papermaking

Introduction

      formation of a matted or felted sheet, usually of cellulose fibres, from water suspension on a wire

screen. Paper is the basic material used for written communication and the dissemination of

information. In addition, paper and paperboard provide materials for hundreds of other uses, such as

wrapping, packaging, toweling, insulating, and photography.

      The word paper is derived from the name of the reedy plant papyrus, which grows abundantly

along the Nile River in Egypt (Egypt, ancient). In ancient times, the fibrous layers within the stem of

this plant were removed, placed side by side, and crossed at right angles with another set of layers

similarly arranged. The sheet so formed was dampened and pressed. Upon drying, the gluelike sap of

the plant, acting as an adhesive, cemented the layers together. Complete defibring, an indispensable

element in modern papermaking, did not occur in the preparation of papyrus sheets. Papyrus was the

most widely used writing material in ancient times, and many papyrus records still survive.

The papermaking process

Historical development

      Papermaking can be traced to about AD 105, when Ts'ai Lun (Cai Lun), an official attached to

the Imperial court of China, created a sheet of paper using mulberry and other bast fibres along with

fishnets, old rags, and hemp waste. In its slow travel westward, the art of papermaking reached

Samarkand, in Central Asia, in 751; and in 793 the first paper was made in Baghdad during the time

of Hārūn ar-Rashīd, with the golden age of Islāmic culture that brought papermaking to the frontiers

of Europe.

      By the 14th century a number of paper mills existed in Europe, particularly in Spain, Italy,

France, and Germany. The invention of printing in the 1450s brought a vastly increased demand for

paper. Through the 18th century the papermaking process remained essentially unchanged, with

linen and cotton rags furnishing the basic raw materials. Paper mills were increasingly plagued by

shortages; in the 18th century they even advertised and solicited publicly for rags. It was evident that

a process for utilizing a more abundant material was needed.

Improvements in materials and processes

      In 1800 a book was published that launched development of practical methods for manufacturing

paper from wood pulp and other vegetable pulps. Several major pulping processes were gradually

developed that relieved the paper industry of dependency upon cotton and linen rags and made

modern large-scale production possible. These developments followed two distinct pathways. In one,

fibres and fibre fragments were separated from the wood structure by mechanical means; and in the

other, the wood was exposed to chemical solutions that dissolved and removed lignin and other wood

components, leaving cellulose fibre behind. Made by mechanical methods, groundwood pulp

contains all the components of wood and thus is not suitable for papers in which high whiteness and

permanence are required. Chemical wood pulps such as soda and sulfite (sulfite process) pulp

(described below) are used when high brightness, strength, and permanence are required.

Groundwood pulp was first made in Germany in 1840, but the process did not come into extensive

use until about 1870. Soda pulp was first manufactured from wood in 1852 in England, and in 1867 a

patent was issued in the United States for the sulfite pulping process.

      A sheet of paper composed only of cellulosic fibres (“waterleaf”) is water absorbent. Hence,

water-based inks and other aqueous liquids will penetrate and spread in it. Impregnation of the paper

with various substances that retard such wetting and penetration is called sizing.

      Before 1800, paper sheets were sized by impregnation with animal glue or vegetable gums, an

expensive and tedious process. In 1800 Moritz Friedrich Illig in Germany discovered that paper

could be sized in vats with rosin and alum. Although Illig published his discovery in 1807, the

method did not come into wide use for about 25 years.

      Discovery of the element chlorine in 1774 led to its use for bleaching paper stock. Lack of

chemical knowledge at the time, however, resulted in production of inferior paper by the method,

discrediting it for some years. Chlorine bleaching is a common papermaking technique today.

Introduction of machinery

      Prior to the invention of the paper machine, paper was made one sheet at a time by dipping a

frame or mold with a screened bottom into a vat of stock. Lifting the mold allowed the water to

drain, leaving the sheet on the screen. The sheet was then pressed and dried. The size of a single

sheet was limited to the size of frame and mold that a man could lift from a vat of stock.

      In 1798 Nicolas-Louis Robert (Robert, Nicolas-Louis) in France constructed a moving screen

belt that would receive a continuous flow of stock and deliver an unbroken sheet of wet paper to a

pair of squeeze rolls. The French government recognized Robert's work by the granting of a patent.

      The paper machine did not become a practical reality, however, until two engineers in England,

both familiar with Robert's ideas, built an improved version for their employers, Henry and Sealy

Fourdrinier (Fourdrinier machine), in 1807. The Fourdrinier brothers obtained a patent also. Two

years later a cylinder paper machine (described below) was devised by John Dickinson, an English

papermaker. From these crude beginnings, modern papermaking machines evolved. By 1875 paper

coated by machinery was being made for use in the printing of halftones by the new photoengraving

process, and in 1884 Carl F. Dahl invented sulfate (kraft) pulp in Danzig, Germany.

      Although the paper machine symbolizes the mechanization of the paper industry, every step of

production, from the felling of trees to the shipment of the finished product, has also seen a dramatic

increase in mechanization, thus reducing hand labour. As papermaking operations require the

repeated movement of large amounts of material, the design and mechanization of materials-

handling equipment has been and continues to be an important aspect of industry development.

      Although modern inventions and engineering have transformed an ancient craft into a highly

technical industry, the basic operations in papermaking remain the same to this day. The steps in the

process are as follows: (1) a suspension of cellulosic fibre is prepared by beating it in water so that

the fibres are thoroughly separated and saturated with water; (2) the paper stock is filtered on a

woven screen to form a matted sheet of fibre; (3) the wet sheet is pressed and compacted to squeeze

out a large proportion of water; (4) the remaining water is removed by evaporation; and (5)

depending upon use requirements, the dry paper sheet is further compressed, coated, or impregnated.

      The differences among various grades and types of paper are determined by: (1) the type of fibre

or pulp, (2) the degree of beating or refining of the stock, (3) the addition of various materials to the

stock, (4) formation conditions of the sheet, including basis weight, or substance per unit area, and

(5) the physical or chemical treatment applied to the paper after its formation.

Fibre sources

      The cell walls of all plants (plant) contain fibres (natural fibre) of cellulose, an organic material

known to chemists as a linear polysaccharide. It constitutes about one-third of the structural material

of annual plants and about one-half that of perennial plants. Cellulose fibres have high strength and

durability. They are readily wetted by water, exhibiting considerable swelling when saturated, and

are hygroscopic—i.e., they absorb appreciable amounts of water when exposed to the atmosphere.

Even in the wet state, natural cellulose fibres show no loss in strength. It is the combination of these

qualities with strength and flexibility that makes cellulose of unique value for paper manufacture.

      Most plant materials also contain nonfibrous elements or cells, and these also are found in pulp

and paper. The nonfibrous cells are less desirable for papermaking than fibres but, mixed with fibre,

are of value in filling in the sheet. It is probably true that paper of a sort can be produced from any

natural plant. The requirements of paper quality and economic considerations, however, limit the

sources of supply.

wood

      Pulped forest tree trunks (boles) are by far the predominant source of papermaking fibre. The

bole of a tree consists essentially of fibres with a minimum of nonfibrous elements, such as pith and

parenchyma cells.

      Forests of the world contain a great number of species, which may be divided into two groups:

coniferous trees, usually called softwoods, and deciduous trees, or hardwoods. Softwood cellulose

fibres measure from about 2 to 4 millimetres (0.08 to 0.16 inch) in length, and hardwood fibres range

from about 0.5 to 1.5 millimetres (0.02 to 0.06 inch). The greater length of softwood fibres

contributes strength to paper; the shorter hardwood fibres fill in the sheet and give it opacity and a

smooth surface.

      When the sulfite process (see below) was the chief method of pulping in the early days of the

pulp industry, spruce and fir were the preferred species. Since that time, advances in technology,

particularly the introduction of the kraft process (described below), have permitted the use of

practically all species of wood, greatly expanding the potential supply.

