In solution, surfactant molecules migrate to the surface of solvents, reducing surface tension (water or other solvents). The polar groups of surfactants are directed directly towards the surface of the solvent and hydrocarbon chains are extended into the air. This distribution makes the surface force balance, and consequently the surface tension decreases. The surface tension remained constant after decreasing to the critical micelle concentration, and the surfactant molecules were distributed on the surface of the solvent until the monolayer structure was formed. Surface tension helps to design effective ink. Because the color coating process and these parameters have a great relationship. Many defects in the coloring layer can be attributed to surface tension.
With the development of water-soluble inks, surfactants are considered to play a special role in ink chemistry. Organic solvent-based ink has the advantages of fine printing quality, smooth printing surface, moderate printing intensity and strong overall color sense, while water-soluble ink does not have these characteristics. For low surface energy matrix, water-soluble ink will cause wetting problem.
The main reason is that as the main component of ink, water produces high surface tension, which is close to the surface tension of water 72 mN / m, while the surface tension of writing matrix is generally 35 mN / M ~ 40 mN / m. Only when the surface tension of the ink is less than that of the writing matrix, can the effective coverage without spots be formed.
The use of surfactants reduces the surface tension of ink. When the contact angle between ink droplets and paper is greater than 140, high quality printing can be obtained, and the lower the surface tension of ink, the greater the contact angle. In addition, the lower the ink surface tension is, the more comprehensive coverage of the writing matrix can be obtained to improve the quality of writing and printing. However, it is difficult to form small uniform ink droplets with too low surface tension. Generally, the surface tension should be greater than 35 mN/m.
The molecular structure of amphoteric surfactants has hydrophobic and hydrophilic groups. The properties related to surface tension can be adjusted by changing the structure of hydrophilic part of surfactant molecule. For example, polymethylsiloxane (silicone resin) has a good application effect. The unique surface activity of silicone resin oil is due to the shielding of hydrophobic Si - O groups by hydrophilic methyl groups. Fluoroalkyl can further reduce surface tension, fluorine atoms have appropriate covalent radius to shield carbon chains from space, and fluoroalkyl seldom interacts with other groups. Fluorinated surfactants can be seen from the example of perfluorooctanoic acid [n-C7F15COOH]. When the mass fraction of perfluorooctanoic acid is 0.01%, the surface tension of water decreases from 72 mN/m to 1512 mN/m.