Timing of polyethylene glycol addition for the control of SiO2 sol structure and sol-gel coating properties

Polyethylene glycol (PEG) was added during the sol process of sol-gel SiO2 coating preparation to investigate the effect of PEG addition timing on particle growth, sol structure, and coating properties. When PEG was added before the addition of the catalyst, the particle size increased from 10 nm (in sol without PEG) to 20 nm, and the necessary aging time for dip coating was shortened from 5 days (in sol without PEG) to similar to 30 h. When PEG was added 24 h later than the catalyst, the uniform sol structure and small particle unit were obtained. With the delay of the PEG addition timing to 120 h, the improvement in the homogeneity of the sol was continued, whereas the porosity of the resulting coatings decreased. The refractive index of PEG-modified SiO2 coatings can be adjusted in a continuous range between 1.14 and 1.20, using the timing when the PEG was added. The highest peak transmittance of glass that was coated with PEG-modified silica coatings reached 99.90%. All the coatings can be changed from hydrophilic to hydrophobic after hexamethyldisilazane vapor modification.