Vapor Sorption of Cellulose at High Relative Humidities G. A. RICHTER, L. E. HERDLE, AND W. E. WAHTERA Wood Cellulose Development Department, Eastman Kodak Co., Rochester 4 , N . Y .
T
H E sorption of water and other vapors by cellulose has been studied over the past 30 years by many investigators who have used their results to gain further knowledge of the structure and properties of cellulose. Early experimenters, including Urquhart and his collaborators (17),Sheppard and Newsome (16), and Maass and his collaborators (15), were concerned primarily with the hysteresis effects obtained in an increasing and decreasing humidity cycle. During the decade following these early studies, the concepts of cellulose structure developed to the point that the water vapor sorption figures were found to correlate with the quantities of crystalline and amorphous cellulose present as measured by x-ray analysis or by the rate of hydrolysis in aqueous acid. It became common practice to accept the moisture regain as a measure of the degree of crystallinity. More recently it has been felt by some investigators (6, 9) that the expression “degree of crystallinity” does not describe the actual physical state of the cellulose being measured and that such factors as crystal size, crystal pattern, and the character of the intermediate regions (termed mesomorphous by sqme early investigators), which are neither crystalline as viewed in the usual sense nor wholly amorphous, so affect the vapor pickup that attempted correlations with physical methods of crystallinity measurement are less convincing. Furthermore, it has been proposed (8), and the authors favor this viewpoint, that the wholly disorganized regions of the cellulose can take on different arrangements that will allow greater or less sorption, depending upon points of chain contacts and freedom of swelling as sorption proceeds. Some experimenters ( 1 , 3) have introduced the concept of “accessibility” to translate the vapor sorption figures into a measure that could be used as a working tool in the study of cellulose reactions. Since it is usually based upon and is directly proportional to the measured water vapor sorption figurei, it is little more than a restatement of their significance. A direct relationship between water vapor sorption and chemical reactivity of cellulose is not always evident, particularly in reactions that depend upon the adoption of dry cellulose as starting material and the use of nonaqueous media. I n many such cases progressive rate of reaction throughout the cellulose wall structure is dependent upon the relative degree to which the reagent can swell the fiber when compared with water swelling. Sorption of nonaqueous solvents must be considered in such cases and widely divergent results may be obtained. Thus, as will be shown, a mercerized cellulose is more accessible than an unmercerized fiber if accessibility is measured by water sorption, but often less accessible as determined by acetic acid vapor pickup, and considerably less so when sorption of propionic or butyric acids is studied. Additional confusion can result if one considers a high accessibility of cellulose as determined with a given reagent to signify high reactivity in reactions involving that reagent. Accessibility merely measures the amount of highly reactive material present, but reveals nothing of the ultimate ease or difficulty with which the nonaccessible portions are converted to the reactive form in processing. Although the authors are fully appreciative of the need of a
more complete understanding of the mechanism of vapor pickup by cellulose, they prefer to restrict the reporting of the results to the original units in which measurements were made. Speculation or postulation that may help to explain the results will be presented for the most part in a q u a h t i v e fashion only. DESCRIPTION O F APPARATUS
The apparatus used for these studies is essentially a thermostated desiccator containing the samples and agitated vapor (see Figure I). The lower half of the desiccator is immersed in water whose temperature is controlled by a coil of copper tubing carrying circulating water thermostated a t 40’ C. The pool for adjustment of relative humidity is placed in the bottom of the desiccator, and the cellulose samples to be tested are held in a rack about 3 inches above the liquid. Air inside the desiccator is agitated by a 4-inch fan blade powered by an electric motor. The desiccator and its water bath are insulated by a ‘/,-inch layer of felt, and the entire apparatus is contained in a hood thermostated at 41’ C. The tendency for water to travel from a wet sample to the pool in experiments using 100% relative humidity is promoted by a small temperature differential between the pool controlled a t 40.1” C. and the air temperature a t the level of the cellulose which runs approximately two tenths of a degree higher. On long exposure this temperature differential is enough so that an equilibrium is approached. PROCEDURES
The individual cellulose samples approximating 1.5 grams each are normally tested in the form of small squares about I 1 / g inches on edge cut from handmade sheets. Twelve such samples are placed in a stainless steel ring slotted to permit them to stand separated from one another but in a fixed vertical position in the desiccator. The samples are not permitted to come in contact with the desiccator wall. The cellulose samples are normally oven dried a t 105’ C., weighed, and placed in a circular support, then oven dried again. The dried samples are transferred to an intermediate desiccator and ultimately t o the thermostated desiccator for exposure. After exposure over the pool comprising 300 to 800 grams of reagent for the desired period of time, the samples are quickly removed to cans fitted with tight covers and weighed on an analytical balance. The samples are then returned to the desiccator for further exposure. As soon as three consecutive daily weights are found to agree with one another, the run is stopped and the cellulose samples are again oven dried and weighed. The cellulose weights obtained at the end of the run are required to check those a t the beginning so that confidence can be placed in the results. Since practically all the exposures are made at high relative humidities, only inappreciable changes in sorbed vapor occur during transfer. This procedure applies to all runs made in atmospheres from which water is picked up by the cellulose. It also applies t o runs made above alcohol solutions. I n these runs with alcohols in the pool no allowance is made for any water that is sorbed during the transfer to and from the weight cans, but it is known
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INDUSTRIAL AND ENGINEERING CHEMISTRY
that any miter so picked up tends to migrate to the pool in the bottom of the desiccator, so that it is felt that the weight figures represent alcohol sorption.
Vol. 44, No. 12
figures might be found above 100%. I n the work presented here, no figures were found above 6‘7%. Because of the two tenths of a degree temperature differential between the levels in the desiccator of the cellulose and the pool, the relative humidity of the air in contact with the cellulose is never quite as high as that at the surface of the pool. Air saturated with moisture at the pool temperature of 40’ C. has a relative humidity of only 99% a t 40.2’ C. Throughout this paper this difference has been neglected, and the relative humidity of the air in a desiccator containing a distilled water pool is considered loo%, and lower relative humidities are considered to be those which equilibrium with the pool should produce. REPRODUCIBILITY OF THE PROCEDURES. I n order to have bases on which to evaluate the accuracy of the individual vapor pickup measurements made in accordance with the above procedures, it seemed advisable to have in the desiccator in each run a sample of a standard cellulose, so that any irregularity in the vapor pickup measurements would be apparent. Bn alkaline refined and bleached cotton linters was chosen as the standard cellulose. Figures compiled from many runs with this standard linters show an average value of 20.7% water at 100% relative humidity and 14.5% a t 95% relative humidity. Average sorptions of acetic acid are 20.7% a t 100% relative humidity and 16.1y0 a t 95% relative humidity. Reproducibility for both o humidity and eveellent solvents is satisfactory a t 1 0 0 ~ relative a t 95% relative humidity. WATER AND ACETIC ACID SORPTION BY COMMERCIAL CELLULOSES
Figure 1.
Vapor Sorption iipparatus
Agitator motor Stuffingbox Insulation Glass desiccator Agitator blade (temperature at this level, 40.4O C.) Cellulose samples Screen on which cellulose support rests (temperature a t this level 40.3’ C.) H . Relative humidity adjustment solution (temperature 40.1° C.) I. Heat transfer liquid a t 40’ C. J . Copper coils carrying circulating water a t 40.0’ C. K. Thermometers Entire apparatus is in a hood with circulating air thermostated a t 41‘ C. A. B. C. D. E. F. G.
