PURIFIED WOOD CELLULOSE
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GEORGE A. RICHTER, Brown Company, Berlin, N . H .
by accepted procedures. As in other analogous cases, however, a more exact definition of terms reveals secondary relations of major importance. T h e pentosan groups that occur in amounts from 8 to 25 per cent in the original wood appear in appreciable quantities in the cellulose that i s o b t a i n e d by analytical procedure, and the unremoved pentosan offsets in a greater or less degree the hexosans of less resistant form that A SPRUCE STAND are dissolved from the wood in the stens of the analvsis. Likewise, in any of the known cooking processes we find a substantial portion of the more resistant pentosans retained in the pulp and experience a loss of some of the more easily solubilized “cellulose”, which finds its way into the spent liquors as hexoses or other products of degradation. In the case of acid liquor cooks the greater portion of the original pentosan groups is dissolved by hydrolysis, and the resulting pulp, if of softwood origin, may contain as little as 2 to 5 per cent of that component as determined by the furfural test. With a hardwood such as birch, the original wood contains over 20 per cent and the resulting acidcooked pulps may show on analysis as high as 10 per cent pentosans. With an alkaline cooking procedure such as typified by the kraft process, the softwood will yield a pulp that contains about 10 per cent pentosans and the corresponding birch pulp will have as high as 20 per cent of that constituent. One might conclude that the alkaline cooks would give a correspondingly higher yield as reflected by pentosan retention, but that does not occur, especially if the comparison is made between pulps of equaI residual lignin content. This may be ascribed to the fact that a substantial portion of the original hexosans does not resist the solvent action of the alkali a t cooking temperatures. I n other words, delignification by an acid liquor is conducive to a major hydrolysis of pentosans into soluble pentoses and to a minor loss of cellulose. On the other hand, the alkaline delignification such as is practiced with sodium hydroxide and sodium sulfide retards the pentosan hydrolysis but exerts strong solvent action on the less resistant cellulose that is present in the wood.
A BIRCHPULPWOOD STAND
HE need for a refined wood cellulose commanded the attention of the cellulose chemist some twenty years ago. The variation in procedures that can be employed in isolating and refining the wood cellulose showed that an opportunity existed to realize in the finished product a combination of physical and chemical properties that was impossible to achieve with cotton except in a limited way and a t far greater cost. Year by year the investigator unfolded a host of possibilities for which he had hardly dared hope in the initial stages of the development. Some of the problems that confronted him are itemized to illustrate more clearly the obj ectives :
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To devise ways and means of removing resinous and ligneous matter from wood substance without causing appreciable inj u r y or degradation to the cellulose itself. To eliminate more completely the pentosan groups that are normally present in wood pulp to an appreciably greater extent than in cotton. To maintain in the cellulose a high percentage of that constituent that is usually designated as alpha-cellulose. To regulate the solution viscosity of the wood cellulose at some desired level that may be extremely high to resemble an undegraded cotton or very low to achieve some desired property in the corresponding conversion products, T o improve and control the chemical activity of the fiber by hydrolysis, oxidation, gelling, or combinations of them. To acquire knowledge of process that will enable the operator to adjust the delignification and the refining steps to compensate for the wide differences in composition and physical structure found in woods of different species and geographic origin.
