EFFECTS OF CERTAIN FACTORS ON THE RADIATION-INDUCED POLYMERIZATION OF METHYL METHACRYLATE C A R L
A .
D E T R I C K '
A N D
J A M E S
1.
KELLY
Nuclear Engineering Department, University of Virginia, Charlottesville,
Vu.
22901
The effects of hydroquinone, oxygen, and acetone on the radiation-induced polymerization of methyl methacrylate were studied. Irradiations were performed in a medium-intensity gamma radiation chamber created by neutron activation of "Mn. Resultant radiation dose rates varied between 1.4 and 3.2 x IO5 rads per hour initially and diminished with a 2.58-hour half life. Hydroquinone acted as a n ideal inhibitor, oxygen as an ideal retarder. Their effects were independent of one another. Solutions of M M A in acetone exhibited the "simple dilution effect." The effects of hydroquinone and oxygen in MMA-acetone were similar to those in bulk M M A . Conversion of the monomer for a given dose was independent of dose rate for dose rates in the range 1 x IO4 to 3.2 x 10' rads per hour.
As PART of a program to study the graft polymerization of methyl methacrylate (MMA) with cellulose (Detrick, 1968), it was first necessary to investigate certain aspects of the radiation-induced polymerization of MMA. Similar studies have been reported (Chapiro, 1962; Kent et al., 1963), but no reference was found on the specific factors and conditions described in this paper. I n this study the effects of the following factors on MMA polymerization were evaluated over a range of absorbed gamma dose: inhibitor, oxygen, and acetone as a solvent for MMA. These results served as a reference for a subsequent graft-polymerization study involving MMA and cellulose.
VALVE
Experimental Procedure
Irradiation Apparatus. All irradiations were made in a medium-intensity (approximately 10' rads per hour) gamma radiation chamber created by neutron activation of the short-lived 5'Mn isotope in the University of Virginia reactor (UVAR). The radiation chamber consisted of a manganese-filled (j5Mn) aluminum annulus, having a height of 6.0 inches, an outside diameter of 2.35 inches, and an inside diameter of 1.25 inches. Neutron activation of the manganese was achieved by lowering the chamber for 2.5 hours into the UVAR core operating with a flux of approximately 2 x 10" n/cm2-sec (corresponding to a power level of about 100 kw). At the end of the activation period, the chamber was removed from the core and repositioned in the pool under 10 feet of water, which provided adequate biological shielding for the experimenter. Samples to be gamma-irradiated were positioned in small aluminum cups in an aluminum capsule (Figure 1) which could be lowered into the central opening of Present address, Bettis Atomic Power Laboratory, West Mifflin, Pa. 15122
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Figure 1. Irradiation capsule
the annular irradiation chamber for the appropriate time interval. Initial absorbed dose rates (in air) varied between 1.4 and 3.2 x 10' rads per hour, depending on location in the chamber (Table I), and diminished with a 2.58hour half life. All irradiations were performed a t the temperature of the UVAR pool, approximately 65" F. Dosage Measurements. The natural isotopic composition of manganese is 100% "Mn, which has a thermal (2200 meters per second) capture cross section of 13.3 b. The activation product, j6Mn, is radioactive, with a 2.58-hour half life. Radioactive decay occurs via a negative beta particle and several gammas, the predominant gamma energy being 0.85 mev. For the purposes of dose calculations, the mass absorption coefficient, of the irradiated samples was assumed to be that corresponding to a monoenergetic 0.85-Mev gamma.. Ind. Eng. Chem. Process Des. Develop., Vol. 9 , No. 2, 1970
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Table 1. Gamma Ray Absorbed Dose Rate in Air for Positions inside the Activated Manganese Source"
Position, Inches fmm Bottom of Source
Absorbed Dose Rate at End of Actiuation, lo" Rads per Hour
0.0 0.5 1.0 1.5 2.0 3 .O 3.5 4.0 4.5 5.0 6.0
1.90 & 0.02 2.39 f 0.02 2.71 =t 0.03 2.98 & 0.03 3.20 & 0.03 3.20 =t 0.03 2.98 & 0.03 2.74 & 0.03 2.50 =t 0.03 2.28 =t 0.02 1.46 =t 0.02
"Based on activation period in UVAR core ut 100 Kw for 2.5 hours.
