Paper is a thin sheet of matted cellulose fibers. Largely derived from lignocellulose, paper is created from a pulp dissolved into a slurry that is drained and dried into sheets. Different types of paper are defined by constituent fiber, paper pulp, sizing, coating, paper size, paper density and grammage. The papermaking process developed in East Asia at least as early as 105 CE by the Han court eunuch Cai Lun, although archaeological evidence exists of 2nd century BCE paper-like material in China. Before the industrialization of paper production, the most common paper was rag paper, made from discarded natural fiber textiles collected by ragpickers. The 1843 invention of wood pulp, coupled with the Second Industrial Revolution, made pulpwood paper the dominant variety to this day.
Etymology
The word paper is etymologically derived from Latin papyrus, which comes from the Greek πᾰ́πῡρος (pápūros), the word for the Cyperus papyrus plant. Papyrus is a thick, paper-like material produced from the pith of the Cyperus papyrus plant, which was used in ancient Egypt and other Mediterranean cultures for writing before the introduction of paper. Although the word paper is etymologically derived from papyrus, the two are separate technological developments that use different materials and production methods. Papyrus is a lamination of natural plant fibre, while paper is manufactured from fibres whose properties have been changed by maceration.
History
Precursors
Paper was preceded by and coexisted with other early writing materials, such as papyrus, parchment, vellum, barkcloth, birch bark, palm leaves, and bamboo and wooden slips. Papyrus, superficially similar to paper, has several downsides that eventually caused it to be replaced by paper: It was geographically limited to a plant primarily grown in Egypt; it was both more expensive and laborious to produce compared to paper; and it was more fragile and sensitive to moisture, making it prone to break apart in damp conditions.
Invention and development
The oldest known archaeological fragments of the immediate precursor to modern paper date to the 2nd century BCE in China. The pulp papermaking process is ascribed to Cai Lun, a 2nd-century CE Han court eunuch. Popular history points to the Battle of Talas in 751 CE as the moment when papermaking spread to the Islamic world, claiming that Tang dynasty papermakers were captured as prisoners and used to extract 'the secrets' of papermaking. However, an archive of 76 paper writings in Sogdian, Arabic, and Chinese has been found in Panjikent, left behind by the ruler Divashtich before the city's capture by Arabs in 722. When Muslims first encountered paper in the 7th and 8th centuries, it had already been in the region for centuries. Sogdian writing on paper has also been found dating to 313 CE in Dunhuang. Paper was being made in Baghdad by the late 8th century. In the 13th century, knowledge of and use of paper spread from the Middle East to medieval Europe, where the first water-powered paper mills were built. Because paper was introduced to the West through the city of Baghdad, it was first called bagdatikos. In the 19th century, industrialization greatly reduced the cost of manufacturing paper. In 1844, the Canadian inventor Charles Fenerty and the German inventor Friedrich Gottlob Keller independently developed processes for pulping wood fibres.
Papermaking
Pulp
Pulp is a lignocellulosic mixture of isolated fibers. Traditional low-lignin pulp sources like rags and paper mulberry can be mechanically broken down; industrial pulpmaking largely makes use of pulpwood, which can be pulped chemically or mechanically.
Chemical pulping
To make pulp from wood, a chemical pulping process separates lignin from cellulose fibre. A cooking liquor is used to dissolve the lignin, which is then washed from the cellulose; this preserves the length of the cellulose fibres. Paper made from chemical pulps is also known as wood-free paper (not to be confused with tree-free paper), because it does not contain lignin, which deteriorates over time. The pulp can also be bleached to produce white paper, but this consumes 5% of the fibres. Chemical pulping processes are not used to make paper made from cotton, which is already 90% cellulose.
There are three main chemical pulping processes: the sulfite process dates back to the 1840s and was the dominant method before the Second World War. The kraft process, invented in the 1870s and first used in the 1890s, is now the most commonly practised strategy; one of its advantages is that the chemical reaction with lignin produces heat, which can be used to run a generator. Most pulping operations using the kraft process are net contributors to the electricity grid or use the electricity to run an adjacent paper mill. Another advantage is that this process recovers and reuses all inorganic chemical reagents. Soda pulping is another specialty process used to pulp straws, bagasse and hardwoods with high silicate content.
Mechanical pulping There are two major mechanical pulps: thermomechanical pulp (TMP) and groundwood pulp (GW). In the TMP process, wood is chipped and then fed into steam-heated refiners, where the chips are squeezed and converted to fibres between two steel discs. In the groundwood process, debarked logs are fed into grinders, where they are pressed against rotating stones to produce fibres. Mechanical pulping does not remove the lignin, so the yield is very high (>95%); however, lignin causes the paper thus produced to turn yellow and become brittle over time. Mechanical pulps have rather short fibres, thus producing weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than the chemical kind.
Recycling and de-inked pulp A process for removing printing inks from recycled paper was invented by German jurist Justus Claproth in 1774. Today this method is called deinking. Paper recycling processes can use either chemically or mechanically produced pulp; by mixing it 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 for the sake of quality; generally speaking, de-inked pulp is of the same quality or lower than the collected paper it was made from. There are three main classifications of recycled fibre:
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