Biermann's Handbook of Pulp and Paper

Volume 2: Paper and Board Making

Pratima Bajpai

• Editore: Elsevier
• Anno: 2018
• ISBN:
  9780128142394
• Edizione: 3
• Argomento: Technologie et ingénierie
• Categoria: Ingénierie de la chimie et de la biochimie

Biermann's Handbook of Pulp and Paper: Paper and Board Making, Third Edition provides a thorough introduction to paper and board making, providing paper technologists recent information. The book emphasizes principles and concepts behind papermaking, detailing both the physical and chemical processes. It has been updated, revised and extended. Several new chapters have been added. Papermaking chemistry has found an adequate scope covering this important area by basics and practical application. Scientific and technical advances in refining, including the latest developments have been presented. The process of stock preparation describes the unit processes. An exhaustive overview of Chemical additives in Pulp and Paper Industry is included. Paper and pulp processing and additive chemicals are an integral part of the total papermaking process from pulp slurry, through sheet formation, to effluent disposal. Water circuits with loop designs and circuit closure are presented. The chapter on paper and board manufacture covers the different sections in the paper machine and also fabrics, rolls and roll covers, and describes the different types of machines producing the various paper and board grades. Coating is dealt with in a separate chapter covering color formulation and preparation and also coating application. Paper finishing gives an insight into what happens at roll slitting and handling. The chapter on environmental impact includes waste water treatment and handling, air emissions, utilization and solid residue generation and mitigation. The major paper and board grades and their properties, are described. Biotechnological methods for paper processing are also presented. This handbook is essential reading for Applied Chemists, Foresters, Chemical Engineers, Wood Scientists, and Pulp and Paper technologist/ Engineers, and anyone else interested or involved in the pulp and paper industry.

• Provides comprehensive coverage on all aspects of papermaking
• Covers the latest science and technology in papermaking
• Includes traditional and biotechnological methods, a unique feature of this book
• Presents the environmental impact of papermaking industries
• Sets itself apart as a valuable reference that every pulp and papermaker/engineer/chemist will find extremely useful

Chapter 1

Refining and Pulp Characterization

Abstract

Pulp refining is a mechanical treatment of pulp fibers to develop their optimum papermaking properties. One action of refining is the “pumping” of water into the cell wall, making it much more flexible. A second action of refining is fibrillation, that is, exposure of cellulose fibrils to increase the surface area of fibers, thereby improving fiber–fiber bonding in the final sheet. A third action is delamination of the cell wall, such as between the primary and secondary layers, which increases the fiber flexibility. The optimum paper properties depend on the product being made. Refining of fibers is very important before making paper from them. It increases the strength of fiber-to-fiber bonds by increasing the surface area of the fibers and making the fibers more pliable to conform around each other. This increases the bonding surface area and leads to a denser sheet. During refining, individual fibers are weakened and made shorter due to cutting action. With very long fibers of a few species, this cutting action improves the formation of the sheet on the paper machine, but in most cases, it is an undesired effect of refining. Consequently, refining is generally a trade-off between improving fiber-to-fiber bonding and decreasing the strength of individual fibers. Most strength properties of paper increase with pulp refining, as they rely on fiber-to-fiber bonding.

Keywords

Delamination; Fiber brushing; Fiber cutting; Fibrillation; Mixed refining; Pulp characterization; Refiner plate; Refiners; Refining; Separate refining; Specific edge load; Strength properties

1.1. Introduction to Refining

Pulp refining is a mechanical treatment of pulp fibers to develop their optimum papermaking properties. The optimum paper properties, of course, depend on the product being made. Furthermore, there is always a trade-off between various properties. Refining of fibers is very important before making paper from them. Refining increases the strength of fiber-to-fiber bonds by increasing the surface area of the fibers and making the fibers more pliable to conform around each other, which increases the bonding surface area and leads to a denser sheet. During refining, however, individual fibers are weakened and made shorter due to cutting action. With very long fibers of a few species, this cutting action improves the formation of the sheet on the paper machine, but in most cases, it is an undesired effect of refining (even if formation is improved somewhat); consequently, refining is generally a trade-off between improving fiber-to-fiber bonding and decreasing the strength of individual fibers. Gentle action in refining (more impacts of lower energy each, for example, by reducing the bar or groove width, thereby increasing the number of bars) leads to higher strength properties for fiber refined to a given freeness.

Refining a pulp increases the fibers' flexibility and results in denser paper, which means that bulk, opacity, and porosity values decrease during the process. Mechanical and hydraulic forces are employed to alter the fiber characteristics. Shear stresses are imposed by the rolling, twisting, and tensional actions occurring between the bars and in the grooves and channels of the refiner. Normal stresses (either tensional or compressive) are imposed by the bending, crushing, and pulling/pushing actions on the fiber clumps caught between the bar-to-bar surfaces. During beating and refining, fibers randomly and repeatedly undergo tensile, compressive, shear, and bending forces. They respond in three ways. Fibers develop new surfaces externally through fibrillation and internally through fiber wall delamination. Fibers deform, resulting in changes in their geometric shape and the fibrillar alignment along their length. Overall, the fibers flatten or collapse. Fiber curl changes and kinks are induced or straightened. On the small scale, dislocations, crimps, and microcompressions are induced or diminished; fibers break, resulting in changes in length distribution and a decrease in mean fiber length. A small amount of fiber wall material also dissolves. All these changes occur simultaneously and are primarily irreversible. The extent of the changes depends on the morphology of the fibers, the temperature, the chemical environment, and the treatment conditions. The conditions depend on the design of the equipment and its operating variables, such as the consistency, intensity, and amount of treatment. Each pulp responds differently to a given set of conditions, and not all fibers within it receive the same treatment. As different types of fibers have different lengths and cell wall thicknesses and also vary in the width of their central lumen (canal), some fibers are fibrillated more by one kind of beating than another. Fibers that have thick cell walls and narrow central canals, such as linen, are less prone to cutting but readily fibrillate. So, choosing the right type of fiber to produce the refining effect appropriate to the desired paper properties is important.

Most strength properties of paper increase with pulp refining, as they rely on fiber-to-fiber bonding. The tear strength, which depends highly on the strength of the individual fibers, actually decreases with refining. After a certain point, the limiting factor of strength is not fiber-to-fiber bonding, but the strength of the individual fibers. Refining beyond this point begins to decrease other strength properties besides tear. Refining of pulp increases their flexibility and leads to denser paper. This means bulk, opacity, and porosity values decrease with refining.

Fig. 1.1 shows four paper types made from increasingly refined fibers. Two of these samples are glassine papers, which are typically refined to a much higher degree than most printing and packaging papers. It is apparent from these figures that fiber-to-fiber bonding increases with refining. Also, the volume of space between the fibers decreases; hence, refining increases the density of the sheet.

Fiber Brushing

Refining at high consistency with a relatively large distance between the refiner plates increases fiber–fiber interactions that are termed fiber brushing. This tends to roughen the fiber surface, with minimal fiber cutting fo...