Pentosan Elimination Although there are wide differences in composition of woods of various species, most of the wood that is available in the United States contains about 50 per cent cellulose as measured 324
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With a given cooking process, the yield of recovered fiber is adversely affected by further elimination of lignin, in more than proportional amount. It is evident, then, that the orthodox bisulfite, soda, kraft, and alkaline sulfite pulps contain appreciable amounts of noncellulosic matter, This imperfect selective removal of secondary matter is not always a liability. I n fact, in the paper industry, particularly, steps are frequently taken to augment the residual pentosan groups in sulfite pulp by less severe cooking; this favors the yield and increases the ease with which such pulps can be hydrated in the preparation of stock for special dense and semitransparent papers. HOW much of the improved hydration properties may be attributed to the greater percentage of pentosans and how much to the lesser hydrolysis of cellulose itself is not altogether clear. Conversion products that have exacting stability requirements, and more particularly such ultimate cellulose composites as the rayons, the cellulose ethers, and the esters in their wide variety of forms, demand for the most part a base wood cellulose of higher purity than can be obtained with present mill cooking procedures. Hence refining steps are necessary, first, to remove the lesser but very objectionable noncellulosic residues and, secondly, to extract the degraded cellulose that cannot be avoided in the delignification. The effectiveness of the refining steps is dependent on the process used and its severity. Improper refining sequence results in uneconomic consumption of chemicals and can, by attack on sound cellulose, cause unsatisfactory yields. Furthermore, the reagents used in the purification steps, as well as the conditions of time, temperature, and pressure, will determine the properties of the refined product. Two products having identical composition, as determined by methods now employed in laboratory identification and each produced from the same unbleached fiber, may possess vastly different physical behavior when subjected to papermaking steps; and they can exhibit such wide differences in chemical activity that one may be quite inert and the other very reactive. The usual procedure for the purification of an unbleached sulfite wood pulp comprises a sequence of chlorine, hot dilute alkali, and a carefully regulated postbleach. The amounts and conditions may be varied over wide limits, but are largely fixed by the composition of the unbleached fiber and the desired combination of properties of the final product. In some cases when paper strength is of paramount importance, it is advisable deliberately to undercook the wood to yield a pulp relatively rich in secondary groups but with a retained cellulose that has suffered little hydrolysis. Additional removal of lignin, pentosans, and resins, can usually be effected with least sacrifice in yield and with least injury to cellulose itself in properly designed posttreatments. I n the event that the final product is intended for esterification, it is often desirable to emphasize a more thorough delignification in the cooking of the wood, but here again there are limitations largely determined by such factors as rayon or film strength as well as solution viscosities. With cooking procedures currently employed there is a constant compromise in the matter of degree of delignification, the yield of product, the reduction of solution viscosity. the hydrolytic effect on cellulose, and the inherent strength of cellulose that remains. Ordinarily if coniferous wood is cooked over a period of 10 hours with the usual calcium bisulfite liquor, a yield of about 45 per cent is obtained. The pulp will have from 1 to 3 per cent lignin and from 3 to 5 per cent pentosans. The solution viscosity will be such that, when refined by common procedure to an acceptable color and a 90 per cent alpha-cellulose content, the value will closely approximate present-day wood fiber that is consumed
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in great amounts by the viscose industry. Further refinement to 95 per cent alpha-content and 1.5 per cent pentosans need not entail additional sacrifice in solution viscosity. Wood cellulose of considerably higher solution viscosity can be obtained if the wood cooking process is altered. To illustrate, if the wood chips are first thoroughly and uniformly impregnated with a 10 to 12 per cent sodium bisulfite solution and if these pretreated chips are then digested with a 5 per cent sulfur dioxide solution for about 5 hours a t 135" C., a pulp will result that is characterized by a low lignin content-namely, 0.2 per cent-and a solution viscosity reaching ten times that of a corresponding pulp that has been cooked by present-day methods. The high solution viscosity of the unbleached fiber is largely retained by the bleached cellulose. Moreover, yields realized by the modified procedure described above are slightly higher than normal, in spite of the fact that the lignin residues are appreciably lower. The protection of the cellulose against acid hydrolysis is also reflected in a greatly increased tearing strength of the paper that is prepared from the pulp, and it is fair to expect that the dual evidence of substantial absence of degraded cellulose-namely, high solution viscosity and high tearing strength of fabricated sheet-should lead to superior strength of the corresponding esters. Such an expectation assumes that the advantages attained in the wood cellulose pulp will not be sacrificed in the name of convenience when esterification takes place.
Increasing the Alpha-Cellulose Content When wood celluloses of higher than 95 per cent alphacellulose content are needed, we must resort to one or more of three alternatives. For instance, it is possible to achieve higher alpha-cellulose by means of a delignification process that differs radically from those now used. These newer procedures are almost necessarily of a multiple-cook type with two or more compositions of liquor, a t relatively high temperatures. The higher alpha-cellulose content can also be obtained by digesting the unbleached or partially bleached wood fiber in a weak alkaline solution a t 125" to 200" C. The higher temperatures claim a sacrifice in yield that is all out of proportion to the increased alpha-cellulose content. As the temperature is increased, the solution viscosity is reduced rapidly, particularly if those temperatures exceed 150" C. In fact, where yields are not a determining factor, digestions in very weak alkaline solutions-for instance, a 0.2 per cent sodium hydroxide or a 1 per cent sodium sulfite liquor- a t temperatures from 175" to 190" C. will yield a product of extremely low viscosity even though the alpha-cellulose content will reach 97 per cent. Such fiber can be used advantageously when low viscosity is desired. The third and, for many purposes, the best method to raise the alpha-cellulose to over 95 per cent is to treat with relatively strong alkaline solutions a t temperatures not over 60" C. This set of conditions preserves yield and also favors retention of the original solution viscosity. The increase in alpha-cellulose is accompanied by a rapid decrease in pentosans and an effective elimination of resins. Recovery and re-use of alkali is assumed. This low-temperature purification can in some instances be effective without loss of papermaking properties. With certain types of raw fiber, particularly those of kraft origin, the papermaking properties are actually enhanced.
