El papel es un material en láminas delgadas producido mediante el procesamiento mecánico o químico de fibras de celulosa derivadas de madera , trapos , pastos u otras fuentes vegetales en agua , drenando el agua a través de una malla fina dejando la fibra distribuida uniformemente en la superficie, seguido de prensado y secado. . Aunque originalmente el papel se fabricaba a mano en hojas sueltas, ahora casi todo se fabrica en máquinas grandes: algunas fabrican bobinas de 10 metros de ancho, que funcionan a 2.000 metros por minuto y hasta 600.000 toneladas al año. [ no verificado en el cuerpo ] Es un material versátil con muchos usos, que incluyen impresión , pintura, gráficos, señalización, diseño, embalaje, decoración, escritura y limpieza . También se puede utilizar como papel de filtro, papel tapiz, guarda libros, papel de conservación, encimeras laminadas, papel higiénico, moneda y papel de seguridad, o en diversos procesos industriales y de construcción.
El proceso de fabricación de papel se desarrolló en el este de Asia, probablemente China , al menos ya en el año 105 d.C. , [1] por el eunuco de la corte Han Cai Lun , aunque los primeros fragmentos arqueológicos de papel se derivan del siglo II a.C. en China. [2] La industria moderna de la pulpa y el papel es global, con China liderando su producción y Estados Unidos siguiéndola.
Los fragmentos arqueológicos más antiguos conocidos del precursor inmediato del papel moderno datan del siglo II a. C. en China . El proceso de fabricación de papel con pulpa se atribuye a Cai Lun , un eunuco de la corte Han del siglo II d.C. [2]
Se ha dicho que el conocimiento de la fabricación de papel pasó al mundo islámico después de la Batalla de Talas en el año 751 EC, cuando dos fabricantes de papel chinos fueron capturados como prisioneros. Aunque la veracidad de esta historia es incierta, poco después se empezó a fabricar papel en Samarcanda . [3] En el siglo XIII, el conocimiento y los usos del papel se extendieron desde Oriente Medio hasta la Europa medieval , donde se construyeron las primeras fábricas de papel accionadas por agua . [4] Debido a que el papel fue introducido en Occidente a través de la ciudad de Bagdad , primero se le llamó bagdatikos . [5] En el siglo XIX, la industrialización redujo considerablemente el coste de fabricación del papel. En 1844, el inventor canadiense Charles Fenerty y el inventor alemán Friedrich Gottlob Keller desarrollaron de forma independiente procesos para la fabricación de pulpa de fibras de madera. [6]
Antes de la industrialización de la producción de papel, la fuente de fibra más común eran las fibras recicladas de textiles usados, llamados trapos. Los trapos eran de cáñamo , lino y algodón . [7] El jurista alemán Justus Claproth inventó un proceso para eliminar las tintas de impresión del papel reciclado en 1774. [7] Hoy en día, este método se llama destintado . No fue hasta la introducción de la pulpa de madera en 1843 que la producción de papel no dependió de los materiales reciclados de los traperos . [7]
La palabra papel deriva etimológicamente del latín papiro , que proviene del griego πᾰ́πῡρος ( pápūros ), palabra para la planta Cyperus papyrus . [8] [9] El papiro es un material grueso parecido al papel producido a partir de la médula de la planta Cyperus papyrus , que se utilizaba en el antiguo Egipto y otras culturas mediterráneas para escribir antes de la introducción del papel. [10] Aunque la palabra papel se deriva etimológicamente de papiro , las dos se producen de manera muy diferente y el desarrollo del primero es distinto del desarrollo del segundo. El papiro es una laminación de fibras vegetales naturales, mientras que el papel se fabrica a partir de fibras cuyas propiedades han sido modificadas mediante la maceración. [2]
Para fabricar pulpa de madera, un proceso químico de pulpa separa la lignina de la fibra de celulosa . Se utiliza un licor de cocción para disolver la lignina , que luego se elimina de la celulosa por lavado ; esto preserva la longitud de las fibras de celulosa. Los papeles elaborados a partir de pulpas químicas también se conocen como papeles libres de madera (no confundir con el papel libre de árboles ); esto se debe a que no contienen lignina, la cual se deteriora con el tiempo. La pulpa también se puede blanquear para producir papel blanco, pero esto consume el 5% de las fibras. Para fabricar papel elaborado a partir de algodón, que ya tiene un 90% de celulosa, no se utilizan procesos químicos de despulpado.
Hay tres procesos químicos principales de fabricación de pulpa: el proceso de sulfito se remonta a la década de 1840 y era el método dominante antes de la Segunda Guerra Mundial. El proceso kraft , inventado en la década de 1870 y utilizado por primera vez en la década de 1890, es ahora la estrategia más practicada; Una de sus ventajas es que la reacción química con la lignina produce calor, que puede utilizarse para hacer funcionar un generador. La mayoría de las operaciones de pulpa que utilizan el proceso kraft contribuyen netos a la red eléctrica o utilizan la electricidad para hacer funcionar una fábrica de papel adyacente. Otra ventaja es que este proceso recupera y reutiliza todos los reactivos químicos inorgánicos. La pulpa con soda es otro proceso especializado que se utiliza para despulpar paja , bagazo y maderas duras con alto contenido de silicato .
