What is the white white titanium dioxide?
1 Overview
Titanium dioxide is commonly known as titanium dioxide, and its excellent white properties make it occupy a large share of the market in the fields of ink, paint, paint and paper making etc. Therefore, it is honored as "the king of white" in our daily life , As long as there are white products or white products, almost all of the participation of titanium dioxide. In addition to titanium dioxide, similar products lead white, zinc white, lithopone, white powder, their market share is far less than titanium dioxide, and the poor performance, especially weatherability, gloss and so on, Therefore, they can only be used in the low-end industries.
At present, the production of titanium dioxide, there are two ways, namely, sulfuric acid and chlorination, worldwide, speaking, the chlorination production capacity slightly more than the sulfuric acid method, and in our country, due to technical reasons, the sulfuric acid method The manufacturer is much larger than the chlorination method, the annual output of chlorination only accounts for 5 ~ 8% of the total output.
The reason why sulfuric acid is widely used because of its long history of development, the process is quite mature, the equipment configuration requirements are low, the requirements of raw materials is not high, titanium, titanium slag can, and this method can produce rutile Titanium dioxide, but also the production of anatase titanium dioxide, low infrastructure requirements, production time is short. Due to the above reasons, sulfuric acid method is widely copied titanium dioxide, titanium dioxide industry occupies a very important position. Compared with the chlorination method, sulfuric acid method has its own shortcomings, mainly in the operation process is very long, the operation of the post complex, low degree of mechanization, process variables, parameter controllability is not high, and "three wastes" emissions, Pollution is serious, energy consumption is huge, belong to high energy consumption, high polluting enterprises. Although sulfuric acid has many disadvantages and is replaced by other methods, its enthusiasm is not diminished at this stage. Its vitality is still tenacious and will still occupy an important position for a long time in the future.
Although the chlorination method is relatively advanced, the process is short, high degree of automation, product quality and other advantages, but its sources of raw materials difficult, high cost, although the "three wastes" less emissions, but will produce high titanium slag and artificial rutile emissions Total waste, including "three wastes" emissions is also quite large. At the same time, its technical complexity, production difficulty, high requirements for equipment and materials, to promote or have a certain degree of difficulty.
Among the indicators of titanium dioxide, whiteness is one of the more important indicators. As the whiteness of titanium dioxide is also the main reason for being called the "white king," the whiteness of titanium dioxide is not a definite value , But according to production methods and process parameters change, of course, the higher the whiteness is better. The factors that affect the whiteness of titanium dioxide are less studied at home and abroad, and the related dissertations and books are also more broadly explained. SUN Li Through the study of titanium dioxide particle size, shape, particle size distribution and impurity types and nucleus defects brief description of their relationship with whiteness, but for the theory to explain less, did not find the root cause; Li Junfeng, Zhang Bingbing, etc. The impact of surface treatment made a related study, that the slurry dispersion, pH value, the order of addition of chemicals have a greater impact on the final product, but very little explanation of the whiteness; although not white beam of titanium dioxide Degree of research, but the purity of soda ash made relevant instructions that the particle size, iron content and production processes have a greater impact on product performance.
2. Definition and classification of whiteness
White is a special color property with a high light reflectance and low color saturation, which is based on the visual perception to determine the degree to which a reflective object can appear white. Generally when the surface reflectance of all wavelengths in the visible spectrum are above 80%, the surface of the object can be considered as white. White is located in a fairly narrow range of color spaces, and like other colors, they can be expressed in three-dimensional quantities (ie, light reflectance Y, purity C, dominant wavelength λ). However, people are accustomed to quantitatively assess the whiteness of different white objects by the one-dimensional whiteness (W), and the ideal white color (such as high-purity barium sulfate with a whiteness value of 100%) as a reference standard. Whether visual assessment of whiteness, or equipment to measure whiteness, must be based on the recognized "standard" basis. So, the so-called white is the distance from the ideal white level.
In order to facilitate the study and comparison, the introduction of tristimulus values Y (lightness), excitatory purity (Pe), and L, a, b value of several physical quantities to regulate whiteness or color. For Y and Pe, different people have different opinions. Some data shows that the surface of object Y> 70, Pe <10 can be seen as white, while others show Y = 70-90. When Pe = 0-10, white.
