PAPER as a SHEET MATERIAL

Post 646 –by Gautam Shah

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Paper’s chief raw material is cellulosic pulp.  Paper is a sheet form of material, and substantially used in sheet-form. It is also used in ‘non-planer’ forms, such as moulded products (egg crates), packing cases (glass), mould dummies, and as Papier-mâché.

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MDF_Sample

Paper as a sheet material is available with many different properties. It can be rough, smooth, grease-proof, water absorbent, water repellent or resistant, soft as cotton, stiff as board, heat resistant, fireproof, combustible, chemically resistant opaque, translucent, transparent, coloured, glossy, dull, strong, weak, tear-able, non-tear-able, light heavy, pulp-able cellular, waxed, sanded, embossed hinged corrugated, easily folded and pierced, coarse, fine or flocked.

Paper is mainly used for writing, printing, drawing, painting signs and images. Paper has many functional uses like wrapping, filtering, absorbing, insulating, protecting (Thai umbrellas), cleaning, mopping, polishing, buffing, toys and product forming, mould making, engraving, etching, embossing, medicare dressing, garment making, and for glazing (Shoji for windows and Fusuma for room dividers). Other uses include mask making, light canoe or boat making for races, single-use construction forms, casting die dummies, kites, lanterns carnival floats, tubes, textile bobbins and cones.

Pen_box,_signed_by_Mohammad-e_Ebrahim,_Iran,_1694_AG D,_papier_mache,_oil_paint_-_Aga_Khan_Museum_-_Toronto,_Canada_-_DSC07051

Pen box of Papier Mache with Oil coating Iran 1694 (Now in AK Museum Canada)

Paper pulp is used in various sheet form composites. Fiber boards are products engineered at a pulp stage. Various products differ in terms of nature and level of ‘pulping’, pressing technologies (pressure, temperature, curing used), wet or dry process of manufacturing, additives (both filler and bonding) and surface treatments. The products include high-medium-low density boards (typically MDF), hardboard, ‘Masonite’ boards, pulp boards with gypsum, cement and other minerals, natural and synthetic fibre additives. These sheet materials are surface treated, coated, tempered, laminated, co-formed or co-extruded.

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Parchment Paper > Pixabay image by Geralt

Inferior plant materials and timber wastes are partly pulped to form a homogeneous mass. Such partly pulped mass, however lack the mutual particle bonding. Boards (and often pre-shaped forms) are created by steam-pressing and with aid of 5% bonding materials (typically Urea or Phenol formaldehyde). Portland cement, Gypsum and polymer emulsion adhesives are also used for forming building boards. Paper pulp boards of extreme light mass are coated with Gypsum, polymers and foam to form acoustic ceiling panels. Layered paper composites with phenolic compounds are used as circuit boards and electric insulation panels.

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Paper making in Hahnemuhle > Wikipedia image by Hahnemuhle PR

Structure of paper as sheet material differs from other sheet materials:

  1. Papers unlike plastic films and metal films are fibrous.
  2. Paper is composed of single short fibres, arranged largely at random instead of a regular array as is the case with woven fabrics.
  3. Unlike cloth, felt or leather it is laminar, that is each fibre is disposed mainly in the plane of the sheet.

Paper, however, resembles other sheet materials in that its structure is anisotropic in its plane and most of the fibres are oriented along the grain or the machine direction.

Paper is mostly made from cellulosic fibres derived from plant sources. The fibres depending on their origin have different types of cell structures, and so provide unique character to the paper. Cellulosic fibres are hygroscopic and swell considerably when wetted, but retain strength and durability. Most plant materials also contain non-fibrous elements or cells. These are less desirable for the paper making, but are useful as a filler material. Until about 19 C. paper was produced by hand processes, and as a result had very distinctive local style, texture and properties. Through the 18th C the paper making process remained essentially unchanged. The linen and cotton rags were the basic raw materials, but increasing demand for paper was posing shortage of pulp raw materials.

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Packaging forms with Papier Mache > Wikipedia images by Berklas

Paper is manufactured from material resources that can be regenerated, and the product is a recyclable material. Major sources of cellulosic fibres for paper manufacturing are wood and cotton. Cotton fibres are used in the form of lints (seed hair left behind after ginning), staples, waste yarn and threads and rags. Lints require no processing, staples need length shortening, but yarns, threads and rags need undoing of all mechanical processes such as spinning and weaving. Cotton fibres offer strength, durability, permanence, fine formation, colour, texture, and feel.

