Evolution of Halides from Halogenated Plastics Exposed to Gamma Radiation JOSEPH BYRNE, T. W. COSTIICYAN', C. B. HANFORW, D. L. JOHNSONS, AND W. L. MANN Massachusetts Institute of Technology, Engineering Practice School, Carbide & Carbon Chemicals Co., Oak Ridge, Tenn. d
T P
HE necessity of processing radioactive fission productls from nuclear reactors has aroused interest in the changes of physical properties and halogen evolution of halogenated plastics subjected to gamma radiation. In equipment designed to handle radioactive materials, parts like gaskets and bushings are frequently made from halogenated plastics that are chosen for their inert chemical behavior. Ln the presence of gamma radiation, a breakdown occurs with release and evolution of chlorine and fluorine, and a normally inert material becomes a source of corrosive agents. Both the corrosion and contamination from corrosion products can be serious problems.
was released from the chemical bond uniformly throughout the solid whereas the halogen evolved a t the surface was a t a small rate initially, increasing to constant value as steady state was attained. Since the diffusion of halogen through the plastic was considered to be very slow, the unsteady-state time was expected to be long. As developed in detail later, the results showed that unsteady state existed throughout some of the tests. The data gathered were the amount of halogen evolved and the changes in physical properties of plastics that had been exposed to measured amounts of gamma radiation. The materials chosen for investigation were plasticized and nonplasticized polymonochlorotrifluoroethylene and a nonplasticized polyvinyl chloride. The polymonochlorotrifluoroethylene was fabricated by Carbide & Carbon Chemicals Co. from Kel-F powder, Grade 300, by Carbide's trade name Fluorothene for convenience. The polyvinyl chloride was obtained from the Van Dorn Iron Works of Cleveland, Ohio, under the trade name Lucoflex. The plasticizer used for Fluorothene was a low molecular weight polymonochlorotrifluoroethylene. I n addition, a few samples of carbon tetrachloride were irradiated. 1.5
c,
i: w
2
RADIA notv EXPOSORF MILLIONS OF ROENTGENS
Figure 1.
11
Previous to this study Watson (6) of the Oak' Ridge National Laboratory conducted an investigation of the evolution of fluorine from polytetrafluoroethylene (Du Pont Teflon) exposed to gamma radiation. Bopp and Sisman ( 8 ) and Burton ( 1 ) have made studies of the effect of reactor radiation on some physical properties of a number of plastics. I n such tests the specimens received a mixed irradiation of neutrons, beta, and gamma rays. The results of Bopp and Sisman on polyvinylidene chloride (Dow saran) and polymonochlorotrifluoroethylene (Carbide and Carbon Fluorothene) indicated a substantial diminution, or a complete loss, of strength after an accumulated exposure of 1017 to 1018 NVT units. The NVT unit is the product of neutron flux, neutrons/(sq. om.) (sec.), and the total time of exposure. The planning of experiments and interpretation of the results required consideration of unsteady-state mass transfer. I n order t o achieve a distinction necessary to discuss unsteady-state condition, release of halogen is used to refer to the destruction of a chemical bond between the halogen and the polymer molecule; evolution of halogen is used to refer to the escape of halogen from the surface of the plastic object. It was assumed that the halogen Present address, Standard Oil Development Co., Linden, N. J. Cambridge, Mass. a Present address, Minnesota Mining and Manufacturing Co., St. Paul, Minn. 1
.I/
Evolution of Chloride from Irradiated Polyvinylchloride
* Present address, Arthur D. Little, Inc.,
0
s 0 5 00
/< 10
RADIA rioN EXPOSURE U i I L I O N S OF ROENTGENS
Figure 2. Evolution of Halide from Irradiated Nonplasticized Fluorothene
No attempt was made t o determine the chemical state of the halogen evolved. Halide is used in reporting the results since the halogens were determined as halides in the analytical procedure. PROCEDURE
The plastic materials were either cut into cubes, approximately 1/8inch on a side, or were made into shavings using a milling machine of '/a-inch width, 0.005- to 0.010-inch thickness, and approximately l/r-inch length. Samples of 1.0 to 2.5 g r a m of 2549
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cubes, and 0.5 gram of shavings were weighed, placed in 10-ml. polystyrene vials, and covered with 10 ml. of sodium hydroxide solution. These samples were exposed for 2 to 28 days to gamma radiation from cobalt-60 a t a nominal intensity of 25,000 roentgens (r.) per minute. At intervals of about 48 hours, two vials of the cubes and one of the shavings were removed from the source. The caustic solution was withdrawn and analyzed for chloride and fluoride in the case of Fluorothene, and chloride only for the polyvinyl chloride samples. The analyses were made after converting any hypohalite to the halide form.