      Because of the enormous and rapidly growing consumption (conservation) of wood for pulp,

concern regarding the depletion of forest resources (ecology) has been expressed, even though yearly

growth often exceeds the annual harvest. In 1962, for example, though new growth exceeded the

harvest by a considerable margin, much of it was inferior in quality and less accessible than the

harvested trees. Moreover, wood is now being harvested at a more rapid pace. Approximately 40

percent of the harvest is going into pulp, and that figure is expected to increase. There is also a rising

public demand for withdrawal of forestland from timber production for recreational use and to

prevent disturbance to the ecology of certain areas. On the other hand, application of new techniques

in fertilization and genetics has brought about enormous increases in the productivity of forestlands

in some areas.

      Two significant trends in pulpwood utilization deserve mention. Until recently, lumbering and

other wood-using industries were operated quite independently of the pulp industry. Since World

War II, however, the waste from the wood-using industries, such as sawdust, has increasingly been

used for pulp. In addition, more abundant and less desirable hardwoods have been used as a source

of pulp. The woodyard of a pulp mill formerly stored pulpwood in the form of roundwood logs, but

recently there has been a trend toward storing in the form of chips.

Rags

      Cotton and linen fibres, derived from textile and garment mill cuttings; cotton linters (the short

fibres recovered from the processing of cottonseed after the separation of the staple fibre); flax

fibres; and clean, sorted rags are still used for those grades of paper in which maximum strength,

durability, and permanence, as well as fine formation, colour, texture, and feel, are required. These

properties are attributed to the greater fineness, length, and purity of rag fibre as compared with most

wood pulp. Rag papers are used extensively for bank note and security certificates; life insurance

policies and legal documents, for which permanence is of prime importance; technical papers, such

as tracing paper, vellums, and reproduction papers; high-grade bond letterheads, which must be

impressive in appearance and texture; lightweight specialties such as cigarette, carbon, and Bible

papers; and high-grade stationery, in which beauty, softness, and fine texture are desired.

      Rags are received at the paper mill in bales weighing from 200 to 500 kilograms (400 to 1,200

pounds). After mechanical threshing, the rags are sorted by hand to remove such foreign materials as

rubber, metal, and paper and to eliminate those rags containing synthetic fibres and coatings that are

difficult to remove. Following sorting, the rags are cut up, then dusted to remove small particles of

foreign materials, and passed over magnetic rolls to remove iron.

      The cut and cleaned rags are cooked (to remove natural waxes, fillers, oils, and grease) in large

cylindrical or spherical boilers of about five-ton capacity. About three parts of cooking liquor, a

dilute alkaline solution of lime and soda ash or caustic soda combined with wetting agents or

detergents, are used with each part of rags. Steam is admitted to the boiler under pressure, and the

contents are cooked for three to ten hours.

      Once cooked, the rags are washed, then mechanically beaten. The beating shortens the fibre,

increases the swelling action of water to produce a softened and plastic fibre, and fibrillates or frays

the fibre to increase its surface area. All of these actions contribute to better formation of the paper

sheet, closer contact between fibres, and the formation of interfibre bonding that gives the paper

strength and coherence.

Wastepaper and paperboard

      By using greater quantities of wastepaper stock, the need for virgin fibre is reduced, and the

problem of solid waste disposal is minimized. The expansion of this source is a highly complex

problem, however, because of the difficulties in gathering wastepaper from scattered sources, sorting

mixed papers, and recovering the fibre from many types of coated and treated papers.

      Wastepaper may be classified into four main categories: high-grades, old corrugated boxes,

printed news, and mixed paper. High-grade and corrugated stocks originate mainly in mercantile and

industrial establishments. White paper wastes accumulate in envelope and printing plants, while

tabulating cards are supplied by large offices. Much magazine stock comes from newsstand returns,

but some comes from homes. Corrugated waste is supplied by manufacturing plants and retail stores.

Printed news is derived from newsstand returns and home collections. Mixed paper comes from

wastebaskets of office buildings and similar sources. In recent years there has been considerable

interest in wastepaper recycling in the interest of ecology.

      Converters of paper and paperboard have also turned to new materials combined with paper and

paperboard to give their products special characteristics. Although these new materials have

broadened the market for paper, their presence has posed new problems in reusing paper stock. The

most common new ingredients are asphalt, synthetic adhesives, metal foils, plastic and cellulose-

derivative films and coatings, and some printing inks.

      Some objectionable materials can be sorted from wastepaper, and packers generally try to

remove them completely. If the producer of wastepaper knows the materials he is using, he can

usually segregate trouble-causing substances at the source. Much depends on good cooperation and

communication among the papermaker, dealer, packer, and producers so that all may understand

what is and what is not acceptable.

      There are two distinct types of paper recovery systems: (1) recovery based upon de-inking and

intended for printing-grade or other white papers, accounting for about 5 to 6 percent of the total, and

(2) recovery without de-inking, intended for boxboards and coarse papers, accounting for the

remainder.

      In the de-inking recovery process, the bales of wastepaper are opened, inspected, and fed into a

pulper, a cylindrical tank with capacity ranging from one to several tons of stock and provided with

agitator blades that circulate and agitate the stock. Hot water and various chemicals help the agitator

separate and disperse the fibres.

      The amount and type of chemicals used vary considerably from mill to mill. Caustic soda is by

far the most generally used, but it is often supplemented with soda ash, silicate of soda, phosphates,

and surfactants (wetting agents). The temperature range is from 65° to 90° C (150° to 190° F).

      The pulpers are aided in the collection and separation of large pieces of trash by special devices.

After the stock leaves the pulper, it is screened to remove finer trash particles and washed to remove

the dispersed ink and chemicals. In some instances the stock is bleached with hypochlorite to

improve its whiteness.

      In pulping paper stock where de-inking is not necessary, the equipment is similar to that already

described. Hot water is also used in the pulper, but the chemicals for dissolving and dispersing the

ink are not needed. The stock is screened and washed to remove trash and dirt.

      The use of paper stock in the paper mill presents difficulties because of the presence of foreign

materials. Miscellaneous trash has always required operators to be watchful, and its presence

depends on the source of the waste and the care with which the paper is prepared for market.

Natural fibres other than wood

      Since cellulose fibre is a major constituent of the stems of plants, a vast number of plants

represent potential sources of paper; many of these have been pulped experimentally. A rather

substantial number of plant sources have been used commercially, at least on a small scale and at

various times and places. Indeed, the use of cereal straws for paper predates the use of wood pulp

and is widely practiced today throughout the world, although on a relatively small scale of

production. Because many parts of the world are deficient in forests, the development of the paper

industry in these areas appears to depend to a considerable degree upon the use of annual plants and

agricultural fibres.

      Nonwoody plant stems differ from wood in containing less total cellulose, less lignin, and more

of other materials. This means that pulps of high cellulose content (high purity) are produced in

relatively low yield, whereas pulps of high yield contain high proportions of other materials. Papers

made from these pulps without admixture of other fibre tend to be dense and stiff, with low tear

resistance and low opacity.

      The morphology (form and structure) of the cells of annual plants also differs considerably from

wood. Whereas the nonfibrous (parenchyma) cells of coniferous wood constitute a minor proportion

of the wood substance, in annual plants this cell type is a major constituent. As hardwoods also often

contain considerable amounts of nonfibrous cells, there is a closer resemblance between hardwood

pulps and pulps from annual plants.

      The preferred pulping reagents for nonwood plants are the alkalis (alkali): caustic soda, lime and

soda ash, and kraft liquor (caustic soda and sodium sulfide). A characteristic of the pulping of annual

plants, compared with wood, is the milder treatment necessary to produce pulp. Straw, for example,

may be pulped with milk of lime in a spherical digester at a steam pressure of about 2 kilograms per

square centimetre (25 pounds per square inch) and a cooking time of 8 to 10 hours. The amount of

lime used is about 10 percent of the amount of dry fibre.

      In the United States straw pulp was formerly used extensively for corrugating medium (i.e., sheet

fluted to form the inner ply of corrugated board). Since then, the use of straw pulp for corrugating

medium has been replaced by semichemical hardwood pulp. Straw pulp is still made in several

European and Asiatic countries on a small scale.

