CELLULOSE and ITS PLACE in INDUSTRY PHILIP C. SCHERER, JR. Virginia Polytechnic Institute, Blacksburg, Virginia
A brief review of some of the more fundamental processes for obtaining pure cellulose from the raw materials i s followed by a very short discussion of the more important chemical properties of the compound. Theapplication of cellulose to industry i s discussed in three main sections: ( a ) natural cellulose fibers; (b) conversion products; (c) decomposition products. A brief survey of the general methods of conversion i s followed by a discussion of the uses of the various products.
the dawn of history to some primitive woman, perhaps, who culled from some half-submerged bundle of rotting reeds the first long fibers of cellulose and, twisting them together, formed the first crude thread, the mother of the mighty spinning industry of today. Cellulose as used by the ancient peoples, however, all came from natural sources and for three thousand years the only progress made was in cultivation of such natural sources to produce stronger fibers more abundantly and thus more cheaply. It is in only very recent ++++++ times that man has been able to rival the silkworm and HISTORICAL spin threads, and rival the wasps in making paper (2). ELLULOSE, the skeleton of living plant life, has No longer must man depend upon cotton and flax for from time immemorial played a very important pure cellnlose but he can grind up trees and extract part in the history of man. As clothing, produced pure cellulose from them. from the cotton or linen plant, it held a high place in the In the centuries that passed since the use of natural liieof ancient Egyptians (1)for such clothing was sacred cellulose began, little progress was made until the nineand pure as it came not from lowly animal or crawling teenth century, in converting natural cellulose-containworm. Its use as paper for the expression of the soaring ing products into useful materials. No doubt the fanciesof thepoet, the measured rhythms of the priestly conversion of cotton and linen into paper forms was caste, and the prosaic facts of the ancient business world known fairly early hut such conversion was probably was also known by the Egyptians who prepared their done slowly and with much labor by hand. The smooth white sheets from the papyrus plant. Only in natural products were improved upon by such processes imagination can we trace the use of cellnlose beyond as mercerization of cotton discovered by John Mercer 131
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in 1850. The first recorded attempts to utilize raw vegetable products, such as wood and straw, for the production of paper, were successful in 1846 ( 3 ) . In this process sodium hydroxide was used as the cooking reagent. The use of nitric acid for the conversion of raw materials to cellulose was patented by Coupier and Miller in 1852 (4) but has never proved a commercially successful process owing to the cost of the acid. Following closely after this came the processes using water alone in 1866 (4), (5), the use of hydrochloric acid ( 4 ) , and themost important process of Tilghman in 1866 (4) in which was introduced the use of sulfurous acid as the decomposing agent. Tilghman in 1866 also introduced the use of bisulfite salts as decomposing agents and i t is upon his work along these lines that the present successful cooking processes are based. Later in this discussion these various processes will be considered in more detail. SOURCES
The sources from which our modern supply of cellulose may be drawn are innumerable since every vegetable material contains cellulose as the skeleton of its structure much in the same way that animal matter requires bony structure to preserve its form. In spite of this very wide distribution of the raw material, the actual sources employed in industry are limited by certain economic and physical properties of the product. In general, the two main sources are the cotton plant and woods of different kinds. In addition to these there are other plants used chiefly for certain special purposes as, for instance, the flax for linen; jute, hemp, manilla, aloe, and other monocotyledon plants for ropes and twines; straw and wood are used in hatmaking; and rags, textile wastes, straw, bagasse, etc., are used for paper and wallboard of different types. In the case of the cotton plant, as is well known, the useful part is the boll. The bolls are picked when ripe and are subjected to a process called "ginning" by which the long cotton fibers are removed from the seeds. These long fibers are simply cooked with dilute caustic soda to remove the natural fats and oils, washed, and, if necessary, bleached and are then ready for use in the textile industry. On the seeds is left a quantity of very short fibers known as the "seed hairs" which recent developments in the industry have enabled us to recover by the aid of a machine known as the "saw gin" which strips these hairs from the seeds and makes them available. Such hairs are called "linters" and, while much too short for use directly in textiles, serve as a very important and cheap source for cellulose in the rayon and explosives industries. Such linters must necessarily be purified in a manner very similar to that employed for the fiber cotton. The only rival to cotton cellulose is the cellulose obtained from woods of various kinds. It will have been noted that in the case of the cotton plant the cellulose obtained from it is practically pure and requires merely a cleaning process to be ready for use. In the case of the wood the cellulose is less than fifty per
cent. of the wood, the remainder being impurities and ligneous matter which is useless for the purposes to which cellulose is put and therefore must be removed. In order to understand the processes used for this purpose i t is necessary to know a little about the nature of cellulose itself and a brief discussion of that compound as to its chief chemical characteristics will be given here. CHEMICAL CHARACTERISTICS