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Modular ceiling Panels of Paper Pulp > Wikipedia image by Adamantios

Wood pulp has been the chief material for paper making, but where forest resources were scant, many alternative sources have been explored. These sources include: Cereal straws, plant stems, linen, jute, hemp, bamboo, cane (rattan), paddy (rice) straws, banana leaf, sugar cane waste bagasse and grasses like esparto. Paper made from such alternative pulps, and without an admixture of other fibre tend to be dense and stiff, with low tear resistance and low opacity. Often such fibres are desired as additives for producing paper for abrasives (sand-paper), cover stock and heavy-duty industrial papers. Such fibres are also used for strength in duplicating and manifold papers. Flax is grown expressly for high-grade cigarette paper.

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Synthetics: Paper like sheets > (https://pixabay.com/en/paper-colorful-color-school-paint-182220/) Image by AlexanderStein

Synthetic or man-made fibres provide certain advantage when compared to plant based materials for paper pulp. Natural cellulose fibres vary considerably in size and shape, whereas synthetic fibres can be made uniform and of selected length and diameter. Long fibres, for example, are necessary in producing strong, durable papers. There are limitations, however, to the length of synthetic fibres that may be formed from suspension in water because of their tendency to tangle and to rope together. Even so, papers have been made experimentally with fibres several times longer than those typical of wood pulp, and these papers have improved strength and softness properties. Natural cellulose fibres have limited resistance to chemical attack and exposure to heat. For such purposes synthetic fibre papers can be made resistant to strong acids, for example in chemical filtration. Paper can even be made from glass fibre, and such paper have great resistance to both the heat and chemicals.

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Golf ball rest pins of dissoluble wood Pulp (http://maxpixel.freegreatpicture.com/Golfing-Tee-Golf-Golfer-880532)

Rags (mainly of cotton) are used extensively where permanence is of prime importance such as for bank notes, legal documents and security certificates. Technical papers include tracing papers, vellums, and reproduction papers, high-grade bond letterheads, cigarettes, carbon, and Bible papers. Khadi (Indian hand made) paper is an example of high rag content paper.

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Rags sorting for paper making > Image by Lewis Hine (1874-1940)

Wastepaper is a major source for cellulose. By recycling the wastepaper the dependency for virgin fibre is reduced and the problem of solid waste disposal is minimized. However the difficulties like, gathering wastepaper from scattered sources, sorting mixed papers, and recovering the fibre from many types of coated and treated papers, make it a very complex problem. Waste Paper treatments for asphalt, synthetic adhesives, metal foils, plastic and cellulose-derivative films and coatings, printing inks, etc. pose acute problems in reuse of paper wastes. Wastepaper is of four main categories: High-grade, old corrugated boxes, printed news papers, and mixed paper. High-grades and corrugated stocks originate mainly in mercantile and industrial establishments. White paper wastes accumulate in paper conversion units and printing plants. Magazine stock comes from newsstand returns, but some comes from homes. Mixed papers come from collectors. Grey Board, cardboard or Packing carton papers are produced from recycled paper wastes. These are as single or multiply boards.

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Laminated-multi-layered paper products > Wikipedia image by Veganbaking.net from USA

In recent years Papers have been coated, layered or co-extruded with many other forms of sheets, films and membranes. These include, metal foils, polymer films, metalized polymer films, films formed through liquid coatings, in-situ foam forming.

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PAPER -part 3

by Gautam Shah

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PAPER FORMING

Paper forming means forming or casting a sheet of paper from the pulp over a surface that will allow removal of water. The pulp nominally is 1 Lt of water and 1 to 10 grams of solid matter. For draining off the water a bed of wire mesh, deckle, fabric, etc. are used. The quality of pulp and the forming process determine the basic nature of paper. Later processes of conversion further modify the surface qualities.

