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Group 4 Paper 2008

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Production and Properties of Paper

        Papyrus, more commonly known as paper, is used commonly for various purposes. The deceivingly simple object that we come into contact with nearly every day in fact goes through a complex process to become the writing paper that we are so familiar with. Common belief holds that paper always comes from trees, but many non-woody plants are also used. Besides writing paper, we also use paper in the form of containerboards, cotton fiber, tissue, newspaper, and money. The process through which various types of paper are created involves many biological, as well as chemical and physical, aspects. It also has many biological properties that result from its makeup and creation process.

        The most important ingredient in the creation of paper is cellulose, which is found in plants. Wood from trees is made up of cellulose fibers, which are connected by a natural adhesive substance called lignin. Cellulose is a polysaccharide fiber, with glucose units connected by beta linkages, that usually forms the framework of most cell walls in plants. It is the most abundant organic compound on the planet. Lignin is a complex polymer that is mostly responsible for rigidity and strength in the structure of plants.

In order to extract the cellulose, the wood goes through a process called pulping, which can be done either mechanically or chemically. Chemically, the wood is cooked with chemicals that serve to dissolve the lignin. Mechanically, the wood is finely ground or chopped to separate the cellulose. However, mechanically processed pulp still contains much of the lignin, which can cause undesirable effects in the resultant paper. The process also tends to produce fibers that are short and stiff, and which yield paper that isn't very strong. For that reason, mechanical pulps are mainly used for packaging, newsprint, and other low-strength applications. Mechanical pulp is sometimes blended with chemical pulp to produce a middle-level product with low cost and reasonable strength and color properties. There is also another method known as the “kraft” process during which lignin is removed in a pressurized vessel called a digester. The pulp is then washed to remove the “black liquor,” which is the waste from the digester, made up of lignin and pulping chemicals. It is usually burned in a recovery boiler, producing energy at the paper mill. Paper made through the “kraft” process is often stronger than paper made from traditional mechanical and chemical pulping methods.

After the pulp goes through these processes, the resulting product is a dark brown unbleached pulp. To achieve the desired whiteness of paper, the pulp must be bleached. Depending on the type of paper desired, the amount of bleach will vary. For writing papers with high brightness, it is often sent to a bleaching tower. Chemically processed pulp is usually easier to bleach because of lower lignin content, while mechanically processed pulp may require more bleaching. Common bleaching agents used are chlorine, sodium hydroxide, and hydrogen peroxide.

Once bleached, the pulp is diluted with water in large amounts. This mixture is then sprayed onto an endless mesh screen, called the forming fabric, on a paper machine. Then the fibers become intertwined, creating a mat, and the majority of the water is extracted. In order to dry the rest of the paper, the continuous sheet is pressed between water-absorbing fabrics at high speeds and moved over heated rolls, called dryers, which remove the residual water by evaporation. Then it passes through the size press that adds starch to the sheet to help fill gaps in the surface. This also seals the surface of the paper so ink cannot bleed through the paper when it is printed on. The process of sizing re-wets the paper, so it must go through another series of dryers. Finally, the sheet is pressed between “calendar” (heavy and polished) rolls to create a smooth sheet. At this point, the paper may be colored or given a glossy coating. Once dry and smoothed out, the paper is wound into a reel to collect the product in a convenient form for further processing. Then it goes through an unwinder and spitter to cut the reel into smaller rolls, which are sent to be packaged.

After going through these processes, paper retains and takes on many different physical properties. The dimensional stability of paper, or the ability to retain its shape and form when subjected to varying degrees of temperature, moisture, or pressure, is affected by the cellulose fibers’ properties and how it dries, either swelling or contracting. Absorption and evaporation of moisture in paper can cause changes in the dimensions of the object, which can affect printing processes and possibly cause undesired paper curl or wrinkling. The variations in dimensional stability seen in different types of paper can be summarized by saying that all paper will expand with increased moisture content and contract with decreased moisture content. The dimensional stability of paper can be increased by decreasing fiber content and therefore decreasing absorbency as well. However, all paper does contain some percentage of moisture, which can depend on humidity, pulp type, chemicals used to process it, and the degree of refining. Water relaxes and weakens bonds between cellulose fibers, and in turn the absorbency of paper is affected by water content. The moisture level in the paper also affects its thermal conductivity. Although there is a negligible amount of water in paper (normally around 6%), that water is still a major factor because water requires high levels of heat to evaporate and has a high specific heat.

Another important property of paper is its pH, which can show some of the residual chemicals from the pulp or atmospheric pollutants. It can be determined in three ways: (1) disintegrate the paper sample in warmed distilled water and measure the pH of an extract; (2) disintegrate the paper sample in cooled distilled water and measure the pH of an extract; (3) directly measure the pH of the paper with a wet electrode on the paper’s surface. There is also a type of paper that contains litmus that is commonly known as pH paper and is used to measure the pH of substances through submersion.

In pulp, the length of fibers affects the strength of the pulp and helps determine its use. Pulps made mechanically generally have longer fibers than those made mechanically. Individual fibers in pulp also have tensile strengths, or the amount of pressure it takes for a material or substance to break, that are higher than or comparable to those of steel. The tensile strength of the entire pulp indicates its maximum strength and durability under ideal conditions. Viscosity, the amount of resistance in a fluid to flowing, gives an estimation of the degree of polymerization, which refers to the average number of glucose units in cellulose molecules of pulp. In this way, it can help indicate the degradation of the cellulose fibers during pulping processes relatively.

Although we often disregard the importance and significance of the seemingly simple substance called paper, it can also be seen as a complex object that affects our lives in many ways. The process of producing paper involves complex biological, chemical, and physical facets that result in the many diverse properties of the different types of paper that we interact with and use. By observing paper, we can also observe core themes of biology at work: the emergent properties of a multiplicity of components and the correlation between structure and function in nature (most notably in the structure of cellulose polymers). Although in the future our use of paper may be largely replaced by electronic communications, it will still remain a vital part of our culture and lives. We must further develop our understanding of this material and expand current methods of production and conservation.


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