Existen dos grandes pastas mecánicas: la pasta termomecánica (TMP) y la pasta de madera triturada (GW). En el proceso TMP, la madera se astilla y luego se introduce en refinadores calentados con vapor, donde las astillas se comprimen y se convierten en fibras entre dos discos de acero. En el proceso de trituración de madera, los troncos descortezados se introducen en trituradoras donde se presionan contra piedras giratorias para convertirlos en fibras. La pulpa mecánica no elimina la lignina , por lo que el rendimiento es muy alto, > 95%; sin embargo, la lignina hace que el papel así producido se vuelva amarillo y quebradizo con el tiempo. Las pastas mecánicas tienen fibras bastante cortas, lo que produce un papel débil. Aunque se requieren grandes cantidades de energía eléctrica para producir pulpa mecánica, cuesta menos que la pulpa química.
Los procesos de reciclaje de papel pueden utilizar pulpa producida química o mecánicamente; mezclándolo con agua y aplicando acción mecánica se pueden romper los enlaces de hidrógeno del papel y separar nuevamente las fibras. La mayoría del papel reciclado contiene una proporción de fibra virgen por motivos de calidad; En términos generales, la pulpa destintada es de la misma calidad o inferior que el papel recolectado con el que se fabricó.
Existen tres clasificaciones principales de fibra reciclada:
Los papeles reciclados pueden fabricarse a partir de materiales 100% reciclados o mezclarse con pulpa virgen, aunque (generalmente) no son tan fuertes ni tan brillantes como los papeles elaborados con esta última.
Además de las fibras, las pastas pueden contener cargas como tiza o caolín , [12] que mejoran sus características para imprimir o escribir. [13] Los aditivos para fines de apresto pueden mezclarse con él o aplicarse a la banda de papel más adelante en el proceso de fabricación; el propósito de dicho dimensionamiento es establecer el nivel correcto de absorbencia de la superficie para adaptarse a la tinta o pintura.
La pulpa se alimenta a una máquina papelera, donde se forma como una banda de papel y se le elimina el agua mediante prensado y secado.
Al presionar la lámina se elimina el agua a la fuerza. Una vez expulsada el agua de la lámina, se utiliza un tipo especial de fieltro, que no debe confundirse con el tradicional, para recoger el agua. Cuando se hace papel a mano, se utiliza una hoja secante.
Drying involves using air or heat to remove water from the paper sheets. In the earliest days of papermaking, this was done by hanging the 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 can reach temperatures above 93 °C (200 °F) and are used in long sequences of more than forty cans where the heat produced by these can easily dry the paper to less than six percent moisture.
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 calcium carbonate or 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 matte, semi-matte 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. Continuous form paper (or continuous stationery) is cut to width with holes punched at the edges, and folded into stacks.
All paper produced by paper machines such as the Fourdrinier Machine are 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.[14]
Paper can be produced with a wide variety of properties, depending on its intended use.
It is estimated that paper-based storage solutions captured 0.33% of the total in 1986 and only 0.007% in 2007, even though in absolute terms the world's capacity to store information on paper increased from 8.7 to 19.4 petabytes.[15] It is estimated that in 1986 paper-based postal letters represented less than 0.05% of the world's telecommunication capacity, with sharply decreasing tendency after the massive introduction of digital technologies.[15]
Paper has a major role in the visual arts. It is used by itself to form two- and three-dimensional shapes and collages.[16][17] It has also evolved to being a structural material used in furniture design.[18] Watercolor paper has a long history of production and use.
The thickness of paper is often measured by caliper, which is typically given in thousandths of an inch in the United States and in micrometres (μm) in the rest of the world.[19] Paper may be between 0.07 and 0.18 millimetres (0.0028 and 0.0071 in) thick.[20]
Paper is often characterized by weight. In the United States, the weight is the weight of a ream (bundle of 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, 8.5 in × 11 in (216 mm × 279 mm) paper weighs 5 pounds because it has been cut from larger sheets into four pieces.[21] In the United States, printing paper is generally 20 lb, 24 lb, 28 lb, or 32 lb at most. Cover stock is generally 68 lb, and 110 lb or more is considered card stock.
In Europe and other regions using the ISO 216 paper-sizing system, the weight is expressed in grams per square metre (g/m2 or usually gsm) of the paper. Printing paper is generally between 60 gsm and 120 gsm. Anything heavier than 160 gsm is considered card. The weight of a ream therefore depends on the dimensions of the paper and its thickness.
Most commercial paper sold in North America is cut to standard paper sizes based on customary units and is defined by the length and width of a sheet of paper.
