I
A. RUDIN, H. P. SCHREIBER, and M. H. WALDMAN Canadian Industries Ltd., Central Research Laboratory, McMasterville, Quebec, Canada
Measurement of Polyethylene Oxidation
. . . by
Differential Thermal Analysis The oxidation resistance of other polymers and rubbers at processing temperatures can also be evaluated by this fast, convenient technique
T H E THER\I.+L STABILITY of polyethylene under fabrication conditions is assessed by various methods ivhich generally involve the use of full scale or simulated processing equipment. There is, hoxvever, no completely satisfactory, rapid laboratory method for follo\ving the reaction betiveen polyethylene and oxygen. Ll'hile most methods produce accurate data under some conditions. they become generally too slow or cumbersome to operate a t the temperatures a t \vhich the polymer is usually processed. .%mong widely used
methods is the measurement of the volume of oxygen absorbed by unit weight of polyethylene a t fixed temperature and pressure. This technique, which is essentially similar to that used by Shtlton ( 8 ) for rubber, has been employed by Lt'ilson (10) and Biggs and Hawkins ( 3 ) and others. .4n oxygen manometer is balanced manually and reduction of the gas volume is recorded as a function of time. Oxygen uptake can also be estimated by measuring carbonyl content, by infrared spectroscopy or by changes in poiver factor ( 7 ) .
Baum ( 7 ) has measured rates o i temperature rise in polyethylene exposed to oxygen and reported results in general agreement with those from the direct measurement of the amount of oxygen reacted. A gravimetric method, in which the weight gain of pol>ethylene exposed to oxygen is measured. has also been used ( 4 ) , as has chemical measurement of peroxide content (2). The present investigation was prompted by the possibility that differential thermal analysis could be useful in the relatively unexplored tem-
REFERENCE TC.
FREEZING OF OXIDIZED POLYETHYLENE
c
? L MIN. ( c w TILPVEL)
L
OXVGEN ADMITTED TO CELL
/---- - -'
--$
/< -MELTING
/
T
This diagram shows details of the differential thermal cell, temperature sensing arrangement, and gas delivery equipment used in measurements of resistance to oxidation of polyethylene
REFERENCE THERMOCOUPLE
30pV. TEST SIGNAL
OF FRESH POLYETHYLENE
THERMOCOUPLE
A typical thermogram obtained when polyethylene is melted, exposed to oxygen at elevated temperatures, and frozen following exposure VOL. 53, NO. 2
FEBRUARY 1961
137
The differential thermal method for measuring degree of oxidation of polyethylene was found to be fast and reproducible.
0 At comparable temperatures, results agreed satisfactorily with those of other, older methods. The advantage of the present technique lies in its fast response and ready modification to operate at the temperatures and pressures used in polyethylene processing.
A comparison of melting and freezing thermograms before and after oxidafion provides some indication of the extent to which the polymer has been damaged.
perature region where the foregoing methods are at a disadvantage. It also seemed possible, in the present case, to heat polyethylene in an inert atmosphere and then measure its stability in oxygen a t the test temperature. eliminating oxidation effects during the preheat period. It was hoped that a com-
parison of the melting curve of the plastic before oxidation and its freezing curve after exposure to oxygen. both of which could be measured along with the main experiment, would provide a useful indication of the degree of physical damage which the material had suffered. It also seemed likely that the use
of powdered polyethylene mixed with an inert diluent in the differential thermal apparatus would prevent the oxidation reaction from becoming diffusion controlled. This latter difficulty plagues some of the tests described above, although it can be avoided (70). The following article is intended mainly to summarize the suitability of differential thermal analysis (d.t.a.) techniques for measuring oxidative resistance of polymers, particularly polyethylene. Apparatus
The apparatus is shown schematically (page 137, left). Stainless steel cups formed the reaction cells. Bare chromelalumel thermocouples were used in this work. It would be a wise precaution to use shielded thermocouples in work of this nature, but unshielded couples were used for convenience here, since comparison of these results with published data showed no evidence of metal catalysis of the oxidation reaction. Thermocouple signals were registered on a Brown-Honeywell two-pen recorder. One pen recorded the temperature of the reference cell and the other the temperature difference betlveen the sample and reference cells. The latter signal was amplified by a Liston-Becker amplifier. set to the required sensitivity
ANTIOXIDANT ADDITION The experiments reported here were confined to a commercial antioxidant, 4,4’-thiobis-(3-methyl-6-tertbutylphenol) (Santanox). This table shows the results, in terms of induction times at different temperatures, of adding this antioxidant to a low and intermediate density polyethylene. It i s evident that antioxidant efficiency decreases drastically with rising temperature, at temperatures higher than about 150” C. The most likely reason for this i s volatility of the antioxidant although a rise in temperature would, of course, shorten the induction period to some extent in any case. These experiments were performed at ambient pressure, whereas polyethylene extrusion and injection moulding at these temperatures generally occurs at much higher pressures. It i s possible that the volatilizaiion loss of antioxidant i s less severe at these higher pressures, and therefore the present finding cannot be applied directly to normal processing. These results do indicate, however, that some antioxidant is likely to b e lost as the polymer extrudes into the atmosphere in a commercial extrusion process. Whether or not these losses are significant will depend on the antioxidant’s volatility and compatibility with molten polyethylene and the extruder temperature and cooling rate after extrusion. While there i s no need to exaggerate the danger of anfioxidant losses from this cause, it could be important
138
INDUSTRIAL AND ENOINEERINOCHEMISTRY
in some applications io know whether polyethylene articles are still protected b y antioxidant after extrusion. The authors are not aware of any data bearing on this problem although it i s evident (hat antioxidants will differ in ihis respect.
