Paper

For other uses, see Paper (disambiguation).

A stack of paper

Paper is thin material mainly used for writing upon, printing upon or packaging. It is produced by the amalgamation of fibers, typically vegetable fibers composed of cellulose, which are subsequently held together by hydrogen bonding. While the fibers 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. Vegetable fiber materials such as cotton, hemp, linen, and rice are also used.

History

Papyrus and parchment

Outside Egypt, parchment or vellum, made of processed sheepskin or calfskin, replaced papyrus as the papyrus plant requires subtropical conditions to grow.

In America, archaeological evidence indicates that a similar paper writing material was invented by the Mayans no later than the 5th century AD. Called amatl, it was in widespread use among Mesoamerican cultures until the Spanish conquest. The paper is created by boiling and pounding the inner bark of trees, until the material becomes suitable for art and writing.

Early papermaking in China

The world's earliest known printed book (using woodblock printing), the Diamond Sutra of AD 868, shows the widespread availability and practicality of paper in China.

Papermaking is considered to be one of the Four Great Inventions of Ancient China, since the first papermaking process was developed in China during the early 2nd century. During the Shang (1600 BC-1050 BC) and Zhou (1050 BC-256 BC) dynasties of ancient China, documents were ordinarily written on bone or bamboo (on tablets or on bamboo strips sewn and rolled together into scrolls), making them very heavy and awkward to transport. The light material of silk was sometimes used, but was normally too expensive to consider. While the Han Dynasty Chinese court official Cai Lun is widely regarded to have invented the modern method of papermaking (inspired from wasps and bees) from wood pulp in AD 105, the discovery of specimens bearing written Chinese characters in 2006 at north-east China's Gansu province suggest that paper was in use by the ancient Chinese military more than 100 years before Cai in 8 BC. [2] Archeologically however, true paper without writing has been excavated in China dating to the reign of Emperor Wu of Han from the 2nd century BC, used for purposes of wrapping or padding protection for delicate bronze mirrors. It was also used for safety, such as the padding of poisonous 'medicine' as mentioned in the official history of the period. Although paper used for writing became widespread by the 3rd century, paper continued to be used for wrapping (and other) purposes.

Toilet paper was used in China by at least the 6th century AD. In AD 589, the Chinese scholar-official Yan Zhitui (531-591 AD) once wrote: "Paper on which there are quotations or commentaries from Five Classics or the names of sages, I dare not use for toilet purposes". An Arab traveler to China once wrote of the curious Chinese tradition of toilet paper in AD 851, writing: "They (the Chinese) are not careful about cleanliness, and they do not wash themselves with water when they have done their necessities; but they only wipe themselves with paper". Toilet paper continued to be a valued necessity in China, since it was during the Hongwu Emperor's reign in AD 1393 that the Bureau of Imperial Supplies (Bao Chao Si) manufactured 720,000 sheets of toilet paper for the entire court (produced of the cheap rice-straw paper). For the emperor's family alone, 15,000 special sheets of paper were made, in light yellow tint and even perfumed. Even at the beginning of the 14th century, during the middle of the Yuan Dynasty, the amount of toilet paper manufactured for modern-day Zhejiang province alone amounted to ten million packages holding 1000 to 10000 sheets of toilet paper each.

The Bencao on traditional Chinese medicine; printed with woodblock in 1249, Song Dynasty

During the Tang Dynasty (AD 618-907) paper was folded and sewn into square bags to preserve the flavor of tea. During the same period, it was written that tea was served from baskets with multi-colored paper cups and paper napkins of different size and shape. During the Chinese Song Dynasty (AD 960-1279) not only did the government produce the world's first known paper-printed money, or banknote (see Jiaozi and Huizi), but paper money bestowed as gifts to deserving government officials were wrapped in special paper envelopes.

Paper spread slowly outside of China; other East Asian cultures, even after seeing paper, could not make it themselves. Instruction in the manufacturing process was required, and the Chinese were reluctant to share their secrets. The paper was thin and translucent, not like modern western paper, and thus only written on one side. The technology was first transferred to Korea in 604 and then imported to Japan by Buddhist priests, around 610, where fibres (called bast) from the mulberry tree were used.

Papermaking arrives in the Middle East

After further commercial trading and the defeat of the Chinese in the Battle of Talas in 751, the invention spread to the Middle East. Production was started in Baghdad, where the Arabs invented a method to make a thicker sheet of paper. The manufacture had spread to Damascus by the time of the First Crusade in 1096; but the wars interrupted production, and it split into two centers. Cairo continued with the thicker paper. Iran became the center of the thinner papers. It was also adopted in India. The first paper mill in Europe was in Spain, at Xátiva (modern Valencia) in 1120. More mills appeared in Fabriano Italy in about the 13th century, as an import from Islamic Spain. They used hemp and linen rags as a source of fibre. The oldest known paper document in the West is the Mozarab Missal of Silos from the 11th century, probably written in the Islamic part of Spain. Paper is recorded as being manufactured in both Italy and Germany by 1400, just about the time when the woodcut printmaking technique was transferred from fabric to paper in the old master print and popular prints.

