Papermaking
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Wikipedia-Article "Papermaking"
- For other uses, see Paper (disambiguation).
Paper is a thin, flat material produced by the compression of fibres. The fibres used are usually natural and based upon cellulose. The most common material is wood pulp from pulpwood (largely softwood) trees such as spruces, but other vegetable fibre materials including cotton, hemp, linen, and rice may be used. A stack of 500 sheets of paper is called a ream. Though generally considered a flexible material, the edges of paper sheets can act as very thin, fine-toothed saws, leading to paper cuts.
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Manufacturing
Whether done by hand or with a paper machine, the paper making process has three simple steps:
Preparation of the fibres
The material to be used for making paper is first converted into pulp, a concentrated mixture of fibres suspended in liquid. The source of fibre is often natural (softwood, hardwood, etc.) or recycled (old corrugated boxes, newsprint, mixed office waste, etc.).
When natural materials are used to make paper, it is essential to break down the lignin inside of the cell walls. This may be done mechanically or the lignin can be removed completely by a chemical process. These processes are not needed when breaking down recycled fibres.
Pulp that has been broken down mechanically is often known as "groundwood pulp". The mechanical process to break down the wood chips into fibers uses no chemicals. Since the lignin is retained in mechanical pulp, there is a relatively high yield (90-98%) However, since the lignin is retained in pulp, paper made from this grade will tend to yellow when exposed to air and light. Because of this, 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 is soluable in a liquid (most often water) so it may be washed from the remaining fibers. Removing the lignin from the wood chips also serves to break apart the chips into fibers that compose the 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 fibres have been extracted, they may also be bleached, dyed, or have additional ingredients added to alter the appearance of the final product. The addition of Kaolin (or calcium carbonate) for example results in the glossy papers typically used for magazines.
Sheet formation
The pulp mixture is then 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 moving web is pressed and dried into a continuous sheet of paper.
In the case of the mould process, a quantity of the pulp is placed into a form, with a wire-mesh base (or other draining device), so that the fibers are left coated on the mesh and excess water can drain away. At this time, pressure may be applied to remove more water through a squeezing action. 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 a continuous (Fourdrinier) process to form a reel or web. When dried, this continuous web may be cut into rectangular sheets by slitting the web vertically and then cutting it horizontally to the desired length. 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 products. 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, there must be another absorbant material to absorb the water removed from the sheet. On a paper machine this is a felt (not to be confused with the traditional felt ) and when handmade, this is often a blotter sheet.
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 a 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
- To write or print on: the piece of paper becomes a document; this may be for keeping a record (or in the case of printing from a computer or copying from another paper: an additional record) and for communication; see also reading.
- To represent a value:
- For entertainment:
- For packaging:
- For cleaning (see also tissue, Kleenex):
- For construction
History
The word paper comes from ancient Egyptian writing material called papyrus, which was woven from papyrus plants. Papyrus was produced as early as 3000 BC in Egypt, and then in ancient Greece and Rome. Further north, parchment or vellum, made of processed sheepskin or calfskin, replaced papyrus which requires subtropical conditions for growth. In China, documents were ordinarily written on bamboo, making them very heavy and awkward to transport. Silk was sometimes used, but was usually too expensive to consider. Most of the above materials were rare and costly.
The Chinese court official Cai Lun described the modern method of papermaking in AD 105; he was the first person to describe the method of making paper from cotton rags. Other sources date back the invention of papermaking in China to 150 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 firstly transferred to Korea in 600 and then imported to Japan by Korean Buddhist priest, Dam Jing, in 625, where fibres (called bast) from the mulberry tree were used. After commercial trades and the defeat of the Chinese in the Battle of Talas, the invention spread to the Middle East, where it was adopted by the Indians and subsequently by the Italians in about the 13th century. They used hemp and linen rags. 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's in ancient times because bamboo (although abundance of materials is generally the primary reason for the use of bamboo as opposed to scientific prowess) 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 the norm for the majority. 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 in those same societies. 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, a process 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 specialist papers such as 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 beneficial than wood papers, being made from renewable materials (the type of cotton fibres used for making paper are in fact discarded as unusable waste from the textile industry) and requiring less chemical and energy intensive processing during manufacture.
Some manufacturers, notably AMD, have started using a new, 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.
See also
- Electronic_paper
- Kaolin
- newspaper
- cardboard
- crêpe
- foxing
- ISO 216
- newsprint
- paper sizes
- paper mill
- paper recycling
- pulp and paper industry
- read or die
- stationery
- substrate (printing)
- washi
External links
- "How is paper made?" at The Straight Dope, 22 November 2005
- No End to Paperwork
- History of Paper and Papermaking
- Paper Online
- How Paper Is Made
- United States Government Printing Office: Government Paper Specification Standards
- Interactive Paper
- Magic Material Paper
- History of Paper Making