This invention relates to a method for baking at a high temperature carbon fiber fabric or rovings which have been laminated or wound up into sheets, tubes or other shapes, in order to adjust their electrical and physical properties, such as their electric resistance, tensile strength, modules of tensile elasticity, and the like.
As taught, for example, by U.S. Pat. Nos. 3,914,395, 4,714,642, 3,964,952 and 4,576,810, it is conventional to prepare and form carbon fibers into various shapes such as tows, tapes, woven sheets, etc. Such shaped carbon fiber products in turn may be utilized for fabricating various goods such as component parts of electric appliances, carriers for active carbon, high-temperatures resistant structural materials or components, and the like.
However, it is also well known that the carbon fibers which constitute such goods have electrical and physical properties which are greatly influenced by the conditions under which the fibers are made. For this reason, in order for the shaped carbon fibers to exhibit desirable such properties, it is widely known to bake the shaped carbon fibers at elevated temperatures, and for such times which will provide them with the desired properties. As taught for example in U.S. Pat. No. 3,628,984, such baking or heating temperatures are generally about 1,800.degree. C. in case of the carbonization of the fibers, and about 2,000.degree. to 3,000.degree. C. in case of graphitization thereof. And, the baking is made in an inert atmosphere employing nitrogen, argon, helium gases, and the like, so that carbon fibers will not be oxidized.
As noted in greater detail hereinafter, in order to achieve this kind of baking, it has been customary in the past to employ baking furnaces utilizing tray-pusher conveyance systems, which are provided with oxygen purging or vacuum chambers. When shaped carbon fiber articles are to be heat-treated in this type of furnace, any oxygen and vapor contained in or adhered to the articles is first removed or purged off the articles before they are fed into an inert gas atmosphere in the furnace. Sometimes when the articles are heated to more than 2,000.degree. C., it is preferable also to remove even hydrogen therefrom.
In order to effect this purging requirement, it heretofore has been necessary to provide at both the inlet and outlet of the tray-pusher type furnace, vacuum chambers which considerably complicate the method of heat treating the articles.