EXPERIMENTS OK THE REDUCTION OF T H E CRYSTALLINITY OF COTTON CELLULOSE’** LEOK SEGAL, MARY L. NELSON, A N D CARL bf. CONRAD
Southern Regional Research LaboratoryP New Orleans, Louisiana Received February 87,1060
In the crystalline regions of cellulose fibers, according to the present concept, the molecular chains of cellulose lie parallel and in three-dimensional arrangements of high geometrical order. In the amorphous regions of the fiber, on the other hand, other sections of the same molecular chains are assumed to lie in less ordered states. There are no sharp boundary lines between the two regions but rather transitional areas through which the same molecular chains pass in gradually more and more disordered arrangements. Secondary valence bonds such as hydrogen bonding are assumed to be the forces that hold the molecular chains close together laterally in the crystalline regions. Thus, anything that would neutralize or substitute for these forces would permit the chains to separate and thus increase the amorphous regions at the expense of the crystalline regions. This transformation should be evidenced by a change in the sharpness of the x-ray pattern and by a change i? the rate of hydrolysis by hydrochloric acid. There are indications in the literature that crystallinity can be reduced by both mechanical and chemical means. On the basis of the disappearance of the characteristic crystalline lattices in the x-ray diffraction pattern, Hem, Steurer, and Fromm (5) and Hermans and Weidinger (7) concluded that the vibratory ball mill completely destroyed the crystallinity of the cellulose in the material. Selson and Conrad (13) concluded that grinding cotton in a Wiley mill to pass a 20-mesh sieve lowered the crystallinity, as indicated by the acid-hydrolysis method, by some 10 per cent. It goes without saying that the grinding caused the fiber to be destroyed. It has been shown by Andress (l),and also discussed by Sisson (15), that mercerization with sodium hydroxide causes an enlargement of the unit crystal cell. Philipp, Nelson, and Ziifle (14) showed that this was accompanied by a lowering of the crystallinity from about 89 per cent to 68 per cent. Mark (9) presented evidence that other strong swelling agents, such as phosphoric acid, calcium rhodanate, and quaternary ammonium bases, produce liquid-like patterns, a result which he interpreted as indicating a large increase of amorphous cellulose at the expense of the crystalline component. He implied that the amorphous patterns were obtained with the swelling medium still present. A greater 1 Presented before the Division of Cellulose Chemistry a t the 116th Meeting of the American Chemical Society, Atlantic City, New Jersey, September 18-23, 1949. Report of a study made under the Research and Marketing Act of 1946. 3 One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research .4dministration, U . S. Department of Agriculture. 326
326
LEON SEGAL, MAItY L. NELSON, A N D OARL !+ CONRAI) l.
observed radial distribution of s-ray diffraction than would be expected on a theoretical basis was interpreted by him as due to superimposed diffraction from the amorphous component. Many investigators, and particularly Mark (lo), have pointed out that the native celluloses suffer from too strong attraction between neighboring chains, which while contributing to high tensile strength at the same time is responsible for an undue stiffness and brittleness. Mark has outlined some of the conditions which should result in improved properties of fibers (11). In view of the apparently very high crystallinity of the cellulose present in cotton fiber, it was believed by the present investigators that the tensile properties, particularly the ability to recover from large deformations, could be improved if this crystallinity could be materially reduced (2). It was believed that this could be accomplished by strong swelling agents, and it was assumed that in the highly swollen condition “blocking” groups, such as methyl, acetyl, or others, would have to be introduced to prevent recrystallization of the cellulose back to its original condition the moment the swelling agent was removed. MATERIALS AND METHODS
