I
GEORGE A. RICHTER’, LLOYD E. HERDLE, and W A I N 0 Eastman Kodak Co., Rochester, N. Y.
E. WAHTERA
Cellulose Swelling Measured by Benzene Retention The benzene retention level accurately reflects the swelling state of a cellulose sample at the time the swelling agent is displaced with benzene
I N THE esterification of cellulose to fatty acid esters it is usually necessary to swell the cellulose a t some stage of the process in order to expose the hydroxy1 groups to the esterifying agent. This expansion of fiber structure is ordinarily carried out as a preswelling step, although some additional swelling takes place in the subsequent stages. I n certain procedures the swelling is accomplished practically wholly during the esterification itself by deliberate inclusion of a special swelling agent in the esterifying mixture. When the sequence calls for preswelling it can be accomplished by water, which is then displaced with fatty acid, or by the fatty acid itself. Both the rate and the degree of swelling by fatty acid are less than can be obtained with water. With either method the degree of preswelling is usually qualitatively judged by the reaction rate and by the ease with which complete esterification takes place. When the reaction yields an ester solution, as typified by most acetylation procedures, the completeness of reaction is judged largely by the absence of unreacted cellulose residues. A better understanding of the fiber swelling mechanism and a better method for measuring the degree of swelling would be helpful in establishing optimum conditions for esterification of a given type of fiber and in developing more suitable types of cellulose than now exist. Swelling of cellulose fiber depends on the release of bonds that tie the glucoside chains together normally in the noncrystalline portion of the cellulose structure. That some types of bonds are stronger than others is illustrated by the difference in behavior of a highly crystalline cotton and a corresponding mercerized product when the dry fibers are exposed to swelling agents slightly less polar than water. The slow and limited sweIling of the dry mercerized fiber in acetic acid, a classic example, suggests that in the amorphous region of such fiber many chains are in closer proximity and possess stronger cross
Present address, 28 Monroe Ave., Pittsford, N. Y.
bonds than in the unmercerized cotton itself. Historically, it has been very difficult to determine the degree to which a cellulose sample has been swollen by any intermediate stage in the preparation of such derivatives as the esters. Sorption of vapors from polar liquids such as water or acetic acid by such partially swollen samples is recognized (3) as a poor measurement of the degree to which they were originally swollen because of swelling or shrinking accompanying the measurement of the sorbed vapor. Similarly, sorption of a polar solute such as sulfuric acid from a fatty acid medium gives a measure of the degree to which the cellulose is swollen a t the completion of the treatment. However, such fatty acid solutions have swelling properties of their own, and sulfuric acid sorption cannot be relied upon to represent accurately the degree of swelling when the cellulose was first immersed in it. The approach used here for determining the degree of swelling in various reagents is based on the displacement of the swelling agent with a nonpolar liquid (normally benzene) that will cause no further swelling and an exposure of the treated fiber in the vapor of the replacement liquid to equilibrium. The concept that this equilibrium liquid retention is a measure of the degree of swelling in the first reagent is based on the assumption that the swelling agent is displaced without causing a change in swollen volume or that any contraction in swollen volume is approximately proportional to the degree to which the sample had been swollen.
