造纸专业英语教材

造纸专业英语教材 Lesson 1 The history of papermaking Paper derives its name from the reedy plant, papyrus. The ancient Egyptians produced the world’s first writing material by beating and pressing together thin layers of plant stem. 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. After a period of several centuries, the art of papermaking extended into the Middle East and later reached Europe, where cotton and linen rags became the main materials. Paper was first made in England in 1496. by he end of the 15th 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. The development of the paper machine is the most important milestone of the industry. Louis Robert, working at the paper mill owned by Ledger Didot, made his first model of the continuous paper machine in 1796 near Paris and received a French patent for his machine in 1799 at the age of 37. In 1803, a patent was issued to Fourdrinier brothers for the improved continuous paper machine designed by Bryan Donkin. At about the same time, John Dickson, a colleague and friend of Donkin, was working his cylinder machine, which was refined by 1809. In 1840, groundwood pulping method was developed in Germany. The first manufacture of pulp from wood using soda process was patented on July 1, 1854 to an England inventor named Hugh Burgess. In 1867, a Philadelphia chemist, Benjamin Tilgham, was awarded the U. S. patent for the sulfite pulping process; the first commercial sulfite pulp was produced in Sweden in 1874. C. F. Dahl is credited with the development of the kraft process was originally patented in 1854. A later patent in 1865 covered the incineration of the spent soda liquor to recover most of he alkali used in the process. These inventions and pioneering prototypes provided the basis for the modern paper industry. The twentieth century has been the rapid refinement and modification of the early and rather crude technology, along with the development of techniques as refiner mechanical pulping, continuous cooking, continuous multistage bleaching, on-machine paper coating, twin-wire forming, and computer process control. Words and Expressions Paper 纸，纸张 papyrus纸莎草 beating 打浆 pressing 压榨，压合 papermaking 抄纸，造纸 fiber 纤维 paper machine 纸机 groundwood pulping 磨木法制浆 soda process 烧碱法制浆 sulfite pulping process亚硫酸盐法制浆 kraft(sulfate) process 硫酸盐法制浆 refiner mechanical pulping 盘磨机械法制浆 continuous cooking 连续蒸煮 continuous multistage bleaching连续多段漂白 on-machine paper coating 机内涂布 twin-wire forming 双网成形 compute process control 计算机过程控制 Lesson 2 Fibrous materials of papermaking Theoretically, pulp fiber can be extracted from almost any vascular plant found in nature. So far, wood is still the most abundant source of papermaking fibers. Besides, about 10% of the fiber used to make paper each year world wide is from non-wood plant, including straws (wheat, rye, rice and barley)，grasses(bamboo， esparto and papyrus)，canes and reeds (bagasse, corn stalks and kenaf), bast (flax, hemp, jute, ramie and 1 mulberry), and seed hairs (cotton). Non vegetable fibers such as polyethylene and glass fibers are also used. In recent years, secondary fiber utilization is increasing at a rapid pace. Botanically, woods are classified into two major groups: softwoods or conifers and hardwoods or broad-leafed-trees, either deciduous or evergreen. The vertical structure of conifers is composed almost entirely of long, tapping cells called tracheids. The wall of a typical trachied or fiber is composed of several layers. The middle lamella with very high lignin content separates two contiguous trachieds. Each trachied has primary wall and a three-layered secondary wall with specific alignments of microfibrils. Microfibrils are bundles of cellulose molecules, and their orientation can influence the characteristics of a pulp fiber. The principal vertical structure of hardwood is composed of both relatively long, narrow cells, called libriform fibers, and much shorter, wide cells, called vessels. Hardwoods also have a vertical parenchyma system and a horizontal or ray parenchyma system. Generally, softwood has higher amount of fibers while hardwood has higher percentage of vessels. Softwood fibers are more than twice as long as hardwood fibers. Technically, wood is xylem tissue, which consists of cellulose, hemicellulose, lignin and extractives, hence a lignocellulosic material. Sapwood is the outer part of the trunk and contains some living cells. Heartwood is found in the centre of older trees, containing only dead cells, and is generally drier than sapwood. Each annual growth ring contains earlywood, which is characterized by large cells with thin cell walls, and latewood, which is characterized by small cells and thick walls. Some of the important pulping variables of wood and wood chips are: moisture content, specific gravity, tension and compression strength, bark content, chemical composition, wood species, chip dimensions, and length of storage. Words and expressions straws 稻麦草 wheat 小麦 rye 黑麦 rice 稻谷 barly 大麦 grasses 草类 bamboo 竹子 esparto 西班牙草 canes and reeds 蔗苇类 bagasse 蔗渣 corn stalks 玉米茎秆 kenaf 洋麻 bast 韧皮类 flax 亚麻 hemp 大麻 jute黄麻 ramie 苎麻 mulberry 桑树 seed hairs 种毛类 polyethylene 聚乙烯 glass fibers 玻璃纤维 softwood 软木，针叶木 conifer 针叶树，针叶木 hardwood 硬木，阔叶木 broad-leafed-tree 阔叶树 tracheid 管胞 middle lammela 胞间层 lignin 木素 primary wall 初生壁 secondary wall 次生壁 microfibril 微纤丝 cellulose 纤维素 libriform fiber 韧皮纤维 vessels 导管 parenchyma system 薄壁组织系统 hemicellulose 半纤维素 extractive 抽提物 sapwood 边材 heartwood 心材 earlywood 早材 latewood 晚材 moisture content 水分含量 specific gravity 比重 tension and compression strength 抗张与抗压强度 2 Reading material: Chemical composition of raw materials Chemical composition of the candidate plant gives an idea of how feasible the plant is as raw material for papermaking. The fibrous constituent is the most important part of the plant. Since plant fibres consist of cell walls, the composition and amount of fibres is reflected in the properties of cell walls. Cellulose is the principal component in cell walls and in fibres. The none-cellulose components of the cell was include hemicelluloses, pectins, lignin and proteins, and in the epidermal cells also certain minerals. The amount and composition of the cell wall compounds differ among plant species and even among plant parts, and they affect the pulping properties of the plant material. Some of non-woody fibre plants contain more pentosans (over 20%), holocellulose (over 70%) and less lignin (about 15%) compared with hardwoods. They have also higher hot water solubility, which is apparent from the easy accessibility of cooking liquors. The low lignin content in grasses and annuals lowers the requirement of chemicals for cooking and bleaching. Except for the fibrous material, plants also consist of other cellular elements, including mineral compounds. While the inorganic compounds are essential for plant growth and development, they are undesirable in pulping and papermaking. Cellulose Cellulose is the principal component of plant fibres used in pulping. It forms the basic structural material of cell walls in all higher terrestrial plants being largely responsible for the strength of the plant cells. Cellulose always has the same primary structure, it is α-1,4 linked polymer of D-glucans. It occurs in the form of long, linear, ribbon-like chains, which are aggregated into structural fibrils. Each fibril contains from 30 to several hundred polymeric chains that run parallel with the laterally exposed hydroxyl groups. The hydroxyl groups take part in hydrogen bonding, with linkages both within the polymeric molecules and between them. This arrangement of the hydroxyl groups in cellulose makes them relatively unavailable to solvents, such as water, and gives cellulose its unusual resistance to chemical attack, as well as its high tensile strength. The first layer of cellulose are formed in the primary cell walls during the extension stage of the cell, but most cellulose is deposited in the secondary walls. The proportion of cellulose in primary cell walls is 20 to 30% of DM and in secondary cell walls 45 to 90%. The cellulose content of a plant depends on the cell wall content, which can vary between plant species and varieties. The age of the plant and plant part also affect the cellulose content. Annual plants generally have about the same cellulose content as woody species, but their higher content of hemicellulose increases the level of pulp yield more than the expected level on the basis of cellulose content alone. The cellulose and alpha-cellulose contents can be correlated with the yields of unbleached and bleached pulps, respectively. Hemicellulose Hemicellulose consist of a heterogeneous group of branched polysaccharides. The specific constitution of the hemicellulose polymer depends on the particular plant species and on the tissue. Glucose, xylose and mannose often predominate in the structure of the hemicelluloses, and are generally termed glucans, xylans, xyloglucans and mannans. Xylans are the most abundant non-cellulose polysaccharides in the majority of angiosperms, where they account for 20 to 30% of the dry weight of woody tissues. They are mainly secondary cell wall components, but in monocotyledons they are found also in the primary cell walls, representing about 20% of both the primary and secondary walls. In dicots they amount to 20% of the secondary walls, but to only 5% of the primary cell walls. Xlans are also different in monocots and in dicots. In gymnosperms, where galactoglucomannans and glucomannans represent the major hemicelluloses, xylans are less abundant (8%). The hemicelluloses in secondary cell walls are associated with the aromatic polymer, lignin. Pectins 3 Pectins, i.e. pectic polysaccharides, are the polymers of the middle lamella and primary cell wall of dicotyledons, where they may constitute up to 50% of the cell wall. In monocotyledons, the proportion of pectic polysaccharides in normally less than this and in secondary walls the proportion of hemicellulose polysaccharides greatly exceeds the amounts of pectic polysaccharides. The pectic substances are characterised by their high content of D-galacturonic acid and methylgalacturonic acid residues. Pectins are more important in growing than in non-growing cell walls, and thus they are not a significant constituent in commercial fibres except in flax fibre, where pectins are found in lamellae between the fibres and account for 1.8% of dry weight. Lignin Lignin is the most abundant organic substance in plant cell walls after polysaccharides. Lignins are highly branched phenolic polymers and constitute an integral cell wall component of all vascular plants. The structure and biosynthesis of lignin has been widely studied. The reason for the great interest is the abundance of lignin in nature, as well as its economical importance for mankind. For papermaking, lignin is chemically dissolved because of the separation of the fibres in the raw material. In cattle feeds, lignin markedly lowers the digestibility. Lignins are traditionally considered to be polymers, which are formed from monolignols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Each of the precursors may form several types of bonds with other precursors in constructing the lignin polymer. A great variation in lignin structure and amount exists among the major plant groups and among species. Great variation in lignin structure and amount exists also among cell types of different age within a single plant and even between different parts of the wall of a single cell. Gymnosperm lignin contains guaiacyl units (G-units), which are polymerized from coniferyl alcohol, and a small proportion of p-hydroxylphenyl units (H-units) formed from p-coumaryl alcohol. Angiosperm lignins are formed from both syringyl units (S-units), polymerized from sinapyl alcohol, and G-units with a small proportion of H-units. Syringyl lignin increases in proportion relative to guaiacyl and p-hydroxylphenyl lignin during maturation of some grasses. In grass species the total lignin content varies from 15 to 26%. For reed canary grass Burrit et all found only 1.2%. in grasses and legumes lignins are predominantly formed from coniferyl and sinapyl alcohols with only small amounts of p-coumaryl alcohol. Lignins are considered to contribute to the compressive strength of plant tissue and water impermeability of the cell wall. Lignins aid cells in resistance to microbial attack, but they do not influence the tensile properties of the cell wall. Monolignols can also form bonds with other cell wall polymers in addition to lignin. Cross-linking with polysaccharides and proteins usually results in a very complex three-dimensional network. This close connection between phenolic polymers and plant cell wall carbohydrates makes the effective separation and utilization of the fibres more complicated. In woody plants relatively few covalent bonds exist between carbohydrates and lignin compared with those in forage legumes and grasses where the lignin component is also covalently linked to phenolic acids, notably 4-hydroxycinnamic acids, p-coumaric acid and ferulic acid. Lignin and hemicelluloses fill the spaces between the cellulose chains in the cell wall and between the cells themselves. This combined structure gives the plant cell wall and the bulk tissue itself structural strength, and improves stiffness and toughness properties. Minerals There are 19 minerals that are essential or useful for plant growth and development. The macro nutrients, such as N, P, S, K(Potassium), Mg and Ca are integral to organic substances such as proteins and nucleic acids and maintain osmotic pressure. Their concentrations in plants vary from 0.1 to 1.5% of DM. The micro nutrients, such as Fe, Mn, Zn, Cu, B, Mo (molybdenum), Cl, and Ni, contribute mainly to enzyme production or activation and their concentrations in plants are low. Silicon (Si) is essential only in some plant species. The 4 amount of silicon uptakes by plants is described by silica (SiO2) concentration. The highest silica concentrations (10-5%) are found in Equisetum ()-species and in grass plants growing in water, such as rice. Other monocotyledons, including cereals, forage grasses and sugarcane contain SiO2 at 1-3% of DM. Si in epidermis cells is assumed to protect the plant against herbivores and in xylem walls, to strengthen the plant as lignin. The concentration of a particular mineral substance in a plant varies depending on plant age or a stage of development, plant species and the concentration of other minerals as well as the plant part. In the pulping process the minerals of the raw materials are considered to be impurities and should be removed during pulping or bleaching. The same elements are found both in non-woody and in woody species, but the concentrations are lower in woody plants. Si is the most deleterious element in the raw material for pulping, because it complicates the recovery of chemicals and energy in pulp mills. Si wears out the installations of paper factories and can lower the paper quality. Other harmful elements for the pulping process include K, Cl, Al, Fe, Mn, Mg, Na, S, Ca and N. Choosing a suitable plant species as the raw material for pulping can minimise the amount of undesirable minerals in process. Moreover, using only the plant parts that contain low amounts of minerals such as Si represents an improvement. Reading material:biosynthesis of the lignin polymer: Lignification, in analogy with other polymerization processes, is the means by which lignin macromolecules grow. Thus the primary reactions of concern are those in which the macromolecule is extended by the coupling of a new monomer to the growing polymer. Branching reactions are also important. They can occur when two sequential reactions are possible at the growing end, i.e. the phenolic end, of the polymer. The primary monomers for lignification are the three p-hydroxycinnamyl alcohols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. These monolignols differ in their degree of methoxylation. In the lignin polymer, these monolignols produce p-hydroxyphenyl, guaiacyl, and syringyl units. Due to the capability of electron-delocalized radicals to couple at various sites, a variety of structural units are found in the resulting polymer. Erdtman was the first to recognize that major lignin structural features were consistent with a process of radical coupling of phenols. Simple chemical dehydrodimerization reactions of coniferyl alcohol, using either the peroxidase-H2O2 system that is implicated in vivo, or other chemical single-electron oxidants including various Fe, Mn, Cu, and Ag salts, produce three dehydrodimeric coupling products incomparable amounts. Most reviews and texts also include other coupling modes for the mono lignols, but these are in fact not observed; 5–5- and 4–O–5- coupled structures in lignins do not arise from monolignol-monolignol coupling reactions. The products involve at least one of the monolignols coupling at its favored β-position. Evidence for further coupling reactions between the monolignol and the initially formed dimmer became available from well-characterized oligomers. The notion that lignification was a process involving ―uncontrolled‖ radical coupling reactions was therefore born, although Erdtman suspected that lignification might be found to be more highly controlled. Synthetic lignins, so-called dehydrogenation polymers(DHPs), can be prepared in vitro from monolignols oxidized by peroxidase-H2O2. Such synthetic lignins are valuable fore xploring and understanding coupling and post-coupling reactions, and as models for spectroscopic and reactivity studies. However, they differ from native or isolated lignins; in particular they have a lower β–O–4-ether content. The primary reason is that dehydrodimerization reactions are over-represented. Dehydrodimerization of coniferyl alcohol yields three primary products, where as sinapyl alcohol yields two. Coupling at the β-position is favored for coniferyl and even more strongly for sinapyl alcohol; the coupling products always result from coupling of atleast one of the monolignols at its β-position. In coupling reactions, including in a biomimetic 5 peroxidase-H2O2 system, the β-ether dimmer is typically produced in less than one third of the yield in the coniferyl alcohol case, and at only about the 9% level for sinapyl alcohol. Monolignol radicals prefer entially couple with like monolignol radicals (when available) rather than cross-couple with dimmers or higher oligomers. Dehydrodimerization reactions are therefore over-represented in synthetic lignins even when attempts are made to introduce the monolignol slowly. Consequently, the β-ether frequency is low, considerably lower than in typical lignins. Limiting the diffusion rates (and there fore monolignol radical concentrations) to favor cross-coupling reactions reveals that β-ethers are strongly favored in cross coupling reactions. Lignin monomers do not have any optical activity–unlike the protein and polysaccharide monomers, they possess no chiral centers. Optical centers are however created in each coupling reaction involving the sidechain β-position, two per event. The result is that the number of isomers of any ―randomly‖ formed lignin structure increases with its degree of polymerization, quickly becoming astronomical. Thus a β-ether dimer, forexample, has 4 optical isomers and half that number (i.e.2 of ―real‖ chemically distinct isomers. A β-ether trimer has 8 chemically distinct isomers, that can be resolved in high-resolution proton NMR spectra. A β-ether tetramer has 32 isomers. And so it progresses geometrically. A pure random β-ether 110-mer was noted to have about the same number of isomers as there are atoms in our galaxy. Lesson 3 Pulp and paper properties and