1. Field of the Invention
The present invention relates to the chemical arts. In particular the present invention relates to a vacuum thermal insulation product and to a method of making the product.
2. Discussion of the Related Art
Thermal insulation products are used to protect a system of interest from energy flow into or out of the system's surroundings. The use of thermal insulation products are prevalent and range from use in refrigerators (for reduced energy consumption or additional internal volume), in shipping containers containing ice or dry ice used for drugs or food (to extend the lifetime of the shipment), in the tiles on the space shuttle (to protect the shuttle from the heat of reentry into the atmosphere).
Thermal insulation materials are porous materials that have an inherently low thermal conductivity. The lower the thermal conductivity, the lower the heat flow through the insulation for a given temperature difference. Since the thermal conductivity of solids and liquids is much higher than that of gases, insulation, except in very specialized cases, is highly porous. The pores must be sufficiently small (<1 mm), such that free convection due to thermal gradients is minimized.
In the absence of free convection, heat flow through the insulation occurs due to the sum of three components—conduction in the solid matrix, infrared radiation, and conduction in the gas contained in the pores of the matrix. Conduction in the solid matrix is minimized by using a low density (high volume fraction of pores) material. Typically, insulation is between 80 and 98% porous. It is also advantageous to use a solid material that has a low inherent thermal conductivity (i.e., plastics and some ceramics/glasses are better than metals) in order to minimize conduction in the matrix.
The relative importance of radiation depends upon the temperature range of interest and becomes more important for a given insulation as the temperature is increased above ambient temperature and/or the magnitude of the other heat transfer modes are minimized. Opacifiers with high infrared extinction coefficients due to absorption (e.g., carbon black, iron oxide) or scattering (e.g., titania) are often added to high performance insulation. Accordingly, with suppression of free convection, use of a low conductivity, highly porous solid matrix, and control of radiation, the thermal conductivity of the insulation approaches that of the gas contained within the pores of the insulation.
Most thermal insulation materials used today are either fibrous materials, such as fiberglass, mineral wool, and asbestos, or polymer foam materials, such as expanded polystyrene, polyurethane, foamed polyethylene and foamed polypropylene. The fibrous materials have drawbacks related to health and safety. The polymer foams have drawbacks related to flammability, recyclability, and release of environmentally unfriendly gases, such as fluorocarbons or hydrocarbons. In addition, the thermal performances of both classes of materials are on the same order or greater than stagnant air (0.026 W/mK at ambient temperature).
Because of increased concern with energy efficiency and the environment, there has been much interest over the last thirty years in the development of new classes of thermal insulation products that have thermal conductivity much less than that of stagnant air. These new products include gas-filled panels, aerogels, also known as nanoporous silicas and vacuum insulation panels.
There are two general approaches to lower conduction in the gas phase in order to lower the total conductivity of the porous insulation products. The first is to trap gases in the pores that have lower thermal conductivity than that of air. Examples of suitable gases include inert gases such as argon, xenon and krypton, as well as carbon dioxide. Depending upon the gas employed, the thermal conductivity of thermal insulation materials filled with the gas can range from 0.009 to 0.018 W/mK. However, it is a drawback of gas-filled panels that the insulation must be packaged such that the gas does not escape from the pores and atmospheric gases (nitrogen, oxygen) do not penetrate into the pores.
The other means for lowering the conduction in the gas phase is to take advantage of the so-called Knudsen effect. When the mean free path of the gas approaches the pore size of the insulation material, the gas phase conductivity is dramatically reduced. When the mean free path is much larger than the pore size, the gas phase conductivity approaches zero.
The mean free path of atmospheric gases is approximately 60 nanometers (nm) at ambient temperature and pressure. In comparison, the pore size of fibrous materials and polymer foams is typically greater than 10 microns. Consequently, there is no lowering of the conduction of the gas due to the Knudsen effect in these insulation materials.
There are two general approaches to take advantage of the Knudsen effect. The first is to use a material with very small pores and low density. A class of materials that fit this description are aerogels. These materials have small pores (<100 nm) and low density and are the only materials which exhibit total thermal conductivity at ambient pressure which is lower than that of the gas contained within the pores. These materials have thermal conductivity in the range of 0.012 W/mK to 0.025 W/mK. However, they are not in widespread commercial use because of high costs.
