Research Progress On Preparation Of Calcium Sulfate Whiskers And Its Application

Calcium sulphate whiskers are one kind of small fibrous sub-nanometer materials with perfectly mechanical and physical properties. At present, calcium sulphate whiskers is mainly prepared by the natural gypsum, with demanding production equipment, high purity and granularity requirement raw material. The application of unmodified calcium sulphate whiskers in organic material is restricted. In order to reduce preparation cost and expand the application scope, it is necessary that the new methods of calcium sulphate whiskers preparation, surface modification, application in adhesive modification and organic synthesis is researched.The preparation of calcium sulfate whiskers, surface modification and the application were investigated in the thesis. Calcium sulfate whiskers and modified calcium sulfate whiskers were prepared under the atmospheric pressure. The morphologies and properties were characterized by means of many kinds of test methods, and the relationships between surface properties and modifier types and dosage were investigated. The application in adhesive modification and the synthesis of carboxylate was studied and satisfactory results were obtained. In particular, based on the reaction kinetics of catalyzing esterification, calcium sulfate whiskers were identified as the ideal heterogeneous catalyst, broadening the research and the application scope of the whiskers. The main conclusions were as follows:1. Preparation of calcium sulfate whiskers(1)Through adding crystallization directors, hemi-hydrate calcium sulfate whiskers were prepared by hydrothermal crystallization under atmospheric pressure. The optimum process conditions were as follows:Both calcium hydroxide suspension concentration and sulfuric acid solution concentration were O.2mol·L-1, adding speed of sulfuric acid solution was1.5mL·min-1, reaction temperature was100℃, stirring speed was400r·min-1, reaction time was40min,and dosage of crystallization directors was2%. Products obtained under the optimized conditions were determined to the mixture of hemi-hydrate calcium sulfate whiskers and anhydrous calcium sulfate whiskers by means of scanning electron microscope and X-ray diffraction, of which the average particle diameter was200nm with aspect ratio of100. Compared to traditional hydrothermal synthesis using gypsum as raw materials, the procedure of gypsum dissolution was unnecessary. With the introduction of crystallization directors, the formation of nucleation was accelerated, the time of hemihydrate crystallization and single crystal growing was shorten, and the whisker size was effectively controlled. The raw materials with abundance of material supply were low-cost, and the prosess industrialization would be great significant to the development of hemi-hydrate calcium sulfate whiskers.(2)Using salt slurry as raw materials, calcium sulfate whiskers were prepared under optimum conditions as following:mass ratio was2:1(sulfuric acid:salt slurry), salt slurry concentration was6%, temperature was105℃, reaction time was30min, under atmospheric pressure. Whisker product obtained under optimum conditions have a characteristic of low impurity and good aspect, whose diameter and length was0.2-4.0μm and35-150μm, respectively. Considered magnesium recovery process and the preparation of calcium sulfate whiskers from salt slurry, high valuable materials would be produced, not only the cost was great reduced, but also the waste residue was handled, achieving environmental protection and resource conservation win-win.2. Surface modification of calcium sulfate whiskersThe oil absorption of modification of calcium sulfate whiskers, viscosity and settling volume were investigated, and it showed:among stearic acid, aluminate coupling agent and complex modifying agent in modifying calcium sulfate whiskers, stearic acid is the worst, complex modifying agent is optimal. To modify200nm and500nm calcium sulfate whiskers, the optimal amount of complex modifying agent was3.0%and2.0%, respectively.Through FT-IR and TG/DTG analysis, it showed that chemical adsorption and physical adsorption of aluminate coupling agent both occurred on the surface of calcium sulfate whiskers by FT-IR and TG/DTG. Through XRD analysis, it showed that stearic acid, aluminum coupling agent only modified whisker surface with constant composition and strcture of calcium sulfate whiskers. 