THE PRODUCTION OF CHEMIOAL CELLULOSE FROM WOOD' BERWYN B. THOMAS Olympic Research Division, Rayonier Incorporated, Shelton, Washington
IN1955 the total world production of
rayon and cellulose acetate fibers topped five billion pounds for the first time. Over half a billion pounds of cellophane film were also made. These products and several smaller ones of similar nature were all made from cellulose, the purified fibrous part of wood. That paper, ljoard for cartons, and many simiiar products are made from wood fibers is a fact familiar to everyone, and many are acquainted with the general methods of making wood pulp from wood. The processes for making the particular kinds of celluloseused in the manufacture of rayon, cellophane, acetate and other derivatives are very similar to those required for making cellulose for paper, but each of the various derivatives requires the cellulose to have certain specific characteristics. As a group these products are made by processes which usually require placing the cellulose in solution by some chemical means. For this reason they are called chemical celluloses or dissolving pulps. In general the paper manufacturer is interested in wood fibers as fiber for their inherent strength and form and in their chemistry only as that affectsthe physical properties. The chemical cellulose user, however, is directly concerned with the chemical nature of the fiber, and in its physical form as that affects the reactivity; often he is entirely dependent on the presence or absence of certain trace materials. True cellulose consists of large numbers of glucose sugar molecules joined together into long chains. From these chains are built larger fibrils and from the fibrils the fibers. But the wood contains also other forms of sugars than the true cellulose itself and there is also a wide variation in the length of the individual chains of the polymer. Thus, the entire sugar fraction, or holocellulose of the wood (amounting to about 60% of its original dry weight), may be described as something of a spectrum (Fig. 1). The number of sugar units in an individual chain is called its degree d polymerization, or "DP," and the various components of the holocellulose can be ranked against approximate DP numbers. The D P is estimated by various analytical methods, and it and related factors are important characteristics of any cellulose sample. Another common classification of the molecules is based on their solubility in aqueous alkalies. If the cellulose is treated with NaOH solution of about 18% 'Presented before the Divisions of Chemical Education and Cellulose Chemistry rut the 133rd Meeting of the Amerioan Chemical Society, San Francisco, April, 1958. Contribution No. 35 from the Olympic Research Division of Rilvanier Incorporated.
VOLUME 35, NO. 10, OCTOBER, 1958
a t 20°C., the cellulose structure will swell, and much of the material which is of shorter, chain length will dissolve. The residue from this treatment is called "alpha cellulose" and the percentage of this component is an important cellulose quality. When the removed solution of shorter chain materials, which are called hemicelluloses, is acidified, part will precipitate. This is called "beta cellulose." The remainder of the substance, soluble in water, is called "gamma cellulose." These divisions are arbitrary but they are founded on differences significant to the industry. The beta and gamma celluloses contain besides short glucose chains many which are composed of other sugars-mannose or xylose or of these sugars mixed with gluco~e,and finally some with sugar acid groups at.tached (17). Other polysaccharide molecules are so soluble that they can often be removed from the wood with simple water treatment. Among these are arabogalactaus, pectins, and in some cases starch. Because the methods of purification for celluloL qe are imperfect, some portions of these dierent products will remain in the pulps. Paper pulps, as shown in Figure 1; often contain greater portions of suchmaterials than dissolving pulps since they do provide certain useful characteristics for the manufacture of paper but are objectionable for the manufacture of chemical products. Also in many cases dissolving pulps will be so prepared that their average DP is somewhat higher than that of paper pulps. There are many kinds of chemical cellulose. Each derivative process requires different characteristics in its raw material and is differently affected by variations of the above types which occur. The pulping, bleaching, and purifying st,epsare therefore varied and closely controlled t o turn out for each class of finished product a uniform starting material which can be used economiWOOD HOLOCELLULOSE DP SPECTRUM
Fig"-
1. Dhtribvtion of Pol-charide. Chain hn*h
in Wood Holoc.llulolu
by
cally. Thus pulps are made for various chemical uses which have alpha contents of 88y0 to 97% and DP averages of 500 to 1700 units. The common requirement for every grade is constant uniformity. Since the requirements for cellulose for the various derivative processes can be stated only against the background of the processes themselves, these must he briefly outlined. VISCOSE PROCESS
The largest proportion of chemical cellulose goes into products made by the viscose process and this is also the most complex of the derivative industries. CARBON DISUIDE
18% NmOH
I- & - m - ~ ! - J J - ~ CELLULOSE SHEETS
STEEPING AlESS
Figvro 2.