The second approach to taking advantage of the Knudsen effect is to encase the insulation material within a vacuum barrier and then to partially evacuate the gas from the pores in the insulation material to form a vacuum insulation panel. This increases the mean free path of the gas by lowering the gas density and lowers the gas phase conduction. At ambient temperature, the thermal conductivity can reach less than 0.002 W/mK. This is an order of magnitude improvement over conventional insulation.
One common approach for encasing vacuum insulation is to use a plastic laminate, a metallized plastic or a metal foil/plastic laminate as the vacuum barrier. Sealing between two sides of the vacuum barrier can be accomplished by heat-sealing the plastic. It is relatively easy to make complex shapes and barrier costs are relatively low. Examples of the use of plastic films are provided in Yamamoto, U.S. Pat. No. 4,529,638. However, these vacuum barrier materials are normally restricted to use at temperatures less than 100° C., because of the increase in barrier permeation rate with temperature of gases such atmospheric nitrogen, oxygen, and water vapor through the plastic. For insulation materials, such as fiberglass, barrier permeation, even at room temperature, is problematic.
Lower gas permeation rates are obtained by using metal foil-based laminates. Examples include aluminum foil-based barriers such as disclosed in Watanabe, U.S. Pat. No. 5,376,424. The thinnest metal foil-based materials include 6 microns of aluminum foil which causes thermal edge effects in panels with the shortest lateral dimension less than 30 cm. Such thermal edge effects cause problems with losses in thermal insulation efficiency (particularly significant with small vacuum panels). When the vacuum barrier layer contains relatively large quantities of thermally conductive metal, energy can flow around the insulation through the barrier and create a thermal short-circuit. The problem is magnified by the fact that a typical barrier material, such as aluminum, can have a thermal conductivity that is over 100,000 times greater than that of the evacuated insulation material. Furthermore, even though metal foil/plastic laminates have excellent gas/vapor permeation resistance over a wide range of temperature, common plastics used for the heat seal layers, such as polyethylene, restrict the use of these barriers to less than 200° C. (even for short lifetimes).
It is also known to encase the insulation in a completely metal barrier envelope. This solution yields excellent product lifetime and the ability of the insulation to be employed at high temperatures. The problems with this approach are two-fold and related. The first is how to seal the barrier around the insulation. The second is how to minimize thermal edge effects. The normal solution is to use a low thermal conductivity metal such as stainless steel and to seal the sides of the barrier by welding. Such an approach is described in Bridges, U.S. Pat. No. 5,252,408. Stainless steel foil is available in thickness down to 12 microns. These foils help to minimize thermal edge effects, but it is difficult to develop leak free welds on foil this thin. Various adhesives and joining materials such as silicones, epoxies, brazes, glasses and ceramics may be used to join the two foil sheets forming the two sides of the vacuum panel. It is difficult to obtain leak-free seals that are stable for long times, are mechanically robust, have high temperature resistance, lend themselves to manufacturing, and do not outgas vapors, solvents, and the like into the vacuum panel.
Another problem with the use of metal foils as a barrier material is that at least one side must be formed to fit around the thermal insulation material. This may cause excessive wrinkling and lead to sealing problems at the seam. This problem becomes more important as the thickness of the vacuum panel increases and for more complex shapes than simple flat panels.
Despite excellent thermal performance, the use of vacuum insulation panels are not widespread because of the high cost of creating the vacuum, lifetime problems associated with maintaining the vacuum (especially at high temperature) and difficulty in producing complex shapes.
Based on the above discussion, it is clear that there is a need for an improved vacuum insulation panel that overcomes many of the disadvantages of the plastic laminate and welded metal foil approaches. Desirable is a vacuum insulation panel which may employ a range of different thermal insulation materials, which may be used over a wide temperature range from cryogenic to high temperature (>900° C.), has a long lifetime, may be produced economically, suffers from minimal thermal edge effects, and may be produced in a range of sizes and shapes.