3. Study on calcium sulfate whiskers in adhesive modificationMixing calcium sulfate whiskers instead of silica could improve performance of unsaturated polyester putty. When200nm calcium sulfate whiskers amount was2.0%, the impact resistance of putty improved by25%, hardness increased by13%, contraction rate decreased by27.8%, and grinding time extended slightly.By in-situ polymerization, when the200nm modified whisker amount was3.0%of solvent-based polyurethane adhesive, the tensile strength of the polyurethane adhesive reached36MPa, initial bonding strength reached95N·(25mm)-1,24h peeling strength arrived213N·(25mm)-1. Thus the tensile strength increased by80%, initial adhesion increased by126%, and the mechanical performance was optimal.Strength of polyurethane sealant improved with200nm modified calcium sulfate whiskers, and hardness and elasticity was changed slightly. Thixotropy increased with the modified whiskers instead of fumed silica, and the cost of raw materials reduced accordingly.The maximum elongation of silicone sealant mixed by calcium sulfate whiskers enhanced, the extrusion of silicone sealant modified by200nm modified whiskers was good, and cost of raw materials was lowered when200nm modified calcium sulfate whiskers substitute for fumed silica as thixotropic agent.The mechanical performance of white latex could significantly improved with calcium sulfate whiskers. The optimal amount of500nm whiskers,200nm whiskers,500nm modified whiskers and200nm modified whiskers was0.2%,0.3%,0.2%and0.3%of total quantity, respectively. Among all, modified calcium sulfate whiskers showed most significant effect that maximum compression-shear strength increased by over160%compared with blank sample.The effect of calcium sulfate whiskers on latex paint was researched, and results indicated that modified calcium sulfate whiskers effected more pronouncedly than unmodified ones. When0.3%200nm modified calcium sulfate whiskers were used, the scrubbing resistance of latex paint increased by275%, arrived7500times.Calcium sulfate whiskers greater impact the performance of colorful polymer water proof clay. With the increase amount of calcium sulfate whiskers, bending and compressive strength increased, bond strength increased, while initial setting and final setting time shortened, alkali resistance kept constant. When adding0.3%calcium sulfate whiskers, impermeability increased by150%, and the performance was optimal. The various indexes of industrialized product were in line with standard requirements, achieving good economic and social benefits.In conclusion, the effect of calcium sulfate whiskers on adhesive modification is obvious. Appropriate specification and dosage of whiskers not only greatly improved the comprehensive performance of adhesive but also replaced expensive thixotropic agent because of their good thixotropy, such as fumed silica, resulting in lower product cost.4. Study on calcium sulfate whiskers in preparation of carboxylic ester(1)The synthesis of n-butyl acetate from glacial acetic acid and n-butanol catalyzed by calcium sulfate whiskers was studied. The esterification of n-butyl acetate is a reversible second-order reaction. The kinetic equation was as follow:-dCA/dt=2153exp (-41808/RT)CA·CB－35.94×10-3exp(-13295/TT)CC·CB(2)When the molar ratio of acetic acid to n-butyl alcohol was1.1:1, the quality of calcium sulfate whiskers was3.0%of n-butanol, reaction temperature was110℃, reaction time was6h, and the quality of toluene(as water carrier) was14.8%of n-butanol, the yield of n-butyl acetate was up to97.6%. The experiments showed that calcium sulfate whiskers almost brought no secondary reaction in this esterifation, and the esterifation catalyzed by calcium sulfate whiskers was green without environmental pollution and corrosion of equipments compared with traditional concentrated sulfurie acid method.(3)The catalytic activity of calcium sulfate whiskers in the synthesis of diethylene glycol dimethacrylate (DEGDMA) via transesterification from methyl methacrylate and glycol ether was investigated, and the optimized conditions were obtained as follows:a molar ratio of methyl methacrylate to diethylene glycol3.5:1, under circumfluence temperature, the amount of calcium sulfate whiskers and nitroxide free radical inhibitor3%,0.1%, respectively. Yield of product ester obtained under above conditions reached above93.3%and its purity was97%. Experimental results indicated:the catalytic activity of calcium sulfate whiskers depended on the structure, not composition. Calcium sulfate whiskers compared with sulfurie acid and p-toluenesulfonic acid showed efficient catalytic activity in the synthesis of diethylene glycol dimethacrylate via transesterification, with no corrosion, low cost, good product quality, light color, less second section, more relaxed reaction conditions, etc. Therefore, the development of the applications of calcium sulfate whiskers in chemical industry has broad market prospects.(4)The synthesis of TMPTA by using calcium sulfate whiskers as catalyst was studied. The optimized conditions were as follows:a molar ratio of acrylate to trimethylolpropane3.4:1, amount of calcium sulfate whiskers2.0%(based on trimethylolpropane), cyclohexane and toluene as water carriers, with hydroquinone and phenothiazines as inhibitors, the final temperature of esterification98℃, a reaction time5h, the air flow15mL·min-1. Under optimized conditions, the yield of TMPTA above96.8%.