Mixing

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A. M. HOFFMAN Solid Propellant Research and Development Laboratories, Aerojet-General Corp., Sacramento, Calif.

Radioactive Tracer Techniques in

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Mixing Radioactive tracers proved to be valuable analytical tools in solving composite solid propellant mixing problems where the materials under investigation were present in very small quantities l w o of the most important factors in making solid polyurethane composite propellants are production time and reliability of ballistic properties. A chief problem is in mixing-i.e., determining the point where the liquid, uncured propellant is reduced to the uniform consistency necessary to obtain reproducible ballistic performance. Before gelation, the propellant is a thick s l u r r y 4 0 to 85 weight % of solids-the principal components of which are the oxidizer, and solid fuel constituen s, together with liquid monomers and plasticizer which make u p the polymeric binder. In addition there are minor components such as catalyst, surface-active agents, and burning-rate modifiers which are present in small quantities-a few tenths or hundredths per cent. I n the work described here, radioactive tracers were used, as analytical tools, to determine the degree and time of dispersion of some of these materials. Further, both the liquid binder and the finely divided solids are readily dispersed in the propellant slurry in approximately the same mixing time; although this time will vary with the solids loading, the upper limit appears to be 30 or 40 minutes. Experimental

Radioactive ingredients. Commercially available monoiodobenzene having a specific activity of 1 mc. of iodine131-half life, 8.08 days-per gram was used as a binder simulant. I t had a melting point of -31.4’ C., a boiling point of 188.6O C . , and a density of 1.863 grams per ml. Silver-111 carbonate of half life, 7.6 days having a specific activity of 1 mc. per gram was prepared by precipitation with sodium bicarbonate from a 1 N solution of silver nitrate in 1 N nitric acid. The silver carbonate had a density of 6.08 grams per ml. and a particle size of 1 to 3 microns. This

material was used to simulate a metallic oxide burning-rate modifier. Iron-59-half life, 45.1 days-chelate curing catalyst in powder form and commercially prepared, had a specific activity of 1 mc. per gram. All of the above isotopes emit gamma radiation. The iron compound was used a t a level of about 0.25 to 0.50 mc. per 60 to 100 pounds of propellant. The other tracers were used at about 1 mc. per 100 pounds. Equipment. The experiments were conducted in stainless steel sigma-blade type mixers with propellant mixing capacities of 60, 600, and 2000 pounds, respectively. The 600- and 2000-pound capacity mixers had a blade rotation rate slightly lower than that of the 60-pound mixer. The radiation determinations were performed with an NRD (Nuclear Research and Development, Inc.) Model B-1600R radiation counter connected to an N R D windowless sample changer which contained the scintillation detector. Sampling Method. The 60-pound capacity mixer, for sampling purposes, was divided, vertically, into four equal compartments. I t was proved in the initial experiments that propellant samples obtained from one of these compartments were representative of the propellant in the other compartments. Eight sample positions were chosen and are shown as positions numbered 1 through 8 in the diagram below. The 600- and 2000-pound capacity mixers, could be divided into only two equivalent sampling sections, because, unlike the 60-pound batch mixer, one of the mixing troughs is deeper than the other (see figure, upper right). Because of the resulting larger section, 13 sample positions were used and are shown as positions numbered 1 through 13 in figure. Complete sets of propellant samples were drawn off a t the above positions

with a 1-inch diameter stainless steel “thief” a t various time intervals during the final mixing cycle of the propellant. Each sample was divided in half, and a specimen for determining the radiation count prepared from each half. Mixing Procedure. Except in specified cases, the mixing procedure for composite polyurethane propellants was as follows: The solid additives and liquid binder components, except for the diisocyanate, were added to the mixer followed by the addition of the crystalline oxidizer. After a mixing period of one half to 1 hour the diisocyanate was admixed into the resulting slurry. A final mixing period followed with the mixer under a vacuum of 29 inches of mercury. Analysis of Results. T h e resulting data from the radiation determinations were subjected to a n analysis of variance based on a two-factor experiment. The full analysis can be found (7, 2). Several other types of analysis are available; however, this type of analysis lent itself to simplicity in obtaining the total testing accuracy-Le., the precision of testing and the variation from sample to sample a t one sampling position. The radiation data from a set of samples taken a t a mixing time interval were statistically analyzed independent of the other sets of samples of the same experiment. The total testing accuracy, ua2,however, was obtained from the error of testing and the sample-to-sample variation for all the sets from an experiment. The variance of the means of the samples at a mixing time interval, urn2,was compared with the variance of the total testing accuracy, uG2, by the ratio, um2/u2. The resulting number was compared with the distribution of F a t the appropriate number of degrees of freedom. The confidence limits for uniform mixing were set a t the 1% level. This VOL. 52. NO. 9

SEPTEMBER 1960

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Sampling locations are shown in one half of a sigma-blade mixer

