Oxygen Uptake of Polyethylene at Elevated Temperatures JOSEPH E. WILSON Development Department, Bakelite Co., Division of Union Carbide and Carbon Corp., Bound Brook, N . J .
T
HE technological importance of the oxidation of high polymers, with its resulting chemical and physical changes, is now widely recognized. I n the case of polyethylene such oxidative degradation is readily produced by processing a t elevated temperature or by outdoor exposure to sunlight. The effects of oxidation on polyethylene include changes in the molecular weight distribution, a deterioration of the mechanical properties, an increase in the power factor, and a change in color to yellow or brown. Such changes could greatly influence the processing behavior of polyethylene and limit the possible applications of the finished product if allowed to proceed unchecked. Informative articles on the oxidation of polyethylene have been published. Myers ( 1 6 ) found an excellent correlation between power factor and carbonyl concentration in polyethylene, and was able to follow carbonyl build-up in the resin during milling on hot metal rolls by power factor measurements. H e demonstrated that a small concentration of an amine antioxidant was effective in preventing carbonyl formation during milling, and computed the energy of activation for carbonyl formation from rate measurements a t several temperatures. Biggs and Hawkins (4)measured the oxygen uptake of polyethylene a t constant pressure using a technique similar to that outlined in this paper. They also showed the inhibiting effect of antioxidants, and found that several types of carbon black slow down the reaction. Biggs and Hawkins noted a tendency for oxygen uptake rate to be proportional to sample surface area rather than mass. They found that the oxygen reacts with the surface before it can diffuse into the center of the polyethylene film, unless the film is very thin. Because of this effect, they referred to their rates as “comparative,” and always used aluminum boats of fixed size to contain the films, so that the same sample area was exposed to oxygen in each case. This paper describes a modified technique that measures oxygen uptake rates which are proportional to resin mass, and presents some of the data obtained. APPARATUS ARD PROCEDURE
The experimental equipment and procedure are essentially the same as those employed by Shelton and Winn (18, $1) in measuring the oxygen uptake of rubber, with minor modifications necessary for the handling of polyethylene resin. The method follows oxygen absorption by measuring volume change a t constznt temperature and pressure. The apparatus consists of three independent absorption units, each with its own sample tube, gas buret, and leveling bulb. Borosilicate glass capillary tubing connects each sample tube to a short length of heavy-walled rubber pressure tubing, which leads in turn to a gas buret. The rubber connecting tubing allows each sample tube to be raised from and lowered into a constant temperature bath of silicone oil controlled within 1 0 . 1 ” C. by means of an Aminco Quickset bimetal thermoregulator in combination with an Emil Greiner electronic relay. Each absorption unit is also connected to a manifold for evacuation and introduction of oxygen. In measuring oxygen uptake the sample tubes, each containing a sample of polyethylene resin, are lowered into the constanttemperature bath. After several alternate evacuations and purgings with oxygen through the manifold, the separate ab-
sorption units are filled with oxygen a t atmospheric pressure and shut o f f from the manifold. The initial volume reading is then taken. From time t o time further volume readings are taken, in each case by raising the leveling bulb to such a height as t o equalize the pressure inside and outside the gas buret. In making a reading the observed volume of oxygen in each buret is corrected to a reference temperature of 25’ C. and a reference pressure of 760 mm. I n computing oxygen uptake rates all volume changes are expressed as milliliters of oxygen absorbed per gram of resin in the sample. Each rate measurement is made in triplicate to obtain an average rate. Before the resin sample is introduced into the sample tube, it is first positioned in a special cartridge as shown in Figure 1. The cartridge is a borosilicate glass tube about 13 em. long and 1.2 em. in inside diameter. The dimensions are not critical. The polyethylene sample is ground to a powder in a Wiley laboratory mill before being distributed between b o r o s i l i c a t e glass wool plugs as indicated. ils the total sample weight is only 50 mg., the individual resin layers are thin. This technique gives the oxygen free access to each resin particle and prevents the sample from coalescing into a lump. As the run proceeds t.he resin floms c down through the glass wool to some extent, resulting in a very thin resin covering on each glass wool fiber, A 1-gram portion of c a l c i u m oxide powder is placed a t each end of the cartridge. The purpose of the calcium oxide is to absorb gaseFigure 1. Cartridge ous carbon dioxide a n d water containing resin to vapor that may be produced as be oxidized oxidation products. The effect of C. Calcium oxide gaseous products would be to deG . Glass wool crease the apparent oxygen uptake R. Resin layers and produce a measured rate of reaction less than the true rate. It was found by successive trials of various amounts of calcium oxide that a total of 2 grams was needed to produce the maximum (true) rate of oxygen uptake.
