Microspheres are small spheres made of various types of materials that are usually less than 100 microns in diameter. These microspheres can be hollow or solid and have been made from a variety of materials including metals, polymers, and ceramics/ glasses.
Hollow glass microspheres have found use in a variety of other applications from defense to transportation to construction. This hollow glass microsphere was unique due to the development of interconnected porosity in the glass wall of the microsphere and is known as a Porous Wall Hollow Glass Microsphere (PWHGM). The addition of pores in the microsphere makes it possible to fill the hollow cavity of the PWHGM with different materials. This development creates many new applications for filtration and encapsulation using PWHGMs. The basic properties of PWHGMs are different compared to other hollow glass microspheres. PWHGMs can have a diameter that ranges from 2 to 100 microns.
PWHGMs have wall thicknesses in the range of approximately 0.5 to 2 microns giving them a diameter to wall thickness ratio range between 4 and 200. By definition thin walled structures have diameter to wall thickness ratios of greater than 10. Unlike many commercial microspheres, such as the soda lime borosilicate ones manufactured, PWHGMs are comprised of around 96% pure silica.
PWHGMs start out as borosilicate HGMs and then are heat treated. Due to their composition, these HGMs undergo spinodal decomposition and phase separate into two interconnecting phases: sodium borate and silica. The sodium borate is leached away with acid leaving mostly porous silica as the wall material. This is how the characteristic nanoporosity of the PWHGMs is created. The wall porosity will generally be 0.01 to 0.1 microns in diameter. Encasing the wall are two layers that are thought to be created during the leaching step. These layers exhibit a different type of porosity than the inner wall porosity.
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