制漿造紙專(zhuān)業(yè)外語(yǔ)大全.doc

125Introduction1.1 SIGNIFICANCE OF PAPERThe significance of paper and paper products in modern life is obvious to everyone: no manufactured product plays a more meaningful role in every area of human activity. Paper provides the means of recording, storage and dissemination of information; virtually all writing and printing is done on paper. It is the most widely used wrapping and packaging material, and is important for structural applications. The uses and applications for paper and paper products are virtually limitless. New specialty products are continually being developed. At the same time, the industry is aware of inroads and competition from other sectors, notably plastics and electronic media for markets traditionally served by paper. As never before, new technology and methodology is being adopted so that the industry can remain competitive in existing markets and be receptive to new opportunities.Aside from the output of products and services, the Pulp and Paper Industry provides employment for vast numbers of people and plays a vital role in the overall economy of these days.1.2DEFINITIONS OF PULP, PAPER AND PAPERBOARDPaper has traditionally been defined as “a felted sheet formed on a fine screen from a water suspension of fibers”. Current paper products generally conform to this definition except that most products also contain non-fibrous additives. Dry forming methods are now utilized for the manufacture of a few specialty paper products.Pulp is the fibrous raw material for papermaking. Pulp fibers are usually of vegetable origin, but animal, mineral or synthetic fibers may be used for special applications. Pulps used for chemical conversion into non-paper products are called dissolving pulps.The distinction between paper and paperboard is based on product thickness. Nominally, all sheets above 0.3 mm thickness are classed as paperboard; but enough exceptions are applied to make the distinction somewhat hazy.1.3 CHRONOLOGY OF TECHNOLOGICAL DEVELOPMENTPaper derives its name from the reedy plant, papyrus. The ancient Egyptians produced the worlds first writing material by beating and pressing together thin layers of the plant stem (see Figure 1-1). However, complete defibering which is characteristic of true papermaking was absent.The first authentic papermaking originated in China as early as 100 AD, utilizing a suspension of bamboo or mulberry fibers. The Chinese subsequently developed papermaking into a highly skilled art, and many beautiful examples of ancient Chinese illustrations on paper are still in existence.After a period of several centuries the art of paper-making extended into the Middle East and later reached Europe, where cotton and linen rags became the main raw materials. By the beginning of the l5th century a number of paper mills existed in Spain, Italy, Germany and France. The first paper mill in North America was established near Philadelphia in 1690.Some of the significant milestones in the historical development of pulp and paper manufacture are summarized in Table 1-1. These inventions and pioneering prototypes provided the basis for the modem paper industry. The twentieth century has seen the rapid refinement and modification of this early and rather crude technology, along with the development of such techniques as refiner mechanical pulping, continuous cooking, continuous multistage bleaching, on-machine paper coating, twin-wire forming, and computer process control, to name just a few. Because pulp and paper operations require the continuous movement of large masses of material, the mechanization of material handling has always been an important aspect of industry development. (Refer to references 1 and 2 for further details on the early history of the industry.)table 1-1. Milestones in pulp and paper industry development. 1.4 MODERN PULP AND PAPER OPERATIONSThe modern pulp and paper mill utilizes wood residuals as the basic raw material. Process operations are highly automated, and many mills now utilize computer control. Overall economics in North America usually favor large-scale units with high operator productivity. Consequently, the construction cost of a viable modern plant is extremely high. For example, a 1000-ton-per-day greenfield bleached kraft pulp mill is now estimated to cost in excess of 1 billion dollars. The high investment cost, typically exceeding one million dollars per worker, qualifies pulp and paper as a capital-intensive industry.Economics also favor integrated forest products operations where logs are brought first to the wood mill for extraction of highest value lumber, plywood and particleboard. Wood residuals are then chipped and conveyed next door for conversion into pulp. Finally, the pulp is transferred to a third division of the plant complex for manufacture into paper. The high value and bulk of raw fiber dictates toward complete and optimal utilization with minimal handling.Those segments of the industry with mechanical pulping operations are the heaviest users of energy. Other types of mills which are able to utilize wood waste materials for fuel are close to energy self-sufficiency. Large volumes of water are used by virtually all conventional pulp and paper operations, and an abundant source of water is required at the plant site. Typically, sophisticated effluent treatment is practiced before the water is returned to the receiving stream.A breakdown of U.S. and Canadian pulp and paper production by general product category is shown in Table 1-2. Some selected industry statistics are given in Table 1-3. Canada is a world leader in the export of newsprint and market pulp. The United States is actually a net importer of paper products because of large purchases of newsprint from Canada, but it is a leading exporter of value-added grades of paper.By virtue of abundant timber，energy and water resources, modern technology, skilled manpower, and accessibility to markets, the North American pulp and paper industry is the world leader in terms of production and diversity of products. It can be noted that the United States and Canada together have 5% of the worlds population. 