500
1000
RA D/AT/ON EXPOSURE MILLIONS O F ROENTGENS
Figure 3. Evolution of Halide from Irradiated Plasticized Fluorothene
For comparative purposes, a few samples of carbon tetrachloride were irradiated to determine the evolution of halogen in a liquid system. The carbon tetrachloride was covered b y a layer of caustic solution and was not agitated during exposure. The changes in physical properties were observed by irradiation of additional samples cut from l/rinch sheets to shapes required for shear, impact, and tensile tests. Bundles of these samples were exposed to gamma radiation and specimens were removed a t 2-day intervals. The physical tests were made b y modifications of the following tests: tensile, ASTM D-638-49T; shear, Federal Specifications 1041, L-P-406a; impact, ASTM D-25647T. RESULTS
EXPERIMENTAL DATA. The experimental results of halide evolution are presented in Figures 1 to 3, which show evolution as a function of the radiation exposure. The choice of radiation exposure, rather than time, as the independent variable was made on the assumption that the rate of release of halogen from the polymer by destruction of the halogen-carbon bond is independent of intensity. The actual intensities in these experiments varied from 20,500 to 35,700 r. per minute, but the variation was not sufficiently controlled to determine an effect of intensity. Originally the data were plotted both against time and against accumulated exposure, and it appeared that use of radiation exposure resulted i n less scatter of the data. Although the evidence was not conclusive, radiation exposure was chosen for presentation. The chloride evolved from polyvinyl chloride (in millimoles halide per gram of plastic) is shown in Figure 1 for radiation exposures up t o 1.5 billion r. for both cubes and shavings. The data for shavings are represented by a straight line, indicating a steady-state condition; the datn on cubes indicate both in amount
Vol. 45, No. 11
TABLII I. LIaxImnr RATEOF HALOGEN EVOLUTION Maximum Rate of Haloeen Evolution hlillimoles Halide/(Gra%) (Billion r.) Carbon Planticieed Nonplasticised Polyvinyl tetrachloride Fluorothene Fluorothene chloride
Halogen Evolved Cubes
Fluoride Chloride Total halide Shavings Fluoride Chloride Total Liquid Maximum rate irrespective of geometry Total halide
.. .. ,
.
..
1 1 2
_.
0.4
0.4 -
0.8 3
..
2.5 2.5 5
-
3 -
9
..
..
..
9
5
6
6
7 -
7
-7 . 7
..