      The residue from the crushing of sugarcane, called bagasse, contains about 65 percent fibre, 25

percent pith cells, and 10 percent water solubles. An essential element in the conversion of bagasse

to a satisfactory paper is the mechanical removal of a substantial proportion of the pith prior to the

pulping operation. Pulping may be carried out either with soda or with kraft cooking liquor and by

batch or continuous systems. Bagasse fibre averages 1.5 to 2 millimetres (0.06 to 0.08 inch) in length

and is relatively fine.

      The use of bagasse is substantial in several Latin American countries and in the Middle East. The

utilization of bagasse for paper in all the sugar-producing countries that are deficient in forest

resources is a practical step.

      A desert plant of the Mediterranean area, especially in southern Spain and northern Africa,

esparto grass has a higher cellulose content than most nonwood plants, with greater uniformity of

fibre size and shape. The use of esparto for papermaking was developed in Great Britain in 1856.

Consumption rose steadily until the mid-1950s but since has steadily declined.

      Esparto held its own against the competition with wood pulp for some time because of its

favourable papermaking properties. The stock forms well on a paper machine because of free

drainage and uniform fibre length, compared with rag or wood pulp. Esparto printing papers possess

good resilience in contact with the printing plate, have good opacity and smoothness, and are

relatively lint-free. Another important characteristic of papers made from esparto is dimensional

stability with changes in moisture content.

      Botanically, bamboo is classified as a grass, even though it attains a considerable size and the

stems or culms resemble wood in hardness and density. It was demonstrated many years ago that

satisfactory pulp could be made from bamboo.

      Because of the abundance of bamboo in Southeast Asia, where increased production of paper is

greatly needed, much interest has been displayed in bamboo pulp development. The growing cycle of

bamboo is favourable, for the culms can be harvested without destroying the root system. Under

ideal conditions of soil fertility and moisture, an established stand of bamboo probably would

produce more fibre per hectare (or acre) per year than any other plant. Wild bamboo, however, is

difficult to harvest and transport economically; so far, the interest in it has not been translated into

any large-scale production. Pulp mills make use of bamboo in India, Thailand, and the Philippines.

Considerable quantities of bamboo pulp are said to be made in China, but details are lacking.

       flax, hemp, jute, and kenaf are characterized by a high proportion of long, flexible bast fibres

that are readily separated and purified from the other materials in the plant. Consequently, such

fibres have long been used for textiles and rope making. Most of this fibre reaching the paper

industry in the past has been secondary or waste fibre. It has been highly prized because of the

strength and durability it imparts to such products as tags, abrasive paper (sandpaper), cover stock,

and other heavy-duty paper. It is also used for duplicating and manifold paper, in which extremely

light weight must be combined with exceptional strength. Flax is grown expressly for high-grade

cigarette paper. Experimental quantities of kenaf have been grown and made into various grades of

paper.

Synthetic fibres (synthetic fibre)

      The development and use of a great variety of man-made fibres (fibre, man-made) have created a

revolution in the textile industry in recent decades. It has been predicted that similar widespread use

of synthetic fibres may eventually occur in the paper industry. Active interest has been evident in

recent years, both on the part of fibre producers and of paper manufacturers. Many specialty paper

products are currently being made from synthetic fibres.

      The advantages of synthetic or man-made fibres in papermaking can be summarized as follows:

      Whereas natural cellulose fibres vary considerably in size and shape, synthetic fibres can be

made uniform and of selected length and diameter. Long fibres, for example, are necessary in

producing strong, durable papers. There are limitations, however, to the length of synthetic fibres

that may be formed from suspension in water because of their tendency to tangle and to rope

together. Even so, papers have been made experimentally with fibres several times longer than those

typical of wood pulp; these papers have improved strength and softness properties.

      Natural cellulose fibres have limited resistance to chemical attack and exposure to heat. Because

synthetic fibre papers can be made resistant to strong acids, they are useful for chemical filtration.

Paper can even be made from glass fibre, and such paper has great resistance to both heat and

chemicals.

      The natural cellulose fibres of ordinary paper are hygroscopic; i.e., they absorb water from the air

and reach an equilibrium depending upon the relative humidity. The moisture content of paper,

therefore, changes with atmospheric conditions. These changes cause swelling and shrinkage of

fibres, accounting for the puckering and curling of papers. Synthetic fibres not subject to these

changes can be used to produce dimensionally stable papers.

      The cheapest man-made fibre, rayon, costs from three to six times as much as an equivalent

amount of wood pulp, whereas most of the true synthetics, such as the polyamides (nylon),

polyesters (Dacron, Dynel), acrylics (Orlon, Creslan, Acrilan), and glass, cost from 10 to 20 times as

much. This difference in cost does not preclude the use of existing synthetics, but it limits their use to

special items in which the extra qualities will justify the additional cost. The cost factor is increased

by the absence in most synthetic fibres of the bonding property of natural cellulose fibres. When

beaten in water, natural fibres swell and cement together as they dry. Paper made from synthetics

must be bonded by the addition of an adhesive, requiring an additional manufacturing step.

      There is a distinct similarity between synthetic fibre “papers” and the class of sheet materials

known as nonwovens. As a step in the manufacture of yarn, staple fibres are carded (i.e., separated

and combed) to form a uniform, lightweight, and fragile web. Subsequently, this web is gathered

together to form a strand or sliver, which is drawn and spun into yarn. If several of these flat webs,

however, are laminated together and bonded with adhesive, a nonwoven fabric that has properties

resembling both paper and cloth results. In this area it is difficult to draw a clear distinction between

what is paper and what is cloth. Processes are now available to form sheet material both by the dry

forming method and by the water forming or paper system. When textile-type fibres are formed into

webs by either of these processes, the resulting products have properties that enable them to compete

in some fields traditionally served by textiles.

Processes for preparing pulp

      Mechanical or groundwood pulp is made by subjecting wood to an abrading action, either by

pressing the wood against a revolving grinding stone or by passing chips through a mill. The wood

fibres are separated and, to a considerable degree, fragmented.

      Chemical wood pulp is made by cooking wood chips with chemical solutions in digesters

operated at elevated temperature and pressure. The chemicals used are (1) sulfite salts with an excess

of sulfur dioxide and (2) caustic soda and sodium sulfide (the kraft process). The lignin of the wood

is made soluble, and the fibres separate as whole fibres. Further purification can be accomplished by

bleaching. Chemical wood pulp that is purified both by bleaching and by alkaline extraction is called

alpha or dissolving pulp. It is used for specialty papers, for rayon and cellulose film production, and

for cellulose derivatives, such as nitrate and acetate.

      Semichemical pulp is made by treating wood chips with sulfite or alkali in amounts and under

conditions that soften the lignin but dissolve only part of it. The softened chips are then defibred.

Mechanical or groundwood pulp

      Pulpwood may arrive at the mill as bolts 1.2 metres (4 feet) in length or as full-length logs. The

logs are sawn to shorter length, and the bolts are tumbled in large revolving drums to remove the

bark. The debarked wood is next sent to grinders (grinding machine), where its moisture content is

important for ease of grinding and quality of pulp. Moisture content should be at least 30 percent and

preferably 45 to 50 percent. Wood of low moisture content is presoaked in a pond or sprayed with

water.

      Early grinders employed round slabs of natural sandstone 69 centimetres (27 inches) wide and

137 centimetres (54 inches) in diameter, often directly connected to water wheels, to produce five or

six tons of pulp per day. The wood was hand-loaded into the grinders.

      Today's much larger pulp grinders are usually powered by electric motors and automatically

loaded. In a recently built mill, each grinder is gear-connected to a 10,000-horsepower motor; the

pulpstone, at 360 revolutions per minute, can handle wood 1.5 to 1.6 metres (60 to 64 inches) long.

Hydraulic cylinders produce a pressure of 14 kilograms per square centimetre (200 pounds per

square inch) against the stone face. Pulp production from each stone is 130 to 150 tons every 24

hours.

