Use of Chromatographic Extraction for Double-Base Propellant Analysis G . K. Landram, A. A. Wickham, and R. J. DuBois Hercules Incorporated, Bacchus Works, Magna, Utah 84044
DOUBLE-BASE PROPELLANTS consist of nitrocellulose and another nitrate ester, usually nitroglycerin. In addition, various inert plasticizers, stabilizers, oxidizers, and metals are present. Consequently, an extraction of the more readily soluble components (i,e.,plasticizers and stabilizers) is necessary prior to analysis. This separation usually is performed by a Soxhlet (I) or Wiley ( 2 ) extraction or by means of filtration after refluxing in an appropriate solvent (2). The MIL-STD-286B Soxhlet extraction used in this laboratory suggests a 6- to 20hour extraction for propellants depending on the solvent, apparatus, and physical preparation of the sample. In addition, the Soxhlet method requires a considerable amount of solvent and rather elaborate glassware and heating units. Recently, microscale thin layer chromatography (TLC) propellant analysis schemes were reported ( 3 , 4 ) . These methods employed a n overnight solvent leach or complete dissolution of all organic components. However, when larger samples (several tenths of a gram or more) are available, micromethods such as TLC would not logically be employed. In an effort to find an improved propellant extraction procedure, liquid-solid column chromatography was explored. Normally, the technique required that the sample be dissolved and then added to a column wet with solvent. However, complete solution of a typical propellant would take a highly polar solvent, one unlikely to provide satisfactory component separations. Preliminary work leaching ground samples placed on top of a wet column showed that too large elution volumes were needed. On the other hand, dry-column techniques have been shown to provide rapid, preparative scale separations with the efficiency of TLC (5). As the name implies, the column packing is added dry. Both ascending and descending elutions have been employed. In the ascending method, the solvent flow is produced by capillary action only. In the descending approach, where a small solvent pressure head is maintained above the adsorbent, the flow is due to a combination of gravity and capillary action. In either case, a capillary-type flow should prevail through a finely ground sample placed at one end of the adsorbent column. Chromatographic separation also might eliminate problems in measuring propellant stabilizers in the presence of their nitration products (6). These nitration products are quite similar chemically to the parent stabilizer. With the Soxhlet extraction some of these products are removed with their parent compounds and interfere in the analysis unless removed in
(1) MIL-STD-286B, Propellants: Sampling, Inspection and Testing, Method 104.1.3, Dec. 1, 1967. (2) F. J. Welcher, Ed., “Standard Methods of Chemical Analysis,” Sixth Ed., Vol. 11, Part B, pp 1372-3, D. Van Nostrand Co., Inc., Princeton, N. J., 1963. (3) J. A. Kohlbeck, ANAL.CHEM., 37, 1282 (1965). (4) R. J. DuBois, J. A. Kohlbeck, and R. S. Lambert, CPIA Publication No. 70, pp 245-58, March 1965. ( 5 ) B. Loev and M. M. Goodman, Chem. Ind., London, 1967,2026. (6) G. F. Macke, J . Clzromatogr., 38, 47 (1968).
