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Paper

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For other uses, see Paper (disambiguation).

Piece of A4 paperPaper is a thin, flat material produced by the amalgamation of plant fibres, which are subsequently held together without extra binder, largely by hydrogen bonds and to a small degree by fiber entanglement. The fibres used are usually natural and composed of cellulose. 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.

Contents [hide]

1 Manufacturing

1.1 Sheet formation

1.2 Drying

2 Applications

3 Types

4 History

5 The future of paper

6 See also

7 External links

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Manufacturing

International Paper Company: Kraft paper mill, located in Georgetown, South CarolinaWhether done by hand or with a paper machine, the paper making process has three simple steps: ===Preparation of the fibres== 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.

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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.

While it is still wet, a watermark woven into the continuous screen may be impressed into the paper. Visible when the finished paper is held to the light, a watermark is used on paper currency and on finer quality paper to identify the manufacturer and establish authenticity.

In the traditional mould process, a quantity of pulp is placed into a form, with a wire-mesh base, so that the fibres form a sheet on the mesh and excess water can drain away. Pressure may be applied to help remove additional water. The paper may then be removed from the mould, wet or dry, and go on to further processing.

Most mass-produced paper is made using the continuous Fourdrinier process to form a reel or web of fibres in a thin sheet. When dried, this continuous web may be cut into rectangular sheets by slicing the web vertically and horizontally to the desired size. Standard sheet sizes are prescribed by governing bodies such as the International Organization for Standardization (ISO).

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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.

Pressing the sheet removes the water by force. Once the water is forced from the sheet, another absorbant material must be used to collect this water. On a paper machine this is called a felt (not to be confused with the traditional felt). When making paper by hand, a blotter sheet is used.

Drying involves using air and or heat to remove water from the paper sheet. In the earliest days of papermaking this was done by hanging the paper sheets like laundry. In more modern times, various forms of heated drying mechanisms are used. On the paper machine, the most common is the steam-heated can dryer. These dryer cans heat to temperatures above 200Ñ"F and are used in long sequences of more than 40 cans. The heat produced by these can easily dry the paper to less than 6% moisture.

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Applications

A paper trimmer.

A bursting machine.

Sheets of coloured gift paper.To write or print on: the piece of paper becomes a document; this may