Homogeneity and Properties H. M. SPURLIN Hercules Powder Company, Wilmington, Del.
precipitation (1-6, 7-15, 18, 20-26, 28, 29). Use of water as a precipitating agent waa undesirable because the decomposition of the nitrocellulose produced a high Tyndall effect. This effect was presumably due to the f&ctthat coagulation was started in the fractions obtained. An example of this behavior was published by McBain (19). Best results were obtained when commercial heptane was used as precipitant and acetone as solvent. With this combination t.he precipitated liquid phase could be readily drawn off (Figure 1). For the fmctiona reported here, a commercial rade of I/+second nitrocellulose (12.05 per cent nitrogen ani? 1.33 centipoises visco5it.y in I per cent butyl acetate solution) was used. (The actual viscosity was 0.28 second by the A. 9. T. M. test method.) An albglass apparatus in a constant temperature beth wa6 used (Fignro 1). Experience soon taught the correct, quantity of heptane to be added in order to secure fractions of the desired size. The following scheme was adopted: Four lots of 120 grams each, as 10 per cent solution in acetone, were separated into six fractions each. The fractions were isolated by dissolvin in acetone and drying as thin films at room temperature. $mct.io& of approximately the same viscosity were mixed, and each mixture was fractionated. The fractions of the mme viscosity were again mixed, and the mixtures again fractionated from similar 10 per cent solutions. This procedure wa8 repeated agsin but from 6 per cent solutions in acetone. The sixty4x fractions so obtained were combined so as t,o obtain seventeen groups of 15 to 40 grams each.
As shown in Figures 2 and 3, a considerable degree of fractionation had been achieved. Tho fractions of highest viscosity had a specific viscosity three times that of the starting material; a rather large portion had a specific 1%cosity which was less than a third of the original. In the last refractionation a considerable separation was still occurring so that even in this series of fractionations, believed to be more complete than any so far reported, nothing a p
I
N 1931 when this investigation was started, the knowledge of cellulose derivat,ivos had progressed to the point where i t was recognized that their degfee of homogeneity with respect to molecular size was one of their important characteristics. It was desired to determine the relation between uniformity of molecular weight and the physical properties of a given cellulose derivative. Because of its commercial importance, as well as its uniformity of substitution, nitrocell~losewas selected for test.
fractionation
In order to secure products of increased homogeneity, i t was necessary to fractionate. Several processes applicable to cellulose derivatives had been described in the literature. Fraotional filtration (10, 18) was rejected for the present purpose because of its slowness. Fractional solution (6) was inapplicable because of the desire to refractionate the fractions. There remained the various processes of fractional
of Nitrocellulose
m
proaching a uniform product was obtained. Still, it is obvious from the difference between the high and low fractions that a great degree of separation had been achieved. The following table compares the viscosity of original nitrocellulose with that of a mixture of its fractions in the proportion in which they were isolated:
Starting material Fraction 1 2 3 4 5 6
4.3 14.2 2.3 19.8 3.0 7.4
Propor-
tion
33.3 Fraction 7 140 8 135 9 120 10 66 11 12 24 Mixture of fractions
11.4 15.2 9.7 4.5 8.2
..
17 11
5.3 4.4 2.1 30.6
The viscosity of the fractions varied over a wide range. On mixing, a viscosity of 30.6 centipoises was obtained for a 5 per cent acetone solution. The viscosity before fractionating was 33.3 centipoises. It is difficult to reconcile the behavior of nitrocellulose on fractionation and remixing of the fractions with a hypothesis which postulates the presence of micelles similar to those in soap solutions. It has been shown that the nitrocellulose molecules are all available for interaction with solvents (16). The forces between the nitrocellulose molecules can hardly be greater than the forces between solvent molecules and nitrocellulose. With such relatively weak forces between the molecules, it seems that there should be a tendency for micelles, if they existed, to dissociate and reform with considerable rapidity. Such a process would preclude the formation of stable uniform fractions, since
[I/,/
m
A sample of nitrocellulose was systematically fractionated by precipitation with heptane from acetone solution. Stress-strain curves and fold resistance were determined on films from such fractions. Results were compared with those from unfractionated nitrocellulose and from blends of high- and low-viscosity nitrocellulose, selected to have the same range of viscosities as the fractions. For each group of material a definite relation existed between viscosity and Schopper fold resistance. There was a sharp limit of viscosity below which the product did not withstand folding. The fractions gave higher fold values than unfractionated material of the same viscosity, and this in turn was better than blends of high- and low-viscosity nitrocellulose. These examples, based on the sensitive fold test, show that physical properties are a function of homogeneity, other factors being constant.
