Hydration and Beating of Cellulose Pulps W. BOYDCAMPBELL Forest Products Laboratories of Canada, Montreal, Canada
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ROM the time of the invention of paper about the year 105 the process of beating the pulp as a preliminary to forming the sheet has been recognized as of prime importance. Early paper makers did not concern themselves greatly with theories; it was enough to know the process necessary to produce the various combinations of long and short, slow and free stocks according to the paper to be made. When chemists began to enter the mills, their attention was naturally drawn to this important operation. The analogy of the imbibition and retention of water by jellies to that of the increased water-holding power of pulp after beating was obvious, and, as the taking up of water by jellies was termed “hydration,” it was natural to apply the same term to pulp beaten in such manner that its water-holding power was increased. The beating process produces two distinct effects: (1) the shortening of fibers necessary for ease of uniform distribution, (2) a change in the character of the stock resulting in increased density and tensile strength of the paper. Either of these effects may, within limits, be caused to predominate by proper manipulation of the beater. The first is obviously a mechanical one and need not be considered here. During beating, the second effect is shown by a progressive change in feel and appearance as the stock becomes more and more soft and slimy. Production of this effect is favored by conditions which causc the fibers to be rubbed or pounded rather than cut. Pulp beaten to emphasize this effect is said to be beaten “wet” or “slow” because it parts with water very slowly on the wire of the paper machine; in other terms, it is “hydrated.” The question is, what phenomena does the term “hydration” indicate when used in the paper-making sense? Since the curve of vapor pressure against water content of either beaten or unbeaten pulp is without sharp breaks, any hydration is obviously not of the type found in crystalline hydrates such as CaS042Hz0. This does not, however, rule out the possibility that hydration may exist in the sense of there being a solid solution of water in cellulose or of an adsorption layer of water on cellulose surfaces. DEDUCTIONS FROM VAPORPRESSURE CURVES Using the term hydration in the broad sense of meaning the binding of water in any way, it is evident that of two samples of pulp, equal in water content and otherwise similar except that one is hydrated to a greater extent than the other, the more highly hydrated one must have the lower vapor pressure. Therefore, if we determine curves of vapor pressure against water content of a pulp before and after beating and find that the two are identical, we are justified in the conclusion that beating makes no change in the degree of any hydration that may exist. Such determinations have been made and independently verified (8,Q). I n view of this proof that no change of degree of hydration is brought about by beating, i t becomes necessary to account for the changes on some other grounds, It is the author’s belief that these changes can be qualitatively accounted for on purely physical grounds. Before attempting such a physical explanation of beating, a few pertinent facts about cellulose-water relationships should be reviewed. The vapor pressure curves do not show that hydration does not exist. They show only that no hydrates analogous to the crystalline hydrates are involved and that no hydration of any kind is produced by beating.
Hydration probably does exist in the sense that there is 8 layer of adsorbed water molecules on the cellulose surfaces. The difference in the apparent specific volume of cellulose when measured in different ways indicates this. In water the specific volume is 0.621; in helium, 0.640 (2,3 ) . Since it seems reasonable to suppose that the helium value is a true volume, the loss of apparent volume in water must be due to some kind of combination. The work of Filby and Maass (3) goes further and indicates, as one would expect, that the loss of apparent volume takes place only in the case of the first few per cent of water sorbed. These few per cent not being water in the normal state may be considered to be water of hydration. Cellulose sorbs water beyond this amount, but, since it is of normal density, it can hardly be called water of hydration. Since x-ray evidence shows no sign of a change in cellulose crystal structure when swollen with water or dilute alkali solution, it is fairly certain that the addition of water is on crystallite surfaces rather tharr as solid solution. CONDITIONS AT CRYSTALLITE SURFACES The attraction of cellulose for water is probably due mainly to its