Durability of Macromolecular Materials - American Chemical Society

5 Long-Term Photo-and Thermal Oxidation of Polyethylene

Downloaded by UNIV OF QUEENSLAND on April 27, 2016 | http://pubs.acs.org Publication Date: April 2, 1979 | doi: 10.1021/bk-1979-0095.ch005

H. M. GILROY Bell Laboratories, Murray Hill, NJ 07974

In this work, the expression "long term" requires defining. For photo-oxidation it refers to natural outdoor exposure up to 38 year duration and artificial accelerated weathering up to two years. In the case of thermal oxidation samples have been maintained at constant temperature for up to seven years. PHOTO-OXIDATION The materials examined in this study include both low den sity polyethylene (LDPE) and high density polyethylene (HDPE) protected by carbon black. These polyethylenes represent sev eral different generations of polymers exposed at various times, the greater bulk of the samples having been exposed from 1946 through 1951. The very early work on the photo-oxidation of polyethylene was carried out in accelerated weathering machines and the compounds selected for outdoor exposure were generally based on the results obtained from accelerated weathering. In 1950 Wallder, et al.(1)established that 100 hours accelerated weathering was equivalent to one year of natural out door exposure for LDPE containing 1% or more of a well dispersed carbon black. Longer exposures outdoors have shown, (2) however, that the relationship is not linear due to a complex relation ship between the photo-initiated and thermal degradation pro cesses that occur under two different sets of conditions. EXPERIMENTAL The compounds examined in this work contained channel carbon blacks varying in size from 9 to 28 mμand furnace blacks from 20 to 78 mμ. The polyethylenes used represented the best commer cial material available at any given time for cable jacket use. Outdoor samples were exposed at three locations in the United States; New Jersey, Florida and Arizona at a 45° eleva tion facing due south. Accelerated weathering was carried out in a modified X1A 0-8412-0485-3/79/47-095-063$05.00/0 © 1979 American Chemical Society Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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DURABILITY OF MACROMOLECULAR MATERIALS

Atlas Weather-Ometer described i n reference 2. Samples were exposed 50 cm from a bare carbon arc (no f i l t e r s ) and with a 90° sector immediately preceeding the water spray blocked off by a metal baffle to simulate night conditions. Evaluation of mechanical damage due to photo-oxidation was monitored by changes i n low-temperature brittleness per ASTM D746 and by decrease i n percent elongation as determined i n a standard tensile strength test. Selected samples were examined for changes i n bulk oxidative s t a b i l i t y by thermal analysis where a specimen i s heated i n an oxygen atmosphere and the time to onset of the exotherm due to oxidation measured. The development of surface oxidation was monitored i n several materials by internal reflectance infra-red techniques described i n reference 3. Melt Index measurements were made per ASTM D1238, Condition

E. Thermal analysis measurements were made using a duPont 990 Thermal Analyzer using aluminum pans and a heating rate of 10°C/ min. Melting endotherms were determined i n the presence of helium. Oxidative induction times were measured isothermally at 200°C i n the presence of oxygen. A detailed description of this technique i s given i n reference 4. WEATHERING RESULTS Low-Density Polyethylene. The data from outdoor weathering are summarized i n Figure 1. The low-temperature brittleness (LTB) measurements show that a well-dispersed carbon black of > 1% concentration and < 35 millimicrons (my) particle size i s necessary for maximum resistance to photo-oxidation. The results from elongation measurements agree with those found for LTB, as would be expected, since both measurements reflect the notch sensitivity of polyethylene to micro cracks caused by photo-oxidation. Accelerated weathering studies on these compounds have been reported previously (2) and indicate the same ranking as found i n outdoor exposure. The data shown i n Figure 1 are representative of many samples containing channel or furnace type carbon blacks and for a given particle size above 1% concentration both blacks afford equal protection. Examination of the surface of these well protected weathered polyethylenes by internal reflectance infrared spectroscopy (3) shows that surface oxidation, as measured by carbonyl formation, occurs within a few months, but after about a year of outdoor exposure further increase i n surface carbonyl i s very s l i g h t . However, although rapid oxidation i s essentially confined to the surface other changes do occur i n the bulk material as time of exposure increases. The melt index of the polyethylene decreases, indicating that crosslinking i s occurring (Table I ) . This i s a result of thermal oxidation of the bulk material, as

Eby; Durability of Macromolecular Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Oxidation


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Ο

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20

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NATURAL OUTDOOR WEATHERING TIME (YEARS) Figure 1.