Viscosity in5% Acetone % by ut. Centipoises
Viscosity Proporin 5 % tion Acetone % b y ut. Centipoises
m
aggregates, such as probably occurs in concentrated solutions, is so rapidly reversible that its equilibrium quickly follows changes of concentration, temperature, and solvent.
Fold Tests and Stress-Strain Curves'
+G
In order to characterize the fractions further, the osmotic pressure of a number of them was measured. Unfortunately it was not possible to do this until their viscosity had dropped t$ 2 somewhat. The molecular weight divided by %$ the original specific viscosity in 1 per cent 3% I butyl acetate solution gave values of 20,000 to 33,000. The graphically extrapolated values .e 4 .6 .8 /O Le 14 /.S /.a PO 2.2 2.6 ,?i T", SP€C/F/C V/SCOS/7Y /N / % BU7YL AC€TA T€ of [q] (17') in 2-methoxyethanol, the solvent employed for the osmotic pressure measurements, FIGURE 3. VISCOSITY DISTRIBUTION CURVE FOR 1 / 2 - NITROCELLULOSE ~ ~ ~ ~ ~ ~ were used. Then molecular weight divided bv [q]gave values of 45,000 and 48,00%. The degrie of polymerization is equal to 180 times the limiting viscosity, micelles on dissociation and re-formation would yield a heterodisaerse svstem similar to the original material. That .1111, , ., if we use these values. the fraciions &e present in the ori&al nitrocellulose is From each of the seventeen groups of fractions, films 0.003 * shown by the fact that, on remixing, the properties of the 0.0005 inch (0.076 * 0.013 mm.) thick were prepared. original material are substantially duplicated. The original The method of casting was as follows: nitrocellulose is made up of a mixture of materials that mainA layer of 20 to 30 per cent solution was spread on a glass tain their identity on fractionation, drying, and redissolving, plate with the aid of a spreader held a definite distance above which makes it appear that the properties of nitrocellulose the plate by runners. The plate was then accurately leveled solutions are determined by the nature of the molecules presin a cabinet about 4 inches (10.2 cm.) high and somewhat larger than the plate. The cabinet was closed except for a slit about ent. Also, any association of these molecules into larger
&$
539
540
INDUSTRIAL AND ENGINEERING CHEMISTRY 1
I
%*
I
/
I/
2 //5COS/TY
3
C€N T/POS€S
/N / % BUTYL ACF7AT€
OF SCHOPPER FOLDTESTS FIGURE 4. RESULTS
1 mm. wide at one end and a canvas cover on the other. This resulted in slow drying and an acetone concentration in the vapor phase which was high enough at all times during drying to revent strains from uneven hardening. The four fractions of Ewest viscosity (26 per cent of the total weight) gave films which were too brittle to be handled. Stress-strain curves and the number of double folds in the Schopper fold tester were determined on unplasticixed films; then the broken pieces were redissolved with four parts of dibutyl phthalate to ten parts of nitrocellulose, and films were cast as before. These films were tested in the same manner as the unplasticized ones.
It was possible to represent the fold tests by fairly good curves on Figure 4; for comparison, the corresponding data for regular commercial nitrocellulose and its various blends are given. As abscissa the viscosity in centipoises of the actual sample or blend is given. The viscosity of the butyl acetate was 0.690 centipoise. For each class of material there was a well-defined point below which it would not stand folding. This is 1.10 centipoises
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for all classes of unplasticiaed film, and from 1.10 for the plststiciaed fractions to 1.17 centipoises for plasticized blends of 20-second and very low-viscosity nitrocellulose. (The sample used had a viscosity of 15 centipoises in 12.2 per cent concentration in the standard A. S. T. M. mixture, and of 0.98 centipoise in 1 per cent butyl acetate solution.) The fractions with the higher viscosities were definitely superior, especially when unplasticized. Only the most viscous fraction and, to a less extent, the second most viscous had a tendency to be similar to the other nitrocelluloses. This was probably associated with the fact that these fractions had all of the original ash and haze, and their high viscosities and relative insolubilities were due largely to a tendency to gel rather than to long molecules. While the superiority of the fractions over the ordinary commercial material was not great, especially in the 1/4-1/2 second region, the blends of high- and low-viscosity material were decidedly inferior, especially when approximately equal quantities of the 20-second (actually 4.05 centipoises in 1 per cent butyl acetate) and 15 centipoises (0.98 