S\VARDHARDNESS. Sward hardness was determined on films of the cellulose propionate valerate both with and without added resins, plasticizers, or wax. As shown in Table V I I , all of the addends produce compositions of lower hardness than unmodified propionate valerate, which has a hardness about equal to that of white shellac (72). Physical Properties. Tensile strength, elongation, and folding endurance were determined on cellulose propionate valerates of increasing degrees of hydrolysis (increasing hydroxyl contents). The properties were determined on films coated onto glass plates from filtered benzene-methanol (90 to 10) solutions. The stripped films were cured 48 hours a t 130” F. to decrease residual solvents to a minimum, and the tests \vere conducted on strips cut both along and across the coating direction ; these measured 15 x 180 mm. with a thickness of 6.5 mils. For tensile and elongation measurements, a S h o p p e r tensile tester with a cross-head speed of 4 inches per minute was used, and the results have been calculated in terms of pounds per square inch of starting cross-sectional area. As shown in Table VI11 and Figure 6: tensile strength increases and folding endurance decreases with increasing hydroxyl content of the esters. The elongation of the films a t the breaking point shows little trend lvith hydrolysis. Physical properties of films. of course, are dependent on the coating conditions-i.e., solvents used and degree of curing. The properties reported thus must be considered to shoiv only trends. Films of these esters coated from acetone, for example, are extremely brittle and may shatter upon removal from the glass plates even when they contain 5 parts of dibutyl sebacate as plasticizer. In the case of sample A (Table VIII), the most
nearly fully esterified ester, greater crystallinity is observed when coated from acetone us. benzene-methanol, as shown by x-ray diagrams (Figure 7). This is not true for the two hydrolyzed esters (samples B and C), since x-ray diagrams of these films, coated from either solvent, showed them to be amorphous. literature Cited ( 1 ) Clarke, H. T., Malm, C. J., U. S. Patent 1,880,808 (Oct. 4,
1932). ( 2 ) Crane, C. L.. Zbid., 3,047,561 (July 31, 1962). ( 3 ) Eastman Chemical Products Bulletin, “Epolenes,” 1959. (4) Field, D. E., Fox. R. B., “Hydrolytic iModification of Cellulose Caprate,” Naval Research Laboratory Rept. 4948 (May 23, 1957). ( 5 ) Malm, C. J., Fordyce, C. R., Tanner, H. A., 2nd. Eng. Chem. 34. 430 11942). (6) Malm, C. J., Fulkerson, B., Mench, J. I V . , U. S. Patent 3,103,506 (Sept. 10, 1963). ( 7 ) Malm, C. J., Hiatt. G. D., Zbzd., 2,324,097 (July 13, 1943). ( 8 ) Malm, C. J., Mench, J. \Y., Kendall, D. L., Hiatt, G. D., Ind. Ene. Chem. 43. 684 f 19511. 9 ) Malm: C. J., Saio, M., Vivian, H. F., Zbid., 39, 168 (1947). 10) Malm, C. J., Tanghe, L. J.: Herzog, H. M., Stewart, M. H., Zbid..50, 1061 (1958). 11) Ott, E., “Cellulose and Cellulose Derivatives,” Vol. V, p. 730. Interscience, S e w York, 1954. 12) Payne, H. F., “Organic Coating Technology,” Val. 1, p. 643, Il’iley, New York, 1954. 13) Tanner, H. A , Rogovin. A , : Lockhart, L., “Cellulose Caprate Cement,” Naval Research Laboratory Rept. P-2691 (Dec. 3, 1945). 14) U. S. 1)epartment of Commerce, “Improved Cellulose Caprate Optical Cement,” Rept. P B 111273 (Sept. 8, 1953). RECEIVED for review November 18, 1965 ACCEPTED April 8, 1966 Division of Cellulose, Wood, and Fiber Chemistry, Winter Meeting, ACS, Phoenix Xriz., January 1966.
DIRECT PROCESS FOR PREPARATION OF SMALL PARTICLE CELLULOSE NITRATE A L A N M. B E L F O R T A N D R O B E R T B. W O R T 2 American Viscose Division, Avicel Department, FMC Corp., Marcus Hook, Pa.
