Preparation Of Ca-doped MgAl-LDHs And Their Effects On Thermal Stability Of PVC

Hydrotalcite compounds(called layered double hydroxides,abbreviated as LDHs)are a kind of two-dimensional anion nano-materials with a layered structure.Because of the unique layered structure and acid-alkaline properties,they can absorb the HCl released from PVC by means of adsorption and ion exchange.Therefore they can be used as a non-toxic and inexpensive thermal stabilizer of PVC and are expected to replace toxic lead salts and expensive organo-tin thermal stabilizers.In this paper,the Ca-doped MgAl-LDHs were prepared by coprecipitation method.The Ca-doped MgAl-LDHs can not only extend the time of Congo red test,but also improve the color appearance of PVC.Firstly,the Ca-doped MgAl-LDHs were prepared by using mixed alkali as the precipitant.The effects of dropping manner,the molar ratio of M2+ to Al3+,pH value,the types of the interlayer anion and precursor,the dosage of calcium x,the crystallization temperature and time,solvents on the performances of Ca-doped MgAl-LDHs were studied.The optimal process condition is chloride salt as precursor,M2+/A13+(mol)=4,pH=10,x=0.8,carbonate-type anion,water as solvent,crystallized at 11? for 24h.The resulting layered Ca-doped MgAl-LDHs exhibit good dispensability,higher regularity and the size of 100nm?200nm.When they were added into the PVC,the Congo red time reaches 72min,which is 33.3%higher than that of the undoped MgAl-LDHs.Next,the Ca-doped MgAl-LDHs were also prepared by using urea as the precipitant.The effects of the types of precursor,the dosage of calcium x,solvent type,crystallization temperature and time on the performances of the Ca-doped MgAl-LDHs were studied.The better process condition is chloride salt as precursor,x=0.6,water as solvent and crystallized at 110? for 18h.The resulting Ca-doped MgAl-LDHs also exhibit good dispensability,regularity and the size of 200nm-500nm.The Congo red time of PVC reaches 53 min.When the solvent is 5%ethylene glycol(mass fraction),the PVC has better color appearance than water.When the solvent is 30%glycerol(mass fraction),the Congo red time of PVC reaches 62min,which is 17%higher than water.The thermal stability of PVC with Ca-doped MgAl-LDHs prepared by two kinds of precipitant was compared.It is found that both of them can improve the thermal stability of PVC.The way of mixed alkali as precipitant is more conducive to improving the Congo red time of PVC significantly,while the way of urea as precipitant is better for improving the PVC coloring.