In order to further improve the same color evaluation method, the International Commission on Illumination (CIE) cites three values of L, a * and b * to represent a certain color, and the L value is the brightness, ranging from 0 to 100, and L = 0 is Completely dark, that is black, L = 100 is bright. a *, b * values have positive and negative points, where a represents red and green, -a * is green, the greater the absolute value, the greater the green saturation, + a * is red, the greater the value, the stronger the red. b * value is yellow-blue, -b * is blue, absolute value is bigger, blue is darker, + b * is yellow, value is bigger, yellow is darker. With these three values can be established in space three-dimensional color coordinate system, shown in Figure 1, L, a, b for the axis, any visible color can be expressed in the coordinate system, this method of representation than Y And Pe more accurate, wider scope of application.
Figure 1 three-dimensional color coordinate system
At present, there are many international formulas for calculating whiteness, such as Ganz whiteness, blue whiteness, Hunt Baidu, Tabble whiteness, whiteness of building materials, and even different industries or enterprises have different ratings The method is to use the frequency of view, Ganz whiteness, blue whiteness, Hunter whiteness is the most widely used of the three, the following brief introduction.
2.1 Ganz white degree
Ganz Whiteness is a whiteness formula published by CIE in the 1980s. It is characterized by the tristimulus value of the object's color. The relevant formula is as follows:
W10 is the dry whiteness value, TW, TW10 is the pale color tone index, Y, x10, y10 is the sample value measured in 10 ° field of view, xn, yn is the coordinate value of D65 standard light source in 10 ° field of view , Where xn = 0.3138, yn = 0.3309. The advantage of this formula is that the measurement results of the material with better whiteness are more accurate, but the measurement accuracy of the other colors is poor, and the evaluation is generally only used to evaluate the white matter.
2.2 blue whiteness
Blu-ray brightness is abbreviated Wb or R457, the formula is:
Wb = KbΣR (λ) F (λ) Δλ
In the formula, Kb is the naturalization coefficient, the value is Kb = ΣF (λ) △ λ, R (λ) is the standard whiteboard blue light whiteness instrument under the same illumination observation spectrum brightness factor, F The relative spectrum of the whiteness meter is correspondingly distributed, and λ is the wavelength. Due to the large reference volume of the formula, the second phase is relatively complicated to calculate, so the value of Z measured by the tristimulus value colorimeter (measured at 10 ° by D65) is usually converted into the following formula:
Wb = 0.925Z + 1.16
The above formula also has a certain range of use, because of its less parameters, easy to calculate in the ink, paint and other fields can be used as a reference for calculation, and high accuracy.
2.3 Hunter whiteness
Hunter whiteness is more common in our country, there are also clearly defined in the national standard, Hunter whiteness expressed by WH, the formula is as follows:
Where, a, b for Hunter color index, L for the brightness index. Hunter whiteness measurement in two ways, one is in the C illumination body 2 ° field of view, the Hunter index L, a, b were:
Another measure of Hunter whiteness is the field of view of the D65 standard illuminator at 10 °, with the Hunter indices L, a, b, respectively:
X2, Y2, Z2 and X10, Y10, and Z10 in the above formulas are tristimulus values in a 2 ° field of view and a 10 ° field of view respectively. The method is characterized by calculating the whiteness value by the color difference form, the whiteness value of the determination result is higher and the difference is smaller, and the method is suitable for the measurement of the sample with the higher whiteness value and is more used in the titanium dioxide industry.
3. whiteness measurement
There are many ways to calculate and measure whiteness. Therefore, what method should be used when comparing the whiteness, because the whiteness values obtained by different calculation methods are quite different. In the market, there are many instruments for measuring whiteness. Different measurement principles may differ. As a result, the results of different instruments using the same sample may be different.
Currently used to measure the whiteness of the instrument MS-350, ND101-DP, ZDB-1 and so on, their light source model and viewing angle is different, Table 1 for the same sample under different test conditions whiteness value, We can see that different test conditions have a greater impact on whiteness, therefore, the choice of test methods have a considerable impact on the whiteness data.
whiteness values under different test conditions
4 factors that affect the whiteness
For now, the factors that affect the whiteness of titanium dioxide impurities, particle size and particle size distribution, particle shape, titanium content, the following brief description of these factors.