Procedures and Apparatus
The apparatus used for measurement of benzene retention (7) is essentially a thermostated desiccator in which the cellulose samples, saturated with liquid benzene, are held in the agitated atmosphere above a pool of benzene. The temperature a t the level of the cellulose samples is a few tenths of a degree above that of the benzene pool (40.0’ C.),
so that a slow migration of free liquid
benzene takes place from the cellulose to the pool below. A small beaker containing phosphorus pentoxide is kept in the desiccator a t cellulose level to remove traces of water inadvertently admitted while the samples are being handled for weighing, Equilibrium benzene contents were determined by requiring three checking figures from the daily weights. All figures in this paper (except as indicated in Table I) are expressed as milliliters of benzene a t equilibrium per 100 grams of oven-dry cellulose. Oven-dry cellulose was determined after each run by washing the samples successively in ethyl alcohol and water and weighing them after overnight drying at 105’ to 110’ C. The alcohol wash was necessary, as direct heating of the cellulose containing benzene under these conditions of drying leaves 1 to 5y0of benzene in the cellulose. The individual samples were circular disks of cellulose about 5.5 cm. in diameter and weighing about 1.5 grams. They were either cut from commercial sheets or handsheeted from fibers slushed in distilled water. After swelling by the procedure under test, the cellulose disk was placed on the screen, 100 to 250 ml. of benzene were drawn through to remove the swelling agent, and the sheet was immersed in a pool of benzene for a day or so before being placed in the desiccator. In all cases discussed here this treatment effectively stopped the swelling action. Where water was the swelling agent under test, a solvent mutually miscible with water and benzene (usually acetic acid) was drawn through the sheet before the benzene. The “benzene retention” as discussed here was measured by taking the difference between the oven-dry cellulose weight and the weight of cellulose plus liquid a t equilibrium over benzene. I n some cases this “benzene” includes small amounts of tenaciously held liquids which have come in contact with the cellulose during the swelling or the displacement of the swelling agent with solvents other than benzene. I n highly swollen samples prepared by flooding VOL. 49,
NO. 5
M A Y 1957
907
with water and subsequent displacement with acetic acid and benzene, the acetic acid retained a t benzene equilibrium will be 1 to 1.5% based on cellulose or about 5% based on benzene To permit uniform treatment of all results, such retention which is not an equilibrium is ignored and is included in the measured benzene retention. The celluloses used throughout the series of tables and graphs are referred to by symbols which have the significance HA. Hot alkali-refined acetylation pulp from western hemlock. CA. Cold alkali-refined acetylation pulp from spruce. L. Acetylation linters. ML. Acetylation linters mercerized by treatment with 18yosodium hydroxide at 20"
c.
Comparison of Retentions Using Three Nonpolar liquids
Retention of the nonpolar solvents, methylcyclohexane, carbon tetrachloride, and benzene, by various waterswollen celluloses is compared in Table I, calculated on both a weight and a volume basis. The results using these solvents of widely different specific gravities strongly suggest that solvent retention by presoaked cellulose is a volume function. For this reason the experiments that follow all have the benzene retentions expressed as milliliters of benzene per 100 grams of cellulose. The precise mechanism by which the nonpolar solvents are retained a t equilibrium in the cellulose is not clearly understood. Being nonpolar, these solvents cannot sorb through hydrogen bonds to accessible cellulose hydroxyls, as is postulated with water and acetic acid. With no presoaking or swelling of the cellulose, benzene retention is less than 1 ml. per 100 grams of cellulose. Since there appears to be a direct correlation in well-swollen celluloses between the benzene retention and the noncrystalline cellulose content as measured, for example, by water vapor sorption, it is evident that the mechanism by which the benzene is held is such that only this fraction of the cellulose participates. This suggests that the benzene retention is a measure of the internal surface of the cellulose. Lauer ( 4 ) believed that benzene was truly sorbed by cellulose and that the plot of such sorption against the degree of vapor saturation would show a typical Langmuir plot. Preliminary experiments seem to show that a Langmuir sorption curve is followed to a vapor saturation of about 95%, above which a sharp rise in pickup of benzene occurs. The authors suspect that, as Lauer used as base material only unswollen cellulose, this greater sorption a t high vapor pressure was within his experimental error and was not detected. The sharp
908
Table 1.
Nonpolar Liquid Retention by Water-Swollen Celluloses"
'CVeight Basis, Grams/100 Grams
Cellulose
Cellulose CHaCeHii 14.4 15.0 12.5 21.7
HA CA L ML
CaHs 17.9 20.0 15.1 26.9
CClr 28.5
...
26.7 47.7
Volume Basis, M1./100 Grams Cellulose CHaCsHit CCL CsHs 18.7 19.5 16.2 28.2 127
17.8 0 . .
