SOME of the NEWER THINGS ABOUT CELLULOSE* GUSTAWS J. ESSELENt 73 Newbury Street, Boston, Massachusetts
Although the use of cellulose in the form of pafir and textiles dates back to antiquity, it i s o d y compara&ely recently that it has assumed importance as a chemical raw material. This situation w s chiefly due to lack of information regarding the chemicel nature of cellulose, for i n point of available supply and ease o f renewal, cellulose i s a n ideal raw material. Recent advances in cellulose study include both new fundamental information re-
garding the structure of the cellulose molecule, as well as new applications and economies in industry. Cellulose ester industries have bettered both quality and price; rayon has improved i n respect to feel, strength, and appearance; a n d cellulose plastics are assu.ming r6les of increasing importance. N m compounds of cellulose are being developed with increasing frequency and smeral of these give promise of becoming industrially important.
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This combination of chemical and physical contributions has resulted in a picture of cellulose configuration, which until recently was the generally accepted one. In this, cellulose is seen to have crystalline properties as revealed by the X-ray diffractionpatterns. The unit crystal cell responsible for the crystalline properties is believed to consist of four glucose residues joined by the 1-4glycosidal oxygen bridges to form two cellobiose residues in each unit crystal cell. The small size of the unit cell may be correlated with the known high molecular weight and colloidal properties by the concept of long primary-valence chains of anhydro-glucose residues bound together by secondary valence forces to form the micelle, two links of each chain appearing in the unit cell. This roughly sketched idea has been extended and elaborated to account for the structure of the cellulose fiber, including its high longitudinal strength, the presence of the outer layer of fibrils in the cellulose fibers from wood, and the orientation about .. the long axis of the fiber. Lately, however, this conception has been attacked by Freudenberg (5) who claims that cellulose cannot be considered as a glucose anhydride inasmuch as the only sterically possible 2,3,6-trimethylglucose anhydride, which he has prepared, is a low-boiling oil entirely different from trimethylcellulose. Moreovei, Willstatter and Zechmeister (6) have shown evidence of cellotriose and cellotetrose from the hydrolysis of cellulose which Freudeuberg feels conflicts with the idea of two cellobiose anhydride residues in the cellulose unit. Likewise mcthyl&ed cellotriose and -tetrose have been isolated by distillation of methvlatcd deuadafion ~roducts of cellulose. From demolition kinetics, Freudenberg calculates that cellulose must wusist solely of long chains of glucose residues joined entirely by primary valences. His demolition data do not satisfy equations dependent on the cellobiose structure. He admits that his conception of the cellulose molecule calls for the presence of tetramethylglucose upon the hydrolysis of methyl-
LTHOUGH the use of cellulose as paper has been known for over a thousand years and its use as a textile fiber even longer than that, i t is only during the last seventy years or so that cellulose has begun to function as a chemical raw material. Even then, those chemical processes which had reached a manufacturing scale, particularly the manufacture of explosives and celluloid, were largely on an empirical basis. The iirst fact about the constitution of the cellulose molecule which became generally recognized was that cellulose is a carbohydrate of the general type of an aliphatic alcohol and it was later found that it could be hydrolyzed quantitatively to glucose. It was not, however, until about eleven years ago that i t was finally recognized that in the elementary cellulose grouping, CsHloOs, there are three and only three hydroxyl groups, one being a primary alcohol group and the other two, secondary. This point, so neatly demonstrated by Denham and Woodhouse ( I ) , is now one of the few points universally accepted from a bewildering assortment of data on structure. The recognition of this fact has been of very real practical value as well as of theoretical importance, for in almost every commercial application of cellulose, the reaction of the hydroxyl groups enters a t some stage of the processing. Further extensive chemical investigations by a number of investigators, together with the results of X-ray studies, have given us in recent years a more detailed picture of the internal structure of the cellulose molecule. The X-ray data, based on the original conceptions of Laue as amplified by the Braggs, was first applied to cellulose by Sponsler and Dore (2) in this country. I t was extended by Mark and Meyer (3) abroad and coniirmed bv Clark (4) . , in this countrv.
