Paper as a Communications Medium
Nicholas Wiseman
In modern society, we live in such a sea of paper as to be almost unaware of it, except when it becomes a nuisance to us. Yet many aspects of modern life depend upon paper - for packaging, for hygiene, for the walls of our houses and the furniture within them, and for cultural and communications purposes. This article deals with the manufacture of paper as a medium for storing and communicating information.
History of paper
Organized society depends upon some medium for storing and conveying information. Earlier societies used a variety of media, including clay tablets, papyrus, and parchment, but these were cumbersome and expensive. It is no coincidence that the first great bureaucracy, Imperial China, relied upon paper.
The Chinese seem to have been using paper since about 200 bc. For the best part of 1,000 years China enjoyed a monopoly of paper, until the secret escaped through the conquests of Islam in central Asia. By the start of the second millennium, paper was being made in the civilized, i.e. Islamic dominated, parts of the Mediterranean region.
For several centuries paper was regarded as being inferior to parchment, a notion that lingers in the fake "parchment" used for significant documents. The oldest known piece of European paper was written on by the queen of a Norman king of Sicily: would the king have used parchment?
Paper came into its own with the advent of printing: the surface of parchment is not resilient enough for printing. But paper continued to be an expensive medium until the invention of the paper-making machine around 1800. The price of paper then dropped dramatically, allowing the growth of our modern culture, which relies so heavily on printed information. The consumption of paper in industrialised countries increased about 200-fold, to the present level of around 200kg per person per year.
Paper structure
Cellulose
The key ingredient in paper is cellulose. Cellulose is nature's superstar. It is one of the strongest materials known, weight for weight - hardly surprising, considering that cellulose is the reinforcing material in trees, grasses and other plants.
Cellulose is a carbohydrate - a compound of carbon, hydrogen,and oxygen. Photosynthesis creates vast amounts of it, by the action of sunlight on water and carbon dioxide. Most of it becomes the food source for a large range of microbiological life forms, which quickly reduce unwanted cellulose to its components of water and carbon dioxide. Human activity uses only a tiny fraction of the annual crop of cellulose. Claims that paper use is outrunning the supply of fibre are ill-informed.
In dark, dry conditions cellulose lasts for centuries, even millennia, as long as it is not acidic. Modern papers, which contain calcium carbonate, are neutral in pH, but papers made from the mid-nineteenth century to the middle-late twentieth century were often made under acid conditions. Cellulosic decay in many of these papers has caused them to become brittle and hence they are a problem for archivists.
Cellulosic fibre
Trees and grasses use cellulose in the form of fibres. Although tiny, each fibre is a marvellous structure, with cellulose spiral-wound into a strong lightweight tube.
In the plant, an organic adhesive called "lignin" binds fibres into strong, flexible bundles.
In paper making, we take these fibres, separate them from one another, suspend them in water, and drain the suspension through a mesh. The mesh traps the fibres in the form of a sheet. The pressed, dried sheet is paper.
A typical paper-making fibre is around 1mm in length, and 0.01mm in diameter. This means that it is just large enough to be visible to the human eye, as you can see by tearing a piece of paper and looking at the fibres that stick out from the torn edge.
Paper made from fibres only, such as newsprint or tissue paper, is therefore somewhat coarse in texture. To give paper a smooth surface,we add clay and other mineral particles. These are typically less than 0.001mm in diameter - well below the size that the human eye can see.
To make paper even more suitable for advertising and other high-quality purposes, we coat the surface with a paint-like material made from clay, starch and water. Most printing and writing papers contain 20 to 40 percent of these mineral materials. This is why printing papers do not burn so well as newsprint.
Another valuable attribute of cellulosic fibre is that the fibre surface has many chemical "hooks" that cause adjacent fibres to stick together when they dry from the wet state. The "hydrogen bond", so formed,dissolves when the paper is rewetted. This means that paper can easily be recycled, and that the fibres liberated from recycled old paper can be used to make new paper.
Waste paper, rather than virgin fibre, now makes up more than half the paper made in the world.
Pulp for paper making
Early paper-makers mainly used hemp and linen fibre. During the nineteenth century, the enormous increase in demand created by the invention of the paper machine meant that the supply of fibre from these sources became completely inadequate. In modern paper making, over 90 percent of the fibre comes from wood.
Most of the wood used in the paper industry is residues from saw milling of plantation trees. Both softwoods and hardwoods are used. Softwood fibres tend to be longer and stronger, while hardwood fibres are finer and softer. The most common softwoods are spruce and pine, while the hardwoods include poplar, beech, birch and eucalyptus.
The two main methods of converting wood into virgin pulp are mechanical pulping and chemical pulping.
Mechanical pulping
Mechanical pulping separates logs or chips of wood into their constituent fibres by mechanical action. The device typically used is a development of the old-fashioned stone grinding mill used to grind wheat into flour.
The process uses hot, wet conditions to soften the lignin,thus allowing fibres to peel apart. It is rather analogous to steaming a postage stamp off an envelope. The lignin is softened, but not removed. The process is therefore very economical in that each tonne of wood produces almost a tonne of fibre, but it uses a lot of energy.
