Cellulose, Raw Material for
Cellophane R. L. iMITCHELL Rayonier Inc., Shelton, Wash.
Figure 1. Raw materia1 “in the raw”
c
ELLULOSE constitutes some 70y0 of the material in a sheet of cellophane. The cellulosic portion comprises the structural framework of the base sheet and of the nitrocellulose in the moistureproof coating as well, where such coating composition is used. Other constituents include glycerol and moisture, also !*ax, xylan, mannan, and numerous other substances in small amount. The installed capacity for cellophane manufacture in this country is equivalent to a cellulose requirement in 1955 of about 170,000 tons of chemical cellulose. Although many forms and types of vegetable matter can be and have been used to supply a suitable cellulose, the most economical source today is wood (Figures 1 and 2 ) . Using critical processes involving complicated equipment and high chemical consumption, cellulose usable for the manufacture of viscose is separated from lignin and other constituents of wood in the form of tiny white fibers only a few millimeters in length (Figure 3). These fibers are formed into a web on a paper machine and wound up as jumbo rolls (Figure 4), which are later sorted and cut into smaller rolls or sheets as required by the cellophane manufacturer. This fibrous sheeted cellulose is taken by the cellophane manufacturer, treated with still more chemicals to effect mercerization, xanthation, and dispersion in alkali to give viscose, then extruded, precipitated, and regenerated in acid to yield a shaped coherent film. Through the years numerous advances have been made in the viscose process, resulting in improved quality and reduced cost of the finished film ( 1 ) . These changes have placed heavy demands on wood cellulose, not only as to increase in quantity but improvement in dissolving properties as well ( 9 ) . The special requirements of the present-day cellophane process need as basic raw material not simply a wood pulp but rather a “chemical cellulose” possessing a number of highly specialized characteristics. Cellulose best suited for film manufacture should have controlled uniformity and purity consistent with lowest possible cost, be capable of giving viscose solutions of good filterability
under unfavorable conditions of low chemical usage, and subsequently yield high quality films of good strength, color, and clarity. Important factors are uniformity, suitable sheet structure, low aging requirement, reactivity, purity, and low cost. QUALITIES NEEDED IN CELLOPHANE PULP
Uniformity in Processing Behavior. I n the production of a viscose-type pulp, cooking and refining operations are carefully controlled by means of such measurements as cuene (cupriethylenediamine) viscosity, TAPPI K number, and unbleached alpha. In addition, the finished product is analyzed for over 20 different items, including moisture, alpha, beta, gamma, degree of polymerization, xylan, mannan, carboxyl, and trace metals. To improve uniformity further, the jumbo rolls of cellulose are selectively
Figure 2. 2320
Load of western hemlock logs
November 1955
Figure 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cellulose wood fibers
Figure 5. Cellulose wood fibers in unswollen state (X 400)
blended during cutting to match out small variations in many of these factors. Because uniformity in analytical characteristics of cellulose does not necessarily ensure uniformity in steeping performance, or in viscose viscosity and filterability, viscose preparations are made daily and evaluated prior to shipment of the pulp. Such evaluation and correlation of analytical properties and performance characteristics are costly, but, in the light of presentday viscose processing practice, are considered essential. Sheet Structure and Fiber Characteristics Suited for Special Types of Steeping. Sheet and fiber properties should be properly balanced so as to give good performance in the shortcycle, high-capacity operations now being wed in cellophane production. For conventional steeping, the sheet should be stif€ and of density sufficient to permit good penetration of steeping liquor between individual sheets in the book; it should have high absorbancy for caustic soda, but be free draining and easy to press. For slurry steeping, the pulp, in either sheet or roll form, should be of low density for easy disintegration. The fibers should give a low elurry