In the production of various types of inks, the proportion of Titanium Dioxide is relatively large, between 25%~50%, some even larger. Therefore, titanium dioxide plays a vital role in the quality of inks. There are three kinds of crystalline titanium dioxide in nature, namely rutile, anatase and plate titanium. Because the crystal form of plate titanium dioxide is very unstable, there are only two types of titanium dioxide suitable for ink, rutile and anatase, both of which are tetragonal. They have stable physical and chemical properties, pure white color, fine particles and high refractive index. In ink production, different types of ink commodities have different quality requirements for titanium dioxide. Practice has proved that rutile titanium dioxide can be widely used in the production of various types of inks. From the comparison of the functional indexes of the two titanium dioxide, rutile titanium dioxide is superior to anatase titanium dioxide in terms of crystal shape, refractive index, chromaticity and fluorescence index. Therefore, rutile titanium dioxide is widely used in ink production. With the rapid progress of packaging and book printing industry at home and abroad, the market demand for ink will increase day by day. As a very important white pigment in ink, titanium dioxide has many functions and effects that can not be replaced by any other material.
As for the baking iron printing white ink, its post printing baking temperature is about 180 ℃, and the baking time is about 120 minutes. Therefore, it is necessary for titanium dioxide in iron printing white ink to have excellent heat resistance, otherwise, the white ink will turn yellow and lose luster during high-temperature baking, and it will also form the invalidation of printing products when it is severe. Therefore, according to the requirements of different types of ink and different printability, reasonable selection of titanium dioxide is the key to making excellent white ink. The quality requirements of titanium dioxide for ink are summarized as follows:
① Good whiteness, durable and non yellowing, which can make the color of printed products bright and bright in various environments;
② Good weather resistance and heat resistance, stable chemical function;
③ Strong hiding power and good coloring power;
④ Small particles, easy to be evenly dispersed in the ink binder;
⑤ Good humidity, easy to grind;
⑥ Good water resistance.
1. influence on ink coverage
(1) The concave and convex refractive index of titanium dioxide crystal will directly affect the size of ink coverage. Generally, the refractive index of titanium dioxide is the best among white pigments. When preparing white ink, titanium dioxide with high refractive index should be used to enhance the hiding power of white ink.
(2) The influence of particle size, particle structure and looseness of titanium dioxide on the covering power of white ink.
In general, in the range greater than 1/2 of the visible light wavelength, the smaller the particle size, the more lubricated the particle surface, the better the looseness of titanium dioxide in the Resin binder, and the stronger the covering force. Because titanium dioxide itself has a significant crystal structure, its refractive index is larger than that of the color developing agent, and the greater the difference between the two refractive indexes, the stronger the hiding power of the titanium dioxide used. Practice has proved that rutile titanium dioxide has better hiding power than anatase titanium dioxide, so it is more widely used in ink production.
2. influence on the color power of ink.
The tinting power of titanium dioxide depends on the scattering ability of titanium dioxide to visible light, and it has a direct impact on the tinting power of ink. The larger the scattering coefficient is, the stronger the tinting power is; The higher the refractive index of titanium dioxide, the stronger the coloring power. Titanium dioxide has the highest refractive index among white pigments, and the refractive index of rutile titanium dioxide is higher than that of anatase titanium dioxide. Therefore, titanium dioxide with strong scattering ability and high refractive index should be selected.
3. influence on laxity function.
The shape of titanium dioxide particles and the uniformity of light reflection directly affect the laxity function of titanium dioxide. It is considered that titanium dioxide particles have smooth surface, uniform reflection, good dispersity, and good gloss and whiteness of the prepared white ink; On the contrary, if the particle surface is rough and diffuse reflection is added, the gloss will be greatly reduced and the looseness will be poor, which will directly affect the whiteness and transfer function of white ink. Therefore, it is necessary to treat titanium dioxide before it can be used.
In addition, titanium dioxide with different functions should be selected according to the detailed characteristics and requirements of various inks in the ink production process.
(1) Offset printing is based on the principle that oil and water repel each other. Therefore, the resin binder used in offset printing ink should not only have excellent anti emulsification function, but also have excellent hydrophobicity. Otherwise, offset white ink is easy to emulsify; In addition, the offset white ink used for long-term printed matter placed in the field requires its titanium dioxide to have excellent light resistance and weather resistance, otherwise the white ink will turn yellow or chalky.
(2) For gravure composite film inks, in order to make the printed products colorful and beautiful, white ink is used for backing first, and then colorful printing is carried out. Therefore, titanium dioxide is required to have excellent coloring power and hiding power.
4. influence on ink whiteness
(1) The influence of the shape, size and dispersion of titanium dioxide particles on whiteness.
Excellent titanium dioxide particles have a smooth shape and no sharp edges. If titanium dioxide with sharp edges on the surface of the particles is used, the reflection effect on light will be greatly weakened and the whiteness of the ink will be reduced. The size of titanium dioxide particles should be controlled within 0.2~0.4 μ m. That is, it is equivalent to about 1/2 of the wavelength of visible light to obtain high scattering ability and make its color appear whiter. When the particle size is less than 0.1 μ M, the crystal is transparent, if the particle size exceeds 0.5 μ m. It will reduce the scattering ability of the pigment to light and affect the whiteness of the ink. Therefore, it is required that the particle size of titanium dioxide is appropriate and evenly distributed to show outstanding whiteness.
(2) The influence of impurities in titanium dioxide on the whiteness of ink.
Generally speaking, if a small amount of iron, chromium, cobalt, copper and other impurities are mixed into titanium dioxide, the prepared ink will have color deviation and whiteness will decrease. This is because the impurity ions in titanium dioxide, especially metal ions, distort the crystal structure of titanium dioxide and lose symmetry. Rutile titanium dioxide is more sensitive to impurities. For example, when the content of Iron Oxide in rutile titanium dioxide is greater than 0.003%, it will show color, while when the content of iron oxide in anatase titanium dioxide is greater than 0.009%, it will show color reaction. Therefore, it is very important to select precise and impurity free titanium dioxide.
