The Synthesis Of Phase Change Microcapsules And Their Application In Thermal Conductive Composites

Volume change and leakage phenomenon of phase change materials restrict their more widely application, during the solid-liquid phase change. A great deal of scientific research has already indicated that the microcapsulation of phase change materials can be a very good method to solve the problem. The microcapsulation is the packaging of phase change materials in compact polymer, so that phase change microcapsules prepared by this method are powdery granular substances with a kind of core-shell structure.In this paper, we first synthesized a kind of phase change microcapsules with paraffin (Tm=48-50℃) as core and melamine formaldehyde resin as shell through in-situ polymerization. The microstructure, surface morphology and phase-change properties of microcapsules were investigated by optical microscope, scanning electronic microscopy (SEM), fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The impact of emulsification time, emulsification stirring rate, stirring rate of the formation of shell material and core/wall ratio on microstructure and properties of phase change microcapsules were studied. Under the optimal synthetic conditions, the content of paraffin in phase change microcapsules was 71% and the enthalpy was 118.69 J/g.Secondly, thermal conductive/phase change elastomer composites were prepared by mixing phase change microcapsules and alumina particles with liquid silicone rubber. The results of SEM and DSC showed that phase change microcapsules were not broken in the composites. Effects of alumina and phase change microcapsules amounts on thermal conductivity of composites were also studied. The results indicated that, when the total volume fraction of alumina and phase change microcapsules was 26%, thermal conductivity of composites presented a decreasing trend with the increase of phase change microcapsules volume fraction. While thermal conductivity of composites rised with increased phase change microcapsules, after the total volume fraction of alumina and phase change microcapsules was 42%.Thirdly, in order to improve the thermal conductivity of the melamine formaldehyde resin wall, an attempt was made to get nano alumina particles to disperse in it. This novel phase change microcapsules were fabricated by mixing alumina with melamine formaldehyde prepolymer solution using the direct addition method (i.e., adding alumina powder into the melamine formaldehyde prepolymer solution directly) and the pre-dispersed addition method (i.e., pre-dispersing the alumina particles homogenously in water through stirring with a high-speed disperse-machine under the assistance of dispersant and wetting agents before mixing with the melamine formaldehyde prepolymer solution). It was demonstrated by SEM that the pre-dispersed addition method yielded the microcapsules that had a better dispersion and less self-agglomeration of alumina, compared to the direct addition method. FTIR spectroscopy, energy dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction imaging (EBSD) confirmed that the nano alumina particles were successfully incorporated in the shell by the pre-dispersed addition method. Furthermore, SEM showed that the larger the amount of alumina added, the rougher the microcapsule surface was. The phase change behavior of microcapsules incorporated with different contents of nano alumina particles in the shell was investigated by DSC. The results revealed that the encapsulation efficiency for this kind of novel microcapsules was above 77% and the incorporation of nano alumina in the shell affected the phase change temperature. Based on DSC and SEM results, it was found that the maximum incorporation content of nano alumina fillers in the microcapsules could reach 12.7% (34.8% of the shell). Thermal gravimetric analysis (TGA) indicated that the addition of alumina improved the thermal stability of phase change microcapsules remarkably.