Don’t Underestimate The Role of Titanium Dioxide in Coatings

Titanium dioxide is an important white pigment, which can be divided into anatase and rutile. Compared with traditional white pigments such as lead white, zinc white, and zinc barium white, titanium dioxide has many excellent properties such as high refractive index, strong achromatic power, large hiding power, and harmlessness, and is known as a very good white pigment. . Used in coatings, plastics, chemical fibers, ceramics and other industries. Among them, the amount of coating is very large, accounting for about 60% of the total amount of titanium dioxide. So how is titanium dioxide used in coatings?

1. The role of titanium dioxide in coatings

Paint is mainly composed of four parts: film-forming substances, pigments, solvents and additives. The pigment in the paint has a certain hiding power. It can not only cover the original color of the object being painted, but also give the coating a bright color. Achieve brightening and beautifying decorative effects. At the same time, the pigment is closely integrated with the curing agent and the base material, which can enhance the mechanical strength and adhesion of the coating film, prevent cracking or falling off, and enhance the thickness of the coating film, prevent the penetration of ultraviolet rays or moisture, and improve the coating quality. The anti-aging and durability properties of the film extend the service life of the coating film and the objects being protected.

Among pigments, white pigments are used in large quantities. The performance requirements for white pigments in coatings are: ① good whiteness; ② good grinding and wettability; ③ good weather resistance; ④ good chemical stability; ⑤ small particle size, covering power and elimination. High color strength, good opacity and gloss.

Titanium dioxide is a white pigment commonly used in coatings. Its output accounts for more than 70% of inorganic pigments, and its consumption accounts for 95.5% of the total consumption of white pigments. At present, about 60% of the world's titanium dioxide is used to make various coatings, especially rutile titanium dioxide, most of which is consumed by the coating industry. Coatings made of titanium dioxide have bright colors, high hiding power, strong tinting power, low dosage, and many varieties. They protect the stability of the medium, enhance the mechanical strength and adhesion of the paint film, prevent cracks, and protect against ultraviolet rays. and moisture penetration, extending the life of the paint film. The color matching of almost every pattern in colorful pattern paints is inseparable from titanium dioxide.

Different types of coatings with different uses have different requirements for titanium dioxide. For example, powder coatings require the use of rutile titanium dioxide with good dispersion. Anatase titanium dioxide has low achromatic power and strong photochemical activity. When used in powder coatings, the coating film is prone to yellowing. The rutile titanium dioxide produced by the sulfuric acid method has the advantages of moderate price, good dispersibility, good hiding power and achromatic power, and is very suitable for indoor powder coatings. In addition to good dispersion, hiding power and achromatic power, titanium dioxide for outdoor powder coatings also requires good weather resistance. Therefore, outdoor powder coatings generally use rutile titanium dioxide produced by the chlorination method.

2. Analysis of the impact of major quality fluctuations of titanium dioxide on coatings

1）Whiteness

As the white pigment of coatings, the whiteness of titanium dioxide is very important and is one of the key quality indicators required for coatings. Poor whiteness of titanium dioxide will directly affect the appearance of the coating film. The main factor affecting the whiteness of titanium dioxide is the content of harmful impurities in it, because titanium dioxide is very sensitive to impurities, especially rutile titanium dioxide.

Therefore, even trace amounts of impurities will have a significant impact on the whiteness of titanium dioxide. The whiteness of titanium dioxide produced by the chloride process is often better than that produced by the sulfuric acid process. This is because the raw material titanium tetrachloride used to produce titanium dioxide by the chloride process has been distilled and purified and has less impurity content, while the titanium dioxide used by the sulfuric acid process The raw materials contain high impurity content, and impurities can only be removed through technological techniques such as washing and bleaching.

2）Covering power

Hiding power is the surface area of the object being coated per square centimeter. When the same area is completely covered, the greater the hiding power of the titanium dioxide used, the thinner the coating film can be, and the smaller the amount of coating required, the The less titanium dioxide is needed. If the covering power of titanium dioxide decreases and the amount of titanium dioxide required increases to achieve the same covering effect, the production cost will increase, and the increased dosage of titanium dioxide will lead to the addition of titanium dioxide in the coating. It is difficult to disperse evenly and aggregation occurs, which will also affect the covering effect of the paint.

3）Weather resistance

Coatings have high requirements on the weather resistance of titanium dioxide, especially outdoor surface coatings, which require high weather resistance or ultra-high weather resistance titanium dioxide. If titanium dioxide with low weather resistance is used, the coating film will suffer from fading, discoloration, powdering, cracking, peeling and other problems. The crystal structure of rutile titanium dioxide is closer than that of anatase titanium dioxide, and its photochemical activity is lower. Therefore, its weather resistance is much higher than that of anatase titanium dioxide. Therefore, titanium dioxide used in coatings is basically rutile titanium dioxide. The main method to improve the weather resistance of titanium dioxide is to perform inorganic surface treatment, that is, coating the surface of titanium dioxide particles with one or more layers of inorganic oxides or hydrated oxides.

4）Dispersion

Titanium dioxide is an ultra-fine particle with a large specific surface area and high surface energy. It is easy to aggregate between particles and is difficult to disperse stably in the coating. Poor dispersion of titanium dioxide will directly affect its optical properties such as achromatic power, hiding power and surface gloss in the coating, and will also affect the storage stability, fluidity, leveling, coating durability and anti-corrosion of the coating. In addition, it will also affect the production cost of coatings, because the energy consumption of grinding and dispersing operations is high, accounting for the majority of the total energy consumption of the coating manufacturing process, and the loss of instruments and equipment is large.