Zinc Cyanamide Pigments

Zinc cyanamide (Zn(CN)2) was developed as a replacement for lead cyanamide (see Section 5.2.4.1) mainly for application in mirror coatings. The electrochemical effectiveness of cyanamides is attributed to a passivation action under alkaline conditions [5.56]. 

Zinc cyanamide is produced on an industrial scale from pure calcium cyanamide in an aqueous medium with zinc salts or a slurry of zinc oxide [5.56]. The pigment has, until now, not reached the importance of lead cyanamide for application in mirror coatings. 

A comparative investigation of MIOs with different shapes, i.e. granular or a more lamellar shape (carried out within a study for the Deutsche Bahn AG ) has shown no difference in the application of either grade in protective coatings with regard to the anticorrosive effect. This has been confirmed not only by accelerated laboratory tests but also by means of outdoor exposure in a coastal climate [5.103]. 

This result leads to the conclusion, that there are other important product properties besides the particle shape that have an influence on the barrier effect. In this connection, it is discussed that effective MIO pigments should have a very high Fe203 content and very lo w water-soluble salt (i.e. chlorides and sulfates) content. 

An ideal MIO pigment should not contain very fine particles in order to keep the wettable pigment surface small. High PVC in the coating can only be achieved when the oil absorption value of the MIO pigment is low [5.103]. In addition to the barrier mechanism, MIOs are recommended for use in topcoats as a protection against ultraviolet degradation [5.55]. 

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Cyanamide pigments are produced from industrial-grade calcium cyanamide which is first dissolved. Sulfide and phosphide impurities are precipitated as iron or lead salts [5.127]—[5.132] or oxidized [5.133] [5.135] and filtered off together with graphite impurities. The pure calcium cyanamide is reacted in an aqueous medium with soluble lead or zinc salts or with a slurry of lead oxide or zinc oxide [5.127], [5.129], [5.133], [5.136]—[5.138]. The pigments are filtered, washed, dried, and ground. Zinc cyanamide [5.139] and pure lead cyanamide are not explosive. An explosion reported during the production of lead cyanamide was caused by contamination with small amounts of acid or nitrates [5.140],... 

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