Synthesis Of Layered Double Hydroxides Based On Red Mud By A Coprecipitation Method And Its Interfacial Characrerization, Flamere Tardant And Mechanical Propertirs For Ethylene Vinyl Acetate Composites

On the basis of alumina industrial solid waste bayer red mud which activated by hydrochloric acid firstly, three kinds of ternary layered double hydroxides（LDHs） Mg/Al/Fe-CO₃ LDHs, Zn/Al/Fe-CO₃ LDHs and Mg/Al/Fe-BO₃ LDHs were synthesized by a coprecipitation method. Mg/Al/Fe-CO₃ LDH-B were modified by the anionic surfactant sodium dodecyl sulfate（SDS） and silane coupling agent KH-550. Moreover, the structure performance and modified effect of LDHs and the dosage of modifying agent were characterized using XRD, FT-IR, SEM, TGA and activation index. In addition, EVA/LDHs composites materials were prepared by means of melt blending. And, the flame-retardant, smoke suppression and thermal degradation properties were studied by limiting oxygen index（LOI）, cone calorimeter test（CCT）, smoke density test（SDT） and TG-FTIR spectrometer（TG-FTIR）.Furthermore, the interfacial properties, mechanical properties were tested and analyzed using SEM and tensile test.The results are shown in the following:（1）The testing results of XRD confirmed that on the basis of red mud activated by hydrochloric acid firstly, high crystallinity LDHs were synthesized successfully using a coprecipitation method by adding different proportions of divalent magnesium ions and zinc ion, then through the introduction of carbonate acid root ions. EVA/LDHs composites materials obtained good flame retardant performance and thermal stability after EVA melt blending with LDHs, not only high LOI values, but low HRR and THR values. Moreover, LOI values of part samples were more than 29%. And, after Mg/Al/Fe-LDH-B added, PHRR of ELDH2 composites was declined to 199.2 kW/m2, and THR was declined to 110.7 MJ/m2. The peak of heat release rate（PHRR） of pure EVA is 1694.8 kW/m2, and the THR is 179.4 MJ/m2. The composites also showed good smoke suppression effect, especially the ELDH2 sample, on the condition of ignition, luminous flux was more than 90% all the test time. In addition, the TGA test results show that the LDHs can improve the thermal stability of composites at high temperature, but at low temperature speed up the decarboxylation process of EVA.（2） The test results of XRD and FT-IR showed that borate successfully inset into the layer of the hydrotalcite, and good intercalated hydrotalcite synthesized successfully. Flame test showed that EVA/BO₃-LDHs composites have good flame and smoke suppression properties, and the EBLDH3 composite has the best smoke suppression, its LOI value of 27.3%, PHRR can be reduced to 238.4 kW/m2, and the flux value always more than 70%. TGA test showed that the insert of borate can improve the thermal stability of the hydrotalcite.（3） The test results of XRD, SEM, FT-IR showed that the SDS intercalated modification and surface modification with KH-550 were successful. The results of XRD, FT-IR, SEM and contact angle test showed that SDS anion was intercalated into the interlayer of LDHs and changed crystal structure of LDHs. KH-550 is attached on the surface of LDHs. Many results of testing showed that the optimum dosage of KH-550 was 3 wt.%, and modified LDHs changed from hydrophilic to hydrophobic. Flame retardant test results showed that the flame retardant, smoke suppression and thermal degradation stability of modified EVA/LDHs composite were improved. Mechanical tensile properties testing results showed that the SDS intercalating modification can significantly increased the elongation at break of the composites, and the elongation at break value of modified composite EVA/LDH-SDS was increased from EVA/LDH-CO₃ with the value of 37.5% to 223.4%. Surface modification with KH-550 can both improve the tensile strength and the elongation at break, the tensile strength of the composite EVA/LDHSSi-3 was increased to 9.8 MP from EVA/LDH-CO₃ with the value of 6.3 MPa, and the elongation at break was up to 426.7%. Organic modification can greatly improve the dispersibility and compatibility of LDHs in EVA, thus to improve the mechanical properties of the composites.（4） LDHs and modified LDHs can be synthesized using a coprecipitation method based on the bayer red mud activated by hydrochloric acid, and can be used as a flame retardant additives applied to EVA. Good flame retardant performance, smoke suppression, thermal decomposition, and mechanical properties of composites provide a new idea for the synthesis of LDHs, and expanding the red mud based hydrotalcite-related field of study.