The cellulose used consisted for the most part of a commercially purified cut cotton fiber having a degree of polymerization of about 1050 and indicated crystallinity.of 89 per cent. A few experiments were performed on a kier-boiled, bleached, 20/1 loose-twist yarn for the determination of the effect of reduced crystallinity on tensile properties. Two types of swelling agents were employed. Trimethylbenzylammonium hydroxide (Triton B) was used as a commercially available representat.ive of a series of quaternary ammonium hydroxides, described especially by Sisson (15), which in sufficiently great concentration completely dissolve cellulose. Only a few results of treatment with this agent are reported. Most of the experiments were carried out with the lower primary alkylamines, alone and in combination. These have been investigated by Davis, Barry, Peterson, and King (4)and, so far as is known, do not cause solution of the cellulose. Crystallinities of the treated products and of the controls were estimated by the acid-hydrolysis residue technique of Philipp, Nelson, and Ziifle (14) with improvements applied by Nelson and Conrad (12). Radial traces across the2quator of the x-ray diffraction patterns were ohtained on untreated and treated samples with a recording x-ray diffraction spectrometer, employing a Geiger-Mueller counter tube. The specimens consisted of samples weighing 0.136 g., ground in a Wiley mill to pass a 20-mesh sieve and compressed in a half-inch ring to form discs of 0.9-mm. thickness. Moisture regains are reported which were obtained on approximately 1-g. samples of material which were placed in a closed system through which air \vas circulated by means of a diaphragm pump. 4 saturated ammonium sulfate solution, producing approximately 81 per cent relative humidity in the atmospherc ovcr n. c~onsidrrnlderange of temperature, wits incorporated in the train. Tensile tests were made on the IP-2 inclined plane tester and the results ex-
REDT~CTION OF THE CRYSTALLINITY OF COTTON CELLIILUSE
0 (control) 30 33 36
89 85 ~
:;
89 80 64 54
327
328
LEON SEGAL, MARY L . NELSON, AND CARL M. CONRAD
The reduction of crystallinity with Triton B was almost all that could have been hoped for, and it would appear from the progressive change with change in concentration of Triton B that by careful control of the concentration, and perhaps temperature, suitable control of final crystallinity might be achieved. But, unfortunately, fiber damage, evident from gelatinization and partial solution of the cellulose, increased with increasing concentration of Triton B. Cotton soaked in 30 per cent Triton B showed no visible fiber damage, whereas that in the 33 per cent solution showed considerable damage. The 36 per cent solution destroyed the fiber structure, though it did not dissolve the cellulose rn did the 38 per cent solution. This damaging effect of Triton B upon the fiber structure made its use seem unpromising, and prompted a study of the other swelling treatment, employing alkylamines. STUDY WITH ALKYLAMINES
Davis, Barry, Peterson, and King (4) found that anhydrous amines, up to and including heptylamine, cause a strong swelling of cellulose without dissolving it, and that the swelling, greater even than with the quaternary ammonium hydroxides, is accompanied by increases in the a and c dimensions of the unit crystal cell, the 101 interplanar distance of which increases with the chain length of the amine. These workers reported that the amines above propylamine required a pretreatment with methylamine or ethylamine before they would enter the crystal lattice, but after this “priming” progressively caused greater distention of.the lattice as the series was ascended. On the basis of these observations, it was thought possible that the lattice rearrangement would be accompanied by a reduction of crystallinity in a way similar to that accompanying mercerization; this might be expected to take place to a progressively greater degree with the higher members, where the greater distortion of the lattice occurs. To obtain further knowledge of this point, the purified cellulose was treated with the desired amine or combination of amines. The residue was then pressed to remove as much of the swelling agent as possible, and extracted for 2 hr. in a Soxhlet with chloroform to remove the residual swelling agent. Chloroform was employed because of its low dipole moment which, it was believed, would favor retention of the increased amorphous component. Recycling of the amine during this extraction was prevented by addition of a few milliliters of 85 per cent phosphoric acid to the distilling flask, where it formed a salt with the amine. The extracted cotton was pressed and allowed to dry in