15% of its paper mills, and produce 36% of its paper. Foreign producers have never been a significant factor in the domestic market, a circumstance enjoyed by few other major North American industries. In 1980, most industry observers agreed that the North American pulp and paper industry was the worlds low cost producer. However, the situation has changed dramatically in a short period of time. Today, some off-shore producers can actually bring table 1-2. 1990 production (000 short tons).USA* Canada*PaperNewsprint 6,610 9,068Other Printing/Writing 22.37 1 3,599Packaging/Wrapping 4,576 497Tissues 5.802 495Total 39,35913,659PaperboardLiner/Corrugating 25,097 2,045Other Boards 14.326 761Total 39,423 2,806Total Paper & Paperboard 78,748 16,465Total Pulp Production57,214 22,835Sources:*API; *CPPAtable 1-3. Selected industry statistics*, 1989.USA CanadaNumber of pulp mills 345179Number of paper/board mills 601129Number of mill employees 246,300 81,000Product Prices, $/short tonBleached softwood kraft pulp 740Bleached hardwood kraft pulp 690Newsprint (30-lb) 540Directory (22.5-lb) 850No. 1 Publication (70-lb) 1600Linerboard 410Corrugating medium 390Solid bleached kraft board 730* Various sources.* One or more pulp mills may be at the site of apaper/board ducts into North America, pay shipping costs, and still be competitive in terms of product cost.In the present era of globalization, competition for export pulp and paper markets is fierce, and the North American industry can hardly afford to rest on its laurels. Short-rotation pine and hardwood plantations are being rapidly developed in sub-temperate regions around the world that will provide vast quantities of low-cost pulpwood in future generations. Countries such as Brazil and Chile are already important producers of pulp fiber. Russia remains a question mark as a forest products competitor; its timber resource is the largest in the world, and the harvest could ultimately double the current North American cut.Fortunately, the long-term outlook indicates continually expanding world demand for all pulp and paper products, so that outside competitive forces should not adversely affect the North American market for the foreseeable future. Per-capita consumption in other parts of the world is far below the North American level and indicates a built-in growth for many decades. Consumption data such as that shown in Figure 1-3 strongly suggest that increased utilization of paper products will be a natural consequence of economic growth among developing nations.1.5 REQUIREMENTS AND SOURCES OF PAPERMAKING FIBERSIn order for fibers to be useful for papermaking， they must be conformable, i.e., capable of being matted and pressed into a uniform sheet. Strong bonds must also develop at the points of contact. For some applications, the fiber structure must be stable over long periods of time. The degree of fiber conformability is characterized and measured as sheet formation, while the degree of bonding is inferred by the tensile or burst strength of the sheet.Some valuable papermaking pulps are unusable in their raw state because the fibers are relatively non-conformable and non-bonding. These pulps must be mechanically treated to develop their papermaking properties. For example, cotton and linen rags (which are still used as pulp sources for the highest-quality durable papers) must be extensively worked to develop the desired fiber properties.table 1-4. Average length, average diameter and length/diameter ratio of various pulp fibers.Length Diameter(mm) (um) Ratio WoodsConiferous (softwood) 4,0 40 100Deciduous (hardwood) 2.0 22 90Straws and GrassesRice 0.5960Esparto 1.110110Misc. (wheat, rye, sabai) 1.5 13120Canes and ReedsBagasse (sugar cane) 1.7 2080Miscellaneous 1.212100BamboosSeveral varieties 2.815180Woody Stalks with Bast Fibers(jute, flax, kenaf, cannabis)Woody stems0.25 1025Bast fibers* 20201000Bast Fibers*Linen 55202600Ramie130 40 3500Leaf FibersAbaca (Manila hemp) 6 24250Sisal2.8 21130Seed FibersCotton30 201500Cotton linters20 201000*Fibers obtained from inner barkPulp fibers can be extracted from almost any vascular plant found in nature. However, a high yield of fibers is necessary if the plant is to have economic importance. The major plant sources of pulp fibers are categorized and characterized in Table 1-4. Wood is far and away the most abundant source of papermaking fibers, and is virtually the only source utilized in North America. As a consequence, this text will concentrate on wood as the principal raw material for pulp.Except for seed hairs, vegetable fibers in their native state are embedded in a matrix of non-fibrous material (mostly lignin, but also containing hemicelluloses, resins and gums). Chemical and mechanical processes in the pulp mill free the fibers from the lignin matrix and leave the ultimate fiber in a relative degree of purity depending on the end use. The greater the requirement for purity, the lower will be the yield of fiber.1.6 INTRODUCTION TO FIBER CHEMISTRYCelluloseIn plant fibers it is the substance cellulose that determines the character of the fiber and permits its use in papermaking. Cellulose is a carbohydrate, meaning that it is composed of carbon, hydrogen and oxygen, with the latter two elements in the same proportion as in