on hydrotalcites are getting more and more. However, the industrial preparation for hydrotalcite has not achieved, because of the restrictions of technology and high productive cost. It is urgent to search for low cost raw materials and technology. In this subject, nano-hydrotalcite was synthesized using brucite as the source of magnesium, surface modified, and the modified nano-hydrotalcite was added into PVC to investigate the heat stability effects. This study established the foundation for low-cost synthesis and application of high-performance hydrotalcites.Nano-hydrotalcite was synthesized by the method of mechanochemistry/crystallization, using brucite, Al(OH)3and Na2CO3as raw materials, and milling time, milling speed, ball to powder weight ratio, molar ratio of Mg and Al, water addition amount, crystallizing temperature, crystallizing time and Na2CO3concentration were investigated to achieve optimum conditions. Meanwhile, the characteristic comparisons between hydrotalcites synthesized by different methods were carried out. The results showed that well crystallized hydrotalcite with average partical size of40nm was synthesized at the dry milling time of6h, wet milling time of2h, milling speed of250r/min, ball to powder weight ratio of50, molar ratio of Mg and Al of3, water addition amount of2mL, crystallization temperature of80℃, crystallization time of lh and Na2CO3concentration of0.lmol/L. As compared with hydrotalcites synthesized by coprecipitation (CHTs), the hydrotalcite synthesized by mechanochemistry/crystallization (MHTs) was with higher crystallinity, more uniform size distribution, and bigger parical size. The synthesis mechanism was suggested by virtue of XRD, FE-SEM, FT-IR, and XPS. It has been indicated that at the first of dry milling process, the parical sizes of brucite and Al(OH)3decrease, while specific surface areas of them increase by the action of mechanical force. Then, plastic deformations of brucite and Al(0H)3particles occur, accompanied by lattice defects, crystal transfer, and amorphous. At last, the whole particle become amorphous, and solid solution of Mg(OH)2-Al(OH)3is formed. In the process of wet milling, metal ions in the laminate of Mg(OH)2-Al(OH)3solid solution, which has high surface energy, can easily react with H2O, and produce the precursor of Mg6Al2(OH)18·4.5H2O, which has the same crystal structure as hydrotalcite. Under the hydrothermal condition, anions of OH-in the interlamination of precursor are exchanged by CO32-in the solution and nano-hydrotalcite is generated at last.Wet surface modification was carried out with surfactants such as sodium stearate, sodium oleate, aluminum distearate and KH-550, etc, and modification effects were characterized by activation index and contact angle. Surfactants types, modification temperature, modification time, surfactant dosage, initial slurry concentration and agitation speed were studied to achieve the optimum conditions. The results showed that better surfactant was aluminium distearate, and under the condition of modification temperature of80℃, modification time of60min, surfactant dosage of4wt.%, initial slurry concentration of5wt.%and agitation speed of1500r/min, the modification effect of nano-hydrotalcite was best, and the activation index and contact angle of modified nano-hydrotalcite were100%and135°, respectively.The modification mechanism of aluminium distearate was suggested in terms of XRD, FE-SEM, TG-DSC and FT-IR characterization results. It has been revealed that the crystal structure of hydrotalcite was not destroyed, and insoluble Mg[(C17H35C00)2A10]2and Al[(C17H35COO)2A1O]3were generated by reactions of (Ci7H35C00)2A10" and Mg2+, Al3+onto the surface of hydrotalcites. Chemical adsorption as well as physical adsorption existed between hydrotalcite and aluminium distearate, the former is very strong and steady, while the latter is faint and easy to desorb.Modified hydrotalcite was added into PVC to investigate the effect of hydrotalcite on thermal stability of composites. The stable thermal stability was characterized by change color experiment in the oven, dynamic stability was characterized by torque rhemometer, and residual stability was characterized by Congo red method. The results have revealed that hydrotalcite is one kind of long-term heat stabilizer, and the dynamic stability time and residual stability time increase with the rise of added hydrotalcite amounts. When the added amounts of MHTs was3wt.%, the dynamic stability time and residual stability time of composites were1150s and43min, respectively. In the composite heat stabilizer, when the mass ratio of calcium stearate (CaSt2) and zinc stearate (ZnSt2) was1:1, the amount of Ca-Zn stabilizer was0.3wt.%, and the amount of MHTs was3wt.%, the dynamic stability time and residual stability time of composites were1350s and82min, respectively.The thermal stability mechanism of nano-hydrotalcite, CaSt2and ZnSt2composite stabilizers can be depicted as follows:the major heat stabilizer of ZnSt2reacted with allyl chloride in PVC at first to restrain thermal degradation of PVC. Then, CO32-in interlamination of hydrotalcite reacted with HCl generated in thermal degradation of PVC. Meanwhile, ZnSt2can take synergistic effect with hydrotalcite and CaSt2, respectively. At last, when CO32-in interlamination of hydrotalcite used up, MgCl2and AICl3were generated by the reaction between HCl and double hydroxides in hydrotalcite laminate, which promoted thermal degradation of PVC.This study has significant guiding meaning for wise utilization and exploitation of brucite resources in China and industrial production of hydrotalcite.