Heat-sensitive materials encapsulation

In this contribution a novel fluidized-bed coating process is introduced to encapsulate heat-sensitive materials with particle sizes below 100 pum. Supercritical carbon dioxide is used as solvent for the coating material as well as carrier fluid for the core material. The behaviour of the high pressure fluidized-bed was investigated for different process parameters and materials. It is shown that the fluidization starts at lower fluid velocities if the pressure is increased and it was possible to fluidized particles with a mean size below 10 xm. The coating of glass beads with stearyl alcohol was carried out and layers with a thickness of 1-8 xm were achieved. 

Lyophilization or freeze-drying is a simple technique that is suitable for the encapsulation of aromas, water-soluble essences, drugs, and importantly used for the dehydration of almost all heat-sensitive materials. It is a process that requires a long dehydration period. The retention of volatile compounds during lyophilization is dependent upon the chemical nature of the system. ... 

Polymers of this nature can be polymerized either in solution or in bulk in the latter case they are normally reacted at high temperatures, e.g., 100-150 C. Since our goal was a casting resin, the formulations were reacted in bulk and at lower temperatures to protect heat sensitive electronic components furthermore, low reaction temperatures minimize side reactions that can lead to crosslinking and polymer insolubility. In this process the polyols and diisocyanates were mixed and allowed to react for about 25 minutes at 71 C to form the prepolymer formation while longer times resulted in material too viscous to cast or deaerate. After the indicated time, 1,4-butanediol was added followed by deaeration and subsequent encapsulation of a preheated (71 C) electronic device. A second deaeration of the encapsulated part is usually necessary. Pot life for such a system is about 15 minutes. Final reaction or "cure" was 24 hours at 71 C. 

Specimens that comprise or simulate special form radioactive material shall be subjected to the impact test, the percussion test, the bending test and the heat test specified in paras 705-709. A different specimen may be used for each of the tests. Following each test, a leaching assessment or volumetric leakage test shall be performed on the specimen by a method no less sensitive than the methods given in para. 710 for indispersible solid material or in para. 711 for encapsulated material. 

The basic chemical technology of silicones and silicone elastomers is covered in Chapter 18 so the discussion here is limited to silicone resins. These materials have many applications, including use in paints and varnishes, molding compounds, encapsulants, electrical insulation, pressure-sensitive adhesives, laminates, and release coatings. Their heat stability, water repellency, and resistance to solvents and weathering favor their use in these applications. 

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