Лако-красочные материалы — производство

Many pigmented coatings are required to retain their original color when they are exposed to daylight during their service life. Light fastness is a pigment’s resistance to discoloration when exposed to daylight. Of the total radiation of daylight, UV is the most damaging portion, though it is not the only responsible factor. Light fastness is primarily governed by the chemistry of the molecule, though other factors that may influence light fastness are structure, defects, par­ticle shape and size, and concentration of the pigment. The binder of the coating in which a pigment is incorporated also influences stability of the pigment to light. Therefore, the performance of a pigment varies considerably from one system to other, as the protec­tion rendered by a binder varies considerably for different pigments. Other pigments may also influence light fastness when blends of pigments are used in coatings. For example, titanium dioxide pro­motes photo-degradation of some organic pigments, resulting in poor light fastness of tints in comparison to full shades of certain pigments. In general, most inorganic pigments are stable to light, while organic pigments vary considerably in their light fastness. Certain inorganic pigments, such as iron oxides, improve light fast­ness of compositions, as they are effective UV absorbers.

Evaluation

As light fastness is a system-related property, it cannot be evaluated for a pure pigment, but rather is evaluated for pigmented coatings. Although natural daylight would be ideal, the test is accelerated by using an intense source of light that corresponds as closely as possi­ble to natural daylight. One common light source used is a xenon arc lamp. Specimens incorporated with the pigment being evaluated are exposed to the radiation in testing cabinets and observed for color fading. The evaluation of color change may be done by stan­dard scales such as the ISO Grey Scale or the Blue Wool Scale. Dis­coloration may also be evaluated by CIE tristimulus values obtained by a spectrophotometric method.

Further information can be found in standard methods such as EN ISO 11341 or ASTM D 4303.