Paper and Papermaking

The earliest writing materials are believed to have been stone and brick. Many examples of the ancient Sumerian cuneiform writing—characters engraved on clay tablets—have been found, and the practice was adopted by other early civilizations such as the Babylonians, the Chaldeans, the Persians, and the Egyptians. Preserving important documents as engravings on metals such as bronze was customary in Ancient Rome. Writing on leaves (usually palm, but those of other trees as well) was also popular in Ancient Rome, and the term "leaf" still denotes the page of a book today. The Romans also wrote on the bark of trees. (The word liber was the Latin word for "inner bark," and soon came to denote a book itself. It is the etymological root of English words such as "library.") Ancient civilizations also made use of parchment, which was made from the split skin of sheep, and vellum, made from calfskin, goatskin, or lambskin (the word "vellum" itself derives from the same etymological root as "veal," denoting a calf). The use of parchment and vellum continued well into the era of the printing press, and still continues for important documents today. It is said that the skins of three hundred sheep were needed to produce one Gutenberg Bible. Papyrus, although bearing similar characteristics, is not paper in the true sense of the term, although it is made by splitting the stalks of the papyrus plant ('Cyperus papyrus'), laying them side by side, and hammering them together to provide a smooth writing surface. Papyrus was popular in Ancient Egypt, where the papyrus plant grew, and later spread elsewhere. Although the species is now nearly extinct in Egypt, it still grows along the upper Nile and in Ethiopia. The first true paper is believed to have been produced in China, by Ts'ai Lun in A.D. 105. The Chinese closely guarded the manufacture of paper, and it took 500 years for the craft to make it even to Korea. By the mid-twelfth century, however, papermaking had reached Europe, most likely by way of Arabia.

Until the late eighteenth century, the bulk of the paper produced in Europe was made from linen and cotton, whose high cellulose content (higher than that of wood pulp) still makes them the best materials for papermaking. (In fact, in the economic boom that followed the ebbing of the Black Death in Europe in the fourteenth century, the rage for new clothes—in particular, underwear—resulted in a lot of discarded old clothes. The all-too-familiar "bone man," who collected the remains of plague victims, became the "rag and bone man" who also collected old clothing, which went to the papermakers. It was this surplus of paper that provided the impetus for the invention of printing. Without an inexpensive material to print on, it would hardly have become as popular as it did.) Eventually, however, the surplus of cotton and linen turned into a shortage. A substitute—wood pulp—was suggested by the French physicist and naturalist René Antoine Ferchault de Réaumer, who got the idea by observing wasps build paper-like nests by chewing wood into pulp. Scientists seized on the idea of using materials other than cotton rags to make paper, and much of the eighteenth century consisted of various suggestions of plants that could be used. These included seaweed, swamp moss, vines, hemp, corn husks, potatoes, reeds, and the bark, leaves, and wood of various trees and shrubs. (A book was even published which was printed on paper made of asbestos.) In the nineteenth century, the technology to produce wood-pulp paper was in place, but a public slow to favor it over cotton- and linen-based paper made it less than economical. It wasn't until the 1880s that wood-pulp paper was accepted, and now, one hundred years later, the only rag-based paper the average person is likely to encounter is United States currency.

Other sources of pulp are also being sought, and include bagasse (a fibrous material left after the processing of sugarcane), esparto (a wild grass native to northern Africa and southern Spain, which has been used in England for a long time as a source of paper pulp), bamboos (any of several types of tall, woody grasses that grow in tropical regions), manila hemp and sisal hemp (obtained from used rope), jute (scraps of burlap), cotton linters (fibers from cotton seeds, left after cotton ginning), flax tow (another source of linen fiber), and kenaf (a plant native to India). Economy and renewability are of prime consideration in the choice of a fibrous source for mass papermaking.