SHREWER AGING
XANTHATal
Brief Block Diemam of ths Viscose Proaess
The products include textile rayons of both continuous filament and staple types, rayon tire oord, cellophane and several minor products. A l t h ~ g hmade by basically similar processes, these products and their cellulose requirements vary widely in character. The viscose process is shown briefly in Figure 2. The cellulose, commonly in a blotter-like sheet form about inch thick, is first steeped in a sodium hydroxide solution of 18% t o 19%. After a suitable period the excess caustic is drained and pressed out, leaving a swollen product called alkali cellulose. The alkali cellulose is ground up in a shredder to a fine fluff, to make the mixture uniform and thoroughly expose it to the air. The crumb is then stored in closed containers for up to two days to allow the oxygen of the enclosed air to react with the cellulose. Carbon disulfide is next added, forming the bright orange cellulose xanthate. This compound is soluble in dilute caustic, giving a honey-like solution called viscose. The viscose is filtered several times in the course of a day or two of ripening, and is then ready for spinning. It is expelled through very fine holes in a spinneret into a bath containing mainly sulfuric acid, sodium and zinc sulfates, and glucose in various amounts. The cellulose is thus regenerated in its original chemical form hut in a new physical shape as a continuous cellulosic filament. The spun yarn is removed from the bath, washed, bleached, and purified before use in textiles or other products. Cellophane is made by extruding the viscose through a slot into a similar bath of acid. The film is washed, bleached, and dried continuously. Several general discussions and reviews of the requirements for, and behavior of, cellulose in the viscose process have been published in recent years (1-8)
I n the first stage, the steeping, the caustic solution makes the cellulose swell greatly, and much of the hemicellulose is diissolved and then removed during the pressing. Hemicelluloses are undesirable in the finished yarn because their short chain lengths and diierent structures would give poor yarn strength; the ability t o dispose of them during the process greatly reduces the requirementsfor purity in the starting cellulose. For the proper operation of this extraction, the physical form of the pulp sheet has the highest significance (2,s). For the older conventional process, the cellulose is steeped in sheet form. The sheets must be uniform in structure, absorptivity, moisture, and resin content, so that as the caustic solution rises in the press they will be evenly wetted, the cellulose will swell uniformly and all parts will be equally reacted with sodium hydroxide. The sheets must not float, or slump down when soaked, because any such motion will leave them out of line with each other and will result in unevenly pressed areas. Finally the sheets must have enough strength to withstand handling even when wet. Recent developments in the industry include a slurry method for the steeping step. Requirements for this process include easy dispersal of the separate fihers and rapid drainage of the caustic solution through them, in addition to several of the above qualities. All of these requirements must be met by correct operation of the pulp sheeting and drying machine. The sheet should be formed of well mixed fibers, d e watered smoothly, properly pressed and dried evenly without excessively high surface temperatures ( 2 ) . I n shredding, the pulp will he intimately mixed, but if it has not all been uniformly treated, this step cannot overcome extreme variations, and particles inactive to the subsequent reaction steps will remain. Very long cellulose chains are no more desired than the short ones, as the viscose will be too thick to spin a t practical concentrations. The long chains are therefore degraded during the aging period by alkaline oxidation. This reaction has the property of reducing the DP sharply but without forming much short chain material. This degradation and the removal of hemicellulose in steeping results in a DP distribution more uniform than could be obtained by any practical means during the pulp manufacture. Pulps of very diierent initial characters can be used to make similar products in this way (4). Metallic contaminants, particularly manganese and cobalt, have a catalytic effect on the alkaline aging reaction, and only a few parts per million will increase the rate of DP reductiou greatly. It