Dr. Amy Fleischer
Professor and Chair of Mechanical Engineering
Director of the NovaTherm Laboratory
Phone: (610) 519-4996
Phase change materials (PCMs) encapsulation is a process of shelling the PCMs with suitable coating materials to ensure the sustainability of the compositions and alter the connection with surrounding, reduce the possibility of environment reaction with PCMs, improve thermal and mechanical stability, enhance heat transfer rate and compatibility with hazardous PCMs that cannot be exposed to the environment or people might be contacted, and decrease the evaporation and diffusion rate and promote the ease of handling as well.
In the encapsulation process, the solid particles, liquid droplets or gas bubbles named as core materials are coated with polymer or copolymer materials called shell materials. PCMs encapsulation can be classified by different size as macro-, micro- and nano-scaled capsules with various shapes off container as spherical, cylindrical, tubular or rectangular. As to microcapsules, the diameter of these kinds of particles was described between 1 µm to 1000 µm, smaller than 1 µm would be nanocapsules and larger than 1000 was called macrocapsules or microgranules.
The main advantages of microencapsulation can be summarized as: (i) protection of unstable, sensitive materials from their environments prior to use, (ii) better processing by improving solubility and dispersibility of core and shell materials, (iii) employment of a variety of core materials, (iv) production with a high concentration and high yield core materials with enhanced shelf-life by preventing degradative reactions and evaporation, (v) safe and convenient handling of core materials, (vi) masking of odor or taste. The desirable microcapsule characteristics are defined with the parameters, such as: (i) the particle size, (ii) the thickness and impermeability of capsule walls, (iii) the mechanical strength of capsule walls to withstand normal handling forces, (iv) the durability of capsule walls to temperature, humidity and various solvents, (v) the functionality over numerous phase transition cycles, (vi) the good thermal conductivity with increasing heat-transfer area, (vii) the resistance to thermal stress for the whole product life, and (viii) the low cost.
The latent heat capacity of an encapsulated PCM is directly changed with respect to kind and weight fraction of the PCM content. In most experimental, the core/shell fraction was fixed from beginning, and is slightly different to the one like encapsulation ratio obtained by DSC profile which described the effective encapsulation of PCMs within the micro/nano capsules, and this also reflects the actual thermal performance and properties. While encapsulation efficiency represents an effective performance of PCM inside the capsules for latent heat storage. Ideally, for a PCM micro/nano capsuled sphere with a specific diameter, the thickness of the shell could have something with core/shell ratio and might be theoretically derived, provided that known the densities of both core and shell materials. However, the size of the micro/nano encapsulated PCM particle was not even at all but with a size distribution that related to each processing techniques explored and the actual manufacturing parameters involved. Even though, the shell thick of each single spherical PCM encapsulated particles could be estimated, and there might be a way to controlled on the polymerization rate of the shell. The thicker of the polymeric shell with lower thermal conductivity, the slower heat conduction rate, resulting in a delay in the phase change process of the micro/nano capsules and leading to a larger difference of the melting and solidification points between the core material and the capsules. In another hand, a thicker polymer shell means a relative well coverage of PCM inside and less possibility of leakage during the solid-liquid phase transition.