Лако-красочные материалы — производство

It is extremely important to realize that weathering involves multiple degradation mechanisms. These can be free radical mechanisms (photolytically or thermally induced), or acid or base-catalyzed chemistry, as well as physically based mecha­nisms. This makes weathering a difficult property to study.

Second, in general, inorganic pigments do not weather (degrade). It is the poly­mer systems that do so. However, inorganic pigments certainly affect the weath­ering of the polymer systems, both chemically and physically. Chemically, a com­ponent of the pigment, such as trace amounts of an unreacted raw material, can be involved in free radical generation or in acid-base chemistry. Physically, inor­ganic pigments have much lower thermal expansion coefficients than the polymer systems, and can therefore affect the physical structure of the polymer system over its numerous heating cycles.

Since it is the polymer system that weathers, the polymer system itself has a huge impact on weathering. In fact, a newer fluoropolymer paint system has been found to give unexpectedly good weathering results with some traditionally poor weathering pigments. On the other hand, alkyd paints are generally poor weath­ering systems.

The properties most commonly used to measure weathering are the color val­ues. Certainly it is desirable to maintain the initial color as much as possible. However, the color values are most affected by surface weathering initially, espe­cially surface roughness, and give no indication of bulk polymer degradation like strength or brittleness measurements do. In fact, the L* value is fairly sensitive to the exact part of the light or dark cycle in which the sample is removed for color measurement. In addition, AL* values are seen to increase and decrease at ran­dom after the initial lightening effect has occurred.

Both indoor and outdoor weathering assessment is routinely performed on pig­ments. Accelerated indoor weathering is mostly done with either a Xenon-Arc apparatus or under UV-lamp irradiation. Outdoor weathering is usually required for verification.

DR pigments weather well because they possess both good UV absorptivity and IR reflectivity. Their UV absorbance removes the UV light so that it cannot degrade the polymer. Their IR reflectivity keeps the polymer at a lower tempera­ture where the polymer degradation is slower. Their biggest drawback is that TiO2 can photo-initiate free radicals. This is one reason why most of the white TiO2 rutile pigments are coated.

Iron oxide has a notorious reputation for causing bad weathering. A recent study showed that the levels of free (acid-leachable) iron oxide correlated better to weathering than the levels of contained iron oxide [41]. The data presented in the plot showed no deleterious weathering effect from samples with <1% free iron oxide. More detailed studies are needed to determine when free metal oxides such as iron oxide are detrimental to weathering. In addition, measurement of proper­ties other than color values may provide a better understanding and correlations.