is not necessary that these elements be entirely absent, but they must be uniform. They may, in fact, be added if rapid controlled aging is desired. Since these elements sometimes occur in water supplies, water selection and treatment are important. I n the xanthation step, the aged alkali cellulose fihers must be uniformly swollen and open in structure, so that the carbon disulfide can diffuse readily to all the internal parts. Any portions not fully xanthated will not dissolve readily into viscose, and thus will remain as more or less gelatinous fragments. Such particles come from fibers incompletely treated with alkali, insufficiently degraded, or unreactive to carbon disulfide. The outer layer of the cellulose fiber, which conJOURNAL O F CHEMICAL EDUCATION
tains much hemicellulose and is less soluble, often causes gel particles if it is not properly hrokeu up in the pulp purification. Other causes include excessive resin or silica. Since such fiber fragmeuts would clog the spinneret,~ or make weak spots in the yarns, they must he filtered out of the viscose (5). And, since they also tend to clog the filters, the quantity of viscose which passes through a standard filter before it clogs is a customary measure of the over-all reactivity of the cellulose. The viscose is pumped through a staudard filter, and the rate of clogging is measured by weight and time recording. After the rayou yarn has been spuu and dried, the final major evaluation of the cellulose is the strength and elasticity of the yarn produced. These are nieasured by appropriate tests (Fig. 3). High strength and long life come from the proper balance of all the st,eps of the process coupled with high pulp quality. Thie is characterized today primarily by high alpha cellulose, low gamma and uniformly high iuitial DP range. Rayon tire cord is a specialiied product first developed about 20 years ago. I n the modern process, developed thru experimental studies over many years, thread emerging from the spinneret is made to coagulate slowly and is grea1,ly stretched while still plastic. This causes the cellulose molecules to line up more nearly in the direction of the filament, as they were in the original wood fiber. Then when the precipitation is completed and numerous secondary bonds form between the chaius, the fiher regains a larger proportion of the crystalline structure possessed by the original cellulose, and much greater strength. And since it can stretch less, loading and flexing the cord as in a tire produces less internal heating. To hring ahout these special conditions, tire cord requires a pulp of higher purity and greater uniformity of D P than textile rayon (6). Sulfite ~voodcellulose was used by the developers of the viscose process in England over 60 years ago. Naturally the process has been improved since t,hen,and the pulping and purifying methods have changed to meet the requirements. Pulps to meet higher requirements for tire cord are made by slightly different cooking conditions, more thorough refining vith hot alkali, and care to prevent degradation of %hecellulose. In recent years further progress has been made in production of super-strong cords. For these, cotton linters r e r e at first largely used to meet the need for purity and high strength, hut by revisiou of the purification methods wood celluloses have been developed to fill even these requirements. And, as the new techniques developed for the super type cord are applied to regular rayon production, we can expect the appearance of products with much improved strength, endurance, and iaunderability. The early choice of sulfite pulp was a necessity. Kraft pulps, eveu afber bleaching methods were developed, were for a long time un~at~isfactory, as the alkaline cook seemed to make the pentosan heinicelluloses of the primary fiher mall resistant to removal by later refining. These then 'aused poor filtering viscose and low yarn strength. It has recently appeared from electron-microscopic work that pulps showing proper reactivity all have a characteristic structure obtained only when a hot acid treatment is the first VOLUME 35, NO. 10, OCTOBER, 1958
step of cooking (4). The kraft process can only be used if the main cook is preceded by a brief acidic prehydrolysis. With this addition, to decompose the pentosans, the kraft method makes an excellent viscose type pulp for the manufacture of high quality end products. Cellulose for cellophane manufacture is made at lower D P t h m for rayon (7, 8). The viscose is pre-
Figure 3.
