Microcellular Polyurea Xerogels for Use in Vacuum Panels

Abstract

The refrigerator industry is faced with regulatory pressure on the one hand to replace CFCs as the blowing agent used in insulative foams and on the other to dramatically increase the energy efficiency of refrigerators by 1998. One of the solutions proposed is the use of vacuum panels as insulating components in the walls of the refrigerator. A variety of materials have been proposed as filler materials for the interior of such panels. A novel class of materials recently developed by Dow Plastics for this application is microcellular polyurea xerogels. These xerogels have been prepared by polymerizing polymeric MDI in solution to provide microcellular materials with pore sizes of about 10 microns and surface areas of about 98 sq meter/gram. Conventional open cell rigid foams, by contrast, have cell sizes of about 120 microns and surface areas of about 0.1 sq meter/gram. Polyurea xerogels represent an alternative filler for vacuum panels to precipitated silica or other inorganic powders, providing lower thermal conductivity under vacuum, lower density (6.5 to 8 pcf versus 12.5 pcf for precipitated silica), and potentially reduced vacuum panel fabrication costs (due to their monolithic form and elimination of powder handling). These microcellular materials were characterized by DMS (dynamic mechanical spectroscopy), DSC (differential scanning calorimetry), TGA (thermal gravimetric analysis) and SEM (scanning electron microscopy). Laboratory scale vacuum panels have been fabricated and thermal conductivities measured. Greater flexibility in vacuum panel fabrication and part integration may be possible due to the ability of these materials to be machined or potentially molded into shapes. Additionally, unlike the technically mature area of open cell rigid foams, which have historically exhibited a lower pore size limit of approximately 50 microns, the potential exists for further pore size reduction in polyurea xerogels (currently at about 10 microns). As pore size is reduced in these novel materials, the corresponding thermal conductivity performance will improve.