      The first artificial grinding stone was produced in 1924; since that time, artificial stones have

replaced natural sandstone. Silicon carbide and aluminum oxide are the abrasives used in the

manufacture of pulpstones. The abrasive material is broken down into a mixture of sizes that are

screened to give fractions of uniform grain size. The abrasive grains are mixed with binder and fired

at high temperature (2,300° C or 4,200° F) in the form of segments that are assembled to form the

abrasive surface of the pulpstone.

      The pulp stock flows from the grinder pit to a series of rifflers and screens, which separate the

heavy foreign material and pieces of unfibred wood (shives), knots, bark, and the like.

      Most groundwood pulp flows directly to an adjacent paper mill for use as stock. When shipped, it

is formed into a sheet on a cylindrical vacuum filter. The sheets are pressed in a hydraulic press to a

moisture content of about 50 percent, and the pressed sheets are formed into bales.

      An important test to control the quality of groundwood pulp is freeness: the readiness with which

water drains from and through a wet pad of pulp. Groundwood pulps are much less “free” than

chemical wood pulps.

      In groundwood pulp, the fibres are fragmented, and there is considerable debris (fines). Also,

groundwood contains all the chemical constituents of wood, including lignin, hemicellulose, resin,

and various colouring materials. This means that papers containing groundwood are subject to

discoloration (yellowing) upon exposure to light and heat and after aging. The yellowing of

newspaper and much book paper is an example of this. Because groundwood fibres are relatively

short and have only a moderate ability to bond to each other, papers containing them do not have

high strength. On the other hand, papers containing groundwood have good opacity; they are bulky

and have good printing qualities.

      Groundwood pulp does not have a high whiteness, being limited in this quality by the colour of

the wood from which it is made. Although often bleached with peroxide or hydrosulfite to improve

whiteness, it does not equal pure cellulose.

Chemical wood pulp

      The effect of sulfurous acid (H2SO3) in softening and defibring wood was observed by B.C.

Tilghman, a U.S. chemist, as early as 1857. Several years later he renewed his experiments and, in

1867, was granted a patent for making paper pulp from vegetable material. He used high temperature

and pressure and observed that the presence of a base such as calcium was important in preventing

burned or discoloured batches of pulp. His work, however, did not result in commercial use of the

process.

      During the 1870s the sulfite process for pulping wood was the subject of experimental work in

Sweden, England, Germany, and Austria. Within a few years the process was in commercial

operation both in Europe and in North America. For many decades the sulfite process was the

leading process for the pulping of wood. Since 1940, however, the kraft process has taken a

predominant position, and sulfite mills are no longer being constructed.

      Sulfite cooking liquor, as it is pumped to the digester at the start of a “cook,” consists of free

sulfur dioxide dissolved in water at a concentration of 4 to 8 percent, together with from 2 to 3

percent in the form of bisulfite. Sulfite digestion is normally carried out as a batch process in a

pressure vessel, a steel shell with an acid-resistant lining of ceramic tile set in acid-proof cement or

stainless steel. A common digester measures five metres (16 feet) in diameter and 15 metres (50 feet)

in height, with a domed top and a conical bottom. It has a capacity of 12 to 15 tons of pulp per batch.

Digesters with a capacity of up to 35 tons have been constructed. Pulp mills normally have a series

of digesters arranged in a digester building.

      After the blow valve is closed at the bottom, the wood chips are allowed to flow into the top

opening and are distributed to fill the digester completely. Hot acid from the accumulator is pumped

into the digester unit, completely filling it and replacing the air. Steam provides the heat.

      At the end of the cook, the contents of the digester are blown to a blowpit by rapid opening of the

bottom valve. The violence of the blow defibres the cooked chips.

      From 1 to 6 percent of the digester charge is undesirable material such as knots, uncooked chips,

dirt, bark, fibre bundles, and shives. The screen room separates the unwanted particles from accepted

fibre, normally on the basis of particle size; there is an increasing use of the centrifugal principle,

which separates particles on the basis of density.

      The sulfite cooking liquor does not “cook out” or disintegrate bark and other foreign material to

the same degree as kraft liquor (described below), and hence more care must be used in selecting and

cleaning wood chips for sulfite.

      In the conventional sulfite cook using softwood, the typical yield is 44 to 46 percent, based on

wood and with a lignin content of 2 to 5 percent. At that point, a relatively light-coloured pulp with

good strength properties is obtained, suitable for use in the unbleached state, especially in mixture

with groundwood for a variety of printing papers. For pulps in which high brightness (whiteness) is

desired, the residual lignin is removed by bleaching.

      In 1851 paper pulp was experimentally produced from wood by cooking it with caustic soda at

elevated temperature and pressure. Although this soda process attained commercial importance, soda

pulp was of relatively low strength; and use of the process was limited to manufacturing filler pulps

from hardwood, which were then mixed with a stronger fibre for printing papers. Because this

process consumed relatively large quantities of soda, papermakers devised methods for recovering

soda from the spent cooking liquor; recovery has remained an integral part of alkaline pulping ever

since.

      In 1884 a German chemist, Carl F. Dahl, employed sodium sulfate in place of soda ash in a soda

pulping recovery system. This substitution produced a cooking liquor that contained sodium sulfide

along with caustic soda. Pulp so produced was stronger than soda pulp and was called “kraft (kraft

process)” pulp, so named from the German and Swedish word for “strong.” The process has also

been termed the sulfate process because of the use of sodium sulfate (salt cake) in the chemical

makeup. Sulfate, however, is not an active ingredient of the cooking liquor.

      Many soda mills were converted to kraft because of the greater strength of the pulp. Kraft pulp,

however, was dark in colour and difficult to bleach; for many years the growth of the process was

slow because of its limitation to papers in which colour and brightness were unimportant. In the

1930s, bleached kraft became commercially important with the discovery of new bleaching

techniques. The availability of pulp of high whiteness and the expanding demand for unbleached

kraft in packaging resulted in rapid growth of the process, making kraft the predominant wood-

pulping method.

      Paper produced by the kraft process is particularly strong and durable. Acceptable pulp can be

produced in the kraft process from many species of wood not suitable for sulfite. The various pines,

for example, especially southern yellow pine, contain large amounts of wood resin or pitch.

Chemically altered and dissolved in the kraft process, this material is removed from the pulp and

becomes a valuable by-product. The wood pitch is not removed to the same degree in the sulfite

process, and hence high-resin woods, such as pine, are not suitable.

      In the cooking operation, wood chips are prepared and fed to the digesting equipment by methods

previously described. The cooking vessels are still widely used as batch digesters. In the past 25

years, however, continuous digesters have been developed and are being widely adopted by the kraft

industry. These huge cylindrical towers, more than 60 metres (200 feet) in height, have a number of

zones or compartments. Wood chips and cooking liquor are fed into the top and injected into

successive zones of high pressure and temperature, where impregnation and cooking takes place as

the chips progress downward. Additional zones wash the spent liquor from the chips. Continuous

digesters are capable of producing 600 tons of pulp per day.

      In batch cooking, after the digester is charged with chips, a mixture of “black liquor,” the spent

liquor from a previous cook, and “white liquor,” a solution of sodium hydroxide and sodium sulfide

from the chemical recovery plant, is pumped in. The digester is heated either by direct injection of

steam or by the circulation of the cooking liquor through a heat exchanger.

      After completion of the cook, the spent cooking liquor is washed from the pulp; the latter is then

screened and sent to the bleach plant or directly to the paper mill if it is to be used unbleached. Some

of the spent liquor (black liquor) is used for an admixture with white liquor to charge new cooks; the

remainder is sent to the recovery plant to reconstitute cooking chemicals.

      All the sodium used for digestion is contained in the spent liquor, mostly in the form of sodium

salts and sodium organic derivatives. The amount of sodium present is such that its reuse is

economically necessary.

Semichemical pulp

      For semichemical pulping, wood preparation and chipping are essentially the same as that for

other wood-pulping processes. The chips are steeped and impregnated with inorganic chemical

solutions similar to those used for full chemical pulping, but in smaller amounts and with less severe

conditions. Probably the most common is the solution of sodium sulfite in the neutral range, between

acidity and alkalinity. Other agents used in some cases are acid sulfite, caustic soda, and kraft

cooking liquor.

