OXIDATIONEFFECTS IN IRRADIATION OF POLYETHYLENE
June, 1959
837
IRRADIATION OF POLYETHYLENE. IV. OXIDATION EFFECTS BY
HIRO~H MSTSUO' I
AND
MALCOLM DOLX
Contribution from the Chemical Laboratory of Northwestern University, Evanston, Illinois Received February B, 1869
The radiolytic oxidation of a linear polyethylene has been studied by measuring changes in total pressure, by observing increases in optical density in the infrared due to carbonyl absorbance and by analyzing gaseous roducts of oxidation when polyethylene film of different thicknesses was exposed to ?-radiation in the initial presence of a pew cm. pressure of oxygen. A steady rate of oxidation was soon established, making valid the use of a solution of the differential equation combining both diffusion and reaction for the steady state. About one fourth of the combined oxygen appeared as carbonyl and one eighth as water. Another eighth of the oxygen formed a mixture of carbon monoxide and dioxide. The rest of the oxygen must have formed mostly peroxides and alcohols. Product yields were linear with amount of oxygen consumed whether during the irradiation or in the dark period subsequent to the irradiation. Hydrogen gas evolution and growth and decay of unsaturation apparently were unaffected by the presence of oxygen, but the dose required to reach the gel point was increased. Chemical mechanisms are discussed.
Introduction The oxidation of polyethylene during pile irradiation was first observed2saby chemical analysis and from infrared absorption measurements of irradiated film a t 5.8 p , the carbonyl absorption band and a t 3.0 p , the hydroxyl absorption band. As the final percentage of oxygen in films 0.06 mm. thick was about four times as great as that in granules of 2.7 mm. diameter, it was concluded that oxidation depended strongly on the surface area. Charlesby4came to the same conclusion by studying weight changes during pile irradiation of polyethylene samples of different sizes and shapes. In the many oxidation studies6-14 that have been carried out since then, there have been no measurements of the rate of oxygen uptake during the irradiations, no calculations of G( - On) although Lawton, Powell and Balwit' did calculate a related G(-02) and G(H20 COZ CO) for post-irradiation oxidation. There have been no kinetic equations developed, no calculation of G-values for the individual oxidation products. Such information is essential for a complete understanding of oxidation mechanisms.
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Experimental Materials.--Tho linear polyethylene, Marlex-50, free of anti-oxidant was kindly supplied by the Phillips Petroleum Co. It was used in the form of films of different thicknesses. Thick films were made by carefully heating and fusing together stacked layers of film between polished aluminum plates. Radiation Source and Cells.-The radiation source and cells were essentially the same as previously described .I6 (1) On leave from the Electrical Communication Laboratory, h'ippon Telegraph and Telephone Public Corporation, Tokyo, Japan. (2) D. G. Rose, M.S. Thesis, Northwestern University, 1948. (3) M. Dole, Report of Symposium IX, "Chemistry and Physics of Radiation Dosimetry," Army Chemical Center, Md., 1950, p. 120. (4) A. Charlesby, PTOC.R o y . SOC.(London),U S A , 187 (1952). ( 5 ) W.C. Sears and W. W. Parkinson, Jr., J . Polyme? Sei., 31, 325 (1950). (6) E.,J. Lawton, J. S. Ralwit and R. S. Powell, abzd., 33,257 (1958) (7) E.J. Lawton, R. 8.Powelland J. S. Balwit, ibid., 32, 277 (1958). (8) K.Schumacher, Kolloid Z., 167, 10 (1958). (9) A. Chapiro, J . chim. p h y s . , 62, 246 (1955). (10) M. Magat, "International Radiation Research Congress," Burlington, Vt., August, 1958. (11) N. Bach, ref. 10. (12) N. A. Bach and V. V. Saraeva. Zhurn. Piz. Khzm., 32, 209 (1958). (13) L. E.St. Pierre and H. A. Dewhurst, J . Chem. Phyx., 29, 241 (1958). (14) P. Alexander and D. Toms, J . Polymer Sci., 22, 343 (1956). (15) M. Dole, D. C. nlilner and T. F. Williams, J . Am. Cham. Soc., 80, 1580 (1958).