testing A large number of pulp testing methods are in common to characterize pulps with respect to quality, processability, and suitability for various end uses. The most ―fundamental‖ measurements provide the means to predict behavior while ―functional‖ tests are designed to measure specific properties. The kappa number test is used in mill control work to indicate the degree of delignification occurring during cooking and the chemical requirement for bleaching. A good indication of cellulose degree of polymerization (DP) can be obtained by measuring viscosity of a cellulose solution of known concentration using cupriathylene diamine hydroxide (CED) as a solvent. Pulp drainability is an important property with respect to pulp processing and papermaking. Measurement of pulp drainage are known as freeness, slowness, wetness, or drain time according to the instrument or method used. Freeness and slowness scales have an inverse relationship. The Canadian Standard Freeness (CSF) and the Schopper-Riegler (?SR) slowness tester are two principal drainage testing devices used respectively for these two properties in North America and Europe. To simulate the type of drainage with microturbulence and oriented shear, the Britt dynamic drainage jar (DDJ) was developed for studying stock drainage phenomenon under conditions more closely approaching those of the paper machine. The wide diversity of paper grades with different functional properties necessities a multiplicity of paper test methods. Some basic properties are important for all grades such as basis weight or grammage and caliper. Others are specifically developed to assess the performance attributes of speciality products and their application. Since paper is a hygroscopic material and will seek equilibrium moisture with the surrounding air, paper samples must be conditioned in a standardized environment to obtain reproducible results. Due to the ―two-sidedness‖, the wire and top-side’s properties must be taken into account for certain end uses. Paper has a definite ―grain‖ caused by the greater orientation of fibers in the machine direction and by the stress/strain imposed during pressing and drying. The directionality of paper must be also taken into account in measuring physical properties. The physical tests on paper can be conveniently divided into four groups: mechanical and strength properties( tensile, burst, tear, folding strength, stiffness, softness, etc); surface properties (roughness, pick strength); optical properties (brightness, opacity, gloss, color); and permeability to fluids (sizing degree, 6 oil resistance, air resistance, and water vapor permeability). Paper chemical properties are important for certain grades of paper such as photographic papers, reproduction papers, anti-tarnish papers, safety papers, electrical papers, food-wrapping papers, and any paper requiring a high degree of performance. Words and expressions kappa number 卡伯值 degree of polymerization 聚合度 cupriathylene diamine hydroxide 铜乙氢氧二铵 Canadian standard freeness 加拿大标准游离度 schopper-riegler slowness 打浆度 basis weight 定量 grammage 克重 two-sidedness 两面性 tensile 抗张强度 burst 耐破度 tear 撕裂度 folding strength 耐折度 softness 柔软度 stiffness 挺度 roughness 粗糙度 pick strength 拉毛强度 brightness 白度，亮度 opacity 不透明度 gloss 光泽度 color 色度，色值，涂料 sizing degree 施胶度 oil resistance 抗油性 air resistance 透气阻力 water vapor permeability 水蒸气渗透性 photographic paper reproduction paper 复印纸 anti-tarnish paper 防锈纸 safety paper 防伪纸 electrical paper 电气用纸 food-wrapping paper 食品包装纸 Lesson 4 Pulping process and pulp end uses Pulp consists of wood or other lignocellulosic materials that have been broken down physically and/or chemically so that discrete fibers are liberated and can be dispersed in water and reformed into a web. Pulping refers to any processes by which wood (or other fibrous raw material) is reduced to fibrous mass. Basically, it is the means by which the bonds are systematically ruptured within the wood structure. The task can be accomplished mechanically, thermally, chemically, or by combination of these treatments. Existing commercial processes are broadly classified as: chemical, semi-chemical, semi-mechanical and mechanical pulping. These are in order of increasing mechanical energy required to separate fibers and decreasing reliance on chemical action. As a result, chemical methods rely on the effect of chemicals to separate fibers, whereas mechanical pulping methods rely completely on physical action. The more that chemicals are involved, the lower the yield and lignin content since chemical degrades and solublizes components of the wood, especially lignin and hemicelluloses. On the other hand, chemical pulping yields individual fibers that is not cut and gives strong papers since the lignin, which interferes with hydrogen bonding of fibers, is largely removed. The typical pulping processes are classified as follows: Mechanical pulping: stone groundwood for logs; RMP and TMP for chips. Chemi-mechanical pulping: chemigroundwood, cold soda, CTMP. Semichemical puling: NSSC, high yield sulfite, high yield kraft. Chemical pulping: kraft, soda and soda –AQ, sulfite (acid and bisulfite). All chemical pulps must be mechanically worked to develop optimum papermaking properties for various applications. Soft wood kraft pulps produced the strongest papers and are preferentially utilized where strength is required. Typical applications are for wrapping, sack, and box-liner papers. Bleached kraft fibers are added to newsprint and magazine grades to provide the sheet with sufficient strength to run on high speed printing process. Bleached grades are also used for toweling and food boards. Sulfite pulps find a major market in bond, writing, and reproducing papers where good formation and 7 moderate strength are required. Kraft or soda hardwood is usually added for improved formation and opacity. Sanitary and tissue papers also use large amounts of sulfite pulp to obtain the requisite softness, bulk and absorbency. Mechanical pulp has traditionally been used primarily for newsprint and coated printing grades where it provides a well-filled and former sheet. Because of improved quality and versatility, markets have opened to a variety of pulps for a wide range of printing grades, tissues, toweling, fluff, coating raw stocks, and food-grade boxboards. Words and expressions: yield 得率 stone groundwood 磨石磨木浆 thermal mechanical pulp热磨机械浆 refiner mechanical pulp 盘磨机械浆 chemigroundwood 化学磨木浆 cold soda 冷减法 chemithermomechanical pulp 化学热磨机械浆 neutral sulfite semichemical cooking pulp 中性亚硫酸盐半化学浆 soda-AQ 烧碱,蒽醌法 wrapping paper 包装纸 sack paper 纸袋纸 box-liner paper 纸盒衬里纸 newsprint 新闻纸 toweling 毛巾纸 food board 食品包装纸板 sanitary papers 卫生纸类 tissue 薄页纸 fluff 绒毛浆 coating raw stock 涂布原纸 Lesson 5 Mechanical pulping The technology of mechanical pulping has undergone dramatic development in the past several decades. In 1960, virtually all mechanical pulp was produced by the basic stone groundwood process. By 1990, well over 50% of the mechanical pulp was being produced by refiner methods. Regardless of the pulping method used, a fundamental determination of mechanical pulp quality is the amount of energy of expended per unit of production, i. e., the specific energy. Because of the difficulty to measure specific energy on a continuous basis, pulp freeness is more commonly used as the main process control parameters. For the nomenclature, in general, any groundwood is designated by the letters GW and any other type of mechanical pulp is designated by MP. Additional letters are used to specify the combinations and sequences of processing steps. The examples are PGW, PRMP, CRMP, CTMP, etc. the line of demarcation between mechanical and semichemical pulps is generally considered to be 85% yield. Mechanical pulp is the pulp produced by using only mechanical attrition to pulp lignocellulosic materials; no chemicals are used. Light colored, non-resinous softwoods and some hardwoods are often the fiber source. The total yield is about 90%-98%. Lignin is retained in the pulp, therefore, high yields of pulp are obtained from wood. Mechanical pulps are characterized by high yield, high bulk, high stiffness, and low cost. But they have low strength since the lignin interferes with hydrogen bonding between fibers when paper is made. The use of mechanical pulps is confined mainly to non-permanent papers like newsprint and catalog paper. Groundwood mechanical pulp is produced by pressing blocks of wood, called bolts, against an abrasive rotating stone surface. A modified process to SGW was developed in a pressurized atmosphere, called pressurized groundwood (PGW). By pressurizing the grinder with steam at temperatures of 105-125?, the wood is heated and softened prior to the grinding process. This process yields a pulp has higher tear strength and freeness and is brighter than SGW, yet has lower power requirements. 8 Refiner mechanical pulp (RMP) is produced by disintegrating chips between revolving metal disks or plates with raised bars at atmospheric pressure, which was an important development in the history of mechanical pulping and usually involves the use of two refining stages operating in series and produces a longer-fibered pulp than conventional groundwood. As a result, the pulp is stronger, freer, bulkier, but usually somewhat darker in color than stone groundwood. Thermomechanical pulping (TMP) was the first major modification of RMP, and is still employed on a large scale to produce high-tear pulps for newsprint and board. This process involves steaming the raw material under pressure for a short period of time prior to and during refining to soften the chips, with the result that the pulp produced has a greater percentage of long fibers and fewer shives than RMP, which produce a stronger pulp than either SGW or RMP. The chemi-mechanical pulping process consists of two stages with yields of 85%-95%. A particularly mild chemical treatment is followed by a drastic mechanical action. The original lignin structure and content is preserved, but extractives and small amounts of hemicellulose are lost. The most common chemi-mechanical process is now CTMP. The pretreatments are hot sulfite or cold soda and are particularly applicable to hardwoods that otherwise do not give mechanical pulps of high quality. Some mills have also begun to use an alkaline peroxide mechanical pretreatment (APMP). Semi-chemical process is also called high yield chemical pulping, which involves two steps with pulp yields of 60,-80%. In the first step a mild chemical treatment is used, which is followed by moderate mechanical refining. The NSSC and the kraft semi-chemical methods are the most common of this category. The residual lignin makes paper from this pulp very stiff, an important property for corrugating medium. Hardwood is usually the fiber source and NSSC hardwood pulp is approximately as strong as NSSC softwood pulp. Words and expressions: specific energy: 单位能耗 pressurized groundwood (PGW) 压力磨木浆 pressure refined mechanical pulp (PRMP) 压力盘磨机械浆 chemi-refiner mechanical pulp (CRMP) 化学盘磨机械浆 alkaline peroxide mechanical pretreatment (APMP) 碱性过氧化氢机械浆 Lesson 6 Alkaline pulping Alkaline pulping process typically refers to the soda pulping and kraft pulping. The soda process utilized a strongly alkaline solution of sodium hydroxide to delignify wood chips. Nowadays, some soda mills are still in operation around the world producing pulp from hardwood and nonwood fibrous raw materials. For the kraft pulping process, the cooking temperature is usually in the 160 – 180 ? range depending on different raw materials, with a corresponding cooking time of roughly 90 minutes. Normally, some 90 minutes are needed to reach cooking temperature. The cooking liquor is an aqueous solution of sodium hydroxide and sodium sulfide with a weight ratio of approximately 5/2 respectively. The cooking process can be either batch or continuous. In the batch process, the chips are cooked in an individual digester with loading, cooking, and dumping done in sequence. In the continuous process, the chips and liquor are fed at a constant rate into the top of the digester and the chips move downward in a continuous manner for discharge from the bottom. Liquor is extracted, circulated through heat exchangers, and reintroduced into the moving chip column to provide the heat necessary for cooking. In both processes, the cooked chips are discharged from the digester under pressure. As the chips are ―blown‖ from the digester, the mechanical force of ejection breaks up the wood chips into individual fibers, producing the wood ―pulp‖ for further processing. 9 During kraft pulping process, the lignin fragments are dissolved as phenolate or carboxylate ions. Carbohydrates, primarily hemicellulose and cellulose, are also chemically attacked and dissolved to some extent. The two driving forces for kraft pulping reactions are alkali concentration (as measured by either effective alkali or active alkali) and temperature. Delignification during kraft cooking proceeds in three distinct phases: a rapid initial lignin removal phase followed by a first order bulk delignificaion and ended up with the residual delignification. The control objective in kraft pulping is to cook to a target kappa number. An offsetting change in the H-factor is usually applied to bring the test values closer to the target value. Because the process has some shortcomings, principally low pulp yield, the comparatively high residual lignin content of bleachable grades, and the malodors caused by reduced sulfur compounds, various pulping modifications have been developed to overcome these shortcomings, including cooking additives, chip pretreatments, and two-stage cooks. Among them, AQ-type process has been extensively applied in the mills around the world for yield increase modification. The digester is a pressure vessel used for cooking chips into pulp. Digesters designed to operate in either a continuous mode in a long and narrow tube shape, or in a batch mode. Rotary spherical digester and vertical cylindrical digester are the two common batch digesters. Typically a mill has a bank of six to eight digesters so that while several are cooking, one is filling, one is blowing, and one might be under repair. Continuous digester tends to be more space efficient, easier to control giving increased yields and reduced chemical demand, labor-saving, and more energy efficient than batch digesters. Kamyr digester, the M&D (Messing and Durkee), and Pandia digester are the commonly used commercial continuous digesters. Recently, two important modifications of kraft pulping to increase uniformity are rapid displacement heating (RDH) for batch digesters and modified continuous cooking (MCC) for continuous digesters such as Kamyr. Words and expressions alkaline pulping 碱法制浆 batch 间歇法 digester 蒸煮器 loading 装料 delignification 脱木素作用 dumping 卸料 H-factor H-因子 continuous digester 连续蒸煮器 rapid displacement heating (RDH) 快速加热置换法 modified continuous cooking (MCC) 改良连续蒸煮 Alkali-oxygen-anthraquinone delifnification of bagasse It is well-known fact that oxygen in an alkaline medium can be used as a delignifying agent in pulp bleaching, both as a substitute for chlorine, and as true cooking agent for chemical pulping. Although it is accepted that molecular oxygen is a specific oxidizing agent for lignin, a first immediate draw back to its use is the low solubility in cooking liquors. This causes serious problems for mass transfer in a heterogeneous chemical process such as wood pulping. Even applying very high oxygen pressure, useful mass transfer of the delignifying agent (molecular oxygen) into the fibre walls, where the reaction should take place, is difficult to obtain in one stage wood pulping. Most efforts have been made in the development of tow –stage pulping of wood to give pulps comparable to kraft in yield and quality. These generally involve a first, more or less mild, cooking stage followed by mechanical defibration. The coarse pulp thus obtained is very suitable for a subsequent alkaline cook in the presence of oxygen due to its higher exposed surface area. But unfortunately the two-stage pulping process is rather cumbersome and likely to be high in capital construction costs. Afterwards, instead of wood chips, thermomechanica fibres have been used for alkali-oxygen pulping, but some strength deficiencies have taken place, originating from mechanical damage caused in the pressurized refining, which is itself an operation high in energy consumption. Environmental control laws on industrial emission, particularly the emission of sulphur compounds, 10 together with the need of more effective utilization of raw materials have contributed to the intense work done in recent years for the development of cooking methods which will give similar pulp qualities and preferably higher yields than the kraft method without the addition of sulphur compounds. One such method might be the old soda method, if only the yield and pulp quality could be improved. Many different additives have been used to improve the yield and quality of soda pulp. Among the additives, anthraquinone has given quite encourage results. It appears that AQ is still the most cost effective sulphur free accelerator for alkaline pulping, albeit of limited applications. The soda-AQ process does offer a direct advantage of eliminating air pollution associated with kraft process. In the case of bagasse, as for other annual plants, but unlike that of wood, the problem of mass transfer of delignifying agent should be much less important, since the plant structure, should permit much easier diffusion and penetration of the delignifying agent into the reactive zone of fiber wall. The loose and open structure and low lignin content of bagasse make it suitable to perform soda-oxygen-AQ pulping. Bagasse is obtained after the extraction of juice from sugar-cane. Each tone of sugarcane yield 150 to 160 kg of air dried bagasse. It is intimated that 20 to 25 million tone air dried bagasse is produced every year in India, and most of it is burnt to generate process heat/power in sugarcane processing unit. A little amount of bagasse is being utilized as raw material in the manufacture of paper and paperboard. The bulk density of bagasse is 65 to 75 kg/m3. Due to the shortage of fibrous raw materials in India, we have always paid particular attention to bagasse as a source of raw material for papermakng. So we thought it to be interesting to investigate bagasse pulping by means of alkali-oxygen cooking process. Lesson 7 Sulfite pulping The sulfite pulping process is a full chemical pulping process, using mixtures of sulfurous acid and/or its alkali salts (Na+, NH3+, Mg2+, K+, or Ca2+) to solubilize lignin through the formation of sulfonate functionalities and cleavage of lignin bonds. By 1900 it had become the most important pulping process, but was surpassed by kraft pulping in the 1940s. The traditional calcium acid sulfite cook must be carried out at a low pH of about 1.5 because of the relative insolubility of calcium. A higher pH would cause scaling compounds to precipitate during cooking, leading to a condition known as ―liming up‖ of the digester. Typically, 80% or more of the SO2 is in the form of free SO2. The use of soluble bases (magnesium, sodium, and ammonium) permits a greater proportion of combined SO2 in the cooking liquor. In so-called acid bisulfite pulping, any pH within the range 1.5 to 4.0 can be achieved by controlling the ratio of free to combined SO2. True bisulfite pulping, defined by equal amount of free and combined SO2, is carried out at a pH of 4.0 to 5.0. the full range of sulfite cooking also includes neutral sulfite and alkaline sulfite. The cooking operation is usually carried out batchwise in a pressure vessel consisting of a steel with acid-resistant lining. The digester is first filled with chips and capped, and then sufficient hot acid from high–pressure accumulator is added to almost fill the vessel. A slow come-up time, a relatively low maximum temperature (130-140?), and a long overall cook(6-8 hours) are typical for acid sulfite cooking in order to avoid undesirable lignin polycondensation reactions. More rapid heating with higher maximum temperatures and pressures can be used as the cooking liquor pH increases for acid bisulfite cooking. It is important to obtain good liquor penetration into the chips well before the maximum temperature is reached. The extent of cooking is generally dictated by the amount of delignification that is desired. Pulp for bleaching should be low in lignin, but if coking proceeds beyond an optimum point, pulp strength, viscosity and yield will be adversely affected. The point at which to stop an individual cook is based on the operator’s judgement. 