CELLULOSIC FIBERS
I
ELIAS KLEIN and JAMES E. SNOWDEN Southern Regional Research laboratory, U.
S. Department of
Agriculture, New Orleans, la.
Replacing Hydroxyl Groups in Cotton Cellulose Several promising derivatives with good fiber properties have been made, particularly one with excellent flame resistance. But before these fibers can be marketed a new method of preparing them must be found
As
an outgrowth of an over-all program to prepare chemically modified cotton fibers, work was begun in July 1955 t o prepare new cellulosic fibers by replacing hydroxyl groups of cotton cellulose. Generally, there are three methods for modifying cotton: by preparing cellulose carboxylic esters, cellulose ethers, and other derivatives. Because during solvolysis, sulfonic acid esters allow cleavage of the carbonoxygen instead of the sulfur-oxygen bonds, they are ideal igtermediates for hydroxyl scission. This is distinctly different from the corresponding carboxylic esters which cleave at the acyl carbonoxygen bond. However, this difference also leads to serious chemical complications involving steric effects. Pretreatment
The best way for preparing cellulosylsulfonates was studied first. Parallel studies were conducted with an aliphatic compound, methanesulfonyl chloride, and an aromatic compound, p-toluenesulfonyl chloride as the sulfonic reagents. There are two paths by which the intermediate sulfonate might be prepared -one utilizes reaction of sulfonyl chloride in a n inert solvent with soda-cellulose and the other uses sulfonylpyridinium halide as the intermediate. Besides the difference in reagents, there is a more significant distinction-the first method is essentially an aqueous system because the aqueous phase associated with the fiber is the reaction site, but the second method, carried out in a pyridine medium, is entirely nonaqueous. Initially, the pyridine method was used because it was logical to suppose that the excess pyridine would hold acid degradation to a minimum. The reaction between cellulose and the sulfonyl chloride liberates hydrochloric acid and if this is not removed, severe acid hydrolysis of the polymer occurs. Preparing cellulose sulfonate was first tried by mer-
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cerizing the fiber to improve its accessibility, neutralizing the sodium hydroxide solution, and then solvent exchanging to pyridine without intermediate drying stages. This process gave comparatively poor yields of ester, and a high degree of discoloration; reducing the reaction temperature from 25' to 0' C. improved the color but did not appreciably change the degree of substitution. Therefore, it was felt that to produce a satisfactory product, three conditions were necessary: First, the cellulose structure had to be opened-Le., accessibility would have to be relatively high; second, because the pyridinium complex is highly sensitive to water, water content of the cellulose had to be low as possible; and third, to retain the initial fiber strength, acid degradation had to be held to a minimum. Previous work in this laboratory (3) indicated that amine decrystallization facilitated cellulose modification; consequently, this method of activation was explored and compared to mercerization. All samples were scoured, then ground to pass a 20-mesh screen, and next swollen for 2 hours in ammonia or primary amines which were then solvent-exchanged with dry pyridine. Liquid ammonia pretreatment gave the best yield of toluenecellulose, ethylamine the second, and mercerization the least. For the methanesuifonyl ester, the mole ratio of sulfonyl chloride to anhydroglucose
INDUSTRIAL AND ENGINEERING CHEMISTRY
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was 9 to 1. Data, shown in Figure 1, was obtained at 0' C. Mercerization is not the most effective pretreatment, although it leads to higher degrees of swelling than ammonia and amine treatments. This apparent contradiction may result from ability of ammonia and amines to combine with the water present and form hydrates which are subsequently removed in the pyridine wash. This would reduce moisture content of the remaining cellulose without permitting collapse of the polymer structure. I t is concluded that ammonia and ethylamine greatly improve yield and purity of cellulose sulfonate if they are prepared from the corresponding sulfonylpyridinium chloride. Aqueous Preparation
An alternate method of preparing the sulfonate is by padding the cellulose with a sodium hydroxide solution and then suspending in an organic solvent containing the sulfonyl chloride ( 6 ) . I n this system, the sulfonyl chloride must dissolve in the aqueous solution about the fiber and then diffuse to the reaction site. The inherent disadvantage here is hydrolysis of the reagent with concurrent loss of base. Using 21% sodium hydroxide as a padding solution on scoured yarns, toluenesulfonyl esters were prepared, having degrees of substitution
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TIME I N HOURS
I Figure 1. Effects of various pretreatments on yield, using a 2 hour swelling period
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6WAVE LENGTH, 8 1 MICRONS 1I0 I
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Residual Esters After forming the sulfonate ester of cellulose, there is the problem of using it as an intermediate for making fibrous cellulose derivatives. The objective here was to form cellulose derivatives having carbon linkages to atoms other than oxygen-e.g., cellulosyl amines and halides. The first complication is that
Table I, Absorption of Sodium Hydroxide by Cotton Tapes from Different Media
70
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14
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16
WAVE LENGTH, MICRONS
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8.