a subsequent separation. This paper compares dry-column chromatographic techniques with the Soxhlet extraction of double-base propellant formulations. EXPERIMENTAL
Materials. The solid supports evaluated were silica gels of both 50- to 200-micron and 200- to 500-micron size. These materials were procured from Brinkmann Instruments Inc., Westbury, N. Y . Chromosorb T (Teflon) 40/60 mesh was obtained from Applied Science Laboratories, State College, Pa. The silica gel was Brockmann activity 11-111 (7). The activity of a support was determined by adding 1 ml of a 0.1 solution of 24trodiphenylamine (2-NDPA) and resorcinol in methylene chloride to the top of a column of typical dimensions filled with the adsorbent. Additional solvent was added to the column to elute the 2-NDPA. If the activity was too high (Brockmann activity Grade I), the 2-NDPA was not eluted at the solvent front. If the activity was too low (Brockmann activity Grade V), resorcinol was eluted along with the 2-NDPA. The resorcinol was detected after TLC separation by a convenient spot test using tetracyanoethylene (8). The optimum activation level of the silica gel support allowed 2-NDPA to be eluted at the solvent front and resorcinol to be adsorbed on the support using methylene chloride. When necessary, the activity was decreased by adding a few per cent water to the support or increased by heating for several hours at 250 “C. The propellants and casting powders used in this study included composite modified double-base, double-base, and cross-linked composite modified double-base types and were taken from typical Hercules Incorporated production lots. All chemicals used were reagent grade. Apparatus. The chromatographic columns were 18 cm long and 1 cm i.d. with tips drawn to 1 mm. About 1 cm of glass wool was placed in the tip ends. The tip was then connected to vacuum and a 10-cm column of adsorbent was added with tapping to obtain a uniform packing. A 0.5-cm layer of glass wool was placed on top of the support. The glass wool may be replaced by a filter paper disk to conveniently remove the insoluble components for analysis. Procedure. The following procedure was applied to the propellants described in this paper. This separation procedure was satisfactorily used for a number of double-base propellants containing nitroglycerin (NG), triacetin (TA), 2-nitrodiphenylamine (2-NDPA), resorcinol, ammonium perchlorate (AP), aluminum, 2,4,6,8-~yclotetramethylenetetranitramine (HMX) and nitrocellulose (NC). Undoubtedly, some variation will be needed for other propellants. Propellant samples were prepared by grinding to approximately 20 mesh in a Wiley mill ( 2 ) or by rasping with a file. For safety reasons, grinding of propellants containing AP was done only on small samples with safety shielding and cooling of the Wiley mill with liquid nitrogen. Chromatographic extraction consisted of placing 0.4 to 1.0 g of material on the upper surface of the glass wool in the wide end of the tube. The sample then was covered with sufficient glass wool to fill the tube. The chromatographic tube was (7) H. Brockmann and H. Schodder,Ber., 74,73 (1941). ( 8 ) G. F. Macke, J. Chromatogr., 36, 537 (1968).
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RESULTS AND DISCUSSION
Table I. Comparison of Teflon and Silica Gel Column Chromatographic Extractionv Extraction efficiencv ("Z of Soxhlet)* Chromosorb Component Silica gel column T column NG
TA 2-NDPA Resorcinol
100.4 100.4
99.6 80.8 98.8 95.3
100.6 98.0
Double-base propellant, 0.6-g samples.
* Average of duplicate determinations.
Table 11. Effect of Propellant Sample Size on Chromatographic Extraction 2 Founda Sample size, g NG TA 2-NDPA Resorcinol 0.4 0.6 0.8 1.o 0
30.9 30.8
30.7 30.6
2.56 2.63 2.65 2.67
0.99
1.03 1.01 0.99
0.59 0.60
0.61 0.59
Average of duplicate determinations.
