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Piece of paper
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Piece of paper

Paper is a thin, flat material produced by the compression of fibers. The fibers used are usually natural and composed of cellulose. The most common source of these kinds of fibers is wood pulp from pulpwood trees, (largely softwoods) such as spruce. However, other vegetable fiber materials including cotton, hemp, linen, and rice may be used. Though generally considered a flexible material, the edges of paper sheets can act as very thin, fine-toothed saws, leading to paper cuts.

Contents

Manufacturing

International Paper Company: Kraft paper mill, located in Georgetown, South Carolina
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International Paper Company: Kraft paper mill, located in Georgetown, South Carolina

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

Preparation of the fibers

The material to be used for making paper is first converted into pulp, a concentrated mixture of fibers suspended in liquid. The source of fiber is often natural (softwood or hardwood trees or other plants) or recycled, such as old corrugated boxes, newsprint, or mixed paper.

When best sheets materials are used to make paper, it is usually necessary to break down the lignin inside of the plant's cell walls. This is done via a chemical process, such as the Kraft process. These processes are not needed when breaking down recycled fibers, as the lignin has already been removed from the source material. If the lignin is retained in the pulp, the paper will yellow when exposed to air and light.

Pulp that has been broken down mechanically is often known as "groundwood pulp." The mechanical process to break down wood chips into pulp requires no chemicals. Since the lignin is not removed from mechanical pulp, yields are relatively high, approximately 90-98%. However, due to the aging issues indicated above, mechanical pulp is most often used for newspapers and other non-permanent goods.

Pulp that is broken down chemically is known as "chemical pulp." The main purpose of a chemical pulping process is to break down the chemical structure of lignin and render it soluble in a liquid (most often water) so it may be washed from the remaining fibers. Removing the lignin from wood chips also serves to break them apart into the fibers that compose pulp.

Recycled fibres do not need to be pulped in the conventional sense. These fibres have already been treated once, so instead they need a more gentle process to break the fibers apart while preserving their integrity.

Once the fibers have been extracted, they may also be bleached, dyed, or have additional ingredients added to alter the appearance of the final product. For example, Kaolin (or calcium carbonate) is added to produce the glossy papers typically used for magazines. The Kappa number indicates how much bleach is required to obtain a given whiteness.

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 watermark may be impressed into the paper at this stage of the process. This mark is used on paper currency and other things. This moving web is pressed and dried into a continuous sheet of paper.

In the mould process, a quantity of pulp is placed into a form, with a wire-mesh base, so that the fibers 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 fibers 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).

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 200ºF 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.

Applications

A paper trimmer
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A paper trimmer

A bursting machine
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A bursting machine

Types

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History

The word paper comes from the ancient Egyptian writing material called papyrus, which was woven from papyrus plants. Papyrus was produced as early as 3000 BCE in Egypt, and in ancient Greece and Rome. Further north, parchment or vellum, made of processed sheepskin or calfskin, replaced papyrus, as the papyrus plant requires subtropical conditions to grow. In China, documents were ordinarily written on bamboo, making them very heavy and awkward to transport. Silk was sometimes used, but was normally too expensive to consider. Indeed, most of the above materials were rare and costly.

The Chinese court official Cai Lun (蔡倫) described the modern method of papermaking in AD 105; he was the first person to describe how to make paper from wood pulp. Paper is considered to be one of the Four Great Inventions of ancient China. Archæologically, true paper had been excavated in China dated from the 2nd-century BC. It spread slowly outside of China; other East Asian cultures, even after seeing paper, could not figure out how to make it themselves. Instruction in the manufacturing process was required, and the Chinese were reluctant to share their secrets. The technology was first transferred to Korea in 604 and then imported to Japan by a Buddhist priest, Dam Jing (曇徴) from Goguryeo, around 610, where fibres (called bast) from the mulberry tree were used. After further commercial trading and the defeat of the Chinese in the Battle of Talas, the invention spread to the Middle East, where it was adopted in India and subsequently in Italy in about the 13th century. They used hemp and linen rags as a source of fiber. The oldest known paper document in the West is the Missel of Silos from the 11th century.

Some historians speculate that paper was the key element in global cultural advancement. According to this theory, Chinese culture was less developed than the West in ancient times prior to the Han Dynasty because bamboo, while abundant, was a clumsier writing material than papyrus; Chinese culture advanced during the Han Dynasty and preceding centuries due to the invention of paper; and Europe advanced during the Renaissance due to the introduction of paper and the printing press.

Paper remained a luxury item through the centuries, until the advent of steam-driven paper making machines in the 19th century, which could make paper with fibres from wood pulp. Although older machines predated it, the Fourdrinier paper making machine became the basis for most modern papermaking. Together with the invention of the practical fountain pen and the mass produced pencil of the same period, and in conjunction with the advent of the steam driven rotary printing press, wood based paper caused a major transformation of the 19th century economy and society in industrialized countries. Before this era a book or a newspaper was a rare luxury object and illiteracy was normal. With the gradual introduction of cheap paper, schoolbooks, fiction, non-fiction, and newspapers became slowly available to nearly all the members of an industrial society. Cheap wood based paper also meant that keeping personal diaries or writing letters ceased to be reserved to a privileged few. The office worker or the white-collar worker was slowly born of this transformation, which can be considered as a part of the industrial revolution.

Unfortunately, the original wood-based paper was more acidic and more prone to disintegrate over time, through processes known as slow fires. Documents written on more expensive rag paper were more stable. The majority of modern book publishers now use acid-free paper.

Recent developments

Paper made in the west since the industrial revolution has been almost exclusively wood based, except for a few specialized papers like those used in banknotes. However, at least one company (Cloudy Bay Cotton) has recently tried to introduce cotton based tissue papers to westernised countries as an alternative to wood based ones. Their reasons for doing this are that the cotton based tissue papers are less abrasive, less likely to cause allergic reactions, and far more environmentally friendly than wood papers, as they are made from renewable materials. The type of cotton fibres used for making paper are discarded as unusable waste from the textile industry, and can be manufactured using fewer chemicals and less energy.

Some manufacturers, notably AMD, have started using a new, slightly more environmentally friendly alternative to expanded plastic packaging made out of paper, known commercially as "paperfoam". The packaging has very similar mechanical properties to some expanded plastic packaging, but is biodegradable and can also be recycled with ordinary paper. [link]

With increasing environmental concerns about synthetic coatings (such as PFOA) and the currently higher prices of hydrocarbon based petrochemicals, there is a recent focus on zein (corn protein) as a coating for paper in high grease applications such as popcorn bags. [link]

Synthetics such as Tyvek and Teslin have been introduced as printing media as a more durable material than paper.

See also

External links

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