The Difference between Composite Cladding and Dual Cladding
Composite coating refers to the simultaneous deposition of two or more coating agents on the surface of TiO2 particles under the same acidic or alkaline conditions. Dual coating refers to the deposition of more than one coating agent under one condition and then the deposition of more than one coating agent a second time under this or another condition.
The typical composite coating process is as follows: the water slurry containing 300g/L TiO2 and pH=9~11 is added to the treatment tank, and two streams of acid and alkaline are introduced at the same time under good stirring. Acid flow content (g/L): 50MgS04, 100TiOSO4, 50H2SO4; Alkaline liquid flow (g/L): 100Na2SiO3, 75NaAlO2. The ratio and rate of addition make the slurry always keep between pH 6~8.
Binary coating is a commonly used method, and the above-mentioned deposition of dense SiO2 under alkaline conditions and then deposition of Al2O3 under acidic conditions is a typical bivariant coating. A more recent improvement is to first deposit a dense envelope of SiO2 and Al2O3 under alkaline conditions (pH=11), and then adjust the pH to about 3 for acid stabilization for 15-30 min (too long for > 1h may cause the first envelope dissolution). After that, the pH of the slurry was raised to 5~5.5, and the first binary coating was carried out with Al2O3, which improved the dispersion and optical properties of TiO2.