There are two basic stages in the papermaking process. The first is "pulping"— extracting fibrous material from the wood or other raw material—and interleaving these fibers together to create paper. Before wood can be pulped, it must undergo debarking. Bark contains little or no fibrous material, and contributes dirt and other contaminants to the pulp, and is removed either mechanically or hydraulically. Debarked logs are then sent to a chipper where they are chopped into small pieces ready for digesting. The purpose of pulping is to separate the cellulose fibers from the other non-fibrous material in the wood, in particular lignin, an organic material that binds fibers of cellulose together. It is the presence of lignin that is primarily responsible for a paper's low durability and yellowing with age. Pulping can be done either by mechanical, chemical, or semichemical (a combination of mechanical and chemical) means. (See Pulping, as well as individual entries on Mechanical Pulping, Semichemical Pulping, and Chemical Pulping.) Mechanical (or groundwood) pulping is typically used for low-quality papers used for newspapers, directories, catalogs, and "pulp" magazines. Most printing and writing papers are made from chemical pulp, in particular that utilizing the kraft process. The advantages of kraft pulping include its ability to handle nearly every known type of wood, its efficient chemical and heat recovery system which lowers processing costs, and its ability to produce a strong, high-brightness pulp. The replacement of batch pulping systems with continuous pulping systems and the use of monitoring operations via computer has also improved, speed, economy, and quality of pulp. Modified chemical processes are used to pulp nonwoody plants. Although extracting cellulose is easier, their fibrous content tends to be less than that of wood. Cotton and other textile remnants generates higher fiber content, and is used for high-quality and permanent writing papers and is also pulped using a modified alkaline chemical process. Recycled paper is also being used increasingly as a source of pulp.

Wood pulp is brown in color (due primarily to the presence of residual lignin), and can often be used as is for brown wrapping paper or paper bags. More often, however, pulp must be bleached in order to produce printing and writing papers. Prolonged chemical pulping results in severe fiber degradation, so bleaching is typically done to remove or alter the lignin without harming the cellulose fibers. Bleaching of mechanical pulps also helps remove some of its lignin content, but is done on a limited basis so as to prevent fiber loss and decreased pulp yield. Bleaching increases the whiteness of mechanical pulps, but not to the extent that it does for chemical pulps. Bleached mechanical pulp also lacks the brightness stability and permanence of chemical pulps. At the end of the pulping process is when loading occurs. Loading is the addition of various fillers, or non-fibrous raw materials that alter the properties of the paper, depending on what qualities the paper is desired to have. Loading is done to modify such paper characteristics as opacity, brightness, printability, texture, weight, color, etc. The most common fillers are clay (refined from naturally-occurring kaolin), titanium dioxide, and calcium carbonate. Fillers account for 5:30% of a paper's total weight, depending on the end-use requirements of the paper. (See Fillers.) Sizing, such as rosin and alum, are often added at this point, as well, which make the paper resistant to water. (See Sizing.) Pulp and non-fibrous additives are known collectively as the papermaking furnish.

Prior to papermaking, the furnish undergoes stock preparation, which includes refining and the mixing of fibrous and non-fibrous materials. Fibers are dissolved in water in a pulper, which is a heavy tank containing high-speed blades which help dissolve the pulp into a slurry. The slurry is sent to undergo beating (or refining), either in a beater and a conical refiner (also called a jordan), which is an older, batch refining system still in use in many small paper mills, or a disk refiner, which is a newer, continuous process. In both systems, the beating process brushes, cuts, frays, and shortens the fibers. It also swells the fibers, increasing their surface area as an aid to bonding. Non-fibrous additives may be added at this point. In the disk refining system, which allows for greater flexibility, the furnish can be altered at will, depending on the final ultimate paper grade or other desired end-use factors.

There are three basic types of paper machines: a fourdrinier machine, a twin-wire former, and a cylinder machine. Each differs primarily in the forming section or wet end. At the beginning of the process, the furnish is diluted with water to a fiber-water ratio of 1 to 200. Centrifugal force removes foreign particles such as bits of metal, dirt, plastic, and other extraneous material. It is also necessary to keep the fibers from clumping together until it is on the forming wire. Premature clots of fibers (or flocs) result in poor paper formation. The paper machine's headbox keeps the fibers from clotting and regulates the rate at which the fiber suspension is sent to the forming section. The slice is an adjustable rectangular slit that regulates the width, thickness, and consistency of the furnish, ensuring that the paper that ultimately forms has properties that are uniform. As the furnish travels through the slice, the individual fibers begin to align in the direction of their flow, forming the grain of the paper.

