Construction of ammonium polyphosphate-modified halloysite nanotubes on phase change material microcapsules for the enhancement of thermophysical performance and flame retardant properties

Organic microencapsulated phase change materials (MPCMs) are regarded as promising for building energy efficiency while the low thermal conductivity and flammability limit their further application. Aiming at improving the thermal response performance and flame retardancy of the MPCMs, the MPCMs embedded by ammonium polyphosphate-modified halloysite nanotubes (a-HNTs) are developed and then applied in the epoxy resin (EP) coatings. Herein, the ammonium polyphosphate (APP) is grafted onto HNTs as a flame retardant for the first time and the resultant nanotubes are successfully embedded into the shell of traditional MPCMs. As a result, the obtained microcapsules (a-MPCMs) possess a regular spherical morphology and the desired chemical composition. Moreover, the a-HNTs with their unique inorganic tubular structure connect the inside and outside of the original polymethylmethacrylate (PMMA) shell and act as the thermally conductive channel, which greatly facilitates the conversion of external heat to the latent heat of the capric acid (CA) core. The phase change data show that the embedding of a-HNTs has little effect on the encapsulation efficiency of MPCMs, and the latent heat of a-MPCMs is 118.3 J/g. Meanwhile, the a-HNTs/PMMA hybrid shell also significantly improves the thermal stability of MPCMs with the highest residual weight of 24.3%. With the addition of a-MPCMs, the resultant temperature-control coatings (a-MPCMs/EP) hold a high thermal cycle stability and excellent temperature control performance with less temperature fluctuation. In addition, a-HNTs also provide superior flame retardant properties for a-MPCMs/EP. Compared to the EP composites with the same mass fraction of pristine MPCMs added (MPCMs/EP), the pHRR and THR of a-MPCMs/EP are reduced by 30.2% and 4.5%, respectively. Through flame retardant modification of HNTs and incorporating them into the shell material of MPCMs, this work provides a promising approach to the development of multifunctional MPCMs with enhanced thermophysical performance and flame retardant properties.