Effect of Antioxidant Is Rapidly Determined by D.T.A. Method at Elevated Temperatures Activation Energy of Induction Induction Santanox Concn., Temp., Time, Time, Resin % (w./w.) C. I f i n , Kcal./Mole “D” 24.2 methyl 0 170 12 24 groups per 1000 C 0 190 0 atoms 0 210 0 0.1 170 48 26 190 13 0.1 0.1 210 4 18.3 methyl groups per 1000 C
‘6C”
atoms
0 0 0 0.05 0.05 0.05
150 170 190
150 170 190
44 12 3 89 23 1
27 29
POLYETHYLENE OXIDATION Method The reference cell was loaded with powdered borosilicate glass which had been dried overnight in a forced draft oven a t 110' C. Glass, 400 mg., was used in the reference cell in each experiment. A 10 I O 1 glass-polyethylene mixture was placed in the sample cell. T h e polyethylene used was a powder produced by sawing a molded plate or grinding in a Wiley mill. Although the two methods do not produce equivalent particle sizes, the measured oxidation resistances were not sensitive to the method of specimen preparation. probably because the experiments were conducted with molten polymer in a dilute system. The polyethylene and glass were mixed lvith a mortar and pestle, and 200 mg. of the mixture was placed in the sample cell. T h e polyethylene-oxygen reaction was not diffusion controlled under these experimental conditions, since induction periods for 20 to 1 glass polyethylene mixtures were identical with those measured Lvith the normal 10 to 1 mixture. T h e induction period \vas more prolonged with a 5 to 1 mixture, when access of oxygen to the polymer apparently became rate determining. After loading both cells, the lid was screwed into the block and the apparatus was flushed thoroughly with nitrogen.
I
2
3 4 567eso
20
3040
amioo
150
TIME(MIN.)AFTER ADMISSION OF OXYGEN
A typical plot of differential temperature vs. time under oxygen for polyethylene Broken lines extrapolated to abscissa give value of induction period for rapid oxidative degradation
CARBON BLACK A few measurements were made of the effect of relatively fine particle size channel black (Monarch 74, Godfrey L. Cabot Inc.) on the resistance of a low density polyethylene to oxidation, with and without added Santanox antioxidant. The results are shown a t right. Carbon black retards oxygen uptake (3). The stabilization by carbon black i s particularly marked in solid polyethylene (6). The present data confirm the aniioxidant effect of Monarch 7 4 carbon black. The protection from the channel black is quite marked, even in the melt, at temperatures lower than 200" C. At 170" to 21 0" C. the normal (2.570)concentration of Monarch 7 4 actually conferred more protection than 0.1% Santanox (compare tables on pages 138 and 139) although this comparison may not b e valid a t higher pressures, such as those in an extruder, where loss of the antioxidant may be less. The synergistic effects of channel black and this antioxidant (5)are also evident in the data in table (right). The synergism i s most marked a t the lowest temperature studied, 170" C.; it i s likely that some antioxidant was lost b y evaporation at higher temperatures. Lowered antioxidant volatility, because of binding of the antioxidant to carbon black, might play a
part in the synergistic activity of this particular combination.
These Data Show the Effects of Carbon Black and Antioxidant Addition on the Oxidative Stability of Polyethylene
Temp., Material
O
c.
Induction Time, Rlin.