Change in low-temperature brittleness for low-density polyethylene protected by various carbon blacks


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the black polyethylene heats up outdoors (polymer temperatures up to 70°C have been recorded(5)).


TABLE I Melt Index Change Produced by Outdoor Weathering LDPE with 2.5% Channel Carbon Black Year Exposure Outdoor, Arizona Melt Index 0 2.0 2 1.7 6 1.6 10 1.3 15 1.1 Annealing effects due to this heating on exposure also occur, and can be seen i n LDPE as a form of secondary crystallization when examined by thermal analysis. Figure 2 shows the presence of a large shoulder on the melting endotherm, representing mat e r i a l that has recrystallized during outdoor exposure. The exposed compounds also show a slow decrease i n r e s i s tance to thermal oxidation at elevated températures» when measured by conventional techniques of oxygen uptake or thermal analy s i s . Table II shows typical thermal analysis data on samples weathered i n New Jersey for 10 years. Similar measurements on compounds exposed for 30 years show that some thermal oxidative s t a b i l i t y s t i l l remains i n the bulk material. TABLE II Decrease i n Thermal Oxidative S t a b i l i t y During Outdoor Weathering of LDPE With 2.5% Channel Carbon Black Outdoor Exposure, N . J . Oxidative Induction Timed) Years Minutes at 200°C (2) (3) 0 66 6 S O 10 25 22 11 30 8 (1) (2) (3)

Measured by Differential Scanning Calorimetry Stabilized with 0.1% phenolic antioxidant

Stabilized with 0.2% amine antioxidant High Density Polyethylene. Extrusion grades of high-density polyethylene protected by 2.5% channel type black of 19 my p a r t i cle size were exposed i n a Weather-O-Meter for 10,000 hours without any evidence of serious mechanical damage, as measured by low temperature brittleness and elongation tests. This represents the useful limit i n our accelerated weathering test, since as described i n an earlier paper,(2) f i l m - l i k e deposits form on



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10 YEARS OUTDOORS.N.J. UNEXPOSED CONTROL

I 200

I 150

Figure 2.

ι 100 TEMPERATURE (°C)

ι 50

I 0

Annealing of low-density polyethylene after ten years outdoors in New Jersey. Thermal analysis heating rate of 10 C/min.



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the surface and after about 8000 hours weathering effectiveness i s l o s t . However, i f we apply the correlation obtained between accelerated and natural outdoor exposure of low-density poly ethylene^) to high-density polyethylene then these high-density polyethylenes w i l l r e s i s t photo-oxidation for w e l l i n excess of forty years. Outdoor exposure of these high-density samples for the l a s t 18 years caused no substantial change due to surface oxidation. Bulk changes similar to those found i n low-density polyethylene were also found i n these high density materials. The melt index slowly decreases and the overall thermal s t a b i l i t y measured at high temperature by thermal analysis decreases. However, the secondary c r y s t a l l i z a t i o n seen i n low-density polyethylene was not detected i n these high density compounds. Thermal Oxidation. The majority of materials tested i n t h i s phase of the program were i n the form of wire insulation (22-26 AWG) but the factors involved i n the thermal oxidation of wire insulation have been shown to be the same as those that occur i n films and sheets, etc. That i s , the polymers rapidly lose sta b i l i z e r , oxidize and mechanical f a i l u r e occurs. Extensive studies of polyolefin thermal oxidation at eleva ted temperatures (above the melt) have been reported, but extra polation of these data to lower temperatures (below the melt) have led to over optimistic estimates of the l i f e - t i m e of these polyolefins.C4) In order to obtain more r e a l i s t i c values for polyolefin l i f e t i m e s , samples were tested at temperatures from 110OC to 40°C at 10°C intervals. Small, individual s t a t i c a i r ovens were used to i s o l a t e individual types of samples and pre vent cross contamination. Samples were maintained at constant temperature and moni tored u n t i l mechanical f a i l u r e due to thermal oxidation occurred. Decrease i n oxidative s t a b i l i t y was followed by thermal analysis and i n i t i a t i o n of oxidation by infrared detection of carbonyl species. Failure was defined as the appearance of cracks pro duced by periodic mechanical stressing. The time to mechanical f a i l u r e due to thermal oxidation for several non-black samples i s shown i n Figure 3. Here i t i s seen that the high-density polyethylene i s more resistant to thermal oxidation than the low-density material. This can be related to the rate of loss of antioxidant, which i s l o s t more slowly from the high density polyethylene. Since these are wire insulation samples i n contact with copper, the addition of a metal deacti vator would further increase their longevity. In actual use, the c a t a l y t i c oxidative degradation of poly olef ins i s controlled, to a large extent, by the additives and contaminants present i n the polymer. Although the s t a b i l i t y of polyethylene i s related i n i t i a l l y to the antioxidant concentra t i o n and type, upon aging the c r i t i c a l factor i s the rate of loss of the antioxidant. Figure 4 shows t y p i c a l examples of effective s t a b i l i z e r loss, due to migration on aging. The samples, shown



