Structure and thermal stability of microencapsulated phase-change materials

A series of microcapsules containing n-octadecane with a urea-melamine-formaldehyde copolymer shell were synthesized by in-situ polymerization. The surface morphology, diameter, melting and crystallization properties, and thermal stability of the microcapsules were investigated by using FTIR, SEM, DSC, TGA and DTA. The diameters of the microcapsules are in the range of 0.2-5.6 mum. The n-octadecane contents in the microcapsules are in the range of 65-78wt%. The mole ratio of urea-melamine has been found to have no effect on the melting temperature of the microcapsules. Two crystallization peaks on the DSC cooling curve have been observed. The thermal damage mechanisms are the liquefied n-octadecane leaking from the microcapsule and breakage of the shell due to the mismatch of thermal expansion of the core and shell materials at high temperatures. The thermal stability of materials can be enhanced up to 10 degreesC by the copolymerization of urea, melamine and formaldehyde in a mole ratio 0.2:0.8:3. The thermal stability of 160 degreesC heat-treated microcapsules containing 8.8% cyclohexane can be further enhanced up to approximately 37 degreesC.