7
and curvature that unsteady state existed. The slope of the curve at 1.1 billion r. is approximately equal to that of the straight line through the data on shavings. The chloride and halide evolved from nonplasticized Fluorothene are shown in Figure 2, the ordinate representing either chloride or fluoride. The ordinates for Figures 2 and 3 are on a scale tenfold that of Figure 1. I n view of the considerable scatter of the data, only one curve was drawn for cubes and one for shavings, in effect taking the rate of evolution of chloride and fluoride to be equal. It is evident that unsteady state existed through the experiments on cubes. The data on shavings show considerable scatter, but it seems likely that steady state was being approached a t the higher exposures. The chloride and fluoride evolution from plasticized Fluorothene are shown in Figure 3. The data for shavings coincide with those for nonplasticized Fluorothene. The cubes, however, showed a higher rate of evolution than for nonplasticized material although steady state was not reached. The changes in impact, shear, and tensile strengths are shown in Figure 4 as a function of radiation exposure. The polyvinyl chloride withstood the radiation better than the Fluorothene. The latter showed almost complete loss of strength a t 200,000,000 r. while polyvinyl chloride was usable after 800,000,000 r. The maximum rates of halide evolution, shown in Table I, were obtained from the slopes of the curves drawn in Figures 1 to 3. The evolution of chloride from carbon tetrachloride is shown for comparison. VISUALOBSERVATIONS.After 2 to 4 days of irradiation, the polyvinyl chloride turned from a translucent light brown to an opaque black. No further change in color occurred. Samples exposed longer than 20 days had blisters a t the surface, some of which had evidently burst. The Fluorothene became brittle after 4 days and remained so for 22 additional days. After still longer but unrecorded exposure, the nonplasticized Fluorothene became pliable and soft, somewhat like limp rubber. DISCUSSION OF RESULTS
ENGINEERING USE OF DATA. The amount of halide evolved during t h e physical lifetime of the materials can be tolerated in some systems and not in others. There are two important reasons for concern about halides evolved: excessive corrosion resulting in physical weakness or leaks and contamination of process materials by corrosion products. The data reported here can be used to estimate the order of magnitude of both the amount of corrosion and the amount of contamination. For example, consider corrosion of an iron surface in contact with a gasket 2 mm. thick. Assume that: 1. The evolution occurs a t the maximum rate reported in Table I-namely, 7 millimoles per gram (billion r.). 2. Each mole of halide removed 0.5 mole of iron.
is small, a determination of the amount of absorption would necessarily require precise or indirect techniques. The assumption of space-uniform release, postulated above, is based on the small absorption of gamma in the plastic material; namely, the gamma intensity is assumed uniform throughout the material, and release is assumed proportional to intensity. The data for Fluorothene summarized in Table I show a nearly equal rate of evolution of chloride and fluoride in each case, which ia somewhat surprising since the ratio of chlorine to fluorine in the original plastic was 1 to 3. To attempt an explanation of the results is unwarranted in view of the untested variables, such a8 surface effects, in these experiments.
3. The attack is uniform and occurs a t point where the halogen is evolved. 4. Only gamma radiation is present a t an intensity of 1000 r. per minute. 5. Evolution is equal on each face of the gasket. The calculation is based on use of 1 square cm. imagined to be in the center of a large area of gasket material:
22 x 0.2 om.
X 1.4grams of plastic/cc. X
0.007 mole of halide/gram billion r. X 56 grams
A
of Fejmole of Fe
1 xcc./grams 7.8
x
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INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1953
x
l/2 mole of Fejmole halide
of Fe X 1000 r./minute
525,600 minutes/year
2x
x 1 inch/cm.
Ld=-u
zoo
= 0.0007 inch/year Dimensionless correction for two surfaces, assuming t h a t half of the halogen released leaves one surface. a
Thus, the calculated corrosion rate of 0.0007 inch per year (0.7 mil) and the contamination is approximately 1 millimole of halide and 0.5 millimole of iron per year. I n this example the use of polyvinyl chloride seems reasonable since the contamination would in most cases be trivial, and the physical life of the material would be more than 2 years. On the other hand, Fluorothene would withstand the radiation only a few months. An inherent assumption made in this calculation is that the rates of evolution and release are equal. During unsteady state when part of the released halogen is being stored within the plastic, the rate of evolution will be less than predicted by the assumption. As long as the halogen is stored within the plastic, corrosion and contamination cannot occur. Therefore, the assumption of equal rates is a conservative approximation for p r e dicting rates of corrosion and contamination. It is recommended that the data presented in Table I of this report be considered when using polyvinyl chloride and Fluorothene, or similar plastics, in construction of equipment to handle gamma-aotive materials. SIGNIFICANCE OF RESULTS.While the objective of the work was to obtain engineering data on evolution, the release of the halogens from the chemical bond is of fundamental interest. There are, however, two principal difficultiesin interpreting these data: the unsteady-state condition and determination of the gamma-energy absorbed. The data obtained give clear indication that unsteady state existed throughout most of the experiments with cubes and in some of the experiments with shavings. Indications of steady state were considered to be: constant slope of the curve of halide evolution as a function of radiation exposure and the slope of the curve for cubes equal t o that of shavings (with both slopes constant). If certain simplifying assumptions are made, it is possible to write a differential equation relating evolution, release, and time, The equation could be solved by numerical methods too tedious for hand calculation. Machine calculation was deemed unwarranted since the assumption of a constant diffusion coefficient is not realistic-bubbles and cracks noted on the exposed plastic would change the diffusion coefficient and surface area, making the equation inappropriate. It is evident that fundamental studies of release of halogen must be based on experiments different in method and precision. No attempt has been made t o estimate the amount of gamma energy actually absorbed in the plastic, and no estimate of the energy involved in breaking the chemical bond can be made from these data. Since the gamma absorptivity of the plastic
o
- cnLomnE r
POL v / N r L
- NON-PLAST/C/ZED FLUOROTHENE A - PLAST/C/ZED Q
.
