Resumen de Development of microcapsules with thermal energy storage (tes) capability for concrete applications
This PhD Thesis focuses on the synthesis and characterization of thermoregulating microcapsules for concrete applications in the framework of a wide research program funded by the Research Council of Norway to introduce thermal storage abilities into Portland cement and geopolymer concrete. Microencapsulation permits the thermal features of phase change materials to be utilized while overcoming disadvantages such as easy release (when melted), flammability and volume change during phase transition. Microcapsules containing different phase change materials: paraffins and fatty acids (linoleic acid, erucic acid, oleic acid and palmitic acid) were prepared by a suspension-like polymerization technique using copolymers of styrene and divinylbenzene P(St-DVB) as the shell material.
Laboratory experiments were performed to investigate the influence of the type of suspending agents, continuous/discontinuous phase mass ratio, agitation rate, toluene/PCM mass ratio, PCM amount and suspending agent amount on the properties of the microcapsules. The optimization process was conducted for those materials which encapsulated paraffins and then evaluated in pilot plant scale. The obtained microcapsules revealed high energy storage capacity (> 100 J/g), encapsulation efficiency (> 80%) and a process yield (> 86%). This optimized formulation also ensures high mechanical and thermal resistance due to the highly consolidated crosslinked structure of the polymer shell.
Fatty acids were assayed as PCMs in order to have greener and more sustainable raw material for the microcapsules formulation. Microcapsules with unsaturated fatty acids had lower latent heat than expected at the same time presenting the high particle yield what indicated that these kind of acids played two different roles, as PCM and also as monomers, in the radical polymerization processes. At high initiator concentrations, the unsaturated fatty acids were observed to react being incorporated into the polymeric backbone of the shell.
Another important problem concerning the incorporation of polymer/paraffin microcapsules into building material is the flammability. To prevent this problem, the microcapsules shell formulation were modified by using the co-monomer hexa(methacryloylethylenedioxy) cyclotriphosphazene (PNC-HEMA) with flame retardant properties. It was found that the incorporation of this flame retardant co-monomer microcapsules exhibit a pomegranate-like structure. Furthermore, the latent heat of the microcapsules increased in the presence of PNC-HEMA. Thermogravimetric analysis performed under atmospheric air confirmed that the PNC-HEMA raised the amount of residue after the burning process, promoting the formation of thermally stable char. Total heat release and peak heat release were lower than for corresponding microcapsules without PNC-HEMA.
Thermal and morphological stability of the microcapsules at high temperatures (up to 240 °C) was observed in-situ. It was concluded that the shape and size of the microcapsules were intact. However, the PCM content decreased with increasing temperature, diminishing to 0 J g-1 at 180 °C. In order to understand this phenomena, a diffusion model of PCM from the microcapsules was developed.
Finally, microcapsules synthesized with the optimal recipe at pilot plant scale were applied in Portland cement concrete (PCC). It was found that the compressive strength of PCC decreased with the addition of microcapsules at temperatures below and above melting temperature of PCM. The thermal conductivity decreased with increasing amount of microcapsules whereas the specific heat capacity increased. A power reduction of 20% could be obtained when 40% of sand was replaced by microcapsules. Cone calorimetry was used in order to evaluate the flammability and the negative effect of incorporation of 10% of MPCM into concrete, however in a heat flux of 50 kW m-2 ignition was not observed, proving little effect of 10% microcapsules on the flammability of concrete.
