Fibre sources
- Key People:
- Nicolas-Louis Robert
The cell walls of all plants contain fibres 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 of wood for pulp, concern regarding the depletion of forest resources 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: 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
The development and use of a great variety of man-made fibres 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.