16.7 29.8 96
20.4 22.7 17.2 30.6 88.5
Molar volume of liquid a Flooded cellulose sheets dewatered by successive pull-throughs of acetic acid and nonpolar liquid at 30' C. before exposure to vapors above pool of pure, nonpolar liquid.
rise in benzene pickup at very high degrees of vapor saturation indicates that the benzene is held under these conditions by additional forces besides those responsible for Langmuir sorption. There appears to be some correlation between the solvent retention of the three liquids by the various celluloses in Table I and the molar volumes. This may support the picture that molecular size is a factor in determining the liquid retention and suggests that small molecules are able to reach cellulose surfaces which are less accessible to slightly larger molecules. Exponents of the capillary theory of sorption may interpret this in a slightly different manner (2). Benzene Retention by Water-Swollen Cellulose
Cellulose swollen by flooding with water and dewatered by a pull-through of acetic acid is extremely reactive toward acetylation. Such water activation has been described frequently in the literature, with special emphasis on its advantage when rapid acetylation is sought or when drying the fiber has made it difficult to regain good activity (mercerized celluloses). Whether undried cellulose is to be used without removal of water by evaporation or the dried fiber is to be rewet preparatory to esterification, some means for water removal is necessary before acylation can be practiced. Ordinarily this is done by displacement of water by fatty acid. Table I1 compares the benzene retentions of a series of celluloses which were sw-ollen by flooding with water under various conditions of time and temperaTable II.
Temp.,
ilcOH ~ See.
0
Sec. 60 3600
30
7 60
7 7
60 3600 24 hr. 7 60
60 60 60 60 60
60 a
7 7
i
~Benzene ~ Retention, , M1./100 Grams Cellulose HA CA L ML 20.3 20.7 16.1 28.8 23.3 22.9 18.4 31.8
20.4 21.3 20.0 20.9 21.9 18.8 20.0
22.7 21.6 21.6 23.1 24.6
17.2 17.5 17.7 18.3 20.0
... ...
... ...
30.6 29.1 29.6
... ... ... ...
Sheets flooded with water dewatered with acetic acid ht 30° C. before benzene displacement
and exposure.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Benzene Retention by Water-Swollen Celluloses"
Tirno,
c.
ture and dewatered with acetic acid. Changes in the time and temperature of flooding with water produce small but probably significant effects on the degree of swelling of the cellulose. Longer time is needed when low temperatures are used to produce maximum swelling. It would be of interest to demonstrate whether periods longer than 60 minutes at 0" C. or even lower temperatures with aqueous solutions will raise the benzene retention above the level found when the water is applied a t room temperature or above. The choice of dewatering agent can have a marked influence on benzene retention (Table 111). Acetic acid is a very efficient agent for removing water from flooded cellulose so that substantially maximum benzene retentions are obtained when very short pullthroughs of acetic acid are used. Experiments not shown here have demonstrated that incomplete removal of water in this dewatering stage results in low benzene retentions, probably because of drying of the cellulose from water during the exposure to benzene vapors in the desiccator. Evidently, benzene is unable to replace water from cellulose when the water is being removed by evaporation but can retain the degree of swelling only when the water has been removed by a solvent miscible with both water and benzene. Of the solvents tested as dewatering agents, methanol is the closest to acetic acid in its behavior, giving benzene retentions substantially the same as those from acetic acid. n-Butyric and isobutyric acids give benzene retentions about the same as acetic acid when unmercerized cellulose is used, but low
CELLULOSE SWELLING Table Ill.
Influence of Dewatering Agent on Benzene Retention" Benzene Retention, M1./100 Grams Cellulose HA CA L ML
Dewatering Agent Methanol 17.8 21.6 16.6 30.2 n-Butyl alcohol 10.7 13.8 13.2 15.9 %Butyric acid 18.1 19.2 15.1 21.5 Isobutyric acid 15.2 19.9 Acetic acid 20.0 21.6 17.7 29.6 a Water-swollen celluloses dewatered using 1-minute pull-through at 30' C. of indicated agents followed by benEene displacements.
...
Table IV. *
Drying after Refinement
...
Effect of Dryng on Benzene Retention" Benzene Retention M1./100 AcOH Retention, M1./100 Grams Cellulose Grams Cellulose Unmerc. Merc. Unmerc. Mere.
