Recently, energy saving has been desired in view of the significance of preventing global warming as a global environmental problem. Thus, consumer apparatuses also have been developed so as to have improved energy saving performance. Refrigerator/freezers in particular have required heat insulating material having superior heat insulation performance from the viewpoint of an efficient use of cold heat.
Generally, heat insulating material uses fiber material such as glass wool or a foam material such as urethane foam. In order to improve the heat insulation performances of these heat insulating materials, the thicknesses of the heat insulating materials must be increased. However, the thick materials cannot be used when a space that can be filled with such heat insulating material is limited and thus the space needs to be saved or must be used effectively.
One of the means for solving the problem as described above is a vacuum heat insulating material composed of a core material for maintaining the space and exterior covering material for blocking the space form outside air. The core material generally includes powder materials, fiber materials, or foam materials containing continuous bubbles, for example. With the recent demand for increased energy saving, a vacuum heat insulating material having a further higher performance has been required.
Generally, heat conduction is represented by the sum of gas heat conduction, solid heat conduction, radiation heat conduction, and convection heat conduction. In a vacuum heat insulating material in which interior of the exterior covering material is depressurized, influence by gas heat conduction and convection heat conduction are almost ignorable. Furthermore, almost no contribution by radiation heat conduction is caused in a temperature region equal to or lower than an ordinary temperature.
Thus, when a vacuum heat insulating material is used for a refrigerator/freezer having a temperature equal to or lower then an ordinary temperature, it is required to consider the reduction of solid heat conduction. Up to now, various fiber materials have been reported as vacuum insulation core materials having superior heat insulation performance.
For example, Japanese Translation of PCT Publication No. H11-506708 discloses a vacuum heat insulating material that uses, as the core material, a fiber assembly structure in which thermoplastic inorganic binder (e.g., low melting glass composition, boric acid) is dispersed in the fiber. As shown in FIG. 4, in conventional fiber two adjacent glass fiber 1 and glass fiber 2 are fixed at intersecting point 4 by inorganic binder 3 via bonding material 5. Japanese Translation of PCT Publication No. H11-506708 discloses that, by using the inorganic binder as bonding material, the individual fibers in fiber assembly are collectively maintained so that the assembly has integrity to provide a resultant product such as a blanket of heat insulating material, a mat, heat heat insulating material, or a panel. Japanese Translation of PCT Publication No. H11-506708 also discloses that this material has superior heat insulation performance because, in contrast with conventional organic binder, no gas is generated by the binder under a vacuum condition in exterior covering material to prevent aged deterioration of the heat insulation performance.
Japanese Patent Unexamined Publication No. H07-167376 discloses a vacuum heat insulating material having core material. This core material is obtained by subjecting inorganic fibers having average fiber diameter of 2 μm or less (preferably 1 μm or less) to an acid aqueous solution processing and compression dehydration processing. Then, eluted components of the inorganic fibers are collected at intersecting points of the inorganic fibers so that they function as binder to cause the inorganic fibers to be bound to one another. Japanese Patent Unexamined Publication No. H7-167376 describes this structure as having superior heat insulation performance because of the following reason. Specifically, this structure is free from aged deterioration of the heat insulation performance due to the structure having no binder for binding fibers to one another, thus eliminating gas caused from the binder under vacuum condition in exterior covering material.
Japanese Patent Unexamined Publication No. H7-139691 also discloses a vacuum heat insulating material having core material. This core material is obtained by the following procedure. First, a plurality of papers obtained by acid sheetmaking of inorganic fibers having average fiber diameter of 2 μm or less (preferably 1 μm or less) are layered in an acid condition. Then, the obtained layers are subjected to a compression process so that eluted components from these fibers are used to bind the inorganic fibers to one another at the respective intersecting points. According to Japanese Patent Unexamined Publication No. H7-139691, this structure can provide a vacuum heat insulating material having superior heat insulation performance due to the following reason. Specifically, this structure is free from aged deterioration of the heat insulation performance and has an arrangement in which the fibers are provided in a direction vertical to heat transmission direction, thereby reducing solid heat conduction.
However, in any one of the above conventional examples, inorganic fibers are bound to one another by binder or bound by using eluted components from the inorganic fibers as binder. This leads to an observation that the above structures increase solid heat conduction among the respective fibers via bound intersecting points, resulting in increased solid heat transmission components compared with a case of fiber material having no binder component.
In the case of fiber in which intersecting points are not bound, on the contrary, solid heat transmission components are suppressed, but the fiber is like cotton and thus is very difficult to handle. Furthermore, this kind of material cannot maintain a shape such as a blanket, a mat, or a panel. Thus, if this material is used as core material of a vacuum heat insulating material, the appearance is deteriorated because of air compression.