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Introduction to printing ink and the application of printing ink in titanium dioxide
About printing ink (I)
Printing ink is mainly made up of pigment, filling, and binder, additives (pigment and filling can be collectively called as coloring materials). Add proper amount of auxiliaries according to the requirements for printing performance. For example, the auxiliaries commonly used in water-based ink are PH value stabilizer, retarder, defoamer, reducer; the auxiliaries commonly used in oil-based ink are abrasion resistance agent, anti-gelling agent, viscosity reducer, etc.; mix, blend or wet grind after matching with certain ratio, and disperse evenly after repeated suppression to produce it.
About printing ink (II)
The manufacturing process of printing ink involves in colorimetry (color matching of printing ink), rheology (system viscosity of printing ink), colloid chemistry (system stability of printing ink), oil chemistry (selection of printing ink solvent), synthetic resin chemistry (flexibility of printing ink), pigment intermediate chemistry (matching of dyes and pigments), analytical chemistry (selection of system formula), photochemistry (UV printing ink), biochemistry (new biological ink) and content of other subjects, which needs comprehensive knowledge to fully master it.
Classification of Ink (I)
· The classification of ink is not strict or clear enough in actual situations due to its miscellaneous varieties. Therefore, it is pretty difficult to classify them scientifically.
· Generalized classification of ink shall be made subject to the printing plate, namely into relief printing plate ink, lithographic plate ink, gravure printing ink and screen printing ink. But this classification is limited by the principle, and cannot cover all practical situations. In recent years, the printing ink types are constantly increasing and more new designs appear. So if the classification is only made subject to the plate, there will be a great limitation.
· Classification by drying type, such as UV printing ink, oxidation drying ink, penetration drying ink, evaporation drying ink, solidification drying ink, and etc.
· Classification by purpose, such as book printing ink, tin printing ink, glass printing ink, plastic printing ink, and etc.
· Classification by product features, such as security printing ink, high gloss ink, photosensitive ink, transparent printing ink, electrostatic printing ink. We also often hear alcohol (soluble) printing ink, water (soluble) printing ink. They are just called by their features. It can be seen that some names of the classified ink are out of date, such as rotary ink. And another example taken is lead printing ink, which is not as strict as it was before.
· When we are talking about the classification of printing ink, we should first establish a concept of classification by plate, and then associate the other classification methods of drying type, product purpose and product features with each other. Only in this way will we establish a complete and systematic concept of classification.
Composition of Ink (I) – Pigment
· Pigments play a function of color development in ink, and have some direct impact on the feature of ink.
· Pigments are colorized, black or white highly dispersed powder that is not soluble in water and organic solvent. According to its source and chemical composition, they are classified into organic pigment and inorganic pigment.
· Dyes are organic compounds that are soluble in water and sometimes in organic solvent. Some insoluble colorized sediment can be prepared from some dye, which is called mordant pigment, and is used for manufacturing of printing ink.
· Inorganic pigments are metal oxides, or insoluble metal salts. The inorganic pigments are also classified into natural inorganic pigments and artificial inorganic pigments, and natural inorganic pigments are mineral pigment.
· Organic pigments are colorized organic compounds. They are also classified into natural type and man-made type. The commonly used type is man-made type. There are many types of organic pigments with complete colors. The performance is better than that of the inorganic pigments.
· The requirements on pigments used in printing ink are pretty high, especially the color, the dispersibility, the light resistance, the transparency and etc. The tone of colorized pigments must approach the spectrum color, and the saturability shall be the maximum, the transparency of pinkish red, green and yellow pigments used for three-primary color printing ink must be high, and all pigments shall be waterproof and can bind to the binder quickly and evenly. The oil absorption shall not be too high, and the pigments are preferably alkali-resisting, acid-resisting, alcohol-resisting and power-resisting.
Composition of Ink (II) – Filling
· The filling is white, transparent, semitransparent or non-transparent powdery substance, which plays the function of filling. Proper adoption of some filling for the filled pigments can not only reduce the amount of pigments used and reduce the cost, but also adjust the nature of printing ink, such as the density, mobility, etc. and improve the flexibility of formula design. The commonly used fillings are kaolin, light calcium carbonate, etc.
· Binders are the main composition of printing ink, which play a function of dispersing the pigment, giving the ink proper binding, flowing and transferring performance, and making the pigment fixed on the surface of printing material by forming a film after printing. Binders are commonly known as varnishes.
