Non-Woody Fibrous Materials
( Originally Published 1920 )
ALL PAPER is today made of cellulose in one form or another, and in different degrees of purity. Cellulose is a carbohydrate having the empirical formula C6-H10-O5, from which its composition may be calculated to be: carbon 44.4%; hydrogen 6.2%; and oxygen 49.4%. It was discovered in 1838 by Payen, and is the common and preponderant component of all vegetable tissues and fibers; it is probably the most important and abundant organic material on the face of the earth. It makes up about 90 per cent of such seed hairs as kapok, cotton and other flosses; about 80 per cent of such bast fibers as hemp, ramie and flax; about 60 per cent of such woods as spruce and pine; and about 35 per cent of straw.
In all plant structures the cellulose is associated with hemicelluloses, which are carbohydrates characterized by solubility in dilute alkaline solutions, and with small amounts of oils, waxes or resins, and of inorganic salts derived from the soil. In woods there is also a very considerable amount of lignin, varying from about 18 to 37 per cent in the various kinds, but in most species ranging from 23 to 30 per cent. The removal of the major portion of these impurities is considered essential in the preparation of good paper-making fibers, and it forms one of the chief functions of a pulp mill. Most fortunately for papermaking, as well as textile work, cellulose is quite resistant to the action of alkaline materials such as those used in washing textile goods and in treating the fibrous raw materials for the removal of impurities. If it were not for this fact papermaking from chemically prepared wood fibers would not be possible. The action of acids on cellulose is more serious and results in a weakening of the fiber; yet, under carefully controlled conditions an acid process, known as the "sulfite" process, may be used with entire success for the production of fibers from many kinds of wood.
It would seem, then, that paper might be made from almost any vegetable material, and the number of things which have been tried with more or less success is very large. As early as 1765 Jacob Christian Schaeffer listed the different plants which he trans-formed into paper without the use of any rag fiber. The materials included poplar down, tree moss, hop tendrils, grapevine bark, hemp, aloe leaves, mulberry, stinging nettle, bulrushes, cabbage stalks, water moss, turf, leaves of the lily of the valley, mallow, orache, indian corn, genista, potato skins, etc., etc. To this list others have added, at different times, their observations on tobacco waste, pea and bean stalks, the barks of various trees and shrubs, seaweed, milkweed silk, peat, sawdust, refuse from canning asparagus, cocoanut husks, cotton stalks and many others. The extent to which such investigations have been carried is shown by a 228-page reading list of references prepared by Dr. C. J. West in 1928.
For a number of reasons the kinds of fiber which have proved serviceable commercially are somewhat limited, and have be-come even more so as the size and productive capacity of the paper mills have increased. Among the qualifications which satisfactory materials must possess are (1) sufficient fiber length to give the desired strength to the paper, (2) ready reducibility to the fiber form, and separation from accompanying impurities or ingredients which might be harmful to the paper, (3) availability in very large quantities and within a reasonable distance from the mill, (4) not too great bulk per unit of fiber produced, and (5) low cost per ton of fiber. There are other minor requirements which are important for some types of paper but not for others; these will appear in the discussion of the different materials.
Rag fibersóboth cotton and linenówere the mainstay of paper-makers until the middle of the 19th century and during this period their preparation departed from the old Chinese methods chiefly in mechanical details. The rags were collected from all parts of the world, and contained all grades, sometimes very imperfectly sorted for quality or cleanliness. The bales or bundles were opened and loose dust and dirt removed by thrashing. The partially cleaned rags were then sorted into grades according to kind ócotton or linenóand according to color and cleanliness. Buttons, hooks and buckles were then cut off, the seams opened to release more dirt and the larger pieces cut up by hand, or later by machines which reduced them to pieces about 2 to 4 inches square. A final dusting was then given before the cut rags went to the boilers. In the old days before proper sanitary precautions were taken, these operations were not particularly healthful and occasional cases of smallpox developed among the sorting girls.
After the final dusting, the rags were placed in rotary kettles, or boilers, with a solution of caustic soda, or a mixture of slaked lime and soda ash, and were heated with steam at a pressure of about 30 pounds per square inch for several hours. The action of the alkali, together with the tumbling and rubbing of the rags, loosened the remaining dirt and destroyed a considerable part of the coloring matter used in dyeing. The pressure in the boiler was then reduced, the cover of the manhole removed and the cooked rags dumped on the floor to drain and season.
At this point the rags were very deep brown and dirty, so they were placed in a "breaking engine," which is very similar to a beater, or Hollander, except that it is fitted with a drum washer which removes dirty water. Fresh water entering at one point is mixed continuously with the dirty rags, which are gradually thrashed to pieces, while the dirty water is being removed by the washer. This operation continues until the water removed is only slightly dirty, and during this time the thrashing action separates the bits of cloth into threads and finally breaks those down into the original fibers. The final step is to bleach the fibers, after which they are ready to go to the beating engines to prepare for the paper machines.