11 Some advantages of sulfite pulping are bright, easily bleached pulps, relatively easily refined pulps, pulp that forms a less porous sheet that holds more water than kraft pulps, and pulps with higher yield than kraft. Disadvantage include a pulp that is weaker than kraft, not all species of wood can be pulped easily, cooking cycles are long, and chemical recovery is fairly complicated. Words and expressions Free SO 游离二氧化硫 combined SO 化合二氧化硫 22 acid bisulfite pulping 酸性亚硫酸盐制浆 acid-resistant lining 防酸衬里 Reading material: Extended delignification Conventional kraft pulping to bleachable kappa no ranges of 28-32 for softwoods and 18-22 for hardwoods has been common in the industry for many years. This high kappa no is detrimental to bleaching. The higher the kappa no is, the more bleaching chemicals be used, the larger the pollution loads are. Because Lignin removal by pulping is economically and ecologically more favorable than by bleaching, extended delignification has been a focus of numerous studies. The rate of delignification can be improved considerably by extended delignification, and the strength properties are not influenced. Softwood pulp has been commercially produced at about kappa no 25 with strength properties comparable to those of conventional kraft pulp at kappa 30 to 35. It is helpful to decrease the consumption of bleaching chemicals and the pollution of bleaching effluent. 1. Rapid displacement heating RDH system is a batch pulping process that recycles black liquor from a completed batch to pretreatment the raw chips in the next batch. White liquor is charged to the digester after the pretreatment. The black liquor pretreatment heats the chips and drives out air, neutralizes wood acids, and fills the chips with high sulfidity liquor. The primary advantages of the RDH process is a decrease in digester steam consumption, with a reduction of 60-80% compared with conventional batch kraft pulping. The energy savings accrue from the shorter steaming time required to reach white liquor cooking temperature. In addition, RDH pulping extends delignification, making it possible to produce pulps with lower kappa no than have been traditionally achieved. Lignin removal also is faster than in conventional batch kraft cooking. This improved pulping performance provides practical economic benefits in terms of increased production, saving in bleach plant chemicals, and reduced bleach plant effluent. Principles of operation —Chip filling: the digester is filled with chips often using a steam packer to increase the charge. Simultaneously, air is extracted via liquor circulation screens. —Warm black liquor fill: warm black liquor (WBL) is pumped into the bottom of the digester, displacing air to the atmospheric tank of the liquor tank farm. After a given volume of WBL is pumped through the chip-filled digester, the top outlet valve is closed. Continued pumping raises the pressure in the liquid-filled digester. Some white liquor may be used as a supplementary alkali source to keep the PH sufficiently high. ,Hot black liquor (HBL) fill: HBL is pumped in from the bottom up. The displaced liquor exits through screens in the dome of the digester and goes to the warm black liquor accumulator. —Hot white liquor (HWL) fill: Preheated HWL often mixed with hot black liquor is pumped into the system. Addition of some HBL afterwards clears the lines of white liquor. —Heating: The heating phase brings the digester contents to the desired maximum cooking temperature often by direct steam heating of recirculating liquor. Normally, only a10-15 increase is necessary 12 in the phase. Steam use is therefore minimal. This provides a big advantage over conventional batch cooking. —Time at cooking temperature and pressure. —Terminal displacement: The digester contents are cooled to stop reactions when the desired level of delignification is reached. This is done by pumping in brownstock washing liquor into the digester bottom, displacing the hot liquor from the top of the digester to pressurized hot liquor accumulators. )are discharged from the digester —Pulp discharge: the contents of the digester (now below 100? using dilution and pumping. Earlier versions of displacement batch cooking systems used compressed air or steam to push (blow) the material out. RDH viscosities averaged about 10% higher than those from conventional batch cooks. Comparing the tear strength for unbleached conventional and RDH pulps, the RDH pulps are significantly stronger than the conventional counterparts. Even more interesting is that the strength improvements of the unbleached pulps carry through the bleach plant. Bleached pulp strength is now approximately 10% higher than it was when the mill was operating on conventional Scandinavian batch and continuous cooking modes. Modified continuous cooking It has been a long time since modified pulping was used in the industry. At the end of 1970s, MCC (modified continuous cooking) was first used in North Europe. It can not only improve the viscosity and strength of pulp significantly, but also decrease the screen rejects. The cooking selectivity can be also improved. It is possible to extended delignification by MCC. EMCC (Extended Modified Continuous Cooking) was developed successfully based on MCC by Kamyr,Inc. at the end of 1980. It further improved the cooking selectivity. In 1993, Kawana, Inc. of Sweden developed ITC (Isothermal Cooking) technology, and the cooking selectivity was improved once more. With the development of the technology and the more severe limitation of environmental protection, extended delignification must be used more and more wildly in the papermaking industry. The improved selectivity of pulping means there has been less carbohydrate degradation for a given amount of lignin removal. This may be due to one, or a combination of two possible mechanisms. The modified process may alter the reactivity of the lignin to make it more easily removed. Alternatively, it could reduce the relative rate of carbohydrate degradation, perhaps because of its modified alkalinity profile. A summary of the principles is the following: —Concentration of dissolved lignin should be as low as possible toward the end of the cook. —Concentration of alkali should be low and uniform throughout the cook. —Concentration of sulfide should be as high as possible especially at the start of bulk delignification. —Ionic strength of liquor should be as low as possible especially toward the end of the cook. The main deviations from normal kraft pulping were the following: —White liquor was added at there different points in the digester system. —A recirculation line for liquor around the impregnation vessel was installed (dotted line in figure ). —The final stage of the cook was carried out in counter-current mode with a gradually decreasing temperature. Compared to conventional kraft cooking with the same wood in the same digester, it is possible to decrease the kappa number in the kraft cook by 10 units without sacrificing the pulp viscosity, when the conventional process is transformed to modified kraft pulping. The bleachability of the modified kraft pulps increased. The strength properties of the modified kraft pulps are slightly better than those of the conventional pulps even though the kappa numbers were much lower. Isothermal cooking The ITC process is similar in concept to the EMCC process. They differ primarily in the equipment used to heat and circulate the white liquor added to the very bottom of the digester. The former uses an additional dedicated heating circulation system. 13 ITC uses the entire digester for cooking. White liquor is added to the circulation in the countercurrent cooking zone, and the temperature is raised to the concurrent cooking zone level. High circulation flow in the lower part equalizes the final temperature and the alkali profile to produce low kappa number variation over the digester cross section and low reject content. Isothermal cooking is selective, this gives a uniform cook with higher pulp yield. For softwood, the yield at a given kappa number is 0.5%-1.5% higher than that of conventionally cooked pulp. The uniform cook produces pulp with good strength and high viscosity. The greatest advantage of isothermal cooking is improved bleachability. Prolonged cooking gives brighter pulp with a good bleaching response. The consumption of chemicals necessary to achieve a given brightness decreases. ITC pulp with its low lignin content is a prerequisite for totally chlorine free (TCF) bleaching. ITC pulps also have a higher brightness ceiling. Low solids cooking Low–Solids Pulping is a modified cooking process that uses split white liquor additions and counter current cooing methods whenever possible to achieve the following: 1. Uniform distribution of temperature and cooking chemical. 2. Even alkali profile. 3. Minimum peak cooking temperatures. 4. Minimum concentration of dissolved lignin toward the end of cooking. To meet these objectives, dissolved wood solids are removed from the system by extracting spent cooking liquors at multiple locations in the vessel. Extraction of these liquors will purge dissolved solids from the system and prevent them from entering subsequent cooking zones. As a result, the amount of by product present in the bulk and residual stages of delignification will decrease. Published results show that conversion to lo-solids pulping gives a 10%-30% decrease in organic solids concentration within the bulk and residual stages of delignification, which should result in a 5%-10%increase in pulp strength (tear), a 2%-5%decrease in alkali consumption, and 1%-3%improvement in bleachability with all other conditions constant. Enhanced delignification selectivity should also occur. Reading material: Semichemical pulping Semichemical pulping processes are characterised in principle by a chemical treatment preceded by a mechanical refining step for defibring or fibrizing. This general definition also applies to the chemimechanical processes and the high-yield chemical pulping processes such as the sulphite and kraft. It is more suitable to hardwoods. The chemical treatment in semichemical and chemimechanical pulping can be effected with reagents like sodium sulphite, caustic soda, and kraft liquor. The sodium sulphite is usually buffered to near neutrality with sodium carbonate or bicarbonate or kraft green liquor. The most important semichemical process is undoubtedly the neutral sulphite semichemical (NSSC) process. The general advantages of the NSSC process are low requirements with regard to wood quality and species, high yields, relatively low consumption of chemicals at a given residual lignin content, low capital investment and profitable small production units as compared to full chemical pulping. Besides single hardwoods and hardwood mixtures, mixtures of hardwoods and softwoods can also be pulped successfully with the NSSC process. NSSC pulping is also used for non-wood plants and residues because of the generally low lignin contents of these materials and the widely variable conditions offered by the NSSC pulping. The principal process involves impregnation with a neutral sodium sulphite pulping liquor at about 125 0C for an hour under pressure after a short steaming of the chips at atmospheric pressure, followed by cooking at temperatures between 160 and 190 0C, depending on the cooking time, which may vary between 15 minutes 14 and 8 hours, which again depending on the type of digester used and the desired pulp type and quality. Defibration is carried out by means of single or multi-stage refinement process using disk refiners. The lignin content of the NSSC pulps is high as compared to those from chemical pulps, and ranges between 10 and 15 per cent. Due to the high lignin and polyoses content the NSSC pulp has low conventional strength properties. The typical NSSC pulp is normally much more rigid and stiff than kraft pulp. Therefore, it is the most typical and suitable fibre material for the production of corrugating medium. The caustic soda in the soda semichemical process reacts with the lignin-carbohydrate complex to form soluble sodium lignate, and the carbohydrates are solubilized by hydrolysis. However, this lignin reaction occurs only after a major portion of the caustic soda has been consumed in neutralizing the readily available acetyl and methoxyl groups and in hemicellulose dissolution. Therefore, lignin removal is the least in alkaline semichemical pulping. The cold-soda or cold caustic process, which is less important than the NSSC process, involves in principle, the treatment of chips with a sodium hydroxide solution at temperatures generally between 20 and 30 0C and a final refiner defibration. In the cold-soda chemimechanical pulping, the caustic soda attacks the fibre bond mainly by reaction with the acetyl and other acid groups, which are reactive even at room temperature. The most important step in the cold-soda pulping is the impregnation with alkaline liquor to reach a very fast but total penetration of the chips, causing the necessary swelling of fibres and avoiding considerable losses of polyoses. Impregnation times are between 15 and 120 minutes with generally short reaction times of 15-30 minutes in pressurized and continuous systems. The concentrations of NaOH are generally low (0.25–2.5%), but up to 10 per cent in the case of some roller mill impregnation systems. Cold-soda pulping requires little installation capital, and despite the cost of the chemicals, the processing costs are actually lower than in stone grinding, because of reduced energy consumption. In some process modifications the liquor is reused up to 20 times. The cold-soda pulp yield ranges between 85 and 92 per cent, whereby the selectivity of lignin and polyoses dissolution is much lower than NSSC pulping. The main disadvantage of cold-soda pulps is a generally a low brightness level (40-50%), which can be effectively increased by a two-stage peroxide-hypochlorite bleaching. As the cold-soda pulps have properties comparable to NSSC pulps, these are used as unbleached coarse grades for corrugating medium production, and as bleached grades for printing papers and newsprint in combination with groundwood pulp and chemical pulp. In the kraft semichemical process, the reactions of kraft liquor are similar except thiolignin is formed and dissolved. Because of the buffering nature of the sodium sulphhydrate in the kraft liquor, attack on the hemicelluloses and cellulose is less in the kraft than in the soda process. In the acid sulphite and bisulphite semichemical pulping, the delignification reaction predominates under acid conditions. As with the NSSC process, the mechanism of delignification probably involves sulphonation of the lignin of the middle lamella in the solid state, followed by hydrolysis to soluble lignosulphonic acid and carbohydrates. The hemicelluloses are less dissolved in these acid procedures than in the others. In the sulphite chemimechanical pulping, the neutral or acid sodium sulphite solutions dissolve mainly carbohydrates. The relatively high temperatures employed have an important weakening effect on the fibre bond. The material from the chemical stage may be partially disintegratedand defibred through the mechanical action of digester discharging, conveying, or deliquoring. The first action in the defibring–refining machine is probably heating the fibre bond and further weakening it to the point that it will split. Temperature is an important factor in fibrizing semichemically softened fibrous material. The second action in the mechanical state is the actual disintegration of the fibre aggregates and separation into individual fibres. The third action in the fibrizing zone of the machine, which is generally superimposed on the defibring action, is the refining or processing of the individual fibres to prepare them for papermaking. This action, which largely involves 15 fibrillation, softening, and formation of colloidal, mucilage-like surfaces, is for the purpose of stock preparation as with any other type of pulp. The actions occurring during the mechanical part of semichemical pulping may take place in one or more stages or passes. This is determined by a number of factors including the kind of fibrous material and its particle size, the degree and kind of chemical treatment, and the requirements for papermaking. The semichemical pulps have chemical and strength properties intermediate between groundwood and full chemical pulps. The brightness of these pulps varies from 35 to 55 per cent which can be improved to 80 to 85 per cent by multi-stage bleaching. The semichemical pulps are characterized by their high lignin and hemicellulose (pentosans) contents. Reading material: High-yield chemical pulping There is no clear border line between semichemical pulping and the so-called high-yield chemical pulping, or between the resulting pulps. From a practical aspect, many high-yield chemical pulps are more or less semichemical pulps with regard to their yields (55-70% and even higher) and typical process design (disk refining after the chemical treatment). High-yield chemical pulps are obtained in principle by modified sulphite and sulphate processes; by applying reduced charges of chemicals and / or reduced cooking time and temperature, and a refining step after cooking. High-yield sulphite pulps are produced by an acidic sulphite, bisulphite or alkaline sulphite process. In high-yield acid sulphite pulping (with calcium, magnesium or sodium bases) the reaction rate is usually decreased by cooking at lower temperatures (120-130 0C) and with lower acidity of the liquor, i.e, less sulphur dioxide, than in full chemical sulphite pulping. Common yields of unbleached pulps are between 60 and 70 per cent. High-yield acidic sulphite pulps are often produced in newsprint sulphite mills, saving up to 30 per cent of wood compared with full chemical pulps. The pulps are mainly used as newsprint furnish in mixtures with groundwood and chemical pulp. Generally the strength values (except burst strength) and the freeness are lower than those of comparable full chemical pulps. In high-yield bisulphite pulping, the cooking liquor has no excess of sulphur dioxide and the maximum cooking temperature is somewhat higher than that in acidic sulphite pulping. The preferred bases are sodium and magnesium. High-yield bisulphite pulps are generally used in the same fields as acidic sulphite pulps (eg., in newsprint). High-yield alkaline sulphite pulping, a modification of alkaline sulphite pulping, is becoming an increasingly interesting alternative to the kraft process. The cooking liquor is buffered between pH 9 and 12, containing sodium sulphite, sodium carbonate, sodium hydroxide and sodium sulphide. The yields lie around 60 per