| Porous microspheres are an important category of microspheres with special morphology and structure, which endowing them with low density, high specific surface area and good flow ability. Porous microspheres can be used in many fields, such as controlled drug delivery and release, adsorption and separation, catalyst carrier, etc. Various methods have been used to prepare porous microspheres. Among them, emulsion method is one of the most common approach, due to its advantages of controllable particle size and morphology and good dispersity. This paper summarizes the current situation of porous microspheres and based on this, we prepare alumina porous microspheres and Fe3O4 nanoparticles/ethyl cellulose composite microspheres by emulsion method. Then we prepare radiation heat-insulated coatings, using alumina porous microspheres as functional pigments, and analyse the effect of pigments on the insulation performane of coatings, the results of our study lay a foundation of the application of porous microspheres. The main results are as follow:(1) Porous alumina microspheres are prepared via an emulsion method accompanied by a sol-gel process, using aluminum sec-butoxide as precursors, ethyl acetoacetate (EAA) as chelating agent, polyvinylpyrrolidone (PVP) as phase separation inducer. The addition of EAA delays the hydrolysis and polycondensation rate of aluminum sec-butoxide. The introduction of PVP induces the onset of phase separation in the emulsion system and make a great difference in the microstructure of the porous microsphere.The increase of of PVP can provide the phase separation domains with a longer time for the coarsening process, resulting in an enlarged pore size and a wider pore size distribution. The gel microspheres after heat treatment experience a phase transformation process in the following sequence: amorphousâ†'γ-Al2O3â†'α-Al2O3.The sample heat-treated at 900℃ retains smooth spherical morphology and internal porous structure as with the as-dried sample. After heat-treated at 1300℃, the microspheres present an internal three-dimensional interconnection, which is a typically crystal structure for α-Al2O3.(2) Fe3O4 nanoparticles/ethyl cellulose composite porous microspheres are prepared via an approach combining emulsion method with solvent diffusion, using ethyl acetate as solvent. The vibrating-sample magnetometry (VSM) test shows that the saturation magnetization of composite microspheres gradually improves with the increasing loading of magnetic nanoparticles and the coercive force reduces with it. A preliminary formation mechanism of porous magnetic composite microspheres is proposed. Water molecules diffuse into the oil phase droplets and induce the phase separation, resulting in a porous structure. Finally, the ethyl acetate diffuses to the water phase, inducing the precipitation of ethyl cellulose and formation of composite porous microspheres. Fe3O4 nanoparticles are distributed uniformly in the ethyl cellulose matrix.(3) Radiation heat-insulated coatings are succefully prepared, using alumina porous microspheres as functional pigments, silicone acrylic emulsion as film formers. The research results show that porous structure and crystalline form have a great influence on the diffuse reflection properties of pigments. And the additive amount and morphology of pigments make a difference in the optical transmittance of coatings.When adding 2.5 wt.% porous alumina microspheres as pigments, the insulation performance of the coating has a significant improvement. The temperature of the sample plate coated with it after 30 minutes irradiation by infrared lamp increases 26.9℃, much lower than that of the plate coated with the coating without pigments, which is 39.6℃. |
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