Paper

Paper is a fat, thick material produced by the amalgamation of plant fibres, which are subsequently held together without extra binder, largely by oxygen bonds and to a large degree by fiber entanglement. The fibres used are usually natural and composed of cellulose. more...

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The most common source of these kinds of fibres is wood pulp from pulpwood trees, largely softwoods such as spruce. However, other vegetable fibre materials including cotton, hemp, linen, and rice may be used.

Manufacturing

Whether done by hand or with a paper machine, the paper making process has three simple steps:

Preparation of the fibres

Wood or plant cell walls are composed of fibers bound together. During pulping, these fibers are separated from each other and carbohydrate surfaces (cellulose or hemicellulose) are exposed. It is hydrogen bonding between these carbohydrate surfaces that provides paper strength. Fibers can be separated either chemically or mechanically.

Most chemical pulp is made using the Kraft process. The purpose of a chemical pulping process is to break down the chemical structure of lignin and render it soluble in the cooking liquor, so it may be washed from the cellulose fibers. Because lignin holds the plant cells together, chemical pulping frees the fibers and makes pulp. After Kraft pulping the pulp can be used directly for bags and boxes or further delignifed, during bleaching, to produce white pulp for printing and writing. Chemical pulps tend to cost more than mechanical pulps, largely due to the low yield, 40-50% of the original wood. Since the process preserves fiber length, however, chemical pulps tend to make stronger paper. Another advantage of chemical pulping is that the majority of the heat and electricity needed to run the process is produced by burning the lignin removed during pulping.

There are two major mechanical pulps, thermomechanical pulp (TMP) and groundwood. In the TMP process wood is chipped and then fed into large steam-heated refiners where the chips are squeezed and fiberized between two steel discs. In the groundwood process debarked logs are fed in into grinders where they are pressed against a rotating stones and fiberized. Mechanical pulping does not remove the lignin, so the yield is very high, > 95%, but also causes paper made from this pulp to yellow and become brittle over time. Mechanical pulps have rather short fiber lengths and produce weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than chemical pulp.

Paper made from either chemical or mechanical pulp can also be recycled. By mixing with water and applying mechanical action the hydrogen bonds in the paper can be broken and fibers separated again.

Sheet formation

The pulp mixture is further diluted with water resulting in a very thin slurry. This dilute slurry is drained through a fine-mesh moving screen to form a fibrous web, a long continuous sheet. Water from the slurry is drained through the screen and the resulting paper is pressed, dried and at the end of the process, is fed into a roll.

Read more at Wikipedia.org