| Phosphogypsum is a kind of by-product which is produced in the process of phosphoric acid production in phosphorus chemical enterprise.The performance of the phosphogypsum is poor because of its acidity and impurities,which greatly limits its applications.In China,the utilization rate of phosphogypsum is very low and it mainly stores up on the ground,which pollutes the environment and presents safety hazards.Phosphogypsum can be formed into a phosphogypsum micropowder?calcium sulfate hemihydrate??PG?after being neutralized,calcined and ground,and the particle size of PG can be controlled by the grinding process.The large and wide applications of PG instead of calcium carbonate?CaCO3?in polymers is one of the important ways to broaden the application field of phosphogypsum and implement the“slag-based production”program.PG can be used as a filler in PVC products instead of CaCO3.Guizhou Kai Phosphorus Gypsum Co.has built two production lines of 500,000 m2/year PVC stone-plastic board and400,000 m2/year PVC foam board for industrial production research.The study found that poor processing stability was the key issue that restricts the industrialization of PVC/PG blends.For this reason,the paper used PG instead of CaCO3 in PVC/CaCO3 blends to investigate the reasons for the poor processing stability of PVC/PG blends compared to PVC/CaCO3 blends.Furthermore,we studied the type of heat stabilizer and selected the best heat stabilizer,then studied the amount of PG and the best heat stabilizer.The research tasks are as follows:Firstly,in order to discuss the reasons for the poor processing stability of PVC/PG blends compared to PVC/CaCO3 blends,we made use of the analysis of PG component,thermal performance?TG-DSC?analysis,static stability?TG-DSC,Congo red method?of PVC/PG and PVC/CaCO3 blends and dynamic stability?torque rheometer?and so on.The results show that the particle size of the phosphogypsum powder is smaller than that of calcium carbonate and the specific surface area is large,so the interface with PVC matrix is strong.However,near the molding temperature of PVC,the phosphogypsum powder releases crystal water,which is acidic and can induce the chlorine radical generated by the homogenization of PVC capturing the hydrogen of the acidic compound in the phosphogypsum micropowder to form HCl,thereby decreasing the initial decomposition temperature and stability of the PVC.It could be concluded that the little amount of crystal water in the phosphogypsum micropowder was the main factor leading to poor processing stability of the PVC/PG blend through the further tests.Secondly,PVC/PG blends were prepared by torque rheometer.And the thermal stability of PG blends was explored by using Pb salt stabilizer,calcium zinc stabilizer,organotin stabilizer and calcium zinc-organotin composite stabilizer as heat stabilizers.Studies have shown that the PVC/PG blends with using calcium-zinc-organotin composite heat stabilizer have the lowest intensity of C=C peak at 1620 cm-1 and HCl peak at 3610cm-1.And its initial decomposition temperature and the fastest decomposition temperature was slightly higher than the temperature of the Pb salt stabilizer.What's more,the static stabilization time was the same as the Pb salt stabilizer,and its thermal stability met the requirements for applications.The calcium-zinc-organotin composite stabilizer was better through the systematic analysis of the effects of Pb salt stabilizer,calcium-zinc stabilizer,organotin stabilizer and calcium-zinc-organotin composite stabilizer on the processing stability of PVC/PG blends.Then,the effects of the amount of PG on the processing stability of PVC/PG blends were discussed.The study found that as the content of phosphogypsum micropowder increased,the compatibility and dispersibility of PVC/PG blends deteriorated.And the initial decomposition temperature,the fastest decomposition temperature and the static stabilization time gradually decreased,and the decreasing amplitude was getting larger and larger.At the same time,the fluidity and tensile properties of the blend deteriorated.What's more,when the amount of PG was 20%,the processing stability of PVC/PG blend could be same as that of pure PVC,and the elongation at break was increased to about 3 times,which could meet the purpose of utilizing phosphogypsum.Finally,the effects of the amount of calcium-zinc-organotin composite stabilizer on the processing stability of PVC/PG blends were discussed.It was found that with the increase of stabilizer content,the initial decomposition temperature,the fastest decomposition temperature,static stabilization time and melt flow rate of the blends firstly increased and then decreased.Moreover,when the amount of stabilizer was 7 phr,the blends had the best processing stability and flow properties.Therefore,when the mount of phosphogypsum micropowder was 20%and the mount of the calcium-zinc-organotin composite stabilizer was 7 phr,the PVC/PG blend had the best processing stability. |
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