Strength Testing of Rayon Yarn.. Tire Cord Yarn Shown on Cones. Textile Yam i n Skeins
pared with a minimum of chemicals, and to facilitate economical processing under these conditions a particularly reactive cellulose is needed. Both the purification and the sheet preparation are involved in filling these requirements. Newer methods of viscose preparation are being investigated in various places; naturally such methods will put different requirements on the cellulose. But as in the super-strength rayon process, wood celluloses tailored to fit the needs of such new processes rill be available when the demand arises. CUPRAMMONIUM PROCESS
The cuprarnrnonium process is another quite different method for making rayon. Cellulose is directly soluble without formation of a derivative in a mixture of copper sulfate, ammonia, and sodium hydroxide, and is regenerated by spinning into water followed by dilute acid. The process is simpler than viscose, but it is more expensive and is now used only for a small amount of high quality textile yam. Cellulose for the cuprammonium process must be well purified, as there is no extraction of hemicelluloses. Satisfactory wood cellulose is available and generally used as the raw material. CELLULOSE ETHERS
The cellulose ethers include a group of products of varying nature and uses. The chief ones are carhoxymethyl cellulose, used as a thickener for food products and a dispersing agent in detergents, and methyl, ethyl, and hydroxyethyl cellulose, used in adhesives, coatings, and medicinal products among other things. The ethers are made in high and low D P ranges, and in different degrees of substitution, which gives some
products soluble in water, some soluble in alkali solutions, and some soluble in organic solvents. Cellulose needs for ether manufacture are as varied as the products, and while for some types a papergrade pulp can be used, others may require special pulps of high purity and DP (9). CELLULOSE ACETATE PROCESS
Cellulose acetate is, for most of its uses, one of the highest quality cellulose derivative products. It requires very pure cellulose, and was made entirely from cotton or linters until 20 years ago. As satisfactory wood pulps have been developed they have replaced the linters to the extent of about 75%. The best modern grades of wood cellulose are practically interchangeable with linters for manufacture of most acetate products. There are several reasons for this high requirement (1&14). First, the acetate is made by a process in which practically all of the starting cellulose goes into the product (Fiq. 4). The pulp is first treated with acetic acid. Then acetic anhydride is added, later more anhydride and a little sulfuric acid as catalyst, ACETIC ANHYDRIDE ACETIC ACID
ACETIC ACID WATER
SULFURIC.AC1D
CELLULOSE
ACETATE Figure 4.
REACTOR
.
HYDROLIZER
WATFR
PRECIPITATOR
)
YARN SPINNING UCETONE SOLUTION)
Brief Block Diagram of the Ccllc1.311 A c e t t t t PPPPP
and the entire mixture becomes a heavy paste. The completed compound is partly hydrolyzed, and then precipitated, by adding water, and subsequent steps are also carried out in dilute acids. Thus except for small amounts soluble in the dilute acids during hydrolysis, precipitation, and washing, there is no opportunity in this process for removal of short chain or nonglucose materials. The completed acetate may be dissolved in acetone and spun into hot air to make fibers, or formed by heating and pressing into molded objects. Secondly, there is considerable degradation in the course of the reaction. Therefore the cellulose must have a very high and rather uniform initial degree of polymerization to give good strength in the finished products. Thirdly, several special problems arise from traces of impurities left in the cellulose. For instance, any remaining lignin, extractives, or other color sources will naturally affect the color of the finished acetate. Pentosans and other nonglucose hemicelluloses form acetates which are less soluble than true cellulose acetate in organic solvent solutions. These less soluble portions create dullness and haze in the otherwise clear acetate yarns or moldings. In some cases minerals present also contribute to haze. Another effect apparently due to residual hemicel-