Sulfite pulp (dried before refinement) 38.6 Prehydrolyzed kraft (undried after pulping) Undried 49.3 Dried and rewet 20.3 Flooded samples dewatered with acetic acid with benzene.
38.0
...
...
50.1 26.3
91.0 43.5
80.3 54.9
(60 min., 30' C.) followed by displacement
Table V. Benzene Retention of Acetic Acid-Swollen Cellulose Time of Soak in Acetic Acid, T ~of ~Benzene ~ Retention, . M1./100 Grams Celldose Batch Min. Soak, C. HA CA L ML A"
B'
C C
5 15 30 60 24 hr. 30 days 60 60 60
38 38 38 38 38
Room
6.1 12.2 13.7 14.2 15.5 14.4
20 60 80
... ... ...
5 15 30 60
38 38 38 38
7.6 10.9 12.3 13.0
60 24 hr. 30 days
38 38
9.7 12.0 11.4
5.7 11.6
...
...
15.6 12.4
... ... ... ... ... ... ...
5.5 11.8
...
9.9 10.7 11.7 12.7 12.9 11.9 9.3 12.5 12.7 9.3 10.2 11.6 11.8 9.6 11.1
0.9 1.6 1.9 5.8 14.9 13.0 1.7 13.3 14.9
... ... ... ...
1.3 4.0 11.0
Room 10.6 Cellulose sheets dried at 50' C. and adjusted to 4.0% H20, soaked in 2.4 parts of 99.8% acetic acid per part of cellulose. Same as A, except cellulose dried at 1 0 5 O C. Same as A, except without water adjustment. a
retentions result when mercerized linters are dewatered with these acids. If nbutyl alcohol is the dewatering agent and is followed directly by benzene, low benzene retentions are found. If the butyl alcohol is itself replaced by ethyl ether or a long exposure to acetic acid before the benzene pull-through, significantly higher benzene retentions are obtained. Something more than mere miscibility with water appears to be necessary for a solvent to remove water from cellulose effectively. Probably the solvent must itself be highly sorbed, so that it can properly replace those bonds by which water is bound to cellulose. Under this picture butyl alcohol must be a solvent which has difficulty in effecting complete removal of water from the cellulose. Other experiments, made with a wide variety of dewatering agents, have led
to the conclusion that activity of the cellulose toward esterification is measurably better where the more polar agents were used (8). The above discussion of benzene retentions would predict that dewatering difficulties with the less polar agents would be exaggerated with mercerized celluloses. More work is justified to show definitely whether the differences in cellulose swelling, measured by either activity in the esterification step or benzene retention, are due to structural changes in the cellulose or to the last traces of water which are more difficult to remove with the less polar solvents. Influence of Drying. The benzene retention of a cellulose which has not been dried after its refinement is the same, whether or not it has been mercerized, supporting the picture that crystallization which would occur upon drying does not
take place during dewatering or impregnation of the sheets with benzene. This is in contrast with the results obtained when a dried cellulose is used, in which case the benzene retentions of the rewet samples always show greater swelling in the celluloses subjected to mercerizing alkali. These dried and rewet celluloses always show lower benzene retentions than the corresponding undried samples. Acetic acid retentions are more difficult to measure, in that equilibrium is approached more slowly, but the available figures seem to show that these same relationships between dried and undried celluloses hold for this liquid also. The ratio of acetic acid to benzene retention in Table IV is approximately 2.0. This ratio is discussed in more detail later.