cent and the pulp is brighter than normal kraft pulp. Due to its good strength properties, the pulp is useful as linerboard. In high-yield kraft or sulphate pulping, the typical karft process is modified either by reducing the charge of chemicals by about one-half or by decreasing the cooking time and temperature. The yields are generally between 55 and 65 per cent, but even up to 80 per cent are also possible. Usually high-yield kraft pulps have lower strength values and are darker than normal kraft pulps and NSSC pulps. The process is used for both hardwoods and softwoods. number of white rot fungi which could act preferentially to degrade middle lamella lignin leading to 16 Lesson 8 Chemical recovery Chemical recovery is the process in which the inorganic chemicals used in pulping are recovered and regenerated for reuse. Starting with ―weak black liquor‖ from the brown stock washers, the steps involved in chemical recovery are as follows: , Concentration of the residual liquor in multiple-effect evaporator to form ― strong black liquor‖ , Black liquor oxidation if required. , Further concentration of the residual liquor to form ―heavy black liquor‖, salt cake can be added at this point to make up soda loss. , Incineration of liquor in the recovery furnace. , Dissolving smelt from the furnace to form green liquor. , Causticizing green liquor with lime to form white liquor. , Burning of lime mud to recover lime. The concentration of black liquor leaving the brown stock washers with a solid content of 13%-17% must be increased to 60%-80% before the liquor can be fried in the recovery boiler. The substances in black liquor derived from two sources: wood and white liquor. The dissolved wood substances consist of ligneous materials, saccharinic acids, lower molecular weight organic acids, and extractives, which are the source of energy and combined chemically with sodium hydroxide in the form of sodium salts. The inorganic constituents include sodium hydroxide, sulfide, carbonate, sulfate, thiosulfate, and chloride, which originate from the white liquor used for pulping. The bulk of the water removal is carried out in the multiple-effect evaporators, a series of reboilers operated at different pressures so that the vapor from one evaporator body becomes the steam supply to the next unit. In those systems where liquor is concentrated to about 50% solids in multiple-effect evaporators and then raised to 65% solids in a direct contact evaporator. The 50% solids liquor is usually called strong black liquor while the liquor fired into the furnace is called heavy black liquor. The climbing-film, long-tube-vertical evaporator is the most widely used type of evaporator in black liquor service. Recent years have seen the application of new type of evaporators to black liquor service, which includes falling-film evaporators, preheat-falling-rising types, forced circulation units and crystallizing evaporators and other variations such as vapor-compression evaporators, waste heat evaporators, and sophisticated condensate treatment systems. Concentrated black liquor is burned in a recovery boiler to complete the combustion of the organic matter in black liquor and recover the sodium and sulfur content in a form of suitable for regenerating the pulping chemicals. During combustion, oxidized sulfur compounds can be converted to sulfide and the inorganic compounds melt and flow out of the furnace as mixture of molten salts called smelt. The smelt leaving the furnace is dissolved in water to produce a solution called green liquor. The green color comes from impurities and the main components in green liquor are NaS and NaCO. The green liquor 223 is clarified and causticized with lime to produce white liquor that is suitable for pulping. The lime mud is washed to remove entrained white liquor and the weak wash liquor sent to the dissolving tank. The washed mud is then calcined in a lime kiln or fluid bed calciner to regenerate lime. The resulting lime is used for causticizing and the calcium cycle is completed. Calcining is a high temperature, heat absorbing (endothermic) reaction. Recent trends have been toward increased thermal efficiencies for kilns, and the use of substitute fuels for oil or natural gas. Some other operations are frequently carried out in the recovery system. These include soap skimming and tall oil recovery, black liquor oxidation to convert sulfide to thiosulfate and ―stabilize‖ sulfur in the liquor, and waste gas incineration for odor control. Words and expressions 17 Chemical recovery 化学品回收 Weak black liquor 稀黑液 Brown stock washer 本色浆洗浆机 Multi-effect evaporator 多效蒸发器 strong black liquor浓黑液 heavy black liquor重黑液 salt cake芒硝 incineration 焚烧 Green liquor绿液 white liquor白液 lime mud 白泥 Causticize苛化 heavy black liquor浓黑液 The climbing-film, long-tube-vertical evaporator 升膜长管立式蒸发器 falling-film evaporators降膜蒸发器 preheat-falling-rising types预热升降膜蒸发器 Smelt:熔融物 罗油 calcine煅烧 calciner煅烧炉 soap皂化物 tall oil塔 Lesson 9 Brown stock washing The purpose of brown stock washing is to economically remove the maximum amount of dissolved organic and soluble inorganic materials presented with the pulp mass and recover the maximum amount of spent chemicals with minimum dilution. The ―dilution‖ of the mother liquor generated during pulping is most conveniently expressed in weight units of water per weight unit of pulp, which is called dilution factor and can be accurately correlated with the degree of washing when all other independent variables are held constant. The standard method of washing was to employ a series of rotary vacuum washers operating in a countercurrent flow sequences. The main element of a vacuum washer is a cloth-covered cylinder that rotates in a vat containing the pulp slurry. By means of internal valving and a sealed drop leg, vacuum is applied as the rotating drum enters the stock. A thick layer of pulp builds up and adheres to the wire face as it emerges from vat. Wash water is applied to displace the black liquor in the sheet as the drum continues to rotate. Finally, the vacuum is cut off and the washed pulp is recovered from the mold. Diffusion washing is characterized by a relatively long period of contact between the pulp and the moving wash liquor. This allows time for the liquor solids to diffuse or leach from the fiber structure. Rotary pressure washers are similar in operation to rotary vacuum washers, but offer some significant advantages. The pulp mat is formed on the surface of the cylinder and dewatwered with the aid of pressure applied inside the wash hood, as opposed to vacuum inside the cylinder. Because the driving force for mat formation, dewatwering and wash liquor displacement is outside the cylinder, the interior of the washer drum can be utilized for a more sophisticated liquor collection system; therefore, a single washer can be operated with two or three displacement stages. The higher pressure also allows higher-temperature wash liquor to be used and greatly reduces foaming. The closed vapor circulation system facilitates collection and treatment of odorous vapors. The horizontal belt washer resembles the fourdrinier section of a paper machine. The pulp suspension is distributed from a headbox onto a traveling filter belt, and formed into a mat. Wash liquor is applied to the top side of that mat while displaced filtrate is removed from the underside of the belt by suction boxes. The washer is operated in a countercurrent mode, with the filtrate from one section being returned as washer liquor to the previous section, and ultimately as dilution for the headbox furnish. In common with the vacuum washer, displacement is the principle washing mechanism, but no mixing and reforming of the pulp mat is required between stages. A total of five displacement stages are easily accommodated along the belt. Norden’s method is a recent proliferation of new washing methods to assign an efficiency number to equipment which is independent of dilution factor. It assumes that a washing stage can be likened to a number of countercurrent mixing stages connected in series. In an individual stage, the pulp and associated solids-containing liquor from the previous stage and mixed with lower-solids wash liquor from the next stage; 18 the stock is then re-thickened to the original consistency and the separated stock and liquor are passed countercurrently to their respective next stages. The Norden efficiency factor is defined as the number of mixing stages which give the same results as the washing equipment under consideration, when operated at the same wash liquor ratio. The total Norden efficiency for a system may be found simply by adding the factors for the various components. Given the system Norden efficiency and the dilution factor, the anticipated washing efficiency of the system can be found. vacuum washers真空洗浆机 valve阀门 drop leg水腿 Diffusion washing扩散洗涤 pressure washers压力洗浆机 headbox 流浆箱 suction box真空箱 Lesson 10 Screening In most pulp and paper processes, some type of stock screening operation is required to remove oversized, troublesome and unwanted particles from good papermaking fibers. The major types of stock screens are vibratory, gravity centrifugal, and pressure (centrifugal or centripetal). They all depend on some form of perforated barrier to pass acceptable fiber and reject the unwanted material. In most instances, it is the size of the perforations (usually holes or slots) that determines the minimum size of debris that will be removed. All screens must be equipped with some type of mechanism to continuously or intermittently clean the openings in the perforated barrier. Otherwise, the screen plate would rapidly plug up. Methods of cleaning employed on current commercial screens include shaking and vibration, hydraulic sweeping action, back-flushing or, most common, pulsing the flow through the openings with various moving foils, paddles and bumps. The typical screen is a relatively simple machine to operate. The most important consideration for a stable, efficient operation is to maintain flow and consistency near optimum levels. Vibratory flat screen at one time were practically the only type used in pulp and paper mills and is capable of efficient separation and concentration of reject material. However, its many disadvantage (e. g. open construction, foam problems, high maintenance, labor intensity, large floor area requirement) have rendered it obsolete for all but specialized applications. The rotary vibratory screen is more compact than the flat screen and requires less operator attention. But, the high maintenance cost has also limited this design for most applications. The gravity centrifugal screen overcame many of the problems of the vibratory screen. The principle of the screen is based partly on the fact that good fiber tends to be thoroughly hydrated and has a specific gravity close to water. When the low-consistency pulp stock is rotated in the centrifugal screen, the fibers align themselves with the direction of flow, which is predominately through the circular holes in the screen plate. The coarse materials are not fully hydrate and have a lower density; this factor limits the effect of centrifugal force and the coarse materials tend to be carried across the screen plate to discharge as rejects. The coarse materials acuminates as it moves axially, and this loose mat also acts to some extent as a screening element. Gravity centrifugal screens have been applied to as a wide range of stock screening applications. The operation principle of pressure screen is similar to gravity centrifugal screens, the distinction is that they operate under full line pressure and the radial flow within the unit can be either centrifugal (outward), centripetal (inward) or a combination depending on design. They have the advantage of high capacity per unit, flexibility of physical location small space requirements and economy of piping and pumping. The totally enclosed design excludes air entrainment and minimizes slime buildup. Because a large rejects flow is required from the primary pulp screens in order to achieve adequate 19 debris removal efficiency, additional stages of screening are performed on the reject stream to concentrate the debris and return the good fiber to the process. A ― cascade arrangement‖ of screens is usually employed. Lesson 11 Centrifugal cleaning High specific gravity contaminates such as sand and dirt solids can be removed from pulp suspension through application of the centrifugal cleaner. This device, also identified by such terms as hydrocycl liquid cyclone, hydrocyclone, vortex cleaner or centricleaner, consists of a conical or cylindrical-conical pressure vessel with a tangential inlet at the largest diameter of the cone or cylinder. Also centered axially at the large diameter end is the vortex finder or accepts nozzle. At the opposite end of minimum-diameter end is the underflow tip or rejects nozzle. The centrifugal cleaner removes unwanted particles from pulp and paper stock by a combination of centrifugal force and fluid shear. Therefore, it separates both on the basis of density difference and particle shape. All centrifugal cleaners work on the particle of a free vortex generated by a pressure drop to develop centrifugal action. The power source is the pump. The stock enters the cleaner tangentially, the inlet scroll guides the flow to impart a rotating motion. As the stock flows inward, the velocity increases, resulting in high centrifugal forces near the center which carry dense particles outward and away from the accepted stock. Good fiber is carried inward and upward to the accepted stock outlet. The dirt, held in the downward current, continues toward the tip. As the diameter narrows, the flow is forced inward against increasing centrifugal force, which concentrates the dirt and release good fiber to the accepts flow. The principal operating problem with centrifugal cleaners is plugging of the reject orifice with foreign materials, fiber flocs, or high-consistency stock. Cleaners are typically employed in a cascade sequence similar to that used for screens. Three types of centrifugal cleaners are used in the pulp and paper industry. The original classic design of centrifugal cleaner for the removal of heavy debris, which is now more precisely designated as ―forward cleaning‖, still accounts for great majority of installations. Reverse-flow and through-flow cleaners are used principally to remove light debris in secondary fiber pulping operations. Lesson 12 Thickening Following low-consistency operations such as cleaning and screening, it is necessary to thicken the stock prior to the next process operation. In many instances, a simple increase in stock consistency to the 4%-8%range is required. For this service, a gravity thickener is commonly used. Water flows into the cylinder by virtue of the difference in liquid level between the vat and cylinder; pulp is retained on the rotating cylinder and is couched off by a rubber roll. For intermediate levels of thickening (10-12%), the so-called valveless washer can be used. When thickening from a low consistency below 0.7% to a level of 10-12%, a two stage operation may be required. To achieve consistency of 12%-16% and also wash the stock, it is necessary to use a vaccum washer. All equipment of this type functions by applying vaccum over the forming and washing zones, then cutting off the vaccum by means of an internal valving arrangement to allow the thickened sheet to be discharged. The suction effect is typically produced by the filtrate drop leg which discharges into a seal tank. The pulp mat is discharged from the washer mould by takeoff rolls or doctors. The mesh face is often shower-cleaned prior to re-submergence in the vat. For simple thickening of very dilute stock up to 12% consistency, multidisc thickeners of the type can be used. Multidisc thickeners are most commonly used as saveall devices, i.e., to recover fine fibers from 20 white water and reuse the water. To achieve stock consistencies above 15%, some type of screw extractor or press arrangement is usually employed. Reading material: Reaction in alkaline cooking During kraft pulping lignin fragmentation is performed by the cleavage of aryl ether bonds and formation of new phenolic groups, which increase the hydrophilicity and solubility of lignin. The reactions of lignin in a kraft cook are complex and still not completely understood, but the main reactions leading to the lignin degradation in alkaline conditions have been well reviewed. The most important delignifying reaction in alkaline conditions is the cleavage of β-aryl ether linkages, which are the most prominent lignin structures. In kraft pulping conditions the reaction of hydrosulphide ions with quinone methides leads to the cleavage of β-O-4-linkages, whereas in the absence of hydrosulphide ions, such as in the soda cook, the dominating reaction is the elimination of γ-hydroxymethyl or β-proton, which leads to the formation of formaldehyde and enol ether structures.In addition to hydrosulphide ions, anthraquinone (AQ) and polysulphide (PS)have also been found to enhance the cleavage of β-aryl ether linkages. The cleavage of non-phenolic β-aryl ether bonds is the dominating and rate determining reaction in alkaline lignin degradation. The etherified β-aryl ether linkages are cleaved by hydroxide ions via an oxirane intermediate. This reaction is not affected by sodium sulphide or anthraquinone, but for example α-carbonyls have been shown to accelerate the cleavage of β-aryl ether linkages in non-phenolic lignin units. Because the reaction is intramolecular, the stereostructure also affects the reactivity and the erythro form of the β-O-4 structure has been found to be more reactive than the corresponding threo form. The phenolic α-aryl ether bonds are cleaved most easily, but since the amount of non-cyclic α-aryl ether structures is low compared to the β-O-4 structures their cleavage does not lead to the major degradation of lignin. The cleavage of the α-ether bonds of phenolic phenyl coumaran structures leads to the formation of alkali stable stilbene structures. The non-phenolic α-aryl ether structures have been reported to be stable. In alkaline conditions of pulping, some undesired carbohydrate degradation also takes place. Owing to the low degree of polymerisation, the amorphous hemicelluloses are more susceptible to degradation and dissolution, but loss of cellulose cannot be avoided either. The end-wise depolymerisation, i.e. peeling, occurs via reducing end groups creating carboxylic acid derivatives. Random alkaline hydrolysis of glycosidic bonds occurs to a lesser extent, but at the same time new end groups are formed and secondary peeling may take place. In modified kraft processes PS and AQ can be used, not only to accelerate delignification, but also to improve the selectivity. Both PS and AQ prevent the peeling of polysaccharides by oxidising their reducing end groups to alkali stable aldonic acids. Proposed obstacles to delignification The low rate of delignification in the residual phase of pulping has often been suggested to be a consequence of alkali stable lignin-carbohydrate linkages as well as less reactive condensed lignin structures. The location and accessibility of residual lignin may also have a significant effect on the delignification. Lignin-carbohydrate complexes Since lignin and carbohydrate components cannot be separated completely by selective chemical treatments or separation methods, a very intimate association between the residual lignin and carbohydrates has been suggested. These so called lignin-carbohydrate complexes (LCC) are most likely native, but the formation of LC-linkages during pulping has also been shown to be possible. Most of the evidence of covalent