inverted and the wide end placed in a 50-ml Erlenmeyer flask containing about 25 ml of methylene chloride. The solvent level was adjusted to ensure capillary flow through the sample and adsorbent. Capillary elution was allowed t o continue until the solvent just traveled the length of the solid adsorbent. Then the tube was turned upright and the glass wool removed. A descending elution with 25 ml of methylene chloride washed N G and 2-NDPA from the tube. These components were collected in a 25-ml volumetric flask and diluted to volume for subsequent measurement. The solid sample residue (NC, HMX, etc.) was removed from the tube with a spatula and prepared for testing without further separation. This was accomplished by dissolving the NC, HMX, and AP in an appropriate volume of dimethyl sulfoxide. A descending 25-ml ethyl acetate elution was used to remove TA and resorcinol. These components also were collected in a 25-ml volumetric flask. Resorcinol was determined directly in ethyl acetate while for T A determination an aliquot was evaporated on a steam bath and the TA rediluted in 1,Z-dichloroethane. All but one of the extracted components were analyzed by spectrophotometry. The spectrophotometric testing was performed using either the Beckman IR-7 or DK-2A spectrophotometer. The NG in methylene chloride was analyzed by infrared (IR) at 1659 cm-l. The TA in dichloroethane was also analyzed by IR at 1745 cm-'. The 2-NDPA in methylene chloride and resorcinol in ethyl acetate were measured at 420 mp and 272 mp, respectively. When measurement of N C and HMX was desired, these components in dimethyl sulfoxide were measured by IR at 1659 cm-l and 1569 cm-l, respectively. Di-n-propyl adipate was measured by a Perkin-Elmer Model 5754 gas chromatograph using a n eight-foot column of 2 0 z SE-54 silicone on Gas Chrom CLH (Applied Science Laboratories). The inorganic components (ammonium perchlorate and aluminum) were not tested for this study. For comparative testing, several ground samples of the desired formulations were extracted by the Soxhlet method and subsequently measured in the same manner as the chromatographically extracted samples. However, a TLC separation was necessary prior t o spectrophotometric measurement of stabilizers t o avoid interferences from stabilizer nitration products (6). 108
Several typical CMDB propellants were used to evaluate both ascending and descending dry-column chromatographic techniques. For these formulations to interface satisfactorily with existing methods, NG, TA, resorcinol, and 2NDPA had to be quantitatively removed from NC, HMX, AP, and Al. I n addition, nitrated stabilizer products had to be left in the residue or on the adsorbent. After extraction N G and TA most frequently are measured by IR. A small correction factor is necessary because of overlap of the analytical peaks. Therefore, removal of N G and TA separately from the column was desirable. The 2-NDPA and resorcinol are determined spectrophotometrically in the visible and UV region, respectively. Resorcinol is interfered with by 2NDPA. Thus, a separation of these two materials also was sought. Preliminary work showed that the ascending dry-column approach was the most suitable for the necessary extractions. Quantitative extractions were possible using either the ascending or descending technique. However, on occasion low extraction efficiencies were obtained using the descending mode. This resulted from allowing too high a flow rate. In the descending mode flow rate can be a function of solvent head, sample size and packing, and adsorbent particle size and packing. For economy of operation in actual laboratory usage, the operator cannot be expected to control these variables carefully. On the other hand, the use of the ascending mode for the extraction step automatically assured satisfactory flow rate control with a minimum of operator attention. Descending elutions were useful for subsequent separations of the extracted components. Obviously, there were many variables possible in the chromatographic extraction method. These included column dimensions, sample size and degree of subdivision, type, activity and particle size of adsorbent, and the elution solvent(s). In this work on the separation of double-base propellants, only a few of these variables were necessarily investigated. Silica gels of 50- to 200- and 200- to 500-micron particle size and Brockmann activity Grade 11-111 were evaluated as adsorbents. Considerably faster ascending separations were obtained with the finer particle size. The coarse material was somewhat faster for the descending elution step. However, because of the possibility of channeling in the coarse material and resultant loss of separating eficiency during the dcscending elutions, the fine material was preferred. Several types of propellant were initially extracted using the 200- to 500-micron silica gel to verify the extraction efficiency of the method. An ascending extraction of 0.6-g samples using diethyl ether was followed by a downward elution with 25 ml of 1,2-dichloroethane. The same propellants were extracted by the Soxhlet method and the results compared. The recoveries of TA and resorcinol were as much as 20% lower for most propellants using the chromatographic procedure. Rather than poor extraction, the low recovery on silica gel was due to poor choice of elution solvent and resulting retention of TA and resorcinol on the rather active support. This was demonstrated by replacing silica gel with a powdered Teflon support which was completely inactive. This support was used to separate a 0.6-g sample of double-base propellant using diethyl ether for upward development and dichloroethane for the downward elution of NG, TA, 2-NDPA, and resorcinol. This separation system and the above silica gel system were compared to the Soxhlet extraction method. The results are shown in Table I. Using the inert column, the recovery of all extractable components was shown to be