T o calibrate the gamma dose rate following neutron activation, the 56Mn activity was allowed to decay until the exposure rate reached the level of a few thousand Roentgens per hour. This permitted the use of a 1000R capacity Landswerk ionization chamber to measure the exposure accumulated in a timed interval of several minutes for a number of positions in the radiation chamber. The measured exposure rates were converted to dose rates in air a t time zero-i.e., the time a t which neutron activation ended. By performing the neutron-activation and sample-irradiation sequences in a standard manner, very reproducible dose rates could be achieved (Table I). Conversion of absorbed doses in air to absorbed doses in the samples was a straightforward calculation (Hine and Brownell, 1958). Reagents. The monomer used was methyl methacrylate (MMA), obtained from Distillation Products Industries as a liquid. As supplied, it contained a small quantity (60 ppm) of hydroquinone to inhibit spontaneous polymerization. Irradiations were made with as-received as well as purified monomer. Purification was achieved by vacuum distillation, the purified monomer being stored under nitrogen until used. In some runs, the MMA was dissolved in laboratory grade acetone obtained from the Fisher Scientific Co. The acetone was used both as received and after degassing by vacuum distillation. The degassed acetone was stored under nitrogen until used. Determination of Extent of Polymerization. The monomer samples being irradiated were contained in small aluminum cups inside the aluminum irradiation capsule. At the end of an irradiation, these cups and their contents were removed, and the net weight of the polymer and monomer in each cup was determined. Then the monomer was allowed to evaporate a t room temperature for 48 hours, leaving solid, nonvolatile poly(methy1 methacrylate) (PMMA), the weight of which was determined. The extent of polymerization was determined from these weights as follows: weight of PMMA So conversion = weight of PMMA MMA
+
The same general procedure was followed in cases where acetone-MMA solutions were irradiated. N o attempt was 192
Ind. Eng. Chern. Process Des. Develop., Vol. 9,No. 2, 1970
made to determine the degree of polymerization-i.e., the size of the PMMA macromolecules-in any of the runs. Experimental Data
Effect of Inhibitor. The commercial monomer, MMA, contained 60 ppm of hydroquinone, a polymerization inhibitor, which could be removed by vacuum distillation. This process also removes oxygen dissolved in the liquid monomer. The purified, degassed monomer and the unpurified monomer were irradiated over a range of absorbed doses under nitrogen in order to determine the effect of the inhibitor (Figure 2). The unpurified monomer exhibited a marked induction period, no polymerization occurring for absorbed doses less than approximately 1.5 x lo' rads. No induction period was observed during irradiation of the purified monomer. Interestingly enough, once polymerization began in the unpurified monomer, the slope of conversion us. absorbed dose was essentially the same as for the purified monomer. These results are consistent with the role of hydroquinone as an ideal inhibitor (Golding, 1959). The exponentially decaying radiation intensity made difficult the evaluation of polymerization rates. Rough estimates of polymerization rates yielded values in the range 2.9 x to 5.5 x mole liter-' sec-' which are higher than polymerization rates reported by other workers (Chapiro, 1962; Kent et al., 1963) a t comparable dose rates and temperatures. Within the range of dose rates investigated (1 x lo4 to 3.2 x lo5 rads per hour) conversion for a given dose was essentially independent of dose rate for all of the runs reported in this paper. Figure 2 indicates that the purified and unpurified monomers both exhibit an autoacceleration in the conversion us. dose relationship at high doses. This has been explained in terms of the well-known gel effect (Chapiro, 1962). The purified monomer was completely converted with a dose of approximately 9 x 10' rads, compared with approximately 11.2 x 10' rads for the unpurified monomer. Effect of Oxygen. Unpurified MMA, containing inhibitor and dissolved oxygen, was irradiated under air as well as nitrogen to evaluate the effect of oxygen on the reaction.
