E D W IN L. L O V E L L AND O . G O L D S C H M ID , R a y o n ier In c o r p o r a te d , S h e lto n , W ash.
R
EGENERATED cellulose filaments are known to be built up of long chain molecules, the chain.lengths of which are readily determined by well established methods. A considerable amount of information (6, 17) exists regarding the effect of chain lengths and their distribu the long chains with respect to one another— that is, on built, up in a definite crystalline pattern (hydrate cellulose), are termed “ micelles” ; the less ordered regions are said to have an“amorphous” structure.
It is known empirically in the industry that certain processes of refining cellulose for viscose purposes lead to yarns and cords of superior properties, but there is relatively little fundamental knowledge of the reasons for these results. The two extreme hy
potheses may be stated briefly as follows: The supermolccular structure of regenerated filaments is determined entirely by physical conditions used in regeneration, such as in coagulation, stretching, and drying. They in
clude related conditions thatdirectly affect the results of these processes, such as viscose and spin-bath com
position. According to this view
point, the structure of cellulose itself plays a minor role as long as mini
mum requirements of chain length and chemical composition are met.
The opposite hypothesis holds that the size and number of crystallites in the original cellulose play an im
portant part in determining the structure of regenerated filaments.
The original micelles might be thought to persist in modified form throughout viscose processing and to
In the experiments reported here, the effect of stretching alone
on the crystallinity of regenerated cellulose filaments was in
vestigated by applying a simplified form of the accessibility deter
mination originated by Nickerson {18). In addition, a tech
nique for the microscopic examination of hydrolyzed residues was used as a new method for parts of the cellulose struc
ture, and the later, slow linear rise to the crystalline por
tion. Percentage hydrolysis was arbitrarily taken to be the per
centage of original material converted to glucose, under the standard experimental conditions using boiling hydrochloric acid- ferric chloride reagent. The amount of glucose formed was deter
mined indirectly by measuring the amount of carbon dioxide evolved and then calculating back by direct comparison with car
bon dioxide evolved from glucose itself at various time intervals.
The method used in the present investigation assumes that the glucose formed may be determined by the actual loss in weight of the sample. Nickerson showed that, for a series of cotton celluloses, the glucose formation calculated indirectly agrees closely with the observed loss in weight, whereas wood
celluloses give much poorer agree
ment. This is probably due to the presence in wood cellulose of components other than glucose;
hence, in such cases the weight loss may be more representative of the accessibility than the carbon dioxide evolved, particularly where the content of ccllulosans and hemi- celluloses is very high. The dis
advantage of making a separate weight-loss determination for each point on the rate curve, rather than continuously observing the carbon dioxide evolution, is partly balanced troublesome, the hydrolyses were carried out in a thermostat at 100° C., and a slow stream of nitro
gen was passed into the hot reaction mixture to maintain proper mixing.
The effect o f stretching on the crystallinity o f regener
ated cellulose filaments has been investigated hy apply
ing a simplified form o f the accessibility determination originated by Nickerson. It appears th at rayon crystal
linity is determined primarily by the process o f coagula
tion and regeneration, the effect o f stretching or o f original cellulose crystallinity being sm all. These conclusions are supported hy photomicrographs o f etch figures of rayons, which reveal that stretching orients the crystalline parts present even in completely unstrctchcd filaments.
TI ME - HOURS
Figure 1. Typical Hydrolysis Curves for Rayon (a b o v e ) and Wood Cellulose (b e lo w )
811
10 M inute« * 25 M inute« 60 M inute« 120 M in u te s Figure 2. Etch Figures o f Tirc-Cord Rayon
Ex p e r i m e n t a l De t a i l s. Cellulose or rayon samples of ap
proximately one gram were digested in 200 ml. of Nickerson’s re
agent (2.4 N hydrochloric acid-0.6 M ferric chloride) at 100.0° C., and the loss in weight of sample was determined after various lengths of time in separate experiments. The all-glass apparatus consisted of a 500-ml. round-bottom two-neck flask fitted with a reflux condenser and nitrogen bubbler. The flask was deeply immersed in a thermostated oil bath. Rayon samples were cut into conveniently short lengths before use; other cellulose samples were formed into loose mats which readily broke up in the hot acid. All samples \yere first dried for 36 hours in vacuo over fresh phosphorus pentoxide and weighed, then quickly added to the preheated reagent at zero time.
The reaction was stopped by, pouring the hot mixture into 2
liters of cool water, with stirring. In each case the residue was collected on a fritted glass filter, washed thoroughly, and weighed after drying in an oven at 105° C.
