Preparation and thermochromic behavior of low-temperature thermochromic microcapsule temperature indicators

Studies have rarely investigated thermochromic materials at refrigeration temperatures (about 8 °C). In this study, such a temperature indicator that can change color at refrigerated temperature was synthesized. Crystal violet lactone (CVL) and lauryl gallate (LG) were dissolved in methyl laurate (ML) at a phase change temperature of 7 °C. A series of low-temperature reversible thermochromic leuco dyes were obtained and used to prepare low-temperature reversible thermochromic microcapsule (TCM) temperature indicators. The FTIR results show that the thermochromic behavior of TCM temperature indicator was determined by LG controlling CVL to open and close the ring for reversible conversion between the lactone structure (colorless) and conjugated structure (colored). The effects of the mass ratio of the three components (wCVL:wLG:wML) on the thermochromic behavior were analyzed according to the CIE Lab chromaticity indexes. The total thermochromic color difference between leuco dye and TCM temperature indicator reached the maximum when the wCVL:wLG:wML was 1:3:125. The TCM temperature indicator gradually changed from dark blue to white from 1 °C to 8 °C. The DSC results show that the decolorization temperature of TCM temperature indicator was very close to the melting point of ML. In addition, microencapsulation prevented the phase separation of leuco dyes and improved the stability of thermochromic materials. The results indicate the enormous potential of TCM temperature indicator for studying thermochromic materials at low temperatures and in temperature monitoring of refrigerated food packages.