Application of thermochromic pigments in inks and coatings

Application of thermochromic pigments in inks and coatings:

1. The choice of connecting material (resin):

1-1. The influence of binder (resin) viscosity and solid content on color density and surface gloss:

Resin in varnish. Since the choice of color-changing ink has a very important influence on the color density and surface gloss of the heat-sensitive color-changing ink, this effect is more significant for black ink. The content of pigment is much higher than that of ordinary ink. As a result, the ink cannot be used due to its high viscosity. Therefore, only reduce the amount of resin and increase the amount of solvent when adjusting the ink, so as to reduce the viscosity of the ink. The consequence of this is that when the ink layer of the printed product dries, the resin content is small, which is not enough to fill the gaps between the pigments, making the surface of the ink layer uneven and non-glossy, thereby forming diffuse reflection of light. Therefore, the apparent color density of the ink layer is low and the gray level is high. For black ink, it looks gray and not black. (If the surface of the printed product is overprinted with a layer of transparent varnish, the blackness can be obviously increased) Therefore, the preparation of the thermochromic ink must use a resin with high solid content and low viscosity (low polymerization degree or low molecular weight). In this way, good results can be obtained. Pearlescent pigments, color changing pigments, fluorescent pigments

1-2. The relationship between the type of binder (resin) and its optical refractive index, the color density of the color-changing ink (paint) and the transparency after decolorization:

Thermal decolorization series (R series) color-changing pigments are often used to print "cover-display" patterns. The color-changing ink layer on the surface of the printed product is required to completely cover the underlying pattern when it is colored. After reaching the decolorization temperature, it should be as transparent as possible to make the lower layer The pattern is clearly displayed. In this type of application, if the resin is not properly selected, "white fog" will be generated after the masking ink layer is decolorized, making the underlying pattern unclear. The basic reason is the inconsistency between the selected resin and the optical refractive index of the color-changing pigment microcapsule wall material. Countless microcapsules are dispersed in the ink layer to form a large number of optical refraction interfaces, just like a white opaque phenomenon formed by crushing a piece of transparent glass. Appropriate selection of resin can greatly reduce the discoloration ink layer

This refraction phenomenon makes the decolorized ink layer more transparent. Generally speaking, the transparency of alkyd resin and acrylic resin is better, while the transparency of polyamide resin and nitrocellulose is extremely poor.

2. The choice of solvent and its influence on discoloration performance:

2-1. The influence of the light refractive index of the solvent on the color density of the color-changing ink (paint):

The optical refractive index of the color-changing pigment microcapsule material is about 1.50-1.54. When the optical refractive index of the solvent is the same or similar, the color density of the ink is higher, such as toluene, xylene and benzyl alcohol (the optical refractive index of which is respectively They are 1.496, 1.493 and 1.540). Conversely, when the optical refractive index difference between the two is large, the apparent color density of the ink is lower. For example, n-propanol (1.386), isopropanol (1.377), acetone (1.359), ethyl acetate (1.372), butyl acetate (1.394), ethylene glycol ether (1.392).

However, the influence of the solvent on the color density of the color-changing ink is temporary. After printing, this effect will gradually disappear as the solvent in the ink layer evaporates.

2-2. The influence of the solvent on the discoloration performance of the pigment:

The resistance of heat-sensitive color-changing pigments to different solvents is different. Soaking in some solvents for 6 months will not affect its color-changing performance, but other solvents can erode the surface of the microcapsule and penetrate into it within a few hours to a few weeks. Internally, the discoloration performance of the pigment fades or even disappears completely. After the solvent evaporates, some of this discoloration performance can be partially recovered, and some are completely unrecoverable. Generally speaking, solvents containing less than 3 carbon atoms have a greater corrosive and destructive effect on color-changing pigments, and solvents containing more than 6 carbon atoms have little effect on pigments. In addition, the corrosive effect of the solvent will be more pronounced at higher temperatures. The following is the approximate time required for the thermochromic pigment to be immersed in different solvents at a temperature of 20°C until the color change performance fades. Pearlescent pigments, color changing pigments, fluorescent pigments

Since the solvent resistance of different colors and different batches of products is different, the specific data is subject to actual measurement. The following data is for reference only.

Based on the above reasons, the following measures should be taken when blending inks and coatings:

--- Use as little as possible or not use alcohol solvents below C4, and replace them with other solvents. For example, cyclohexanone can be used instead of propanol.

---When solvents such as propanol must be used, the ink (paint) should be prepared at the time of use and avoid storage.

Because the color-changing pigments produced by our company are easily dispersed in the resin liquid, all the ink (paint) components except for the color-changing pigments can be well distributed. Add the specified amount of the color-changing pigments before printing, and then add them to the ink fountain after they are evenly stirred. print. It is also possible to distribute ink components other than propanol, isopropanol, etc. (including color-changing pigments), add a prescribed amount of the above-mentioned solvents just before use, and start printing after mixing evenly. Note that the amount of ink should not be too much at one time.

Special note: Finished diluents/mixed solvents and some resin solutions commonly used in the ink industry may contain ethanol, acetone, propanol and other ingredients that damage the performance of color-changing pigments. Even if the content of harmful ingredients is very small, it will cause serious effects.