The pulp is soft and absorbent, reasonably light in colour,and reasonably strong. It makes good kitchen towelling and newsprint. Much magazine paper also consists principally of mechanical pulp. However, the lignin yellows in sunlight. Mechanical pulp is therefore not suitable for papers used for permanent storage.
Chemical pulping
Chemical pulping processes "cook" wood chips with chemical solutions in big pressure vessels, to dissolve the lignin. The predominant"sulphate" or "kraft" process uses alkaline solutions.
Kraft pulp is naturally brown in colour. In unbleached form it makes good, strong brown-paper bags; hence the name "kraft" (strong,in German). Bleaching makes the pulp white. In recent years, the use of chlorine and chlorine compounds for bleaching has been phased out. Modern mills use less polluting chemicals such as peroxide, oxygen and ozone.
Kraft pulping dissolves most of the lignin, and some of the cellulose, amounting to more than half the wood substance. The dissolved material is a valuable source of chemicals and of energy. Each pulp mill therefore has a chemical recovery plant. This plant produces enough energy to run the process, with some left over and for sale. It also regenerates the chemical solution for use in the pulping process.
Modern kraft pulp mills produce no effluent, except clean water. They are extremely large, complex and expensive. Each one costs in the order of $1 billion, and produces more than 1,000 tonnes of pulp each day.
Paper making
The fundamental process of paper making has not changed since the first sheet of paper was produced in China. What has changed is the amount of labour involved. In an eighteenth century mill, a team of three skilled craftsmen, making paper by hand, could produce one or two reams of paper each day. The chief paper-maker worked bent over a vat, and had his hands in and out of water all day. An unhealthy, laborious job! In a modern paper-mill, a single machine may produce 100 tonnes of paper daily, and the paper- makers work in clean air-conditioned control rooms.
The paper-maker's raw material is pulp. This can be either virgin pulp, freshly produced from wood, or recycled pulp from recovered paper. In either case, the paper-maker first turns it into a suspension in water. Paper making uses lots of water, typically 200-300 tonnes for every tonne of paper produced. In modern industry most of this is recycled internally, so that the paper-mill discharges only a few tonnes of water for each tonne of paper. Discharged water must be stringently purified, so as not to cause pollution.
The fibre suspension is treated to remove any impurities, and to prepare the fibres for the paper-making process. Clay, calcium carbonate, and other filler materials are then added. The paper-makers might also add any of a wide variety of dyes and other additives. Each grade of paper has its own special mix.
A special type of nozzle then lays the fibre suspension on to a moving belt of an open weave plastic mesh. Most of the water drains through,leaving behind a newly formed sheet of wet paper. In a modern machine, the belt may be moving at 90km/hour.
Large roller presses consolidate the wet sheet and remove further water. The wet paper now contains about 1.5 tonnes of water for every tonne of dry fibre. Steam-heated dryers then evaporate most of this water. Dry paper has around 8 percent moisture. At this point in the process, many printing papers have their surfaces treated with a sizing agent. Sizing prevents ink from spreading, and so makes a crisp print job.
Then the paper is calendered, by squeezing it between two smooth-surfaced iron rollers. Calendering is rather like ironing a shirt. It provides a smooth and compact surface for printing or writing upon.
For many grades of paper, the manufacturing process is now complete. All that is required is to package the paper, often after cutting it into sheets according to the customer's requirements. Coated grades of paper undergo a further process, which smears coating material on to each side of the paper, and then dries it. Coated paper is often super-calendered, to make it very glossy and suitable for high-quality advertising.
The future of paper as a communications medium
Since the emergence of the computer, many commentators have voiced opinions on the future of paper. One common opinion is that paper is doomed, and will be replaced by computer screens, by electronic paper, or by some other new medium.
The contrary opinion, just as often voiced, is that paper cannot be replaced, and will be with us for ever. There is no doubt that the consumption of paper, in general, has increased substantially since computers were first introduced into homes and offices.
Will demand for paper collapse, as people switch in large numbers to other media? Perhaps the collapse will take place, as the current generation of youngsters, brought up with computers in the home, enter the work-force.
Little if any factual evidence exists on which accurate forecasts can be based. Research has begun in the last few years to examine, on a scientific basis, the role of paper in society. A new book, The Myth of the Paperless Office (Sellen and Harper, 2001), draws conclusions from research carried out during the 1990s. It uses the concept of "affordances" to explain what paper and other media do for the user. A great deal more research must be done, in a variety of environments. The paper industry has a vital interest in this type of research. A modern paper machine costs $200 million. It can produce only a few of the thousands of grades of paper used in modern industry, and it cannot readily be converted to produce other grades. The industry is understandably nervous about the demand for a given grade of paper suddenly evaporating.
Whatever the future of paper, it is sure to be interesting.
ReferenceSellen, A. and Harper, R. (2001), The Myth of the Paperless Office, The MIT Press, Cambridge, MA.
Nicholas Wiseman(n.wiseman@umist.ac.uk) is Professor, Department of Paper Science, University of Manchester Institute of Science and Technology, UK.