      After the impregnation operation, the chips are fed into one or more disk refiners (described

below) in series. The attrition action of refiners reduces the softened chips to pulp. The yield of

semichemical pulp based on wood is 66 to 90 percent. The higher fibre yield pulps are usually

termed chemimechanical pulps.

      The semichemical pulps have chemical and strength properties intermediate between softwood,

groundwood, and full chemical pulps. These are used in a wide range of papers and boards. The

major tonnage of semichemical pulps goes into the light board, termed corrugating medium, which is

fluted to serve as the interior layer of corrugated boxboard in heavy-duty containers. Stiffness and

adequate strength are the important properties. Semichemical pulp is used in many low-cost printing

papers.

Bleaching and washing

      The use of calcium and sodium hypochlorites to bleach paper stock dates from the beginning of

the 19th century. In the early days of sulfite pulp manufacture, a single-stage treatment of pulp at low

consistency, using calcium hypochlorite (chlorinated lime), satisfied most requirements.

      This simple bleaching treatment, however, is not practical for kraft that is difficult to bleach, nor

can it retain maximum pulp strength. Accordingly, multistage bleaching systems have evolved in

which various sequences of chemical treatment are employed, depending upon the type of

unbleached pulp and special requirements.

      During the normal first stage in a modern bleach plant, the unbleached pulp is chlorinated. Three

to four percent of gaseous chlorine is rapidly mixed with pulp at a temperature of 21° to 27° C (70°

to 80° F); the mixture is quite acid due to the acidity of the chlorine. Chlorine is absorbed largely by

reaction with the noncarbohydrate components of pulp, with no brightening effect and with only

slight dissolution of lignin.

      In the following stage an alkaline extraction with dilute caustic soda dissolves chlorinated

compounds, which are then washed out.

      In its simplest sequence the final stage consists of a treatment with a very alkaline hypochlorite to

neutralize the solution, followed by a final wash.

      In recent years the compound chlorine dioxide (ClO2) has become available for on-site

preparation; it is too unstable to be shipped for wood pulp bleaching. By the use of small amounts of

ClO2 in later bleaching stages, it is possible to achieve high degrees of purification and brightness

without the degradation of cellulose.

      The brightness of paper and other materials is determined by special reflection meters containing

photoelectric cells that measure the amount of light of selected wavelength reflected by the surface.

Freshly prepared pure magnesium oxide is considered to be 100 on the brightness scale. On this scale

unbleached sulfite and groundwood cover the range from about 50 to about 62; peroxide bleached

groundwood, 66 to 72; single-stage hypochlorite sulfite, 80 to 85; multistage bleached pulp, 85 to 88;

and multistage with chlorine dioxide, 90 to 94.

Manufacture of paper and paperboard

Preparation of stock

      Mechanical squeezing and pounding of cellulose fibre permits water to penetrate its structure,

causing swelling of the fibre and making it flexible. Mechanical action, furthermore, separates and

frays the fibrils, submicroscopic units in the fibre structure. Beating reduces the rate of drainage from

and through a mat of fibres, producing dense paper of high tensile strength, low porosity, stiffness,

and rattle.

      An important milestone in papermaking development, the Hollander beater consists of an oval

tank containing a heavy roll that revolves against a bedplate. The roll is capable of being set very

accurately with respect to the bedplate, for the progressive adjustment of the roll position is the key

to good beating. A beater may hold from 135 to 1,350 kilograms (300 to 3,000 pounds) of stock, a

common size being about 7 metres (24 feet) long, 4 metres (12 feet) wide, and about 1 metre (3.3

feet) deep. A centre partition provides a continuous channel.

      Pulp is put into the beater, and water is added to facilitate circulation of the mass between the roll

and the bedplate. As the beating proceeds, the revolving roll is gradually lowered until it is riding full

weight on the fibres between it and the bedplate. This action splits and mashes the fibres, creating

hairlike fibrils and causing them to absorb water and become slimy. The beaten fibres will then drain

more slowly on the paper machine wire and bond together more readily as more water is removed

and the wet web pressed. Much of the beating action results from the rubbing of fibre on fibre. Long

fibres will be cut to some extent.

      The beater is also well-adapted for the addition and mixing of other materials, such as sizing,

fillers, and dyes. By mounting a perforated cylinder that can rotate partially immersed in the beater

stock, water can be continuously removed from the beater, and the stock therefore can be washed.

      Although many design modifications have been made in the Hollander beater over the years, the

machine is still widely used in smaller mills making specialty paper products. For large production

modern mills have replaced the beater by various types of continuous refiners.

      In mills that receive baled pulp and use refiners, the pulp is defibred in pulpers. While there are a

number of variations in basic design, a pulper consists essentially of a large, open vessel, with one or

more bladed, rotating elements that circulate a pulp-water mixture and defibre or separate fibres. The

blades transform the pulp or wastepaper into a smooth mixture. Unlike beaters and refiners, pulpers

do not reduce freeness and cause fibrillation in the fibres. A typical pulper has a capacity of 900

kilograms (2,000 pounds) of fibre in 6 percent solution and requires 150 horsepower to drive it.

      The original continuous refiner is the Jordan, named after its 19th-century inventor. Like the

beater, the Jordan has blades or bars, mounted on a rotating element, that work in conjunction with

stationary blades to treat the fibres. The axially oriented blades are mounted on a conically shaped

rotor that is surrounded by a stationary bladed element (stator).

      Like other refiners, the disk refiner consists of a rotating bladed element that moves in

conjunction with a stationary bladed element. The disk refiner's plane of action, however, is

perpendicular to the axis of rotation, simplifying manufacture of the treating elements and

replacement. Since the disk refiner provides a large number of working edges to act upon the fibre,

the load per fibre is reduced and fibre brushing, rather than fibre cutting, may be emphasized.

       sizing has been described above as the treatment given paper to prevent aqueous solutions, such

as ink, from soaking into it. A typical sizing solution consists of a rosin soap dispersion mixed with

the stock in an amount of 1 to 5 percent of fibre. Since there is no affinity between rosin soap and

fibre, it is necessary to use a coupling agent, normally alum (aluminum sulfate). The acidity of alum

precipitates the rosin dispersion, and the positively charged aluminum ions and aluminum hydroxide

flocs (masses of finely suspended particles) attach the size firmly to the negatively charged fibre

surface.

      Paper intended for writing or printing usually contains white pigments or fillers to increase

brightness, opacity, and surface smoothness, and to improve ink receptivity. Clay (aluminum

silicate), often referred to as kaolin or china clay, is commonly used, but only in a few places in the

world (Cornwall, in England, and Georgia, in the United States) are the deposits readily accessible

and sufficiently pure to be used for pigment. Another pigment is titanium dioxide (TiO2), prepared

from the minerals rutile and anatase. Titanium dioxide is the most expensive of the common

pigments and is often used in admixture with others.

      Calcium carbonate (CaCO3), also used as a filler, is prepared by precipitation by the reaction of

milk of lime with either carbon dioxide (CO2) or soda ash (sodium carbonate, Na2CO3). Calcium

carbonate as a paper filler is used mainly to impart improved brightness, opacity, and ink receptivity

to printing and magazine stocks. Specialty uses include the filling of cigarette paper, to which it

contributes good burning properties. Because of its reactivity with acid, calcium carbonate cannot be

used in systems containing alum.

      Other fillers are zinc oxide, zinc sulfide, hydrated silica, calcium sulfate, hydrated alumina, talc,

barium sulfate, and asbestos. Much of the filler consumed is used in paper coatings (see below).

      Since most fillers have no affinity for fibres, it is necessary to add an agent such as alum to help

hold the filler in the formed sheet. The amount of filler used may vary from 1 to 10 percent of the

fibre.

      The most common way to impart colour to paper is to add soluble dyes (dye) or coloured

pigment to the paper stock. Many so-called direct dyes with a natural affinity for cellulose fibre are

highly absorbed, even from dilute water solution. The so-called basic dyes have a high affinity for

groundwood and unbleached pulps.