Swelling by Immersion in Fatty Acid A common method for preswelling cellulose that is to be acylated is to mix the fiber containing from 2 to 670 water in 2 or 3 parts of fatty acid a t 30' to 40' C. for a period of time dictated by local conditions of manufacture. Water content of fiber, temperature, ratio of acid to cellulose, time of mixing, and in a very important manner the processing history of the cellulose determine the degree to which the fiber is swollen and hence activated. Preliminary swelling is usually judged by measuring the esterification rate and the completion of reaction after a given period in the acylation mixture. As the degree of swelling is considered to parallel the benzene retention when determined by the method described above, a series of experiments was made (Table V). T o avoid differences in behavior as influenced by sheet drying conditions used in the manufacture of commercial wood pulps, all samples were prepared as handmade sheets that were dried as indicated. Except where ovendried cellulose was used with no water adjustment, the water-containing pulps were held to 4% moisture. After swelling, the acetic acid was displaced by benzene and equilibrium benzene retention was determined. The unmercerized celluloses (4% water content) show a rapidly increasing benzene retention through the first 15 minutes of soaking in acetic acid, with slightly higher values when the soaking period is extended up to 24 hours. With a mercerized cellulose swelling occurs much more slowly, but with extended time the values equal those of the unmercerized fiber. The influence of temperature of acetic acid can be important in accelerating the swelling of celluloses, especially if they are mercerized. Where very short activation times are advantageous, the VOL.49,
NO. 5
M A Y 1957
909
higher temperatures could play an important role in making such a process successful. The importance of drying history as a factor determining the rate of swelling is shown by the finding that unmercerized cellulose sheets, oven-dried and then adjusted to 4% water, show benzene retentions which rise more slowly during the first hour of soak than retentions of the corresponding samples that had been dried a t a lower temperature. When the oven-dried pulps are soaked without water adjustment, the rate of swelling in the activation step as indicated by benzene retention is further reduced, the effect being especially pronounced in mercerized samples. All these figures are in line with acetylation experience obtained with celluloses preswollen in the manner shown. Table V I demonstrates the lesser reactivity and swelling of fiber that has been dried commercially under conditions more drastic than those used in drying the remade handsheets. Pulp manufacturers have long recognized the deteriorating effects of their standard drying practices on reactivity of their products. Influence of Sulfuric Acid. hctivation of cellulose is retarded if sulfuric acid is present in the preswelling acetic acid, if short immersions are used (Table VII). These lower benzene retentions when sulfuric acid is present may be attributed to lesser swelling of the cellulose because of dehydration of the system. Just as anhydrous acetic acid is known to be a poorer swelling agent for cellulose than acetic acid containing 0.1 to 0.2% water, so acetic acid containing such small quantities of water but having this water bound to the sulfuric acid solute is also a poorer swelling agent, if only short time periods are considered. When the sulfuric-acetic acid treatments were extended to 24 hours, the benzene retentions rose to levels above those for corresponding treatments with acetic acid. However, mercerized linters required the presence of moisture to reach high benzene levels. In prolonged treatment with sulfuricacetic acid solutions a small percentage of acetyl is introduced into the cellulose (6). This change in surface might be expected to affect the retention of benzene on the cellulose. The higher benzene retentions of samples presoaked for 24 hours in such solutions could be explained either on the basis of this partial acetylation or on the premise that ultimately greater swelling results from the use of the sulfuric-acetic acid system. Swelling in Polar Organic liquids
The relative effectiveness of a series of organic liquids as swelling agents when used in the same manner as acetic acid is shown in Table VIII. The fatty
9 10
Table VI. Time of Soak in AcOH, Min.
Machine Drying vs. Drying at 50' C." Benzene Retention, M1./100 Grams Cellulose Machine dry (water extracted as sheet Hand sheet .Machine dry and redried)
15 60
14.1 14.8
10.1 12.0
14.0 14.4
a HA sheets dried at 50' C., adjusted to 4.0% water, soaked at 3 acetic acid per part of cellulose.
C. in 2.4 parts of 99.8y0
8 O
Table VII.
H2SO4, % 1
Influence of Sulfuric Acid on Acetic Acid-Swelling of Cellulose" Benzene Retention, M1./100 Grams Cellulose Time of HA ML -___ L Ib IIC Ib II C I* IIC Soak, Hr. 1 24
11.5 15.8
3.2 14.5
11.2 14.0
3.0 14.1
3.1 16.0
0.2 3.6
3
1 24
10.4 14.9
2.3 17.5
10.4 14.1
2.7 18.1
6.1 20.4
0.8
0
1 24
14.2 15.5
9.7 12.0
12.7 12.9
9.6 11.1
5.8 14.9
1.3 4.0
4.0
* Cellulose sheets soaked in 2.4 parts by weight of acetic acid containing indicated percentage of sulfuric acid based on cellulose. Cellulose dried at 50" C., water adjusted to 4.0%. Cellulose dried at 1 0 5 O C., no water adjustment. Table VIII.
