THE MANUFACTURE OF “BEMBERG”
Rayon made by the cuprammonium process is known as “Bemberg”. Manufacturers generally agree that among the rayon group, the yarn made under the “Bemberg” process is ideal for the manufacture of Women’s Stockings.The point from which manufacture begins is the Outers. These come to Great Britain from the U.S.A. and South America in tightly packed bales, which must be loosened to undergo a mechanical cleansing. Both processes are carried out simultaneously in a special apparatus which, by contrivances for tearing apart and beating out the compressed linters, reduces it to its original short fibres and separates all foreign matter (i.e., not only dirt, but pieces of cotton seed and other woody vegetable matter) from the loose fibres. As they are heavier than the linters, they can be separated and removed from the machine by compressed air or suction. The waste is then collected and sold to manufacturers of paper. The raw linters have a pale brownish appearance and require further cleansing, which is done by chemical means in three stages—washing, boiling, and bleaching. By “boiling” is understood the boiling of the linters under high pressure in caustic soda. The pressure raises the boiling point of the caustic soda and so permits the use of higher temperatures; which makes the effect of the boiling more intensive, removing fat. wax and colouring matter.
The appearance of the boiled linters shows very clearly the effect of the cleansing. Even then, however, it is not sufficient, as the cotton still appears yellowish brown. It must be entirely freed from all impurities which cause discolouration; and that is achieved by bleaching.
Very mild chlorine solution is used for bleaching; and the bleached linters become pure white. They are then ready for transformation into rayon.
Before we discuss that stage, let us glance at the other materials required. Cellulose can only be brought into solution, by indirect means; and one of them is the use of certain copper salts. The common crystalline blue copper sulphate, with the aid of soda and ammonia, can be transformed into a fine dark blue cuprammonium liquid. That liquid is capable of dissolving cellulose.
The solution of the purified linters is of great importance, since the
quality of the finished product depends to a considerable extent on proper attention to that process. The cotton is put into the cuprammonium liquid in large iron dissolvers and stirred continuously. Eventually, a thick and viscous solution is obtained, of a deep blue colour, smelling strongly of ammonia and easily drawn into threads. However, it is not yet ready for spinning. It is inevitable that the linters should still contain traces of dirt which, although too fine to be removed by the initial cleansing processes, would stop up the fine holes of the spinning apparatus and produce faulty filaments. In addition, a certain amount of air is present in the solution which would interfere with the spinning process. The solution is therefore filtered and the air extracted. Now it is ready for use and comes into the stock container, from which it is withdrawn for spinning.After passing through a pipe, the solution arrives at a metal headpiece—the nozzle head—on which is fixed a metal disc with small holes. The whole thing, headpiece and nozzle, is fitted into a glass cylinder which has a metal ring top and bottom. A glass funnel, the lower end of which is connected with a small glass thimble, is also set into the cylinder and the complete apparatus forms the spinning funnel. This is fed continuously with a coagulating liquid called the spinning bath, which, after filling the entire spinning-funnel, runs off through the small glass thimble. With suitable pressure, the spinning solution is forced through fine orifices in a metal disc and arrives in the spinning bath. The stream of coagulating solution draws the separate filaments down the funnel; they pass through the small glass thimble and are then wound onto a rotating wheel in the form of thread. The thickness of the thread depends on the volume of spinning solution supplied and on the length which is drawn away from the nozzle by the reel.
The spinning bath acts on the filaments in such a way as to wash out the ammonia and copper. Both chemicals, however, are necessary to keep the cellulose in its dissolved form, for as soon as the ammonia and copper are removed, the filaments coagulate. They must, however, pass through the intermediate stages between these two extremes; they must harden slowly as they sink deeper into the funnel. Their downward movement is assisted and speeded up by the flowing spinning bath. By this increase of speed, the filaments are drawn out very fine and leave the funnel with a diameter of about 0.01 mm.