A SUMMARY OF RECENT DEVELOPMENTS IN CELLULOSE AND CELLULOSE DERIVATIVES
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JOHN S. TINSLEY Hercules Powder Company, Wilmington, Delaware
ITIS FITTING that we should consider recent developments in the chemistry of cellulose and its derivatives a t this time since it marksihe 100th anniversary of the discovery of a reproducible compound of cellulose. While it was known some years earlier that strong nitric acid produced a flammable substance from polysaccharides, it was in 1847 (1) that it was recognized that three nitrate groups had replaced hydroxyls on the glucose residue. This discovery has been challenged a t several times during the intervening years hut when the final accepted molecular configuration of cellulose was established, this hypothesis was substantiated. The utilization of cellulose in its various forms is many centuries old, and the efforts of scientifically inclined investigators to solve its chemical and physical behavior is a t least 100 years old. With this background, it is rather difficult to define the term "recent." Purves (3) chooses to date the era of modern cellulose chemistry from 1919, a t which time the labors of many investigators had established the composition and structure of the simple sugars. Even with this background and the knowledge that pure cellulose could be hydrolyzed to almost pure glucose, it was not until a p proximately ten years later'that the organic structure of cellulose was finally established. This structure is exemplified in Figure 1which represents the labor of 80 years, most of u-hich was contributed by organic chemists.
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that cellulose is polymolecular in nature and that any expression of molecular weight or degree of polymerization represents the average of a wide range of chain lengths. The defining and measurement of average molecular weights have been arrived a t by measurements of viscosity pioneered by Staudinger ($), by perfection of osmotic pressure methods in ~vhichSchulz (4) was a leader, and by the development of the ultracentrifuge technique by Svedberg (5). While these various methods arrive a t a different average molecular weight due to the nature of the determination, they are each useful in correlating the physical properties of cellulose and cellulose derivatives. The most frequently used of t,he various methods is the viscosity measurement in- very dilute solution. We shall not go into the detail of the techniques involved in those measurements, but it has been established that the average degree of polynwrization of cellulose as it occurs in nature is in the range of 3000 to 3500 anhydroglucose units. The average degree of polymerization of the cellulose derivatives encountered in plastics is between 200 to 600, and of derivatives for lacquers and protective coatings may drop as low as 80. In an effort to explain the macrostructure of the cellulose fiber, the microscope has been used extensively in connection with various physical and chemical methods of disintegration. Figure 2 shom a cotton linters fiber
Confiwnltion of Portion of CoUuloss Moleode
The behavior of cellulose in its chemical reactions is auite simule and exdained bv the ordinary eauations ised for the reactionsof primary and secondary ilcohols. The alcohol groups may be esterified, etherified, or oxidized by standard techniques. However, the rates of reaction are freauentlv extremelv slow. which is usually attributed to 'the dhysical form. The era from T?., 1930 to date on the fundamental studies of cellulose rig- a. 8..ten cotton~i"t.=. ~ ~ ~ i f i ~ (OX . t i . ~ chemistry might be called the era of physicists and physical chemists, who have devoted a great deal of attentim to the ~hvsicalstructure of cellulose fibers. nartiallv disinteerated bv the familiar Drocess of beatine By the time the chemical formula had been well as used in paper making. The fibrils in evidence give a established, physical measurements had demonstrated partial picture of the fiber structure. The electron A
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FEBRUARY, 1948
Pigure 3.
Electron Micrograph of Beaten Cotton Linters. Magnifioetion 22.000X
microscope has been used in more recent years producing a picture, as shown in Figure 3, similar to the light microscope. Figure 4 demonstrates a still later technique using fibers similarly disintegrated, then evaporating a film of chromium metal onto the fibrils in order to produce a greater contrast. It is readily seen that the fibril coated with the chromium metal is impervious to the electron beam, and much better definition of the individual filaments is obtained. While this work on the macrostructure of the cellulose fiber is interesting and important to the paper industry, it has made little contribution to the question of chemical reaction rates. It was noted early that cellulose fibers exhibited the properties of producing an x-ray diagram similar to that obtained by examination of crystalline substances. This diagram was much less distinct than that obtained from truly crystalline materials and various hypotheses have been proposed to explain the occurrence of points of orientation o r . "local order" in fibers and to estimate the per cent of the total fiber involved in the orientation. Figure 5A represents an parly concept of the micellar arrangement
Pigurc 4 .