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Figure 2. Effect of inhibitor on radiation-induced polymerization of MMA in nitrogen
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Purified MMA Unpurified MMA
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Also, purified MMA was irradiated under air (Figure 3). As expected, induction doses were noted for both unpurified MMA cases. These induction doses are essentially the same, indicating that air (oxygen) behaves as an ideal retarder (Golding, 1959) and not as an inhibitor. The action of oxygen as an ideal retarder is further demonstrated by comparing the lines in Figure 3 for purified and unpurified MMA under air. I n the former case, no induction dose is noted, indicating the inability of oxygen to inhibit the reaction. However, the curves of conversion us dose for the two cases are essentially parallel, indicating the same degree of retardation in each case. I t may be concluded that the effects of hydroquinone and oxygen overpressure are independent of one another. The effectiveness of oxygen as a retarder may be evaluated by comparing the case of purified MMA under nitrogen (Figure 2) with the case of purified MMA under air (Figure 3). Conversions for a given absorbed dose for the “under air” case were approximately one third the conversions for the “under nitrogen” case. N o other data are reported elsewhere with which to compare this result directly. MMA-Acetone Solutions. The effect of acetone as a solvent for MMA was also examined. Acetone was chosen as the solvent because it produces approximately the same number of radicals as MMA during irradiation (Seitzer and Tobolsky, 1955). Various concentrations of MMA in acetone were examined to determine if an optimum concentration could be found which would yield a maximum conversion for a given absorbed dose (Chapiro, 1962). Polymerizations were performed a t a constant absorbed dose (7.63 x 10’ rads) under air and nitrogen using purified reagents (Figure 4 ) . Conversions decreased linearly as the acetone concentration increased, conversions in air being about 29’5 of those in nitrogen for all MMAacetone compositions. Since no inflection was observed in the conversion as a function of solvent concentration, a solution of ’755 MMA-25‘; acetone was arbitrarily selected for further investigation.
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Solutions of 7 5 5 M M A - Z S acetone were irradiated under different experimental conditions (Figure 5 ) . As indicated, solutions of purified as well as unpurified monomer and solvent were irradiated under nitrogen and air. As in the case of the monomer alone (Figures 2 and 3), the hydroquinone in the unpurified solutions required an induction dose of about 1.5 x 10’ rads before any polymerization was noted. The presence of the solvent had little effect on the induction period. Oxygen acted as an ideal retarder for both purified and unpurified solutions. Conversions for a given absorbed dose in air were about one third those in nitrogen.
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ABSORBED DOSE
8
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Figure 3. Polymerization of pure a n d impure M M A in air and nitrogen
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ACETONE CONCENTRATION ( % BY WEIGHT) Figure 4. Effect of acetone concentration on polymerization of dissolved M M A Absorbed dose = 7.63 X 10’ rads
El
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Figure 5 . Effect of oxygen on polymerization of pure and impure solutions of MMA-acetone
0
Unpurified M M A in air Unpurified M M A in N,
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Purified M M A in air
h
75% 75% 75% 75%
purified MMA-25%
acetone in N,
purified MMA-25%
acetone in air
unpurified MMA-25%
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unpurified MMA-25%
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lnd. Eng. Chern. Process Des. Develop., Vol. 9 , No. 2 , 1970
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Conversions of 1 0 0 7 in air were not attained because of the very high dosages required. The gel effect was observed for high conversions of the monomer-solvent system, becoming noticeable a t conversions of about 50”;. This is similar to the case of the bulk monomer. Sum ma ry