* Presented before the Division of Chemical Education at the
eiphty-fifth meeting of the A.C.S., Washineon, D. C.. March 28. 1933, as a contribution to the symposium-on "'Recent Develop: m a t s in Various Chemical Industries." t President. Gustavus 1. Esselen. Inc.. Consultine Chemists
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ated glucose due to the methylation of the end glucose residue of the chain, but points out that in a chain of 100 units the amount would be too small to isolate, or that the failure may be explained if we assume a loop structure for the cellulose chain or an anhydride structure for the open aldehyde end of the chain. These ideas are supported in a measure by Staudinger (7) who also declares that the idea of secondary valences in a cellulose micelle must be abandoned and that the macromolecule is joined solely by primary valences. Staudinger and Schweitzer (8) have offeredmethods for determination of molecular weight based on the viscosity of solutions of cellulose in Schweitzet's reagent or solutions of cellulose derivatives in suitable solvents. These methods are in accord with Staudinger's work with other highly polymerized bodies. He gives a molecular weight of 190,000 to cellulose from purified cotton but has evidence to show that some degradation has occurred in purification and that the figure is too low. This value is higher than that calculated by Freudenberg from demolition kinetics (about 50 glucose residues, mol. wt. 8100) which Freudenberg thinks is too low, or that obtained by Stamm (9) using the ultra-centrifuge (40,000) and much higher than any of the values obtained by freezing-pointmethods which are now open to considerablequestion. In this connection, i t is of interest to note that Staudinger and Schweitzer have found that rayon made by the cuprammonium process has a molecular weight of only 35,000 or a little more than one-sixth that of purified cotton. No discussion of recent work on the structure of celIulose would be complete without mention of Hibbert's (10) successful synthesis of cellulose by the action of bacteria on lower carbohydrates, which is not only of chemical significance but bears on the discussions concaning the mode of formation of cellulose in the plant. The true significance of work such as that briefly outlined above becomes apparent when we consider that such a foundation of knowledge is bound to result not only in improvements in the long-established industries which are based upon cellulose as raw material, but in all probability will ultimately result in the establishment of entirely new industries. Of the established cellulose industries, the paper and cotton and linen textile industries are the oldest. In the latter part of the nineteenth century there was laid the foundation for the nitroceUuloseindustries,Gz., explosives andpyroxylin plastics such as celluloid; and also forCthe rayon industry, which, in the present century, has become established as a fifth definite division of the textile industry. Lacquers with a cellulose base have effected remarkable economies in industrial finishes; and thin transparent sheetings of cellulose and cellulose acetate have revolutionized the packaging of a wide variety of products. In addition to these there is a wide range of other cellulose products familiar to you all, such as lumber, the synthetic boards for heat insulation, artificial leather and other coated fabrics, vulcanized fiber, and photographic films. As mentioned before, the oldest use of cellulose is that
as a textile fiber and this dates far back into antiquity. It may be for this very reason that it has been so difficult to persuade the cotton manufacturer that his raw material is a chemical compound and that a great many of its properties and changes in properties may be understood and controlled only when viewed in the light of chemical reactions. That this realization is slowly dawning on the textile manufacturers is perhaps demonstrated in the recently established work of the Institute of Textile Research a t one of our eastern colleges which is investigating the chemistry of the cotton textile fiber. To mention only one case where a clearer understanding of chemical changes might be of value from a practical textile standpoint, we might mention changes in the mechanical properties of textile fibers which take place in processing, such as changes in strength, flexibility, and feel. The changes in these properties are undoubtedly due to changes in the chemical and colloidal situation in the molecules or micelles. For instance, Freudenberg and Staudinger have both recently shown that, during the processes of purification of cellulose, the molecular weight decreases due, apparently, to breaking of the chain of glucose residues. It should not be difficult to correlate this change in molecular weight with loss in strength if our ideas are correct that the strength of the cellulose fiber is, for the most part, due to the long primary-valence chains. If these relationships between chemical changes and mechanical properties can be discovered and elaborated, the possibilities of technical