The ISO 216 system used in most other countries is based on the surface area of a sheet of paper, not on a sheet's width and length. It was first adopted in Germany in 1922 and generally spread as nations adopted the metric system. The largest standard size paper is A0 (A zero), measuring one square metre (approx. 1189 × 841 mm). A1 is half the size of a sheet of A0 (i.e., 594 mm × 841 mm), such that two sheets of A1 placed side by side are equal to one sheet of A0. A2 is half the size of a sheet of A1, and so forth. Common sizes used in the office and the home are A4 and A3 (A3 is the size of two A4 sheets).
The density of paper ranges from 250 kg/m3 (16 lb/cu ft) for tissue paper to 1500 kg/m3 (94 lb/cu ft) for some specialty paper. Printing paper is about 800 kg/m3 (50 lb/cu ft).[22]
Paper may be classified into seven categories:[23]
Some paper types include:
Much of the early paper made from wood pulp contained significant amounts of alum, a variety of aluminium sulfate salt that is significantly acidic. Alum was added to paper to assist in sizing,[24] making it somewhat water resistant so that inks did not "run" or spread uncontrollably. Early papermakers did not realize that the alum they added liberally to cure almost every problem encountered in making their product would be eventually detrimental.[25] The cellulose fibres that make up paper are hydrolyzed by acid, and the presence of alum eventually degrades the fibres until the acidic paper disintegrates in a process known as "slow fire". Documents written on rag paper are significantly more stable. The use of non-acidic additives to make paper is becoming more prevalent, and the stability of these papers is less of an issue.
Paper made from mechanical pulp contains significant amounts of lignin, a major component in wood. In the presence of light and oxygen, lignin reacts to give yellow materials,[26] which is why newsprint and other mechanical paper yellows with age. Paper made from bleached kraft or sulfite pulps does not contain significant amounts of lignin and is therefore better suited for books, documents and other applications where whiteness of the paper is essential.
Paper made from wood pulp is not necessarily less durable than a rag paper. The aging behaviour of a paper is determined by its manufacture, not the original source of the fibres.[27] Furthermore, tests sponsored by the Library of Congress prove that all paper is at risk of acid decay, because cellulose itself produces formic, acetic, lactic and oxalic acids.[28]
Mechanical pulping yields almost a tonne of pulp per tonne of dry wood used, which is why mechanical pulps are sometimes referred to as "high yield" pulps. With almost twice the yield as chemical pulping, mechanical pulps is often cheaper. Mass-market paperback books and newspapers tend to use mechanical papers. Book publishers tend to use acid-free paper, made from fully bleached chemical pulps for hardback and trade paperback books.
The production and use of paper has a number of adverse effects on the environment.
Worldwide consumption of paper has risen by 400% in the past 40 years[clarification needed] leading to increase in deforestation, with 35% of harvested trees being used for paper manufacture. Most paper companies also plant trees to help regrow forests. Logging of old growth forests accounts for less than 10% of wood pulp,[29] but is one of the most controversial issues.
Paper waste accounts for up to 40% of total waste produced in the United States each year, which adds up to 71.6 million tons of paper waste per year in the United States alone.[30] The average office worker in the US prints 31 pages every day.[31] Americans also use in the order of 16 billion paper cups per year.
Conventional bleaching of wood pulp using elemental chlorine produces and releases into the environment large amounts of chlorinated organic compounds, including chlorinated dioxins.[32] Dioxins are recognized as a persistent environmental pollutant, regulated internationally by the Stockholm Convention on Persistent Organic Pollutants. Dioxins are highly toxic, and health effects on humans include reproductive, developmental, immune and hormonal problems. They are known to be carcinogenic. Over 90% of human exposure is through food, primarily meat, dairy, fish and shellfish, as dioxins accumulate in the food chain in the fatty tissue of animals.[33]
The paper pulp and print industries emitted together about 1% of world greenhouse-gas emissions in 2010[34] and about 0.9% in 2012.[35]
In the 2022−2024 edition of the annual "Pulp and paper capacites survey", the Food and Agriculture Organization of the United Nations (FAO) reports that Asia has superseded North America as the top pulp and paper producing continent. [36]
FAO figures for 2021 show the production of graphic papers continuing its decline from a mid-2000s peak to hover below 100 million tonnes a year. By contrast, the production of other papers and paperboard – which includes cardboard and sanitary products – has continued to soar, exceeding 320 million tonnes.[36]
FAO has documented the expanding production of cardboard in paper and paperboard, which has been increasing in response to the spread of e-commerce since the 2010s.[36] Data from FAO suggest that it has been even further boosted by COVID-19-related lockdowns.[37]
Some manufacturers have started using a new, significantly more environmentally friendly alternative to expanded plastic packaging. Made out of paper, and known commercially as PaperFoam, the new packaging has mechanical properties very similar to those of some expanded plastic packaging, but is biodegradable and can also be recycled with ordinary paper.[38]
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.[39]
Also, synthetics such as Tyvek and Teslin have been introduced as printing media as a more durable material than paper.
Research by the Library of Congress has demonstrated that cellulose itself generates acids as it ages, including formic, acetic, lactic, and oxalic acids
More than 90% of human exposure is through food
