Controllable Synthesis Of Mg-Al-LDH And Its Application In PVC Composites

Hydrotalcite was developed as a new-fashioned inorganic thermal stabilizer in 1980 s.Compared with the existing thermal stabilizers,hydrotalcites have always been popular and have the advantages of non-toxic,clean,inexpensive and high-efficient.But the present hydrotalcites have the defects of aggregation,irregular morphology,high production costs and so on.The most widely used in PVC thermal stabilizer is MgAl-LDHs.Mg Al-LDHs belongs to long-term thermal stabilizer and its long-term thermal stability is excellent,but its early coloring is serious.In addition,according to the research results at home and abroad,the BET of MgAl-LDHs has great influence on the thermal stability of PVC.Only when the BET is less than 30 m²/g can play a role of thermal stabilizer,otherwise it will promote the decomposition of PVC.Unfortunately,there is little research about the relationship between BET and thermal stability up to now.The synthesis methods of MgAl-LDHs are variety and the most common process used in industry is the coprecipitation method,hydrothermal method and urea method.These methods have limitations and weaknesses to hinder the large-scale continuous industrialization.This paper focuses on realizing the controllable preparation of MgAl-LDHs in BET by controlling the synthesis conditions.The goal is to try to get the product in BET between 8 m²/g and 30 m²/g.On these bases,this paper studies the relationship between the BET of MgAl-LDHs and thermal stability.In addition,we composited the early-type heat stabilizer MgAl-LDHs with ZnSt₂ and CaSt₂ to get the excellent PVC composites.In this paper,we adopted low supersaturated precipitation-hydrothermal method to get MgAl-LDHs with using MgCl2 or Mg（OH）₂ as magnesium source,Al2（SO4）3·18H2O as aluminium source,Na₂CO₃ to provide intercalated anions,and NaOH as pH regulator.In this dissertation,the effect of hydrothermal temperature,hydrothermal time,synthesis conditions of Mg（OH）₂ and seed load on the crystal structure,microstructure and specific surface area of LDHs was investigated by the means of X-ray diffraction（XRD）,scanning election microscopy（SEM）,physical adsorption instrument and laser particle analyzer.The most suitable processing method of magnesium source is using MgCl2 and NaOH to carry out by one-step hydrothermal reaction.The optimum synthesis conditions were 170 oC,12 h,and 300 r/min.Furthermore,adding seed could significantly increase layer thickness and reduce the BET of MgAl-LDHs.And the optimum seed load was 0.50%.Eventually,a series of regular shape products between the minimum BET of 8.48 m²/g and the maximum BET of 28.75 m²/g were synthesized.Simultaneously,this paper adopted high supersaturated precipitation-hydrothermal method and the optimum synthesis conditions were 190 oC,12 h.The suitable magnesium source is using MgCl2 and NaOH to carry out by one-step hydrothermal reaction.And the optimum seed load was 0.50%.Eventually,a series of clean and regular shape products between the minimum BET of 10.33 m²/g and the maximum BET of 20.57 m²/g were synthesized.In addition,using low-cost Qinghai salt lake MgCl2 to replace analytical material to synthesize MgAl-LDHs successfully in the end and the minimum BET reached 11.03 m²/g.And the cost is greatly reduced.Ultimately,congo red and thermal aging oven methods were employed to explore the relationship between the BET of MgAl-LDHs and thermal stability.The result indicated that the smaller BET of MgAl-LDHs,the better performance of thermal stability.At the same level of BET,the thermal stability of MgAl-LDHs in low supersaturation coprecipitation hydrothermal method was better than in high supersaturation coprecipitation hydrothermal method.What’s more,the thermal stability results revealed that the optimum proportion of LDHs,ZnSt₂ and CaSt₂ was MgAl-LDHs / ZnSt₂ /CaSt₂=10∶1∶4.Under the condition of optimal proportion,the congo red time of PVC composite reached 46 min and the thermal aging oven time came to 600 min.