Small particle cellulose nitrate was prepared by a direct process starting with microcrystalline cellulose. Nitrations were performed in a high solids reactor using low weight ratios of nitrating acid mixtures. Nitrating acid mixtures were added to the cellulose raw material. Some other conventional steps were eliminated: nitrating acid recovery, kiering viscosity control, and the solvent emulsion technique for preparing densified particles from fibrous cellulose nitrate. The small particle cellulose nitrate product had an average particle size of 2.4 microns, a high bulk density, and a homogeneous molecular weight distribution.
nitrate has been a versatile product in plastic, and propellant applications for more than a century. “Soluble” grade cellulose nitrate was used for photographic film, artificial leather, and quick-drying lacquers for automobiles and is still a major ingredient in formulations for furniture lacquers, multicolor lacquers, and cloth coatings. “Smokeless” grade cellulose nitrate was first used in large volume as a ballistic propellant during TYorld War I. Recently, major solid propellants containing cellulose nitrate have been perfected for long-distance ballistic missiles and for space rockets. Two small particle forms of cellulose nitrate were recently introduced : Plastisol grade nitrocellulose (E.I. d u Pont de Nemours BL Co.) is recommended for commercial and miliELLULOSE
C lacquer,
tary uses; nitrocellulose in organosol form (Hercules Powder Co.) is used in a iviping filler for furniture. Densified forms of cellulose nitrate are made from fibrous cellulose nitrate by solvent emulsion techniques (3, 9, 70). Fibrous cellulose nitrate is dissolved or swollen in a solventnonsolvent mixture, the mixture is subjected to high shear to form a globular dispersion, and small particles of cellulose nitrate are precipitated by evaporation of the solvent. Fibrous cellulose nitrate raw material for densification is manufactured by a process which has changed little since 1845. Improved continuous nitration (5) was recently introduced but processing disadvantages still persist. Fifty parts of nitrating acid are used for each part of cellulose to obtain intimate contact (5) of the reactants. T h e nitrating acid mixture VOL. 5
NO. 2
JUNE 1 9 6 6
115
must be reclaimed, fortified, and re-used. These operations of nitration, acid recovery, purification, solvent dispersion, and nonsolvent precipitation to prepare densified cellulose nitrate consume great amounts of energy and time. Avicel microcrystalline cellulose (2) (American Viscose Division, FMC Corp., Marcus Hook, Pa.) offers an opportunity to simplify the manufacture of small particle cellulose nitrate. Since the physical form of cellulose is not altered during nitration reactions, the cellulose nitrate product has the same shape and size as the starting cellulose. Microcrystalline cellulose is a small particle raw material from which small particle cellulose nitrate should be prepared in one step. This new cellulosic raw material offers improved chemical purity, narrow molecular weight distribution, small particle size, high bulk density, and increased surface area. The object of this work was to design a direct economical method for preparing small particle soluble cellulose nitrate from microcrystalline cellulose.
I
n
11
I
h
SIGMA MIXER
DROWNING TANK
1
Preparation
Materials. hlicrocrystalline cellulose was technical grade cellulose powder (Avicel) designed for industrial applications. General Nitric acid (reagent grade, minimum 907, " 0 3 , Chemical Division, Allied Chemical) and sulfuric acid (c.P., 95.5-96.57, H2S04, General Chemical Division, Allied Chemical) were premixed with cooling and diluted with deionized water where necessary to prepare nitrating acid mixtures of the desired composition. Method. Nitration reactions were performed in a 3-quart sigma-blade mixer equipped with a water jacket through which brine, at - 3' C., was circulated for cooling. The reactor was located in a well ventilated hood. The nitrating acid, composed of H?SOa, " 0 3 , and HzO (66.1, 28.1; 5.8 w . / w . ) , was mixed, cooled, and used at 5' C. Five to nine parts of nitrating acid mixture were used for nitration of each part of microcrystalline cellulose. The flow chart of Figure 1 describes the preparation of microcrystalline cellulose nitrate. Nitration. Microcrystalline cellulose (500 grams, 0.5% moisture) was charged into a 3-quart sigma-blade mixer and mixing and cooling were commenced. A premeasured quantity of 1250 grams of cooled nitrating acid mixture was added rapidly, and the whole was mixed for 1 minute. Maximum reaction temperature was 40' C. The reaction mixture gradually changed from a dry powder to wet lumps. A remaining quantity of 1250 grams of nitrating acid mixture was added in increments of 250 grams during the next 10 minutes. The reaction mixture changed to a stiff paste, then to a smooth, chalk-white gel, and finally to a smooth slurry. The circulation of brine was stopped. Reaction time was 30 minutes. Average reaction temperature was 25' to 30' c. Purification. The product was separated and washed in quick succession. The product mixture was dispersed rapidly by high speed stirrer agitation into 45 gallons of ice-cooled water, contained in a 50-gallon drum, and stirred for 2 minutes. The water slurry was pumped to and separated in a basket centrifuge equipped with a stainless steel screen containing 10micron openings. The product was washed in the centrifuge Ivith several changes of water and stabilized by the boiling water technique. The wet cake from the centrifuge was stirred into 20 to 30 parts of water, and the slurry was adjusted to pH 7 by the addition of NaHCOa and heated to reflux during five 8-hour periods. Each boiling period was followed by filtration and washing with water. T h e product, in 907, yield, was stored water-wet in jars. Properties
