Лако-красочные материалы — производство

Aftertreatment or coating of the pigment particles improves the weather resistance and lightfastness of the pigmented organic matrix, and improves dispersibility of the pigments in this matrix. The treatment consists of coating the individual pigment particles with colorless inorganic compounds of low solubility by precipitating them onto the surface. However, this reduces the optical performance of the pigment approximately in proportion to the decrease in the TiO2 content. The surface coatings prevent direct contact between the binder matrix and the reactive surface of the TiO2. The effect of these coatings largely depends on their composition and method of application, which may give porous or dense coatings. The treatment process also affects the dispersibility of the pigment, and therefore a compromise often has to be made. High weather resistance and good dispersibility of the pigment in the binder or matrix are usually desired. These effects are controlled using different coating densities and porosities. In addition organic substances can be added during the final milling of the dried pigment.

Several types of treatment can be used:

1. Deposition from the gas phase by hydrolysis or decomposition of volatile sub­stances such as chlorides or organometallic compounds. Precipitation onto the pigment surface is brought about by adding water vapor. This method is especially applicable to chloride pigments, which are formed under dry condi­tions.

2. Addition of oxides, hydroxides, or substances that can be adsorbed onto the surface during pigment grinding. This can produce partial coating of the pigment surface.

3. Precipitation of the coating from aqueous solutions onto the suspended TiO2 particles. Batch processes in stirred tanks are most common; various com­pounds are deposited one after another under specific conditions. There are hundreds of patents on this subject. Continuous precipitation is sometimes

used in mixing lines or cascades of stirred tanks. Coatings of widely varying compounds are produced in a variety of sequences. The most common are oxides, oxide hydrates, silicates, and/or phosphates of titanium, zirconium, silicon, and aluminum. For special applications, boron, tin, zinc, cerium, manganese, antimony, or vanadium compounds can be used [2.42], [2.43].

Typical groups of inorganic coatings are:

1. Pigments with dense surface coatings for paints or plastics made by:

— Homogeneous precipitation of SiO2 with precise control of temperature, pH, and precipitation rate [2.44].

— Aftertreatments, with tempering at 500-800 °C in between or at the end of the aftertreatment [2.45].

— Aftertreatment with Zr, Ti, Al, and Si compounds [2.46].

— Aftertreatment with merely 1-3% of alumina.

2. Pigments with porous coatings for use in emulsion paints obtained by simple treatment with Ti, Al, and Si compounds, giving a silica content of 10% and a TiO2 content of 80-85%

3. Lightfast pigments with dense surface coatings for the paper laminate industry that have a highly stabilized lattice and a surface coating based on silicates or phosphates of titanium, zirconium, and aluminum: ca. 90% TiO2

Coprecipitation of special cations such as antimony or cerium can improve light­fastness further [2.47].

For coloring of plastics usually smaller TiO2 particles are used with typically less than 3% of inorganic coating (TiO2 typically >95%).

After treatment in aqueous media, the pigments are washed on a rotary vacuum filter or filter press until they are free of salt, and then dried in, e. g., belt, spray, or fluidized-bed dryers.

Before micronizing the pigment in air-jet or steam-jet mills, and sometimes also before drying, the pigment surface is further modified by adding organic substances to improve dispersibility and facilitate further processing. The nature of the com­pounds used depends on the intended use of the pigment. The pigment surface can be made either hydrophobic (e. g., using silicones, organophosphates, and alkyl phtha — lates) or hydrophilic (e. g., using alcohols, esters, ethers and their polymers, amines, organic acids). Also combinations of hydrophobic and hydrophilic substances may be used for obtaining tailor-made surface properties [2.48].

Problems arising from the temperature (>>100 °C) of the micronized pigments (packaging problems, low pouring weight, decomposition of organic additives) can be overcome by indirect or direct cooling of the pigments prior to packaging, especially when using PE or PP packaging materials [2.49].

Chloride or Sulfate Pigments?

Pigments produced by the chloride process (chloride pigments) typically have better brightness and a more neutral hue than pigments produced by the sulfate process (sulfate pigments). Often chloride pigments show better scattering power and better durability.

For a wide range of applications, however, chloride and sulfate pigments are ex­changeable; and for various applications sulfate pigments may offer superior prop­erties (e. g. for printing inks due to the less abrasive character of sulfate pigments).

Pigments used in demanding applications are almost always subjected to inorganic aftertreatment.

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