Activation Energy of Induction Time, Kcal./Jlole
Sample D
150 170 190 210
32 12 0 0
24
D plus 2.5%
170 190 210
21
32
D plus 0.1% Santanox antioxidant
170 190 210
59 20 0 48 13 4
D plus
170 190 210
170 30 9
Monarch 74 channel black
0.1%
Santanox and 2.57, Monarch 74
VOL. 53, NO. 2
26
FEBRUARY 1961
139
Table I. Induction Times from Differential Thermal and Oxygen Uptake Measurements Are in Reasonable Agreement
Temp.,
c.
140 150 160 170
D.T.S. for DYNH-3 Run 3 , Min. >300 32
...9 . 5
Activation Energies of Induction Periods in Oxidation of Polyethylene Comparison with published figures shows satisfactory agreement Activation Enern.
Material Marlex 50"
Oxwen Uptake for DYNH-1
Blend B (IO), 1Iiu.
300 90 46
...
A slow stream of nitrogen, indicated by a bubble tube, \vas maintained during the heating period. The melting thermogram of the polymer was recorded as the furnace and block were warmed to the required temperature. When the block and furnace were at the same temperature and the differential thermocouple indicated a steady base line. the nitrogen stream was turned off. The vacuum pump was operated for about a minute to flush nitrogen from the cell block and gas delivery train and oxygen was then admitted to the system a t a sloiv. steady rate. Since the lid and block were not air-tight the apparatus was essentially under one atmosphere of oxygen. Reference and differential temperatures Xvere recorded automatically as the oxidation reaction proceeded. .4 typical reaction thermogram is shown on page 137, right. The melting and freezing thermograms before and aftei oxidation are also shown in this figure. Sonie estimate of the extent the polymer has been altered by oxidation can be made from a comparison of melting and freezing temperatures or areas under the corresponding thermograms, if care is taken to heat and cool the cell at a standard rate. This follows from the fact that structural changes due to oxidation affect the ability of the polymer to crystallize and reduce the energy change associated with fusion or freezing. This estimate will not necessarily correlate with results of other measurements of oxidative change, such as variation of tensile properties ( J ) . The primary data were sometimes replotted with the temperature difference between cells as a function of exposure time to oxygen. A typical plot is shown on page 139. This curve is similar to those obtained in oxygen uptake studies. where the intercept on the time axis of the extrapolated rapid part of the reaction is usually recorded as the induction period. The same procedure was followed in this work. The standard deviation of the mean of two replicate measurements of induction times was less than 2 minutes in the experiments reported here.
1 40
Table II.
KcaP IIole 33 26
DYNH-3, Run 3b DYNH-1, Blend B*
23.5 25
DYNKb
21.6
Low pressure, linear polyethylene.
d.t.a. 0 2 uptake of thin molded slab d.t.a. 02 uptake of polyethylene powder 02 uptake of thin molded slab
Reference
150-190 80-140
This paper
150-190 110-160
This paper
(6) (10)
80- 140
(6)
High preasure, branched polyethylene.
Comparison with Results of Other Methods
Induction periods measured by differential thermal analysis for polyethylene DYNH-3 are roughly the same as those reported by \Yilson (10) for DYNH-1 Blend B (Table I). Both materials are polyethylenes from the same source and with about the same degree of branching. Closer agreement than this should not be expected since the two methods used different properties of the reaction system to indicate the beginning of rapid oxidation. and the polymer samples \yere not identical. Activation energies of induction times are compared Lvith literature values in Table 11. The agreement is satisfactory. considering the differences in materials and experimental temperature ranges. The lower values of activation energies calculated from the oxygen uptake measurements may perhaps reflect some diffusion control of the reaction in these experiments (6). The activation energy for slow oxidation of DYNH-1 Blend B during the induction period was reported by Wilson (70) from oxygen uptake measurements a t 110' to 160' C. to he 34 kcal. per mole. Differential thermal analysis of the reaction of DYNH-3 Run 3 a t 140' to 170' C. resulted in a corresponding activation energy of 33 kcal. per mole. I t seems likely, from the foregoing comparisons, that differential temperature measurements provide a reliable means for following the reaction between oxygen and polyethylene. Unstabilized Polyethylenes Most of the experiments reported here were performed a t 150' to 210' C. because this is the usual temperature range for polyethylene processing and use of the differential thermal method is particularly convenient in this region, A few trials \$ere made at 140' C. for comparison with published figures. as summarized above. I n these latter experiments, with polyethylenes containing 25.5 methyl groups per thousand carbon atoms, no induction time could generally be assigned ; oxidation pro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Temp. Range, O C.
Method
Table Ill. Induction Periods Vary with Branching Frequency, but the Effect Is Very Temperature Sensitive Sample A B C D E 1 I e t h y l Groups per I O U 0 Temp.,
c.
150 170 190