Solution Viscosity In the characterization of wood cellulose material that is to be used for esterification and allied conversions, the solu-
(Right) PULPWOOD AWAITING CONVERSION
tion viscosity value assumes major importance. For instance, i t is of fundamental importance in the preparation of xanthates and is a determining factor in the selection of a cellulose for nitration, acetylation, and etherification. More recently i t has been recognized as an indicative property in papermaking when strength of paper is sought and also when waterleaf paper is to undergo subsequent parchmentization, vulcanization, or other gelling treatments which give special effects such as wet strength, impermeability, and grease resistance. I n fact, the so-called cuprammonium test is being used regularly by a t least one manufacturer to control the refining steps in the fabrication of practically all types of papermaking pulps. I n the esterification industries the initial solution viscosity of the cellulose determines in a large degree the solution viscosity or the plasticity of the ester. Most investigators accept the thought that depolymerization of the cellulose molecule is responsible for the reduction of viscosity of the resulting cellulose solution. So far as the author knows, all processes of esterification and etherification are attended by depolymerization and a reduction in solution viscosity of the cellulose or the cellulose compound. The extent to which the initial viscosity of the cellulose is reduced when processed is subject t o some control. Important determinants include elements of expediency, favorable processing conditions, incidental effect of acid catalysts, as well as purposeful endeavor to capitalize the increased fluidity of solution of resulting products. The initial solution viscosity of a cellulose determines the highest viscosity level that can be attained in the conversion
product itself. Hence, cellulose with high solution viscosity is a t a premium when viscous products are sought, except
in those cases where that initial advantage is offset by need of more severe conditions of esterification to restore chemical activity that was sacrificed by reason of the pulp treatment. On the other hand, when an end product of low solution viscosity becomes a deliberate objective, then it is often possible to assist in that endeavor by causing proper depolymerization of the base fiber before it is subjected to the esterification sequence. Thus we may produce wood cellulose of very high and also of very low solution riscosity. Numerically, these limits can be illustrated by values of 30 and 0.1 poise, respectively, as measured by the standard A. C. S.procedure. Although it is more difficult to maintain a combination of high alphacellulose and lorn viscosity, it is possible to reach industrially a combination of 94 per cent alpha-cellulose and a solution viscosity of 0.25 poise, in comparison with figures of 90 per cent and 3.0 poises in the case of the wood fiber that is regularly supplied to the rayon industry. Another example will clarify the significance of the above figures. Soda cellulose prepared from the standard sulfite wood fiber and aged in the usual manner at 20" C. for about 72 hours will then show an alpha-cellulose of 90* per cent and a cuprammonium viscosity of about 0.25 poise. Similar relations could be cited in the case of etherification, nitration, and acetylation. The full significance of the solution viscosity of the cellulose deserves further attention. As has been indicated, the test is generally carried out in a cuprammonium solution under 326
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standard conditions. Such a test is easily reproduced and is for the most part satisfactory in reflecting expectation of behavior of the fiber in subsequent treatments. There are, however, a few interesting exceptions which, if not recognized, will cause serious disappointment in the laboratory or in the factory. The procedure by which the cellulose is brought to a given viscosity level, as measured in the cuprammonium solution, can have a profound effect on the ultimate solution viscosity of the conversion product in its own solvent. Thus, two wood celluloses, each having the same viscosity in the cuprammonium solvent, may upon nitration yield nitrates that differ tenfold when the nitrate viscosity is measured in a typical solvent. The difference can be attributed to a secondary effect of sulfuric acid hydrolysis that accompanies the nitration and is explained by a differing sensitivity t o such hydrolysis of the two celluloses in question. The final cellulose nitrate viscosity can be generally predicted if the history of the wood delignification and of the purification steps is known. With a risk of repetition it must be emphasized that the known cuprammonium viscosity of a base cellulose can be used as a criterion of ultimate solution viscosity of the conversion product only when the manufacturing steps for attaining the cellulose viscosity are understood and maintained constant and when the conditions of the esterification steps are for the most part known.