Cellulose is a definite chemical compound with an empirical formula CSHIOO.by which is indicated that there are three elements (carbon, hydrogen, and oxygen) present in the compound in the ratio of 6:10:5 respectively. These elements are necessarily arranged in a definite way within the smallest particle of the substance, known as its molecule. If this were not so then another well-known substance, starch, which has identically the same empirical formula, should be identical with cellulose. The difference between the two substances is immediately apparent when we consider that one of the most important and easily digested foods is starch, while cellulose is not digested in the human stomach and is useless for food. Since both substances possess the same three elements in the same proportion the difference in properties must be due to the arrangement of those elements within the molecule. Chemists have been working many years in an attempt to find out the arrangement of the atoms within the molecule of cellulose and have not, as yet, definitely decided upon a structure for the compound though much evidence tends to give us considerable knowledge as to the way in which the compound is put together. One great difficulty confronting the chemist when he attempts to learn something about cellulose is that all the usual methods for handling a compound fail in the case of cellulose. Whereas most of our knowledge as to compounds of various types comes from studying the pure compound, all the usual methods for purifying substances fail when applied to cellulose. It is not possible to distil the compound, it cannot be dissolved and recrystallized since no known substance actually dissolves cellulose, and all attempts to purify i t by washing, or dissolving, out impurities apparently also attack the cellulose and change its nature. These are very great handicaps to overcome but by studying the way in which cellulose reacts with other compounds and reasoning by analogy with other compounds of known constitution i t is possible to obtain a reasonable picture as to what the structure of the compound might be. Recent evidence obtained by X-ray analysis also tends to give us some information as to the way in which cellulose is built up (16). As has already been shown, cellulose is built up of carbon, hydrogen, and oxygen in the ratio 6:lO: 5, or to use the chemical terms it has six atoms of carbon for ten atoms of hydrogen, and five atoms of oxygen. The molecule, or smallest portion of the compound having the properties of the compound, may thus contain twenty-one atoms all told, or forty-two or
sixty-three, and so on, as long as the ratio is as given above. It is a t this point that our knowledge fails us since we have no definite way in which to determine just how many atoms are in the fundamental unit of cellulose. Much work is being done on this point and when a definite answer to this problem is obtained a large stride will have been made in our understanding of the raw material with which we are working. The chief chemical property of cellulose is the similarity of its reactions with the class of compounds known as alcohols. Whereas cellulose and ethyl alcohol are similar to one another in certain ways, there is little similarity when taken into the human system, so that as far as cellulose is directly concerned the Eighteenth Amendment is safe. Alcohols also consist of carbon, hydrogen, and oxygen, arranged in a very definite manner. This arrangement is characterized by a group known as the hydroxyl group so that ethyl alcohol is represented by C2Hs(OH) with the hydroxyl group in parentheses. Compounds possessing this group react with organic acids according to the following equation: R.OH ALCOHOL
+
HX
+
ROX ESTER
ACID
+
HsO WATER
This has been found to be true no matter how many hydroxyl groups an alcohol may have and, what is more, by determining how much of the "X" of the acid has entered the compound it is possible to find how many hydroxyl groups a compound may have. In the case of cellulose it was found that for each six carbon atoms there were three hydroxyl groups. Further evidence has shown that there must be in cellulose a molecular structure very similar to that in the sugar, glucose, C6H1206. It is not the purpose of this paper to proceed further into the discussion of the structure of cellulose and if the reader will bear in mind the fact that cellulose is very similar in its reactions to an ordinary alcohol, except that i t has three hydroxyl groups instead of one, most of the ordinary industrial applications can be explained. For further discussion of this very fascinating problem of the structure of cellulose the interested reader is referred to the textbook of Heuser (6). WOOD
As bas already been pointed out, alcohols can react with various acids to furnish neutral compounds of characteristics entirely different from the original. Since cellulose possesses an alcoholic nature, i t also has the power of combining with acids. This is exactly what has happened in the formation of wood, which apparently consists of cellulose chemically combined with an as yet not fully understood acid called "lignic acid." If we compare a sample of cotton (which is nearly pure cellulose) with a sample of wood the difference is immediately apparent both to the eye, the sense of feeling, and also to chemical tests. Wood is very inert and neutral toward chemical reagents just as we might expect a compound of cellulose with some acid to be. It is apparent, then, that in order to obtain
the cellulose in a pure form from wood i t would be necessary to break up the union between i t and the lignic acid wherewith i t is combined. No ordinary method suffices for this but resort must be had to rather drastic chemical means to remove the lignic acid part of the wood. We will discuss later some of the methods devised for this purpose. There are other factors which make difficult the way of the pulp producer and which should be mentioned here. Among other things there are present in the wood certain natural volatile oils such as are very evident in the case of the pitch pine from which turpentine is obtained. The amount is small and we know very little as to the way in which they are formed in wood. Natural oxidation converts some of these oils to balsams and resins, and also to certain gums. The amounts of these products run as high as five per cent. of the wood in some cases. If these products were allowed to remain in the pulp they would contaminate i t so as to render i t useless for many purposes. Necessarily then such substances must be removed, and, if such removal can be combined with the removal of the lignic part of the wood, great economy of time and labor can be effected. Besides such chemical constituents of wood as lignin and resins there is also the problem presented by bark and knots and decay. The bark and knots apparently resist the chemical action of the substance used to free the cellulose and would contaminate the pulp seriously if they were not completely removed before treatment. The decayed wood which results from the action of certain fungus growths must also be removed before treatment and, since this is a labor- and time-consuming operation, precautions are usually taken to prevent the growth of such fungi. This is usually done by keeping the moisture content of the wood a t about sixty per cent., or by spraying with cymene or naphthalene. Preliminary Treatment of Wood. The first step in preparing the wood for treatment consists of removing the bark by means of special machines. These consist either of large rotating disks carrying sharp knives which slice the bark from the log in a manner similar to
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