Percentage hydrolysis (weight loss) vs. time curves (Figure 1) were constructed and were found to show an initially rapid rise (for about 2 hours) followed by a slower, linear rise. Extrapola
tion of this linear part to percentage hydrolyzed at zero time (ac
cessibility) was made using 3- and 6-hour values.' The slope of the linear part (percentage per hour) was arbitrarily taken as the reactivity of the crystalline parts.
A C C E S S I B I L I T Y R E S U L T S
Strictly comparable rayons, differing only in degree of stretch during regeneration, were prepared from a single sample of com
August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 813' mercial rayon-grade wood
cellulose from southern pine. The viscose (7.5%
cellulose, 6.5% caustic) and spin-bath (11% sul
furic acid, 23% sodium sulfate, 0.85%zinc sulfate, 4 % glucose at 50° C.) compositions were repre- sentativeof those normally used in the rayon semi
works in this laboratory.
An extreme difference in the amount of stretching was employed in this series. The most highly stretched rayon was a high tenacity tirc-cord type; considerably less stretch was given to the normal rayon, and an en
tirely unstretched spin
ner’s fluff was prepared by ejecting the viscose into the spin bath and allowing free shrinkage without ten
sion. The results for these three rayons and the orig
inal wood cellulose are shown in the first part of Table I.
Stretching under these normal conditions of re
generation produced no significant effect on the crystallinity of the fila
ments, despite the extreme differences obtained in tenacity and elongation.
The spinner’s fluff, having almost no strength, ap
peared to be slightly more crystalline than the high strength rayons. There
fore, the increased orienta
tion of cellulose chains which accompanies me
chanical stretching of the filaments (7, IB) does not seem sufficient to bring ad
ditional chain segments in
to the crystal lattices that are evidently formed by regeneration alone.
It must be emphasized that this lack of effect on
crystallinity may not always be the case if the conditions during coagulation and regeneration are varied so that different swollen states of the newly formed filaments are obtained. Conrad and Scroggie (S) examined the accessibilities of a number of rayons in Nickerson reagent and found a range of values from 15 to 31.5%.
Thus, a large amount of stretch, which appears from the present results to have no effect on crystallinity under normal conditions, may increase crystallinity if the swollen state of the filaments is markedly different when this stretch is being applied.
The effect of these changed conditions on yarn strength may be due to an actual change in orientation or crystallinity, or it may be due to an improvement in uniformity of structure of the filaments In cross section. A second kind of spinner’s fluff listed
10 M in utes 33 M in u te s 180 M in u te s
Figure 3. Etch Figures o f Spinner’ s Fluff from Regular Bath
in Table I illustrates this kind of divergence between acces
sibility and strength; the sample was obtained by coagulating and regenerating in two separate steps (2), a process that was supposed to give the least possible orientation in the filaments, such as might arise from accidental mechanical stresses. The accessibility of this product was essentially the same as that of the other unstretched rayon, but the strength was considerably better and the elongation greater.
Tire-eord type rayon from cotton linters (Table I) was spun under the same conditions as those used for the corresponding wrood cellulose rayon. Although the original linters showed a significantly smaller accessibility than the wood cellulose, the final rayon accessibilities were not very different, and the
tenaci-Ta b l e I. Ac c e s s i b il it i e s o f Ce l l u l o s e s t o Nic k e r s o n Re a g e n t
Accessi Reac Grams/Denier Elongation, %
Material bility, % tivity® Dry Wet Dry Wet
Wood cellulose
a Arbitrary units, per cent per hour for crystalline portion.
ties were the same. From this result it would again appear that rayon crystallinity is primarily determined by the process of coagulation and regeneration— at least under normal spinning conditions; thus any effect of original cellulose crystallinity is small. The advantage of the greater reactivity of wood cellu
lose in viscose processing, which is reflected in accessibility and reactivity figures for wood cellulose compared with cotton lintcrs, may, therefore, be utilized without prejudice in this respect to the supermolccular structure of the final rayon.
A P P E A R A N C E O F H Y D R O L Y Z E D R E S ID U E S
The total rate of hydrolysis of cellulose by strong acids is the sum of two separate rates as a consequence of the micellar net
work structure (13). Such rate curves show that after a certain time the initial rapid rate is largely spent, and only a slow attack on the crystalline micellar region is evident. In other words, the acid seems to destroy the noncrystalline sections, and leave only the micelles.