Where acetic acid is the vapor being sorbed by the cellulose samples, the sorption figures are checked by titration when equilibrium has been reached. After titration the cellulose is filtered from the titrating liquor, washed, dried, and weighed. rlgain the cellulose weight should check that of the original cellulose. Titration values for the acetic acid content are normally slightly below the nTeight figures, and it is customary to regard the differences as being due to water in the cellulose samples. This difference is very small, amounting to 1 to 2 % of the total sorbed weight. Many exposures have been made of initially wet celluloses to atmospheres of various relative humidities. I n such runs it is not feasible to get an accurate initial cellulose weight, and the sorption figures are based upon the weight of cellulose found a t the end of the run only. For water vapor pickups, the pool was composed of aqueous sulfuric acid of the concentration required to give the desired relative humidity. Reduced “relative humidities” for alcohols and acetic acid were obtained by using solutions of dibutyl phthalate in the pool. A 95% acetic acid relative humidity is obtained in these studies by using a solution of 95 moles of acetic acid per 5 moles of dibutyl phthalate. Dibutyl phthalate was used as the diluent in all nonaqueous runs of less than 100% relative humidity, for which data are presented. All figures shown in the tables are expressed as per cent vapor based on dry cellulose. It is thus conceivable that sorption
Sorptions of ivater and acetic acid by a variety of commercial celluloses are shown in Table I. A fully mercerized laboratory sample is included as a comparison. Higher vater vapor pickups are found with celluloses that have had a strong cold alkali treatment in their history, with kraft-type wood pulps, and with unbleached sulfite pulp. Hot alkaline refined sulfite pulps and cotton linters have vapor pickups appreciably lox-er than the aforementioned groups. The ram cotton linters pulp listed among the miscellaneous materials shown a t the bottom of Table I has approximately the same vapor pickup as the refined product, thusindicatingvery littlechange in structureof fiber in the processing steps. For the most part, acetic acid pickup parallels the sorption of water, provided strong alkali was not used in the preparation of the sample. Strong alkali treatment of the fiber reduces the direct pickup of acetic acid vapor. The unbleached kraft pulp differs from the other chemical pulps in that it shows high sorptions of both water and acetic acid. Subsequent data support the viewpoint that the lower acetic acid sorption in the case of the mercerized pulps is explained by their lesser swelling by acetic acid than by water. It may be pointed out here that the equilibrium water content of dry celluloses exposed a t 100% relative humidity is approximately 1.5 times the content after exposure a t 95% relative humidity. This approximation is useful in estimating equilibrium sorption contents where measurements have been made a t only one relative humidity. It may also be useful in estimating 100yo relative humidity figures \There quick results are wanted, equilibrium being approached much more quickly and the results being much less sensitive to minor variations in exposure conditions when 95% relative humidity studies are made, An approximation of this type cannot be made for measurements using acetic acid. Different types of celluloses and celluloses having relatively minor variations in their pulp histories react in quite a different manner to this change in acetic acid relative humidity. It should also be noted that the vapor pickup levels obtained experimentally at 95 and 100% relative humidity are always higher than would be calculated using the simplified version of the Hailm-ood and Horrobin ( 5 ) isotherm from constants deter-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1952
TABLE I, EQUILIBRIUM VAPORSORPTIONLEVELSOF SEVERAL TYPES OF CELLULOSE (Figures show sorption in per cent based on oven dry cellulose) Water Acetic Acid Sorption, Yo Sorption, % Alpha Sample Cellulose, % Acetylation grade of refined linters 99 Acetate - grade wood pulp (cold, alkaline refined) 97 Acetate grade pulp of western hemlock origin 96 Acetate grade pulp of western hemlock origin 96 Acetate grade pulp of pine origin 96 Spruce wood pulp produced by hot alkali refining 94 Hemlock wood pulp produced by hot alkali refining 95 Gumwood-base pulp refined by cold alkali extraction 97 Hemlock pulp made.by hot alkaline refining 92 Bleached kraft pulp of Swedish softwood ori in 88 Bleaoied semirefined hardwood base pulp 91 Viscose-grade hemlock 91 PULP High alpha cellulose pulp (cold alkaline refined) 98 Washed unbleached northern suruce kraft
ters Miscellaneous materials Groundwood Hemlock shavings Viscose rayon Acetate rayon Solvent-extraoted unbleached linters
100%
95%
14.4
20 7
16.1
30.5
20.2
24 6
20.3
25.7
16.2
26 6
18.8
100%
95%
rei:
rel. humid,
humid.
20.7
rel. rel. humid. humid.
23.6
16.2
24 8
17.6
25.3
16.3
27 6
20.0
22.8
16.3
24 3
19.5
22.6
16.8
22 1
19.5
..
18.9
..
21.8
23.5
17.4
28 2
21.1
27.6
18.8
24 8
21.0
22.6
16.9
21 1
18.7
22.1
16.3
24 9
19.8
29.7
20.9
20 9
20.3
89
32.9
19.9
31 8
24.2
91
27.5
18.3
27 6
20.5
88
25.3
18.6
31.1'
25.5
99
33.7
22.0
21.5
..
33.3 26.3 41.3 14.1
24.2 20.6 29.7 11.4
39 5 28 6
35.0 26.5 22.2
20.0
14.4
19.1
15.3
.. .. .. ..
..
..
..
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from the list of miscellaneous materials included in this table. Groundwood has a higher water vapor pickup than any of the refined pulps, and the acetic acid sorption is considerably out of the range of that of the refined cellulose. If one applies a correction for the approximately 30% lignin present in the raw wood fiber, the sorption based on cellulose and pentosan groups present is correspondingly greater. Hemlock wood shavings behave in much the same manner as the groundwood but in a lesser degree. The higher water and acetic acid sorptions by the groundwood may be partly explained by a maceration of fiber wall structure during the grinding operation. It may also be that the sorption of acetic acid by the lignin in the wood substance is very high or that the native cellulose initially present in the undigested wood responds to acetic acid swelling more than does the. chemically processed cellulose. It may be significant that both the kraft pulp and the groundwood have high percentages of pentosans that have not undergone acid hydrolysis, and these pentosan groups may have higher sorptive power than cellulose itself. These differences may in turn determine the differences in structure as brought about when the respective cellulosic materials are ultimately dried. A viscose-type rayon is thought to be about as sorptive a cellulose as is available. The water vapor pickups recorded in Table I support this viewpoint. The very high water sorption obtained is probably paralleled by other types of regenerated cellulose (16). Acetic acid sorption by viscose rayon is also relatively high, but there is evidence that just as in the case of mercerized cellulose, it is limited by the degree to which the rayon filaments can be swollen by direct contact with the acetic acid vapor. Thus, when the rayon is first preswollen by water vapor, the acetic acid sorption is raised from 28 to 40%. SORPTION SEQUENCE
WATERFOLLOWED BY ACETICACID. I n all cases studied the acetic acid sorption by cellulose as determined both by weight and by titration is higher when the fiber is first swollen by water vapor. This higher sorption is magnified in cases where the fiber has undergone severe alkali treatment. I n general, with the mercerized cellulose the acetic acid so sorbed approximates the same level (31 t o 34%) as is obtained when the respective dry cellulose is exposed directly to water under equilibrium conditions. Just as in the case of the single stage acetic acid exposure discussed earlier, the surprisingly high acetic acid sorptions obtained when the kraft and groundwood pulps are subjected to the two-stage sorption are difficult to explain. The presence of the unhydrolyzed pentosan groups in those particular pulps may again be significant.
mined a t lower relative humidities. The differential between experimental and calculated values varies with the cellulose under consideration. At 100% relative humidity the actual sorption level found for linters was 5% above that calculated by the formula, and in the case of a wood pulp that had undergone strong alkali treatment this differential was 9%. It seems evident that the assumption that only the monohydrate need be considered in treatment of water sorption by cellulose is not justified when high relative huTABLE 11. INFLUENCD OF PRESWELLING ON SUBSEQUENT SORPTION midities are being used. Exposuresa The water vapor sorption WaterWaterlevels found here at 100% Water Acetic acetic Butyric butyric Watersorption, acid, acidb, acid, acidb, Pyridine, pyridineb relative humidity are of the Cellulose % % % % % % 70 same order of magnitude as Bleached cotton linters 20.7 20.7 23.3 2.8 16.0 25.1 26.0 Acetate-grade pulp of western the total nonfreezing water 26.6 hemlock origin 3.3 25.7 28.5 26.8 17.8 28.7 contents previously measured An almost fully mercerized wood oellulosec 3 3 . 2 2 4.9 32.3 2.1 2.9 18.2 35.0 calorimetrically near 0 ' C. Unbleached northern spruce kraft 32.9 31.8 47.7 . . .. .. Pulp using cotton and viscose rayon Unbleached hemlock sulfite pulp 25.3 23.9 36.6 .. .. fibers (IO). It is unfortuBleached high aloha-cellulose pulp made b y cold alkaline renate that the materials used fining process 29.7 20.9 34.6 .. .. .. .. Viscose grade hemlock pulp 22.1 22 2 29.1 .. .. .. .. in the two studies were so Groundwood 33.3 39:5 65.6 .. .. different that close comparison Viscose rayon 41.3 28.6 30.3 .. .. .. .. of the results is difficult. a Butyric acid and pyridine runs used 72-hour exposures and equilibrium was not fully established. b Oven dry cellulose was first exposed to water vapor substantially to equilibrium point and then to solvent Some further insight into the vapor. mechanism by which these C Treatment made with 16% NaOH solution at 50' C . vapors are sorbed is gained I
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Vol. 44, No. 12
TABLE111. VAPORPICKUP ABOVE ~IISCELLANEOCS SOLVENTS ProWater. % 100% 95% re]. rel. humid. humid,
Acetic
Acid,
100%
rel. humid.