the air. Preliminary treatments, using ethylamine followed by either heptylamine or hexylamine, are described and their effects shown in table 2. The 33 per cent aqueous amine mas without appreciable effect in reducing the crystallinity of the control, whereas heptylamine, and to a slightly lesser degree, hexylamine, preceded by anhydrous ethylamine, caused a marked reduction of the crystallinity. The hexylamine, being commercially available, should he a more practicable reagent for general use even if slightly less effective. I t was next sought to determine whether a mixture of lower and higher amines in a single-bath treatment could be substituted for the two-bath treatment used
REDUCTION OF THE CRYSTALLINITY O F COTTON CELLL'LOSE
329
to obtain the data of table 2. The results are shown in table 3. I t will be seen that no decrease, but rather, if anything, a slight increase of crystallinity resulted when the ethylamine content of the mixture amounted to 50 per cent or less; hut that when the ethylamine amounted to 75 per cent of the total weight, the crystallinity was reduced to 68 per cent in one case and 50 per cent in the second. This led to control experiments with ethylamine alone, \\.here the crystallinity was reduced by treatment at room temperature to 38 per cent and by treatment at 10°C. to 23 per cent. The large reduction with ethylamine alone had not been anticipated and occurred apparently without any loss of the fiber structure. TABLE 2 Crysfallinities resiilting from treatment of cellitlose icilk efhiilnnzinc ,iollowrd hi! higher amines I TPEANENI
CPYSIALLINITY
I
per (e11
Untreated cotton (control) . . . . . . . . . . . . . . . . . . . . . . . . . , . . . Heptylamine, preceded by 33 per cent aqueous ethylarnine . . . , I Heptylamine, preceded by anhydrous ethylamine . . . . , .. Hexylamine, preceded by anhydrous ethylnniine . . . . , . . .. .
89
,
Xi 40
, , ,
, , ,
46
~
TABLE 3 Cryslaflinilies rrsitlling f r o m lhe iise of diflerent proportions of clh!iloviinc nnrl ker{ilaniine, and of elhylainine alone, i n Q single-balk trealntertl COYPOSIIION OF Y I X I U P C EIEYLAYINE: HEXYLAMINE
per ccnl
25:i5 50:m 75:25 75:25
1oO:O (at room tempPrature) 1oO:o (at 10°C.)
I
CRYSTALLININ
I
per cent
i
91
91
68 50
38 23
Further work is being done t o determine the crystallinities ohtainable with proportions of amines other than those shQwn in the table. According to Sisson (l5), when a swelling agent is removed by means of solvents less polar than water, as was done intentionally here, the dried swollen cellulose retains to a greater or less degree its amorphous condition. However, the original crystallinity presumably will be restored upon wetting with water, followed by drying in air, owing to the hydrogen bonding thus induced. This effect had resulted when material whose crystallinity had been reduced in a Wiley mill (13) or in a vibratory ball mill ( 5 , 7) was treated with water. A series of experiments was accordingly carried out on material swollen in ethylamine and hexylamine to determine whether, and to what extent, the crystallinity reduced by the amine treatments would be restored by immersion in boiling
330
LEON SEGAL, MARY L . NELSOS, AND CARL M. CONRAD
mater, followed by drying in air. In view of the very surprising results with this series, a second series was carried out with greater attention to detail. Table 4 presents the results of the two series. It is evident that only slight restoration of crystallinity occurs in both series, although its extent is rather insignificant, and that longer boiling before drying seems to cause a reversion. In order to obtain additional information concerning the interpretation of crystallinities determined by the acid-hydrolysis method, the amine-treated products were investigated by means of x-rays. If the crystallinity is actually reduced by the amine treatments, this should be made evident by a reduction of diffraction by the crystalline lattices and an increase in scattered radiation due to the amorphous component. Figure 1 presents the results of a series of treatments. Trace A is that of a sample before treatment. Here the characteristic lattice diffractions from the 101, lOi, 021, and 002 planes are strong and relaTABLE 4 Pnrfrnl resfoi ntion of cr?lslalltnity of ancane-swollen cottons bil botlzng zn I N Z I P T for diflerent periods CRYSTALLINITY OF PPEPAPATIOI
T I Y E I I BOILISG WATER
Seriep I bourr
0 (control) 0.5 1.0 1.5 2.0 3.0
per
CL"l
!
