Лако-красочные материалы — производство

Weathering of paints and coatings containing TiO2 leads to pigment chalking [2.65]. If weathering occurs in the absence of oxygen, or in binders with low permeability to oxygen (e. g., in melamine-formaldehyde resins), no chalking is observed, but graying takes place, which decreases on exposure to air. Graying is greatly reduced in the absence of water. Both effects are more severe with anatase pigments. Empirical stabilization processes have been developed by pigment producers, e. g., doping with zinc or aluminum prior to calcination or during oxidation.

According to recent understanding, impairment of the lightfastness and weather resistance of TiO2 pigments proceeds according to the following cycle [2.66]:

1. Molecules of water are bound to the TiO2 surface, forming hydroxyl groups on the surface.

2. Absorption of light of short wavelength (anatase <385 nm, rutile <415 nm) occurs, producing an electron and an electron defect or “hole” (exciton) in the crystal lattice, which migrate to the surface of the pigment.

3. At the surface of the pigment, an OH — ion is oxidized to an OH’radical by an electron “hole”. The OH — ion is then desorbed and can oxidatively break down the binder. A Ti3+ ion is simultaneously produced by reduction of Ti4+ with the remaining electron of the exciton.

4. The Ti3+ ion can be oxidized by adsorbed oxygen with formation of an O2- ion. The latter reacts with H+ and is converted into an HO2- radical.

5. The cycle ends with the binding of water to the regenerated TiO2 surface.

The chalking process can be regarded as the reaction of water and oxygen to form OH’and HO2’radicals under the influence of shortwave radiation and the catalytic activity of the TiO2 surface:

H2O + O2 + hv —^OH’ + HO2′

The enthalpy requirement for this reaction (312 kJ mol-1) is provided by radiation of wavelength 385 nm. The cycle (1)-(5) is broken by excluding air or water. If oxygen is excluded or a binder is chosen in which the diffusion of oxygen is rate determining, a concentration of Ti3+ ions builds up. Graying then takes place, but this decreases with gradual exposure to oxygen. If water is excluded, rehydration and formation of surface hydroxyl groups do not take place; breakdown of the binder therefore ceases. Despite this photochemical breakdown of the binder, treated rutile pigments are used to stabilize many binders. This is because nonpigmented coatings are degraded by exposure to light and weathering; the added TiO2 pigments prevent light from penetrating the deeper layers of the coating film and thus inhibit breakdown of the binder. High-quality TiO2 pigments must satisfy high requirements with respect to weather resistance. These pigments must withstand the severe climatic conditions of the Florida test, resisting a two-year exposure without appreciable chalking or deterioration of gloss.

2.1.6