Preparation And Properties Of Microencapsulated Phase Change Materials(MPCM) With Waterborne Polyurethane As Shell Material

Microcapsules are a class of particles or clusters with core-shell structure,whose particle sizes are nanometers or micrometers.The shell of the microcapsules generally uses polymer materials,and the core uses phase change materials.The microcapsules usually has high heat storage density and latent heat of fusion,and is easy to be coated,thus showing various excellent properties.Therefore,it is widely used in military field,construction field,textile field,functional thermal fluid field and thermal insulation materials.The particle sizes and morphologies of the microcapsules depend on the choice of shell and core materials and the preparation conditions.In this paper,three types of water-borne polyurethanes,non-ionic polyurethanes,anionic polyurethanes and cationic polyurethanes were prepared,and then they were used as shell materials to prepare microencapsulated phase change materials(MPCM)by using suspension polymerization when paraffin and n-octadecane was selected as the core material respectively,and realized their application in coatings and thermal insulation materials.(1)A special structure of nonionic polyurethane was synthesized by using polyethylene glycol(PEG)and isophorone diisocyanate(IPDI)as the main raw materials,and its chemical structure and molecular weight were characterized by FTIR,1HNMR and GPC.The emulsification mechanism of the non-ionic polyurethane was discussed by tracking the resistivity changes of during the emulsification process.MPCM of nonionic polyurethane/paraffin and nonionic polyurethane/n-octadecane were prepared with the nonionic polyurethane as the shell material by the emulsion polymerization method.The particle sizes,latent heat of phase change and thermal stability of microcapsule were characterized by a Malvern laser particle size analyzer(LDSA),differential scanning calorimetry(DSC)and thermogravimetric analysis(TG).Furthermore,the microstructure and re-dispersion was studied by electron microscopy(TEM).The results showed that there no isocyanate groups existed in nonionic polyurethane and carbamate groups appeared,with the number average molecular weight being about 35000 and a narrow molecular weight distribution.The conductivity of nonionic polyurethane emulsion corresponded to the emulsification mechanism.The particle size and melting phase change enthalpy of MPCM increased with the content of paraffin and n-octadecane increased,and their heat-resistant temperature microcapsules was 200 °C.The average particle size of the MPCM was in the range of 240-485 nm showed a clear core-shell structure under the observation of TEM.MPCM had a good stability under high speed shear,and a good re-dispersibility for the dried microcapsule powders.A stable dispersant can be obtained by stirring the dried powder with water,with the agglomeration rate being less than 1.8 wt% and the average particle sizes being 420-675 nm.The melt enthalpy and the phase change temperature of the re-dispersed MPCM were almost unchanged,which indicated that the nonionic polyurethane is a good material to protect the core material.(2)Anionic polyurethane prepolymers with dimethylolbutanoic acid(DMBA)as a hydrophilic chain extender were synthesized.The chemical structures of anionic polyurethanes were characterized by FT-IR and GPC.And then they were used to encapsulate the paraffin to prepare the anionic polyurethane/paraffin microcapsule.The emulsifying process of paraffin was tracked by the testing of conductivity,and effects of paraffin contents on the particle sizes and emulsion stability was studied.The microstructure and phase change latent heat of microcapsules were also discussed.And then the nanometer microencapsulated phase change material(NMPCM)of anionic polyurethane/octadecane were abstained when n-octadecane was used as the core material and the anionic polyurethane was used as the shell material.The results showed that the conductivity of the emulsion showed a peak when the oil phase concentration was 50%,and the conductivity paraffin emulsion was smaller than that of self-emulsification,indicating a good thermal conductivity.The phase transition enthalpy of NMPCM was 80 J/g.NMPCM had an obvious nuclear shell structure and had almost no overcoldomg phenomenon.(3)Cationic polyurethane was prepared by using castor oil modified glycol(AC006),hexamethylene diisocyanate(HDI)and N-methyldiethanolamine(MDEA)as the chain extenders.The chemical structure and molecular weight of cationic polyurethane was characterized by FT-IR and GPC.Cationic polyurethane/paraffin microcapsules were prepared by suing paraffin as the core material,and The sizes and latent heat of the microcapsules were studied by using LDSA and DSC.The results showed that the cationic polyurethane has a good emulsifying ability and the emulsion is uniform and transparent.The particle sizes of the microcapsules gradually increased with the increase of paraffin contents,and reached the best distribution when the amount of the initiator is about 0.125%.The microcapsules using styrene St as crosslinker has smaller particle size than which cross-linked by tripropylene glycol diacrylate(TPGDA)when the paraffin content was the same.with the increase of paraffin content,the position of endothermic peak was almost unchanged,while the area of the endothermic peak increases,indicating the melting enthalpy(ΔHm)and crystallization enthalpy(ΔHc)also increased.