INDUSTRIAL A.VD ENGINEERING CHEMISTRY
354
calculated and is less uncertain, because the values a t 298.1 "
K. used in evaluating the constants of integration are now known with more accuracy. Table I1 contains values of M" and K , the equilibrium constant, a t various temperatures which may be compared with the former results. where J is the thermodynamic pressure or fugacity of the substance designated by the subscript, and is calculated from the equation M" = -RT In K . Table I1 ToK 300 400 500 600 700 800
900
AF' Cal. 9,650 4,200 1,330 6,920 12,620 +18,090 23,560
-
+ + + +
K 895 196 262 303 124 116 191
x
104
X 10-3 X 10-6 X 10-6 x 10-7 X 10-8
The values given in this table substantiate the qualitative conclusions which were drawn previously. They are not, however, in agreement with the recent results of Lewis and Frolich,l6 Audibert and Raineau,17 Brown and Galloway,'* and Morgan, Taylor, and Hedley,lg all of whom worked a t Lewis and Frolich, IND.END.CHEM.,20, 285 (1928). Audibert and Raineau, Ann. Office Combustibles Liquids, Vol. IV (1927). Abstracted, IND. E N G . CHEM.,20, 1105 (1928). 18 Brown and Galloway, I b i d . , 20, 960 (1928). 19 Morgan, Taylor, and Hedley, J . SOL.Chem. I n d . , 47, 117T (1928). 16
17
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high pressures where correction from pressure to fugacity could not be made with any certainty owing to inadequate knowledge concerning the equations of state of the gases, Then, too, these workers were interested primarily in practical yields and not in studying the equilibria. The results of these investigations have been summarized by Fieldner and Brown.*o Nore recently Smith and Branting*l have studied the methanol equilibrium a t 576.9" K. and 1 atmosphere total pressure and have obtained K = 5.57 X 10-4. This value is probably correct to 5 per cent and is the only value in which any confidence should be placed for purely theoretical purposes. The calculations in this paper are not in agreement with this value, but the situation has been much improved and the new experiment31 data on methanol and carbon monoxide bring the calculations considerably closer to Smith and Branting's experimental value than were the former results. Obviously, something is wrong somewhere. Part of the difference may be due to the value chosen for the entropy of hydrogen, part to lack of sufficient heat-capacity data on methanol vapor, etc. The author feels, however, that the entropy value of methanol liquid a t 298.1" K. cannot be in any great error. This would also be true of the free energy if the question of the entropy of hydrogen were satisfactorily settled. Certainly more experimental work is necessary in order to clear up these difficulties. 20
21
Fieldner and Brown, IND. E N G .CuEM., 20, 1110 (1928). Smith and Branting, J . Am. Chem. Soc., 61, 129 (1929).
Specifications for Cellulose for Use in the Manufacture of Smokeless Powder',' Fred Olsen PICATINNY ARSENAL, DOVER, N. J.
T
H E type of cellulose ordinarily employed in the manufacture of nitrocellulose for use in smokeless powders is a high grade of cotton linters. This is perhaps due to the fact that the feature which has dominated nitrocellulose for the past two generations has been high stability. To this end an elaborate system for processing nitrocellulose has been developed, in the belief that the best results can be achieved only through the use of a high grade of cellulose. During the World War the demand for cotton linters became so heavy that the cutting of lint made a t the gins became closer and closer. Normally about 80 pounds of linters are removed from a ton of seed which has previously yielded about a ton of spinnable cotton and about 25 pounds of firstcut linters. The cut of second-cut linters was increased to about 150 pounds per ton, with a correspondingly larger amount of hull included with the linters. A still further supply of cotton was obtained by grinding the hulls after they had been cracked open to permit the recovery of the meat from which cottonseed oil is made, the ground hulls being separated by air into bran and hull fiber. This hull fiber is, of course, very short and indeed quite dusty. The purification process to which the cotton linters or hull fibers were subjected during the war period was by no means so thorough as is now practiced. Consequently there may be some justification in attributing the reduced stability of 1 Presented before the Division of Cellulose Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14, 1928. * Published by permission of the Chief of Ordnance, United States Army.