%
95% rel. humid.
Methanol, % 100% 95% rel. rel. humid. humid,
Ah. -Ethyl (Ah.), % 100%
95%
rel. humid,
rel. humid.
$:I%: %, 1;y% humid.
Acetic hnhyButyric Benaeneb, drideb, Acida, %, %, Imp%, 100% 100%
Cellulose Bleached cotton linters 20.7 141 20.7 16.1 9.9 8.6 9.4 8.6 15.3 Rleaohed hieh alnha-ee1lulose pulp^ made- by--cold alkaline refining process 29.7 20.9 20.9 20.3 12.9 13.8 10.8 4.6 Viscose-grade hemlock pulp 221 16.3 24.9 19.8 ii:o 11.0 10.1 10.0 19.3 Acetate-grade pulp of western hetnlook origin 22.7 16.1 2G.F 18.8 120 11.0 103 9 9 20.1 Hemlock-base pulp made by hot refining 22.6 168 22.1 19.5 12.5 10.7 10.5 9.1 20.8 Unbleached northern spruce kraft pulp 32.9 19.9 31.8 24.2 13 5 13.3 12.2 24.0 Bleached kraft pulp 27.6 18.8 24.8 21.0 13:s 12.2 11.5 10.2 204 A n almost fully-mercerized wood I cellulose 33.2 22.0 24.9 20.3 15.1 14.8 12.2 12.2 5 1 I n this case a final equilibrium was n o t fully established. b In these cases a 30' C. sorption temperature was used instead of the usual 40" C. level a n d exposures were stopped
The influence of preswelling is even more marked when the mercerized pulp is exposed to vapors that have more limited swelling properties than acetic. acid. Results with butyric acid are typical (see Table 11). I n such instances also, greatly increased rates and levels of sorption are found Tvhen the exposure to the reagent is preceded by preswelling with water. Pyridine is of particular interest, showing an increase from 2.2 to 35.101, with mercerized fiber, although with unmercerized cellulose the influence of preswelling is negligible. I n results not reported here, it w-as shown that sorption rate of pyridine by refined cotton linters is higher than found m-ith a commercial grade of wood cellulose intended for acetylation. This higher sorption rate m-ith cotton is not paralleled in the case of other vapors sorbed. V.IPOR SORPTIOK ABOVE >hSCELLANEOUS SOLVEXTS. The vapor pickup levels shown by a variety of celluloses exposed over the Ion-er fatty acids aiid alcohols are shown in Table 111 together with a few miscellaneous measurements of sorption of acetic anhydride and benzene. Rlethanol and ethyl alcohol are sorbed by cellulose to a lesser degree than water, but if the celluloses are arranged in the order of decreasing vapor pickup, their order will be about the same whether alcohols or water are being considered. T i t h the alcohols studied, mercerized celluloses and kraft pulps show higher vapor pickups than do the other nood pulps or linters. I n the sorption of alcohols the difference resulting from exposure t o a 95% saturatedatniosphere rather than 1007, is far less than was experienced with either acetic acid or water. This may mean that marginall?. accessible regions in the celluloses are not opened up by these somewhat poorer swelling agents. Propionic and butyiic acids are sorbed to a lesser degrec than is acetic acid, evidently because of their poorer capacity for swelling cellulose. Mercerized samples, which are more difficult for such reagents to swell, show greatly reduced sorptions with the higher fatty acids. The high sorption level of the kraft pulps when compared with other celluloses is again shon-n with propionic acid. Segligible sorptions of benzene, acetic anhydride, and butyl alcohol were found in 31 hours a t 30" C. It is doubtful that these levels would go much higher on continued exposure. RATE OF VAPOR SORPTION
The sorption of water by celluloie is very rapid and although there is some evidence that rz mercerized cellulose exhibits a faster rate than do the unmercerized products, there appears to be no great difference as influenced by history of cellulose preparation. On the other hand, the late of acetic acid sorption by cellulose is greatly influenced by mercerization, as well as bv small amounts of water present in the celluloses when exposed. Figure 2 show3
Butyl Alcoholh,
%,
100%
humid.
humid. rel.
humid. rel.
humid. rel.
2.8
1.3
1 0
1.5
..
1.1
1.0
1.8
3.3
2.0
1.4
1.2
..
.,
..
..
2 0
1 2
..
..
..
..
..
1.2
2 9
short of full equilibrium.
this behavior graphically. These curves s h o ~that acetic aut1 is sorbed by the unmercerized cellulose rapidly and that the presence of a small percentage of water in those celldoses has no great effect. Sorption by dry mercerized wood pulp is very slon but is greatly accelerated when the initial cellulose has been adjusted to contain 4% water. A post-treatment of the mercerized pulp with weak sodium sulfite solution at high temperature brought about a soinewhat faster sorption rate, especially in the comparisons made with the initially dry fiber. The lower rate with the dry mercerized fiber can be explained by a dower swelling of the shrunken cellulose by acetic acid alone. Bcetic acid sorption rate of a dry mercerized type of cellulose can be greatly improved by displacement of Tqater initially preqent in the undried fiber with organic solvent and with a subsequent removal of the displacement solvent (see Table IV). If the conception that drying a water-wet fiber sets up crossbonding of inolecular chains is a correct one, it seems reasonable to believe that the solvent dewatering sequence lessens bonding that occurs during drying from water and ensures a greater immediate accessibility as n-ell as more rapid su-elling. The rapid sorption found n ith the solvent-extracted cellulose is consistent with the observation that in the acetylation of dry mercerized cellulose much 1cds activation time in acetic acid is required if the cellulose is dried irom isopropyl alcohol rather than from water.
- 24 yi
yl
:22 D
-20
n S
._ 0
14
PI2
vi
:10 u
-
.-" 8 s 6
E 4 " u - 2 a
a
0
0
5
Figure 2.
$0
15 20 E x p o s u r e , hours
30
Rate of Acetic Acid Sorption by Cellulose
I,.
Cotton linters '4. Acetate-grade wood p u l p M. Viscose-grade sulfite t r e a t e d w i t h 16% N a O H for 1 h o u r a t 500
c.
After t h e alkali t r e a t m e n t , M was t r e a t e d w i t h 3% NnrSOs s o l u t i o n for 2 hours at 1703 C. D. Signifies that the cellulose w a s o v e n d r y at s t a r t of exposure Cellulose c o n t a i n e d 4 % w a t e r a t start of e x p o s u r e to acetic a c i d .'pI
MS.
December 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
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2887
IXFLUENCE OF HIGHTEMPERATURE ALKALINEEXTRACTION.lactone formation, which in turn influences the sorption power. I n certain wood pulp-refining procedures a sulfite pulp base is There is also the question of carboxyl groups formed, preferensubjected to high temperature alkaline digestion which causes tially in the noncrystalline region, with consequent loss of sorptive the elimination of degraded cellulose and results in a product capacity. Cation cross bridges seem out of the picture, since characterized by higher alpha cellulose. Maximum temperaweak hydrochloric acid was usedinthe washing stagesand distilled tures used in the digestion determine the yield, the alpha cellulose water in the final wash. level attained, and the vapor sorption of the resulting product as shown in Table V. In many ways this relationship of vapor sorption and yield parallels the data obtained with acid hydrolysis of cellulose, suggesting that the extraction of cellulose with hot alkaline solutions as a method for determining crystallinity deserves some further attention with particular emphasis in the field of higher temperatures than were used in the tabulated set of experiments, Whether the hot alkali digestion reduces sorptive power mainly because of elimination of amorphous material or whether the digestion allows or causes an additional number of stable crossbonds to be formed by promoting orientation is not evident. 14 I I I I I I 0
I
2
3
4
5
6
Copper Number
TABLE
ITr.