      Various agents are added to paper stock to enhance or to modify the bonding and coherence

between fibres. To increase the dry strength of paper, the materials most commonly used are starch,

polyacrylamide resins, and natural gums such as locust bean gum and guar gum. The most common

type of starch currently used is the modified type known as cationic starch. When dispersed in water,

this starch assumes a positive surface charge. Because fibre normally assumes a negative surface

charge, there is an affinity between the cationic starch and the fibre.

      The natural cellulose interfibre bonding that develops as a sheet of paper dries is considered to be

due to interatomic forces of attraction known to physical chemists as hydrogen bonding or van der

Waals forces. Because these attractive forces are neutralized or dissolved in water, wet paper has

practically no strength. Although this property is convenient for the recovery of wastepaper, some

papers require wet strength for their intended use. Wet strength is gained by adding certain organic

resins to the paper stock that, because of their chemical nature, are absorbed by the fibre. After

formation and drying of the sheet, the resins change to an insoluble form, creating water-resistant

bonds between fibres.

Formation of paper sheet by machines

      In a paper machine, interrelated mechanisms operating in unison receive paper stock from the

beater, form it into a sheet of the desired weight by filtration, press and consolidate the sheet with

removal of excess water, dry the remaining water by evaporation, and wind the traveling sheet into

reels of paper. Paper machines may vary in width from about 1.5 to 8 metres (5 to 26 feet), in

operating speed from a few hundred metres to 900 metres (about 3,000 feet) per minute, and in

production of paper from a few tons per day to more than 300 tons per day. The paper weight (basis

weight) may vary from light tissue, about 10 grams per square metre (0.03 ounce per square foot), to

boards of more than 500 grams per square metre (1.6 ounces per square foot).

      Traditionally, paper machines have been divided into two main types: cylinder machines

(cylinder machine) and Fourdrinier machines (Fourdrinier machine). The former consists of one or

more screen-covered cylinders, each rotating in a vat of dilute paper stock. Filtration occurs by flow

action from the vat into the cylinder, with the filtrate being continuously removed. In the Fourdrinier

machine a horizontal wire-screen belt filters the stock. In recent years a number of paper machines

have been designed that depart greatly from traditional design. These machines are collectively

referred to as “formers.” Some of these formers retain the traveling screen belt but form the sheet

largely on a suction roll. Others eliminate the screen belt and use a suction cylinder roll only. Still

others use two screen belts with the stock sandwiched between, with drainage on both sides.

      In a typical modern Fourdrinier machine the various functional parts are the headbox; stock

distribution system; Fourdrinier table, where sheet formation and drainage of water occur; press

section, which receives the wet sheet from the wire, presses it between woolen felts, and delivers the

partially dried sheet to the dryer section; dryer section, which receives the sheet from the presses and

carries it through a series of rotating, steam-heated cylinders to remove the remaining moisture; size

press, which permits dampening the sheet surface with a solution of starch, glue, or other material to

improve the paper surface; calender stack, for compressing and smoothing the sheet; and the reel.

      The function of the headbox is to distribute a continuous flow of wet stock at constant velocities,

both across the width of the machine and lengthwise of the sheet, as stock is deposited on the screen.

Equal quantities of properly dispersed stock should be supplied to all areas of the sheet-forming

surface. The early headbox, more commonly called a flowbox or breastbox, consisted of a

rectangular wooden vat that extended across the full width of the machine behind the Fourdrinier

breast roll. The box was provided with baffles to mix and distribute the stock. A flat metal plate

extending across the machine (knife slice) improved dispersion of the fibre suspension, providing

distribution of flow across the machine, and also metered the flow to produce a sheet of uniform

weight. To accommodate increased speed in modern headboxes, the knife slice is designed to

develop a jet of liquid stock on the moving wire. Modern headboxes are enclosed, with pressure

maintained by pumping.

      The Fourdrinier table section of a paper machine is a large framework that supports the table

rolls, breast roll, couch roll, suction boxes, wire rolls, and other Fourdrinier parts. The wire mesh

upon which the sheet of fibre is formed is a continuous rotating belt that forms a loop around the

Fourdrinier frame. The wire, not a permanent part of the machine, is delicate and requires periodic

replacement. It is a finely woven metal or synthetic fibre cloth that allows drainage of the water but

retains most of the fibres. The strands of the Fourdrinier wire are usually made of specially annealed

bronze or brass, finely drawn and woven into a web commonly in the range of 55 to 85 mesh (strands

per inch). Even finer wires are used for such grades as cigarette paper, coarser wires for heavy

paperboard and pulp sheets. Various types of weave are used to obtain maximum wire life.

      The table rolls, in addition to supporting the wire, function as water-removal devices. The rapidly

rotating roll in contact with the underside of the wire produces a suction or pumping action that

increases the drainage of water through the wire.

      The dandy roll is a light, open-structured unit covered with wire cloth and placed on the wire

between suction boxes, resting lightly upon the wire and the surface of the sheet. Its function is to

flatten the top surface of the sheet and improve the finish. When the dandy roll leaves a mesh or

crosshatch pattern, the paper is said to be “woven.” When parallel, translucent lines are produced, it

is said to be “laid.” When names, insignia, or designs are formed, the paper is said to be

“watermarked.” Paper watermarks (watermark) have served to identify the makers of fine papers

since the early days of the art. A watermark is actually a thin part of the sheet and is visible because

of greater transmission of light in its area compared with other areas of the sheet. Because light

transmission can be varied by degrees, it is possible to produce watermarks in the form of portraits or

pictures.

      The final roll over which the formed sheet passes, before removal from the Fourdrinier wire, is

the couch roll. Prior to the transferring operation, the couch roll must remove water from and

consolidate the sheet to strengthen it. In modern machines the couch roll is almost always a suction

roll.

      The press section increases the solids content of the sheet of paper by removing some of the free

water contained in the sheet after it is formed. It then carries the paper from the forming unit to the

dryer section without disrupting or disturbing sheet structure and reduces the bulk or thickness of the

paper.

      The first two functions are always necessary. Pressing always results in compaction, and this may

or may not be desirable depending upon the grades being made.

      Felts for the press section act as conveyor belts to assist the sheet through the presses, as porous

media to provide space and channels for water removal, as textured cushions or shock absorbers for

pressing the moist sheet without crushing or significant marking, and as power transfer belts to drive

nondriven rolls or parts.

      Woven felts of wool, often with up to 50 percent synthetic fibres, are made by a modified woolen

textile system. Selected grades of wool are scoured, blended, carded, and spun into yarn. The yarn is

woven into flat goods, leaving a fringe at each end. The ends are brought together and joined to

produce an endless, substantially seamless belt.

      Paper machine felts have a limited life ranging from about a week to several months. Their

strength and water-removal ability is gradually lost through wear and chemical and bacterial

degradation and by becoming clogged with foreign material.

      Press rolls must be strong, rigid, and well-balanced to span the wide, modern machines and run at

high speed without distortion and vibration. Solid press rolls consist of a steel or cast iron core,

covered with rubber of various hardnesses depending upon the particular service required. Suction

press rolls consist of a bronze or stainless steel shell two inches (five centimetres) or more in

thickness and usually covered with one inch of rubber.

      Paper leaving the press section of the machine has a solids content or dryness of 32 to 40 percent.

Because of the relatively high cost of removing water by evaporation, compared with removing it by

mechanical means, the sheet must be as dry as possible when it enters the dryers. The dryer section

of a conventional paper machine consists of from 40 to 70 steam-heated drying cylinders. After

passing around the cylinders, the sheet is held in intimate contact with the heated surfaces by means

of dryer felts.

      Until recent years, relatively heavy, rather impermeable cloths composed of wool, cotton,

asbestos, or combinations of these materials covered the dryer portion of the paper machine. Such

cloths are termed dryer felts, though felting or fulling process is rarely used in their manufacture.

Relatively lightweight, highly permeable cloths called dryer fabric also are employed.

      For conventional dryer felts, cotton is still the most commonly used fibre, although it is seldom

used alone. The main difference between the conventional dryer felt and the open-mesh dryer fabric

is air or vapour permeability. High permeability is desirable because it allows the escape of the water

vapour from the sheet.

