174. What are the dry forms of offset inks? 176. What is a driers? 177. What are the commonly used driers? What are the characteristics of each? 178. What should be taken care of when using driers? 179. What is imprint crystallization? What harm does it have? 180. What is osmotic drying? 181. What is printing? 182 how to avoid print through? 183. What is imprinting powder? 184. What are the factors affecting the imprinting powder?
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Drying of ordinary sheetfed offset inks mainly takes two forms of oxidative polymerization drying and permeation drying.
175. What is oxidative polymerization drying?
The dry vegetable oil molecules in the ink binder contain unsaturated double bonds, and they react with oxygen in the air to cause oxidative polymerization reactions. Many dry three-dimensionally distributed plant oil molecules turn into a three-dimensional network structure. The huge molecules lose their fluidity and dry on the surface of the substrate. This dry form is called oxidative polymerization drying.
Dry vegetable oils are mixed with trace amounts of organic antioxidants, which affect the absorption of oxygen. As a result, its oxidative polymerization reaction rate is slow and cannot meet the needs of normal printing. In order to speed up the drying of ink, driers must be added. The driers on the one hand react with the antioxidants to form precipitates, which exclude the influence of antioxidants; on the other hand, they also directly promote the polymerization of dry vegetable oil molecules. Therefore, the drying speed can be greatly accelerated.
The commonly used driers are white dry oil and red dry oil. White dry oil is a creamy white fluid made of metal aluminum, manganese, and cobalt. It is characterized by promoting the oxidative polymerization of the ink layer from top to bottom, inside and outside to form a solid film. It is a commonly used driers in offset printing. Dry red oil is a purple-brown liquid fluid characterized by the surface oxidation of the main conjunctiva, gradually drying inwards, drying faster, but because of its own color, generally only put in magenta, black ink used.
A driers is a strong emulsifier. Adding a certain amount of driers can accelerate the drying of the ink, but excessive addition can cause serious emulsification of the ink. This can not only accelerate the drying, but can even cause the ink roller to slip and cause ink transfer difficulties. . In this case, only the original ink can be added to dilute the concentration of the driers or to switch directly to the new ink.
2 Excessive driers may cause ink crystallization.
The blots on the paper were dried and solidified completely, and the surface was bright and smooth, with no fine unevenness and glassy appearance, and was called crystallization. The main hazard of crystallization is that if there is a need to overprint other colors on this color, poor adhesion will occur. Nowadays, resin-based offset printing inks should not crystallize under normal use conditions. However, if excessive driers are added, this may occur.
After the ink is transferred to the paper, due to the effect of the printing pressure and the capillary adsorption of the paper fiber, some of the ink in the ink penetrates into the paper, and the other part of the ink is fixed on the surface of the paper together with the pigment. This drying method is called osmotic drying. .
When printing on thin, loose, uncoated paper, the ink may permeate through the paper and reach the back of the paper, causing smudges on the back, called through-print.
Overprinting is caused by the excessive penetration of the binder in the ink. The methods to avoid it are:
1 The penetration depth of the linking material is related to the size of the capillary aperture of the paper. For printing thin uncoated paper, the paper should not be too loose.
2 Because the depth of penetration is inversely proportional to the square root of the viscosity of the ink, the viscosity of the ink should be controlled and the ink cannot be adjusted too thin.
(3) The pressure permeation caused by the printing pressure is proportional to the square root of the pressure, so the printing pressure should not be too large.
When printing with coated paper, the linking material in the ink penetrates excessively into the paper, and the excess of pigment particles remains on the surface of the print and causes the imprinted powder to become powdery.
The most important factors affecting chalking are the size of the capillary pore size of the paper and whether the ink pigment particle size distribution is reasonable. So choosing the right paper and ink is the key. In addition, pulverization is also related to the excessive penetration of the binder. To reduce excessive penetration of the binder, it is necessary to increase the viscosity of the ink and reduce the printing pressure.
185. What are the factors affecting the speed of imprinting drying?
The factors that affect the drying rate of the imprint are: ink, paper, ink layer thickness, wetting fluid, ambient temperature and humidity, and production process.
186. What are the influencing factors on the drying speed of the imprinting ink?
1 The composition and ratio of the binder. The different ingredients in the binder dry in different ways. Dry vegetable oils are dried by oxidative polymerization, and the drying rate is slow. Other ingredients are dried by infiltration and dry quickly. Therefore, if the amount of oil in the ink is large, the drying speed is slow. The dry vegetable oil contains many different components. The greater the degree of unsaturation of the oil, the faster the oxidative polymerization will dry.
2 pigment ingredients. Some inorganic pigments are metal salts and oxides, which generally have a humectant effect on dry vegetable oils and dry quickly. Organic pigments for the production of color inks do not have a squeezing effect, and sometimes they also inhibit oxidation drying and slow drying.
3 ink viscosity. The ink has a large viscosity, shortens the time for thickening of the ink, accelerates the oxidation polymerization, and is advantageous for drying.
4 Diluter effect. The ink blended with virgin oil as diluting agent is not conducive to drying.
5 driers content. The ink itself contains a driers. In order to adjust the printability, in order to accelerate the drying rate, driers are sometimes added. However, the excessive amount of driers will cause the ink to emulsify seriously, and it will not be conducive to drying.
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