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Hand-Paper making

The differences between various grades and types of paper are determined by:

1. Type of fibre or pulp

2. Degree of beating or refining of the pulp

3. Addition of various materials to the stock

4. Sheet formation method, basis weight or substance per unit area

5. Physical or chemical treatments after paper formation.

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Paper is produced by three basic processes:

a. Hand made or traditional way

b. Mould made or semi-industrial way

c. Machine made or fully industrialized way

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HAND MADE PAPERS

The basic process of making paper has not changed in more than 2,000 years. It involves two stages, the pulp forming and the formation of felted sheets. The pulp suspension is spread on a porous surface, to drain out the excess water. The pulp forming process may be partly mechanical or chemical but sheet formation is a hand operation.

In making paper pulp by hand, an appropriate mix of raw materials is placed in a vat or trough and is pounded with a heavy pestle or hammer to separate the fibres. During this phase the material is washed with running water to remove impurities. When the fibres are sufficiently broken up, they are kept in water suspension. At this stage the pulp, called half stuff, is ready for the actual process of paper making.

Paper is formed over a mould, a reinforced sheet of metal mesh having either a square mesh pattern, called a wove-pattern, or a pattern of more widely spaced longitudinal wires held together with smaller transverse wires, called a laid pattern. The mould pattern imprints itself on the finished sheet of paper. Handmade papers that are not given special finishes are identified as wove or laid papers, depending on the style of mould that is used in their making.

There are TWO processes for manufacturing hand made papers.

1. A framed wire mesh -mould, called a deckle is dipped into a steam heated pulp tub. When the frame is removed from the vat, the surface of the mould is coated with a thin film of fibre-water mixture. It is given a shake to orient the deposition of fibres in all directions. It also causes the individual fibres to interlock with those adjacent ones, giving strength to the sheet and allow drainage of much of the water from the mixture through the mould mesh. The frame along with the deposited mass is allowed to settle until the paper is sufficiently cohesive to permit its removal from the deckle. The paper is then transferred onto a felt blanket. Several layers of blankets are pressed together to remove the excess water. The sheets are then separated and allowed to dry in a natural or forced air ventilation system.

2. In the second process, the wire mesh frame is nearly similar. But instead of dipping the frame in the pulp tub, the pulp is allowed to float in horizontally, or is poured over a submerged mesh. The pulp is levelled with jerks. The entire frame is placed in a shaded area to drain and dry. Pressing if any occurs much later and often without interleaving of felt blankets.

A major structural feature of a paper web is the lack of uniformity in weight per unit area. Minimizing these variations involves skill. Dilute suspension of fibres (1 to 10 grams of dry solids per litre) in water is allowed to drain through a net (woven polyester fabric) to leave behind a coherent layer 6 to 18 mm deep and several metres wide are formed. A visible change occurs in the appearance of the stock, as it reaches a concentration of 20 g, dry solids per 1 litre, its surface ceases to be mobile, loses the liquid sheen and becomes matt. At this stage natural drainage stops, further forming is done by vacuum removal of water and pressing by a roller.

The sheets of paper are separated from the felts, stacked, and pressed. The process of pressing the stack of paper is repeated several times, and each time, and the stack is built up with the individual sheets in different orientations relative to one another. This procedure improves the surface of the finished paper and is called exchanging. The final stage in paper making is drying. The paper is hung, in groups of four or five sheets, over ropes in a special drying room until its moisture has almost completely evaporated.

Hand made papers are produced in small quantities, with local raw materials (often rare or scarce), and techniques (traditional, proprietary, rudimentary). Even where machine or chemical pulps are used, the unique style of forming gives a special flavour.

All hand made papers are unique in quality, texture and colour. Batch to batch acute variations gives a variegated character. Hand made papers have deckled edge on all the four sides, lesser thickness over the border regions, and multidirectional nonuniform grain structure. Hand made papers are less compact or dense. Hand made papers are naturally fluffy and absorbent unless heavily sized and severely calendered. Handmade paper is a reflection of its maker’s personality.

Hand made Papers are widely used for craft and decorative purposes. For writing or printing, however, it needs additional treatments following drying to make it less absorbent. The treatment consists of sizing the paper. Rough-textured papers are pressed lightly for a comparatively short period, and smooth-surfaced papers are pressed heavily for comparatively long periods.

Museum of Handmade Paper (Museo della Carta) View of Piazza del Duomo.

Museum of Handmade Paper (Museo della Carta) View of Piazza del Duomo.

