Traditional insulation materials for appliances, transportation vehicles and industrial equipment include mineral fibers and foamed materials. For the insulation of refrigerators, freezers, or water heaters, mineral fiber insulation can provide an R-value of about four R's per inch, where an R-value equals one HrFt.sup.20 F/Btu. Foamed materials containing chlorinated fluorocarbons in the foam cells can provide an R-value of 8 R's per inch. Due to desires to eliminate the fluorocarbons from the environment, and desires to provide more efficient insulation products to save energy, manufacturers of appliances, transportation equipment and industrial equipment, are seeking more efficient insulation products. One possible solution is to merely increase the thickness of the insulation product, thereby increasing the overall R-value. This is an undesirable solution because it makes the insulated object rather bulky.
In order to provide a very efficient insulation product ("super insulation"), some sophisticated insulation systems have been developed. Some of these systems have been developed for space applications for NASA and other governmental bodies.
One of the known super insulation products is that of finely divided inorganic particulate material compressed into a board-like structure, and encapsulated in order to contain the material. The material resembles a flour-like powder. The predominate material used is silicon dioxide or forms thereof, particularly fumed silica or precipitated silica. Preferably, the material is amorphous in structure rather than crystalline. It is known that these products can be compressed into boards having an R-value of 15 R's per inch at 75.degree. F. mean temperature and a 40.degree. F. temperature driving force when the encapsulation envelope is evacuated. The particulate material does a good job of stopping the gas conduction component of thermal heat transfer but does not do a good job of stopping the radiation component of thermal heat transfer. Also, the greater the vacuum (i.e., the lower the gauge pressure) the more the particulate material is compressed, thereby providing more particle-to-particle contact and a better path for solid conduction heat transfer. Thus, an upper limit to R-value is reached dependent on the substantial solid conduction heat transfer of the compacted particulate material. Further increases of vacuum do not improve the R-value of wholly particulate material insulations because of solid conduction. In addition, non-opacified powders are relatively inefficient in blocking the radiative heat transfer.
Another sophisticated super insulation product involves the use of layers of highly reflective material to stop the radiant component of heat transfer. The reflective layers are typically foils, and these must be separated by thermally efficient spacers, such as thin glass fiber mats, or glass beads. The product must be encapsulated and evacuated to prevent gas conduction heat transfer. The drawback with these foil systems is that the foils are difficult to work with, and the product has high material costs. Further, an inherent problem with any evacuated insulation system is that the system must be able to withstand the atmospheric pressure pressing on the sides of the panel. The greater the vacuum, the greater the pressure produced from outside the panel. The thin layers of foil, even though kept separated by spacers, are subject to deformation by the atmospheric pressure. In the event layers of foil touch each other, a path of solid thermal conduction will result, thereby providing a thermal bridge.
It can be seen that the prior art super insulation products lack many of the desirable features of an ideal product. These features include resistance to compressibility, ease of manufacture, relatively low cost of materials, avoidance of high vacuums, and maintenance of a high R-value during a long life of the product.
A new product comprising a board formed from a predominant amount of mineral fibers and between 5 and 40 percent by weight of particulate material has been proposed. This product is encapsulated and evacuated. The mineral fibers provide a good barrier to radiant energy heat transfer. The particulate material provides a good barrier to heat transfer by gas conduction. The entire structure is sufficiently resistant to compression so as to provide a good structure for the pressures of the atmosphere when the panel is encapsulated and evacuated. The materials should be relatively straightforward to work with, and easy to form into the product of the invention. Further, the materials are relatively inexpensive. Finally, the insulation product of the invention should be stable for a long product life of constant R-value. There is a need for a suitable method to manufacture this new product.