Cellulose Swelling by Organic Liquids Other than Acetic Acid" Benzene Retention, M1./100 Grams Cellulose _HA CA L M Organic Liquid Time Ib 11" Ib IIc Ib 11' Ib IXc 60min. 24hr. 30days
8.4 12.0
0.9 4.8 9.3
4.2 11.0
0.8
... 10.2 .........
8.1
0.9 7.6 9.0
n-Butyric acid
60min. 24hr. 30days
1.8 8.3
1.4 2.6
0.5
...
1.8 8.7
0.3 0.6 8.2
0.7 0.7
0.3
...
0.3 0.6 6.7
10.0
16.7
3.9
12.9
11.4
8.0
1.3 1.!jd 5.0 19.4
...
1.3 4.1
... ...
Propionic acid
...
4.2
Isobutyric acid
60min. 24hr.
Methanol
60min. 24hr. 30days
Isopropyl alcohol
60 min. 30days
Pyridine
60min. 24hr. 3Odays
......... ... 0... .9 1 . 9 ... 1.8... 1 . 4 ... 1.4 ... 2 . 1 ... 1 . 6 ... 2.3 ... 2 . 3 ... 10.0 10.0 ... 8 . 2 12.0 11.7 ... 14.0 12.4 . . . . . . . . . 12.7 11.4 ...... ... 8 . 9 . . . . . . . . . 10.5 ... 13.6 . . . . . . 3 . 0 ... 1 . 3 ... 2.7 . ... 4 . 2 . 0 . 7 ... 6 . 8 ... 0 . 8 12.2 12.4 14.5 3 . 2 12.9 11.4 ... 0 . 8 1 4 . 3 ... 16.8 ... 14.3 ... 6.1 3.2 15.9 13.5 17.3 ... 13.6 12.1 18.7 18.0
Dimethylformamide
60min.
14.4
24 hr. 30days
17.4
Monomethylformamide
60min.
Formamide
60 min.
. . . . . . . . . . . . 15.5 12.5 ... 14.1 17.5 ... 14.2 11.6 21.7 16.6 14.4 . . . . . . 15.2 14.2 23.4 .....................
16.2 22. 73 27.2 28.4d
a Cellulose sheets soaked at 38' C. (samples in 30-day tests soaked at room temperature) using 2.3 ml. of liquid per gram of cellulose. Cellulose dried at 50' C., water adjusted to 4.0%. Cellulose dried at 105' C., no water adjustment. Cellulose water content adjusted to 4.0%.
acids above acetic acid are much less effective as swelling agents. Methanol, pyridine, formamide, and the methylsubstituted formamides are about as effective as acetic acid as swelling agents for unmercerized celluloses. Methanol, formamide, and monomethylformamide are unique among organic liquids in their ability at room temperature to swell thoroughly dried mercerized celluloses quickly. Like
INDUSTRIAL A N D ENGINEERING CHEMISTRY
water, these solvents have high dielectric constants and a pronounced tendency toward hydrogen bond formation. These properties are probably both necessary in good swelling agents for cellulose. None of these solvents, with the possible exception of formamide, swell cellulose to the degree found when water is the swelling agent. The behavior of pyridine is noteworthy. With unmercerized celluloses
CELLULOSE SWELLING
c
.11 8c C
w
-
c
?2
6-
Q)
c
w N
-
m5
c
4-
-
2 1
OO
Acetic acid pickup (m1./100g. cellulose 1 Figure 1 . vapor
a
0 A
it is able to swell the samples fairly rapidly to about the same level to which they should be swollen by acetic acid. I t will ultimately swell mercerized cellulose beyond the limit attainable with acetic acid, but this swelling is a very slow process at ordinary temperatures. This probably contributes to the fact that pyridine is an excellent medium in which to perform certain esterifications of cellulose (7, 5 ) . Even in 30 days a t room temperature, pyridine does not swell cellulose to the degree obtained by immersion in water. In measuring the benzene retention of celluloses given a preliminary treatment with amides or their substituted derivatives, special measures are necessary to assure their complete removal before exposure in benzene. Apparently they can be satisfactorily removed by rapid washing in acetic acid containing a trace of perchloric acid. They are strongly sorbed by cellulose, and it is difficult to remove them completely by simple washing and soaking in benzene.