LC-bonds is indirect, but lignin and carbohydrates have frequently been proposed to be linked by benzyl ester, benzyl ether and glycosidic linkages. Ester linkages are known to 21 be readily hydrolysed in alkali,whereas the α-ether LC-linkages have been shown to be relatively stable under alkaline pulping conditions. According to model compound experiments the α-ether LC-linkages, which have been suggested to exist between lignin and all types of wood polysaccharidesalso retard the cleavage of adjacent β-aryl ether linkages, having thus a negative impact on the delignification. The covalent LC-linkages also increase the extent of cross-linking in lignin molecules and thus decrease the solubility of degraded lignin. Condensed lignin structures Condensed aromatic lignin structures have also been suggested to contribute to the incomplete delignification, since the relative amount of mainly C5 substituted guaiacyl structures has been found to increase in residual lignins during kraft pulping and oxygen delignification. This may be a consequence of the formation of new condensed structures as well as the enrichment of original condensed lignin structures into fibres due to their less reactive nature. Condensed 5-5’-biphenyl and 5-O-4 lignin structures have been shown to be less reactive and to accumulate into the fibres during kraft pulping. Recently it was also found that the β-O-4 linkages connected to the condensed structures are more stable under kraft pulping conditions than those connected to non-condensed moieties. However, with model compounds it has been shown that a variety of condensation reactions are possible in alkaline pulping, leading to the formation of more stable carbon-carbon bonds between lignin units. New condensed lignin structures, e.g. α-1 and α-5-linkages can be formed by the reactions of quinone methides with carbanions formed in alkaline conditions. Diphenyl methane (DPM) structures have also been suggested to form by the reaction of carbanions with formaldehyde released during pulping by the cleavage of hydroxylated γ-carbon. Formation of DPM structures is more prominent in the absence of HS ions, but it has been shown to occur also during kraft pulping.As already mentioned, condensation reactions with carbohydrates are also possible. Especially the C5 substituted DPM and 5-5’-biphenyl structures but also some C6 substituted lignin structures have been shown to be fairly resistant towards oxygen delignification. The corresponding resistance of condensed structures has not been observed towards peroxide or chlorine dioxide. Using model compounds it has been shown that the biphenyl structure degrades in oxygen delignification conditions much slower than the corresponding monophenolic structure. This was suggested to be due to the formation of an intramolecular hydrogen bond after ionisation of the other phenolic hydroxyl group, thus making the electron transfer to oxygen more difficult. However, the formation of condensed 5-5’-biphenyl structures by radical coupling reactions during oxygen delignification have also been shown to be possible. Heterogeneous structure and distribution of lignin in softwoods The accessibility of lignin as well as its reactivity may vary in different morphological regions due to the heterogeneous structure and distribution of lignin within the cell wall and within various constituents of the wood. It is well known that in the middle lamella and the primary wall the concentration of lignin is higher than in the secondary wall, which still contains most of the lignin. Ray cells and compression wood are also known to be very lignin-rich. Furthermore, a distinction in the chemical structure of lignin also exists which depends on the origin of the lignin. The content of phenolic lignin units has been reported to be higher in the secondary wall lignin than in the primary wall or middle lamella lignin. The molar mass of lignin is also higher in the middle lamella. It has been shown that the etherified lignin units are more condensed than their phenolic counterparts, which is consistent with the results, according to which the lignin structure in the middle lamella is more condensed than in the secondary wall. Also in ray cells lignin has been found to be more condensed and less phenolic than in the other constituents of wood. Since the degradation of lignin is mainly driven by the cleavage of aryl ether bonds, whose reaction mechanisms are dependent on whether they are phenolic or non-phenolic, and the condensed lignin structures are supposed to be less reactive, the distribution of those functionalities may have a significant effect on the delignification. It has been shown that 22 in kraft pulping conditions the dissolution rate for lignin in the highly lignified middle lamella and ray cells is lower than that for whole wood. After kraft pulping, residual lignin is still unevenly distributed and the surface layers of the kraft fibres and the ray cells contain more lignin than the long fibres. It has been suggested that two types of LC-complexes exist in the kraft fibres: a high molar mass lignin-galactan complex, which is assumed to originate from the outer layer of the cell wall or compression wood, and a lower molar mass lignin-carbohydrate complex, whose origin is mainly the secondary cell wall. The lignin fraction with the higher molar mass was also found to be slightly more condensed than that with the lower molar mass. In addition to the structural differences, the content of transition metal ions detrimental to oxygen and peroxide bleaching has also been found to be higher in ray cells and on the fibre surface. These factors may greatly affect the uniformity of further delignification. It has been reported that kraft pulp without ray cells consumed considerably less active chlorine in order to obtain a certain brightness level than the original pulp. Higher viscosity and lower peroxide consumption have been reported for a pulp, which was peeled before TCF-bleaching in order to remove lignin rich ray cells and fibre surface material. The lower content of transition metals after removal of surface material also results in the reduced degradation of peroxides and formation of detrimental hydroxyl radicals which induce cellulose degradation. Lesson 13 Pulp bleaching Bleaching is the treatment of wood and other lignocellulosic pulps with chemical agents to increase their brightness. Brightness is increased by decolorizing or dissolving the colored components in pulp, primarily lignin. Two approaches are used in the chemical bleaching of pulps. One is to almost totally remove residual lignin, which is used for bleaching of chemical pulps and leads to greater fiber-fiber bonding strength in paper but weaker fibers due to the strong chemical used to decrease the length of cellulose molecules. The other approach is to utilize chemicals that selectively destroy some of the chromophoric groups in mechanical pulps, which is referred to as lignin-preserving or brightening.0 Since lignin is a complex molecule with different linkages, the breakage of various types of bonds requires the use of different chemicals. Therefore, all the colored material in pulps can not be eliminated by any single chemical and bleaching is a multi-step process. The principal commercial bleaching chemicals consists of the oxidants: chlorine (C), oxygen (O), hypochlorite (H), chlorine dioxide (D), hydrogen peroxide (P), and ozone (Z); the only reductant: hydrosulfite; and the only alkali: sodium hydroxide (E, or caustic extraction). It is possible to rank each of the oxidizing chemicals for economy, selectivity, and particle bleaching, although it must be remembered that the particular conditions of use will greatly effect on ranking. The rankings are: Economy: O>C>D>H>P>Z; Selectivity: Delignification D>C>O>Z Brightening: D, P>H Particle bleaching: D>H, C, O, P>Z Because of their greater economy, chlorine and oxygen are used in the early stages of bleaching for lignin removal, but can not be used for more extensive treatment because their poor selectivity results in cellulose degradation. Chlorine dioxide, hypochlorite, and peroxide can be used in the later stage due to their selectivity and also their higher cost. We can divide bleaching sequences into two parts: delignification and brightening. In the delignification part, lignin removal is the objective and kappa number is used as the principal control parameter. In the brightening part, higher brightness is the objective and brightness is the main control parameter. Oxygen can 23 be used either before chlorination or after as an additive to the E stage (Eo). In the delignification part of the sequence, 80%-85% of the residual lignin in unbleached pulp is removed. The brightening part of the sequence can be single stage, i. e., H to reach 70% brightness. Or this part can be 3 or 4 stages to reach 90% brightness. Hypochlorite and chlorine dioxide are the principal bleaching agents in this part. Sequences more typical of modern mills for bleaching softwood kraft pulp are (D+C)(E+O)DED and O(D+C)(E+O)D. The most widely adopted short sequence is (D+C)(EO)D. In most cases, each bleaching stage is followed by a washing stage to remove reaction products. Sulfite and hardwood kraft pulps are ―easier bleaching‖ than softwood kraft pulps. Both have lower lignin content, and the lignin residues in sulfite pulps are partially sulfonated and therefore more readily solubilized. Consequently, a somewhat simpler process can be used to achieve a comparable brightness level. For mechanical pulp bleaching or pulp brightening, in order to retain the advantage of high yield, pulps must be decolored or brightened by methods that do not solubilize any appreciable amount of lignin. Commercially used chemicals for this purpose today are sodium hydrosulfite which has a reductive action, and hydrogen peroxides which have an oxidative action. Single-stage treatments are sufficient for improving the eye appeal of newsprint-type pulps. Brightness level over 80 can often be achieved by a modern two-stage peroxide or peroxide / hydrosulfite sequence. Reading material: General lignin reactions in pulp bleaching The chemistry of bleaching is concerned mainly with the response of lignin to the action of various bleaching agents because successful bleaching requires that lignin be removed or modified from the pulp. In theory sites ortho and para to the hydroxyl and alkoxyl substituent on the aromatic ring should be preferentially attacked by electrophiles. Attack of bleaching chemicals on the carbohydrate components of pulp is unavoidable and has the potential for causing deterioration of pulp strength and yield. 1. Electrophilic substitution and oxidation of aromatic structures The general sequence is comprised of three distinct pathways: electrophilic substitution at an unsubstituted ring carbon, electrophilic substitution of the 1-carbon with displacement of a benzyl alcohol side chain, and dealkylation. All of these transformations proceed through a complex to three distinct cyclohexadienoyl intermediates. Additional substitution may take place at other electron-deficient ring sites to a degree depending on the type and availability of the cationic species and the incidence of competing reactions. The effect of the introduced ring substuituent on lignin solubility depends on the hydrophilicity of the attacking cationic species; for example, chloro substituents have an adverse effect on water solubility; hydroxyl groups have the opposite effect. 2. Addition reaction of olefinic structures Ethylenic groups are often present in the lignin component of pulp as parts of larger structural types, for example, stilbenes and vinyl aryl ethers. As electron-rich entities, these groups react initially with cationic bleaching species. Following the formation of a π-complex, a carbonium or oxnium ion is formed which is attacked by weak nucleophiles, Y-, in the system to yield vicinal disubstitution products. Addition reactions do not directly lead to the degradation of lignin, the physical properties and chemical reactivity of the modified lignin may be significantly altered. 3. Free reactical-initiated cleavage of ortho-quinonoid rings Cationic bleaching species are unable to oxidatively cleave aromatic rings or o-quinonoid intermediates. Instead, ring rupture is proposed to occur following the coupling of an initially formed phenoxyl radical or cationic radicals with a reactant radical. The initial products of the ring opening are muconic 24 acid(2,4-hexadienedioic acid) esters. The formation type of structures is noteworthy because they contribute enhance water and alkali solubility to lignin residues. Moreover, subsequent hydrolysis of the alkyl or aryl ester groups leads to cleavage of an interunitary lignin linkage. Muconic acid derivatives are also subject to further oxidative fragmentation reactions by bleaching chemical such as oxygen, ozone, and chlorine dioxide. Coupling of lignin free radicals 4. The coupling is preceded by the abstraction of a hydrogen aton from a phenolic hydroxyl group by a reactant free radical to form a phenoxyl radical. A cyclohexadienonyl radical mesomer of the phenoxyl radical then couples with a different or identical free radical to form a biphenyl compound. The coupling of lignin radicals may negate the desirable effect of competing lignin-degrading reactions. 5. Homolytic rearrangement of initially formed free radicals Initially formed cation and hydroxylcyclohexadienonyl radicals are postulated as undergoing a rearrangement in which the bond between the αandβcarbon atoms of the side chins is homolytically cleaved. This reaction scheme is based on the observed response of lignin model compounds, particularly those having benzylic alcohol groups, to various reactant radicals. 6. Free radical abstraction of hydrogen atoms Hydrogen atom abstraction from both lignin and carbohydrates by free radical has been postulated. For lignin, the hydrogen atom may be directly and preferentially removed from the alpha carbon atom of the side chain. In this process, a benzylic hydroxyl group is converted to an α- keto group. Alternatively, anα- keto group may arise following abstraction of a hydrogen atom from the benzyl carbon of a cationic radical or cyclohexadienonyl radical. The α- keto group activated the side chain in reactions with nucleophiles in later bleaching stages. The introduction of keto groups into hemicellulose and cellulose chain units makes them susceptible to β–elimination (i.e., peeling) reactions which often translated into losses in pulp yield and fiber strength. Reading material:Recation of carbohydrates with bleaching agents and alkali Although the chemistry of pulp bleaching mainly involves the reactions of lignin with bleaching chemicals, carbohydrates nevertheless undergo concurrent transformations during bleaching that often have an important bearing on the strength properties and yield of the resulting pulp. The impact of these reactions becomes increasingly more significant as the lignin content of the pulp decreases. The reactive sites for attack by electrophilic bleaching species on carbohydrates are acetal (glycosidic) linkages, reducing end-groups and enone structures formed initially by attack of an electrophilic bleaching species. The reactions of carbohydrates with bleaching chemicals may be conveniently classified according to whether they occur at acetal sites or elsewhere. 1. reactions at acetal sites In a free radical chain process, the chlorine radical abstracts a hydrogen atom the C-1 atom of the glycoside forming an organic radical which then reacts with elemental chlorine to regenerate chlorine radicals and form a chlorinated anhydroglucose unit. In an ensuing hydrolysis step, the acetal linkage is directly cleaved and an aldonic acid end group is ultimately formed. In the ozonation of carbohydrate model compounds and cellulose, the dominant reactions in the initial phase are ionic and involve a direct attack on the acetal linkage. The C-H bond is attacked by ozone and a hydrotrioxide is formed by a 1,3-dipolar addition mechanism. The hydrotrioxide undergoes fragmentation to the lactone with release of oxygen which may be in the singlet form. 2. Reaction at non-acetal sites 25 Oxidation of hydroxyl groups The oxidation of a secondary alcohol group is initiated by abstraction of a hydrogen atom from a hydroxyl-substituted carbon (i.e., C-2). An ensuing attack by molecular oxygen or other electrophiles (chlorine, hypochlorite, hydroxyl, or hydroperoxide radicals) on the hydroxyalkyl radical leads ultimately to the formation of the ketol. This sequence may also be used to rationalize the formation of 3-keto and 6-aldehydo groups. β-alkoxy elimination: cleavage of polysaccharide chains Ketols formed by the oxidation of polysaccharide alcohol groups readily undergo alkali-induced β-alkoxy elimination reactions. β-elimination leads to chain rupture that is manifested as a decrease in pulp viscosity and frequently by a loss of pulp strength. The role of carbonyl groups in the viscosity loss is indicated by the finding that their removal by reduction with sodium borohydride before reaction with alkali greatly decrease the extent of viscosity loss. Oxidation of reducing end groups The aldehyde form of a terminal polysaccharide unit can be oxidized to a carboxylic acid by chlorine, hypochlorite ion, and ozone. Lesson 14 Beating and refining The terms beating and refining are often used interchangeably. More precisely, beating refers to the mechanical action of rotating bars opposing a stationary bedplate on a circulating fiber suspension where the individual fibers are oriented perpendicular to the bars. This batch operation is exemplified in the traditional Hollander beater which is still used in some older paper mills, especially for handling such difficult furnishers as jute, hemp, flax and cotton. Refining refers to the mechanical action carried out in continuous conical or disk-type refiners where the fibers move parallel to the crossings. In all cases, the objective is to develop or modify the pulp fibers in an optimal manner for the demands of the particular papermaking furnish. During refining, both mechanical and hydraulic forces are employed to alter the fiber characteristics. Shear stresses are imposed on the fibers by the rolling, twisting, and tensional actions occurring between the bars and the grooves and channels of the refiner. Normal stresses are imposed by the bending, crushing and pulling/pushing action on the fiber clumps caught between the bar-to-bar surfaces. The initial action is to partially remove the primary (P) wall. Removal of P-layer exposes the secondary (S) wall and allows water to be absorbed into the molecular structure. The consequent loosening of the internal structure (internal fibrillation) promotes fiber swelling and renders the fiber soft and flexible. The further action of external fibrillation involves loosening of the fibrils and raising of the fiber microfibrils on the surface of the fibers, resulting in a very large increase in surface area for the beaten fibers. As the fibers become more flexible, the cell walls collapse into lumens, thus creating ribbon-like elements of great conformability. Fiber shortening or cutting always occurs to some extent during refining, mainly due to the shearing action of the bar crossings. Refining also produces fines consisting of fragments of broken fibers and particles removed from fiber walls. One obvious effect of refining is the dramatic change in the drainage or dewatering properties of the pulp. Pulp drainability is rapidly reduced as the refining proceeds, mainly due to the increased concentration of fines. Two factors are of primary importance in analyzing refiner