In a traditional fourdrinier machine, the water-fiber mixture flows through the slice onto a moving wire mesh belt. As the mixture moves forward, water drains through the mesh and the fibers begin to interlace, forming a mat. More fibers are deposited on top of the previous layer. The belt is supported by suction-cup-shaped foils or turning table rolls that also aid in drainage, usually by suction. Some paper machines increase drainage by oscillating the wire, producing a shake in the direction perpendicular to the direction the wire is moving. The side of the fiber mat that forms on top of the belt is called the wire side of the paper while the reverse is called the felt side of the paper. Since some amount of fine fibers and fillers drain through the wire with the water, the felt side of the paper will have a somewhat different composition and texture than the wire side. Recent innovations have reduced the two-sidedness of paper. As the wire continues with the newly-formed paper web, it passes over vacuum boxes, which suck out water that is beyond the reach of the foils, shake, table rolls, or gravity itself. In many machines, the web now passes under a dandy roll, a hollow, wire-covered roller that improves paper formation. Designs on the dandy roll also add a watermark or the markings typical of laid finish paper. At the end of the forming section of the paper machine, the web passes the couch roll, a perforated cylinder that uses a vacuum to remove even more water. At this point, the web is about 80:85% water, and is ready to be sent to the press section which uses pressure and suction to remove and evenly distribute as much moisture as possible, and to increase fiber bonding and consequently sheet strength. The press section also affects the paper's final bulk and finish. When it leaves the press section, the web is about 60:70% water by weight. It is further dried in the drying section, where heated cylinders evaporate residual moisture. It is necessary to keep the web under tension to prevent distortions and shrinkage. When the web is dried, its water content will be 2:8%, depending on its end-use requirements. The method of drying also depends on the paper's specific end-use requirements. (See also Yankee Dryer and Air Drying.) External sizing, materials added to improve the paper's resistance to fluids and to seal the surface fibers so as to increase sheet strength, is also added at the size press, which is located within the drying section. (See also Surface Sizing).

The final step for the paper web on the machine is calendering. The machine's calender comprises several steel rollers which impart a dense, smooth surface with consistent thickness. The degree of calendering depends on the desired finish of the paper. Paper is wound into a full-width machine roll, and rewinders reduce the roll to the desired width and diameter. The creation of the paper roll is a more complex operation than one would expect. The requirements of any paper roll are printability and runnability, and as press speeds increase, new demands are placed on the roll-building technology. Tests are made that generate cross-machine direction profiles of the paper as it comes out of the slice and monitor variations in moisture content and caliper or, in other words, variations in the thickness of the forming web. Any variation beyond certain acceptable tolerances will prevent the web from winding correctly. It is necessary for printers to utilize rolls that unwind with even tension across the roll. If it is wound too tightly, the paper will be stretched beyond its ability to return to its original dimensions. Winding that is slack near the core but tight on the outer portion of the roll will result in a telescoped or starred roll, specific distortions and defects in the paper. New innovations, such as electronic drives and computer controls have contributed greatly to the creation of optimally-wound rolls.

Finishing refers to the final preparations of the paper, depending on the specific end-use requirements. It can include rewinding, sheeting, trimming, or altering the paper's finish by embossing, supercalendering, or coating.

The many properties a particular paper possesses affect its printability, print quality, and runnability. Once produced, paper is tested by methods produced by the Technical Association of the Pulp and Paper Industry. Each paper property has a specific test and guideline prescribed in the TAPPI Official Test Method. Tests that do not have an Official Method, but have practical applications, are known as TAPPI Useful Test Methods. Since paper responds to environmental conditions such as temperature and humidity, tests must be conducted in a tightly controlled environment.