In order to carry out irradiations a t about one eighth the usual dose rate, a holder for the cells was constructed in such a way that the irradiation cell could be supported above the stainless steel blocks containing the co-60 rather than between them. Gas Analysis and Pressure Measurements.-Before the irradiations, the cells containing the Marlex-50 were evacuated and then filled with pure oxygen at pressures varying from one to six em. By means of a mercury manometer in a side arm the total pressure could be read a t any desired time to &0.02 cm. using a cathetometer. The cell was immersed in a constant temperature bath a t 27" during the pressure measurements. The cell contained a second sidearm which could be immersed in liquid nitrogen to freeze out all gaseous products, leaving 0 2 , Hz and CO uncondensed. The pressure was then read and corrected to 27'. to give the non-condensable gas pressure a t 27". The sidearm was next immersed in a Dry Ice-acetone bath to freeze out or condense only water, methyl alcohol, acetone, etc. This pressure was read and corrected to 27'. The difference between this pressure and the total was assumed to be water vapor pressure. Sometimes a trace of liquid condensed, a t which time the partial pressure of the water became constant. During the time that these pressure measurements were being made, the irradiation cell was removed from the zone of the radiations. The three measurements required about 30 minutes; in this period consumption of oxygen by the post-irradiation effect introduced a negligible error. Several samples of the gas uncondensable a t liquid nitrogen temperature as well as samples of the condensable gas were analyzed in the mass spectrometer either by Dr . D. RII. Mason of the Department of Chemical Engineering, Northwestern University or by the Institute of Gas Technology, Chicago. The volume of the uncondensahle gas was measured in a Toepler pump and could be calculated also after a calibration of the volume of the cell. The volume of condensable gases was also measured, when it was possible to do so, in the Toepler pump. Measurement of Gel Content and Infrared Technique.Estimates of the sol-gel ratio and of vinyl and vinylene group concentrations16 were carried out as previously described.16 For estimates of the carbonyl group concentration a value17 of 188 1. cm.-l mole-' was taken for the carbonyl molar extinction coefficient'? at 5.8 p . Although growth in the infrared absorption band due to OH had been previously observed during irradiation of polyethylene in ai1-,23~ in this work OH could not be detected due to the relatively small doses used and the small pressure of oxygen.
Results The general trend of the results in the case of the thinnest film used, 0.0045 em. thick, is shown in Fig. 1. The top curve represents the total pressure as a function of time. The initial drop in pressure was due to the excess "clean-up" or consumption of oxygen over the hydrogen evolved; the minimum represents the time when most of the oxygen (16) T. F. Williams and M. Dole, paper submitted for publication. (17) L. H. Cross, R. B. Richards and H . A. Willis, Disc. Faradicu Soc., 9, 235 (1950).
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experiment, 27.8 mm., less the pressure (PH, Pco) at liquid nitrogen temperature (corrected to
room temperature) 19.6 mm., less the pressure (chiefly PH,o)of the products condensable at Dry Ice temperature, 5.4 mm., should be the pressure of carbon dioxide, or 2.8 mm. This estimate is somewhat greater than the previous estimate of Pco Pco,, 2.5 mm., which suggests that PCO could not have been very significant. However, in another experiment where the initial oxygen pressure, P0o,was much greater, the data of Table I were obtained (estimated for 27'). The pressure of hydrogen was obtained by calculation, assuming G(H2) to be 3.8. The pressure of CO could then be obtained by subtracting P H from ~ P H ~ Pco. In this experiment the pressure of carbon monoxide was seen to be about equal to that of carbon dioxide and 8% of the noli-condensable gas pressure a t liquid nitrogen temperature. A mass spectrometric analysis of the lion-condensable gas yielded 6 and 4% carbon monoxide in two different experiments.