water. Cellulose is also a polysaccharide， indicating that it contains many sugar unitsThe chemical formula for cellulose is (C6H10O5)n, where n is the number of repeating sugar units or the degree of polymerization (DP). The value of n varies with the different sources of cellulose and the treatment received (see Table 1-5). Most paper-making fibers have a weight-averaged DP in the 600-1500 range.table 1-5. Degree of polymerization values (weighted averages).Native cellulose (in situ) 3500Purified cotton linters 1000 - 3000Commercial wood pulps 600-1500Regenerated cellulose (e.g. rayon) 200 - 600 The recurring unit is actually two consecutive glucose anhydride units, known as a cellobiose unit. Pure cellulose can be rather easily hydrolyzed to glucose (C10H10O6) under controlled (acidic) conditions.The polymeric linkages during cellulose synthesis are such that the chains form in an extended manner. As a consequence, cellulose molecules fit snugly together over long segments, giving rise to powerful associative forces that are responsible for the great strength of cellulosic materials.Cellulose in plant fibers is found in several ordered levels of orientation, as illustrated in Figure 1-4. Where the molecules fit together over long segments, regions of crystallinity develop which are difficult to penetrate by solvents or reagents. By contrast, the relatively more amorphous regions are readily penetrated and are therefore more susceptible to hydrolysis reactions. The microscopic and submicroscopic structure of cellulose is further illustrated in Figure 1-5.figure 1-5. Microscopic and submicroscopic structure of cellulose (Bruley).The properties of cellulosic materials are related to the DP of the constituent cellulose molecules. Decreasing the molecular weight below a certain level will cause deterioration in strength.Long-chain cellulose is known as alpha cellulose. A number of shorter-chain polysaccharides, known collectively as hemicelluloses, also form pan of the woody structure of plants. Hemicellulose (along with degraded cellulose) is further conveniently categorized (by chemical means) according to DP:beta cellulose - DP between 15 and 90gamma cellulose - DP less than 15HemicellulosesBy contrast to cellulose which is a polymer only of glucose, the hemicelluloses are polymers of five different sugars:hexoses: glucose, mannose, galactosepentoses: xylose, arabinoseDepending on the plant species, these sugars along with uronic acids form various polymeric structures: some are associated with the cellulosic portion of the plant, while others are more closely associated with lignin.During chemical treatment of wood to produce pulp, the amounts, locations, and structures of the various hemicelluloses usually change dramatically. The hemicelluloses are more easily degraded and dissolved than cellulose, so their percentage is always less in the pulp than in the original wood.LigninThe term holocellulose is used to describe the total carbohydrate content of fibers. In addition to holocellulose, woody plant materials contain an amorphous, highly-polymerized substance called lignin. Its principal role is to form the middle lamella, the intercellular material which cements the fibers together. Additional lignin is also contained within the remaining cross-section of the fiber.The chemistry of lignin is extremely complex (see Figure 1-6). The structure consists primarily of phenyl propane units linked together in three dimensions. The three linkages between the propane side chains and the benzene rings are broken during chemical pulping operations to free the cellulosic fibers.A full treatment of cellulose and lignin chemistry is obviously beyond the scope of this book. For a comprehensive treatment, readers are referred to standard textbooks (references 3 and 4).ExtractivesIn addition to holocellulose and lignin, a number of diverse substances may be present in native fibers, depending on the plant source, e.g. resin acids, fatty acids, turpenoid compounds and alcohols. Most of these substances are soluble in water or neutral organic solvents, and are collectively called extractives. Among North American wood species, many have less than 1 % extractives content based in moisture-free weight. The southern pines have a notably higher content, which provides substantial amounts of raw tall oil and turpentine as by-products from alkaline pulping operations.1.7 BEHAVIOR OF CELLULOSIC FIBERSCellulosic fibers exhibit a number of properties which fulfill the requirements of papermaking (summarized in Table 1-6). In general, the best balance of papermaking properties occurs when most of the lignin is removed from the fibers while retaining substantial amounts of hemicellulose. Properties are also greatly optimized by a mechanical treatment (e.g., beating or refining). which causes removal of the primary fiber walls and allows the fibers to hydrate (i.e. take water into the structure) and swell, increasing their flexibility and bonding power. The typical behavior of chemical pulp handsheet strength properties during beating is illustrated in Figure 1-8.table 1-6. Properties of cellulosic fibers.* high tensile strength* suppleness (flexibility, conformability)* resistance to plastic deformation* water insoluble* hydrophillic* wide range of dimensions* inherent bonding ability* ability to absorb modifying additives* chemically stable* relatively colorless (white)The hydrophillic nature of cellulosic fibers plays an important role because the papermaking process occurs in an aqueous medium. The fibers readily absorb water and are easily dispersed in a water suspension. When wet fibers are brought together during the sheet-forming operation, bonding is promoted by the p