Analysis. Microcrystalline cellulose nitrate had 11.67, nitrogen (Coleman nitrogen analyzer, Coleman Instruments, Inc., based on the Dumas nitrogen method). Subsequent analyses showed that the Du Pont nitrometer method gave results 0.57,higher in nitrogen content than the Dumas method. All nitrogen results were adjusted to the higher level; the product prepared above had an adjusted nitrogen content of 12.17c. 116
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
CENTRIFUGE
STABILIZATION STORAGE
Figure 1 . Flow diagram of preparation of microcrystalline cellulose nitrate
Sulfur content was determined by Parr bomb ignition, followed by amperometric titration with lead nitrate. Sulfur content was 0.047,. Particle Size Distribution. Particle size and particle size distribution were determined for another microcrystalline cellulose nitrate product (12.6yc N), prepared by the described method with seven parts of nitrating acid mixture. Particle size was measured from photomicrographs cf the product by an ASTM method ( I ) . Photomicrographs were taken with 4mm. (numerical aperture 0.66) and 8-mm. (numerical aperture 0.50) B-phase objectives. A total of 1000 particles was measured by the Martin method (4).which defines the statistical diameter of the particle as the distance between opposite sides of the particle, measured crosswise of the particle, and on a line bisecting the projected area. The results of the analysis, micron diameter us. cumulative fraction per cent, are plotted in Figure 2. Density. The densities of A, microcrystalline cellulose nitrate (12.1% N), and two commercial cellulose nitrates were determined by the water immersion method ( 8 ) . The commercial cellulose nitrates were B, Plastisol grade nitrocellulose (HC15, l2.070 N, fine particle), and C, RS grade cellulose nitrate (Hercules Powder Co., 12.4% N ) . Duplicate measurements of each sample were performed. The densities 0.011 grams per ml.; B, 1.681 0.017 were: A, 1.683 grams per ml.; and C, 1.661 0.011 grams per ml. Amounts of 3.0 grams (dry basis) of each of these forms of cellulose nitrate were dispersed in 60 ml. of distilled water contained in Nessler tubes, shaken vigorously, and allowed to stand a t 25' C . Figure 3 is a visual comparison of the cellulose nitrate bulk densities after standing for approximately one month. Sedimentation Studies. Ultracentrifugations of 0.4 weight acetone solutions of microcrystalline cellulose nitrate (12.67, N) and of AS grade cellulose nitrate (Hercules Powder Co., 11.57, N) were performed. The differential distribution factor, g (s), and the sedimentation constant (S, Svedberg units) were determined for each sample ( 7 7 ) . Plots of these
*
*
-
J
0.25
MICROCRYSTALLINE CELLULOSE NITRATE
n
il F
6
0.05
0
CUWLATIM FRACTDN.Y.
Figure 2. Particle size distribution of microcrystalline cellulose nitrote
E CELLULOSE IITRATE
5
m SEDMENTATION CONS
Figure 4. Sedimentation distribution for microcrystalline cellulose nitrate and AS grade cellulose nitrate
Number average particle size distribution of microcrystalline cellulose nitrate is shown by Figure 2. In this typical sample, medium particle diameter is 2.4 microns and 98Y0 of the particles are below 40 microns. T h e limit of resolution of the light microscope used in this study is 0.23 micron; only particles larger than 0.25 micron are counted. There may be many uncounted particles less than 0.25 micron. The number average particle size method favors the small particles in the distribution and is probably different from the weight average distribution usually reported for cellulose nitrate samples. Densities of microcrystalline cellulose nitrate, Plastisol grade nitrocellulose, and RS grade nitrocellulose differed only slightly. Figure 3 shows that microcrystalline cellulose nitrate is denser than either the Plastisol grade nitrocellulose or the RS grade cellulose nitrate. Sedimentation studies provide information for measurement of molecular weight (and degree of polymerization), estimation of purity (shape of the distribution of molecular weights), and comparison of the degree of polymerization and purity of two or more samplcs. The degree of polymerization of microcrystalline cellulose nitrate (12.67, N)and of .4S grade cellulose nitrate, calculated from sedimentation and intrinsic viscosity data ( 6 ) , were 130 and 180 D.P.. respectively. A cellulose nitrate degree of polymerization of 150 corresponds to a viscosity grade of 2.5 to 3.0 seconds. Sedimentation rate and differential distribution factor vary together; a narrow symmetrical curve represents a homogeneous molecular weight distribution. hlicrocrystalline cellulose nitrate has a narrower molecular weight distribution than AS grade nitrocellulose. T h e properties of the microcrystalline cellulose nitrate will closely approximate those of the average molecular weight.