lulose is a tendency for the viscosity of acetate solutions to be higher than they properly should be. This makes difficulties in handling the solution. The viscosity effectseems to be most closely related to the amount of mannan hemicellulose retained by the cellulose (11, 18). Nearly a11 photographic films are now made from cellulose acetate or butyrate, and the film industry has a specially critical requirement. The cellulose must be free from any impurity which would be photographically active and cause spots in the emulsions mounted on the film. Thus cellulose for acetate filaments, films, and clear moldmg compounds must be free from mineral contaminants, lignin, and hemicelluloses, especially those containing mannose, and it must have a high initial DP and high brightness. To produce wood cellulose of this quality requires careful cooking of the unbleached pulp, and specially adjusted conditions in the alkaline refining step ( I S , 14). Chlorine dioxide is often used for the later bleachings to give very high brightness while retaining a high DP. With such procedures the total hemicelluloses can be reduced to a very few per cent. There are some exceptions to these requirements. Obviously, if the product is to be a rug yam, or a colored utility molding, haze and trace color are not significant. And, for thermoplastic molding applications, viscosity effects are much less important. For these purposes more economical pulp grades, such as those made for rayon tire cord, can be used. However, even the purest cellulose can be made less reactive toward the acetate process, with resulting haze and filtration difficulty (lo),if it is not properly dried. Low drying temperatures are advisable, and sometimes surfactants are added to overcome this effect. The sheets must also be kept soft to permit easy and rapid dispersal in the reaction mixture. CELLULOSE NITRATE PROCESS
Cellulose nitrate is, like the acetate, an ester of an acid with the cellulose. Unlike the acetate, nitrate is normally produced and often used in the original fibrous form, without being put into solution a t all. Early experiments on nitration of wood cellulose were not very successful, and the product was made from cotton for many years. The trouble was found to be with the pentosans in wood pulp. If nitrating acids of high water content were used, the pentosans were hydrolyzed but not nitrated, and the residues were insoluble in nitrate solvents. With concentrated acids the pentosans were nitrated to soluble products, but these were unstable and gave brittle films (16). During the Second World War the demand for explosive nitrate forced development of satisfactory wood cellulose grades. By using a hot alkaline refining stage after the chlorination, the pentosan content wae reduced to a tolerable level. The pulp sheets must also be uniform, absorptive to the acids, and not too dense. Sulfite pulps have been used in the past, but some h a f t pulps are now also acceptable. CHEMICAL CELLULOSES FOR PAPER
Pulps having the high purity of chemical cellulose are also needed for some uses as fiber. Chief among these are photographic papers and plastic filling or impregnating pulps. JOURNALO F CHEMICAL EDUCATION
Photographic papers must have high strength, both wet and dry, dimensional stability, permanence, and especially freedom from those impurities which damage the photographic emulsion. Well purified chemical cellulose meets these needs suitably. Plastic filling pulps are used in pulverized form in such products as melamine dishware and refrigerator lining panels. They must he very bright and above all free from a tendency to turn yellow on heating. Since lignin and hemicelluloses contribute to such darkening, highly purified and thoroughly washed cellulose is needed. For other purposes such as electrical insulators, a paper is made and impregnated with resin. Here strength, absorbency, and freedom from ionic materials are important. And other industries have yet other combinations of requirements. CHEMICAL CELLULOSE FROM COTTON
Cotton as a cellulose source has been mentioned se~eraltimes in this discussion. As Figure 5 show
Figure 5.
