the contribution of Allen Noshay, who studied this sulfonation procedure a n d obtained the data on simultaneous addition of reactants, a n d the assistance of Clifford Storms a n d Joseph Cusumano who performed many of the experiments. literature Cited
(1) Baumgarten, P., Chemie 55, 116 (1942). (2) Blaser, B., et al., U. S. Patent 2,764,576 (Sept. 25, 1956). (3) Boundy, R. H., Boyer, R. F., “Styrene: Its Polymers, Copolymers and Derivatives,” p. 335, Reinhold, New York, 1962. 1926, 684. (4) Burkhardt, N., Lapworth, A,, J . Chem. SOC. (5) Gilbert, E. E., “The Reactions of Sulfur Trioxide and of its Complexes. with Organic Compounds,” Chem. Reo. to be published.
(6) Hanson, M. \V., Bouck, J. B., J . Am. Chem. Sot. 79, 5631 (1957). (7) Kittila, R. S., ‘.Dimethylformamide, A Review of Catalytic Effects and Synthetic Applications,” E. I. du Pont de Nemours and Co., Inc., Wilmington, Del., 1957. (8) Mitchell, J. E. (to Colgate-Palmolive Co.), U. S. Patent 2,694,086 (Nov. 9, 1954). (9) Roth, H. H., Ind. Eng. Chern. 46, 2435 (1954). (10) Ibid., 49, 1820 (1957). (11) Suter, C. M., J . Am. Chem. SOC.60, 538 (1938). (12) Weber, R., Ber 20, 86 (1887). RECEIVED for review September 4, 1962 ACCEPTED October 18, 1962 DiLision of Polymer Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
SWELLING OF NITROCELLULOSE B Y PLASTICIZERS A
LAN L
.
W 00 DMA N A ND A RN0 L D A D I C0 FF
, I’.
S. >\‘az,al Ordnance Test Station, China LaXe, Calif
The plasticizing ability of a compound i s assumed to be related to its swelling ability. The equilibrium volume swelling of nitrocellulose films cross linked b y toluene diisocyanate was obtained for three solvating compounds and six plasticizers. Dimensional changes of the films were followed until equilibrium was attained, usually in 3 to 5 days. No apparent relation between cohesive energy density and swelling ability was observed. Some plasticizer bleed-out problems are explained in terms of degree of cross linking, degree o f swelling, and the orientation tendency of the polymer. Infrared absorption spectra indicate that reaction between the isocyanate (NCO) and hydroxyl groups i s not complete. After reaction of one of the NCO groups, the second NCO does not always have enough mobility to find a hydroxyl group on an adjacent polymer chain.
HE ISVESTIGATIONS described here Lvere carried out to Tdetermine a n order of plasticizer efficiency based on the equilibrium volume swelling of cross linked nitrocellulose films. Some past investigations of plasticizer efficiency involving rates of solution (77) of polymers in plasticizers fail to give a reliable index because of the number of other variables. such as plasticizer viscosity, convection. physical state, and surface area of the polymer, jvhich enter into the detailed story of polymer solution. Studies of mechanical behavior, such as Gehman temperature, brittle temperature. glass transition temperature, and Clash-Berg determinations, in general are useful but involve much time and ofren require the use of many samples and conditions. T h e variation of intrinsic viscosity with solvent ( 7 , 72: 27: 23) gives good relative values for plasticizers and solvents (27). However. some plasticizers swell but do not dissolve the polymer. These viscosity methods require the use of relatively large amounts of plasticizers or solvent, and large quantities of nitrate esters may cause difficulty because of their sensitivity to shock o r impact. Interpretation of the effect of a plasticizer in the presence of another cosolvent or diluent upon viscosity may be open to question, particularly if the solvents solvate different groups o r if there is a large size difference ( 6 ) between the plasticizer and solvent molecules. The effect of solvent change upon cellulosic molecules is relatively small ( 7 ) . Addition of a nonsolvent such as hexane to the solution of polymer in a solvent or plasticizer to the point of phase separa-
278
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