'Surface properties'. Cleanliness of a paper's surface is an important consideration. Various surface contaminants are an unwanted byproduct of the papermaking process, however. Loosely bonded surface fibers, particles of fibers, fillers, or coatings detached by cutting and trimming operations performed with inadequately sharpened blades, lint, dust from the air, or fabric fibers from machine felts all contribute to generating surface debris which cause blanket piling and various printing defects. Dirt is a term that in papermaking specifically refers to specks or particles embedded in the paper that contrasts with the color of the paper itself. Ink absorbency and its opposite property, ink holdout, are functions of a paper's porosity, as well as other surface and structural properties. A lack of uniform ink absorbency results in a mottled or galvanized appearance, or visible inconsistency in ink density, color, gloss, or other aspect. Variable color density and gloss result from variations in the paper's gloss and point-to-point variations in the capacity for ink holdout. (See Ink Holdout and Ink Absorbency.) Printing smoothness refers to how complete the contact between the paper and the printing surface is. The smoothness of paper affects the appearance of the printed image, involving as it does the variations in surface contour. Smoothness is primarily a formation issue, and the type and length of fibers used, the extent of wet pressing, the amount of filler, the extent of calendering, and the use of coatings all affect the ultimate levelness of the paper's printing surface. (See Smoothness.) A paper's conformability is the degree of contouring that will take place when a printing surface applies pressure to the paper to bring it in more complete contact with the ink on the printing plate. Poor formation of the paper or low compressibility will result in incomplete image transfer to the paper. The surface strength, also called pick resistance, is the ability of paper to resist rupturing under forces that act at right angles to its surface, as when the film of ink splits between the paper surface and the plate or blanket during printing. Picking refers to any ink-transfer damage, such as the pulling off of coating fragments, the separation of paper plys below the printed surface, or any blistering or rupturing. (See Picking.) Also affecting the smoothness and printability of paper is the two-sidedness of paper. Coatings applied to papers usually reduce the differences in the two sides. Paper manufactured on newer twin-wire formers also ensure that both sides of the paper are roughly similar in texture and smoothness. (See also individual entries for Wire Side and Felt Side.) The compressibility of a paper is a function of its hardness, density, moisture content, and the relative humidity, and refers to its change in thickness under pressure. Resiliency is the paper's ability to return to its original thickness and surface contour when pressure is removed. Hardness and softness are, respectively, the extent to which a paper will resist or allow indentations made by a pen, printing plate, or other surface. All of these structural characteristics are collectively described as its printing cushion. Although high softness is preferred, different end-use requirements necessitate differing degrees of hardness and resiliency.

'Structural properties'. Formation is a paper quality that describes the uniformity of its minute surface contours. "Hills" and "valleys," seen as light and dark regions when paper is held up to a light source, affect the smoothness and levelness of the paper's surface. Calendering, or compressing the "hills," is only a partial solution, as the "valleys" still remain, and printing on such paper will have a blotchy or mottled appearance. Grain direction is the result of the fibers aligning themselves with direction of their travel through the paper machine (called ["machine direction [MD])"]. A paper's grain direction is called long-grain when the grain direction is parallel to its longer dimension. Short-grain paper has its grain direction parallel to its shorter dimension. Grain direction is important, particularly for sheetfed printing, as long-grain paper will be more dimensionally stable than short-grain paper. (See Grain.) A paper's strength is described not only in terms of its grain and cross-grain directions, but also in a direction perpendicular to the plane of the paper's surface. The internal bond strength—also called the plybond strength—refers to the force needed to separate the two plys of a single sheet of paper. Internal bond strength is a consideration when forces in printing act on a paper's surface, such as tacky inks and blanket-to-blanket web presses. (See Internal Bond Strength.) A paper's porosity is described as the amount of interfiber space that exists in the paper. A paper's porosity affects its ability to absorb ink. High-porosity paper is needed for rapid ink vehicle penetration and setting, and to decrease ink holdout and to resist smudging during folding processes. Other printing methods require low-porosity paper to increase ink holdout, decrease ink penetration and reduce strike-through. (See Porosity.) Also important is a paper's dimensional stability, which is a paper's ability to retain its original dimensions under printing stresses and changing environmental conditions. All papers will expand or shrink in size with changing moisture content, but a lesser degree of refining (producing paper with a loose fibrous structure) and the use of fillers can increase dimensional stability. A paper's tendency to absorb or release moisture to the atmosphere is an important consideration when doing multiple-color press work where the paper must run through the press two or more times. (See Dimensional Stability.) Low dimensional stability can result in bad register of successive colors. A paper's visoelasticity refers to its ability to return to its original dimensions after being stretched. Beyond a certain tensile strength, however, some permanent distortion is inevitable. (See Waffling.) An important consideration in web printing is a coated paper's blister resistance. During heatset drying, water vapor generated will increase a paper's tendency to blister, especially if a paper's porosity does not adequately allow the release of vapor pressure that is built up within the paper as it is exposed to heat. Related to blistering is fiber puffing, in which fibers in coated groundwood paper explode during drying, which mars the surface of the paper.