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5
0
10 15 Time, hr. Fig. 1.-Gas pressures in irradiation cell during 7-ray irradiation: upper curve, total pressure; middle curve, pressure corrected to 27" with sidearm in liquid nitrogen trap; lower curve, water partial vapor pressure.
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TABLE I SUMMARY OF P R E ~ S U R ESTIMATES E Anti-oxidant free Marlex-50 film 0.0388 cm. thick after a total dose of 6.47 X 1020 e.v.g.-l Initial 01pressure, mm. Final pressure, mrn. PES
0.3
pco pH10 PCOa
Total
-4
\
5 0.2
r'
04.0 94.7
66.7 5 7 18.1 4.2
94.7mm.
Fraction of On converted to
C=O (carbonyl) H20 COa
CO
0.25 .14
.06 .05 -
Total 0.50 Peroxides and alcoliols by difference 0.50
-
a,
1.00
0.1
5 10 15 Irradiation time, hr. Fig. 2.-Total gas pressure in cell during irradiation multiplied by cell volume and divided by total surface area of polyethylene film. Film thicknesses are given a t the right and the numbers over the vertical lines represent the length of the dark period (Le., no irradiation).
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had reacted; and the final linear rising portion of the curve was due to a constant evolution of hydrogen. If this latter curve is extrapolated back to zero time, the intercept on the pressure axis represents the final total pressure of the gaseous oxidation products. In the case of the experimental data of Fig. 1, the extrapolated pressure was about 7.5 mm. while the estimated pressure of water vapor and other gases condensable a t Dry Ice temperature was 5.0 0.5 mm. The difference in these pressures, 2.5 mm. can be ascribed to carbon monoxide and carbon dioxide. The total pressure (PH, Pco Pco, PH,o)at the end of the
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Figure 2 illustrates the effect of film thickness on the rate of pressure change as well as the post-irradiation oxidation effect. The thicker the film the greater the rate of evolution of hydrogen per cm.2, and the thicker the film, the more pronounced is the post-irradiation oxidation effect. In Fig. 2 the vertical lines represent the decrease in oxygen pressure during resting periods when the film was not being irradiated. Note that the post-irradiation effect declines as the pressure of the oxygen drops. It is interesting to plot the initial slopes of the curves of Fig. 2 as a function of the film thickness. This is done in Fig. 3 where it can be seen that as the films become thicker, the initial slopes change from negative to positive values, and the curves become linear with the thickness 6. This is to be expected because per sq. cm. of surface area, the rate of hydrogen evolution should increase linearly with the thickness. At zero thickness there should be no hydrogen evolution, the latter being a volume effect. Actually, the experimentally determined data fail to follow the linear relation because as the film thickness is reduced the negative rate of pressure increase due to oxygen consumption outweighs the positive rate for hydrogen evolution and the ordinates of Fig. 3 become negative. However, at zero thickness there should also be no oxygen consumption. By extrapolating the straight lines of Fig. 3 to
L
OXIDATION EFFECTS IN IRRADIATION OF POLYETHYLENE
June, 1939
539
0.04 I
0.02
h
s
o
t
3 -0 h v
-0.02 r3
v
s" -0.04
0
0.04 0.06 0.08 .10 Thickness. cm. Fig. 4.-Steady-state initial flux of oxygen into film plotted as a function of film thickness: open circles, high intensity experiments; closed circles low d e n s i t y experiments corrected to dose rate of high intensity experiments; vertical lines growth of oxygen uptake during resting periods subseouent to the irradiation. Initial Oxygen pressure about 0 om. Ior all polnts.
0
-0.06 0
Fig. 3.-Initial
with tima n l n t t d
0.05 0.10 Thickness, cm. rate of change of the function PTVIA QFI Q
fiinotinn nf
film t,hipknpnR for dif-
ferent initial oxygen pressures. 'l'he lowest curve represents results obtained in the low intensity experiments, normalized to the same dose rate as the data of the other curves.