Acetylated
Traces of infrared spectra for cotton yarn
ranging from 0.34 to 0.48 ( 5 3 and 6.5% sulfur, respectively) with the highest obtained in isolated cases. When preparation of fabrics was attempted by this method, the product contained less than 1% sulfur. This reduction in yield was presumably caused by swelling restrictions imposed by the fabric structure. When scoured cotton tapes were padded with aqueous sodium hydroxide solutions or similar solutions saturated with dioxane, adding dioxane caused preferential absorption of sodium hydroxide (Table I). In the tosylation reaction, adding dioxane produced parallel trends in yield. Although this absorption is not the sole determinant in the degree of reaction obtained, it is an important factor. Further studies relating the absorption of base to formation of cellulose modifications are under way.
4.0 5.0 6.0 7.0 8.0
o
Mesylated and prepared by ethylamine pretreatment
Figure 2.
NaOH,
121
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20
Absorption, Moles per Mole Anhydroglucose DioxaneWater Water 0.675 0.690 0.725 0.740 0.770
0.245 0.273 0.325 0.390 0.452
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Simultaneously acetylated and mesylated
those sulfonyl groups, which have esterified the secondary hydroxyls, are not amenable to cleavage without opening of the glucose ring. Several explanations have been suggested for this, but the most probable one is that replacement of sulfonyl groups at secondary carbon atoms normally requires a Walden inversion. Because of ring bonding of the glucose secondary carbon atoms and the polymeric bonding of these ring units, a Walden inversion of the Cz or CSatoms is difficult to reconcile. Consequently, only primary carbon-substituted sulfonates cleave in replacement reactions. This results in retention of sulfonyl content in the final product, depending on preferential orientation encountered in forming sulfonates. When cellulosyl sulfonates are formed by amine pretreatment and reaction in pyridine, subsequent hydrolysis with 0 . 5 N alcoholic potassium hydroxide results in retention of 6 0 to 70% of the initial sulfur content, depending on the degree of oxidation allowed during hydrolysis. Because the objective was to form cellulose derivatives having other than carbon-oxygen bonds, attempts to circumvent this seemed worth while. This was partly done by simultaneously acetylat-
5
6 7 8 9 IO I1 WAVE LENGTH, MICRONS
__i__lj 14 15 16
ing and methanesulfonating in a pyridine medium, using the corresponding chlorides. Traces of the infrared spectrum for methanesulfonated cotton yarns prepared by ethylamine pretreatment show bands a t 12.0 and 3.25 microns, which are characteristic of the methanesulfonyl group (Figure 2 4 , curve A). Curve B is a trace of the same material after 24-hour hydrolysis, using the Eberstadt method (7). Intensity of the rnethanesulfonyl band is reduced but quantitative analysis shows that 70% of the original sulfur content remains. In spectral traces of an acetylated yarn, both before and after Eberstadt analysis, the characteristic carbonyl bands (5.66 microns) have disappeared (Figure 2,B). Spectral traces of a yarn simultaneously acetylated and methanesulfonated contain both carbonyl and methane-sulfonyl bands (Figure 2,C). Curve B is for the same sample after Eberstadt analysis. I n contrast to Figure 2,A where Eberstadt saponification caused only a small decrease in methanesulfonyl band intensities, this sample shows no evidence of residual methanesulfonyl groups; as expected the carbonyl bands have also disappeared. There are two explanations for this behavior: First, in the presence of the VOL. 50, NO. 1
JANUARY 1958
81
acetylating reagent, orientation of the methanesulfonyl substitution reactionmay be directed principally toward primary hydroxyls of the cellulose. Second, presence of the acetyl groups on neighboring carbon atoms activates the methanesulfonyl groups and increases their lability. The first explanation is hard to reconcile with experiments of Malm (4) on regenerated cellulose. Lower diffusion rate of reagents through cotton cellulose as compared to regenerated celluloses indicates that distribution between primary and secondary groups is random. The second explanation agrees with data recently published by Haskins and Sunderwirth (2) who reported that a cellulose acetate sample containing 2.18 acetyl groups per anhydroglucose unit, when tosylated underwent stoichiometric replacement of the sulfonate ester residues. When 2.0 or more acetyl groups per anhydroglucose unit are contained, only two types of free hydroxyl groups can occur; primary groups which undergo exchange, and secondary groups having an acetyl group on neighboring carbons. At a degree of substitution less than 2, secondary hydroxyls not adjacent to a n acetylated carbon may also occur. For Figure 2,C, the acetyl degree of substitution of the sample was slightly below 2.0, and wet analysis showed that 8% of the original sulfur was retained. Therefore, those secondary sulfonate ester groups having an acetyl group on the neighboring carbon atom are subject to it’alden inversion and may be cleaved.
