Why is the "titanium dioxide" you use not white enough?

Titanium dioxide is the common name for titanium dioxide used in pigments. The excellent white performance of titanium dioxide makes it occupy a large market share in the ink, coating, papermaking and other industries. Many white products or off-white products in life are mostly involved in titanium dioxide. Therefore, it is known as the "king of white". Ignoring the many excellent qualities of titanium dioxide as an additive, titanium dioxide is the king of white, so its "white" characteristics should also be one of its important signs, which is also the main content of this article.

According to empirical analysis, when the whiteness of titanium dioxide increases, its hiding power and color-reducing power indicators will also increase accordingly. Therefore, to achieve the same effect, the amount of titanium dioxide used is less, and the cost will be reduced. During the use of titanium dioxide, users will reflect that titanium dioxide is not white enough. In fact, the lack of whiteness of titanium dioxide refers to the "covering ability and whiteness brightness of titanium dioxide" is not enough. If you want to change the whiteness of titanium dioxide, it is necessary to "deepen" the factors that affect its whiteness. At present, the research work on the influence of titanium dioxide whiteness mainly starts from the types and contents of impurities in titanium dioxide; the size and shape of titanium dioxide particles; the content of titanium dioxide; the defects of the lattice of titanium dioxide particles; the dispersibility of titanium dioxide, etc.

1. Types and contents of impurities In the titanium dioxide process, especially the sulfuric acid process, most of the work is to remove impurities in the product. The mature sulfuric acid process route has very low requirements for equipment configuration and raw materials. Titanium ore and titanium slag are both acceptable. Moreover, this method can produce both rutile titanium dioxide and anatase titanium dioxide. It has low infrastructure requirements and short production time. In the titanium dioxide industry, especially in my country's titanium dioxide industry, it still occupies a very important position. The titanium liquid obtained by sulfuric acid acid hydrolysis is an extremely complex suspension system, which contains undecomposed ilmenite, some fine particles that do not participate in the reaction, colloidal impurity particles formed by elements such as silicon and aluminum, and a large number of soluble impurities such as sulfates. Even if the content of some impurities is very small, it will have a significant adverse effect on its whiteness. These impurities are mainly some oxides. The reduction of the whiteness of titanium dioxide caused by oxides can be divided into two situations: one is the color of impurity oxide pollution, and the other is the coloring caused by lattice doping. Coloring metal oxide impurities can affect whiteness at very low content. These elements include iron, manganese, chromium, copper, etc. These impurities themselves have colors and are very easy to show color in white titanium dioxide. Common impurities and coloring concentrations are shown in the table below.

In actual production, the limit content of impurity contaminated titanium dioxide is lower than that in the above, and impurities can show color at a lower concentration. Taking iron as an example, when the red Fe3+ enters the position of Ti4+, it is deformed due to polarization, which absorbs more red than normal iron. In addition to the impurities brought by the above process, impurities caused by mechanical wear, such as in Raymond, gas powder and other equipment, due to severe friction, the material on the surface of the machine falls off, and impurities will also be brought into the titanium dioxide, thereby increasing the absorption of light by titanium dioxide and reducing its whiteness. 2. Particle size and particle size distribution Particle size and particle size distribution are also the main factors affecting the whiteness of titanium dioxide, which is mainly affected by the reflection and scattering of light by titanium dioxide particles. Within a certain particle size range, the particle size of titanium dioxide particles decreases, the surface area increases, the scattering ability of light increases, and the whiteness is higher. From the optical effect point of view, when the light scattering force is the largest, the relationship between the pigment diameter and the wavelength λ of the incident light, the refractive index n1 of the pigment, and the refractive index n2 of the paint can be expressed by the formula: D=2λ[π(n1-n2)]. In the above formula, the wavelength of visible light is 400-700nm, the refractive index of rutile titanium dioxide is 2.71, and the refractive index of most paints (resins) in the paint is 1.45-1.60. Therefore, the particle size of rutile titanium dioxide is best controlled at 0.2~0.3μm. Within this particle size range, the pigment has the greatest scattering force on visible light, and can obtain higher hiding power, color-reducing power and pure whiteness. However, when the particle size is less than 0.1μm, due to the diffraction of light, the light passes through the surrounding of the pigment particles without reflection, which reduces the covering power and even tends to be transparent. This type of ultrafine titanium dioxide is transparent and can be used to make transparent sunscreen skin cream. This skin cream has a delicate paste and a natural skin feel. It is currently very popular in Japanese skin care products.