At present, the industrial production of nano-calcium carbonate usually adopts limestone calcination, then digestion, carbonization and other processes, and the obtained calcium carbonate has a particle size of 20-100 nm.
The calcination of limestone is mostly prepared by mixing anthracite and limestone. The temperature fluctuates greatly during calcination, resulting in low activity of lime, so that when lime undergoes digestion reaction, the digestion speed is slow, and the particle size of the Ca(OH)2 slurry obtained by digestion is Large; if the particle size of the Ca(OH)2 slurry is large, the carbonized nano-calcium carbonate will agglomerate (although the original particle size reaches nano-scale, the final product is easy to agglomerate to form a secondary aggregate of 1-5 μm, When applied, the strength and thixotropy become worse, and the expected target cannot be achieved, so the particle size of Ca(OH)2 must be controlled.
According to the application effect, the particle size of calcium carbonate is not as fine as possible. When a filler with too small particle size is used for polymer processing, the viscosity is large and the dispersion is not good. Therefore, the difficulty of synthesizing calcium carbonate is not to reduce the particle size of the crystal. , but to achieve the appropriate range, while good dispersion.
The full name of MS glue is silane-modified polyether sealant. It has the comprehensive advantages of silicone glue and polyurethane glue. However, nano-calcium carbonate with particle size ≤100nm is easy to agglomerate and has poor dispersibility. When used in sealant, it has poor extrusion performance, In order to improve these shortcomings, Liu Yaxiong studied the influence of key parameters such as the activity of lime, the particle size of calcium hydroxide in lime pulp, different carbonization rates during carbonization reaction and the amount of slurry on the synthesis of cubic calcium carbonate. the result shows:
(1) The higher the calcination temperature of limestone, the lower the activity of lime, the coarser the particle size of the digested Ca(OH)2, and the more uneven the particle size. 330～350mL/4N-HCl, the average particle size of the digested Ca(OH)2 is 3.3～3.7μm.
(2) The raw slurry Ca(OH)2 obtained by high-speed disperser, D50=2.12μm, has a larger particle size; using colloid mill D50=1.90μm, high-energy planetary ball mill D50=1.96μm, vertical stirring sand mill D50 =1.93μm, the particle size is small, and the colloid mill is a medium-free grinding, which is easy to operate and has a low cost. Therefore, the colloid mill is selected for grinding raw pulp with the best performance and industrial operability.
(3) After the colloid mill grinds the raw pulp, adjust the raw pulp mass fraction to 8%, the temperature is 20 °C, and the crystal shape control agent citric acid is added in an amount of 0.3%, and the kiln gas containing CO2 volume fraction of 32% is introduced, and the flow rate is 3m3/h, the higher the carbonization rate when the slurry is replenished, the more Ca(OH)2 is added, and the larger the particle size of the generated calcium carbonate is. 20%-50%, the specific surface area of the prepared cubic calcium carbonate is 10-15m2/g, the equivalent particle size is 141-218nm, the oil absorption value (DOP) is 18.2-20.8g, and the particle size detected by electron microscope is 100-400nm. At this time, the particle size is the most regular and the dispersion performance is good. After making MS glue, the extrudability is 220~290mL/min, the strength is 0.93~0.98MPa, the elongation is 380%~430%, and the modulus is 0.32~0.36 MPa, the comprehensive performance is the best, and each performance meets the standard of low-modulus sealants in GB14683-2017 “Silicone and Modified Silicone Construction Sealants”.