Preparation And Surface Mdification Of Nano Calcium Carbonate By Carbonization Cethod And Application In Aqueous Emulsion

Calcium carbonate as an environmental friendly inorganic material has been applied to the field of paint,rubber,ink,adhesive and paper since it can be obtained from a variety of sources and it has the characteristic of low cost,high whiteness and excellent color.In some certain specific areas,however,there are special requirements for the crystal form,size and surface properties of the calcium carbonate.Therefore,the preparation,modification and application of calcium carbonate become one of research hotspot in the field of materials.In this paper,cubic nano calcium carbonate was prepared by the low temperature carbonization method and the calcium carbonate/silicon dioxide?CaCO₃@SiO₂?composite particles were synthesized by surface modification technology with sodium silicate.Furthermore,the composite particles were also applied to modify the polyvinyl acetate emulsion.Firstly,for obtaining the size-controllabe,well-dispersed cubic nano calcium carbonate,the processeses,including digestion temperature and time,the types and additive contents of crystal controlling agents,the content of calcium hydroxide,carbonation temperature,the flow rate of CO2,and stirring speed were studied.The effects of the processeses on the properties of the product are also disscussed.Then the optimum process was obtained as follow.The digestion temperature and time are 80 ? and 2.0 h,respectively.Sulfuric acid can be chose as crystal controlling agent and the additive amount is 0.6%.The content of calcium hydroxide suspension is 8.0wt%.Carbonization temperature is 10 ? and the flow rate of CO2 is 300 mL/min with the stirring speed of 800rpm.The obtained product has calcite crystal type with the average particle size of 30-35 nm,and the specific surface area is 44 m²/g.Secondly,the cubic nano calcium carbonate was modified with sodium silicate as the silicon source.The effects of pH value and the addition amount of silicon source on the modification are mainly researched.By the characteration of the property of acid resistance,XRD,FT-IR and TEM of CaCO₃@SiO₂ composite particles,the optimal process was obtained.The best process of modification is as following.The pH value is 9.5^-10.0 and the additive content of sodium silicate is 5.0%.The coating layer of composite particles is amorphous silica with 5^-10nm,and the property of acid resistance was better than that of the unmodified calcium carbonate.Finally,the composite particles of CaCO₃@SiO₂ were dispersed by ball milling,and then applied to the polymerization of polyvinyl acetate?PVAc?emulsion.The effects of dispersant types and additive contents on the dispersion of the composite particles and the effects of composite particles on the stability of emulsion were studied.Sodium polyacrylate?5040?was chose as the dispersing agent and the adding amount was 2.0%.It was extremely beneficial to refine the particle size and improve the properties of water resistance,viscosity,shear compression strength and hardness of the emulsion with the composite particles.At the same time,it was found that the effect of the addition of the composite particles on the polymerization and storage stability of emulsion was negligible.The results showed that the emulsion viscosity was 5205 cp,the compressive shear strength reached the maximum value of 58.76MPa and the emulsion film showed excellent transparency and had a hardness up to 4H grade,when the additive amount of composite particles was 5.0%.