Preparation And Characterization Of Novel Carbon Nanotube-modified Melamine-formaldehyde (MF) Resin Phase-change Microcapsules

Nowadays,energy shortage and environmental degradation have become the global focus,and saving energy,changing the mode of energy use,and developing new types of energy are becoming the important strategies to solve this problem.Phase-change material is a kind of material which can store and release thermal energy by its own phase transition,and thus has a wide application prospect in the field of energy storage and temperature regulation.This study mainly studies the application and modification of phase-change microcapsules.In view of the problems,such as poor thermal conductivity,insufficient mechanical properties and supercooling of traditional phase-change microcapsules,various inorganic filler particles with good thermal conductivity and excellent mechanical properties were introduced to prepare organic-inorganic composite phase-change microcapsules.The morphology,roughness and surface electrical property of microcapsules were characterized using optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),atomic force microscope(AFM)and Zeta Sizer,the heat storage performance and thermal stability of the microcapsules were analyzed using differential scanning calorimetry(DSC)and thermo gravimetric analyzer(TGA),and the composition and structure of the microcapsules were evaluated using fourier transform infrared spectroscop e(FTIR)and X-ray diffraction(XRD).In addition,the temperature regulation performance and thermal conductivity of microcapsules were characterized by infrared thermograph(FLIR)and Hot Disk thermal constant analyzer(TCA).The mechanical properties of microcapsules were characterized by nanoindenter.The main conclusions are listed as follows:(1)A new type of microenca psula ted phase-change materia ls(MPCMs)with double-walled shells(melamine-formaldehyde(MF)resin/carbon nanotube(CNT)-poly(4-styrenesulfonic acid)sod ium(PSS))was successfully prepared through a facile layer-by-layer(Lb L)self-assembly technique using the prefabricated MF resin-encapsulated n-octadecane as template.The results show that the assembly of carbon nanotubes can promote the melting and crystallization process of phase-change microcapsules,inhibit the supercooling effectively and allow the latent heat to be stored and released more quickly.Besides,carbon nanotubes are proved to be effective for improving the mechanical properties and thermal conductivity of phase-change microcapsules.When the assembly of 4(PSS/A-CNT)bilayers was adopted,the average hardness,Young's modulus and thermal conductivity were increased by 230%,32.1% and 57.89%,respectively,compared with pure melamine resin microcapsules.(2)When the shell is doped with carboxylated-carbon nanotubes(C-CNTs),its surface becomes rough,containing a large number of vermicular carbon nanotubes,consequently,the thermal stability of the phase-change microcapsules is obviously improved.In addition,the C-CNTs can not only promote the thermal conductivity,but also can significantly improve the compressive capacity and strength of phase-change microcapsules.It was found that the maximum load pressure,average hardness and Young's modulus were increased by 55.1%,60% and 30.9%,respectively,compared to pure melamine resin microcapsules.(3)When the core is doped with isocyanate-treated carbon nanotubes(i-CNTs),the morphology and mechanical properties of phase-change microcapsules are not affected,but the vermicular CNTs,clearly seen in the broken microcapsules,can inhibit the supercooling of microcapsules effectively.(4)After comparing of the thermal conductivities of epoxy resin composite filled with MF MPCMs,CNT-assembled MPCMs,core-doped i-CNTs MPCMs,shell-doped C-CNTs MPCMs and as well as pure epoxy resin(control),we can find that CNT-assembled and CNT-doped modification can improve the thermal conductivity of phase-change microcapsules effectively.In addition,from the characterization of infrared thermography,it can be concluded that the MPCMs exhibited good thermal regulation capacity,which can play a certain role in delaying the process of heating and cooling of composite materials,and thus effectively reducing the fluctuation of environmental temperature.