0.02
search, hydrogen evolution, vinyl group decay and vinylene growth and decay were unaffected by the presence of the oxygen. One difficulty in testing zero thickness, the intercept on the ordinate can the effect of oxygen on changes in unsaturation or be obtained. This gives the maximum rate per cm.2 hydrogen evolution is that the films must be rather of oxygen consumption less the rate of products thick to give significant sensitivity; on increasing formed for infinitely thick films. The points a t the thickness of the films the ratio of oxidized rewhich the extrapolated lines meet the actual curves gions of the polyethylene to unoxidized decreased. give the thicknesses at the indicated initial oxygen Hence, for a really accurate test of the effect of oxypressures beyond which no further significant oxy- gen on hydrogen evolution, vinyl or vinylene degen consumption can be gained. This last point is cay and vinylene growth, much greater pressures of confirmed by the data of Fig. 4 where the initial oxygen should have been used. rate of oxygen uptake a t about 5 cm. pressure is In agreement with the observation of many plotted as a function of film thickness. It is seen others, the dose required to reach the gel point was that the rate approaches a limiting value. The about doubled in the case of films 0.03 cm. thick, upper curve demonstrates that a t an eightfold but hardly affected for films 0.09 cm. thick. lower intensity the uptake of oxygen per cm.2 per A blank experiment with a mixture of hydrogen unit of radiation energy absorbed is about twice as and oxygen gas in the irradiation cell showed no great, with the limiting ratio at infinite thickness change of pressure with time; Le., there was no even greater than this. The middle curve represents perceptible radiolytically induced reaction between the oxygen uptake including both the oxygen con- oxygen and hydrogen gas. The density of the gas sumption during the irradiation and during any phase was so low relative to that of the polyethylene subsequent dark period. that no effect would have been expected unless a If the optical density at 5.8 p , the carbonyl ab- chain reaction ensued. sorption band, is plotted as a function of the oxygen As will be seen below, G-values for oxygen conuptake, the relationship appears to be linear within sumption, G(-02), depend very strongly on many the experimental uncertainties both for low and factors, especially on the film thickness, radiation high intensities and demonstrates that the nature intensity and oxygen pressure. Because of the of the reaction products did not depend on the increase in the rate of oxygen consumption with amount of oxygen reacted, at least for the extent film thickness as ahown in Fig. 4,G(-OJ at first of reaction involved here. Furthermore, since the increased with film thickness. As the thickness of ratio of oxygen consumed to product gas formed the film was increased beyond about 0.02 cm. in our during the dark periods (periods of no irradiation) experiments, G( -02)decreased, because a greater seemed to be equal to the ratio for the periods of fraction of the y-ray energy was absorbed in the inirradiation ( *15%), the conclusion also could be terior of the film where no or very little oxygen was reached that the reactions involving oxygen were available for reaction. The maximum observed the same during both irradiation and no irradia- G(- 0 2 ) was 9.9 for a film thickness of 0.021 cm. and tion. an initial oxygen gas pressure of 5.5 cm. Not As far as could be told from the data of this re- enough experiments were done using the low inten-
HIROSHI MATSUO AND MALCOLM DOLE
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sity radiation to estimate the maximum G( - 0,) a t the optimum film thickness, but under comparable conditions G( - 02)for the low intensity experiments was greater by a factor of about 1.2 than G( 02) for the high intensity experiments. Table I1 contains estimates of the maximum Gvalues of the oxidation products as well as can be calculated from the data.
