The color change principle and structure of reversible thermochromic pigments:
Thermochromic pigment is a kind of microcapsules that change color repeatedly with temperature rise or fall.
The reversible thermochromic pigment is prepared from an electron transfer type organic compound system. Electron transfer type organic compound is a kind of organic coloring system with special chemical structure. At a specific temperature, the molecular structure of the organic substance changes due to electron transfer, thereby realizing a color transition. This color-changing substance is not only bright in color, but also can realize the color change from the state of “colored === colorless” and “colorless === colored”. It is a heavy metal complex salt complex type and liquid crystal type reversible temperature change What the substance does not possess.
The microencapsulated reversible thermochromic substance is called reversible thermochromic pigment (commonly known as: thermochromic pigment, thermopowder or thermochromic powder). The particles of this pigment are spherical, with an average diameter of 2 to 7 microns (one micron is equal to one thousandth of a millimeter). The inside is a discoloration substance, and the outside is a transparent shell about 0.2~0.5 microns thick that neither dissolves nor melts. It is it that protects the discoloration substance from the erosion of other chemical substances. Therefore, it is very important to avoid damaging this shell during use.
Color change temperature of thermochromic pigment
1. Sensitive temperature change color temperature
In fact, the color change temperature of thermochromic pigments is not a temperature point, but a temperature range, that is, the temperature range (T0~T1) included from the start of the color change to the end of the color change. The width of this temperature range is generally 4~6℃. Some varieties with higher discoloration accuracy (narrow range varieties, denoted by “N”) have a narrow discoloration temperature range, only 2~3℃.
Generally, we define the temperature T1 corresponding to the completion of the color change during the constant temperature heating process as the color change temperature of the thermochromic pigment.
2. Cycle times of temperature change color:
Take a small amount of the tested color-changing pigment, mix it with 504 epoxy glue, scrape the sample (thickness 0.05-0.08 mm) on white paper and let it stand at room temperature above 20°C for one day. Cut a 10×30 mm paper pattern. Take two 600 mL beakers and fill them with water. The water temperature is 5~20℃ above the upper limit (T1) of the color change temperature range of the tested sample and not less than 5℃ below the lower limit (T0). (For the RF-65 series ink, the water temperature is set as T0=35℃, T1=70℃. ), and keep the water temperature. The sample is immersed in two beakers in turn, and the time to complete each cycle is 3 to 4 seconds. Observe the color change and record the reversible color cycle number (usually, the color change cycle number of the thermal decolorization series is greater than 4000-8000 times).
Conditions of use of thermochromic pigments:
The reversible thermochromic pigment itself is an unstable system (stability is difficult to change), so its light resistance, heat resistance, aging resistance and other properties are far inferior to ordinary pigments, and attention should be paid in use.
1. Light resistance:
Thermochromic pigments have poor light resistance and will quickly fade and become invalid under strong sunlight, so they are only suitable for indoor use. Avoid strong sunlight and ultraviolet light, which will help extend the life of the color-changing pigment.
2. Heat resistance:
The thermochromic pigment can withstand high temperature of 230℃ in a short time (about 10 minutes), and can be used for injection molding and high temperature curing. However, the thermal stability of the color-changing pigments is different in the color-developing state and the achromatic state, and the stability of the former is higher than that of the latter. In addition, when the temperature is higher than 80°C, the organic matter constituting the discoloration system will also begin to degrade. Therefore, color-changing pigments should avoid long-term working at temperatures higher than 75°C.
Storage of thermochromic pigments:
This product should be stored in a cool, dry and completely dark condition. Since the stability of the color-changing pigment in the color-developing state is higher than that in the achromatic state, the varieties with a lower color-changing temperature should be stored in the freezer. Under the above conditions, the performance of most types of color-changing pigments has not been significantly degraded after 5 years of storage
Post time: Apr-08-2021