The Museum of Handmade Paper, located in Mill Valley in the northern part of the modern town, celebrates the long-established paper making tradition in Amalfi. The town was one of the first centres of paper making in Europe, the skill having been acquired by the Amalfitans from the Arabs. The museum is housed in an ancient paper mill which was once owned by the Milano family, a family famous in Amalfi for its involvement in the production and manufacture of paper. In 1969 the building was converted into a museum as a result of the will of Nicholas Milano, the mill’s then owner. The museum contains the machinery and equipment (restored and fully functional) that was once used to manufacture paper by hand.

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MOULD MADE PAPER

Mould made papers are also produced on machines that are fairly automatized. The basic paper formation technique is like any hand made paper, but these are continuous one. A rotating mesh drum with part of the surface submerged in a pulp tub, draws in the pulp due to a partial vacuum. The drum continuously transfers the mass on the other side to felt-covered rollers which press the mass to drain out the excess water. The process is not only fast and continuous but provides a uniform output. Mould made papers have grain orientation along the length. Only two edges of a mould made paper are deckled. Mould made papers are fairly smooth, compact, of even thickness and uniform quality. Straw board sheets, rag papers, card papers, card boards, carton box papers, ledger paper, art work, graphics and lithography papers etc. are produced by this process. Most mould papers have one face with mesh impression while the other, face is comparatively smoother.

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MACHINE-MADE PAPER

Machine paper making is more complex, but basic processes are similar to hand-papermaking. Nominally paper machines can be divided into two main types: Cylinder machines and Fourdrinier’s machines.

Machine-made papers are produced in very large scale plants. Paper output is of very uniform quality, colour and thickness. Production of very wide (6 mts) and in large continuous lengths is possible. For the cheapest grades of paper, such as newsprint, ground-wood pulp alone is used. For better grades, chemical wood pulp, or a mixture of pulp and rag fibres, are employed. For the finest papers, such as the highest grades of writing papers, rag fibre alone is used.

Additives like colouring matter, sizing material such as rosin or glue, and fillers such as sulphate of lime or kaolin, which give added weight and body to the finished paper, are mixed into the pulp.

The face touching the wire mesh shows impression of wire compared to even face of the top side. In twin wire mesh processes two thin sheets are cast and their wire faces are joined together (usually without gums or sizing materials) to achieve both side smooth paper. Such sheets, because of natural lamination and disorientation of fibres show high strength, stability and tear resistance, as in currency notes.

Water-Mark

The dandy roll is a light unit lightly upon the wire and the surface of the sheet. Its function is to flatten the top surface of the sheet and improve the finish. Dandy rolls help create woven. laid and imprints names, insignia, or designs called watermarks. Paper watermarks have served to identify the makers of fine papers since the early day. A watermark is actually a thinner part of the sheet and is visible because of greater transmission of light.

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PAPER -part 2

by Gautam Shah

Earliest paper was made in China around 200 BC from a mass of entangled silk. Later (AD 105) it was produced using mulberry and other bast fibres, old rags, and hemp waste. The art of paper making was introduced in Japan in 610 AD, and into Central Asia about 750 AD. The first paper In Baghdad was made during 793, the golden age of Islamic culture,. Earliest paper in Europe was made in Spain, around 1036 AD., from linen and cotton rags. . By 14th C Several paper factories came up in Spain, Italy, France, and Germany. The demand for paper increased with the introduction of printing.

Till 18th C each region relied on local raw materials and so produced paper of local quality and style . By 19th C many pulping processes were innovated to overcome shortage of raw materials. The pulping processes related to separating fibres from wood by mechanical as well as chemical means.

Paper making

Nicolas-Louis Robert (1798) of France created first practical machines for paper manufacturing. Machine By 1875 papers created by machinery formed papers were used for new photoengraving process for printing halftones. Fourdrinier machines, (1884) produced long rolls of paper dried by suction, pressure, and heat. Pulp production by grinding wood, and chemical pulp processes offered cheaper raw materials.

Fourdrinier process

Earlier paper sheets were sized by a tedious impregnation process using animal glue or vegetable gums. But by 1800, paper sheets were sized with rosin and alum and bleached with chlorine.

Handmade Paper forming

Paper is mainly made from cellulosic fibres, derived from plant sources. The cell walls of all plants contain fibres of cellulose. It constitutes about one-third of the structural material of annual plants and about one-half that of perennial plants. The fibres depending on their origin have different types of cell structures and so provide unique character to the paper. Paper of some sort can be produced from almost natural plant, but qualitative requirements and economics of manufacturing, limit the sources of supply.