8
12
16
20
24
Acetic acid pickup (m1./100 g. cellulose) Figure 2. Swelling of linters in acetic acid vapor
Swelling of unmercerized pulp in acetic acid
Oven-dried HA exposed to acetic acid vapor followed by immediate displacement with benzene Degree of Saturation AcOH Vapor Symbol Sheet Form of AcOH Vapor, % Exposure, Hr. 0 Mill sheet 100 1I 2 , 4 , 7 72 Hand sheet 100 1,4, 24,72 0 Hand sheet 95 1, 2, 4, 7, 72 Hand sheet 75 72 Hand sheet 35
4
Oven-dried 1, exposed to acetic acid vapors, followed by immediate displacement with benzene Sheet Degree of Saturation AcOH Vapor Sym bo1 Form of AcOH Vapor, % Exposure, Hr. 100 1, 2 , 4 , 7 0 Mill sheet A Hand sheet 100 72 Hand sheet 95 1,2,4,7,24,72 Hand sheet 75 1, 2 , 4 , 7 , 7 2 Hand sheet 50 1,2,4,7 rn A Hand sheet 35 72
::
swelling agent. Plots of benzene retention us. acetic acid picked u p from the
atmosphere are shown for linters, an acetylation wood pulp, and for mer-
14
0
12 -
Degree of Saturation of AcOH Vapor, Symbol
70
0
100 95 75 35
0
0
a
c 10-
0 .-c
c
W +
? W
c
-
-
8-
-
-
AcOH Vapor Symbol Exposure, Hr.
o 0
0 A
24.48.72 1,4,24,72 1, 2, 48, 7 2 72
-
Swelling of Cellulose with Reagents Condensed from Vapors .
If swelling is accomplished with the limited acetic acid or water that is picked up by dry cellulose from an atmosphere containing vapor, somewhat lower benzene retentions will be found than from flooding the sheet with
4
20
24
Acetic acid pickup (m1./100 g. cellulose 1 Figure 3. Swelling of mercerized linters in acetic acid vapor Oven-dried ML exposed to acetic acid vapors; benzene
acetic acid picked .up immediately disploced with
VOL. 49, NO. 5
MAY 1957
91 1
ments are in progress to show whether or not this actually happens. The lower degree of swelling produced by sorption of water vapor by mercerized samples contrasts with the higher swelling found if the samples are flooded.
P c 0 .c
C e,
Celluloses Resistant to Swelling
c
?? Q,
C e, N
C
aJ
m
Water vapor pickup (rn1./100g. cellulose) *
4.
Swelling of celluloses by sorbed water vapor
Oven-dried sheets conditioned for 24 hours at 40’ C. over water vapors a t 100, 95, 85, and 35% R.H., followed by dewatering with n-butyric acid (20’ C.) and displacement with benzene A,A. Benzene retention of water-flooded sample dewatered with acetic ocid vs. water retention of same water-flooded sample
cerized linters in Figures 1 to 3. The fact that these plots are practically linear indicates that the swelling produced by limited acetic acid under these conditions is roughly proportional to the acetic acid taken from the air. The ratio of benzene retention to acetic acid picked up is slightly different for the several types of cellulose, but in each case is slightly below 2.0. Evidently the swelling produced by such limited acetic acid varies between celluloses and is dependent on the fiber history. The close correspondence between the acetic acid-benzene ratio here and the acetic acid retentionbenzene retention of water-swollen celluloses referred to earlier is very significant, as it shows a distinct difference in the manner in which benzene and acetic acid are retained by cellulose. The real reason for the difference is not clear. Displacement of the more polar acid by benzene may be accompanied by partial collapse of the expanded fiber, part of the swelling agent may be
Table IX.