DRYING MEDIUMON RATEO F ACETIC ACID VAPORPICKUP
INFLUEX‘CE O F
(Base fiber used was a viscose grade wood pulp treated with 16% N a O H solution a t 60° C ’ handsheets were treated as indicated and oven dried for 32 hours before Zxposure for the indicated time period above 0.95 mole fraction acetic acid; sorptions are expressed as per cent acetic acid based on cellulose) Time Of Exposure, Hr. Equilibrium Treatment 4 7 Sorption Level A . Successive passage of water methanol. a n d benzene throueh the sheet; vacuum dried a t 500 c. 23.9 23.5 23.2 B. As in (A) b u t air dried 23.2 25.3a 2 3 . 8 C . Successive passage of water and isopropyl alcohol (large volume) through the sheet: vacuum dried a t 60° C. 23.9 23.2 22.8 D. As in (C) but air dried 21.9 23.7‘J 2 2 . 8 E. Water wetting; air dried 11.2 11.3 21.0 F. Bleached linters comparison 14.4 14.6 16.4 dried from water a I n cases of rapid sorption, higher “acid” contents measured by weight are sometimes found after short exposures than are present a t equilibrium because of the slowness with which residues of water or solvent migrate to the pool in the bottom of the desiccator.
TABLE V.
KFFECT
OF
HOT ALKALINE EXTRACTION ON VAPOR PICKUP
(Unbleached sulfite pulp was extracted with 1 % NaOH solution a t the indicated temperature for 2 hours a n d washed: product8 were oven dried) Vapor Sorption at 96% Rel. Humid., 0 Alkali Treatment Alpha Temp., C. Yield, % Cellulose, % Water Acetic acid Orig. cellulose 88.2 18.6 22.6 100 si 14 96.7 17.6 21.3 125 74.6 96.8 16.9 19.8 160 72.4 97.1 16.5 19.7 173 70.5 97.0 16.3 19.3
It is interesting to note that acetic acid vapor sorption follows the same general pattern in this series as found with water.
Figure 3.
Effect of Oxidation on Vapor Sorption
Vapor sorption is measured at 95% relative humidity Vapor sorbed i s shown i n parentheses A . Acetate-grade wood pulp L . Cotton linters
Additional evidence of lessened sorptive power of cellulose is found in experiments in which cellulose sheets were oxidized by exposure to ultraviolet light for long periods. The data appear in Table VI. As in the caRe of chemical oxidation already described, there was an appreciable increase in copper number and a definite decrease in sorption of both water and acetic acid. Subsequent washing with water did not restore the original sorption levels, but a hot alkaline extraction of the light-exposed cellulose went a long way toward restoring the original values. Whether this can be accounted for by simple extraction of alkalisoluble material that has low sorptive capacity or whether there is so&e secondary physical change that takes place in the cellulose residue on alkaline extraction is not clear. Restoration of sorption level by alkaline extraction throws some doubt on the abovementioned possibility that shorter chains resulting from oxidation orient more easily and thus reduce sorptive power. If the ultraviolet light effect is confined to the surface layer of the sheet only, it would indicate that this outer layer of modified cellulose is tremendously important in determining the extent to which vapor is sorbed. It seems more likely that more than the surface layer is affected, even though observation showed that major color changes appeared in the surface layer only. KO experiments were made with sheets that had been exposed to the light and then peeled to remove the outer layers. Such experiments should demonstrate whether the interior of the exposed sheet possesses higher sorption properties than the outer layers. Experiments of this nature should prove of interest.
OXIDATION OF CELLULOSE
HYDROLYSIS
Bn acetylation-grade wood pulp and bleached cotton linters were further treated with oxidizing agents and the resulting products then exposed to water and to acetic acid vapors. Data as plotted in Figure 3 show a direct relationship between copper number and water pickup. Acetic acid figures are directionally in the same order, but the relationship is not so pronounced. KO satisfactory explanation occurs for this reduced vapor pickup by celluloses that have undergone oxidation. It is difficult to explain the phenomenon by the presence of aldehydic end groups. There is some likelihood that the more accessible cellulose is rendered soluble by oxidation and thus removed from the system by extraction or that depolymerization attending oxidation makes it easier for the shorter chains to line up in crystalline formation. One can also postulate that oxidation results in a
Many experiments have been listed in the literature shon ing that the sorption of water vapor by cellulose is greatly decreased when the cellulose has been hydrolyzed in dilute hot aqueous acid. A few experiments showing the same effect with sorption measured a t high relative humidities are listed in Table VII, where 1% sulfuric acid solutions at 85’ C. were used for 1 hour. Such relatively mild acid treatment does not produce the rapid elimination of the so-called amorphous fraction of the cellulose that has been studied so thoroughly in the literature ( $ , I $ , I S ) , but results are directionally the same. In the sulfite pulp cooking process, delignification is progressively accompanied by hydrolysis of the cellulose. The effects of various time periods in a typical sulfite cooking stage are shown in experiments in which hemlock chips were digested in the acid
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 12
fying agent. The effects of such exposure are indicated in [Ultraviolet light exposures were made for 24 hours a t 50' C. o n each side of the sheet, all vapor pickup figures Table IX. I n this set of esare expressed in per cent based on d r y cellulose a n d were made a t 95% relative humidit; (over 0.95 mole fraction periments air-dry acetylationacetic acid) ] grade cellulose was rewet with Acetate-Grade Partially hfercerized Bleached Cotton Wood Pulp Cellulose0 Linters water, the water displaced with Copper Acetic Copper Acetic Copper Acetic acetic acid, and the acetic acid number Water Acid number Water acid number Water acid then displaced with acetic acid Untreated 0.45 16.6 18.5 0.66 21.6 19.7 0.36 14.2 16.1 Exposed t o ultraviolet containing 1% sulfuric acid. 15.1 3.20 12.0 11.0 light 2.90 15.2 16.4 2.66 19.0 The treated samples were Exposed t o ultraviolet light, then water washed .. 15.3 16,Q .. .. .. .. .. .. washed and air dried. Khen Xxnored t o ultrnviolet light, then extrrtbted the exposure to thc sulfuric with 1% N a O H 90111acid-acetic acid solution was tion a t 100' C. .. 16.6 18.1 .. 20.6 19.2 .. 13.9 14.5 prolonged, the samples were A viscose-grade cellulose was treated with 16% N a O H solution a t 50' C stabilized by overnight soaking in a saturated magnesium carbonate solution and rinsed with distilled water. The water sorption by this cellulose was considerably increased above that obtained by the original cellulose, indicating that the exposure to the sulfuric acid-acetic acid solution changes the cellulose in an entirely different manner from 33 that found with aqueous acid hydrolysis. Differences in crystal size, shape, or pattern may offer some explanation for this radi32 cally different behavior. If it is imagined that shorter molecular chains ordinarily formed on hydrolysis can be responsible for lower sorption because of an easier orientation of the shortened chains, speculation can be made on differences in mobility of the shortened chains depending upon whether hydrolysis takes place in a water or an acetic acid medium. There is also the .00 80 -223 possibility that small amounts of ester formed during the exposure in the acetic acid solution may have an important influence \ upon the manner in which the structural pattern of the treated o n 2 60-221 \Cuprcrmmonium viscosity cellulose changes on subsequent drying. This is being investiga.5 .y '.\ ted further. \ The higher sorption that is found when cellulose is hydrolyzed in an acetic acid medium may mean that part of the benefit derived from pretreatment of the acetylation-grade cellulose with catalyst-fatty acid solutions prior to addition of anhydride stems from the cleavage of the larger cellulose aggregates which resist penetration by acetylation agents. INFLCEI~CE OF FORMALDEHYDE TREATXENT ox CELLULOSE SORPTIOK.Both water and acetic acid sorptions can be greatly reduced by formaldehyde treatment of cellulose. Such treatments were carried out by immersing the cellulose in a 5% formaldehyde Time at 14OOC in cook solution in the presence of oxalic acid catalyst and by subsequent Figure 4. Effect of Pulping Time on Vapor Sorption drainage and oven heating at 100' C. Table X summarizes Delignification of western hemlock chips was carried out with some of the more important results. A11 three typcs of cellulose 8770 free SO1 and 1% combined SO2 a t 140' C. for the time indiAFTER EXPOSURE OF THE CELLULOSE TO ULTRAVIOLET LIGHT TABLE VI. VAPORSORPTION
Q
t
cated after a 3-hour heating-up period; products were washed and oven dried before exposure
