The method of coating an electroconductive powder together with a binder on a substrate such as paper thereby to form thereon an electroconductive layer for the purpose of forming a paper for duplication such as an electrothermosensitive paper or an electrostatic recording paper is being widely practiced. The term "electro-thermosensitive paper" is herein used to designate a laminated composite sheet comprising a substrate layer, an electroconductive layer supported thereon, and a thermosensitive color-producing layer disposed on the electroconductive layer. The electroconductive layer is utilized as one electrode to cooperate with a recording head constituting another electrode in generating Joule heat by which a recording is made on the thermosensitive layer. An electroconductive powder to be used for such purposes is required to have, in addition to high and stable electroconductivity, characteristics such as a white color tone, uniformity of properties such as particle size and composition, excellent dispersibility, nontoxic nature, and low price.
However, copper iodide (CuI) powder, which is being widely used at present for the electroconductive layers of electro-thermosensitive papers, does not afford a perfectly white color tone although it does have good electroconductivity. Furthermore, because its production is unstable, it is difficult to produce this copper iodide powder in a uniform and stable manner. Still another problem arises from the toxicity of iodine.
In the case of an electrostatic recording paper, the electroconductivity of its electroconductive layer may be lower than that of an electro-thermosensitive paper. That is, it may have a high resistance. For this reason, organic electrolytes are being used for the electroconductive substance. However, because these electrolytes have the characteristic of adsorbing moisture in the air and in the substrate paper, this adsorbed moisture causes the resistance of the electroconductive layer to vary, making it difficult to secure stable electroconductivity. Zinc oxide (ZnO) whose resistance has been lowered by doping with aluminum is also being used, but it has poor resistance to humidity and lacks stability.
As additives to be added for imparting electroconductivity to resins or for lowering their tendency to be electrostatically charged, (a) powders of metals such as Ni, Cu, and Al, (b) Ag powder, and (c) carbon black, carbon fibers, etc., are known. However, oxide films are produced on the surfaces of powders (a), and their contact resistances increase; Ag powder (b) is expensive; and carbon materials (c) give black resins. Thus, none of these additives can be said to be fully satisfactory as a practical agent for imparting electroconductivity, especially, to a white resin.