/
Other techniques, such as manometric and gravimetric methods have been used in the past to investigate oxidation rates. But Shelton (18) noted that simultaneous volume and pressure changes in the manometric method make it difficult to obtain precise data on the actual amount of oxygen absorbed. The reaction rate also changes with pressure, and it is impossible t o determine the extent to which the observed decrease in rate results from the decreased pressure or from other factors. The gravimetric method is not so sensitive as the volumetric method, and it. gives no information on the amount of oxygen which first combines and then escapes in the form of carbon dioxide or water. Dufraisse ( I S ) has point,ed out that the volumetric method is more convenient for studying the kinetics of the oxidation reaction, and that the dat,a are more reliable; hence there were several reasons €or choosing the volumetric method in the present investigation.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 41, No. 10
GENERAL SHAPE OF OXYGEN UPTAKE CURVE
The typical oxygen uptake behavior of polyethylene is illustrated by the upper curve in Figure 2. The particular type of polyethylene used here is DYNH-1, a general-purpose unstabilized resin. The oxygen uptake starts out slowly, reaches a constant rate for a short period, and then slows down again. In this paper the calculated rates refer to the center, linear portion of the curve. For the upper curve in Figure 2, extrapolation of the linear portion t o the time axis indicates an induction period of about 55 minutes. The length of the induction period is sensitive to the temperature, the presence of antioxidants, and the degree of purity of the sample. The explanation for the decrease in rate a t the upper end of the curve is not known with certainty. It is probably due to the fact that the concentration of hydrocarbon available for oxidation has become depleted. Biggs and Hawkins ( 4 ) also obtained S-shaped curves of this same general type, with a n induction period, a linear portion, and an upper section of slowly decreasing rate.
dDD-
CALCIUM OXIDE h
u'
tu v
3007
Table I. Temp., C. 160
150 140 130 120 110
Oxidation of DYNH-1, Blend B
(Oxygen presbure approximately 1 atm.) Rate LogMl./G./Hr. Log Rate P , Min. ( 1 / P X 104) 256 2.41 46 2.34 115 2.06 90 2.05 34.0 1.53 300 1.52 12.5 1.10 610 1.29 3.96 0.60 1000 1.00 1.29 0.11 1960 0.71
1/K 0.00231 0.00237 0.00242 0,00248 0.00255 0.00261
Figure 4 presents the usual Arrhenius plot of the logarithm of the rate against the reciprocal of the absolute temperature. The energy of activation computed from the slope of the least squares line through the points is 35 kcal. per mole. The fact that this plot is linear indicates that the rate-controlling reaction is the same over the temperature range from 110' t o 160' C. Several explanations have been offered for the well-known induction period observed in the oxidation of all types of hydrocarbons. One point of view (16) associates the induction period with the gradual rise in concentration of an intermediate substance, the subsequent reaction or decomposition of which causes the reaction to proceed. Evidence has been presented that this intermediate substance is a hydroperoxide in the oxidation of unsaturated hydrocarbons of low molecular weight (6). Others have felt t h a t the induction period is due to the presence of small amounts of impurities or natural antioxidants in the hydrocarbon ( l a ) . I n the latter view the induction period is taken to be the time required for an amount of free radicals to be produced t h a t is equivalent to the amount of inhibitor originally present. Whatever the process t h a t occurs during the induction period, its rate should be inversely proportional to the length of the induction period. On this basis an Arrhenius plot of the logarithm of 1 / P against the reciprocal of the absolute temperature has been drawn for polyethylene oxidation (Figure 5 ) . The linear nature of the plot indicates that the same process is rate-controlling during the induction period a t all temperatures studied. The slope of the line corresponds to an energy of activation of 25 kcai mole.
