Abstract: The present invention provides a process for producing molded articles of a polyimide precursor by a use of a usual extrusion technique, and physical properties of polyimide products prepared by the heat treatment of the molded articles of the polyimide precursor at 400.degree. C. are excellent. This process is characterized by drying a solid obtained by pouring a dilute solution of a polyimide precursor including an aromatic tetracarboxylic acid component and an aromatic diamine component 1 into a liquid in which the precursor is only poorly soluble to obtain a dry solid having a polyimide precursor concentration of 50-70% by weight, and then extruding this dry solid at 80.degree.-130.degree. C.

Abstract: A mixture comprising (i) an oxide, hydroxide or oxyhydroxide of aluminum, (ii) an aluminum-containing sulfate, (iii) an oxide or oxyacid of boron or an alkali metal salt thereof and (iv) an alkali metal sulfate is heated at a temperature of 900 to 1200.degree. C. to grow an aluminum borate whisker. In this process, the reaction is advanced apparently in the solid phase, and this process is advantageous over the conventional processes in that the yield is high and the isolation and purification of the reaction product is facilitated.

Abstract: An aluminum borate whisker is prepared by heating and reacting aluminum sulfate and an oxide of boron, an oxyacid of boron or an alkali metal salt thereof in the presence of an alkali metal sulfate and an alkali metal carbonate. In this process, the reaction is advanced apparently in the solid phase, and this process is advantageous over the conventional processes in that the yield is high and the withdrawal of the reaction product is facilitated.