· Binders can be made from various substances, such as various dry vegetable oil, most of which can be used to make binders for ink. Mineral oil can also be made into binders; solvents and water as well as various synthetic resins can be made into binders.
· The rheological property, viscosity, neutral, acid value, luster, water resistance and the printing performance of ink depend on the binders. Same pigments with different binders may be made into different types of ink; same binders with different pigments can be made into same type of ink because it does not change the fundamental property of ink. Therefore, the quality of ink mainly depends on the binders rather than the pigments.
· Composition of Ink (IV) – Additives
· Additives are some materials added to improve the inherent property of ink during the printing. Printing ink formulated based on the basic composition cannot meet the requirements on some feature aspects, or cannot meet the requirements for printing use due to the changes of conditions, which can be solved with addition of a little auxiliary materials.
· There are many additives, such as drying agent, anti-drying agent, reducer, viscosity reducer, plasticizer and etc.
· Application of Titanium Dioxide in Ink (I)
· As a kind of pigment, titanium dioxide is used in ink, which is first dispersed in the film forming matter (binder), and then coated on the surface of object, making the object colorized. The molecules of titanium do not bind with the molecules of colorized object, including the molecules of film forming matter. This sets a high requirement on various physical indexes and pigment indexes of titanium dioxide; another matter is whether the titanium dioxide has enough inertia when formulated, that is to say, whether it can maintain the nature of the system or the nature of the finished products during the application so as to keep the color and luster of ink permanently.
1. Tinting Strength
Tinting strength refers to the ability to be colored by coloring materials. The tinting strength of titanium dioxide determines its significance as a pigment used and its economic value. The tinting strength depends on the structure, the preparation method, the dispersing performance of the pigment itself. And the tinting strength of the pigment impacts the properties of the ink directly: If the tinting strength is low, the amount used will be great, the solid content in the ink will be increased, which will reduce the adjustable scope of the ink performance and is adverse to the adjustment of ink properties.
For the pigments used as coloring materials, their first feature is being bright and saturated in color. This is also the first condition for the selection of pigments for manufacturing of ink. Saturation level refers to the purity degree of a tone. The purer the tone is, the brighter the people feel about it. And titanium dioxide is currently the whitest pigment among all the other white pigment found in the world. This determines the irreplaceable position of titanium dioxide as a white pigment in the raw materials used in ink.
3. Transparency and Covering Power
Transparency refers to the light transmitting degree of the film formed on the coated object after the pigment is formulated into ink; if refractive index of the pigment in the coloring ink approximately equals to that of the binders, it is a transparent pigment.
Covering power refers to the covering ability of ink firm on the printed material containing pigment for the ground layer; the higher the refractive index of the pigment and the refractive index of the binders in ink are, the lower the transparency is, and the higher the covering power is.
4. Tinting Strength
Tinting strength refers to the ability to be colored by the coloring materials. The tinting strength of titanium dioxide determines its significance as a pigment used and its economic value.
5. Oil Absorption
Oil absorption refers to the ability to blend the pigment and the oil. There are oil absorptions in the printing ink industry:
The first oil absorption means the quantity of oil needed to make certain amount of pigment into thick liquid;
The second oil absorption means the quantity of oil needed to make certain amount of pigment into printing ink;
The oil absorption is closely related with the particle size, the dispersibility and wetting ability of the pigment; it is also related with the moisture of the pigment and the acidity of the oil;
When the oil absorption is high, the ink concentration is hard to improve, the ink properties are hard to adjust;
The lipophilicity, wetting ability, particle shape, surface static electricity of the pigment will also intensify the impact of oil absorption on the printing ink.
The dispersibility of titanium dioxide indicates its ability to disperse in the binder; the fine particles of titanium dioxide are dispersed in the coloring material during the coloring process. Its dispersibility is directly related to the play of pigment performance; the dispersibility is usually related to the surface wetting ability, the size of surface, the size of crystal and the cluster status of particles of titanium dioxide;
Requirements on titanium dioxide in ink:
Stability of various indicators, good inertia, even and fine distribution of strength, low oil (water) absorption, the lowest water-soluble salt, neutral PH value, good heat resistance, good post processing
Common impact of titanium dioxide on printing oil:
Thixotropy of printing oil, viscosity of printing oil, luster of printing oil