performance: the amount of effective energy applied per unit weight of pulp (net specific energy), and the rate at which the energy is applied (refining intensity). The first factor can be precisely measured. The specific edge load is widely applied to evaluate how intensively the fibers are hit, which is calculated by dividing the rate of net energy application (net power) by 26 the total length of bar edges contacting the stock per unit time. Two major types of continuous refiners used for stock preparation are disc refiners and conical refiners. The conical refiners can be further differentiated into low-angle types (Jordans) and high-angle types (Claflins). In conical refiners, the rotating plug (rotor) and its housing (stator) are fitted with metal bars oriented lengthwise. The fiber flow parallel to the bars. The position of the plug determines the clearance of the bars and controls the amount of work done on the fibers for a constant stock throughout. Disc refiners are more recent development and offer significant advantage over conical refiners. There are three basic types of designs: rotating opposing stationary disc; two opposing rotating discs; and rotating double-sided disc between two stationary discs. For the last design, the stock flow can follow either a parallel arrangement (duoflow) or a series arrangement (monoflow). Typically, tear strength always decreases with refining due to the strength attrition of individual fibers; other strength parameters such as burst, tensile, folding endurance increases due to improved fiber-to-fiber bonding. The paper stock become slower to drain and resultant sheets become denser with reduced porosity, lower opacity, and decreased dimensional stability. Lesson 15 Nonfibrous additives and wet-end chemistry A wide range of chemicals is utilized in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes. Functional additives such as alum, internal sizing agents, mineral fillers, starch, and dyes are used to impart certain qualities to the paper product and must be retained on the sheet to be effective. Control additives such as drainage aids, deformers, retention aids, pitch dispersants, biocides, and corrosion inhibitors are added as required to improve the papermaking process, but do not directly affect the product and are not necessary retained on the product. Many additives have several effects at the same time; for example, alum is required for rosin sizing under acid conditions but also serves as a drainage and retention aid. The order of addition must be taken into account to prevent interaction at the wrong time and enhance retention in the paper sheet. For the economic perspective on the chemical and mineral contribution to papermaking, an average 10% of the cost of making paper can be attributed to chemicals. Wet end chemistry deals with all the interactions between furnish materials and the chemical/physical process occurring at the wet end of the paper machine. The major interactions at the molecular and colloidal level are surface charge, flocculation, coagulation, hydrolysis, time-dependent chemical reactions and microbiological activity. These interactions are fundamental to the papermaking process. There are three major groups involved in wet-end chemistry: solid, colloids and solubles. Most attentions is focused on the solids and their retention. Some colloidal materials, referred as cationic demand, anionic trash, or interfering substances, are introduced from either the carryover of pulping mill and bleaching plant, broke, or process water and will interfere with the action of cationic chemical additives. The zeta potential is a colloidal effect having to do with charge distributions on the surface of suspended particles, which is the charge density on the surface of colloids and suspended particles. It varies from about -50mV to +50 mV. Retention is often at a maximum when the charge density is near zero. Retention of nonfibrous additives occurs through the mechanisms of filtration, chemical bonding, colloid phenomenon, and adsorption. Filtration is important for retaining only larger particles. The aluminum polymer has a significant flocculation effect by bridging from particle to particle and thereby forming large ionically-attracted flocs. However, the retention effect is sensitive to shearing forces of strong agitation, the synthetic polymers have been developed with good shear resistance and they are available either as cationic or anionic retention aids. The retention mechanism is a combination of ionic charge and long molecular chains 27 linking fibers and particles together (patch and bridge model). Reading materials :WET-END CHEMICALS Pilot plant, laboratory analyses provide better understanding of how common additives influence sizing efficiency THE EFFECTS OF PRECIPITATED CALCIUM carbonate (PCC) and wet-end cationic starch addition levels on ketene dimer sizing efficiency are well known.1 Increasing wet-end cationic starch addition level generally increases ketene dimer sizing efficiency. Increasing PCC addition level or surface area generally decreases efficiency. Two fundamental studies were carried out to gain a better understanding of how PCC and wet-end cationic starch affect ketene dimer sizing and reaction efficiency. In the first study, the effects of PCC and ketene dimer addition points on sizing efficiency were evaluated on the Western Michigan University pilot paper machine. A second study was then carried out in the laboratory under high shear conditions to determine how PCC and wet-end cationic starch affect ketene dimer first pass retention and reaction efficiency PCC, KETENE DIMER ADDITION POINT STUDY. The effects of PCC and cationic starch addition points on ketene dimer sizing efficiency were evaluated in a typical fine paper furnish. A commercial liquid ketene dimer made from a mixture of oleic and linoleic acids was used for the evaluation.2,3 Ketene dimer addition level was varied from 0.100% to 0.175%. Outlines of the addition sequences that were used are shown in Table 1.The sizing performance of addition sequence 1-PCC (12.0%), followed by cationic starch (0.5%), ketene dimer, and alum (0.2%)-was chosen as a baseline for determining the effects of moving PCC and ketene dimer addition points. The results of natural aged Hercules Size Test (HST) testing for each of the four addition sequences are shown in Figure 1. A direct comparison of the sizing data obtained for addition sequences 1 and 2 showed that moving the liquid ketene dimer addition point closer to the fan pump had little or no effect on sizing efficiency (approximately a 13-sec difference in contact time). Moving the PCC addition point closer to the fan pump, however, had a large, negative effect on ketene dimer efficiency. Based on these results, PCC addition before cationic starch and liquid ketene dimer addition gave higher levels of natural aged sizing than post addition of PCC. A comparison of the sizing results for sequences 1 and 4 shows that increasing the time separation between PCC and liquid ketene dimer addition points, while maintaining the preferred addition order, gave an additional increase in natural aged HST sizing. KETENE DIMER RETENTION, REACTION EFFICIENCY STUDY. A second study was then carried out in the laboratory to determine how PCC and several commonly used wet-end additives affect ketene dimer retention and reaction efficiency. Handsheets for retention testing were made in a dynamic handsheet mold ()HM)4 under high shear conditions over a wide range of ketene dimer (0.075% to 0.150%), PCC (10% to 20%), wet-end cationic starch (0% to 1%), alum (0% to 0.5%), and anionic polyacrylamide retention aid (0.01% to 0.05%,APAM) addition levels. A cationic starch based emulsion of a ketene dimer made from a mixture of palmitic and stearic acids (AKD) was used for the evaluation.1,5 Addition sequence 3 was chosen as the most difficult test of sizing efficiency . The total first pass retention obtained in the DHM under these conditions ranged from 82% to 95% at the 10% PCC addition level, and from 69% to 89% at the 20% PCC addition level.Total first pass retention generally increased as cationic starch, alum, and APAM addition levels increased. 28 The amounts of retained and bound AKD (percent of total amount added) in each handsheet were then measured using an extraction/gas chromatography method similar to that reported by Dart and McCaHey.6 The median AKD retention was 54%. Only a very rough correlation between AKD retention and total first pass retention was obtained (R2 = 37%). Based on these results, it is likely that the best charge conditions for PCC retention are not the best conditions for retention of a cationic AKD sizing emulsion. Simple model equations based on PCC, cationic starch, AKD, alum, and APAM addition levels were then developed for the retained and bound AKD data sets. The most important predictions of the model equation developed from the AKD retention data set are shown in Figures 2 and 3. Increasing the PCC addition level was predicted to have a significant, negative effect on AKD retention. Increasing the wet-end cationic starch addition level was predicted to have a significant, positive effect on AKD retention (compare Figures 2 and 3).And, increasing APAM addition level was predicted to increase AKD retention over the entire range of cationic starch and PCC addition levels tested. Similar relationships between cationic starch and ADAM addition levels and the amount of bound AKD in the sheet were found. For example, the regression model developed from the percent bound AKD data predicted that increasing cationic starch or ADAM addition level should increase the amount of bound AKD in the sheet. For both additives, the increase in percent bound AKD can he attributed, at least in part, to an increase in AKD retention.As shown in Figure 4, a rough correlation was found between the amount of AKD retained in the sheet and the amount of bound AID (R^sup 2^ = 59%). However, no statistically significant correlation between % PCC and the amount of bound AKD in the sheet was found (R^sup 2^ = #1%), even though the PCC addition level had an important effect on the total amount of AKD retained in the sheet. These results suggest that ketene dimer associated with PCC does not contribute to the formation of bound AKD. Additional work is needed to confirm this hypothesis. It is clear from these results that a significant portion of the effects of PCC, cationic starch, and retention aids on AKD sizing efficiency can be attributed to changes in first pass retention. Cationic starch and retention aids increase sizing by increasing the total amount of ketene dimer, both bound and unbound, in the sheet. Both bound and unbound ketene dimer are known to contribute to sizing.7,8 PCC affects sizing by reducing ketene dimer retention. PCC may also affect sizing by increasing the surface area or reducing the average pore diameter of the sheet. A regression analysis of the ratio of bound/retained AKD data, however, suggests that cationic starch may have a second beneficial effect on AKD efficiency. (This ratio represents the percentage of retained AKD that becomes bound AKD.) The model equation developed from the percent bound/retained AKD data predicted that the percentage of retained AKD that became bound AKD increased as the cationic starch addition level increased. A graph of the raw percent bound/retained data versus cationic starch addition level confirmed this prediction (see Figure 5, R^sup 2^ = 50%). Based on previous work by Bottorff on the hydrolysis rate of AKD on PCC,7 it is possible that cationic starch increases the percentage of bound AKD by passivating the surface of the PCC. Based on previous work by the author on the effects of size press starch on AKD efficiency, it is also possible that cationic starch forms a substrate for reaction of the AKD.9 EXPERIMENTAL. A general description of both of the papermaking methods used in these evaluations is provided. Papermaking on pilot machine: Paper for the addition point evaluation was made on the pilot paper machine at WWL= (23.5 mpm). A typical copy paper furnish was used (75 g/m^sup 2^). The pulp furnish (70% hardwood/30% softwood bleached kraft pulp) was refined to 425 ml Csf using a double disk refiner. The pH (7.6 to 8.0), alkalinity (150 to 200 ppm), and hardness (100 ppm) of the papermaking stock were adjusted at the machine chest using the appropriate amounts of H^sub 2^SO^sub 4^, NaHCO^sub 3^, and NaOH.Wet-end additions of PCC (12%, 11 mz/g), liquid ketene dimer, quaternary-amine-substituted cationic 29 potato starch (0.5%), and alum (0.2%) were made as shown in Table 1. Stock temperature at the headbox and the white water tray was controlled to 52 deg C. The wet presses were set at 40 psi. Sheet moisture before the size press and at the reel were controlled to 2% and 5%, respectively. Starch concentration (oxidized corn starch) in the puddle size press was fixed at 5.1%rb, giving a starch addition level of 50 kg/mton (pH 7.5 to 8, 50 to 55 deg C). Size press addition of NaCl was fixed at 2.5 kg/mton. Calendar pressure was adjusted to obtain a Sheffield smoothness of 150 flow units at the reel (column No. 2, felt side up). Papermaking on dynamic handsheet mold: Handsheets for AKD retention testing were made in a dynamic handsheet mold4 using the following procelure. A 100 ml aliquot of wood pulp (0.45% consistency) was poured into a small beaker then placed inside a large crystallizing dish containing 900 ml of dilution water of the desired alkalinity (100 ppm), pH (7.5 to 8.0), and hardness (100 ppm).The pulp slurry and dilution water were then heated to 40 deg C in a microwave oven. The desired amounts of cationic potato starch, AKD, and PCC (11 m^sup 2^/g) were then added sequentially to the heated pulp at 10-sec intervals. An overhead stirrer (1,000 rpm) was used to agitate the slurry during these additions. After these additions were made, the thick stock was added to the heated 900 ml aliquot of dilution water and loaded into the DHM. Alum was added ten seconds after the addition of the stock to the DHM. Ten seconds later, the desired amount of high molecular weight anionic polyacrylamide retention aid was added. Drainage was initiated ten seconds after the addition of retention aid.The stirrer was left on during drainage. A five-pulse cycle was used (0.2 seconds vacuum on; 0.4 seconds vacuum off; 3.0 seconds total cycle time). The sheet was then pressed between two sheets of blotter paper using a Cobb roll and dried for five minutes on a hot plate set at 110 deg C. Size retention and reactivity testing were measured using a method simitar to that reported by Dart and McCalley.6 Sizing agent emulsions were made using the methods described in Reference 5. HST testing: All HST testing-All HST testing was carried out at 22 deg C and 50% relative humidity, using Hercules Test Ink #2 at 80% reflectance (average of felt and wire sides). Lesson 16 Dry and wet strength additives A number of natural and synthetic polymeric substances may be admixed with the stock at the wet end to improve the physical properties of the dry sheet. Their action is to reinorece fiber-to-fiber bonds and thereby improve the burst and tensile strength, provide greater resistance to erasure, reduce ―fuzz‖ or lint on the paper surface, reduce the rate of water penetration. The traditional internal strength additives are natural and modified starches and gums. Both are usually cooked at low concentration prior to use to promote swelling and dispersion. The trend today is toward increased use of such synthetic polymers as latexes and polyacrylamides either alone or in combination with starches and gums. The action of wet-strength resins is to tie fibers and fines together with additional bonds that are not taken apart by water. Wet-strength paper is defined as such if it retains more than 15% of its tensile strength when wet. The most common wet-strength agents are urea-formaldehyde, melamine-formaldehyde, and polyamide resins; they are water-soluble and available in both anionic and cationic forms. These agents are applied at an intermediate degree of polymerization, so that the final ―cure‖ is obtained in the dryers. 30 Reading material: wet-chemistry theory Wet strength of paper is often referred to as relative wet-strength, the ratio between wet strength and dry strength. Normally, 10 to 15% is considered as a wet-strength paper. In contrast to dry strength, the origins in the hydrogen bonds present in natural cellulose, wet strength requires water stable (covalent) bonds. Naturally, cellulose contains few covalent bonds, and therefore wet-strength chemicals are necessary. Polymer resins are often used to increase wet strength. Two different wet-strength mechanisms exist, the protection mechanism and the reinforcement mechanism. The protection mechanism involves diffusion of the wet-strength polymer to the fibre surface where it corss-links through and around the fibres. Such cross-linked networks prevent fibre swelling and helps to preserve covalent bonds when the paper is exposed to water. In contrast, the reinforcement mechanism means that new bonds between the wet-strength polymer and the fibres are formed. Fiber charge is an important factor that influences the papermaking process as well as the paper properties. Cellulose fibres contain various ionisable groups: carboxyl-, sulfonic acid-, phenolic- and hydroxyl groups and are therefore negatively charged at all pH values. Under normal papermaking conditions, the negative fibre charge is due to carboxyl and sulfonic acid groups. These charged groups are located either on the fiber surfaces or inside the cell wall; hence they are referred to as surface- and bulk charges, respectively. Surface charges are important for fibre- and paper strength. Adsorption of polymers is used in many applications. The driving force is a strong polymer-surface interaction. If the solvent is poor, then adsorption is more favourable than the polymer-solvent interaction. A condition for most cationic additives is the ability to adsorb to the fibre surface. That is why polymer additives used in papermaking often are cationic. The density and distribution of charges are also very important. Other factors that affect the adsorption are molecular weight of the polymer and the presence of fibre segments called ―fines‖. Fines have large specific area and their capacity to adsorb polymers is therefore higher than for whole fibres. Polymer adsorption is often reversible and there is a probability that they will detach from the surface. This probability is generally low since all segments have to detach at the same time. However, changes in pH, ionic strength etc. might cause desorption of polymers from the fibre surface. The natural biopolymer: A natural biopolymer is generally characterized in terms of its: quality (heavy metal/protein content, pyrogenicity, cytotoxy, clarity etc), intrinsic properties (molecular weight, viscosity, degree of deacetylation, stability) and physical form (size etc). The commercial prospect of the actual biopolymer is good since its natural resources are abundant. The natural biopolymer has the following properties: - availability and ability to be used in varying form: powder, solution and gels. - Non-toxicity - Biocompatibility - Good absorption capacity - Non-water solubility Its larges use is as flocculent and chelator of toxic and radioactive metals but some other important applications can be cited: as a component in wound dressings and drug delivery systems thanks to its biocompatibility, non-toxicity and wound-healing effect. The natural biopolymer is also and excellent moisturizer and is therefore used in hair- and skin care producst. The actual