Surface Modification Of Phosphogypsum Powder And Its Use In PVC

Phosphorus gypsum is the solid waste in the phosphate fertilizer industry.Generally,4.5～5.0 tons of phosphorus gypsum are let off when 1 ton of phosphoric acid produced,and 2.5～5.0 tons of phosphorus gypsum are discharged when 1 ton of diammonium phosphate produced.In China,the cumulative emission load of phosphorus gypsum exceeds 300 million tons,most of which are treated by stockpiling,and the actual utilization rate is about 30%.The discharged phosphorus gypsum residue occupies a large amount of land,forming a slag mountain and seriously polluting the environment.The document issued by Guizhou Provincial People’s Government(Guizhou Government [2018]10)puts forward the tasks and requirements for the comprehensive implementation of "Control production according to usage".And one of the key tasks is to accelerate the pace of green upgrading and transformation,ensuring that there is no newly added stock of phosphorus gypsum in the province,and consuming the existing stock year by year.Replacing calcium powder with phosphorus gypsum powder massively and widely in polymer is one of the important methods to expand the application field of phosphorus gypsum and to implement the plan of "control production according to usage"In this paper,the influence of the type and dosage of coupling agent for surface modification of hemihydrate phosphorus gypsum on the structure and properties of hemihydrate phosphorus gypsum/PVC blends was discussed,and optimal modification scheme of phosphorus gypsum was determined.On this basis,the effects of the dosage of hemihydrate phosphorus gypsum and anhydrous phosphorus gypsum on the structure and properties of hemihydrate phosphorus gypsum/PVC blends and anhydrous phosphorus gypsum/PVC blends studied.Results show that:Unmodified hemihydrate phosphorus gypsum/PVC blends are hydrophilic,and its contact angle is 68.752°.When modified by stearic acid,hemihydrate phosphorus gypsum/PVC blends are hydrophobic,and its contact angle is 91.803°.The addition of aluminate coupling agent improves the hydrophobicity of hemihydrate phosphorus gypsum/PVC blends.The properties of hemihydrate phosphorus gypsum/PVC blends modified by stearic acid and aluminate coupling agent are better than hemihydrate phosphorus gypsum/PVC blends modified by single modifier and unmodified hemihydrate phosphorus gypsum/PVC blends.When the content of the aluminate coupling agent reaches 1.5%,its hydrophobicity is the best with the contact angle of 95.825°.More content of hemihydrate phosphorus gypsum powder will bring better hydrophobicity of hemihydrate phosphorus gypsum/PVC blends.However,the other properties of blends show different change.The addition of a small amount of hemihydrate phosphorus gypsum powder will promote the stability and mechanical properties of hemihydrate phosphorus gypsum/PVC blends.When the content of hemihydrate phosphorus gypsum powder is 5%,the initial decomposition temperature of hemihydrate phosphorus gypsum/PVC blends is highest and the stability is the best.At the same time,blends have the best mechanical property with the elongation of 86.9%,the tensile strength of 37.15 mpa,and the tensile modulus of 2145.00 Mpa.Compared with pure PVC,the elongation at break increased by 47.8% and the modulus increased by 285 Mpa,but the tensile strength doesn’t change significantly.With the content of anhydrous phosphorus gypsum powder increased,the contact angle increases gradually.When the content of anhydrous phosphorus gypsum powder is 10%,the melt flow rate of anhydrous phosphorus gypsum powder/PVC blends is highest.When the content of anhydrous phosphorus gypsum powder is 5%,the initial decomposition temperature of blends reaches the maximum value and the stability is the best.At the same time,mechanical properties of blends are the best with elongation rate of 87.4%,tensile strength of 36.12 Mpa and tensile modulus of 1919.50 MPa.Compared with pure PVC,elongation at break increased by 48.3%,and modulus increased by 59.5Mpa,but tensile strength slightly decreases.After comprehensive consideration,the content of phosphorus gypsum powder should be 5%.