viscosity at a normal consistency or conversely permit use of higher consistency without undue flocculation or power requirement. The fibers should also be free draining and easy to press, giving low fiber loss and permitting high capacity operation. Low Aging Requirement. Because of low chemical usage and high cellulose content of the usual cellophane-type viscose, the cellulose raw material should have a low aging requirement. This does not necessarily mean low degree of polymerization, as the use of added oxidation-acceleration catalysts will now permit the preparation of economical viscose compositions from cellulose of almost any level of degree of polymerization. Reactivity Leading to Good Viscose Filterability. Fiberx should be uniformly reactive, so as to produce good filtering viscose under unfavorable conditions of low degree of xanthate substitution, high celhlose content, and low alkalinity in viscose. Use in the cellulose of certain types of added surface active materials often is advantageous in promoting reactivity in the various stages of viscose preparation and is beneficial in subsequent casting operations. Purity Sufficient to Yield Film of Good Strength and Appearance. I n the manufacture of cellophane, emphasis is placed on the need to produce the viscose solution and resultant, film at minimum cast. In the manufacture of tire cord, emphasis is placed on attaining maximum strength in the finished product,
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Figure 6. Highly swollen softwood fibers ( X 400)
Just as the end use requirements for a sneet of cellophane differ from tire cord, so do the specilk requirements in raw materials differ. For tire cord of high strength a premium is placed on cellulose purity and uniformity of chain length, because they contribute so greatly to the degree of crystallinity and uniaxial orientation that may subsequently be attained by heavy stretching. For cellophane, however, where random orientation is preferred and strength requirements are in two or three dimensions rather than concentrated in one dimension, purity of raw material may be sacrificed to gain economy in processing costs.
Figure 4.
Jumbo rolls of cellulose in the roll pit
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 7. Highly swollen hardwood fibers (X 400)
Figure 8. Swollen xanthate fiber after steeping in 10% sodium hydroxide (X400)
Sufficient purity is needed, however, to ensure a high yield of film having good strength and appearance. To give high film yield, the cellulosic pulp should be low in gamma content, not necessarily high in alpha. Use of hemi-pressings in the mixer charge helps t o improve the yield value without seriously impairing film quality. Factors other than the cellulose raw material have much to do with film strength, clarity, and color-for example, operating conditions such as level of acid strength, casting speed, and viscose salt index. However, high degree of polymerization level and good degree of polymerization uniformity of the cellulose contribute to film strength both in the gel state and in the finished product. A pulp of high brightness, free from lignin residues and other colored compounds, contributes to the attainment of good film color after only mild bleaching. Low Cost. It is obvious that all the above qualities cannot be coupled with low cost. Of necessity, the best cellulose for any given product represents some compromise among factors of cost, uniformity, purity, reactivity, and yield. It is believed furthermore that a cellophane-type pulp should represent a different compromise than a tire cord-type pulp. The difficulty experienced in trying to attain a combination of all the above qualities in one sample of cellulose may not be fully appreciated by superficial examination of a sheet of chemical wood cellulose, or, for that matter, any other kind of cellulose. Only when the individual cellulose fibers are carefully examined with the microscope, under conditions which reveal their complex nature and structural nonuniformity, can one even begin to realize why he has been so completely betrayed and obtained zi viscose dispersion having a plugging value of 50 rather than the 60,000 that had been hoped for; or why conditions needed to open the fiber and give uniform accessibility for reacting chemicale may be so drastic that strength of the residual cellulose chains is impaired.