Removal of Objectionable Residues Residues of resin, waxes, and lignin in wood cellulose are for the most part objectionable in their effect on color and color stability of the papers or the esters produced therefrom. Often the color is exaggerated by reason of secondary chemicals used or by the severity of treatment. For example, nitrating acids will increase the depth of color of lignin traces so that solutions made from the cellulose nitrate take on a deep yellow hue. Likewise, in the molding of products that contain cellulose and urea resin, the molding temperature will cause pronounced discoloration if lignin or alcoholsoluble material is present in the wood fiber. Bleached sulfite wood pulps of a decade ago averaged from 0.3 to 0.5 per cent lignin and from 0.3 to 1 per cent extractables. Present-day specially refined wood fibers of both softwood and hardwood origin will contain not more than 0.054.1 per cent lignin and 0.1-0.2 per cent extractables, and compare favorably with the best of purified cotton. Removal of lignin is usually accomplished by a judicious use of chlorine in the refining sequence. The resinous bodies are removed effectively by saponification, by segregation of resin-rich wood cells, as well as by application of soaps and other suitable detergents. In the case of wood cellulose that is prepared from alkaline-cooked raw fiber, the removal of resinous bodies is less difficult and the final extraction of lignin more difficult. The reverse is true of an acid-cooked wood pulp. Some types of wood resin are particularly troublesome in that they become especially tacky when chlorinated and in such form resist saponification as well as removal by detergents. This behavior sometimes necessitates removal of resin before chlorination may be carried out and also limits the extent to which chlorine can be used to remove lignin when both impurities are to be brought to lowest levels. I n such cases the purification steps become more complicated than usual. The reduction of pentosans in wood fiber is one of the more difficult problems that confront the chemist. For many purposes the elimination beyond a 2 or 3 per cent level is not important and less so if the furfural-yielding compounds simulate the related alpha-glucosan in its resistance to the solvent action of mercerizing caustic soda solution. A hydrolyzed
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or an oxidized pentosan group if allowed to remain in the wood cellulose, makes for instability or embrittlement of paper or other products that are made by procedures that do not eliminate the degraded pentosans during the process. A wood cellulose that contains more than one per cent pentosans does not acetylate satisfactorily. Hence, for such purposes its removal is essential. Most effective removal is accomplished by a combination of steps in which the pentosans are first hydrolyzed by treatment that has a minor hydrolytic effect on the cellulose itself, and are then subjected to the solvent action of alkaline solutions. Purified wood fiber that contains less than one per cent pentosans is commercially available. Industrial acceptance is determined mostly by the prevailing price of cotton linters.
Keed for Physical as Well as Chemical Characterizations Many examples could be cited to demonstrate conclusively that we cannot predict the behavior of a cellulose, whether it be of wood or cotton origin, by chemical characterization alone. Suffice it to say, experience has taught the converter to stipulate that purchase specifications not only embody laboratory analysis in the usual sense but also defend the purchaser against those unpredictable physical differences that often occur, by insistence that the fiber be evaluated in a miniature unit process such as is actually carried out in the conversion steps. The internal structure of the individual fiber and its surface density are important factors. In the fabrication of papers this physical structure is definitely related to the manner in which the fiber will swell and hydrate. It is well known that a strong, cold alkaline treatment, especially in the case of previously hydrolyzed fiber, will render the subsequently washed fiber almost incapable of hydration on beating. Sheets formed of such a beaten mass of alkali-treated fiber are soft and weak and exhibit almost complete absence of cementitious bonds of fiber to fiber. Moreover, this resistance to hydration is further magnified if the washed fiber is allowed to undergo the extreme shrinkage and densification that takes place if the washed fiber is first dried. That this substantially complete sacrifice in hydration property cannot be explained by alkaline extraction of cement-forming constituent from the fiber is indicated by the lesser known fact that we can successfully fabricate a sheet from a suspension of unwashed cellulose in the strong alkaline liquor, and that such a sheet can be washed without disintegration. When the washed sheet is then dried, a paper of excellent strength results which possesses any desired degree of hardness, depending upon the detailed conditions of fabrication. All cellulose reactions are of the heterogeneous type. The reaction must take place progressively from the surface to the interior of the fiber unit. Any change in the physical composition of the fiber will alter the penetrability of the reactants and may allow preferential diffusion of the watersoluble volatile ingredients of the reaction mixture. A mercerized wood fiber that has been freed of alkali is highly distended and in that form is readily reacted upon by reagents that esterify in alkaline solutions, and also by the usual chemicals employed in etherifications. If such alkali-containing swollen cellulose is washed, it becomes less swollen, and if the alkali-free cellulose is then dried, it shrinks to dimensions appreciably less than those of the original fiber. The shrunken fiber is considerably denser and has a hard smooth surface. Contrary to casual expectation, it reacts favorably to nitration. The dense fiber wall does not obstruct free penetration of the highly concentrated nitric acid mixture. Nitration proceeds normally, and upon subsequent transfer to the centrifuge the excess acid is released from the
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fiber much more effectively than in the case of a similar product that has been prepared from the corresponding unmercerized wood fiber. On the other hand, a dry mercerized fiber prepared as above will resist the penetration of acetic anhydride and the acetylating catalyst, and demands a special means to reswell the fiber wall in order to facilitate the entry of the acetylating reagent. Otherwise the acetylation is incomplete and unsatisfactory. A dry, alkali-free, mercerized wood fiber swells less rapidly and to a lesser degree when reimmersed in strong alkaline
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solutions than it does the first time the cellulose is mercerized. This fact is sometimes reflected in a less complete xanthation if a purification of wood fiber has been attained by such means. Space does not allow further elaboration of .the behavior of the hardened mercerized wood fiber. Only one additional caution is offered to those who attempt to predict its behavior. Unless they know by what process the wood has been delignified and how the raw fiber has been bleached or refined, prediction becomes nothing more than speculation. PRESENTED before the Division of Cellulose Chemistry a t the 97th Meeting of the Amerioan Chemical Society, Baltimore, Md.
