Лако-красочные материалы — производство

Mixed metal oxides benefit from a low surface porosity (BET surfaces are typically in the range 3-6 m2 g-1 for rutile yellows and 3-10 m2 g-1 for cobalt greens and blues), resulting in low oil absorption numbers. As a consequence, high pigment loadings in color concentrates (pigment pastes, colorants and masterbatches) can be achieved. A high pigment concentration is often one of the few practical ways to overcome settling issues related to the high specific gravity (ca. 4-5 g cm-3). Inorganic mixed metal oxide pigments are relatively easy to disperse, though shear stability issues may create difficulties in some particular resins and applications.

Most mixed metal oxide pigments show infrared reflectance, which is important for camouflage applications as well as applications with a defined minimum total solar reflectance [3.97]. Special pigments designed for infrared reflectance are the brownish to black pigments Pigment Brown 29, Pigment Green 17, Pigment Brown 35 and Pigment Black 30 (Table 3.4).

Rutile yellow pigments are often used in combination with high performance organic pigments in order to formulate brilliant colors that are non-toxic (as opposed to lead chromates, lead molybdates and supposedly cadmium pigments). Compared with titanium dioxide, nickel titanates give deeper colors when combined with organic pigments. Especially in combination with higher performance and more expensive organic pigments, the use of rutile yellow lowers the demand for organic pigment contents and thus overall pigmentation cost involved. Apart from enhancing color depth, chrome titanates also provide an increase in opacity. Rutile yellows are used in formulations of many standard industry colors (like RAL tone colors), which are less clean than the pure organic pigment. The main reason for formulating with rutile yellows is the improvement in weatherability due to the UV absorbing properties of nickel and chrome titanate yellows. Rutile yellows are also used as toners in pastel tone colors.

Zinc ferrites are excellent general-purpose pigments of yellowish brown color. As opposed to iron oxide yellows, zinc ferrites offer higher heat stability and can be used in plastics and coatings cured at temperatures above 120 °C. Zinc ferrites are coloristically not as clean as rutile yellows. Zinc ferrites can be slightly magnetic.

Manganese titanate brown pigments are mainly used because of their weather­ing properties. Particularly in rigid-PVC (vinyl siding), these iron-free browns have become important. Iron-containing pigments, particularly if highly soluble iron is involved, are known to catalyse the degradation of PVC by UV radiation [3.98].

Cobalt blue pigments exist in colors from a red shade to very greenish-turquoise­like. This bathochromic shift in the maximum absorption is obtained by substituting aluminum for chromium. Opacity is strongly increased and, as a consequence, the weatherability of pigmented coatings also tends to be better for Pigment Blue 36 than Pigment Blue 28. However, color and appearance (higher gloss) considerations often force the choice for Pigment Blue 28. Important progress has been reported by several manufacturers in providing high strength cobalt blues, with improved UV opacity and weathering properties. Furthermore, lowering of the pigmentation level to be used in engineering plastics minimizes any impact on mechanical strength properties. The use of cobalt blues and greens in plastics is driven by fastness properties as well as preventing warpage in polyolefins. Special areas are camouflage applications, which explicitly involve the cobalt chromite Pigment Green 26.

The role of brown and black pigments like Pigment Brown 29 (identical to Pigment Green 17), Pigment Brown 35 and Pigment Brown 30 is dominated by their ability to reflect in the near infrared as opposed to other inorganic brown pigments like iron oxides or other black pigments like copper chromite or carbon black [3.99]. The nickel-iron-chrome complex Pigment Brown 30 does not reflect as much in the near infrared as an iron chromite, but offers a more bluish tint. Brown colored iron chrome hematites are used in PVC pipe and window profiles as a non-toxic alternative to color formulations based on lead chromates, lead molybdates and some black (normally carbon black). Heat build-up on PVC profiles can be measured in accordance with ASTM D4803-96.

Copper chromite blacks are general-purpose pigments with outstanding durability and temperature resistance. Copper chromite blacks exhibit no infrared reflectance. The main application is in coatings for both dark grays and light colors. The more expensive cobalt chromites are only used in niche applications requiring higher heat stability. Manganese ferrites do not possess the weatherability and acid stability (Kesternich test) of copper chromite blacks, but can present a cost effective alternative if these properties are not required.

3.1.3.4