/50
.
-
-
I
100
E P
1-1
I %I I
0
I
I
1
150
/oo
0 0
5 00
1000
RA DIA T/ON EXPOSURE U / L L / O N S OF ROENTGENS
Figure 4.
Changes i n Physical Properties of Irradiated Plastics
The data for the shavings, being less complicated by storage of halide in the plastic, may be compared on a maximum rate basis shown in Table I. I n view of the diversity of materials, a reasonably constant value of rate of evolution was found. Moreover, the rate for carbon tetrachloride is only 30% higher than the average for the plastic materials. A number of explanations could be proposed, but these are insufficient data to make speculation fruitful. The mechanisms of release and evolution cannot be deduced from these data, and further experiments would be necessary to resolve such questions as the equality of the rate of evolution of chloride and fluoride and the possibility of surface reactions. From an engineering viewpoint, the findings demonstrate that use of halogenated plastics in gamma-active systems requires consideration of the evolution of halogen, and these data permit a n estimation of the order of magnitude of such evolution. CHANGES IN PHYSICAL PROPERTIES. The polyvinyl chloride survived exposure better than Fluorothene and actually showed substantial improvement in shear and impact strength during part of the experimentation. The improvement possibly resulted from plasticization by smaller molecules produced by splitting of the polymer chain. Similar improvement in impact strength was notedby Boppand Sisman (3) for polytetrafluoroethylene (Teflon). The changes in the physical properties of nonplasticized Fluorothene with gamma radiation are similar to those found by Bopp
INDUSTRIAL AND ENGINEERING CHEMISTRY
2552
and Sisman for reactor radiation ( 9 ) . Complete loss of strength occurred in the reactor after exposure estimated to be 50,000,000 r. of gamma, plus beta and neutron irradiation. I n these experiments with gamma alone, complete failure occurred a t 100,000,000 to 200,000,000 r., indicating that beta and neutron irradiation as well as gamma are harmful. It is worth noting that and ( 6 )found Some materials-e.g*, Were unchanged in strength after prolonged radiation. Consequently the data reported here on physical properties must be considered specific for the materials studied. CONCLUSIONS
Polyvinyl chloride retained physical strength better than polymonochlorotrifluoroethyleneduring exposure to gamma radiation. ACKNOWLEDGMENT
The authors wish to express appreciation to Carbide & Carbon Chemicals Co. for making available the facilities and materials for this study. Personal thanks are given to C. D. Watson of the Oak Ridge National Laboratory for his guidance and aid. REFERENCES
(1) Burton, M., J . Phys. & Colloid Chem., 51, 786 (1947). ( 2 ) Gisman, O., and Bopp, C. D., "Physical Properties of Irradiated Plastics," R e p t . ORNL-928, p. 9, Oak Ridge National Labora-
tory, Carbide and Carbon Chemicals Co., 1951. (Copies available a t Depository Libraries of the iitomic Energy Commission.)