and oxygen in MMA-acetone irradiations are similar to those in the irradiations of MMA alone. Because of the nature of the gamma source, dose rates varied between 1 x 10‘ and 3.2 x 10’ rads per hour. For bulk monomer and monomer-acetone irradiations with this source, conversion for a given dose was independent of dose rate. literature Cited
Hydroquinone, present in the trace quantity of 60 ppm, served as an effective ideal inhibitor, necessitating an induction dose of roughly 1.5 x 10’ rads before polymerization began. Beyond this, conversions varied linearly with absorbed doses initially. The gel effect, causing an autoacceleration in conversions, became noticeable a t roughly 50% conversion in bulk monomer irradiations as well as in monomer-acetone irradiations. Oxygen served as an effective ideal retarder for both monomer and monomer-acetone polymerizations. Conversions in air were about one third the values in nitrogen. Irradiations in oxygen required no induction doses to initiate polymerization. The effects of hydroquinone and oxygen are independent of one another. Solutions of MMA in acetone exhibit the “simple dilution effect” (Chapiro, 1962). The effects of hydroquinone
Chapiro, A., “Radiation Chemistry of Polymeric Systems,” pp. 17548,250-65, Interscience, New York, 1962. Detrick, C. A., “Exploratory Study of Radiation-Induced Graft Polymerization of Methyl Methacrylate in Paper,” M.S. thesis, University of Virginia, 1968. Golding, B., “Polymers and Resins,” pp. 41-4, Van Nostrand, Princeton, N.J., 1959. Hine, G. J., Brownell, G. L., “Radiation Dosimetry,” pp. 81-5, Academic Press, New York, 1958. Kent, J. A., Winston, A., Boyle, W. R., “Preparation of Wood-Plastic Combinations Using Gamma Radiation t o Induce Polymerization,” West Virginia University, ORO-612 (1963). Seitzer, W. H., Tobolsky, A. V., J . A m . Chem. SOC.77, 2687-92 (1955).
RECEIVED for review August 9, 1968 ACCEPTED January 14, 1970
REDUCTION OF IRON ORE WITH HYDROGEN IN A DIRECT CURRENT PLASMA JET HERBERT
1.
GILLESI
A N D
CURTIS
W .
CLUMP
Chemical Engineering Department, Lehzgh L‘nlLersitx, Bethlehem, Pa
ELECTRICAL energy has long been used
in the chemical and metallurgical process industries. Recent trends in the development of electric power generation, particularly by the use of nuclear fuel, are expected to bring substantially decreased electrical costs in the near future. The exploration of new methods for the utilization of electrical energy in commercial processing, such as the d.c. plasma jet, has therefore become increasingly attractive. One of the most important characteristics of an electrically produced plasma is that energy can be supplied to a gas without loss of potential for chemical reaction. Energy supplied by combustion, for example, requires a substantial loss in reactivity, since part of the gas is oxidized to supply the required energy. With the 100cc reactive gases and extremely high temperatures possible, rapid processing rates and low cost electricity should give plasma-metallurgical processes significant potential importance. Recent reviews (Gilles, 1968; Huska, 1965; Kubanek and Gauvin, 1967) of work related t o chemical reactions I Present address, Homer Research Laboratories, Bethlehem Steel Corp., Bethlehem, Pa. 18016
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Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970
18015
in plasmas reveal that most effort has been directed a t evaluatjng feasibility, rather than quantitative studies which lead to developing a better understanding of the process mechanism. The greatest attention has been directed at gaseous reactions in plasmas, whereas solidgas metallurgical type reactions have been given the least. The purpose of this work is to further the understanding of high-temperature solid-gas reactions in the d.c. plasma jet, primarily, to determine important process parameters and their quantitative effects; secondly, to uncover some of the phenomena occurring when solids react with extremely high-temperature gases. The reduction of iron oxide with hydrogen was chosen because iron ore is a material of great commercial importance for which new processing routes are constantly being sought. Two types of reactions involving solids in plasmas have been investigated: The plasma gas supplies energy but does not take part in the reaction, and a gaseous reactant, either the plasma gas or a second gas added with the solid, reacts with the solid. Recently a quantitative study has been made of the thermal decomposition of solid molybdenum disulfide in an argon, induction-coupled radiofrequency plasma (Huska