Replacement Reactions Samples without acetyl groups were selected for actual replacement of the methanesulfonyl and toluenesulfonyl groups because of both time sequence and analytical complications. As pointed out (7) sulfonyl groups are easily replaced with halides, especially bromides and iodides. Infrared spectra are most convenient for illustrating the variety of other reaction which cellulosyl methanesulfonate undergoes. The spectra in Figure 3 were obtained using the potassium bromide disk technique, as described by O’Connor ( 5 ) . Curve A is for methanesulfonated cotton yarns prepared by amine pretreatment and reaction in a pyridine medium; the others are for this yarn after reaction with a variety of nucleophilic reagents. The characteristic bands a t 3.25 and 12.03 microns in curve A are retained to varying degrees in the other curves, depending on amount of exchange obtained. Curve B is for 6-deoxyphthalimidomethanesulfonyl cellulose, if the nomenclature can be extended this far. This compound was obtained by refluxing the methanesulfonated yarn in a buffered aqueous solution of the potassium salt of phthalimide.
82
100.-
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4
’ 6 Q ’ I b I h ’ I d ’ WAVE LENGTH, MICRONS
ucts of the reaction of methanesulfonyl cellulose with the sodium salt of p-methylthiophenol. This reached a relatively high degree of substitution as evidenced by intensity of the 6.70-micron band. The band a t 12.45 microns is also present in the reagent and does not represent new linkage. Curve F is a trace from the saccharine derivative of cellulose obtained by the usual exchange reaction. The band at 5.75 microns is characteristic of the carbonyl group. Curve G is a trace of a rather unique derivative obtained by refluxing the methanesulfonyl cellulose in a solution of potassium thiocyanate. Both elemental analysis and intensity of the bands a t 4.62 and 4.85 microns indicate substantial degree of exchange. I n addition to the derivatives illustrated by spectral traces, derivatives of methanesulfonyl cellulose have also been obtained using the reagents, pentabromophenol, bis(isopropy1)-dithiophosphoric acid, bis(dibromopropy1) phosphoric acid, nitropropane, and potassium cyanide. All of these reagents have an electron pair to donate. This common denominator and their solubility in a suitable solvent of high dielectric seems to be the only criteria for utilizing them in exchange reactions with cellulosyl sulfonates. The work described here is incomplete and is being continued.
1
Figure 3. Traces of infrared spectra using potassium bromide disk for mesylated cotton yarns prepared b y amine pretreatment and reaction in a pyridine medium (A) compared with this mesylated yarn after reaction with a variety of nucleophilic reagents
Acknowledgment Appreciation is expressed to R. T. O’Connor, Elsie DuPrB, and E. R. McCall of this laboratory, who cooperated so fully in the preparation of the infrared spectra. literature Cited
The band at 5.75 microns is characteristic of the carbonyl band, and that a t 6.12 microns is characteristic of the double bond (C=C) in the ring. Curve Cis for the product obtained by treating methanesulfonated yarn with the sodium salt of p-toluenesulfonamide. New absorption bands appear a t 6.25 (benzene ring), 12.30, and 14.20 microns. This last band is not found in either the reagent or the methanesulfonyl cellulose and appears to arise from the new bond formed. Curve D is for the product from exchanging the sulfonate groups with npropylamine. Substitution obtained was relatively low and presumably caused by weak basicity of the reagent. Kjeldahl analysis and appearance of the 3.45micron band caused by carbon-hydrogen stretch showed a low degree of substitution. Curve E was obtained from the prod-
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Genung, L. B., Mallatt, R. E., IND. EKG. CHEM., A N A L . ED. 13, 369
(1941). (2) Haskins, J. F., Sunderwirth, S. G., J . Am. Chem. SOC. 79, 1492 (1957). ( 3 ) Loeb, L., Segal, L., Texas Research J . 24,654 (1 954). (4) Malm, C. J . , Tanghe, L. J., Laird, C. B., J . Am. Chem. Suc. 70, 2740
(1948). ( 5 ) O’Connor, R. T., DuPr6, F,. F. McCall. E. R.. Anal. Chem. 29, 998 (1957). (6) Sakurada. IC.. Nakashima. T.. Scz. \
I
Papets inst. Phys. Research’( T o k y o ) 6 , 197 (1927).
17) Schwenker. R. I?.. Jr.. Pascu. E., Div. of Celldose Chemis’try, 13ist Meeting, ACS, Miami, Fla., April 1957. \
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RECEIVED for review June 6, 1957 ACCEPTEDOctober 4, 1957 Division of Cellulose Chemistry, Symposium on New, Chemicallv Modified Cellulose Fibers, 131st Meeting, .4CS, Miami, Fla., April 1957.