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The areas numbered 1 to 13 a r e the sampling positions for propellant in one half of the mixers of 600 and 2000 pounds propellant mixing capacity; areas 1 to 8 a r e for sampling the 60pound capacity mixer

throughout thc propellants during 15 to 20 minutes of mixing. Fine powders therefore appear to require somewhat less mixing time than do liquids. Dispersion of the Burning-Rate Modifier. Concurrence with the above study was obtained when the burningrate modifier simulant, silver-1 11 carbonate, was added to both 60- and 2000pound batches of propellant-78.5 weight YOsolids-immediately after the addition of the diisocyanate. The modifier simulant was shown to be completely dispersed during 10 to 15 minutes of mixing in both mixers as indicated by the results of the analysis of variance of the radiation data. The results of these two studies have indicated that solids, even finely divided powders, were more readily dispersed in the propellant mixture than was the liquid material. Discussion

limit was obtained from the knowledge of the maximum allowable variation in ingredient dispersion that would not affect eirher the ballistic properties or the polymerization rate of the propellant. The Binder

Effect of Solids Loading on Binder Dispersion. Four propellants with varying solids loading of 75 to 80 weight 7 0 were prepared in the 60-pound capacity mixer. The radioactive monoiodobenzene, the binder simulant, was added to the propellant with the diisocyanate in the last mixing step. Samples of propellant were taken at mixing time increments of 20 minutes, in a total mixing time of 100 minutes. The resulting radiation data, reduced by the analysis of variance, indicated that within the scope of the experiment all four batches achieved uniformity within the first 20 minutes of mixing. Further evaluation was made in the production mixer (2000 pounds capacity) with two propellants containing 75 and 78.5 weight solids, respectively. As in the small scale tests, the tracer was added with the diisocyanate during the final mixing of the propellant. The results of these two tests indicated a change from the small scale experiments-Le., the propellant with the 75 weight % solids required 20 minutes of mixing to reach the desired uniformity, whereas the higher solids propellant, 78.5 weight 70, needed at least 30 to 40 minutes.

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This may indicate that the slower rotation of the mixing blades, in the larger mixer, with resulting decrease in rate of shear of the propellant, reduced the rate of wetting bemeen the liquid fuel and the oxidizer. Effect of Mixer Size. That the slower rotation of the mixer blades lengthens the time of mixing was shown clearly when propellants of the same solids loading (78.5 weight 70) were mixed under the same conditions in three mixers of 60, 600. and 2000 pounds capacity, respectively. At least 10 minutes of additional mixing time were required to obtain uniformity of the 600- and 2000-pound batches than of the 60-pound batch.

In sigma blade-type mixers, the batch of propellant was not mixed uniformly. Slower mixing occurred a t the blade’s axis and at the top surface of the propellant. The least mixing occurred at the top front and back walls, because in almost all cases a binder-rich layer or “roll” of propellant formed at the top sides of the mixer parallel to the bladrs. Also, a particular sample position, taken from the bulk-below the surface- of the propellant, at any time interval, showed nonuniformity with respect to the remaining material. These nonuniform areas usually occurred at random throughout ail the sampling intervals. These regions of either liquid-rich or liquid-deficient material cannot be readily eliminated, as they depend on the rheological properties of the propellant and mixer geometry. However, these regions do not affect the ballistic prcperties of the cured propellant, because their deficiency in either oxidizer or fuel was only 1 or 2% greater than the allowed limits, and the regions involved were small.

Solid Additives

Acknowledgment

Dispersion of Iron Chelate. The main criterion for evenly cured propellant was the uniform dispersion of the iron chelate curing agent. This material is soluble in the binder system and was added to the niixer as a dry powder. The study consisted of making two 60pound batches of a propellant containing 78.5 weight Yo of solids. In the firat batch, the radioactive iron chelate was added to the fuel in the mixer: in the second batch, the radioactive chelate was added 15 minutes after the addition of the final component, the diisocyanate. Both batches were sampled at various time intervals during the final stages of mixing. I n both cases the curing agent was thoroughly dispersed and dissolved

Thanks are given to C . M . Seabourn, P. K. Myers, and J. W . Lk‘. Walker, for

INDUSTRIAL AND ENGINEERING CHEMISTRY

assistance in the investigations and to

F. R. Hepner and R . L. Parrette, who supervised this program. Literature Cited ( I ) Davies, 0. L.? ”The Design and Analysis of Industrial Experiments,” pp. 99-144. Hafner, New York, 13.54. ( 2 ) Mood, A. M., “Introduction to the Theory of Statistics,” pp. 34-337, McGraw-Hill, New York, 1950.

RECEIVED for review October 28, 1959 ACCEPTEDMay 26, 1960 Division of Paint, Plastics, and Printing Ink Chemistry, 136th Meeting, ACS, Atlantic City, 1v. J., September 1959.