||Paper Making Process|
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Here is the paper making process.|
The principal functions of pulping are to dissolve the lignin that holds the cellulose fibers together and to separate the fibers. The logs that will be reduced to pulp go through one of two processes: either they are mechanically ground into pulp, or they are reduced to a pulp by being chipped and then cooked in a chemical solution. Cheaper grades of paper are generally produced from mechanically made pulp, which often contains some unwanted residues. Chemical methods remove more of the residues. In the chemical process, wood chips are first cooked and heated in a digester, a closed tank operated at high temperature and pressure. In the sulfite process, the chips are pulped under steam pressure in a solution of sulfite salts. The chemical solution consists of caustic soda and sodium sulfide. In both processes, the lignin, the material that holds wood cells together, is dissolved, and the cellulose fibers separate. Cooking time may be as long as 12 hours. The cooked pulp is then washed to remove the chemicals and screened to separate out undigested wood knots and other unwanted materials. Combining a brief chemical cook with mechanical treatment to separate the fibers produces a higher yield but sacrifices some of the quality of chemically pulped paper. Other machines used to clean the pulp include the vortex machine, in which the pulp is whirled rapidly so that heavy pieces of foreign matter fall to the bottom, and the centrifugal machine, in which the pulp is filtered by means of a vacuum through a wire drum that revolves in the pulp vat, making the pulp cleaner.
The pulp may be bleached at this point, although some bleaching may take place as part of the digestion process. Bleaching must be carefully controlled so that the cellulose fibers in the pulp is not weakened by the process. However, it is good to buy unbleached paper, for bleach is hazardous.
In order to make the fibers more flexible, thereby increasing their matting, or felting, capacity, the pulp next goes through a mechanical pounding and squeezing process called beating. The Hollander, an oval tub within which are heavy knife-edged bars, was the original industrial beater, invented in the Netherlands in 1680. It has largely been replaced by high-speed conical or disk beaters(refiners). Pigments or dyes are added to the pulp at the beating stage. You can give the paper color at this stage. The pulp is also mixed with filler materials that help to preserve the paper or give it a better opacity and finish The most common fillers are white chalks, clays, and titanium dioxide. Sizing materials, such as rosins, starches, and gums, that will make the paper resistant to the water in water-based writing inks may also be added during beating. Paper intended for printing or other purposes may require special sizing materials.
The cylinder machine differs from the Fourdrinier principally in the "wet end," or forming operation. Instead of the moving wire screen, a screen-covered rotary cylinder is half-submerged in the pulp vat. As the cylinder rotates, a sheet of matted pulp is formed on its exterior surface and is then picked up by a moving belt, where it is treated to remove the remaining water, as in the Fourdrinier process. A series of cylinders may be used, each one depositing an additional layer of pulp on the belt, so that multilayer sheets are built up. Cylinder machines are used for making thicker papers and paperboard.