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Paper is a thin material produced by the amalgamation of fibres, typically vegetable fibers composed of cellulose, which are subsequently held together by hydrogen bonding. more...

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While the fibres used are usually natural in origin, a wide variety of synthetic fibers, such as polypropylene and polyethylene, may be incorporated into paper as a way of imparting desirable physical properties. The most common source of these kinds of fibers is wood pulp from pulpwood trees, largely softwoods and hardwoods, such as spruce and aspen respectively. Other vegetable fibre materials including those of cotton, hemp, linen, and rice may be used.

Processing of the fibers

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 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.

Drying

After the paper web is produced, the water must be removed from it in order to create a usable product. This is accomplished through pressing and drying. The methods of doing so vary between the different processes used to make paper, but the concepts remain the same.

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