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Table 111. Comparison of Chromatographic Extraction and Soxhlet Extraction for Several Propellants Difference, %, Found chromatographic Soxhletb 1;s. Soxhlet Chromatographic" Component Propellant 29.9 30.0 -0.1 NG Double-base No. 1 0.95 0.93 f0.02 2-NDPA 2.76 2.85 -0.09 TA 0.59 0.59 0.00 Resorcinol 26.5 26.9 -0.4 NG Double-base No. 2 0.96 0.93 f0.03 2-NDPA 4.74 4.88 -0.14 TA 0.66 0.70 -0.04 Resorcinol 9.9 10.2 -0.3 NG Casting powder 0.96 0.92 fO.04 2-NDPA 0.90 0.94 -0.04 Resorcinol 41.3 40.8 fO. 5 NG Double-base No. 3 1.93 1.92 f O . 01 2-NDPA 3.74 3.87 -0.13 Di-n-propyl adipate" 33.6 33.1 f0.5 NG Cross-linked 0.95 0.92 2-NDPA +O. 03 double-base 0.36 0.35 Resorcinol +o. 01 Average of six determinations. Average of two determinations. c Gas chromatographic method. 0
b
essentially complete based on Soxhlet results. Thus the poor recovery with silica gel was due only to difficulties in removing the components from the adsorbent and not in extraction from the propellant. Consequently, a suitable extraction/separation scheme was developed using silica gel adsorbent of 50- to 200-micron size and a Brockmann activity of 11-111. However, methylene chloride was used for the initial ascending extraction step instead of diethyl ether. The latter was too volatile for use in the spectrophotometric analysis and would require evaporation after this step. (While HMX is slightly soluble in methylene chloride, the quantity dissolved in the small amount of solvent used here was negligible.) Two descending elutions were used. One descending elution with methylene chloride removed NG and 2-NDPA from the column. The residue containing N C and HMX was then removed for analysis, After purging the column dry of methylene chloride, ethyl acetate was used to remove TA and resorcinol. Existing test methods required that the resorcinol be determined directly in ethyl acetate. For TA measurement an aliquot of ethyl acetate was removed, the solvent evaporated, and the TA rediluted in dichloroethane. Stabilizer nitration products remained on the column. A series of double-base propellant samples ranging from 0.4 to 1.0 g were extracted by the above method. The results are shown in Table 11. These results show that the increased sample size has essentially no effect on the recovery of NG, TA, 2-NDPA, and resorcinol from this propellant. Sample sizes of up to 1.0 g may be extracted without overloading the column. The optimum sample size will depend on the uniformity of the sample and the sensitivity of the methods used to determine the various components. Several types of double-base propellants and a base grain
were extracted by the Soxhlet method and with the above procedure. The results are shown in Table 111. The data show that the chromatographic extraction was comparable to the Soxhlet extraction for separation of NG, 2-NDPA, TA, and resorcinol in the double-base propellants. The chromatographic extraction was slightly lower in efficiency for extraction of N G and resorcinol from ground samples of casting powder. This may have been due to the increased density and hardness of the ground casting powder particles as compared to the ground propellant. A second ascending separation with methylene chloride was subsequently found to ensure complete extraction of these components. The same chromatographic procedure was used for the testing of a ground sample of sheet propellant. The N G and 2NDPA were separated in one step and di-n-propyl adipate in the second step. The residue remaining in the column after the methylene chloride elution quantitatively contained NC and the organometallic burning rate modifiers which may be analyzed readily is desired. A cross-linked propellant was analyzed similarly. The chromatographic recovery data for NG, 2-NDPA, and resorcinol were equivalent to those obtained from extraction by the Soxhlet method for 72 hours. Thus, chromatographic extraction seems applicable to any number of double-base propellants. In addition, the general approach of dry-column chromatographic extraction should prove useful for testing many nonpropellant polymer systems. For the formulations studied here, extraction time was cut from 1 to 4 days by Soxhlet to about 1 hour using chromatography. Solvent consumption was also reduced by about 90%, and on-column separation of several mutually interfering extractants simplified measurements. RECEIVED for review August 14, 1969. Accepted October 15, 1969.
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