OF HYDROLYSIS OF CELLULOSE o s VAPOR TABLE 1711. EFFECT sulfite liquor a t a maximum temperature of 140' C. (Figure 4). SORPTION When the time of the digestion was prolonged, the cuprammoWater Sorption a t 95% nium solution viscosity of the wood pulp dropped radically and the Starting Cellulose Hydrolysis Conditions Rel. Humid., water vapor pickup showed a progressive decrease, following the Bleached cotton linters 1% HzSOasolution, 8.5' C., 1 hr. 13.4 same general pattern as demonstrated when a given wood pulp S o treatment 14.4 is hydrolyzed in a subsequent operation. A similar decrease is 15.6 Acetate-grade wood pulp 1%HzSOa solution, 8.5' C., 1hr. No treatment 16.2 found in the sorption of acetic acid vapor when the respective celluloses are similarly exposed. I n another experiment, cellulose was exposed to hydrochloric OF ROOM TEMPERATURE HYDROLYSIS OF TABLE VIII. EFFECT acid vapors (Table VIII) with a purpose of avoiding extracCELLULOSE ON VAPORSORPTIOK tion of degraded cellulose by the hydrolyzing medium. The (Cotton linters exposed t o the vapors above a n aqueous solution containing exposed samples were then washed and dried. Yield was found 20% HC1,and 9 % H2SOa for 3 days a t room temperaturg was washed 5 times as indicated, using a trace of ammonia in t h e third wash) to be substantially 100%. Severe depolymerization had taken Equilibrium place as evidenced by very low solution viscosity. Both water ~~~~~~~~~i~~ Vapor Sorption at 95% Viscosity, Rel. Humid., % and acetic acid sorption were greatly reduced by such hydrolysis. Washed in Poises Water Acetic acid PRETREATMENT WITH SULFURIC-ACETICACID SOLCTIONS. 8.7 8.7 Isopropyl alcohol 0.07 Most procedures for preparation of cellulose esters of fatty 8.6 8.6 Water 0.07 14.5 16.1 Untreated cotton linters 74 acids include an exposure to a catalyst-containing fatty acid solution before actual treatment with anhydride or other esteri-
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
December 1952
TABLEIx. WATER VAPOR SORPTION B Y CELLULOSE8 PRETREATED WITH SULFURIC-ACETIC ACID [Liquids at 30° C. were pulled through. the sheets for short periods only; a!l samples were washed with water and air dried after the final treatment indicated to free the cellulose from remoyable residues of acetic acid or sulfuric acid; samples immersed in sulfuric acid solutions for 3 hours were stabilized by soakin overnight in a saturated magnesium carbonate solution and rinsing w i 8 distilled water before drying and exposure; all exposures were made in a 95% relative humidity (water) atmosphere] Partially Mercerized Treatment Cotton Acetate-Grade Wood of Cellulose Sheets Linters Wood Pulp Cellulose= 15.6 17.0 23.3 A. Water sucked through sheet
B. As in (A) followed by water disolacement
with
acetic
16.7
23.8
16.4
22.4
C.
D. 0
3-hour immersion in the HzSOn solution 16.9 20.5 26.4 A viscose-grade wood pulp treated with 16% NaOH solution a t 50° C.
were greatly modified. Water sorption decreased to two thirds and acetic acid sorptions t o one third of those found with untreated comparisons. Washing of the treated cellulose had no influence on the water sorption but restored acetic acid sorption t o somewhat higher levels. Hot, weak alkali digestion or a treatment with 12% sodium hydroxide solution a t 40" C. had no significant effect on the formaldehyde-treated fiber except an increase of the acetic acid sorption t o a point about two thirds t h a t found with the untreated cellulose. Attempts to increase the acetic acid sorption by a pre-exposure to water vapor were only partially successful. By such a sequence, the acid sorbed was no higher than the' water levels established in a single exposure. The classical picture-that the change in character of cellulose resulting from such formaldehyde treatment is due t o formation of methylene linkages from one chain to another-seems adequate to explain the results. Such cross linkages between chains in the dried state probably occur in the disorganized regions. This type of cross bond should greatly deter swelling by either sorbed water or acetic acid, and the observed reduction in sorption of these agents is readily understood. Such linkages should also be rather stable toward the action of hot dilute alkali or cold strong alkali, and one might have predicted the observed result that such treatments did not have the effects on vapor sorption found when they were practiced on celluloses not treated with formaldehyde. INFLUENCE OF SODIUMCARBONATE DURINQ DRYINQ.The presence of very small percentages of sodium carbonate left in a, cellulose that is dried from the dilute carbonate solution brings about a marked increase in vapor sorption. The increase is far beyond that which can be accounted for by salt formation
2889
with acetic acid or by formation of salt hydrates with water. The effect is magnified with an unbleached kraft pulp (see Table XI). Washing the dried sample free of carbonate and redrying returns the cellulose to its original sorption level. Evidently the phenomenon is fugitive in nature and the data suggest a temporary interference of cross bonding that probably takes place when the cellulose is dried in the presence of the carbonate, Certain minimum amounts of carbonate are needed to bring about these changes in behavior. When a 0.02% solution is substituted for the 0.0570 solution listed in Table XI, no special effects are noted. Furthermore, all sodium salts do not have the same effect. This failure to respond was demonstrated with sodium sulfate, which a t the same concentration (0.05%) produced no change in sorptive power of the original cellulose. Although experimental evidence is not complete, preliminary work shows that other alkaline buffer salts bring about results similar to those noted with sodium carbonate. Very small amounts of acid solution (0.01% sulfuric acid) used in the same manner bring about an opposite effect, with a marked reduction in sorption. The parallel result obtained with cellulose that was hydrolyzed in aqueous suspension suggests that the cellulose has been similarly hydrolyzed when dried in the presence of a small amount of free acid. VAPORPICKUP BY COLDALKALI-TREATED CELLULOSE.It has long been known that the water sorption of cellulose is greatly enhanced by strong cold alkali treatment, Data given in Table XI1 are typical of such effects. A viscose grade of wood cellulose and an acetylation grade of cotton linters were used as base materials. The major change in water sorption takes place a t about 10% sodium hydroxide concentration when the treatment is made a t 25" C. and a t 12% concentration when made a t 50" C. These figures are in substantial agreement with evidence of critical treating conditions as established by other methods ( 1 4 ) and with moisture regain data previously obtained a t a lower relative humidity (11). There is very little change in acetic acid sorption with increased severity of alkali treatment when oven dry cellulose is exposed. On the other hand, preswelling by water vapor followed by exposure to acetic acid vapor shows the characteristic high sorption usually associated with mercerization. Sorption figures established with linters when treated with 18% sodium hydroxide solution a t three different temperatures show the expected progressive increase in sorption as the treating temperature is lowered. It is also significant in results shown in Tables XI1 and XI11 that all celluloses, including cotton linters, refined pulp, and kraft-type pulps, tend to show the same water vapor pickup level after mercerization. This implies that the dserences in degree of crystallinity present before mercerization tend to equalize to a lower level when mercerization is carried out. This equality in sorption behavior is evident both with the washed but undried product and also with the subsequently dried fiber, indicating the same resultant orientation of molecular chains on
TABLEX. VAPORSORPTION BY FORMALDEHYDE-TREATED CELLULOSES (All vapor sorption measurements started with oven-dry cellulose and used 95% saturated atmospheres) Partially Mercerized Acetate-Grade Wood Pulp Bleached Cotton Linters Wood Cellulose Acetic acid Acetic acid Acetic acid following following following Acetic water vapor Acetic water vapor Acetic water vapor Formaldehyde Treatment Water, % acid, % exposure, 5% Water, % acid, % exposure, % ' Water, % acid, % exposure, % 18.8 Untreated Comparisons 16.2 23.9 14.4 16.6 19.8 18.6 29.5 22.0 10.5 5.9 9.9 10.1 A. d 5.4 9.6 5.0 11.5 14.5 B. As (A) but washed after 3-h.our heating 10.2 6.9 10.9 9.9 14.4 6.4 C. As (A) then treated with 1% NaOH solution at 125O C. for 2 hours 10.7 11.4 D. As (A) then treated with 12% NaOH solution at 40° C. for 1 hour 11.0 9.1 *. .. a I n case A, cellulose was immersed in 5% formaldehyde solution containing 0.1% oxalic acid, then drained and dried a t 100° C. for 3 hours. All samplw were subjected to the usual 16-houroven heating to complete drying.