suppose that the oxygen initially dissolved in the film must have reacted fairly rapidly, thereby making possible the establishment of a steady state of oxygen consumption and diffusion, a t least within an hour or two. However, a steady state could be expected, theoretically, only if the ambient oxygen pressure remained constant, but if the latter was constant, no rate of oxidation could have been observed by pressure measurements. One of the surTABLE I1 prising results of this research was the initially unESTIMATED MAXIMUMG-VALUES,M O L E C U L E SE.v. / ~ ~ ~ changing rate of oxygen consumption despite the continually declining ambient oxygen' pressure; a GC-02) 10.0 G(carbony1) 5.0 marked change of the oxygen uptake rate occurred G(Hz0) 2.5 usually only after zl period of no irradiation. PreG(C0) 1.0 sumably, a new steady-state concentration gradient G(C0z) 0.6 of oxygen in the polyethylene was established on renewing the irradiation. Figure 5 illustrates Kinetic Interpretation of Data hypothetical curves of the oxygen concentration From the foregoing and from previous work of in the film as a function of distance through the film others it is obvious that an interpretation combin- at different times. It is seen readily that as the ing simultaneous diffusion theoryI8 and reaction oxygen in the film is cleaned up, the concentration must be developed. Let us assume that competi- gradient in the surface layer of the polyethylene intive reactions exist for free radicals between oxy- creases. Thus, if the radiation intensity is not too gen and other free radicals, then we can write high, an increase with time in the rate of oxygen uptake should have been observable because the flux 2 k'c of oxygen into the film depends on the concentraat =iq-n where c is the oxygen concentration in the film and tion gradient in the surface layer. Figure 6 illust is the time. Combining (1) with the diffusion trates just such an effect in the case of an experiequation for unidirectional diffusion and assuming ment a t the lower -pray intensity. a, the diffusion coefficient to be independent of to If a steady state does exist, eq. 2 can be simplified concentration and irradiation dosage, eq. 2 results
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The boundary conditions adopted are s c = co at x = f - at all t
(34
=~ a t = x ~atallt
(3b)
3
For high enough concentrations so that k"c >> 1, (4) reduces to an equation for diffusion coupled with a zero-order reaction a2c
bX
c = co a t all .z at t = 0
(3c)
In eq. 2 and 3, x is the distance through the film measured from the mid-point, k' and k" are reaction rate constants and co the oxygen concentration in the surface layer. We assume that the rate of attainment of oxygen solubility equilibrium in the surface was rapid compared to reaction and diffusion. If this were not the case, and if the rate of diffusion of oxygen into the film were rate limiting, then the oxygen consumption rate would have been independent of film thickness. The oxygen pressure decreased during the irradiation, so co must also have decreased with time. For this reason, we shall be chiefly concerned with the initial rate of oxygen uptake. The data indicated that in the high intensity experiments an initial steady-state rate of oxygen uptake was established. The solubility of oxygen at room temperature and 7 em. pressure (the highest pressure used) was about 2 X lo-' mole of oxygen per gram of polyethylene.lS As the maximum observed rate of oxygen consumption was of the order 10-6 mole/g. hr. in the high intensity experiments, it is reasonable to (18) For theories combining both diffusion and reaction see, for example, J. Crank, "The Mathematics of Diffusion," Oxford, 1956, Sec. 8.3. (19) Unpublished meaaurementa of M. FaIlgatter in this Laboratory.
PG2 =
k
(5)
Integrating twice and evaluating the constants of integration by means of the boundary conditions c
- Cm
k 25.)
= -22
(6)
where cm is the oxygen concentration in the middle of the film at x equals zero. I n the boundary layer at z equal to 6 / 2 co
k - Cm = 2cp (i)'
(7)
Thus the difference in concentration, co - Cm, depends on the film thickness. Obviously (7) becomes invalid when the film thickness is so great that a zero-order reaction cannot prevail throughout the film. Differentiating (6) with respect to x, and multiplying 6c/6x by a, to obtain the ffux, J, one can obtain either J = kx
(sa)
or J = (c]-
cm)'/2
(2k-)'/a
(8b)
Equation 8a requires that Jo,the flux in the surface layer, depend only on the film thickness Jo = k 28
(9)
This means that the concentration difference (cg
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OXIDATION EFFECTS IN IRRADIATION OF POLYETHYLENE
June, 1969
c,) is independent of concentration a t any fixed value of 6 but that it increases as the square of 6 / 2 . If k”c