All of these surface and structural properties determine a paper's printability, or how well a printed image is transferred to the paper, and a paper's runnability, or its ability to run through a press or other printing means without affecting the printing process itself, such as contaminating a press's fountain solution, transferring paper debris to the plate or blanket, generating the need to frequently adjust the press, or any other defect in the paper that reduces press speeds. On a web press, runnability is described in terms of web breaks per 100 rolls. Runnability is a fairly quantifiable property, but so far various attempts at quantifying the printability of paper has been elusive, but devices such as the Prüfbau, IGT, and Huck gravure printability testers are occasionally used toward this end, as are printing presses themselves, but the problems of evaluating printability stem from the more subjective aspects of the term, which defy objective quantification.

'Chemical Properties'. A paper's chemical composition determines not only the texture of the paper but its performance on the press. If a paper is to be used on a sheetfed offset press, it will need to be water-resistant, while paper running on a web offset press runs more quickly, so does not need to be. Coatings must also be compatible with ink-drying systems, and must not transfer any of the coating materials to the printing system, which would contaminate the chemical balance of that system. The type of fibers used in the paper (cotton vs. wood, for instance) affects a paper's attributes, as does the pulping method used. Groundwood pulp provides high opacity and ink absorbency, but low strength, brightness and permanency. The type and quantity of fillers also affects paper characteristics. Fillers, as their name implies, fill up pores and capillaries between paper fibers, dividing large capillaries into many smaller ones. Ink (and other fluid) absorbency works by capillary action. Increasing the number of capillaries increases the absorbency of the paper, although the capacity of the paper to hold moisture is decreased. (Many smaller pores will absorb more but hold less than fewer large pores.) Fillers also make paper less susceptible to changes in moisture content, improving dimensional stability. (The determination of the amount of filler is called its ash content. See Ash Content.) Paper is able to absorb moisture from the air. As cellulose fibers take on water, they expand primarily in width (but not greatly in length), and when they lose water, they shrink primarily in width (but not greatly in length), which affects the dimensional stability of paper, causing distortion problems in printing. Moisture loss results in paper curling by disrupting the stresses between the felt side and the wire side of the paper, as the wire side has greater fiber alignment in the grain direction than the felt side. The moisture content of paper itself must be varied depending on the end-use requirements. Paper that is too dry is hard and brittle, reducing the quality of the printed impression. Paper that is too moist can blister during heatset drying. Book papers that have varying moisture contents and varying abilities to accept or lose moisture can change dimensions after binding and trimming, forming uneven edges among different signatures. The equilibrium moisture content of paper is the moisture content of paper when it neither loses nor gains moisture from the surrounding air, and depends on the relative humidity and the fiber and filler content of the paper. (A variation is the equilibrium relative humidity, or the level of humidity in the atmosphere at which paper will neither lose nor gain moisture.) Interestingly, the equilibrium moisture content of a paper will be lower if the paper starts at a low relative humidity and progressively is conditioned to higher ones than if the paper starts at a high relative humidity and is conditioned to lower ones. This difference is called hysteresis, and describes a paper's equilibrium moisture content in the context of its entire moisture history. Another problem involving a paper's moisture content involves the buildup of electrical charges, which occurs if the relative humidity of the paper falls below 35%. If the paper cannot dissipate its electrical charges, static cling occurs. (See Moisture Content.) Another important chemical property is a paper's pH, which refers to its acidity or alkalinity. The term "pH" itself means "potential of the hydrogen ion (H⁺)". A solution that contains an excess of hydrogen ions is said to be an "acid," while a solution that contains an excess of hydroxyl (OH^-) ions is said to be a "base," or an "alkaline" solution. A paper's pH is regulated by the internal sizing used, and any changes in pH contributed by a paper coating. Most paper is currently acid paper, but alkaline paper is receiving great interest, as alkaline papers retain their brightness and strength over time, unlike acid papers which tend to deteriorate, sometimes quite rapidly. The life expectancy for alkaline papers is about 200 years, compared to 50 years for acid papers. (See pH.) A paper's resistance to water is also important, and therefore the desired amount of sizing, or treatment to prevent water penetration (and whether it will be internal sizing or external sizing), depends on the end-use requirements of the paper. Waterleaf is unsized paper that absorbs water readily, and is used for towels, blotting paper, and other papers designed expressly for the absorption of fluids. Slack-sized paper has a minimal amount of internal sizing, and hard-sized paper has a large amount of internal sizing. Internal sizing hinders the penetration of fluids into the paper, but it does not make the paper waterproof or hamper its ink absorbency. Internal sizing is typically used to protect the paper from press dampening systems, and usually consists of a rosin added to the papermaking furnish during the stock preparation phase. Acid-rosin sizing is increasingly being replaced by alkaline-rosin sizing, as the interest in alkaline papers increases. External sizing (also called surface sizing) usually consists of a starch that is added to the web before it is completely dry. External sizing contributes little water resistance; its purpose is primarily to seal the surface fibers, increasing surface strength. (See Sizing)