.. .. ..
.. ..
.. ..
..
.. ..
..
.. ,.
..
..
INDUSTRIAL AND ENGINEERING CHEMISTRY
2890
TABLE XI. EFFECT OF SODIUM CARBOXATE RESIDUE IN CELLULOSE DURING DRYIKG ON SORPTION (All samples were oven dried before exposure)
Treatment Water washed Oven dried after wash with 0.05% KazCOa solutiona C. As in (B) but water washed after drying and redried 8. Water washed
Cellulose Acetate-grade A. woodpulp B.
Unbleached kraft wood Pulp
Water Sorption, % 85% 100% rel. rel. humid. humid. 10.5 22.4
Acetic Acid Sorption, %, humid. 19.9
10.8
25.9
23.0
9.9 12.9
22.2 32.1
19.5 27.4
Oven dried after *ash with 0.05% NazCOa solution 13.0 40.3 44.0 C. As in (B) b u t water washed after drying and 12.4 31.3 28.0 redried a After exposure to the sodium carbonate solution, the cellulose was pressed to 50% fiber concentration before drying; residue sodium carbonate in dry fiber is 0.03 to 0.07%.
B.
drying. Furthermore, with all mercerized celluloses cited, rewetting of oven dry fiber before establishing sorption equilibrium results in almost identically higher sorption levels which are intermediate between those found vith the undried and the fully dried samples. This d l be discussed in greater length in a following section in this report. That each of the mercerized products from the widely different cellulose bases has the same approximate sorptive capacity as determined under three conditions of trial is probably no accident and suggests an ultimate structural pattern that is reached by the common mercerization step. I n view of the like behavior of all types of cellulose that have undergone mercerization, it appears that the study of any fully mercerized cellulose should give results which apply to others. The increased water vapor sorption brought about by mercerization can be destroyed by a subsequent acid hydrolysis of the mercerized fiber with 1% sulfuric acid solution a t 100" C. On remercerization, the higher sorption levels were largely restored; the three figures are, respectively, 30, 22, and 27%. Thus, according to one viewpoint, it nrould appear that the original cellulose first undergoes some bond breakage during mercerization, The acid hydrolysis allows some orientation of the shorter
Vol. 44, No. 12
chains, and remercerization forms new amounts of the partially or wholly unoriented substance. By Hermans' theory of hydrate formation ( 6 ) ,one can explain at least part of the observed changes from the viewpoint that crystals in mercerized cellulose can be penetrated by water and that on subsequent hydrolysis the crystals again become less penetrable. I N F L U E N C E O F DRYIUG ON SORPTION
Equilibrium sorption level of cellulose is greatly affected by its physical condition as determined by drying history and by the direction from which the equilibrium is reached. This has long been recognized in the literature as evidenced by the hysteresis studies, but the degree to which vapor pickup a t high relative humidity depends on these conditions has not been fully demonstrated. If, for instance (Table XIV), undried celluloses are exposed to high relative humidities, equilibrium sorption levels are very much higher than those established with the same initially ovendried cellulose, indicating that the normal drying process causes considerable loss of sorptive power. A definite but lesser loss of sorptive power occurs when water is removed from an undried cellulose by solvent displacement. Thus, when exposed to 100% relative humidity conditions, the undried cellulose comes to equilibrium a t from 54 t o 66% water, the solvent-treated fiber from 40 to 43%, and the ovendried fiber from 25 to 33%, depending upon the chemical history of the fiber. Lower relative humidity conditions with solventtreated samples show no measurable difference in sorption from that found with the undried material, thereby suggesting that there is a moisture threshold below which the undried fiber undergoes substantial physical change, xvhether brought to that point by direct evaporation of water or by replacement of water by solvent. Drying from the methanol c a u m additional loss in sorption, bringing the level to a point approaching that observed when the undried cellulose is dried directly. No test was made to determine vhether a drying from a nonpolar solvent such as benzene mill avoid the secondary change found with the alcohol drying. The undried unmercerimd cellulose (Table XIV) shows more retained water than found with the fiber that had been treated with strong alkali. This may possibly be explained by the fact that the partial mercerization which was carried out on an originally dry cellulose did not completely destroy the crystalline structure set up during that first drying. Later experiments have
VAPOR SORPTION B Y ALKBLI-TREATED CELLCLOSES TABLE XII. EQUILIBRIUM (After 1 hour in the alkaline solutions samples were washed, acidified with 0.5% HC1 for 1 hour a n d again washed; figures show equilibrium vapor pickup levels in per cent based on pulp starting from the oven-dried state) Viscose-Grade Wood Pulp Bleached Cotton Linters Acetic acid Acetic acid sorption sorption following following llcetic Bcid Acetic llcid ,Tater vapor water vapor Water Sorption Sorption exposure, Water Sorption Sorption exposure, Conditions of 95% 95% 100% 100% 100% 100% 967T 100% 95% 100% NaOH Treatment rel. rel. rel. rel. rel. rel. rel. rel. rel. rel. humid. humid. humid. humid. humid. humid. humid. humid. humid. humid. Temp., C: NaOH, To .. 0 22.2 19.8 14.4 19.7 16.2 22.1 16,3 20.7 4 25 .. 18.6 29'. 4 2i:g 22.5 16.3 19.3 13.4 17.2 14.1 8 .. 21.7 23.5 17.1 19.2 25 13.2 17.1 14.0 19.9 28.6 ,. 9 .. .. 19.8 25 18 3 .. .. .. 10 .. .. 19 4 20.2 25 11 .. 23.2 20.7 2.5 12 32:4 36:l 2a:2 18:3 20 ' 0 17:o 26:F 22.9 20.3 26 33.8 .. 33.7 21.5 18.8 32.8 16 22.0 23 5 20.3 25 35.7
.. ..
I
18 18 18 0
0 15 40
..
..
..
..
..
.. ..
..
..
.. ..
..
..
..
..
.. ..
..
..
.. ..
33.6 32.4 31.1 20.0
I
..
..
22.7 22.2 21.9 18.7
19.3
.
.
.. ..
..
22.6
.. 21.2 13.5
..
19.2 15.1
33.9 32.7 31.1 22.3
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1952
2891
by the undried cellulose. I n the case of partially mercerAlkali Treatment Water Vapor Sorption a t 95% Rel. Humid., % ized fiber, the values for unNaOH Oven Dried and rewet, dried, dried and rewet, and Base Cellulose soh, % Temp., C. Undrieda dried undriedb dried samples are, respectively, Bleached cotton linters .. .. 20.8 15.0 19.2 16 50 32.8 22.1 27.3 56, 40, and 33%. The same 18 20 34.3 24.7 30.2 relationship holds with unmerAcetate-grade wood pulp .. .. 26.6 17.3 23.4 16 50 34.3 23.7 30.0 cerized fiber. I n other words, 18 20 35.0 24.3 30.2 loss in accessibility caused by Viscose-grade wood pulp .. .. 25.7 17.3 23.9 full drying is partly but not 29.5 16 50 33 7 22 6 18 20 34.7 23.4 29.7 completely reversible even by Unbleached kraft wood pulp .. .. 30.2 20.7 27.8 complete and long-time soak16 50 35.3 24 4 30.5 18 20 33.3 24 9 30.1 ing. Undried products were exposed directly to water vapor. The vapor pickup after solb The dried products were rewet in water and the wetted samples exposed. vent displacement of water from an undried cellulose is approximately a t the same point as found when the dry fiber has been rewet by immersion in water. This would indicate that the same net loss of initial accessibility is experienced by solvent displacement as when a cellulose is fully dried and rewet. It would be expected that the reactivity of the celluloses obtained by the two methods could differ in important respects as indicated by chemical tests, but the degree of crystallinity as measured by sorption is probably comparable by the two routes.