The interpretation may be made more general by the state- meiit that the acid first penetrates the more accessible regions of the supermolccular structure and, by hydrolysis, solubilizes those parts of the cellulose chains most open to attack; that is, the acid etches out disordered regions of the network structure.
This interpretation, moreover, is supported by the fact (16) that the x-ray diffraction pattern becomes sharper after hydroly
sis.
If such a mechanism does exist, it should result in visible etch figures that have some relation to the orientation in cellulose structures. Rayon can be studied by this means more easily than natural fibers because its characteristic network structure is uncomplicated by a superimposed morphological structure.
The supposition that hot acids must etch out the cellulose structure, in a manner depending on the degree of orientation of the structure, leads to the conclusion that the etching takes place
in three dimensions. An experimental problem exists, therefore, of rendering visible the obtained planes of weakness; the problem consists in making the etched product appear in two dimensions, so that it may be examined under high magnification with a microscope. Furthermore, if a two-dimensional view is ob
tained, two possible planes of weakness are revealed: longi
tudinal (along the filament axis) and transverse (across it).
. Hydrolyzed rayon filaments are not changed from their original appearance until they are mechanically treated; that is, planes of weakness become planes of cleavage, not spontaneously but under mechanical stress. This is a well known characteristic of tendered fabrics in general (4). Such behavior offers the possi
bility of revealing planes of cleavage separately as well as together.
The technique described below was designed with all three factors in mind. W ien hydrolyzed filaments are allowed to dry on a glass slide, their shrinkage acts against their adhesion to the glass surface and exerts a force that splits the filaments trans
versely at any weakened spots. Application of a crushing ac
tion to the wet filaments forces an increase in width that splits any weak planes longitudinally.' This is the source of so-called fibrillation in regenerated filaments. Finally, by crushing and drying, both pianos of cleavage become evident. The actual cleavage obtained for a given hydrolyzed sample is largely de
pendent on the mechanical treatment used to bring it into evi
dence. However, use of a standardized mechanical treatment leads to results for different hydrolytic processes with different yams which are thought to give a good picture of the differences in supermolecular structure in the series. enough water to form a thin,suspension; one drop of the suspen
sion was transferred to a microscope slide by means of a wide
August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 815
Rf.s u i.t s. The appear
ance of hydrolyzed resi
dues of tirc-cord rayon, at different stages of hy
d r o ly sis, is show n in Figure 2. A progressive increase in tran sverse sp littin g (after drying) and in longitudinal split
ting (after crushing) is clearly ev id en t. Un- stretched rayon (spinner’s flu ff) shows the same general increase in amount of splitting, but the ap
pearance of the cracks (Figure 3) is markedly different. In stea d of sharp transverse breaks and a fibrillated appear
ance longitudinally, the unstretclied rayon shows irregular cracking in every direction. The difference is particularly great in the case of crushed and dried samples; the marked reg
ularity of the bricklike elements in hydrolyzed tire-cord rayon contrasts strongly with the irrcgu- larit.y o f unstretched rayon.
The unstretched rayon from a two-stage coagula- tion-regeneration process, which leads to a uniformly rounded cross section (Figure 4), shows much less fragmentation (Figure 5) than the unstretchcd sample regenerated in the regular bath; however, the cracks are character
istically irregular.
The effect of time of hy
drolysis on the appear
ance of these hydrolyzed residues suggested that different rayons could be most clearly differentiated by useofastandardhydrol- ysis time of 45 minutes.
Figure 6 shows the results obtained by this method with three rayons and a
commercial grade of cellophane. For example, normally stretched rayon is clearly different from the highly stretched rayon; the transverse splits are more numerous and irregular, the fibrillation is less clearly defined, and the crushed-dried figures show more numerous, smaller, and irregular bricklike elements.
All these experimental results seem to be in full agreement with the hypothesis previously discussed, regarding the etching effect of hot acid in revealing micellar orientation. The regular
ity of the etch figures obtained from the highly stretched rayons and their obvious difference from the unstretched rayons may be attributed to structural differences implicit in high micellar orien
tation in the one case and lack of it in the other. This view is further supported by color photomicrographs1 showing the
10 M in u te s 30 M in u te s 120 M in u te s
Figure 5. Etch Figure o f Spinner’ s Fluff from Tw o-Stage Coagulation-Regeneration
polarization colors of crushed and dried filaments of
hydrolyzed
tire-cord rayon and spinner’s fluff. In the two positions
of
maximum brightness, the cellulose crystallites add to, or sub
tract from, the birefringence of the first-order red plate; they appear blue or yellow, depending on the position of the crystal
lites with respect to the planes of vibration of the
plate
(14)- Uniformity of color, therefore, indicates uniformity of crystallite orientation. In the case of stretched rayon, the bricklike elements appeared completely uniform in color in both
positions;
the spinner’s fluff, on the other hand, was irregularly
colored,
in agreement with the irregular etch figure.