      For every ton of paper dried on the paper machine, approximately two tons of water are

evaporated into the atmosphere. About 50 to 60 tons of air are required to remove the water vapour,

with about 2,700 kilograms (6,000 pounds) of steam required by the dryers.

Finishing and converting

      The rolls of paper produced by the paper machine must still undergo a number of operations

before the paper becomes useful to the consumer. These various operations are referred to as

converting or finishing and often make use of intricate and fast-moving machinery.

      There are two distinct types of paper conversion. One is referred to as wet converting, in which

paper in roll form is coated, impregnated, and laminated with various applied materials to improve

properties for special purposes. The second is referred to as dry converting, in which paper in roll

form is converted into such items as bags, envelopes, boxes, small rolls, and packs of sheets. A few

of the more important converting operations are described here.

      Paper has been coated to improve its surface for better reproduction of printed images for over

100 years. The introduction of half-tone and colour printing has created a strong demand for coated

paper. Coatings are applied to paper to achieve uniformity of surface for printing inks, lacquers, and

the like; to obtain printed images without blemishes visible to the eye; to enhance opacity,

smoothness, and gloss of paper or paperboard; and to achieve economy in the weight and

composition of base paper stock by the upgrading effect of coating.

      The chief components of the water dispersion used for coating paper are pigment, which may be

clay, titanium dioxide, calcium carbonate, satin white, or combinations of these; dispersants to give

uniformity to the mixture or the “slip”; and an adhesive binder to give coherence to the finished

coating. The latter may be a natural material such as starch or a synthetic material such as latex.

      Equipment installed between dryer sections on the paper machine can apply the coating (on-

machine coating), or it can be done by a separate machine, using rolls of paper as feed stock (off-

machine coating).

      The extrusion-coating process, a relatively new development in the application of functional

coating, has gained major importance in the past 20 years. The process is used to apply polyethylene

plastic coatings to all grades of paper and paperboard. Polyethylene resin has ideal properties for use

with packaging paper, being waterproof; resistant to grease, water vapour, and gases; highly stable;

flexible in heat sealing; and free from odour and toxicity.

      In the extrusion-coating machine, the polyethylene resin is melted in a thermoplastic extruder

that consists of a drive screw within an electrically heated cylinder. The cylinder melts and compacts

the resin granules and extrudes the melt in a continuous flow under high pressure. The resin is

discharged through a film-forming slot die. The die has electric heaters with precision temperature

controls to give uniform temperature and viscosity to the plastic melt. The slot opening can be

precisely adjusted to control film uniformity and thickness.

      The hot extruded film is then stretched and combined with paper between a pair of rolls, one of

which is a rubber-covered pressure roll and the other a water-cooled, chromium-plated steel roll. The

combination takes place so rapidly that a permanent bond is created between the plastic film and the

paper before they are cooled by the steel roll.

      The most widely used package for commodities and manufactured products is the corrugated

shipping container. A corrugated box consists of two structural elements: the facings (linerboard) and

the fluting structure (corrugating medium).

      Linerboard facings are of two general types: the Fourdrinier kraft liner is made of pine kraft pulp,

usually unbleached, in an integrated mill as a continuous process from the tree to the paper web; and

the cylinder liner is made from reprocessed fibres, generally from used containers, providing a

content of about two-thirds kraft.

      The operation begins by unwinding the single-face liner and corrugating medium from holders,

threading the medium into the fluting rolls, applying adhesive to the tips, and bringing the medium in

contact with the liner to form a single-face web. Next, the single-face web passes another glue roll

that applies adhesive to the exposed flute tips of the medium. The second face liner is brought in

contact with the single-face web, and the combined board travels through a hot plate section between

belts to set the bond, to a cooling section, and then to a slitter-scorer.

The world paper industry

      The paper industry tends to be concentrated in those countries that are industrially advanced and

have abundant supplies of fibrous raw material, especially wood. There is a large-scale international

trade in wood pulp, pulpwood, and paper flowing from those countries with large forest resources to

those countries with less or that are as yet undeveloped.

Paper properties and uses

      Used in a wide variety of forms, paper and paperboard are characterized by a wide range of

properties. In the thousands of paper varieties available, some properties differ only slightly and

others grossly. The identification and expression of these differences depend upon the application of

standard test methods, generally specified by industry and engineering associations in the

papermaking countries of the world.

Substance and quantity measurement

      Weight or substance per unit area, called basis weight, is a fundamental property of paper and

paperboard products. From the first uses of paper in the printing trades, it has been measured in

reams, originally 480 sheets (20 quires) but now more commonly 500 sheets (long reams). The term

ream weight commonly signifies the weight of a lot or batch of paper. Since the printing trades use a

variety of sheet sizes, there can be numerous ream weights for paper having the same basis weight.

      The table gives basis weight ranges for some common papers.

      To determine basis weight, the sample is brought to equilibrium under standard conditions (24° C

or 75° F; 50 percent relative humidity). The paper specimens must consist of at least 10 sheets with a

total area of not less than about 600 square centimetres (100 square inches). Since the properties of

paper change with moisture content, all tests are conducted under standard conditions.

      The caliper (thickness) of paper or paperboard in fractions of a millimetre or inch is measured by

placing a single sheet under a steady pressure of 0.49 to 0.63 kilogram per square centimetre (seven

to nine pounds per square inch) between two circular and parallel plane surfaces, the smaller of

which has an area of 1.6 square centimetres (0.25 square inch).

      The density or specific gravity of paper is calculated from the basis weight and caliper and may

vary over wide limits. Glassine, for example, may be 1.4 grams per cubic centimetre and creped

wadding, used for packaging breakables, only 0.1 gram per cubic centimetre. Most common papers

are in the range of 0.5 to 0.7 gram per cubic centimetre.

Strength and durability

      The strength of paper is determined (materials testing) by the following factors in combination:

(1) the strength of the individual fibres of the stock, (2) the average length of the fibre, (3) the

interfibre bonding ability of the fibre, which is enhanced by the beating and refining action, and (4)

the structure and formation of the sheet.

      Resistance to rupture when subjected to various stresses is an important property in practically all

grades of paper. Most papers require a certain minimum strength to withstand the treatment received

by the product in use; but even where use requirements are not severe, the paper must be strong

enough to permit efficient handling in manufacture. tensile strength is the greatest longitudinal stress

a piece of paper can bear without tearing apart. The stress is expressed as the force per unit width of

a test specimen.

      Since the weight of the paper and the width of the test specimen affect the force of rupture, a

conventional method of comparing inherent paper strength is the breaking length—that is, the length

of a paper strip in metres that would be just self-supporting. This value varies from about 500 metres

for extremely soft, weak tissue to about 8,000 metres for strong kraft bag paper, and to about 14,000

metres for sheets of paper made under ideal laboratory conditions.

      Because some paper products such as towels, sanitary tissues, and filter paper are subjected to

wetting by water in their normal use, wet tensile testing has become important. This test is essentially

the same as that for dry tensile strength, except that the specimen is wetted. Paper that has not been

specifically treated to produce wet strength possesses from about 4 to about 8 percent of its dry

strength when completely wetted. By treating paper as described above, wet strength can be raised to

about 40 percent of the dry strength.

      One of the oldest and most widely used strength tests for paper and paperboard is the bursting

test, or Mullen test. It is defined as the hydrostatic pressure (caused by liquids at rest) necessary to

cause rupture in a circular area of a given diameter. Other strength tests for which standard methods

exist are tearing strength and folding endurance.

      The resistance of paper to a bending force is evident in the various operations of its manufacture

and in its many uses. The range in this property extends from very soft, flexible tissues to rigid

boards. Thicker and heavier sheets tend to be stiff, whereas soft, flexible sheets are light and thin.

Even at the same weight there is a considerable difference in stiffness, chiefly due to the

compactness and the amount of bonding of the sheet.