TABLE XIII.
VAPOR SORPTIONBY MERCERIZEDCELLULOSES O
-
Q
TABLE XIV.
EFFECT OF CONDITIONS OF WATERREMOVAL ON WATERVAPORSORPTION
Water content before exposed (Yebased on pulp)
Figure 5.
Degree of Drying and Equilibrium Water
Content Never-dried alkali-treated viscose-grade sulfite wood pulp was dried slowly over sulfuric acid solutions at room temperature to the moisture levels indicated and then exposed over water at 100 % relative humidity
demonstrated that an undried mercerized fiber that was made from a never-dried base has the same water sorption as the undried unmercerized base itself, both in the neighborhood of 67y0, It is significant that with the unmercerized fiber the equilibrium vapor sorption a t the higher humidity after solvent displacement is at the same approximate level as in the case of the alkalitreated cellulose. At the lower relative humidity, however, there is a slight but real difference in the sorption, which probably indicates a greater loss of accessibility in the case of unmercerized fiber at this ultimate lower moisture level. The higher water sorption level, which is so often reported in the literature and which is also reported in Tables XI1 and XI11 for mercerized cellulose, when compared with an unmercerized fiber is obviously due to different effects produced on drying. With a mercerized fiber, the loss in accessibility to water set up on drying is less than found with the unmercerized cellulose, or it may be that part of the latticework formed on drying is more easily broken when the structure is reswollen with sorbed water. One explanation might be that disorientation that occurs when fiber is swollen by mercerizing liquor interferes to a greater degree with the bonding of molecular chains that have retained their parallel relationship than occurs in the unmercerized fiber. When oven dry cellulose is rewet by soaking in water and then exposed to water vapor, the water retained a t equilibrium is appreciably greater than when the dry fiber is similarly exposed. The increased sorption level caused by full immersion of the dry fiber is much greater than evidenced by the usual hysteresis curves reported in the literature, but the rewetting of the dry fiber does not restore sorptive levels to those originally possessed
Oven dried 32 hours a t 105O C. Undried cellulose Water displaced with acetic acid (no drying) Water displaced with acetone (no drying. Water isplaced with CHsOH (no drying) Water displaced with CHaOH (methanol evaporated a t 50' C.) Dried over 95% Has04 Vacuum dried to 1.8% Hz0 Oven dried, then soaked 36 d a w in water Oven dried, then soaked 1 hour in water Dried a t 50° C. to about 3%:water and soaked 36 days in water Dried to 3% water and soaked 14 days in distilled water a t 0" C. As above but soaked in water at 40' C. a A viscose-grade wood pulp treated b A wholly unmercerized cellulose.
33.2 55.5
15.5 19.4
25.7 66.8
12.0 16.8
43.1
20.1
43.6
17.6
41.4
18.7
37.2
16.3
41.8
19.4
40.7
17.3
34.8 31.2 34.5
17.6 16.3 15.9
29.7 23.6 25.5
13.7 12.7 12.6
39.8
17.2
..
..
40.8
16.8
..
..
44.0
17.4
..
..
39.1 38.5
.. ..
with 16% NaOH solution at 50" C.
Some further support for the concept that the early stages of drying are responsible for the irreversible part of crystallization is given in Figure 5, where it is shown that after drying over sulfuric acid solutions to intermediate moisture contents, the eventua! sorption equilibrium a t 100% relative humidity is dependent upon the degree to which the cellulose has been dried. Moisture contents of the initially undried cellulose are, of course, unimportant, the equilibrium sorption level approached from the high side being the same regardless of the excess water present. This is the meaning of the horizontal line in Figure 5. Further evidence of the marked difference in sorption properties of an undried and the corresponding oven-dried cellulose appears in Figure 6, which shows hysteresis curves established with the respective samples. The conditions are in some respects similar to those described by Hermans ( 7 ) , except that his experiments were made with a mercerized fiber. The dotted line marks the path of water sorption obtained with the dry fiber
.
2892
INDUSTRIAL AND ENGINEERING CHEMISTRY
as it was exposed to increasing conditions of humidity and finally returned progressively t o a dry atmosphere. I n contrast, the solid line traces the water sorption level as the initially undried cellulose is brought to successively lower conditions of relative humidity and then returned stepwise to an ultimate humidity of 100%.
Per cent relative h u m i d i t y
Figure 6. Equilibrium Water Content of OvenDried and Undried Acetylation-Grade Wood Pulp a t Various Relative Humidities Samples were exposed successively a t w a t e r vapors at different relative humidities, s t a r t i n g w h e r e i n d i c a t e d with an arrowhead Solid line. U n d r i e d cellulose D a s h e d line. Oven-dried cellulose
.
Vol. 44, No. 12
is calculatedas held in the samedegree by both theoriented and the unoriented portions. If one accepts the viewpoint that an undried cellulose is 100% water accessible, then it is possible, by use of the vapor sorption data, to calculate the fraction of a cellulose accessible to water. Thus, with the dry unmercerized cellulose of Table XIV the accessible fraction is 26/66.7 or 39%. After displacement of water from this undried cellulose with acetic acid, the calculated figure is 44/66.7 or 66% accessible to water. It is doubtful that the greater water accessibility of the fiber that was dried by solvent displacement can be attributed to a disorientation of cellulose chains that are aligned in the undried or the directly dried fiber. It seems more likely that in the process of removing water by displacement with acetic acid some of the aligned chains become associated, while others which do undergo crystallization on direct drying retain their accessibility to water. This behavior may be explained by a condition of imperfect alignment or by failure of certain chains to approach each other sufficiently to allow association t o occur. This reasoning would indicate the presence of three types of cellulose in this solvent treated fiber: an unoriented cellulose with full accessibility having an equilibrium water content of 6 moles per glucose unit, a substantially oriented but accessible cellulose also capable of sorbing 6 moles of water, and an oriented crystallized cellulose that sorbs no water beyond the small amount that can attach to the peripheral hydroxyls. Such a premise is adopted in the calculation appearing in Table XV. The lower water content obtained with the undried alkalitreated cellulose than found with the undried unmercerized fiber is explained by the fact that the alkali-treated cellulose had been dried before it was subjected to the strong alkali treatment. Evidently only 84% of the dry fiber (56/66.7) had been converted to an accessible form by the mercerizing alkali, leaving 16% of the cellulose in its original crystalline state. As mentioned before, this reasoning is supported by later data, not shown, from which it was demonstrated that the 66.7% level is reached when the same type of mercerization is carried out with a cellulose that has undergone no previous drying. As shown in Table XIV, drying of the alkali-treated fiber reduced the water sorption to 33%. Accepting Hermans' finding that crystalline mercerized cellulose readily sorbs 1/8 mole of water per glucose unit, this figure can be broken down into oriented and unoriented cellulose fractions as indicated in Table XV, showing 48% of the cellulose to be unoriented and capable
The round-trip sorption curves for the two celluloses are quite different, signifying again the profound influence of the oven drying and its effect on the loss of sorptive power. Similar experiments m-ith a fully dried but resoaked fiber should prove interesting, as would a case in which the undried cellulose is dewatered by solvent displacement and then subjected to the down-and-up travel. Just as the amount of sorption by dry cellulose is influenced by the degree of swelling brought about by the sorbed vapor, so the desorption of an undried cellulose is in part dependent on the shrinkage in fiber structure that occurs as vapor escapes to establish the new level. The TABLE XV. DISTRIBUTION OF SORBED WATERIN ORIENTEDAND UNORIENTED FRACTIOXS higher water levels found with (An analysis of the d a t a of Table XIV showing a possible distribution of water between oriented and unoriented the undried cellulose in desorpphases in equigbrium a t 100% relative humidity a n d the percentages of these phases present on the basis of such distribution; unoriented" cellulose here is t h a t fraction still capable of forming a hexahydrate after oven drying, tion, as compared with figures and "crystallized" cellulose is t h a t which cannot form such a hexahydrate) found a t the corresponding Laboratory-Prepared points of relative humidity Unmercerized Sulfite Pulp Partially Mercerized Cellulosea Solvent Solvent when the fullysaturated butiniNever Oven displaced Never Oven displaced tially dry fiber is progressively dried dried (AcOH) dried dried Rewet (AcOH) Unoriented cellulose retuinedto a point O ~ O %r e i 47 47 47 39 39 39 47 Per cent of total tive humidity, suggeets that the Moles HzO/glucose unit 6 6 6 6 6 6 6 26 32 32 32 32 Per cent HzO based on cellulose 26 26 hydrated cellulose of the unOriented Uncrystallized cellulose dried material is shrunken less 61 0 28 37 .. .. 12 Per cent of total bhan would correspond to the 6 0 6 6 .. .. 6 Moles HzO/glucose unit 41 0 18 24 .. .. 8 swelling of the dry fiber as it Per cent based on cellulose Crystallized (unmercerized) 16 climbs the sorption curve or to Per cent of total 61 33 16 16 16 Moles HzO/glucose unit 0 0 0 0 0 0 its shrunkenvolumeduring subPer cent HzO based on cellulose 0 0 0 0 0 0 Crystallized (mercerized) sequent desorption. The equiPer cent of total .. .. 37 37 25 librium water content a t 100yo Moles HzO/glucose unit .. .. I/* l'/a l'/a relative humidity of the unPer cent Hz0 based on cellulose .... .. 1 5 3 Total Hz0 dried unmercerized cellulose Calculated .. .. .. .. .. 37 .. corresponds almost exactly to a Experimental 67 26 44 56 33 40 43 0 A viscose-grade wood pulp treated with 16% NaOH solution a t 50° C. hexahydrate (66.775 water based on cellulose) if the water
....
