Preparation Of Phosphogypsum-Based Calcium Sulfate Short Whisker And The Application Research In PVC

In recent years,phosphor chemical industry in China has developed rapidly,and the increase in the emission of solid waste phosphogypsum has gradually received attention around the world.As the province with the largest amount of domestic phosphogypsum,Hubei Province,the effective and rational utilization of phosphogypsum is a major issue facing the phosphate industry in Hubei and even the entire country.Whisker is a kind of fibrous crystals prepared by single crystal growth and is generally grown under controlled conditions.Its L/D ratio is high,crystal stucture are complete,atoms are highly ordered,and the material strength is close to the theoretical crystal strength and modulus.It has excellent physical and chemical properties such as light weight,high thermal stability,high strength,high elastic modulus,and high hardness.It can fullfill both functions of fiber reinforcement and filler filling as a new type of composite modifier.The use of phosphogypsum to produce low-cost,high-performance calcium sulfate whiskers is a key to high-value utilization of phosphogypsum.In the research of building materials at home and abroad,calcium sulfate whiskers can be used as fillers and reinforcing fibers in plastics,rubbers and other polymers to improve the mechanical properties,dimensional stability and fire retardation of polymer materials.However,for calcium sulphate whiskers containing crystal water,the dehydration temperature is generally lower than the temperature for polymer molding processing.In the process of calcium sulfate whiskers used as inorganic filler filled polymer materials,the crystal water of whiskers decomposed at high temperature resulted in a calcium sulphate powder disorderly distributed in the matrix as flaws,which would cause interface failure and reduce the mechanical strength of the composite material.In recent years,the study of calcium sulfate whiskers has mainly focused on the growth mechanism in different reaction systems.It has been neglected by researchers that the thermal dehydration temperature and crystal structure stability of whiskers for high temperature processing.This study relied on the Hubei Province Science and Technology Support Program“Phosphate Mine Associated Calcium Resources Comprehensive Development and Utilization Technology Research”?YSF2015000837?.Phosphogypsum was used as raw material,and calcium sulfate whiskers were prepared by hydrothermal method under normal pressure in different inorganic chloride electrolyte solutions.The crystal morphology and phase composition of the calcium sulfate short crystals in different alkali metal chloride electrolyte solutions were investigated,as well as the effects of thermodynamic properties.The calcium sulfate short crystals selected in the alkali metal salt solution system were functionally modified,and the effects of modification process parameters and modifiers on the modification effect were studied.The theoretical basis is provided for the processing and application of calcium whiskers,and the functionally modified short crystals of calcium sulfate are applied to PVC products.The results show that:Under the mass fraction of 10wt%-30wt%in different alkali solution systems,most of the synthesized products were CaSO4·0.5H2O without impurity ions under the CaCl₂ solution system.Under the NaCl solution system,CaSO₄·2H₂O was mostly converted to omongwaite?OMW?which dehydration temperature is 259.4°C;Under the KCl electrolyte solution system,CaSO4·2H2O was converted into gorgeyite?GOY?which dehydration temperature achieved 489.8°C.Under certain concentration of alkali metal solution,the alkali metal can enter into the calcium sulfate-based crystal lattice according to the double salt reaction to form a more stable crystal structure with increased dehydration temperature of the calcium sulfate short crystal.Among them,the crystal structure of the synthesized product under the NaCl solution system is complete,the thermal stability is high,and it is not easy to hydrate.It is suitable for the modification of the polymer processing.The surface modification technical factors were optimized:temperature 90°C,time 25min,interface modifier 8%,drying temperature 105°C and initial material mass fraction 15%.For the selection of interfacial modifiers,stearic acid has the best effect in the surface modification of short crystals of calcium sulfate.Stearic acid undergoes chemical and physical adsorption on the surface of short crystals.In the process of surface modification of stearic acid,surface modification of calcium sulfate does not change the crystal structure and composition of the short crystal.Due to the small lattice and tightly packed atoms of rutile TiO₂,as well as higher index of refraction and opacity,its reflectivity in the band from 400 to 2500 nm is higher,and it is suitable for the infrared heat reflection modification of calcium sulfate short crystals.By adding titanium dioxide-coated modified whiskers in a certain proportion,the visible light section and the near-infrared section have higher reflectivity.The reflectivity of the 400-2500 nm wavelength segment is calculated to be 80.34%.The infrared heat reflectivity can reache 84.72%.In the calcium sulfate short whiskers/PVC composite products,the mechanical strength of the calcium sulfate short whisker modified by stearic acid composite PVC is the highest,and the mechanical strength of the filler-modified PVC composite material has been improved to some extent.With the increase of the short whiskers with certain L/D ratio in certain range,the impact strength and tensile strength of PVC composites tend to increase linearly.As the calcium sulfate short whiskers are dispersed in the PVC resin matrix to form a composite interface,when the content of calcium sulfate short whiskers exceeds a certain threshold,the interface is destroyed,and the effect of short whiskers reinforcement is reduced.Due to the modified short whiskers have a hydrophobic layer modified on the surface,the interfacial bonding force is stronger during the composite process with PVC,which makes it difficult for the filler and the matrix to de-bond when the whisker/PVC composite sheet is stretched and impacted and the mechanical properties of composite sheet improved.There are two forms mechanism of calcium sulfate short whisker reinforced PVC material:load transfer,pull-out effect,the effective connection of short whiskers and PVC matrix on interface buffers the stress concentration and the energy generated by crack propagation inside the matrix under external force.