certain lots of propellent powders to the poorer quality of the cellulose employed. Cellulose for use by the Ordnance Department of the United States Army was expected to meet the following requirements: It shall contain not more than (1) 7 per cent material soluble in 10.0 * 0.1 per cent potassium hydroxide or in 7.14 * 0.1 per cent sodium hydroxide solution; (2) traces of lime, chlorides, sulfates, or hypochlorites; (3) 7 per cent moisture; (4) 0.4 per cent etherextractive matter; (5) 0.8 per cent ash. Alkali-Soluble Limit The purpose of the alkali-soluble limitation was to restrict the amount of oxy- and hydrocellulose. However, as most of the wartime cotton was of poor color, it is doubtful if very much of the material contained an excess amount of oxycellulose due to overbleaching. The nitrates of oxy- and hydrocellulose are reported in the literature as being highly unstable, although little is actually known about their properties. As heavier cuts of linters were made the cooking in the digesters became correspondingly more severe in order to effect the suitable disintegration of the highly colored hulls. However, little trouble was experienced in securing material complying with this requirement. Post-war developments of nitrocellulose lacquers have resulted in the production of cotton linters having an alkalisoluble content of less than 3.5 per cent. Apparently, cellulose of this quality can be obtained readily, and Ordnance Department specifications are being revised to require not more than 3.5 per cent material soluble in 7.14 per cent sodium
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1929
hydroxide solution. This peculiar concentration of caustic soda solution is due to the fact that earlier methods of testing prescribed the use of a 10 per cent solution of potassium hydroxide. These are equivalent concentrations of sodium and potassium hydroxides. T a b l e I-Effect of t h e Soda-Soluble a n d Ether-Extractive M a t e r i a l in Cellulose upon t h e Yield a n d S t a b i l i t y of Nitrocellulose SAMPLES: A-Wartime cotton linters B-High-grade second-cut linters obtained from Buckeye Cotton Oil Company, August, 1927 C S u l f i t e pulp prepared from a misture of spruce and hemlock D-Cellulose from sugar cane (Celulosa Lubana) E-Tissue from rags NITRATIKG COKDITIOKS: Moisture content of cotton less than 1 per cent Comuosition of nitratinz acid: HeSOa, 61.0; HKoa, 2 2 . 5 ; NO?,1.5; H&, 15.0 per cent Temperature of nitration 2 8 O C. Time of nitration, 25 m(nutes for samples A and B ; 60 minutes for samples C, D, and E h-itrated with mechanical agitation PURIFICATION OF SIT ROC ELL CLOSE:^ 16 hours' sour boiling in water containing not less than 0.1 per cent of sulfuric acid; 24 hours' boiling in three changes of water; pulped i e beater or refining engine as used in paper mills; boiled for 6 hours in dilute sodium carbonate solution; boiled 6 hours w-ith five changes of water; ten cold-water washes u. SAX- SAM- SAM- SAM-SAMORDKANCE DEPT. PLE PLE PLll PLE PLE SPECIFICATIONA B C D E Soda-soluble, per cent Original 7 6.8 2 . 8 1 1 . 5 18.6 6 . 4 Present 3 . 5 Ether-extractive, per cent 0.4 0.25 0.1 0 3 0.4 1.7 No discoloration in 1 hour KI test a t 65.5' C., minutes 35 135' C. normal methyl violet test: Salmon pink coloration, 50 40 40 45 30 45 minutes 110 90 90 95 110 Fumes. minutes None in 5 hours Explosion 1 . 5 8 1 . 6 0 1 4 8 1.36 1 . 6 0 Yield.. Dounds = - ~ - - ~ Details of purification and methods of testing given in Pamphlet 450. Ordnance Department E . S. Army.
s.
~~~~
~
When samples of the currently produced high-grade linters and of wartime linters, in which the soda-soluble contents are about 3 and 7 per cent, respectively, are nitrated and subjected to the same conditions of purification, little, if any, difference in stability is encountered. (Table I) Further, certain samples of wood cellulose containing 10 per cent of soda-soluble material and of cellulose from sugar cane and various straws containing about 20 per cent of soda-soluble material have been nit,rated and purified without difficulty to give stable nitrocellulose. It would seem, therefore, that the stabilit,y of nitrocellulose is not dependent upon the sodasoluble content of the cellulose t.0 any marked extent. The main objection to a high soda-soluble content in cellulose which is to be nitrated seems t o be the reduced yields obtained. From 1.3 to 1.5 pounds of nitrocellulose are obtained per pound of celluloses containing 20 to 10 per cent of soda-soluble material, respectively, whereas 1.6 pounds or slightly better, of nitrocellulose of nitrogen content 12.6 per cent can be obtained from high-grade cotton linters.