Some historians speculate that paper was a key element in 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 subsequent centuries due to the invention of paper; and Europe advanced during the Renaissance due to the introduction of paper and the printing press.

Nineteenth Century advances in papermaking

Paper remained expensive, at least in book-sized quantities, 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. With the introduction of cheaper paper, schoolbooks, fiction, non-fiction, and newspapers became gradually available by 1900. Cheap wood based paper also meant that keeping personal diaries or writing letters became possible and so, by 1850, the clerk, or writer, ceased to be a high-status job.

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. Mass-market paperback books still use these cheaper mechanical papers (see below), but book publishers can now use acid-free paper for hardback and trade paperback books.

Papermaking

Main article: Papermaking

Chemical pulping

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 that it may be washed from the cellulose fibers. Because lignin holds the plant cells together, chemical pulping frees the fibres and makes pulp. The pulp must be bleached to produce white paper for printing, painting 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 fibre 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. Some chemicals used in chemical pulping are also known to be carcinogenic. [3]

Papers made from chemical wood-based pulps are also unhelpfully known as woodfree papers.

The Kraft process is the most commonly practiced strategy for pulp manufacturing and produces especially strong, unbleached papers that can be used directly for bags and boxes but are often processed further, e.g. to make corrugated cardboard.

Mechanical pulping

There are two major mechanical pulps, thermomechanical pulp (TMP) and mechanical pulp. The latter is known in the USA as groundwood pulp. In the TMP process, wood is chipped and then fed into large steam-heated refiners where the chips are squeezed and fibreized between two steel discs. In the groundwood process, debarked logs are fed into grinders where they are pressed against rotating stones and fibreized. 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 fibre lengths and produce weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than chemical pulp.

Recycled paper

Paper recycling processes can use either chemical or mechanical pulp. By mixing with water and applying mechanical action the hydrogen bonds in the paper can be broken and fibres separated again. Most recycled paper contains a proportion of virgin fibre in the interests of quality.

There are three main classifications of recycled fibre:.

• Mill Broke or Internal Mill Waste - this incorporates any substandard or grade-change paper made within the paper mill which then goes back into the manufacturing system to be repulped back into paper. Such out-of-specification paper is not sold and is therefore often not classified as genuine reclaimed recycled fibre. However, most paper mills have been recycling their own waste fibre for many years, long before recycling become popular.

• Preconsumer Waste - this is offcuts and processing waste, such as guillotine trims and envelope blank waste. This waste is generated outside the paper mill and could potentially go to landfill, and is a genuine recycled fibre source. Also includes deinked preconsumer (recycled material that has been printed but did not reach its intended end use, such as waste from printers and unsold publications).

• Postconsumer waste - this is fibre from paper which has been used for its intended end use and would include office waste, magazine papers and newsprint. As the vast majority of this paper has been printed (either digitally or by more conventional means such as litho or gravure), it will either be recycled as printed paper or go through a de-inking process first.

Recycled Papers can be made from 100% recycled materials or blended with virgin pulp. Recycled papers are (generally) not as strong nor as bright as papers made from virgin pulp.

Additives

Besides the fibres, pulps may contain fillers such as chalk or china clay, which improve the characteristics of the paper for printing or writing. Additives for sizing purposes may be mixed into the pulp and/or applied to the paper web later in the manufacturing process. The purpose of sizing is to establish the correct level of surface absorbency to suit the ink or paint.

Drying

After the paper web is produced, the water must be removed from it by pressing and drying.

Pressing the sheet removes the water by force. Once the water is forced from the sheet, felt (not to be confused with the traditional felt) is used to collect the water. 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 dryers can heat to temperatures above 200°F (93°C) 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.

Finishing

The paper may then undergo sizing to alter its physical properties for use in various applications.

Paper at this point is uncoated. Coated paper has a thin layer of material such as china clay applied to one or both sides in order to create a surface more suitable for high-resolution halftone screens. (Uncoated papers are rarely suitable for screens above 150 lpi.) Coated or uncoated papers may have their surfaces polished by calendering. Coated papers are divided into matt, semi-matt or silk, and gloss. Gloss papers give the highest optical density in the printed image.

The paper is then fed onto reels if it is to be used on web printing presses, or cut into sheets for other printing processes or other purposes. The fibres in the paper basically run in the machine direction. Sheets are usually cut "long-grain", i.e. with the grain parallel to the longer dimension of the sheet.