Pulp for Paper making

Cellulose for paper manufacturing is derived from many sources. Cellulose fibres have high strength and durability. They are readily wetted by water, exhibiting considerable swelling when saturated, and are hygroscopic. Even in the wet state, natural cellulose fibres show no loss in strength. Most plant materials also contain non-fibrous elements or cells. The non-fibrous cells are less desirable for the paper-making, than fibres, but mixed with fibre, are of some value such as the filling material in the paper sheet. It is the combination of these qualities with strength and flexibility that makes cellulose of unique value for paper manufacturing.

Fibre sources: Major sources of cellulose for paper manufacturing is wood. Cotton is the next important source of cellulose for paper making. Cotton fibres are used in the form of lints (seed hair left behind after ginning), staples, waste yarn and threads and rags. Lints require no processing, staples need length shortening, but yarns, threads and rags need undoing of all mechanical processes such as spinning and weaving. Cotton fibres are used where maximum strength, durability, and permanence, as well as fine formation, colour, texture, and feel, are required. These properties are attributed to the greater fineness, length, and purity of rag fibre as compared with most wood pulp. Rag papers are used extensively for bank note and security certificates, life insurance policies and legal documents, for which permanence is of prime importance, technical papers, such as tracing paper, vellums, and reproduction papers, high-grade bond letterheads, which must be impressive in appearance and texture, lightweight specialities such as cigarette, carbon, and Bible papers, and high-grade stationery, in which beauty, softness, and fine texture are desired. Khadi paper is an example of high rag content paper.

Linen, Jute, Hemp, various type of grass, bamboo, cane (rattan), paddy (rice) straw, banana leaf, sugar cane waste (bagasse), are some of the other sources for cellulose.

The cut and cleaned rags are cooked (to remove natural waxes, fillers, oils, and grease) in large cylindrical or spherical boilers. About three parts of cooking liquor, a dilute alkaline solution of lime and soda ash or caustic soda combined with wetting agents or detergents, is used with each part of rags. Steam is admitted to the boiler under pressure, and the contents are cooked for three to ten hours. Once cooked, the rags are washed, then mechanically beaten. The beating shortens the fibre, increases the swelling action of water to produce a softened and plastic fibre, and fibrillates or frays the fibre to increase its surface area. All of these actions contribute to better formation of the paper sheet, closer contact and inter-fibre bonding that gives the paper strength and coherence.

Waste-paper is a major source for cellulose. By recycling the waste-paper the dependency for virgin fibre is reduced and the problem of solid waste disposal is minimized. However, the difficulties like, gathering waste-paper from scattered sources, sorting mixed papers, and recovering the fibre from many types of coated and treated papers, make it a very complex problem. Paper treatments such as asphalt, synthetic adhesives, metal foils, plastic and cellulose-derivative films and coatings, printing inks, etc. pose acute problems in reuse of paper wastes.

Waste-paper is of four main categories: High-grade, old corrugated boxes, printed news papers, and mixed paper. High-grades and corrugated stocks originate mainly in mercantile and industrial establishments. White paper wastes accumulate in paper conversion units and printing plants. Magazine stock comes from news-stand returns, but some comes from homes. Mixed papers come from collectors. There are two distinct types of paper recovery systems: 1. recovery based upon de-inking and intended for printing-grade or other white papers, and 2. recovery without de-inking, intended for box-boards and coarse papers. Caustic soda, soda ash, silicate of soda, phosphates, and surfactant are used to remove the inks.

Natural fibres other than wood: Alternative sources for paper pulp have been in use even before wood pulp was available. Cereal straws, plant stems, paddy husk, grasses like esparto, bagasses (sugar cane), etc. are used in many parts of the world, that are deficient in forests and where such products are abundantly available. Non-woody plant stems differ from wood, as these contain less total cellulose, less lignin, and more of other materials. Papers made from these pulps without an admixture of other fibre tend to be dense and stiff, with low tear resistance and low opacity.

Flax, Hemp, Jute. Kenaf have been highly prized because of the strength and durability it imparts to such products as tags, abrasive paper (sandpaper), cover stock, and other heavy-duty paper. It is also used for duplicating and manifold paper, in which extremely light weight paper must have exceptional strength. Flax is grown expressly for high-grade cigarette paper.