Electron Micrograph of,Beaten Cotton Linters. Chromium Shadowed. Magnification 22,000X.
of cellulose molecules arranged in order, so that they would produce areas of apparent crystallinity. This hypothesis has been displaced because, among other things, it does not explain the high tensile strength of fibers since there would be obvious weak spots a t the spaces between the crystal structure. Figure 5B is a schematic diagram which more nearly fits the currently accepted theory of the structure of a micelle. In this figure it will be seen that there are local areas, indicated by dotted lines, in which a. high degree of order is permitted by the occurrence of a numberof chains in parallel. Holyever, since the chains are not all of equal length, they may readily extend from an area of high degree of order to a high degree of disorder, thus giving rise to the presence of so-called amorphous areas. These amorphous areas are presumed to be much more readily penetrated by chemical reagents and therefore are much more reactive than the ordered or oriented areas Many methods have b ~ e nproposed for estimating the per cent of available surface and the extent of the
A.
Early Micellar Theory.
o, b, inns. strvstu.e of rnicelle: c. d, spaces l,etw.en
crystal.
B.
Figures SA and SB.
curmnt1y Accept.d Theory Sshemmtic Representation of Theories of Fiber St~uctur.
amorphous areas. The first group of methods included the sorption of vapors or solutes onto the surface of cellulose from dilute solution. Another method designed to measure the same factors involves the reactions of dry cellulose with thallous ethylate, then estimating the reaction product (6). At present there is considerable interest in methods of dissolving out the so-called amorphous areas with hot hydrochloric acid, assuming that the ordered or crystalline areas will remain substantially unattacked. One such method (7) depends on the subsequent oxidation of the hydrolyzed cellulose by ferric chloride to glucuronic acid, which is rapidly converted to carbon dioxide in the strong acid solutions. More recently, techniques have been evolved for estimating the amount of unreacted cellulose by isolation and weighing (8). While varying degrees of precision can be obtained by
JOURNAL OF CHEMICAL EDUCATION
these techniques, the shortcoming of the methods appears to he the assumption that the areas classified as crystalline will not be changed if the amorphous areas are attacked. This is probably not the case, and various investigators are active in this-field a t present. As seen in the above discussion, it is difficult to define the term "recent" in dealing with developments in cell lulose chemistry. It is equally difficult to establish a basis for discussion of cellulose derivatives. Since this symposium is primarily devoted to plastics, we shall direct our discussion along that line.
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POWDER HOPPER 2 4 6 8 10 I2 14 16 IS NUMBER OF CAREON ATOMS IN SUBSTITUENT GROUP Figur. 7. Tensile Strength and Elongation of Cellulou, Trieste. Film lHagedorn & Moller. C=llulo*eshemir 12. 31 (193111 0
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PLUNGER MOLD Pigar. 6.