      Because paper is composed of a randomly felted layer of fibre, the structure has a varying degree

of porosity (permeability). Thus, the ability of fluids, both liquid and gaseous, to penetrate the

structure is a property both highly significant to the use of paper and capable of being widely varied

by the conditions of manufacture.

       sizing paper with vegetable materials and rosinlike substances has already been described. When

paper began to be used for wrapping, consumers demanded sizing treatments that could protect the

contents of the package (packaging) from the effects of fluid transfer through the paper wrapping. In

some instances complete impermeability was required. In another direction the use of paper as an

absorbent medium for wiping up liquids, for filtering, and for saturating has created a demand for

maximum wettability and permeability toward water and other fluids.

      In certain types of packaging, paper must resist grease and oil penetration. The resistance of

paper to the penetration of water can be increased by treatment of fibre with materials that lack

affinity for water, with little effect upon sheet porosity, but the penetration of oil materials is little

affected by such treatment. Oil and grease resistance is attained, in fact, by reduction in porosity. So-

called greaseproof paper is made by beating an easily hydrated pulp to extremely low freeness,

which results in a dense sheet with very little void space.

      Absorbent papers such as toweling, sanitary tissue, and blotting and filter paper are normally

made from lightly beaten stock. Since cellulose is naturally hydrophilic (i.e., has a strong affinity for

water), absorbent papers have a minimum of foreign materials associated with the fibre. Of particular

importance are the wood rosins that may be present in pulp and produce a self-sizing effect,

especially upon aging.

Optical properties

      The most important optical properties of paper are brightness, colour, opacity, and gloss.

      The term brightness has come to mean the degree to which white or near-white papers and

paperboard reflect the light of the blue end of the spectrum (i.e., their reflectance). This reflectance is

measured by an instrument that illuminates paper at an average angle of incidence of 45° and a

wavelength of 457μ (microns). Brightness measured in this way is found to correlate closely with

subjective estimates of the relative whiteness of paper.

      Opacity is one of the most desired properties of printing and writing papers. Satisfactory

performance of such papers requires that there be little or no “show-through” of images from one

side of the sheet to the other. Satisfactory opacity in printing papers requires that white mineral

pigments be incorporated with the paper stock or applied as a coating.

      The terms gloss, glare, finish, and smoothness are used in describing the surface characteristics

of paper. The broad term finish refers to the general surface characteristics of the sheet. Smoothness

refers to the absence of surface irregularities under either visual or use conditions. Gloss refers to

surface lustre and connotes a generally pleasing aspect. Glare is used for a more intense reflection

and a more unpleasant effect. Calendering and coating are important paper-treating methods that

affect gloss. Gloss of paper is determined by measuring percent reflectance at a low angle of

incidence, 15 degrees (75 degrees from the perpendicular).

Paper grades

Bond paper

      Bond is characterized by a degree of stiffness, durability for repeated handling and filing,

resistance to the penetration and spreading of ink, bright colour, and cleanliness. There are two

groups of bond papers: rag content pulp and chemical wood pulp. Rag content bond may vary from

25 to 100 percent cotton fibre content. The principal uses of bond paper are for letterhead stationery,

advertising pieces, announcements, leases, deeds, writs, judgments and other legal documents,

currency, certificates, and insurance policies.

Book paper

      Most book papers are made of various combinations of chemical wood pulp; for lower-priced

grades groundwood, semichemical, and de-inked wastepaper are also used. In addition to pulp, the

“furnish” from which book papers are made contains various amounts of sizing, fillers, and dyes.

      Uncoated book paper comes in four finishes: (1) antique or eggshell, (2) machine finish, MF, (3)

English finish, EF, and (4) supercalendered. Antique has the roughest surface. High bulking pulps,

such as soda pulp, are used and only slightly beaten in stock preparation. The sheet is lightly

calendered (pressed between rollers) to provide a degree of surface smoothness while preserving the

antique or eggshell appearance. Machine finish has a medium-smooth surface obtained for this finish

from a calender stack at the dry end of the machine. Machine finish book is a relatively inexpensive

general utility paper. It is principally used for books, catalogs, circulars, and other matter using line

etchings. Machine finish book may be used for halftones up to a 100-line screen. English finish is a

step higher in the book paper scale; this finish is distinguished from machine finish by a higher

degree of stack beating, by greater pressure between the rollers of a machine calender, and by

calendering at a greater moisture content of the sheet. Supercalendered book is the smoothest surface

that can be obtained without coating. The finish is obtained by a special calendering operation after

the paper leaves the paper machine. The supercalender presses the paper between successive sets of

iron and compressed fibre rolls that make a smooth, compact printing surface. It is used for books,

brochures, and magazines where halftone printing in the range of a 100–120 line screen is required.

      Coated book papers are produced to create surfaces suitable for the printing of fine-screen

halftones. Coated book paper must be uniformly smooth, receptive to printing inks, have high

brightness and gloss, and be capable of folding without cracking.

      Bible paper, as the name implies, was developed for lightweight, thin, strong, opaque sheets for

such books as bibles, dictionaries, and encyclopaedias. Bible papers are pigmented (loaded) with

such pigments as titanium dioxide and barium sulfate and contain long fibres and artificial bonding

agents to maintain strength.

Bristol

      The general term bristol refers to a group of stiff, heavy papers with thicknesses ranging from

0.15 millimetre (0.006 inch) upward. These grades are made from various combinations of chemical

wood pulp. The stock is beaten to a medium degree and usually well sized to prevent penetration of

moisture. Increasingly important in recent years has been the use of bristols for the punch cards used

in tabulating and sorting machines.

Groundwood and newsprint papers

      These are printing and converting grades containing varying amounts of groundwood pulp,

together with small percentages of chemical wood pulp for strength and durability.

      For many years newsprint was virtually the only use for groundwood pulp, but more recently,

due to improvements in the pulping process and to the introduction of a bleaching process for this

pulp, a class of printing papers of broad utility has been developed. Magazines, paperbound books,

catalogs, directories, and general commercial printing consume large quantities of these papers.

      Groundwood papers are noted for an even, uniform formation and a high degree of opacity.

These papers tend to be bulky and are receptive to printing ink. They do not have high whiteness and

tend to turn yellow when exposed to light and after long aging.

Kraft wrapping

      Kraft wrapping, a heavy stock used for paper bags, is used in greater volume than all other

wrapping papers combined. It is composed of wood pulp in unbleached condition made from

softwoods, usually pine. It is distinguished by outstanding tensile and tearing strength. Kraft

wrapping is sized to retard wetting when exposed to water. For wrapping of wet materials, the paper

may be given wet strength by treatment with special resins. Multiwall sacks of kraft paper are used

for shipment of bulk materials.

Paperboards

      The term paperboard is a general term that is descriptive of products which are 0.30 millimetre

(0.012 inch) or more in thickness, made of fibrous materials on paper machines. Paperboard is

commonly made from wood pulp, straw, wastepaper, or a combination of these materials.

      There are three main types of paperboard: (1) boxboards, used for such products as food board,

food trays, plates, and paper boxes, (2) container boards, for the manufacture of corrugated and solid

fibre shipping containers, and (3) paperboard specialties, including such items as binders board,

electrical pressboard, and building boards.

Sanitary papers

      The group of papers known collectively as the sanitary grades include toilet tissue, toweling,

facial tissue, and napkins. These grades are made from various proportions of sulfite and bleached

kraft pulps with relatively little refining of the stock to preserve a soft, bulky, absorbent sheet. This

sheet is further softened by machine creping, in which the wet sheet is pressed upon a smooth drying

roll and subsequently removed by running against a flat stationary metal blade (doctor blade). The

sheet is piled up upon itself, thus producing a creped effect. Facial tissue is dry-creped; that is, drying

is complete on the drying roll before the creping doctor blade. Toweling is generally of heavier

weight than the tissues and is usually creped while still wet. Napkins are of somewhat heavier weight

than tissues. The plastic nature of paper fibres when slightly moist permits the reproduction of

surface patterns by embossing to a remarkable degree. Paper napkins are an example of this art.

      Because of the soft, bulky texture of sanitary papers, they are relatively weak. Since they are

often exposed to wetting in use, they are often treated with resins to increase wet strength.