All paper produced by Fourdrinier-type machines is wove paper, i.e. the wire mesh that transports the web leaves a pattern that has the same density along the paper grain and across the grain. Textured finishes, watermarks and wire patterns imitating hand-made laid paper can be created by the use of appropriate rollers in the later stages of the machine.

Wove paper does not exhibit "laidlines", which are small regular lines left behind on paper when it was handmade in a mould made from rows of metal wires or bamboo. Laidlines are very close together. They run perpendicular to the "chainlines", which are further apart. Handmade paper similarly exhibits "deckle edges", or rough and feathery borders.

Applications

A Paper cutter

• 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.

Paper can be produced with a wide variety of properties, depending on its intended use.

• To represent a value: paper money, bank note, cheque, security (see Security paper), voucher and ticket
• For entertainment: book, magazine, newspaper, art, zine,
• For packaging: corrugated box, paper bag, envelope, wrapping tissue, Charta emporetica and wallpaper
• For cleaning: toilet paper, handkerchiefs, paper towels, facial tissue and cat litter
• For construction: papier-mâché, origami, quilling, Paper honeycomb, used as a core material in composite materials, paper engineering, construction paper and clothing
• Other uses: emery paper, sandpaper, blotting paper, litmus paper, universal indicator paper, paper chromatography and Capacitor Dielectrics (Permittivity 1.5 - 3)

Types and weight

Card and paper stock for craft use comes in a wide variety of textures and colors.

Paper is often characterized by weight. The weight assigned to a paper is the weight of a ream, 500 sheets, of varying "basic sizes", before the paper is cut into the size it is sold to end customers. For example, a ream of 20 lb, 8½ x 11" paper weighs 5 pounds, because it has been cut from a larger sheet into four pieces. In the United States, printing paper is generally 20 lb, 24 lb, or 32 lb at most. Cover stock is generally 68 lb, and 110 lb or more is considered card stock.

In Europe the weight is expressed in grammes per square metre (gsm or usually just g) of the paper. Printing paper is generally between 60g and 120g. Anything heavier than 160g is considered card. The weight of a ream with therefore depends on the dimensions of the paper; One ream of A4 (210mm x 297mm) size (approx 8.27" x 11.7") weighs 2.5 kilogrammes (approx 5.5 pounds).

The density of paper ranges from 250 kg/m³ (16 lb/ft³) for tissue paper to 1500 kg/m³ (94 lb/ft³) for some speciality paper. Printing paper is about 800 kg/m³ (50 lb/ft³).

• Bank paper
• Bond paper
• Book paper
• Construction paper/sugar paper
• Cotton paper
• Electronic paper
• Fish paper (Vulcanized fibres for electrical insulation)
• Inkjet paper
• Kraft paper
• Laid paper
• Leather paper
• mummy paper

• Tyvek paper
• Paper towels
• Wallpaper
• Washi
• Wax paper
• Wet and Dry Paper
• Wove paper
• Coated paper: glossy and matt surface

The future of paper

Some manufacturers, notably AMD, have started using a new, significantly 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. [4]

With increasing environmental concerns about synthetic coatings (such as PFOA) and the higher prices of hydrocarbon based petrochemicals, there is a focus on zein (corn protein) as a coating for paper in high grease applications such as popcorn bags. [5]

Besides paperfoam, paper made from rocks rather than trees is also emerging as a more ecological alternative to regular paper made from trees or other alternatives as paperfoam. This Rock paper is available from companies as ViaStone and John Su.

Invisible paper is being developed by Gaskell Industries Ltd., to be used as a substitute to more visible alternatives.

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

References and Notes

• Needham, Joseph (1986). Science and Civilization in China: Volume 5, Chemicals and Chemical Technology, Part 1, Paper and Printing. New York: Cambridge University Press, 1985. (also published in Taipei: Caves Books, Ltd., 1986.)

also referred to as:

• Tsien, Tsuen-Hsuin, '"Paper and Printing," vol. 5 part 1 of Needham, Joseph Science and Civilization in China:. Cambridge University Press, 1986. ISBN 0521086906. (also published in Taipei: Caves Books, Ltd., 1986.)
• "Document Doubles" in Detecting the Truth: Fakes, Forgeries and Trickery, a virtual museum exhibition at Library and Archives Canada

See also

• Arches paper
• Buckypaper
• Graphene Oxide Paper
• Paperclip
• Paper recycling
• Paper size

External links

• TAPPI Technical Association of the Pulp and Paper Industry
• How is paper made? at The Straight Dope, 22 November 2005
• How Paper Is Made
• United States Government Printing Office: Government Paper Specification Standards