MINUTES
Figure 2. Typical oxygen uptake curve for DYNH-1, blend A 160' C.
The lower curve of Figure 2 shows the effect of omitting calcium oxide from the oxidation cartridge. The apparent decrease in rate is evidently due t o t h e evolution of gaseous oxidation products. These products were not definitely indentified, but probably consisted of carbon dioxide and water. Biggs found it necessary t o place barium oxide in his sample tubes t o absorb such gaseous products. I n his work on rubber oxidation Shelton (18) used calcium oxide t o absorb the water vapor and "acidic gas" that were produced in considerable amount. Examination of Figure 2 shows that after 150 minutes a t 160' C. the resin had taken up 320 ml. of oxygen per gram and evolved 240 ml. of gaseous products per gram. Hence all of the reacted oxygen does not remain combined with the resin. EFFECT OF TEMPERATURE
The rate of polyethylene oxidation decreases rapidly as the temperature is reduced. This is shown clearly in Figure 3 where oxygen uptake is plotted against time a t several temperatures. As the temperature decreases, the induction period, P, increases markedly, changing from about 1 hour a t 160' C. t o about 33 hours a t 110"C. The computed rates and induction periods a t the various temperatures are given in Table I.
0
20
40
80
80
HOURS
Figure 3.
Effect of temperature on oxygen uptake of DYNH-1, blend B
Paraffin oil is similar in chemical composition t o polyethylene. Brook and Matthews (12)measured the oxygen uptake of paraffin oil and observed a n induction period followed by a linear uptake related to time as in the case of polyethylene. The energy of activation for the constant-rate portion of the curve was 36 kcal. in good agreement with the corresponding figure of 35 kcal. for polyethylene. The authors also plotted the logarithm of 1/P against l / K and obtained E = 30 kcal. for the induction period reaction, in comparison with 25 kcal. for the same reaction in
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INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1955
polyet.liylene. The agreement between the activation energies for paraffin oil and polyethylene suggests the possibility that a similar oxidation mechanism may be followed for the two substances.
a 23
2.4
I/K
25
2.6
The derived and experimentally confirmed rate expression shows over-all oxidation rate proportional to hydrocarbon concentration and independent of oxygen pressure a t pressures of the order of atmospheric. It is quite possible that this mechanism also holds in the case of polyethylene oxidation. As in other oxidation reactions, a change in experimental conditions can profoundly affect the characteristics of polyethylene oxidation. For example, Myers (16)found an activation energy of only 16 kcal. for carbonyl formation in polyethylene milled on hot metal rolls. This value is considerably lower than the 35 kcal. found in the present work. It appears possible t h a t the hot metal surface may have catalyzed the oxidation and reduced the activation energy, an effect commonly observed in catalyzed reactions (14). An additional piece of evidence for catalysis under Myers' experimental conditions is the fact that no induction period was observed (26, Figure 1). INHIBITION BY PHENOLIC ANTIOXIDANT
2.7
,
x lo3
Figure 4. Arrhenius plot for oxidation of DYNH-1, blend B
Shelton and his coworkers have revealed the essential features of rubber oxidation in an extensive series of investigations. The oxygen uptake curve for GR-S gum stock a t 130' C. is reproduced from Shelton's work ( 5 ) in Figure 6. For both Hevea and GR-S rubber stocks Shelton found a slow constant-rate reaction, followed by a fast constant-rate reaction a t temperatures above 100" C. (19). At 130" C. the slow constant-rate reaction for GR-S lasts approximately as long as the induction period for DYNH-1 (Figure 6). There is evidence that the slow constantrate stage in rubber oxidation is due to the presence of minute amounts of impurities which function as antioxidants, and t h a t the slow stage would not be observed if the antioxidants were not present (10). It has been shown (20) that the energy of activation for the slow stage of the oxidation of GR-S black stock and Hevea black stock is about 26 kcal., in close agreement with the value found for the induction period reaction in polyethylene oxidation. For the fast reaction in GR-S and Hevea stocks (6, 20) the value of E estimated from Shelton's data ranges from 16 to 29 kcal., considerably less than the 35 kcal. obtained for the constant rate stage in polyethylene. The lower energy of activation for Ilevea and GR-S polymers is in line with their unsaturated character and agrees well with the findings for olefinic materials of low molecular weight. This lowered energy of activation for unsaturated materials can be traced t o the activating influence of the carbon-carbon double bond. One of the rate-determining steps in the oxidation of an olefin is the removal of a hydrogen atom from the parent hydrocarbon t o form a radical (8). The presence of a double bond in the chain renders the hydrogen atom on the alpha-carbon labile and facilitates its removal, in this way resulting in a lowered energy of activation for the over-all reaction. Olefins of low molecular weight are also known to exhibit welldefined induction periods in their oxygen uptake reactions ( 1 1 ) . Energies of activation for the oxidation of 16 such olefins have been measured ( 7 , IO), the values found ranging from 21 to 28 kcal. A reaction mechanism for the oxidation of these materials has been presented on the basis of convincing evidence by Bolland, Bateman, and Gee (3, 8, 10). This well-known and widely accepted mechanism involves an oxidation chain reaction with free radical carriers and termination by radical recombination.