nitrating acid. A continuous organic nitration process Tvas evolved with a few changes of equipment. Xovel features of the nitration process are reverse addition of nitration reactants (nitrating acid added to cellulose), use of low amounts of nitrating acid, and elimination of nitrating acid recovery, kiering viscosity control, and solvent emulsion densification. Improved properties of soluble microcrystalline cellulose nitrate include median particle diameter of 2.4 microns, high bulk packing density, and narrow, homogeneous molecular \\-eight distribution.
Conclusions
(11) Weissberger, A , , “Physical Methods of Organic Chemistry,” p. 632, Interscience, New York, 1949. RECEIVED for review November 30, 1965 ACCEPTED March 15, 1966
Acknowledgment
The work reported was aided greatly by the suggestions and assistance of many associates. The authors thank M, J. Danzig and D. C. Nelson for helpful suggestions about processing; J. Hermans, M. R . Edelson, and N. J. TVegemer for gathering and analyzing sedimentation data; and C. D. Felton for measuring particle size distribution. literature Cited (1) Am. Sac. Testing Materials, Philadelphia. Pa.: “Analysis by Microscopic Methods for Particle Size Distribution of Particulate Substances of Subsieve Size,” XSTM Designation E 20-62T, 1962. ( 2 ) Battista, 0. A,, Smith, P.. Znd. Eng. Chem. 54,20-9 (1962). (3) Cook, R. L., Andrew, E. .A. (to Olin Matheson Chemical Carp.), U. S. Patent 2,888,713(1959). ( 4 ) Dalla-Valle, J. M.. “Micromeritics, T h e Technology of Fine Particles.” p. 69, Pitman Publishing Co., New York, 1948. ( 5 ) Hercules Chemist 37, 1 (1959). ( 6 ) Jullander: I., Arkiu Kemi dMineral. Geol. 21A, No. 8 (1945). ( 7 ) Miles, F. D.: “Cellulose Kitrate,” p. 48, Interscience, New York: 1955. 18) Petiuas. T.. Mathieu. M.. Trans. Faradav SGC.42B. 17 11946’1. ( 9 ) ReiAhardt,’C. M. (to O l b Matheson Chemical Corp.), U. S. Patent 2,919,181 (1959). (10) Voris, R. S. (to Hercules Powder Co.), Zbid., 2,843,582
--
1195R) ,~, ,.
Small particle cellulose nitrate was made from microcrystalline cellulose by a method utilizing five to nine parts of
A MODEL FOR MOLECULAR WEIGHT
DISTRIBUTIONS FERDINAND RODRIGUEZ’ AND 0. K . CLARK2 Tonawanda Laboratories, Silicones Diuision, Union Carbide Corp., Tonawanda, N . Y .
u
recently, the experimental measurement of molecular weight distribution was most often approached in two ways. In one, the polymer is precipitated or dissolved into fractions by a solvent-nonsolvent mixture. The fractions yield a cumulative molecular weight distribution curve which can be differentiated (2). The other, abbreviated way, is to measure the iveight average, and number average, J i n , molecular weights-by light scattering and osmometry, for example-and to estimate the polydispersity by using the ratio M,/M, in some mathematical model. NTIL
-vu,
Gel Permeation Chromatography
A new tool, gel permeation chromatography (GPC), gives a type of differential distribution directly (6). I n this method, 1 Present address, School of Chemical Engineering, Cornell University, Ithaca, N. Y . 2 Present address, Long Reach Laboratories, Silicones Division, Union Carbide Corp., Sistersville, FV. Va.
118
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
a small sample of a polymer solution is deposited a t one end of a packed column and then eluted by a steady flow of solvent. The raw data obtained are in the form of a diagram of polymer concentration us. solution volume, V : c =
g ( V ) = k(dTV/dV)
(1)
where g ( V ) signifies some function of V as in Figure 1, concentration G is the increment of total sample weight, TV, per unit volume, and the arbitrary constant, k , is included to show that this is not normalized. Generally, we calibrate the system as log M (molecular weight) us. V (Figure 2) :
In many cases (4, 6), the relationship between elution volume,