biopolymer has, like cellulose, a linear structure but instead of the hydroxyl group there is 31 another reactive group. At acidic pH these groups become positively charged groups with high reactivity. Thus ,they are very efficient in neutralizing the negatively charged fibres. The possibility of the natural biopolymer to be used as wet-strength additive lies within its structure, which is very similar to that of cellulose or CMC (carboxyl methylcellulose); the glucose units are linked together by β-1,4 glycoside bonds. The close similarity to cellulose probably makes it possible for the natural biopolymer to form strong hydrogen bonds to cellulose. By studying the irreversible adsorption of CMC to cellulose some general wet-strength mechanisms can be understood. Irreversible adsorption of CMC CMC is the mose widely used water-soluble derivate of cellulose. It is produced by reacting cellulose with monochloracetate. Irreversible adsorption of CMC is an alternative to improve paper strength. CMC can be irreversibly attached to cellulose at high temperature and in presence of an electrolyte. A salt is used to shield the repulsion between negatively charged fibres and CMC, making it possible for CMC to approach the fibre surface and attach. The irreversible adsorption is believed to be a matter of co-crystallisation mechanisms. An irreversible adsorption is thermodynamically stable and cannot be washed away. A comon technique for wet-strength development of paper is to add CMC together with PAC to the pulp. Cationic PAE helps to retain the anionic CMC to the fibre surface. Both dry and wet strength are improved. The surface conformation and properties of such fibres will be different compared to when only CMC is added. Temperature has strong effect on CMC adsorption. The adsorption increases rapidly with temperature up to 120?C. High electrolyte concentration also promotes the adsorption. At low electrolyte concentration no CMC is attached between pH6 and 11 whereas high electrolyte concentration makes the adsorption less pH dependent. Acidic conditions are more favourable and the adsorption increases when a divalent-instead of a nomovalent ion is used. Another factor is pulp consistency; high consistency promotes the attachment when divalent ions are used. The attachment also depends on the DS ( degree of substitution); namely, it decreases for higher DS, due to the charge repulsion between CMC and fibres. Co-crystallisation mechanisms are also more efficient with purer cellulose fibres than if lignin and hemicelluloses are present. The great advantage of CMC is that it can improve wet strength wighout beating of the fibres. This is very useful for the production of tissue paper that requires softness and good adsorption since beating gives stiffer paper with less porosity. Lesson 17 Internal sizing and alkaline papermaking The purpose of sizing is to enzble paper products to resist penetration by fluids. Sizing can be achieved either by using wet-end additives (―internal sizing‖) or by applying a suitable coating to the surface of the dried paper (―surface sizing‖). Internal sizing is accomplished by adding materials to the stock before the headbox to retard water penetration into the final paper. Water penetration is retarded by the nonpolar portions of the size molecule. A reactive protion of the size molecule anchors it to the surface of the fiber. Surface sizing works by different mechanism and occur at the size press where an application of starch (or other material) fills the capillaries of paper, making water penetration much more difficult. Starch is not hydrophobic as are internal sizing agents. There are two common methods used in internal sizing: rosin sizing, which must be carried out at pH4~6 and alkaline sizing, which is carried out at pH8 or higher with alkyl ketene dimer (AKD) and alkyl 32 succinic anhydride (ASA). Paper may be hard-sized (high resistance to liquid penetration such as printing and packaging papers), slack-sized (low penetration such as newsprint), or no-sized paper (toweling and sanitary papers). For rosin sizing with alum, the rosin is precipitated onto the fibers by the action of alum as an oriented monolayer of aluminum resinate molecules. Rosin may be used in the salt form as a solution at pH 10-11 (soap size), the salt form with 20%-30% water (paste rosin size) or in the free acid form as an emulsion (emulsion size), which is effective at slightly higher headbox pH than the others. Normally the rosin is added before the alum. If alum is added before alum, the process is called reverse sizing, which is useful in water containg high amounts of calcium. For alkaline sizing, synthetic sizing agents react chemically with the cellulose hydroxyl groups to form stable ester linkages to give sizing effects. These agents are not water soluble and must be used as emulsions. Cationic starch is used to stabilize these emulsions. With most internal sizing methods, hardwood pulps are much easier to size than softwood pulps; sulfate pulps are easier to size than sulfite pulps, which are both easier to size than thermomechanical pulps. The explanation centers around the number of carboxylate groups as anchor points in sizing. There are several important advantages to alkaline papermaking. The papers have longevity; calcium carbonate (inexpensive and bright filler) can be used; paper is stronger and less brittle; there is less corrosion on the paper machine; and there are few problems using recycling fiber containing calcium carbonate filler. Reading material: filling and loading The process of adding mineral matter to paper stock prior to the formation of the sheet is called filling or loading. This process is extremely old, having been practiced in the ancient days of papermaking. As the uses for paper became greatly expended, many papers were developed in which fillers were considered to be highly beneficial. Today, many kinds of fillers are commonly used as an integral part of certain grades of paper. The process is call ―filling‖ or ―loading‖ be considered as adulteration of the paper, since the pigments can improve the properties of the paper when used in the proper proportion, it is well known that it would be almost impossible to make certain grades of paper without use of fillers. For example fillers are highly desirable in pringing paper where they increase the opacity and improve the surface and printability of the sheet. Fillers also improve the appearance and absorbency of paper, as well as increasing the density. The principal fillers used are clay, talc (agality and asbestine), calcium carbonate, titanium dioxide, zinc sulphide, calcium sulfate (gypsum), diatomaceous silica, calcium sulfite, and blanc fixe. Of all these, clay and calcium carbonate are by far the most widely used. Pigments must meet certain requirements to be suitable for filling. They should have a high degree of wihteness, a high index of refraction, small particle size, low solubility in water, and low specific gravity. Moreover, it is desirable that the pigment be chemically inert so that it will not result in unfavorable reactions with the other constituents of the sheet. Furthermore, unless the pigments have very unusual properties, it must be cheap. Clay meets most of the above requirements (except high refractive indes), and hence is admirably suited for filling. This accounts for the large quantities of clay which are consumed in the manufacture of newsprint, book and pringing papers. Titanium and zinc pigments are too expensive for general use, but they have the ability to impart a high degree of whiteness and opacity in low percentages, and consequently they are often used in the higher grades of book, offset, writing paper. But zinc oxide is not suitable at all because of its reactivity with alum. Pearl hardening has the disadvantage of imparting low opacity, althou it does produce high wihteness. Barium pigment have a very high specific gravity and are very poorly retained, but barium 33 sulphate is sometimes used because it imparts an exceptionally high brilliance to the sheet. Calcium solfate pigments have the disadvantage of high solubility. Ordinarily, the methods of adding filler are no very complicated. In many cases, the pigment is added dryly to the stock in which it is dispersed by the action of the beater. When this method is used, all the grit and foreign matter are added with the pigment. Consequently, it is considered better practice to disperse the pigment in water first and then pass the slurry through a screen before adding to the stock. Fillers are added to paper to improve both the optical and physical properties. The principal object in filling is to increase the opacity and brightness of the paper, and the next most important object is to improve the smoothness, finish, and printability of the sheet, particularly after calendering. The pringability of the paper is improved because clay particles are more readily wetted by ink than the fibers are and because the clay produces more and finer capillaries in the shett. Other characteristics imparted to the paper through loading are increased weight, improved softness and improved absorption properties. There may be other special reasons for adding fillers as, for example, the use of calcium carbonate in cigarette papers to regulate the rate of combustion, and the use of carbon as a conducing agent in electrical conducing papers. The use of high percentage of filler in paper results in certain undesirable effect, chief of which is a decrease in the strength and in sizing. Fillers also decreases the bulk, since the pigment is heavier than the fiber and consequently increase the weight more than the thickness of the sheet. So the amounts of pigment used in filling should be less than 20% to 25% on the weight of the fiber. Lesson 18 Sheet Formation The paper machine is a device for continuously forming, dewatering, pressing, and drying a web of paper fibers. Up to the 1950’s, all paper and paperboard products were formed on conventional fourdrinier and cylinder vat machine. Twin wire formers become popular since the late 1960s for printing and lightweight papers. In a fourdrinier, a dilute suspension of fibers (typically 0.3-0.6% consistency) is applied to an endless wire screen or plastic fabric, which is usually called wires by papermakers. Water is removed by gravity, or the pressure difference developed by table rolls, foils or suction equipment, and the drilled couch.. The web at this point is 18%-23% consistency. Unlike laboratory sheet forming, the sheet-forming process on a fourdrinier wire is far more complicated, involving, in addition to drainage, such effects as the generation and decay of turbulence, formation and breakdown of fiber networks, retention and transport of fine particles in the mat, compact of the mat, and shear forces between the mat and free suspension. The most important effect of the drainage process is the dewatering of the fiber suspension to form the mat. When the fibers are free to move independently of one another, drainage proceeds by the mechanism of filtration and the fibers are deposited in discrete layers. When the fibers in suspension are immobilized, they floc together in coherent networks; drainage then occurs by thickening, and a more felted floccy sheet structure results. In each design of commercial sheet-forming machine (whether fourdrinier or twin wire), the three elementary forming effects of dilution, turbulence, and oriented shear are applied to different degrees in an attempt to optimize sheet quality. The forming medium of fourdrinier wire part is an endless finely woven belt and plastic mesh fabrics are used almost exclusively today due to their longer service life. The fabric travels between two large rolls; the breast roll near the headbox and the couch roll at the other end. Typically, the breast roll is solid and serves only to support the fabric. The couch roll is a hollow perforated shell containing one or two stationary high-vaccum suction boxes for dewatering the sheet. The various elements between the breast roll and couch roll serve the dual functions of wire support and water removal. A number of different arrangements can be used depending on the particular requirements. 34 Most machines today utilize a forming board immediately after the breast roll, followed by a number of foil assemblies. The wire then passes over a series of vaccum-augmented devices, from low vaccum (wet boxes) to high vaccum (dry boxes), and finally over the high vaccum couch.. Slower paper machines (up to 400m/min) are often equipped with a shake mechanism which imparts a cross-oscillation motion to the wire to improve sheet formation. Some fourdriniers are equipped with a dandy roll mounted above the wire and riding on the stock in the suction box area. The dandy roll has a wire cloth covering and services to compact the sheet and improve formation in the top portion. It is customary to trim off a narrow strip from each edge as the sheet leaves the couch. These edges are usually weak due to lower fiber weight and more erratic formation, and could be a source of breaks during web transfers, the cutting is performed by high-pressure water jets, called squirts, and will be re-slurred and later added to the broke system. Lesson 19 Pressing The primary functions of the press section are to remove water, consolidate the sheet, impart favorable sheet properties and promote higher wet web strength for good runablity in the dryer section. The fragile paper web is transferred from the forming section and conveyed on specially-constructed felts through a series of roll press nips and into the dryer section. It is far more economical to remove water by mechanical means than by evaporation, so the papermakers are always looking for methods to improve pressing efficiency and reduce the evaporative load into the dryer section. Water removal should be uniform across the machine so that the pressed sheet has a level moisture profile entering the dryer section. Virtually all continuous wet pressing of paper webs is carried out in a two-roll press nip. The sheet is carried into the nip and supported during pressing by a specially constructed felt. It is convenient to consider the pressing as occurring in four phases: In phase 1, compression of sheet and felt begins; air flow out of both structures until the sheet is saturated; no hydraulic pressure is built up and therefore no driving force for dewatering. In phase 2, the sheet is saturated and hydraulic pressure within the sheet structure causes water to move from the paper into the felt. Phase 2 continues up to the mid-nip where the hydraulic pressure and total pressure reaches maximum. In phase 3, the nip expands until the hydraulic pressure in the paper is zero, corresponding to the point of maximum paper dryness. In phase 4, both paper and felt expand and the pressure is created in both structures, a number of factors cause water to return from the felt to the paper. The reabsorption of water from the felt in phase 4, called rewetting, is recognized as serious limitation to idealized water removal. Various mechanisms from this behavior are postulated, including capillary absorption and mechanical absorption. Additional rewetting occurs following the nip unless sheet and felt are immediately separate; all modern press configuration provide for this separation. It has shown that the most important requirements in press design is to provide the shortest path for the water to follow in escaping from the nip. The main water flow should be perpendicular to the felt, and lateral flow should be minimized. The original presses had plain rolls, which were severely flow-limited because the water only leaves on the entering side of the nip by lateral movement. The suction press, developed as the first approach toward transverse-flow pressing, utilizes a suction roll as a receptor for the expressed water. The holes in the shell provides an easier escape route for the water. Some water is able to flow directly to the holes, but most of the expressed water also must travel laterally to escape the nip. For the grooved-roll transverse-flow press, the 35 grooves in the roll cover provide easily-accessible receptacles for expelled water. The maximum lateral distance for water travel in the grooved press is only 1.3 mm as compared to a typical figure of 5 mm and 200 mm respectively for the suction and plain presses. The water caught in the grooves is thrown off by centrifugal force at high roll surface speeds and the roll is cleaned by the action of sprays and doctor blades. Another innovation toward true transverse-flow pressing is the utilization of a blind-drilled receptor roll. Only the cover of the solid roll is drilled with small, closely-spaced holes. The blind-drilled roll has greater void volume and the patterns can be installed in soft roll covers. The wells tend to self-clean by the action of centrifugal force.. The fabric press is another development of transverse press which is used more in Europe and North America. A totally new type of press was introduced in 1981. The extended-nip press features a very wide nip to give the sheet a long dwell time at high pressure. When used as last nip, this press provides not only a much drier sheet, but also a stronger sheet due to improved consolidation of the web structure. Pressing at sheet temperature in the 60-90? is a modern technique for increased press dewatering and improved sheet consolidation. Lesson 20 Paper drying After pressing, the sheet is conveyed through the dryer section where the residual water is removed by evaporation. On conventional paper machines, the thermal energy for drying is transferred to the paper by wrapping it on series of large-diameter, rotating, and steam-filled cylinders. Two indices are important in assessing the performance of a dryer section: evaporation rate and steam economy. The evaporation drying rate is measured as pounds of water evaporated per hour per square foot of dryer surface contacted and is greatly influenced by the steam pressure used inside the drying cylinders. Steam economy is measured as thousands of BTU’s per pound of water evaporated or as mass of per unit mass of water evaporated. The drying rate varies along the machine. The first two or three cylinders into the drying section serve principally to raise the temperature of the sheet (―warm-up zone‖). Evaporation then quickly reaches a peak rate which is maintained as long as water is present on the fiber surfaces or within the large capillaries (―constant rate zone‖). At this point where the remaining free moisture is concentrated in the smaller capillaries, the rate begins to decrease (―falling rate zone‖).Finally, at about 9% moisture held by physicochemical forces, and the evaporation rate is further reduced (―bound water zone‖). The wet web from the press section containing 55%-60% moisture(40%-45% dryness) is passed over a series of rotating steam-heated cylinders where water is evaporated and carried away by ventilation air. The wet web is held tightly against the cylinder by a synthetic, permeable fabric called a dryer felt. The fabric also serves to support and guide the sheet through the dryer section. Most paper machines have three to five independently-felted dryer sections, each with independent speed control to maintain sheet tension between sections and adjust for any sheet shrinkage that occurs. All top and bottom felt runs are equipped with tensioning and positioning rolls. Usually, three to five sections are also grouped for independent steam pressure control. Lesson 21 Calendaring and supercalendaring Calendaring is a general term meaning pressing with a roll. Most paper grades are calendered to obtain a smooth surface for