tion is a useful technique, but the result is interpretable only in terms of the volume of a particular nonsolvent used and may vary from sample to sample and depend upon the molecular weight distribution of the sample. Changing from one nonsolvent to another may alter the results and cause inversions? as can be seen in Table I taken from Moore (20). Ferry (8) has discussed the loirering of the polymer glass transition temperature as a function of the amount of plasticizer added: according to a n equation by Jenckel and Heusch (76). in terms of the iso-free-volume concept. Thus, equilibrium volume sivelling measurements can give a n order of plasticizer efficiency (4>7, 7S> 24) comparable to that obtained from a lowering of the glass transition temperature. I n volume swelling measurements of other polymers such as the various rubbers (4, 74, 75, 25), the polymer-plasticizer interaction terms have been evaluated from a knowledge of the swelling of a polymer in a series of solvents and the knowledge of the interaction parameter, X I , for one of the systems. This scheme, as Flory ( 7 7 ) has indicated, may not give a n absolute value for x1 but will establish a n order of solvent type. Although some x1 values are reported in the literature (27) for nitrocellulose-solvent systems, the work here was resiricted to a simple listing of diluents and swelling ratios. The list of diluents used was not as broad as desired because of limitations of time but serves as a useful index. Work is under way to evaluate the structure parameter by retractive force measurements and then to evaluate X I for each of the plasticizers.
3
4
5
7
6
8
IO
9
12
I1
13
14
WAVELENGTH (MICRONS) Figure 1.
Infrared spectrum of nitrocellulose cross linked with toluene diisocyanate
Flory (70) has given the following relationship between swelling ratio qm and the polymer-solvent interaction term X I : q,5’3
=
(i‘WC)(1
- 2 .W,/M)-1(1/2
-
X1)Vl
where ~zrl, is the distance between cross link sites, M is the primary polymer molecular weight, B is the specific volume of the polymer, and VI is the molar volume of the solvent. If polymer specimens from the same stock are prepared. the structure parameters a r r the same and q,5’3 is proportional to ( l / 2 - x1)Vl where x1 ( 9 ) is defined as BV,/RT and B represents the interaction energy density characteristic of a solventsolute pair.
agreed, indicating no significant sol fraction. Films similarly prepared but containing no cross linking agent dissolved completely in acetone. n-butyl acetate, and several o l the plasticizers. Results
Results of the swelling measurements are given in Table I1 T h e two plasticizers containing a high nitrate ester density are among the poorest plasticizers. This is particularly strange because they contain ester groupings similar to those found in nitrocellulose. Calculation of the square roots of the cohesive energy densities, (CED)”*, of pentaerythritol trini-
Experimental
Preparation of Films. Kitrocellulose (2.5 grams): containing 0.00505 mole of hydroxyl groups, having a nitrogen content of 12.67, and a viscosity average molecular weight of 91,600, was dissolved i n 50 nil. of dried methyl ethyl ketone. T h e required amount of toluene diisocyanate (0.00063 mole for film A and 0.000!26 mole for film R j was added from a standard solution. Solutions A and €3 were shaken for several minutes, allowed to settle free of air bubbles, and poured onto mercury contained in a dish in a desiccator. Dry air was passed through the desiccator a t a rate of less than 0.2 liter per minute. After 3 days the films were taken from the desiccator and the last traces of the methyl ethyl ketone were removed by heating the film a t 70’ C. for 24 hours. The films obtained were 0.1 m m . thick. Measurements. T h e swelling measurements were made on rectangular 2 X 3 cm. film sections of uniform thickness. T h e film was placed under two flat glass plates in a beaker to prevent curling when placed in a plasticizer. T h e dimensions of each section were measured accurately to *0.01 mm., and the volume was calculated. Plasticizer or solvent was added ; volume determinations were made until swelling equilibrium was achieved, usually three to five days. Because of the possibility of very slow diffusion of large, bulky molecules such as metriol trinitrate (2-hydroxymethyl-2-methyl-1, 3-propanediol trinitratej into the nitrocellulose, one film sample was left in this plasticizer for three weeks. No significant change was observed in the swelling. KO corrections were made for the anisotropic swelling caused by compression of the film by the glass plates. After the swelling experiment, each film was freed of plasticizer by washing with ethyl alcohol, dried, and weighed. ‘The weight before and after swelling
Table 1. Volume of Precipitant Liquids Required to Produce Incipient Turbidity When Added to 5 MI. of 1% Nitrocellulose Solution (20)
Solvent
.Amyl acetate Ethyl acetate -4cetone Cyclohexanone
16 18 15 12
8 5 (1)
0 (2) O(1) 0 (3) 5 (4)
“ Figures in parenthfsts denote Table II.