4.1 Impurities
In the titanium dioxide process, especially the sulfuric acid process titanium dioxide process, most of the work is to remove impurities in the product, as impurities seriously affect the performance of titanium dioxide, especially whiteness. Colorless metal oxide impurities at very low levels can affect the whiteness, these elements are iron, manganese, chromium, copper, etc., these impurities itself with a color, white titanium dioxide easily color, common The impurities and color concentrations shown in the table.
common impurities and color limits
In actual production, the content of titanium dioxide contaminated by impurities is much more serious than that in Table 2, ie it can develop color at lower concentration. Take iron as an example. When the red Fe3 + enters Ti4 + Polarized and deformed, which absorbs more red than normal iron. Therefore, the replacement of Ti4 + with Fe3 + results in a disproportionate absorption of light, which means the effect of iron on the whiteness of titanium white is much more serious than the data listed in the table. This is mainly due to the strong ability of Fe3 + to be polarized. When encountering polarized atoms, iron atoms will be polarized and deformed. Moreover, the titanium atom has a small radius and strong polarization ability. When the concentration of Fe3 + increases, mutual attraction will occur between each other due to the oxygen defect, and the outside of the nucleus is elongated, further losing electrons due to light energy, resulting in strong light Mutual color phenomenon. According to the color elements of different light color interdependencies are also different, iron red light color interdependencies, manganese light gray color interdependencies. Cerium yellow light color interdependence phenomenon.
4.2 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 phenomenon of light reflection and scattering by titanium dioxide particles. Figure 2 is the particle size distribution of five samples of titanium dioxide, the graph abscissa is the particle size of the particle, the ordinate is the proportion of the titanium dioxide in the system, ie the frequency, the integral below the curve is 1. In the graph, the larger the peak value of the curve, the narrower the peak area, the more left of the curve. The smaller the titanium dioxide particle size of the kind of particles is, the more concentrated the particle size distribution is and the better the performance of the titanium dioxide powder is.
Figure 2 titanium dioxide sample size distribution
The average particle sizes of the five samples in Fig. 2 are 0.4, 0.55, 0.6, 0.8, and 1.0 μm. These samples have exactly the same composition except for their particle diameters. These samples were measured with the MS-350 Whiteness Tester , The result is shown in Figure 3.
Figure 3 the relationship between average particle size and whiteness
According to the data in the figure, the smaller the titanium dioxide particle size, the higher the whiteness value. This is mainly due to the smaller titanium dioxide particle size, larger surface area, and enhanced light reflection and diffuse reflection. According to the characteristics of the light wave, when the particle size of the pigment particles is smaller than half of the light wave, the maximum scattering of the light of this wavelength can be obtained. From this analysis, the best light scattering wavelength is about 0.2 μm and the longer wavelength Red light scattering the largest particle size of about 0.35μm, therefore, the small size of titanium dioxide scattered light blue phase, while the transmitted light is blue complementary color red and yellow phase, on the contrary, large size titanium dioxide scattering The light is red and the light is blue.
In the above five samples, the average particle size is above 0.4μm, which far exceeds the blue light scattering particle size of 0.2μm and the red light scattering particle size of 0.35μm, but there is still blue and red light scattering, The average particle size in the sample is only an average value. There are a large number of particles of 0-0.35 μm in the dispersion system. These particles can produce strong reflection, scattering and absorption. When the average particle size is small, particles of 0-0.2 μm , The blue light scattering is strong, the average particle size is large, 0 ~ 0.2μm particle size less particles, 0.2 ~ 0.35μm particle size of the particles on the strong red light scattering, whiteness decreased significantly.
4.3 Particle shape
Particle shape refers to the presence of titanium dioxide particles can be divided into spherical, rod-shaped and layered, as shown in Figure 4. The spherical particles are ball-shaped or spherical, the rod-shaped particles have a rectangular cross-section and a circular cross-section, or the like. The layered particles are thinner in grain size but larger in area on one side. These three particle shapes are common in titanium dioxide, spherical ideal, but due to the envelope or gas powder may produce rod-shaped and layered. Artificially controlled gas powder conditions, to produce three types of titanium dioxide, whiteness measured with the MS-350 whiteness tester, the results shown in Table 3.
Figure 4 Titanium dioxide particles electron micrographs
the relationship between particle shape and whiteness
As can be seen from , the whiteness of the layered titanium dioxide is slightly lower, and the whiteness of the spherical and rod-shaped white is very similar. Therefore, although the shape of the titanium dioxide particle slightly affects the whiteness, it is not a major factor. In production, the shape of the particles mainly affect the gloss, hiding power, dispersion and other indicators of titanium dioxide, whiteness index may not have to consider this factor.
