Improvement of properties of mica titanium pearlescent pigment by surface modification

Titanium-mica pearlescent pigment is a new type of non metallic decorative paint. It is made of mica powder and coated with a high refractive index metal oxide transparent film. Through the interference or reflection of light, the pearly luster from soft satin to dazzling flicker is produced. Developed by DuPont in 1963, this new pearlescent pigment has been rapidly developed due to its weather resistance, non-toxicity, acid resistance, alkali resistance, non-conductivity, non-magnetic properties, and chemical stability. It is widely used in the automotive industry. Coatings, plastics, leather, ink printing, ceramics and cosmetics, toys, rubber, decorative and other fields.
At present, with the expansion of the application range of mica pearlescent pigments, its performance is higher, and its hiding power, coloring, transparency, stability and light resistance are unsatisfactory, and it is limited in application. Therefore, improving its performance is currently the main direction of pearl pigment research.
Surface modification of inorganic materials is a new high-tech in the field of materials science in recent years. It changes its surface properties by physical, chemical and mechanical methods. People use these modification principles to conduct in-depth research on the development of high-grade mica pearlescent pigments.
2 method for improving the performance of mica titanium pearlescent pigment
2.1 Multi-cladding structure for improving hiding performance The mica pearlescent pigment developed in the early stage is mainly composed of a single-clad structure, and the technology is relatively mature. Due to the poor covering performance of the single-cladding structure, there are certain restrictions on the application in some specific occasions. Many cladding structures have been developed. The multi-clad structure has a multi-layer colorless transparent film, which combines the natural metal oxide film and the nano-scale metal particles to form a pearlescent effect on the light, and a strong scattering effect on the light. A metallic luster is obtained, which improves the hiding power. The synthesis process is as follows:
Mica→mica suspension→coated titanium dioxide→low temperature anoxic calcination→coated Cr(OH) ₃
The low temperature oxygen-free calcination is carried out in an argon atmosphere below 200 ° C, and the surface of the TiO ₂ film on the surface of the mica is reduced to a low-valent titanium oxide and a metal titanium. The TiO ₂ film is further coated with a Cr(OH) ₃ film, and a layer of low-valent titanium oxide and titanium metal distributed in a spot shape are deposited between the TiO ₂ and Cr(OH) ₃ films. The three-coated mica pearlescent pigment has high hiding power and coloring ability, but the pearlescent effect is reduced compared with the double-coated mica pearlescent pigment. To solve this problem, the current method is to increase the transparency of the metal oxide film or to cover two thick layers of a colorless transparent metal oxide film. There are also reports on four-clad and multi-coated pearlescent pigments.
2.2 Improving the stability of pearlescent pigments Pearlescent pigments are easily damaged by the dual effects of ultraviolet radiation and moisture. The US patent reported that the improved pearlescent pigments have good gloss stability and pure color. The pigment is prepared as follows:
Based on metal oxide TiO ₂ coated mica flakes, disperse it in water to prepare a slurry, and then slowly add one or more solutions containing dissolved metal salts or mixtures (such as chromium methacrylate and Na ₂ HPO ₄ • 2H ₂ 0), adjust the pH to 4-7, precipitate the metal salt on the surface of the pigment under the action of precipitant (FeS0 ₄ •7H ₂ 0), wash by filtration, dry at 130 ° C, and the obtained pigment is pure. No agglomerates, dense coating, and its stability is significantly better than the original pearlescent pigment. [next]
2.3 Improving transparency and tinting strength For coated mica pearlescent pigments, the quality of the metal oxide deposited on the mica plate, especially the crystal size of the precipitate, plays a key role in the degree of light scattering. If the crystal is too small, the pearl luster and the interference color will disappear, and the transparency and tinting strength of the pearl pigment will decrease. According to optical theory, when the particle size falls to half the wavelength of visible light, the light penetrates the particle without being refracted. Such particles will be transparent. The Al ₂ 0 ₃ particles are much smaller than this value, so the coating is substantially transparent. The methods for improving transparency and coloring properties reported in the U.S. patent are as follows:
The method is prepared by coprecipitating TiO ₂ with AL ₂ O ₃ and SiO ₂ on mica flakes. The mica flakes (particle size 10-70 μm, thickness 0.1-5 μm) are suspended in water to prepare a slurry, and the pH is adjusted to 1.5-2.5 with a suitable acid (such as hydrochloric acid), and then heated to 70 ° C - 80 ° C while measuring Add TiCL4 solution, ALCL ₃ •6H ₂ 0 solution (containing hydrochloric acid) and alkaline Na ₂ SiO ₃ solution (containing NaOH). The speed and quantity of addition should be kept constant at pH 0.2, and the corresponding hydroxide or The hydrated oxide is completely precipitated on the surface of the mica plate. After the required blue interference color thickness is reached, the coating is stopped, washed by filtration, dried at 120 ° C, calcined at 700 ° C - 1000 ° C for 30 min, and the metal hydroxide or hydrated oxide is transformed. As a corresponding oxide, the mica has excellent transparency and pearlescent properties.