Benzene Retention by Celluloses Swollen after Treatment with Ultraviolet Light or Formaldehydea
Time of Soak in AcOH, Min. 5 15 30 60
24 hr. 30
attached in some manner that does not produce swelling, or the nonpolar liquid may be attached to the interior surfaces of the swollen cellulose as a monomolecular layer whereas the swelling agent is associated in such a manner that, in effect, more than a monomolecular layer is held. The linear relationship between swelling agent and degree of swelling as indicated by benzene retention is not found if water vapor is used as the swelling agent (Figure 4). The authors suspect that the sorption of limited water vapor, especially a t low relative humidities, does not produce enough swelling to permit penetration by the somewhat larger benzene molecules. I t is possible, however, that in the removal of the water by such a dewatering agent as butyric acid there is an actual shrinkage of the swollen fiber, so that in this case the benzene retention does not measure the degree to which swelling had taken place in the swelling agent. Experi-
Ultraviolet Light-Treatedb L
Hd
3.2 8.5 10.0 10.8
...
9.9d
3.4 7.3 8.9 9.4 m..
8.QJ
Formaldehyde Treatedc L 4.5 4.4 4.9 4.8 6.1
...
Samples dried at 50’ C., adjusted t o 4.0% H20, soaked in 2.4 parts of 99.8% acetic acid at 38’ C . * Mill sheets received 24-hour ultraviolet light exposure on each side of sheet. Shredded cellulose soaked in solution of 10% HCHO 0.1% (C0OH)s for 24 hours at room temperature, followed by successive oven-drying at 54O and 105’ C. Treated cellulose washed in distilled water and sheeted by hand. Direct comparison value of corresponding sample without exposure to ultraviolet light but oven-dried at 105’ C.
+
912
INDUSTRIAL A N D ENGINEERING CHEMISTRY
It is difficult to esterify celluloses which have been heated with formaldehyde, because such products resist swelling in the activating stages of the acetylation sequence. The very low benzene retentions (Table IX) found after an activation which is normal for acetylation confirm the picture that the formal cross linkages have greatly reduced the capacity of these celluloses to swell. The benzene retention of cellulose exposed to ultraviolet light is about the same as that of cellulose dried in the oven a t 105” C. Apparently the drying which accompanies the exposure to ultraviolet light is similar to oven drying in its effect on ease of swelling. Summarizing, the level of benzene retention shown by celluloses appears to reflect accurately the state of swelling of a cellulose sample at the time of displacement of the swelling agent with benzene. As far as direct comparison with esterification experience is possible, the henzene retention correlates very well with the degree of swelling qualitatively considered to have taken place. The measurement offers, for example, a method of evaluating the relative effectiveness of swelling agents, of determining the influence of conditions of manufacture of celluloses on their swelling properties, and of measuring the swelling effects produced by the several preliminary stages before esterification. I n short, measurement of the benzene retention is a new tool which should be valuable wherever the degree to which cellulose is swollen is important.
Literature Cited (1) Blouin, F. A., Reeves, R. E., Hoffpauir, C. L., Textile Research J . 26, 272
(1956). ( 2 ) Hermans, P. H., “Colloid Science,” H. R. Kruyt, ed., p. 522, Elsevier, New York, 1949. (3) Howsmon, J. A., in “Cellulose and Cellulose Derivatives,” Emil Ott and H. M. Spurlin, eds., p. 418, Interscience Publishers, Kew York, 1954. (4) Lauer, K., Kolloid-Z. 107, 86 (1944). ( 5 ) Malm, C. J., Mench, J. W., Kendall, D. L.. Hiatt. G. D.. IND. ENG. CHEh1.‘43, 684’(1951). ’ ( 6 ) Richter, G. A., Herdle, L. E., Gage, I. L., Ibid., 45, 2773 (1953). ( 7 ) Richter, G. A,, Herdle, L. E., Wahtera, W. E., Ibid., 44, 2883 (1952). (~, 8 ) Richter. G. A.. Perkins. E. L., Herdle. L. E.; U. S.‘Patent 2,622,080 (Dec: 16,1952). RECEIVED for review June 5 , 1956 ACCEPTED October 17, 1956