4.4 Titanium content
The titanium content refers to the total content of titanium dioxide in titanium dioxide. The content of titanium dioxide in pigment grade titanium dioxide is generally above 90%, specifically between 92 and 98%, and the other substances are mainly the coating agent and a small amount of impurities. Currently according to different applications can choose different types of coating agent, commonly used coating agent aluminum coating, silicon coating, zirconium coating, there are two or more coating agent used in common, such as zirconium aluminum package Membrane, aluminum-silicon coating, the amount of these coating agent manufacturers vary. In order to facilitate the study, here is the use of aluminum coated products, that is, titanium dioxide in the post-processing stage wrapped with a layer of water and alumina film, coating chemicals are aluminum sulfate and sodium aluminate, the coating amount was (in alumina Meter) 2,3,4,5,6,7%, due to the presence of impurities and organic treatment, the absolute content of titanium dioxide in the final product between 92 ~ 97%. The whiteness of these samples was measured with the MS-350 Whiteness Tester under the same conditions. The results are shown in FIG. 5.
Figure 5 Titanium dioxide content and whiteness
As can be seen from FIG. 5, as the content of titanium dioxide increases, the whiteness value also increases, that is, the aluminum content increases, and the whiteness value decreases, that is to say, the aluminum element has the property of reducing whiteness. In addition to aluminum, zirconium and silicon are also commonly used coatings, and they also have similar properties except for the degree of whiteness reduction.
5. Production methods to improve the whiteness
In the production and customer use process, we all hope that the whiteness of titanium dioxide can be higher. According to empirical analysis, when the whiteness of titanium dioxide increases, its hiding power and achromaticity index will increase accordingly. Therefore, To achieve the same effect using less titanium dioxide, the cost will be reduced. There are several main ways to improve the whiteness in production (for the sulfuric acid method).
(1) raw material selection
Raw materials are the first step in the production. Only qualified raw materials can produce qualified products. Titanium dioxide is very sensitive to the content of impurities and sometimes accurate to 1ppm. Therefore, choosing a good source of minerals is the most important. Iron, manganese, vanadium and niobium are the more important impurities whose content needs to be rigorously tested and precisely controlled.
(2) hydrolysis technology
Hydrolysis is an important process in the production of titanium dioxide by sulfuric acid method and also an important location that affects the whiteness of titanium dioxide. Generally, the uniformity of pressurized hydrolyzate is poor, the washing speed is slow, and the whiteness of the finished product is poor. However, the atmospheric hydrolysis can overcome this disadvantage when the particles have a uniform shape and whiteness increases.
(3) calcination conditions
Calcination is dehydration, desulfurization, crystalline conversion, crystal growth process, and the finished product has a direct relationship between whiteness, do not owe or over-burning. Generally set in front of the rotary kiln combustion chamber to prevent direct contact with high temperature flame material, and control the temperature, speed and calcination time, based on experience to control the various parameters.
(4) sanding conditions
Sander is the first operation in the post-treatment stage, the task of which is to break the material for later coating treatment. When the material is too thick, the coating uneven, too thin will lead to reduced hiding finished product, and waste grinding resources, therefore, the average control of the average particle size between 0.3 ~ 0.6μm.
(5) envelope process
Envelope, the amount of coating, the uniformity of the coating and so on can affect the brightness of the product, under normal circumstances, phosphorus, aluminum, zirconium will have the whiteness has increased trend, and silicon package will make white The degree of change or even decline, and when the uneven coating will seriously affect the finished product whiteness.
(6) gas powder conditions
Air powder is crushed titanium dioxide to a suitable particle size, the larger the particle size, the worse the uniformity, the lower the whiteness, while the whiteness increases, therefore, according to experience, the finished product size is generally pulverized to 0.2 ~ 0.35μm between.
6. Summary
Through the above analysis of the whiteness of titanium dioxide, we can draw two conclusions:
(1) Titanium white whiteness commonly used Ganz white, blue white and Hunter whiteness to indicate different whiteness by different applications, and the whiteness of the instrument there are differences in the contrast need to determine the instrument model ;
(2) The influencing factors of whiteness of titanium dioxide are impurity, particle size and particle size distribution, particle shape and titanium content, among which the influence of impurities is the most serious, followed by particle size distribution and titanium content, and the influence of particle shape on whiteness is the smallest The role of the lower the impurity content, the more concentrated the particle size distribution, the smaller the average particle size, the higher the titanium content, the greater the whiteness value, whereas the smaller the whiteness.
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