Precipztant Vol., MI. Hexane
Alcohol
Ben= 8 15 48 44
4 0 (3) (4)
5 0 0 0
(4) (3) (1) (2)
best ( 7 ) t o poorest (4)solvent in series
Swelling of Nitrocellulose in Various Solvents
(Cohesiw Enerrv Plasticizer (Solcent) Film A
n-Butyl acetate
31.7
Acetone
30.0
Triacetin Triethylene glycol dinitrate Adiponitrile Ethyl alcohol Diethyl phthalate
28,l 21.6 20 4 20.4 13.0
Metriol trinitrate Pentaerythritol trinitrate
0.3
0.2
8.28 8.5 10.0
12.7
(5) (20)
.
.
9.6
(22)
9.4 10.8 10.5
(20)
Exptl. Exptl.
Film B
n-Butyl acetate Ethyl alcohol
49.3
25.9
VOL. 1
NO. 4
DECEMBER 1 9 6 2
279
I
I
I
I
I
-
-
3
4
5
6
7
8
9
10
II
1 2 1 3 1 4
WAVELENGTH ( M I C R O N S ) Figure 2.
infrared spectrum of nitrocellulose cross linked with toluene diisocyanate after methanol treatmenl
trate, metriol trinitrate, a n d triacetin, based upon heat of vaporization values measured in these laboratories, gives 10.5, 10.8, and 11.3 (cal./cc.)1'2, respectively. Acetone has a cohesive density of 10.0 (79). Nitrocellulose has a reported (CED)'l2 of 11.5 (2). There appears to be little correlation between swelling ability a n d the solvent cohesive energy density. Table I1 explains why polymer moderately plasticized with poor plasticizers such as pentaerythritol trinitrate o r metriol trinitrate tends to bleed plasticizer when cross linked. The uncross linked polymer is swollen by plasticizer, a n d Lvhen cross linking occurs the restraints imposed upon the system are such that the polymer must exude the plasticizer to come to its equilibrium swelling ratio. If a column of the plasticized polymer is placed o n its end, it is known that additional constraints are placed upon the system, increasing the exudation of plasticizer. Similar phenomena may occur with nitrocellulose of the proper plasticizer content if orientation of polymer segments occurs. These oriented sites, if crystallite (13) in nature, may form the equivalent of cross link sites and lead to bleeding. Since orientation of segments may be diffusion- and time-dependent, a good casting may, in time, bleed. To prevent this bleeding phenomenon: the obvious recommendations are to use very lightly cross linked systems a n d to work a t a plasticizer level below that required to give equilibrium swelling for that particular system. I t is also of interest to determine the extent to which the isocyanate reacts with the polymer. T h e infrared spectrum in Figure 1 shows that a t least one isocyanate (NCO) group has reacted with the polymer because of the amide peak observed a t 6.55 microns, but not all of the second NCO groups ( N C O peak a t 4.4 microns) have reacted. Extraction of the film with benzene or methyl ethyl ketone produced very little change in the N C O absorption. Heating of the film a t 61' C. for 86 hours had no significant effect on the N C O absorption, indicating virtually complete reaction. Swelling the film before heating for long times affected the N C O absorption very slightly. If the film is treated with methanol vapor, the N C O absorption is decreased (Figure 2) and the amide absorption is increased. A solution in which the diisocyanate had been allowed to react with the polymer until a viscosity increase was apparent ( 5 hours a t room temperature) showed about the same amount of N C O absorption as a portion of the same solution cast when still very fluid. 280