However, if the
spinner’s fluff had not possessed at least molecular
orientation in
1 It waa not possible to reproduce these color photoMicrographa here.
Tiro-C ord R ayon from Cotton L in t era Tiro-C ord R a yon N orm a l R a yon Cellophane
Figure 6. Etch Figures o f Various Rayons and Cellophane, Hydrolyzed 43 M inutes in 2.33 iV Hydrochloric Acid at 100° C .
regions unoriented with respect to the fiber axis or to one another, no color would have been found. This indicates the presence of a structure of unoriented micelles, as already deduced from ac
cessibility results. Application of the hydrolysis-crushing technique to commercial regenerated cellulose films leads to a similar appearance of splitting that suggests the orientation of cellulose chains in the machine direction.
The interpretation of these results indicates that essentially the same thing is shown by the etch figures and by the technique of swelling and crushing (5 , 11) carried out to obtain fibrillation of highly stretched rayon threads. The orientation of crystalline regions is revealed on the one hand by preferential weakening of the amorphous parts by hydrolysis, and on the other hand by
preferential swelling of the amorphous parts. The hydrolysis method appears to reveal, fibrillation in greater detail than does the swelling method, because of the natural limitations inherent in the latter. The swelling method reveals fibrillne in the swollen state, whereas the hydrolysis method gives etch figures of fibrillae with enhanced sharpness. One result of this work, therefore, is that it makes available a new technique for the study of micellar orientation in rayon yarn; this method offers a greater range of applicability and interpretation than has hitherto been possible.
Most of the early work on orientation in cellulose fibers and filaments was done with x-ray diffraction methods (9). Ad
mittedly (IS), however, those methods are not well suited to quantitative measurement of the relative amounts of crystalline
August, 1946 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 817
and noncrystalline components. Chemical methods, such as the one used in this work, permit quantitative estimations to be made, although it must be kept in mind that “ accessibility”
is determined by the physical and chemical conditions imposed during the test (l).
Some support for the present results may be taken from the data of Lauer and co-workers {10), who found that both isotropic and highly stretched rayons of many different types give essen
tially the same specific heat of wetting. If heat of wetting is a measure of the internal accessible hydroxyls, the conclusion must again be drawn that accessibility (or crystallinity) is not affected by stretching, but rather is determined primarily in the process of coagulation and regeneration. .
LITER ATU R E CITED
(1) Assaf, A. G., HaaS(«It. H., and Purves, C. B., J. Am. Chcm.
Soc., 66, 59-65(1944).
(2) Cliarch, W. H., and Underwood, W. F. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,249,745 (July 22, 1941).
(3) Conrad, C. C., and Scroggie, A. G., In d. En g. Ch e m., 37.
592-8 (1945).
(4) Dorfie, C., “ Methods of Cellulose Chemistry” , p. 212, New York, D. Van Nostrand Co., Inc., 1933.
(5) Franz, E „ Die Chemie, 56, 113-20, 132-6, especially 134. Fig. 25 (1943).
(6) Gral6n, N., and Samuolson, O., Svensk Papperstidn., 48, 1-5 (1945).
(7) Hermans, P. H., in Röhrs, Staudinger, and Vieweg’s “ Fort
schritte der Chemie, Physik, und Technik der makromoleku
laren Stoffe” , Vol. II, pp. 17-35, Berlin, J. F. Lehmanns Ver
lag, 1942.
(8) Kratky, O., Angew. Chem., 53, 153-62 (1940).
(9) Kratky, O., in Rohrs, Staudinger, and Vieweg’s “ Fortschritte der Chemie, Physik, und Technik der makromolekularen Stoffe” , Vol. I, pp. 172-91, Berlin, J. F. Lehmanns Verlag,
tives", p. 226, New York, Interscience Publishers, Inc., 1943.
(17) Spurlin, H. M., Ibid., pp. 934-42.
P r e s e n t e d at the Northwest Regional Meeting o f the A m e r i c a n C h e m i c a l S o c i e t y , Seattle, Wash., October 20, 1945. Contribution No. 4 from t h e
Central Chemical Laboratory of Rayonier Incorporated.