What Is Vacuum Insulation?
Vacuum insulation refers to insulation that has been vacuumed by decompressing the inside of the insulation material.
Specifically, it refers to a material with heat-insulating properties, such as glass wool that is covered with a laminated film and then vacuumed by further depressurizing the interior.
Uses of Vacuum Insulations
Vacuum insulation is used in household refrigerators, electric hot water pots, vending machines, cold storage boxes for transportation, etc. With the conservation law revisions, companies are actively considering the introduction of vacuum insulation to improve the thermal efficiency of various equipment. In addition, vacuum heat insulators are used as heat insulators.
Vacuum insulation is also advantageous in that the insulation part is vacuumed and its thickness is extremely thin, allowing it to be placed in narrow spaces. Therefore, it can be used for a wide range of products, from small to large in size.
Principle of Vacuum Insulation
Heat conduction, in which heat is transferred from one object to another, requires a medium (gas, liquid, or solid) to transfer heat. When these materials heat up, the molecules inside the medium vibrate, which propagates the heat to the neighboring molecules.
Thermal conductivity expresses the ease with which heat propagates. If this thermal conductivity is small, it means that heat is difficult to propagate. In other words, the smaller the thermal conductivity of the medium, the less heat is transferred and the better the insulation. In general, glass wool and polystyrene are used as materials with thermal insulation properties. The thermal conductivity of these materials themselves is relatively small, but since they contain air inside, heat conduction by the air also occurs.
Vacuum insulation uses glass wool, which is conventionally used as the core material, and then covers it with laminated film to remove the air inside, resulting in almost no heat conduction by the air. Vacuum insulation is therefore less prone to thermal conduction than conventional glass wool insulation and has extremely high thermal insulation properties.
However, if a hole develops in the laminated film of vacuum insulation, air can enter the interior, resulting in a rapid loss of performance as an insulator. In addition, since there is a possibility of vacuum breakage due to deterioration over time, care must be taken when using the material for long periods of time.
Other Information on Vacuum Insulation
1. Improvement of Heat Resistance and Durability in Vacuum Insulation
In recent years, as attention has focused on countermeasures against global warming and energy issues, products with superior heat resistance and durability have been developed for vacuum insulation. One example is a vacuum insulation material with a glass wool core inside and stainless steel foil as an outer wrapping material.
The heat resistance of stainless steel foil is 300°C or higher, which is a significant improvement over conventional vacuum insulation materials that use a resin laminate film as the outer packaging material. In addition, in conventional vacuum insulation, the resin laminate film allows a slight permeation of gas, and the heat conduction of the gas inside the glass wool, which serves as the insulating portion, causes the insulation performance to deteriorate.
However, by using stainless steel foil as the outer wrapping material, gas permeability has been improved and the durability of vacuum insulation has been greatly enhanced.
2. Vacuum Insulation for Residential Use
Vacuum insulation is also used in housing. For example, a core material consisting of glass wool is used for the interior, which is wrapped with a laminated film and has a porous structure with a vacuum inside. Vacuum insulation materials are only a few millimeters thick but can achieve the same thermal insulation effect as glass wool which is several dozen times thicker.
Such excellent heat insulation performance is achieved by making each porous material pore a large space. This is because the larger each pore is, the greater the vacuum area in the area of the glass wool, and thus the thermal conduction by the air, can be suppressed.
To make each porous hole a large space, the core material is compressed under pressure. During this process, the fibers of the core material are fixed in layers, forming a structure in which the fibers are in contact with each other at points, and large spaces (gaps) are formed in the core material. This increases the vacuum portion of the glass wool core, which results in a higher thermal insulation performance.
Vacuum insulation is made by placing the core material in a laminated film bag, creating a vacuum, and thermocompressing the open end. Vacuum insulation materials have also been developed by applying this structure, where a single sheet of vacuum insulation material is divided into multiple blocks by thermo-compression bonding of laminated film, and each block is vacuumed.