Use of Wood Cellulose end Cotton Linters for Rayon and Cellulose Acetate
(16) for the past 25 years, linters have always furuished a part of the raw material. Since 1932, however, the proportion of the supply coming from linters has dropped from near 50% to about 15%. This shift has had three causes: quality, price, and availability. While cotton linters are easily purified to 99% alpha cellulose, and have high Dl?, these are not the most desirable characteristics for all uses. Wood cellulose is superior for some purposes, such as cellophane, and is a t the least competitive on quality whenever it can be converted by an economical process to a satisfactory finished product. Development of the wood celluloses has brought improved materials to make desired products economically and practically. Prices and availability of linters are highly variable. The linter is a secondary product, dependent first on the production of fiber cotton and next on cottonseed milling for oil. Weather, the cotton and food markets, and the complex politics of government farm controls all therefore affect linters supply and price. Wood pulp prices, on the other hand, have changed little and only gradually through several years, and are normally lower than linters for most pulp grades. Supplies of wood pulps, being independent of other materials, have grown as needed to meet the demand of this large industry. VOLUME 35, NO. 10, OCTOBER, 1958
CONCLUSION
It is evident from the foregoing requirements for various derivatives that a large number of different grades of chemical cellulose are needed. A summary of alpha and D P requirements of the various processes described is given in the table. The major manufactnrer of chemical celluloses in North America has vell over 20 distinct cellulose grades, each made by a specified procedure to standard results. Other producers in the industry also make numerous different pulp grades.
Summary of Cellulose Requirements
for Derivative
Processes A lnhn
np
Viscose process product,s Cellophane Textile rayon Staple fiber rayon Tire cord rayon Super high tenacity rayon Cuprammoniumrayon Celluloseethers Cellulose acetate Clear and textile Colored and bulk Cellulose nitrate
Maintenance of uniform processes and results requires close control. From the composition of the raw water and selection of the logs to the weight, moisture, and density of the finished package of pulp, every step must be specified, and frequent tests made to hold the specification. As each jumbo roll of 3to 10 tons comes off the pulp drying machine, it is sampled, then stored until the sample is analyzed to ascertain that the pulp meets specifications (Figure 6). Acceptable rolls are then chmen and cut in groups (2, 8) to blend further their minor differences, so that analytical uniformity is maintained in each lot of pulp shipped to customers. I t must be evident that the industries based on the processes described here are highly technical in all their phases. All of them, from cellulose manufacturer to the user of completed rayon or acetate products,
Figure 6. Chemical Cellulose, in ROIIEof Dry Sheet, in Storage Pending Analysis .nd Selection fop cuttins
497
carry on extensive research to improve the processes and products. They therefore offer excellent opportunities to students interested in a varied and dynamic field of study. LITERATURE CITED (1) RICHTER, G. A,, Tappi, 38, 129-50 (1955). (2) WALKER,F., Paper Trade J., 139, NO. 17, 22-26 (April 25, 1955). (3) MORE,J. W., Paper presented a t Third Symposium on Viscose Technical Questions, Swedish Forest Products Research Laboboratory, 1956. AND D. TEVES, Tappi, 35, (4) HAAS,H.,E. BATTENRERCI, 116-24 (19521. M., ~ a p p i38,507-12 , (1955). (5) GOLREN, ~ ,D., I. K. MILLER,AND W. D. WHITE,Tappi, (6) B A C H L OD. 38,503-07 (1955).
R. L., Ind. Eng. C k (8) MITCHELL, (9) ANDERSON. A. W., AND R. W. I 53 (1956j. A,, Tappi, 40,429-441 (1957). (10) RICHTER,GEORGE H. W., AND B. B. WHITE,Tappi, 37, 225-32 (11) STEINMANN, 119541. ~-~--,.
(12) WATSON, J. K.,
- -- . .
AND
D. R. HENDERSON, Tappi, 40, 68690
11467) \ ,
(13) PROFFITT,3. R., H. M. GRAHAM, E. R. PURCHASE, AND R. C. BLUME,Tappi, 37, 2832 (1954). (14) BoRarN, K., Tappi,36, 284-88 (1953). D.. "Cellulose Nitrate." Interscience Pub(15) . . MILES. FRANK lish&s, Inc., 1955, pp. 5-7. (16) Teztile Organon, 28,39 (1957). J. K., E. V. PARTLOW, AND N. S. THOMPSON, (17) HAMILTON, unpublished work in press.
JOURNAL OF CHEMICAL EDUCATION