hydroxyl groups since the hygroscopicity is greatly reduced when these are replaced as in the nitrates and acetate5 If the cellulose molecule were of short length it would probably be soluble in water, but the extreme length of chain makes a difference. Solubility of a substance is always a matter of balance between the rate of crystallization and rate of solution. With such a long molecule it is readily conceivable that one portion of a molecule may be surrounded b j water and therefore in solution while another portion remains still attached to the parent lattice. The attraction of cellulose for water would have to exceed that of cellulose for cellulose very greatly to allow of the whole length surrounding itself with water a t one moment and thus gaing into solution like a shorter molecule. If this is true, we must conceive of the surface layers of wet cellulose crystallites as being in a peculiar state of two-dimensional solution. Such a condition would account for many of its properties. A natural cellulose fiber is made up of crystallites disposed in a structure, the form of which depends on the origin of the fiber. The crystallites have a regular internal lattice structure as demonstrated by x-ray analysis, but these crystallites are not all orientated alike in the fiber. Where crystallite meets crystallite there must be a greater or lesser degree of discontinuity in the lattice structure, and along these surfaces the crystallizing force holding the two crystallites together will be weak; how weak depends on the dissimilarity of orientation of the two. Though water is unable to separate a molecule from a crystallite, it may separate one crystallite from another along such planes. Likewise, on removal of water by evaporation, crystallites will unite if sufficiently close to each other. When cellulose pulp is put into water, a large proportion of these intercrystallite attractions axe dissolved by water and a degree of swelling takes place. Mechanical action, as in a beater, can then further disrupt the structure in a manner analogous to fraying out a piece of wool. Surfaces which, previous to the mechanical action, were inside the fiber are brought out to increase the external surface, yet without causing any increase in the totai surface &rea. This increase of
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IN D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
February, 1934
external surface is microscopically visible a3 fibrillation. The fibrils probably consist of portions of cellulose in which crystallites are very similarly orientated, or they may in fact be crystallites having indefinite length though having a definite lattice in cross section. The author believes this fibrillation of a swollen cellulose structure to be the essential action of the beating process. Ron. FIBRILLATION CAUSESINCREASED STRENGTHAND DENSITYOF THE RESCLTANT PAPER Consider a layer of liquid between two parallel plates of material perfectly wetted by the liquid. Owing to surface tension the plates are pulled toward each other with a force: A
(T T ~+ R 2aR2 -)
dynes
2
where A = surface tension of liquid R = radius of liquid layer (presumed circular) in the E
=
plane of the plates distance between plates
The first part of this expression is proportional to the periphery of the layer and may for convenience be termed the “skin effect.” It is independent of the distance between the plates. The second part is proportional to the area and inversely proportional to the distance between the plates. It may be termed the “internal tension effect.” The value of the second part may be calculated from the lowering of the vapor pressure without reference to the surface tension which may have abnormal values when the layer is very thin: 1.95 X 1O6Tug P = log
(F) dynes
where u = density of liquid M = mol. wt. of liquid as vapor p o / p = relative vapor pressure Now consider a layer of fibers laid down as in forming a sheet of paper. As soon as the water level falls below the surface set by the fiber structure, either by evaporation or by drainage, a large number of concave menisci will form between the fibers, and the surface tension of these will set up a compacting force. Such force is appreciable but not large. Its total value is proportional to the lineal amount of fiber in the surface and is therefore increased by fibrillation. As further evaporation occurs, the water becomes distributed as drops bridging spaces between fibers. Surface tension now acts to deform the individual fibers and draw them to each other. The rigidity of each fiber is proportional to the fourth power of its cross-sectional dimension, so that, while coarse unbeaten fibers are rigid enough to withstand the bending force, slim ones and fibrils are not. The latter give way with collapse