Myers ( 1 6 ) and Biggs (4)have discussed the effect of aminetype antioxidants on polyethylene oxidation. Figure 7 illustrates the effect of small concentrations of a phenolic antioxidant. The addition of antioxidant prolongs the induction period, but has no effect on the uptake rate in tho linear portion of the curve. A similar result was obtained by Biggs with an amine-type antioxidant (4,Figure 2 ) . This finding is also in line with the work of Shelton ( 5 ) on rubber oxidation, where it was shown t h a t t h e antioxidant is used up in the slow stage of the reaction and has little or no effect on the rate in the fast stage.
3t n
0 X
n
'1v
s 0 J
2.2
2.3
2.4
I/K
2.5
2.8
2.7
x io3
Figure 5. Arrhenius plot for reciprocal of induction period DYNH-1. blend B
Figure 8 presents a plot of the increase in the induction period vs. the concentration of antioxidant added. A similar proportionality between induction period and concentration of antioxidant was observed by Robertson and Waters ( 1 7 ) in their work on the oxidation of Tetralin with phenol as the inhibitor. The most convincing explanation for the action of phenolic antioxidants has been offered by Bolland ( 9 ) , who postulatea a~ chain termination reaction between the antioxidant and one of the free radicals involved in the oxidation chain. This explanstion would imply the continuous formation of free radicals and reaction with antioxidant until all antioxidant molecules had becn destroyed, the length of time required for this destruction being proportional to the concentration of antioxidant. T h e proportionality demonstrated in Figure 8 is consistent with this hypothesis.
INDUSTRIAL AND ENGINEERING CHEMISTRY
2204
LIMITATION BY DIFFUSION
I n kinetic studies involving the rate of oxygen absorption by high polymers it is essential t o know whether the observed rate is a true measure of the chemical reaction or is limited by the relatively slow diffusion of oxygen into the sample under the particular conditions employed. When oxygen diffuses into and reacts with a polymer film, the dissolved oxygen in the center of the film may react so rapidly that it becomes completely exhausted before more oxygen can diffuse in from the outside. When such a situation holds true after a steady state has been
HOURS
Figure 6.
Oxygen uptake of GR-S gum stock and DYNH-1, Blend B 130' C .
reached, the surface laycr will continue to oxidize while the center portion of the film remains unreacted. This condition causes a substantial reduction in the measured oxygen uptake rate per gram below the theoretical value for an infinitely thin film. Wilson ( 2 2 ) has shown that it is possible t o derive equations expressing reaction rate as a function of film thickness, diffusion constant, and solubility OP oxygen in the film.