control in the textile industry should be infinitely extended. One of the recent examples of the use of the chemical characteristics of cellulose in the textile industry is that of immunized and amidated cotton (11). Immunized cotton is cotton which has been so treated that it resists dyeing by ordinary direct dyestuffs to about the same extent as does cellulose acetate. For this reason i t is used to weave with cotton to give interesting cross-dyeing effects. It is prepared by partial esterification of the cotton with p-toluenes~fochlodein an organic solvent. The esterificatiou takes place in layers from the outside inward and is carried only far enough to affect a very thin outside layer. The resulting product is not very different from ordinary cotton in appearance or feel, although i t is quite resistant to water. If the immunized cotton is further treated with ammonia or an organic base, the result is amidated cotton, a material which cannot be told from cotton in appearance even under the microscope but is entirely different in chemical properties and shows an even greater affinity for the common acid dyestuffs than does wool. Chief among the recently established cellulose iudustries is the manufacture of rayon, which now ranks as a fifth generally accepted textile fiber. There are four general methods in use today for the manufacture of rayon. These processes all have one common feature in that they all involve the conversion of the cellulose into a soluble form, following which, the resulting cellulosic solution is forced through exceedingly fine orifices into
a hardening medium, which solidifiesthe fine streams of cellulose-containing liquid into still finer continuous filaments. This hardening medium may be either warm air, which serves to evaporate a volatile solvent as in the nitrocellulose and cellulose acetate process, or i t may be a solution of some suitable chemical which reacts with the soluble cellulosic derivative and regenerates a material quite similar to the original cellulose as in the viscose and cuprammonium processes. Although, as just stated, there are four processes being operated in this country today for the mannfacture of synthetic fibers, a single one of these, namely the viscose process, accounts for over 80% of the total production. In this process, the cellulosic material in the form either of purified cotton linters or wood pulp, is first saturated with a solution of sodium hydroxide of about 18% strength. The excess of this solution is squeezed out and the wet cellulose is then shredded into a finely divided form known as crumbs. After an interval of several days under carefully controlled temperature conditions, these crumbs are then treated with carbon bisulfide which converts them into a form which is soluble in dilute caustic soda solution. The resulting solution is about the consistency and color of molasses. This also is stored for several days under conditions of carefully regulated temperature and, during this period, a change occurs in the dispersed cellulose which makes it more suitable for the spinning operation. In the viscose process the spinning operation consists of extruding the cellulose solution through very fine orifices in a platinum spinneret into a bath of dilute sulfuric acid. The fine streams of cellulose solution are hardened almost immediately on coming in contact with the acid, forming fine filaments which are removed from the bath continuously and wound either on a revolving bobbin or inside of a small rapidly spinning centrifugal container. A single thread may contain anywhere from sixteen to sixty-four individual filaments. After spinning, the resulting thread is carefully washed free from acid. Slight residues of sulfur or sulfur compounds are removed by treatment with a dilute alkaline sulfide bath and the threads are again carefullywashed. Sometimes the product is bleached as well. The conditions of final drying are important and are carefully controlled. The history of the rayon industry has been one of constant improvement. About five years ago a process was announced which seemed a t the time to offer the long-sought method of producing synthetic%bers as strong as natural silk. It consisted essentially in using, in the viscose process, a hardening or precipitating bath containing a very much larger proportion of sulfuric acid than normal. In fact, the percentage of sulfuric acid was such that if the rayon filaments were permitted to remain in contact with this bath for more than a very short period of time, they were completely dissolved. Material produced in this way showed remarkable strength. It was materially stronger than natural silk when it was in the normal dry condition and about equal in strength when wet. However, i t was found to have more tendency to break when bent than