.. .. .. ..
.. ..
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1952
of sorbing 6 moles of water per glucose unit, 36% in the form of crystalline mercerized cellulose capable of picking up ‘/a mole of water, and the 16% of unchanged unmercerized crystalline cellulose that is not water accessible. When this cellulose is rewet by immersion in water and brought t o equilibrium in a saturated atmosphere, the crystalline mercerized cellulose should, according t o Hermans, contain 1 1 / ~ mole of water per glucose unit, and the predicted water content of this cellulose would be 37%. Actually, about 40% water was found. The water sorption of the undried alkali-treated fiber that had been dewatered with acetic acid can be treated in the same manner as was done with the unmercerieed base, and by such procedure the components can be classified in four groupsnamely, a fully oriented portion capable of forming a hexahydrate, one that is substantially oriented but water accessible, an oriented mercerhed cellulose that is not capable of forming a hexahydrate, and the oriented inaccessible portion that has escaped mercerieation. Mathematical results of such an analysis also appear in Table XV. It is very significant that the equilibrium moisture contents of dried cellulose and of cellulose rewet after drying differ appreciably. This difference occurs not only in mercerized cellulose, where in the case shown in Table XV it can be almost wholly accounted for by a difference in degree of hydration of the crystalline fraction, but also in unmerceriaed cellulose, where it is thought by Hermans and others on the basis of x-ray evidence that no hydration occurs in the oriented regions. It would appear that such differences in equilibrium water content must still be accounted for on the basis that marginally accessible fractions upon flooding with water have much higher equilibrium water contents than they do when the dry fibers are exposed directly to water vapor.
2893
The lower accessibility obtained by water vapor sorption of dry linters (20.7/66.7 or 39%) when compared with that by deuterium exchange (61%) is partially explained by the fact that measurements in the latter ease are made using a flooded sample. Thoroughly water-soaked linters have an equilibrium water content of about 29%, giving an accessibility of 43% (29/66.7). This figure would correspond to that obtained by Frilette, Hanle, and Mark ( 4 ) by deuterium exchange using cotton as the base but is well below their value for linters, and would suggest that the earlier chemical history of the samples may be of great importance in determining the accessibility figures. LITERATURE CITED
(1) Assaf, Haas, and Purves, J . Am. Chem. SOC.,66, 66 (1944). (2) Battista, IND.ENO.CHEM.,42, 502 (1950). (3) Conrad and Soroggie,Ibid., 37,592 (1946). (4) Frilette, Hanle, and Mark, J . Am. Chem. Soc., 70, 1107 (1948). (5) Hailwood and Horrobin, Trans. Faraday Soc., 42B, 84 (1946). (6) Hermans, “Contribution to the Physics of Cellulose Fibers,’: pp. 51-2, New York, Elsevier Publishing Co., 1946. (7) Hermans, “Physics and Chemistry of Cellulose Fibers,” p. 181,New York, Elsevier Publishing Co., 1949. (8) Ibid., pp. 189-90. (9) Howsmon, Teztile Research J., 19,152 (1949). (10) Magne, Portas, and Wakeham, J . Am. Chem. SOC.,69, 1896 (1947). (11) Mitchell, IND.ENG.C H ~ M43, . , 1786 (1951). (12) Nickerson, Ibid., 34, 1480 (1942). (13) Philipp, Nelson, and Ziifle, Tezlile Reeeurch J., 17,585 (1947). ENG.CHEM.,32,480, 1122 (1940). (14) Richter, IND. (15) Russell, Maass, and Campbell, Can. J. Research, B15, 13 (1937). (16) Sheppard and Newsome, J . Phys. Chem., 33,1817 (1929). (17) Urquhart et aE., J. Teztile Inst., 15T,138 (1924). RECEIVIOD for review March 1, 1952.
ACCEPTEDAugust 9, 1952.
Reaction of Vanillin with Silver Oxide IN THE PRESENCE OF TECHNICAL CAUSTIC IRWIN A. PEARL
AND
DONALD L. BEYER
The Institute of Paper Chemistry, Appleton, Wis.
S
EVERAL years ago ( 8 ) it was shown that vanillin could be converted to vanillic acid by reaction with alkali and 0.5 mole of silver oxide in substantially quantitative yield according t o the following equation: 2RCHO
+ Ag20 + 2NaOH
-+ NaOH
2RCOONa
2Ag
+ HZ + HzO
It was also shown that a Canniezaro reaction mechanism was responsible for this deviation from the classical oxidation of aldehydes with alkali and 1 mole of silver oxide. The alkali employed in this study was C.P. sodium hydroxide. S u b s e quently (4), it was brought to the authors’ attention that this process was somewhat sensitive t o obscure differences in experimental conditions. A preliminary reinvestigation of the process led to the discovery that vanillin can be transformed into vanillic acid in accordance with the above equation only in the presence of C.P. or U.S.P.grades of sodium hydroxide; technical grades of caustic soda give anomalous results. With the recent demand for vanillic acid and its derivatives there arose the possibility
of commercial production of vanillic acid by this method. This process could be practiced commercially only if technical caustic soda was employed as the alkali. The present paper reports the results of studies on the possible use of technical caustic in the transformation of vanillin t o vanillic acid by means of silver oxide. CONTROL EXPERIMENT
Silver oxide was prepared by mixing aqueous solutions of 170 grams (1 0 mole) of silver nitrate and 40 grams (1 0 mole) of c.p. hydroxide. The silver oxide (o.5 mole) filtered, washed with hot water, transferred t o a large beaker, and covered with 1700 ml. of hot water (60’ (2.). Solid sodium hydroxide (120 g r a m ) was added with stirring, raising the temperature t o 72” C. At this point 152 grams (1.0 mole) of vanillin were added. A reactiontook place, raising the temperature to 860 c. The mixture was stirred until cool and filtered, and the silver residue was washed with hot water. The filtrate and washings were acidified with sulfur dioxide and cooled. The heavy white precipitate was filtered, washed with cold water, and dried t o yield 154 gram ( 9 2 ~ of ~ )pure vanillic acid melting at 2 1 0 ~to 2110 c. Ether extraction of the filtrate yielded an additional 10 grams (6%) of vanillic acid melting a t 206’ t o 208” C.