Vol. 47, No. 11
Figure 10. Swollen xanthate fiber after steeping in 22% sodium hydroxide ( X 400)
reveal the complex structure of the primary walI and of the inner layers of the fiber. Swelling in this and subsequent photographs was obtained by treating incompletely xanthated fibers with water. Figure 7 shows swollen hardwood fibers (black gum) which, in contrast t o softwood fibers, are short and spindle-shaped. Figure 8 shows a swollen xanthate fiber typical of the condition arising when steeping liquor is below optimum in sodium hydroxide content-e.g., 10% sodium hydroxide. Figure 9 shows an incompletely dispersed xanthate fiber reeidue (typical “gel”), a few of which still remain even when steeping conditions are optimum and sodium hydroxide concentration of the steeping liquor is normal a t about 18%. With the sodium hydroxide concentration a t too high a level in steeping liquor-e.g., 22% (Figure 10)-poor solution also results. The shape of the residue, however, is entirely different from that resulting from a low concentration of sodium hydroxide in steeping liquor (Figure 8). Figure 11 shows the influence of steeping liquor concentration on viscose filterability. Optimum sodium hydroxide in steeping liquor is 18 to 19% for conventional sheet steeping and 16 t o 17% for slurry steeping at, say, 30” C. At higher temperatures in slurry, 40” to 50” C., a correspondingly higher sodium hydroxide concentration of 18 t o 19% should be used. Figure 12 shows the effect of added metals in reducing aging
NATURE OF CELLULOSE
A set of fiber photomicrographs has been assembled to illustrate the complex nature of the cellulosic raw material with which we are dealing. Viscose process curves have also been selected to indicate means sometimes useful in judging optimum conditione under which the cellulose should be made to react. Cellulose wood fibers in the unswollen state are shown in Figure 5. Highly ewollen softwood fibers (hemlock), shown in Figure 6,
Figure 9. Typical “gel” residue remaining after steeping in optimum 18% sodium hydroxide ( X 800)
November 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
7, SODIUM
HYDROXIDE
IN
STEEPING
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LIQUOR
Figure 11. Influence of steeping liquor concentration on viscose filterability I
0
20
0 HOURS
4GlNG
30 4T
40
J
50
309 C
Figure 12. Effect of manganese and cobalt in reducing aging requirement
0'
I
I
6
14
1 22
% SODIUM HYDROXIDE
Figure 13.
I
I
30
IN STEEPING LIQUOR
Effect of mercerization o n distribution of sulfur in xanthation
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92 94 96 ALPHA-CELLUUJSE CONTENT OF CELLULOSE RAW MATERIAL
Figure 15. Relative effect of pulp purity on quality of film and tire cord
be seen that higher cellulose and particularly lower alkalinity in the viscose, although economical, adversely affect filterability. Figure 15 shows the relative effect of pulp purity on quality of film and cord. Because of differences in physical form, film is much less sensitive than cord to changes in pulp purity as measured by a-cellulose content. SUMMARY VISCOSE
COMPOSITION
Figure 14. Effect of viscose composition on viscose filterability Shaded portions indicate correction to equivalent 7.5% cellulose content
requirement. The acceleration effect is due to the fact that the metals promote the consumption of oxygen. Figure 13 shows the distribution of sulfur in xanthation. Parallelism with the earlier steeping curve, Figure 11, shows that inadequate xanthation following poor mercerization may account for the low filterability of the resultant viscose. The effect of viscose composition on viscose filterability is shown in Figure 14. Shaded portions of bars indicate correction to equivalent cellulose content in the viscose solution. It can
Wood cellulose has been proved by use t o be a relatively lowcost raw material suitable for the production of cellophane films. However, in pulp purification sequences as well as in viscose solution processes, this cellulosic raw material must be recognized as not a simple chemical compound but rather a structurally complex native polymer. T o be able, in the future t o transform a section of a tree more efficiently into a sheet of dexible transparent film will require much additional cooperative thinking and planning by both the cellulose producer and the cellulose consumer. LITERATURE CITED
(1) Inskeep, G. C., and Van Horn, P., IND. ENG.CHEY.,44, 2511 (1952). (2) Samuelson, o., Svensk.Papperstidn., 29, 866 (1953). RECEIVEDfor review dpril 14, 1955. ACCEPTED August 1, 1955. Division of Cellulose Chemistry, Symposium on Regenerated Cellulose Film, 125th Meeting, ACS, Kansas City, Mo., March-April 1954. Contribution 13 from the Research Division of Rayonier Ino.