and shrinking of the structure. With still further evaporation of water so that the water layers between cellulose particles become very thin and the reduction of vapor pressure becomes appreciable, the internal tension begins to increase and applies intense force to bring these cellulose surfaces together. This force acts between the crystallites in the fibers for the most part and causes individual fiber shrinkage, but i t also acts between fibrils of different fibers where these are nearly in contact already. The force due to such internal liquid tension is of the order of hundreds of atmospheres. For water i t amounts to 145 atmospheres a t a relative vapor pressure of 90 per cent and rises t o 955 atmospheres at a relative vapor pressure of 50 per cent of saturation. Liquid forces thus bring the cellulose surfaces sufficiently close to allow their own crystallizing forces to act on each other, and a bond is formed. I n short, surface tension forces cause shrinkage even to the point of introducing stresses in the solid particles, and the crystallizing forces hold the compressed structure together after the liquid is evaporated. Replacing the liquid
2 19
allows swelling to the extent that the crystallizing forces are dissolved and internal stresses relieved. Any further dispersion of the structure by mechanical action of beating does not create a new surface but only rearranges that already existing. Experiments directed to throw more definite light on these points have been under way in these laboratories for some time. The results of these experiments so far are in agreement with the ideas just stated but further work must be done before these can be published as definite findings. Measurements have been made of vapor pressures of methyl and propyl alcohols in equilibrium with pulp containing various amounts of alcohol sorbed under differing conditions. According to the work of Kress and Bialkowsky (5) cellulose swells in methyl alcohol about 62 per cent of its swelling in water, which may be interpreted as being in line with the solubility effects of the two liquids on carbohydrates. But the sorption of methyl alcohol a t relative vapor pressures below about 80 per cent is considerably greater than that of water a t similar relative vapor pressure. However, when the amounts sorbed are plotted against internal pressure calculated from the vapor pressure lowering, the curves are without inflection and practically identical with each other, indicating that compression brought about in this way is an important factor in determining the amount of liquid retained during drying. CONCLUSION I t is by no means claimed that proof of these ideas has been presented. They must be looked upon only as the author’s interpretation of the evidence available. I n such a short summary it has been impossible to acknowledge in detail the indebtedness to various workers from whom inspiration has been drawn but some mention should be made of the papers of Urquhart and Williams (11, 1W), Mark and Meyer (6),Clark (I), McBain ( 7 ) ,Kistler (I),and Terzaghi (10) for definite suggestions which have been used in formulating these conceptions. LITERATURE CITED Clark, G. L., IND. ENG.CHEM.,22, 474 (1930). Davidson, G . F., ShirleyZnst. hlem., 6 , 41 (1927). Filby, Edgar, and Maass, O., Can. J. Research, 7, 162 (1932). Kistler, S. S., J . Phys. Chem., 36, 52 (1932). Kress, 0.. and Bialkowsky, H., Paper Trade J . , 93, No. 20, 35-44 (1931). Mark, H., and Meyer, K. H., Cellulosechem., 11, 91 (1930). McBain, J. W., J . Phys. Chem., 31, 564 (1927). Pidgeon, L. M., and Campbell, W. B., Proc. Tech. Sect. Can. Pulp Paper Assoc., 16, 74 (1930). Seborg, C. O., and Stamm, -4.J., IND. ENQ. CHEM.,23, 1271 (1931). Tereaghi, Charles, Fourth Colloid Synposium Monograph, 1926, 5 8 ; Jerome Alexander’s “Colloid Chemistry,” Vol. 111. Chemical Catalog, 1931. Urquhart, A. R . , Shirley Znsl. Mem., 5 , 303 (1926); 8, 19, 27 (1929). Urquhart, A. R., and Williams, A. M., Zbid., 3, 49, 197, 307 (1924); 4, 5, 167 (1925). RECEIVED May 20, 1933. Presented before the Division of Cellulose Chemistry at the 85th Meeting of the American Chemical Society, Washington, D. C., March 26 to 31, 1933.
P A P E R DESTRUCTION HASTENED BY HIGHTEMPERATURE .4ND HUMIDITY.The Bureau of Standards has found that the deteriorative effect of acidic atmospheres on paper is greatly accelerated when the temperature or humidity of the air is increased. Sulfur dioxide is a distinct hazard to books and manuscripts in congested areas. In a 10-day exposure, sulfur dioxide decreases the folding endurance considerably at 86” F. and a relative humidity of 65 per cent. At 104’ F., the rate of loss of folding endurance is doubled, and a similar or greater effect occurs when the relative humidity is raised to 80 per cent.