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Figure 3.

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Polyethylene

Mechanical failure caused by thermal oxidation. Both samples stabilized with 0.1% phenolic antioxidant.




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30,

800 DAYS AT CONSTANT TEMPERATURE Figure 4.

Loss of effective stabilization in low-density polyethylene. time measured by differential scanning calorimetry.

Induction



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i n Figure 4, are low density polyethylene wire insulation heated i n s t a t i c a i r ovens. They contained an i n i t i a l concentration of 0.1% phenolic antioxidant and 0.1% metal deactivator. The loss of effective s t a b i l i z a t i o n i s predominantly physi c a l , not chemical, as i d e n t i f i c a t i o n of s t a b i l i z e r on the surface of aged polyolefins has shown.(6,2) The concentration of a n t i oxidant dissolved i n the polymer and the resulting oxidative s t a b i l i t y decrease with time and approach equilibrium for the temperature of concern. The entire process i s complex, with s t a b i l i z e r s o l u b i l i t y only a few parts per m i l l i o n at room tem perature (8) and d i f f u s i o n of s t a b i l i z e r from the polymer rapid. At higher temperatures (40-70C) both s o l u b i l i t y and d i f f u s i o n increase with opposite effects on s t a b i l i z e r retention and more rapid loss i n oxidative s t a b i l i t y . Impurities and contaminants can also affect the l i f e t i m e of polyolefins, generally i n an adverse manner. The adverse effect of copper on the long-term s t a b i l i t y of polyolefins i s w e l l known. (9,10) Other active metals have also been shown to accel erate the oxidation process.(11, 12) Additives can also affect the longevity of polyethylene. Processing aids, such as polypropylene or low molecular weight polyethylene decrease the long-term s t a b i l i t y of wire insulation. Pigments can also be detrimental. Titanium dioxide has been shown to interact with phenolic type antioxidants(13) and sever a l investigators(14,15) have shown titanium dioxide and some red pigments(16) to be i n f e r i o r to other colors, insofar as long term thermal oxidative s t a b i l i t y i s concerned. One additive that improves both long-term photo and thermal s t a b i l i t y of polyethylene i s carbon black. The a b i l i t y of car bon black to retard destructive thermal oxidation i n polyethy lenes at elevated temperatures i s w e l l known(17) and, as was seen i n the e a r l i e r section of this paper on photo oxidation, i t i s effective at lower temperatures also. Our studies of the thermal oxidation of low-density polyethylene show black samples to be outstanding, even i n the presence of copper. For example black low-density polyethylene wire insulation i s s t i l l intact after 7 years at 80°C while a l l other colors, including unpigmented, f a i l e d mechanically due to oxidation after only about 3 months· Mechanical f a i l u r e during the thermal oxidation of nonblack polyethylene changes during the oxidative process. Samples oxidize slowly with l i t t l e change i n mechanical properties u n t i l the onset of autocatalytic oxidation. During the early part of t h i s rapid oxidation both scission and crosslinking occur and mechanical f a i l u r e of the material can be induced by s t r a i n or impact. As thermal oxidation progresses, crosslinking predomin ates and mechanical properties improve u n t i l , as the material approaches 30-50% gel (as measured by extration i n b o i l i n g xylene) the polymer becomes highly resistant to mechanical f a i l ure. Upon further oxidation the gel content nears 70-80% and



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150

LU 3

DAYS AT 80°C Figure 5.