Schematic Diagram of Infn~tionMolding-Machine
The first well-defined derivative of cellulose also became the forerunner of the thermoplastic industry when Hyatt (9) developed the celluloid billiard ball. The cellulose nitrate plastic field reached a peak in 192%24 of approximately 28 million pounds per year and it still remains one of the best plastics from the standpoint of physical properties in spite of its fabrication limitations. The patent and technical literature were filled with accounts of the preparation and properties of other cellulose derivatives even long before the chemical structure of cellulose was established. Only one of these derivatives, cellulose acetate, had developed to industrial importance by 1930. It was about this time that a new principle in plastic fabrication was introduced using the nonflammable cellulose acetate. This was the introduction of the injection-molding machine and can well be considered the beginning of the modern thermoplastic industry. Figure 6 is a simple diagram outlining the principle involved by which a molten plastic material is injected into a cold mold, the plastic hardens and assumes the shape of the mold cavity, thus reducing the production cycle of molded articles to a matter of seconds. This development stimulated investigators to prepare derivatives of cellulose suitable for plastics instead of the conventional film and textile outlets. It is evident from the chemical structure, as shown in Figure 1, that almost an infinite number of esters, ethers, and mixed combmations could be prepared from cellulose. The extreme versatility of this starting base, coupled with the inherent
toughness whicli it imparts to its derivatives, justifies the wide fundamental and industrial research devoted to it. In 1931, Hagedorn and Mollef (10) published a description of the properties of cellulose esters extending from the acetate through the stearate. As shown in Figure 7, the tensile strength drops off markedly as one follows the homologous series beyond the butyrate, while the elongation increases markedly in the same proportion. While this chart is very informative and indicates that for most industrial purposes the simple triesters of the higher fatty acids are of little interest because of their physical properties, it should be noted that the triesters, generally, are not thermoplastic and cannot be used in injection-molding compositions. Also, for any given derivative of cellulose, it is possible to show a marked variation in properties as one proceeds upward from the initial plastic range of substitution equivalent to about 2.2 groups per glucose unit. Figure 8 illustrates this change in various properties for ethyl cellulose over the range 42 to 51 per cent ethoxyl. Of especial interest is the fact that with this derivative the hardness of a film continues to decrease with in(Catinued on page 117)
Fi(rure 8. Vpristion oL Propsrtiy of Ethyl Cellulos~with Chanains D e w e of Substitution
FEBRUARY, 1948 A SUMMARY OF RECENT'DEVELOPMENTS IN CELLULOSE AND CELLULOSE DERIVATIVES (ContinueZ from page 96)
creasing substitution, but the softening temperature turns sharply upward as one approaches the trisuhstituted derivative. This sharp increase in softening point contributes to the inability to mold high-substitution products. With the advent of themany synthetic polymers over the past 15 years, the casual observer has had a tmdency to look upon these products as neuer and therefore more glamorous than cellulose derivatives which formed the basis of our modern plastics industry. H o w ever, as the plastics industry became more diversified and the requirements more clearly defined, the research chemists in industrial laboratories have been extremely active in preparihg "tailor-made" cellulose derivatives to meet the requirements of each industry. During this period, several types of cellulose acetate have been developed to improve moisture resistance and to resist deformation a t elevated temperatures. Other cellulose esters, such as cellulose acetate hutyrate and cellulose propionate, have been developed for se!ected properties, especially high impact strength, good moisture resistance, and ease of molding. Also, to fit into this field, ethyl cellulose has been developed from the lahoratory stage into the commercial applications x~hereextreme functional properties are rcquired. I t should be understood that cellulose derivatives are not used in the vlastic industrvrvithoutcom~oundine with other ingredknts. ~n all cases, modifieis (callei plasticizers) are required to simplify fabrication and to increase the versatility of the compositions. Thus,
with the same cellulose derivative, it is possible to pfoduce soft, low-melting compositions as well as hard, high-melting mixtures. An enormous amount of effort has been expended in the preparation and evaluation of plasticizers as well as the nature of polymer plasticizer interaction. As a result of the work of investigators over the past 100 years, there is a wealth of Information from which we can predict the properties of nev cellulose derivatives. Many industrial laboratories are engaged in the study of these materials, and commercial development awaits the demands of the consuming industry and availability of intermediate chemicals necessary for cellulose convcrsion. LITERATURE CITED (1) C R ~W., , Ann., 62, 233 (1847). (2) PDRVES,C. B., "Cellulose and Cellulose Derivat,ives," edited by E. Om, Interscience Publishers, New York, 1943. o. 43. STAUDINGER, H., "Die Hochmolek'llaren Organischen Verbindungen," J. Springer, Berlin, 1932. SCHULE, G. V., Z. Physik. Chem., A158, 237 (1932). SVEDBERG, T.,AND K. 0. PEDERSEN, "The Ultracentrifuge," Clarendon Press, Oxford, 1940. HARRIS,C. A., AND C. B. PURVES,Papet Trade J., 110, 63 (1940). NICKERSON, R. F., I n d . Eng. Chem., Anal. Ed., 13, 423 (1941). (8) PRILLIP, H. J., plc6ented before A.C.S., April, 1946, (9) HUAR',J. w., U.S. Patent 105,338 ( J ~ 12, I ~1870). (10) HAGEDDRN A N D MOLLER, Celluloseehemie., 12,31 (1931)
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