In the old days when few dyes were used, the preparation of rag stock was not too difficult, though the bleaching of indigo blues was not wholly satisfactory. With the advent of aniline dyes, and especially with some of the very fast colors, the preparation of white fibers has become more difficult, and in some cases impossible. In addition to this there are the new artificial fibers woven with cotton or used alone and which cannot always be sorted out. Further complication is caused by the use of resin treatments of textiles to water-proof, or to wrinkle-proof, or to accomplish other things which are desirable from the standpoint of the textile maker, but which cause all sorts of trouble in the paper mill. The use of very fine strands of rubber in some grades of cloth cuttings is another source of trouble. Such rubber threads are difficult to detect, are not removed by any of the treatments in the preparation of rag stock, and may ruin the paper if their presence is not discovered in time.
For these reasons the users of rag fibers have turned more and more to raw cotton in various forms and to flax for their supply. Cotton, as picked in the field, is first treated by ginning to remove the long fiber suitable for spinning. The short fibers then remaining attached to the seed are removed by passing the seed through a linter machine, which resembles a gin, except for the closer setting of the saw blades. The blades can be set to take off nearly all the fiber in a single pass, or to give a relatively long and a shorter grade by passing the seed through the machine twice at different blade settings. The longer of these two grades is used for upholstery work but the shorter finds some use in papermaking. After passing the linter machine the seeds are split to recover the kernels for oil making, and the hulls are ground in hammer mills, the chaff separated from the very short fibers, and these are cooked with caustic soda much as wood is cooked in the soda process. The very short fibers thus obtained are used in papermaking and for the production of cellulose compounds by chemical treatment.
As a substitute for linen fibers from rags, the flax obtained from plants grown for linseed oil production is now largely used. For many years it was considered practically impossible to prepare fibers from such flax straw without so injuring the strength of the bast fiber that it would be of little value, but modern methods of chlorination and bleaching have overcome the difficulties, and flax fiber is now extensively used in high-grade products, especially cigarette paper, tissues and paper of the so-called Oxford India type. For these papers the ability of flax to break down during the beating process into very fine, slender fibrils makes it especially valuable. In this respect hemp fiber, which is prepared from rope, rags and cordage in much the same way that rags are treated, maybe classed as even better than linen.
Strangely enough rag fiber, besides being used in the papers mentioned, as well as in the highest quality of writing and ledger papers, is also used in blottings, fiber board, and in roofing papers which are to be impregnated with asphalt. The amount used in these products far exceeds that used in the high-grade papers.
Esparto grass, which grows in Spain and northern Africa, has long been esteemed in Great Britain as a source of fibers, but is seldom used in America. After dusting and sorting out useless material, the grass is cooked with a solution of caustic soda in stationary digesters in which good circulation of the liquor is maintained by pumping. At the end of the cook the spent liquor is drawn off, the grass washed several times with hot water and finally transferred to storage chests from which it goes to washers for a final washing and bleaching.
Esparto fiber is short, uniform and somewhat wiry in appearance, and the true fibers are accompanied by peculiarly-shaped, small cells, which make its identification particularly easy. It is an excellent fiber for bulky antique papers, and gives an ideal base for coated papers. It imparts pliability, ease of sizing, good opacity, excellent evenness of formation and fine printing qualities.
Straw is present in all grain-producing countries in enormous quantities and is often burned to get it out of the way. Except for cotton and linen, straw is probably the oldest source of fibers used by modern papermakers. In 1800, Mathias Koops in England printed a book on paper made from straw without the admixture of any other fiber, and at the same time claimed to have made paper from wood alone. Just how this was accomplished is not known at the present time.
Straw is used for making two distinctly different products; a coarse, yellowish fiber which is used for strawboards and cheap wrapping papers, and a bleached fiber which has some of the properties of esparto and the shorter fibered wood pulps, and which can be used in many high-grade products. The coarse grade of pulp is made by cooking the straw in rotary digesters with milk of lime at a steam pressure of 35ó45 pounds. The straw is fed into these digesters without cutting or dusting, and at the end of the cook the liquid is drained off under pressure and the charge is dumped on the floor, where it is seasoned for three to five days before it is used. The entire time for a cookócharging, cooking and coolingóis about 24 hours. The cooked straw is washed for about 4 hours in engines very similar to those used for rag stock, and it is then ready to use. Much of this pulp finds its way into corrugated and plain boxboards. It is said that the possibilities of making board from straw were discovered in 1829 while experimenting with straw for wrapping paper. The web of paper accumulated on the press roll of the paper machine and when it was cut off it was noticed that a solid board had formed.
High-grade bleached fiber can be prepared from straw by any of the usual chemical cooking processes, and for such work the straw is cut into short lengths and freed from dust, grain and chaff before filling into the digesters. The soda process is most generally used, but a considerable amount of straw is now treated by the Pomilio process which employs caustic soda and chlorine in separate treatments. Such fiber is more often used in Europe than in America. It contains a wide variety of forms and sizes of fibers, including some very thin-walled cells which tend to gelatinize on beating and make the paper hard and rattly. For this reason straw cellulose is not much used for book papers but is more suitable for drawing paper and similar grades.