centipoise in 1 per cent butyl acetate) were blended. For example, a blend of 51 per cent of the low with 49 per cent of the high, with a 1per cent butyl acetate viscosity of 1.93 centipoises, gave only 20 folds; on the other hand, a straight commercial nitrocellulose of 1.62 centipoises gave 30 folds, and fractions of 1.611.79 centipoises gave 3 8 4 7 folds. Comparing the blend, the straight material, and the 1.6-centipoise fraction, we obtain values of 18,30, and 38 folds (from a smooth curve). Calculated on the same specific viscosity basis as was used in Figure 3, the low-viscosity material used had qsp. = 0.42; the high-viscosity had qSp. = 4.9. If both materials had a distribution curve similar to Figure 3, the two curves would scarcely overlap. It appears, therefore, that as long as there is a reasonably smooth distribution curve for viscosity of fractions, the number of folds in the Schopper tester is little worse than for a very uniform product, even with a flat curve such as Figure 3; on the other hand, if blends are made so that there are two peaks of frequency of portions (one with some ten times the viscosity of the other), only about half the number of Schopper folds are obtained. None of the other tests applied to the samples showed as great a degree of superiority of the fractions. This is due to the fact that other properties do not vary so rapidly with viscosity as does the Schopper fold test in the range studied here. For example, tensile strength rises from about 550 kg. per sq. cm. for '/c-second nitrocellulose to 700 kg. for 5-second nitrocellulose, a ratio of 1.27; the Schopper fold test rises from 10 to 36, a ratio of 3.6 (for films cast under the conditions used for these tests). For this reason, the stress-strain curves (Figure 5) do not show a definite superiority for the fractions except in the low-viscosity range where the fold test shows little difference. I n practically all cases, however, where a fraction shows lower tensile strength than a commercial nitrocellulose or blend of the same viscosity, its elongation is better. This may be due to greater solvent retention by the fractions. It was desired to carry out these tests as rapidly as possible in order to avoid degradation of the nitrocellulose between the first and second series of tests, made on the same material. Therefore, the films were kept only about 3 days a t 25" C. and GO-70 per cent humidity before the tests.
Conclusions
It is believed that this series of tests was comprehensive enough to give a true picture of the variation of properties of free films of nitrocellulose with degree of homogeneity, with respect to molecular weight. The author is aware that other have not demonstrated the same experimenters (14,22,26,27) superiority of the more uniform fractions over commercial
MAY, 1938
I
INDUSTRIAL AND ENGINEERING CHEMISTRY Y/SC05-/ZY / N / A BU, AC.
541
I L.l/C#
3.27 CA:L?% f5~?,S% 20"
2.36 2.8 3
z00 2.24
"t'
FIGURE 5. STRESS-STRAIN CVRVES
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cellulose derivatives and blends. This would appear to be due to two factors: Others do not seem to have carried fractionation far enough to secure fractions of decidedly greater uniformity than the starting material; and they have placed most reliance on the stress-strain curves, for which it is necessary to run a great number of samples in order to show small differences. The work of Medvedev (22) would have been of more value if his fraction IVB had been compared with unfractionated and blended nitrocelluloses of the same viscosity, The writer had enough material available only for six checks on each point for the fold test, and for three to six checks on the stress-strain curves. It is doubtful if other investigators had samples as large as this. Differences were more pronounced for the unplasticized than for the plasticized films. This is believed to be generally true. There seems to be no commercial advantage in securing a greater degree of uniformity for products to be used in protective coatings and plastic masses. One property, adhesion, seems to be improved by blending high- and low-viscosity material. Certain of the blends lifted the surface of the glass on which films were dried in the same manner as gelatin.
Acknowledgment The author would like to express his appreciation to R. F. Kingery for his assistance in this investigation.
Literature Cited (1) Beck, A., ClBment, L., and RiviBre, C., Chimie et industrie, 24, 1068 (1930). ( 2 ) Berl, C., and Hefter, O., Cellulosechem., 14,65 (1933). (3) Breguet, M. A.,thesis, Lyon, 1924; Rev. gen. colloides, 3,200-6, 230-5 (1925). (4) Caille, A., Chimie et industrie, 19, 402 (1928).