I&EC P R O D U C T RESEARCH A N D D E V E L O P M E N T
Thus, there appears to be a maximum extent of reaction possible for these nitrocellulose polymers. and the diisocyanate reacts in such a fashion that, after reacting once, it often finds no other OH groups close enough with which to react a second time. Some of the difficulties of other workers ( 3 ) in cross linking nitrocellulose with isocyanates can be explained by these facts. If too high a n NCO to OH ratio is used. the OH will react with the first N C O of the diisocyanate and fewer OH'S will be lefr to react with a second. T h e optimum N C O j O H ratio for maximum cross linking has not been determined. literature Cited
(1) Xlfrey, T., Bartovics. A , , Mark, H., J . A m . Chem. SOL.64, 1557 (1942). Characterization." (2) , , Allen. P. FV.. "Techniaues of Polvmer , p. 8, Butterworths, London: 1959. (3) Bouchez, E., Champetier. G., Compt. rend. 242, 635 (1956). (4) Boyer, R. F., Spencer, R. S.. J . Polymer Sci. 3, 97 (1948). (5) Bristow, G. M., \Vatson: FV. F., Trans. Faraday Sac. 54, 1731 (1958). (6)' Clement, P., Ann. chim. (Paris) [12] 2, 420 (1947). (7) Doty, P., Zable, H. S., J . Polymer Sci. 1, 90 (1946). (8) Ferry. J. D.. "Viscoelastic Properties of Polymers." p. 357, LYiley, New York, 1961. (9) Floty, P. J.. "Principles of Polymer Chemistry," p. 509, Cornell Univ. Press, Ithaca, S . Y.,1953. (10) Zbid., p. 580. (11) Zbid., p. 583. (12) Zbid., p. 612. (13) Francis, P. S., J . Appl. Polymer Sci.5 , 261 (1961). 14) Gee, G., Trans. Faraday Soc. 38, 418 (1942) ; 40, 468 (1944). 15) Zbid., 42B, 33, 585 (1946). (16) Jenckel, E., Heusch, R., Kolloid-Z. 130, 89 (1953). (17) Kraus, A , , Farben-Chem. 10, 236 (1939) ; Farbe u . Lack 1939, p. 493; Zbid., 57, 95 (1951). (18) Lel'chuk, Sh. L., Sedlis, V. I., J . Appl. Chem. C.S.S.R. 31, 875 (1958) (Engl. trans.). (19) Mark, H., Tobolsky, A. V., "The Chemistry of High Polymeric Systems," p. 261, Interscience: New York, 1955. (20) Moore, I V . R., Trans. Faraday SOL.43, 543 (1947). (21) Moore, FV. R., Epstein, J. A , J . Appl. Chem. (London) 5 , 34 (1955). (22) Pitman, H. FV., U. S. Naval Ordnance Test Station, China Lake, Calif., unpublished work. 1954. Chem. Ind. Japan 37, Suppl. (23) Sakurada, I., Shojino, M., J . SOC. Binding, 603 (1934). (24) Valentine, L.. J . Polymer Sci. 23, 297 (1957). (25) !$'hitby, G. S., Evans, A. B. A , , Pasternack, D. S., Trans. Faraday SOC. 38, 269 (1942). (26) FYoodman, A. L., Murbach, \Ir. J., Kaufman, M. H., J . Phys. Chem. 64, 658 (1960). (27) Zubov, P. I., Z\.erev, M. P., Kolloid Zhur. 18, 679 (1956).
I
RECEIVED for review .4pril 16, 1962 ACCEPTED August 9, 1962 San Diego Regional Meeting, ACS, December 1961.