Series I1 per cent
54* 58
52
65*
52
46
60 53
50
* These t\ro samples of series I were from the same batch, while the remainder of t h r series I samples u e r e taken from another similarly treated batch of cotton. tively sharp. A parallel sample after treatment with anhydrous ethylamine for 5 min. is represented by trace B. The acid-hydrolysis determination had indicated 44 per cent crystallinity. I t will be noted that the characteristic diffractions are diminished, a result which indicates a decrease in the amount of crystalline material present. Trace C represents the diffraction of a sample treated for 4 hr. with ethylamine. The crystallinity of this sample was estimated to be 24 per cent. Here the characteristic diffractions are greatly diminished and the overall appearance of the trace strongly indicates an increase of the amorphous component. Trace D, obtained from a sample treated with ethylamine and then hexylamine (crystallinity about 20 per cent), does not differ much in appearance from trace C. These first experiments indicate that a certain minimum time in ethylamine is necessary in order to reduce crystallinity to low vahes. The effect on the lowered crystallinity of immersing the treated cotton for 2 hr. in boiling water, followed by drying in air, is shown in figure 2. Traces h and B were obtained from two samples of the treated cotton listed
REDUCTIOS O F THE CRYSTALLINITY O F COTTOS CELLCLOSE
33 1
FIG.1. Radial traces of diffraction intensities along the equators of the x-ray pntternx of amine-treated cottons. A , untreated purified cut cotton; D , cotton immersrd 5 mill. i n anhydrous ethylamine; C, cotton immersed 4 hr. in anhydrous ethylamine; D ,cotton immersed 4 hr. i n anhj.drous ethylamine arid then ovrrnight in anhydrous hexylamine.
in table 4 under series 11. Trace B, from the boiled cotton, shows a slight improvement of the crystalline pattern as evidenced by the sharpening of the 002 lattice diffraction, but there is no indication that water induced a significant restoration of the crystalline lattice to that of the untreated cotton, as might have been expected. The acid-hydrolysis data for this hoiled sample indicirtc a relatively small increase of crystallinity.
332
LEON SEQAL, MARY L. NELSOS, AND CARL M. COSRAD
FIG.2. Radial traces of diffraction intensities along the equators of the x-my ptittrrns of amine-treated cottons, showing the effect of boiling water. A , cotton briefly immersed in anhydrous ethylamine and then overnight in anhydrous hexylamine; B , the above cotton immersed 2 hr. in boiling water; C, cotton immersed 4 hr. in anhydrous ethylamine and then overnight in anhydrous hexylamine; D, the above cotton immersed 2 hr. in boiling water.
Trace C is the same as the bottom trace in figure 1, and trace D was obtained from the water-boiled portion of the treated cotton that was used for obtaining that trace. It is still evident, even in this “low crystalline” cotton, that boiling water has not effected any marked restoration of the original crystalline lattice. In general, then, it may be said that the x-ray analysis qualitatively supports the conclusions drawn from the crystallinity determinations.
REDUCTION OF THE CRYSTALLINITY OF COTTON CELLULOSE
333
If we regard the amorphous regions of cellulose as that portion in which moisture adsorption from a moist atmosphere takes place, then an increase of mois. ture adsorption under standardized conditions should indicate a decrease of crystallinity, other things being equal. This type of relationship has been found to hold, for example, in mercerized cellulose (15, 16, 17) in which acid-hydrolysis determinations (14) have indicated a decrease of crystallinity. It seemed desirable to examine this relationship in amine-swollen cotton. The reaults of such a
other samples to
series of experiments, carried out according to the moisture regain method described above, are shown in figure 3. It can be seen that a good linear relationship exists between the moisture regains a t equilibrium at 81 per cent relative humidity and the crystallinities of the various samples, including the amine-treated ones, as determined by the acid-hydrolysis methods. Three of the points at the upper part of the curve (indicated by half-black circles) were not produced by amine treatment but are points, adjusted slightly to 81 per cent relative humidity, obtained by Nelson and Conrad (13) on grinding portions of the same untreated cotton used in
334
L E O N SEGAL, MARY L. NELSON, AND CARL M. CONRAD
these studies. The full-black circle is for the untreated control sample. That all the points lie about the same straight line tends to confirm the validity of the relationship between the crystallinities and the moisture regains of the aminetreated samples. The appearance of the cross-sections of fibers swollen with the amines is shown in figure 4. There is evidence that the cross-sections have been enlarged and the lumens opened in the treated samples. The swelling seen in figure 4, B and
c:
B
A
FIG.4. Cross-sections of untreated and amine-swollen cotton fibers. A , untreated cotton; B, cotton swollen with anhydrous ethylamine; C, cotton swollen with ethylamine and hexylamine.