normal rayon fibers, which interfered with its use in weaving and knitting, and this fact, combined with the high acid costs involved in its manufacture, seems to have prevented its becoming a commercial success. While this particular development did not live up to the early expectations, nevertheless real advances have been made in the industry in recent years with the result that we now have artificial silk with all of the outward characteristics of feel and appearance which have so long been peculiar to real silk. The fabrics made from these new fibers are soft and have the scroop of natural silk; and improvements in delustering have removed the objectionable shine while retaining a pleasing and natural-appearing sheen. I t has long been known that a shiny or lustrous surface may be changed to a dull or delustered one by the simple expedient of roughening it. This was the principle on which some of the early attempts a t delustering rayon were based, usually involving the precipitation or deposition of some finely divided substances on the surface of the rayon filaments. Such a process usually sufferedfrom the objection that the deposited substance had a tendency to wear off or rub off. However, the optical situation in rayon is such that a second method of delustering is available which still leaves the fiber with its smooth surface unchanged, and its soft, smooth "feel" unimpaired. In order to understand this, i t is necessary to consider what makes ordinary rayon so lustrous. The individual rayon filament is essentially transparent and has a smooth surface. As a result, when a beam of light hits the surface, a small part of the light is reflected specularly and the balance passes through. Some is also reflected from the under surface of the filament and still more from the surfaces of the -next succeeding filament. Inasmuch as in a rayon thread or bundle of threads there is a very considerable number of filaments, this process of reflection from succeeding surfaces continues until all of the light has been reflected back to the eye, and since this reflection is to a large degree specular reflection, we have the appear.. ance of sheen or luster. In order to reduce the luster or gloss, it is necessary to arrange things so that a considerable proportion of the light reflected from the substance is of the diffuse or scattered type. In other words, the higher the proportion of diffuse reflection, compared with the total reflection,both specular and diffuse,the more delustered will the material appear. Since rayon filaments are transparent, this result can be brought about by introducing inside the material of the filament small amounts of an oil, finely dispersed, or of a finely divided white pigment, or both. It has been found that the amounts required are relatively small and accordingly have only a minor effect on the strength of the thread. Furthermore, since the added material is entirely inside the filaments, there is no disturbance of the smooth feel. In fact, in some instances there is a decided improvement in the "feel" or "hand." Up until the present, the chief use for the various synthetic fibers has been in ladies' garments and fancy
fabrics though some has gone into underwear for men. A new use is promised for this summer in the announcement that men's suits will be on the market made of cellulose acetate synthetic fibers. In the recent flare of technocracy, another cellulose fiber, namely ramie, came in for somewhat more than its due share of publicity. The exaggerated claims for ramie's superiority form a good example of the danger of a little technical knowledge. Ramie is a bast fiber of unusual length, strength, and beauty, and when separated from the other parts of the plant, offersan almost pure native cellulose. It has long been used for fish lines and nets because of its high strength. However, its entry into competition with other textile fibers has until recently been limited by the fact that the separation of the fiber from the other portions of the plant is difficult and for high-grade fibers this had always been carried on by hand, since no satisfactory machinery had been developed for this purpose. Recently, however, mechanical means have been perfected.by which it is claimed that much of the woody part of the pIant can be readily separated from the true fiber, thus doing away with the slow and costly hand operations. Furthermore, chemical methods have recently been developed which complete the purification and make the ramie fiber ready for textile purposes. This does not mean, however, that i t is ready for immediate and universal adoption in the textile industry. On the other hand, there are purposes for which it is very suitable, as witness the fact that there is a mill in Germany which has been manufacturing ramie fabrics for over fiftyyears. The spinning of ramie is complicated by the fact that there is little tendency for the long smooth fibers to cliig together, but this has recently been overcome to a considerable extent by a chemical treatment whichimparts a twist to the fiber. In any use of ramie it must be recognized that although the fibers are unusually strong, they also tend to be brittle. As a result, fabrics have to be chosen which are not subject to sharp and repeated bending, as for example laces, draperies, curtains, and furniture coverings. In spite of the fact that such fabrics often develop a certain roughness due to the breaking