2
4
6
Vol. 47, No. 10
t o minimize the safe thickness of film allowable are: low diffusion constant, low solubility of oxygen in the film, high rate of oxidation, and high temperature. Generally speaking, high temperature tends to accentuate the limiting effect of diffusion because the oxidation rate climbs more rapidly then the diffusion rate as the temperature increases. Thus the energy of activation for oxidation ranges from 16 to 35 kcal. or more, while the energy of activation for gaseous diffusion in polymer films is of the order of 10 kcal. (1). The decreased solubility of oxygen in the film a t high temperature also magnifies the diffusion limitation ( 2 2 ) . An example of the limiting effect of slow oxygen diffusion is found in the work of Shelton ( 5 )on the oxidation of carbon blackfilled GR-S. At 150" C. the film thickness required to give a rate proportional to mass was 0.014 inch, while a t 100' C. it could be safely increased to 0.051 inch. Below 100' C. the absorption curves for different thicknesses were found to coincide, and diffusion did not limit the oxidation for films as thick as 0.075 inch or more. The diffusion limitation is far more pronounced in polyethylene resin, apparently because of the very low diffusion constant and solubility of oxygen in this material. Biggs ( 3 ) found that a polyethylene film thickness between 0.001 and 0.005 inch will permit oxidation a t a rate proportional to mass rather than surface area a t 150" C. I n the present work 50 mg. of resin powder was distributed in four layers having a total area of about 4.5 sq. em. If the same weight of resin were pressed into a film of this area, the film thickness would be about 0.005 inch. The use of powder allowed more efficient access of oxygen to each resin particle than would be possible in a solid, continuous film, As the resin came up to reaction temperature it was observed t o melt and flow down through the glass wool plugs to some extent, resulting in a further diminution of the average resin film thickness. In order to select the proper sample size, successive oxygen uptake runs were made on 1.0-, 0.3-, 0.1-, 0.05-, and 0.02-gram resin samples a t 160" C., in each case by arranging the sample as in Figure 1. The computed rates per gram rose as the weight dropped from 1.0 t o 0.1 gram, and then remained constant for the 0.1-, 0.5-, and 0.02-gram samples. This indicates that the 0.05gram sample size can be used safely t o obtain rates proportional to resin mass. Biggs made some comparative observations on the oxygen uptake of polyethylene samples 0.075 inch thick contained in shallow aluminum boats of fixed area. H e pointed out that the curves obtained were not exact indexes of the differences in rate of reaction a t the various temperatures used, because the depth of penetration of adequate supplies of oxygen was not constant. Computation of nominal oxygen uptake rates per gram of sample from his curves gave rates of 1.81 ml. per gram per hour a t 130" C. and 0.62 a t 110" C. Reference to Table I shows that
8
HOURS
Figure 7. Oxygen uptake of samples containing antioxidant DYNH-1, blend C. 160'" C . 0 5
0
I n order t o circumvent this limitation due to slow diffusion rates, it is often feasible t o reduce the film thickness below a certain critical value, so t h a t chemical reaction, rather than diffusion, will be the controlling factor. The conditions which tend
10
PERCENT ANTIOXIDANT
Figure 8.
Effect of antioxidant on induction period
DYNH-1, blend C. 1 6 0 O C.
October 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
these rates are considerably less than the corresponding absorption-per-mass rates obtained in the present investigation. The agreement is better at the lower temperature, as expected from diffusion theory. SUMMARY
2205
LITERATURE CITED
Barrer, R. M., “Diffusion in and Through Solids,” p. 419, Cambridge University Press, Cambridge, 1941. Bateman, L., and Gee, G., Proc. Roy. Soc., A195, 376 (1949). Biggs, B. S., Natl. Bur. Standards, Circ. 525, 137 (1953). Biggs, B. S., and Hawkins, W. L., Modern Plastics, 31, No. 1, 121 (1953).
Oxygen uptake measurements on polyethylene resin a t approximately 1-atm. pressure of oxygen have revealed the following facts. The oxidation reaction has a pronounced induction period, followed by a constant-rate stage and eventually by a gradual decrease in rate. The energy of activation for the constant-rate stage of the reaction is 35 kcal. per mole in the range from 110’ t o 160’ C. The energy of activation for the reaction occurring during the induction period is 25 kcal. per mole in the same range. The induction period can be greatly increased by the addition of antioxidants, the amounts of the increase being proportional t o the concentration of the antioxidant. The reaction is severely diffusion-limited, and care must be taken to obtain rates proportional to sample mass rather than surface area. ACKNOWLEDGMENT
For permission to publish this work the author is grateful to the Bakelite Company, a Division of the Union Carbide and Carbon Corp. The author also wishes to mention the contribution of June Stershic, who carried out most of the oxygen uptake measurements.
Blum. G. W.. Shelton. J. R.. and Winn, H.. IND. ENG.CHEM.,
43,464 (1951).
