The low-viscosity solvent components (vegetable oil, mineral oil) in the printing ink binder are squeezed into the paper depressions and paper pores during printing, and then the solvent penetrates into the paper through the capillary between the paper fibers, the ink viscosity It quickly increases and dries.
Of course, the oxidative polymerization reaction will be carried out afterwards, and it will take a considerable time to completely dry and form a strong ink film.
1) Volatile dry type
Ink such as thermosetting ink, flexo ink, silk screen ink, etc., after printing on the substrate, the solvent is separated and volatilized, and the solid content of the ink is left on the substrate. This drying method is called volatile drying.
The ink binder is composed of polymer (resin) and solvent. After printing, the solvent is volatilized by a heating process such as hot air, and the remaining resin and pigment form an ink film, so the drying is completed in seconds.
For example, solvent-based gravure inks use low-boiling solvents (benzene, boiling point 110 ° C; ethyl acetate, boiling point 77 ° C; ethanol, boiling point 78 ° C), and water-based flexo inks use water as the solvent. Both are dried due to solvent evaporation after printing.
On the other hand, there are many types of paper suitable for high-speed web rubber roller thermoset ink from coated paper to wood pulp paper, but there are many ink rollers on the printing machine, which requires the ink to have on-machine stability. Compared with the gravure and flexo inks described above, the thermoset ink uses a high-boiling mineral oil solvent (boiling point 250 ° C to 300 ° C). Therefore, a high temperature type hot oven (paper temperature 100 ° C to 150 ° C) is used to dry the ink by volatilization of the solvent, and then a strong ink film is formed by oxidative polymerization reaction.
In the past, heat-fixed ink prints were prone to problems such as scalding, blistering, hot wrinkles, and paper breaks due to high paper temperature. Recently, after the improvement of the ink, the paper temperature has dropped to about 100 ° C, and the above-mentioned failures have been very rare.
2) Thermal polymerization drying type
For metal ink, etc., linseed oil-modified alkyd resin or castor oil-modified alkyd resin is used, which is heated in a drying process to cause thermal polymerization reaction.
On the other hand, there is also a heat-curing solvent-free ink, which can be combined with a variety of drying methods (including Diels-Alder reaction of oxidative polymerization). The ink is composed of the following three basic reaction systems:
â‘ Polycondensation reaction occurs from unsaturated polyester resin system and amino resin system including OH group and COOH group.
â‘¡ Crosslinking reaction occurs between epoxy resin and amino resin.
â‘¢ Free radical polymerization reaction between unsaturated polyester resin and unsaturated monomer.
Especially for the purpose of achieving quick-drying, it is necessary to add substances based on different reaction systems such as organic peroxides or metal desiccants and other polymerization aids or catalysts. By adding this special catalyst, to maintain the stability of printing ink storage and on-board stability. Once heating exceeds the specified temperature range in the drying process, the catalyst will act instantaneously and quickly produce a curing reaction.
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