of individual fibers, there are nevertheless occasional reports, apparently well authenticated, that ramie has been made into a linen-like fabric with unusual wearing qualities. This makes it appear to be well worth further study and research to find a way to overcome the inherently brittle characteris% of the ramie fiber. While admittedly this looks like a large order, nevertheless there is no obvious reason why it should not ultimately be accomplished. In fact, it appears already to have been done, though not under the control necessary for uniform repetition. There is an added incentive for the successful solution of this problem in the realization that ramie grows readily in several of our southern states. In so far as concerns the chemical compounds of cellulose which have reached industrial significance, the nitrate was the earliest and, indeed for many years, the .. only one to assume any importance. More recently
another ester, the acetate, has achieved commercial importance. There are three general types of cellulose nitrates. The type containing about 11% nitrogen is used in the manufacture of the pyroxylin plastic materials such as celluloid, pyralin, and fiberloid; the type containing about 12'% nitrogen is the basis of the nitrocellulose lacquer industry; while the type with still higher nitrogen content furnishes our smokeless powder. All three are made by modifications of the same general process which consists in treating dry, purified cotton linters with a mixture of nitric and sulfuric acids, followed by a thorough purification and stabilization treatment with both hot and cold water. Cellulose acetate, on the other hand, is made in a somewhat different way. Purified cotton linters are treated with a mixture of acetic anhydride and glacial acetic acid in the presence of a catalyst and under carefully controlled temperature conditions. This primary reaction results in an ester in which all of the available hydroxyl groups are esterified but which has proved to be of little commercial value. In order to convert it into the commercially useful form it has to be partially hydrolyzed, which is accomplished by adding to the thick, sirupy mass produced in the primary stage, a little water containing a small amount of mineral acid. When that degree of hydrolysis has been attained which corresponds to the desired properties in the cellulose acetate, the sirup is mixed with an excess of water which precipitates the cellulose acetate in a granular form. I t is then carefully washed free from acetic acid and finally dried. In spite of the economic set-up which a few years ago seemed distinctly to favor the nitrate of cellulose over the acetate in every case where its inflammability would permit its use, the acetate is now rapidly forging to the front. This is due to steady improvements in every stage of the manufacture of the acetate plastic which have not only brought about marked cost reductions, but have also resulted in greatly improved quality of the product. At present cellulose a q t a t e is not only competing with the nitrate for its share of the cellulose plastics field, but i t is also for many purposes a strong competitor of synthetic resins. As manufactured a t present, cellulose acetate may be made water-white or of any desired shade, it is tough and strong, and is relatively nou-inflammable. The uses of the cellulose plastics are sd familiar that it seems almost redundant to mention even a few which may range from watch crystals through photographic films and toilet articles to plates for false teeth. One of the uses of cellulose esters which is familiar to everyone but which has been developed and grown to large proportions during the last few years is in the manufacture of laminated or safety glass. In this material the cellulose ester takes the r81e of the meat in the sandwich not only because of the analogy due to its position between the pieces of glass but also becauseit is the cellulose ester which, due to its tough and pliable nature, serves to prevent the shattering of the glass cemented to it. Extend the analogy until you have a
club sandwich of substantial proportions and the glass advantages as a plastic and it would not be surprising becomes bulletproof, though in this case it is desirable to see it on the market in the near future. Another familiar and recent development in the use to have a thick piece of glass in the center. The early faults in safety glass offered a challenge to the cellulose of cellulose and its compounds is in the field of thin, ester technologist and we now see very little safety transparent wrapping materials, such as Cellophane glass with hazy, bubbly or badly discolored sheets in and similar materials. This use, which was originally the center. At &-st cellulose nitrate with its price ad- developed abroad, was brought to industrial fruition vantage practically monopolized the safety-glass busi- through American engineering skill. The chemical ness. Recently, however, there has been developed a part of the process is similar to that for making viscose new process for the manufacture of cellulose acetate rayon (12), the chief difference being that instead of sheeting which has practically overcome this advantage being extruded through tiny round orifices, the celluand has brought the manufacture of cellulose acetate losic solution is forced out through a long narrow slot into a chemical bath which decomposes the soluble sheets to a high degree of efficiency. As a result, much of the laminated safety glass today cellnlose compound and regenerates the cellulose in the is made with cellulose acetate plastic sheets instead of form of a thin, tough, continuous sheet. Constant the nitrate or pyroxylin plastic, with an accompany- improvement in the process coupled with mass producin^ marked improvement in the permanency of the tion has brought about a steady succession of price reductions in these wrapping materials until today the in light: One cannot leave the field of cellulase esters without humblest product can be dressed in a wrapping develmentioning, however briefly, the phenomenal develop- oped only a few years ago. In this field, regenerated ment of the nitrocellulose lacquers and finishes. At the cellulose took the lead but has recently been receiving close of the war nitrocellulose was almost unknown in competition from cellulose acetate. As is naturally to be expected from the aliphatic the field of industrial finishes. Now i t has for several years commanded a position of utmost importance not alcohol nature of cellulose, ethers are readily formed only in the field of automobile finishing, for which it and a number of these have been known for some time. was originally developed, but in every other branch of With recent refinements in their manufacture some of finishing save outdoor structural work and the special these are beginning to hold promise of industrial develresistant finishes. This development alone has been opment. The methyl ether of cellulose, for example, is responsible for a new industry using from twelve to soluble in water and is finding use in the textile industry fifteen million pounds of nitrocellulose annually. for a thickening agent. It does not sour or mold and About 80% of this is of the so-called low-viscosity type, possesses the unique property of becoming insoluble in which simply means that when dissolved in an organic the hot or in the presence of fixed alkalies but easily resolvent in reasonable amounts, i t produces a solution dissolving in cold water. This is a valuable property which is still sufficiently low in viscosity so that it can in textile printing. Some of the higher ethers of cellnbe applied by the usual methods of spray or brush. -lose show remarkable resistance to both alkali and acid This lowering of the viscosity is ordinarily accomplished and offer industrial promise for many purposes. This by an after-treatment under heat and pressure, one of is especially true of benzyl cellnlose which is receiving the larger manufacturers having developed a continuous marked attention from the standpoint of plastics. So, with astonishing rapidity, cellulose moves on to process for the purpose. Perhaps the latest, and a t the present time the most fulfil the predictions made less than a decade ago, that rapidly growing, use of cellulose nitrate, is as an ad- in view of its almost universal availability and the relahesive. I t has long been used for this purpose in the tive ease with which it can be purified, cellulose will, manufacture of leather belting but, within the last few with increasing knowledge of its behavior, form the basis years, processes have been developed for making shoes of entirely new and diversified chemical industries. by sticking them together, rather than by sewing or nailing and the special adhesives used for this purpose LITERATURE CITED ' have a nitrocellulose base. The degree of adhesion at(1) DENHAM AND WOODHOUSE, I. Chem. Soc.. 1927,244. tainable in this way is quite surprising to ouh testing (2) SPONSLERAND DORE, ''F0UTth colloid symposium monoit for the first time, and already many thousand pairs graph," The Chemical Catalog Co., New York City, 1926. of shoes are being made in this manner annually. (3) MARKAND METER,Ber., 61, 593 (1928); Z. physik. Chem., While cellulose acetate and nitrate have led the field 7~ ~ ? *,A"-",. r~o?a\ --, in commercial developments, chemists have not been (4) CLARK, ~ n dEng. . Chew., 22,474 (1930). ( 5 ) FREUDENBERG, J. SSO Chem. Ind., 50; 2 8 7 ~(1931); see tardy in the preparation of other esters and a great also FREUDENBERG, "The relation of cellulose to lignin many patents have been issued on the manufacture of in wood,u J. CHEM.EDUC.,9, 1171-80 ( ~ u l y 1932). , ( 6 ) WILLSTATTTER AND ZEC~MEISTER, Ber., 628, 722 (1929). esters of cellulose. These have raneed from the simple
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esters of many of the cyclic series acids and a great many mixed esters, ~h~~~have been mmors recently that cellnlose acetopropionate in particular offers special
(10) HIBBERT AND BARSHA, ibid., 53,3097 (193i). (11) TROTMAN. DYW,59.92, 117 (1928); MULLIN,Textile Colorist, 53, 85, 127, 460, 542 (1931). (12) HYDEN,IS^. Eng. Chem., 21,405 (1929).