Catalytic Alkylation of Isobutane with Gaseous Olefins J
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F. H. BLUNCK AND D. R. CARMODY Standard Oil Company (Indiana), Whiting, Ind.
present work, which has been H E union of a paraffin At 1000 pounds per square inch pressure conducted in the vicinity of with an olefin to produce and about 400" F., isobutane reacts with 400" F., a pressure of 1000 a paraffin has been degaseous olefins under the influence of pounds was sufficient to obtain scribed by several investigators. double chlorides of aluminum and alkali favorable equilibrium conditions. Ruthruff and Kuentzel (6, 7 ) metals, particularly sodium aluminum Practically all the experiments passed refinery mixtures of conto be described were carried out densable gaseous hydrocarbons chloride and lithium aluminum chloride. a t pressures not far from 1000 over certain double halides of The alkylation reaction is accompanied by pounds. aluminum'with other metals a t a varying but considerable amount of 750 pounds per square inch. At polymerization and by extensive rearrangevarious t e m p e r a t u r e s a b o v e Materials ments, which lead to the production of room temperature the liquid The double compounds of product obtained was greater products not explicable on any simple alkali halides and aluminum in amount than the olefins in theory. The life of the catalyst is short, halides were employed as catathe charge, and they concluded and the alkylation reaction declines more lysts. They are not like orthat combination of olefins rapidly than polymerization. Higher temdinary double s a l t s ; their and paraffins was taking place. properties differ markedly from perature favors alkylation but further Ipatieff and others (4, 6) althose of the constituents. They kylated isoparaffins with decreases catalyst life. The potassium exhibit none of the volatility olefins a t room temperature compound is not very active. of the aluminum halides, and and above, using boron trithe melting points are generally fluoride, nickel, and water, and in the vicinity of 400" F. or below. As-used in these experialkylated n-hexane and isobutane with ethylene using alumiments they were usually liquids and were suspended on num chloride a t room temperature. Several investigators pumice, which was shown by a blank experiment to have no ( I , $ ) have shown that strong sulfuric acid will produce alkylacatalytic action under the prevailing conditions. tion at low temperatures and pressures. Frey and Hepp The compounds were prepared by heating equal molar (3) effected the paraffin-olefin junction noncatalytically a t quantities of anhydrous aluminum chloride and the halide, high temperature and pressure. The work here described previously mixed, to 500" F. in a glass tube placed inside a utilizes catalysts and conditions generally similar to those sealed steel bomb. A slight excess of aluminum chloride was employed by Ruthruff and Kuentzel, but pure hydrocarbons added t o replace any loss of aluminum chloride due to subwere used as a charge. The paraffin in all cases was isobulimation in the early stages of the heating period, After the tane, and the olefins employed were ethylene, propylene, and temperature of 500" F. had been obtained, the bomb was alisobutylene. lowed t o cool slowly. The complex salt was removed from The alkylation reaction is favored by low temperatures and the bomb and remelted in the glass tube to sublime off any high pressures. At room temperature the equilibrium is in unreacted aluminum chloride. In most of the experiments favor of the alkylated product even a t atmospheric pressure. the catalyst was prepared by pouring the molten double salt At 900' F. pressures of several thousand pounds per square onto an equal weight of oven-dry 8-mesh pumice. The inch are required to obtain a substantial conversion. In the
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