'Optical Properties'. A paper's whiteness refers to the extent to which all wavelengths of light are reflected from a paper's surface, and brightness refers to the degree that blue light is reflected from a paper's surface. (See Whiteness and Brightness.) Color refers to the extent to which paper selectively absorbs light of certain wavelengths, the remaining wavelengths being reflected back to the observer, imparting to the paper surface its color. (See Color, Paper.) Gloss refers to a paper's specular reflectance, or the condition of a paper surface that causes it to be shiny or glossy. The opposite of a glossy surface, a matte surface, reflects light diffusely, imparting a dull finish to the surface. (See Gloss.) Opacity refers to the extent to which light passes through a paper, which is an important consideration in printing. Low opacity results in a greater degree of show-through, or the visibility of the printing on the reverse of a sheet of paper. (See Opacity.)

'End-Use Properties'. One important criterion is a paper's ability to accept adhesives, either an adhesive backing for labels and tapes, or the various adhesives used in binding processes. The composition of a coated paper's coating must allow the application of adhesives if it is to be used for such purposes. (The binding strength of an adhesive-bound book is determined using a head and tail tester.) A related property important in the production of cartons and packaging is a paperboard's glueability, or the speed and strength with which an adhesive bond is formed when two sections of a paperboard are glued together. Papers and paperboards used in packaging must be produced in line with FDA (Food and Drug Administration) regulations if they are going to be used to package foods and pharmaceuticals. A paper used in the packaging of edible products cannot emanate strong odors, while papers used for packaging silver- or iron-based metals should not contain chemicals that will corrode or chemically react with the materials being packaged. Papers used to package soaps, detergents, or other alkaline materials should be able to resist discoloration. Other types of packaging should also be able to withstand penetration by various types of liquids such as oil, grease, blood, etc. Paper used in electrical work (such as for wrapping cables) must be chemically neutral. Some papers need to possess flame resistance. (See Flame Resistance.) Paper produced with a high lignin content, such as groundwood-based paper, is, as we saw earlier, susceptible to yellowing upon exposure to sunlight or fluorescent lighting. Papers that are likely to be exposed to such lighting will need to maintain a high level of lightfastness, or be able to resist yellowing and fadi