Physico-chemical and mechanical properties of microencapsulated phase change material

Abstract

Microencapsulated phase change materials (MPCM) are well known in advanced technologies for the utilization in active and passive systems, which have the capacity to absorb and slowly release the latent heat involved in a phase change process. Microcapsules consist of little containers, which are made of polymer on the outside, and paraffin wax as PCM in the inside. The use of microencapsulated PCM has many advantages as microcapsules can handle phase change materials as core allowing the preparation of slurries. However there are some concerns about cycling of MPCM slurries because of the breakage of microcapsules during charging/discharging and the subsequent loss of effectiveness. This phenomenon motivates the study of the mechanical response when a force is applied to the microcapsule. The maximum force that Micronal® DS 5001 can afford before breaking was determined by Atomic Force Microscopy (AFM). To simulate real conditions in service, assays were done at different temperatures: with the PCM in solid state at 25 °C, and with the PCM melted at 45 °C and 80 °C. To better understand the behavior of these materials, Micronal® DS 5001 microcapsules were characterized using different physic-chemical techniques. Microcapsules Fourier Transform Infrared Spectroscopy (FT-IR) results showed the main vibrations corresponding to acrylic groups of the outside polymer. Thermal stability was studied by Thermogravimetrical Analysis (TGA), and X-ray Fluorescence (XRF) was used to characterize the resulting inorganic residue. The thermal properties were determined using Differential Scanning Calorimetry (DSC) curves. Particles morphology was studied with Scanning Electron Microscopy (SEM) and Mie method was used to evaluate the particle size distribution. Samples had a bimodal distribution of size and were formed by two different particles sizes: agglomerates of 150 μm diameter formed by small particles of 6 μm. Atomic Force Microscopy in nanoindentation mode was used to evaluate the elastic response of the particles at different temperatures. Different values of effective modulus Eeff were calculated for agglomerates and small particles. It was observed that stiffness depended on the temperature assay and particle size, as agglomerates showed higher stiffness than small particles, which showed an important decrease in elastic properties at 80 °C.