TABLE 5 Eflect of amine treatments on the tenacity and elongation of a cotton Iiarn TREATMESI
TESACITY
-1
1. Bleached and kiered 20/1 yarn, untreated control. . . . . . . . . . . . . . . . . . . . . . 2. Treated with ethylamine, without tension. 3. Treated with eth,ylamine and hexylamine, . . . . . . . without tension. . . , . . 4. Same as 3, then in boiling water 2 hr.. . . . . . ,
I
1.70 i 0.05 1.60 3= 0.03
5.8 13.3
1.50 1.50
17.1 14.3
0.05 0.04
C, differs from that observed after mercerization, where the lumens are generally tightly closed. The effect on the tensile properties of the reduced crystallinity obtained by the amine treatment was shown by treating a series of samples of bleached and kier-boiled 20/1 yarn in the same way as the purified cut cotton. At least ten, and in some cases more, observations were made on each set of treated yarns. The results are presented in table 5 . Tenacity was decreased in all cases, a result that had been anticipated with lowered crystallinity. Boiling with water for 2 hr. resulted in no increase in tenacity. All amine treatments resulted in a considerable increase in elongation, a result which would be anticipated on the basis of an increased proportion of the amorphous component. DISCUSSION
These results seem to support the view that the swelling treatments employed cause a vety material reduction of the crystalline component of native cellulose
REDVCTIOS OF T H E CRYSTALLINITY O F COTTON CELLULOSE
335
with, of course, an increase of the amorphous component. While it might be questioned whether we are not dealing here merely with a different lattice pattern, similar, say, to that produced by mercerization, the evidence seems to be against such an explanation. The special lattice diagrams characteristic of the amine-cellulose compounds were obtained by Davis, Barry, Peterson, and King (4) with the liquids present in the sample, whereas in the samples employed in the present study the amines had been removed. This was shown by qualitative tests for nitrogen and for amines. The radial traces shown in figures 1 and 2 do not sho\v any ne\\- diffraction lattices characteristic of a new crystalline rompound, nor do they sho\v even a hydrate cellulose diagram. The results obtained in the present study differ someivhat from those obtained by Ingersoll (8) on regenerated celluloses. Ingersoll found that viscose yarns slvollen \vith liquid ammonia, freed of the sivelling agent \vith dry ether, rewet [vith water, and again dried, became much improved in lateral order. In the present study lateral order was only slightly improved by exposure to water for periods up to 2 hr. and then drying. It should be noted, ho\vever, that the native rather than thr hydrate lattice \vas involved in the present studirs, and certain other conditions likewise tvere different, The results of the present study have a number of implications, diose importance can scarcely be overemphasized in view of the generally accepted concepts of Mark (9) and of Hermans (6) on the structure of cellulose f i l i ~ r s .If cryEtallinity in the natural cellulose contributes to stiffne this crystallinity in fillers can he reduced to sufficient and controllable degrees, it is felt that real progress has been made. \\'hen, moreover, this reduced crystallinity van lit, retained in the fiber, even after rather severe treatment with water at high temperature, t h e \Yay appears open for a major advance without the difficult introduction of "blocking groups." The relatively small tiecrease in strength, ivhich is less than might be expected. is in itself a promising result; and it may, ivith the combined use of cross-linking and stretching techniques, lead to fibers Ivith substantially new characteristics. STMMART A S D COSCLUSIOSS