It was concluded that: when polymonochlorotrifluoroethylene (Fluorothene) and polyvinyl chloride are exposed to gamma radiation, the quantities of halogen evolved may be significant from corrosion and contamination viewpoints, For engineering DurDoses, the rate of evolution can be taken as 7 millimoles of halide per gram of material per billion roentgens of exposure.
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Vol. 45, No, 11
(3) Ibid., P. 85.
ps 166. (5) Watson, C. D., personal communication (April 1952). (4)
I
RECEIYED for review january 21, 1953.
ACCEPTED
July 15, 1053.
Large-Scale Laboratory Preparation of 2,5-Dichlorostyrene HENRY POLLOCK AND H. W. DAVIS Department of Chemistry, University of South Carolina, Columbia, S . C .
E
RICKSON and Michalek (7) prepared 2,5-dichlorostyrene by means of the side-chain chlorination of 2,5-dichloroethylbensene followed by dehydrochlorination. Dehydrochlorination of l-chloroethyl-2,5-dichlorobenzeneby means of steam and calcium sulfate to produce the desired styrene has been reported by Basdekis ( 9 ) . Another method ( 5 ) utilized the simultaneous reduction and dehydration of 2,5-dichloroacetophenone in the presence of ethyl alcohol and silica gel. Brooks ( 3 , 4 ) prepared thia dichlorostyrene by the synthesis of 2,5-dichlorobenzaldehyde from 2-chloro-5-nitrobenzaldehyde and treatment with methylmagnesium bromide to give the corresponding carbinol which was dehydrated over potassium acid sulfate. The crtrbinol has been dehydrated by Michalek ( 1 1 ) over hot alumina a t reduced pressure. A survey of analogous reactions that had possibilities for application to this problem showed that Walling and Wolfstirn (14) had prepared 3,Pdichlorostyrene from the decarboxylation of 3,4-dichlorocinnamic acid. Marvel et al. (10) prepared the acetate of 3,4-dichlorophenylmethylcarbinol,which produced the corresponding styrene on pyrolysis. Overberger and Saunders ( I d ) treated 3-chlorophenylmagnesium bromide with acetaldehyde to produce the carbinol, which was dehydrated over potassium acid sulfate to yield rn-chlorostyrene. Procedures were selected for study on the basis of availability and cost of starting materials, ease of synthesis, and equipment required, excluding a t once those involving dehydrohalogenation of a side-chain halogen. Although the initial over-all yield for reaction 1, about 16%, waa low as compared to 34% for reaction 2, and 35'% for reaction 3, i t was decided to investigate further the preparation of 2,5dichloroacetophenone because of the simplicity of synthesis leading to the corresponding styrene. Several acetylations of p-dichlorobenzene gave the data of Tables I and 11.
The results indicated that it was necessary to acetylate in a n excess of p-dichlorobenzene, since the conventional solvents used for preparations of this type proved unsatisfactory. A comparison of the reactivities of acetyl bromide and acetyl chloride shows no difference within the limit,s of experimental error. It mas necessary to use a large excess of aluminum chloride, probably because of the formation of an aluminum chloride complex with the ketone. The over-all yields of 2,5-dichlorostyrene for the procedures in Figure 1are given below: % Yield
Procedure Experimental
1 2 3 4 6
43 34
35 0 19
Literature . t .
... 51
...
...
Thus the most suitable of the above schemes for the preparation of 2,5-dichlorostyrene, based on the over-all yield and simplicity of synthesis, consists of the acetylation of p-dichlorobenzene and its reduction to the carbinol, followed by dehydration over activated alumina. EXPERIMENTAL
Procedure 1. ACETYLATION. Acetyl chloride (6.13 moles, 480 grams) is added dropwise for 20 minutes, with stirring, to a mixture of 12.25 moles (1630 grams) of aluminum chloride in 17.5 moles (2570 grams) of molten p-dichlorobenzene and a t a temperature of 70" C. The p-dichlorobenzene is dried previously by distillation at atmospheric pressure. The temperature is raised to 100" C . when addition is complete, and the mixture is heated with continued stirring for 3 hours, or until the evolution of hydrogen chloride is slight. The reaction mass is cooled to
