Preparation And Characterization Of Microencapsulated Reversibly Thermochromic Phase Change Materials

With the rapid growth of global industry, energy shortage is becoming a critical issue to restrain the development of society and economy. As a new resource for energy storage and utilization, phase change materials (PCMs) can be used to improve energy efficiency and as a media for application of new renewable energy. However, a PCM can only handle a fixed phase change latent heat. When the PCM has storage or released its maximum phase change latent heat (completely melting or solidifying state), it will not be able to continue storaging or releasing any more energy. Therefore, it is essential to have an approach to directly demonstrate the states of energy storage or release in the PCMs. Meanwhile, microencapsulating technology is one of the most effective ways for solving existing problems in the PCMs, such as liquid migration of melted PCMs, volume change during phase change process, and uneven dispersion in applications. In order to avoid health issues by using formaldehyde resins to produce the microcapsules, environmentally benign materials should be considered as the shell materials for the PCM microcapsules. In this thesis, a reversibly thermochromic phase change material (RTPCM) was developed, which can perform as an indicator of energy storage or release by its color change. The RTPCM was microencapsulated via complex coacervation with nature polymers, gelatin and gum arabic. A crosslinking reaction between modified gelatin (MGE) having vinyl groups and diene monomers as a crosslinker on the shell of the microcapsules was employed for improving the thermal stability and encapsulation efficiency of the microcapsules.The main research contents are listed as following:(1) The RTPCM compositing of2-Phenylamino-3-methyl-6-di-n-butylamino-fluoran as a color former,2,2-bis(4-hydroxy-phenyl)propane as a color developer and hexadecanol or tetradecanol as a solvent showed a distinct color change from black (energy emptying) to white (energy saturation) upon heating and inverse change upon cooling. The color change in the RTPCM was synchronous with the phase change. The phase change temperature of the RTPCM using hexadecanol or tetradecanol was49.2or36.8℃, respectively, and the enthalpy of the two RTPCM systems exceeded200J/g. The RTPCM has a high potential to be developed as a latent heat energy storage material with color change as an indicator of its energy states.(2) Modified gelatins having vinyl groups were successfully synthesized using methacrylic anhydride via acylation reaction. The degrees of substitution in modified gelatins were42%or68%, respectively. The RTPCM microcapsules were successfully prepared via complex coacervation with the modified gelatin and gum arabic as shell materials and diene monomers as crosslinkers. Thermal stability and encapsulation efficiency of the microcapsules were improved when a crosslinking reaction was employed. The RTPCM microcapsules with good morphology, high latent heat energy (>72J/g), high encapsulation efficiency (>85%), and uniform particle size (9.3μm) have been successfully prepared.(3) There were minor changes in particle size, morphology and thermal properties of the RTPCM microcapsules after50of repeated thermal cycles, indicating the RTPCM microcapsules have good thermal stability. Meanwhile, the color change of the RTPCM microcapsules was visibly obvious, and a stable white and gray at the completely melting and solidifying state was shown (energy saturation and emptying, respectively) after50of repeated thermal cycles. The RTPCM microcapsules had stable and repeatable color changes during heating and cooling. Therefore, the RTPCM microcapsules can be used as a latent heat storage material and also as an indicator for the energy states.