printing. Another common objective is to improve the cross-direction (CD) uniformity of certain properties, particularly of thickness, which is important for reel-building and converting. It must be noted that where calendaring pressure is varied to compensate for non-uniformity in one CD property (e. g., 36 caliper), then the CD profiles of other properties are made less uniform. Generally, the induced non-uniformities are small, and caliper control remains an important aspect of calendaring. Calendaring changes the surface and interior properties of the sheet by passing the web through one or more two-roll nips where the rollers may or may not be of equal hardness. The pressures are extreme and the time that any section of the web actually spends in the nip is infinitesimally small. The basic objective is to press the paper against the smooth surface with sufficient force to deform the paper plastically and replicate the calendar roll surface onto the paper. The replication process can be enhanced by the application of greater pressure and/or shear forces can be enhanced by the application of greater pressure and/or shear forces and by heating or moistening the fibers to make them more pliable. For reasons of economy and efficiency, most calendaring operations are carried out on-machine. For many grades of paper, the required degree of smoothness can be achieved with hard steel nips. However, if a smooth, highly-glazed surface is required without over compaction, then a different type of calendaring action is required. On-machine soft-nip calendars are now being successfully utilized intermediate quality grade. However, for production of the highest-quality printing papers, supercalendering is required. The typical stand alone supercalender consists of a series of rolls arranged vertically, with alternating hard metal rolls and soft rolls made from compressed fibrous material. The web of paper is fed from an unwind stand into the top of the stack, through each nip, and out the bottom into a rewind unit. Quite often, the web is fed around lead rolls into each nip to prevent air entrapment that could cause creasing. The unique results achieved with supercalendering are due to the intermediate fibrous rolls which possess elastic or plastic properties. With the application load on the nips, the metal rolls cause a depression or deformation on the fibrous rolls at the point of contact, and the deformation spreads out on either side of the nip. When rotated, this spread area will creep because of the constant effort of the material to return to its normal shape. This flow causes a relative motion of the filled roll surface against the metal roll surface, thus producing rolling friction, which helps to give the polishing and smoothing effects. Supercalendering is almost always carried out as a separate (i. e., off machine) operation because of the delicate nature of the filled rolls. These rolls are easily damaged or dented by torn paper or lumps from any source getting into the nips. To prevent marking of the paper surface, any damaged rolls must be immediately replaced. Lesson 22 Reeling and winding After drying and calendaring, the paper product must be collected in a convenient form for subsequent processing off-machine. Typically, a paper machine is equipped with a drum reel which collects the product to a specific diameter. The reel drum (called pope reel on some models) is motor-driven under sufficient load (amperage) to ensure adequate tension on the sheet from the calendars. During normal operation, the web wraps around the reel drum and feeds into the nip formed between the drum and the collecting reel. While the reel builds up, an empty spool is positioned on the primary arms. just before the reel has built up to the required diameter, the new spool is accelerated to machine speed by a rubber wheel and then loaded by the primary arms against the reel drum. When the reel build is complete, the secondary arms release their pressure against the drum, causing the paper reel to slow down; a loop of paper then billows out between the reel and the drum, which is blown upward by air jets. At the proper moment, the backtender breaks the sheet at the loop so that it wraps the new spool. The full paper reel is then removed from the reel rails by a crane. Winding is usually defined as process which changes a material manufactured in web form into roll form for further processing. The assembly of equipment for carrying out winding or rewinding is the winder. 37 The purpose of the winder is to cut and wind the full width, large-diameter paper reel into suitable size rolls. These rolls may then be wrapped and sent directly to the customer or they may be processed through subsequent coating, calendaring or sheet operations. Lesson 23 paper recycling Secondary fiber is defined as any fibrous material that has already undergone a manufacturing process and is being recycled as the raw material for another manufactured product. Obtaining fiber from recycled paper is a matter of separating impurities from the usable fiber. Five basic grades of wastepaper are defined and generally acceptable by the paper industry: 1. mixed paper. Office waste, boxboard cuttings and mill wrappers. 2. old newspapers (ONP) 3. old corrugated containers (OCC) 4. pulp substitutes. Unprinted paper and boart that has not been coated or adulterated in any way; included are tabulating cards, white and semi-bleached sheets, cutting, shavings or trim. 5. high-grade deinked. Deinked waste paper. A prime objective in processing wastepaper stocks is to remove enough contaminants and/or upgrade the material so that the second fiber is suitable to make the finished product within specifications. The major process steps that may be used are screening, cleaning, washing, bleaching, dispersion and deinking. All extraneous constituents are considered contaminants, including dirt, rocks, and tramp metal. Some of the more difficult product-related contaminants such as glues, hot melts and latexes are lumped together or ―tackies‖. into the category of ―stickies‖ Most of the primary pulping equipment and secondary in-line pulpers and/or deflakers have the ability to remove much of gross contamination. Some are augmented with a so-called flotation purge system to help remove the lighter weight contaminants. Beyond this, all systems have suitable screens for removing particles and centrifugal separators (forward and reverse cleaners) for removing all types of fine particle contamination. A dispersion system is sometimes incorporated into a secondary pulping system to control certain stickies. The objective is to disperse the contaminants thoroughly over the surfaces of the fibers and thereby nullify any adverse effects. Both hot and cold systems are available; the hot systems generally yield a cleaner appearance, but the old systems are adequate for stocks used as filler plies in multi-ply products. Deinking of pulp fibers is essentially a laundering or cleaning process where the ink is considered to be the dirt. Chemicals, along with heat and mechanical energy, are used during repulping to dislodge the ink particles from the fibers and disperse them in the stock suspension. The ink particles are then separated from the so-called ―grey stock‖ by a series of washing or folatation steps, or by applying a hybrid process that utilizes both separation techniques. The key chemicals used for stock deinking are surface active agents (surfactant), which affect the surface tension of liquids and solids. The washing and flotation methods have respective advangates and the principles involved and the chemical utilized have been quite different. The objective in washing is to break the ink down into particles under 15 microns, render them hydrophilic, and keep them finely dispersed. For effective flotation removal, the ink particles must form hydrophobic folcs, ideally in the size range from 30-60 microns. In the washing process, detergents and dispersants are utilized in the pulper to remove the ink constituents from the fibers, break them down, and disperse them into very fine particles. The ink dispersion is subsequently separated from the pulp, typically by a multistage dilution/thickening washing sequence. In the floatition process, chemicals are introduced during the repulping operation to promote 38 flocculation of the ink particles and the formation of foam. The grey stock is subsequently aerated at low foltation cells, causing the light flocs of ink particles to rise to the surface where they are skimmed off. Reading material: Brightness Reversion YELLOWING OF PAPER. A certain amount of yellowing is a perfectly normal process and is actually the patina of an old print. A print that is exposed in favourable conditions will not become excessively yellow. The only way to avoid any change at all is to keep a print in a portfolio but in such a case the print should, every once in a while, get an airing. Furthermore, if a print is kept in a portfolio one must make sure that it gets neither too dry nor too damp, that it does not get too dusty, and that it not be rubbed excessively. The yellowing of paper is particularly problematic when it is irregular. Humidity can provoke two types of damage. The first type of damage is a directly caused one since the humidity will dissolve the sizing, make the paper buckle, destroys the assembled pieces (glued backings, etc.), and provokes the formation of spots and water marks. The second type of damage is an indirect consequence in that the humidity will favour the growth of bacteria of various sorts and various types of plant life. The damage done by humidity is even greater if the prints are kept tightly packed and covered with tissue paper to set them off and prevent them from staining each other. Excessive humidity can, at times, cause nothing less than a glueing together of these prints especially if damp periods are followed up by dry ones. Actually it is a rather artificial distinction when one separates the damage done by humidity from the damage done by heat especially since the destructive effects of heat are often closely associated to excessive dryness and to excessive humidity. In the first case, when heat is very dry, paper becomes very brittle and friable while the coloured areas and the inks become scaly. In the second case, when heat is very damp, various types of plant life may begin to develop. On the other hand, the minute the temperature rises excessively the paper is burned and turns a reddish tinge or is actually burned leaving a well known brownish border area. The heat-induced yellowing of TCF bleached pulps is an extraordinarily complex process. A high number of interacting factors influence the brightness stability of the materials. The yellowing process proceeds predominantly in the carbohydrate part of the pulps. Oxidation processes seem to play a role in the formation of chromophores only in cases of relatively high lignin content. Except for the temperature, the most important factors are humidity and intrinsic pH of the pulps, indicating that the hydrolytic processes are mainly responsible for discoloration. The yellowing tendency of the pulp is stronger the higher the oxidation state of the carbohydrate matrix. During heating, the carbohydrates undergo reactions which lead to the formation of furan-type heteroaromatics. The formation of furan derivatives indicates that hemicelluloses may also be involved in these processes. A weak influence of the carbonyl content on the brightness stability was found. Ketone and aldehyde groups could be analytically separated. Carboxyl groups act as proton donors which catalyze hydrolytic reactions and the dehydration of the carbohydrates, thus accelerating the formation of chromophores. Heavy-metal ions seem to act in the same manner as carboxylic protons. They can be masked by complexing agents. In this case, yellowing is less marked. Both natural biopolymer materials and synthetic polymers undergo UV induced discoloration, usually an increase in the yellowness on exposure. Lignocellulosic materials such as wood and paper readily undergo light-induced yellowing. While both cellulose and lignin constituents of wood can photoyellow, it is the latter that is mostly responsible for the phenomenon. Lignin, which comprises 29-33% by weight of softwood, contains numerous chromophores that efficiently absorb UV radiation (Heitner, 1993). As much as 80-95% of 39 the absorption coefficient of wood can be ascribed to the lignin fraction. The complex photochemistry of yellowing in lignin-containing materials is not completely understood; the present understanding of the process was succinctly summarized recently (Forsskåhl et al., 1993) and at least four pathways of photodamage have been recently discussed. The practical interest in discoloration relates specially to newsprint paper made of groundwood pulp that yellows rapidly on exposure to sunlight. Action spectra for photoyellowing of these pulps have been reported, and a recent study confirms the solar UV wavelengths to cause yellowing while the wavelengths in the region of 500 nm to 600 nm was shown to photobleach the pulp. The cellulose fraction in wood also undergoes a free radical mediated degradation on exposure to wavelengths < 340 nm. PREVENTIVE CONSERVATION: Environmental Factors and Deterioration Temperature and Relative Humidity With every 10 degrees Celsius (18 degrees F) increase in temperature the useful life of paper is reduced by one half, since the rate of adverse chemical reactions increases with rising temperature. Therefore reducing the temperature in the archives storage will help minimize the rate of deterioration and prolong the life of the records. For a mixed media collection, a temperature set point between 18 and 20 degrees Celsius is recommended. Relative humidity has an even more critical impact on the rate of deterioration. Deterioration will occur if the relative humidity is either too high or too low. Dangerously high relative humidity (generally accepted to be 65% or greater) accelerates many adverse chemical reactions and when combined with high temperature and poor air circulation, mould and mildew growth will occur. Insect infestation is also encouraged since insects prefer a warm, humid environment. The presence of transition metals promotesthermal yellowing. In particular, the presence of ferric (Fe3+) ions appears to promote the thermal yellowing of unbleached and peroxide-bleached pulps. Treatment of pulps with chelating agents, EDTA or DTPA, decreases thermal yellowing. Light Exposure to light speeds up oxidation, which leads to a chemical breakdown in many archival materials. ALL LIGHT (both visible and ultra violet) causes damage, but ultra violet light is particularly harmful to archival materials. Common sources of ultra violet light are sunlight and fluorescent lighting. Light damage is both cumulative and irreversible. Short exposure under high light levels is just as harmful as long exposure under low light levels. Some common effects of over-exposure to light include, bleaching of coloured papers, light-sensitive inks and pigments; darkening or yellowing of paper (especially paper with a high lignin content such as newsprint); and embrittlement of paper and photographic materials. The approaches used to inhibit light-induced yellowing reaction were modification of lignin by etherification or esterification of the phenolic hydroxyl groups, reduction of the aromatic carbonyl groups, combined etherification of phenolic hydroxyl and reduction of aromatic carbonyl groups, and hydrogenation of vinyl groups. Paper additives affect the degree and rate of yellowing caused by exposure of paper to near-UV light. Ultraviolet absorbers, that is, compounds that absorb UV light and dissipate the light energy by a non-yellowing mechanism, have been used extensively to prevent the light-induced degradation of a wade variety of polymers. Hydroxybenzophenones have been widely used as UV light absorbers. This type of compound absorbs UV light and forms an o-quinonemethide which dissipates the light energy as hear by tautomerizing to the starting material. Polyethylene glycol has been used to prevent light-induced yellowing of wood surfaces and paper. Although the mechanism of polyethylene glycol inhibition of light-induced yellowing is not well understood, it has been suggested that it inhibits yellowing by excluding air from the paper fibers. The most successful methods for inhibiting light-induced yellowing of lignin-containing papersare those that scavenge free radicals. Ascorbates, thiols, and thioethers all stabilize lignin in paper by quenching peroxyl 40 and alkoxyl free radicals, species that oxidize the phenoxyl free radical and convert it to an o-quinonoid structure. Pollution Gaseous pollutants harmful to archival materials include, sulphur dioxide, nitrogen dioxide, hydrogen sulfide, and ozone. These pollutants are all quite common in urban, industrialized areas. Sulphur dioxide and nitrogen dioxide combine with moisture in the air to form sulphuric and nitric acids. These gases will attack book leathers and paper. Leather damaged by this form of pollutant turns into a dry, powdery surface known as ―red rot‖, which is beyond repair. Ozone is particularly harmful to pho tographic materials and magnetic media. A common source of ozone is sunlight combining with nitrogen dioxide from automobile exhaust. Other sources include electrostatic photocopier machines and air filter systems. The primary damage caused by particulate pollutants (dirt, dust and other solid particles) is surface abrasion. However, with time these particles may become imbedded in paper fibres and photographic emulsions, causing more extensive deterioration. Dust particles combined with moisture from the air can cause staining or contribute to mould growth. Dirt also helps deposit acidic gases from the air onto materials, contributing to further chemical deterioration. Biological Agents (Insects, Rodents and Mould) Mould spores are always in the air and will germinate and grow as soon as the right environmental conditions are present. For many types of mould growth the optimum environment is over 22 degrees Celsius (70 degrees F) and over 70% relative humidity combined with poor air circulation. However many moulds will grow at significantly lower temperatures and R.H. as the biology projects found in the back of your fridge can attest. Mould and mildew can both weaken and permanently stain archival records. Mould spores feed on the many nutrients present, such as cellulose in paper, photographic emulsions, leather book covers, etc. Mould spores are also thought to react with trace metals (usually iron salts) found in machine-made paper, causing a reddish-brown spot to form known as ―foxing‖. In addition to weakening the paper this foxing, can be very disfiguring. The most common insect pests harmful to archival materials are cockroaches, silverfish, termites, carpet beetles and wood-borer beetles. Insects will feed on a variety of substances found in archival holdings, such as cellulose in paper, starch adhesives, glue, sizing, photograph emulsions, bookbinding cloth, and leather. Damage from a rodent infestation can also be enormous. Mice and rats will shred and chew paper to make nests and their droppings are quite corrosive and cause disfiguring stains. In addition to the physical damage to records resulting from biological agents of deterioration there is the added problem of health and safety issues to consider. Exposure to mould and rodent droppings can be extremely hazardous to your health. Mould exposure can provoke allergic reactions, other respiratory afflictions and even death in extreme cases. Rodent droppings can carry the Hanta virus. 41