Nano composite phase change materials microcapsules

Abstract

MicroPCMs with nano composite structures have been systematically studied. A series of MicroPCMs were fabricated by the in situ polymerization together with the addition of silver nano-scale particles into the whole core-shell structures. A full factorial experiment was designed, including three factors, i.e. (1) the weight ratio of microcapsule core/shell (1:5 and 1:7), (2) the molar ratio of formaldehyde/melamine (1:1.8 and 1:2.3) and (3) nano-material addition (0, 1 and 3%). In total, 12 MicroPCMs samples with different combination of parameters were prepared. The content of the hexadecane (PCMs) present in the MicroPCMs is approximately 83% (C/S=5) and 88% (C/S=7). The effects of these factors as well as their interactions on the morphology, structural features and physical properties of the MicroPCMs were investigated by using statistical software packages of SPSS and Minitab. The structural and morphological features of the newly developed MicroPCMs were evaluated by a series of modern instruments and characterization technologies, including the DSC, TG, FT-IR, Raman spectroscopy, TEM, SEM, XRD and EDX. As confirmed by the SEM and Laser Particle Analyser, the size of the NC-MicroPCMs was found in the range of 3.5 um to 4.0 um. The appearance of coarse surface is mainly attributed to the distribution of nano-particles on the shell. The SEM and TEM observations and analysis of surface elemental analysis (EDX) revealed that silver nano particles are distributed not only on the surface of the shell, but also within the shell structure and core materials. Comparing with the conventional MicroPCMs, the NC-MicroPCMs contain new chemical components and molecular groups as identified by the Raman and FT-IR spectroscopy, which could be due to the formation of chemical bonds after the pretreatment of silver nano-particles. Moreover, extra X-ray diffraction peaks of silver were found in the XRD spectra, indicating that silver nano-particles were formed into an integral structure together with the microcapsule core and shell by means of chemical bonds and strong physical linkages. Extra functionalities arising from the addition of silver nano-particle were found in addition to the thermal regulating property, including: (1) the enhancement of IR radiation properties in terms of reflecting, scattering and absorbing; (2) the depression of super-cooling of PCM in the core, and (3) the increase of thermal stabilities by strengthening the microcapsule shell as the result of the complex nano-composite structure. The effects of SERS (Surface Enhanced Raman Spectroscopy) arising from the silver nano-particles were observed as a strong interference background occurred during the testing process of Raman spectroscopy. The Raman scattering intensity of the NC-MicroPCMs were magnified more than 100 times comparing with the conventional MicroPCMs. This strong interference may be attributed to the polarization effects, causing an abnormal Raman spectra for some composite microcapsules, especially those with higher addition load of silver nano material. These effects were also exhibited in macroscopic level in the fabric coatings that show an enhanced IR scattering and absorption phenomenon as tested by the "Fabric Infrared Radiation Management Tester" (FRMT). In order to evaluate it quantitatively, an index "Integral Intensity of Raman Scattering" (IIR) was defined to present the level of IR scattering enhancement. It was found that the indices are closely related to the IR radiation properties of the fabrics coated by the NC-MicroPCMs. Structural features including "Degree of Crystallinity" (DOC) were measured by the XRD pattern and DSC for heat of fusion of shell polymers. It was found that the three factors (F/M, C/S and Add%) have a strong influence on DOC. DOC is increased with the formation of the nano composite structure. Also, DOC is closely related to the property of thermal stability. It was further confirmed that the MicroPCMs incorporated with silver nano-particles also show unique high temperature resistance during curing. The thermal regulating effects of the MicroPCMs coatings on fabrics were studied. Temperature regulating "plateau regions" was detected around the temperature of phase change, showing the function of phase change materials. Addition of silver nano-particles to the MicroPCMs has a positive influence on thermal regulating properties of the MicroPCMs coatings. The test results revealed that the addition of silver nano-particles into the MicroPCMs structure increases Temperature Regulating Capacity (TRC) as high as 13.5% comparing with the conventional coating. In this research, a novel type of smart functional materials, i.e. NC-MicroPCMs, has been studied systematically together with the effects of processing parameters to produce the microencapsulated phase change materials with introducing the silver nano particles into the MicroPCMs structure. The NC-MicroPCMs have shown excellent multifunctional thermal properties and thermal stabilities that are far beyond those of the conventional MicroPCMs. The novel NC-MicroPCMs can be used to develop advanced smart materials and products with prosperous and promising applications in a number of industries.