Лако-красочные материалы — производство

In large-scale production, lead or lead oxide is reacted with nitric acid to give lead nitrate solutions, which are then mixed with sodium dichromate solution. If the precipitation solutions contain sulfate, lead sulfochromate is formed as a mixed — phase pigment. After stabilization the pigment is filtered off, washed until free of electrolyte, dried, and ground.

The color of the pigment depends on the ratio of the precipitating components and other factors during and after precipitation (e. g., concentration, pH, temperature, and time). According to Wagner [3.117], the precipitated crystals are orthorhombic, but change very readily to the monoclinic form on standing; higher temperatures accelerate this conversion. Almost isometric particles that do not show any dichroism may be obtained by appropriate control of the process conditions. Needle-shaped monoclinic crystals should be avoided because they lead to disadvantages such as low bulk density, high oil absorption, and iridescence in the coating film.

Unstabilized chrome yellow pigments have poor lightfastness, and darken due to redox reactions. Recent developments have led to improvements in the fastness properties ofchrome yellow pigments, especially towards sulfur dioxide and temper­ature. This has been achieved by coating the pigment particles with compounds of titanium, cerium, aluminum, antimony, and silicon [3.118-3.126].

Carefully controlled precipitation and stabilization provide chrome yellow pig­ments with exceptional fastness to light and weathering, and very high resistance to chemical attack and temperature, enabling them to be used in a wide field of applications. The following qualities are commercially available:

1. Unstabilized chrome yellows (limited importance)

2. Stabilized chrome yellows with higher color brilliance, stable to light and weathering

3. Highly stabilized chrome yellow pigments

— very stable to light and weathering

— very stable to light and weathering, and resistant to sulfur dioxide

— very stable to high temperature, light, and weathering

— very stable to high temperature, sulfur dioxide, light, and weathering

4. Low-dust products (pastes or powders)

Lightfast chrome yellow pigments that are coated with metal oxides (e. g., of alu­minum, titanium, manganese) are produced by DuPont [3.119]. A chrome yellow that is coated with large amounts of silicate and alumina and which shows improved stability to temperature, light, and chemicals is also produced by DuPont [3.120].

Bayer described pigments containing lead chromate stabilized in aqueous slurry with silicate-containing solutions and antimony(III), tin(II), or zinc compounds [3.121].

ICI produces light — and weatherfast chrome yellow pigments stabilized with an­timony compounds and silicates in the presence of polyhydric alcohols and hydrox — yalkylamines [3.122].

Ten Horn describes a process for the production of lead sulfochromate containing at least 50% lead chromate [3.123]. This has a low acid-soluble lead content (<5% expressed as PbO, by BS 3900, Part B3, 1965).

BASF produces temperature-stable lead chromate pigments with a silicate coating obtained by hydrolysis of magnesium silicofluoride [3.124].

Heubach has developed a process for the alternative precipitation of metal oxides and silicates [3.125, 3.126]. A homogenizer is used to disperse the pigment particles during stabilization. Products obtained have a very good temperature resistance and very low lead solubility in acid (<1% PbbyDIN 55 770, 1986 or DIN/ISO 6713,1985).

Continuous processes for the production of chromate pigments have been devel­oped in the United States and Hungary [3.127, 3.128].