Synthetic fibres: Synthetic or man-made fibres provide certain advantage when compared to plant based materials for paper pulp. Natural cellulose fibres vary considerably in size and shape, whereas synthetic fibres can be made uniform and of selected length and diameter. Long fibres, for example, are necessary in producing strong, durable papers. There are limitations, however, to the length of synthetic fibres that may be formed from suspension in water because of their tendency to tangle and to rope together. Even so, papers have been made experimentally with fibres several times longer than those typical of wood pulp, these papers have improved strength and softness properties. Natural cellulose fibres have limited resistance to chemical attack and exposure to heat. For such purposes synthetic fibre papers can be made resistant to strong acids, for example in chemical filtration. Paper can even be made from glass fibre, and such paper has great resistance to both the heat and chemicals. Natural fibres swell when beaten in water and cement together as they dry, synthetic fibres must be bonded by the addition of an adhesive, requiring an additional manufacturing step. Synthetic fibres are not affected by changes in moisture and produce dimensional ly stable papers.

Synthetic fibre felts (non-woven) are very similar to paper in construction. Yarn staples, carding waste, and filament cuts often re-carded, i.e., separated, combed and pressed to form a uniform, lightweight, and fragile web. This web, felt or the non woven-blankets if assimilated with heat or adhesive, a product similar to paper results.

Pulp is produced by both mechanical and chemical (alkali & acid type) processes. Wood is debarked cut into pieces, pulverized and pulped with water as the main suspending agent. A mechanical pulp sometimes contains several impurities like lignin, woody matter etc. Mechanical processes reduce the fiber length due to heavy handling of the mass. On the other hand chemical processes being less severe, do not reduce the fiber length. Removal of impurities’ is very good. However, adjustment of PH level is sometimes a problem. Chemical pulps are sometimes called wood free pulps, because there is a substantial removal of lignin and other woody matter. During pulping the mass is cleaned and bleached.

Mechanical or ground-wood pulp is made by subjecting wood to an abrading action, either by pressing the wood against revolving grinding stones or by passing the chips through a mill. The pulp stock flows from the grinder to a series of riffles and screens, which separate the heavy foreign material and pieces of non- fiber wood (shives), knots, bark, and the like. The wood fibres are separated. In ground wood pulp, the fibres are fragmented, and there is considerable debris (fines). Ground wood pulps contain all the chemical constituents of wood, including lignin, hemi-cellulose, resin, and various colouring materials. This debris on exposure to light and heat and after ageing, cause discolouration (yellowing) of paper. Ground wood pulps are not very white, so are often bleached with peroxide or hydro sulphite to improve the whiteness, yet it does not equal to whiteness of pure cellulose. Ground wood pulp fibres are relatively short and have only a moderate ability to bond each other, so papers made from such raw materials have low strength. However, papers containing ground wood have good opacity, high bulk and good printing qualities.

Chemical wood pulp is made by cooking wood chips with chemical solutions in digester operated at elevated temperature and pressure. The chemicals used for two prime purposes 1. To make the lignin of wood soluble, and 2 for purification and bleaching. Paper produced by the kraft process is particularly strong and durable. Chemical wood pulp that is purified both by bleaching and by alkaline extraction is called an alpha or dissolving pulp, and is used for speciality papers.

Semi chemical pulp is made by treating wood chips with sulphite or alkali in amounts and under conditions that soften the lignin, but dissolve only part of it. The softened chips are then de-fibred. The chips are steeped and impregnated with inorganic chemical solutions similar to those used for full chemical pulping, but in smaller amounts and with less severe condition.

The high fiber yield pulps are usually termed chem-mechanical pulps. The semi chemical-pulps have chemical and strength properties intermediate between softwood, ground wood, and full chemical pulps. These are used in a wide range of papers and boards. The major tonnage of semi chemical pulps goes into the light board (termed corrugating medium), which is fluted to serve as the interior layer of corrugated box board in heavy-duty containers. Stiffness and adequate strength are the important properties. Semi chemical pulp is used in many low-cost printing papers.

Various agents are added to paper pulp to enhance or to modify the bonding and coherence between fibres. To increase the dry strength of paper, the materials most commonly used are starch, poly acrylamide resins, and natural gums such as locust bean gum and guar gum.

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