355
mination can replace the somewhat cruder method for sodasoluble material. At this time, however, it appears to be impossible to state definitely the desired alpha-cellulose content just as it is impossible to lay down appropriate sodasoluble requirements. Cotton linters can readily be procured today with a soda-soluble content of less than 3.5 per cent which corresponds probably to about 95 per cent alphacellulose. Certainly no difficulty would be met in obtaining cotton linters having an alpha-cellulose content of 90 per cent. On the other hand, stable nitrocelluloses have been made from cellulose from wood and other sources, where the alphacellulose content was only 80 per cent or even as low as 60 per cent. In fact, one of the highest grades of nitrating tissue paper available on the market fails to meet the present tentative cellulose specifications for soda-soluble material, it having practically twice the maximum limit. (Sample E, Table I) It is probable that under a general specification for cellulose it would be necessary to state several grades depending upon the source of the cellulose, and that the alpha-cellulose content, for example, might vary considerably according to the grade of cellulose employed. Lime, Chloride, Sulfate, and Hypochlorite Content The requirement that not more than traces of lime, chlorides, sulfates, or hypochlorites may be present was designed to avoid the use of carelessly washed cellulose from which the purification agents might not have been completely removed. The requirement is, however, of little significance, and no trouble has been experienced from lots failing to comply. It is altogether probable that traces of these substances would not affect either the yield or the stability of the product, since the action of the mixed acids would be to destroy the chlorides and hypochlorites; the lime would be converted to calcium sulfate which, apart from increasing the ash content slightly, would be without effect upon stability; and finally the sulfates could not be injurious, since the cellulose is immersed in a nitrating bath very rich in sulfuric acid. It would be quite feasible to omit such a specification, relying upon the ash content requirement t o take care of the amount of lime which could be present. Moisture Content The requirement that cotton shall contain not more than 7 per cent moisture is mainly for the purpose of avoiding payment for water instead of for cellulose, and also to lessen the burden upon the driers a t the nitrocellulose factory, where the cellulose is usually required to contain less than 1 per cent moisture prior to nitration. Ether-Extractive Matter
The purpose of the restriction of the material extractable by ether to not more than 0.4 per cent is to avoid such materials as oils and waxes. The digestion commonly applied, however, is usually sufficiently thorough to permit easy compliance with this limit. Very little exact information is available as to the significance of the presence of ether-extractable material, and it is exceedingly doubtful if twice as much material as is permitted by this requirement would have any detrimental effect upon the stability of the nitrocellulose. It is significant that the highest grade of nitrating tissue paper employed in the manufacture of films or highly transparent nitrocellulose plastics was found to have more than four times the amount of ether-extractive material It is recognized that it would probably be more satisfactory permitted by the present specifications. (Table I) It is to establish a requirement for an alpha-cellulose content of probable, however, that if locally high concentrations of oily cellulose for nitration and, with the issuance of an appropriate materials existed, these might react so vigorously with the method by the Cellulose Division of the AMERICAN CHEMI- nitrating acids that the heat evolved might cause the rapid CAL SOCIETY,~ i t is probable that the alpha-cellulose deter- decomposition of the charge, possibly to the extent of spon* IND.END.CHBM.,Anal. Ed., 1, 52 (January 15, 1929). taneous combustion. rote-It is probable that the presence of materials other than alphacellulose in cellulose used in the manufacture of nitrocellulose for smokeless powder has little effect upon the stability of the nitrocellulose, because of the drastic hydrolytic treatment which the nitrocellulose receives during purification. Such treatment is frequently not appropriate for nitrocellulose to be used in celluloid or in certain types of lacquers, etc., where undesirable changes in the viscosity of the nitrocellulose solutions may be effected by the action of the acid employed in the hydrolytic boiling process. Atsuki [ J . Coil. Eng. T o k y o Imp. Uniu., 11, 201 (1922); C. A , , 17, 1138 (1923); Rev. g i n . mat. plastigues, 4, 194 (192811 has discussed the purification of pyroxylin for celluloid made from cellulose of various degrees of purity. The conclusions are in general not applicable to the preparation of nitrocellulose for use in the manufacture of smokeless powder because the methods of purification of pyrocotton are more drastic and give quite different stability results.