.I study has been made of the application of certain swelling agents to cotton cclliilose and of their removal by nonaqueous liquids, \vith a vie\\. to reducing the crystallinity of the cellulose. 'The crystallinity of the resulting products has heen cstimatcd by the nrid-hydrolysis residue method. The study serms to pcr-
mit the folloiving conclusions: Triton I3 is capahle of loivering the .tallinity of cotton fiber progrrssivrly lilt i: acrornpanied by severe damage ovcr its critical sivelling range, hut t h t o thc struc.trii,r of the fiber. l'i.c:itmrnt of cotton filiers \vith anhydrous primary alkylamines causes :I re.tdlinity in varying degree, drpending on the amine or mixture of amines i i d :mtl othei. conditions employed. Thrsc amines d o not : ~ p p r a rt o tlam:igr the htni(-tiireof the fihcr. 1Vliei.ras thr aminr homologs a h v c propy1:imine :ire not c~apal)lrof s\vclling vot ton fil)rr :lii,ectly ivith Iwluc*tion of c,r>-stallinity,they ('iillsr iwlrichn of
33ti
LEOK SEGAL, MARY L. NELSON, AND CARL Y. CONRAD
crystallinity after preliminary treatment with ethylamine. At a ratio of 75 per cent by n.eight ethplamine to 25 per cent by weight hesplamine the action of the higher amine can be accomplished in a single-bath mixture. When an amine-treated cotton, from which the amine has been removed by means of a nonaqueous solvent, is dried and then immersed in boiling water for various periods up to 3 hr., the restoration of the original crystallinity is only slight. X-ray diffraction radial traces qualitatively confirm the conclusions drawn on the basis of crystallinity determinations made by the acid-hydrolysis residue method. Crystallinity is inversely related to moisture regain at 81 per cent relative humidity over a considerable range of moisture regain for amine-treated and mechanically decrystallised cottons. In preliminary experiments, treatments of a kier-boiled cotton yarn with amines without tension caused slight decreases in tenacity and large increases in elongation at break. Observations of cross-sections of untreated and amine-treated cotton fibers indicate that the amine treatment results in considerable swelling of the cell walls. Appreciation is expressed to h4iss V. C. Dwyer for her assistance with the preparations and for the moisture regain determinations, to Mr. J. W. Beck for the crystallinity determinations, to Miss H. M. Ziifle for the tracings and the computations, to Mr. J. J. Creely for the x-ray diffraction recordings, and t o Miss I. V. deGruy for the photomicrographs. REFERENCES (1) (2) (3) (4)
AKDRESS, K. R . : Z.physik. Chem. B4, 190 (1929). BAKER,W . o.,A N D FULLER, C. S.: J. Am. Chem. Soc. 66, 1120 (1943). ROCK,L. H . : Ind. Eng. Chem. 29,985 (1937). D A V I SW , . E . , BARRY, A . J., PETERSON, F. C., ASD K I N G A , . J . : J. Am. Chem. SOC.66.
1294 (1943). (5) HESS,K . , STEURER, E., A N D F R O M M , H.: Kolloid-Z. 98, 149-59, 290-304 (1942). P. €1.: J . Phys. Chem. 46, 827 (1941). (6) HERMANS, (7) H E a x . ~ s sP. , H . , A N D WEIDINGER, A , : J. Am. Chem. SOC.63,2547 (1946). H. G . : J. Applied Phys. 17, 921 (1946). (8) ISGERSOLL, (9) MARK,H . : J . Phys. Chem. 44, 764 (1940). , Unisylva 1, 18 (1947). (10) M A R K H.: , . : Chem. Eng. News 27, 138 (1949). (11) M A R KH A I . L . , A N D CONRAD, C. h f . : Textile Research J . 16, 149 (1948). (12) XELSON, M. L., A N D CONRAD, C. M,: Textile Research J. 18, 155 (1948). (13) SELSOS, M. L . , A N D ZIIFLE, H . M . : Textile Research J . 17, 585 (1947). (14) P H I I . I m , H. J., SELSON, (15) SISSON,W . A , : In Ce!lrrlose and C e l l d o s e Derivatives, edited t>y Emil O t t , Chap 111 A . Interscience Publishers, Inc., New York (1946). (16) VALKO, E. I.: In Cellulose and Cellulose Derivatives, edited by Emil O t t , Chap. 111D. Interscience Publishers, Inc., S e w York (1946). (17) VALKO, E. I.: Reference 16, Chap. I11 G.
