Modified stannic oxide sol, stannic oxide-zirconium oxide composite sol, coating composition and optical element

A process for producing a modified metal oxide sol, comprising:

• producing a stannic oxide aqueous sol comprising stannic oxide particles, wherein sizes of the stannic oxide particles are from 4 to 50 nm, and a SnO2concentration of from 0.5 to 50 wt %;
• hydrothermally treating the stannic oxide aqueous sol at a temperature of from 100 to 300° C.;
• mixing the hydrothermally treated stannic oxide aqueous sol with an aqueous solution of an oxyzirconium salt having a concentration of from 0.5 to 50 wt % as calculated as ZrO2;
• heating the obtained mixed liquid at a temperature of from 60 to 100° C. for from 0.1 to 50 hours to prepare a stannic oxide-zirconium oxide composite aqueous sol having particle sizes of from 4 to 50 nm;
• mixing the heated stannic oxide-zirconium oxide composite aqueous sol with an aqueous medium comprising Sb2O5 colloidal particles comprising an alkylamine, an oligomer thereof or a mixture thereof;
• aging the aqueous mixture of the stannic oxide-zirconium oxide composite aqueous sol and aqueous medium comprising Sb2O5 colloidal particles comprising an alkylamine at a temperature of from 20 to 300° C. for from 0.1 to 50 hours;
• and
• contacting the aged aqueous mixture with an anion exchanger to remove anions present in the sol;
• wherein
• a weight ratio of ZrO2/SnO2 in the stannic oxide-zirconium oxide composite aqueous sol is from 0.05 to 0.50,
• an alkylamine/Sb2O5 molar ratio in the Sb2O5 colloidal particles comprising an alkylamine is from 0.02 to 4.00, and
• a weight ratio of Sb2O5/SnO2+ZrO2) in the aqueous mixture of the stannic oxide-zirconium oxide composite aqueous sol and aqueous medium comprising Sb2O5 colloidal particles comprising an alkylamine is from 0.01 to 0.50, calculated as metal oxides.