Post 646 –by Gautam Shah
Paper is a sheet form of material, and substantially used in sheet-form. Paper’s chief raw material is cellulosic pulp. It is also used in ‘non-planer’ forms, such as moulded products (egg crates), packing cases (glass), mould dummies, and as Papier-mâché.
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.
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.
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.
Structure of paper as sheet material differs from other sheet materials:
- Papers unlike plastic films and metal films are fibrous.
- Paper is composed of single short fibres, arranged largely at random instead of a regular array as is the case with woven fabrics.
- 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.
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.
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.
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.
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.
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.
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.