3 56
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ash Content The purpose of the ash limitation of 0.8 per cent is simply to insure the use of reasonably clean cellulose. Very little trouble is experienced in meeting this requirement, and in much of the use to which nitrocellulose is put an ash content higher than 0.8 per cent would probably be of no significance. Proposed Additional Requirements Two additional requirements have been proposed recently which are designed to control the quality of the cellulose from the standpoint of ready absorption of the nitrating acids, on the one hand, and to insure a reasonable degree of uniformity in the viscosity of solutions prepared from the nitrated cellulose, on the other. I n the first case suitable absorbency is said to be achieved when a sample of approximately 1 gram of the dried cellulose rolled into a ball in the palm of the hand sinks below the surface of the nitrating acid in not more than 2 minutes. The composition of the nitrating
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viscosity of its solutions. As a consequence a specification has been formulated which requires cotton linters to have a viscosity of 50 to 750 seconds in cuprammonium solutions a t 25' C., and that for any particular purchase the viscosity shall be within plus or minus 50 per cent of the time specified within that range. Note-Viscosity was determined by measuring the time required for a 0 2 mm. in diameter to fall through a 2.5 per cent solution glass sphere 3.0 of cellulose in cuprammonia at 25' C. The apparatus and technic are specified in detail in Ordnance Department Specification to be published. f
At present the practice in the Ordnance establishments favors the use of cellulose, the viscosity of whose solutions is a t the lower extreme in the range given. The main difficulty, however, has been experienced in connection with the method of determining the viscosity. An endeavor has been made to prescribe in detail the exact manner in which the cuprammonium solutions shall be prepared, the manner in which the cellulose shall be dissolved in such solutions, and the exact details in making the viscosity measurements. A subcommittee of the Cellulose Division of the AMERICAN CHEMICAL SOCIETY is now working on the standardization of methods for the determination of viscosity, and the findings of this committee will in all probability be incorporated in future revisions of the cellulose specifications.4 Conclusion
It is believed that the formulation of specifications covering cellulose for conversion to nitrocellulose can properly be made only after a considerable change in orientation of our viewpoints has been effected. It appears that much traditional misinformation encumbers our present specifications. It is Yiscority o f Ether-olrohol Jsluhon of N!trocrllu/ore - C e n f ~ p p a , r r l the opinion of some of us that cotton linters, for example, IJ,scwtt#er ore rworfcd in obsoiutr u m W instead of being the most suitable form of cellulose for nitraFigure I-Relation b e t w e e n Viscosity of Solutions of Nitrocellulore and of Solutions of t h e Cellulose f r o m Which t h e Nitrocellulose tion, perhaps constitute one of the least desirable materials. W a s Prepared Its convenience in handling is due rather to the fact that the acid is, approximately, sulfuric acid 63 per cent, nitric acid present equipment has been developed to meet its particular 22 per cent, and water 15 per cent. Normal samples of characteristics. The heavy-walled cotton hair with its cotton linters readily comply with this requirement, but the natural twist forms knotted masses or clumps, which, although highest grade of nitrating tissue available required 30 minutes readily manipulated, offer considerable resistance to uniform for the cellulose to sink below the acid, and yet it is known penetration by the nitrating acids, and also by the wash that the paper is readily nitrated. It would seem preferable waters used in the subsequent purification stages. This conto regard this test as one to be carried out only when estab- dition would probably be avoided by using other types of lishing the suitability of a certain type of cellulose, rather cellulose in different physical form. One of the first obthan as a test which is applied to each lot of material pur- jections offered to other types of cellulose, such as sheets of chased after the suitability of the type of cellulose has been purified wood pulp, is that these types of cellulose do not established. It would probably be better to modify the test comply with the existing specifications for cellulose, which to require that when a nitration of the sample is conducted have been built upon the characteristics of cotton linters. under ordinary nitrating conditions a suitable grade of nitro- This is true, but it should be possible, by concentrating attencellulose result therefrom, it being recognized that morlifica- tion upon the character of the nitrocellulose to be produced, tion of many of the factors influencing the degree of nitration to ascertain those properties of cellulose which are really might have to be made in order to achieve the best results. responsible for the properties of the derived nitrocellulose. For example, the composition of the nitrating acid might The existing U. S. Army specifications are such as permit a require modification, or a different ratio of nitrating acid to high grade of cotton linters t o be procured, and it seems cellulose might have to be employed depending upon the reasonable to assume that the high purity of the raw material contributes to the production of nitrocellulose of good bulkiness of the cellulose. The viscosity test has proved t o be a very difficult one to stability. It is one of the purposes of this paper, however, to show standardize. I n general, under a given set of conditions the viscosity of nitrocellulose in an appropriate solvent such as that the requirements for cellulose t o be used in the prepaacetone or a 2 : l ether-alcohol solution is directly proportional ration of nitrocellulose are not so rigorously met by current to the viscosity of a cuprammonium solution of the cellulose specifications for cotton linters as to preclude the successful from which the nitrocellulose was prepared. (Figure 1) use of other grades of cotton or of other forms of cellulose. At the present time, however, very wide variations in practice It is the hope aIso that those now engaged in fundamental exist in the United States with reference to the viscosity values studies on methods of processing cotton or other forms of for nitrocellulose t o be used in the manufacture of smokeless cellulose may thus be a little less hampered by the onus of powder, in some cases relatively high viscosity material and striving to make their product conform t o the present limitain other cases low viscosity material being regarded as the tions relating to the short seed hairs which comprise cotton most desirable product. The practice in the Army has not linters, and that they may seek more freely t o determine the been clearly defined, but a distinct disadvantage is recognized suitability of their product by more trustworthy criteria. 4 IND. ENG.CHBM, Anal. Ed., 1, 49 (January 15, 1929). in the use of material that is not uniform with respect to the