VOL. 30, NO. 5
Ibid., 25, 276-85 (1931). Clark, J., and Miles, F. D., Trans. Faraday SOC.,27, 757-67
(1931). Deschiens, M., Chimie et industrie, 20, 1023 (1928). Duclaux, J., and Barbihre, J.. Bull. SOC. chim., 53,564 (1933). Duclaux, J., and Nodsu, R., Rev. gen. colloides, 7 , 241-50, 38592 (1929). Duclaux, J., and Wollman, E., Bull. S O C . chim., 27, 414 (1920). Elod, E., and Schmid-Bielenberg, H., 2.physik. Chem., B25,27 (1934). Glikman, S. A.,Plasticheskie Massy, No. 21 (1934); J . chim. phys., 31,458 (1934). Herz, W., Cellulosechem., 15, 95 (1934). Herzog, R. O., and Deripasko, A., Cellulosechem., 13,25 (1932). Iwasaki, S.,J. SOC. Chem. I n d . J a p a n , 34, Sugpl. Binding, 9 (1931). Kats, J. R., “Die Rontgenspektrographie aIs Untersuchungsmethode bei hochmolekularen Substanzen,” Berlin, Urban & Schwarzenberg, 1934. Kraemer, E. O., and Lansing, W. D., J. Phus. Chem., 39, 156 (1935). Kumichel, W., Kolloidchem. Beihefte, 26, 161-98 (1928). McBain, J. W., and Scott, D. A., IND. ENG.CHEM.,28, 473 (1936). MoNally, J. G., and Godbout, A. P., J. A m . Chem. SOC.,51, 3095 (1929). Mardles, E. W. J., S. Chem. Soc., 123, 1951 (1923). Medvedev, A. J., Kunststofle, 23, 249-51, 273-6 (1933); J . Applied Chem. (0.5. S.R.), 6, 880 (1933). Obogi, R., and Broda, E., Kolloid-Z., 69, 172 (1934). Okamura, I.,Cellulosechem., 14, 135-8 (1933). Rocha, H.J., Kolloidchem. Beihefte, 30,230 (1930). Bogovin, 2. A.,and Glazman, S., J. Applied Chem. (U. S. 9. R.) 8, 1237-49 (1935). Schneider, G., Canadian Patent 360,989 (Oct. 6, 1936). Sheppard, S. E., J. IND.ENG.CHEM.,13,1017 (1921). Ulmann, M.,Ber., 68, 134-45, 1217-24 (1935). RECEIVED January 14, 1938. Presented before the Division of Cellulose Chemistry at the 94th Meeting of the American Chemioal Society, Rooheeter, N. Y . , September 6 to 10, 1937.
Suitability of Plastics xor Airplane Dopes r e
GORDON M. KLINE AND C. G. MALMBERG National Bureau of Standards, Washington, D. C.
T
HE problem of producing a fire-resistant doped fabric to replace the hazardous cellulose nitrate product was investigated in 1933 a t the National Bureau of Standards for the National Advisory Committee for Aeronautics ( 2 ) . It was established that none of the synthetic resins then available would produce films of satisfactory tautness when used alone on the fabric, and that their addition to either cellulose nitrate or cellulose acetate dope resulted in a corresponding lowering of the fabric tension. Data were also obtained on the comparative rates of burning of fabrics doped with various cellulose nitrate and cellulose acetate compositions, both with and without the addition of fireretarding salts to the fabric. It was shown that cellulose acetate dope yields a relatively nonhazardous product when applied to untreated fabric and that, when applied to fabric treated with boric acid-borax mixture, a product which is nonflammable under ordinary conditions is obtained. However, the investigation was not continued a t that time to include a study of the effect of varying the ingredients of a cellulose acetate dope on the tautening characteristics and particularly on the moisture sensitivity of the doped fabric. Cellulose acetate has now become available commercially in several grades, representing materials of different degrees
of acetylation and viscosity. Likewise, other cellulose derivatives, possessing characteristics which indicate t h a t they might make satisfactory dope bases, are being manufactured. Among these more recently developed products are ethylcellulose, cellulose acetobutyrate, cellulose acetopropionate, and cellulose nitroacetate. These products are also available in various grades denoting differences in chemical composition and in viscosity. The proper formulation of dopes based on these new materials to obtain the requisite balance of solvents, diluents, and plasticizers, both as to types and amounts, can be accomplished only by a thorough laboratory study of the various factors involved. The Bureau of Aeronautics of the United States Navy Department has, therefore, requested the National Bureau of Standards to undertake such an experimental study in order to develop a dope, based on these comparatively nonflammable cellulose derivatives, which will compare favorably with or surpass cellulose nitrate dope with respect to the effect of high relative humidity on the tautness of the doped fabric. I n the first phase of this work a variety of cellulose derivatives and synthetic resins were applied to airplane fabric to study the relation of tautness to the type of plastic base used in the dope, the percentage of acyl or alkyl substitu-