Mechanical failure during thermal oxidation of wire insulation at 80° C



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crosslinking stops. Then scission again predominates since there are very few sites left to crosslink and mechanical failure again occurs. Data for this sequence of events for wire insulation at 80°C is shown in Figure 5. The entire process is faster at higher temperatures and slower at lower temperatures. Black low density polyethylenes have been found to behave differently during thermal oxidation at low temperatures (80 to 40°C). The black samples oxidize and crosslink but they do not fail mechanically, as does non-black LDPE. Hawkins(17) has shown the greater thermal stability of black polyethylene as compared to non-black polyethylene in the solid state. Other in vestigators (18,19) have demonstrated the activity of carbon black as a free radical trap and others (20) have suggested that polymer radicals form carbon-polymer bonds with the carbon black. The exact reason why the black samples do not fail mech anically even though they oxidize and crosslink is not clear and any one, or a combination of a l l the above factors could be the cause. It appears that the lack of mechanical failure after oxidation indicates very l i t t l e or no scission during the oxi dative process and finally no further oxidation after the devel opment of the *v* 70% gel found in oxidized samples. ACKNOWLEDGMENTS In a study as extended as the one reported here many indi viduals are necessarily involved. The author gratefully acknowledges a l l who contributed, and particularly G. F. Brown, M. G. Chan, J . B. DeCoste, L. Dorrance, C. R. Glenn, J . B. Howard, L. Johnson, V. J . Kuck, L. G. Rainhart and V. T. Wallder. LITERATURE CITED 1. Wallder, V. T., Clarke, W. J., DeCoste, J . B. and Howard, J. B., Ind. Eng. Chem., (1950), 42, 2320. 2. Howard, J. B. and Gilroy, Η. Μ., Poly. Eng. & Sci., (1969), 9, (No. 4), 286. 3. Chan, M. G., and Hawkins, W. L., Poly. Preprints, (1968) 9, (No. 2), 1938. 4. Howard, J . B., Poly. Eng. & Sci., (1973), 13, (No. 6), 429, 5. Gardner, B. L. and Papillo, P. J., Ind. Eng. Chem. Prod. Res. Dev., (1962), 1, 249. 6. Heyward, I. P., Chan, M. G., Lewis, L . , 35th ANTEC, Soc. Plast. Eng., Montreal, (1977). 7. Bair, H. E., Poly. Eng. & Sci., (1973), 13, 435. 8. Roe, R. J . Bair, H. E., & Gieniewski, C., J . Appl. Poly. Sci., (1974), 18, 843. 9. Hansen, R. Η., Russell, C. Α., DeBenedictis, T., Martin, W. H. & Pascale, J . V., paper presented at 139th Meeting of ACS, 1961. 10. Hawkins, W. L. & Winslow, F. Η., Plast, Inst. (London) Trans. (1961), 29, 82.



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11. Shelton, J. R., J . Appl. Poly. Sci. (1959, 2, 345. 12. Tobolsky, Α. V. & Mesroliam, R. B., "Organic Ploxides" p. 101, Interscience, New York, 1954. 13. Holtzen, D. Α., Soc. Plast. Eng. Tech., Papers, (1976) 22, 488. 14. Pusey, B., Chen, M. and Roberts, W., Proc. 20th Int. Wire and Cable Symp., (1971), 209. 15. Howard, J . B., Proc. 21st Int. Wire and Cable Symp. (1972) 16. Gilroy, Η. Μ., Proc. 23, Int. Wire and Cable Symp., (1974), 42. 17. Hawkins, W. L. "Polymer Stabilization", p. 91, Wiley-Interscience, New York, 1972. 18. Donnet, J . B. and Henrich, G., Compt. Re., (1955) 246, 3230. 19. Watson, W. F., Ind. Eng. Chem., (1955), 47, 1281. 20. Szware, M., J . Poly. Sci., (1955), 16, 367. RECEIVED

December 8,

1978.


Durability of Macromolecular Materials - American Chemical Society

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