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 – Quality determinants

Post 210 –by Gautam Shah

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A paper is valued for its quality, form and suitability for a purpose. There are many factors that determine these. The raw material composition, manufacturing and post processing methods mark the papers’ quality, strength, feel and appearance. The form chiefly relates to the post production processes and treatments and its size conversion. The paper becomes suitable for a purpose after various treatments, shape forming and combinative make-up with other materials.

 Paper-cutting by Bettina von Arnim, titled “Jagdszene” (German for “hunting scene”) WSikipedia Image by Bettina von Arnim (* 1785-04-04, † 1859-01-20)

One of the most important factors of paper raw materials is the fibre quality, its length and integrity. Chemical pulps in general have higher fiber length compared to semi chemical pulp and mechanical pulp, when made from same wood. Mechanical pulping, though has greater yield, it damages or shortens the fibre. Next in importance is the proportion of rag versus the cellulose. Higher rag content provides greater strength and better quality.

Paper made from unbeaten or inadequately beaten fibres is characterized by a lack of tensile strength, by high porosity, high absorbency, full opacity, by its irregular surface and in many cases by a wild uneven formation. A sheet from a well-beaten stock, exhibits greater mechanical strength, a higher density, a measure of grease-proof, translucency and in extreme cases transparency, a smoother surface and a more regular formation. A waste recycled pulp apparently has lower capacity to swell compared to a virgin-pulp due to prime `beating’. Pulp from better quality recycled waste is used for newsprint paper and straw board or packing carton paper. Card board and card have small amount of rags. Writing papers have better rag content that is contain substantial linen or cotton fibres.

Waste paper recycling

Waste paper recycling

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Recycling Paper as bags Bag seller India

Grease-proof (resistant to oil and fat penetration) and ‘glassive’ grades require a paper as dense and homogeneous as possible. This is achieved by reducing the length of fibres. On the other hand craft paper, where strength and anti tear properties are important, fiber shortening is minimized.

Water Absorbency of a paper depends on the quality of the pulp and also on the degree and method of sizing. Papers are internally sized (i.e. sized during pulp formation stage), tub-sized or surface-sized after a paper formation stage. Sizing materials include glues, casein, starch, rosin, gelatin and many synthetic polymeric emulsions and suspensions.

Toilet paper

Toilet paper

PH Value of a Paper determines permanency of a paper, i.e. deterioration of a paper due to ageing. Papers with a tendency to yellow and become brittle have acidic content. Documents, account books, art work prints, maps, survey sheets, require acid free papers. Addition of alkaline substances like calcium or a magnesium carbonate can reduce acidity of a paper, but also increase the weight of a paper, and make it impermeable and smooth.

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Ph value of paper and ageing Old books Yellowed brittle pages Wikipedia Image by Tom Murphy VII

Density of a Paper is also governed by many other factors, such as, quality of pulp, sizing, pressing, post manufacturing treatments like calendaring.

Hardness and Softness of Paper are related to many factors such as the constituents of the pulp, grain orientation, pressing, calendaring, sizing, loading substances, presence of residual chemicals (bleaching and optical brightening agents, acids). Brittleness occurs due to over handling of pulp, presence of residual chemicals and the moisture content. Papers often are treated with silicone emulsions, soaps, to provide a soft feel.

Old books Yellowed brittle pages Wikipedia Image by Tom Murphy VII

Optical Properties include opacity, brightness, gloss and colour. Opacity of a paper relates to legibility of an image through a paper or from the backside imprint on the paper. Tracings have very smooth grain structure with show through, achieved by very through beating of pulp or by impregnation with oil or waxy substances. While some papers are mildly translucent or have strike-through properties (legibility of an image printed on the back face). Opacity is often a desired property of copying, printing, writing papers. Opacity of a paper is achieved by loading substances, calendering processes, impregnation, coatings, sizing, dyeing etc.

Brightness shows as the degree to which white or near-white papers and paper-board can reflect the light of the blue end of the spectrum.

Gloss, Glare, Finish, and Smoothness, are used in describing the surface characteristics of paper. Smoothness refers to the absence of surface irregularities under visual and feel conditions. Gloss refers to surface lustre. Glare, is used for a more intense reflection and often an unpleasant effect of high gloss. Calendaring and coating, are important paper-treating methods that affect gloss.

Colour of paper at natural level is dull or off-white to bleached or super white. The most common way to impart colour to paper is to add soluble dyes or coloured pigment to the paper stock. Direct dyes with a natural affinity for cellulose fibre are highly absorbed, even from dilute water solution. Basic dyes have a high affinity for ground-wood and unbleached pulps.

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This article in continuation of earlier articles on same subject >>

https://interiordesignassist.wordpress.com/2014/07/21/paper-part-1/

https://interiordesignassist.wordpress.com/2014/07/22/paper-part-2/

https://interiordesignassist.wordpress.com/2014/07/23/paper-part-3/

https://interiordesignassist.wordpress.com/2014/07/27/paper-finishing-and-converting/

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