Bolland, 5. L., Proc. Roy. Soe., 186, 230 (1946). Bolland, J. L., Trans. Faraday Soc., 46, 358 (1950). Bolland, J. L., and Gee, G., Ibzd., 42,244 (1946). Bolland, J. L..and Ten Have, P., Dascussions Faraday Soe., No. 2,252 (1947). Bolland, J. L., and Ten Have, P., Trans. Faradag Soc., 45, 93 (1949).
Booser, E. R., and Fenske, M. R., IND. ENG.CHEM.,44, 1860 (1952).
Brook, J. H. T., and Matthews, J. B., Discussions Faraday Soc., No. 10, 302 (1951).
Dufraisse, in Davis and Blake’s “Chemistry and Technology of Rubber,” Reinhold, New York, 1937. Hinshelwood, C. N ., “Kinetics of Chemical Change in Gaseous Systems,” 3rd ed., p. 227, Clarendon Press, Oxford, 1933. Mulcahy, M. F. R., Trans. Faraday Soc., 45, 576 (1949). Myers, C. S., IND. ENG.CHEM., 44, 1095 (1952). Robertson and Waters, Trans. Faraday Soc., 42, 201 (1946). Shelton, J. R., Am. SOC.Testing hIaterials, Spec. Tech. Pub., 89,12 (1949).
Shelton, J. R., and Cox, W. L., Rubber Chem. and Technol., 26, 632 (1953).
Shelton, J. R., Wherley, F. J., and Cox, W. L., IND.ENO. CHEM.,45,2080 (1353). Shelton, J. R., and Winn, H., Ibid.,38, 71 (1946). Wilson, J. E., J . Chem. Phyco., 22, 334 (1954). RECEIVED for review February 9, 1955.
ACCEPTED April 25, 1955.
Diffusion Coefficients in Hvdrocarbon Systems J
n-Heptane in Gas Phase of Methane-n-Heptane System L. T. CARMICHAEL, H. H. REAMER, B. H, SAGE, AND W. N. LACEY California Institute of Technology, Pasadena, Calij,
M
ATERIAL transport in the gas phase by molecular diffusion has been the subject of many investigations. Maxwell ( 1 2 ) and Stefan (21-24) carried out much of the early work and laid a surprisingly satisfactory basis for describing the behavior where the simple kinetic theory is applicable. Chnpman and Cowling (8) extended the treatment and included the effects of composition on the diffusion characteristics of the components of gaseous mixtures. Sutherland ( 2 5 ) proposed a rather satisfactory empirical expression describing the effect of temperature and molecular weight on the transport characteristics of the components of a gas phase a t low pressure. Sherwood and Pigford (20) reviewed some concepts of molecular diffusion and outlined the application of such processes to the prediction of absorption and extraction operations. Babbitt discussed the work of Maxwell and Stefan and presented a treatment of diffusion from their standpoint (1, 2 ) . Jost reviewed the status of diffusion in solids, liquids, and gases (10). Kirkvood and Crawford (11)presented the basic transport characteristics of homogeneous systems. Schlinger (17‘) considered the transport of one component through an essentially stagnant gas with fugacity and with partial pressure as the potential and reviewed the literature concerning
the experimental measurements of molecular diffusion of gases. The present discussion is concerned with the measurements of the Maxwell diffusion coefficients for n-heptane in the gas phase of the methane-n-heptane system a t temperatures from 100’ to 220” F. and a t pressures from 14 to 60 pounds per square inch absolute. Throughout this discussion and in the figures all pressures are presented in pounds per square inch or pounds per square foot absolute. METHODS AND APPARATUS
The methods used in this study involved an adaptation of the classical diffusion cell of Stefan (21). A schematic drawing of t h e apparatus constitutes Figure 1. The gas phase filled diffusion cell A and the n-heptane was introduced as a liquid in chamber B. It entered the lower part of cell A through fritted glass disk C. Thermocouples at D and D’were provided in order to determine the temperature of the upper surface of the fritted-glass disk. An electric heater a t E was provided t o maintain the temperature of the disk substantially the same as that of the gas in the d 8 u sion path. Steady-state condition was determined by the constancy of the capillary depression of the a-heptane liquid in t h e glass disk.
