This invention relates to a carbonaceous granular heat insulator for use in a high-temperature oven in a non-oxidizing atmosphere and a process for preparing the same.
A carbon material is excellent in a high-temperature stability and a chemical stability in a non-oxidizing atmosphere and, therefore, is used as a refractory material and a heat insulator in a high-temperature oven.
In general, an insulating performance required of an insulating material, i.e., thermal conductivity, greatly varies depending on the porosity of the insulating material. As the porosity increases, the thermal conductivity decreases while the insulating performance is improved.
Examples of the carbonaceous heat insulator which has been put to practical use include a porous carbonaceous material prepared by carbonizing a foamed resin through baking or a bulky carbon molding prepared by impregnating a felt made of a carbon fiber with a thermosetting resin, molding the impregnated felt, curing the resulting molding and carbonizing the cured molding through baking. All of these carbonaceous heat insulators are foamed carbon material and, therefore, take advantage of an insulating function of large voids present therein.
However, the raw materials of these foamed carbon materials are disadvantageous in that the applications thereof are limited because the cost in high due to expensive raw materials used and complicated steps of production.
Carbon black comprises an aggregate having a chain structure formed by fusion and secondary aggregation of very fine spherical fundamental particles having a diameter of 10 to 200 nm, and many voids are present among the aggregate.
Although the surface layer and its vicinity of the fundamental particle comprise a carbon hexagonal network, the fundamental particle comprises amorphous carbon on the whole. The carbon black itself, therefore, has an excellent insulating property and is advantageously inexpensive, which makes it possible to put the carbon black into practical use, e.g., as an insulating filler of a high-temperature oven.
However, since the carbon black is a finely divided powder comprising an aggregate of fine fundamental particles, it tends to scatter during charging of an oven therewith and discharging it from the oven, which brings about environmental pollution. Further, it is poor in fluidity, which makes it very difficult to handle. In general, carbon black to be compounded with rubber is granulated for the purpose of controlling the environmental pollution and improving the fluidity. However, pellets prepared by granulation are fragile and do not have a pellet hardness sufficient to prevent the occurrence of attrition loss and breakage of the granules. Further, the pellets tend to break into finely divided powder, which raises problems such as scattering of the powder and lowering in fluidity.