Лако-красочные материалы — производство

Titanate pigments are ultimately formed by solid-state reactions at high tempera­tures. Whether the process involves wet or dry blending of the raw batch, raw batch milling, or precipitation of the raw batch, the pigments are only formed upon high temperature calcination. They are then milled to fine and narrow parti­cle size distributions. Other steps such as surface treatments or microgranulation may also be included for various purposes, depending on the manufacturer.

Solid-state reactions generally require high temperatures. In particular, bulk dif­fusion, such as the diffusion of Ni ions into a TiO2 lattice, is a relatively slow pro­cess and will control the reaction rate for pigment formation. Therefore, ideal raw batches should consist of intimately blended nano-sized particles to minimize dif­fusion distances for the dopants and to generate homogeneous particles. Al­though manufacturers work towards this ideal, the perfect raw batch has yet to be achieved in a cost-effective manner.

There is a common misconception that coprecipitation generates a mix that is homogeneous on an atomic level. However, most coprecipitation reactions precip­itate the metal ions sequentially, not simultaneously. Also, while atomically mixed batches would form crystals at much lower temperatures than mixtures of coarser

particles, the coprecipitated batches seldom lower the reaction temperature by more than 100 Celsius degrees.

The raw materials for the raw batch can be simple metal oxides such as NiO or, alternatively, species that yield metal oxides upon thermal treatment, such as basic nickel carbonate. Since solid-state reactions are involved, the choice of these raw materials is critical. Considering that each 100 pm particle or agglomerate repre­sents one million 1pm particles that could be much more intimately blended, the importance of raw material particle size and raw batch preparation cannot be over­stated. Other ingredients for redox, particle growth control, or blending purposes may also be formulated into the raw batch. Depending on the manufacturer, either wet or dry blending of the raw batch is done. Of course, the wet-milled raw batches will need to be dewatered to a large degree before calcination.

The raw batch calcination can be accomplished in almost any kind of furnace. Tunnel kilns, box kilns, shuttle kilns, and rotary kilns have been used for many years, each with its own advantages, disadvantages, and limitations. In some instances, patents have been issued for the calcination technique [16, 18]. Special firing profiles have been developed for some pigments to guide the reaction chemistry.

Grinding can be done in a variety of different mills. A high degree of particle size classification is needed to minimize grit and achieve a high degree of gloss in coatings. Vibratory and jet mills are most commonly used.

6.3.1