2.4 Improving the Lightfastness of Pearlescent Pigments When pearlescent pigments are placed for a long time or applied to paint films and coatings, the pearlescent effect will be weakened, discolored, powdered, and peeled off. The reason is that the surface coated Ti0 ₂ has strong photochemical activity. When it absorbs light of less than 405 nm wavelength, it will change from electron valence band to valence band, thereby forming electron holes and causing chemical changes in the surrounding medium. . In order to prevent this phenomenon, a uniform protective film may be coated on the surface of the mica titanium to seal the photoactive dots of the original TiO 2 to improve the light resistance.
A method for improving the light resistance of pearlescent pigments is as follows: 4 g of silver- white mica titanium pearlescent pigment is placed in a 250 ml four-necked flask, 100 ml of deionized water is added, and the pH is adjusted to 7-8 with 2 mol/L NaOH, and then Place in a constant temperature water bath, stir to warm to 80 ° C, and then add a mass fraction of 0.5% ALCL ₃ solution, in order to maintain the pH value of the solution, while adding 2mol / L NaOH solution, the end of the feed, continue to heat stirring for 1h, filtration, The filter cake was dried to 110 ° C until it was washed, and calcined at 500 ° C for 1 h in a muffle furnace to obtain a mica pearlescent pigment having greatly improved light resistance.
2.5 Using a crystal form conversion agent to increase the refractive index of pearlescent pigments When preparing a mica titanium pearlescent pigment, the titanium salt hydrolyzes the TiO ₂ deposited on the mica plate, and in the high temperature calcination, an anatase type and a rutile type crystal structure can be formed, The refractive index is higher than the former, so the rutile-type mica titanium pigment has a good pearlescent effect. However, in the crystallization process of mica titanium, it is difficult to ensure that the titanium dioxide is completely converted to the rutile type by controlling the temperature and time, so the rutile accelerator can be added during the preparation process. At present, a mixture of divalent tin salt and hydrogen peroxide is mainly used, and the optimum amount of tin salt is preferably 20% of the amount of titanium salt.
2.6 Enhancing the coloring ability of mica pearlescent pigments with organic colorants Organic pigments have a vivid color and can be coated on the surface of mica by chemical adsorption. At present, the main method of coating organic pigments is chemical adsorption, and there are two different methods depending on the treatment agent:
(1) Inorganic color precipitation coating method The preparation process of the European patent report is as follows:
Mica pearlescent pigment→organic pigment or dye→slaker coating→filtering→drying→product Adding a coloring agent to the organic pigment or dye to fix the adsorption. The lake former used is generally a compound such as AL, Zr, Ca, Ba, Sr or the like. This process has the advantage of being able to be carried out at higher mass fractions and lower temperatures. [next]
(2) Organic coupling agent coating method The coupling agent has an amphoteric structure, and a part of the molecules in the molecule or react with various functional groups on the surface of the inorganic material to form a strong chemical bond, and another part of the group may react with the organic substance. A chemical or physical reaction that produces a "molecular bridge" with special functions between inorganic materials and organic pigment molecules. For example, using an organic chromate coupling agent, the coupling agent can be first reacted with mica, and then reacted with an organic pigment, or the mica pearlescent pigment can be directly mixed with a coupling agent or an organic pigment to react. Generally, the amount of coupling agent is 0.01%-2.0% of the mica mass fraction.
3 Mica pearl pigment research direction prediction The development history of mica titanium pearlescent pigment is short, but because of its unique decorative effect and non-toxic, acid-resistant, alkali-resistant, non-conductive, non-magnetic, chemically stable and other excellent characteristics, application The prospects are very broad.
(1) With the continuous improvement of the appearance quality requirements of articles, the application field of mica titanium pearlescent pigments is expanding day by day, so higher requirements are placed on its performance. At present, little work has been done on the surface modification of pearlescent pigments. its influence future performance will focus mainly used surface treating agent such as silicon oxide, aluminum oxide pigments for more in-depth study;
(2) Multi-coating coloring mica pearlescent pigments have better performance than single-coating pigments due to their structural improvement, and can meet the needs of some special occasions, which will be the focus of another research in the future;
(3) Due to the high pearlescent color of rutile type, the development of Jinhong Petrochemical Accelerator is of great significance for the preparation of high quality pearl mica.