The thermochromic pigment (microcapsule) is a spherical particle, and the finer the particle, the stronger the scattering effect on light. Therefore, under the same conditions, the pigment with a fine average particle size is lighter in appearance. The thick pigments are brighter in color. After adjusting the ink, the opposite is true. The finer the pigment, the better the gloss of the ink, the brighter the color, and the higher the color density on the printed product.
First, the relationship between particle size and apparent hue and ink:
Apparent Hue: Thermochromic pigments (microcapsules) are spherical particles. The finer the particles, the stronger the scattering effect on light. Therefore, under the same conditions, the pigment with a fine average particle size has a brighter appearance. Light, but the thicker pigments are brighter. After adjusting the ink, the situation is the opposite. The finer the pigment, the better the gloss of the ink, the brighter the color, the higher the color density on the printed product. The pigments have no gloss after printing, the print marks are rough and unclear, and the fastness is not good. This situation is more obvious in gravure and offset printing.
Ink: The finer the pigment, the larger the specific surface area. Therefore, the fine pigment is more dry than the crude pigment, and the fluidity is poor. Therefore, the use of fine pigment should be high in solid content, low in viscosity, and good in fluidity. Tinker oil.
Solvent resistance: The smaller the average particle size of the color-changing pigment, the thinner the wall of the capsule constituting the microcapsule, and the solvent resistance is also lowered.
Second, the choice of resin and the apparent color density and surface gloss of the color-changing ink:
The choice of resin in varnish has a significant influence on the apparent color density and surface gloss of thermochromic inks, and this effect is more pronounced for black inks. Since the content of the pigment in the color-changing ink is much higher than that of the ordinary ink, if the color-changing ink is prepared by using the resin of the ordinary ink, the ink may be unusable due to the high viscosity. Therefore, when the ink is adjusted, the amount of the resin can be reduced and the amount of the solvent can be increased to lower the viscosity of the ink. The consequence of this is that when the ink layer of the printed product is dried, since the resin content is small, it is not enough to fill the gap between the pigments, so that the surface of the ink layer is uneven and has no luster, thereby forming a diffuse reflection on the light. Therefore, the ink layer has a low apparent color density and a high gray scale. For black ink, it looks gray and not black. (If you print a layer of transparent varnish on the surface of the print, you can clearly feel the blackness is improved)
Therefore, the color-changing ink preparation must use a resin having a high solid content and a low viscosity (low polymerization degree or low molecular weight) in order to obtain a good effect.
Third, the choice of solvent:
Color-changing pigments are well tolerated for gasoline, kerosene, mineral oil, benzene, toluene, xylene, cyclohexanone, benzyl alcohol, turpentine, DBP/DOP plasticizers and aqueous solutions with pH = 2-7.5. Soaking at room temperature for 2 months does not affect its discoloration performance. When butyl acetate is used as a solvent, the storage period is shorter than the above solvent. Low-boiling solvents such as methanol, ethanol, acetone, propanol, isopropanol, DMF, methyl ethyl ketone, butanol and ethyl acetate will affect the color change performance of the pigment within a few days, reducing the color development concentration (C6 or higher). The solvent has little effect on the pigment). Therefore, the following measures should be taken when dispensing ink and printing:
(A) Use as little or no alcohol solvent as possible and replace it with other solvents. For example, cyclohexanone can be used in place of butanol.
(B) When it is necessary to use a solvent such as propanol to adjust the ink, the ink should be used as needed to avoid storage.
Since the company’s color-changing pigments are easily dispersed in the resin liquid, all ink sets other than the color-changing pigments can be dispensed, and a predetermined amount of color-changing pigment is added before printing. After stirring, the ink can be added to the ink fountain to start printing. It is also possible to dispense inks other than propanol, isopropanol, butanol, etc. (including color-changing pigments), and to add a predetermined amount of the above solvent before use, and to start printing after stirring. Note that the amount of ink to be dispensed at one time should not be too much.