Any of the cereal straws may be used for pulp making, but wheat is the one most often employed. Barley, rye, oat and rice straw are also used at times, and there is probably no marked difference in the grade of pulp obtained. While the supply of all these straws is very great there are a number of reasons why they have not found wider application in high-grade papers. They are all annual crops and the entire supply for a year's operation of a mill must be gathered 'within a very short period. This means a very large storage capacity for baled straw at the mill, and unless the bales are protected from rain and snow considerable deterioration may take place. In most instances the pulp mills are not located near the straw producing areas, so transportation costs are high. The bulk of the straw is so much greater than that of wood that the amount of pulp obtainable from a given digester capacity is much less than for wood. All the straws are cultivated field crops and so tend to bring to the mill much fine dirt, as well as weeds which cannot be sorted out, and which do not cook as readily as the straw, and hence show in the pulp as partly cooked pieces of poor color. To these objectionable features may be added the quality of the paper, which, as already noted, is hard and rattly, as well as less opaque than wood pulp paper. Nevertheless, straw is a possible source of paper pulp which can be more extensively used when necessity forces it upon us.
Corn stalks and sugar cane are two other cultivated crops which might serve as sources of high-grade pulp and are already used to a considerable extent in the production of coarse products such as bulky, light-weight wall boards, and sometimes, alone or in admixture with straw, in making strawboards. Corn stalks are produced annually to the extent of millions of tons in the corn growing regions and sugar cane refuse, or "bagasse," after pressing out the juice, was formerly used as a low grade fuel.
These two materials can be reduced to fibers easily by the alkaline cooking processes. The pulp includes long, thick-walled fibers and shorter fibers of similar structure. Also present are cells of various shapes and sizes resembling those in straw, though some-what larger, and an objectionable feature is the large number of thin-walled pith cells. These fill the spaces between the fiber bundles in the stalks, and normally are irregularly globular in shape, but during the cooking and washing processes they collapse and become flat. This makes the stock prepared from these sources hard to drain and imparts hard, rattly and translucent characteristics to paper made from it. If the long fibers could be economically separated from the short fibers and pith cells they would form an excellent material for high-grade paper, but at least four attempts to use bagasse on a commercial scale have failed, chiefly because the papers were of poor quality.
Both of these raw materials suffer from the same shortcomings mentioned under straw, and in addition the stalks are much more difficult to dry, and introduce the danger of fermentation and heating if improperly stored. There is no question that they can be made into acceptable paper when the increasing scarcity of wood makes it economically feasible.
Another source of fibers, which it would seem should be more extensively used, includes the bamboos, and the very similar Cuban cane and the giant American cane from the southern United States. There are many species of bamboo, but not all are equally easy to pulp. If the right kinds are selected they can be cooked easily by the alkaline processes and the pulp has very excellent papermaking qualities. The fibers are much longer than those from deciduous woods, though not as long as those from the conifers, and pith cells are not numerous enough to cause any appreciable trouble.
It is estimated that in North Burma there are about 100,000,000 tons of bamboos, and there are large amounts available in India, Siam, the Philippines and Indo China. It grows rapidly and if properly handled the supply may be considered inexhaustible. If these sources of bamboo were more easily accessible a large sup-ply of very desirable fibers would be available.
Manila hemp is used for certain very strong, tough papers. Its use was started at least as early as 1837 when Lyman Hollings-worth., of South Braintree, Mass., was obliged to use old ropes because of lack of funds to purchase rags. The unusual strength obtained from this fiber has continued its use for flour and cement sacks, cable insulating papers, sandpaper, shipping tags, etc., but for many of these purposes it is being replaced by strong kraft fiber made by the sulfate process from coniferous woods.
Manila rope is treated with alkali, washed and bleached some-what after the manner of rags. The color is usually not as good as highly bleached rags, so "manila" paper has come to be associated with a yellowish color, and imitation or "bogus" manila paper is colored to about the same degree.
Other fibers of a somewhat similar nature and use are obtained from jute and sisal hemp. The former is prepared from old burlap bags, twine, cordage and woven goods, while the latter comes chiefly from old cordage. The use of materials other than jute for bags has considerably cut into the available supply of this material.
Many other possible sources of fiber have been investigated from time to time, either because of their obvious possibilities, or at the behest of some Congressman who wishes to do a good turn for his constituents. Such a project has been the use of cotton stalks, of which there are large quantities available. In spite of repeated studies of this material no process has been devised to date for producing from it good, clean, white fiber. One of the chief reasons is that nature neglected to put any long fiber into the woody portion of the stalks, and the longer fibers in the bark are not sufficiently numerous to counterbalance defects of the wood.